Aruna Goswami

A high-resolution spectral analysis of three carbon-enhanced metal-poor stars ⋆

We present results of an analysis of high-resolution spectra (R � 50000), obtained with the Subaru Telescope High Dispersion Spectrograph, of two CarbonEnhanced Metal-Poor (CEMP) stars selected from the Hamburg/ESO prism survey, HE 1305+0007 and HE 1152 0355, and of the classical CH star HD 5223. All of these stars have relatively low effective temperatures (4000–4750K) and high carbon abundances, which results in the presence of very strong molecular carbon bands in their spectra. The stellar atmospheric parameters for these stars indicate that they all have surface gravities consistent with a present location on the red-giant branch, and metallicities of [Fe/H] = 2.0 (HE 1305+0007, HD 5223) and [Fe/H] = 1.3 (HE 1152 0355). In addition to their large enhancements of carbon ([C/Fe] = +1.8, +1.6 and +0.6. respectively), all three stars exhibit strong enhancements of the sprocess elements relative to iron. HE 1305+0007 exhibits a large enhancement of the 3rd-peak s-process element lead, with [Pb/Fe] = +2.37, as well as a high abundance of the r-process element europium, [Eu/Fe] = +1.97. The 2nd-peak s-process elements Ba, La, Ce, Nd, and Sm are found to be more enhanced than the 1st-peak s-process elements Zr, Sr and Y. Thus, HE 1305+0007 joins the growing class of the so-called “Lead Stars”, and also the class of objects that exhibit the presence of both r-process and s-process elements, the CEMP-r/s stars. The large enhancement of n-capture elements exhibited by HE 1152 0355 and HD 5223 are more consistent with the abundance patterns generally noticed in CH stars, essentially arising from pure s-process nucleosynthesis. The elemental abundance distributions observed in these stars are discussed in the light of existing theories of CH star formation, as well as the suggested formation scenarios of the CEMP-r/s group.

CH stars at high Galactic latitudes

Carbon-rich stars of population II, such as CH stars, can provide direct information on the role of low to intermediate-mass stars of the halo on the early Galactic evolution. Thus an accurate knowledge of CH stellar population is a critical requirement for building up scenarios for early Galactic chemical evolution. In the present work we report on several CH stars identified in a sample of Faint High Latitude Carbon stars from Hamburg survey and discuss their medium resolution spectra covering a wavelength range 4000 - 6800 u Estimation of the depths of bands (1,0) 12 C 12 C �4737 and (1,0) 12 C 13 C �4744 in these stars indicate isotopic ratio 12 C/ 13 C � 3, except for a few exceptions; these ratios are consistent with existing theories of CH stars evolution. The stars of Hamburg survey, a total of 403 objects were reported to be carbon star candidates with strong C2 and CN molecular bands. In the first phase of observation, we have acquired spectra of ninety one objects. Inspection of the objects spectra show fifty one objects with C2 molecular bands in their spectra of which thirteen stars have low flux below about 4300 u Twenty five objects show weak or moderate CH and CN bands , twelve objects show weak but detectable CH bands in their spectra and there are three objects which do not show any molecular bands due to C2, CN or CH in their spectra. Objects with C2 molecular bands and with good signals bluewards of 4300 u which show prominent CH bands in their spectra are potential candidate CH stars. Thirty five such candidates are found in the present sample of ninty one objects observed so far. The set of CH stars identified could be the targets of subsequent observation at high resolution for a detail and comprehensive analysis for understanding their role in early Galactic chemical evolution.

Comparative modelling of the spectra of cool giants

Context. Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. Various model codes and strategies for the analysis of stellar spectra are available today. Aims. We aim to compare the results of deriving stellar parameters using different atmosphere models and different analysis strategies. The focus is set on high-resolution spectroscopy of cool giant stars. Methods. Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. Results. We present the results from the different groups, as well as an additional experiment comparing the synthetic spectra produced by various codes for a given set of stellar parameters. Similarities and differences of the results are discussed. Conclusions. Several valid approaches to analyze a given spectrum of a star result in quite a wide range of solutions. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method. This clearly shows how far from a definitive abundance analysis we still are.

High-resolution spectroscopy of QY Sge: an obscured RV Tauri variable?

ABSTRA C T The first high-resolution optical spectra of QY Sge are presented and discussed. Menzies & Whitelock, on the basis of photometry and low-resolution spectra, suggested that this G0I supergiant was obscured by dust and seen only by scattered light from a circumstellar reflection nebula. The new spectra confirm and extend this picture. Photospheric lines are unusually broad indicating scattering of photons from dust in the stellar wind. The presence of very broad, Na D emission lines is confirmed. Sharp emission lines from low levels of abundant neutral metal atoms are reported for the first time. An abundance analysis of photospheric lines shows that the stellar atmosphere is of approximately solar composition but with highly condensable (e.g. Sc and Ti) elements depleted by factors of 5 ‐ 10.

HD 209621: Abundances of neutron-capture elements

High resolution spectra obtained from the Subaru Telescope High Dispersion Spectrograph have been used to update the stellar atmospheric parameters and metallicity of the star HD 209621. We have derived a metallicity of [Fe/H] = 1.93 for this star, and have found a large enhancement of carbon and of heavy elements, with respect to iron. Updates on the elemental abundances of four s-process elements (Y, Ce, Pr, Nd) along with the first estimates of abundances for a number of other heavy elements (Sr, Zr, Ba, La, Sm, Eu, Er, Pb) are reported. The stellar atmospheric parameters, the effective temperature, Teff, and the surface gravity, logg (4500 K, 2.0), are determined from LTE analysis using model atmospheres. Estimated [Ba/Eu] = +0.35, places the star in the group of CEMP-(r+s) stars; however, the s-elements abundance pattern seen in HD 209621 is characteristic of CH stars; notably, the 2nd-peak s-process elements are more enhanced than the first peak s-process elements. HD 209621 is also found to show a large enhancement of the 3rd-peak s-process element lead (Pb) with [Pb/Fe] = +1.88. The relative contributions of the two neutron-capture processes, rand s- to the observed abundances are examined using a parametric model based analysis, that hints that the neutron-capture elements in HD 209621 primarily originate in s-process.

The CH fraction of Carbon stars at high Galactic latitudes

CH stars form a distinct class of objects with characteristic properties like iron deficiency, enrichment of carbon and overabundance of heavy elements. These properties can provide strong observational constraints for the theoretical computation of nucleosynthesis at low metallicity. An important issue is the relative surface density of CH stars, which can provide valuable input to our understanding of the role of low- to intermediate-mass stars in early Galactic chemical evolution. Spectroscopic characterization provides an effective way of identifying CH stars. The present analysis aims at a quantitative assessment of the fraction of CH stars in a sample using a set of spectral classification criteria. The sample consists of 92 objects selected from a collection of candidate faint high-latitude carbon stars from the Hamburg/ESO survey. Medium-resolution (λ/δλ ∼ 1300) spectra for these objects were obtained using the Optomechanics Research (OMR) spectrograph at the Vainu Bappu Observatory (VBO), Kavalur and the Himalaya Faint Object Spectrograph (HFOSC) at the Himalayan Chandra Telescope (HCT), Indian Astronomical Observatory, Hanle, during 2007-2009, spanning a wavelength range 3800-6800 A. Spectral analysis shows 36 of the 92 objects to be potential CH stars; combined with our earlier studies this implies ∼37 per cent (of 243 objects) as the CH fraction. We present spectral descriptions of the newly identified CH star candidates. Estimated effective temperatures, 12 C/ 13 C isotopic ratios and their locations on the two-colour J - H versus H - K plot are used to support their identification.

Chemical analysis of CH stars – II. Atmospheric parameters and elemental abundances

We present detailed chemical analyses for a sample of 12 stars selected from the CH star catalogue of Bartkevicius. The sample includes two confirmed binaries, four objects that are known to showradial velocity variations and the rest with no information on the binary status. A primary objective is to examine if all these objects exhibit chemical abundances characteristics of CH stars, based on detailed chemical composition study using high-resolution spectra. We have used high-resolution (R similar to 42 000) spectra from the ELODIE archive. These spectra cover 3900 to 6800 angstrom in the wavelength nge. We have estimated the stellar atmospheric parameters, the effective temperature T-eff, the surface gravity log g, and metallicity [Fe/H] from local thermodynamic equilibrium analysis using model atmospheres. Estimated temperatures of these objects cover a wide range from 4200 to 6640 K, the surface gravity from 0.6 to 4.3 and metallicity from -0.13 to -1.5. We report updates on elemental abundances for several heavy elements, Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu and Dy. For the object HD 89668, we present the first abundance analyses results. Enhancement of heavy elements relative to Fe, a characteristic property of CH stars is evident from our analyses in the case of four objects, HD 92545, HD 104979, HD 107574 and HD 204613. A parametric-model-based study is performed to understand the relative contributions from the s- and r-process to the abundances of the heavy elements.

On the Hydrogen Deficient Nature of Z UMi

Z Ursa Minoris was classified by Benson et al. (1994) a a R Coronae Borealis (RCB) variable star from its light variations. Hydrogen deficiency, which is a defining feature of RCB stars, was not established. To investigate this aspect we have obtained high resolution spectra in both blue (4200-4630A) and red (5050-7950A) regions. Lines of the CH molecule (G band) at about 4300A, which are present in spectra of N-type carbon stars are weak or absent in the spectrum of Z UMi indicating its hydrogen deficient nature and membership of the rare class of RCB variables.

THE SPECTRUM OF THE COOL R CORONAE BOREALIS VARIABLE S APODIS IN A DEEP DECLINE

A high resolution spectrum (5575 - 8875A) of the cool R Coronae Borealis variable S Apodis in its 1993 deep decline is discussed. Narrow or chromospheric emission lines of Na I, K I, Ca II, and Ba II and broad emission lines of He I 7065A and Na I D are seen, as expected from published reports of warmer RCBs in decline. The surprise of the S Aps spectrum is the considerable blue shift of the broad emissions component to the Na D lines. The blue shift may result from lines formed in a bipolar flow which is obscured asymmetrically by soot clouds along the line of sight.

CONSTRAINTS OF THE PHYSICS OF LOW-MASS AGB STARS FROM CH AND CEMP STARS

We analyze a set of published elemental abundances from a sample of CH stars which are based on high resolution spectral analysis of ELODIE and SUBARU/HDS spectra. All the elemental abundances were derived from local thermodynamic equilibrium analysis usingmodel atmospheres, and thus, they represent the largest homogeneous abundance data available for CH stars up to date. For this reason, we can use the set to constrain the physics and the nucleosynthesis occurring in low mass AGB stars. CH stars have been polluted in the past from an already extinct AGB companion and thus show s-process enriched surfaces. We discuss the effects induced on the surface AGB s-process distributions by different prescriptions for convection and rotation. Our reference theoretical FRUITY set fits only part of the observations. Moreover, the s-process observational spread for a fixed metallicity cannot be reproduced. At Fe/H]>-1, a good fit is found when rotation and a different treatment of the inner border of the convective envelope are simultaneously taken into account. In order to increase the statistics at low metallicities, we include in our analysis a selected number of CEMP stars and, therefore, we compute additional AGB models down to [Fe/H]=-2.85. Our theoretical models are unable to attain the large [hs/ls] ratios characterizing the surfaces of those objects. We speculate on the reasons for such a discrepancy, discussing the possibility that the observed distribution is a result of a proton mixing episode leading to a very high neutron density (the so-called i-process)