Related papers: On the binary helium star DY Centauri: Chemical co…
It has recently been noted that there seems to be a strong correlation between planetary nebulae with close binary central stars, and highly enhanced recombination line abundances. We present new deep spectra of seven objects known to have…
We report on a detailed study of the X-ray spectrum of the nearby eclipsing spectroscopic binary YY Gem. Observations were obtained simultaneously with both large X-ray observatories, XMM-Newton and Chandra. We compare the high-resolution…
For the purpose of deriving the helium abundances in chemically peculiar stars, the importance of assuming a correct helium abundance has been investigated for determining the effective temperature and gravity of main sequence B-type stars,…
White dwarf stars are traditionally found to have surface compositions made primarily of hydrogen or helium. However, a new family has recently been uncovered, the so-called Hot DQ white dwarfs, which have surface compositions dominated by…
Context: There is considerable interest in the helium variable a Cen as a bridge between helium-weak and helium-strong CP stars. Aims: We investigate Ce III and other possible lanthanides in the spectrum the of hottest chemically peculiar…
95% of all stars end their lives as white dwarfs. About 20% of the hot post-AGB stars are hydrogen deficient. Most of these are the result of a late helium-shell flash, but the evolutionary status of a fraction of about 10-20% of the…
The photospheric metal pollution of white dwarfs is now well-established as the signature of the accretion of planetary debris. However, the origin of the trace hydrogen detected in many white dwarfs with helium atmospheres is still…
Using a high resolution spectrum of the secondary star in the neutron star binary {Cen X-4}, we have derived the stellar parameters and veiling caused by the accretion disk in a consistent way. We have used a $\chi^{2}$ minimization…
A photometric and spectroscopic analysis of 152 cool white dwarf stars is presented. The discovery of 7 new DA white dwarfs, 2 new DQ white dwarfs, 1 new magnetic white dwarf, and 3 weak magnetic white dwarf candidates, is reported, as well…
Recent pieces of evidence have revealed that most, and possibly all, globular star clusters are composed of groups of stars that formed in multiple episodes with different chemical compositions. In this sense, it has also been argued that…
We conducted a high-precision elemental abundance analysis of the twin-star comoving pair HIP34407/HIP34426. With mean error of 0.013 dex in the differential abundances (D[X/H]), a significant difference was found: HIP34407 is more…
The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB) stars are discussed in terms of the merger of a carbon-oxygen white dwarf with a helium white dwarf. The model is expressed as a linear mixture of the individual…
In this review we present a brief discussion on the observational evidence in favor of the presence of temperature variations, and conclude that many planetary nebulae show spatial temperature variations that are larger than those predicted…
The detailed composition of most metal-poor halo stars has been found to be very uniform. However, a fraction of 20-70% (increasing with decreasing metallicity) exhibit dramatic enhancements in their abundances of carbon - the so-called…
We analyze the reddening, surface helium abundance and mass of 115 horizontal branch (HB) and blue hook (BH) stars in OmegaCentauri, spanning the HB from the blue edge of the instability strip to Teff~50000K. The mean cluster reddening is…
We study C/O white dwarfs with masses of 1.0 to 1.4 Msun accreting solar-composition material at very high accretion rates. We address the secular changes in the WDs, and in particular, the question whether accretion and the thermonuclear…
In response to the proposed high helium content stars as an explanation for the double main sequence observed in Omega Centauri, we investigated the consequences of such stars elsewhere on the color-magnitude diagram. We concentrated on the…
The question of whether genuinely young high-$\alpha$ stars exist has been discussed for over a decade since their discovery from asteroseismology of giant stars as it is challenging to break the degeneracy between the binary interaction…
One of the high-level goals of Galactic archaeology is chemical tagging of stars across the Milky Way to piece together its assembly history. For this to work, stars born together must be uniquely chemically homogeneous. Wide binary systems…
Spectrophotometric data of the young planetary nebula M 3-27, from 2004 to 2021, are presented and discussed. We corroborate that the H I Balmer lines present features indicating they are emitted by the central star, therefore He I lines…