[IUR1,IUR2]Ian U. Roederer

IUR1]Department of Astronomy, University of Michigan, Ann Arbor, MI 48104, USA

IUR2]Department of Physics, North Carolina State University, Raleigh, NC
27695, USA

JAG]Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, 14482, Germany

CAP1, DA1]Instituto de Astrofísica de Canarias La Laguna, Tenerife 38205,
Spain

CAP2, DA2]Departamento de Astrofísica Universidad de La Laguna La Laguna,
Tenerife 38206, Spain

VA]Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP,
Rua das Estrelas, 4150-762 Porto, Portugal

PJA, CRL]Instituto de Astrofísica de Andalucía (IAA-CSIC), Granada, 18008,
Spain

EMAG]Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, 14482, Germany

MBA1]Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, 14482, Germany

MBA2]European Southern Observatory (ESO), Chile, Santiago, 19001, Chiles

SAB]Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, 14482, Germany

TB]Lund Observatory, Division of Astrophysics, Department of Physics, Lund
University, Box 43, 22100, Lund, Sweden

LB]Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange,
Nice, France

AC]Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange,
Nice, France

AD]Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange,
Nice, France

CJH]Goethe University Frankfurt, Institute for Applied Physics, Germany

SPJ]Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, 14482, Germany

AJK]Department of Physics and Astronomy, Uppsala University, Uppsala, 75120,
Sweden

SL]Osservatorio Astronomico di Padova, INAF, Padova, 35122, Italy

LM]Osservatorio Astrofisico di Arcetri, INAF, Firenze, I50125, Italy

RM]Kavli Institute for Cosmology, University of Cambridge Cambridge, United
Kingdom

PM]Osservatorio Astronomico di Trieste, INAF, Trieste, I-34143, Italy

AMa]Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze,
Sesto Fiorentino, Firenze, I-50019, Italy

JRM]Departamento de Física Teórica e Experimental, Universidade Federal do Rio
Grande do Norte, Campus Universitário Natal, 59078-970, Brasil

AM1]Dipartimento di Fisica e Astronomia “Augusto Righi”, Alma Mater Studiorum,
Università di Bologna, Bologna, 40129, Italy

AM2, LO, DR]Osservatorio di Astrofisica e Scienza dello Spazio, INAF, Bologna,
40129, Italy

NNa]Department, Organization, City, 100190, State, Country

CP]European Southern Observatory, Garching bei München, 85748, Germany

KP1]Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, 14482, Germany

KP2]Potsdam University, Institute for Physics and Astronomy, Potsdam-Golm,
14476, Germany

SN]Università degli Studi di Firenze, Dipartimento di Fisica e Astronomia,
Firenze, 50125, Italy

PT]Sorbonne Universités, UPMC Univ Paris 6 et CNRS, UMR 7095, Institut
d’Astrophysique de Paris Paris, France

KV1]Department of Physics and Astronomy, University of Victoria PO Box 1700,
STN CSC Victoria, BC, V8W 2Y2, Canada

GAW]Department of Physics and Space Science, PO Box 17000, Station Forces
Kingston, Ontario, K7K 7B4, Canada

AZ]Osservatorio Astronomico di Brera, INAF, Merate, Italy

# The discovery space of ELT-ANDES. Stars and stellar populations

<EMAIL_ADDRESS>Julián D. Alvarado-Gómez<EMAIL_ADDRESS>Carlos
Allende Prieto<EMAIL_ADDRESS>Vardan Adibekyan<EMAIL_ADDRESS>David
Aguado<EMAIL_ADDRESS>Pedro J. Amado<EMAIL_ADDRESS>Eliana M. Amazo-
Gómez<EMAIL_ADDRESS>Martina Baratella<EMAIL_ADDRESS>Sydney
A. Barnes<EMAIL_ADDRESS>Thomas Bensby<EMAIL_ADDRESS>Lionel
Bigot<EMAIL_ADDRESS>Andrea Chiavassa<EMAIL_ADDRESS>Armando
Domiciano de Souza<EMAIL_ADDRESS>Camilla Juul Hansen
<EMAIL_ADDRESS>Silva P. Järvinen<EMAIL_ADDRESS>Andreas J.Korn
<EMAIL_ADDRESS>SaraLucatello<EMAIL_ADDRESS>Laura Magrini
<EMAIL_ADDRESS>Roberto Maiolino<EMAIL_ADDRESS>Paolo Di Marcantonio
<EMAIL_ADDRESS>Alessandro Marconi<EMAIL_ADDRESS>José R. De Medeiros<EMAIL_ADDRESS>AlessioMucciarelli
<EMAIL_ADDRESS>Nicolas Nardetto<EMAIL_ADDRESS>LiviaOriglia<EMAIL_ADDRESS>CelinePeroux<EMAIL_ADDRESS>Katja Poppenhäger<EMAIL_ADDRESS>Cristina Rodríguez-López<EMAIL_ADDRESS>Donatella Romano<EMAIL_ADDRESS>Stefania Salvadori
<EMAIL_ADDRESS>Patrick Tisserand<EMAIL_ADDRESS>Kim Venn
<EMAIL_ADDRESS>Gregg Wade<EMAIL_ADDRESS>Alessio Zanutta
<EMAIL_ADDRESS>[  [  [  [  [  [  [  [  [  [  [  [  [  [  [  [  [  [
[  [  [  [  [  [  [  [  [  [  [  [  [  [  [  [  [

###### Abstract

The ArmazoNes high Dispersion Echelle Spectrograph (ANDES) is the optical and
near-infrared high-resolution echelle spectrograph envisioned for the European
Extremely Large Telescope (ELT). We present a selection of science cases,
supported by new calculations and simulations, where ANDES could enable major
advances in the fields of stars and stellar populations. We focus on three key
areas, including the physics of stellar atmospheres, structure, and evolution;
stars of the Milky Way, Local Group, and beyond; and the star-planet
connection. The key features of ANDES are its wide wavelength coverage at high
spectral resolution and its access to the large collecting area of the ELT.
These features position ANDES to address the most compelling and potentially
transformative science questions in stellar astrophysics of the decades ahead,
including questions which cannot be anticipated today.

###### keywords:

Star clusters (1567), Stellar atmospheres (1584), Stellar evolution (1599),
Stellar physics (1621), Stellar populations (1622), High resolution
spectroscopy (2096), Galactic archaeology (2178)

##  1 Introduction

The European Extremely Large Telescope (ELT) will be the world’s largest
optical and near-infrared (NIR) telescope once it is commissioned, and it will
retain that status for the foreseeable future. Design work is underway for the
ArmazoNes high Dispersion Echelle Spectrograph (ANDES), which will be the
workhorse optical and NIR high-resolution spectrograph for the ELT. ANDES is
poised to capitalize on major revolutions that will occur in the field of
stellar astrophysics in the decades ahead. These revolutions are enabled by
new views of the infrared sky from JWST, new cadences of the changing optical
sky from the Rubin Observatory, new enormous multiplexing capabilities of
instruments such as the Multi Object Optical and Near-infrared Spectrograph
(MOONS) and 4-meter Multi-Object Spectroscopic Telescope (4MOST), and other
observational facilities. ANDES will be the right instrument to build on these
discoveries by enabling new stellar physics experiments and probing stellar
populations in new environments in the 2030s and beyond.

Studies on the physics of stars, their interactions with exoplanets, and the
stellar populations of the Milky Way and external galaxies benefit enormously
from general-purpose high-resolution spectrographs that cover a broad
wavelength coverage. High-resolution ultraviolet (UV), optical, and NIR
spectrographs routinely serve a wide range of applications across all major
fields of astrophysics. Such instruments on UV-optical-NIR telescopes have led
to a steady stream of discoveries for generations.

The baseline design for ANDES includes three fiber-fed spectrographs that
simultaneously record wavelengths, $\lambda$, from roughly 0.40 to 1.80 $\mu$m
at a spectral resolving power, $R\equiv\lambda/\Delta\lambda$, of 100,000.
Design goals include extensions to the $U$ (0.35–0.41 $\mu$m) and $K$
(1.80–2.40 $\mu$m) bands, where the $K$ channel is contained within a separate
spectrograph, as well as an intermediate spectral resolving power of $R$ =
50,000. The ANDES design includes a two-fiber seeing-limited observing mode
(fiber diameter = 0.76” on sky) and a 61-spaxel fiber matrix mode (sizes
ranging from 5 to 100 mas/spaxel) that functions like an integral field unit
(IFU) fed by a single conjugate adaptive optics (SCAO) system for $\lambda>$
0.95 $\mu$m.

The design and capabilities of ANDES are described in greater detail in
Marconi et al [126]. The large collecting area of the ELT and the capable,
general-purpose ANDES design will play a transformative role in shaping
stellar astrophysics in the coming decades.

In this paper, we highlight key open questions in the field of stellar
astrophysics today where ANDES is poised to enable major advances. We focus on
three key areas: the physics of stellar atmospheres, structure, and evolution
(Section 2); individual stars and resolved stellar populations in the Milky
Way, Local Group, and beyond (Section 3), and the connections between stars
and their planetary systems (Section 4). Other papers in this series focus on
the transformative role that ANDES may play in advancing our understanding of
exoplanets and protoplanetary disks, galaxies and the intergalactic medium,
and cosmology and fundamental physics. We recognize that the most compelling
science cases of tomorrow may differ from the ones presented here. Yet these
science cases underscore the sustained demand from the community for a capable
high-resolution spectrograph on the ELT with broad wavelength coverage, and
the enormous discovery potential that ANDES will enable. All the calculations
made in the following sections have been made with the latest version of the
ANDES ETC111https://andes.inaf.it/instrument/exposure-time-calculator/ (v1.1).

##  2 The Physics of Stellar Atmospheres, Structure, and Evolution

###  2.1 Asteroseismology of stellar populations across the Milky Way and
beyond

Asteroseismology is the study of stellar oscillations. It can reveal
information about the interiors of stars that is otherwise inaccessible. It
also enables accurate stellar ages to be determined, providing a new dimension
of our view of the formation of our Galaxy and its accretion history [e.g.,
30, 137]. Previous asteroseismic studies performed with photometric space
missions, such as CoRoT, Kepler, and TESS [67, 112, and references therein]
were only able to observe relatively bright nearby main sequence stars or red
giants [e.g., 41]. These observations have been limited to a small fraction of
the stellar populations found throughout the Milky Way [e.g., 136, 206].

ANDES will be able to detect and measure small ($\sim$ 10–30 cm/s) Doppler
shifts of spectral lines. These oscillations offer the promise of being able
to determine ages accurately to $\approx$ 1–2 Gyr for large numbers of
individual field stars, which also has not been possible previously. ANDES
will also enable measurements of oscillations in much more distant main
sequence stars in the disk of our galaxy, faint halo stars, and red giants in
Local Group galaxies for the first time. The observed eigenmodes can be used
to determine the age of individual extragalactic stars, providing a unique
opportunity to date components of Local Group galaxies for the first time and
provide new constraints on galaxy formation mechanisms.

At the other end of the Hertzsprung-Russell (HR) diagram, asteroseismology
with ANDES will be pivotal in our understanding of the fundamental physical
parameters of M dwarfs, in particular mass and radius. Thermodynamic and
Solar-like pulsations have been predicted for young and main sequence M dwarfs
[175, 176], but they have not yet been detected (see Rodríguez-López 174 for a
review). The predicted pulsation periods are short, on the order of tens of
minutes, and the expected radial velocity amplitudes of oscillations are
small, below 1 m s-1. The intrinsic faintness of M dwarfs (Gaia $G_{\rm
BP}\gtrsim 10.0$ mag) presents an observational challenge that ANDES can
overcome, because it will be able to obtain time-resolved spectroscopic data
of M dwarfs. These measurements of line profiles and radial velocity
variations at a consistently high cadence and precision are necessary to
discover and characterize the expected pulsation modes of M dwarfs.

###  2.2 Backbone measurements of stellar activity and magnetism

In the context of late-type stars, stellar activity can be broadly summarized
as any observed manifestation of their magnetism. It underlies many of the
complex behaviors in those stars, including the relationships between internal
processes, such as convection and dynamos, with external ones, such as stellar
winds and angular momentum loss [74, 221, 93]. The stellar activity must be
characterized when searching for exoplanets. It is especially critical when
searching for Earth-mass planets orbiting Sun-like stars, because the lack of
such information can easily lead to false positives, and it currently
represents the limiting factor when pushing the radial velocity method to the
required precision limit at the cm s-1 level [78, 79, 151, 86]. Moreover,
several studies have demonstrated how activity in young stars ($t<100$ Myr)
can shape the stellar spectrum in ways we still cannot account for, affecting
the atmospheric parameters and the abundances we infer with a classic
spectroscopic analysis [227, 202, 16, 17]. Understanding which mechanism(s) of
activity dominate such alterations is pivotal.

ANDES has the potential to be the most powerful astronomical instrument to
collect data on stellar atmospheric activity and magnetic fields, because of
its broad wavelength coverage and the large collecting area of the ELT. Table
1 shows a number of current and proposed future instruments, some with the
capability to study stellar magnetic fields simultaneously in the whole
wavelength range with the most widely used activity indicators. Compared to
other facilities, ANDES excels because of its wide spectral coverage from the
blue to the red (0.35–2.4 $\mu$m). The collecting area of the ELT and spectral
resolving power of ANDES will provide the completeness required to study the
complex connection between stellar atmospheric activity and magnetic physics,
and they will allow us to simultaneously disentangle sensitive phenomena in
the blue and red parts of the spectrum. For example, magnetic features at
different atmospheric layers in the blue and red respond differently to
exoplanet transits [see 165, 166].

Instrument (Telescope) |  $\lambda$ ($\mu$m) | Res. | Ca iiH&K | H$\alpha$ | Ca iiIRT | BZB | SIM
---|---|---|---|---|---|---|---
ANDES (ELT 39 m) [baseline] | 0.40-1.80 | 100 k | ✗ | ✓ | ✓ | ✓ | ✗
ANDES (ELT 39 m) [goal] | 0.35-2.40 | 100 k | ✓ | ✓ | ✓ | ✓ | ✓
CARMENES (CAHA 3.5m) | 0.52-1.71 | 90 k | ✗ | ✗ | ✓ | ✓ | ✓
CRIRES+ (VLT 8.2 m) | 0.95-5.30 | 100 k | ✗ | ✗ | ✗ | ✓ | ✗
ESPRESSO (VLT 8.2 m) | 0.38-0.68 | 140 k | ✓ | ✓ | ✗ | ✗ | ✓
G-CLEF (GMT 25.4 m) | 0.35–0.90 | 108 k | ✓ | ✓ | ✓ | ✗ | ✓
GIANO-B (TNG 3.6 m) | 0.95-2.45 | 50 k | ✗ | ✗ | ✗ | ✓ | ✓
HARPS (ESO 3.6 m) | 0.38-0.69 | 120 k | ✓ | ✓ | ✗ | ✗ | ✓
HARPS-N (TNG 3.6 m) | 0.37-0.69 | 120 k | ✓ | ✓ | ✗ | ✗ | ✓
HIRES (Keck 10 m) | 0.30-1.00 | 85 k | ✓ | ✓ | ✓ | ✓ | ✗
NIRPS (ESO 3.6 m) | 0.95-1.80 | 100 k | ✗ | ✗ | ✓ | ✓ | ✓
PEPSI (LBT 2x8.4 m) | 0.38-0.91 | 250 k | ✓ | ✓ | ✓ | ✓ | ✗
UVES (VLT 8.2 m) | 0.30-1.10 | 80 k | ✓ | ✓ | ✓ | ✓ | ✗
VISION (CFHT 3.6 m) | 0.37-2.44 | 75 k | ✓ | ✓ | ✓ | ✓ | ✓
Number of ADS references: | 256 | 130 | 70 | 75 |
Table 1: Scope of capabilities comparison for different facilities to study
the most commonly used stellar activity indicators (the Ca ii H&K lines at
393.3 and 396.8 nm, the H$\alpha$ line at 656.3 nm, and the Ca ii NIR triplet
(IRT) lines at 849.8, 854.2, and 866.2 nm) and magnetic field measurements via
Zeeman Broadening (ZB) at $\sim$1$\mu$m. The second column lists the
wavelength range covered by each instrument. The third column lists the
spectral resolving power (Res.) in units of 1000 (k). The last column
indicates the simultaneity (SIM) in the observable ranges. The bottom row
lists the number of refereed publications in the Astrophysics Data System
(ADS) as of 2023 with an explicit mention of each activity indicator.

This potential could be only achieved through the implementation of the
proposed ANDES $U$ channel extension. The emission in the cores of
collisionally dominated Ca ii H&K Fraunhofer lines at 393.3 and 396.7 nm (Ca
iiH&K) constitutes the most consistent and reliable indicator of chromospheric
magnetic activity observable using ground-based instruments. This measure also
provides the longest observational time baseline [169, 24]. Ca iiH&K is also
well correlated to the coronal activity observed in X rays [see 225, 8, 191].
Other less sensitive activity indicators, such as the Ca ii infrared triplet
(Ca iiIRT) or H$\alpha$, have been used in the past, mainly due to the lack of
Ca iiH&K measurements. The Ca iiIRT lines can also be strongly affected by
changes in Teff and metallicity as shown by analyses of model spectra [see 10,
127]. In solar observations, the prominent Balmer series lines respond mostly
to the photosphere, exhibiting a broad scatter that correlates with sunspot
transits but not chromospherically sensitive lines [see 125, 42]. Similarly, a
strong scatter from H$\alpha$ is observed in Sun-like stars when compared with
Ca iiH&K lines and X rays [see 9]. Therefore, the proposed $U$ channel
extension to the baseline design of ANDES would enable these exciting new
measurements and robustly characterize stellar activity for the different
atmospheric layers across spectral types. It would enable the new measurements
of phenomena spanning timescales of seconds to decades, to be benchmarked
alongside legacy data from other facilities. It is no coincidence that this
activity indicator has had the largest impact on different sub-fields of
astrophysics, as shown by the number of references listed in Table 1.

ANDES will also offer the opportunity to study stellar magnetism directly in
unprecedented detail for stars all across the HR diagram. Among cool stars of
spectral types F to M, Zeeman Broadening (ZB) measurements use Ti i and FeH
lines around $\lambda\approx$ 1 $\mu$m [195, 170], while Zeeman
Intensification (ZI) measurements use a set of Fe i and Ti i lines spread
throughout the $\lambda\approx$ 0.42–0.85 $\mu$m range [107, 108]. Only
$\lesssim 400$ of those stars have measurements of their magnetic fields via
ZB or ZI methods, yet the expectation is that all such stars host dynamo-
driven magnetic fields of varying levels of intensity. With magnetic fields
being the main driver of the space weather environment of cool stars (i.e.
high-energy radiation, stellar winds, energetic transient events; see Drake et
al 52), any exoplanet-related observing programme on ANDES will allow direct
measurements of the stellar magnetic field. Apart from the information on the
field itself, this will enable a more realistic characterization of the
exoplanet habitability conditions, by placing constrains for models
investigating processes such as atmospheric escape and photon- and particle-
driven atmospheric chemistry [see e.g. 6].

Magnetic fields are also crucial actors affecting the observational properties
and evolution of intermediate- and high-mass stars [e.g. 50]. Those magnetic
fields are broadly considered to be “fossil fields,” which are slowly decaying
remnants from flux advection, mass transfer, or mergers at earlier phases of
evolution. The presence of magnetic fields in intermediate-mass main sequence
stars influences atmospheric microscopic diffusion processes, leading to
overall peculiar chemistry and nonuniform vertical and lateral distributions
of chemical abundances. Magnetic fields channel and confine stellar winds in
the hottest and most massive main sequence stars of spectral types B and O,
leading to efficient shedding of rotational angular momentum and mass-loss
quenching [157]. These effects fundamentally impact the evolution of the star,
including its subsequent supernova characteristics and remnant properties
[158, 104]. Studies of the magnetic properties of such stars, based on
resolving the wavelength separations of magnetically split lines, have mostly
been conducted using optical wavelengths [e.g. 128, 90]. Only recently has
Chojnowski et al [36] conducted the first serious exploitation of this
phenomenon using the $H$ band.

ANDES will be well-positioned to revolutionize this field, because the
majority of the current data on stellar magnetic fields in both low- and high-
mass stars has been obtained with instrumentation on $\leq 4$ m class
telescopes. ANDES will be able to distinguish subtle line profile differences
among intermediate and massive stars on the main sequence. ANDES will also be
superior to existing facilities in its ability to better resolve lines in more
rapidly rotating stars for a given magnetic field strength, and resolve
Zeeman-split components in stars with weaker magnetic fields for a given
Doppler line width.

Finally, ANDES will also provide an unprecedented opportunity to understand
how stellar activity and magnetism vary as a function of stellar type and age
by performing studies of stars in open clusters. The overwhelming majority of
previous activity and magnetic field measurements have been made for field
stars, where trends with other parameters are nearly impossible to discern.
The large collecting area of the ELT will enable ANDES to study intrinsically
fainter stars at larger distances, potentially revealing how stellar mass,
age, and environmental aspects influence the behavior of stellar activity and
magnetism, with implications for multiple fields of modern astrophysics.

###  2.3 Gravity darkening: a new measure of stellar rotation

Rotation is a fundamental parameter that governs the physical structure and
evolution of stars. For example, rotation induces internal mixing, which in
turn impacts the stellar lifetime and the surface chemical abundances. Two
important consequences of stellar rotation for the stellar photosphere are
geometrical distortion (flattening) and gravity darkening. Both effects can
influence the observed stellar flux and emergent spectrum because flattening
induces a local effective gravity that is stronger at the poles than at the
equator. Similarly, gravity darkening induces a latitudinal dependence on the
photospheric flux emission, with a local effective temperature higher at the
poles than at the equator. This latitudinal dependence of local effective
gravity and temperature breaks the spherical symmetry of a fast-rotating star,
so the observed spectrum reveals the viewing inclination angle, $i$. Once $i$
is known, more fundamental physical parameters can be derived, including the
true rotation velocity and mass [116]. These quantities are intimately linked
to the internal circulation of matter, energy, and angular momentum, as well
as angular momentum loss and non-spherical mass loss [121, 64, 65].

High spectral resolution and ultra-high signal-to-noise (S/N $>1000$) data
enable direct measurements of these quantities, which are directly encoded in
the continuum and lines of a spectrum of a rotating star. Spectra of this
quality is not generally obtainable today, except for the very brightest stars
in the sky [211, 212]. ANDES will enable new measurements of these quantities
for fast-rotating stars, because of its high spectral resolution, wide
wavelength coverage spanning the optical to the NIR regimes, and the large
collecting area of the ELT. Gravity darkening and rotation velocities measured
by ANDES would be able to provide novel constraints on models of stellar
structure and evolution. Measuring gravity darkening in many fast-rotating
stars across the HR diagram has the potential to provide critical constraints
that lead to a more profound understanding of stellar physics.

###  2.4 Lithium in stars: primordial versus evolutionary and environmental
effects

Hydrogen, helium and lithium are produced in the first few minutes after the
Big Bang. Reconciling predictions for the primordial lithium abundance with
observations remains one of the major open questions in astrophysics today.
The primordial lithium abundance can be predicted from standard Big Bang
nucleosynthesis, where it emerges as a function of the cosmological baryonic
density, $\Omega_{B}$, a parameter that quantifies the amount of ordinary
matter. $\Omega_{B}$ can be derived to high precision from observations of the
cosmic microwave background [e.g., 38]. This predicted abundance is in
conflict with the lithium abundances observed in the oldest stars. The vast
majority of unevolved Population II (Pop II) dwarf stars in the Milky Way with
metallicities, [Fe/H], between $\sim-3.0$ and $\sim-1.0$ share a similar
surface lithium abundance, the so-called Spite Plateau value [204]. This
derived lithium abundance is a factor of 3–4 lower than that predicted by the
standard Big Bang nucleosynthesis model.

A complementary view of the initial A(Li) of Pop II stars relies on stars on
the lower red giant branch [140]. These giants are characterised by a constant
Li abundance, drawing a Plateau that mirrors the A(Li) Plateau observed with
the dwarf stars but at a lower abundance (A(Li)$\sim$0.9 dex) as shown in
Figure 1. The lower Li content is due to the first dredge-up episode and the
increasing size of the convective envelope. Stellar models including diffusion
and additional mixing are able to reconcile the measured A(Li) in both dwarf
and giant stars with the cosmological value [142]. The key upside of this
approach is that these stars are about 4 magnitudes brighter than the main
sequence turnoff at the wavelength of the Li i line (at 670.7 nm), enabling to
investigate A(Li) in stars more distant than those usually observed with the
dwarf stars.

ANDES will be able to collect high-resolution spectra of the Li i line in
individual lower red-giant branch stars in extragalactic systems—including the
Magellanic Clouds and the Sagittarius dwarf galaxy—for the first time. Lithium
abundances in these stars could be derived with a precision of 0.06 (0.03) dex
from ANDES spectra with S/N = 50 (100) per pixel. ANDES will be capable of
collecting spectra of this quality for stars in these Local Group galaxies in
about 1 (5) hr of observing time for $V$ = 18.5, and around 2 (8) hr for $V$ =
19.0. This new capability will enable investigations into whether the lithium
abundance discrepancy exists in extragalactic systems, which experienced
different star formation histories and whose stars exhibit different chemical
abundance patterns. ANDES will thus have the potential to establish the
environmental dependence of the lithium abundance discrepancy that continues
to vex the cosmological and stellar astrophysics communities.

Figure 1:  Surface lithium (Li) abundance as a function of [Fe/H] for Milky
Way dwarf stars [blue circles 23, 15, 11, 88, 134] and stars on the lower red-
giant branch [red circles 142]. The green square marks the lithium abundance
derived from the stacked spectrum of 51 lower RGB stars in the Sagittarius
globular cluster M54 [141]. The blue solid line denotes the lithium abundance
(and $\pm 1\sigma$ uncertainties) inferred from Big Bang nucleosynthesis [38].
The grey shaded area indicates the range of [Fe/H] of the old Sagittarius and
Large Magellanic Cloud globular clusters that are ideal targets for ANDES.

###  2.5 Atomic diffusion in low-mass stars

Atomic diffusion is the slow transport of chemicals due to temperature,
pressure and abundance gradients occurring in the radiative stellar layers.
Specifically, pressure and temperature gradients push the ions toward the
center, decreasing the surface abundances (gravitational settling), while the
interaction with the radiation field and the concentration gradients lead to
the opposite effect (radiative levitation). The surface abundance variations
along the evolutionary sequence on the interplay of these two competitive
processes, which are maximized in stars around the main sequence turnoff point
[e.g., 135, 185]. Subsequent evolution along the red giant branch restores the
initial abundances, as the deepening outer convection zone mixes elements back
to the surface. Observations show that the abundance differences between dwarf
and giant stars are significantly smaller than those expected from stellar
models that include atomic diffusion [72, 39, 109, 139], indicating that
diffusion is strongly inhibited by additional mixing processes (on top of
convection) that operate in main sequence stars. The nature of this additional
mixing is still unknown, but its correct characterization is crucial to better
constrain stellar evolution models including these processes, understand
potential biases affecting the chemical abundances of unevolved field stars,
and precisely quantify the cosmological lithium problem (Section 2.4; Korn et
al 109, Gavel et al 69). At the very least, the joint analysis of dwarfs and
giants in Galactic archaeology requires an empirical understanding of the
surface abundance corrections caused by atomic diffusion.

The efficiency of the atomic diffusion and the unidentified source of
additional mixing along the main sequence depend on the stellar mass and
temperature, so precise chemical abundances of different elements at different
masses are crucial to constrain these effects. Globular clusters offer a
unique opportunity to investigate these effects, by comparing abundances in
relatively unevolved stars (turnoff point, subgiants) with more evolved stars
(red giants). A handful of globular clusters and a couple of open clusters
have previously been studied in search for element-specific diffusion
features, such as for Li [110, 120, 148]. The main limitation in the
spectroscopic studies performed so far has been the faintness of unevolved
stars in Milky Way globular clusters, where magnitudes at the main sequence
turnoff are $V\approx$ 16.5–19.0, so high-resolution spectroscopic
observations are time-consuming to prohibitive. To this day, the main
sequences of globular clusters remain unexplored at high spectral resolution.

ANDES will provide an unprecedented opportunity to study atomic diffusion by
deriving precise chemical abundances of Li, Fe, and other metals in unevolved
and evolved stars of globular clusters. In the closest clusters, such as NGC
6397 (Figure 2), ANDES will be able to collect spectra of stellar masses as
low as $\sim$ 0.6 $M_{\odot}$, which are not obtainable using any other
existing high-resolution spectrograph.

Figure 2:  Color-magnitude diagram of the nearby globular cluster NGC 6397,
color-coded according to [Fe/H] abundance ratios predicted by a theoretical
model of atomic diffusion calibrated according to the results of Korn et al
[110]. Stellar masses along the main sequence are labeled, together with the
expected S/N ratios at 550 nm with three hours of exposure time using ANDES
and UVES.

###  2.6 Constraining the initial mass function using CNO isotopes in
unevolved stars

Determining the shape of the initial mass function (IMF) and its dependence on
the physical conditions of the medium out of which the stars form is a major
open question of modern astrophysics. Isotopic abundance ratios provide a
powerful diagnostic of the IMF in systems where direct measurements, such as
star counts, are impossible. Stars of different masses produce different
isotopic ratios among carbon, nitrogen, and oxygen, which are collectively
referred to as CNO elements. Measuring isotopic ratios among the CNO elements
provides a critical test of the possible non-universality of the IMF [82, 154,
182]. Unevolved stars whose abundances have not been altered by internal
evolutionary processes hold the key to studying the chemical evolution of rare
isotopes of CNO in the Milky Way. The current sample sizes are far too small
to provide meaningful constraints on the IMF. The 12C/13C and 16O/18O ratios
have been measured only in a few nearby objects [see 181, for a recent review]
and it is unclear whether the 14N/15N ratio can be measured at all in
unevolved stars.

If ANDES includes a $K$ channel, this capability would enable measurements of
the 12C/13C ratios in a large sample of unevolved stars for the first time
(see Fig. 3). According to ANDES ETC, one hour of science exposure shall
suffice to reach S/N $\simeq$ 200, sufficient to measure the faintest 13CO
features at a few hundredths of the continuum in the spectrum of a fully
convective M dwarf as faint as $K\simeq 13.5$ (in Vega system), which
corresponds to a distance of about 200 pc (for comparison, CRIRES+ mounted on
UT3 resolves the 13CO features of such an object only up to $\sim$50 pc
distance). Longer integration times of, say, 3 hours, will allow to reach $d=$
275 pc, thus greatly expanding the sample of potential targets unobservable at
high spectral resolution using current facilities. Although the probed
metallicity range will still be limited to nearly solar metallicities, it is
worth noticing that ANDES will permit the determination of the pristine C
isotopic ratio in a significant number of open clusters. A query to the Hunt
and Reffert [92] catalogue returns that faint 13CO features can be measured in
M dwarf spectra of 1, 25, and $\sim$60 open clusters with log(Age) $>$ 8.2 yr
within, respectively, 60, 200, and 275 pc distance from us. Determinations of
the pristine 12C/13C ratio in open clusters have implications for both stellar
evolution (e.g., investigation of extra-mixing processes) and Galactic
chemical evolution studies (existence and extent of inhomogeneities, if any,
informing on the most likely stellar polluters).

Figure 3: Synthetic spectra in the $K$ band for a fully-convective M dwarf
star ($T_{\mathrm{eff}}=$ 3000 K, log g = 4.5 dex, [M/H] = 0) observed with
ANDES ($R=$ 100,000) at S/N = 200. The blue, red, and black curves refer to
different isotopic ratios (12C/13C = 89, 60, and 30, respectively). The CO
features that enable these measurements are detectable in several wavelength
ranges, in addition to this one, across the $K$ band.

###  2.7 Nucleosynthesis of the heaviest elements: the rapid neutron-capture
process

Understanding the origins of the elements all across the periodic table is one
of the major challenges of modern astrophysics. The rapid neutron-capture
process, or r-process, is one of the fundamental ways that stars and stellar
remnants produce the elements listed along the bottom two-thirds of the
periodic table [40, 13], including the actinides, transuranic elements, and
potentially superheavy elements [54, 87]. Merging pairs of neutron stars have
been confirmed as one site of r-process nucleosynthesis, through the
spectacular detection of both electromagnetic radiation and gravitational
waves from a relatively nearby event observed in 2017 (e.g., Abbott et al 1,
Drout et al 53, Watson et al 222). Yet key open questions remain, including
how many additional r-process sites are required by observations, which
elements are made by the r-process, and in what amounts, and what are the
specific conditions required—or range of conditions permissible—for r-process
nucleosynthesis to occur [61, 75].

ANDES will advance our understanding of these questions because it will be
highly efficient at collecting high-resolution, high-S/N optical spectra of
low-metallicity r-process-enhanced stars identified from photometric or
medium-resolution spectroscopic surveys. Many of these stars are located far
beyond the Solar neighborhood [4, 130]. ANDES will also expand the range of
potential stellar samples in Milky Way dwarf galaxies beyond the handful of
brightest red giants that are marginally accessible using current facilities
[168, 100]. Key absorption lines in late-type stars are found throughout the
$U$, $B$, and $V$ channels [e.g., 178], including Sr ii at 407.7 nm, Y ii at
360.0 nm, Zr ii at 414.9 nm, Mo i at 386.4 nm, Ru i at 379.9 nm, Ba ii at
455.4 nm, La ii at 398.8 nm, Eu ii at 381.9 nm, Dy ii at 394.4 nm, Yb ii at
369.4 nm, Ir i at 380.0 nm, Pb i at 405.7 nm, Th ii at 401.9 nm, and U ii at
385.9 nm.

###  2.8 Rare phases of stellar evolution

The merger of two white dwarfs (WDs) is a rare astrophysical phenomenon that
concludes the life of a close stellar binary system. Many WD mergers will lead
to Type Ia supernova events, yet rare classes of WDs offer novel opportunities
to explore uncommon and short-lived states of evolved stars. The fusion of two
massive (M${}_{\rm total}>$ 1.44 M⊙) carbon-oxygen WDs, for example, may
generate a Type Ia thermonuclear supernova explosion [196] or create a neutron
star after a gravitational collapse. Understanding the relationships between
these various classes of stars, and the physics that drives them, is critical
to improving population synthesis models that are widely employed across many
fields of astronomy.

The hydrogen-deficient carbon (HdC) stars [223] are supergiants with
atmospheres that are composed almost entirely of helium, and they are strong
candidates to be the descendants of mergers of helium and carbon-oxygen WDs.
HdC stars are rare objects. Only about 150 are known in the Milky Way [215,
216], and about 30 are known in the Magellanic Clouds [214]. Population
synthesis models predict the existence of two ensembles of HdC stars [217],
which emerge $\approx$ 1 and $\gtrsim$ 5 Gyr after star formation events.
Abundance studies strongly support a double-degenerate scenario for their
origin [97], yet none of them have ever been detected in a binary system.

ANDES will open a new window to study HdC stars in the Milky Way thin disk,
bulge, halo, and other galaxies. Observations of HdC stars require a high
spectral resolution to obtain reliable abundances. HdC stars have very
peculiar and varied abundances for many elements, as revealed by lines of,
e.g., Li i (670.7 nm), F i (690.3, 739.9, and 775.5 nm), Sr ii (407.7 and
421.5 nm), Ba ii (455.4 and 649.6 nm), and others. Wide wavelength coverage
from the $B$ to $I$ channels is necessary to cover this set of lines, and the
$U$ channel would also be beneficial. In addition, the 18O/16O is an important
diagnostic that can be measured using CO bands observable in the $K$ channel.
The large collecting area of the ELT enables these evolved giants to be
observed even in the Magellanic Clouds in only $\approx 10$–30 min. ANDES will
be capable of generating unprecedented samples of HdC stars in our Galaxy and
beyond. This potential advance represents one of the many new windows into the
exotic phases of stellar evolution that will be enabled by ANDES.

##  3 Stars of the Milky Way, Local Group, and Beyond

###  3.1 The first stars

The formation of the first stars signaled the end of the cosmic Dark Ages. The
first stars, which are often referred to as Population III (Pop III) stars,
formed from metal-free gas at redshift $z>$ 15–30 [e.g., 27, 105]. These first
stars are predicted to have been relatively massive, with characteristic
masses $\geq$ 10 $M_{\odot}$ [e.g., 84, 210, 153] and most likely $\leq$ 100
$M_{\odot}$[111]. They produced the first ionizing photons, the first
supernovae, the first metals, and the first stellar-mass black holes. The
first stars played a central role in shaping the nature of the first galaxies
and the earliest metal enrichment. Despite their significance, these objects
have not been observed directly. ANDES will have the ability to reveal the
nature and end states of the first stars in two important ways. First, ANDES
will be able to search for long-lived, low-mass metal-free Pop III stars,
should any exist today. The key observational signature of these stars would
be the presence of H Balmer lines in absorption and the absence of absorption
lines of any metals or molecules, including the CH G band near 430 nm, the Na
i D doublet near 589 nm, the Mg i b triplet near 518 nm, the Ca ii H & K
doublet at 393.3 nm and 396.7 nm, and Fe i lines at 358.1 nm and 382.0 nm.

ANDES would be able to detect one or more Pop III stars—or exclude their
existence at high confidence—by collecting spectra of several hundred
candidate metal-free stars with initial metallicity estimates [Fe/H] $<-4$
[77, 177]. These data would enable new, stronger constraints on the
characteristic mass and low-mass end of the Pop III IMF for the first time
[183].

Secondly, ANDES will be well-suited to characterize the surviving descendants
of extreme Pop III stars by determining the chemical compositions of hundreds
of second-generation stars born from the remnants of Pop III stars. Those
chemical abundance patterns could be interpreted using new state-of-the-art
models to constrain the masses and explosion energies of the elusive Pop III
supernovae themselves [184, 111, 220]. The predicted end states of these
extreme Pop III stars include energetic pair-instability supernovae [PISNe;
81, 189, 5, 226]; high-energy hypernovae [161, 199]; and low-energy faint
supernovae that led to the formation of stars highly enhanced in carbon and
deficient in iron, the so-called carbon-enhanced metal-poor stars [e.g., 96,
47, 184].

Figure 4:  Wavelengths of absorption lines detected in the optical spectra of
six stars known at present with [Fe/H] $<-4$. Each point represents one line
in one star, and different elements are marked by different colors. The design
of ANDES will enable the detection of the vast majority of these lines,
covering all major nucleosynthesis groups, in many more stars that retain the
fossil records of metals produced by the first stars. Figure modified from
Roederer et al [177].  Figure 5: A simulated 1-hour spectrum of a subgiant
second-generation star with a total metallicity of $10^{-6}$ times the Solar
metallicity in the $U$ channel around Ca ii H & K lines. Random noise has been
added to simulate the S/N for ANDES (black), UVES (red), and X-SHOOTER (blue).

The key observational signatures to be examined include numerous metal
absorption lines, spanning 370 to 670 nm, in the most metal-poor stars. Figure
4 illustrates the wavelengths of these lines in known metal-poor stars, and it
highlights the importance of the ANDES $U$ and $B$ channels to detect many of
these lines. About 40% more lines would be detectable in these stars with the
inclusion of the $U$ channel. High-resolution and high-S/N spectra are
necessary to enable the detection of weak metal lines or place meaningful
upper limits on non-detections of these lines.

Similar work can be conducted with existing high-resolution spectrographs on
4–10 m class telescopes, but only modest-size samples of targets are within
reach of high-resolution optical spectrographs on these telescopes. Figure 5
shows simulated spectra around the Ca ii H & K lines for a subgiant star at a
distance of $\approx 20$ kpc where the total amount of elements heavier than
helium is $<10^{-6}$ Solar. Observations using X-SHOOTER and UVES at the VLT
would lack sufficient S/N and spectral resolution to conclusively identify the
unique nature of this hypothetical star, whereas ANDES at the ELT could easily
resolve the stellar Ca ii lines. Ongoing and planned surveys—including ones
conducted with 4MOST, DECAM, DESI, Gaia, LAMOST, MOONS, Pristine, the Rubin
Telescope, SkyMapper, S-PLUS, and more—will identify thousands of targets with
$V$ magnitudes fainter than 18 that are too faint for high-resolution
spectroscopic follow-up with the current generation of optical telescopes. The
large collecting area of the ELT will greatly expand the samples of first-star
candidates and their descendants that can be observed in the Milky Way disk,
halo, bulge, and Local Group dwarf galaxies.

###  3.2 Stellar populations in the Galactic bulge

The age distribution of stars in the Galactic bulge is a key ingredient to
understanding the processes that shaped the formation and evolution of the
bulge [18]. Observational evidence has, however, presented conflicting views
on this subject. Photometric studies have found that the bulge is an almost
entirely old stellar population [e.g., 229, 37, 171]. In contrast, studies
based on isochrone ages of dwarf, turnoff, and subgiant stars magnified by
gravitational microlensing events have reported that the fraction of young-
and intermediate-age stars may be as high as 50% at super-solar metallicities
[19, 20].

The large collecting area of the ELT will enable ANDES to obtain high-
resolution spectra of turnoff and subgiant stars in the bulge, even when they
are not being magnified in gravitational microlensing events. Turnoff and
subgiant stars located in the bulge have apparent magnitudes of 18 $\leq
V\leq$ 19\. ANDES will be able to obtain a spectrum with $R=$ 100,000 (50,000)
and $S/N\approx$ 30–40 (50–60) for a bulge turnoff star in 1–2 hr.

The optical spectrum (500–900 nm) of each star contains numerous lines of Fe
and $\alpha$ elements—including O, Mg, Si, Ca, and Ti—that are necessary to
determine parameters, metallicities, and other detailed chemical abundances.
With this information, stellar ages can be inferred with a precision of about
$25$%, depending on the evolutionary state of the star, from theoretical
isochrones [20]. By systematically mapping the age distribution of the bulge
across a wide range of Galactic longitude and latitude, ANDES would enable
novel constraints on the the formation and nature of the bulge and its
connections to other stellar populations.

ANDES will also be uniquely capable of studying the oldest stellar populations
buried deep within the bulge for the first time. In the $\Lambda$CDM paradigm,
galaxies like the Milky Way formed the largest concentration of old stars in
their innermost regions (e.g., Tumlinson 219, Salvadori et al 187, El-Badry et
al 55, Pagnini et al 153). The old stars that survive in these innermost
regions today are inaccessible to optical or UV spectroscopy, but they can be
observed in the NIR spectral range. High-resolution and high-S/N spectroscopy
covering $\sim$ 950–2400 nm is generally not possible for large samples of
candidate metal-poor stars within the innermost few kpc of the Galaxy with
existing facilities, because the extinction is so high. Studies using ANDES
would surpass previous efforts that have helped to establish the potential of
this approach, such as the APOGEE [164], the EMBLA [89], and the PIGS [14]
surveys. There would be synergy with the MOONS survey and ELT’s HARMONI
instrument. Their large multiplexing capabilities could identify the most
promising targets for ANDES follow-up to derive the compositions of a few
dozen elements that trace the nucleosynthesis of the earliest generations of
stars in the bulge. Key spectral lines of interest include Na i 1638.8 nm, Mg
i 1208.3 nm, Ti ii 1587.3 nm, Fe i 1287.9 nm, Fe i 1507.7 nm, Zn i 1105.4 nm,
Sr ii 1032.7 nm, Ce ii 1578.4 nm, and Eu ii 1016.5 nm [43, 129, 200, 58].

###  3.3 Reaching the edge of the Galactic disk with open clusters

Open clusters in the Galactic disk span a wide distribution of ages and
distances, and the properties of their stars can be determined both accurately
and precisely. These features place open clusters among the best tracers of
the properties and stellar populations of the disk. For example, our knowledge
of the shapes of chemical abundance gradients in the outer part of the Galaxy
is still unsettled, and these gradients hold important implications for
understanding the formation of the Galactic disk and the impact of stellar
migration [e.g., 123].

Studies using data from the Gaia mission have characterized the ages and
distances of all known open clusters, and these data have revealed many new
ones. Large spectroscopic surveys such as Gaia-ESO, APOGEE, and GALAH have
derived the chemical properties of some of them, but these surveys—and future
ones with WEAVE and 4MOST—are limited in magnitude. These surveys are unable
to chemically characterize clusters across the full radial extent of the
Galactic disk.

ANDES will overcome these limitations and be able to collect high-resolution
spectra of open cluster stars all the way to the outer edge of the Galactic
disk. In Fig. 6, we show the distribution of the apparent $V$ magnitudes,
$m_{V}$, of red clump stars in open clusters with age $>$ 1 Gyr. ANDES can
easily reach the farthest clusters with short exposure times (1800 s for S/N
$\sim$ 120 at 650 nm with $R=$ 100,000 for a star with $m_{V}=16$), such that
the chemical abundances of a few dozen elements can be characterized in 4–5
stars in $<$ 2 hr per cluster.

Figure 6: Distribution of apparent $V$ magnitudes of red clump stars in open
clusters as a function of their galactocentric radius, Rgc. Distances and ages
are adopted from Cantat-Gaudin et al [29]. ANDES will be able to collect high-
resolution spectra to chemically characterize stars in clusters all the way to
the outer edge of the Galactic disk, which remains out of reach of current
surveys and large optical telescopes.

###  3.4 The mysterious nature of M subdwarfs

Cool subdwarf stars have lifetimes much longer than the Hubble time [115].
These faint, low-mass stars are relatively rare in the solar neighborhood, but
they are predicted to be the most numerous stellar community within the Milky
Way’s halo [22]. Studies of cool subdwarf spectra show that they are metal
poor when compared with the more common cool dwarfs with near-solar
metallicity [71, 117]. The nature of subdwarfs has been explored over the last
six decades by a large number of authors [190, 76, 71, 228], but fundamental
questions about their origins and kinematics remain. For example, searches for
cool subdwarfs have been limited to high Galactic latitudes, which favors the
detection of halo and thick disk objects. Whether or not a cool subdwarf
population exists within the Galactic thin disk remains an open question for
observations and a challenge for stellar evolution theory.

ANDES will be capable of collecting spectra of cool subdwarfs in unprecedented
detail. The broad spectral coverage of ANDES will enable derivation of
accurate atmospheric parameters and chemical compositions of cool subdwarfs in
all Galactic populations where they are found. These quantities are essential
ingredients to understand the origin and nature of this mysterious class of
objects.

###  3.5 Resolved stellar populations across the dwarf galaxies in the Local
Group

The Local Universe provides a unique window into the process of hierarchical
mass assembly on all scales. The Local Group of galaxies spans a range of more
than a million in stellar mass, from the Large Magellanic Cloud stellar mass
of $\sim 3\times 10^{9}$ $M_{\odot}$ to numerous dwarf galaxies with stellar
masses $<10^{3}$ $M_{\odot}$ (e.g., McConnachie 132, Simon 197), and it
includes many gas-poor, isolated systems and several gas-rich interacting
galaxies. These galaxies will be prime targets of the next generation of
multi-object spectrographs, such as 4MOST and MOONS. Those surveys will
identify thousands of stars in these galaxies that are too faint for high-
resolution and high-S/N optical spectroscopy with existing facilities. Yet
those stars will hold the promise of unlocking our understanding of galaxy
formation and evolution at the low mass scale [224, 12] and the epochs of the
earliest chemical enrichment [63].

Most Local Group dwarf galaxies are intrinsically metal-poor with a dominant
old stellar population, so they are ideal fossils with which to study the
imprints of the first stars and discover the nature and properties of the
earliest nucleosynthesis events [95, 106, 199]. The fundamental physical
conditions of major nucleosynthetic sites remain obscure, even after decades
of study. The key to breaking degeneracies in this field is to investigate the
chemical compositions of stars in galaxies with a variety of star formation
and chemical enrichment histories. For example, these chemical compositions
provide insight into the progenitors of Type Ia supernovae [124], the
production of heavy elements in AGB stars (via the s- and i-processes; Ritter
et al 173, Skúladóttir et al 198), the products of the kilonovae that
accompany neutron star mergers (via the r-process; e.g., Kasen et al 101, Ji
et al 100), and the source(s) of the extreme carbon enhancement observed in
many extremely metal-poor stars [149, 188, 99].

Furthermore, some ultra faint dwarf galaxies are thought to be surviving first
galaxies [25, 186, 62, 205]. They offer a complementary perspective on high
redshift systems, such as the ones expected to be observed with JWST.
Theoretical simulations based on $\Lambda$CDM theory predict a large number of
low-mass satellites for galaxies with masses comparable to the Magellanic
Clouds. Unfortunately, most of these satellites are too faint to be directly
detected, and others will have already dissolved to unrecognizable levels
after merger events with more massive dwarfs. Fortunately, stars with peculiar
abundances in the more massive dwarfs, such as the Magellanic Clouds and the
Sagittarius dwarf, retain chemical signatures of their now-dissolved birth
galaxies [35, 167, 143]. The unprecedented statistical scales of stars
observed by 4MOST and MOONS around the Magellanic Clouds, for example, will
reveal numerous rare stars in these systems with [Fe/H]$<-2.5$.

ANDES will provide the opportunity to perform high-resolution spectroscopic
follow-up of all of these interesting stars that will be discovered in the
Milky Way satellites in the next few decades. Virtually all stars in Local
Group galaxies beyond the Milky Way have Gaia magnitudes $G\gtrsim 19$ (Figure
7), so high-resolution spectroscopy of their stars will require the collecting
area of telescopes on the scale of the ELT. Many of the spectral lines
necessary for these analyses are found at relatively blue wavelengths
($\lesssim$ 460 nm), where detector efficiencies and spectrograph throughputs
decrease. Even relatively bright stars in dwarf galaxies will require the
higher throughput at all wavelengths that will be available with ANDES.

Figure 7: Left: phase-space diagram for the Milky Way dwarf galaxies (from
Pace et al 152), where the x-axis shows the distance, $r$, to the Galactic
center (GC) and the y-axis shows the 3-dimensional velocity relative to the
GC. Positive (negative) velocities indicate that the dSph is moving away from
(toward) the GC, and the black lines represent the escape velocity of the MW.
Right: Gaia color-magnitude diagram for the ultra-faint dwarf galaxy Reticulum
II, showing stars with $>20\%$ membership probability, $p$ [98]. The blue
square and red diamond show two bright stars recently observed using the GHOST
spectrograph on the 8 m Gemini-South Telescope [80]. Most stars in Reticulum
II are fainter than $G$ = 19, which is one of the nearest dwarf galaxies,
demonstrating the need for ANDES to collect high-resolution spectra of most
stars in Local Group dwarf galaxies.

###  3.6 Evolved stars in other galaxies

Evolved cool stars of various masses are major cosmic engines, providing
strong mechanical and radiative feedback on their host environment [114]
through stellar winds and supernova mass ejections [85]. These M-type stars of
are among the largest and most luminous stars in the Universe. The brightest M
supergiants should be excellent extragalactic distance indicators for a wide
range of galaxy types and luminosities [e.g., 91] and they provide robust
chemical abundance estimates that constrain models of galaxy formation and
chemical evolution within the Milky Way [e.g., with JWST; 118] and nearby
galaxies [46]. Massive evolved stars have been used to examine the metal
content and distribution in Local Universe galaxies out to distances of $\sim
20$ Mpc using $J$-band spectroscopy [70, 155, 26] and to study the distances
and properties of Galactic, young massive star clusters [45, 34, 60, 59, 147].
Metallicities of individual massive stars, asymptotic giant branch stars, and
red supergiant stars that are bright in the mid infrared can be estimated at
distances of tens of Mpc [57].

However, these stars are a cauldron of complexity for all the physics involved
from the mixing in the interior towards the transport into the highly
dynamical surfaces (Fig. 8, left), and, eventually the extremely vigorous
(still not completely understood) mass loss mechanism [85]. The emerging
stellar spectra and intensity map are characterised by broad, asymmetric and
shifted lines (Fig. 8, right) with deviations steamed by a temporal dependent
and episodic convection pattern consisting of large granules and (super)-sonic
velocities [e.g., 73]. Theoretically confirmed by state-of-the-art numerical
simulations [e.g., 32].

---
Figure 8: _Left:_ Example of synthetic intensity map from a 3D radiation-
hydrodynamics simulation of stellar convection [34]. _Right:_ zoom-in view
around an Fe i line in the J-band taken from the same simulation, at the
spectral resolving power of ANDES. The light gray displays spectra at
timesteps offset by $\sim$ 21 d, while the solid black line is the temporal
average. ANDES will be capable of resolving these differences in the line
profiles.

ANDES offers a new opportunity to obtain high-resolution spectroscopy of
evolved stars beyond the Local Group. Spectral diagnostics include atomic
lines of Mg, Si, Ti, and Fe in the J-band, where we can obtain uncertainties
on the abundances of $\pm$0.1 dex (at solar metallicities) with S/N $>55$ at
resolving power lower than 10000 [57]. However, due to the complex dynamics of
those stars, higher spectral resolution are needed to resolve the shape and
temporal shifts of the lines (Fig. 8, right panel). This represents an
important step to address fundamental questions about the evolution of massive
stars, their environments, as well as their metallicity in other galaxies
[56].

###  3.7 Calibrating properties of Cepheid variables for the cosmic distance
ladder

Cepheid variables are the backbone of the extragalactic distance ladder. The
discovery of the accelerating expansion of the Universe, which was awarded the
Nobel Prize in 2011, is based on our understanding of Cepheid variables [172].
However, there is currently a 5-$\sigma$ tension between the expansion rate of
the universe, $H_{0}$, depending on whether it is derived from the cosmic
microwave background or from the distance ladder [49]. If confirmed, this
tension would indicate that the $\Lambda$CDM model of the universe should be
refined.

ANDES observations will enable a new route to determine $H_{0}$ using the
Baade-Wesselink method of Cepheid distance determination [103, 146]. This
method measures distances to Cepheid variables as follows. The variation of
the stellar angular diameter, which can be obtained from surface brightness-
color relations or by interferometric measurements, is compared to the
variation of the linear diameter, which can be derived from radial velocity
measurements multiplied by the projection factor. The distance to the Cepheid
can then be obtained by dividing the linear and angular diameters. This method
has the potential to test the tension in $H_{0}$ by determining the distances
to individual Cepheids in the Local Group and beyond, from which the period-
luminosity-metallicity relations in various galaxies can be calibrated
consistently.

This method takes advantage of the unmatched capabilities of ANDES in three
ways. First, ANDES will enable the study of the dynamical structure of the
atmospheres of extragalactic Cepheids [e.g., 144, 145]. Secondly, ANDES will
introduce new anchors as constraints on $H_{0}$ by providing Baade-Wesselink
distances [207, 208, 66, 218] to individual Cepheids throughout the Local
Group, including ones in the Large and Small Magellanic Clouds, IC 1613, and
NGC 6822. Finally, ANDES will enable the derivation of Cepheid metallicities
and other chemical abundances, such as $\alpha$-element abundances, which will
help to calibrate and interpret the dependence of the period-luminosity
relationship on chemical composition [179, 180].

##  4 The Star-Planet Connection

###  4.1 Stellar chemical composition as a driver of exoplanet formation

The field of exoplanets has been rapidly growing since the discovery of the
first exoplanet around a main-sequence star almost 30 years ago [131]. As the
sample of exoplanets has grown, it has been possible to statistically compare
their properties with those of their host stars—including age, mass, chemical
composition, effective temperature ($T_{\rm eff}$), surface gravity
($\log~{}g$), and more. High-resolution spectroscopy with ANDES will advance
our understanding of these connections.

A higher occurrence frequency of Jupiter-like planets has been observed around
high-metallicity stars than low-metallicity stars, supporting the scenario
that giant planets formed from the core accretion process [e.g., 2, 150].
Rocky Earth-like planets, in contrast, exhibit a weaker correlation with host
star metallicity [138, 113]. The abundances of many elements, in addition to
iron, are predicted to affect planet formation, so detailed chemical abundance
studies of the stellar hosts are required to formulate a more complete picture
of how planets form [e.g., 3, 83, 48, 21]. The abundance of elements such as
carbon, nitrogen, oxygen, and sulfur in exoplanet hosts also provides
fundamental constraints on the mechanisms of planetary formation and migration
in protoplanetary disks. ANDES will play a fundamental role in characterizing
the chemical composition of exoplanet hosts. High-resolution spectroscopy
across a broad spectral range (0.35–2.45 $\mu$m) of many stars will reveal how
the abundances of individual elements and isotopes map to planetary
properties.

The atmospheres of host stars are covered with complex, stochastic patterns
associated with convective heat transport, such as granulation, that can
severely complicate the interpretation of exoplanet spectra [33, 31]. The
impact of granulation on planetary signal extraction depends on the stellar
parameters and stellar metallicity. The wide wavelength coverage of ANDES will
be well suited to disentangle these effects. Molecules such as CO, H2O, and
HCN are readily observable in the NIR spectra of main-sequence and M-dwarf
stars, for example, and TiO, VO, and numerous atomic lines are found in the
optical spectra of these stars. All of these lines provide diagnostic
abilities critical to characterizing the internal stellar dynamics and
extracting reliable results from planetary atmospheres.

In addition to the chemical composition affecting the architecture of
planetary systems, the formation of planets will have an impact on the
composition of the host star. Formation of the rocky planets in the Solar
System may be responsible for the subtle, but measurable, differences between
the composition of the Sun and most nearby solar twins [133], which exhibit
clear correlations with condensation temperature. The opposite behavior may
occur when rocky planets, after orbit destabilization due to dynamical
interactions with other bodies, are accreted by the host star [203].

Chemical abundances can be derived from atomic lines and molecular bands,
including the [O i] line at 630.0 nm, the [C i] line at 872.7 nm, C i
transitions at 505.2 nm and 538.0 nm, and the S i line at 675.7 nm. Planetary
host stars are generally close and bright, and S/N ratios of 100–150 Å-1 yield
excellent chemical characterisations. The large collecting area of the ELT
will enable ANDES to greatly expand the number of exoplanet host stars where
studies such as these can be conducted. ANDES spectra will complement data
collected by the Ariel satellite [213], which will characterize the
atmospheres of exoplanets and their stellar hosts at low spectral resolution
[44, 122].

###  4.2 Star-planet interactions

Many known exoplanets orbit quite close to their host stars, with orbital
periods of only a few days. This close proximity can give rise to star-planet
interactions (SPI), tidal or magnetic interactions through which the exoplanet
influences its host star.

Tidal SPI refers to the tidal influence a planet has on its host star, which
can lead to an exchange of angular momentum between the planet’s orbit and the
stellar spin. Manifestations of tidal SPI include elevated stellar rotation
[162, 28], enhanced overall magnetic activity [102, 163, 94], and disruption
of magnetic activity [159].

Magnetic SPI concerns the interplay between the stellar magnetic field and a
close-in orbiting planet, with or without a magnetic field [209]. Magnetic SPI
can yield detectable radio emission via the electron-cyclotron maser
instability. In the optical spectral range, the magnetic interaction is
thought to enhance local activity in the star, but these signals have not been
explored as well at optical wavelengths as they have in the radio. The SPI
indicators would be modulated in both wavelength domains with the planet’s
orbit, not the stellar rotation period. Early indications of magnetic SPI in
the optical remain unconfirmed, suggesting a mechanism that does not operate
continuously [193, 194]. Several recent studies have revitalized the field by
detecting radio emission from nearby stars (Proxima Cen, Pérez-Torres et al
156; YZ Cet, Pineda and Villadsen 160) at certain phases of the orbit of their
planets, making these systems the best candidates to be nominated as the first
detection of SPI at radio wavelengths, and obvious candidates for optical
followup with ANDES.

Observations constraining both tidal SPI and magnetic SPI are critical to our
understanding of exoplanet habitability. For example, ANDES observations of
stellar spectral signatures could reveal the rotation periods of exoplanets
and their magnetic field strengths, which are thought to be an important
ingredient in exoplanet habitability [192, 68, 51]. The high spectral
resolution and broad wavelength coverage of ANDES will probe stellar rotation
through line broadening, with a wealth of stellar absorption lines
particularly in the UBV and RIZ channels; it will measure chromospheric
activity through simultaneous observations of the Ca ii H & K lines, the
H$\alpha$ line, the Ca ii NIR triplet, and the He i NIR triplet; and
characterize properties of the upper convection zone of stars through resolved
line profiles. The unmatched light-collecting area of the ELT and the broad
spectral coverage provided by ANDES will enable—for the first time—searches
for the rotation period of exoplanets based on different methods, including
rotational broadening and Doppler shift of lines in the planetary atmospheres
[201] and analysis of starlight reflected from the planet [119, 7].

ANDES will enable greater sensitivity of these effects in much larger samples
of exoplanet host stars than is accessible at present. The combination of the
planet rotation period, obtained from ANDES observations, and the stellar
rotation and planet orbital periodicities, obtained from all-sky photometric
space missions, such as TESS and PLATO, will produce a unique portrait of SPI.

##  5 Summary

ANDES will enable a wide range of potentially transformative science across
the field of stellar astrophysics. These exciting opportunities include
expanding our understanding of the physics of how stars work, the unique roles
that stars play in shaping the formation and evolution of galaxies, and the
intimate connections between stars and their systems of planets. The science
cases presented here often rely on more than one of the wide diversity of the
proposed capabilities of ANDES, and they represent only a fraction of the
potential science that ANDES could enable while in operation on the ELT in the
decades ahead. The takeaway message may be summarized as follows: Let’s build
ANDES together, and the potential for transformative stellar astrophysics
science will be virtually unlimited.

Acknowledgments

IUR acknowledges support from United States National Science Foundation grants
AST 1815403 and AST 2205847. CAP is thankful to the Spanish Ministry of
Science and Innovation (MICINN) for funding under the project
PID2020-117493GB-I00. TB acknowledges support from project grant No.
2018-04857 from the Swedish Research Council. E.M.A.G. acknowledges support
from the German Leibniz-Gemeinschaft under project number P67/2018. RM
acknowledges support from the Science and Technology Facilities Council
(STFC), by the ERC Advanced Grant 695671 ‘QUENCH,’ and by the UKRI Frontier
Research grant RISEandFALL. RM also acknowledges funding from a research
professorship from the Royal Society. AC acknowledges support from the French
National Research Agency (ANR) funded project PEPPER (ANR-20-CE31-0002). AJK
acknowledges support by the Swedish National Space Agency (SNSA). VA is
supported by FCT (Fundação para a Ciência e Tecnologia) through national funds
by the following grants: UIDB/04434/2020, UIDP/04434/2020, and
2022.06962.PTDC. CRL and PJA acknowledge financial support from the Agencia
Estatal de Investigación of the Ministerio de Ciencia e Innovación through
projects PID2019-109522GB-C52, PID2022-137241NB-C43 and the Severo Ochoa grant
CEX2021-001131-S all funded by MCIN/AEI/10.13039/501100011033. DR acknowledges
support from the Italian National Institute for Astrophysics through Theory
Grant 2022, Fu. Ob. 1.05.12.06.08. JRM acknowledges continuous support from
the Universidade Federal do Rio Grande do Norte and Brazilian Agencies CNPq
and CAPES.

Author contributions.

ADD HERE FROM THE GOOGLE DOC

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