Studies of Helium-rich and Hydrogen-deficient stars to explain their origin and evolution

Abstract

The existence of hydrogen-deficient stars in a hydrogen-dominated universe in itself, sparks a considerable curiosity to explore their origin and evolution. Very rare in nature, the peculiar characteristics of these enigmatic objects suggest an evolutionary path very different from that of a normal star. However, due to the extreme rarity of these exotic objects, it is difficult to arrive at a definitive evolutionary history. A thorough knowledge about their chemical composition across the different samples of hydrogen-deficient stars enables us to tackle this issue. In this thesis, we mainly focus on finding an evolutionary connection between different groups of low mass, carbon-rich hydrogen-deficient stars through spectroscopic analyses of their observed spectra. The low-mass, carbon-rich, hydrogen-deficient supergiants are divided into four groups − the extreme helium stars or the EHes, the R Coronae Borealis Stars or the RCBs, the hydrogen-deficient carbon stars or the HdCs, and the DY Persei variables. They span a wide range of effective temperature − from the coolest DY Persei variables (Tef f ∼ 3500 K) to the hot extreme helium stars (EHes) (Tef f > 14000 K). Apart from hydrogen-deficiency, the majority of these stars are also characterized by peculiar chemical abundances connecting them in a common evolutionary sequence. The two most notable chemical peculiarities observed in these stars are (i) enhancement of 18O with relative to 16O and depletion of 13C with respect to 12C in the atmosphere of HdCs and cool RCBs (Clayton et al. 2007) and, ii) enrichment of fluorine (F) with respect to iron (Fe) by 800-8000 times than solar in the atmospheres of warm RCBs and cool EHes (Pandey 2006; Pandey et al. 2008). Based on the observed surface properties, two formation scenarios were proposed − 1) The Double Degenerate (DD) merger scenario involving the merger of two low mass double-degenerate white-dwarfs and, the 2) FF scenario involving a late or final He shell flash in a post-AGB star. However, the observed chemical peculiarities favour the DD scenario. The status of these chemical peculiarities was not explored in cool DY Persei variables and hot EHes. Hence the main aim of this thesis is to investigate the evolutionary connection of the hot EHes and cool DY Persei variables with the rest of the group by exploring the chemical signatures pertaining to the abundance anomalies. If the abundance peculiarities are found to be common across the entire range − DY Persei, HdCs, RCBs and EHes, a common formation scenario can be finally established. Below, we briefly describe the studies undertaken. In the first investigation, we perform a comparative, low-resolution NIR spectroscopic study on a sample of cool RCBs, HdCs, DY Persei variables and few DY Persei suspects with data obtained using TIRSPEC, mounted on 2m Himalayan Chandra Telescope (HCT). We obtain estimates of 12C/13C and 18O/16O from the relative strengths of the 12C 16O, 13C 16O and 12C 18O molecular bands observed in the NIR K- band region. The results confirm high 12C/13C and 18O/16O ratios in the sample of RCBs and HdCs. Among the samples of DY Persei variables, a quartet including DY Persei itself shows isotopic ratios consistent with RCBs and HdCs indicating that DY Persei variables may be related to RCBs and HdCs in the sense of their evolution. However, high-resolution spectroscopic analysis on a larger sample of DY Persei variables is required for firmly confirming their status as the cooler counterparts of RCBs. In the second study, we investigate the status of fluorine abundances in hot extreme helium stars (EHes) through high-resolution optical spectroscopic analyses. The spectroscopic data were obtained using HESP mounted on HCT and from ESO archives for ten hot EHes. Fluorine estimates were obtained using the F ii lines in two windows centered at 3505 ˚A and 3850 ˚A . The results show that six of the ten stars have 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. We find a surprising relation between F enrichment and the trend of N (nitrogen) with Ne (neon) abundances. Nevertheless, theoretical predictions suggest that Ne and F are both produced from N through different nucleosynthetic processes. However, while the observed Ne abundances suggest that Ne is a result of a complete conversion of N through successive α captures, the simultaneous enrichment of Ne and F without any visible depletion of N raises a need for further theoretical studies. In the third project, we carry out an LTE abundance analysis of the highresolution optical spectrum of the extreme helium star, V2205 Oph, obtained using HESP. Using a grid of LTE model atmospheres, we determine the atmospheric parameters of V2205 Oph: Tef f = 23900 ± 900 K, log g = 2.65 ± 0.1 (cgs) and ξ = 28 ± 2 kms−1 , from the imposition of ionization equilibrium of ion pairs such as C ii/C iii and N ii/N iii and using spectroscopic indicators like He i line profiles. We report chemical abundances of 12 elements from H to Fe, which agrees with previous determinations. By exploring the discrepancy of abundances determined from ionic pair of same species, we infer that ions of Si and S are heavily affected by departures from LTE. Hence, future studies using non-LTE abundance analysis of this star is recommended. With the results obtained from observational spectroscopic studies dedicated mainly to two extreme members of the group, the cool DY Persei variables and the hot EHes, we successfully establish the evolutionary connection between the entire sequence of H-deficient supergiants − the DY Pers, the HdCs, the RCBs, the cool and hot EHes. Below, we mention a brief structure of the thesis. In Chapter 1, we give a brief introduction to different groups of hydrogendeficient stars. We discuss in detail the observed properties of the low mass hydrogen deficient stars and review the two proposed formation scenarios − the DD merger and the FF scenario, in light of the evolutionary sequence followed by H-normal, low and intermediate-mass stars. In Chapter 2, we discuss the spectroscopic observations, data acquisition, and processing techniques. In Chapter 3, we discuss about the investigation of 12C/13C and 18O/16O ratios in low-resolution NIR spectra of DY Persei variables. In Chapter 4, we report about the spectroscopic investigation of F abundances from high-resolution optical spectra of hot EHes. In Chapter 5, we discuss about the LTE abundance analysis of the hot EHe, V2205 Oph. In Chapter 6, we conclude and provide some future prospects.