1,014 research outputs found
Cooling of young stars growing by disk accretion
In the initial formation stages young stars must acquire a significant
fraction of their mass by accretion from a circumstellar disk that forms in the
center of a collapsing protostellar cloud. Throughout this period mass
accretion rates through the disk can reach 10^{-6}-10^{-5} M_Sun/yr leading to
substantial energy release in the vicinity of stellar surface. We study the
impact of irradiation of the stellar surface produced by the hot inner disk on
properties of accreting fully convective low-mass stars, and also look at
objects such as young brown dwarfs and giant planets. At high accretion rates
irradiation raises the surface temperature of the equatorial region above the
photospheric temperature T_0 that a star would have in the absence of
accretion. The high-latitude (polar) parts of the stellar surface, where disk
irradiation is weak, preserve their temperature at the level of T_0. In
strongly irradiated regions an almost isothermal outer radiative zone forms on
top of the fully convective interior, leading to the suppression of the local
internal cooling flux derived from stellar contraction (similar suppression
occurs in irradiated ``hot Jupiters''). Properties of this radiative zone
likely determine the amount of thermal energy that gets advected into the
convective interior of the star. Total intrinsic luminosity integrated over the
whole stellar surface is reduced compared to the non-accreting case, by up to a
factor of several in some systems (young brown dwarfs, stars in quasar disks,
forming giants planets), potentially leading to the retardation of stellar
contraction. Stars and brown dwarfs irradiated by their disks tend to lose
energy predominantly through their cool polar regions while young giant planets
accreting through the disk cool through their whole surface.Comment: 14 pages, 6 figures, submitted to Ap
Multidimensional hydrodynamic simulations of the hydrogen injection flash
The injection of hydrogen into the convection shell powered by helium burning
during the core helium flash is commonly encountered during the evolution of
metal-free and extremely metal-poor low-mass stars. With specifically designed
multidimensional hydrodynamic simulations, we aim to prove that an entropy
barrier is no obstacle for the growth of the helium-burning shell convection
zone in the helium core of a metal-rich Pop I star, i.e. convection can
penetrate into the hydrogen-rich layers for these stars, too. We further study
whether this is also possible in one-dimensional stellar evolutionary
calculations. Our hydrodynamical simulations show that the helium-burning shell
convection zone in the helium core moves across the entropy barrier and reaches
the hydrogen-rich layers. This leads to mixing of protons into the hotter
layers of the core and to a rapid increase of the nuclear energy production at
the upper edge of the helium-burning convection shell - the hydrogen injection
flash. As a result a second convection zone appears in the hydrogen-rich
layers. Contrary to 1D models, the entropy barrier separating the two
convective shells from each other is largely permeable to chemical transport
when allowing for multidimensional flow, and consequently, hydrogen is
continuously mixed deep into the helium core. We find it difficult to achieve
such a behavior in one-dimensional stellar evolutionary calculations.Comment: 8 pages, 8 figures - accepted for publication in Astronomy and
Astrophysics. Animations related to the manuscript can be downloaded from
http://www-astro.ulb.ac.be/~mocak/index.php/Main/AnimationsHeFlas
The evolution of stars in the Taurus-Auriga T association
In a recent study, individual parallaxes were determined for many stars of
the Taurus-Auriga T association that are members of the same moving group. We
use these new parallaxes to re-address the issue of the relationship between
classical T Tauri stars (CTTSs) and weak-emission line T Tauri stars (WTTSs).
With the available spectroscopic and photometric information for 72 individual
stars or stellar systems among the Taurus-Auriga objects with known parallaxes,
we derived reliable photospheric luminosities, mainly from the Ic magnitude of
these objects. We then studied the mass and age distributions of the stellar
sample, using pre-main sequence evolutionary models to determine the basic
properties of the stellar sample. Statistical tests and Monte Carlo simulations
were then applied to studying the properties of the two T Tauri subclasses. We
find that the probability of CTTS and WTTS samples being drawn from the same
parental age and mass distributions is low; CTTSs are, on average, younger than
WTTSs. They are also less massive, but this is due to selection effects. The
observed mass and age distributions of both T Tauri subclasses can be
understood in the framework of a simple disk evolution model, assuming that the
CTTSs evolve into WTTSs when their disks are fully accreted by the stars.
According to this empirical model, the average disk lifetime in Taurus-Auriga
is 4 10**6 (Mstar/Msun)**0.75 yr.Comment: accepted by A&A Letter
Binary evolution using the theory of osculating orbits: conservative Algol evolution
Our aim is to calculate the evolution of Algol binaries within the framework
of the osculating orbital theory, which considers the perturbing forces acting
on the orbit of each star arising from mass exchange via Roche lobe overflow
(RLOF). The scheme is compared to results calculated from a `classical'
prescription. Using our stellar binary evolution code BINSTAR, we calculate the
orbital evolution of Algol binaries undergoing case A and case B mass transfer,
by applying the osculating scheme. The velocities of the ejected and accreted
material are evaluated by solving the restricted three-body equations of
motion, within the ballistic approximation. This allows us to determine the
change of linear momentum of each star, and the gravitational force applied by
the mass transfer stream. Torques applied on the stellar spins by tides and
mass transfer are also considered. Using the osculating formalism gives shorter
post-mass transfer orbital periods typically by a factor of 4 compared to the
classical scheme, owing to the gravitational force applied onto the stars by
the mass transfer stream. Additionally, during the rapid phase of mass
exchange, the donor star is spun down on a timescale shorter than the tidal
synchronization timescale, leading to sub-synchronous rotation. Consequently,
between 15 and 20 per cent of the material leaving the inner-Lagrangian point
is accreted back onto the donor (so-called `self-accretion'), further enhancing
orbital shrinkage. Self-accretion, and the sink of orbital angular momentum
which mass transfer provides, may potentially lead to more contact binaries.
Even though Algols are mainly considered, the osculating prescription is
applicable to all types of interacting binaries, including those with eccentric
orbits.Comment: A&A in press. Minor typos correcte
Spectroscopic Detection of a Stellar-like Photosphere in an Accreting Protostar
We present the first spectrum of a highly veiled, strongly accreting
protostar which shows photospheric absorption features and demonstrates the
stellar nature of its central core. We find the spectrum of the luminous (L_bol
= 10 L_sun) protostellar source, YLW 15, to be stellar-like with numerous
atomic and molecular absorption features, indicative of a K5 IV/V spectral type
and a continuum veiling r_k = 3.0. Its derived stellar luminosity (3 L_sun) and
stellar radius (3.1 R_sun) are consistent with those of a 0.5 M_sun
pre-main-sequence star. However, 70% of its bolometric luminosity is due to
mass accretion, whose rate we estimate to be 1.6 E-6 M_sun / yr onto the
protostellar core. We determine that excess infrared emission produced by the
circumstellar accretion disk, the inner infalling envelope, and accretion
shocks at the surface of the stellar core of YLW 15 all contribute signifi-
cantly to its near-IR continuum veiling. Its projected rotation velocity v sin
i = 50 km / s is comparable to those of flat-spectrum protostars but
considerably higher than those of classical T Tauri stars in the rho Oph cloud.
The protostar may be magnetically coupled to its circumstellar disk at a radius
of 2 R_*. It is also plausible that this protostar can shed over half its
angular momentum and evolve into a more slowly rotating classical T Tauri star
by remaining coupled to its circumstellar disk (at increasing radius) as its
accretion rate drops by an order of magnitude during the rapid transition
between the Class I and Class II phases of evolution. The spectrum of WL 6 does
not show any photospheric absorption features, and we estimate that its
continuum veiling is r_k >= 4.6. Together with its low bolometric luminosity (2
L_sun), this dictates that its central core is very low mass, ~0.1 M_sun.Comment: 14 pages including 9 figures (3 figures of 3 panels each, all as
separate files). AASTeX LaTex macros version 5.0. To be published in The
Astronomical Journal (tentatively Oct 2002
Planet Shadows in Protoplanetary Disks. I: Temperature Perturbations
Planets embedded in optically thick passive accretion disks are expected to
produce perturbations in the density and temperature structure of the disk. We
calculate the magnitudes of these perturbations for a range of planet masses
and distances. The model predicts the formation of a shadow at the position of
the planet paired with a brightening just beyond the shadow. We improve on
previous work on the subject by self-consistently calculating the temperature
and density structures under the assumption of hydrostatic equilibrium and
taking the full three-dimensional shape of the disk into account rather than
assuming a plane-parallel disk. While the excursion in temperatures is less
than in previous models, the spatial size of the perturbation is larger. We
demonstrate that a self-consistent calculation of the density and temperature
structure of the disk has a large effect on the disk model. In addition, the
temperature structure in the disk is highly sensitive to the angle of incidence
of stellar irradition at the surface, so accurately calculating the shape of
the disk surface is crucial for modeling the thermal structure of the disk.Comment: 14 pages, 14 figures. To appear in Ap
A new stellar mixing process operating below shell convection zones following off-center ignition
During most stages of stellar evolution the nuclear burning of lighter to
heavier elements results in a radial composition profile which is stabilizing
against buoyant acceleration, with light material residing above heavier
material. However, under some circumstances, such as off-center ignition, the
composition profile resulting from nuclear burning can be destabilizing, and
characterized by an outwardly increasing mean molecular weight. The potential
for instabilities under these circumstances, and the consequences that they may
have on stellar structural evolution, remain largely unexplored. In this paper
we study the development and evolution of instabilities associated with
unstable composition gradients in regions which are initially stable according
to linear Schwarzschild and Ledoux criteria. In particular, we explore the
mixing taking place under various conditions with multi-dimensional
hydrodynamic convection models based on stellar evolutionary calculations of
the core helium flash in a 1.25 \Msun star, the core carbon flash in a
9.3\,\Msun star, and of oxygen shell burning in a star with a mass of
23\,\Msun. The results of our simulations reveal a mixing process associated
with regions having outwardly increasing mean molecular weight that reside
below convection zones. The mixing is not due to overshooting from the
convection zone, nor is it due directly to thermohaline mixing which operates
on a timescale several orders of magnitude larger than the simulated flows.
Instead, the mixing appears to be due to the presence of a wave field induced
in the stable layers residing beneath the convection zone which enhances the
mixing rate by many orders of magnitude and allows a thermohaline type mixing
process to operate on a dynamical, rather than thermal, timescale. We discuss
our results in terms of related laboratory phenomena and associated theoretical
developments.Comment: accepted for publication in Astrophysical Journal, 9 pages, 8 figure
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