119 research outputs found
The Thermal Evolution of Ices in the Environments of Newly Formed Stars: The CO_2 Diagnostic
Archival data from the Infrared Spectrometer of the Spitzer Space Telescope are used to study the 15 μm absorption feature of solid CO_2 toward 28 young stellar objects (YSOs) of approximately solar mass. Fits to the absorption profile using laboratory spectra enable categorization according to the degree of thermal processing of the ice matrix that contains the CO_2. The majority of YSOs in our sample (20 out of 28) are found to be consistent with a combination of polar (H_2O-rich) and nonpolar (CO-rich) ices at low temperature; the remainder exhibit profile structure consistent with partial crystallization as the result of significant heating. Ice-phase column densities of CO_2 are determined and compared with those of other species. Lines of sight with crystallization signatures in their spectra are found to be systematically deficient in solid-phase CO, as expected if CO is being sublimated in regions where the ices are heated to crystallization temperatures. Significant variation is found in the CO2 abundance with respect to both H_2O (the dominant ice constituent) and total dust column (quantified by the extinction, AV ). YSOs in our sample display typically higher CO_2 concentrations (independent of evidence for thermal processing) in comparison to quiescent regions of the prototypical cold molecular cloud. This suggests that enhanced CO_2 production is driven by photochemical reactions in proximity to some YSOs, and that photoprocessing and thermal processing may occur independently
Ice and Dust in the Quiescent Medium of Isolated Dense Cores
The relation between ices in the envelopes and disks surrounding YSOs and
those in the quiescent interstellar medium is investigated. For a sample of 31
stars behind isolated dense cores, ground-based and Spitzer spectra and
photometry in the 1-25 um wavelength range are combined. The baseline for the
broad and overlapping ice features is modeled, using calculated spectra of
giants, H2O ice and silicates. The adopted extinction curve is derived
empirically. Its high resolution allows for the separation of continuum and
feature extinction. The extinction between 13-25 um is ~50% relative to that at
2.2 um. The strengths of the 6.0 and 6.85 um absorption bands are in line with
those of YSOs. Thus, their carriers, which, besides H2O and CH3OH, may include
NH4+, HCOOH, H2CO and NH3, are readily formed in the dense core phase, before
stars form. The 3.53 um C-H stretching mode of solid CH3OH was discovered. The
CH3OH/H2O abundance ratios of 5-12% are larger than upper limits in the Taurus
molecular cloud. The initial ice composition, before star formation occurs,
therefore depends on the environment. Signs of thermal and energetic processing
that were found toward some YSOs are absent in the ices toward background
stars. Finally, the peak optical depth of the 9.7 um band of silicates relative
to the continuum extinction at 2.2 um is significantly shallower than in the
diffuse interstellar medium. This extends the results of Chiar et al. (2007) to
a larger sample and higher extinctions.Comment: Accepted for publication in The Astrophysical Journa
Ices in the Quiescent IC 5146 Dense Cloud
This paper presents spectra in the 2 to 20 micron range of quiescent cloud
material located in the IC 5146 cloud complex. The spectra were obtained with
NASA's Infrared Telescope Facility (IRTF) SpeX instrument and the Spitzer Space
Telescope's Infrared Spectrometer. We use these spectra to investigate dust and
ice absorption features in pristine regions of the cloud that are unaltered by
embedded stars. We find that the H2O-ice threshold extinction is 4.03+/-0.05
mag. Once foreground extinction is taken into account, however, the threshold
drops to 3.2 mag, equivalent to that found for the Taurus dark cloud, generally
assumed to be the touchstone quiescent cloud against which all other dense
cloud and embedded young stellar object observations are compared. Substructure
in the trough of the silicate band for two sources is attributed to CH3OH and
NH3 in the ices, present at the ~2% and ~5% levels, respectively, relative to
H2O-ice. The correlation of the silicate feature with the E(J-K) color excess
is found to follow a much shallower slope relative to lines of sight that probe
diffuse clouds, supporting the previous results by Chiar et al. (2007).Comment: 13 pages, 13 figures with multiple parts, accepted for publication in
Astrophysical Journal, Feb. 201
Desorption of CO and O2 interstellar ice analogs
Solid O2 has been proposed as a possible reservoir for oxygen in dense clouds
through freeze-out processes. The aim of this work is to characterize
quantitatively the physical processes that are involved in the desorption
kinetics of CO-O2 ices by interpreting laboratory temperature programmed
desorption (TPD) data. This information is used to simulate the behavior of
CO-O2 ices under astrophysical conditions. The TPD spectra have been recorded
under ultra high vacuum conditions for pure, layered and mixed morphologies for
different thicknesses, temperatures and mixing ratios. An empirical kinetic
model is used to interpret the results and to provide input parameters for
astrophysical models. Binding energies are determined for different ice
morphologies. Independent of the ice morphology, the desorption of O2 is found
to follow 0th-order kinetics. Binding energies and temperature-dependent
sticking probabilities for CO-CO, O2-O2 and CO-O2 are determined. O2 is
slightly less volatile than CO, with binding energies of 912+-15 versus 858+-15
K for pure ices. In mixed and layered ices, CO does not co-desorb with O2 but
its binding energies are slightly increased compared with pure ice whereas
those for O2 are slightly decreased. Lower limits to the sticking probabilities
of CO and O2 are 0.9 and 0.85, respectively, at temperatures below 20K. The
balance between accretion and desorption is studied for O2 and CO in
astrophysically relevant scenarios. Only minor differences are found between
the two species, i.e., both desorb between 16 and 18K in typical environments
around young stars. Thus, clouds with significant abundances of gaseous CO are
unlikely to have large amounts of solid O2.Comment: 8 pages + 2 pages online material, 8 figures (1 online), accepted by
A&
The Relationship between the Optical Depth of the 9.7 micron Silicate Absorption Feature and Infrared Differential Extinction in Dense Clouds
We have examined the relationship between the optical depth of the 9.7 micron
silicate absorption feature (tau_9.7) and the near-infrared color excess,
E(J-Ks) in the Serpens, Taurus, IC 5146, Chameleon I, Barnard 59, and Barnard
68 dense clouds/cores. Our data set, based largely on Spitzer IRS spectra,
spans E(J-Ks)=0.3 to 10 mag (corresponding to visual extinction between about 2
and 60 mag.). All lines of sight show the 9.7 micron silicate feature. Unlike
in the diffuse ISM where a tight linear correlation between the 9.7 micron
silicate feature optical depth and the extinction (Av) is observed, we find
that the silicate feature in dense clouds does not show a monotonic increase
with extinction. Thus, in dense clouds, tau_9.7 is not a good measure of total
dust column density. With few exceptions, the measured tau_9.7 values fall well
below the diffuse ISM correlation line for E(J-Ks) > 2 mag (Av >12 mag). Grain
growth via coagulation is a likely cause of this effect.Comment: 11 pages including 2 figures, 1 table. Accepted for publication in
ApJ Letters, 23 July 200
The 9.7 and 18 um silicate absorption profiles towards diffuse and molecular cloud lines-of-sight
Studying the composition of dust in the interstellar medium (ISM) is crucial
in understanding the cycle of dust in our galaxy. The mid-infrared spectral
signature of amorphous silicates, the most abundant dust species in the ISM, is
studied in different lines-of-sight through the Galactic plane, thus probing
different conditions in the ISM. We have analysed 10 spectra from the Spitzer
archive, of which 6 lines-of-sight probe diffuse interstellar medium material
and 4 probe molecular cloud material. The 9.7 um silicate absorption features
in 7 of these spectra were studied in terms of their shape and strength. In
addition, the shape of the 18 um silicate absorption features in 4 of the
diffuse sightline spectra were analysed. The 9.7 um silicate absorption bands
in the diffuse sightlines show a strikingly similar band shape. This is also
the case for all but one of the 18 um silicate absorption bands observed in
diffuse lines-of-sight. The 9.7 um bands in the 4 molecular sightlines show
small variations in shape. These modest variations in the band shape are
inconsistent with the interpretation of the large variations in
{\tau}_9.7/E(J-K) between diffuse and molecular sightlines in terms of silicate
grain growth. Instead, we suggest that the large changes in {\tau}_9.7 / E(J-K)
must be due to changes in E(J-K).Comment: 14 pages, 19 figures, accepted for publication in A&
High-resolution SOFIA/EXES Spectroscopy of Water Absorption Lines in the Massive Young Binary W3 IRS 5
We present in this paper mid-infrared (5-8~m) spectroscopy toward the
massive young binary W3~IRS~5, using the EXES spectrometer in high-resolution
mode (50,000) from the NASA Stratospheric Observatory for Infrared
Astronomy (SOFIA). Many (180) =1--0 and (90) =2-1
absorption rovibrational transitions are identified. Two hot components over
500 K and one warm component of 190 K are identified through Gaussian fittings
and rotation diagram analysis. Each component is linked to a CO component
identified in the IRTF/iSHELL observations (=88,100) through their kinematic
and temperature characteristics. Revealed by the large scatter in the rotation
diagram, opacity effects are important, and we adopt two curve-of-growth
analyses, resulting in column densities of cm. In one
analysis, the model assumes a foreground slab. The other assumes a
circumstellar disk with an outward-decreasing temperature in the vertical
direction. The disk model is favored because fewer geometry constraints are
needed, although this model faces challenges as the internal heating source is
unknown. We discuss the chemical abundances along the line of sight based on
the CO-to-HO connection. In the hot gas, all oxygen not locked in CO
resides in water. In the cold gas, we observe a substantial shortfall of oxygen
and suggest that the potential carrier could be organics in solid ice.Comment: Accepted for publication in ApJ. 34 pages, 13 figures, and 14 tables.
Comments are more than welcome
The Nature of Carbon Dioxide Bearing Ices in Quiescent Molecular Clouds
The properties of the ices that form in dense molecular clouds represent an
important set of initial conditions in the evolution of interstellar and
preplanetary matter in regions of active star formation. Of the various
spectral features available for study, the bending mode of solid CO2 near 15
microns has proven to be a particularly sensitive probe of physical conditions,
especially temperature. We present new observations of this absorption feature
in the spectrum of Q21-1, a background field star located behind a dark
filament in the Cocoon Nebula (IC5146). We show the profile of the feature be
consistent with a two-component (polar + nonpolar) model for the ices, based on
spectra of laboratory analogs with temperatures in the range 10-20K. The polar
component accounts for 85% of the CO2 in the line of sight. We compare for the
first time 15 micron profiles in three widely separated dark clouds (Taurus,
Serpens and IC5146), and show that they are indistinguishable to within
observational scatter. Systematic differences in the observed CO2/H2O ratio in
the three clouds have little or no effect on the 15 micron profile. The
abundance of elemental oxygen in the ices appears to be a unifying factor,
displaying consistent behavior in the three clouds. We conclude that the ice
formation process is robust and uniformly efficient, notwithstanding
compositional variations arising from differences in how the O is distributed
between the primary species (H2O, CO2 and CO) in the ices
Infrared H2O absorption in massive protostars at high spectral resolution: full spectral survey results of AFGL 2591 and AFGL 2136
Interstellar matter and star formatio
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