438 research outputs found
Water in low-mass star-forming regions with Herschel (WISH-LM): High-velocity H2O bullets in L1448-MM observed with HIFI
Herschel-HIFI observations of water in the low-mass star-forming object
L1448-MM, known for its prominent outflow, are presented, as obtained within
the `Water in star-forming regions with Herschel' (WISH) key programme. Six
H2-16O lines are targeted and detected (E_up/k_B ~ 50-250 K), as is CO J= 10-9
(E_up/k_B ~ 305 K), and tentatively H2-18O 110-101 at 548 GHz. All lines show
strong emission in the "bullets" at |v| > 50 km/s from the source velocity, in
addition to a broad, central component and narrow absorption. The bullets are
seen much more prominently in HO than in CO with respect to the central
component, and show little variation with excitation in H2O profile shape.
Excitation conditions in the bullets derived from CO lines imply a temperature
>150 K and density >10^5 cm^-3, similar to that of the broad component. The
H2O/CO abundance ratio is similar in the "bullets" and the broad component, ~
0.05-1.0, in spite of their different origins in the molecular jet and the
interaction between the outflow and the envelope. The high H2O abundance
indicates that the bullets are H2 rich. The H2O cooling in the "bullets" and
the broad component is similar and higher than the CO cooling in the same
components. These data illustrate the power of Herschel-HIFI to disentangle
different dynamical components in low-mass star-forming objects and determine
their excitation and chemical conditions.Comment: Accepted for publication in A&
Warm SiO gas in molecular bullets associated with protostellar outflows
In this paper we present the first SiO multiline analysis (from J=2-1 to
J=11-10) of the molecular bullets along the outflows of the Class 0 sources
L1448-mm and L1157-mm, obtained through observations with IRAM and JCMT. We
have computed the main physical parameters in each bullet and compared them
with other tracers of warm and dense gas and with models for the SiO excitation
in shocks. We find that the bullets close to L1448--mm, associated with high
velocity gas, have higher excitation conditions (n(H2) ~ 10^{6} cm^{-3}, T >
500 K) with respect to the L1157 bullets (n(H2) ~1-5 10^{5} cm^{-3}, T ~
100-300 K). In both the sources, there is a clear evidence of the presence of
velocity components having different excitation conditions, with the denser
and/or warmer gas associated with the gas at the higher speed. In L1448 the
bulk of the emission is due to the high-excitation and high velocity gas, while
in L1157 most of the emission comes from the low excitation gas at ambient
velocity. The observed velocity-averaged line ratios are well reproduced by
shocks with speeds v_s larger than ~ 30 km/s and densities ~ 10^{5} - 10^{6}
cm^{-3}. Plane-parallel shock models, however, fail to predict all the observed
line profiles and in particular the very similar profiles shown by both low and
high excitation lines. The overall observations support the idea that the L1157
clumps are shock interaction events older than the L1448 bullets close to the
driving source. In the latter objects, the velocity structure and the
variations of physical parameters with the velocity resemble very closely those
found in optical/IR jets near the protostar, suggesting that similar launching
and excitation mechanisms are also at the origin of collimated jets seen at
millimetre wavelengths.Comment: 11pages, 9 figures, A&A accepte
Spatially resolved H_2 emission from a very low-mass star
Molecular outflows from very low-mass stars (VLMSs) and brown dwarfs have
been studied very little. So far, only a few CO outflows have been observed,
allowing us to map the immediate circumstellar environment. We present the
first spatially resolved H2 emission around IRS54 (YLW52), a ~0.1-0.2 Msun
Class I source. By means of VLT SINFONI K-band observations, we probed the H2
emission down to the first ~50 AU from the source. The molecular emission shows
a complex structure delineating a large outflow cavity and an asymmetric
molecular jet. Thanks to the detection of several H2 transitions, we are able
to estimate average values along the jet-like structure (from source position
to knot D) of Av~28 mag, T~2000-3000 K, and H2 column density N(H2)~1.7x10^17
cm^-2. This allows us to estimate a mass loss rate of ~2x10^-10 Msun/yr for the
warm H2 component . In addition, from the total flux of the Br Gamma line, we
infer an accretion luminosity and mass accretion rate of 0.64 Lsun and ~3x10^-7
Msun/yr, respectively. The outflow structure is similar to those found in
low-mass Class I and CTTS. However, the Lacc/Lbol ratio is very high (~80%),
and the mass accretion rate is about one order of magnitude higher when
compared to objects of roughly the same mass, pointing to the young nature of
the investigated source.Comment: accepted as a Letter in A&
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
Physical properties of outflows: Comparing CO and H2O based parameters in Class 0 sources
Context. The observed physical properties of outflows from low-mass sources
put constraints on possible ejection mechanisms. Historically, these quantities
have been derived from CO using ground-based observations. It is thus important
to investigate whether parameters such as momentum rate (thrust) and mechanical
luminosity (power) are the same when different molecular tracers are used.
Aims. We aim at determining the outflow momentum, dynamical time-scale, thrust,
energy and power using CO and H2O as tracers of outflow activity. Methods.
Within the framework of the WISH key program, three molecular outflows from
Class 0 sources have been mapped using the HIFI instrument aboard Herschel. We
use these observations together with previously published H2 data to infer the
physical properties of the outflows. We compare the physical properties derived
here with previous estimates based on CO observations. Results. Inspection of
the spatial distribution of H2O and H2 confirms that these molecules are
co-spatial. The most prominent emission peaks in H2 coincide with strong H2O
emission peaks and the estimated widths of the flows when using the two tracers
are comparable. Conclusions. For the momentum rate and the mechanical
luminosity, inferred values are independent of which tracer that is used, i.e.,
the values agree to within a factor of 4 and 3 respectively.Comment: Accepted for publication in A&A, 5 pages, 2 figure
The Herschel HIFI water line survey in the low-mass proto-stellar outflow L1448
As part of the WISH (Water In Star-forming regions with Herschel) key
project, we report on the observations of several ortho- and para-H2O lines
performed with the HIFI instrument towards two bright shock spots (R4 and B2)
along the outflow driven by the L1448 low-mass proto-stellar system, located in
the Perseus cloud. These data are used to identify the physical conditions
giving rise to the H2O emission and infer any dependence with velocity. These
observations provide evidence that the observed water lines probe a warm
(T_kin~400-600 K) and very dense (n 10^6 - 10^7 cm^-3) gas, not traced by other
molecules, such as low-J CO and SiO, but rather traced by mid-IR H2 emission.
In particular, H2O shows strong differences with SiO in the excitation
conditions and in the line profiles in the two observed shocked positions,
pointing to chemical variations across the various velocity regimes and
chemical evolution in the different shock spots. Physical and kinematical
differences can be seen at the two shocked positions. At the R4 position, two
velocity components with different excitation can be distinguished, with the
component at higher velocity (R4-HV) being less extended and less dense than
the low velocity component (R4-LV). H2O column densities of about 2 10^13 and 4
10^14 cm^-2 have been derived for the R4-LV and the R4-HV components,
respectively. The conditions inferred for the B2 position are similar to those
of the R4-HV component, with H2O column density in the range 10^14 - 5 10^14
cm^-2, corresponding to H2O/H2 abundances in the range 0.5 - 1 10^-5. The
observed line ratios and the derived physical conditions seem to be more
consistent with excitation in a low velocity J-type shock with large
compression rather than in a stationary C-shock, although none of these
stationary models seems able to reproduce all the characteristics of the
observed emission.Comment: Accepted for publication in A&
Water emission from the chemically rich outflow L1157
In the framework of the Herschel-WISH key program, several ortho-H2O and
para-H2O emission lines, in the frequency range from 500 to 1700 GHz, were
observed with the HIFI instrument in two bow-shock regions (B2 and R) of the
L1157 cloud. The primary aim is to analyse water emission lines as a diagnostic
of the physical conditions in the blue (B2) and red-shifted (R) lobes to
compare the excitation conditions. A total of 5 ortho- and para-H216O plus one
o-H218O transitions were observed in B2 and R with a wide range of excitation
energies (27 K<=Eu<=215 K). The H2O spectra, observed in the two shocked
regions, show that the H2O profiles are markedly different in the two regions.
In particular, at the bow-shock R, we observed broad (~30 km s-1 with respect
to the ambient velocity) red-shifted wings where lines at different excitation
peak at different red-shifted velocities. The B2 spectra are associated with a
narrower velocity range (~6 km s-1), peaking at the systemic velocity. The
excitation analysis suggests, for B2, low values of column density NH2O
<=5{\times}1013 cm-2, a density range of 105 <=nH2 <=107 cm-3, and warm
temperatures (>=300 K). The presence of the broad red-shifted wings and
multiple peaks in the spectra of the R region, prompted the modelling of two
components. High velocities are associated with relatively low temperatures
(~100K),NH2O{\simeq}5{\times}1012-5{\times}1013 cm-2 and densities
nH2{\simeq}106-108 cm-3.Lower velocities are associated with higher excitation
conditions with Tkin>=300 K, very dense gas (nH2 ~108 cm-3) and low column
density (NH2O<5{\times}1013 cm-2).Comment: 9 pages, 8 figures, A&A in pres
POISSON project - III - Investigating the evolution of the mass accretion rate
As part of the POISSON project (Protostellar Optical-Infrared Spectral Survey
on NTT), we present the results of the analysis of low-resolution NIR spectra
0.9-2.4 um) of two samples of YSOs in Lupus and Serpens (52 and 17 objects),
with masses 0.1-2.0 Msun and ages from 10^5 to a few 10^7 yr. After determining
the accretion parameters of the Lup and Ser targets by analysing their HI
near-IR emission features, we added the results to those from previous regions
(investigated in POISSON with the same methodology). We obtained a final
catalogue (143 objects) of mass accretion rates (Macc) derived in a homogeneous
fashion and analysed how Macc correlates with M* and how it evolves in time. We
derived the accretion luminosity (Lacc) and Macc for Lup and Ser objects from
the Br_gamma line by using relevant empirical relationships from the literature
that connect HI line luminosity and Lacc. To minimise the biases and also for
self-consistency, we re-derived mass and age for each source using the same set
of evolutionary tracks. We observe a correlation MaccM*^2.2, similarly to what
has previously been observed in several star-forming clouds. The time variation
of Macc is roughly consistent with the expected evolution in viscous disks,
with an asymptotic decay that behaves as t^-1.6. However, Macc values are
characterised by a large scatter at similar ages and are on average higher than
the predictions of viscous models. Although part of the scattering may be
related to the employed empirical relationship and to uncertainties on the
single measurements, the general distribution and decay trend of the Macc
points are real. These findings might be indicative of a large variation in the
initial mass of the disks, of fairly different viscous laws among disks, of
varying accretion regimes, and of other mechanisms that add to the dissipation
of the disks, such as photo-evaporation.Comment: 18 pages, 10 figures, accepted by A&
HH 223: a parsec-scale H2 outflow in the star-forming region L723
The dark cloud Lynds 723 (L723) is a low-mass star-forming region where one
of the few known cases of a quadrupolar CO outflow has been reported. Two
recent works have found that the radio continuum source VLA 2, towards the
centre of the CO outflow, is actually a multiple system of young stellar
objects (YSOs). Several line-emission nebulae that lie projected on the
east-west CO outflow were detected in narrow-band Halpha and [SII] images. The
spectra of the knots are characteristic of shock-excited gas (Herbig-Haro
spectra), with supersonic blueshifted velocities, which suggests an optical
outflow also powered by the VLA 2 YSO system of L723. We imaged a field of ~5'
X 5' centred on HH 223, which includes the whole region of the quadrupolar CO
outflow with nir narrow-band filters . The H2 line-emission structures appear
distributed over a region of 5.5' (0.5 pc for a distance of 300 pc) at both
sides of the VLA 2 YSO system, with an S-shape morphology, and are projected
onto the east-west CO outflow. Most of them were resolved in smaller knotty
substructures. The [FeII] emission only appears associated with HH 223. An
additional nebular emission from the continuum in Hc and Kc appears associated
with HH 223-K1, the structure closest to the VLA 2 YSO system, and could be
tracing the cavity walls. We propose that the H2 structures form part of a
large-scale near-infrared outflow, which is also associated with the VLA 2 YSO
system. The current data do not allow us to discern which of the YSOs of VLA 2
is powering this large scale optical/near-infrared outflow.Comment: Accepted for A&A http://dx.doi.org/10.1051/0004-6361/201015125 12
pages, 9 figure
Heavy water around the L1448-mm protostar
Context: L1448-mm is the prototype of a low-mass Class 0 protostar driving a
high-velocity jet. Given its bright H2O spectra observed with ISO, L1448-mm is
an ideal laboratory to observe heavy water (HDO) emission. Aims: Our aim is to
image the HDO emission in the protostar surroundings, the possible occurrence
of HDO emission also investigating off L1448-mm, towards the molecular outflow.
Methods: We carried out observations of L1448-mm in the HDO(1_10-1_11) line at
80.6 GHz, an excellent tracer of HDO column density, with the IRAM Plateau de
Bure Interferometer. Results: We image for the first time HDO emission around
L1448-mm. The HDO structure reveals a main clump at velocities close to the
ambient one towards the the continuum peak that is caused by the dust heated by
the protostar. In addition, the HDO map shows tentative weaker emission at
about 2000 AU from the protostar towards the south, which is possibly
associated with the walls of the outflow cavity opened by the protostellar
wind. Conclusions: Using an LVG code, modelling the density and temperature
profile of the hot-corino, and adopting a gas temperature of 100 K and a
density of 1.5 10^8 cm^-3, we derive a beam diluted HDO column density of about
7 10^13 cm^-2, corresponding to a HDO abundance of about 4 10^-7. In addition,
the present map supports the scenario where HDO can be efficiently produced in
shocked regions and not uniquely in hot corinos heated by the newly born star.Comment: Accepted by A&A as Letter; 5 pages, 3 figure
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