365 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
Very Large Array Observations of Ammonia in Infrared-dark Clouds. I. Column Density and Temperature Structure
We present Very Large Array observations of NH 3 (1,1) and (2,2) in a sample of six infrared-dark clouds (IRDCs) with distances from 2 to 5 kpc. We find that ammonia serves as an excellent tracer of dense gas in IRDCs, showing no evidence of depletion, and the average abundance in these clouds is 8.1 _ 10 –7 . Our sample consists of four IRDCs with 24 _m embedded protostars and two that appear starless. We calculate the kinetic temperature of the gas in IRDCs and find no significant difference between starless and star-forming IRDCs. We find that the bulk of the gas is between 8 and 13 K, indicating that any embedded or nearby stars or clusters do not affect the gas temperature dramatically. Though IRDCs have temperatures and volume densities on par with local star formation regions of lower mass, they consist of much more mass which induces very high internal pressures. In order for IRDCs to survive as coherent structures, the internal pressure must be balanced by a confining pressure provided by the high concentration of molecular clouds in the spiral arm in which they reside. The high molecular concentration and pressure are roughly consistent with gas dynamics of a bar galaxy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90758/1/0004-637X_736_2_163.pd
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
Water, O2 and Ice in Molecular Clouds
We model the temperature and chemical structure of molecular clouds as a
function of depth into the cloud, assuming a cloud of constant density n
illuminated by an external FUV (6 eV < E < 13.6 eV) flux G_0 (scaling factor in
multiples of the local interstellar field). Extending previous
photodissociation region models, we include the freezing of species, simple
grain surface chemistry, and desorption (including FUV photodesorption) of
ices. We also treat the opaque cloud interior with time-dependent chemistry.
Here, under certain conditions, gas phase elemental oxygen freezes out as water
ice and the elemental C/O abundance ratio can exceed unity, leading to complex
carbon chemistry. Gas phase H2O and O2 peak in abundance at intermediate depth
into the cloud, roughly A_V~3-8 from the surface, the depth proportional to
ln(G_0/n). Closer to the surface, molecules are photodissociated. Deeper into
the cloud, molecules freeze to grain surfaces. At intermediate depths
photodissociation rates are attenuated by dust extinction, but photodesorption
prevents total freezeout. For G_0 < 500, abundances of H2O and O2 peak at
values ~10^(-7), producing columns ~10^(15) per cm^2, independent of G_0 and n.
The peak abundances depend primarily on the product of the photodesorption
yield of water ice and the grain surface area per H nucleus. At higher values
of G_0, thermal desorption of O atoms from grains enhances the gas phase H2O
peak abundance and column slightly, whereas the gas phase O2 peak abundance
rises to ~10^(-5) and the column to ~2x10^(16) per cm^2. We present simple
analytic equations for the abundances as a function of depth which clarify the
dependence on parameters. The models are applied to observations of H2O, O2,
and water ice in a number of sources, including B68, NGC 2024, and Rho Oph.Comment: 70 pages including 17 figure
Disk Imaging Survey of Chemistry with SMA (DISCS): I. Taurus Protoplanetary Disk Data
Chemistry plays an important role in the structure and evolution of
protoplanetary disks, with implications for the composition of comets and
planets. This is the first of a series of papers based on data from DISCS, a
Submillimeter Array survey of the chemical composition of protoplanetary disks.
The six Taurus sources in the program (DM Tau, AA Tau, LkCa 15, GM Aur, CQ Tau
and MWC 480) range in stellar spectral type from M1 to A4 and offer an
opportunity to test the effects of stellar luminosity on the disk chemistry.
The disks were observed in 10 different lines at ~3" resolution and an rms of
~100 mJy beam-1 at ~0.5 km s-1. The four brightest lines are CO 2-1, HCO+ 3-2,
CN 2_3-1_2 and HCN 3-2 and these are detected toward all sources (except for
HCN toward CQ Tau). The weaker lines of CN 2_2-1_1, DCO+ 3-2, N2H+ 3-2, H2CO
3_03-2_02 and 4_14-3_13 are detected toward two to three disks each, and DCN
3-2 only toward LkCa 15. CH3OH 4_21-3_12 and c-C3H2 are not detected. There is
no obvious difference between the T Tauri and Herbig Ae sources with regard to
CN and HCN intensities. In contrast, DCO+, DCN, N2H+ and H2CO are detected only
toward the T Tauri stars, suggesting that the disks around Herbig Ae stars lack
cold regions for long enough timescales to allow for efficient deuterium
chemistry, CO freeze-out, and grain chemistry.Comment: 29 pages, 4 figures, accepted for publication in Ap
Antiproteases as Therapeutics to Target Inflammation in Chronic Obstructive Pulmonary Disease
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
Disk Imaging Survey of Chemistry with SMA: II. Southern Sky Protoplanetary Disk Data and Full Sample Statistics
This is the second in a series of papers based on data from DISCS, a
Submillimeter Array observing program aimed at spatially and spectrally
resolving the chemical composition of 12 protoplanetary disks. We present data
on six Southern sky sources - IM Lup, SAO 206462 (HD 135344b), HD 142527, AS
209, AS 205 and V4046 Sgr - which complement the six sources in the Taurus star
forming region reported previously. CO 2-1 and HCO+ 3-2 emission are detected
and resolved in all disks and show velocity patterns consistent with Keplerian
rotation. Where detected, the emission from DCO+ 3-2, N2H+ 3-2, H2CO 3-2 and
4-3,HCN 3-2 and CN 2-1 are also generally spatially resolved. The detection
rates are highest toward the M and K stars, while the F star SAO 206462 has
only weak CN and HCN emission, and H2CO alone is detected toward HD 142527.
These findings together with the statistics from the previous Taurus disks,
support the hypothesis that high detection rates of many small molecules depend
on the presence of a cold and protected disk midplane, which is less common
around F and A stars compared to M and K stars. Disk-averaged variations in the
proposed radiation tracer CN/HCN are found to be small, despite two orders of
magnitude range of spectral types and accretion rates. In contrast, the
resolved images suggest that the CN/HCN emission ratio varies with disk radius
in at least two of the systems. There are no clear observational differences in
the disk chemistry between the classical/full T Tauri disks and transitional
disks. Furthermore, the observed line emission does not depend on measured
accretion luminosities or the number of infrared lines detected, which suggests
that the chemistry outside of 100 AU is not coupled to the physical processes
that drive the chemistry in the innermost few AU.Comment: accepted for publication in ApJ, 41 pages including 7 figure
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
- …