2,104 research outputs found
Water emission from the high-mass star-forming region IRAS 17233-3606. High water abundances at high velocities
We investigate the physical and chemical processes at work during the
formation of a massive protostar based on the observation of water in an
outflow from a very young object previously detected in H2 and SiO in the IRAS
17233-3606 region. We estimated the abundance of water to understand its
chemistry, and to constrain the mass of the emitting outflow. We present new
observations of shocked water obtained with the HIFI receiver onboard Herschel.
We detected water at high velocities in a range similar to SiO. We
self-consistently fitted these observations along with previous SiO data
through a state-of-the-art, one-dimensional, stationary C-shock model. We found
that a single model can explain the SiO and H2O emission in the red and blue
wings of the spectra. Remarkably, one common area, similar to that found for H2
emission, fits both the SiO and H2O emission regions. This shock model
subsequently allowed us to assess the shocked water column density,
N(H2O)=1.2x10^{18} cm^{-2}, mass, M(H2O)=12.5 M_earth, and its maximum
fractional abundance with respect to the total density, x(H2O)=1.4x10^{-4}. The
corresponding water abundance in fractional column density units ranges between
2.5x10^{-5} and 1.2x10^{-5}, in agreement with recent results obtained in
outflows from low- and high-mass young stellar objects.Comment: accepted for publication as a Letter in Astronomy and Astrophysic
The abundances of nitrogen-containing molecules during pre-protostellar collapse
We have studied the chemistry of nitrogen--bearing species during the initial stages of protostellar collapse, with a view to explaining the observed longevity of N2H+ and NH3 and the high levels of deuteration of these species. We followed the chemical evolution of a medium comprising gas and dust as it underwent free--fall gravitational collapse. Chemical processes which determine the relative populations of the nuclear spin states of molecules and molecular ions were included explicitly, as were reactions which lead ultimately to the deuteration of the nitrogen--containing species N2H+ and NH3. The freeze-out of `heavy' molecules onto grains was taken into account. We found that the timescale required for the nitrogen--containing species to attain their steady--state values was much larger than the free--fall time and even comparable with the probable lifetime of the precursor molecular cloud. However, it transpires that the chemical evolution of the gas during gravitational collapse is insensitive to its initial composition. If we suppose that the grain--sticking probabilities of atomic nitrogen and oxygen are both less than unity (S less than 0.3), we find that the observed differential freeze--out of nitrogen- and carbon--bearing species can be reproduced by the model of free--fall collapse when a sufficiently large grain radius (a_{g}= 0.5 micron) is adopted. Furthermore, the results of our collapse model are consistent with the high levels of deuteration of N2H+ and NH3 which have been observed in L1544 providing that 0.5<a_{g}<1 micron. We note that the o/p H2D+ ratio and fractional abundance of ortho-H2D+ should be largest when ND3 is most abundant
Rovibrationally resolved photodissociation of HeH+
Accurate photodissociation cross sections have been obtained for the A-X
electronic transition of HeH+ using ab initio potential curves and dipole
transition moments. Partial cross sections have been evaluated for all
rotational transitions from the vibrational levels v"=0-11 and over the entire
accessible wavelength range 100-1129 Angstrom. Assuming a Boltzmann
distribution of the rovibrational levels of the X state, photodissociation
cross sections are presented for temperatures between 500 and 12,000 K. A
similar set of calculations was performed for the pure rovibrational
photodissociation in the X-X electronic ground state, but covering photon
wavelengths into the far infrared. Applications of the cross sections to the
destruction of HeH+in the early Universe and in UV-irradiated environments such
as primordial halos and protoplanetary disks are briefly discussed
The Structure, Kinematics and Physical Properties of the Molecular Gas in the Starburst Nucleus of NGC 253
We present 5.2" x 2.6" resolution interferometry of CO J=1-0 emission from
the starburst galaxy NGC 253. The high spatial resolution of these new data, in
combination with recent high resolution maps of 13CO, HCN and near-infrared
emission, allow us for the first time to link unambiguously the gas properties
in the central starburst of NGC 253 with its bar dynamics. We confirm that the
star formation results from bar-driven gas flows as seen in "twin peaks"
galaxies. Two distinct kinematic features are evident from the CO map and
position-velocity diagram: a group of clouds rotating as a solid body about the
kinematic center of the galaxy, and a more extended gas component associated
with the near-infrared bar. We model the line intensities of CO, HCN and 13CO
to infer the physical conditions of the gas in the nucleus of NGC 253. The
results indicate increased volume densities around the radio nucleus in a
twin-peaks morphology. Compared with the CO kinematics, the gas densities
appear highest near the radius of a likely inner Linblad resonance, and
slightly lead the bar minor axis. This result is similar to observations of the
face-on, twin-peaks galaxy NGC 6951, and is consistent with models of starburst
generation due to gas inflow along a bar.Comment: To appear in the ApJ, 28 pages, 12 figure file
A pair of planets around HD 202206 or a circumbinary planet?
Long-term precise Doppler measurements with the CORALIE spectrograph reveal
the presence of a second planet orbiting the solar-type star HD202206. The
radial-velocity combined fit yields companion masses of m_2\sini = 17.4 M_Jup
and 2.44 M_Jup, semi-major axes of a = 0.83 AU and 2.55 AU, and eccentricities
of e = 0.43 and 0.27, respectively. A dynamical analysis of the system further
shows a 5/1 mean motion resonance between the two planets. This system is of
particular interest since the inner planet is within the brown-dwarf limits
while the outer one is much less massive. Therefore, either the inner planet
formed simultaneously in the protoplanetary disk as a superplanet, or the outer
Jupiter-like planet formed in a circumbinary disk. We believe this singular
planetary system will provide important constraints on planetary formation and
migration scenarios.Comment: 9 pages, 14 figures, accepted in A&A, 12-May-200
HD 5388 b is a 69 M_Jup companion instead of a planet
We examined six exoplanet host stars with non-standard Hipparcos astrometric
solution, which may be indicative of unrecognised orbital motion. Using
Hipparcos intermediate astrometric data, we detected the astrometric orbit of
HD 5388 at a significance level of 99.4 % (2.7 sigma). HD 5388 is a
metal-deficient star and hosts a planet candidate with a minimum mass of 1.96
M_J discovered in 2010. We determined its orbit inclination to be i = 178.3
+0.4/-0.7 deg and the corresponding mass of its companion HD 5388 b to be M_2 =
69 +/- 20 M_J. The orbit is seen almost face-on and the companion mass lies at
the upper end of the brown-dwarf mass range. A mass lower than 13 M_J was
excluded at the 3-sigma level. The astrometric motions of the five other stars
had been investigated by other authors revealing two planetary companions, one
stellar companion, and two statistically insignificant orbits. We conclude that
HD 5388 b is not a planet but most likely a brown-dwarf companion. In addition,
we find that the inclinations of the stellar rotation axis and the companion's
orbital axis differ significantly.Comment: 5 pages, 4 figures, Accepted for publication in Astronomy and
Astrophysics Letter
Detection of FeO towards SgrB2
We have observed the J=5-4 ground state transition of FeO at a frequency of
153 GHz towards a selection of galactic sources.
Towards the galactic center source SgrB2, we see weak absorption at
approximately the velocity of other features towards this source (62 km
s LSR).
Towards other sources, the results were negative as they were also for
MgOH(3-2) and FeC(6-5). We tentatively conclude that the absorption seen toward
SgrB2 is due to FeO in the hot ( 500 K) relatively low density absorbing
gas known to be present in this line of sight.
This is the first (albeit tentative) detection of FeO or any iron--containing
molecule in the interstellar gas. Assuming the observed absorption to be due to
FeO, we estimate [FeO]/[SiO] to be of order or less than 0.002 and
[FeO]/[H] of order . This is compatible with our negative
results in other sources.
Our results suggest that the iron liberated from grains in the shocks
associated with SgrB2 remains atomic and is not processed into molecular form.Comment: 1 postscrit figure,10 page
The ortho-to-para ratio of ammonia in the L1157 outflow
We have measured the ortho-to-para ratio of ammonia in the blueshifted gas of
the L1157 outflow by observing the six metastable inversion lines from (J, K) =
(1, 1) to (6, 6). The highly excited (5, 5) and (6, 6) lines were first
detected in the low-mass star forming regions. The rotational temperature
derived from the ratio of four transition lines from (3, 3) to (6, 6) is
130-140 K, suggesting that the blueshifted gas is heated by a factor of ~10 as
compared to the quiescent gas. The ortho-to-para ratio of the NH3 molecules in
the blueshifted gas is estimated to be 1.3--1.7, which is higher than the
statistical equilibrium value. This ratio provides us with evidence that the
NH3 molecules have been evaporated from dust grains with the formation
temperature between 18 and 25 K. It is most likely that the NH3 molecules on
dust grains have been released into the gas phase through the passage of strong
shock waves produced by the outflow. Such a scenario is supported by the fact
that the ammonia abundance in the blueshifted gas is enhanced by a factor of ~5
with respect to the dense quiescent gas.Comment: 16 pages, including 3 PS figures. To appear in the ApJ (Letters).
aastex macro
Low, Milky-Way like, Molecular Gas Excitation of Massive Disk Galaxies at z~1.5
We present evidence for Milky-Way-like, low-excitation molecular gas
reservoirs in near-IR selected massive galaxies at z~1.5, based on IRAM Plateau
de Bure Interferometer CO[3-2] and NRAO Very Large Array CO[1-0] line
observations for two galaxies that had been previously detected in CO[2-1]
emission. The CO[3-2] flux of BzK-21000 at z=1.522 is comparable within the
errors to its CO[2-1] flux, implying that the CO[3-2] transition is
significantly sub-thermally excited. The combined CO[1-0] observations of the
two sources result in a detection at the 3 sigma level that is consistent with
a higher CO[1-0] luminosity than that of CO[2-1]. Contrary to what is observed
in submillimeter galaxies and QSOs, in which the CO transitions are thermally
excited up to J>=3, these galaxies have low-excitation molecular gas, similar
to that in the Milky Way and local spirals. This is the first time that such
conditions have been observed at high redshift. A Large Velocity Gradient
analysis suggests that molecular clouds with density and kinetic temperature
comparable to local spirals can reproduce our observations. The similarity in
the CO excitation properties suggests that a high, Milky-Way-like, CO to H_2
conversion factor could be appropriate for these systems. If such
low-excitation properties are representative of ordinary galaxies at high
redshift, centimeter telescopes such as the Expanded Very Large Array and the
longest wavelength Atacama Large Millimeter Array bands will be the best tools
for studying the molecular gas content in these systems through the
observations of CO emission lines.Comment: 5 pages, 4 figures. ApJ Letters in pres
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