Nelemans, G.; Napiwotzki, R.; Karl, C.; Marsh, T. R.; Voss, B.; Roelofs, G.; Izzard, R. G.; Montgomery, M.; Reerink, T.; Christlieb, N.; Reimers, D.
We present results from our ongoing follow-up observations of double
white dwarf binaries detected in the ESO Supernova-Ia Progenitor SurveY
(SPY). We discuss our observing strategy and data analysis and
present the orbital solutions of five close double white dwarf
binaries: HE0320-1917, HE1511-0448, WD0326-273, WD1013-010
and WD1210+140. Their periods range from 0.44 to 3.22 days. In none
of these systems we find any spectral lines originating from the
companion. This rules out main sequence companions and indicates that
the companion white dwarfs are significantly older and cooler than
the bright component. Infrared photometry suggests the presence
of a cool, helium-rich white dwarf companion in the binary
WD 0326-273. We briefly discuss the consequences of our
findings for our understanding of the formation and evolution of
double white dwarfs.
Astronomy and Astrophysics, Volume 440, Issue 3, 2005, pp.1087-1095
A long standing problem with asymptotic giant branch (AGB) star models has been their inability to produce the low-luminosity carbon stars in the Large and Small Magellanic Clouds (LMC and SMC, respectively). Dredge-up must begin earlier and extend deeper. We find this for the first time in our models of LMC metallicity. Such features are not found in our models of SMC metallicity.
The fully implicit and simultaneous stellar evolution code STARS has been used to calculate the evolution of AGB stars with metallicities of Z=0.008 and Z= 0.004, corresponding to the observed metallicities of the LMC and SMC, respectively. Third dredge-up occurs in stars of 1 Msolar and above and carbon stars were found for models between 1 M⨀; and 3 M⨀. We use the detailed models as input physics for a population synthesis code and generate carbon star luminosity functions. We now find that we are able to reproduce the carbon star luminosity function of the LMC without any manipulation of our models. The SMC carbon star luminosity function still cannot be produced from our detailed models unless the minimum core mass for third dredge-up is reduced by 0.06 M⨀.
Monthly Notices of the Royal Astronomical Society: Letters, Volume 356, Issue 1, pp. L1-L5