337 research outputs found
Spectroscopic observations of ices around embedded young stellar objects in the Large Magellanic Cloud with AKARI
The aim of this study is to understand the chemical conditions of ices around
embedded young stellar objects (YSOs) in the metal-poor Large Magellanic Cloud
(LMC). We performed near-infrared (2.5-5 micron) spectroscopic observations
toward 12 massive embedded YSOs and their candidates in the LMC using the
Infrared Camera (IRC) onboard AKARI. We estimated the column densities of the
H2O, CO2, and CO ices based on their 3.05, 4.27, and 4.67 micron absorption
features, and we investigated the correlation between ice abundances and
physical properties of YSOs.The ice absorption features of H2O, CO2, 13CO2, CO,
CH3OH, and possibly XCN are detected in the spectra. In addition, hydrogen
recombination lines and PAH emission bands are detected toward the majority of
the targets. The derived typical CO2/H2O ice ratio of our samples (~0.36 +-
0.09) is greater than that of Galactic massive YSOs (~0.17 +- 0.03), while the
CO/H2O ice ratio is comparable. It is shown that the CO2 ice abundance does not
correlate with the observed characteristics of YSOs; the strength of hydrogen
recombination line and the total luminosity. Likewise, clear no correlation is
seen between the CO ice abundance and YSO characteristics, but it is suggested
that the CO ice abundance of luminous samples is significantly lower than in
other samples.The systematic difference in the CO2 ice abundance around the
LMC's massive YSOs, which was suggested by previous studies, is confirmed with
the new near-infrared data. We suggest that the strong ultraviolet radiation
field and/or the high dust temperature in the LMC are responsible for the
observed high abundance of the CO2 ice. It is suggested that the internal
stellar radiation does not play an important role in the evolution of the CO2
ice around a massive YSO, while more volatile molecules like CO are susceptible
to the effect of the stellar radiation.Comment: 12 pages, 8 figures, 5 tables, accepted for Astronomy & Astrophysics
journa
High Resolution 4.7 um Keck/NIRSPEC Spectra of Protostars. I: Ices and Infalling Gas in the Disk of L1489 IRS
We explore the infrared M band (4.7 um) spectrum of the class I protostar
L1489 IRS in the Taurus Molecular Cloud. This is the highest resolution wide
coverage spectrum at this wavelength of a low mass protostar observed to date
(R=25,000; Dv=12 km/s). Many narrow absorption lines of gas phase 12CO, 13CO,
and C18O are detected, as well as a prominent band of solid 12CO. The gas phase
12CO lines have red shifted absorption wings (up to 100 km/s), likely
originating from warm disk material falling toward the central object. The
isotopes and the 12CO line wings are successfully fitted with a contracting
disk model of this evolutionary transitional object (Hogerheijde 2001). This
shows that the inward motions seen in millimeter wave emission lines continue
to within ~0.1 AU from the star. The colder parts of the disk are traced by the
prominent CO ice band. The band profile results from CO in 'polar' ices (CO
mixed with H2O), and CO in 'apolar' ices. At the high spectral resolution, the
'apolar' component is, for the first time, resolved into two distinct
components, likely due to pure CO and CO mixed with CO2, O2 and/or N2. The ices
have probably experienced thermal processing in the upper disk layer traced by
our pencil absorption beam: much of the volatile 'apolar' ices has evaporated
and the depletion factor of CO onto grains is remarkably low (~7%). This study
shows that high spectral resolution 4.7 um observations provide important and
unique information on the dynamics and structure of protostellar disks and the
evolution of ices in these disks.Comment: 11 pages, 6 figures Scheduled to appear in ApJ 568 n2, 1 April 200
Formation of hydrogen peroxide and water from the reaction of cold hydrogen atoms with solid oxygen at 10K
The reactions of cold H atoms with solid O2 molecules were investigated at 10
K. The formation of H2O2 and H2O has been confirmed by in-situ infrared
spectroscopy. We found that the reaction proceeds very efficiently and obtained
the effective reaction rates. This is the first clear experimental evidence of
the formation of water molecules under conditions mimicking those found in cold
interstellar molecular clouds. Based on the experimental results, we discuss
the reaction mechanism and astrophysical implications.Comment: 12 pages, 3 Postscript figures, use package amsmath, amssymb,
graphic
NMR relaxation in half-integer antiferromagnetic spin chains
Nuclear relaxation in half-integer spin chains at low temperatures (T << J,
the antiferromagnetic exchange constant) is dominated by dissipation from a gas
of thermally-excited, overdamped, spinons. The universal low temperature
dependence of the relaxation rates and is computed.Comment: 7 pages, 1 uuencoded postscript figure appende
The ERE of the "Red Rectangle" revisited
We present in this paper high signal-to-noise long-slit optical spectra of
the Extended Red Emission (ERE) in the "Red Rectangle" (RR) nebula. These
spectra, obtained at different positions in the nebula, reveal an extremely
complex emission pattern on top of the broad ERE continuum. It is well known
that three features converge at large distance from the central object, in
wavelength and profile to the diffuse interstellar bands (DIBs) at 5797, 5849.8
and 6614 ang., (e.g. Sarre et al., 1995). In this paper we give a detailed
inventory of all spectral subfeatures observed in the 5550--6850 ang. spectral
range. Thanks to our high S/N spectra, we propose 5 new features in the RR that
can be associated with DIBs. For the 5550--6200 ang. spectral range our slit
position was on top of the NE spike of the X shaped nebula. A detailed
description of the spatial profile-changes is given of the strongest features
revealing that even far out in the nebula at 24 arcsec from the central star,
there remains a small shift in wavelength of 1 respectively 2 ang between the
ERE subfeatures and the DIB wavelengths of 5797.11 and 5849.78 ang.Comment: 8 pages, 9 figures accepted by Astronomy and Astrophysic
Chemistry in a gravitationally unstable protoplanetary disc
Until now, axisymmetric, alpha-disc models have been adopted for calculations
of the chemical composition of protoplanetary discs. While this approach is
reasonable for many discs, it is not appropriate when self-gravity is
important. In this case, spiral waves and shocks cause temperature and density
variations that affect the chemistry. We have adopted a dynamical model of a
solar-mass star surrounded by a massive (0.39 Msun), self-gravitating disc,
similar to those that may be found around Class 0 and early Class I protostars,
in a study of disc chemistry. We find that for each of a number of species,
e.g. H2O, adsorption and desorption dominate the changes in the gas-phase
fractional abundance; because the desorption rates are very sensitive to
temperature, maps of the emissions from such species should reveal the
locations of shocks of varying strengths. The gas-phase fractional abundances
of some other species, e.g. CS, are also affected by gas-phase reactions,
particularly in warm shocked regions. We conclude that the dynamics of massive
discs have a strong impact on how they appear when imaged in the emission lines
of various molecular species.Comment: 10 figures and 3 tables, accepted for publication in MNRA
SURFRESIDE2: An ultrahigh vacuum system for the investigation of surface reaction routes of interstellar interest
A new ultrahigh vacuum experiment is described to study atom and radical addition reactions in interstellar ice analogues for astronomically relevant temperatures. The new setup – SURFace REaction SImulation DEvice (SURFRESIDE2) – allows a systematic investigation of solid state pathways resulting in the formation of molecules of astrophysical interest. The implementation of a double beam line makes it possible to expose deposited ice molecules to different atoms and/or radicals sequentially or at the same time. Special efforts are made to perform experiments under fully controlled laboratory conditions, including precise atom flux determinations, in order to characterize reaction channels quantitatively. In this way, we can compare and combine different surface reaction channels with the aim to unravel the solid state processes at play in space. Results are constrained in situ by means of a Fourier transform infrared spectrometer and a quadrupole mass spectrometer using reflection absorption infrared spectroscopy and temperature programmed desorption, respectively. The performance of the new setup is demonstrated on the example of carbon dioxide formation by comparing the efficiency through two different solid state channels (CO + OH → CO_2 + H and CO + O → CO_2) for which different addition products are needed. The potential of SURFRESIDE2 to study complex molecule formation, including nitrogen containing (prebiotic) compounds, is discussed
NMR relaxation rates for the spin-1/2 Heisenberg chain
The spin-lattice relaxation rate and the spin echo decay rate
for the spin- antiferromagnetic Heisenberg chain are
calculated using quantum Monte Carlo and maximum entropy analytic continuation.
The results are compared with recent analytical calculations by Sachdev. If the
nuclear hyperfine form factor is strongly peaked around the
predicted low-temperature behavior [, ] extends up to temperatures as high as . If has significant weight for there are large
contributions from diffusive long-wavelength processes not taken into account
in the theory, and very low temperatures are needed in order to observe the
asymptotic forms.Comment: 9 pages, Revtex 3.0, 5 uuencoded ps figures To appear in Phys. Rev.
B, Rapid Com
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