73 research outputs found
CaII Infrared triplet line models in Classical T Tauri stars
We study the formation of the calcium II infrared triplet lines 8498\AA,
8542\AA and 8662\AA, in the accreting magnetospheric flows of Classical T Tauri
stars (CTTS), and present a grid of models for a large range of magnetospheric
conditions. We apply our models to the interpretation of multi epoch
observations of the CTTS DI Cep. We find that these lines form in the
magnetospheric infall and that the variability of the CaII triplet lines in DI
Cep can be explained in the context of changes in the mass accretion
rate/temperature of the accretion column gas
The Magnetic Fields of Classical T Tauri Stars
We report new magnetic field measurements for 14 classical T Tauri stars
(CTTSs). We combine these data with one previous field determination in order
to compare our observed field strengths with the field strengths predicted by
magnetospheric accretion models. We use literature data on the stellar mass,
radius, rotation period, and disk accretion rate to predict the field strength
that should be present on each of our stars according to these magnetospheric
accretion models. We show that our measured field values do not correlate with
the field strengths predicted by simple magnetospheric accretion theory. We
also use our field strength measurements and literature X-ray luminosity data
to test a recent relationship expressing X-ray luminosity as a function of
surface magnetic flux derived from various solar feature and main sequence star
measurements. We find that the T Tauri stars we have observed have weaker than
expected X-ray emission by over an order of magnitude on average using this
relationship. We suggest the cause for this is actually a result of the very
strong fields on these stars which decreases the efficiency with which gas
motions in the photosphere can tangle magnetic flux tubes in the corona.Comment: 25 pages, 5 figure
Unveiling the Inner Disk Structure of T Tauri Stars
We present near-infrared spectra of the excess continuum emission from the
innermost regions of classical T Tauri disks. In almost all cases, the shape of
the excess is consistent with that of a single-temperature blackbody with T ~
1400 K, similar to the expected dust sublimation temperature for typical dust
compositions. The amount of excess flux roughly correlates with the accretion
luminosity in objects with similar stellar properties. We compare our
observations with the predictions of simple disk models having an inner rim
located at the dust sublimation radius, including irradiation heating of the
dust from both the stellar and accretion luminosities. The models yield inner
rim radii in the range 0.07-0.54 AU, increasing with higher stellar and
accretion luminosities. Using typical parameters which fit our observed sample,
we predict a rim radius ~ 0.2 AU for the T Tauri star DG Tau, which agrees with
recent Keck near-infrared interferometric measurements. For large mass
accretion rates, the inner rim lies beyond the corotation radius at (or within)
which magnetospheric accretion flows are launched, which implies that pure
gaseous disks must extend inside the dust rim. Thus, for a significant fraction
of young stars, dust cannot exist in the innermost disk, calling into question
theories in which solid particles are ejected by a wind originating at the
magnetospheric radius.Comment: accepted by ApJ letter
No disks around low-mass stars and brown dwarfs in the young sigma Orionis cluster?
We report on the analysis of 2MASS near-infrared data of a sample of low-mass
stars and brown dwarfs in the sigma Orionis cluster. Youth and cluster
membership have been spectroscopically confirmed using the Li I spectral line.
We find little evidence in the JHKs colour-colour diagram for near-infrared
excess emission for these cluster members. By comparison with model
expectations, at most 2 out of 34 stars show (H-K) colour consistent with a
near-infrared excess. This scarcity of near-infrared signatures of
circumstellar disks in the lower-mass and substellar regimes of this cluster
contrasts with findings in younger clusters, hinting at an age dependence of
the disk frequency. Taking into account the apparent cluster age, our result
supports the idea of a relatively fast (few Myr) disk dissipation and extends
this conclusion to the substellar regime. We also find some evidence that, in
this cluster, the disk frequency as measured by the Ks-band excess may be mass
dependent.Comment: 4 pages, 2 eps figures, accepted by Astronomy and Astrophysics
Letter
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Stellar Properties of Pre-Main Sequence Stars from High Resolution Near-IR Spectra
We present high resolution (R=50,000) spectra at 2.2 um of 16 young stars in
the rho Ophiuchi dark cloud. Photospheric features are detected in the spectra
of 11 of these sources, all Class II young stellar objects. In 10 of these
sources, we measure effective temperatures, continuum veiling, and vsini
rotation from the shapes and strengths of atomic photospheric lines by
comparing to spectral synthesis models at 2.2 um. We measure surface gravities
in 2 stars from the integrated line flux ratio of the 12CO line region at 2.3
um and the Na I line region at 2.2 um. Although the majority (8/10) of the
Class II stars have similar effective temperatures (3530 K +/-100 K), they
exhibit a large spread in bolometric luminosities (factor ~8), as derived from
near-IR photometry. In the two stars where we have surface gravity measurements
from spectroscopy, the photometrically derived luminosities are systematically
higher than the spectroscopic luminosities. Our spectroscopic luminosities
result in older ages on the H-R diagram than is suggested by photometry at J or
K. Most of our sources show a substantially larger amount of continuum excess
than stellar flux at 2.2 um. The derived veiling values at K appear correlated
with mid-IR disk luminosity, and with Brackett gamma equivalent width,
corrected for veiling. The derived vsini rotation is substantial (12-39 km
s-1), but systematically less than the rotation measured in Class I.5 (flat)
and Class I sources from other studies in Ophiuchus.Comment: Accepted for publication in the Astronomical Journal. 37 pages, 8
figure
HST/STIS Observations of the Bipolar Jet from RW Aurigae: Tracing Outflow Asymmetries Close to the Source
We have observed the bipolar jet from RW Aur A with STIS on board the HST.
After continuum subtraction, morphological and kinematic properties of this
outflow can be traced to within 0."1 from the source in forbidden emission
lines. The jet appears well collimated, with typical FWHMs of 20 to 30 AU in
the first 2" and surprisingly does not show a separate low-velocity component
in contrast to earlier observations. The systemic radial outflow velocity of
the blueshifted lobe is typically 50% larger than that of the redshifted one
with a velocity difference of about 65 km/s. Although such asymmetries have
been seen before on larger scales, our high spatial resolution observations
suggest that they are intrinsic to the "central engine" rather than effects of
the star's immediate environment. Temporal variations of the bipolar jet's
outflow velocities appear to occur on timescales of a few years. They have
combined to produce a 55% increase in the velocity asymmetry between the two
lobes over the past decade. In the red lobe estimated mass flux and momentum
flux values are around one half and one third of those for the blue lobe,
respectively. The mass outflow to mass accretion rate is 0.05, the former being
measured at a distance of 0."35 from the source.Comment: Accepted by ApJ, 16 pages, 5 figure
Unveiling the Structure of Pre-Transitional Disks
In the past few years, several disks with inner holes that are empty of small
dust grains have been detected and are known as transitional disks. Recently,
Spitzer has identified a new class of "pre-transitional disks" with gaps; these
objects have an optically thick inner disk separated from an optically thick
outer disk by an optically thin disk gap. A near-infrared spectrum provided the
first confirmation of a gap in the pre-transitional disk of LkCa 15 by
verifying that the near-infrared excess emission in this object was due to an
optically thick inner disk. Here we investigate the difference between the
nature of the inner regions of transitional and pre-transitional disks using
the same veiling-based technique to extract the near-infrared excess emission
above the stellar photosphere. We show that the near-infrared excess emission
of the previously identified pre-transitional disks of LkCa 15 and UX Tau A in
Taurus as well as the newly identified pre-transitional disk of ROX 44 in
Ophiuchus can be fit with an inner disk wall located at the dust destruction
radius. We also model the broad-band SEDs of these objects, taking into account
the effect of shadowing by the inner disk on the outer disk, considering the
finite size of the star. The near-infrared excess continua of these three
pre-transitional disks, which can be explained by optically thick inner disks,
are significantly different from that of the transitional disks of GM Aur,
whose near-infrared excess continuum can be reproduced by emission from
sub-micron-sized optically thin dust, and DM Tau, whose near-infrared spectrum
is consistent with a disk hole that is relatively free of small dust. The
structure of pre-transitional disks may be a sign of young planets forming in
these disks and future studies of pre-transitional disks will provide
constraints to aid in theoretical modeling of planet formation.Comment: Accepted for publication in ApJ on May 10, 2010; 29 page
Accretion and ejection properties of embedded protostars: the case of HH26, HH34 and HH46 IRS
We present the results of a near-IR spectroscopic analysis on 3 young
embedded sources (HH26IRS, HH34IRS and HH46IRS) belonging to different
star-forming regions and displaying well developed jet structures. The aim is
to investigate the source accretion and ejection properties and their
connection. We used VLT-ISAAC spectra (R~9000, H and K bands) to derive in a
self-consistent way parameters like the star luminosity, the accretion
luminosity and the mass accretion rate. Mass loss rates have also been
estimated from the analysis of different emission features. The spectra present
several emission lines but no photospheric features in absorption, indicating a
large veiling in H and K. We detected features commonly observed in jet-driving
sources (HI,[FeII],H_2,CO) and also a number of emission lines due to permitted
atomic transitions, like NaI and TiI. The NaI 2.2um doublet is observed along
with CO(2-0) band-head emission, indicating a common origin in an inner gaseous
disc heated by accretion. We find that accretion provides ~50% and ~80% of the
bolometric luminosity in HH26IRS and HH34IRS, as expected for accreting young
objects.Mass accretion and loss rates spanning 10^-8 - 10^-6 Msun/yr have been
measured. The derived Mloss/Macc is ~0.01 for HH26IRS and HH34IRS, and >0.1 for
HH46IRS, numbers that are in the range of values predicted by MHD jet-launching
models and found in the most active classical T Tauri stars. Comparison with
other similar studies seems to indicate that Class Is actually having
accretion- dominated luminosities are a limited number. Although the analysed
sample is small, we tentatively present some criteria to characterise such
sources. Studies like the one presented here but on larger samples of
candidates should be performed in order to test and refine these criteria.Comment: Accepted for A&A - 13 pages, 7 figure
V2494 cyg: A unique FU ori type object in the cygnus OB7 complex
A photometric and spectral study of the variable star V2494 Cyg in the L 1003 dark cloud is presented. The brightness of the star, formerly known as HH 381 IRS, increased by 2.5 mag in R (probably in the 1980s) and since then has remained nearly constant. Since the brightness increase, V2494 Cyg has illuminated a bipolar cometary nebula. The stellar spectrum has several features typical of the FU Ori (FUor) type, plus it exhibits very strong Ha and forbidden emissionlines with high-velocity components. These emission lines originate in the Herbig-Haro (HH) jet near the star. The kinematic age of the jet is consistent with it forming at the time of the outburst leading to the luminosity increase. V2494 Cyg also produces a rather extended outflow; it is the first known FUor with both an observed outburst and a parsec-sized HH flow. The nebula, illuminated by V2494 Cyg, possesses similar morphological and spectral characteristics to Hubble's variable nebula (R Monocerotis/NGC 2261). © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
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