84 research outputs found
Orbital motion in T Tauri binary systems
Using speckle-interferometry we have carried out repeated measurements of
relative positions for the components of 34 T Tauri binary systems. The
projected separation of these components is low enough that orbital motion is
expected to be observable within a few years. In most cases orbital motion has
indeed been detected. The observational data is discussed in a manner similar
to Ghez et al. (1995). However, we extend their study to a larger number of
objects and a much longer timespan. The database presented in this paper is
valuable for future visible orbit determinations. It will yield empirical
masses for T Tauri stars that now are only poorly known. The available data is
however not sufficient to do this at the present time. Instead, we use short
series of orbital data and statistical distributions of orbital parameters to
derive an average system mass that is independent of theoretical assumptions
about the physics of PMS stars. For our sample this mass is 2.0 solar masses
and thus in the order of magnitude one expects for the mass sum of two T Tauri
stars. It is also comparable to mass estimates obtained for the same systems
using theoretical PMS evolutionary models.Comment: Accepted by Astronomy and Astrophysic
The near-infrared and ice-band variability of Haro 6-10
We have monitored the angularly resolved near infrared and 3.1 micron
ice-band flux of the components of the young binary Haro 6-10 on 23 occasions
during the years 1988 to 2000. Our observations reveal that both the visible
star Haro 6-10 (Haro 6-10S) and its infrared companion (Haro 6-10N) show
significant variation in flux on time scales as short as a month. The
substantial flux decrease of Haro 6-10S over the last four years carries the
reddening signature of increased extinction. However, a comparable K-band flux
increase observed in the IRC is associated with a dimming in the H-band and
cannot be explained by lower extinction. Absorption in the 3.1 micron water-ice
feature was always greater towards the IRC during our observations, indicating
a larger amount of obscuring material along its line of sight. We detect
variability in the ice-band absorption towards Haro 6-10S and Haro 6-10N,
significant at the 3.5 sigma and 2.0 sigma levels, respectively.Comment: 8 pages, 6 Figures, Accepted for Publication in Astronomy &
Astrophysic
The Orbit of GG Tau A
We present a study of the orbit of the pre-main-sequence binary system GG Tau
A and its relation to its circumbinary disk, in order to find an explanation
for the sharp inner edge of the disk. Three new relative astrometric positions
of the binary were obtained with NACO at the VLT. We combine these with data
from the literature and fit orbit models to the dataset. We find that an orbit
coplanar with the disk and compatible with the astrometric data is too small to
explain the inner gap of the disk. On the other hand, orbits large enough to
cause the gap are tilted with respect to the disk. If the disk gap is indeed
caused by the stellar companion, then the most likely explanation is a
combination of underestimated astrometric errors and a misalignment between the
planes of the disk and the orbit.Comment: 5 pages, 6 figures, accepted by Astronomy and Astrophysics, new
version contains changes suggested by language edito
Further Indications of Jet Rotation in New Ultraviolet and Optical HST/STIS Spectra
We present survey results which suggest rotation signatures at the base of
T-Tauri jets. Observations were conducted with the Hubble Space Telescope
Imaging Spectrograph at optical and near ultraviolet wavelengths (NUV). Results
are presented for the approaching jet from DG Tau, CW Tau, HH 30 and the
bipolar jet from TH 28. Systematic asymmetries in Doppler shift were detected
across the jet, within 100 AU from the star. At optical wavelengths, radial
velocity differences were typically 10 to 25 (+/-5) km/s, while differences in
the NUV range were consistently lower at typically 10 (+/-5) km/s. Results are
interpreted as possible rotation signatures. Importantly, there is agreement
between the optical and NUV results for DG Tau. Under the assumption of steady
magnetocentrifugal acceleration, the survey results lead to estimates for the
distance of the jet footpoint from the star, and give values consistent with
earlier studies. In the case of DG Tau, for example, we see that the higher
velocity component appears to be launched from a distance of 0.2 to 0.5 AU from
the star along the disk plane, while the lower velocity component appears to
trace a wider part of the jet launched from as far as 1.9 AU. The results for
the other targets are similar. Therefore, if indeed the detected Doppler
gradients trace rotation within the jet then, under the assumption of steady
MHD ejection, the derived footpoint radii support the existence of magnetized
disk winds. However, since we do not resolved the innermost layers of the flow,
we cannot exclude the possibility that there also exists an X-wind or stellar
wind component.Comment: 22 pages, 21 figures, accepted by The Astrophysical Journa
Observational Constraints on the Formation and Evolution of Binary Stars
We present a high spatial resolution UV to NIR survey of 44 young binary
stars in Taurus with separations of 10-1000 AU. The primary results include:
(1) The relative ages of binary star components are more similar than the
relative ages of randomly paired single stars, supporting coeval formation. (2)
Only one of the companion masses is substellar, and hence the apparent
overabundance of T Tauri star companions relative to main-sequence star
companions can not be explained by a wealth of substellar secondaries that
would have been missed in main-sequence surveys. (3) Roughly 10% of T Tauri
binary star components have very red NIR colors (K-L > 1.4) and unusually high
mass accretion rates. This phenomenon does not appear to be restricted to
binary systems, however, since a comparable fraction of single T Tauri stars
exhibit the same properties. (4) Although the disk lifetimes of single stars
are roughly equal to their stellar ages, the disk lifetimes of binary stars are
an order of magnitude less than their ages. (5) The accretion rates for both
single and binary T Tauri stars appear to be moderately mass dependent. (6)
Although most classical T Tauri star binaries retain both a circumprimary and a
circumsecondary disk, there are several systems with only a circumprimary disk.
Together with the relative accretion rates, this suggests that circumprimary
disks survive longer, on average, than circumsecondary disks. (7) The disk
lifetimes, mass ratios, and relative accretion signatures of the closest
binaries (10-100 AU) suggest that they are being replenished from a
circumbinary reservoir with low angular momentum. Overall, these results
support fragmentation as the dominant binary star formation mechanism.Comment: 67 pages including 11 figures, LaTeX2e, accepted for publication in
Ap
MHD simulations of jet acceleration from Keplerian accretion disks: the effects of disk resistivity
Accretion disks and astrophysical jets are used to model many active
astrophysical objects, viz., young stars, relativistic stars, and active
galactic nuclei. In this paper we present self-consistent time-dependent
simulations of supersonic jets launched from magnetized accretion disks, using
high resolution numerical techniques. In particular we study the effects of the
disk magnetic resistivity, parametrized through an alpha-prescription, in
determining the properties of the inflow-outflow system. Moreover we analyze
under which conditions steady state solutions of the type proposed in the self
similar models of Blandford and Payne can be reached and maintained in a self
consistent nonlinear stage. We use the resistive MHD FLASH code with adaptive
mesh refinement, allowing us to follow the evolution of the structure for a
time scale long enough to reach steady state. A detailed analysis of the
initial configuration state is given. We obtain the expected solutions in the
axisymmetric (2.5D) limit. Assuming a magnetic field around equipartition with
the thermal pressure of the disk, we show how the characteristics of the disk
jet system, as the ejection efficiency and the energetics, are affected by the
anomalous resistivity acting inside the disk.Comment: 20 pages, 18 figures, accepted for publication in Astronomy and
Astrophysic
Rotating molecular outflows: the young T Tauri star in CB26
The disk-outflow connection is thought to play a key role in extracting
excess angular momentum from a forming proto-star. Though jet rotation has been
observed in a few objects, no rotation of molecular outflows has been
unambiguously reported so far. We report new millimeter-interferometric
observations of the edge-on T Tauri star - disk system in the isolated Bok
globule CB26. The aim of these observations was to study the disk-outflow
relation in this 1Myr old low-mass young stellar object. The IRAM PdBI array
was used to observe 12CO(2-1) at 1.3mm in two configurations, resulting in
spectral line maps with 1.5 arcsec resolution. We use an empirical
parameterized steady-state outflow model combined with 2-D line radiative
transfer calculations and chi^2-minimization in parameter space to derive a
best-fit model and constrain parameters of the outflow. The data reveal a
previously undiscovered collimated bipolar molecular outflow of total length
~2000 AU, escaping perpendicular to the plane of the disk. We find peculiar
kinematic signatures that suggest the outflow is rotating with the same
orientation as the disk. However, we could not ultimately exclude jet
precession or two misaligned flows as possible origin of the observed peculiar
velocity field. There is indirect indication that the embedded driving source
is a binary system, which, together with the youth of the source, could provide
the clue to the observed kinematic features of the outflow. CB26 is so far the
most promising source to study the rotation of a molecular outflow. Assuming
that the outflow is rotating, we compute and compare masses, mass flux, angular
momenta, and angular momentum flux of disk and outflow and derive disk
dispersal timescales of 0.5...1 Myr, comparable to the age of the system.Comment: 14 pages, 6 figures, to appear in Astronomy & Astrophysic
Mass ratios of the components in T Tauri binary systems and implications for multiple star formation
Using near-infrared speckle interferometry we have obtained resolved
JHK-photometry for the components of 58 young binary systems. From these
measurements, combined with other data taken from literature, we derive masses
and particularly mass ratios of the components. We use the J-magnitude as an
indicator for the stellar luminosity and assign the optical spectral type of
the system to the primary. On the assumption that the components within a
binary are coeval we can then place also the secondaries into the HRD and
derive masses and mass ratios for both components by comparison with different
sets of current theoretical pre-main sequence evolutionary tracks. The
resulting distribution of mass ratios is comparatively flat for M(2)/M(1) >
0.2, but depends on assumed evolutionary tracks. The mass ratio is neither
correlated with the primary's mass or the components' separation. These
findings are in line with the assumption that for most multiple systems in T
associations the components' masses are principally determined by fragmentation
during formation and not by the following accretion processes. Only very few
unusually red objects were newly found among the detected companions. This
finding shows that the observed overabundance of binaries in the Taurus-Auriga
association compared to nearby main sequence stars should be real and not the
outcome of observational biases related to infrared observing.Comment: accepted by Astronomy and Astrophysics, contains extensive tables and
figures that will only be published electronically at CDS (in total: 34
pages, 11 figures
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