42 research outputs found
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
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
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
Keck Interferometer observations of classical and weak line T Tauri stars
We present observations of the T Tauri stars BP Tau, DG Tau, DI Tau, GM Aur,
LkCa 15, RW Aur and V830 Tau, using long baseline infrared interferometry at K
band (2.2 microns) from the Keck Interferometer. The target sources have a
range of mass accretion rates and excess near-infrared emission. The
interferometer is most sensitive to extended emission on characteristic size
scales of 1 to 5 millarcseconds. All sources show evidence for resolved K band
emission on these scales, although a few of the sources are marginally
consistent with being unresolved. We calculate the infrared excess based on
fitting stellar photosphere models to the optical photometry and estimate the
physical size of the emission region using simple geometric models for the
sources with a significant infrared excess. Assuming that the K band resolved
emission traces the inner edge of the dust disk, we compare the measured
characteristic sizes to predicted dust sublimation radii and find that the
models require a range of dust sublimation temperatures and possibly optical
depths within the inner rim to match the measured radii.Comment: 19 pages, 2 figures, to appear in the Astrophysical Journa
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Globalisation from Above? Corporate Social Responsibility, the Workers' Party and the Origins of the World Social Forum
In its assessment of the origins and early development of the World Social Forum this article challenges traditional understandings of the Forum as representing âglobalisation from belowâ. By tracing the intricate relations among elements of business, civil society, and the Workersâ Party in the first years of the Forum, this article reveals the major role played by a corporate movement stemming from the Brazilian democratisation process in the 1980s, and how this combined with the transformed agenda of the Workersâ Party as it gained higher political offices to constrain the Forumâs activities from the outset. In so doing, this article challenges not only widespread conceptions of the Forum as a counterâhegemonic alternative but also current critiques concerning its subsequent limitations. Furthermore, it reveals how traditional understandings of the World Social Forum and of global civil society are underpinned by flawed assumptions which typecast political activities in the global âSouthâ
Magnetically channeled accretion in T Tauri stars : a dynamical process
We review observational evidence and open issues related to the process of
magnetospheric accretion in T Tauri stars. Emphasis is put on recent numerical
simulations and observational results which suggest that the interaction
between the stellar magnetosphere and the inner accretion disk is a highly time
dependent process on timescales ranging from hours to months.Comment: To appear in Open Issues in Local Star Formation and Early Stellar
Evolution, eds. J. Lepine, J. Gregorio-Hete
Adaptive Optics Spectroscopy of the [Fe II] Outflows from HL Tauri and RW Aurigae
We present new results of [Fe II] 1.644-micron spectroscopy toward the jets
from HL Tau and RW Aur carried out with the Subaru Telescope combined with the
adaptive optics system. We observed the regions within 2" - 3" from the stars
with the sub-arcsecond resolutions of 0."5 and 0."2 for HL Tau and RW Aur,
respectively. In addition to the strong, high velocity emission extended along
each jet, we detected a blueshifted low velocity emission feature seen as a
wing or shoulder of the high velocity emission at each stellar position.
Detailed analysis shows that the position-velocity diagrams (PVDs) of HL Tau
and RW Aur show a characteristic similar to those of the cold disk wind and
X-wind models in that the [Fe II] line width is broad in the vicinity of the
stellar position and is narrower at the extended jet. A closer comparison
suggests, however, that the disk wind model tends to have too large line width
at the jet while the X-wind model has excess emission on the redshifted side at
the stellar position. The narrow velocity width with symmetric line profiles of
the observed high velocity emission supports an X-wind type model where the
launching region is localized in a small radial range, while the low velocity
emission located away from the star favors the presence of a disk wind. The [Fe
II] emission from the HL Tau jet shows a gap of 0."8 between the redshifted jet
and the star, indicating the presence of an optically thick disk of ~ 160 AU in
radius. The [Fe II] emission from the RW Aur jet shows a marked drop from the
redshifted peak at Y ~ -0."2 toward the star, suggesting that its disk radius
is smaller than 40 AU.Comment: Accepted in the ApJ (October 2006, v649n2), AAS LaTEX macros v 5.2,
Total 25 pages with 7 figure
Multisite observations of SU Aurigae
We present results from the 1996 MUSICOS (MUlti-SIte COntinuous Spectroscopy)
campaign on the T Tauri star SU Aurigae. We find a 2.7-d periodicity in the He
I (587.6 nm) line and somewhat longer, less well-pronounced periodicities in
the Balmer lines and in Na D. Our observations support the suggestion that the
wind and infall signatures are out of phase on SU Aur. We present Doppler
images of SU Aur that have been obtained from least-squares deconvolved
profiles. Images taken about one rotation apart show only limited overlap, in
particular at low latitudes. This is in part due to limitations in
signal-to-noise, and in part due to line profile deformations that arise from
short-lived and/or non-surface features. The agreement at high latitudes is
better and suggests that at least some longer-lived features are present. The
analysis of Stokes V profiles yields a marginal magnetic field detection during
one of the phases.Comment: 22 pages, 20 figures, accepted for publication in MNRA