47 research outputs found
The Rotation of Young Low-Mass Stars and Brown Dwarfs
We review the current state of our knowledge concerning the rotation and
angular momentum evolution of young stellar objects and brown dwarfs from a
primarily observational view point. Periods are typically accurate to 1% and
available for about 1700 stars and 30 brown dwarfs in young clusters.
Discussion of angular momentum evolution also requires knowledge of stellar
radii, which are poorly known for pre-main sequence stars. It is clear that
rotation rates at a given age depend strongly on mass; higher mass stars
(0.4-1.2 M) have longer periods than lower mass stars and brown dwarfs.
On the other hand, specific angular momentum is approximately independent of
mass for low mass pre-main sequence stars and young brown dwarfs. A spread of
about a factor of 30 is seen at any given mass and age. The evolution of
rotation of solar-like stars during the first 100 Myr is discussed. A broad,
bimodal distribution exists at the earliest observable phases (1 Myr) for
stars more massive than 0.4 M. The rapid rotators (50-60% of the
sample) evolve to the ZAMS with little or no angular momentum loss. The slow
rotators continue to lose substantial amounts of angular momentum for up to 5
Myr, creating the even broader bimodal distribution characteristic of 30-120
Myr old clusters. Accretion disk signatures are more prevalent among slowly
rotating PMS stars, indicating a connection between accretion and rotation.
Disks appear to influence rotation for, at most, 5 Myr, and considerably
less than that for the majority of stars. If the dense clusters studied so far
are an accurate guide, then the typical solar-like star may have only 1
Myr for this task. It appears that both disk interactions and stellar winds are
less efficient at braking these objects.Comment: Review chapter for Protostars and Planets V. 15 page and 8 figure
IR diagnostics of embedded jets: velocity resolved observations of the HH34 and HH1 jets
We present VLT-ISAAC medium resolution spectroscopy of the HH34 and HH1 jets.
Our aim is to derive the kinematics and the physical parameters and to study
how they vary with jet velocity. We use several important diagnostic lines such
as [FeII] 1.644um, 1.600um and H2 2.122um. In the inner jet region of HH34 we
find that both the atomic and molecular gas present two components at high and
low velocity. The [FeII] LVC in HH34 is detected up to large distances from the
source (>1000 AU), at variance with TTauri jets. In H2 2.122um, the LVC and HVC
are spatially separated. We detect, for the first time, the fainter red-shifted
counterpart down to the central source. In HH1, we trace the jet down to ~1"
from the VLA1 driving source: the kinematics of this inner region is again
characterised by the presence of two velocity components, one blue-shifted and
one red-shifted with respect to the source LSR velocity. In the inner HH34 jet
region, ne increases with decreasing velocity. Up to ~10" from the driving
source, and along the whole HH1 jet an opposite behaviour is observed instead,
with ne increasing with velocity. In both jets the mass flux is carried mainly
by the high-velocity gas. A comparison between the position velocity diagrams
and derived electron densities with models for MHD jet launching mechanisms has
been performed for HH34. While the kinematical characteristics of the line
emission at the jet base can be, at least qualitatively, reproduced by both
X-winds and disc-wind models, none of these models can explain the extent of
the LVC and the dependence of electron density with velocity that we observe.
It is possible that the LVC in HH34 represents gas not directly ejected in the
jet but instead denser ambient gas entrained by the high velocity collimated
jet.Comment: A&A accepte
HST FUV C IV observations of the hot DG Tauri jet
Protostellar jets are tightly connected to the accretion process and regulate
the angular momentum balance of accreting star-disk systems. The DG Tau jet is
one of the best-studied protostellar jets and contains plasma with temperatures
ranging over three orders of magnitude within the innermost 50 AU of the jet.
We present new Hubble Space Telescope (HST) far ultraviolet (FUV) long-slit
spectra spatially resolving the C IV emission (T~1e5 K) from the jet for the
first time, and quasi-simultaneous HST observations of optical forbidden
emission lines ([O I], [N II], [S II] and [O III]) and fluorescent H2 lines.
The C IV emission peaks at 42 AU from the stellar position and has a FWHM of 52
AU along the jet. Its deprojected velocity of around 200 km/s decreases
monotonically away from the driving source. In addition, we compare our HST
data with the X-ray emission from the DG Tau jet. We investigate the
requirements to explain the data by an initially hot jet compared to local
heating. Both scenarios indicate a mass loss by the T~1e5 K jet of ~1e-9
Msun/year, i.e., between the values for the lower temperature jet (T~1e4 K) and
the hotter X-ray emitting part (T>1e6 K). However, a simple initially hot wind
requires a large launching region (~1 AU), and we therefore favor local
heating.Comment: 5 pages, 2 figures, accepted by A&A letter
On the ALMA observability of nascent massive multiple systems formed by gravitational instability
This is the final version. Available from Oxford University Press via the DOI in this record.Massive young stellar object (MYSOs) form during the collapse of high-mass pre-stellar cores, where infalling molecular material is accreted through a centrifugally-balanced accretion disc that is subject to efficient gravitational instabilities. In the resulting fragmented accretion disc of the MYSO, gaseous clumps and low-mass stellar companions can form, which will influence the future evolution of massive protostars in the Hertzsprung-Russell diagram. We perform dust continuum radiative transfer calculations and compute synthetic images of disc structures modelled by the gravito-radiation-hydrodynamics simulation of a forming MYSO, in order to investigate the Atacama Large Millimeter/submillimeter Array (ALMA) observability of circumstellar gaseous clumps and forming multiple systems. Both spiral arms and gaseous clumps located at ~a few 100 au from the protostar can be resolved by interferometric ALMA Cycle 7 C43-8 and C43-10 observations at band 6 (1.2 mm), using a maximal 0.015" beam angular resolution and at least 10-30 min exposure time for sources at distances of 1-2 kpc. Our study shows that substructures are observable regardless of their viewing geometry or can be inferred in the case of an edge-viewed disc. The observation probability of the clumps increases with the gradually increasing efficiency of gravitational instability at work as the disc evolves. As a consequence, large discs around MYSOs close to the zero-age-main-sequence line exhibit more substructures than at the end of the gravitational collapse. Our results motivate further observational campaigns devoted to the close surroundings of the massive protostars S255IR-NIRS3 and NGC 6334I-MM1, whose recent outbursts are a probable signature of disc fragmentation and accretion variability.European Research Council (ERC)Science and Technology Facilities Council (STFC)Russian Science FoundationSwiss National Science Foundatio
The kinematic relationship between disk and jet in the DG Tauri system
We present high angular resolution millimeter wavelength continuum and
13CO(2-1) observations of the circumstellar disk surrounding the TTauri star DG
Tauri. We show that the velocity pattern in the inner regions of the disk is
consistent with Keplerian rotation about a central 0.67 Msun star. The disk
rotation is also consistent with the toroidal velocity pattern in the initial
channel of the optical jet, as inferred from HST spectra of the first
de-projected 100 AU from the source. Our observations support the tight
relationship between disk and jet kinematics postulated by the popular
magneto-centrifugal models for jet formation and collimation.Comment: 4 pages, A&A Letter accepte
[OI]63micron jets in class 0 sources detected by Herschel
We present Herschel PACS mapping observations of the [OI]63 micron line
towards protostellar outflows in the L1448, NGC1333-IRAS4, HH46, BHR71 and
VLA1623 star forming regions. We detect emission spatially resolved along the
outflow direction, which can be associated with a low excitation atomic jet. In
the L1448-C, HH46 IRS and BHR71 IRS1 outflows this emission is kinematically
resolved into blue- and red-shifted jet lobes, having radial velocities up to
200 km/s. In the L1448-C atomic jet the velocity increases with the distance
from the protostar, similarly to what observed in the SiO jet associated with
this source. This suggests that [OI] and molecular gas are kinematically
connected and that this latter could represent the colder cocoon of a jet at
higher excitation. Mass flux rates (\.M(OI)) have been measured from
the [OI]63micron luminosity adopting two independent methods. We find values in
the range 1-4 10 Mo/yr for all sources but HH46, for which an order of
magnitude higher value is estimated. \.M(OI) are compared with mass
accretion rates (\.M) onto the protostar and with \.M derived
from ground-based CO observations. \.M(OI)/\.M ratios are in
the range 0.05-0.5, similar to the values for more evolved sources.
\.M(OI) in HH46 IRS and IRAS4A are comparable to \.M(CO), while
those of the remaining sources are significantly lower than the corresponding
\.M(CO). We speculate that for these three sources most of the mass
flux is carried out by a molecular jet, while the warm atomic gas does not
significantly contribute to the dynamics of the system.Comment: 37 pages and 12 figures, accepted for publication on Astrophysical
Journa
The near-infrared excitation of the HH211 protostellar outflow
The protostellar outflow HH211 is of considerable interest since it is
extremely young and highly collimated. Here, we explore the outflow through
imaging and spectroscopy in the near-infrared to determine if there are further
diagnostic signatures of youth. We confirm the detection of a near-infrared
continuum of unknown origin. We propose that it is emitted by the driving
millimeter source, escapes the core through tunnels, and illuminates features
aligning the outflow. Narrow-band flux measurements of these features contain
an unusually large amount of continuum emission. [Fe II] emission has been
detected and is restricted to isolated condensations. However, the
characteristics of vibrational excitation of molecular hydrogen resemble those
of older molecular outflows. We attempt to model the ordered structure of the
western outflow as a series of shocks, finding that bow shocks with J-type
dissociative apices and C-type flanks are consistent. Moreover, essentially the
same conditions are predicted for all three bows except for a systematic
reduction in speed and density with distance from the driving source. We find
increased K-band extinctions in the bright regions as high as 2.9 magnitudes
and suggest that the bow shocks become visible where the outflow impacts on
dense clumps of cloud material. We propose that the outflow is carved out by
episodes of pulsating jets. The jets, driven by central explosive events, are
responsible for excavating a central tunnel through which radiation temporarily
penetrates.Comment: 12 pages, 9 figure
POISSON project - I - Emission lines as accretion tracers in young stellar objects: results from observations of Chamaeleon I and II sources
We present the results of the analysis of LR optical-NIR spectra (0.6-2.4 um)
of a sample 47 YSOs in the ChaI and II star-forming clouds. These data are part
of the POISSON project (Protostellar Optical-Infrared Spectral Survey on NTT).
The aim is to determine the accretion luminosity (Lacc) and mass accretion rate
(Macc) of the sources through the analysis of the detected emission features.
We also aim at verifying the reliability and consistency of the existing
empirical relationships connecting emission line luminosity and Lacc. We employ
five tracers (OI-6300A, Ha, CaII-8542A, Pab, and Brg) to derive the accretion
luminosity. The tracers provide Lacc values showing different scatters when
plotted as a function of L*. The Brg seems to be the most reliable, because it
gives the minimum Lacc dispersion over the entire range of L*, whereas the
other tracers provide much more scattered Lacc values, which are not expected
for our homogeneous sample. The comparison between Lacc(Brg) and Lacc obtained
from the other tracers also shows systematic differences among the empirical
relationships. These may probably be ascribed to different excitation
mechanisms contributing to the line emission, which may vary between our sample
and those where the relationships were calibrated. Adopting the Lacc derived
from Brg, we find Lacc=0.1L*-1L* for all sources, and Macc of the order of
10^-7-10^-9 Msun/yr. The Macc derived in ChaI are proportional to M*^2, as
found in other low-mass star-forming regions. The discrepancies observed in the
case of Lacc(Brg) and Lacc(Pab) can be related to different intrinsic Pab/Brg,
ratios. The derived ratios show the existence of two different emission
modalities, one that agrees with predictions of both wind and accretion models,
the other suggesting optically thick emission from relatively small regions
(10^21-10^22 cm^-3) with gas at low temperatures (<4000K).Comment: 22 pages, 8 figures, accepted for publication in A&A; institute
affiliations and typos correcte
Radio outburst from a massive (proto)star. When accretion turns into ejection
Context. Recent observations of the massive young stellar object S255 NIRS 3
have revealed a large increase in both methanol maser flux density and IR
emission, which have been interpreted as the result of an accretion outburst,
possibly due to instabilities in a circumstellar disk. This indicates that this
type of accretion event could be common in young/forming early-type stars and
in their lower mass siblings, and supports the idea that accretion onto the
star may occur in a non-continuous way. Aims. As accretion and ejection are
believed to be tightly associated phenomena, we wanted to confirm the accretion
interpretation of the outburst in S255 NIRS 3 by detecting the corresponding
burst of the associated thermal jet. Methods. We monitored the radio continuum
emission from S255 NIRS 3 at four bands using the Karl G. Jansky Very Large
Array. The millimetre continuum emission was also observed with both the
Northern Extended Millimeter Array of IRAM and the Atacama Large
Millimeter/submillimeter Array. Results. We have detected an exponential
increase in the radio flux density from 6 to 45 GHz starting right after July
10, 2016, namely about 13 months after the estimated onset of the IR outburst.
This is the first ever detection of a radio burst associated with an IR
accretion outburst from a young stellar object. The flux density at all
observed centimetre bands can be reproduced with a simple expanding jet model.
At millimetre wavelengths we infer a marginal flux increase with respect to the
literature values and we show this is due to free-free emission from the radio
jet. Abridged.Comment: 14 pages, 9 figures, 3 tables, accepted by Astronomy & Astrophysic