128 research outputs found
The Core Mass Growth and Stellar Lifetime of Thermally Pulsing Asymptotic Giant Branch Stars
We establish new constraints on the intermediate-mass range of the
initial-final mass relation by studying white dwarfs in four young star
clusters, and apply the results to study the evolution of stars on the
thermally pulsing asymptotic giant branch (TP-AGB). We show that the stellar
core mass on the AGB grows rapidly from 10% to 30% for stars with = 1.6 to 2.0 . At larger masses, the core-mass growth
decreases steadily to 10% at = 3.4 . These
observations are in excellent agreement with predictions from the latest TP-AGB
evolutionary models in Marigo et al. (2013). We also compare to models with
varying efficiencies of the third dredge-up and mass loss, and demonstrate that
the process governing the growth of the core is largely the stellar wind, while
the third dredge-up plays a secondary, but non-negligible role. Based on the
new white dwarf measurements, we perform an exploratory calibration of the most
popular mass-loss prescriptions in the literature. Finally, we estimate the
lifetime and the integrated luminosity of stars on the TP-AGB to peak at
3 Myr and = 1.2 10 yr for 2 ( 2 Myr for luminosities brighter than
the RGB tip at 3.4), decreasing to = 0.4 Myr and
= 6.1 10 yr for stars with
3.5 . The implications of these results are discussed with
respect to general population synthesis studies that require correct modeling
of the TP-AGB phase of stellar evolution.Comment: 14 pages, 7 figures, 4 tables. Accepted for publication in Ap
The Spectral Types of White Dwarfs in Messier 4
We present the spectra of 24 white dwarfs in the direction of the globular
cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs.
Determining the spectral types of the stars in this sample, we find 24 type DA
and 0 type DB (i.e., atmospheres dominated by hydrogen and helium
respectively). Assuming the ratio of DA/DB observed in the field with effective
temperature between 15,000 - 25,000 K, i.e., 4.2:1, holds for the cluster
environment, the chance of finding no DBs in our sample due simply to
statistical fluctuations is only 6 X 10^(-3). The spectral types of the ~100
white dwarfs previously identified in open clusters indicate that DB formation
is strongly suppressed in that environment. Furthermore, all the ~10 white
dwarfs previously identified in other globular clusters are exclusively type
DA. In the context of these two facts, this finding suggests that DB formation
is suppressed in the cluster environment in general. Though no satisfactory
explanation for this phenomenon exists, we discuss several possibilities.Comment: Accepted for Publication in Astrophys. J. 11 pages including 4
figures and 2 tables (journal format
The Masses of Population II White Dwarfs
Globular star clusters are among the first stellar populations to have formed
in the Milky Way, and thus only a small sliver of their initial spectrum of
stellar types are still burning hydrogen on the main-sequence today. Almost all
of the stars born with more mass than 0.8 M_sun have evolved to form the white
dwarf cooling sequence of these systems, and the distribution and properties of
these remnants uniquely holds clues related to the nature of the now evolved
progenitor stars. With ultra-deep HST imaging observations, rich white dwarf
populations of four nearby Milky Way globular clusters have recently been
uncovered, and are found to extend an impressive 5 - 8 magnitudes in the
faint-blue region of the H-R diagram. In this paper, we characterize the
properties of these population II remnants by presenting the first direct mass
measurements of individual white dwarfs near the tip of the cooling sequence in
the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS
multiobject spectroscopic observations, our results indicate that 0.8 M_sun
population II main-sequence stars evolving today form 0.53 +/- 0.01 M_sun white
dwarfs. We discuss the implications of this result as it relates to our
understanding of stellar structure and evolution of population II stars and for
the age of the Galactic halo, as measured with white dwarf cooling theory.Comment: Accepted for Publication in Astrophys. J. on Aug. 05th, 2009. 19
pages including 9 figures and 2 tables (journal format
- …