430 research outputs found
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
NICMOS Images of the GG Tau Circumbinary Disk
We present deep, near-infrared images of the circumbinary disk surrounding
the pre-main-sequence binary star, GG Tau A, obtained with NICMOS aboard the
Hubble Space Telescope. The spatially resolved proto-planetary disk scatters
roughly 1.5% of the stellar flux, with a near-to-far side flux ratio of ~1.4,
independent of wavelength, and colors that are comparable to the central
source; all of these properties are significantly different from the earlier
ground-based observations. New Monte Carlo scattering simulations of the disk
emphasize that the general properties of the disk, such as disk flux, near side
to far side flux ratio and integrated colors, can be approximately reproduced
using ISM-like dust grains, without the presence of either circumstellar disks
or large dust grains, as had previously been suggested. A single parameter
phase function is fitted to the observed azimuthal variation in disk flux,
providing a lower limit on the median grain size of 0.23 micron. Our analysis,
in comparison to previous simulations, shows that the major limitation to the
study of grain growth in T Tauri disk systems through scattered light lies in
the uncertain ISM dust grain properties. Finally, we use the 9 year baseline of
astrometric measurements of the binary to solve the complete orbit, assuming
that the binary is coplanar with the circumbinary ring. We find that the
estimated 1 sigma range on disk inner edge to semi-major axis ratio, 3.2 <
Rin/a < 6.7, is larger than that estimated by previous SPH simulations of
binary-disk interactions.Comment: 40 pages, 8 postscript figures, accepted for publication in Ap
Accretion-powered chromospheres in classical T Tauri stars
(Abridged) Optical spectra of classical T Tauri stars (cTTS) are rich in
emission lines of low-excitation species that are composed of narrow and broad
components, related to two regions with different kinematics, densities, and
temperatures. The photospheric spectrum is often veiled by an excess continuous
emission. This veiling is usually attributed to radiation from a heated region
beneath the accretion shock. The aim of this research is to clarify the nature
of the veiling, and whether the narrow chromospheric lines of Fe I and other
metals represent a standard chromosphere of a late-type star, or are induced by
mass accretion. From high-resolution spectroscopy of DR Tauri we found that the
amount of veiling in this star varies from practically nothing to factors more
than 10 times the stellar continuum intensity, and that the veiling is caused
by both a non-photospheric continuum and chromospheric line emission filling in
the photospheric absorption lines. This effect can be shown to exist in several
other T Tauri stars. We conclude that enhanced chromospheric emission in cTTS
is linked not only to solar-like magnetic activity, but is powered to a greater
extent by the accreting gas. We suggest that the area of enhanced chromospheric
emission is induced by mass accretion, which modifies the local structure of
stellar atmosphere in an area that is more extended than the hot accretion
spot. The narrow emission lines from this extended area are responsible for the
extra component in the veiling through line-filling of photospheric absorption
lines.Comment: 11 pages, 13 figure
Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk
Using the Hubble Space Telescope, the 4 m Blanco telescope at the Cerro
Tololo Inter-American Observatory, and the Spitzer Space Telescope, we have
performed deep imaging from 0.8 to 8 um of the southern subcluster in the
Chamaeleon I star-forming region. In these data, we have discovered an object,
Cha 110913-773444, whose colors and magnitudes are indicative of a very
low-mass brown dwarf with a circumstellar disk. In a near-infrared spectrum of
this source obtained with the Gemini Near-Infrared Spectrograph, the presence
of strong steam absorption confirms its late-type nature (>=M9.5) while the
shapes of the H- and K-band continua and the strengths of the Na I and K I
lines demonstrate that it is a young, pre-main-sequence object rather than a
field dwarf. A comparison of the bolometric luminosity of Cha 110913-773444 to
the luminosities predicted by the evolutionary models of Chabrier and Baraffe
and Burrows and coworkers indicates a mass of 8+7/-3 M_Jup, placing it fully
within the mass range observed for extrasolar planetary companions (M<=15
M_Jup). The spectral energy distribution of this object exhibits mid-infrared
excess emission at >5 um, which we have successfully modeled in terms of an
irradiated viscous accretion disk with M'<=10e-12 M_sun/year. Cha 110913-773444
is now the least massive brown dwarf observed to have a circumstellar disk, and
indeed is one of the least massive free-floating objects found to date. These
results demonstrate that the raw materials for planet formation exist around
free-floating planetary-mass bodies.Comment: 5 pages, accepted to Astrophysical Journal Letter
Searching for sub-stellar companion into the LkCa15 proto-planetary disk
Recent sub-millimetric observations at the Plateau de Bure interferometer
evidenced a cavity at ~ 46 AU in radius into the proto-planetary disk around
the T Tauri star LkCa15 (V1079 Tau), located in the Taurus molecular cloud.
Additional Spitzer observations have corroborated this result possibly
explained by the presence of a massive (>= 5 MJup) planetary mass, a brown
dwarf or a low mass star companion at about 30 AU from the star. We used the
most recent developments of high angular resolution and high contrast imaging
to search directly for the existence of this putative companion, and to bring
new constraints on its physical and orbital properties. The NACO adaptive
optics instrument at VLT was used to observe LkCa15 using a four quadrant phase
mask coronagraph to access small angular separations at relatively high
contrast. A reference star at the same parallactic angle was carefully observed
to optimize the quasi-static speckles subtraction (limiting our sensitivity at
less than 1.0). Although we do not report any positive detection of a faint
companion that would be responsible for the observed gap in LkCa15's disk
(25-30 AU), our detection limits start constraining its probable mass,
semi-major axis and eccentricity. Using evolutionary model predictions, Monte
Carlo simulations exclude the presence of low eccentric companions with masses
M >= 6 M Jup and orbiting at a >= 100 AU with significant level of confidence.
For closer orbits, brown dwarf companions can be rejected with a detection
probability of 90% down to 80 AU (at 80% down to 60 AU). Our detection limits
do not access the star environment close enough to fully exclude the presence
of a brown dwarf or a massive planet within the disk inner activity (i.e at
less than 30 AU). Only, further and higher contrast observations should unveil
the existence of this putative companion inside the LkCa15 disk.Comment: 6 pages, 4 figures, accepted for publication in A&
Possible detection of a magnetic field in T Tauri
Medium-resolution circular spectropolarimetry of T Tauri is
presented. The star was observed twice: on November 11, 1996 and January 22,
2002. Weak circular polarization has been found in photospheric absorption
lines, indicating a mean surface longitudinal magnetic field of
G and G at the epoch of the first and second
observations respectively. While these values are near the detection limit of
our apparatus, we belive that they are real. In any case one can conclude from
our data that of T Tau does not significantly exceed 200 G, which is
much less than surface magnetic field strength of the star ( kG) found by
Guenther et al. (1999) and Johns-Krull et al. (2000). We discuss possible
reasons of this difference.Comment: 5 pages, 3 figure
Constraints on the disk geometry of the T Tauri star AA Tau from linear polarimetry
We have simultaneously monitored the photometric and polarimetric variations
of the Classical T Tauri star AA Tau during the fall of 2002. We combine these
data with previously published polarimetric data covering two earlier epochs.
The phase coverage is complete, although not contiguous. AA Tau clearly shows
cyclic variations coupled with the rotation of the system. The star-disk system
produces a repeatable polarisation curve where the polarisation increases with
decreasing brightness. The data fit well with the model put forward by Bouvier
et al. (1999) where AA Tau is viewed almost edge-on and its disk is actively
dumping material onto the central star via magnetospheric accretion. The inner
edge of the disk is deformed by its interaction with the tilted magnetosphere,
producing eclipses as it rotates and occults the photosphere periodically. From
the shape of the polarisation curve in the QU-Plane we confirm that the
accretion disk is seen at a large inclination, almost edge-on, and predict that
its position angle is PA~90 deg., i.e., that the disk's major axis is oriented
in the East-West direction.Comment: Astron. Astrophys., in pres
An assessment of Li abundances in weak-lined and classical T Tauri stars of the Taurus-Auriga association
Accurate measurements of lithium abundances in young low-mass stars provide
an independent and reliable age diagnostics. Previous studies of nearby star
forming regions have identified significant numbers of Li-depleted stars,often
at levels inconsistent with the ages indicated by their luminosity. We aim at a
new and accurate analysis of Li abundances in a sample of ~100 pre-main
sequence stars in Taurus-Auriga using a homogeneous and updated set of stellar
parameters and model atmospheres appropriate for the spectral types of the
sample stars.We compute Li abundances using published values of the equivalent
widths of the Li 6708 A doublet obtained from medium/high resolution spectra.
We find that the number of significantly Li-depleted stars in Taurus-Auriga is
greatly reduced with respect to earlier results. Only 13 stars have abundances
lower than the interstellar value by a factor of 5 or greater. All of them are
weak-lined T Tauri stars drawn from X-ray surveys; with the exception of four
stars located near the L1551 and L1489 dark clouds, all the Li-depleted stars
belong to the class of dispersed low-mass stars, distributed around the main
sites of current star formation. If located at the distance of Taurus-Auriga,
the stellar ages implied by the derived Li abundances are in the range 3-30
Myr, greater than the bulk of the Li-rich population with implication on the
star formation history of the region. In order to derive firm conclusions about
the fraction of Li-depleted stars of Taurus-Auriga, Li measurements of the
remaining members of the association should be obtained, in particular of the
group of stars that fall in the Li-burning region of the HR diagram.Comment: Accepted for publication in Astronomy & Astrophysics. 20 pages, 5
figure
Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
We present a unique, homogeneous database of photometric measurements for
Classical T Tauri stars extending up to 20 years. The database contains more
than 21,000 UBVR observations of 72 CTTs. All the data were collected within
the framework of the ROTOR-program at Mount Maidanak Observatory (Uzbekistan)
and together they constitute the longest homogeneous, accurate record of TTS
variability ever assembled. We characterize the long term photometric
variations of 49 CTTs with sufficient data to allow a robust statistical
analysis and propose an empirical classification scheme. Several patterns of
long term photometric variability are identified. The most common pattern,
exhibited by a group of 15 stars which includes T Tau itself, consists of low
level variability (Delta(V)<=0.4mag) with no significant changes occurring from
season to season over many years. A related subgroup of 22 stars exhibits a
similar stable long term variability pattern, though with larger amplitudes (up
to Delta(V)~1.6 mag). Besides these representative groups, we identify three
smaller groups of 3-5 stars each which have distinctive photometric properties.
The long term variability of most CTTs is fairly stable and merely reflects
shorter term variability due to cold and hot surface spots. Only a small
fraction of CTTs undergo significant brightness changes on the long term
(months, years), which probably arise from slowly varying circumstellar
extinction.Comment: 16 pages, 11 figures. Astron. Astrophys., in pres
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