122 research outputs found
HST/STIS observations of the RW Aurigae bipolar jet: mapping the physical parameters close to the source
We present the results of new spectral diagnostic investigations applied to
high-resolution long-slit spectra of the RW Aur bipolar jet obtained with
HST/STIS. The spectra include the forbidden doublets [O I] 6300,6363 \AA, [S
II] 6716,6731 \AA, and [N II] 6548, 6583 \AA that we utilized to determine
electron density, electron temperature, hydrogen ionisation fraction, total
hydrogen density, radial velocity and the mass outflow rate. We were able to
extract the parameters as far as 3".9 in the red- and 2".1 in the blueshifted
beam. The RW Aur jet appears to be the second densest outflow from a T Tauri
star studied so far, but its other properties are quite similar to those found
in other jets from young stars. The overall trend of the physical parameters
along the first few arcseconds of the RW Aur jet is similar to that of HH 30
and DG Tau and this can reflect analogies in the mechanisms operating in that
region, suggesting the same engine is accelerating the jets in the T Tauri
stars with outflows. Our study of the RW Aur jet indicates for the first time
that, despite the detected marked asymmetries in physical and kinematic
properties between the two lobes, the mass outflow rates in the two lobes are
similar. This appears to indicate that the central engine has constraining
symmetries on both sides of the system, and that the observed asymmetries are
probably due to different environmental conditions.Comment: 24 pages, 10 figures, accepted for publication in the Astronomy and
Astrophysic
Visual orbit for the low-mass binary Gliese 22 AC from speckle interferometry
Based on 14 data points obtained with near-infrared speckle interferometry
and covering an almost entire revolution, we present a first visual orbit for
the low-mass binary system Gliese 22 AC. The quality of the orbit is largely
improved with respect to previous astrometric solutions. The dynamical system
mass is 0.592 +- 0.065 solar masses, where the largest part of the error is due
to the Hipparcos parallax. A comparison of this dynamical mass with
mass-luminosity relations on the lower main sequence and theoretical
evolutionary models for low-mass objects shows that both probably underestimate
the masses of M dwarfs. A mass estimate for the companion Gliese 22 C indicates
that this object is a very low-mass star with a mass close to the hydrogen
burning mass limit.Comment: Accepted by Astronomy and Astrophysics, 6 pages, 2 figure
A fourth component in the young multiple system V 773 Tau
I report on a new component in the pre-main sequence multiple system V 773
Tauri. This second visual companion, V 773 Tau C, with a projected separation
of about 0."2 has been detected using speckle interferometry in the
near-infrared. Repeated observations from 1996 to 2002 show significant orbital
motion and thus confirm the character of the new companion as a gravitationally
bound star. Together with the two components of the spectroscopic binary V 773
Tau A and the previously known visual companion V 773 Tau B, the V 773 Tau
system appears as a young "mini-cluster" of four T Tauri stars within a sphere
of a radius less than 100 AU. V 773 Tau, A, B and C form a triple system that
is not hierarchic, but is apparently stable despite of this. The brightness of
V 773 Tau C has probably increased over the last years, which may explain its
non-detection in previous binary surveys.Comment: Accepted by Astronomy and Astrophysics, 5 pages, 2 figure
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
Emission lines from rotating proto-stellar jets with variable velocity profiles. I. Three-dimensional numerical simulation of the non-magnetic case
Using the Yguazu-a three-dimensional hydrodynamic code, we have computed a
set of numerical simulations of heavy, supersonic, radiatively cooling jets
including variabilities in both the ejection direction (precession) and the jet
velocity (intermittence). In order to investigate the effects of jet rotation
on the shape of the line profiles, we also introduce an initial toroidal
rotation velocity profile, in agreement with some recent observational evidence
found in jets from T Tauri stars which seems to support the presence of a
rotation velocity pattern inside the jet beam, near the jet production region.
Since the Yguazu-a code includes an atomic/ionic network, we are able to
compute the emission coefficients for several emission lines, and we generate
line profiles for the H, [O I]6300, [S II]6716 and [N II]6548 lines. Using
initial parameters that are suitable for the DG Tau microjet, we show that the
computed radial velocity shift for the medium-velocity component of the line
profile as a function of distance from the jet axis is strikingly similar for
rotating and non-rotating jet models. These findings lead us to put forward
some caveats on the interpretation of the observed radial velocity distribution
from a few outflows from young stellar objects, and we claim that these data
should not be directly used as a doubtless confirmation of the
magnetocentrifugal wind acceleration models.Comment: 15 pages, 8 figures. Accepted to publication in Astronomy and
Astrophysic
Jet rotation: launching region, angular momentum balance and magnetic properties in the bipolar outflow from RW Aur
Using STIS on board the HST we have obtained a spectroscopic map of the
bipolar jet from RW Aur. We find signatures of rotation within the first 300 AU
of the jet (1.''5 from RW Aur). Both lobes rotate in the same direction (i.e.
with different helicities). Toroidal velocities are in the range 5 - 30 km/s at
20 (30) AU from the symmetry axis in the blueshifted (redshifted) lobe, in line
with other STIS observations and with theoretical simulations. The sense of
rotation is anti-clockwise looking from the tip of the blue lobe down to the
star. Rotation is more evident in the [OI] and [NII] lines and at the largest
sampled distance from the axis. We derive that the mass ejected in the observed
part of the outflow is accelerated from a region in the disk within about 0.5
(1.6) AU from the star for the blue (red) lobe. We estimate that the angular
momentum transport rate of the jet can be two thirds or more of the estimated
rate transported through the relevant portion of the disk. The magnetic lever
arm (ratio rA/r0 between the Alfv`en and footpoint radii) is between 3.5 and
4.6 (accuracy 20-25%), or, the ejection index = d ln() / d r
is in the range 0.025 - 0.046 (same accuracy).These values are in the range
predicted by the models, and suggest that some heating must be provided at the
base of the flow. We also derive the ratio B_phi / B_p of the toroidal and
poloidal components of the magnetic field at the observed location. B_phi / B_p
= 3.8 +- 1.1 at 30 AU from the axis in the red lobe and -8.9 +- 2.7 at 20 AU
from the axis in the blue lobe. The toroidal component is dominant, as
predicted by magnetic collimation models.Comment: 13 pages, 6 figures. Accepted for publication in A&
The Empirical Mass-Luminosity Relation for Low Mass Stars
This work is devoted to improving empirical mass-luminosity relations and
mass-metallicity-luminosity relation for low mass stars. For these stars,
observational data in the mass-luminosity plane or the
mass-metallicity-luminosity space subject to non-negligible errors in all
coordinates with different dimensions. Thus a reasonable weight assigning
scheme is needed for obtaining more reliable results. Such a scheme is
developed, with which each data point can have its own due contribution.
Previous studies have shown that there exists a plateau feature in the
mass-luminosity relation. Taking into account the constraints from the
observational luminosity function, we find by fitting the observational data
using our weight assigning scheme that the plateau spans from 0.28 to 0.50
solar mass. Three-piecewise continuous improved mass-luminosity relations in K,
J, H and V bands, respectively, are obtained. The visual
mass-metallicity-luminosity relation is also improved based on our K band
mass-luminosity relation and the available observational metallicity data.Comment: 8 pages, 2 figures. Accepted for publication in Astrophysics & Space
Scienc
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