305 research outputs found
Spitzer spectral line mapping of supernova remnants: I. Basic data and principal component analysis
We report the results of spectroscopic mapping observations carried out
toward small (1 x 1 arcmin) regions within the supernova remnants W44, W28,
IC443, and 3C391 using the Infrared Spectrograph of the Spitzer Space
Telescope. These observations, covering the 5.2 - 37 micron spectral region,
have led to the detection of a total of 15 fine structure transitions of Ne+,
Ne++, Si+, P+, S, S++, Cl+, Fe+, and Fe++; the S(0) - S(7) pure rotational
lines of molecular hydrogen; and the R(3) and R(4) transitions of hydrogen
deuteride. In addition to these 25 spectral lines, the 6.2, 7.7, 8.6, 11.3 and
12.6 micron PAH emission bands were also observed. Most of the detected line
transitions have proven strong enough to map in several sources, providing a
comprehensive picture of the relative distribution of the various line
emissions observable in the Spitzer/IRS bandpass. A principal component
analysis of the spectral line maps reveals that the observed emission lines
fall into five distinct groups, each of which may exhibit a distinct spatial
distribution: (1) lines of S and H2 (J > 2); (2) the H2 S(0) line; (3) lines of
ions with appearance potentials less than 13.6 eV; (4) lines of ions with
appearance potentials greater than 13.6 eV, not including S++; (5) lines of
S++. Lines of group (1) likely originate in molecular material subject to a
slow, nondissociative shock that is driven by the overpressure within the
supernova remnant, and lines in groups (3) - (5) are associated primarily with
dissociative shock fronts with a range of (larger) shock velocities. The H2
S(0) line shows a low-density diffuse emission component, and - in some sources
- a shock-excited component.Comment: 43 pages, including 21 figures. Accepted for publication in Ap
SWAS observations of comet 9P/Tempel 1 and Deep Impact
On 4 July 2005 at 1:52 UT the Deep Impact mission successfully completed its
goal to hit the nucleus of 9P/Tempel 1 with an impactor, forming a crater on
the nucleus and ejecting material into the coma of the comet. The 370 kg
impactor collided with the sunlit side of the nucleus with a relative velocity
of 10.2 km/s. NASA's Submillimeter Wave Astronomy Satellite (SWAS) observed the
1(10)-1(01) ortho-water ground-state rotational transition in comet 9P/Tempel 1
before, during, and after the impact. No excess emission from the impact was
detected by SWAS. However, the water production rate of the comet showed large
natural variations of more than a factor of three during the weeks before the
impact.Comment: to appear in the proceedings of the IAU Symposium No. 231:
"Astrochemistry - Recent Successes and Current Callenges". Typo corrected in
author affiliation lis
Distribution of Water Vapor in Molecular Clouds
We report the results of a large-area study of water vapor along the Orion
Molecular Cloud ridge, the purpose of which was to determine the
depth-dependent distribution of gas-phase water in dense molecular clouds. We
find that the water vapor measured toward 77 spatial positions along the
face-on Orion ridge, excluding positions surrounding the outflow associated
with BN/KL and IRc2, display integrated intensities that correlate strongly
with known cloud surface tracers such as CN, C2H, 13CO J =5-4, and HCN, and
less well with the volume tracer N2H+. Moreover, at total column densities
corresponding to Av < 15 mag., the ratio of H2O to C18O integrated intensities
shows a clear rise approaching the cloud surface. We show that this behavior
cannot be accounted for by either optical depth or excitation effects, but
suggests that gas-phase water abundances fall at large Av. These results are
important as they affect measures of the true water-vapor abundance in
molecular clouds by highlighting the limitations of comparing measured water
vapor column densities with such traditional cloud tracers as 13CO or C18O.
These results also support cloud models that incorporate freeze-out of
molecules as a critical component in determining the depth-dependent abundance
of water vapor
Herschel/HIFI Spectral Mapping of C, CH, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH Formation
The CH ion is a key species in the initial steps of interstellar carbon
chemistry. Its formation in diverse environments where it is observed is not
well understood, however, because the main production pathway is so endothermic
(4280 K) that it is unlikely to proceed at the typical temperatures of
molecular clouds. We investigation CH formation with the first
velocity-resolved spectral mapping of the CH rotational
transitions, three sets of CH -doubled triplet lines, C and
C, and CHOH 835~GHz E-symmetry Q branch transitions, obtained
with Herschel/HIFI over 12 arcmin centered on the Orion BN/KL
source. We present the spatial morphologies and kinematics, cloud boundary
conditions, excitation temperatures, column densities, and C optical
depths. Emission from C, CH, and CH is indicated to arise in the
diluted gas, outside of the explosive, dense BN/KL outflow. Our models show
that UV-irradiation provides favorable conditions for steady-state production
of CH in this environment. Surprisingly, no spatial or kinematic
correspondences of these species are found with H S(1) emission tracing
shocked gas in the outflow. We propose that C is being consumed by rapid
production of CO to explain the lack of C and CH in the outflow, and
that fluorescence provides the reservoir of H excited to higher
ro-vibrational and rotational levels. Hence, in star-forming environments
containing sources of shocks and strong UV radiation, a description of CH
formation and excitation conditions is incomplete without including the
important --- possibly dominant --- role of UV irradiation.Comment: Accepted for publication in The Astrophysical Journa
Ultraluminous infrared galaxies: mergers of sub-L* galaxies?
A sample of 27 low-redshift, mostly cool, ultraluminous infrared galaxies (ULIRGs) has been imaged at 1.6 μm with the Hubble Space Telescope (HST) Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). The majority (67%) of the sample's galaxies are multiple-nucleus galaxies with projected separations of up to 17 kpc, and the rest of the sample (33%) are single-nucleus galaxies, as determined by the NICMOS angular resolution limit. The average observed, integrated (host+nucleus) H magnitude of our HST H sample ULIRGs is -24.3, slightly above that of an L* galaxy (MH = -24.2), and 52% of the sample's galaxies have sub-L* luminosities. The ULIRGs in the HST H sample are not generated as a result of the merging of two luminous (i.e., ≥L*) spiral galaxies. Instead, the interactions and mergers occur in general between two, or in some cases more, less massive sub-L* (0.3-0.5L*) galaxies.
Only one out of the 49 nuclei identified in the entire HST H sample has the properties of a bright quasar-like nucleus. On average, the brightest nuclei in the HST H sample galaxies (i.e., cool ULIRGs) are 1.2 mag fainter than warm ULIRGs and low-luminosity Bright Quasar Survey quasars (BQS QSOs) and 2.6 mag fainter than high-luminosity BQS QSOs. Since the progenitor galaxies involved in the merger are sub-L* galaxies, the mass of the central black hole in these ULIRGs would be only about (1-2) × 107 M☉, if the bulge-to-black hole mass ratio of nearby galaxies holds for ULIRGs. The estimated mass of the central black hole is similar to that of nearby Seyfert 2 galaxies but at least 1 order of magnitude lower than the massive black holes thought to be located at the center of high-luminosity QSOs. Massive nuclear starbursts with constant star formation rates of 10-40 M☉ yr-1 could contribute significantly to the nuclear H-band flux and are consistent with the observed nuclear H-band magnitudes of the ULIRGs in the HST H sample. An evolutionary merging scenario is proposed for the generation of the different types of ULIRGs and QSOs on the basis of the masses of the progenitors involved in the merging process. According to this scenario, cool ULIRGs would be the end product of the merging of two or more low-mass (0.3L*-0.5L*) disk galaxies. Warm ULIRGs and low-luminosity QSOs would be generated by a merger involving intermediate-mass (0.5 L*) disk galaxies. Under this scenario, warm ULIRGs could still be the dust-enshrouded phases of UV-bright low-luminosity QSOs, but cool ULIRGs, which are most ULIRGs, would not evolve into QSOs
The water abundance behind interstellar shocks: results from /PACS and /IRS observations of HO, CO, and H
We have investigated the water abundance in shock-heated molecular gas,
making use of measurements of far-infrared CO and HO line
emissions in combination with measurements of mid-IR H rotational
emissions. We present far-infrared line spectra obtained with 's PACS
instrument in range spectroscopy mode towards two positions in the protostellar
outflow NGC 2071 and one position each in the supernova remnants W28 and 3C391.
These spectra provide unequivocal detections, at one or more positions, of 12
rotational lines of water, 14 rotational lines of CO, 8 rotational lines of OH
(4 lambda doublets), and 7 fine-structure transitions of atoms or atomic ions.
We first used a simultaneous fit to the CO line fluxes, along with H
rotational line fluxes measured previously by , to constrain the
temperature and density distribution within the emitting gas; and we then
investigated the water abundances implied by the observed HO line fluxes.
The water line fluxes are in acceptable agreement with standard theoretical
models for nondissociative shocks that predict the complete vaporization of
grain mantles in shocks of velocity km/s, behind which the
characteristic gas temperature is K and the HO/CO ratio is 1.2Comment: 42 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Water vapor emission from IRC+10216 and other carbon-rich stars: model predictions and prospects for multitransition observations
We have modeled the emission of H2O rotational lines from the extreme C-rich
star IRC+10216. Our treatment of the excitation of H2O emissions takes into
account the excitation of H2O both through collisions, and through the pumping
of the nu2 and nu3 vibrational states by dust emission and subsequent decay to
the ground state. Regardless of the spatial distribution of the water
molecules, the H2O 1_{10}-1_{01} line at 557 GHz observed by the Submillimeter
Wave Astronomy Satellite (SWAS) is found to be pumped primarily through the
absorption of dust-emitted photons at 6 m in the nu2 band. As noted by
previous authors, the inclusion of radiative pumping lowers the ortho-H2O
abundance required to account for the 557 GHz emission, which is found to be
(0.5-1)x10^{-7} if the presence of H2O is a consequence of vaporization of
orbiting comets or Fischer-Tropsch catalysis. Predictions for other
submillimeter H2O lines that can be observed by the Herschel Space Observatory
(HSO) are reported. Multitransition HSO observations promise to reveal the
spatial distribution of the circumstellar water vapor, discriminating among the
several hypotheses that have been proposed for the origin of the H2O vapor in
the envelope of IRC+10216. We also show that, for observations with HSO, the
H2O 1_{10}-1_{01} 557 GHz line affords the greatest sensitivity in searching
for H2O in other C-rich AGB stars.Comment: 35 pages, 12 figures, to be published in The Astrophysical Journa
Spitzer Observations of CO2 Ice Towards Field Stars in the Taurus Molecular Cloud
We present the first Spitzer Infrared Spectrograph observations of the 15.2
micron bending mode of CO2 ice towards field stars behind a quiescent dark
cloud. CO2 ice is detected towards 2 field stars (Elias 16, Elias 3) and a
single protostar (HL Tau) with anabundance of ~15-20% relative to water ice.
CO2 ice is not detected towards the source with lowest extinction in our
sample, Tamura 17 (A_V = 3.9m). A comparison of the Elias 16 spectrum with
laboratory data demonstrates that the majority of CO2 ice is embedded in a
polar H2O-rich ice component, with ~15% of CO2 residing in an apolar H2O-poor
mantle. This is the first detection of apolar CO2 towards a field star. We find
that the CO2 extinction threshold is A_V = 4m +/- 1m, comparable to the
threshold for water ice, but significantly less than the threshold for CO ice,
the likely precursor of CO2. Our results confirm CO2 ice forms in tandem with
H2O ice along quiescent lines of sight. This argues for CO2 ice formation via a
mechanism similar to that responsible for H2O ice formation, viz. simple
catalytic reactions on grain surfaces.Comment: Accepted by Astrophysical Journal Letter
Detection of Extended Hot Water in the Outflow from NGC 2071
We report the results of spectroscopic mapping observations carried out
toward a ~1 min x 1 min region within the northern lobe of the outflow from NGC
2071 using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope.
These observations covered the 5.2-37 um spectral region and have led to the
detection of a number of ionic, atomic, and molecular lines, including
fine-structure emission of Si+, Fe+, S++, S, the S(0)-S(7) pure rotational
lines of H2, the R(3) and R(4) transitions of HD, and at least 11 transitions
of H2O. In addition, the 6.2, 7.4, 7.6, 7.9, 8.6 and 11.3 um PAH emission bands
were also observed and several transitions of OH were tentatively detected.
Most of the detected line transitions were strong enough to map including, for
the first time, three transitions of hot H2O. We find that: (1) the water
emission is extended; (2) the extended emission is aligned with the outflow;
and, (3) the spatial distribution of the water emission generally follows that
observed for H2. Based on the measured line intensities, we derive an HD
abundance relative to H2 of 1.1-1.8 10^-5 and an H2O number density of 12-2
cm^3. The H2 density in the water-emitting region is not well constrained by
our observations, but is likely between 3 10^4 and 10^6 cm^3, yielding an H2O
abundance relative to H2 of between 2 10^-5 and 6 10^-4. Future observations
planned for the Herschel Space Observatory should greatly improve the density
estimate, and thus our knowledge of the H2O abundance, for the water-emitting
regions reported here. Finally, we note a possible departure from the H2O
ortho-to-para ratio of 3:1 expected for water formed in hot post-shocked gas,
suggesting that a significant fraction of the water vapor we detect may arise
from H2O sputtered from cold dust grains.Comment: 35 pages, 15 figures, 4 tables, accepted for publication in Ap
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