1,043 research outputs found
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
The progenitors of magnetic white dwarfs in open clusters
According to the fossil-field hypothesis magnetic fields are remnants of the
previous stages of evolution. However, population synthesis calculations are
unable to reproduce the magnetic white dwarf (MWD) sample without binary
interaction or inclusion of a population of progenitor with unobservable
small-scale fields. One necessary ingredient in population synthesis is the
initial-to-final-mass relation (IFMR) which describes the mass-loss processes
during the stellar evolution. When white dwarfs are members of open clusters,
their evolutionary histories can be assessed through the use of cluster
properties. In this work, we assess the cluster membership by correlating the
proper-motion of MWDs with the cluster proper-motion and by analyzing the
candidates spectroscopically with our magnetic model spectra in order to
estimate the effective temperature and radii. We identified SDSS
J085523.87+164059.0 to be a proper-motion member of Praesepe. We also included
the data of the formerly identified cluster members NGC 6819-8, WD 0836+201 and
estimated the mass, cooling age and the progenitor masses of the three probable
MWD members of open clusters. According to our analysis, the newly identified
cluster member SDSS J085523.87+164059.0 is an ultra-massive MWD of mass 1.12
0.11 Msolar. We increase the sample of MWDs with known progenitor masses
to ten, with the rest of the data coming from the common proper motion
binaries. Our investigations show that, when effects of the magnetic fields are
included in the diagnostics, the estimated properties of these cluster MWDs do
not show evidence for deviations from the IFMR. Furthermore, we estimate the
precision of the magnetic diagnostics which would be necessary to determine
quantitatively whether magnetism has any effect on the mass-loss.Comment: 8 pages, 4 figures, accepted for publication in A&
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 Age of the Milky Way Inner Halo
The Milky Way galaxy is observed to have multiple components with distinct
properties, such as the bulge, disk, and halo. Unraveling the assembly history
of these populations provides a powerful test to the theory of galaxy formation
and evolution, but is often restricted due to difficulties in measuring
accurate stellar ages for low mass, hydrogen-burning stars. Unlike these
progenitors, the "cinders" of stellar evolution, white dwarf stars, are
remarkably simple objects and their fundamental properties can be measured with
little ambiguity from spectroscopy. Here I report observations and analysis of
newly formed white dwarf stars in the halo of the Milky Way, and a comparison
to published analysis of white dwarfs in the well-studied 12.5 billion-year-old
globular cluster Messier 4. From this, I measure the mass distribution of the
remnants and invert the stellar evolution process to develop a new relation
that links this final stellar mass to the mass of their immediate progenitors,
and therefore to the age of the parent population. By applying this technique
to a small sample of four nearby and kinematically-confirmed halo white dwarfs,
I measure the age of local field halo stars to be 11.4 +/- 0.7 billion years.
This age is directly tied to the globular cluster age scale, on which the
oldest clusters formed 13.5 billion years ago. Future (spectroscopic)
observations of newly formed white dwarfs in the Milky Way halo can be used to
reduce the present uncertainty, and to probe relative differences between the
formation time of the last clusters and the inner halo.Comment: Published in Nature, 2012, 486, 90. Second version corrects a missing
reference (#10) in the third paragraph and Figure 1 captio
A Bound on the Light Emitted During the TP-AGB Phase
The integrated luminosity of the TP-AGB phase is a major uncertainty in
stellar population synthesis models. We use the white dwarf initial final mass
relation and stellar interiors models to demonstrate that a significant
fraction of the core mass growth for intermediate (1.5 < Msun < 6) mass stars
takes place during the TP-AGB phase. We find evidence that the peak fractional
core mass contribution for TP-AGB stars is ~20% and occurs for stars between 2
Msun and 3.5 Msun. Using a simple fuel consumption argument we couple this core
mass increase to a lower limit on the TP-AGB phase energy output. Roughly half
of the energy released in models of TP-AGB stars can be directly accounted for
by this core growth; while the remainder is predominantly the stellar yield of
He. A robust measurement of the emitted light in this phase will therefore set
strong constraints on helium enrichment from TP-AGB stars, and we estimate the
yields predicted by current models as a function of initial mass. Implications
for stellar population studies and prospects for improvements are discussed.Comment: Submitted to the Astrophysical Journal. 25 pages, 2 figures
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