Flexure compensation simulation tool for TMT-WFOS Spectrograph

The Wide Field Optical Spectrograph (WFOS) is one of the first-light instruments of Thirty Meter Telescope. It is a medium resolution, multi object, wide field optical spectrograph. Since 2005 the conceptual design of the instrument has focused on a slit-mask based, grating exchange design that will be mounted at the Nasmyth focus of TMT. Based on the experience with ESI, MOSFIRE and DEIMOS for Keck we know flexure related image motion will be a major problem with such a spectrograph and a compensation system is required to mitigate these effects. We have developed a flexure Compensation and Simulation (FCS) tool for TMT-WFOS that provides an interface to accurately simulate the effects of instrument flexure at the WFOS detector plane (e.g image shifts) using perturbation of key optical elements and also derive corrective motions to compensate the image shifts caused by instrument flexure. We are currently using the tool to do mote-carlo simulations to validate the optical design of a slit-mask concept we call Xchange-WFOS, and to optimize the flexure compensation strategy. We intend to use the tool later in the design process to predict the actual flexure by replacing the randomized inputs with the signed displacement and rotations of each element predicted by global FEA model on the instrument

Field blue straggler stars: Discovery of white dwarf companions to blue metal-poor stars using UVIT/AstroSat

Blue metal-poor (BMP) stars are the main-sequence stars that appear bluer and more luminous than normal turn-off stars of metal-poor globular clusters. They are believed to be either field blue straggler stars (FBSS) formed via post-mass transfer mechanism or accreted from dwarf satellite galaxies of the Milky Way. A significant fraction of BMP stars are discovered to be potential binaries. We observed 27 BMP stars using UVIT/\textit{AstroSat} in two FUV filters, F148W and F169M. We report the discovery of white dwarf (WD) companions of 12 BMP stars for the first time. The WD companions have estimated temperatures T$_{eff}$ $\sim$10500 $-$ 18250 K, and masses 0.17 M$_{\odot}$ $-$ 0.8 M$_{\odot}$. Based on [Fe/H] and space velocity, we group the 12 BMP/FBSS stars as the thick disk (5) and halo (5), whereas two stars appear to be in-between. All the 5 thick disk BMP/FBSS have extremely low-mass (M $<$ 0.2 M$_{\odot}$) WDs as companions, whereas the 5 halo BMP/FBSS have low (0.2 M$_{\odot}$ $<$ M $<$ 0.4 M$_{\odot}$), normal (0.4 M$_{\odot}$ $<$ M $<$ 0.6M$_{\odot}$), and high mass (M $>$ 0.6 M$_{\odot}$) WD companions. Our analysis suggests that at least $\sim$44 $\%$ of BMP stars are FBSS, and these stars hold the key to understand the details of mass transfer, binary properties, and chemical enrichment among the FBSS

Decoding the compositions of four bright rr-process-enhanced stars

There has been a concerted effort in recent years to identify the astrophysical sites of the $r$-process that can operate early in the Galaxy. The discovery of many $r$-process-enhanced (RPE) stars (especially by the $R$-process Alliance collaboration) has significantly accelerated this effort. However, only limited data exist on the detailed elemental abundances covering the primary neutron-capture peaks. Subtle differences in the structure of the $r$-process pattern, such as the relative abundances of elements in the third peak, in particular, are expected to constrain the $r$-process sites further. Here, we present a detailed elemental-abundance analysis of four bright RPE stars selected from the HESP-GOMPA survey. Observations were carried out with the 10-m class telescope Gran Telescopio Canarias (GTC), Spain. The high spectral signal-to-noise ratios obtained allow us to derive abundances for 20 neutron-capture elements, including the third $r$-process peak element osmium (Os). We detect thorium (Th) in two stars, which we use to estimate their ages. We discuss the metallicity evolution of Mg, Sr, Ba, Eu, Os, and Th in $r$-II and $r$-I stars, based on a compilation of RPE stars from the literature. The strontium (Sr) abundance trend with respect to europium (Eu) suggests the need for an additional production site for Sr (similar to several earlier studies); this requirement could be milder for yttrium (Y) and zirconium (Zr). We also show that there could be some time delay between $r$-II and $r$-I star formation, based on the Mg/Th abundance ratios.Comment: 33 pages, 22 figures, Accepted for publication in MNRA

Li distribution, kinematics and detailed abundance analysis among very metal-poor stars in the Galactic halo from the HESP-GOMPA survey

We present a study on the detailed elemental abundances of newly identified bright very metal-poor stars with the detection of lithium, initially observed as part of the SDSS/MARVELS pre-survey. These stars were selected for high-resolution spectroscopic follow-up as part of the HESP-GOMPA survey. In this work, we discuss the Li abundances detected for several stars in the survey, which include main-sequence stars, subgiants, and red giants. Different classes of stars are found to exhibit very similar distributions of Li, which point towards a common origin. We derive a scaling relation for the depletion of Li as a function of temperature for giants and main-sequence stars; the majority of the samples from the literature were found to fall within 1sigma (0.19 and 0.12 dex/K for giants and dwarfs respectively) of this relationship. We also report the existence of a slope of the Li abundances as a function of distances from the Galactic plane, indicating mixed stellar populations. Most Li-rich stars are found to be in or close to the galactic plane. Along with Li, we have derived detailed abundances for C, odd-Z, alpha-, Fe-peak, and neutron-capture elements for each star. We have also used astrometric parameters from Gaia-EDR3 to complement our study, and derived kinematics to differentiate between the motions of the stars; those formed in situ and accreted. The stellar population of the Spite plateau, including additional stars from the literature, is found to have significant contributions from stars formed in situ and through accretion. The orbits for the program stars have also been derived and studied for a period of 5 Gyr backward in time.Comment: 19 pages, 16 figures. Accepted for publication in the Astrophysical Journa

Carbon-enhanced Metal-poor Stars in SDSS/SEGUE. I. Carbon Abundance Estimation and Frequency of CEMP Stars

We describe a method for the determination of stellar [C/Fe] abundance ratios using low-resolution (R = 2000) stellar spectra from the SDSS and SEGUE. By means of a star-by-star comparison with a set of SDSS/SEGUE spectra with available estimates of [C/Fe] based on published high-resolution analyses, we demonstrate that we can measure [C/Fe] from SDSS/SEGUE spectra with S/N > 15 to a precision better than 0.35 dex. Using the measured carbon-to-iron abundance ratios obtained by this technique, we derive the frequency of carbon-enhanced stars ([C/Fe] > +0.7) as a function of [Fe/H], for both the SDSS/SEGUE stars and other samples from the literature. We find that the differential frequency slowly rises from almost zero to about 14% at [Fe/H] ~ -2.4, followed by a sudden increase, by about a factor of three, to 39% from [Fe/H] ~ -2.4 to [Fe/H] ~ -3.7. We also examine how the cumulative frequency of CEMP stars varies across different luminosity classes. The giant sample exhibits a cumulative CEMP frequency of 32% for [Fe/H] < -2.5, 31% for [Fe/H] < -3.0, and 33% for [Fe/H] < -3.5. For the main-sequence turnoff stars, we obtain a lower cumulative CEMP frequency, around 10% for [Fe/H] < -2.5. The dwarf population displays a large change in the cumulative frequency for CEMP stars below [Fe/H] = -2.5, jumping from 15% for [Fe/H] < -2.5 to about 75% for [Fe/H] < -3.0. When we impose a restriction with respect to distance from the Galactic mid-plane (|Z| < 5 kpc), the frequency of the CEMP giants does not increase at low metallicity ([Fe/H] < -2.5), but rather, decreases, due to the dilution of C-rich material in stars that have undergone mixing with CNO-processed material from their interiors. The frequency of CEMP stars near the main-sequence turnoff, which are not expected to have experienced mixing, increases for [Fe/H] < -3.0. [abridged]Comment: 19 pages, 10 figures, 6 tables, accepted for publication in AJ on August 20, 201