989 research outputs found
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&
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
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
Substructure in Tidal Streams; Tributaries in the Anticenter Ring
We report on the detection in Sloan Digital Sky Survey data of at least
three, roughly parallel components in a 65 degree-long stellar stream complex
previously identified with the Anticenter or Monoceros Ring. The three-stream
complex varies in width from 4 to 6 degrees along its length and appears to be
made up of two or more narrow substreams as well as a broader, diffuse
component. The width and complexity of the stream indicate that the progenitor
was likely a dwarf galaxy of significant size and mass. The stream is 8.9 kpc
distant and is oriented almost perpendicularly to our line of sight. The
visible portion of the stream does not pass near any known dwarf galaxies and a
preliminary orbit does not point to any viable progenitor candidates. Orbits
for the narrower substreams can be modeled with velocity offsets from the broad
component of about 8 km/s. We suggest that the broad component is likely to be
the remains of a dwarf galaxy, while the narrower streams constitute the
remnants of dynamically distinct components which may have included a native
population of globular clusters. While the color of the main sequence turn-off
is not unlike that for the Monoceros Ring, neither the visible stream nor any
reasonable projection of its orbit passes through Monoceros or Canis Major, and
we conclude that this stream is probably unrelated to the overdensities found
in these regions.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
The effect of episodic accretion on the phase transition of CO and CO_2 in low-mass star formation
We study the evaporation and condensation of CO and CO_2 during the embedded
stages of low-mass star formation by using numerical simulations. We focus on
the effect of luminosity bursts, similar in magnitude to FUors and EXors, on
the gas-phase abundance of CO and CO_2 in the protostellar disk and infalling
envelope. The evolution of a young protostar and its environment is followed
based on hydrodynamical models using the thin-disk approximation, coupled with
a stellar evolution code and phase transformations of CO and CO_2. The
accretion and associated luminosity bursts in our model are caused by disk
gravitational fragmentation followed by quick migration of the fragments onto
the forming protostar. We found that bursts with luminosity on the order of
100-200 L_sun can evaporate CO ices in part of the envelope. The typical
freeze-out time of the gas-phase CO onto dust grains in the envelope (a few
kyr) is much longer than the burst duration (100-200 yr). This results in an
increased abundance of the gas-phase CO in the envelope long after the system
has returned into a quiescent stage. In contrast, luminosity bursts can
evaporate CO_2 ices only in the disk, where the freeze-out time of the
gas-phase CO_2 is comparable to the burst duration. We thus confirm that
luminosity bursts can leave long-lasting traces in the abundance of gas-phase
CO in the infalling envelope, enabling the detection of recent bursts as
suggested by previous semi-analytical studies.Comment: 12 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Evolution of very low mass pre-main sequence stars and young brown dwarfs under accretion: A phenomenological approach
In the poster presented in Cool Star 15, we analyzed the effect of disk
accretion on the evolution of very low mass pre-main sequence stars and young
brown dwarfs and the resulting uncertainties on the determination of masses and
ages. We use the Lyon evolutionary 1-D code assuming a magnetospheric accretion
process, i.e., the material falls covering a small area of the radiative
surface, and we take into account the internal energy added from the accreted
material as a free parameter . Even if the approach to this problem
is phenomenological, our formalism provides important hints about
characteristics of disk accretion, which are useful for improved stellar
interior calculations. Using the accretion rates derived from observations our
results show that accretion does not affect considerably the position of
theoretical isochrones as well as the luminosity compared with standard
non-accreting models. See more discussions in a forthcoming paper by Gallardo,
Baraffe and Chabrier (2008).Comment: Poster contribution Cool Star 15, St. Andrews, U
Planetary internal structures
This chapter reviews the most recent advancements on the topic of terrestrial
and giant planet interiors, including Solar System and extrasolar objects.
Starting from an observed mass-radius diagram for known planets in the
Universe, we will discuss the various types of planets appearing in this
diagram and describe internal structures for each type. The review will
summarize the status of theoretical and experimental works performed in the
field of equation of states (EOS) for materials relevant to planetary interiors
and will address the main theoretical and experimental uncertainties and
challenges. It will discuss the impact of new EOS on interior structures and
bulk composition determination. We will discuss important dynamical processes
which strongly impact the interior and evolutionary properties of planets (e.g
plate tectonics, semiconvection) and describe non standard models recently
suggested for our giant planets. We will address the case of short-period,
strongly irradiated exoplanets and critically analyse some of the physical
mechanisms which have been suggested to explain their anomalously large radius.Comment: 24 pages, 8 figures, Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. Klessen, C. Dullemond, Th. Henning.
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