2,484 research outputs found
Pulsating young brown dwarfs
We present the results of a nonadiabatic, linear stability analysis of models
of very low-mass stars (VLMSs) and brown dwarfs (BDs) during the deuterium
burning phase in the center. We find unstable fundamental modes with periods
varying between ~5 hr for a 0.1 Msun star and ~1 hr for a 0.02 Msun BD. The
growth time of the instability decreases with decreasing mass and remains well
below the deuterium burning time scale in the mass range considered (0.1--0.02
Msun). These results are robust against variations of the relevant input
physics in the evolutionary models. We identify possible candidates for
pulsational variability among known VLMSs and BDs in nearby star forming
regions whose location in the HR diagram falls within or close to the boundary
of the instability strip. Finally, we discuss the possibility that the
variability observed in a few objects with periods of ~1 hr can be interpreted
in terms of pulsation.Comment: 5 pages, 3 figures, A&A Letters (in press
Effect of episodic accretion on the structure and the lithium depletion of low-mass stars and planet-hosting stars
Following up our recent analysis devoted to the impact of non steady
accretion on the location of young low-mass stars or brown dwarfs in the
Herzsprung-Russell diagram, we perform a detailed analysis devoted to the
effect of burst accretion on the internal structure of low-mass and solar type
stars. We find that episodic accretion can produce objects with significantly
higher central temperatures than the ones of the non accreting counterparts of
same mass and age. As a consequence, lithium depletion can be severely enhanced
in these objects. This provides a natural explanation for the unexpected level
of lithium depletion observed in young objects for the inferred age of their
parent cluster. These results confirm the limited reliability of lithium
abundance as a criterion for assessing or rejecting cluster membership. They
also show that lithium is not a reliable age indicator, because its fate
strongly depends on the past accretion history of the star. Under the
assumption that giant planets primarily form in massive disks prone to
gravitational instability and thus to accretion burst episodes, the same
analysis also explains the higher Li depletion observed in planet hosting
stars. At last, we show that, depending on the burst rate and intensity,
accretion outbursts can produce solar mass stars with lower convective envelope
masses, at ages less than a few tens of Myr, than predicted by standard (non or
slowly accreting) pre-main sequence models. This result has interesting,
although speculative, implications for the recently discovered depletion of
refractory elements in the Sun.Comment: 8 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Scenarios to explain extreme Be depletion in solar-like stars: accretion or rotation effects ?
Studies of beryllium abundance in large samples of solar-type stars show a
small fraction of extremely beryllium-deficient stars, which challenges our
current understanding of light element depletion in these stars. We suggest two
possible scenarios that may explain this high level of Be depletion: early
accretion and rotational mixing. We show that in both cases, the conditions
required to reach the observed level of Be depletion are quite extreme, which
explains the very small fraction of detected Be outliers. We suggest that
substantial Be depletion can be obtained in stars if they were fast rotators in
the past, with high initial rotational velocities and short disc lifetimes. Our
analysis suggests that rotational mixing may not be efficient enough to deplete
Be in less than 10 Myr. Consequently, the detection of strongly Be-deficient
stars in clusters younger than 10 Myr may provide a genuine signature of
accretion process and the proof that some protostars may undergo many extreme
bursts of accretion during their embedded phases of evolution.Comment: 7 pages, 6 figures, accepted for publication in A&
Secondary Stars in CVs: The Theoretical Perspective
We apply the new generation of theoretical models of low-mass stars to
secondaries in CVs, focussing on systems above the period gap. The models
confirm that the spectral type should be a good indicator of the donor mass.
The orbital period-spectral type diagram can potentially constrain the
long-term mean mass transfer rate. A transfer rate that increases with
decreasing period is most easily reconciled with the observational data.Comment: 6 pages; to appear in proceedings of Warner Symposium on Cataclysmic
Variable
Significant uncertainties from calibrating overshooting with eclipsing binary systems
The precise measurement of the masses and radii of stars in eclipsing binary
systems provides a window into uncertain processes in stellar evolution,
especially mixing at convective boundaries. Recently, these data have been used
to calibrate models of convective overshooting in the cores of main sequence
stars. In this study we have used a small representative sample of eclipsing
binary stars with to test how precisely this
method can constrain the overshooting and whether the data support a universal
stellar mass--overshooting relation. We do not recover the previously reported
stellar mass dependence for the extent of overshooting and in each case we find
there is a substantial amount of uncertainty, that is, the same binary pair can
be matched by models with different amounts of overshooting. Models with a
moderate overshooting parameter (using the
scheme from Herwig et al. 1997) are consistent with all eight systems studied.
Generally, a much larger range of is suitable for individual
systems. In the case of main sequence and early post-main sequence stars, large
changes in the amount of overshooting have little effect on the radius and
effective temperature, and therefore the method is of extremely limited
utility.Comment: Accepted for publication in A&
Deuterium-burning in substellar objects
We consider the depletion of primordial deuterium in the interior of
substellar objects as a function of mass, age and absolute magnitude in several
photometric passbands. We characterize potential spectroscopic signatures of
deuterium in the lines of deuterated water HDO. These results will serve as a
useful, independent diagnostic to characterize the mass and/or the age of young
substellar objects, and to provide an independent age determination of very
young clusters. These results can serve to identify objects at the
deuterium-burning limit and to confront the theoretical prediction that
D-burning is a necessary condition to form star-like objects.Comment: 13 pages, Latex file, uses aasms4.sty, accepted for publication in
ApJ Letter
The Double-Lined Spectroscopic Binary Haro 1-14c
We report detection of the low-mass secondary in the spectroscopic binary
Haro 1-14c in the Ophiuchus star forming region. The secondary/primary mass
ratio is . With an estimated photometric primary mass of 1.2
, the secondary mass is and the projected
semi-major axis is AU. The system is well-suited for astrometric
mapping of its orbit with the current generation of ground-based IR
interferometers. This could yield precision values of the system's component
masses and distance.Comment: Accepted by ApJ Letter
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