42 research outputs found
Detection of abundant solid CO in the disk around CRBR 2422.8-3423
We present direct evidence for CO freeze-out in a circumstellar disk around the edge-on class I object CRBR 2422.8-3423, observed in the M band with VLT-ISAAC at a resolving power R~10,000. The spectrum shows strong solid CO absorption, with a lower limit on the column density of 2.2E18 cm-2. The solid CO column is the highest observed so far, including high-mass protostars and background field stars. Absorption by foreground cloud material likely accounts for less than 10% percent of the total solid CO, based on the weakness of solid CO absorption toward nearby sources and the absence of gaseous C18O J=2-1 emission 30'' south. Gas-phase ro-vibrational CO absorption lines are also detected with a mean temperature of 50 +/-10 K. The average gas/solid CO ratio is ~1 along the line of sight. For an estimated inclination of 20 +/- 5 degree, the solid CO absorption originates mostly in the cold, shielded outer part of the flaring disk, consistent with the predominance of apolar solid CO in the spectrum and the non-detection of solid OCN-, a thermal/ultraviolet processing of the ice mantle. The gaseous CO comes from the warm upper layers closer to the star
Ices in Star-Forming Regions: First Results from VLT-ISAAC
The first results from a VLT-ISAAC program on L- and M-band infrared
spectroscopy of deeply-embedded young stellar objects are presented. The advent
of 8-m class telescopes allows high S/N spectra of low-luminosity sources to be
obtained. In our first observing run, low- and medium-resolution spectra have
been measured toward a dozen objects, mostly in the Vela and Chamaeleon
molecular clouds. The spectra show strong absorption of H2O and CO ice, as well
as weak features at `3.47' and 4.62 mu. No significant solid CH3OH feature at
3.54 mu is found, indicating that the CH3OH/H2O ice abundance is lower than
toward some massive protostars. Various evolutionary diagnostics are
investigated for a set of sources in Vela.Comment: 8 pages, 4 figures, to appear in The Origins of Stars and Planets:
the VLT View, eds. J. Alves, M. McCaughrean (Springer Verlag
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Infrared, UV/VIS and Raman Spectroscopy of Comet Wild-2 Samples Returned by the Stardust Mission
Results from the preliminary examination of Stardust samples obtained using various spectroscopic methods will be presented
Fully Sampled Maps of Ices and Silicates in Front of Cepheus A East with Spitzer
We report the first fully sampled maps of the distribution of interstellar
CO2 ices, H2O ices and total hydrogen nuclei, as inferred from the 9.7 micron
silicate feature, toward the star-forming region Cepheus A East with the IRS
instrument onboard the Spitzer Space Telescope. We find that the column density
distributions for these solid state features all peak at, and are distributed
around, the location of HW2, the protostar believed to power one of the
outflows observed in this star-forming region. A correlation between the column
density distributions of CO2 and water ice with that of total hydrogen
indicates that the solid state features we mapped mostly arise from the same
molecular clumps along the probed sight lines. We therefore derive average CO2
ice and water ice abundances with respect to the total hydrogen column density
of X(CO2)_ice~1.9x10^-5 and X(H2O)_ice~7.5x10^-5. Within errors, the abundances
for both ices are relatively constant over the mapped region exhibiting both
ice absorptions. The fraction of CO2 ice with respect to H2O ice is also
relatively constant at a value of 22% over that mapped region. A clear
triple-peaked structure is seen in the CO2 ice profiles. Fits to those profiles
using current laboratory ice analogs suggest the presence of both a
low-temperature polar ice mixture and a high-temperature methanol-rich ice
mixture along the probed sightlines. Our results further indicate that thermal
processing of these ices occurred throughout the sampled region.Comment: 26 pages, 8 figures, accepted for publication in Ap
The c2d Spitzer spectroscopic survey of ices around low-mass young stellar objects II: CO2
This paper presents Spitzer-IRS spectroscopy of the CO2 15.2 micron bending
mode toward a sample of 50 embedded low-mass stars in nearby star-forming
clouds, taken mostly from the ``Cores to Disks (c2d)'' Legacy program. The
average abundance of solid CO2 relative to water in low-mass protostellar
envelopes is 0.32 +/- 0.02, significantly higher than that found in quiescent
molecular clouds and in massive star forming regions. It is found that a
decomposition of all the observed CO2 bending mode profiles requires a minimum
of five unique components. Roughly 2/3 of the CO2 ice is found in a water-rich
environment, while most of the remaining 1/3 is found in a CO environment.
Ground-based observations of solid CO toward a large subset of the c2d sample
are used to further constrain the CO2:CO component and suggest a model in which
low-density clouds form the CO2:H2O component and higher density clouds form
the CO2:CO ice during and after the freeze-out of gas-phase CO. It is suggested
that the subsequent evolution of the CO2 and CO profiles toward low-mass
protostars, in particular the appearance of the splitting of the CO2 bending
mode due to pure, crystalline CO2, is first caused by distillation of the
CO2:CO component through evaporation of CO due to thermal processing to ~20-30
K in the inner regions of infalling envelopes. The formation of pure CO2 via
segregation from the H2O rich mantle may contribute to the band splitting at
higher levels of thermal processing (>50 K), but is harder to reconcile with
the physical structure of protostellar envelopes around low-luminosity objects.Comment: Accepted for Ap
Laboratory evidence for efficient water formation in interstellar ices
Even though water is the main constituent in interstellar icy mantles, its
chemical origin is not well understood. Three different formation routes have
been proposed following hydrogenation of O, O2, or O3, but experimental
evidence is largely lacking. We present a solid state astrochemical laboratory
study in which one of these routes is tested. For this purpose O2 ice is
bombarded by H- or D-atoms under ultra high vacuum conditions at astronomically
relevant temperatures ranging from 12 to 28 K. The use of reflection absorption
infrared spectroscopy (RAIRS) permits derivation of reaction rates and shows
efficient formation of H2O (D2O) with a rate that is surprisingly independent
of temperature. This formation route converts O2 into H2O via H2O2 and is found
to be orders of magnitude more efficient than previously assumed. It should
therefore be considered as an important channel for interstellar water ice
formation as illustrated by astrochemical model calculations.Comment: 15 pages, 4 figures. ApJ, in pres
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
Ice chemistry in embedded young stellar objects in the Large Magellanic Cloud
We present spectroscopic observations of a sample of 15 embedded young
stellar objects (YSOs) in the Large Magellanic Cloud (LMC). These observations
were obtained with the Spitzer Infrared Spectrograph (IRS) as part of the
SAGE-Spec Legacy program. We analyze the two prominent ice bands in the IRS
spectral range: the bending mode of CO_2 ice at 15.2 micron and the ice band
between 5 and 7 micron that includes contributions from the bending mode of
water ice at 6 micron amongst other ice species. The 5-7 micron band is
difficult to identify in our LMC sample due to the conspicuous presence of PAH
emission superimposed onto the ice spectra. We identify water ice in the
spectra of two sources; the spectrum of one of those sources also exhibits the
6.8 micron ice feature attributed to ammonium and methanol. We model the CO_2
band in detail, using the combination of laboratory ice profiles available in
the literature. We find that a significant fraction (> 50%) of CO_2 ice is
locked in a water-rich component, consistent with what is observed for Galactic
sources. The majority of the sources in the LMC also require a pure-CO_2
contribution to the ice profile, evidence of thermal processing. There is a
suggestion that CO_2 production might be enhanced in the LMC, but the size of
the available sample precludes firmer conclusions. We place our results in the
context of the star formation environment in the LMC.Comment: Minor corrections to Table 2. Accepted for publication in ApJ, 66
pages, 9 figures (some in color), 4 table
The c2d Spitzer Spectroscopic Survey of Ices Around Low-Mass Young Stellar Objects: I. H2O and the 5-8 um Bands
With the goal to study the physical and chemical evolution of ices in
solar-mass systems, a spectral survey is conducted of a sample of 41 low
luminosity YSOs using 3-38 um Spitzer and ground-based spectra. The long-known
6.0 and 6.85 um bands are detected toward all sources, with the Class 0-type
YSOs showing the deepest bands ever observed. In almost all sources the 6.0 um
band is deeper than expected from the bending mode of pure solid H2O. The depth
and shape variations of the remaining 5-7 um absorption indicate that it
consists of 5 independent components, which, by comparison to laboratory
studies, must be from at least 8 different carriers. Simple species are
responsible for much of the absorption in the 5-7 um region, at abundances of
1-30% for CH3OH, 3-8% for NH3, 1-5% for HCOOH, ~6% for H2CO, and ~0.3% for
HCOO- with respect to solid H2O. The 6.85 um band likely consists of one or two
carriers, of which one is less volatile than H2O because its abundance relative
to H2O is enhanced at lower H2O/tau_9.7 ratios. It does not survive in the
diffuse interstellar medium (ISM), however. The similarity of the 6.85 um bands
for YSOs and background stars indicates that its carrier(s) must be formed
early in the molecular cloud evolution. If an NH4+ salt is the carrier its
abundance with respect to solid H2O is typically 7%, and low temperature
acid-base chemistry or cosmic ray induced reactions must have been involved in
its formation. Possible origins are discussed for the carrier of an enigmatic,
very broad absorption between 5 and 8 um. Finally, all the phenomena observed
for ices toward massive YSOs are also observed toward low mass YSOs, indicating
that processing of the ices by internal ultraviolet radiation fields is a minor
factor in the early chemical evolution of the ices. [abridged]Comment: Accepted for publication in ApJ. 22 pages, 18 b&w figure