191 research outputs found
Are Stars with Planets Polluted?
We compare the metallicities of stars with radial velocity planets to the
metallicity of a sample of field dwarfs. We confirm recent work indicating that
the stars-with-planet sample as a whole is iron rich. However, the lowest mass
stars tend to be iron poor, with several having [Fe/H]<-0.2, demonstrating that
high metallicity is not required for the formation of short period Jupiter-mass
planets. We show that the average [Fe/H] increases with increasing stellar mass
(for masses below 1.25 solar masses) in both samples, but that the increase is
much more rapid in the stars-with-planet sample. The variation of metallicity
with stellar age also differs between the two samples. We examine possible
selection effects related to variations in the sensitivity of radial velocity
surveys with stellar mass and metallicity, and identify a color cutoff
(B-V>0.48) that contributes to but does not explain the mass-metallicity trend
in the stars-with-planets sample. We use Monte Carlo models to show that adding
an average of 6.5 Earth masses of iron to each star can explain both the
mass-metallicity and the age-metallicity relations of the stars-with-planets
sample. However, for at least one star, HD 38529, there is good evidence that
the bulk metallicity is high. We conclude that the observed metallicities and
metallicity trends are the result of the interaction of three effects;
accretion of about 6 Earth masses of iron rich material, selection effects, and
in some cases, high intrinsic metallicity.Comment: 19 pages 11 figure
Direct measurement of the 14N(p,g)15O S-factor
We have measured the 14N(p,g)15O excitation function for energies in the
range E_p = 155--524 keV. Fits of these data using R-matrix theory yield a
value for the S-factor at zero energy of 1.64(17) keV b, which is significantly
smaller than the result of a previous direct measurement. The corresponding
reduction in the stellar reaction rate for 14N(p,g)15O has a number of
interesting consequences, including an impact on estimates for the age of the
Galaxy derived from globular clusters.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
High Angular Resolution JHK Imaging of the Centers of the Metal-Poor Globular Clusters NGC5272 (M3), NGC6205 (M13), NGC6287, and NGC6341 (M92)
The Canada-France-Hawaii Telescope (CFHT) Adaptive Optics Bonnette (AOB) has
been used to obtain high angular resolution JHK images of the centers of the
metal-poor globular clusters NGC5272 (M3), NGC6205 (M13), NGC6287, and NGC6341
(M92). The color-magnitude diagrams (CMDs) derived from these data include the
upper main sequence and most of the red giant branch (RGB), and the cluster
sequences agree with published photometric measurements of bright stars in
these clusters. The photometric accuracy is limited by PSF variations, which
introduce systematic errors of a few hundredths of a magnitude near the AO
reference star.
The clusters are paired according to metallicity, and the near-infrared CMDs
and luminosity functions are used to investigate the relative ages within each
pair. The near-infrared CMDs provide the tightest constraints on the relative
ages of the classical second parameter pair NGC5272 and NGC6205, and indicate
that these clusters have ages that differ by no more than +/- 1 Gyr. These
results thus support the notion that age is not the second parameter. We
tentatively conclude that NGC6287 and NGC6341 have ages that differ by no more
than +/- 2 Gyr. However, the near-infrared spectral energy distributions of
stars in NGC6287 appear to differ from those of stars in outer halo clusters,
bringing into question the validity of this age estimate.Comment: 22 pages, 17 figures. To be published in the Astronomical Journa
Stellar models of evolved secondaries in CVs
In this paper we study the impact of chemically evolved secondaries on CV
evolution. We find that when evolved secondaries are included a spread in the
secondary mass-orbital period plane comparable to that seen in the data is
produced for either the saturated prescription for magnetic braking or the
unsaturated model commonly used for CVs. We argue that in order to explain this
spread a considerable fraction of all CVs should have evolved stars as the
secondaries. The evolved stars become fully convective at lower orbital
periods. Therefore, even if there was an abrupt decrease in magnetic braking
for fully convective stars (contrary to open cluster data) it would not be
expected to produce a sharp break in the period distribution for CVs. We also
explore recent proposed revisions to the angular momentum loss rate for single
stars, and find that only modest increases over the saturated prescription are
consistent with the overall observed spindown pattern. We compare predictions
of our models with diagnostics of the mass accretion rate in WDs and find
results intermediate between the saturated and the older braking prescription.
Taken together these suggest that the angular momentum loss rate may be higher
in CV secondaries than in single stars of the same rotation period, but is
still significantly lower than in the traditional model. Alternative
explanations for the CV period gap are discussed.Comment: 24 pages, 9 figures. Submitted to Ap
On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: II- Application to light elements in population I main-sequence stars
In the two previous papers of this series, we have discussed the importance
of t he -gradients due to helium settling on rotation-induced mixing,
first in a n approximate analytical way, second in a 2D numerical simulation.
We have found that, for slowly rotating low mass stars, a process of ``creeping
paralysis" in which the circulation and the diffusion are nearly frozen may
take place below the convective zone. Here we apply this theory to the case of
lithium and beryll ium in galactic clusters and specially the Hyades. We take
into account the rota tional braking with rotation velocities adjusted to the
present observations. We find that two different cells of meridional
circulation appear on the hot side of the "lithium dip" and that the "creeping
paralysis" process occurs, not dir ectly below the convective zone, but deeper
inside the radiative zone, at the to p of the second cell. As a consequence,
the two cells are disconnected, which ma y be the basic reason for the lithium
increase with effective temperature on thi s side of the dip. On the cool side,
there is just one cell of circulation and t he paralysis has not yet set down
at the age of the Hyades; the same modelisatio n accounts nicely for the
beryllium observations as well as for the lithium ones .Comment: 13 printed pages, 10 figures. ApJ, in press (April 20, 2003
The Angular Momentum Evolution of Very Low Mass Stars
We present theoretical models of the angular momentum evolution of very low
mass stars (0.1 - 0.5 M_sun) and solar analogues (0.6 - 1.1 M_sun). We
investigate the effect of rotation on the effective temperature and luminosity
of these stars. We find that the decrease in T_eff and L can be significant at
the higher end of our mass range, but becomes negligible below 0.4 M_sun.
Formulae for relating T_eff to mass and v_rot are presented.
We compare our models to rotational data from young open clusters of
different ages to infer the rotational history of low mass stars, and the
dependence of initial conditions and rotational evolution on mass. We find that
the qualitative conclusions for stars below 0.6 M_sun do not depend on the
assumptions about internal angular momentum transport, which makes these low
mass stars ideal candidates for the study of the angular momentum loss law and
distribution of initial conditions. We find that neither models with solid body
nor differential rotation can simultaneously reproduce the observed stellar
spin down in the 0.6 to 1.1 M_sun mass range and for stars between 0.1 and 0.6
M_sun. The most likely explanation is that the saturation threshold drops more
steeply at low masses than would be predicted with a simple Rossby scaling. In
young clusters there is a systematic increase in the mean rotation rate with
decreased temperature below 3500 K (0.4 M_sun). This suggests either
inefficient angular momentum loss or mass-dependent initial conditions for
stars near the fully convective boundary. (abridged)Comment: To appear in the May 10, 2000 Ap
Identification of the LMXB and Faint X-ray Sources in NGC 6652
We have detected three new x-ray point sources, in addition to the known
low-mass x-ray binary (LMXB) X1832-330, in the globular cluster NGC 6652 with a
Chandra 1.6 ksec HRC-I exposure. Star 49 (M_{V}~4.7), suggested by Deutsch et
al.(1998) as the optical candidate for the LMXB, is identified (<0.3") not with
the LMXB, but with another, newly detected source (B). Using archival HST
images, we identify (<0.3") the LMXB (A) and one of the remaining new sources
(C) with blue variable optical counterparts at M_{V}~3.7 and 5.3 respectively.
The other new source (D) remains unidentified in the crowded cluster core. In
the 0.5-2.5 keV range, assuming a 5 keV thermal bremsstrahlung spectrum and
N_{H}=5.5*10^{20}, source A has intrinsic luminosity L_{X}~5.3*10^{35} ergs/s.
Assuming a 1 keV thermal bremsstrahlung spectrum, B has L_{X}~4.1*10^{33}
ergs/s, while C and D have L_{X}~8*10^{32}$ ergs/s. Source B is probably a
quiescent LMXB, while source C may be either a luminous CV or quiescent LMXB.Comment: 14 pages, 3 figures, accepted by Astrophysical Journa
An explanation for metallicity effects on X-ray Binary properties
We show that irradiation induced stellar winds can explain two important
metallicity effects in X-ray binaries - the higher numbers and the softer
spectra of the X-ray binaries in metal rich globular clusters compared to the
metal poor ones. As has been previously noted by Iben, Tutukov and Fedorova,
the winds should be stronger at lower metallicity due to less efficient line
cooling. This will speed up the evolution of the LMXBs in metal poor clusters,
hence reducing their numbers. These winds can also provide extra material near
the accreting object which may create an intrinsic absorber to harden the X-ray
spectra of the metal poor cluster systems relative to the metal rich ones, as
suggested by observations. We outline some additional observational predictions
of the model.Comment: 6 pages, no figures, accepted to Ap
3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
We examine the stability and observational consequences of mixing induced by
3He burning in the envelopes of first ascent red giants. We demonstrate that
there are two unstable modes: a rapid, nearly adiabatic mode that we cannot
identify with an underlying physical mechanism, and a slow, nearly radiative
mode that can be identified with thermohaline convection. We present
observational constraints that make the operation of the rapid mode unlikely to
occur in real stars. Thermohaline convection turns out to be fast enough only
if fluid elements have finger-like structures with a length to diameter ratio
l/d > 10. We identify some potentially serious obstacles for thermohaline
convection as the predominant mixing mechanism for giants. We show that
rotation-induced horizontal turbulent diffusion may suppress the 3He-driven
thermohaline convection. Another potentially serious problem for it is to
explain observational evidence of enhanced extra mixing. The 3He exhaustion in
stars approaching the red giant branch (RGB) tip should make the 3He mixing
inefficient on the asymptotic giant branch (AGB). In spite of this, there are
observational data indicating the presence of extra mixing in low-mass AGB
stars similar to that operating on the RGB. Overmixing may also occur in
carbon-enhanced metal-poor stars.Comment: 25 pages, 6 figures, modified version, accepted by Ap
Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low Mass White Dwarfs
We present the first detailed study of the properties (temperatures,
gravities, and masses) of the NGC 6791 white dwarf population. This unique
stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~
0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that
exhibits both a red giant clump and a much hotter extreme horizontal branch.
Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS
spectra, suggests that most of these stars are undermassive, = 0.43 +/-
0.06 Msun, and therefore could not have formed from canonical stellar evolution
involving the helium flash at the tip of the red giant branch. We show that at
least 40% of NGC 6791's evolved stars must have lost enough mass on the red
giant branch to avoid the flash, and therefore did not convert helium into
carbon-oxygen in their core. Such increased mass loss in the evolution of the
progenitors of these stars is consistent with the presence of the extreme
horizontal branch in the CMD. This unique stellar evolutionary channel also
naturally explains the recent finding of a very young age (2.4 Gyr) for NGC
6791 from white dwarf cooling theory; helium core white dwarfs in this cluster
will cool ~3 times slower than carbon-oxygen core stars and therefore the
corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well
measured main-sequence turnoff age. These results provide direct empirical
evidence that mass loss is much more efficient in high metallicity environments
and therefore may be critical in interpreting the ultraviolet upturn in
elliptical galaxies.Comment: 15 pages, 9 figures, 2 tables. Accepted for publication in Astrophys.
J. Very minor changes from first versio
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