Highly rprocessenhanced RPE stars are rare and usually metal poor FeH lt 1.0 and they mainly populate the Milky Way halo and dwarf galaxies. This study presents the discovery of a relatively bright V  12.72 highly RPE rII star EuFe  1.32 BaEu  0.95 LAMOST J020623.21494127.9. This star was selected from the Large Sky Area MultiObject Fiber Spectroscopic Telescope mediumresolution R  7500 spectroscopic survey followup highresolution R  25000 observations were conducted with the High Optical Resolution Spectrograph installed on the Gran Telescopio Canarias. The stellar parameters T SUBeffSUB  4130 K inlineformula mmlmath overflowscrollmmlmilogmmlmimmlmspace width0.25emmmlmspacemmlmi mathvariantnormalgmmlmimmlmath inlineformula  1.52 FeH  0.54   1.80 km sSUP1SUP have been inferred taking into account nonlocal thermodynamic equilibrium effects. The abundances of CeFe PrFe and NdFe are 0.19 0.65 and 0.64 respectively relatively low compared to the Solar rprocess pattern normalized to Eu. This star has a high metallicity FeH  0.54 compared to most other highly RPE stars and has the highest measured abundance ratio of Eu to H EuH  0.78. It is classified as a thindisk star based on its kinematics and does not appear to belong to any known stream or dwarf galaxy.
['Stellar abundances', 'Fundamental parameters of stars', 'Chemically peculiar stars', '1577', '555', '226', 'Astrophysics - Solar and Stellar Astrophysics', 'Astrophysics - Astrophysics of Galaxies']
Discovery of an Extremely rprocessenhanced Thindisk Star with EuH  0.78
{'Discovery of an Extremely r -process-enhanced Thin-disk Star with [Eu/H] = +0 . 78': "<!-- image -->  \nRENJING XIE  \n1, 2  \n1 CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Science, Beijing 100101, China  \n2 School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China  \n3 School of Physics and Technology, Nantong University, Nantong 226019, China  \n4 Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China  \n5 Instituto de Astrof'ısica de Canarias, V'ıa L'actea S/N, E-38205 La Laguna, Tenerife, Spain  \n6 Universidad de La Laguna, Departamento de Astrof'ısica, E-38206 La Laguna, Tenerife, Spain  \n7 Department of Physics and JINA Center for the Evolution of the Elements, University of Notre Dame, Notre Dame, IN 46556, USA  \n(Accepted July 6, 2024)  \nSubmitted to ApJL", 'ABSTRACT': 'Highly r -process-enhanced stars are rare and usually metal-poor ([Fe/H] < -1.0), and mainly populate the Milky Way halo and dwarf galaxies. This study presents the discovery of a relatively bright ( V = 12 . 72 ), highly r -process-enhanced ( r -II) star ([Eu/Fe] = + 1.32, [Ba/Eu] = -0.95), LAMOST J020632.21 + 494127.9. This star was selected from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) mediumresolution ( R ∼ 7500 ) spectroscopic survey; follow-up high-resolution ( R ∼ 25 , 000 ) observations were conducted with the High Optical Resolution Spectrograph (HORuS) installed on the Gran Telescopio Canarias (GTC). The stellar parameters ( T eff = 4130 K, log g = 1.52, [Fe / H] = -0.54, ξ = 1.80 kms -1 ) have been inferred taking into account non-local thermodynamic equilibrium (NLTE) effects. The abundances of [Ce/Fe], [Pr/Fe], and [Nd/Fe] are + 0.19, + 0.65 and + 0.64, respectively, relatively low compared to the Solar r -process pattern normalized to Eu. This star has a high metallicity ([Fe/H] = -0.54) compared to most other highly r -process-enhanced stars, and has the highest measured abundance ratio of Eu to H ([Eu/H] = +0 . 78 ). It is classified as a thin-disk star based on its kinematics, and does not appear to belong to any known stream or dwarf galaxy.  \nKeywords: stars: abundances - stars: chemically peculiar - stars: fundamental parameters', '1. INTRODUCTION': "The rapid neutron-capture process ( r -process) is one of the fundamental nucleosynthetic pathways for producing the heaviest elements. Several astrophysical sites have been proposed to satisfy the physical conditions required for the r -process, such as core-collapse supernovae (e.g., Sato 1974; Witti et al. 1994; Farouqi et al. 2010; Mirizzi 2015; Tsujimoto 2021), magneto-rotational jet-driven supernovae (e.g., Symbalisty et al. 1985; Fujimoto et al. 2008; Nishimura et al. 2015; Obergaulinger et al. 2018; Reichert et al. 2023), common envelop jet supernovae (e.g., Grichener & Soker 2019; Jin & Soker 2023), collapsars (Siegel et al. 2019), and neu-  \ntron star mergers (NSMs, Lattimer & Schramm 1974; Rosswog et al. 2014; Thielemann et al. 2017). The detection of GW170817 (Abbott et al. 2017) and its electromagnetic counterpart shows that NSMs can indeed produce r -process elements (Drout et al. 2017; Kasen et al. 2017; Pian et al. 2017; Watson et al. 2019). Research is still ongoing to determine whether NSMs are the sole source of r -process elements; some studies suggest that the observed r -processelement abundance patterns requires multiple sites (e.g., Cˆot'e et al. 2018, 2019; Hotokezaka et al. 2018; Siegel 2019; Brauer et al. 2021; Farouqi et al. 2021; Tsujimoto 2021; Yamazaki et al. 2021; Naidu et al. 2022; Ekanger et al. 2023; Kobayashi et al. 2023).  \nGenerally, r -process-enhanced (RPE) stars are metal-poor, with [Fe/H] < -1.0 (e.g., McWilliam et al. 1995; Sneden  \net al. 2008; Holmbeck et al. 2020), and they are mainly restricted to the Milky Way (MW) halo and dwarf galaxies (e.g., Hirai et al. 2015; Ji et al. 2016; Jeon et al. 2021; Matsuno et al. 2021). These stars are classified as r -I, r -II, or limitedr , depending on the level of r -process enrichment (Beers & Christlieb 2005; Frebel 2018; Holmbeck et al. 2020). The dominant r -process patterns (Ba to Hf) of these stars are nearly identical (sometimes referred to as 'universaility'), although some star-to-star differences between the lightest and heaviest elements remain (e.g., Sneden et al. 1994; Aoki et al. 2017; Sakari et al. 2018; Roederer et al. 2022a). Information on the elemental abundances and kinematics of these stars constrain their early enrichment history, which is crucial to understanding the origin of the r -process.  \nThis work reports on the discovery of an RPE star with [Fe/H] > -0.6 that is kinematically associated with the thin disk of the MW. Section 2 introduces the observations, and Section 3 describes the determination of stellar atmospheric parameters. A detailed abundance analysis and estimates of uncertainties is presented in Section 4. A discussion of the implications of the existence of this star, and an old (but uncertain) estimate of its nucleosynthetic age based on the observed [Th/Eu] ratio, is provided in Section 5, followed by a summary of our conclusions in Section 6.", '2. OBSERVATIONS': 'The RPE candidate LAMOST J020632.21 + 494127.9 (hereafter J 0206 + 4941) was selected from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Cui et al. 2012; Zhao et al. 2012; Liu et al. 2020; Yan et al. 2022) medium-resolution ( R ∼ 7500) spectroscopic survey by matching the observed spectra to synthetic templates of the region around the Eu II line at 6645.1 ˚ A (Chen et al. 2021). This star is relatively bright ( V < 13) and substantially more metal-rich compared to previously observed r -II stars.  \nThe high-resolution spectrum was taken with the High Optical Resolution Spectrograph (HORuS) installed on the Gran Telescopio Canarias (GTC, Tabernero et al. 2020; Allende Prieto 2021). The spectral wavelength coverage is from 3800 to 6900 ˚ A with a resolving power of R ∼ 25 , 000 . The multiorder echelle spectra were extracted and wavelength calibrated based on Th-Ar hollow-cathode lamp exposures, using the HORuS automated pipeline the chain 1 . The signal-tonoise ratio (S/N) at 5600 ˚ A is estimated to be 119, using the code DER SNR (Stoehr et al. 2008). Table 1 lists the basic information of this target, including the details of the observations, kinematics, and stellar atmospheric parameters, as described in detail below.  \nTable 1. Target Information  \n- a Gaia Collaboration et al. (2023)\n- b Determined by a photometric method.\n- c Determined by a spectroscopic approach.  \nThe high-resolution spectrum of J 0206 + 4941 around 6645 ˚ A is shown in Figure 1, along with the spectrum of HD 124897, a star with similar atmospheric parameters ( T eff = 4286 K, log g = 1.6 cgs, [Fe/H] = -0.52, and ξ = 1.58 km s -1 , Blanco-Cuaresma et al. 2014). The slightly higher T eff of HD 124897 leads to stronger Fe-group lines. The lines of heavy elements are labeled in red. HD 124897 has a moderate r -process enhancement of [Eu/Fe] = +0.45 (Forsberg et al. 2019a), and is classified as an r -I star. As shown in the figure, the Eu II line of our candidate star is significantly stronger than that of HD 124897. Many lines of heavy elements are weak or absent in the spectra of HD 124897, but they are strong in the spectra of our candidate star, indicating that J 0206 + 4941 has significant r -process enhancement.', '3. STELLAR ATMOSPHERIC PARAMETERS': 'We first derive initial estimates of effective temperature and surface gravity of J 0206+4941 using a photometric approach. For comparison, we also determine the atmospheric  \nFigure 1. Comparison of high-resolution spectra of J 0206 + 4941 and a moderately r -process-enhanced ( r -I) star, HD 124897 ([Eu/Fe] = +0.45, Forsberg et al. 2019a) with similar stellar parameters. The lines of heavy elements are labeled in red, and lines of lighter elements are labeled in black. The stellar parameters ( T eff , log g, [Fe/H], ξ ) are shown in the legend.  \n<!-- image -->  \nparameters using a spectroscopic approach: effective temperature ( T eff ), surface gravity (log g ), metallicity ([Fe/H]), and microturbulent velocity ( ξ t ).', '3.1. Photometric Approach': "We estimate the effective temperature of our target star based on the photometric color index ( V -K ) 0 with the empirical calibration relations given by Alonso et al. (1999). An adopted reddening E ( B -V ) of 0 . 1518 ± 0 . 0083 is derived using the GALEXTIN online tool (Amˆores et al. 2021) with the Green et al. (2019) 3D dust map. The E ( V -K ) reddening is determined using the conversions from Ram'ırez & Mel'endez (2005). The surface gravity is calculated using the online code PARAM 1.3 (da Silva et al. 2006), based on the relationships between bolometric flux, temperature, mass, and log g . The calculation requires estimates of T eff , [Fe/H], V magnitude, and parallax, where we use [Fe/H] measured by the spectroscopic approach described below, the V magnitude from Zacharias et al. (2013), and the parallax from Gaia DR3 (Gaia Collaboration et al. 2023).", '3.2. Spectroscopic Approach': 'The line list and atomic data of Fe I and Fe II lines are taken from Yan et al. (2018). Only lines with equivalent widths (EWs) between 20 m ˚ A and 120 m ˚ A, and not heavily blended with other species, have been used. This selection results in 45 Fe I and 7 Fe II lines, respectively. The abundances and EWs of individual Fe lines are analyzed by the interactive IDL code Spectrum Investigation Utility ( SIU , Reetz 1999) using the MARCS stellar atmospheric models (Gustafsson et al. 2008). The effects of non-local thermodynamic equilibrium (NLTE) are considered specifically for each Fe I line, based on Mashonkina et al. (2011), following the method described in Sitnova et al. (2015).  \nA spectroscopic estimate of T eff is obtained from the excitation equilibrium of Fe I lines with excitation energies  \n(E exc ) higher than 2.0 eV (Sitnova et al. 2015), log g is estimated from the ionization balance of Fe I and Fe II, and ξ t is obtained by forcing the iron abundances from Fe I lines to be independent of their EWs. We adjust T eff , log g , and ξ t by restricting the slope of log ϵ (Fe I) versus E exc lower than 0.001 dex eV -1 , the difference between [Fe II/H] and [Fe I/H] within 0.1 dex, and the slope of log ϵ (Fe I) versus log(EW) less than 0.01.  \nThe results determined by both methods are listed in Table 1. The difference between the photometric and spectroscopic effective temperatures is only 52 K, and the difference in surface gravities between the two methods is 0.10 dex. We adopt the spectroscopic parameters in the following analysis.', '4.1. Abundance Estimates': 'We use a spectrum-synthesis approach for the abundance determination. The line list and atomic data are taken from Zhao et al. (2016) and Roederer et al. (2018b); we have slightly revised the log gf values by fitting the Solar spectrum (Kurucz et al. 1984). The influence of hyperfine structure (HFS) is considered for Sc (Zhang et al. 2008), Cu (Shi et al. 2014), Sr (Borghs et al. 1983), Ba (Kurucz & Bell 1995), and Eu (Lawler et al. 2001). We calculate the theoretical line profiles by SIU , using the MARCS stellar model atmospheres (Gustafsson et al. 2008). The Solar abundances are adopted from Asplund et al. (2009).  \nThe carbon abundance is estimated from the molecular CH G -band region around 4310 ˚ A; the line list we adopted comes from Masseron et al. (2014). Among the light oddZ elements, we can obtain abundance estimates of Na and Al. Four Na I lines at 5682 ˚ A, 5688 ˚ A, 6154 ˚ A, and 6160 ˚ A, and two Al I lines at 6696 ˚ A and 6698 ˚ A have been used. The α -elements with detectable lines include Mg, Si, Ca, and Ti, while the [O I] line at 6300 ˚ A is too weak to derive a reli-  \n<!-- image -->  \n<!-- image -->  \n<!-- image -->  \n<!-- image -->  \n<!-- image -->  \nFigure 2. Spectral synthesis for the abundance determinations of Sr, Ba, La, Ce, Eu and Th. The red line indicates the best-fit synthesis for the line(s) of interest. The blue shaded regions indicate ± 0.30 dex deviations from the best fit. The gray-dashed lines indicate synthetic spectra in the absence of the element of interest.  \n<!-- image -->  \ndance. NLTE effects have been taken into consideration, based on Gehren et al. (2004, Na), Baumueller & Gehren (1996, Al), Mashonkina (2013, Mg), Shi et al. (2008, Si), Mashonkina et al. (2007, Ca), Sitnova et al. (2016, Ti), and Shi et al. (2014, Cu). In addition, the NLTE corrections for lines of Cr I, Mn I, and Co I are obtained from an online spectral tool 2 .  \nThe neutron-capture elements frequently have their absorption lines blended with other elements. This is especially the case for relatively metal-rich stars, as strong Fe lines of-  \nten lead to strong blends. Figure 2 shows a sample of spectral synthesis for the Sr I, Ba II, La II, Ce II, Eu II, and Th II lines. We use the Sr I line at 4607 ˚ A to determine the Sr abundance. There are numerous very strong absorption lines for elements, such as Fe and Ti lines around 4077 ˚ Aand 4161 ˚ A, making it difficult to distinguish the absorption lines of Sr II. The abundance of Ba is derived from the Ba II lines at 4554 ˚ A, 4934 ˚ A, 6141 ˚ A, and 6496 ˚ A. The NLTE effects have been taken into account (Mashonkina et al. 1999). The abundance of Eu is derived from the Eu II lines at 4129 ˚ A, 4205 ˚ A, 6049 ˚ A, and 6645 ˚ A, with the NLTE effects considered (Mashonkina & Gehren 2000). As the lines at 4129 ˚ A and 4205 ˚ A are heavily blended, only upper limits on Eu  \nare obtained from these two lines. The abundance of Ce is derived from the Ce II lines at 4418 ˚ A, 4628 ˚ A, 5247 ˚ A. The abundance of Th is estimated by using the Th II line at 4019 ˚ A. The abundances for a total of 15 neutron-capture elements, including Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Lu, Hf, and Th have been determined; the results are listed in Table 2.', '4.2. Uncertainties': 'To estimate the uncertainty of the abundance determinations, two types of error sources are considered: systematic errors ( σ sys , due to errors in the stellar atmospheric parameters), and statistical errors ( σ stat , due to the line measurements themselves). Table 2 lists the abundance differences when changing T eff by +100 K, log g by +0 . 2 dex, [Fe/H] by +0 . 1 dex, and ξ t by +0 . 2 km s -1 . The σ sys is calculated by taking the square root of the quadratic sum of the errors associated with the atmospheric parameters. The σ stat is the line-to-line dispersion divided by √ N , where N is the number of lines used for a given element. Finally, we computed the total error, represented as σ Total , by summing σ sys and σ stat in quadrature.', '5.1. The Light and Fe-peak Elements': 'The observed carbon abundance of J 0206 + 4941 is deficient, relative to the Solar value ([C/Fe] = -0.20). However, this star falls in the red giant region, therefore, its carbon has been depleted during the normal course of giant-branch evolution. Using the procedure described by Placco et al. (2014), the carbon depletion for J 0206 + 4941 is ∼ 0.2 dex, yielding a natal [C/Fe] ≈ 0.00, indicating that this star was likely never C-enhanced. From inspection of Figure 3, the abundances of Mg, Si, and Ti appear similar to those of the MW field stars. Although the [Ca/Fe] ratio is slightly lower, it is still close to the boundary of other field stars. The [Na/Fe] and [Al/Fe] ratios (not shown in the figure) are both slightly super-Solar.  \nAmong the Fe-group elements, [V/Fe], [Cr/Fe], and [Mn/Fe] are slightly over-abundant, while [Co/Fe], [Ni/Fe], [Cu/Fe], and [Zn/Fe] are slightly under-abundant. All of these abundance ratios are normal compared to stars of similar metallicity (e.g., Battistini & Bensby 2015).', '5.2. The r -process Enhancement and Pattern': "The lower two panels of Figure 3 show the [Ba/Fe] and [Eu/Fe] ratios of J 0206 + 4941 compared to MW field stars. From inspection, this star exhibits a very strong enhancement of [Eu/Fe] ( + 1.32), and a slight enhancement of [Ba/Fe] ( + 0.37). The low [Ba/Eu] ratio ( -0 . 95 ) indicates that its neutron-capture elements are mainly due to the r -process.  \nIn order to put J 0206 + 4941 in the context of other highly r -process-enhanced stars, we have listed the known RPE  \nstars with [Eu/Fe] ≥ +1.0 in Table A1, ordered by decreasing [Eu/H]. As shown in this table, most of these stars have low metallicity ([Fe/H] < -1.0) and are identified as members of the MW halo, while a few such stars are found in dwarf galaxies or globular clusters. J 0206 + 4941 is the first strongly r -process-enhanced star associated with the MW disk, and has a relatively high metallicity ([Fe/H] = -0.54). The [Eu/H] of J 0206 + 4941 is +0.78, which is the highest among known RPE stars.  \nMany previous studies have found that the main r -process abundance patterns of metal-poor RPE stars are very close to that of the Solar r -process. This agreement is most noticeable for the elements from Ba to Hf, although there are some differences in light neutron-capture elements (e.g., Sneden et al. 1994; Roederer et al. 2014b; Sakari et al. 2018), and starto-star variations among the actinides. A comparison of the heavy-element abundances in J 0206 + 4941 with the Solar r -process and two RPE 'template' stars (HD 222925; Roederer et al. 2022b, and 2MASS J22132050-5137385; Roederer et al. 2024) is shown in the top panel of Figure 4, where the Solar r -process, HD 222925, and 2MASS J221320505137385 r -process patterns are normalized to the Eu abundance of J 0206 + 4941. The residuals are presented in the bottom panels.  \nThe abundance pattern of J 0206 + 4941 is not perfectly consistent with the Solar r -process. The elements Sr, Y, Ce, Pr, and Nd are significantly lower compared to the Solar pattern, while only Sr and Ce are significantly lower compared to that of HD 222925. When compared to 2MASSJ22132050-5137385, the Ce abundance remains lower and the Y and Zr are higher. This difference suggests that the production of r -process elements may require multiple sites or multiple mechanisms, or that this star's abundances have a different contribution for these elements.", '5.3. Kinematics': "We measure the heliocentric radial velocity ( RV helio = -30.86 ± 0.67 km s -1 ) from the high-resolution GTC spectrum; it shows no obvious variation from the LAMOST DR7 ( -30.94 ± 1.00 km s -1 ; Luo et al. 2022), and the Gaia DR3 measurements ( -30.00 ± 0.40 km s -1 ; Gaia Collaboration et al. 2023). As a result, J 0206 + 4941 is not likely to be in a binary system.  \nWe calculate the local standard of rest (LSR) Galactic space velocities ( U LSR , V LSR , W LSR ) for J 0206 + 4941, adopting the coordinates, proper motions, and parallax with zero-point offset corrected 3 from the Gaia DR3catalog (Gaia Collaboration et al. 2023), as well as the radial velocity de-  \nX. J. XIE ET AL.  \nTable 2. Elemental Abundances and Uncertainties  \nrived from the GTC spectrum. The distance derived from the corrected parallax is d = 3 . 072 ± 0 . 148 kpc , which is consistent with the photogeometric distance from Bailer-Jones et al. (2021, d = 3 . 128 ± 0 . 151 kpc ). We use a righthanded Galactocentric Cartesian coordinate system and assume that the Sun is located at ( X ⊙ , Y ⊙ , Z ⊙ ) = ( -8.2, 0.0, 0.0208) kpc (McMillan 2017; Bennett & Bovy 2019), the circular velocity at the Solar position is v 0 = 232.8 km s -1 (McMillan 2017), and the Solar peculiar motion is ( U ⊙ , V ⊙ , W ⊙ ) = (11.1, 12.24, 7.25) km s -1 (Schonrich et al. 2010). We adopt the MW potential from McMillan (2017), and employ the AGAMA routine (Vasiliev 2019) to derive the orbital energy ( E ), three-dimensional action ( J = ( J r , J ϕ , J z ) ), pericentric radius ( r peri ), apocentric radius ( r apo ), maximum height from the Galactic midplane ( Z max ), and eccentricity  \n( ecc = r apo -r peri r apo + r peri ). We assume Gaussian distributions for its observables, and additionally account for the correlated observational uncertainties in proper motions and parallax. We run a Monte Carlo re-sampling procedure based on 100,000 draws to estimate the uncertainties of the dynamical parameters.  \nJ 0206 + 4941 exhibits typical thin-disk velocities (see Figure 5) with a very low orbital eccentricity ( ecc = 0 . 054 ± 0 . 004 ) and limited orbital height ( Z max = 0 . 66 ± 0 . 03 kpc). Many previously discovered highly r -process-enhanced stars have accretion origin (e.g., Roederer et al. 2018a; Xing et al. 2019; Sakari et al. 2019). However, J 0206 + 4941 does not appear to be clustered in dynamical phase space with any known accreted structures (Naidu et al. 2020; Reichert et al. 2021; Hattori et al. 2023; Shank et al. 2023), thus it may well  \nFigure 3. Comparison of abundance ratios in J 0206 + 4941 (red star) with MW field stars (gray circles). The abundance ratios of the MW stars come from Venn et al. (2004), Bensby et al. (2014), Battistini & Bensby (2016), Forsberg et al. (2019b), and Holmbeck et al. (2020). The black-dashed line is the Solar reference line. The orange-dashed line in the [Eu/Fe] vs. [Fe/H] panel indicates the r -II limit (+0.7) as defined by Holmbeck et al. (2020).  \n<!-- image -->  \nFigure 4. The heavy-element abundances of J 0206 + 4941 are shown in the top panel, with the Solar r -process pattern, abundances of HD 222925 (Roederer et al. 2022b), and 2MASS J22132050-5137385 (Roederer et al. 2024) normalized to match the Eu abundance of J 0206 + 4941. Residuals between J 0206 + 4941 and the Solar r -process pattern, HD 222925, and 2MASS J22132050-5137385 are shown in the lower panels.  \n<!-- image -->  \nhave been formed in-situ in the MW's thin disk. This star may have formed from an ISM enriched in r -process elements by a single binary neutron star merger and/or CCSNe, indicating that mixing may not be isotropic, even in the thin disk. We also note that Hirai et al. (2022) found, in their simulation of the origin of RPE stars in a MW-like galaxy, that all simulated r -II stars with [Fe/H] > -1 were formed in-situ, rather than via accretion from a parent dwarf satellite.", '5.4. Age Estimation': 'The presence of the radioactive element Th allows us to estimate the age of this star, albeit with a large error bar.  \nThe age can be estimated using the following relation (Cayrel et al. 2001):  \nt = 46 . 67 Gyr [ log ϵ (Th / Eu) 0 -log ϵ (Th / Eu) obs ] ,  \nwhere log ϵ (Th / Eu) 0 is the initial production ratio (PR) corresponding to the element formation at t = 0, and log ϵ (Th / Eu) obs is the observed ratio after the radioactive element Th has decayed within a time t . We adopted an initial PR from Schatz et al. (2002), and the calculated age is 12.32 Gyr. The uncertainty is estimated only from the propagation of the abundance-measurement uncertainty. Only one  \nFigure 5. Toomre Diagram for J 0206 + 4941. The MW thin-disk stars (pink triangles) and thick-disk stars (blue circles) are from Bensby et al. (2003) and Venn et al. (2004), respectively.  \n<!-- image -->  \nTh line can be measured, and the Th abundance has a systematic error of 0.16 dex, which leads to a large age uncertainty of 9.08 Gyr.', '6. CONCLUSIONS': "We describe the discovery of an extremely r -processenhanced thin-disk star, J 0206 + 4941. It was originally selected from the LAMOST medium-resolution survey as a candidate RPE star, based on its unusually strong lines of Eu. A high-S/N, moderately high-resolution ( R ∼ 25 , 000 ) follow-up observation was obtained using HORuS on the GTC. We derive the abundances of 30 elements, and determine the star's kinematics. Our main findings can be summarized as follows:  \n- 1. J 0206 + 4941 is the most metal-rich ([Fe/H] = -0.54) highly r -process-enhanced star found to date, and exhibits the highest known abundance ratio of the r -process element Eu relative to H ([Eu/H] = +0.78).\n- 2. The abundances of the light elements in this star with Z ≤ 30 are commensurate with other stars of similar metallicity and evolutionary phase, with a strong enhancement of Eu, ([Eu/Fe] = +1.32) and modest enhancement of Ba ([Ba/Fe] = +0.37). The observed [Ba/Eu] ratio ([Ba/Eu] = -0.95) indicates that there is relatively little contribution from the s -process to the enrichment of the neutron-capture elements.\n- 3. The abundance pattern of the elements with Z ≥ 30 is not perfectly consistent with the scaled Solar r -process pattern, as demonstrated by the lower abundances of Ce, Pr, and Nd. This difference provides some support for multiple origins of the r -process.\n- 4. J 0206 + 4941 does not exhibit significant radialvelocity variations, indicating it is not likely to be in a binary system.\n- 5. The Galactic space velocities and orbital parameters show that J 0206 + 4941 is restricted to the thin disk of the MW, and apparently was not accreted from an external dwarf galaxy.\n- 6. The presence of the radioactive element Th allows us to estimate the age of J 0206 + 4941 as 12.32 ± 9.08 Gyr.  \nClearly, a higher-resolution ( R > 50 , 000 ) spectrum of this star with better S/N in the blue region would prove illuminating, and enable the measurement of numerous other r -process elemental abundances, possibly including uranium, enabling derivation of a more accurate age estimate. In addition to J 0206 + 4941, we have also found a few other candidate RPE stars with [Fe/H] > -1.0 from the LAMOSTMRS spectra. These stars are excellent candidates for highresolution follow-up spectroscopic observations with HORuS and other instruments in the near future.  \nWe thank the anonymous referee for valuable suggestions that have improved the manuscript. This research is supported by the National Natural Science Foundation of China under Grant Nos. 12090040, 12090044, 12373036, 12022304, 11973052, the National Key R & D Program of China No.2019YFA0405502, the Scientific Instrument Developing Project of the Chinese Academy of Sciences, Grant No. ZDKYYQ20220009, and the International Partnership Program of the Chinese Academy of Sciences, Grant No. 178GJHZ2022047GC. H.-L.Y. acknowledges the support from the Youth Innovation Promotion Association, Chinese Academy of Sciences. CAP is thankful for financial support from the Spanish Ministry MICINN projects AYA2017-86389-P and PID2020-117493GB-I00. T.C.B. acknowledges support from grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), and from OISE-1927130: The International Research Network for Nuclear Astrophysics (IReNA), awarded by the US National Science Foundation.  \nThis paper is based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrof'ısica de Canarias, on the island of La Palma. The Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences.", 'REFERENCES': 'doi: 10.1051/0004-6361/201220761', 'APPENDIX': "Here we assemble a list of highly r-process-enhanced stars, chosen to have [Eu/Fe] ≥ +1 . 0 , sorted by decreasing [Eu/H]. Note that the abundances listed in the table are LTE results. The [C/Fe] listed is the 'as measured' quantity, not including corrections for evolutionary state. The column labeled LOC indicates the likely stellar population with which each star is associated.  \nTable A1 . Highly r-process-enhanced Stars ([Eu/Fe] ≥ +1 . 0 and [Ba/Eu] < 0.0).  \nTable A1 continued  \nX. J. XIE ET AL. Table A1 (continued)  \nTable A1 continued  \nTable A1 (continued)  \nNOTE-RYA96: Ryan et al. (1996); SHE01: Shetrone et al. (2001); SNE03: Sneden et al. (2003); BAR05: Barklem et al. (2005); LAI08: Lai et al. (2008); HAY09: Hayek et al. (2009); AOK10: Aoki et al. (2010); ALL12: Allen et al. (2012); COH13: Cohen et al. (2013); ROE14: Roederer et al. (2014a); JAC15: Jacobson et al. (2015); LIH15: Li et al. (2015); HOW16: Howes et al. (2016); JI16: Ji et al. (2016); PLA17: Placco et al. (2017); HAN18: Hansen et al. (2018); HAW18: Hawkins & Wyse (2018); ROE18: Roederer et al. (2018b); SAK18: Sakari et al. (2018); XIN19: Xing et al. (2019); CAI20: Cain et al. (2020); EZZ20: Ezzeddine et al. (2020); HOL20: Holmbeck et al. (2020); RAS20: Rasmussen et al. (2020); GUL21: Gull et al. (2021); REI21: Reichert et al. (2021); PLA23: Placco et al. (2023); GAR24: Cabrera Garcia et al. (2024); ROE24: Roederer et al. (2024); SHA24: Shah et al. (2024)."}