447 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
Polarimetric variations of binary stars. VI. Orbit-induced variations in the pre-main-sequence binary AK Sco
We present simultaneous UBV polarimetric and photometric observations of the
pre-main-sequence binary AK Sco, obtained over 12 nights, slightly less than
the orbital period of 13.6 days. The polarization is a sum of interstellar and
intrinsic polarization, with a significant intrinsic polarization of 1% at
5250A, indicating the presence of circumstellar matter distributed in an
asymmetric geometry. The polarization and its position angle are clearly
variable on time scales of hours and nights, in all 3 wavelengths, with a
behavior related to the orbital motion. The variations have the highest
amplitudes seen so far for pre-main-sequence binaries (~1%, ~30deg) and are
sinusoidal with periods similar to the orbital period and half of it. The
polarization variations are generally correlated with the photometric ones:
when the star gets fainter, it also gets redder and its polarization increases.
The color-magnitude diagram B-V, V exhibits a ratio of total to selective
absorption R=4.3 higher than in normal interstellar clouds (R=3.1). The
interpretation of the simultaneous photometric and polarimetric observations is
that a cloud of circumstellar matter passes in front of the star, decreasing
the amount of direct, unpolarized light, and hence increasing the contribution
of scattered (blue) light. We show that the large amplitude of the polarization
variations can not be reproduced with a single scattering model and axially
symmetric circumbinary or circumstellar disks.Comment: 24 pages, 10 figures, accepted for publication in the Astronomical
Journa
A Third Star in the T Tauri System
New speckle-holographic images of the T Tauri Infrared Companion (T Tauri
IRC; T Tauri S) reveal it to be a double system with a sky-projected separation
of 0".05, corresponding to a linear distance of 7 AU. The presence of this
third star may account for the relative paucity of dust surrounding the IRC.Comment: 5 pages in AASTeX preprint form, including one grayscale figur
A Long-Lived Accretion Disk Around a Lithium-Depleted Binary T Tauri Star
We present a high dispersion optical spectrum of St 34 and identify the
system as a spectroscopic binary with components of similar luminosity and
temperature (both M3+/-0.5). Based on kinematics, signatures of accretion, and
location on an H-R diagram, we conclude that St 34 is a classical T Tauri star
belonging to the Taurus-Auriga T Association. Surprisingly, however, neither
component of the binary shows LiI 6708 A, absorption, the most universally
accepted criterion for establishing stellar youth. In this uniquely known
instance, the accretion disk appears to have survived longer than the lithium
depletion timescale. We speculate that the long-lived accretion disk is a
consequence of the sub-AU separation companion tidally inhibiting, though not
preventing, circumstellar accretion. Comparisons with pre-main sequence
evolutionary models imply, for each component of St 34, a mass of 0.37+/-0.08
Msun and an isochronal age of 8+/-3 Myr, which is much younger than the
predicted lithium depletion timescale of ~ 25 Myr. Although a distance 38%
closer than that of Taurus-Auriga or a hotter temperature scale could reconcile
this discrepancy at 21-25 Myr, similar discrepancies in other systems and the
implications of an extremely old accreting Taurus-Auriga member suggest instead
a possible problem with evolutionary models. Regardless, the older age implied
by St 34's depleted lithium abundance is the first compelling evidence for a
substantial age spread in this region. Additionally, since St 34's coeval
co-members with early M spectral types would likewise fail the lithium test for
youth, current membership lists may be incomplete.Comment: 4 pages, including 2 figures. Accepted for publication in ApJ Let
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
The First Detection of Spatially Resolved Mid-Infrared Scattered Light from a Protoplanetary Disk
We report spatially resolved 11.8 micron images, obtained at the W. M. Keck
10 m telescope, of the protoplanetary disk around the pre--main-sequence star
HK Tau B. The mid-infrared morphology and astrometry of HK Tau B with respect
to HK Tau A indicate that the flux observed in the mid-infrared from HK Tau B
has been scattered off the upper surface of its nearly edge-on disk. This is
the first example of a protoplanetary disk observed in scattered light at
mid-infrared wavelengths. Monte Carlo simulations of this disk show that the
extent (FWHM =0."5, or 70 AU) of the scattered light nebula in the mid-infrared
is very sensitive to the dust size distribution. The 11.8 micron measurement
can be best modelled by a dust grain population that contains grains on the
order of 1.5-3 micron in size; grain populations with exclusively sub-micron
grain sizes or power law size distributions that extend beyond 5 micron cannot
reproduce the observed morphology. These grains are significantly larger than
those expected in the ISM implying that grain growth has occurred; whether this
growth is a result of dust evolution within the disk itself or had originally
occurred within the dark cloud remains an open question.Comment: 11 pages, 1 postscript figure, accepted for publication in ApJ
High Angular Resolution Mid-infrared Imaging of Young Stars in Orion BN/KL
We present Keck LWS images of the Orion BN/KL star forming region obtained in
the first multi-wavelength study to have 0.3-0.5" resolution from 4.7 to 22
microns. The young stellar objects designated infrared source-n and radio
source-I are believed to dominate the BN/KL region. We have detected extended
emission from a probable accretion disk around source-n but infer a stellar
luminosity on the order of only 2000 Lsun. Although source-I is believed to be
more luminous, we do not detect an infrared counterpart even at the longest
wavelengths. However, we resolve the closeby infrared source, IRc2, into an arc
of knots ~1000 AU long at all wavelengths. Although the physical relation of
source-I to IRc2 remains ambiguous, we suggest these sources mark a high
density core (10^7-10^8 pc^-3 over 1000 AU) within the larger BN/KL star
forming cluster. The high density may be a consequence of the core being young
and heavily embedded. We suggest the energetics of the BN/KL region may be
dominated by this cluster core rather than one or two individual sources.Comment: 13 pages including 3 color figures. Accepted to The Astrophysical
Journal Letters pending slight reduction in length. High resolution figures
(jpeg) may be found at
http://cfa-www.harvard.edu/~lincoln/keck.bnkl.midir.ppr
X-ray Diagnostics of Grain Depletion in Matter Accreting onto T Tauri Stars
Recent analysis of high resolution Chandra X-ray spectra has shown that the
Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this
result, we point out the utility of the Ne/O ratio as a discriminant for
accretion-related X-rays from T Tauri stars, and for probing the measure of
grain-depletion of the accreting material in the inner disk. We apply the Ne/O
diagnostic to the classical T Tauri stars BP Tau and TW Hya--the two stars
found to date whose X-ray emission appears to originate, at least in part, from
accretion activity. We show that TW Hya appears to be accreting material which
is significantly depleted in O relative to Ne. In constrast, BP Tau has an Ne/O
abundance ratio consistent with that observed for post-T Tauri stars. We
interpret this result in terms of the different ages and evolutionary states of
the circumstellar disks of these stars. In the young BP Tau disk (age 0.6 Myr)
dust is still present near the disk corotation radius and can be ionized and
accreted, re-releasing elements depleted onto grains. In the more evolved TW
Hya disk (age 10 Myr), evidence points to ongoing coagulation of grains into
much larger bodies, and possibly planets, that can resist the drag of
inward-migrating gas, and accreting gas is consequently depleted of
grain-forming elements.Comment: 13 pages, 1 Figure, ApJ Letters, in pres
VLBA determination of the distance to nearby star-forming regions II. Hubble 4 and HDE 283572 in Taurus
The non-thermal 3.6 cm radio continuum emission from the naked T Tauri stars
Hubble 4 and HDE 283572 in Taurus has been observed with the Very Long Baseline
Array (VLBA) at 6 epochs between September 2004 and December 2005 with a
typical separation between successive observations of 3 months. Thanks to the
remarkably accurate astrometry delivered by the VLBA, the trajectory described
by both stars on the plane of the sky could be traced very precisely, and
modeled as the superposition of their trigonometric parallax and uniform proper
motion. The best fits yield distances to Hubble 4 and HDE 283572 of 132.8 +/-
0.5 and 128.5 +/- 0.6 pc, respectively. Combining these results with the other
two existing VLBI distance determinations in Taurus, we estimate the mean
distance to the Taurus association to be 137 pc with a dispersion (most
probably reflecting the depth of the complex) of about 20 pc.Comment: 21 pages, 4 figues, accepted in ApJ (Dec 20, 2007 issue
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