91 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
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 one that got away: a unique eclipse in the young brown dwarf Roque 12
We report the discovery of a deep, singular eclipse of the bona fide brown dwarf Roque 12, a substellar member of the Pleiades. The eclipse was 0.6mag deep, lasted 1.3h, and was observed with two telescopes simultaneously in October 2002. No further eclipse was recorded, despite continuous monitoring with Kepler/K2 over 70d in 2015. There is tentative (2σ) evidence for radial velocity variations of 5km/s, over timescales of three months. The best explanation for the eclipse is the presence of a companion on an eccentric orbit. The observations constrain the eccentricity to e>0.5, the period to P>70d, and the mass of the companion to to ~0.001-0.04Msol. In principle it is also possible that the eclipse is caused by circum-sub-stellar material. Future data releases by Gaia and later LSST as well as improved radial velocity constraints may be able to unambiguously confirm the presence of the companion which would turn this system into one of the very few known eclipsing binary brown dwarfs with known age
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
An S-shaped outflow from IRAS 03256+3055 in NGC 1333
The IRAS source 03256+3055 in the NGC 1333 star forming region is associated
with extended sub-millimeter emission of complex morphology, showing multiple
clumps. One of these is found to coincide with the driving source of a bipolar
jet of S-shaped morphology seen in the emission lines of H_alpha and [SII] as
well as in the H2 emission lines in the K-band. Detailed images of the driving
source at the wavelengths of H_alpha and [SII] and in the I, J, H, and K bands
as well as a K-band spectrum and polarimetry are discussed. The near-infrared
morphology is characterized by a combination of line emission from the jet and
scattered light from a source with a steep continuum spectrum. The morphology
and proper motion of the jet are discussed in the context of a binary system
with a precessing disk. We conclude that the molecular core associated with
IRAS 03256+3055 consists of several clumps, only one of which shows evidence of
recent star formation at optical and near-infrared wavelengths.We also briefly
discuss a second, newly found near-infrared source associated with a compact
sub-millimeter continuum source near IRAS 03256+3055, and conclude that this
source may be physically unrelated the cluster of molecular clumps.Comment: 25 pages, including 5 figures. Accepted for publication in The
Astronomical Journa
The rotation of very low-mass stars and brown dwarfs
The evolution of angular momentum is a key to our understanding of star
formation and stellar evolution. The rotational evolution of solar-mass stars
is mostly controlled by magnetic interaction with the circumstellar disc and
angular momentum loss through stellar winds. Major differences in the internal
structure of very low-mass stars and brown dwarfs -- they are believed to be
fully convective throughout their lives, and thus should not operate a
solar-type dynamo -- may lead to major differences in the rotation and activity
of these objects. Here, we report on observational studies to understand the
rotational evolution of the very low-mass stars and brown dwarfs.Comment: 8 pages, 5 figures, Proceedings for IAU Symposium No. 243, 2007,
'Star-disk interaction in young stars
Toward Resolving the Outflow Engine: An Observational Perspective
Jets from young stars represent one of the most striking signposts of star
formation. The phenomenon has been researched for over two decades and there is
now general agreement that such jets are generated as a by-product of
accretion; most likely by the accretion disk itself. Thus they mimic what
occurs in more exotic objects such as active galactic nuclei and micro-quasars.
The precise mechanism for their production however remains a mystery. To a
large degree, progress is hampered observationally by the embedded nature of
many jet sources as well as a lack of spatial resolution: Crude estimates, as
well as more sophisticated models, nevertheless suggest that jets are
accelerated and focused on scales of a few AU at most.
It is only in the past few years however that we have begun to probe such
scales in detail using classical T Tauri stars as touchstones. Application of
adaptive optics, data provided by the HST, use of specialised techniques such
as spectro-astrometry, and the development of spectral diagnostic tools, are
beginning to reveal conditions in the jet launch zone. This has helped
enormously to constrain models. Further improvements in the quality of the
observational data are expected when the new generation of interferometers come
on-line. Here we review some of the most dramatic findings in this area since
Protostars and Planets~IV including indications for jet rotation, i.e. that
they transport angular momentum. We will also show how measurements, such as
those of width and the velocity field close to the source, suggest jets are
initially launched as warm magneto-centrifugal disk winds. (abridged)Comment: 14 pages, 8 figures, contributed chapter for Planets and Protostars V
meeting (October 2005
A HST study of the environment of the Herbig Ae/Be star LkHa 233 and its bipolar jet
We present the results of HST/STIS and WFPC2 observations of LkHa 233 and its
environment. LkHa233 is a Herbig Ae/Be star with a collimated bipolar jet. We
investigate optical forbidden lines along the LkHa 233 jet to determine
physical parameters of this jet (electron density n_e, hydrogen ionisation
fraction x_e, electron temperature T_e, and mass density n_H). The knowledge of
these parameters allows us a direct comparison of a jet from a Herbig star with
those from T Tauri stars. The WFPC2 images in broad-band filters clearly show a
dark lane caused either by a circumstellar disk or a dust torus. In the
blueshifted lobe, n_e is close to or above the critical density for [SII] lines
(2.5x10^4 cm^-3) in the first arcsecond and decreases with distance from the
source. The ionisation x_e~0.2-0.6 gently rises for the first 500 AU of the
flow and shows two re-ionisation events further away from the origin. The T_e
varies along the flow between 10^4 K and 3x10^4 K. The (radial) outflow
velocities are ~ 80-160 km s^(-1), and they appear to increase with distance
from the source. In the redshifted lobe, T_e, n_e, x_e, and n_H are all lower
than in the blueshifted lobe, but have the same order of magnitude. All these
derived parameters are just beyond or at the upper limits of those observed for
classical T Tauri star jets. This may indicate that the flows from the higher
mass Herbig stars are indeed scaled-up examples of the same phenomenon as in T
Tauri stars.Comment: 21 pages, 5 figures, accepted for publication in the Astronomy and
Astrophysic
Accretion bursts in high-mass protostars: a new testbed for models of episodic accretion
It is well known that low mass young stellar objects (LMYSOs) gain a
significant portion of their final mass through episodes of very rapid
accretion, with mass accretion rates up to ~yr. Recent observations of high mass young stellar objects
(HMYSO) with masses uncovered outbursts with
accretion rates exceeding ~yr. Here we
examine which scenarios proposed in the literature so far to explain accretion
bursts of LMYSOs can apply to the episodic accretion in HMYSOs. We utilise a 1D
time dependent models of protoplanetary discs around HMYSOs to study burst
properties. We find that discs around HMYSOs are much hotter than those around
their low mass cousins. As a result, much more extended regions of the disc are
prone to the thermal hydrogen ionisation and MRI activation instabilities. The
former in particular is found to be ubiquitous in a very wide range of
accretion rates and disc viscosity parameters. The outbursts triggered by these
instabilities, however, always have too low , and are one to several
orders of magnitude too long compared to those observed from HMYSOs so far. On
the other hand, bursts generated by tidal disruptions of gaseous giant planets
formed by the gravitational instability of the protoplanetary discs yield
properties commensurate with observations, provided that the clumps are in the
post-collapse configuration with planet radius Jupiter
radii. Furthermore, if observed bursts are caused by disc ionisation
instabilities then they should be periodic phenomena with the duration of the
quiescent phase comparable to that of the bursts. This may yield potentially
observable burst periodicity signatures in the jets, the outer disc, or the
surrounding diffuse material of massive HMYSOs. (abridged)Comment: 8 pages, 6 figures, Accepted to A&A Letter
SK 1: A Possible Case of Triggered Star Formation in Perseus
According to a triggered star formation scenario (e.g. Martin-Pintado &
Cernicharo 1987) outflows powered by young stellar objects shape the molecular
clouds, can dig cavities, and trigger new star formation. NGC 1333 is an active
site of low- and intermediate star formation in Perseus and is a suggested site
of self-regulated star formation Norman & Silk 1980. Therefore it is a suitable
target for a study of triggered star formation (e.g. Sandell & Knee 2001, SK
1). On the other hand, continuum sub-mm observations of star forming regions
can detect dust thermal emission of embedded sources (which drive outflows),
and further detailed structures. Within the framework of our wide-field mapping
of star formation regions in the Perseus and Orion molecular clouds using SCUBA
at 850 and 450 micrometers, we map NCG 1333 with an area of around 14' x 21'.
The maps show more structure than the previous maps of the region observed in
sub-mm. We have unveiled the known embedded SK 1 source (in the dust shell of
the SSV 13 ridge) and detailed structure of the region, among some other young
protostars. In agreement with the SK 1 observations, our map of the region
shows lumpy filaments and shells/cavities that seem to be created by outflows.
The measured mass of SK 1 (~0.07 Msun) is much less than its virial mass
(~0.2-1 Msun). Our observations support the idea of SK 1 as an event triggered
by outflow-driven shells in NGC 1333 (induced by an increase in gas pressure
and density due to radiation pressure from the stellar winds, that have
presumably created the dust shell). This kind of evidences provides a more
thorough understanding of the star formation regulation processes.Comment: 5 pages, 1 Postscript figure. Contributed paper delivered at IAU
Symposium 237 "Triggered Star Formation in a Turbulent ISM", Bruce Elmegreen
and Jan Palous (eds.
- …