Astronomers Discover Stars With Carbon Atmospheres Released at November 21, 2007 P. Dufour, J. Liebert, G. Fontaine, N. Behara published the results in the Nov. 22 issue of Nature

Introduction Normal white dwarf : Normal white dwarf : the DAs and the non-DAs George P. McCook and Edward M. Sion, The Astrophysical Journal Supplement Series 121, #1 (March 1999), pp. 1 – 130.

Introduction DQ white dwarfs DQ white dwarfs - carbon maximum contamination at T eff ~ - carbon maximum contamination at T eff ~10, 000 K because of convection - atmosphere behavior of the hot side had not been fully studied - the discovery of several new hot white dwarfs with carbon lines in the Sloan Digital Sky Survey (Liebert et al. 2003)

Hot DQ stars Most stars are very similar to the previously known DQ stars G227-5 and G35-26 and exhibited mostly CI lines. a few showed a more complex and rich spectrum that is attributed to CII absorption lines. no combination of carbon and helium could successfully reproduce the observed features by assuming a helium-dominated atmophere

Fit to the optical spectra (left panel) and energy distribution (right panel) for carbon-rich white dwarfs with log g fixed at 8. Hydrogen and helium abundances are constrained (or dermined in two cases) from the absence of the H(4861) and HeI 4471 lines. Photometric distances, based on the assumption that log g = 8, are indicated in each panel.

H and evolutionay scenarios these WDs are the progenies of objects such as the unique hot PG1159 star H Artists' concept of the surface of the white dwarf star H , believed to have somehow expelled all its hydrogen and all but a very small trace of its helium, leaving an essentially bare stellar nucleus with a surface of 50 percent oxygen and 50 percent carbon. When this star cools, it may have a carbon atmosphere.

H and evolutionay scenarios It could also be the result of a heavy- weight intermediate-mass star (8 M ⊙ ≤M≤10M ⊙ ) that has gone through carbon burning and has a O-Ne-Mg core. for some stars, data fits with models with high surface gravity have lines that are much too broad

Conclusion All carbon-rich white dwarfs they have found so far have T eff between 18,000 and 24,000 K a star like H , showing initially a mixed C and O atmosphere, would temporarily disguise itself as a helium-rich star before transforming itself into a carbon dominated atmosphere star at an effective temperature whose exact value is not yet known due to a current lack of proper models (hopefully around 24,000 K !) these stars must again go through a drastic spectral change as they cool

Reference 7/11/ htm 7/11/ htm 7/11/ htm 7/11/ htm arXiv: v1 [astro-ph] 21 Nov 2007

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