Sunetra Giridhar

Abundance Analyses of Field RV Tauri Stars. VI. An Extended Sample

An abundance analysis is presented and discussed for a sample of 14 RV Tauri stars. The present abundance data and those from our previous papers and by other workers are combined in an attempt to further understanding of the dust-gas separation process that afflicts many RV Tauri variables. We propose that a stars intrinsic (i.e., initial) metallicity is given by the photospheric zinc abundance. Variables warmer than about 5000 K and with an initial metallicity [Fe/H] ≥ -1 are affected by dust-gas separation. Variables of all metallicities and cooler than about Teff 5000 K are unaffected by dust-gas separation. The RV Tauri variables show a spread in their C abundances, with the lower boundary of the points in the C versus Zn plane falling close to the predicted trend for giants after the first dredge-up. The upper boundary is inhabited by a few stars that are carbon-rich. The O abundances in the mean follow the predicted trend from unevolved stars, in line with the expectation that photospheric O abundance is unaffected by the first dredge-up. An evolutionary scenario involving mass loss by a first-ascent or early-AGB red giant, the primary star of a binary, is sketched.

Abundance Analyses of Field RV Tauri Stars. V. DS Aquarii, UY Arae, TW Camelopardalis, BT Librae, U Monocerotis, TT Ophiuchi, R Scuti, and RV Tauri

Abundance analyses are presented and discussed for eight RV Tauri variables. The RVB star UY Ara shows the abundance anomalies seen in other RVB 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 RVC stars with earlier results for other RVC stars show that these intrinsically metal-poor stars do not show effects of dust-gas separation. Analyses of five RVA 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. Possible sites for dust formation and dust-gas separation—the dusty wind off the RV Tauri variable or a dusty circumbinary disk—are reviewed and observational tests suggested.

Comprehensive abundance analysis of red giants in the open clusters NGC 752, 1817, 2360 and 2506

ABSTRACT We have analyzed high-dispersion echelle spectra (R & 50,000) of red giant mem-bers for four open clusters to derive abundances for many elements. The spread intemperatures and gravities being very small among the red giants nearly the samestellar lines were employed thereby reducing the random errors. The errors of averageabundance for the cluster were generally in 0.02 to 0.07 dex range. Our present samplecovers galactocentric distances of 8.3 to 10.5 kpc. The [Fe/H] values are −0.02±0.05for NGC 752, −0.07±0.06 for NGC 2360, −0.11±0.05 for NGC 1817 and −0.19±0.06for NGC 2506. Abundances relative to Fe for elements from Na to Eu are equal withinmeasurement uncertainties to published abundances for thin disk giants in the field.This supports the view that field stars come from disrupted open clusters.Keywords: – Galaxy: abundances – Galaxy: open clusters and associations – stars:abundances: general – open clusters: individual: NGC 752, NGC 1817, NGC 2360,NGC 2506 1 INTRODUCTIONOpen clusters (OCs) are believed to be coeval groups of starsborn from the same proto-cluster cloud which may have beenpart of a larger star-forming region in the Milky Way.Ages ofclusters range from very young where stars are still formingto nearly 10 Gyr (Dias et al. 2002). Since all stars in mostOCs are at the same distance and have the same chemicalcomposition, basic stellar parameters like age, distance, andmetallicity can be determined more accurately than for fieldstars. Thus, OCs provide an excellent opportunity to mapthe structure, kinematics, and chemistry of the Galactic diskwith respect to Galactic coordinates and time.The presence of chemical homogeneity among clustermembers has been shown by the study of OCs, see, for ex-ample, spectroscopic analyses of the Hyades (Paulson et al.2003; De Silva et al. 2006) and Collinder 261 (Carretta etal. 2005; De Silva et al. 2007). This observed homogeneitysignifies that the proto-cloud is well mixed, and hence, theabundance pattern of a cluster bears the signature of chem-ical evolution of the natal cloud. Chemical evolution of theMilky Way has, of course, been well studied using field stars.A large fraction of field stars are from disrupted OCs (Lada

Comprehensive abundance analysis of red giants in the open clusters NGC 752, NGC 1817, NGC 2360 and NGC

ABSTRACT We have analyzed high-dispersion echelle spectra (R & 50,000) of red giant mem-bers for four open clusters to derive abundances for many elements. The spread intemperatures and gravities being very small among the red giants nearly the samestellar lines were employed thereby reducing the random errors. The errors of averageabundance for the cluster were generally in 0.02 to 0.07 dex range. Our present samplecovers galactocentric distances of 8.3 to 10.5 kpc. The [Fe/H] values are −0.02±0.05for NGC 752, −0.07±0.06 for NGC 2360, −0.11±0.05 for NGC 1817 and −0.19±0.06for NGC 2506. Abundances relative to Fe for elements from Na to Eu are equal withinmeasurement uncertainties to published abundances for thin disk giants in the field.This supports the view that field stars come from disrupted open clusters.Keywords: – Galaxy: abundances – Galaxy: open clusters and associations – stars:abundances: general – open clusters: individual: NGC 752, NGC 1817, NGC 2360,NGC 2506 1 INTRODUCTIONOpen clusters (OCs) are believed to be coeval groups of starsborn from the same proto-cluster cloud which may have beenpart of a larger star-forming region in the Milky Way.Ages ofclusters range from very young where stars are still formingto nearly 10 Gyr (Dias et al. 2002). Since all stars in mostOCs are at the same distance and have the same chemicalcomposition, basic stellar parameters like age, distance, andmetallicity can be determined more accurately than for fieldstars. Thus, OCs provide an excellent opportunity to mapthe structure, kinematics, and chemistry of the Galactic diskwith respect to Galactic coordinates and time.The presence of chemical homogeneity among clustermembers has been shown by the study of OCs, see, for ex-ample, spectroscopic analyses of the Hyades (Paulson et al.2003; De Silva et al. 2006) and Collinder 261 (Carretta etal. 2005; De Silva et al. 2007). This observed homogeneitysignifies that the proto-cloud is well mixed, and hence, theabundance pattern of a cluster bears the signature of chem-ical evolution of the natal cloud. Chemical evolution of theMilky Way has, of course, been well studied using field stars.A large fraction of field stars are from disrupted OCs (Lada

Difference image analysis: extension to a spatially varying photometric scale factor and other considerations

We present a general framework for matching the point-spread function (PSF), photometric scaling and sky background between two images, a subject which is commonly referred to as difference image analysis (DIA). We introduce the new concept of a spatially varying photometric scale factor which will be important for DIA applied to wide-field imaging data in order to adapt to transparency and airmass variations across the field-of-view. Furthermore, we demonstrate how to separately control the degree of spatial variation of each kernel basis function, the photometric scale factor and the differential sky background. We discuss the common choices for kernel basis functions within our framework, and we introduce the mixed-resolution delta basis functions to address the problem of the size of the least-squares problem to be solved when using delta basis functions. We validate and demonstrate our algorithm on simulated and real data. We also describe a number of useful optimizations that may be capitalized on during the construction of the least-squares matrix and which have not been reported previously. We pay special attention to presenting a clear notation for the DIA equations which are set out in a way that will hopefully encourage developers to tackle the implementation of DIA software.

Comprehensive abundance analysis of red giants in the open clusters NGC 2527, 2682, 2482, 2539, 2335, 2251 and 2266

We have analyzed high-resolution echelle spectra of red giant members for seven open clusters in the Galactic anticentre direction to explore their chemical positions. Cluster membership has been confirmed by radial velocity. The spread in temperatures and gravities being very small among the red giants, nearly the same stellar lines were employed for all stars thereby reducing the abundance errors: the errors of the average abundance for a cluster were generally in the 0.02 to 0.05 dex range. Our present sample covers Galactocentric distances of 8.3 to 11.3 kpc and an age range of 0.2 to 4.3 Gyr. A careful comparison of our results for the cluster NGC 2682 (M 67) to other high-resolution abundance studies in the literature shows general good agreement for almost all elements in common.

CCD time‐series photometry of the globular cluster NGC 6981: variable star census and physical parameter estimates

We present the results from 10 nights of observations of the globular cluster NGC 6981 (M72) in the V , R and I Johnson wavebands. We employed the technique of difference image analysis to perform precision differential photometry on the ti me-series images, which enabled us to carry out a census of the under-studied variable star population of the cluster. We show that 20 suspected variables in the literature are actually non-v ariable, and we confirm the variable nature of another 29 variables while refining their ephemeri des. We also detect 11 new RR Lyrae variables and 3 new SX Phe variables, bringing the total confirmed variable star count in NGC 6981 to 43. We performed Fourier decomposition of the light curves for a subset of RR Lyrae stars and used the Fourier parameters to estimate the fundamental physical parameters of the stars using relations available in the literature. Mean values of these physical parameters have allowed us to estimate the physical parameters of the parent cluster. We derive a metallicity of [Fe/H]ZW � 1.48±0.03 on the Zinn & West (1984) scale (or [Fe/H]UVES � 1.38±0.03 on the new Carretta et al. (2009) scale) for NGC 6981, and distances of �16.73±0.36 kpc and �16.68±0.36 kpc from analysis of the RR0 and RR1 stars separately. We also confirm the Oosterhoff type I classification for the cluster, and show th at our colour-magnitude data is consistent with the age of �12.75±0.75 Gyr derived by Dotter et al. (2010).

Abundance Analyses of Field RV Tauri Stars. IV. AD Aquilae, DS Aquarii, V360 Cygni, AC Herculis, and V453 Ophiuchi

Abundance analyses are presented and discussed for five RV Tauri variables. Three stars—DS Aqr, V360 Cyg, and V453 Oph—are RV C stars by spectroscopic classification, i.e., metal lines are weak. They are shown to be metal poor with [Fe/H] from -1.0 to -2.2 with normal relative abundances of other elements. By contrast, AD Aql and AC Her are RV B stars with an odd abundance pattern: elements that condense into grains at a high temperatures are underabundant (i.e., [Fe/H] = -2.1 for AD Aql) but elements with a low condensation temperatures are much less underabundant (i.e., [S/H] = 0.0 and [Zn/H] = -0.1 for AD Aql). This abundance pattern is ascribed to a separation of dust and gas in the upper atmosphere of the star. The present analyses with previously published results are used to investigate the systematics of the dust-gas separation in RV Tauri variables. The process is apparently inoperative in stars with an initial metallicity of about [Fe/H] -1.0; RV C stars and similar variables in globular clusters are immune to the dust-gas separation. The process achieves more severe effects in RV B than in RV A stars. The strength of the abundance anomalies attributed to dust-gas separation is not correlated with reported infrared excesses. After correction for the effects of the dust-gas separation, there is no strong evidence from the abundances that evolution along the AGB and experience of the third dredge-up preceded the formation of the majority of the RV Tauri variables.

Exploring the variable stars in the globular cluster NGC 5024 (M53): new RR Lyrae and SX Phoenicis stars★

We report CCD V and I time series photometry of the globular cluster NGC 5024 (M53). The technique of difference image analysis has been used which enables photometric precisions better than 10 mmag for stars brighter than V ∼ 18.5 mag even in the crowded central regions of the cluster. The high photometric precision has resulted in the discovery of two new RR1 stars and 13 SX Phe stars. A detailed identification chart is given for all the variable stars in the field of our images of the cluster. Periodicities were calculated for all RR Lyrae and SX Phe stars and a critical comparison is made with previous determinations. Out of the four probable SX Phe variables reported by Dekany & Kovacs, the SX Phe nature is confirmed only for V80; V81 is an unlikely case while V82 and V83 remain as dubious cases. Previous misidentifications of three variables are corrected. Astrometric positions with an uncertainty of ∼0.3 arcsec are provided for all variables. The light-curve Fourier decomposition of RR0 and RR1 is discussed, we find a mean metallicity of [Fe/H] = ―1.92 ± 0.06 in the scale of Zinn & West from 19 RR0 stars. The true distance moduli 16.36 ± 0.05 and 16.28 ± 0.07 and the corresponding distances 18.7 ± 0.4 and 18.0 ± 0.5 kpc are found from the RR0 and RR1 stars, respectively. These values are in agreement with the theoretical period-luminosity relations for RR Lyrae stars in the I band and with recent luminosity determinations for the RR Lyrae stars in the Large Magellanic Cloud (LMC). The age of 13.25 ± 0.50 Gyr, for NGC 5024, E(B ― V) = 0.02 and the above physical parameters of the cluster, as indicated from the RR0 stars, produce a good isochrone fitting to the observed colour-magnitude diagram (CMD). The period-luminosity (PL) relation for SX Phe stars in NGC 5024 in the V and I bands is discussed in the light of the 13 newly found SX Phe stars, and their pulsation mode is identified in most cases.

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 deciency. 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 stratication. 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