871 research outputs found
ISO Spectroscopy of Young Stellar Objects
Observations of gas-phase and solid-state species toward
young stellar objects (YSOs) with the spectrometers
on board the Infrared Space Observatory
are reviewed. The excitation and abundances of
the atoms and molecules are sensitive to the changing
physical conditions during star-formation. In
the cold outer envelopes around YSOs, interstellar
ices contain a significant fraction of the heavy element
abundances, in particular oxygen. Different ice
phases can be distinguished, and evidence is found for
heating and segregation of the ices in more evolved
objects. The inner warm envelopes around YSOs are
probed through absorption and emission of gas-phase
molecules, including CO, CO_2, CH_4 and H_2O. An
overview of the wealth of observations on gas-phase
H_2O in star-forming regions is presented. Gas/solid
ratios are determined, which provide information on
the importance of gas-grain chemistry and high temperature
gas-phase reactions. The line ratios of molecules
such as H_2, CO and H_2O are powerful probes
to constrain the physical parameters of the gas. Together
with atomic and ionic lines such as [0 I]
63 µm, [S I] 25 µm and (Si II] 35 µm, they can also
be used to distinguish between photon- and shock-heated
gas. Finally, spectroscopic data on circumstellar
disks around young stars are mentioned. The
results are discussed in the context of the physical
and chemical evolution of YSOs
A Search for Hydroxylamine (NH_2OH) toward Select Astronomical Sources
Observations of 14 rotational transitions of hydroxylamine (NH_2OH) using the NRAO 12 m telescope on Kitt Peak
are reported toward IRC+10216, Orion KL, Orion S, Sgr B2(N), Sgr B2(OH), W3IRS5, and W51M. Although
recent models suggest the presence of NH_2OH in high abundance, these observations resulted in non-detection.
Upper limits are calculated to be as much as six orders of magnitude lower than those predicted by models. Possible
explanations for the lower-than-expected abundance are explored
A Search for Hydroxylamine (NH2OH) toward Select Astronomical Sources
Observations of 14 rotational transitions of hydroxylamine (NH2OH) using the
NRAO 12 m Telescope on Kitt Peak are reported towards IRC+10216, Orion KL,
Orion S, Sgr B2(N), Sgr B2(OH), W3IRS5, and W51M. Although recent models
suggest the presence of NH2OH in high abundance, these observations resulted in
non-detection. Upper limits are calculated to be as much as six orders of
magnitude lower than predicted by models. Possible explanations for the lower
than expected abundance are explored.Comment: 18 pages, 3 figures, 3 table
Distribution of HNCO 5 in Massive Star-forming Regions
The goal of this paper is to study the spatial distribution of HNCO in
massive star-forming regions, and investigate its spatial association with
infrared sources, as well as physical conditions in region of HNCO emission. We
have mapped nine massive star-forming regions in HNCO 5 with the
Purple Mountain Observatory 13.7m telescope. The C18O maps of these sources
were obtained simultaneously. The HNCO emission shows compact distribution,
with emission peak centred on water masers. Nearly all the HNCO clumps show
signs of embedded mid-infrared or far-infrared sources. The FWHM sizes of HNCO
clumps are significantly smaller than C18O clumps but rather similar to HC3N
clumps. We also found good correlation between the integrated intensities,
linewidths and LSR velocities of HNCO and HC3N emission, implying similar
excitation mechanism of these two species. As such, collisional excitation is
likely to be the dominant excitation mechanism for HNCO 5
emission in galactic massive star-forming regions.Comment: 9 pages, 4 figures, accepted by A&
Infrared Observations of Hot Gas and Cold Ice toward the Low Mass Protostar Elias 29
We have obtained the full 1-200 um spectrum of the low luminosity (36 Lsun)
Class I protostar Elias 29 in the Rho Ophiuchi molecular cloud. It provides a
unique opportunity to study the origin and evolution of interstellar ice and
the interrelationship of interstellar ice and hot core gases around low mass
protostars. We see abundant hot CO and H2O gas, as well as the absorption bands
of CO, CO2, H2O and ``6.85 um'' ices. We compare the abundances and physical
conditions of the gas and ices toward Elias 29 with the conditions around
several well studied luminous, high mass protostars. The high gas temperature
and gas/solid ratios resemble those of relatively evolved high mass objects
(e.g. GL 2591). However, none of the ice band profiles shows evidence for
significant thermal processing, and in this respect Elias 29 resembles the
least evolved luminous protostars, such as NGC 7538 : IRS9. Thus we conclude
that the heating of the envelope of the low mass object Elias 29 is
qualitatively different from that of high mass protostars. This is possibly
related to a different density gradient of the envelope or shielding of the
ices in a circumstellar disk. This result is important for our understanding of
the evolution of interstellar ices, and their relation to cometary ices.Comment: 18 pages and 14 figures, accepted for publication in A&
The near-infrared reflected spectrum of source I in Orion-KL
Source I in the Orion-KL nebula is believed to be the nearest example of a
massive star still in the main accretion phase. It is thus one of the best
cases for studying the properties of massive protostars to constrain high-mass
star formation theories. Near-infrared radiation from source I escapes through
the cavity opened by the OMC1 outflow and is scattered by dust towards our line
of sight. The reflected spectrum offers a unique possibility of observing the
emission from the innermost regions of the system and probing the nature of
source I and its immediate surroundings. We obtained moderately high
spectral-resolution (R~9000) observations of the near-infrared diffuse emission
in several locations around source I/Orion-KL. We observed a widespread rich
absorption line spectrum that we compare with cool stellar photospheres and
protostellar accretion disk models. The spectrum is broadly similar to strongly
veiled, cool, low-gravity stellar photospheres in the range Teff~3500-4500 K,
luminosity class I-III. An exact match explaining all features has not been
found, and a plausible explanation is that a range of different temperatures
contribute to the observed absorption spectrum. The 1D velocity dispersions
implied by the absorption spectra, sigma~30 km/s, can be explained by the
emission from a disk around a massive, mstar~10 Msun, protostar that is
accreting at a high rate, mdot~3x10^{-3} Msun/yr. Our observations suggest that
the near-infrared reflection spectrum observed in the Orion-KL region is
produced close to source I and scattered to our line of sight in the OMC1
outflow cavity. The spectrum allows us to exclude source I being a very large,
massive protostar rotating at breakup speed. We suggest that the absorption
spectrum is produced in a disk surrounding a ~10 Msun protostar, accreting from
its disk at a high rate of a few 10^{-3} Msun/yr.Comment: Accepted for publication on A&
Physical and chemical differentiation of the luminous star-forming region W49A - Results from the JCMT Spectral Legacy Survey
The massive and luminous star-forming region W49A is a well known Galactic
candidate to probe the physical conditions and chemistry similar to those
expected in external starburst galaxies. We aim to probe the physical and
chemical structure of W49A on a spatial scale of ~0.8 pc based on the JCMT
Spectral Legacy Survey, which covers the frequency range between 330 and 373
GHz. The wide 2x2 arcminutes field and the high spectral resolution of the HARP
instrument on JCMT provides information on the spatial structure and kinematics
of the cloud. For species where multiple transitions are available, we estimate
excitation temperatures and column densities. We detected 255 transitions
corresponding to 60 species in the 330-373 GHz range at the center position of
W49A. Excitation conditions can be probed for 16 molecules. The chemical
composition suggests the importance of shock-, PDR-, and hot core chemistry.
Many molecular lines show a significant spatial extent across the maps
including high density tracers (e.g. HCN, HNC, CS, HCO+) and tracers of
UV-irradiation (e.g. CN and C2H). Large variations are seen between the
sub-regions with mostly blue-shifted emission toward the Eastern tail, mostly
red-shifted emission toward the Northern clump, and emission peaking around the
expected source velocity toward the South-west clump. A comparison of column
density ratios of characteristic species observed toward W49A to Galactic PDRs
suggests that while the chemistry toward the W49A center is driven by a
combination of UV-irradiation and shocks, UV-irradiation dominates for the
Northern Clump, Eastern tail, and South-west clump regions. A comparison to a
starburst galaxy and an AGN suggests similar C2H, CN, and H2CO abundances (with
respect to the dense gas tracer 34CS) between the ~0.8 pc scale probed for W49A
and the >1 kpc regions in external galaxies with global star-formation.Comment: Proposed for acceptance in A&A, abstract abridge
Emission from Water Vapor and Absorption from Other Gases at 5-7.5 Microns in Spitzer-IRS Spectra of Protoplanetary Disks
We present spectra of 13 T Tauri stars in the Taurus-Auriga star-forming
region showing emission in Spitzer Space Telescope Infrared Spectrograph (IRS)
5-7.5 micron spectra from water vapor and absorption from other gases in these
stars' protoplanetary disks. Seven stars' spectra show an emission feature at
6.6 microns due to the nu_2 = 1-0 bending mode of water vapor, with the shape
of the spectrum suggesting water vapor temperatures > 500 K, though some of
these spectra also show indications of an absorption band, likely from another
molecule. This water vapor emission contrasts with the absorption from warm
water vapor seen in the spectrum of the FU Orionis star V1057 Cyg. The other
six of the thirteen stars have spectra showing a strong absorption band,
peaking in strength at 5.6-5.7 microns, which for some is consistent with
gaseous formaldehyde (H2CO) and for others is consistent with gaseous formic
acid (HCOOH). There are indications that some of these six stars may also have
weak water vapor emission. Modeling of these stars' spectra suggests these
gases are present in the inner few AU of their host disks, consistent with
recent studies of infrared spectra showing gas in protoplanetary disks.Comment: 33 pages, 9 figures, to appear in the 20 August, 2014, V791 - 2 issue
of the Astrophysical Journa
The first results from the Herschel-HIFI mission
This paper contains a summary of the results from the first years of
observations with the HIFI instrument onboard ESA's Herschel space observatory.
The paper starts by outlining the goals and possibilities of far-infrared and
submillimeter astronomy, the limitations of the Earth's atmosphere, and the
scientific scope of the Herschel-HIFI mission. The presentation of science
results from the mission follows the life cycle of gas in galaxies as grouped
into five themes: Structure of the interstellar medium, First steps in
interstellar chemistry, Formation of stars and planets, Solar system results
and Evolved stellar envelopes. The HIFI observations paint a picture where the
interstellar medium in galaxies has a mixed, rather than a layered structure;
the same conclusion may hold for protoplanetary disks. In addition, the HIFI
data show that exchange of matter between comets and asteroids with planets and
moons plays a large role. The paper concludes with an outlook to future
instrumentation in the far-infrared and submillimeter wavelength ranges.Comment: Invited review paper to appear in Advances of Space Research; 32
pages, 9 figures. See http://www.sron.rug.nl/~vdtak/adspres.pdf for a 14-page
journal-style versio
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