471 research outputs found
The Formation of Low-Mass-Protostars and Proto-Brown Dwarfs
The formation of low-mass protostars and especially of brown dwarfs currently
are ``hot topics'' in cool star research. The talks contributed to this
splinter session discussed how low in mass and how low in luminosity objects
might exist, if these substellar objects show evidence for circum(sub)stellar
disks, and how the bottom of the mass function in young clusters after the
formation process looks like. In a lively open discussion, a vast majority of
the speakers and the audience expressed why, given the available data, a
stellar-like formation mechanism down to the lowest masses should be preferred.Comment: Summary of Splinter Session on "The Formation of Low-Mass-Protostars
and Proto-Brown Dwarfs" at the 14th Cambridge Workshop on Cool Stars, Stellar
Systems and the Sun in Pasadena, 6 - 10 November 200
Generation of radiative knots in a randomly pulsed protostellar jet I. Dynamics and energetics
HH objects are characterized by a complex knotty morphology detected mainly
along the axis of protostellar jets in a wide range of bands. Evidence of
interactions between knots formed in different epochs have been found,
suggesting that jets may result from the ejection of plasma blobs from the
source. We aim at investigating the physical mechanism leading to the irregular
knotty structure observed in jets in different bands and the complex
interactions occurring among blobs of plasma ejected from the stellar source.
We perform 2D axisymmetric HD simulations of a randomly ejected pulsed jet. The
jet consists of a train of blobs which ram with supersonic speed into the
ambient medium. The initial random velocity of each blob follows an exponential
distribution. We explore the ejection rate parameter to derive constraints on
the physical properties of protostellar jets by comparison of model results
with observations. Our model takes into account radiative losses and thermal
conduction. We find that the mutual interactions of blobs ejected at different
epochs and with different speed lead to a variety of plasma components not
described by current models. The main features characterizing the random pulsed
jet scenario are: single high speed knots, showing a measurable proper motion
in nice agreement with observations; irregular chains of knots aligned along
the jet axis and possibly interacting with each other; reverse shocks
interacting with outgoing knots; oblique shocks produced by the reflection of
shocks at the jet cocoon. All these structures concur to determine the
morphology of the jet in different bands. We also find that the thermal
conduction plays a crucial role in damping out HD instabilities that would
develop within the cocoon and that contribute to the jet breaking.Comment: 10 pages, 10 figures, accepted for publication in A&
TEST - The Tautenburg Exoplanet Search Telescope
The Tautenburg Exoplanet Search Telescope (TEST) is a robotic telescope
system. The telescope uses a folded Schmidt Camera with a 300mm main mirror.
The focal length is 940mm and it gives a 2.2 square degree field of view. Dome,
mount, and CCD cameras are controlled by a software bundle made by Software
Bisque. The automation of the telescope includes selection of the night
observing program from a given framework, taking darks and skyflats, field
identification, guiding, data taking, and archiving. For the search for
transiting exoplanets and variable stars an automated psf photometry based on
IRAF and a lightcurve analysis based on ESO-Midas are conducted. The images and
the results are managed using a PostgreSQL database.Comment: 4 pages, 2 figures, to appear in Proc. of'Transiting Planets', IAU
Symposium 25
The low-mass diskless population of Corona Australis
We combine published optical and near-infrared photometry to identify new low-mass candidate members in an area of about 0.64 deg^2 in Corona Australis with the S-parameter method. Five new candidate members of the region are selected. They have estimated ages between 3 and 15 Myr and masses between 0.05 and 0.15 M_⊙. With Spitzer photometry we confirm that these objects are not surrounded by optically thick disks. However, one of them is found to display excess at 24 μm, thus suggesting it harbors a disk with an inner hole. With an estimated mass of 0.07 M_⊙ according to the SED fitting, this is one of the lowest-mass objects reported to possess a transitional disk. Including these new members, the fraction of disks is about 50% among the total Corona Australis population selected by the same criteria, lower than the 70% fraction reported earlier for this region. Even so, we find a ratio of transitional to primordial disks (45%) very similar to the value derived by previous authors. This ratio is higher than for solar-type stars (5–10%), suggesting that disk evolution is faster in the latter, and/or that the “transitional disk” stage is not such a short-lived step for very low-mass objects. However, this impression needs to be confirmed with better statistics
Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS
We derive the physical properties at the base of the jet from DG Tau both
along and across the flow and as a function of velocity. We analysed seven
optical spectra of the DG Tau jet, taken with the Hubble Space Telescope
Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel
to the jet axis and stepping it across the jet width. The resulting
position-velocity diagrams in optical forbidden emission lines allowed access
to plasma conditions via calculation of emission line ratios.
We find at the base of the jet high electron density, 10, and
very low ionisation, , which combine to give a total
density up to 3 10. This analysis confirms previous reports of
variations in plasma parameters along the jet, (i.e. decrease in density by
several orders of magnitude, increase of from 0.05 to a plateau at 0.7
downstream at 2 from the star). Furthermore, a spatial coincidence is
revealed between sharp gradients in the total density and supersonic velocity
jumps. This strongly suggests that the emission is caused by shock excitation.
The position-velocity diagrams indicate the presence of both fast accelerating
gas and slower, less collimated material. We derive the mass outflow rate,
, in the blue-shifted lobe in different velocity channels, that
contribute to a total of 8 4 10 M
yr. We estimate that a symmetric bipolar jet would transport at the low
and intermediate velocities probed by rotation measurements, an angular
momentum flux of 2.9 1.5 10 M yr
AU km s.
The derived properties of the DG Tau jet are demonstrated to be consistent
with magneto-centrifugal theory. However, non-stationary modelling is required
in order to explain all of the features revealed at high resolution.Comment: 16 pages, 18 figure
First detection of acceleration and deceleration in protostellar Jets? Time variability in the Chamaeleontis II outflows
Context. Kinematical and time variability studies of protostellar jets are fundamental for understanding the dynamics and the physics of these objects. Such studies remain very sporadic, since they require long baselines before they can be accomplished. Alms. We present for the first time a multi-epoch (20 years baseline) kinematical investigation of HH 52, 53, and 54 at optical and near-IR wavelengths, along with medium (optical) and high resolution (NIR) spectroscopic analyses, probing the kinematical and physical time variability conditions of the gas along the flows. Methods. By means of multi-epoch and multi-wavelength narrow-band images, we derived proper motions (PMs), tangential velocities, velocity and flux variability of the knots. Radial velocities and physical parameters of the gas were derived from spectroscopy. Finally, spatial velocities and inclination of the flows were obtained by combining both imaging and spectroscopy. Results. The PM analysis reveals three distinct, partially overlapping outflows. Spatial velocities of the knots vary from 50 km s -1 to 120 km s-1. The inclinations of the three flows are 58 ± 3°, 84 ± 2°, and 67 ± 3° (HH 52, HH 53, and HH 54 flows, respectively). In 20 years, about 60% of the observed knots show some degree of flux variability. Our set of observations apparently indicates acceleration and deceleration in a variety of knots along the jets. For about 20% of the knots, mostly coincident with working surfaces or interacting knots along the flows, a relevant variability in both flux and velocity is observed. We argue that both variabilities are related and that all or part of the kinetic energy lost by the interacting knots is successively radiated. The physical parameters derived from the diagnostics are quite homogeneous along and among the three outflows. The analysis indicates the presence of very light (NH � 103 cm-3), ionised (Te,. � 0.2-0.6), and hot (Te � 14000-26000 K) flows, impacting a denser medium. Several knots are deflected, especially in the HH 52 flow. At least for a couple of them (HH 54 G and GO), the deflection originates from the collision of the two. For the more massive parts of the flow, the deflection is likely the result of the flow collision with a dense cloud or with clumps. Finally, we discuss the possible driving sources of the flows. ©ESO 2009
Magnetic activity and accretion on FU Tau A: Clues from variability
FU Tau A is a young very low mass object in the Taurus star forming region
which was previously found to have strong X-ray emission and to be anomalously
bright for its spectral type. In this study we discuss these characteristics
using new information from quasi-simultaneous photometric and spectroscopic
monitoring. From photometric time series obtained with the 2.2m telescope on
Calar Alto we measure a period of ~4d for FU Tau A, most likely the rotation
period. The short-term variations over a few days are consistent with the
rotational modulation of the flux by cool, magnetically induced spots. In
contrast, the photometric variability on timescales of weeks and years can only
be explained by the presence of hot spots, presumably caused by accretion. The
hot spot properties are thus variable on timescales exceeding the rotation
period, maybe due to long-term changes in the accretion rate or geometry. The
new constraints from the analysis of the variability confirm that FU Tau A is
affected by magnetically induced spots and excess luminosity from accretion.
However, accretion is not sufficient to explain its anomalous position in the
HR diagram. In addition, suppressed convection due to magnetic activity and/or
an early evolutionary stage need to be invoked to fully account for the
observed properties. These factors cause considerable problems in estimating
the mass of FU Tau A and other objects in this mass/age regime, to the extent
that it appears questionable if it is feasible to derive the Initial Mass
Function for young low-mass stars and brown dwarfs.Comment: 10 pages, 7 figures, accepted for publication in MNRAS, 'Note added
in proof' include
A combined optical/infrared spectral diagnostic analysis of the HH1 jet
Complete flux-calibrated spectra covering the spectral range from 6000 A to
2.5 um have been obtained along the HH1 jet and analysed in order to explore
the potential of a combined optical/near-IR diagnostic applied to jets from
young stellar objects. Important physical parameters have been derived along
the jet using various diagnostic line ratios. This multi-line analysis shows,
in each spatially unresolved knot, the presence of zones at different
excitation conditions, as expected from the cooling layers behind a shock
front. In particular, a density stratification in the jet is evident from
ratios of various lines of different critical density. In particular, [FeII]
lines originate in a cooling layer located at larger distances from the shock
front than that generating the optical lines, where the compression is higher
and the temperature is declining. The derived parameters were used to measure
the mass flux along the jet, adopting different procedures, the advantages and
limitations of which are discussed. dM/dt is high in the initial part of the
flow but decreases by about an order of magnitude further out. Conversely, the
mass flux associated with the warm molecular material is low and does not show
appreciable variations along the jet. We suggest that part of the mass flux in
the external regions is not revealed in optical and IR lines because it is
associated with a colder atomic component, which may be traced by the far-IR [O
I]63 um line.
Finally, we find that the gas-phase abundance of refractory species is lower
than the solar value suggesting that a significant fraction of dust grains may
still be present in the jet beam.Comment: Accepted on Astronomy & Astrophysic
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