Observational studies of hydrogen-deficient stars for investigating their evolutionary connections

Abstract

The group of hydrogen-deficient (H-deficient) and carbon rich supergiants spanning a range in their effective temperatures are: hydrogen deficient carbon (HdC) stars, R Coronae Borealis (RCB) stars, and extreme helium (EHe) stars, in the order of their increasing effective temperature. The origin and evolution of these stars is not yet clear. There are two proposed scenarios for their origin. In one dubbed the double degenerate (DD) scenario, a helium white dwarf merges with a carbon–oxygen (C–O) white dwarf. A merger of these two white dwarfs, both having a very thin outer H-rich layer, makes the resulting star H-deficient. An alternative scenario dubbed the final flash (FF) scenario involves a single postasymptotic giant branch (AGB) star experiencing a final helium shell flash which causes the H-rich envelope to be ingested by the He shell. The result is that the star becomes a H-deficient supergiant for a brief period and is sometimes referred in this condition as a ‘born-again’ AGB star. In this thesis, the origins and evolution of H-deficient supergiants are investigated: First, by conducting a survey for identifying H-deficient stars in the Galactic globular cluster ω Centauri, and to pin-point their positions on the HR-diagram, and second, by deriving the Galactic RCB/HdC stars C-abundances and the 12C/13C ratios that are potential clues to their origin. The distances are not accurately known to any of the Galactic H-deficient stars. The position of a star on the HR-diagram, gives us an idea about its evolution and possibly its origin. To place the H-deficient stars on the HR-diagram, one of the best ways would be to search for these stars in the globular clusters of the Galaxy. Hence, a survey was conducted to identify new H-deficient stars in the largest and brightest globular cluster of the Galaxy: ω Centauri. Our survey is based on the Str¨omgren photometric studies of red giant stars in ω Cen by Calamida et al. (2009). From the photometric and the spectroscopic studies of the red giant stars of ω Cen, it is clear that they show a large spread in their metallicity: −2.5<[Fe/H]<+0.5. This spread in metallicity, not as expected for the globular cluster, is taken as a clue for the presence of H-deficient stars in ω Cen. By applying the photometric and the spectroscopic characteristics of the RCB/HdC stars, the program stars were selected. For these program stars, the low-resolution spectra were obtained from the Vainu Bappu Observatory (VBO), Kavalur, India. The analyses were carried out based on the strengths of the (0,0) MgH band extending from 5330 to 4950˚A, with the band head at 5211˚A, and the Mg b lines at 5167.32˚A, 5172.68˚A and 5183.60˚A. Based on the strengths of these features, the three groups were identified in our sample: (i) the metal rich giants with strong Mg b lines and the MgH band, (ii) the metal poor giants with weak Mg b lines and no MgH band, and (iii) the metal rich giants with strong Mg b lines, but no MgH band. By comparing the observed MgH bands among the stars of (i) and (iii) group, with similar stellar parameters, four stars were identified having weaker or absent MgH band. Two stars: 178243 and 73170 are from the first group showing the strong Mg b lines, but weaker MgH band than expected for their stellar parameters. The other two stars: 262788 and 193804 are from the third group showing strong Mg b lines, but absent MgH band not as expected for their stellar parameters. The MgH band strengths in the observed spectra of these four stars along with all the first and third group stars, were further analyzed by synthesis. The Mg abundances derived for these four stars are much lower than that expected for the red giants of ω Cen as given by Norris & Da Costa (1995) for their metallicities. The weak/absent MgH band in the observed spectra of these four giants inspite of the presence of strong Mg b lines, may not be due to the stellar parameters or a lower Mg abundance. The only plausible reason is a relatively lower abundance of hydrogen in their atmosphere. Hence, from our survey, we report the discovery of four giants with relatively lower abundance of hydrogen in their atmospheres. In this survey we have not found any H-deficient star of RCB-type. This result is in agreement with our prediction for the number of H-deficient stars formed by the DD and FF scenario in the globular cluster, ω Cen. To explore the origin of the Galactic RCB stars, the carbon abundances and the 12C/13C ratios of RCB and HdC stars were determined by synthesizing the (0, 0), (1, 0) and (0, 1) C2 Swan bands, and matching them with the observed spectrum. High-resolution optical spectra of RCB/HdC stars at maximum light were obtained from the W. J. McDonald Observatory, USA, and the Vainu Bappu Observatory, India. The carbon abundances determined from the Ci lines are a factor of four lower than that adopted for the model atmosphere, and is dubbed as the ‘carbon problem’ (Asplund et al. 2000). This discrepancy persists with the change in the input carbon abundance of the adopted model atmosphere. Whereas, the carbon abundance derived from the C2 Swan bands is about the same for the adopted models constructed with different carbon abundances over the range 8.5 (C/He = 0.1%) to 10.5 (C/He = 10%). The 12C/13C ratios determined for the majority RCBs and all the HdCs are much higher than the CN-cycle equilibrium value of 3.4. These high values are consistent with that predicted for the cold merger of a He white dwarf with a CO white dwarf. The two minority RCB stars (stars which are metal poor and having high [Si/Fe] and [S/Fe] ratios, relative to the majority RCB stars) are having low values of 12C/13C ratios, that are close to the CN-cycle equilibrium value. These low values of 12C/13C ratios remain unaccounted due to their distinctive pattern of elemental abundances. The carbon abundance and the 12C/13C ratio were also determined for the final flash object, V4334 Sgr. The carbon abundance of V4334 Sgr is about 10-100 times higher than the RCB/HdC stars, and the 12C/13C ratio is about 3.4, the CN-cycle equilibrium value. These values are as expected for the final flash object.