1Indian Institute of Astrophysics, Koramangala, Bangalore 560034, India

# Abundances of neutron-capture elements in CH and Carbon-Enhanced Metal-Poor
(CEMP) stars

Meenakshi Purandardas1 and Aruna Goswami1

###### Abstract

All the elements heavier than Fe are produced either by slow (-s) or rapid
(-r) neutron-capture process. Neutron density prevailing in the stellar sites
is one of the major factors that determines the type of neutron-capture
processes. We present the results based on the estimates of corrected value of
absolute carbon abundance, [C/N] ratio, carbon isotopic ratio and [hs/ls]
ratio obtained from the high resolution spectral analysis of six stars that
include both CH stars and CEMP stars. All the stars show enhancement of
neutron-capture elements. Location of these objects in the A(C) vs. [Fe/H]
diagram shows that they are Group I objects, with external origin of carbon
and neutron-capture elements. Low values of carbon isotopic ratios estimated
for these objects may also be attributed to some external sources. As the
carbon isotopic ratio is a good indicator of mixing, we have used the
estimates of 12C/13C ratios to examine the occurance of mixing in the stars.
While the object HD 30443 might have experienced an extra mixing process that
usually occurs after red giant branch (RGB) bump for stars with log(L/L⊙) $>$
2.0, the remaining objects do not show any evidence of having undergone any
such mixing process. The higher values of [C/N] ratios obtained for these
objects also indicate that none of these objects have experienced any strong
internal mixing processes. Based on the estimated abundances of carbon and the
neutron-capture elements, and the abundance ratios, we have classified the
objects into different groups. While the objects HE 0110$-$0406, HD 30443 and
CD$-$38 2151 are found to be CEMP-s stars, HE 0308$-$1612 and HD 176021 show
characteristic properties of CH stars with moderate enhancement of carbon. The
object CD$-$28 1082 with enhancement of both r- and s-process elements is
found to belong to the CEMP-r/s group.

###### keywords:

Stars—Individual; Stars—Abundances; Stars—Carbon; Stars—Nucleosynthesis.

<EMAIL_ADDRESS><EMAIL_ADDRESS>—- March 2020———

12.3456/s78910-011-012-3 #### 000 0000 1– 1

##  1 Introduction

Chemical analysis of metal-poor stars such as CH stars and CEMP stars can
provide important clues about the nature of nucleosynthesis processes occured
in the early Galaxy. Especially, the abundances of neutron-capture elements
can be used to constrain the Galactic chemical evolution due to heavy
elements. Various sky survey programmes (HK survey and Hamburg/ESO survey,
Beers et al. 1985; Wisotzki et al. 2000; Christlieb et al. 2001) were
conducted in the past to find metal-poor stars. All these surveys show that
the fraction of carbon enhanced objects increases with decreasing metallicity
(Beers & Christlieb 2005; Frebel et al. 2005; Norris et al. 2007; Spite et al.
2013; Yong et al. 2013).

CH stars are FGK giants that show strong carbon molecular bands in their
spectra. They are high radial velocity objects, mostly found in the halo of
our Galaxy. CEMP stars are the metal-poor ([Fe/H] $<$ -1) counter parts of CH
stars. Both the CH stars and CEMP stars show enhancement of carbon and
neutron-capture elements. Hence these objects are ideal candidates to study
the origin and evolution of these elements. Based on the type of enhancement
of neutron-capture elements CEMP stars are classified into different groups,
such as CEMP-s, CEMP-r, CEMP-r/s and CEMP-no stars (Beers & Christlieb 2005).
The evolutionary status of CH and CEMP stars do not support the enhancement of
carbon and heavy elements observed in these stars. The widely accepted
scenario to explain this enhancement is that these objects are in a binary
system. The primary companion once passed through the Asymptotic Giant branch
(AGB) phase and synthesized carbon and heavy elements. The synthesized
materials are then transferred to the secondary companion through some mass
transfer mechanisms. The radial velocity variations exhibited by CH and CEMP
stars (McClure 1983, 1984; McClure & Woodsworth 1990; Hansen et al.2016a)
support this idea.

In this paper, we have presented the results from the high-resolution analysis
of six stars that include four CEMP stars and two CH stars. The paper is
organized as follows. In section 2, we have presented a brief discussion on
the new results obtained for our programme stars as some of the results from
abundance analysis of these objects were presented in Purandardas et al.
(2019), and Purandardas, Goswami & Doddamani (2019). Section 3 presents the
sample selection, observations and data reductions. Section 4 describes the
determination of radial velocity and stellar atmospheric parameters. Details
of the abundance anaysis are presented in section 5. In section 6,
interpretation of our results are presented. Conclusions are drawn in Section
7.

##  2 Novelty of this work

We have presented the abundance analysis results for 26 elements in our
programme stars in Purandardas et al. (2019), and Purandardas, Goswami &
Doddamani (2019). In these works, we have also reported the mass and age of
these stars as well as the results from the kinematic analysis. The location
of these objects in the H-R diagram shows that they are either sub-giants or
in the ascending stage of the giant branch. Various mixing processes have been
found to operate in giant stars. It is therefore important to understand
whether the stars have undergone any internal mixing processes before
interpretting the observed abundances. We had not addressed this problem in
our previous works. In this paper, we have checked whether any internal mixing
processes have altered the surface chemical composition of these stars based
on [C/N] and carbon isotopic ratios. While HD 30443 might have experienced an
extra mixing process that usually occurs after red giant branch (RGB) bump for
stars with log(L/L⊙) $>$ 2.0, the remaining objects do not show any evidence
of having undergone any such mixing process. In our previous works, we have
not checked the possible source of enrichment of neutron-capture elements
observed in our programme stars. In the present work, we have tried to
understand the possible source of neutron-capture elements based on the
location of the absolute carbon abundance values of the programme stars, in
the A(C) vs. [Fe/H] diagram. Based on the estimated value of carbon abundances
together with the observed enhancement of neutron-capture elements, we have
classified them as Group I objects following Yoon et al. (2016) classification
scheme. As the Group I objects are all binaries, it is likely that our
programme stars that belong to this group are also in binary systems with
external origin of carbon and neutron-capture elements. Low values of carbon
isotopic ratios estimated for these objects may also be attributed to external
sources. In the present work, we have re-calculated the [hs/ls] ratio for our
programme stars without considering the contribution from samarium which is an
r-process element which we had taken into account for this estimation in our
previous works.

##  3 Observations and Data reduction

Programme stars are selected from the CH star catalogue of Bartkevicious
(1996), Goswami (2005) and Goswami et al. (2010). In the later papers,
potential CH star candidates are identified based on the low resolution
(R=$\lambda/\delta\lambda$$\sim$1330) spectroscopic studies of faint high
latitude carbon stars. In these works, two of our programme stars HE
0110$-$0406 and HE 0308$-$1612 are identified to be potential CH star
candidates. We have obtained high resolution (R $\sim$ 60 000) spectra of
these objects along with HD 30443 using the high-resolution fiber fed Hanle
Echelle Spectrograph (HESP) attached to the 2 m Himalayam Chandra Telescope
(HCT) at the Indian Astronomical Observatory, Hanle. The spectra cover the
wavelength range from 3530 to 9970 Å. The spectrograph allows a resolution of
60 000 with slicer, and a resolution of 30 000 without slicer. The spectrum is
recorded on a CCD with 4096$\times$4096 pixels of 15 micron size. For the
programme stars, CD$-28$ 1082 and HD 176021, we have used the high resolution
FEROS spectra (Fiber-fed Extended Range Optical Spectrograph (FEROS) of 1.52 m
telescope of Europian Southern Observatory at La Silla). The wavelength
coverage of the FEROS spectra is from 3500 - 9000 Å with a spectral resolution
of $\sim$ 48 000. The detector is a back-illuminated CCD with 2948$\times$4096
pixels of 15 $\mu$m size. For the object CD$-38$ 2151, a high resolution (R
$\sim$ 72 000) spectrum was obtained using the high-resolution fiber fed
Echelle spectrometer attached to the 2.34 m Vainu Bappu Telescope (VBT) at the
Vainu Bappu Observatory (VBO), Kavalur. The spectrum covers the wavelegth
region from 4100 to 9350 Å with gaps between orders. The spectrometer operates
in two modes. It allows a resolution of 72 000 with a 60 micron slit and a
resolution of 27 000 without the slit. The spectrum is recorded on a CCD with
4096$\times$4096 pixels of 12 $\mu$m size. The data reduction is carried out
using various spectroscopic reduction packages such as IRAF. Examples of a few
sample spectra of the programme stars are shown in Figure 1.

Figure 1:  Sample spectra of the programme stars in the wavelength region
7980-8030 Å.

##  4 Determination of radial velocity and stellar atmospheric parameters

Radial velocity of the programme stars are determined by measuring the shift
in the wavelength for a large number of unblended and clean lines in their
spectra. The radial velocities range from $-26.7$ to 139.7 km s-1. The
estimated radial velocities of the programme stars are presented in Table 1.

Stellar atmospheric parameters are determined from the measured equivalent
widths of clean and unblended Fe I and Fe II lines using the local
thermodynamic equilibrium (LTE) analysis. We made use of the recent version of
MOOG of Sneden (1973) for our analysis. Model atmospheres are selected from
Kurucz grid of model atmospheres with no convective overshooting
(http://cfaku5.cfa.hardvard.edu/). Solar abundances are taken from Asplund,
Grevesse & Sauval (2009). Effective temperature is taken to be that value for
which the trend between the abundance derived from Fe I lines and the
corresponding excitation potential gives a zero slope. At this temperature,
microturbulent velocity is determined for which the abundance derived from Fe
I lines do not exhibit any dependence on the reduced equivalent width.
Corresponding to these values of effective temperature and microturbulent
velocity, log g is determined in such a way that the abundances obtained from
Fe I and Fe II lines are nearly the same. Only those lines with excitation
potential from 0.0 - 5.0 eV and equivalent widths from 20 - 180 mÅ are
considered for the analysis. The derived atmospheric parameters and the radial
velocities are listed in Table 1.

Table 1: Derived atmospheric parameters and radial velocities of the programme
stars.

Star | Teff | log g | $\zeta$ | [Fe I/H] | [Fe II/H] | Vr
---|---|---|---|---|---|---
| (K) | (cgs) | (km s-1) |  |  | (km s-1)
HE 0110$-$0406 | 4670 | 1.00 | 1.92 |  $-1.31$$\pm$0.09 |  $-1.29$$\pm$0.12 |  $-44.40$$\pm$3.8 (HESP)
HE 0308$-$1612 | 4600 | 1.70 | 1.42 |  $-0.72$$\pm$0.19 |  $-0.73$$\pm$0.15 | 85.5$\pm$1.22 (HESP)
CD$-28$ 1082 | 5200 | 1.90 | 1.42 |  $-2.46$$\pm$0.08 |  $-2.44$$\pm$0.02 |  $-26.7$$\pm$0.3 (FEROS)
HD 30443 | 4040 | 2.05 | 2.70 |  $-1.68$$\pm$0.05 |  $-1.69$$\pm$0.11 | 66.61$\pm$0.20 (HESP)
CD$-38$ 2151 | 4600 | 0.90 | 2.30 |  $-2.03$$\pm$0.10 | $-2.03$ | 139.7$\pm$1.9 (VBT)
HD 176021 | 5900 | 3.95 | 1.02 |  $-0.62$$\pm$0.08 |  $-0.65$$\pm$0.05 | 109.1$\pm$0.5 (FEROS)

##  5 Abundance analysis

The detailed discussion on the abundance analysis results for the six
programme stars are presented in Purandardas et al. (2019), and Purandardas,
Goswami & Doddamani (2019). Here we present a brief summary of these results.
However here we give more emphasize to the results based on the absolute
carbon abundance, [C/N] ratio, carbon isotopic ratio and the [hs/ls] ratio
which is recalculated in this work.

The abundances of various elements are determined from the measured equivalent
widths of absorption lines due to neutral and ionized elements. We have used
only the symmetric and clean lines for our analysis. Lines are identified by
overplotting the arcturus spectra upon the individual spectra of our programme
stars. Then a master linelist is prepared using the measured equivalent widths
and other line information such as lower excitation potential and the loggf
values taken from the Kurucz database. We have also consulted VALD database.
We could estimate the abundances of 24 elements which include the light
elements C, N, O, odd-Z element Na, $\alpha$ \- and Fe-peak elements Mg, Si,
Ca, Ti, V, Cr, Mn, Co, Ni and Zn and the neutron-capture elements Sr, Y, Zr,
Ba, La, Ce, Pr, Nd, Sm and Eu. We have also used spectrum synthesis
calculations for elements such as Sc, V, Mn, Ba, La and Eu taking their
hyperfine structures into considerations. The hyperfine structures of Sc, V
and Mn are taken from Prochaska & McWilliam (2000). For Ba, La and Eu, the
hyperfine structures are taken from McWilliam (1998), Jonsell et al. (2006)
and Woorley et al. (2013) respectively.

We could estimate oxygen abundance only for CD$-38$ 2151 and HD 30443. For
these objects, the abundance of oxygen is determined from the spectrum
synthesis calculations of the [OI] line at 6300.3 and OI line at 6363.8 Å. The
carbon abundance could be determined for all the objects from the spectrum
synthesis calculations of the C2 molecular band at 5165 Å. We could estimate
the carbon isotopic ratio for all of our programme stars except HD 176021
using the spectrum synthesis calculation of CN band at 8005 Å (Figure 2). The
values lie in the range from 7.4 to 45. Abundance of nitrogen is estimated
using the spectrum synthesis calculation of CN band at 4215 Å. Nitrogen is
found to be enhanced in CD$-28$ 1082 and CD$-38$ 2151\. Other objects exhibit
moderate enhancement in nitrogen. The molecular lines for C2 and CN are taken
from Brooke et al. (2013), Sneden et al. (2014) and Ram et al. (2014). The
estimated values of carbon and nitrogen are presented in Table 2.

Table 2: Abundance results for carbon, nitrogen, C/O and carbon isotopic
ratios.

Star |  log$\epsilon$(C) |  log$\epsilon$(C)∗ | [C/Fe] |  log$\epsilon$(N) | [N/Fe] | C/O |  12C/13C
---|---|---|---|---|---|---|---
HE 0110$-$0406 | 7.85 | 7.98 | 0.73 | 7.15 | 0.63 | - | 45.0
HE 0308$-$1612 | 8.48 | 8.50 | 0.78 | 7.25 | 0.15 | - | 15.6
CD$-28$ 1082 | 8.16 | 8.23 | 2.19 | 8.10 | 2.73 | - | 16.0
HD 30443 | 8.43 | 8.47 | 1.68 | 6.55 | 0.40 | 1.02 | 7.40
CD$-38$ 2151 | 7.90 | 8.02 | 1.50 | 7.20 | 1.40 | 2.95 | 11.2
HD 176021 | 8.33 | 8.33 | 0.52 | 7.80 | 0.59 | - | -

∗ Corrected value of carbon.

Figure 2:  Synthesis of CN band around 8005 Å. Synthesized spectra is shown in
red colour and the observed spectra is represented in black colour. Synthetic
spectra corresponding to 12C/13C $\simeq$ 12 (blue) and 1 (magenta) are also
shown.

Sodium is moderately enhanced in all our programme stars except HD 176021 in
which Na is near solar. HE 0110$-$0406 and HD 176021 show near solar
abundances of alpha elements. While, HE 0308$-$1612 shows slight enhancement
of these elements. We could not estimate Si in these objects. Among the alpha
elements, we could estimate only Mg and Ca in CD$-28$ 1082 with [Mg/Fe] $\sim$
0.45 and [Ca/Fe] $\sim$ 0.27. In HD 30443, Si and Ca are moderately enhanced
with [Si/Fe] $\sim$ 0.82 and [Ca/Fe] $\sim$ 0.51. While Magnesium, Sc and Ti
are near solar. Magnesium and Ca are found to be moderately enhanced in
CD$-38$ 2151\. While Si is found to be enhanced with [Si/Fe] $\sim$ 1.62.
Scandium and Ti are found to be near solar in this object.

HE 0110$-$0406 shows near solar abundance of Fe peak elements except Ni which
is slightly enhanced with [Ni/Fe] $\sim$ 0.45. While Fe-peak elements are
slightly enhanced in HE 0308$-$1612\. We could not estimate Co in HE
0308$-$1612\. Among the Fe peak elements, we could estimate only Mn in CD$-28$
1082 which is found to be enhanced with [Mn/Fe] $\sim$ 1.48. In HD 30443, Mn
and Co are near solar and Ni is moderately enhanced with [Ni/Fe]$\sim$ 0.59.
Cobalt and Ni are near solar in CD$-38$ 2151\. While chromium is slightly
enhanced and Mn is underabundant with [Mn/Fe] $\sim$ $-0.20$. In HD 176021,
all the Fe peak elements are found to be near solar.

All of our programme stars exhibit enhancement of neutron-capture elements.
From our detailed analysis, we found that CD$-28$ 1082 is a CEMP-r/s star and
the objects HE 0110$-$0406, CD$-38$ 2151 and HD 30443 are CEMP-s stars. We
could not estimate Eu in HD 30443 and CD$-38$ 2151\. Hence, it is not possible
to classify these stars based on the criteria as given by Beers & Christlieb
(2005). In this case, we have used the criteria for the classification of
CEMP-s stars as given by Hansen et al. (2019) based on [Sr/Ba] ratio.
According to this classification scheme, [Sr/Ba] $>$ $-0.5$ can be used to
seperate CEMP-s stars from CEMP-r/s stars. HD 30443 and CD$-38$ 2151 show
[Sr/Ba] $\sim$ $-0.27$ and [Sr/Ba] $\sim$ 0.88 respectively. HE 0308$-$1612
and HD 176021 exhibit the properties of CH subgiants. All the programme stars
show the enhancement of heavy s-process elements more than the light s-process
elements except for CD$-38$ 2151 and HD 176021. The abundance results for
neutron-capture elements are listed in Table 3. In this table, ls stands for
light s-process elements (Sr, Y and Zr) and hs represents the heavy s-process
elements (Ba, La, Ce, and Nd).

Table 3: Ratios of light and heavy s-process elements

Star | [Fe/H] | [ls/Fe] | [hs/Fe] | [hs/ls]
---|---|---|---|---
HE 0110$-$0406 | $-1.30$ | 1.03 | 1.36 | 0.33
HE 0308$-$1612 | $-0.73$ | 1.11 | 1.62 | 0.51
CD$-28$ 1082 | $-2.45$ | 1.52 | 1.90 | 0.38
HD 30443 | $-1.69$ | 1.24 | 1.93 | 0.69
CD$-38$ 2151 | $-2.03$ | 1.24 | 1.10 | $-0.14$
HD 176021 | $-0.64$ | 1.50 | 1.37 | $-0.13$

##  6 Interpretation of results

The interpretation of the results based on the corrected value of absolute
carbon abundance, [C/N], carbon isotopic ratios and [hs/ls] ratios obtained
for our programme stars are presented here in detail. Understanding the
possible source of origin of the enhancement of neutron-capture elements is
very important to understand the type of nucleosynthesis process which
produced it. One of the best ways to get any clues regarding the source is to
locate the stars in the A(C) vs. [Fe/H] diagram. It is found that CEMP stars
exhibit a bimodal distribution in the A(C) vs. [Fe/H] diagram. Spite et al.
(2013) claims the occurance of two plateau at A(C) = 8.25 and another at A(C)
= 6.50. While Yoon et al. (2016) confirmed two peaks at A(C) = 7.96 and A(C) =
6.28 respectively for high and low carbon regions corresponding to the
corrected carbon values. The stars that occupy the high carbon region are
mostly found to be in binary systems. Hence the observed enhancement of carbon
is attributed to the binary companion. Stars that occupy the low carbon region
are found to be single and the observed carbon abundance is intrinsic in
origin. CEMP stars are classified by Yoon et al. (2016) into three groups
based on the morphology in the A(C) vs. [Fe/H] diagram. Group I objects are
mainly composed of CEMP-s and CEMP-r/s stars. These objects show a weak
dependence of A(C) on [Fe/H]. The absolute carbon abundance, A(C) of Group II
objects show a clear dependence on [Fe/H]. While for Group III objects, A(C)
is found to be independent of [Fe/H], Group II and Group III objects are
mainly composed of CEMP-no stars. In Figure 3, the location of our programme
stars are shown in the A(C) vs. [Fe/H] diagram. We have applied corrections to
the estimated carbon abundances using the public online tool by Placco et al.
(2014) available at http/://vplacco.pythonanywhere.com/. The corrected carbon
values are listed in Table 2. Figure 3 shows that all of our programme stars
are Group I objects. Hence we assume that the observed enhancement of carbon
may be attributed to the binary companion.

Figure 3: Corrected A(C) vs. [Fe/H] diagram for the compilation of CEMP stars
taken from Yoon et al. (2016). CEMP-s stars are represented by Open circles.
CEMP-r/s and CEMP-no stars are shown by open squares and open triangles
respectively. Programme stars are represented by black coloured symbols, HE
0110$-$0406 (cross), HE 0308$-$1612 (filled circle), CD$-28$ 1082 (filled
triangle), HD 30443 (open pentagon), CD$-38$ 2151 (filled square) and HD 17602
(filled pentagon)

The low values of 12C/13C ratio observed in our programme stars also support
the extrinsic origin of carbon. Various mixing processes such as first dredge
up, thermohaline mixing and rotation induced mixing (Charbonnel 2005; Dearborn
et al. 2006; Eggleton et al. 2006) can cause a low carbon isotopic ratio.
Vanture (1992) suggests that certain nucleosynthetic reactions of the accreted
material can reduce the 12C abundance. We have estimated the [C/N] ratio of
our programme stars to derive any clues if they have undergone any mixing
processes. Since nitrogen is produced at the expense of carbon, [C/N] ratio
acts as a sensitive indicator of mixing. Spite et al. (2005) analysed CNO and
Li abundances for a sample of extremely metal-poor stars. Their analysis shows
that the stars that exhibit clear evidence of mixing have a low value of [C/N]
ratio ($<$ $-0.60$), and they belong to upper RGB or horizontal branch. The
stars that do not show any evidence of mixing have [C/N] $>$ $-0.60$ and lie
on the lower RGB. Figure 4 shows that all of our programme stars have [C/N]
$>$ $-0.6$. This implies that none of our programme stars have undergone any
significant internal mixing processes.

Figure 4: Position of the programme stars in the [C/N] vs. Teff diagram.
Programme stars are represented using red coloured symbols. The symbols used
for the programme stars are same as in Figure 3. Open squares represent the
stars from literature (Spite et al. 2006, Aoki et al. 2007, Goswami et al.
2016, Hansen et al. 2016b and Hansen et al. 2019).

Stellar models predict that when a low mass star ascends the red giant branch,
the outer convective envelope expands inwards and penetrates into the region
of CN processed materials (First dredge up (FDU)). The luminosity at which the
FDU occurs in low mass field stars is log(L/L⊙) $\sim$ 0.80 (Gratton et al.
2000). All of our programme stars have log(L/L⊙) $>$ 0.80, except HD 176021.
Hence they might have undergone first dredge up. However, these stars fall in
the unmixed category. Even though in solar mass/metallicity stars, 12C/13C
ratio decreases by a factor of 20-30 from the original value and surface
abundance of nitrogen increases after FDU (Iben & Renzim 1984), it is found to
be less efficient in metal-poor stars (VandenBerg & Smith 1988, Charbonnel
1994). This implies that the changes in the surface compositions of C and N
after the occurance of FDU are very small in metal-poor stars. The carbon
abundance is found to be decreased only by about 0.05 dex and the decrease in
the 12C/13C ratio is not large enough for these stars (Gratton et al. 2000).

A second mixing episode can happen when the star becomes brighter than the RGB
bump at a luminosity of log(L/L⊙) $\sim$ 2.0. In this process, 12C abundance
decreases by a factor of $\sim$ 2.5 and 12C/13C reaches a value of $\sim$ 6 to
10. Nitrogen abundance also increases by a factor of nearly 4 (Gratton et al.
2000). Two of our programme stars HD 30443 and CD$-38$ 2151 have log(L/L⊙)
$\sim$ 2.2 and 2.87 respectively. Hence they are expected to show the
signatures of mixing. But these stars show no signatures of mixing based on
[C/N] ratio. Spite et al. (2006) suggest that [C/N] raio is not a clean
indicator of mixing as the abundance of C and N in the interstellar medium
from which these stars are formed show large variations. In that case, one can
use the 12C/13C as a good indicator of mixing, since it is high in primordial
matter ($>$ 70) and the determination of carbon isotopic ratio is found to be
insenstive to the choice of atmospheric parameters for the stars (Spite et al.
2006). We have plotted 12C/13C ratio and Teff of our programme stars as shown
in Figure 5. From the figure, it is clear that the object HD 30443 share the
region occupied by the stars that have undergone internal mixing processes.
This means that HD 30443 had experienced internal mixing processes that might
have altered its initial surface chemical compositions. CD$-38$ 2151 lies
close to the boundary of seperation of mixed and unmixed stars. The low value
of carbon isotopic ratio in other programme stars may be interpretted as the
result of strong processing in the AGB progenitors (Hansen et al. 2016b) and
not in the star itself.

Figure 5: Position of the programme stars in 12C/13C vs. Teff diagram.
Programme stars are represented using red coloured symbols. The symbols used
for the programme stars are same as in Figure 3. Open squares represent the
stars from Spite et al. (2006) and Aoki et al. (2007).

From the estimated carbon isotopic ratio and the location of our programme
stars in the A(C) vs. [Fe/H] diagram, we assume that the observed enhancement
of neutron-capture elements may be attributed to an extrinsic source. As Group
I objects are mostly found to be associated with binary systems, the
enhancement may be justified by the mass transfer from the binary companion.

According to Busso et al.(2001), the light s-process elements such as Y, Zr
and Sr are predominantly produced in AGB stars with near solar metallicity.
Hence, mass transfer from such a companion can produce more enhancement of
light s-process elements than the heavy s-process elements. The AGB stars with
[Fe/H] $\sim$ $-1.0$ can produce more heavy s-process elements such as Ba, La,
Ce and Nd than the light s-process elements, and this leads to lower [ls/Fe]
than [hs/Fe] in the stars that have accreted materials from the metal-poor AGB
stars. The reason is that, in the metal-poor AGB stars, the number of Fe seed
nuclei available for neutron-capture process is less and hence the neutron
exposure for each Fe seed nuclei will be more. This leads to the formation of
more heavier elements. The binary mass transfer scenario can similarly be
applied to justify the observed enhancement of neutron-capture elements in
CD$-28$ 1082 which is a CEMP-r/s star. Several authors (Hampel et al. 2016;
Hansen et al. 2016a and references therein) have shown that the observed
abundance patterns of CEMP-r/s stars are consistent with the model
calculations of i-process in low-mettalicity AGB stars.

##  7 Conclusion

The possible source of the origin of enhancement of neutron-capture elements
in six carbon stars are examined in the light of absolute carbon abundances.
Location of our programme stars in the A(C) vs. [Fe/H] diagram shows that
these objects belong to Group I category. Various studies on radial vecities
of these objects show that most of them exhibit radial velocity variations.
That is, these objects are mostly found to be associated with binary systems.
Hence the observed enhancement of carbon and neutron-capture elements in our
programme stars may be attributed to the binary companion. But none of the
programme stars are known to be confirmed binaries. The low values of carbon
isotopic ratio also supports the extrinsic origin.

Elemental abundance ratios also bear important signatures about the source of
enrichment of neutron-capture elements. We have re-estimated the [hs/ls] ratio
for our programme stars. Abate et al. (2015) predict an [ls/hs] ratio less
than zero for AGB models. All the programme stars except CD$-38$ 2151 and HD
176021 show an [hs/ls] ratio characteristics of AGB progenitors.

We have also examined whether the programme stars have undergone any internal
mixing based on [C/N] and carbon isotopic ratios. Because it is important to
understand whether the mixing have modified the surface composition of the
star before interpretting the observed abundance patterns. We found that none
of the proramme stars have experienced internal mixing, except HD 30443. The
estimated values of [C/N] ratio also show that the objects have not gone
through any significant mixing processes. Thus, the observed values of low
carbon isotopic ratio in the unmixed stars may be due to the strong mixing
processes occured in the AGB progenitors. In other words, the observed surface
chemical compositions of our programme stars except HD 30443, preserve the
fossil records of the materials synthesised in the AGB stars from which they
have accreted the materials.

## Acknowledgements

We thank the staff members at IAO, CREST and VBO for their assistance and
cooperation during the observations. Funding from DST SERB project No.
EMR/2016/005283 is gratefully acknowledged. This work made use of the SIMBAD
astronomical database, operated at CDS, Strasbourg, France, the NASA ADS, USA
and data from the European Space Agency (ESA) mission Gaia
(https://www.cosmos.esa.int/gaia), processes by the Gaia Data Processing and
Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium).

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