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Determining the He/H ratio in cool stars presents a fundamental astrophysical challenge. While this ratio is established for hot O and B stars, its extrapolation to cool stars remains uncertain due to the absence of helium lines in their observed spectra. We address this knowledge gap by focusing on the Sun as a representative cool star. We conduct spectroscopic analyses of the observed solar photospheric lines by utilizing a combination of MgH molecular lines and neutral Mg atomic lines including yet another combination of CH and C_2 molecular lines with neutral C atomic lines. Our spectroscopic analyses were further exploited by adopting solar model atmospheres constructed for distinct He/H ratios to determine the solar photospheric helium abundance. The helium abundance is determined by enforcing the fact that for an adopted model atmosphere with an appropriate He/H ratio, the derived Mg abundance from the neutral Mg atomic lines and that from the MgH molecular lines must be the same. Ditto holds for the C abundance derived from neutral C atomic lines and that from CH lines of the CH molecular band and C_2 lines from the C_2 Swan band. The estimated He/H ratio for the Sun is discussed based on the one-dimensional local thermodynamic equilibrium (1D LTE) model atmosphere. The helium abundance (He/H = 0.091(+0.019/+0.014)) obtained for the Sun serves as a critical reference point to characterize the He/H ratio of cool stars across the range in their effective temperature. Using this derived He/H ratio, the solar mass fractions are determined to be X_sun=0.7232(+0.0305/-0.0377), Y_sun=0.2633(+0.0384/-0.0311), and Z_sun=0.0135(+0.0006/-0.0007)., Comment: 13 Pages, 3 Figures and 2 Tables, Accepted for publication in the Astrophysical Journal (ApJ)

HISPEC is a new, high-resolution near-infrared spectrograph being designed for the W.M. Keck II telescope. By offering single-shot, R=100,000 between 0.98 - 2.5 um, HISPEC will enable spectroscopy of transiting and non-transiting exoplanets in close orbits, direct high-contrast detection and spectroscopy of spatially separated substellar companions, and exoplanet dynamical mass and orbit measurements using precision radial velocity monitoring calibrated with a suite of state-of-the-art absolute and relative wavelength references. MODHIS is the counterpart to HISPEC for the Thirty Meter Telescope and is being developed in parallel with similar scientific goals. In this proceeding, we provide a brief overview of the current design of both instruments, and the requirements for the two spectrographs as guided by the scientific goals for each. We then outline the current science case for HISPEC and MODHIS, with focuses on the science enabled for exoplanet discovery and characterization. We also provide updated sensitivity curves for both instruments, in terms of both signal-to-noise ratio and predicted radial velocity precision., Comment: 25 pages, 9 figures. To appear in the Proceedings of SPIE: Techniques and Instrumentation for Detection of Exoplanets XI, vol. 12680 (2023)

We present the first far-UV (FUV) imaging results of the intermediate-age Galactic open cluster NGC 2818 that has a Planetary nebula (PN) within the field using images taken from the Ultra-violet Imaging Telescope (UVIT) aboard AstroSat. We identify cluster members by combining UVIT-detected sources with Gaia EDR3 data. We detect four bright and hot blue straggler stars (BSSs) and two yellow straggler stars (YSSs) based on their location in the optical and FUV-optical color-magnitude diagrams. Based on the parameters estimated using Spectral Energy Distribution (SED), we infer that BSSs are either collisional products or might have undetectable white dwarf (WD) companions. Our photometric analysis of YSSs confirms their binarity, consistent with the spectroscopic results. We find YSSs to be formed through a mass-transfer scenario and the hot components are likely to be A-type subdwarfs. A comparison of the radial velocity (RV), Gaia EDR3 proper motion of the PN with the cluster, and reddening towards the PN and the cluster does not rule out the membership of the PN. Comparing the central star's position with theoretical pAGB models suggest that it has already entered the WD cooling phase, and its mass is deduced to be ~0.66Msun. The corresponding progenitor mass turns out to be ~2.1Msun, comparable to the turn-off mass of the cluster, implying that the progenitor could have formed in the cluster. We suggest that the NGC 2818 might be one of the few known clusters to host a PN, providing a unique opportunity to test stellar evolution models., Comment: 20 pages, 12 figures, 4 tables, Accepted for publication in ApJ

We present the first comprehensive study of the most massive globular cluster Omega Centauri in the far-ultraviolet (FUV) extending from the center to ~ 28% of the tidal radius using the Ultraviolet Imaging Telescope aboard AstroSat. A comparison of the FUV-optical color-magnitude diagrams with available canonical models reveals that the horizontal branch (HB) stars bluer than the knee (hHBs) and the white dwarfs (WDs) are fainter in the FUV by ~ 0.5 mag than model predictions. They are also fainter than their counterparts in M13, another massive cluster. We simulated HB with at least five subpopulations, including three He-rich populations with a substantial He enrichment of Y up to 0.43 dex, to reproduce the observed FUV distribution. We find the He-rich younger subpopulations to be radially more segregated than the He-normal older ones, suggesting an in-situ enrichment from older generations. The Omega Cen hHBs span the same effective temperature range as their M13 counterparts, but some have smaller radii and lower luminosities. This may suggest that a fraction of Omega Cen hHBs are less massive than those of M13, similar to the result derived from earlier spectroscopic studies of outer extreme HB stars. The WDs in Omega Cen and M13 have similar luminosity-radius-effective temperature parameters, and 0.44 - 0.46 M$_\odot$ He-core WD model tracks evolving from progenitors with Y = 0.4 dex are found to fit the majority of these. This study provides constraints on the formation models of Omega Cen based on the estimated range in age, [Fe/H] and Y (in particular), for the HB stars., Comment: Accepted for publication in ApJL; 13 pages, 5 figures, 1 table

We present the first results of eight Globular Clusters (GCs) from the AstroSat/UVIT Legacy Survey program GlobULeS based on the observations carried out in two FUV filters (F148W and F169M). The FUV-optical and FUV-FUV color-magnitude diagrams (CMDs) of GCs with the proper motion membership were constructed by combining the UVIT data with HST UV Globular Cluster Survey (HUGS) data for inner regions and Gaia Early Data Release (EDR3) for regions outside the HST's field. We detect sources as faint as F148W $\sim$ 23.5~mag which are classified based on their locations in CMDs by overlaying stellar evolutionary models. The CMDs of 8 GCs are combined with the previous UVIT studies of 3 GCs to create stacked FUV-optical CMDs to highlight the features/peculiarities found in the different evolutionary sequences. The FUV (F148W) detected stellar populations of 11 GCs comprises 2,816 Horizontal Branch (HB) stars (190 Extreme HB candidates), 46 post-HB (pHB), 221 Blue Straggler Stars (BSS), and 107 White Dwarf (WD) candidates. We note that the blue HB color extension obtained from F148W$-$G color and the number of FUV detected EHB candidates are strongly correlated with the maximum internal Helium (He) variation within each GC, suggesting that the FUV-optical plane is the most sensitive to He abundance variations in the HB. We discuss the potential science cases that will be addressed using these catalogues including HB morphologies, BSSs, pHB, and, WD stars., Comment: Accepted for publication in MNRAS; 24 pages, 20 Figures, 6 Tables

We present the far-UV (FUV) photometry of images acquired with UVIT on AstroSat to probe the horizontal branch (HB) population of the Galactic globular cluster NGC 2298. UV-optical color-magnitude diagrams (CMDs) are constructed for member stars in combination with HST UV Globular Cluster Survey (HUGS) data for the central region and Gaia and ground-based photometric data for the outer region. Blue HB (BHB) sequence with a spread and four hot HB stars are detected in all FUV-optical CMDs and are compared with theoretical updated BaSTI isochrones and synthetic HB models with a range in helium abundance, suggesting that the hot HB stars are helium enhanced when compared to the BHB. The estimated effective temperature, radius, and luminosity of HB stars, using best SED fits, were compared with various HB models. BHB stars span a temperature range from 7,500-12,250 K. The three hot HB stars have 35,000-40,000 K, whereas one star has around 100,000K. We suggest the following evolutionary scenarios: two stars are likely to be the progeny of extreme HB (EHB) stars formed through an early hot-flasher scenario; one is likely to be an EHB star with probable helium enrichment, the hottest HB star is about to enter the WD cooling phase, could have evolved from BHB phase. Nevertheless, these are interesting spectroscopic targets to understand the late stages of evolution., Comment: 25 pages, 12 figures, 5 tables, Accepted for publication in ApJ

Surface abundances of 14 (11 majority class and 3 minority class) R Coronae Borealis stars (RCBs) along with the final flash object, V4334 Sgr (Sakurai's object) are revised based on their carbon abundances measured from the observed C2 bands; note that the earlier reported abundances were derived using an assumed carbon abundance due to the well known ``carbon problem''. The hot RCB MV Sgr is not subject to a carbon problem; it is remarkable to note that MV Sgr's carbon abundance lies in the range that is measured for the majority and minority class RCBs. The revised iron abundances for the RCBs are in the range log E(Fe)=3.8 to log E(Fe)=5.8 with the minority class RCB V854 Cen at lower end and the majority class RCB R CrB at the higher end of this range. Indications are that the revised RCBs' metallicity range is roughly consistent with the metal poor population contained within the bulge. The revised abundances of RCBs are then compared with extreme helium stars (EHes), the hotter relatives of RCBs. Clear differences are observed between RCBs and EHes in their metallicity distribution, carbon abundances, and the abundance trends observed for the key elements. These abundances are further discussed in the light of their formation scenarios., Comment: Accepted for publication in the Astrophysical Journal (ApJ), 31 pages, 14 figures

We present the UV photometry of the globular cluster NGC 1261 using images acquired with the Ultraviolet Imaging Telescope (UVIT) on-board ASTROSAT. We performed PSF photometry on four near-UV (NUV) and two far-UV (FUV) images and constructed UV colour-magnitude diagrams (CMDs), in combination with HST, Gaia, and ground-based optical photometry for member stars. We detected the full horizontal branch (HB) in NUV, blue HB in the FUV and identified two extreme HB (EHB) stars. HB stars have a tight sequence in UV-optical CMDs well-fitted with isochrones generated (12.6 Gyr age, [Fe/H] = -1.27 metallicity) using updated BaSTI-IAC models. Effective temperatures (Teff), luminosities and radii of bright HB stars were estimated using spectral energy distribution. As we detect the complete sample of UV bright HB stars, the hot end of the HB distribution is found to terminate at the G-jump (Teff ~ 11500 K). The two EHB stars, fitted well with single spectra, have Teff= 31,000 K and a mass = 0.495Msun and follow the same Teff-Radius relation of the blue HB stars. We constrain the formation pathways of these EHB stars to extreme mass loss in the RGB phase (either due to rotation or enhanced Helium), OR early hot-flash scenario., Comment: 13 pages, 9 figures, 3 tables, Accepted for publication in MNRAS

We present the UV photometry of the old open cluster NGC188 obtained using images acquired with Ultraviolet Imaging Telescope (UVIT) on board the ASTROSAT satellite, in two far-UV (FUV) and one near-UV (NUV) filters. UVIT data is utilised in combination with optical photometric data to construct the optical and UV colour-magnitude diagrams (CMDs). In the FUV images, we detect only hot and bright blue straggler stars (BSSs), one hot subdwarf, and one white dwarf (WD) candidate. In the NUV images, we detect members up to a faintness limit of ~22 mag including 21 BSSs, 2 yellow straggler stars (YSSs), and one WD candidate. This study presents the first NUV-optical CMDs, and are overlaid with updated BaSTI-IAC isochrones and WD cooling sequence, which are found to fit well to the observed CMDs. We use spectral energy distribution (SED) fitting to estimate the effective temperatures, radii, and luminosities of the UV-bright stars. We find the cluster to have an HB population with three stars (Teff = 4750 - 21000 K). We also detect two yellow straggler stars, with one of them with UV excess connected to its binarity and X-ray emission., Comment: 11 pages, 7 figures, 3 tables, Accepted for publication in Journal of Astrophysics and Astronomy

In this work, the helium-enhancement (He-enhancement) in the lithium-rich (Li-rich) K-giant HD 77361 is investigated using the strengths of the MgH band and the MgI lines. The detailed abundance analysis and also the synthesis of the MgH band and the Mg I lines has been carried out for HD 77361. One would expect, within uncertainties, same Mg abundance from both the MgH and Mg I lines. But, we found that Mg abundance derived from MgH lines is significantly less than the abundance from Mg I lines, and this difference cannot be reconciled by changing the stellar parameters within the uncertainties, implying He enhancement in star's photosphere. The He enhancement in the atmospheres is estimated by using models of different He/H ratios so that both the lines, MgH as well as Mg I, return the same Mg abundance for the adopted model's He/H ratio. We found He/H=0.4+/-0.1 as the value for HD 77361, the normal value of He/H=0.1. Knowing the amount of He-enhancement in the Li-rich giants is a strong clue for understanding the scenarios responsible for the Li and He enrichment. The analysis and results are discussed., Comment: 8 Pages and 7 Figures, Accepted for Publication in the Journal of Astrophysics and Astronomy. arXiv admin note: text overlap with arXiv:2005.06807

We present the results obtained from the UV photometry of the globular cluster NGC 1261 using Far-UV (FUV) and Near-UV (NUV) images acquired with the Ultraviolet Imaging Telescope (UVIT) onboard the Astrosat satellite. We utilized the UVIT data combined with HST, GAIA, and ground-based optical photometric data to construct the different UV colour-magnitude diagrams (CMDs). We detected blue HB (BHB), and two extreme HB (EHB) stars in FUV, whereas full HB, i.e., red HB (RHB), BHB as well as EHB is detected in NUV CMDs. The 2 EHB stars, identified in both NUV and FUV, are confirmed members of the cluster. The HB stars form a tight sequence in UV-optical CMDs, which is almost aligned with Padova isochrones. This study sheds light on the significance of UV imaging to probe the HB morphology in GCs., Comment: 7 Pages, 4 Figures, Accepted for publication in JAA

The helium-enriched (He-enriched) metal-rich red giants of Omega Centauri, discovered by Hema and Pandey using the low-resolution spectra from the Vainu Bappu Telescope (VBT) and confirmed by the analyses of the high-resolution spectra obtained from the HRS-South African Large Telescope (SALT) for LEID 34225 and LEID 39048, are reanalysed here to determine their degree of He-enhancement/hydrogen-deficiency (H-deficiency). The observed MgH band combined with model atmospheres with differing He/H ratios are used for the analyses. The He/H ratios of these two giants are determined by enforcing the fact that the derived Mg abundances from the MgI lines and from the subordinate lines of the MgH band must be same for the adopted model atmosphere. The estimated He/H ratios for LEID 34225 and LEID 39048 are 0.15+/-0.04 and 0.20+/-0.04, respectively, whereas the normal He/H ratio is 0.10. Following the same criteria for the analyses of the other two comparison stars (LEID 61067 and LEID 32169), a normal He/H ratio of 0.10 is obtained. The He/H ratio of 0.15-0.20 corresponds to a mass fraction of helium (Z(He)=Y) of about 0.375-0.445. The range of helium enhancement and the derived metallicity of the program stars are in line with those determined for Omega Cen blue main-sequence stars. Hence, our study provides the missing link for the evolutionary track of the metal-rich helium-enhanced population of Omega Centuari. This research work is the very first spectroscopic determination of the amount of He-enhancement in the metal-rich red giants of Omega Centauri using the MgI and MgH lines., Comment: 22 pages, 3 fugures, 1 table, Accepted for publication in the Astrophysical Journal

The main objective of this paper is to explore abundances of fluorine in hot Extreme Helium Stars (EHes). Overabundance of fluorine is a characteristic feature for cool EHes and R Coronae Borealis (RCB) stars and further enforces their close connection. For hot EHes this relationship with the cooler EHes, based on their fluorine abundance is unexplored. We present in this paper the first abundance estimates of fluorine determined from singly ionised fluorine lines (F\,{\sc ii}) for 10 hot EHe stars from optical spectra. Fluorine abundances were determined using the F\,{\sc ii} lines in two windows centered at 3505 \AA \ and 3850 \AA . Six of the 10 stars show significant enhancement of fluorine similar to the cool EHes. Two carbon-poor hot EHes show no signature of fluorine and have a significant low upper limit for the F abundance. These fluorine abundances are compared with the other elemental abundances observed in these stars which provide an idea about the formation and evolution of these stars. The trends of fluorine with C, O, and Ne show that significant helium burning after a CO-He white dwarf merger can account for a majority of the observed abundances. Predictions from simulations of white dwarf mergers are discussed in light of the observed abundances., Comment: 41 pages, 18 figures, 5 tables, Accepted for publication in ApJ

HISPEC (High-resolution Infrared Spectrograph for Exoplanet Characterization) is a proposed diffraction-limited spectrograph for the W.M. Keck Observatory, and a pathfinder for the MODHIS facility project (Multi-Object Diffraction-limited High-resolution Infrared Spectrograph) on the Thirty Meter Telescope. HISPEC/MODHIS builds on diffraction-limited spectrograph designs which rely on adaptively corrected single-mode fiber feeds. Seeing-limited high-resolution spectrographs, by virtue of the conservation of beam etendue, grow in volume following a D^3 power law (D is the telescope diameter), and are subject to daunting challenges associated with their large size. Diffraction-limited spectrographs fed by single mode fibers are decoupled from the telescope input, and are orders of magnitude more compact and have intrinsically stable line spread functions. Their efficiency is directly proportional to the performance of the adaptive optics (AO) system. AO technologies have matured rapidly over the past two decades and are baselined for future extremely large telescopes. HISPEC/MODHIS will take R>100,000 spectra of a few objects in a 10" field-of-view sampled at the diffraction limit (~10-50 mas), simultaneously from 0.95 to 2.4 microns (y-K). The scientific scope ranges from exoplanet infrared precision radial velocities, spectroscopy of transiting, close-in, and directly imaged exoplanets (atmospheric composition and dynamics, RM effect, spin measurements, Doppler imaging), brown dwarf characterization, stellar physics/chemistry, proto-planetary disk kinematics/composition, Solar system, extragalactic science, and cosmology. HISPEC/MODHIS features a compact, cost-effective design optimized to fully exploit the existing Keck-AO and future TMT-NFIRAOS infrastructures and boost the scientific reach of Keck Observatory and TMT soon after first light.

(Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faint Universe and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale structures in the Universe. Major pillars in the science program for MSE include (i) the ultimate Gaia follow-up facility for understanding the chemistry and dynamics of the distant Milky Way, including the outer disk and faint stellar halo at high spectral resolution (ii) galaxy formation and evolution at cosmic noon, via the type of revolutionary surveys that have occurred in the nearby Universe, but now conducted at the peak of the star formation history of the Universe (iii) derivation of the mass of the neutrino and insights into inflationary physics through a cosmological redshift survey that probes a large volume of the Universe with a high galaxy density. MSE is positioned to become a critical hub in the emerging international network of front-line astronomical facilities, with scientific capabilities that naturally complement and extend the scientific power of Gaia, the Large Synoptic Survey Telescope, the Square Kilometer Array, Euclid, WFIRST, the 30m telescopes and many more., Comment: 9 chapters, 301 pages, 100 figures. This version of the DSC is a comprehensive update of the original version, released in 2016, which can be downloaded at arXiv:1606.00043. A detailed summary of the design of MSE is available in the MSE Book 2018, available at arXiv:1810.08695

High-resolution optical spectra are analyzed for two of the four metal rich mildly hydrogen-poor or helium-enhanced giants discovered by Hema and Pandey (2014) along with their comparison normal (hydrogen-rich) giants of Omega Cen. The strengths of the MgH bands in the spectra of the program stars are analyzed for their derived stellar parameters. The observed spectra of the sample (hydrogen-poor) stars (LEID 39048 and LEID 34225) show weaker MgH bands unlike in the spectra of the normal comparison giants (LEID 61067 and LEID 32169). The magnesium abundance derived from MgH bands is less by 0.3 dex or more for LEID 39048 and LEID 34225, than that derived from Mg I lines. This difference, cannot be reconciled by making the changes to the stellar parameters within the uncertainties. This difference in the magnesium abundances derived from Mg I lines and from the MgH band is unacceptable. This difference is attributed to the hydrogen-deficiency or helium-enhancement in their atmospheres. These metal rich hydrogen-poor or helium-rich giants provide an important link to the evolution of the metal-rich sub population of Omega Cen. These stars provide the first direct spectroscopic evidence for the presence of the He-enhancement in the metal rich giants of Omega Cen., Comment: 49 pages, 13 Figures, Accepted for Publication in the Astophysical Journal

In this paper we present for the first time, the study of low resolution $H$- and $K$- band spectra of 7 DY\,Per type and suspects stars as well as DY\,Persei itself. We also observed $H$- and $K$- band spectra of 3 R Coronae Borealis (RCB) stars, 1 hydrogen-deficient carbon (HdC) star and 14 cool carbon stars including normal giants as comparisons. High $^{12}$C/$^{13}$C and low $^{16}$O/$^{18}$O ratios are characteristic features of majority RCBs and HdCs. We have estimated $^{16}$O/$^{18}$O ratios of the programme stars from the relative strengths of the $^{12}$C$^{16}$O and $^{12}$C$^{18}$O molecular bands observed in $K$- band. Our preliminary analysis suggest that a quartet of the DY\,Per suspects along with DY\,Persei itself seems to show isotopic ratio strength consistent with the ones of RCB/HdC stars whereas two of them do not show significant $^{13}$C and $^{18}$O in their atmospheres. Our analysis provides further indications that DY\,Per type stars could be related to RCB/HdC class of stars., Comment: 16 pages including 4 tables and 4 figures, Accepted for publication in ApJ ,typos corrected

Optical high-resolution spectra of V652 Her and HD 144941, the two extreme helium stars with exceptionally low C/He ratios, have been subjected to a non-LTE abundance analysis using the tools TLUSTY and SYNSPEC. Defining atmospheric parameters were obtained from a grid of non-LTE atmospheres and a variety of spectroscopic indicators including He I and He II line profiles, ionization equilibrium of ion pairs such as C II/C III and N II/N III. The various indicators provide a consistent set of atmospheric parameters: $T_{\rm eff}$=25000$\pm$300K, $\log g$ = 3.10$\pm$0.12(cgs), and $\xi=13\pm2 {\rm km\,s^{-1}}$ are provided for V652 Her, and $T_{\rm eff}$=22000$\pm$600K, $\log g$ = 3.45$\pm$0.15 (cgs), and $\xi=10 {\rm km\,s^{-1}}$ are provided for HD 144941. In contrast to the non-LTE analyses, the LTE analyses - LTE atmospheres and a LTE line analysis - with the available indicators do not provide a consistent set of atmospheric parameters. The principal non-LTE effect on the elemental abundances is on the neon abundance. It is generally considered that these extreme helium stars with their very low C/He ratio result from the merger of two helium white dwarfs. Indeed, the derived composition of V652 Her is in excellent agreement with predictions by Zhang & Jeffery (2012) who model the slow merger of helium white dwarfs; a slow merger results in the merged star having the composition of the accreted white dwarf. In the case of HD 144941 which appears to have evolved from metal-poor stars a slow merger is incompatible with the observed composition but variations of the merger rate may account for the observed composition. More detailed theoretical studies of the merger of a pair of helium white dwarfs are to be encouraged., Comment: 23 pages including 5 Tables and 14 Figures, Accepted for publication in ApJ

Abundance analyses of the two newly discovered R Coronae Borealis (RCB) stars ASAS-RCB-8 and ASAS-RCB-10 were conducted using high-resolution optical spectra and model atmospheres. Their chemical compositions place the pair among the majority class of RCBs. ASAS-RCB-10 is one of the most N-poor majority RCBs with an above average O abundance. Relative to ASAS-RCB-10, ASAS-RCB-8 is H poor by 1.6 dex, O-poor by 0.7 dex but N-rich by 0.8 dex suggesting a higher contamination by CNO-cycled material., Comment: Accepted for publication in PASP

A fiber-fed Echelle spectrograph at 2.3 m Vainu Bappu Telescope (VBT), Kavalur, has been in operation since 2005. Owing to various technological advancements in precision spectroscopy in recent years, several research avenues have been opened in observational astronomy. These developments have created a demand to improve the Doppler precision of our spectrograph. Currently, the stability of the instrument is compromised by the temperature and pressure fluctuations inside the Echelle room. Further, a better wavelength calibration approach is needed to carefully track and disentangle the instrumental effects from stellar spectra. While planning a possible upgrade with an Iodine absorption gas cell, we measured the raw stability of the spectrograph using a series of calibration frames taken with the ThAr gas discharge lamp. The time series data were analysed with cross-correlation method and the shift in ThAr emission lines was accurately measured across different Echelle orders. In this paper, we present our stability analysis methodology and results for the Kavalur spectrograph. We also identify possible sources of error and discuss our strategy to mitigate them., Comment: 12 pages, 7 figures, Accepted in Journal of Optics (India)

MSE is an 11.25m aperture observatory with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. More than 3200 fibres will feed spectrographs operating at low (R ~ 2000 - 3500) and moderate (R ~ 6000) spectral resolution, and approximately 1000 fibers will feed spectrographs operating at high (R ~ 40000) resolution. MSE is designed to enable transformational science in areas as diverse as tomographic mapping of the interstellar and intergalactic media; the in-situ chemical tagging of thick disk and halo stars; connecting galaxies to their large scale structure; measuring the mass functions of cold dark matter sub-halos in galaxy and cluster-scale hosts; reverberation mapping of supermassive black holes in quasars; next generation cosmological surveys using redshift space distortions and peculiar velocities. MSE is an essential follow-up facility to current and next generations of multi-wavelength imaging surveys, including LSST, Gaia, Euclid, WFIRST, PLATO, and the SKA, and is designed to complement and go beyond the science goals of other planned and current spectroscopic capabilities like VISTA/4MOST, WHT/WEAVE, AAT/HERMES and Subaru/PFS. It is an ideal feeder facility for E-ELT, TMT and GMT, and provides the missing link between wide field imaging and small field precision astronomy. MSE is optimized for high throughput, high signal-to-noise observations of the faintest sources in the Universe with high quality calibration and stability being ensured through the dedicated operational mode of the observatory. (abridged), Comment: 210 pages, 91 figures. Exposure draft. Appendices to the Detailed Science Case can be found at http://mse.cfht.hawaii.edu/docs/

DY Cen has shown a steady fading of its visual light by about 1 magnitude in the last 40 years suggesting a secular increase in its effective temperature. We have conducted non-LTE and LTE abundance analyses to determine the star's effective temperature, surface gravity, and chemical composition using high-resolution spectra obtained over two decades. The derived stellar parameters for three epochs suggest that DY Cen has evolved at a constant luminosity and has become hotter by about 5000 K in 23 years. We show that the derived abundances remain unchanged for the three epochs. The derived abundances of the key elements, including F and Ne, are as observed for the extreme helium stars resulting from a merger of an He white dwarf with a C-O white dwarf. Thus, DY Cen by chemical composition appears to be also a product of a merger of two white dwarfs. This appearance seems to be at odds with the recent suggestion that DY Cen is a single-lined spectroscopic binary., Comment: Accepted to ApJ with 8 Figures and 5 Tables

In this letter, the results of our low-resolution spectroscopic survey for identifying the hydrogen-deficient (H-deficient) stars in the red giant sample of the globular cluster Omega Cen are reported. Spectral analyses were carried out on the basis of the strengths of (0,0) MgH band and the Mg b triplet. In our sample, four giants were identified with weak/absent MgH bands in their observed spectra not as expected for their well determined stellar parameters. The Mg abundances for the program stars were determined from subordinate lines of the MgH band to the blue of the Mg b triplet, using the spectral synthesis technique. The derived Mg abundances for the program stars were as expected for the red giants of Omega Cen (Norris & Da Costa 1995), except for the four identified candidates. Determined Mg abundances of these four candidates are much lower than that expected for the red giants of Omega Cen, and are unacceptable based on the strengths of Mg b triplet in their observed spectra. Hence, the plausible reason for the weak/absent MgH bands in the observed spectra of these stars is a relatively lower abundance of hydrogen in their atmospheres. These giants may belong to the group of helium enriched red giants of Omega Cen., Comment: 13 pages, 3 figures, Accepted for publication in ApJ Letters

The high resolution optical spectra of H-deficient stars, R Coronae Borealis stars and H-deficient carbon stars are analyzed by synthesizing the C2 Swan bands (0,1), (0,0), and (1,0) using our detailed line-list and Uppsala model atmosphere, to determine the C-abundances and the 12C/13C ratios which are potential clues to the formation process of these stars. The C-abundances derived from C2 bands are about the same for the adopted models constructed with different carbon abundances over the range 8.5 (C/He = 0.1%) to 10.5 (C/He = 10%). The carbon abundances derived from C I lines are a factor of four lower than that adopted for the model atmosphere over the same C/He interval, as reported by Asplund et al.: 'the carbon problem'. In principle, the carbon abundances obtained from C2 Swan bands and that adopted for the model atmosphere can be equated for a particular choice of C/He that varies from star to star (unlike C I lines). Then, the carbon problem for C2 bands is eliminated. However, such C/He ratios are in general less than those of the extreme helium stars, the seemingly natural relatives to the RCB and HdC stars. The derived carbon abundances and the 12C/13C ratios are discussed in light of the double degenerate (DD) and the final flash (FF) scenarios. The carbon abundance and the 12C/13C ratios for the FF product, Sakurai's Object is derived. The carbon abundance in the Sakurai's object is 10 times higher than in the RCB star VZ Sgr. On an average, the carbon abundance in the Sakurai's Object is about 10 to 100 times higher than in RCB stars. The 12C/13C ratio in Sakurai's Object is 3.4, the equilibrium value, as expected for FF products., Comment: Accepted for publication on Proceedings of Science

Observed spectra of R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC) stars are analyzed by synthesizing the C2 Swan bands (1,0), (0,0), and (0,1) using our detailed line list and the Uppsala model atmospheres. The (0,1) and (0,0) C2 bands are used to derive the 12C abundance, and the (1,0) 12C13C band to determine the 12C/13C ratios. The carbon abundance derived from the C2 Swan bands is about the same for the adopted models constructed with different carbon abundances over the range: 8.5 (C/He = 0.1%), to 10.5 (C/He = 10%). Carbon abundances derived from C I lines are about a factor of 4 lower than the carbon abundance of the adopted model atmosphere over the same C/He interval, as reported by Asplund et al. (2000), who dubbed the mismatch between adopted and derived C abundance the 'carbon problem'. In principle, the carbon abundances obtained from C2 Swan bands and that assumed for the model atmosphere can be equated for a particular choice of C/He that varies from star to star. Then, the carbon problem for C2 bands is eliminated. However, such C/He ratios are in general less than those of the EHe stars, the seemingly natural relatives to the RCB and HdC stars. A more likely solution to the C2 carbon problem may lie in a modification of the model atmosphere's temperature structure. The derived carbon abundances and the 12C/13C ratios are discussed in light of the double degenerate (DD) and the final flash (FF) scenarios., Comment: Accepted for Publication in ApJ

We have conducted an LTE abundance analysis for SAO 40039 a warm post-AGB star whose spectrum is known to show surprisingly strong He I lines for its effective temperature and has been suspected of being H-deficient and He-rich. High-resolution optical spectra are analyzed using a family of model atmospheres with different He/H ratios. Atmospheric parameters are estimated from the ionization equilibrium set by neutral and singly ionized species of Fe and Mg, the excitation of Fe I and Fe II lines and the wings of the Paschen lines. On the assumption that the He I lines are of photospheric and not chromospheric in origin, a He/H ratio of approximately unity is found by imposing the condition that the adopted He/H ratio of the model atmosphere must equal the ratio derived from the observed He I triplet lines at 5876, 4471 and 4713 Angstrom, and singlet lines at 4922 and 5015 Angstrom. Using the model with the best-fitting atmospheric parameters for this He/H ratio, SAO 40039 is confirmed to exhibit mild dust-gas depletion, i.e., the star has an atmosphere deficient in elements of high condensation temperature. The star appears to be moderately metal-deficient with [Fe/H]=-0.4 dex. But the star's intrinsic metallicty as estimated from Na, S and Zn, elements of a low condensation temperature, is [Fe/H]_o {[Fe/H]_o refers to the star's intrinsic metallicity} ~ -0.2. The star is enriched in N and perhaps O too, changes reflecting the star's AGB past and the event that led to He enrichment., Comment: 15 Pages, 3 Figures, 2 Tables, Accepted for publication in the Astrophysical Journal Letters

A non-LTE (NLTE) abundance analysis was carried out for three extreme helium stars (EHes): BD+10 2179, BD-9 4395, and LS IV+6 002, from their optical spectra with NLTE model atmospheres. NLTE TLUSTY model atmospheres were computed with H, He, C, N, O, and Ne treated in NLTE. Model atmosphere parameters were chosen from consideration of fits to observed He I line profiles and ionization equilibria of C and N ions. The program SYNSPEC was then used to determine the NLTE abundances for Ne as well as H, He, C, N, and O. LTE neon abundances from Ne I lines in the EHes: LSE 78, V1920 Cyg, HD 124448, and PV Tel, are derived from published models and an estimate of the NLTE correction applied to obtain the NLTE Ne abundance. We show that the derived abundances of these key elements, including Ne, are well matched with semi-quantitative predictions for the EHe resulting from a cold merger (i.e., no nucleosynthesis during the merger) of a He white dwarf with a C-O white dwarf., Comment: Astrophysical Journal, accepted

Context. Emission and absorption line profiles which are formed in the interaction of pre-main sequence (PMS) stars and their circumstellar environment are found to be variable at various timescales. Aims. We investigate the patterns and timescales of temporal line profile variability in order to explore the dynamical circumstellar environment of the PMS Herbig Ae star V351 Ori. Methods.We obtained 45 high-resolution (R~28,000) spectra of V351 Ori at timescales of hours, days, and months.We analysed the H\alpha line profiles and also examined the H\beta, NaD1 and NaD2 line profiles to explore the nature of the spectroscopic variability. Results. The H\alpha line profiles showed strong variations over all timescales. The shape of the profiles changed over timescales of a day. Single as well as simultaneous event(s) of blue-shifted and red-shifted transient absorption components (TACs), i.e. signatures of outflow and infall, were also observed in the H\alpha line profiles. The shortest period of variation in the TACs was < 1 hour. All transient absorption events were found to decelerate with a rate of a few to fractions of m s-2. The depth and width of the TACs were also changing with time. The presence of elongated red-shifted components at some epochs supports the episodic nature of accretion. Conclusions. Variable emission and absorption components detected in H\alpha line profiles show the dynamic nature of interaction between V351 Ori and its circumstellar environment. The H\alpha non-photospheric profiles of the star most probably originate in the disk wind. Episodic accretion of gaseous material at a slow rate and outflow of clumpy gaseous material are still occurring in V351 Ori at an age of ~6.5 Myr. Dynamic magnetospheric accretion and disk wind emerge as the most satisfactory model for interpreting the observed line profile variations of V351 Ori., Comment: Accepted for publication in A&A

Neutral fluorine lines are identified in the optical spectra of several R Coronae Borealis stars (RCBs) at maximum light. These lines provide the first measurement of the fluorine abundance in these stars. Fluorine is enriched in some RCBs by factors of 800 to 8000 relative to its likely initial abundance. The overabundances of fluorine are evidence for the synthesis of fluorine. These results are discussed in the light of the scenario that RCBs are formed by accretion of an He white dwarf by a C-O white dwarf. Sakurai's object (V4334 Sgr), a final He-shell flash product, shows no detectable neutral fluorine lines., Comment: To appear in proceedings of "Hydrogen-Deficient Stars" conference, held in Tuebingen, Germany, Sept. 17-21, 2007. 4 pages

Neutral fluorine (F I) lines are identified in the optical spectra of several R Coronae Borealis stars (RCBs) at maximum light. These lines provide the first measurement of the fluorine abundance in these stars. Fluorine is enriched in some RCBs by factors of 800 to 8000 relative to its likely initial abundance. The overabundances of fluorine are evidence for the synthesis of fluorine. These results are discussed in the light of the scenario that RCBs are formed by accretion of an He white dwarf by a C-O white dwarf. Sakurai's object (V4334 Sgr), a final He-shell flash product, shows no detectable F I lines., Comment: 25 pages, 7 figures, accepted to ApJ

Neutral fluorine (F I) lines are identified in the optical spectra of cool EHe stars. These are the first identification of F I lines in a star's spectrum, and provide the first measurement of fluorine abundances in EHe stars. The results show that fluorine is overabundant in EHe stars. The overabundance of fluorine provides evidence for the synthesis of fluorine in these stars, that is discussed in the light of asymptotic giant branch (AGB) evolution, and the expectation from accretion of an He white dwarf by a C-O white dwarf., Comment: 14 pages, 3 figures, accepted to ApJ letters

Abundance analysis of the cool extreme helium (EHe) star LSS 3378 is presented. The abundance analysis is done using LTE line formation and LTE model atmospheres constructed for EHe stars. The atmosphere of LSS 3378 shows evidence of H-burning, He-burning, and s-process nucleosynthesis. The derived abundances of iron-peak and alpha-elements indicate absence of selective fractionation or any other processes that can distort chemical composition of these elements. Hence, the Fe abundance (log epsilon(Fe) = 6.1) is adopted as an initial metallicity indicator. The measured abundances of LSS 3378 are compared with those of R Coronae Borealis (RCB) stars and with rest of the EHe stars as a group., Comment: 7 pages, 4 figures, 3 tables, MNRAS format, accepted for publication in MNRAS

Abundances of about 18 elements including the heavy elements Y and Zr are determined from Hubble Space Telescope Space Telescope Imaging Spectrograph ultraviolet spectra of seven extreme helium stars (EHes): LSE 78, BD+10 2179, V1920 Cyg, HD 124448, PV Tel, LS IV -1 2, and FQ Aqr. New optical spectra of the three stars -- BD+10 2179, V1920 Cyg, and HD 124448 were analysed. The abundance analyses is done using LTE line formation and LTE model atmospheres especially constructed for these EHe stars. The stellar parameters derived from an EHe's UV spectrum are in satisfactory agreement with those derived from its optical spectrum. Adopted abundances for the seven EHes are from a combination of the UV and optical analyses. Published results for an additional ten EHes provide abundances obtained in a nearly uniform manner for a total of 17 EHes, the largest sample on record. The initial metallicity of an EHe is indicated by the abundance of elements from Al to Ni; Fe is adopted to be the representative of initial metallicity. Iron abundances range from approximately solar to about one-hundredth of solar. Clues to EHe evolution are contained within the H, He, C, N, O, Y, and Zr abundances. Two novel results are (i) the O abundance for some stars is close to the predicted initial abundance yet the N abundance indicates almost complete conversion of initial C, N, and O to N by the CNO-cycles; (ii) three of the seven stars with UV spectra show a strong enhancement of Y and Zr attributable to an s-process. The observed compositions are discussed in light of expectations from accretion of a He white dwarf by a CO white dwarf., Comment: 126 pages, 15 figures, 20 Tables, accepted for publication in the ApJ

Spectra of several R Coronae Borealis (RCB) stars at maximum light were examined for the [C I] 9850 A and 8727 A absorption lines. The 9850 A line is variously blended with a Fe II and CN lines but positive identifications of the [C I] line are made for R CrB and SU Tau. The 8727 A line is detected in the spectrum of the five stars observed in this wavelength region. Carbon abundances are derived from the [C I] lines using the model atmospheres and atmospheric parameters used by Asplund et al. (2000). Although the observed strength of a C I line is constant from cool to hot RCB stars, the strength is weaker than predicted by an amount equivalent to a factor of four reduction of a line's gf-value. Asplund et al. dubbed this 'the carbon problem' and discussed possible solutions. The [C I] 9850 A line seen clearly in R CrB and SU Tau confirms the magnitude of the carbon problem revealed by the C I lines. The [C I] 8727 A line measured in five stars shows an enhanced carbon problem. The gf-value required to fit the observed [C I] 8727 A line is a factor of 15 less than the well-determined theoretical gf-value. We suggest that the carbon problem for all lines may be alleviated to some extent by a chromospheric-like temperature rise in these stars. The rise far exceeds that predicted by our non-LTE calculations, and requires a substantial deposition of mechanical energy., Comment: 11 pages (embedded 5 figures and 3 tables), accepted for publication in MNRAS

Analysis of HST STIS ultraviolet spectra of two hot extreme helium stars (EHes): V1920 Cyg and HD 124448 provide the first measurements of abundances of neutron-capture elements for EHes. Although the two stars have similar abundances for elements up through the iron-group, they differ strikingly in their abundances of heavier elements: V1920 Cyg is enriched by a factor of 30 in light neutron-capture elements (Y/Fe, Zr/Fe) relative to HD 124448. These differences in abundances of neutron-capture elements among EHes mirrors that exhibited by the RCrB stars, and is evidence supporting the view that there is an evolutionary connection between these two groups of hydrogen-deficient stars. Also, the abundances of Y and Zr in V1920 Cyg provide evidence that at least one EHe star went through a s-process synthesis episode in its earlier evolution., Comment: 4 pages (embedded figure and table), accepted for publication in ApJ Letters

Infrared Space Observatory 3 - 25 $\mu$m spectra of the R Coronae Borealis stars V854 Cen, R CrB, and RY Sgr are presented and discussed. Sharp emission features coincident in wavelengths with the well known Unidentified Emission Features are present in the spectrum of V854 Cen but not of R CrB or RY Sgr. Since V854 Cen is not particularly H-poor and has a 1000 times more H than the other stars, the emission features are probably from a carrier containing hydrogen. There is a correspondence between the features and emission from laboratory samples of hydrogenated amorphous carbon. A search for C$_{60}$ in emission or absorption proved negative. Amorphous carbon particles account for the broad emission features seen between 6 - 14 $\mu$m in the spectrum of each star., Comment: 11 pages with 2 tables and 4 figures in ApJ print page form. Accepted for publication in ApJ

We present the chemical compositions of four K giants CS 22877-1, CS 22166-16, CS22169-35 and BS 16085 - 0050 that have [Fe/H] in the range -2.4 to -3.1. Metal-poor stars with [Fe/H] < -2.5 are known to exhibit considerable star - to - star variations of many elements. This quartet confirms this conclusion. CS 22877-1 and CS 22166-16 are carbon-rich. There is significant spread for [$\alpha$/Fe] within our sample where [$\alpha$/Fe] is computed from the mean of the [Mg/Fe], and [Ca/Fe] ratios. BS 16085 - 0050 is remarkably $\alpha$ enriched with a mean [$\alpha$/Fe] of $+$0.7 but CS 22169-35 is $\alpha$-poor. The aluminium abundance also shows a significant variation over the sample. A parallel and unsuccessful search among high-velocity late-type stars for metal-poor stars is described., Comment: 14 pages (text), 4 tables, 3 figures, Accepted for publication in PASP

Extreme helium stars (EHe) with effective temperatures from 8000K to 13000K are among the coolest EHe stars and overlap the hotter R CrB stars in effective temperature. The cool EHes may represent an evolutionary link between the hot EHes and the R CrBs. Abundance analyses of four cool EHes are presented. To test for an evolutionary connection, the chemical compositions of cool EHes are compared with those of hot EHes and R CrBs. Relative to Fe, the N abundance of these stars is intermediate between those of hot EHes and R CrBs. For the R CrBs, the metallicity M derived from the mean of Si and S appears to be more consistent with the kinematics than that derived from Fe. When metallicity M derived from Si and S replaces Fe, the observed N abundances of EHes and R CrBs fall at or below the upper limit corresponding to thorough conversion of initial C and O to N. There is an apparent difference between the composition of R CrBs and EHes; the former having systematically higher [N/M] ratios. The material present in the atmospheres of many R CrBs is heavily CN- and ON-cycled. Most of the EHes have only CN-cycled material in their atmospheres. There is an indication that the CN- and ON-cycled N in EHes was partially converted to Ne by $\alpha$-captures. If EHes are to evolve to R CrBs, fresh C in EHes has to be converted to N. If Ne is found to be normal in R CrBs, the proposal that EHes evolve to R CrBs fails. The idea that R CrBs evolve to EHes is ruled out; the N abundance in R CrBs has to be reduced to the level of EHes, as the C/He which is observed to be uniform across EHes, has to be maintained. Hence, the inferred [N/M], C/He, [Ne/M], and the H-abundances of these two groups indicate that the EHes and the R CrBs may not be on the same evolutionary path., Comment: 24 pages (MNRAS Journal style), 7 tables, 19 figures, Accepted for publication in MNRAS

A set of high-resolution optical spectra of RCrB acquired before, during, and after its 1995-1996 decline is discussed. All of the components reported from earlier declines are seen. This novel dataset provides new information on these components including several aspects not previously seen in declines of RCrB and other RCBs. In the latter category is the discovery that the decline's onset is marked by distortions of absorption lines of high-excitation lines, and quickly followed by emission in these and in low excitation lines. This 'photospheric trigger' implies that dust causing the decline is formed close to the star. These emission lines fade quickly. After 1995 November 2, low excitation narrow (FWHM ~12 km s-1) emission lines remain. These appear to be a permanent feature, slightly blue-shifted from the systemic velocity, and unaffected by the decline except for a late and slight decrease of flux at minimum light. The location of the warm, dense gas providing these lines is uncertain. Absorption lines unaffected by overlying sharp emission are greatly broadened, weakened, and red-shifted at the faintest magnitudes when scattered light from the star is a greater contributor than direct light transmitted through the fresh soot cloud. A few broad lines are seen at and near minimum light with approxiamately constant flux: prominent among these are the He I triplet series, Na I D, and [N II] lines. These lines are blue-shifted by about 30 km s(-1) relative to the systemic velocity with no change in velocity over the several months for whicht he lines were seen. It is suggested that these lines, especially the He I lines, arise from an accretion disk around an unseen compact companion, which may be a low-mass white dwarf. If so, R CrB is similar to the unusual post-AGB star 89 Her., Comment: 31 pages, 26 figures