29 research outputs found
Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution
Boron abundances have been derived for seven main sequence B-type stars from
HST STIS spectra around the B III 2066 A line. In two stars, boron appears to
be undepleted with respect to the presumed initial abundance. In one star,
boron is detectable but it is clearly depleted. In the other four stars, boron
is undetectable implying depletions of 1 to 2 dex. Three of these four stars
are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only
rotationally induced mixing predicts that boron depletions are unaccompanied by
nitrogen enrichments. The inferred rate of boron depletion from our
observations is in good agreement with these predictions. Other boron-depleted
nitrogen-normal stars are identified from the literature. Also, several
boron-depleted nitrogen-rich stars are identified, and while all fall on the
boron-nitrogen trend predicted by rotationally-induced mixing, a majority have
nitrogen enrichments that are not uniquely explained by rotation.
The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni)
abundances. The seven B-type stars have near solar iron-group abundances, as
expected for young stars in the solar neighborhood. We have also analysed the
halo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absence
of the B III line gives the upper limit [B/H]<2.5. These and other published
abundances are used to infer the star's evolutionary status as a post-AGB star.Comment: 31 pages, 14 figures. accepted to Ap
The Origins and Evolutionary Status of B Stars Found Far From the Galactic Plane II: Kinematics and Full Sample Analysis
This paper continues the analysis of faint high latitude B stars from Martin
(2004). Here we analyze the kinematics of the stars and combine them with the
abundance information from the first paper to classify each one. The sample
contains 31 Population I runaways, fifteen old evolved stars (including five
BHB stars, three post-HB stars, a pulsating helium dwarf, and six stars of
ambiguous classification), one F-dwarf, and two stars which do not easily fit
in one of the other categories. No star in the sample unambiguously shows the
characteristics of a young massive star formed in situ in the halo. The two
unclassified stars are probably extreme Population I runaways. The low binary
frequency and rotational velocity distribution of the Population I runaways
imply that most were ejected from dense star clusters by DES (dynamic ejection
scenario). However we remain puzzled by the lack of runaway Be stars. We also
confirm that PB 166 and HIP 41979 are both nearby solar-metallicity BHB stars.Comment: 59 pages including 14 Figures, 12 Tables; Accepted for publication in
the Astronomical Journa
Evolutionary models of white dwarfs with helium cores
We present seven evolutionary tracks for low-mass white dwarfs with helium cores, ranging in mass from 0.179 to 0.414 M_sun. We generated the pre-white dwarf models from a 1 M_sun sequence extending up to the tip of its red-giant branch by applying high mass-loss rates at appropriate positions, and we followed their evolution across the Hertzsprung-Russell diagram and down the cooling path. We discuss the internal structures and cooling properties of these new models and compare them with those of recently published models for low-mass white dwarfs which are based on simplified initial configurations. We also demonstrate that our new models seem to remove the apparent discrepancies between the characteristic ages of millisecond pulsars and the cooling ages of their white dwarf companions. (orig.)Proceedings of the 11. European workshop on white dwarfs, 19 refs.Available from TIB Hannover: RR 7310(98-25) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
The role of convective boundaries
We investigate the influence of convective overshoot on stellar evolution models of the thermal pulse AGB phase with M_Z_A_M_S = 3M_sun. An exponential diffusive overshoot algorithm is applied to all convective boundaries during all evolutionary stages. We demonstrate that overshooting at the bottom of the pulse-driven convective zone, which forms in the intershell during the He-shell flash, leads to more efficient third dredge-up. Some overshoot at the bottom of the convective envelope removes the He-H discontinuity, which would otherwise prohibit the occurrence of the third dredge-up for this stellar mass. However, no correlation between the amount of envelope overshoot and the efficiency of the third dredge-up has been found. Increasingly efficient third dredge-up eventually leads to a carbon star model. Due to the partial mixing efficiency in the overshoot region a "1"3C-pocket can form after the third dredge-up event which may be crucial for n-capture nucleosynthesis. (orig.)15 refs.Available from TIB Hannover: RR 7310(98-38) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman