71 research outputs found
Two Boehm-Vitense gaps in the main sequence of the Hyades
Hipparcos proper motions and trigonometric parallaxes allow the derivation of
secular parallaxes which fix the distances to individual stars in the Hyades
cluster to an accuracy of \sim 2 percent. The resulting color-absolute
magnitude diagram for 92 high-fidelity single members of the cluster displays a
very narrow main sequence, with two turn-offs and associated gaps. These occur
at the locations where the onset of surface convection affects the B-V colors,
as predicted by Boehm-Vitense thirty years ago. The new distances provide
stringent constraints on the transformations of colors and absolute magnitudes
to effective temperatures and luminosities, and on models of stellar interiors.Comment: 4 pages, 2 PostScript figures, LaTeX using aastex and
emulateapj5.sty; accepted for publication in Astrophysical Journal Letter
Measuring ^{12}C(&alpha,&gamma)^{16}O from White Dwarf Asteroseismology
During helium burning in the core of a red giant, the relative rates of the
3&alpha and ^{12}C(&alpha,&gamma)^{16}O reactions largely determine the final
ratio of carbon to oxygen in the resulting white dwarf star. The uncertainty in
the 3&alpha reaction at stellar energies due to the extrapolation from
high-energy laboratory measurements is relatively small, but this is not the
case for the ^{12}C(&alpha,&gamma)^{16}O reaction. Recent advances in the
analysis of asteroseismological data on pulsating white dwarf stars now make it
possible to obtain precise measurements of the central ratio of carbon to
oxygen, providing a more direct way to measure the ^{12}C(&alpha,&gamma)^{16}O
reaction rate at stellar energies. We assess the systematic uncertainties of
this approach and quantify small shifts in the measured central oxygen
abundance originating from the observations and from model settings that are
kept fixed during the optimization. Using new calculations of white dwarf
internal chemical profiles, we find a rate for the ^{12}C(&alpha,&gamma)^{16}O
reaction that is significantly higher than most published values. The accuracy
of this method may improve as we modify some of the details of our description
of white dwarf interiors that were not accessible through previous
model-fitting methods.Comment: 8 pages, 4 figures, 3 tables, uses emulateapj5.sty, Accepted for
publication in the Astrophysical Journa
Two-Dimensional Hydrodynamics of Pre-Core Collapse: Oxygen Shell Burning
By direct hydrodynamic simulation, using the Piecewise Parabolic Method (PPM)
code PROMETHEUS, we study the properties of a convective oxygen burning shell
in a SN 1987A progenitor star prior to collapse. The convection is too
heterogeneous and dynamic to be well approximated by one-dimensional
diffusion-like algorithms which have previously been used for this epoch.
Qualitatively new phenomena are seen.
The simulations are two-dimensional, with good resolution in radius and
angle, and use a large (90-degree) slice centered at the equator. The
microphysics and the initial model were carefully treated. Many of the
qualitative features of previous multi-dimensional simulations of convection
are seen, including large kinetic and acoustic energy fluxes, which are not
accounted for by mixing length theory. Small but significant amounts of
carbon-12 are mixed non-uniformly into the oxygen burning convection zone,
resulting in hot spots of nuclear energy production which are more than an
order of magnitude more energetic than the oxygen flame itself. Density
perturbations (up to 8%) occur at the `edges' of the convective zone and are
the result of gravity waves generated by interaction of penetrating flows into
the stable region. Perturbations of temperature and electron fraction at the
base of the convective zone are of sufficient magnitude to create angular
inhomogeneities in explosive nucleosynthesis products, and need to be included
in quantitative estimates of yields. Combined with the plume-like velocity
structure arising from convection, the perturbations will contribute to the
mixing of nickel-56 throughout supernovae envelopes. Runs of different
resolution, and angular extent, were performed to test the robustness of theseComment: For mpeg movies of these simulations, see
http://www.astrophysics.arizona.edu/movies.html Submitted to the
Astrophysical Journa
Stellar Envelope Convection calibrated by Radiation Hydrodynamics Simulations: Influence on Globular Clusters Isochrones
One of the largest sources of uncertainty in the computation of globular
cluster isochrones and hence in the age determination of globular clusters is
the lack of a rigorous description of convection. Therefore, we calibrated the
superadiabatic temperature gradient in the envelope of metal-poor low-mass
stars according to the results from a new grid of 2D hydrodynamical models,
which cover the Main Sequence and the lower Red Giant Branch of globular
cluster stars. In practice, we still use for computing the evolutionary stellar
models the traditional mixing length formalism, but we fix the mixing length
parameter in order to reproduce the run of the entropy of the deeper adiabatic
region of the stellar envelopes with effective temperature and gravity as
obtained from the hydro-models. The detailed behaviour of the calibrated mixing
length depends in a non-trivial way on the effective temperature, gravity and
metallicity of the star. Nevertheless, the resulting isochrones for the
relevant age range of galactic globular clusters have only small differences
with respect to isochrones computed adopting a constant solar calibrated value
of the mixing length. Accordingly, the age of globular clusters is reduced by
0.2 Gyr at most.Comment: 9 pages, 3 figures Accepted for publication in ApJ Letter
Fine Grid Asteroseismology of R548 and G117-B15A
We now have a good measurement of the cooling rate of G117-B15A. In the near
future, we will have equally well determined cooling rates for other pulsating
white dwarfs, including R548. The ability to measure their cooling rates offers
us a unique way to study weakly interacting particles that would contribute to
their cooling. Working toward that goal, we perform a careful
asteroseismological analysis of G117-B15A and R548. We study them side by side
because they have similar observed properties. We carry out a systematic, fine
grid search for best fit models to the observed period spectra of those stars.
We freely vary 4 parameters: the effective temperature, the stellar mass, the
helium layer mass, and the hydrogen layer mass. We identify and quantify a
number of uncertainties associated with our models. Based on the results of
that analysis and fits to the periods observed in R548 and G117-B15A, we
clearly define the regions of the 4 dimensional parameter space ocuppied by the
best fit models.Comment: The first author would love to hear from you if you found this paper
interesting. email [email protected]
Observable form of pulses emitted from relativistic collapsing objects
In this work, we discuss observable characteristics of the radiation emitted
from a surface of a collapsing object. We study a simplified model in which a
radiation of massless particles has a sharp in time profile and it happens at
the surface at the same moment of comoving time. Since the radiating surface
has finite size the observed radiation will occur during some finite time. Its
redshift and bending angle are affected by the strong gravitational field. We
obtain a simple expression for the observed flux of the radiation as a function
of time. To find an explicit expression for the flux we develop an analytical
approximation for the bending angle and time delay for null rays emitted by a
collapsing surface. In the case of the bending angle this approximation is an
improved version of the earlier proposed Beloborodov-Leahy-approximation. For
rays emitted at the accuracy of the proposed improved approximations
for the bending angle and time delay is of order (or less) than 2-3%. By using
this approximation we obtain an approximate analytical expression for the
observed flux and study its properties.Comment: 13 pages, 10 figures;Typos in equations and refrences are corrected.
No change in the results and discussion
Convection, Thermal Bifurcation, and the Colors of A stars
Broad-band ultraviolet photometry from the TD-1 satellite and low dispersion
spectra from the short wavelength camera of IUE have been used to investigate a
long-standing proposal of Bohm-Vitense that the normal main sequence A- and
early-F stars may divide into two different temperature sequences: (1) a high
temperature branch (and plateau) comprised of slowly rotating convective stars,
and (2) a low temperature branch populated by rapidly rotating radiative stars.
We find no evidence from either dataset to support such a claim, or to confirm
the existence of an "A-star gap" in the B-V color range 0.22 <= B-V <= 0.28 due
to the sudden onset of convection. We do observe, nonetheless, a large scatter
in the 1800--2000 A colors of the A-F stars, which amounts to ~0.65 mags at a
given B-V color index. The scatter is not caused by interstellar or
circumstellar reddening. A convincing case can also be made against binarity
and intrinsic variability due to pulsations of delta Sct origin. We find no
correlation with established chromospheric and coronal proxies of convection,
and thus no demonstrable link to the possible onset of convection among the A-F
stars. The scatter is not instrumental. Approximately 0.4 mags of the scatter
is shown to arise from individual differences in surface gravity as well as a
moderate spread (factor of ~3) in heavy metal abundance and UV line blanketing.
A dispersion of ~0.25 mags remains, which has no clear and obvious explanation.
The most likely cause, we believe, is a residual imprecision in our correction
for the spread in metal abundances. However, the existing data do not rule out
possible contributions from intrinsic stellar variability or from differential
UV line blanketing effects owing to a dispersion in microturbulent velocity.Comment: 40 pages, 14 figures, 1 table, AAS LaTex, to appear in The
Astrophysical Journa
Variation in the frequency separations with activity and impact on stellar parameter determination
Frequency separations used to infer global properties of stars through
asteroseismology can change depending on the strength and at what epoch of the
stellar cycle the p-mode frequencies are measured. In the Sun these variations
have been seen, even though the Sun is a low-activity star. In this paper, we
discuss these variations and their impact on the determination of the stellar
parameters (radius, mass and age) for the Sun. Using the data from maximum and
minimum activity, we fitted an age for the Sun that differs on average by 0.2
Gyr: slightly older during minimum activity. The fitted radius is also lower by
about 0.5% for the solar effective temperature during minimum.Comment: to be published in JPCS to be published in JPC
An Absence of Gaps in the Main Sequence Population of Field Stars
Using high precision parallaxes from the Hipparcos catalog, we construct H-R
diagrams for two samples of bright stars. The first is a magnitude-limited
sample that is over 90% complete and uses uniform photometry from the Catalog
of WBVR Magnitudes of Northern Sky Bright Stars (declination above -14 deg).
This sample shows a smooth distribution of stars along the main sequence, with
no detectable gaps. The second contains all of the stars closer than 100
parsecs in the Hipparcos catalog with declination less than -12 deg. Uniform
spectroscopy from the Michigan Spectral Survey shows that some stars which
appear on the main sequence in the H-R diagram, particularly those in the 0.2 <
B-V < 0.3 region that has been labeled the Bohm-Vitense gap, are classified as
giants by the MK system of spectral classification. Other gaps that have been
identified in the main sequence are also affected by such classification
criteria. This analysis casts doubt on the existence of the Bohm-Vitense gap,
which is thought to result from the sudden onset of convection in stars. The
standard identification of main sequence stars with luminosity class V, and
giants with luminosity class III, must be reconsidered for some spectral types.
The true nature of the stars that lie on the main sequence in the H-R diagram,
but which do not have luminosity class V designations, remains to be
investigated.Comment: 8 pages, 4 figures, Ap J Lett (accepted
A large stellar evolution database for population synthesis studies. II. Stellar models and isochrones for an alpha-enhanced metal distribution
[Abridged] We present a large, new set of stellar evolution models and
isochrones for an alpha-enhanced metal distribution typical of Galactic halo
and bulge stars; it represents a homogeneous extension of our stellar model
library for a distribution already presented in Pietrinferni et al.(2004). The
effect of the alpha-element enhancement has been properly taken into account in
the nuclear network, opacity, equation of state and, for the first time, the
bolometric corrections, and color transformations. This allows us to avoid the
inconsistent use - common to all alpha-enhanced model libraries currently
available - of scaled-solar bolometric corrections and color transformations
for alpha-enhanced models and isochrones. We show how bolometric corrections to
magnitudes obtained for the U,B portion of stellar spectra for T_{eff}<=6500K,
are significantly affected by the metal mixture, especially at the higher
metallicities. We also provide complete sets of evolutionary models for
low-mass, He-burning stellar structures covering the whole metallicity range,
to enable synthetic horizontal branch simulations. We compare our database with
several widely used stellar model libraries from different authors, as well as
with various observed color magnitude and color-color diagrams (Johnson-Cousins
BVI and near infrared magnitudes, Stromgren colors) of Galactic field stars and
globular clusters. We also test our isochrones comparing integrated optical
colors and Surface Brightness Fluctuation magnitudes with selected globular
cluster data. We find a general satisfactory agreement with the empirical
constraints.Comment: 46 pages, 20 figures, ApJ in press, the whole database presented in
this paper can be foud at http://www.te.astro.it/BASTI/index.ph
- âŠ