arXiv subjects

Sunetra Giridhar

Publications and source records attributed to Sunetra Giridhar.

43 records · Page 3Linked to original sources

arXiv2003

HD 34700: A new T Tauri double-lined spectroscopic binary

We find the star HD 34700 to be a double-lined spectroscopic binary. We also identify it as a weak-line T Tauri object The spectra of both components are very similar and both show the Li I feature at 6708 Å. Strong arguments in favour of the binary nature of the star as opposed to other possibilities are offered. It is very likely that the companion is also a T Tauri star of similar mass. A projected rotational velocity, $v$sin$i$, of 25 and 23 km/s$^{-1}$ has been estimated for the blue and red components. We present a list of the lines identified and the radial velocities of the two components on three spectra obtained on consecutive nights.

astro-ph↗

arXiv2002

A revised calibration of the Mv-W(OI 7774) relationship using Hipparcos data: Its application to Cepheids and evolved stars

A new calibration of the Mv-W(OI 7774) relationship has been calculated using better estimates of reddenings and distances to a sample of 27 calibrator stars of A-G spectral types, based on accurate parallaxes and proper motions from the Hipparcos and Tycho catalogues. The present calibration predicts absolute magnitude with accuracies of $\pm$ 0.38 mag. for a sample covering a large range of Mv, from -9.5 to $+0.35$ mag. The colour term included in a previous paper has been dropped since its inclusion was not bringing any significant improvement to the calibration. The variation of the OI 7774 feature in the classical cepheid SS Sct has been studied. We calculated a phase-dependent correction to random phase OI feature strengths in Cepheids, such that it predicts mean absolute magnitudes using the above calibration. After applying such correction, we could increase the list of calibrators to 58 by adding Mv and OI triplet strength data for 31 classical Cepheids. The standard error of the calibration using the composite sample was comparable to that obtained from the primary 27 calibrators, showing that it is possible to calculate mean Cepheid luminosities from random phase observations of the OI7774 feature. We use our derived calibrations to estimate Mv for a set of evolved objects to be able to locate their positions in the H-R diagram.

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arXiv2002

Spectroscopic Survey of Field stars: A search for Metal-Poor stars

We have undertaken a spectroscopic survey of field stars to find metal-poor objects among them. Observations are carried out using OMR spectrograph attached to 2.3m telescope at VBO Kavalur, India. The sample of candidate stars are chosen from prismatic survey of Beers and his collaborators covering a large part of the Galaxy. At the first phase of this project, the analysis is completed for a set of 19 relatively hot stars (Teff in 6000 to 8000K range). The metallicities of the program stars are derived by synthesizing the spectrum in the wavelength range 4900 to 5400A for different metallicities and matching them with the observed spectra. More than half of the candidate stars were found to show [Fe/H] in -0.7 to -1.2 range. Two most metal-poor stars have [Fe/H] values of -1.3 and -1.8. It appears that metal-poor candidates suggested by Beers et al. from their prismatic survey has a very significant fraction of metal-poor stars. The significantly metal-poor stars found so far would be studied in detail using high resolution spectra to understand nucleosynthesis processes that might have occurred in early Galaxy.

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arXiv2001

Atmospheric Abundances in Post-AGB candidates of Intermediate Temperature

Detailed atmospheric abundances have been calculated for a sample of A-G supergiant stars with IR fluxes and/or high galactic latitudes. HD 172481 and HD 158616 show clear indications of being post-AGB stars that have experienced third dredge-up. HD 158616 is carbon-rich while the abundance pattern of HD 172481 and its large Li enhancement gives support to the hot bottom burning scenario that explains paucity of carbon-rich stars among AGB stars. HD 172324 is very likely a hot post-AGB star that shows a strong carbon deficiency. HD 725, HD 218753 and HD 331319 also appear to be evolved objects between the red giant and the AGB. HD 9167, HD 173638 with a few exceptions, reflect solar abundances and no signs of post red giant evolution. They are most likely young massive disk supergiants. Further analysis of proto-Planetary Nebula HDE 341617 reveals that He lines show signs of velocity stratification. The emission lines have weakened considerably since 1993. The envelope expands at 19 km s$^{-1}$ relative to the star. Atmospheric abundances, evolutionary tracks and isochrones are used to estimate masses and ages of all stars in the sample.

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arXiv2001

Chemical Compositions of Four Metal-poor Giants

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 [$α$/Fe] within our sample where [$α$/Fe] is computed from the mean of the [Mg/Fe], and [Ca/Fe] ratios. BS 16085 - 0050 is remarkably $α$ enriched with a mean [$α$/Fe] of $+$0.7 but CS 22169-35 is $α$-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.

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arXiv1999

Abundance Analyses of Field RV Stars, V: DS Aquarii, UY Arae, TW Camelopardis, BT Librae, U Moncerotis, TT Ophiuchi, and RV Tauri

Abundance analyses are presented and discussed for eight RV Tauri variables. The RV B star UY Ara shows the abundance anomalies seen in other RV B stars, namely elements that condense into grains at high temperature are underabundant but elements of low condensation temperature are much less underabundant. This pattern is ascribed to a separation of dust from gas with accretion of gas but not dust by the atmosphere. Abundances for two RV C stars with earlier results for other RV C stars show that these intrinsically metal-poor stars do not show effects of dust-gas separation. Analyses of five RV A stars show that these cooler stars are very largely unaffected by dust-gas separation. It is proposed that the deeper convective envelope of cooler stars dilutes anomalies resulting from dust-gas separation - the dusty wind off the RV Tauri variable or a dusty circumbinary disk - are reviewed and observational tests are suggested.

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arXiv1999

The Chemical Compositions of the SRd Variable Stars-- II. WY Andromedae, VW Eridani, and UW Librae

Chemical compositions are derived from high-resolution spectra for three stars classed as SRd variables in the General Catalogue of Variable Stars. These stars are shown to be metal-poor supergiants: WY And with [Fe/H] = -1.0, VW Eri with [Fe/H] = -1.8, and UW Lib with [Fe/H] = -1.2. Their compositions are identical to within the measurement errors with the compositions of subdwarfs, subgiants, and less evolved giants of the same FeH. The stars are at the tip of the first giant branch or in the early stages of evolution along the asymptotic giant branch (AGB). There is no convincing evidence that these SRd variables are experiencing thermal pulsing and the third dredge-up on the AGB. The SRds appear to be the cool limit of the sequence of RV Tauri variables.

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