100 research outputs found
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
Effect of the 3D distribution on water observations made with the SWI: I. Ganymede
Context. Characterising and understanding the atmospheres of Jovian icy moons is one of the key exploration goals of the Submillimetre Wave Instrument (SWI), which is to be flown on ESA\u27s Jupiter Icy Moons Explorer (JUICE) mission.Aims. The aim of this paper is to investigate how and under which conditions a 3D asymmetric distribution of the atmosphere may affect the SWI observations. In this work we target the role of phase angle for both nadir and limb geometries for unresolved and partially resolved disc observations from large distances.Methods. We adapted the LIME software package, a 3D non-local thermodynamical equilibrium radiative transfer model, to evaluate ortho-H2O populations and synthesise the simulated SWI beam spectra for different study cases of Ganymede\u27s atmosphere. The temperature and density vertical distributions were adopted from a previous work. The study cases presented here were selected according to the distance and operational scenarios of moon monitoring anticipated for SWI during the Jupiter phase of the JUICE mission.Results. We demonstrate that nadir and limb observations at different phase angles will modify the line amplitude and width. Unresolved observations where both absorption against surface continuum and limb emission contributes within the beam will lead to characteristic line wing emission, which may also appear in pure nadir geometry for specific phase angles. We also find that for Ganymede, the 3D non-local thermodynamical equilibrium populations are more highly excited in the upper atmosphere near the sub-solar region than they are in 1D spherically symmetric models. Finally, the 3D radiative transfer is better suited to properly simulate spectral lines for cases where density or population gradients exist along the line of sight
H2O line mapping at high spatial and spectral resolution - Herschel observations of the VLA1623 outflow
Apart from being an important coolant, H2O is known to be a tracer of
high-velocity molecular gas. Recent models predict relatively high abundances
behind interstellar shockwaves. The dynamical and physical conditions of the
H2O emitting gas, however, are not fully understood yet. We aim to determine
the abundance and distribution of H2O, its kinematics and the physical
conditions of the gas responsible for the H2O emission. The observed line
profile shapes help us understand the dynamics in molecular outflows. We mapped
the VLA1623 outflow, in the ground-state transitions of o-H2O, with the HIFI
and PACS instruments. We also present observations of higher energy transitions
of o-H2O and p-H2O obtained with HIFI and PACS towards selected outflow
positions. From comparison with non-LTE radiative transfer calculations, we
estimate the physical parameters of the water emitting regions. The observed
water emission line profiles vary over the mapped area. Spectral features and
components, tracing gas in different excitation conditions, allow us to
constrain the density and temperature of the gas. The H2O emission originates
in a region where temperatures are comparable to that of the warm H2 gas
(T\gtrsim200K). Thus, the H2O emission traces a gas component significantly
warmer than the gas responsible for the low-J CO emission. The H2O column
densities at the CO peak positions are low, i.e. N(H2O) \simeq (0.03-10)x10e14
cm-2. The H2O abundance with respect to H2 in the extended outflow is estimated
at X(H2O)<1x10e-6, significantly lower than what would be expected from most
recent shock models. The H2O emission traces a gas component moving at
relatively high velocity compared to the low-J CO emitting gas. However, other
dynamical quantities such as the momentum rate, energy and mechanical
luminosity are estimated to be the same, independent of the molecular tracer
used, CO or H2O.Comment: 14 pages, 13 figures, 4 table
Mapping water in protostellar outflows with Herschel: PACS and HIFI observations of L1448-C
We investigate on the spatial and velocity distribution of H2O along the
L1448 outflow, its relationship with other tracers, and its abundance
variations, using maps of the o-H2O 1_{10}-1_{01} and 2_{12}-1_{01} transitions
taken with the Herschel-HIFI and PACS instruments, respectively. Water emission
appears clumpy, with individual peaks corresponding to shock spots along the
outflow. The bulk of the 557 GHz line is confined to radial velocities in the
range \pm 10-50 km/s but extended emission associated with the L1448-C extreme
high velocity (EHV) jet is also detected. The H2O 1_{10}-1_{01}/CO(3-2) ratio
shows strong variations as a function of velocity that likely reflect different
and changing physical conditions in the gas responsible for the emissions from
the two species. In the EHV jet, a low H2O/SiO abundance ratio is inferred,
that could indicate molecular formation from dust free gas directly ejected
from the proto-stellar wind. We derive averaged Tkin and n(H2) values of about
300-500 K and 5 10^6 cm-3 respectively, while a water abundance with respect to
H2 of the order of 0.5-1 10^{-6} along the outflow is estimated. The fairly
constant conditions found all along the outflow implies that evolutionary
effects on the timescales of outflow propagation do not play a major role in
the H2O chemistry. The results of our analysis show that the bulk of the
observed H2O lines comes from post-shocked regions where the gas, after being
heated to high temperatures, has been already cooled down to a few hundred K.
The relatively low derived abundances, however, call for some mechanism to
diminish the H2O gas in the post-shock region. Among the possible scenarios, we
favor H2O photodissociation, which requires the superposition of a low velocity
non-dissociative shock with a fast dissociative shock able to produce a FUV
field of sufficient strength.Comment: 16 pages, 13 figures, accepted for publication on Astronomy &
Astrophysic
Resolving the shocked gas in HH54 with Herschel: CO line mapping at high spatial and spectral resolution
The HH54 shock is a Herbig-Haro object, located in the nearby Chamaeleon II
cloud. Observed CO line profiles are due to a complex distribution in density,
temperature, velocity, and geometry. Resolving the HH54 shock wave in the
far-infrared cooling lines of CO constrain the kinematics, morphology, and
physical conditions of the shocked region. We used the PACS and SPIRE
instruments on board the Herschel space observatory to map the full FIR
spectrum in a region covering the HH54 shock wave. Complementary Herschel-HIFI,
APEX, and Spitzer data are used in the analysis as well. The observed features
in the line profiles are reproduced using a 3D radiative transfer model of a
bow-shock, constructed with the Line Modeling Engine code (LIME). The FIR
emission is confined to the HH54 region and a coherent displacement of the
location of the emission maximum of CO with increasing J is observed. The peak
positions of the high-J CO lines are shifted upstream from the lower J CO lines
and coincide with the position of the spectral feature identified previously in
CO(10-9) profiles with HIFI. This indicates a hotter molecular component in the
upstream gas with distinct dynamics. The coherent displacement with increasing
J for CO is consistent with a scenario where IRAS12500-7658 is the exciting
source of the flow, and the 180 K bow-shock is accompanied by a hot (800 K)
molecular component located upstream from the apex of the shock and blueshifted
by -7 km s. The spatial proximity of this knot to the peaks of the
atomic fine-structure emission lines observed with Spitzer and PACS ([OI]63,
145 m) suggests that it may be associated with the dissociative shock as
the jet impacts slower moving gas in the HH54 bow-shock.Comment: 6 pages, 5 figure
Water distribution in shocked regions of the NGC1333-IRAS4A protostellar outflow
We present the study of the H2O spatial distribution at two bright shocked
regions along IRAS4A, one of the strongest H2O emitters among the Class 0
outflows. We obtained Herschel-PACS maps of the IRAS4A outflow and HIFI
observations of two shocked positions. The largest HIFI beam of 38 arcsec at
557 GHz was mapped in several key water lines with different upper energy
levels, to reveal possible spatial variations of the line profiles. We detect
four H2O lines and CO (16-15) at the two selected positions. In addition,
transitions from related outflow and envelope tracers are detected. Different
gas components associated with the shock are identified in the H2O emission. In
particular, at the head of the red lobe of the outflow, two distinct gas
components with different excitation conditions are distinguished in the HIFI
emission maps: a compact component, detected in the ground-state water lines,
and a more extended one. Assuming that these two components correspond to two
different temperature components observed in previous H2O and CO studies, the
excitation analysis of the H2O emission suggests that the compact (about 3
arcsec) component is associated with a hot (T~1000 K) gas with densities
~(1-4)x10^5 cm^{-3}, whereas the extended one (10-17 arcsec) traces a warm
(T~300-500 K) and dense gas (~(3-5)x10^7 cm^{-3}). Finally, using the CO
(16-15) emission observed at R2, we estimate the H2O/H2 abundance of the warm
and hot components to be (7-10)x10^{-7} and (3-7)x10^{-5}. Our data allowed us,
for the first time, to resolve spatially the two temperature components
previously observed with HIFI and PACS. We propose that the compact hot
component may be associated with the jet that impacts the surrounding material,
whereas the warm, dense, and extended component originates from the compression
of the ambient gas by the propagating flow.Comment: 13 pages, 11 figures. Accepted for publication in Astronomy and
Astrophysic
Herschel observations of the Herbig-Haro objects HH52-54
We are aiming at the observational estimation of the relative contribution to
the cooling by CO and H2O, as this provides decisive information for the
understanding of the oxygen chemistry behind interstellar shock waves. Methods.
The high sensitivity of HIFI, in combination with its high spectral resolution
capability, allows us to trace the H2O outflow wings at unprecedented
signal-to-noise. From the observation of spectrally resolved H2O and CO lines
in the HH52-54 system, both from space and from ground, we arrive at the
spatial and velocity distribution of the molecular outflow gas. Solving the
statistical equilibrium and non-LTE radiative transfer equations provides us
with estimates of the physical parameters of this gas, including the cooling
rate ratios of the species. The radiative transfer is based on an ALI code,
where we use the fact that variable shock strengths, distributed along the
front, are naturally implied by a curved surface. Based on observations of CO
and H2O spectral lines, we conclude that the emission is confined to the HH54
region. The quantitative analysis of our observations favours a ratio of the
CO-to-H2O-cooling-rate >> 1. From the best-fit model to the CO emission, we
arrive at an H2O abundance close to 1e-5. The line profiles exhibit two
components, one of which is triangular and another, which is a superposed,
additional feature. This additional feature likely originates from a region
smaller than the beam where the ortho-water abundance is smaller than in the
quiescent gas. Comparison with recent shock models indicate that a planar shock
can not easily explain the observed line strengths and triangular line
profiles.We conclude that the geometry can play an important role. Although
abundances support a scenario where J-type shocks are present, higher cooling
rate ratios than predicted by these type of shocks are derived.Comment: Accepted for publication in A&
Evidence for the start of planet formation in a young circumstellar disk
The growth of dust grains in protoplanetary disks is a necessary first step
towards planet formation. This growth has been inferred via observations of
thermal dust emission towards mature protoplanetary systems (age >2 million
years) with masses that are, on average, similar to Neptune3. In contrast, the
majority of confirmed exoplanets are heavier than Neptune. Given that young
protoplanetary disks are more massive than their mature counterparts, this
suggests that planet formation starts early, but evidence for grain growth that
is spatially and temporally coincident with a massive reservoir in young disks
remains scarce. Here, we report observations on a lack of emission of carbon
monoxide isotopologues within the inner ~15 au of a very young (age ~100,000
years) disk around the Solar-type protostar TMC1A. By using the absence of
spatially resolved molecular line emission to infer the gas and dust content of
the disk, we conclude that shielding by millimeter-size grains is responsible
for the lack of emission. This suggests that grain growth and millimeter-size
dust grains can be spatially and temporally coincident with a mass reservoir
sufficient for giant planet formation. Hence, planet formation starts during
the earliest, embedded phases in the life of young stars.Comment: Accepted for publication in Nature Astronomy, 3 figures, 3 extended
figure
Episodic infall towards a compact disk in B335?
Previous observations of B335 have presented evidence of ongoing infall in
various molecular lines, e.g., HCO, HCN, CO. There have been no confirmed
observations of a rotationally supported disk on scales greater than ~12~au.
The presence of an outflow in B335 suggests that also a disk should be present
or in formation. To constrain the earliest stages of protostellar evolution and
disk formation, we aim to map the region where gas falls inwards and
observationally constrain its kinematics. Furthermore, we aim to put strong
limits on the size and orientation of any disk-like structure in B335. We use
high angular resolution CO data from ALMA, and combine it with
shorter-baseline archival data to produce a high-fidelity image of the infall
in B335. We also revisit the imaging of high-angular resolution Band 6
continuum data to study the dust distribution in the immediate vicinity of
B335. Continuum emission shows an elliptical structure (10 by 7 au) with a
position angle 5 degrees east of north, consistent with the expectation for a
forming disk in B335. A map of the infall velocity (as estimated from the
CO emission), shows evidence of asymmetric infall, predominantly from
the north and south. Close to the protostar, infall velocities appear to exceed
free-fall velocities. 3D radiative transfer models, where the infall velocity
is allowed to vary within the infall region, can explain the observed
kinematics. The data suggests that a disk has started to form in B335 and that
gas is falling towards that disk. However, kinematically-resolved line data
towards the disk itself is needed to confirm the presence of a rotationally
supported disk around this young protostar. The measured high infall velocities
are not easily reconcilable with a magnetic braking scenario and suggest that
there is a pressure gradient that allows the infall velocity to vary in the
region.Comment: 14 pages, 11 figure
The ALMA Protostellar Interferometric Line Survey (PILS): First results from an unbiased submillimeter wavelength line survey of the Class 0 protostellar binary IRAS 16293-2422 with ALMA
The inner regions of the envelopes surrounding young protostars are characterised by a complex chemistry, with prebiotic molecules present on the scales where protoplanetary disks eventually may form. This paper introduces a systematic survey, "Protostellar Interferometric Line Survey (PILS)" of the Class 0 protostellar binary IRAS 16293-2422 using the Atacama Large Millimeter/submillimeter Array (ALMA). The survey covers the full frequency range from 329 to 363 GHz (0.8 mm) with additional targeted observations at 3.0 and 1.3 mm. More than 10,000 features are detected toward one component in the protostellar binary. Glycolaldehyde, its isomers, methyl formate and acetic acid, and its reduced alcohol, ethylene glycol, are clearly detected. For ethylene glycol both lowest state conformers, aGg' and gGg', are detected, the latter for the first time in the ISM. The abundance of glycolaldehyde is comparable to or slightly larger than that of ethylene glycol. In comparison to the Galactic Center, these two species are over-abundant relative to methanol, possibly an indication of formation at low temperatures in CO-rich ices. Both 13C and deuterated isotopologues of glycolaldehyde are detected, also for the first time ever in the ISM. For the deuterated species, a D/H ratio of approximately 5% is found with no differences between the deuteration in the different functional groups of glycolaldehyde. Measurements of the 13C-species lead to a 12C:13C ratio of approximately 30, lower than the typical ISM value. This low ratio may reflect an enhancement of 13CO in the ice due to either ion-molecule reactions in the gas before freeze-out or differences in the temperatures where 12CO and 13CO ices sublimate. The results reinforce the importance of low-temperature grain surface chemistry for the formation of prebiotic molecules seen here in the gas after sublimation of the entire ice mantle
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