arXiv subjects

Aruna Goswami

Publications and source records attributed to Aruna Goswami.

At least 19 recordsLinked to original sources

arXiv2026

Three Extremely Metal-Poor stars: discovery of a new CEMP-no star

We have conducted detailed high-resolution spectroscopic studies on three extremely metal-poor (EMP) stars HE 0401-0138, HE 1153-0518 and HE 1246-1344. For the stars HE 0401-0138 and HE 1246-1344, we have estimated the abundances of C, Na, Mg, Ca, Sc, Ti, Cr, Mn, Co, Ni, Sr and Ba along with the upper limits for Li, O, La, Ce, Pr, Nd, Sm, and Eu. For HE 1153-0518, abundances of seven light elements from C through Ni and two heavy elements, Y and Ba, have been derived, together with upper limits for Li, O, and La. Based on their observed abundance patterns, HE 0401-0138 and HE 1246-1344 are classified as normal EMP stars, whereas HE 1153-0518 is identified as a newly discovered CEMP-no star. HE 1153-0518 shows strong carbon enhancement with a high absolute carbon abundance, extreme sodium enrichment, very low neutron-capture element abundances, and a very low carbon isotopic ratio (12C/13C = 2.0). Its spectral energy distribution shows clear infrared excess, indicating the presence of circumstellar dust. The abundance pattern of HE 1153-0518 suggests enrichment by early nucleosynthetic processes, such as faint core-collapse supernovae with mixing and fallback, while the possibility of binary interaction and subsequent internal mixing cannot be ruled out. The discovery and detailed study of HE 1153-0518 add an important object to the small population of high-A(C) CEMP-no stars and provide valuable constraints on early chemical enrichment pathways and the nature of the first generations of stars.

astro-ph.SR↗

arXiv2024

Thirty Meter Telescope International Observatory Detailed Science Case 2024

The Thirty Meter Telescope (TMT) International Observatory (TIO) will be a revolutionary leap forward in astronomical observing capabilities, enabling us to address some of the most profound questions about the universe. From unraveling the mysteries of dark matter and dark energy to exploring the origins of stars and planets, TMT will transform our understanding of the cosmos. The TIO Detailed Science Case (DSC) presents science goals that inform the top-level requirements for the observatory's design and operations, including the telescope, enclosure, instruments, and adaptive optics system.

astro-ph.IM↗

arXiv2024

Asteroseismology of the mild Am $δ$ Sct star HD 118660 : TESS photometry and modelling

We present the results of an asteroseismic study of HD 118660 (TIC 171729860), being a chemically peculiar (mild Am) star exhibiting $δ$ Scuti ($δ$ Sct) pulsations. It is based on the analysis of two sectors of time-series photometry from the space mission TESS and seismic modelling. It yielded the detection of 15 and 16 frequencies for TESS sectors 23 and 50, respectively. The identified pulsation modes include four radial ($\ell=0$) and five dipolar ($\ell=1$) ones. The radial modes are overtones with order $n$ ranging from $3$ and $6$. Such high values of $n$ are theoretically not expected for stars with the effective temperature of HD 118660 ($\rm T_{\rm eff}\approx 7550 \rm K$ ) located near the red edge of the $δ$ Sct instability strip. To estimate the asteroseismic parameters, we have generated a grid of stellar models assuming a solar metallicity ($Z=0.014$) and different values for the convective overshooting parameter ($0.1\leq α_{\rm ov}\leq 0.3$). We conclude that the analysis of the radial modes is insufficient to constrain $α_{\rm ov}$ and $Z$ for $δ$ Sct stars. The value for the equatorial velocity of HD 118660 derived from the seismic radius and the rotational frequency is consistent with values found in the literature.

astro-ph.SR↗

arXiv2024

Classification and characterization using HCT/HFOSC spectra of carbon stars selected from the HES survey

We present results from the analysis of 88 carbon stars selected from Hamburg/ESO (HES) survey using low-resolution spectra (R$\sim$1330 \& 2190). The spectra were obtained with the Himalayan Faint Object Spectrograph Camera (HFOSC) attached to the 2-m Himalayan Chandra Telescope (HCT). Using a well-defined spectral criteria based on the strength of carbon molecular bands, the stars are classified into different groups. In our sample, we have identified 53 CH stars, four C-R stars, and two C-N type stars. Twenty-nine stars could not be classified due to the absence of prominent C$_{2}$ molecular bands in their spectra. We could derive the atmospheric parameters for 36 stars. The surface temperature is determined using photometric calibrations and synthesis of the H-alpha line profile. The surface gravity log g estimates are obtained using parallax estimates from the Gaia DR3 database whenever possible. Microturbulent velocity ($ζ$) is derived using calibration equation of log g \& $ζ$. We could determine metallicity for 48 objects from near-infrared Ca II triplet features using calibration equations. The derived metallicity ranges from $-$0.43$\leq$[Fe/H]$\leq$$-$3.49. Nineteen objects are found to be metal-poor ([Fe/H] $\leq$$-$1), 14 very metal-poor ([Fe/H]$\leq$$-$2), and five extremely metal-poor ([Fe/H]$\leq$$-$3.0) stars. Eleven objects are found to have a metallicity in the range $-$0.43 $\leq$[Fe/H]$\leq$$-$0.97. We could derive the carbon abundance for 25 objects using the spectrum synthesis calculation of the C$_{2}$ band around 5165Å. The most metal-poor objects found will make important targets for follow-up detailed chemical composition studies based on high-resolution spectroscopy, that are likely to provide insight into the Galactic chemical evolution.

astro-ph.SR↗

arXiv2023

Peculiar abundances of the cool carbon star HE 1104-0957

We present, for the first time, a detailed abundance analysis of the carbon star HE 1104$-$0957 based on high resolution (R${\sim}$ 50\,000) spectra. Our analysis shows that the object is an extremely metal-poor star with [Fe/H] $\sim$ $-$2.96. We find that the object shows enhancement of carbon with [C/Fe] $\sim$ 1.82. However, it does not fall into any of the sub-groups of carbon-enhanced metal-poor (CEMP) stars based on the characteristic elemental abundance ratios used for the classification of various CEMP sub-groups. HE 1104$-$0957 is also found to exhibit an enhancement of oxygen and nitrogen with [O/Fe], and [N/Fe] $\sim$ 1.54, and 2.54 respectively. In HE 1104$-$0957, $α$-elements are found to be slightly enhanced with [$α$/Fe] $\sim$ 0.46. Fe-peak elements are also moderately enhanced in HE 1104$-$0957 with a value 0.63 with respect to Fe. Our analysis shows that HE 1104$-$0957 exhibits enhancement of neutron-capture elements, particularly r-process elements. The low-resolution spectra of this object shows the spectral features characteristics of a typical C-R star. However, We find that the surface chemical compositions of this object is contradictory to that expected for a C-R star. It requires a detailed analysis to better understand the abundance anomalies exhibited by this object.

astro-ph.SR↗

arXiv2023

Formation scenarios of Ba stars: new evidence from the masses of the companion AGBs

The abundances of the slow neutron-capture (s-) process elements observed in barium stars can be explained by considering mass transfer in a binary system from an asymptotic giant branch (AGB) star onto a smaller mass and less evolved binary companion star. Based on the abundances of neutron-capture elements the barium stars are broadly divided into two categories, "strong" and "mild" barium stars. However, an understanding of the role of the characteristic properties of the companion AGBs on the formation of the different types of barium stars is still lacking. This work focuses on investigating the role of the mass of the companion AGB stars in this context. In a recent high-resolution spectroscopic study, we studied in detail the chemical composition of a few barium stars that we have identified. In order to understand the nature and the mass of the companion stars, we first estimated the mass of these objects and then used a parametric model-based analysis to derive the masses of the primary companion stars. The calculation is extended to 205 barium stars taken from literature. An analysis of the mass distribution revealed that both the strong and the mild barium stars occupy the same range of masses. However, their companion AGB stars' mass distributions peak at two different values 2.5 M$_{\odot}$ and 3.7 M$_{\odot}$, for the strong and the mild barium stars respectively. This provides clear evidence that the formation of mild and strong Ba stars is greatly influenced by the initial masses of the companion AGB stars. It remains, however, to be seen the possible impact of other factors such as orbital period and metallicity on the formation scenarios of barium stars.

astro-ph.SR↗

arXiv2023

On the potential of Carbon-Enhanced Metal-Poor stars for Galactic Archaeology

The low-mass metal-poor stars in the Galaxy that preserve in their atmosphere, the chemical imprints of the gas clouds from which they were formed can be used as probes to get insight into the origin and evolution of elements in the early galaxy, early star formation and nucleosynthesis. Among the metal-poor stars, a large fraction, the so-called carbon-enhanced metal-poor (CEMP) stars exhibits high abundance of carbon. These stars exhibit diverse abundance patterns, particularly for heavy elements, based on which they are classified into different groups. The diversity of abundance patterns points at different formation scenarios. Hence, accurate classification of CEMP stars and knowledge of their distribution is essential to understand the role and contribution of each group. While CEMP-s and CEMP-r/s stars can be used to get insight into binary interactions at very low metallicity, CEMP-no stars can be used to probe the properties of the first stars and early nucleosynthesis. To exploit the full potential of CEMP stars for Galactic archaeology a homogeneous analysis of each class is extremely important. Our efforts towards, and contributions to providing an improved classification scheme for accurate classification of CEMP-s and CEMP-r/s stars and in characterizing the companion asymptotic giant branch (AGB) stars of CH, CEMP-no, CEMP-s and CEMP-r/s binary systems are discussed. Some recent results obtained based on low- and high-resolution spectroscopic analysis of a large number of potential CH and CEMP star candidates are highlighted.

astro-ph.SR↗

arXiv2023

Chemodynamical study of two CEMP-no stars from the Hamburg/ESO Survey

The Carbon-Enhanced Metal-Poor (CEMP) stars with no enhancement of neutron-capture elements, the so-called CEMP-no stars are believed to be the direct descendants of first-generation stars and provide a unique opportunity to probe the early Galactic nucleosynthesis. We present a detailed chemical and kinematic analysis for two extremely metal-poor stars HE1243$-$2408 and HE0038$-$0345 using high-resolution (R${\sim}$86,000) HERMES spectra. For the object HE1243$-$2408, we could make a detailed comparison with the available literature values; however, only limited information is available for the other object HE0038$-$0345. Our estimated metallicity for these two objects are $-$3.05 and $-$2.92 respectively. With estimated [C/Fe] (1.03 and 1.05) and [Ba/Fe] ($-$0.18 and $-$0.11) respectively, the objects are found to be bonafide CEMP-no stars. From the observed abundances of C, Na, Mg, and Ba (i.e., A(C), A(Na), A(Mg), A(Ba)), the objects are found to belong to Group II CEMP-no stars. A detailed abundance profile analysis indicates that the objects are accreted from dSph satellite galaxies that support hierarchical Galaxy assembly. Further, our analysis shows that the progenitors of the stars are likely Pop II Core-Collapse Supernovae. The object HE0038$-$0345 is found to be a high-energy, prograde, outer-halo object, and HE1243$-$2408 is found to be a high-energy, retrograde, inner-halo object. Our detailed chemodynamical analysis shows that HE1243$-$2408 is related to I'itoi structure, where as HE0038$-$0345 is likely related to Sgr or GSE events. The mass of the progenitor galaxies of the program stars inferred from their dynamics is at par with their likely origin in massive dSph galaxies.

astro-ph.SR↗

arXiv2023

Optical characterization and Radial velocity monitoring with Belgian and Indian Telescopes (ORBIT): the eclipsing binaries EPIC 211982753 and EPIC 211915147

The K2 eclipsing binary candidates EPIC 211982753 (hereinafter called EPIC2753) and EPIC 211915147 (hereinafter called EPIC5147) are characterized with the help of photometric and high-resolution spectroscopic data. The light curve analysis uses the R-band photometric data from the 1.3-m Devasthal Fast Optical Telescope (DFOT, India), ASAS-3 and K2 observations. High-resolution echelle spectra are collected using the HERMES spectrograph at the 1.2-m MERCATOR telescope (La Palma, Spain). The synthetic light and radial velocity curves are generated with the help of the modeling package PHOEBE 1.0. The orbital period analysis based on the ~3.2 years of K2 observations does not show any change in the orbital period of both targets. The component masses M1,2 are estimated as 1.69(0.02) and 1.59(0.02) solar mass for EPIC2753, and 1.48(0.01) and 1.27(0.01) solar mass for EPIC5147. Both systems are high mass-ratio eclipsing binaries with q>0.85. The component radii R1,2 are found to be 1.66(0.02) and 1.53(0.02) solar radius for EPIC2753, and 1.80(0.05) and 1.42(0.05) solar radius for EPIC5147. The distances of EPIC2753 and EPIC5147 are determined as 238(4) and 199(5) pc, respectively. MESA Isochrones and Stellar Tracks are used to understand the evolutionary status of both systems.

astro-ph.SR↗

arXiv2022

Spectroscopic study of Ba and CEMP-s stars: Mass distribution of AGB progenitors

We have performed detailed high-resolution spectroscopic analysis on seven metal-poor stars (BD+75 348, BD+09 3019, HD238020, HE0319-0215, HE0507-1653, HE0930-0018, HE1023-1504) and derived their atmospheric parameters T$_{eff}$, log$g$, [Fe/H], and mictroturbulent velocity ($ξ$). The metallicity range is found to be -2.57$<$[Fe/H]$<$-0.42. The elemental abundances of 17 light elements and 12 heavy elements are estimated. We have classified BD+75 348 and BD+09 3019 as strong Ba stars, HD238020 as a mild Ba star, and the remaining four objects as CEMP-s stars. We have estimated the masses of the stars from Hertzsprung-Russel (HR) diagram, and, compiling the data of 205 Ba stars from literature, estimated the mass distribution of Ba stars. We have also estimated the initial masses of the companion AGBs of the program stars as well as the masses of the companion AGBs of 159 Ba and 36 CEMP-s stars from literature, with the help of a parametric-model-based analysis using FRUITY models. While the primary mass distribution of mild Ba stars peaks at 3.7 M$_{\odot}$, for the strong Ba stars the peak appears at 2.5 M$_{\odot}$. We, therefore, propose that the initial masses of the progenitor AGBs dominantly control the formation of mild and strong Ba stars. However, a clear overlap, in the range 1.3-4.0 M$_{\odot}$, noticed between the progenitor masses of both the subclasses of Ba stars, may indicate that other factors, such as metallicities and the orbital periods, may also have significant contributions. The progenitor AGBs' mass distribution of CEMP-s stars is found to peak at 2.03 M$_{\odot}$.

astro-ph.SR↗

arXiv2022

Spectroscopic study of four metal-poor carbon stars from the Hamburg/ESO Survey: On confirming the low-mass nature of their companions

Elemental abundances of extrinsic carbon stars provide insight into the poorly understood origin and evolution of elements in the early Galaxy. In this work, we present the results of a detailed spectroscopic analysis of four potential carbon star candidates from the Hamburg/ESO Survey (HES) HE~0457$-$1805, HE~0920$-$0506, HE~1241$-$0337, and HE~1327$-$2116. This analysis is based on the high-resolution spectra obtained with Mercator/HERMES (R$\sim$86,000) and SUBARU/HDS (R$\sim$50,000). Although the abundances of a few elements, such as, Fe, C, and O are available from medium-resolution spectra, we present the first ever detailed high-resolution spectroscopic analysis for these objects. The object HE~0457$-$1805 and HE~1241$-$0337 are found to be CEMP-s stars, HE~0920$-$0506 a CH star, and HE~1327$-$2116 a CEMP-r/s star. The object HE~0457$-$1805 is a confirmed binary, whereas the binary status of the other objects is unknown. The locations of program stars on the absolute carbon abundance A(C) vs [Fe/H] diagram point at their binary nature. We have examined various elemental abundance ratios of the program stars and confirmed the low-mass nature of their former AGB companions. We have shown that the i-process models could successfully reproduce the observed abundance pattern in HE~1327$-$2116. The parametric model based analysis performed for HE~0457$-$1805, HE~0920$-$0506, and HE~1241$-$0337 based on the FRUITY models confirmed that the surface chemical composition of these three objects are influenced by pollution from low-mass AGB companions.

astro-ph.SR↗

arXiv2022

LAMOST J045019.27+394758.7, with peculiar abundances of N, Na, V, Zn, is possibly a Sculptor dwarf galaxy escapee

We present the results of the high-resolution (R$\sim$60,000) spectroscopic analysis of the star LAMOSTJ045019.27+394758.7 (hereafter J045) from the list of carbon stars of LAMOST DR2. From our analysis, we find that J045 does not exhibit the spectral characteristics of carbon stars. It is found to be a metal-poor ( [Fe/H] = $-$1.05) giant that shows very unusual elemental abundances, particularly for N, Na, V, and Zn. J045 shows $α$-elements (Mg, Si, Ca) with near-solar values ($<$[$α$/Fe]$>$ = 0.09) in contrast to Galactic stars that show [$α$/Fe] in the range 0.2 to 0.3 dex. In J045, Sc and Ti are under abundant with [X/Fe] $\le$ $-$0.25. Vanadium gives [V/Fe] = 0.51 and zinc is under-abundant with [Zn/Fe] = $-$0.62. The object exhibits near-solar abundances for Sr, Y, Ba, Pr, and Sm. The La is marginally enhanced, and Ce and Nd are marginally under-abundant in J045. With [Ba/Eu] = $-$0.38, the object falls into the category of neutron-capture rich r-I stars. The estimated abundances of various elements show that the observed abundance pattern is not compatible with the abundances characteristic of Galactic metal-poor stars but matches quite closely with the abundance pattern of Sculptor Dwarf galaxy stars of similar metallicity. Based on the above observational evidences, we suggest that the object is a possible Sculptor Dwarf Galaxy escapee.

astro-ph.GA↗

arXiv2021

The peculiar abundances of HE 1005-1439. A carbon-enhanced extremely metal-poor star contaminated with products of both s-and i-process nucleosynthesis

We performed a detailed, high-resolution spectroscopic analysis of HE 1005-1439 based on SUBARU/HDS spectra with an R ~ 50000. Abundances of ten light elements from C through Ni and twelve heavy elements Sr, Y, Ba, La, Ce, Pr, Nd, Eu, Dy, Er, Hf, and Pb were determined. We also performed a parametric-model-based analysis of the abundances of the heavy elements to understand the origin of the observed abundance pattern. For the first time, we came across an object with a surface chemical composition that exhibits contributions from both slow (s) and intermediate (i) neutron-capture nucleosynthesis. The observed abundance pattern is unique and has never been observed in any CEMP stars. The star is found to be a CEMP-s star based on the CEMP stars' classification criteria. However, the observed abundance pattern could not be explained based on theoretical s-process model predictions. On the contrary, our parametric-model based analysis clearly indicates its surface chemical composition being influenced by similar contributions from both the s- and i-process. We critically examined the observed abundances and carefully investigated the formation scenarios involving s-process and i-process that are available in literature, and we found that none of them could explain the observed abundances. We note that the variation we see in our radial velocity estimates obtained from several epochs may indicate the presence of a binary companion. Considering a binary system, we, therefore, propose a formation scenario for this object involving effective proton ingestion episodes (PIEs) triggering i-process nucleosynthesis followed by s-process asymptotic giant branch (AGB) nucleosynthesis with a few third-dredge-up (TDU) episodes in the now extinct companion AGB star. Results obtained from the parametric-model-based analysis are discussed in light of existing stellar evolutionary models.

astro-ph.SR↗

arXiv2021

Observational evidence points at AGB stars as possible progenitors of CEMP-s & r/s stars

Origin of enhanced abundance of heavy elements observed in the surface chemical composition of carbon-enhanced metal-poor (CEMP) stars still remain poorly understood. Here, we present detailed abundance analysis of seven CEMP stars based on high resolution (R${\sim}$ 50\,000) spectra that reveal enough evidence of Asymptotic Giant Branch (AGB) stars being possible progenitors for these objects. For the objects HE0110$-$0406, HE1425$-$2052 and HE1428$-$1950, we present for the first time a detailed abundance analysis. Our sample is found to consists of one metal-poor ([Fe/H]$<$$-1.0$) and six very metal-poor ([Fe/H]$<$$-2.0$) stars with enhanced carbon and neutron-capture elements. We have critically analysed the observed abundance ratios of [O/Fe], [Sr/Ba] and [hs/ls] and examined the possibility of AGB stars being possible progenitors. The abundance of oxygen estimated in the programme stars are characteristics of AGB progenitors except for HE1429$-$0551 and HE1447$+$0102. The estimated values of [Sr/Ba] and [hs/ls] ratios also support AGB stars as possible progenitors. The locations of the programme stars in the absolute carbon abundance A(C) vs. [Fe/H] diagram along with the Group I objects hint at binary nature of the object. We have studied the chemical enrichment histories of the programme stars based on abundance ratios [Mg/C], [Sc/Mn] and [C/Cr]. Using [C/N] and $^{12}$C/$^{13}$C ratios we have examined if any internal mixing had modified their surface chemical compositions. Kinematic analysis shows that the objects HE 0110$-$0406 and HE1447$+$0102 are thick disk objects and the remaining five objects belong to the halo population of the Galaxy.

astro-ph.SR↗

arXiv2021

Probing the nucleosynthetic contribution of low-metallicity, low-mass star companions of CEMP stars

The observed abundance diversities among the CEMP stars can shed light on the formation and evolution of elements in the early Galaxy. In this work, we present results obtained from a detailed abundance analysis of a sample of seven extrinsic carbon stars. The analysis is based on high-resolution spectra obtained with HCT/HESP (R$\sim$60,000) and SUBARU/HDS (R$\sim$50,000). We present, for the first time, the elemental abundance results for the objects BD$-$19 132, BD$-$19 290, HE~1304$-$2111, HE~1354$-$2257, and BD+19 3109. Abundances of a few elements are available in literature for HE~1157$-$0518 and HD~202851, we present an update on the abundances of these elements along with new abundance estimates for several other elements. Our analysis confirms the object HD~202851 to be a CH star. While BD$-$19 132, HE~1354$-$2257, and BD+19 3109 are found to be CEMP-s stars, the objects BD$-$19 290, HE~1157$-$0518, and HE~1304$-$2111 are found to belong to CEMP-r/s group. The observed abundance patterns of the three CEMP-r/s stars are well reproduced with the i-process model predictions. While the objects BD+19 3109 and HD~202851 are confirmed binaries, the binary status of the remaining objects are not known. Analysis based on different elemental abundance ratios confirms low-mass former AGB companions for all the objects. Kinematic analysis shows that BD$-$19 290, HE~1157$-$0518, HE~1354$-$2257, and BD+19 3109 belong to the Galactic halo, whereas BD$-$19 132, HE~1304$-$2111, and HD~202851 are members of Galactic thin disk.

astro-ph.SR↗

arXiv2021

Chemical analysis of two extremely metal-poor stars HE 2148-2039 and HE 2155-2043

We present elemental abundance results for HE 2148-2039 and HE 2155-2043 based on a detailed high-resolution spectroscopic analysis. The high-resolution SUBARU/HDS spectra used for our analysis have a resolution of R~60 000. Although limited information based on photometry and low-resolution spectroscopy are available, we present for the first time an abundance analysis based on high-resolution spectra for both the objects. Our analysis shows that the two objects are extremely metal-poor with [Fe/H]< -3. Among the neutron-capture elements, abundances of only Sr and Ba could be determined in our programme stars. For both the objects [Ba/Fe] is found to be < 0. While strontium is under abundant in HE 2148-2039 with [Sr/Fe]~ -2.02, Sr is near solar in HE 2155-2043. The locations of the programme stars in the absolute carbon abundance, A(C) vs. [Fe/H] diagram show that HE 2148-2039 is a CEMP-no Group II object and HE 2155-2043 is a CEMP-no Group III object. Observed [Sr/Ba] ratios are characteristics of a fast rotating massive star progenitor for HE 2155-2043 and a metal-poor Asymptotic giant branch (AGB) star for HE 2148-2039. The estimated [Sc/Mn] as well as [C/Cr] ratios in HE 2155-2043 show that the surface chemical composition of this object is mono-enriched. The surface chemical composition of HE 2148-2039 is also found to be mono-enriched based on [Mg/C] ratio. With respect to their locations in the [C/N] vs. T eff diagram, HE 2148-2039 shows signatures of mixing, and HE 2155-2043 falls in the unmixed region of [C/N] vs. Teff plot. Kinematic analysis shows that both the objects belong to Galactic halo population.

astro-ph.SR↗

arXiv2021

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

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/L0) > 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.

astro-ph.SR↗

arXiv2021

HCT/HESP study of two carbon stars from the LAMOST survey

Carbon stars, enhanced in carbon and neutron-capture elements, provide wealth of information about the nucleosynthesis history of the Galaxy. In this work, we present the first ever detailed abundance analysis of carbon star LAMOSTJ091608.81+230734.6 and a detailed abundance analysis of neutron-capture elements for the object LAMOSTJ151003.74+305407.3. Updates on the abundances of elements C, O, Mg, Ca, Cr, Mn and Ni for LAMOSTJ151003.74+305407.3 are also presented. Our analysis is based on high resolution spectra obtained using Hanle Echelle Spectrograph (HESP) attached to the Himalayan Chandra Telescope (HCT), IAO, Hanle. The stellar atmospheric parameters (T$_{eff}$, logg, micro-turbulance $ζ$, metallicity [Fe/H]) are found to be (4820, 1.43, 1.62, $-$0.89) and (4500, 1.55, 1.24, $-$1.57) for these two objects respectively. The abundance estimates of several elements, C, N, O, Na, $α$-elements, Fe-peak elements and neutron-capture elements Rb, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm and Eu are presented. Our analysis shows the star LAMOSTJ151003.74+305407.3 to be a CEMP-r/s star, and LAMOSTJ091608.81+230734.6 a CH giant. We have examined if the i-process model yields ([X/Fe]) of heavy elements could explain the observed abundances of the CEMP-r/s star based on a parametric model based analysis. The negative values obtained for the neutron density dependent [Rb/Zr] ratio confirm former low-mass AGB companions for both the stars. Kinematic analysis shows that LAMOSTJ151003.74+305407.3 belongs to the Galactic halo population and LAMOSTJ091608.81+230734.6 to the disc population.

astro-ph.SR↗