arXiv subjects

Bacham E. Reddy

Publications and source records attributed to Bacham E. Reddy.

At least 19 recordsLinked to original sources

arXiv2026

Unveiling the nature of barium stars. I. Asteroseismic masses and the evolutionary link between Ba dwarfs and giants

Barium star systems are excellent sites for studying AGB nucleosynthesis, binary evolution, and mass transfer processes. However, an accurate estimation of their fundamental stellar parameters is still lacking. Using TESS data, we made the first extensive asteroseismic mass measurements of 31 Ba giants and 13 Ba dwarfs. For some, we were able to measure $ΔP$, ascertaining their evolutionary phase. We then constructed a grid of stellar models across the relevant mass range, where we accreted AGB material using composition from existing yields. We found that the average masses of the Ba dwarfs and Ba giants are significantly different ($1.29\pm0.09~\rm{M}_\odot$ versus $1.96\pm0.16~\rm{M}_\odot$, respectively). However, their mass distributions peak around $1.3~\rm{M}_\odot$. While our sample of Ba giants spans the low- and intermediate-mass regime, we found no intermediate-mass Ba dwarfs. The abundance trends of $s$-process elements show an overall anti-correlation with stellar mass, particularly in the low-mass regime. The stellar models adopting Monash AGB yields can satisfactorily reproduce the observed light elements, $s$, and heavy-$s$ abundance trends, with an accreted mass of $0.1-0.5~\rm{M}_\odot$, but fail to explain the [hs/ls] ratio. Our results support an evolutionary scenario in which Ba giants evolve from Ba dwarfs, with mass accretion occurring while the progenitor Ba star is still on the main sequence. In this scenario, a substantial number of intermediate-mass Ba dwarfs are expected. We found that post-accretion additional mixing in our models is critical to explain the observed $s$-process abundances in Ba dwarfs and the low C isotopic ratio ($<30$) in Ba giants. The mismatch between the model and the observed [hs/ls] ratio suggests that the chemical enrichment of Ba stars cannot be explained by standard single-star AGB yields alone (abridged for arXiv).

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arXiv2025

Asteroseismology of Carbon-Deficient Red Giants: Merger Products of Hierarchical Triple Systems?

Carbon-deficient giants (CDGs) are a rare and chemically peculiar class of stars whose origins remain under active investigation. We present an asteroseismic analysis of the entire known CDG population, selecting 129 stars observed by $Kepler$, K2, and TESS to obtain seismic constraints. We detect solar-like oscillations in 43 CDGs. By measuring $ν_{\rm max}$ and applying seismic scaling relations, we determine precise masses for these stars, finding that 79\% are low-mass ($M \lesssim 2~M_\odot$). The luminosity distribution is bimodal, and the CDGs separate into three chemically and evolutionarily distinct groups, characterized by clear trends in sodium and CNO abundances, $α$-element enhancement, and kinematics. We find that two of these groups are only distinguished by their initial $α$-element abundances, thus effectively reducing the number of groups to two. Lithium enrichment is common across all groups, linking CDGs to lithium-rich giants and suggesting a shared evolutionary origin. We find that spectroscopic $\log g$ is systematically offset from seismic values. Group~1 CDG patterns are most consistent with formation through core He-flash mixing, while the more massive and more chemically processed Groups~2 and 2$α$ likely formed through mergers involving helium white dwarfs, possibly in hierarchical triples. Pollution from AGB stars appears very unlikely, given the unchanged [C+N+O] abundance across all groups.

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arXiv2025

Li-enrichment in red clump giants: Clues for past binary interaction or merger events

To understand the underlying mechanisms for high lithium abundances among core He-burning or red clump (RC) giants, we analyzed a sample of 5227 RC giants of mass M $\leq$ 2~M$_{\odot}$ using spectra and asteroseismic data. We found 120 RC giants ($\sim$2~$\%$) with a lower limit of A(Li) = 0.7~dex, a factor of 40 more than their predecessors close to the RGB tip. Of the 120 RC giants, we could measure actual rotations for 16 RC giants using stellar spots from the Kepler light curve analysis. We found that most of the high rotation RC giants are also very high Li-rich RC giants, and the rotation seems to decline rapidly with Li abundance depletion, suggesting that both the high rotation and high Li abundance are transient phenomena and associated with a single source. Further, we found a significantly high occurrence of 15~$\%$ and 12~$\%$ of Li-rich RC giants among extremely low-mass RC giants and RC giants with anomalous [C/N] ratios, respectively. The extremely low mass, fast rotation and anomalous [C/N] values of RC giants are attributed to their past binary interaction/merger history. The results pose a question of whether the binary interaction/merger is a prerequisite along with the He-flash for Li-enhancement among RC giants.

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arXiv2025

High Lithium Abundance Connection with the Chromospheric Helium in Red Giants: Spectroscopic and Asteroseismic analyses

We present a study of correlations between high Li abundances and strong chromospheric He I 10830 Å absorption line strengths in Kepler field giant stars. Our sample includes 84 giants with detectable solar-like oscillations in their lightcurves, and their Li abundances come from the literature or were measured here using LAMOST medium-resolution spectra. Evolutionary phases are determined through asteroseismic analysis, with mixed-mode period spacing (ΔP) used to infer the time evolution of RC giants. Near-infrared observations of the He I λ10830 line were obtained with the high-resolution Habitable-zone Planet Finder (HPF) spectrograph on the Hobby-Eberly Telescope (HET). We find high Li abundances and strong He I lines exclusively among red clump (RC) giants, with their absence in red giant branch stars suggesting a shared origin linked to the He-flash. Additionally, a steady decline in He I strength with decreasing Li abundance among RC giants indicates a correlation between these properties. Older, Li-normal RC giants are He-weak, while most younger super-Li-rich giants are He-strong, suggesting temporal evolution of both phenomena. We hypothesize that the core He-flash and subsequent sub-flashes may enhance Li abundances in RC giant photospheres and trigger heightened chromospheric activity, leading to stronger He I λ10830 Å lines in younger RCs. Over time, post-He-flash, chromospheric activity diminishes, resulting in weaker He I lines in older, Li-normal RCs.

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arXiv2024

Study of a red clump giant, KIC~11087027, with high rotation and strong infrared excess -- Evidence of tidal interaction for high lithium abundance

This paper presents results from Kepler photometric light curves and high-resolution spectroscopic study of a super Li-rich giant KIC11087027. Using the light curve analysis, we measured the star's rotational period P$_{\rm rot}$=30.4$\pm$0.1~days, which translates to rotational velocity V$_{\rm rot}$=19.5 $\pm$ 1.7~km s$^{-1}$. Star's location in the HR-diagram, derived values of $^{12}C/^{13}C$ = 7$\pm$1 and $[C/N]=-0.95\pm 0.2$, and the inferred asteroseismic parameters from secondary calibration based on spectra suggest star is a low-mass red clump giant in the He-core burning phase. Using Gaia data, we found evidence of variation in radial velocity and proper motion, indicative of presence of an unresolved binary. The large V$_{\rm rot}$ is probably a result of tidal synchronization combined with the after-effects of He-flash, in which the size of the star is reduced significantly. The simultaneous presence of features like high rotation, very high Li abundance, strong dust shell, and strong flares in a single star is relatively uncommon, suggesting that the star experiencing tidal synchronization has recently undergone He-flash. The results pose a question whether the binary interaction, hence the high rotation, is a prerequisite for dredging-up of the high amounts of Li from the interior to the photosphere during or immediately after the He-flash event.

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arXiv2024

Mining the GALAH data I: Study of five Super lithium-rich metal-poor giants

The presence of a large amount of Li in giants is still a mystery. Most of the super Li-rich giants reported in recent studies are in the solar metallicity regime. Here, we study the five metal-poor super Li-rich giants (SLRs) from GALAH Data Release 3 with their [Fe/H] ranging from -1.35 to -2.38 with lithium abundance of A(Li) $\geq$ 3.4~dex. The asteroseismic analysis reveals that none are on the red giant branch. The average period spacing ($ΔP$ ) values indicate giants are in the core He-burning phase. All of them are low-mass giants (M $<$ 1.5M$_{\odot}$). The location in the HR diagram suggests one of them is in the red clump phase, and interestingly, the other four are much brighter and coincide with the early AGB phase. The abundance analysis reveals that C, O, Na, Ba, and Eu are normal for giants of respective metallicities and evolutionary phases. Further, we didn't find any strong evidence for the presence of dust in the form of infrared excess or binarity from the available radial velocity data. We discussed a few scenarios for the existence of SLRs at higher luminosity, including past merger events. The findings will help to understand the production and evolution of Li among giants, in particular, during and the post-red clump phase.

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arXiv2023

Asteroseismology sheds light on the origin of carbon-deficient red giants: likely merger products and linked to the Li-rich giants

Carbon-deficient red giants (CDGs) are a peculiar class of stars that have eluded explanation for decades. We aim to better characterise CDGs by using asteroseismology (Kepler, TESS) combined with spectroscopy (APOGEE, LAMOST), and astrometry (Gaia). We discovered 15 new CDGs in the Kepler field, and confirm that CDGs are rare, being only $0.15\%$ of our background sample. Remarkably, we find that our CDGs are almost exclusively in the red clump (RC) phase. Asteroseismic masses reveal that our CDGs are primarily low-mass stars ($M \lesssim$ 2~M$_{\odot}$), in contrast to previous studies which suggested they are intermediate mass ($M = 2.5 - 5.0~\rm M_{\odot}$) based on HR diagrams. A very high fraction of our CDGs ($50\%$) are also Li-rich giants. We observe a bimodal distribution of luminosity in our CDGs, with one group having normal RC luminosity and the other being a factor of two more luminous than expected for their masses. We find demarcations in chemical patterns and luminosities which lead us to split them into three groups: (i) normal-luminosity CDGs, (ii) over-luminous CDGs, and (iii) over-luminous highly-polluted CDGs. We conclude that a merger of a helium white dwarf with an RGB star is the most likely scenario for the two groups of over-luminous stars. Binary mass-transfer from intermediate-mass AGB stars is a possibility for the highly-polluted over-luminous group. For the normal-luminosity CDGs, we cannot distinguish between core He-flash pollution or lower-mass merger scenarios. Due to the overlap with the CDGs, Li-rich giants may have similar formation channels.

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arXiv2023

A large sample of newly-identified carbon-deficient red giants from APOGEE

Based on the APOGEE survey we conducted a search for carbon-deficient red giants (CDGs). We found 103 new CDGs, increasing the number in the literature by more than a factor of 3. CDGs are very rare, representing $0.03$~per cent of giants. They appear as an extended tail off the normal carbon distribution. We show that they are found in all components of the Galaxy, contrary to previous findings. The location of CDGs in the Hertzsprung-Russell diagram (HRD) shows that they are primarily intermediate-mass stars ($2-4~\rm{M}_{\odot}$). Their extended distribution may indicate that CDGs can also sometimes have $M < 2.0~\rm{M}_{\odot}$. We attempted to identify the evolutionary phases of the CDGs using stellar model tracks. We found that the bulk of the CDGs are likely in the subgiant branch or red clump phase, whereas other CDGs may be in the red giant branch or early asymptotic giant branch phases. Degeneracy in the HRD makes exact identification difficult. We examined their C, N, and O compositions and confirmed previous studies showing that the envelope material has undergone extensive hydrogen burning through the CN(O) cycle. The new-CDGs have [C+N+O/Fe] that generally sum to zero, indicating that they started with scaled-solar composition. However, the previously known-CDGs generally have [C+N+O/Fe$] > 0.0$, indicating that some He-burning products were added to their envelopes. As to the site(s) in which this originally occurred, we do not find a convincing solution.

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arXiv2023

Lithium abundances in giants as a function of stellar mass: An evidence for He-flash as the source of Li enhancement in low mass giants

In this work, we studied the distribution of lithium abundances in giants as a function of stellar mass. We used a sample of 1240 giants common among Kepler photometric and LAMOST medium resolution (R $\approx$ 7500) spectroscopic survey fields. The asteroseismic $Δ$P - $Δν$ diagram is used to define core He-burning red clump giants and red giant branch stars with inert He-core. Li abundances have been derived using spectral synthesis for the entire sample stars. Directly measured values of asteroseismic parameters $Δ$P(or $ΔΠ_1$) and $Δν$ are either taken from the literature or measured in this study. Of the 777 identified red clump giants, we found 668 low mass ($\leq$ 2~M$_{\odot}$) primary red clump giants and 109 high mass ($>$ 2~M$_{\odot}$) secondary red clump giants. Observed Li abundances in secondary red clump giants agree with the theoretical model predictions. The lack of Li-rich giants among secondary red clump giants and the presence of Li-rich, including super Li-rich giants, among primary red clump stars reinforces the idea that Helium-flash holds the key for Li enrichment among low-mass giants. The results will further constrain theoretical models searching for a physical mechanism for Li enhancement among low-mass red clump giants. Results also serve as observational evidence that only giants with mass less than $\approx$ 2~M$_{\odot}$ develop degenerate He-core and undergo He-flash.

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arXiv2022

The Active Chromospheres of Lithium-Rich Red Giant Stars

We have gathered near-infrared $zyJ$-band high resolution spectra of nearly 300 field red giant stars with known lithium abundances in order to survey their \species{He}{i} $λ$10830 absorption strengths. This transition is an indicator of chromospheric activity and/or mass loss in red giants. The majority of stars in our sample reside in the red clump or red horizontal branch based on their $V-J,M_V$ color-magnitude diagram and their Gaia \teff, \logg\ values. Most of our target stars are Li-poor in the sense of having normally low Li abundances, defined here as \eps{Li}~$<$~1.25. Over 90\% of these Li-poor stars have weak $λ$10830 features. But more than half of the 83 Li-rich stars (\eps{Li}~$>$~1.25) have strong $λ$10830 absorptions. These large $λ$10830 lines signal excess chromospheric activity in Li-rich stars; there is almost no indication of significant mass loss. The Li-rich giants also may have a higher binary fraction than do Li-poor stars, based on their astrometric data. It appears likely that both residence on the horizontal branch and present or past binary interaction play roles in the significant Li-He connection established in this survey.

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arXiv2022

Probing infrared excess connection with Li enhancement among red clump giants

We have performed a search among low mass red giants for finding evidence for merger scenario for triggering He-flash and subsequent Li enhancement. We chose a sample of red giants from GALAH survey with well-measured Li abundances, and near and mid-IR fluxes from 2MASS and WISE surveys, respectively. The sample contains 418 cool red clump giants and 359 upper red giant branch giants. Most of the giants and majority of super Li-rich giants show no IR excess. Only five red clump giants and one RGB giant show IR excess. Notably, of the five red clump giants with IR excess, three are super Li-rich (A(Li) > = 3.2 dex), and two are Li-rich (A(Li) > = 1.0 dex). Results suggest Li enhancement among red clump giants may be due to two channels: one resulting from in-situ He-flash in single star evolution and the other due to He-flash triggered by events like merger of He-white dwarfs with giants He-inert core on RGB. In the latter case, IR excess, as a result of mass loss, is expected from merger events. We have modelled IR excess in all six giants using DUSTY code and derived dust parameters. The estimated kinematic ages and time scales of dust envelopes of the super Li-rich phase suggest Li enhancement took place very recently. Further, the analysis shows a significantly higher proportion (four out of five red clump giants) of rapid rotators (vsini > = 8 km/s) among Li-rich giants with IR excess compared to Li-normal and Li-rich giants with no IR excess.

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arXiv2021

Tracking the evolution of lithium in giants using asteroseismology: Super-Li-rich stars are almost exclusively young red-clump stars

We report novel observational evidence on the evolutionary status of lithium-rich giant stars by combining asteroseismic and lithium abundance data. Comparing observations and models of the asteroseismic gravity-mode period spacing $ΔΠ_{1}$, we find that super-Li-rich giants (SLR, A(Li)~$> 3.2$~dex) are almost exclusively young red-clump (RC) stars. Depending on the exact phase of evolution, which requires more data to refine, SLR stars are either (i) less than $\sim 2$~Myr or (ii) less than $\sim40$~Myr past the main core helium flash (CHeF). Our observations set a strong upper limit for the time of the inferred Li-enrichment phase of $< 40$~Myr post-CHeF, lending support to the idea that lithium is produced around the time of the CHeF. In contrast, the more evolved RC stars ($> 40$~Myr post-CHeF) generally have low lithium abundances (A(Li)~$<1.0$~dex). Between the young, super-Li-rich phase, and the mostly old, Li-poor RC phase, there is an average reduction of lithium by about 3 orders of magnitude. This Li-destruction may occur rapidly. We find the situation to be less clear with stars having Li abundances between the two extremes of super-Li-rich and Li-poor. This group, the `Li-rich' stars ($3.2 >$~A(Li)~$> 1.0$~dex), shows a wide range of evolutionary states.

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arXiv2021

Host star metallicity of directly imaged wide-orbit planets: implications for planet formation

Directly imaged planets are self-luminous companions of pre-main sequence and young main sequence stars. They reside in wider orbits ($\sim10\mathrm{s}-1000\mathrm{s}$~AU) and generally are more massive compared to the close-in ($\lesssim 10$~AU) planets. Determining the host star properties of these outstretched planetary systems is important to understand and discern various planet formation and evolution scenarios. We present the stellar parameters and metallicity ([Fe/H]) for a subsample of 18 stars known to host planets discovered by the direct imaging technique. We retrieved the high-resolution spectra for these stars from public archives and used the synthetic spectral fitting technique and Bayesian analysis to determine the stellar properties in a uniform and consistent way. For eight sources, the metallicities are reported for the first time, while the results are consistent with the previous estimates for the other sources. Our analysis shows that metallicities of stars hosting directly imaged planets are close to solar with a mean [Fe/H] = $-0.04\pm0.27$~dex. The large scatter in metallicity suggests that a metal-rich environment may not be necessary to form massive planets at large orbital distances. We also find that the planet mass-host star metallicity relation for the directly imaged massive planets in wide-orbits is very similar to that found for the well studied population of short period ($\lesssim 1$~yr) super-Jupiters and brown-dwarfs around main-sequence stars.

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arXiv2020

Discovery of ubiquitous lithium production in low-mass stars

The vast majority of stars with mass similar to the Sun are expected to only destroy lithium over the course of their lives, via low-temperature nuclear burning. This has now been supported by observations of hundreds of thousands of red giant stars (Brown et al. 1989, Kumar et al. 2011, Deepak et al. 2019, Singh et al. 2019, Casey et al. 2019). Here we perform the first large-scale systematic investigation into the Li content of stars in the red clump phase of evolution, which directly follows the red giant branch phase. Surprisingly we find that all red clump stars have high levels of lithium for their evolutionary stage. On average the lithium content increases by a factor of 40 after the end of the red giant branch stage. This suggests that all low-mass stars undergo a lithium production phase between the tip of the red giant branch and the red clump. We demonstrate that our finding is not predicted by stellar theory, revealing a stark tension between observations and models. We also show that the heavily studied (Brown et al. 1989, Reddy et al. 2005, Kumar et al. 2011, Singh et al. 2019, Casey et al. 2019) very Li-rich giants, with A(Li) $> +1.5$ dex, represent only the extreme tail of the lithium enhancement distribution, comprising 3% of red clump stars. Our findings suggest a new definition limit for Li-richness in red clump stars, A(Li) $> -0.9$ dex, which is much lower than the limit of A(Li) $> +1.5$ dex used over many decades (Brown et al. 1989, Castilho et al. 1995, Reddy et al. 2005, Carlberg et al. 2016, Casey et al. 2019, Holanda et al. 2020).

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arXiv2019

Concerning Li-rich status of KIC~9821622: A Kepler field RGB star reported as Li-rich Giant

Given the implications for the origin of Li enhancement in red giants we have reviewed Li-rich classification of KIC~9821622, the only bonafide RGB giant with He inert-core till date, reported as a Li-rich giant by reanalyzing the high-resolution spectra. We have obtained $A(Li)_{LTE} = 1.42 \pm 0.05$ dex. After correcting for non-LTE it is $A(Li)_{NLTE} = 1.57 \pm 0.05 $ dex which is significantly less than the reported A(Li) = $1.80 \pm 0.2$~dex. We found the sub-ordinate line at 6103 Å is too weak or absent to measure Li abundance. The derived abundance is normal for red giants undergoing dilution during the 1st dredge-up. Since all the known Kepler field Li-rich giants belong to the red clump region, this clarification removes the anomaly and strengthens the evidence that the Li enhancement in low mass giants may be associated only with the He-core burning phase. The Li excess origin, probably, lies during He-flash at the RGB tip, an immediate preceding phase to red clump.

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arXiv2019

Survey of Li-rich giants among Kepler and LAMOST fields: Determination of Li-rich giants Evolutionary Phase

In this letter, we report the discovery of 24 new super Li-rich (A(Li) $\ge$ 3.2) giants of He-core burning phase at red clump region. Results are based on systematic search of a large sample of about 12,500 giants common to the LAMOST spectroscopic and Kepler time resolved photometric surveys. The two key parameters derived from Kepler data; average period spacing ($Δp$) between $l=1$ mixed gravity dominated g-modes and average large frequency separation ($Δν$) $l=0$ acoustic p-modes, suggest all the Li-rich giants are in He-core burning phase. This is the first unbiased survey subjected to a robust technique of asteroseismic analysis to unambiguously determine evolutionary phase of Li-rich giants. The results provide a strong evidence that Li enhancement phenomenon is associated with giants of He-core burning phase, post He-flash, rather than any other phase on RGB with inert He-core surrounded by H-burning shell.

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arXiv2019

Chemical compositions of giants in the Hyades And Sirius superclusters

An abundance analysis for 20 elements from Na to Eu is reported for 34 K giants from the Hyades supercluster and for 22 K giants from the Sirius supercluster. Observed giants were identified as highly probable members of their respective superclusters by Famaey et al. (2005, A&A, 430, 165). Three giants each from the Hyades and Praesepe open clusters were similarly observed and analysed. Each supercluster shows a range in metallicity: $-0.20 \leq$ [Fe/H] $\leq +0.25$ for the Hyades supercluster and $-0.22 \leq $ [Fe/H] $\leq +0.15$ for the Sirius supercluster with the metal-rich tail of the metallicity distribution of the Hyades supercluster extending beyond that of the Sirius supercluster and spanning the metallicity of the Hyades and Praesepe cluster giants. Relative elemental abundances [El/Fe] across the supercluster giants are representative of the Galactic thin disc as determined from giants in open clusters analysed in a similar way to our approach. Judged by metallicity and age, very few and likely none of the giants in these superclusters originated in an open cluster: the pairings include the Hyades supercluster with the Hyades - Praesepe open clusters and the Sirius supercluster with the U Ma open cluster. Literature on main sequence stars attributed to the two superclusters and the possible relation to the associated open cluster is reviewed. It is suggested that the Hyades supercluster's main sequence population contains few stars from the two associated open clusters. As suggested by some previous investigations, the Sirius supercluster, when tightly defined kinematically, appears to be well populated by stars shed by the U Ma open cluster.

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arXiv2018

Spectroscopic study of two new super Li rich red clump K giants

In this paper, we report the discovery of two new super Li-rich K giants: HD~24960 and TYC~1751-1713-1. Based on high resolution ($R\approx 60,000$) spectroscopy, we have derived Li abundance of A(Li)$\approx$4.0~dex for both stars. Other elemental abundances are normal of typical K giants. Low ratios of [C/N] $\leq$ $-$ 0.25 and $^{12}C/^{13}C \leq10$ suggest that the stars are in upper RGB phase. Further, based on Gaia astrometry and secondary calibrations using Kepler asteroseismic and LAMOST spectroscopic data, we argue that both stars belong to red clump (RC) phase with core He-burning. Results add to half a dozen already known red clump Li-rich K giants, and support the growing evidence that the origin of Li excess in RC giants seems to be associated with either He-flash at the tip of the red giant branch (RGB) or recent planet/ brown dwarf merger events closer to the RGB tip.

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