270 research outputs found
CO observations of water-maser post-AGB stars and detection of a high-velocity outflow in IRAS 15452-5459
Many aspects of the evolutionary phase in which Asymptotic Giant Branch stars
(AGB stars) are in transition to become Planetary Nebulae (PNe) are still
poorly understood. An important question is how the circumstellar envelopes of
AGB stars switch from spherical symmetry to the axially symmetric structures
frequently observed in PNe. In many cases there is clear evidence that the
shaping of the circumstellar envelopes of PNe is linked to the formation of
jets/collimated winds and their interaction with the remnant AGB envelope.
Because of the short evolutionary time, objects in this phase are rare, but
their identification provides valuable probes for testing evolutionary models.
We have observed (sub)millimeter CO rotational transitions with the APEX
telescope in a small sample of stars hosting high-velocity OH and water masers.
These targets are supposed to have recently left the AGB, as indicated by the
presence of winds traced by masers, with velocities larger than observed during
that phase. We have carried out observations of several CO lines, ranging from
J=2-1 up to J=7-6. In IRAS 15452-5459 we detect a fast molecular outflow in the
central region of the nebula and estimate a mass-loss rate between 1.2x10^{-4}
Msun yr^{-1} (assuming optically thin emission) and 4.9x10^{-4} Msun yr^{-1}
(optically thick emission). We model the SED of this target taking advantage of
our continuum measurement at 345 GHz to constrain the emission at long
wavelengths. For a distance of 2.5 kpc, we estimate a luminosity of 8000 Lsun
and a dust mass of 0.01 Msun. Through the flux in the [CII] line (158 um), we
calculate a total mass of about 12 Msun for the circumstellar envelope, but the
line is likely affected by interstellar contamination.Comment: 12 pages, 9 figures, accepted for publication on A&
The APEX-CHAMP+ view of the Orion Molecular Cloud 1 core - Constraining the excitation with submillimeter CO multi-line observations
A high density portion of the Orion Molecular Cloud 1 (OMC-1) contains the
prominent, warm Kleinmann-Low (KL) nebula that is internally powered by an
energetic event plus a farther region in which intermediate to high mass stars
are forming. Its outside is affected by ultraviolet radiation from the
neighboring Orion Nebula Cluster and forms the archetypical photon-dominated
region (PDR) with the prominent bar feature. Its nearness makes the OMC-1 core
region a touchstone for research on the dense molecular interstellar medium and
PDRs. Using the Atacama Pathfinder Experiment telescope (APEX), we have imaged
the line emission from the multiple transitions of several carbon monoxide (CO)
isotopologues over the OMC-1 core region. Our observations employed the 2x7
pixel submillimeter CHAMP+ array to produce maps (~ 300 arcsec x 350 arcsec) of
12CO, 13CO, and C18O from mid-J transitions (J=6-5 to 8-7). We also obtained
the 13CO and C18O J=3-2 images toward this region. The 12CO line emission shows
a well-defined structure which is shaped and excited by a variety of phenomena,
including the energetic photons from hot, massive stars in the nearby Orion
Nebula's central Trapezium cluster, active high- and intermediate-mass star
formation, and a past energetic event that excites the KL nebula. Radiative
transfer modeling of the various isotopologic CO lines implies typical H2
densities in the OMC-1 core region of ~10^4-10^6 cm^-3 and generally elevated
temperatures (~ 50-250 K). We estimate a warm gas mass in the OMC-1 core region
of 86-285 solar masses.Comment: 11 pages, 9 figures, accepted by A&
Excitation of the molecular gas in the nuclear region of M82
We present high resolution HIFI spectroscopy of the nucleus of the
archetypical starburst galaxy M82. Six 12CO lines, 2 13CO lines and 4
fine-structure lines are detected. Besides showing the effects of the overall
velocity structure of the nuclear region, the line profiles also indicate the
presence of multiple components with different optical depths, temperatures and
densities in the observing beam. The data have been interpreted using a grid of
PDR models. It is found that the majority of the molecular gas is in low
density (n=10^3.5 cm^-3) clouds, with column densities of N_H=10^21.5 cm^-2 and
a relatively low UV radiation field (GO = 10^2). The remaining gas is
predominantly found in clouds with higher densities (n=10^5 cm^-3) and
radiation fields (GO = 10^2.75), but somewhat lower column densities
(N_H=10^21.2 cm^-2). The highest J CO lines are dominated by a small (1%
relative surface filling) component, with an even higher density (n=10^6 cm^-3)
and UV field (GO = 10^3.25). These results show the strength of multi-component
modeling for the interpretation of the integrated properties of galaxies.Comment: Accepted for publication in A&A Letter
Strong CH+ J=1-0 emission and absorption in DR21
We report the first detection of the ground-state rotational transition of
the methylidyne cation CH+ towards the massive star-forming region DR21 with
the HIFI instrument onboard the Herschel satellite. The line profile exhibits a
broad emission line, in addition to two deep and broad absorption features
associated with the DR21 molecular ridge and foreground gas. These observations
allow us to determine a CH+ J=1-0 line frequency of 835137 +/- 3 MHz, in good
agreement with a recent experimental determination. We estimate the CH+ column
density to be a few 1e13 cm^-2 in the gas seen in emission, and > 1e14 cm^-2 in
the components responsible for the absorption, which is indicative of a high
line of sight average abundance [CH+]/[H] > 1.2x10^-8. We show that the CH+
column densities agree well with the predictions of state-of-the-art C-shock
models in dense UV-illuminated gas for the emission line, and with those of
turbulent dissipation models in diffuse gas for the absorption lines.Comment: Accepted for publication in A&
Sensitive limits on the abundance of cold water vapor in the DM Tau protoplanetary disk
We performed a sensitive search for the ground-state emission lines of ortho-
and para-water vapor in the DM Tau protoplanetary disk using the Herschel/HIFI
instrument. No strong lines are detected down to 3sigma levels in 0.5 km/s
channels of 4.2 mK for the 1_{10}--1_{01} line and 12.6 mK for the
1_{11}--0_{00} line. We report a very tentative detection, however, of the
1_{10}--1_{01} line in the Wide Band Spectrometer, with a strength of
T_{mb}=2.7 mK, a width of 5.6 km/s and an integrated intensity of 16.0 mK km/s.
The latter constitutes a 6sigma detection. Regardless of the reality of this
tentative detection, model calculations indicate that our sensitive limits on
the line strengths preclude efficient desorption of water in the UV illuminated
regions of the disk. We hypothesize that more than 95-99% of the water ice is
locked up in coagulated grains that have settled to the midplane.Comment: 5 pages, 3 figures. Accepted for publication in the Herschel HIFI
special issue of A&
The distribution of water in the high-mass star-forming region NGC 6334I
We present observations of twelve rotational transitions of H2O-16, H2O-18,
and H2O-17 toward the massive star-forming region NGC 6334 I, carried out with
Herschel/HIFI as part of the guaranteed time key program Chemical HErschel
Surveys of Star forming regions (CHESS). We analyze these observations to
obtain insights into physical processes in this region.
We identify three main gas components (hot core, cold foreground, and
outflow) in NGC 6334 I and derive the physical conditions in these components.
The hot core, identified by the emission in highly excited lines, shows a
high excitation temperature of 200 K, whereas water in the foreground component
is predominantly in the ortho- and para- ground states. The abundance of water
varies between 4 10^-5 (outflow) and 10^-8 (cold foreground gas). This
variation is most likely due to the freeze-out of water molecules onto dust
grains. The H2O-18/H2O-17 abundance ratio is 3.2, which is consistent with the
O-18/O-17 ratio determined from CO isotopologues. The ortho/para ratio in water
appears to be relatively low 1.6(1) in the cold, quiescent gas, but close to
the equilibrium value of three in the warmer outflow material (2.5(0.8)).Comment: 7 pages, 3 figures, accepted by A&
HERSCHEL-HIFI spectroscopy of the intermediate mass protostar NGC7129 FIRS 2
HERSCHEL-HIFI observations of water from the intermediate mass protostar
NGC7129 FIRS 2 provide a powerful diagnostic of the physical conditions in this
star formation environment. Six spectral settings, covering four H216O and two
H218O lines, were observed and all but one H218O line were detected. The four
H2 16 O lines discussed here share a similar morphology: a narrower, \approx 6
km/s, component centered slightly redward of the systemic velocity of NGC7129
FIRS 2 and a much broader, \approx 25 km/s component centered blueward and
likely associated with powerful outflows. The narrower components are
consistent with emission from water arising in the envelope around the
intermediate mass protostar, and the abundance of H2O is constrained to \approx
10-7 for the outer envelope. Additionally, the presence of a narrow
self-absorption component for the lowest energy lines is likely due to
self-absorption from colder water in the outer envelope. The broader component,
where the H2O/CO relative abundance is found to be \approx 0.2, appears to be
tracing the same energetic region that produces strong CO emission at high J.Comment: 6 pages, 4 figures, accepted by A&
Hydrides in Young Stellar Objects: Radiation tracers in a protostar-disk-outflow system
Context: Hydrides of the most abundant heavier elements are fundamental
molecules in cosmic chemistry. Some of them trace gas irradiated by UV or
X-rays. Aims: We explore the abundances of major hydrides in W3 IRS5, a
prototypical region of high-mass star formation. Methods: W3 IRS5 was observed
by HIFI on the Herschel Space Observatory with deep integration (about 2500 s)
in 8 spectral regions. Results: The target lines including CH, NH, H3O+, and
the new molecules SH+, H2O+, and OH+ are detected. The H2O+ and OH+ J=1-0 lines
are found mostly in absorption, but also appear to exhibit weak emission
(P-Cyg-like). Emission requires high density, thus originates most likely near
the protostar. This is corroborated by the absence of line shifts relative to
the young stellar object (YSO). In addition, H2O+ and OH+ also contain strong
absorption components at a velocity shifted relative to W3 IRS5, which are
attributed to foreground clouds. Conclusions: The molecular column densities
derived from observations correlate well with the predictions of a model that
assumes the main emission region is in outflow walls, heated and irradiated by
protostellar UV radiation.Comment: Astronomy and Astrophysics Letters, in pres
Water abundances in high-mass protostellar envelopes: Herschel observations with HIFI
We derive the dense core structure and the water abundance in four massive
star-forming regions which may help understand the earliest stages of massive
star formation. We present Herschel-HIFI observations of the para-H2O 1_11-0_00
and 2_02-1_11 and the para-H2-18O 1_11-0_00 transitions. The envelope
contribution to the line profiles is separated from contributions by outflows
and foreground clouds. The envelope contribution is modelled using Monte-Carlo
radiative transfer codes for dust and molecular lines (MC3D and RATRAN), with
the water abundance and the turbulent velocity width as free parameters. While
the outflows are mostly seen in emission in high-J lines, envelopes are seen in
absorption in ground-state lines, which are almost saturated. The derived water
abundances range from 5E-10 to 4E-8 in the outer envelopes. We detect cold
clouds surrounding the protostar envelope, thanks to the very high quality of
the Herschel-HIFI data and the unique ability of water to probe them. Several
foreground clouds are also detected along the line of sight. The low H2O
abundances in massive dense cores are in accordance with the expectation that
high densities and low temperatures lead to freeze-out of water on dust grains.
The spread in abundance values is not clearly linked to physical properties of
the sources.Comment: 8 pages, 5 figures, accepted for publication the 15/07/2010 by
Astronomy&Astrophysics as a letter in the Herschel-HIFI special issu
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