184 research outputs found
Discovery of Interstellar Hydrogen Fluoride
We report the first detection of interstellar hydrogen fluoride. Using the
Long Wavelength Spectrometer (LWS) of the Infrared Space Observatory (ISO), we
have detected the 121.6973 micron J = 2 - 1 line of HF in absorption toward the
far-infrared continuum source Sagittarius B2. The detection is statistically
significant at the 13 sigma level. On the basis of our model for the excitation
of HF in Sgr B2, the observed line equivalent width of 1.0 nm implies a
hydrogen fluoride abundance of 3E-10 relative to H2. If the elemental abundance
of fluorine in Sgr B2 is the same as that in the solar system, then HF accounts
for ~ 2% of the total number of fluorine nuclei. We expect hydrogen fluoride to
be the dominant reservoir of gas-phase fluorine in Sgr B2, because it is formed
rapidly in exothermic reactions of atomic fluorine with either water or
molecular hydrogen; thus the measured HF abundance suggests a substantial
depletion of fluorine onto dust grains. Similar conclusions regarding depletion
have previously been reached for the case of chlorine in dense interstellar
clouds. We also find evidence at a lower level of statistical significance (~ 5
sigma) for an emission feature at the expected position of the 4(3,2)-4(2,3)
121.7219 micron line of water. The emission line equivalent width of 0.5 nm for
the water feature is consistent with the water abundance of 5E-6 relative to H2
that has been inferred previously from observations of the hot core of Sgr B2.Comment: 11 pages (AASTeX using aaspp4.sty) plus 2 figures; to appear in ApJ
Letter
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Detection of [O I] 63 <i>μ</i>m in absorption toward Sgr B2
A high signal-to-noise 52-90 μm spectrum is presented for the central part of the Sagittarius B2 complex. The data were obtained with the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The [O I] 63 μm line is detected in absorption even at the grating spectral resolution of 0.29 μm. A lower limit for the column density of atomic oxygen of the order of 1019 cm-2 is derived. This implies that more than 40% of the interstellar oxygen must be in atomic form along the line of sight toward the Sgr B2 molecular cloud
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Thermal H<sub>2</sub>O emission from the Herbig-Haro flow HH 54
The first detection of thermal water emission from a Herbig-Haro object is presented. The observations were performed with the LWS (Long Wavelength Spectrograph) aboard ISO (Infrared Space Observatory). Besides H2O, rotational lines of CO are present in the spectrum of HH 54. These high-J CO lines are used to derive the physical model parameters of the FIR (far-infrared) molecular line emitting regions. This model fits simultaneously the observed OH and H2O spectra for an OH abundance X(OH)=10-6 and a water vapour abundance X(H2O)=10-5.
At a distance of 250pc, the total CO, OH and H2O rotational line cooling rate is estimated to be 1.3x10-2 L⊙, which is comparable to the mechanical luminosity generated by the 10km s-1 shocks, suggesting that practically all of the cooling of the weak-shock regions is done by these three molecular species alone
Oxygen-rich dust production in IC 10
We report the detection of oxygen-rich circumstellar envelopes in stars of
the nearby (700 kpc) starburst galaxy IC 10. The star formation history and the
chemical environment of this galaxy makes it an ideal target to observe dust
production by high-mass stars in a low-metallicity environment. The goal of
this study is to identify oxygen-rich stars in IC 10 and to constrain their
nature between asymptotic giant branch stars (AGBs), red supergiants (RSGs),
and other infrared bright sources. We examine the mass-loss rate of the stars
and compare to results obtained for the Magellanic Clouds. Our objectives are
to (1) assess whether RSGs can be significant dust producers in IC 10, and (2),
solve the discrepancy between the star formation history of IC 10 and the
relatively low number of RSGs detected in the optical. We search for silicate
dust in emission by using the spectral map observed with the Infrared
Spectrograph on board the Spitzer Space Telescope. The optical (UBVRI) and
infrared (JHK, Spitzer/IRAC and Spitzer/MIPS) photometry is used to assert the
membership of the stars to IC 10 and disentangle between AGBs and RSGs.
Radiative models are used to infer mass-loss rates and stellar luminosities.
The luminosity and colors of at least 9 silicate emission sources are
consistent with stars within IC 10. Furthermore, the photometry of 2 of these
sources is consistent with RSGs. We derive dust mass-loss rates similar to the
values found in the Magellanic Clouds. Accounting for the sample completeness,
RSGs are not important contributors to the dust mass budget in IC 10.Comment: Accepted for publication in A&
Stellar Populations and Mass-Loss in M15: A Spitzer Detection of Dust in the Intra-Cluster Medium
We present Spitzer Space Telescope IRAC and MIPS observations of the galactic
globular cluster M15 (NGC 7078), one of the most metal-poor clusters with a
[Fe/H] = -2.4. Our Spitzer images reveal a population of dusty red giants near
the cluster center, a previously detected planetary nebula (PN) designated
K648, and a possible detection of the intra-cluster medium (ICM) arising from
mass loss episodes from the evolved stellar population. Our analysis suggests 9
(+/-2) x 10^-4 solar masses of dust is present in the core of M15, and this
material has accumulated over a period of approximately 10^6 years, a timescale
ten times shorter than the last galactic plane crossing event. We also present
Spitzer IRS follow up observations of K648, including the detection of the
[NeII] 12.81 micron line, and discuss abundances derived from infrared fine
structure lines.Comment: Accepted for publication in AJ. 20 pages, 10 figures, 6 tables. Full
resolution versions of figures 1, 5, 7, and 8 are available in a PDF version
of this manuscript at http://ir.astro.umn.edu/~mboyer/ms_060906.pd
A cAMP-binding ectoprotein in the yeast Saccharomyces cerevisiae
tides 10, 593-595
The optically bright post-AGB population of the LMC
The detected variety in chemistry and circumstellar shell morphology of the
limited sample of Galactic post-AGB stars is so large that there is no
consensus yet on how the different objects are linked by evolutionary channels.
The evaluation is complicated by the fact that their distances and hence
luminosities remain largely unknown. Via cross-correlation of the Spitzer SAGE
catalogue with optical catalogues we selected a sample of LMC post-AGB
candidates based on their [8]-[24] colour index and estimated luminosity. We
determined the fundamental properties of the central stars of 105 of these
objects using low-resolution, optical spectra that we obtained at Siding Spring
Observatory and SAAO, and constructed a catalogue of 70 high probability and
1337 candidate post-AGB stars that is available at the CDS. The sample forms an
ideal testbed for stellar evolution theory predictions of the final phase of
low- and intermediate-mass stars, because the distance and hence luminosity and
also the current and initial mass of these objects is well constrained. About
half of the objects in our sample of post-AGB candidates show a spectral energy
distribution (SED) that is indicative of a disc rather than an expanding and
cooling AGB remnant. Like in the Galaxy, the disc sources are likely associated
with binary evolution. Important side products of this research are catalogues
of candidate young stellar objects, candidate supergiants with circumstellar
dust, and discarded objects for which a spectrum was obtained. These too are
available at the CDS
The Kr85 s-process Branching and the Mass of Carbon Stars
We present new spectroscopic observations for a sample of C(N)-type red
giants. These objects belong to the class of Asymptotic Giant Branch stars,
experiencing thermal instabilities in the He-burning shell (thermal pulses).
Mixing episodes called third dredge-up enrich the photosphere with newly
synthesized C12 in the He-rich zone, and this is the source of the high
observed ratio between carbon and oxygen (C/O > 1 by number). Our spectroscopic
abundance estimates confirm that, in agreement with the general understanding
of the late evolutionary stages of low and intermediate mass stars, carbon
enrichment is accompanied by the appearance of s-process elements in the
photosphere. We discuss the details of the observations and of the derived
abundances, focusing in particular on rubidium, a neutron-density sensitive
element, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The
critical reaction branching at Kr85, which determines the relative enrichment
of the studied species, is discussed. Subsequently, we compare our data with
recent models for s-processing in Thermally Pulsing Asymptotic Giant Branch
stars, at metallicities relevant for our sample. A remarkable agreement between
model predictions and observations is found. Thanks to the different neutron
density prevailing in low and intermediate mass stars, comparison with the
models allows us to conclude that most C(N) stars are of low mass (M < 3Mo). We
also analyze the C12/C13 ratios measured, showing that most of them cannot be
explained by canonical stellar models. We discuss how this fact would require
the operation of an ad hoc additional mixing, currently called Cool Bottom
Process, operating only in low mass stars during the first ascent of the red
giant branch and, perhaps, also during the asymptotic giant branch.Comment: 54 pages + 6 figures + 6 tables. ApJ accepte
Luminosities and mass-loss rates of SMC and LMC AGB stars and Red Supergiants
(Abridged) Dust radiative transfer models are presented for 101 carbon stars
and 86 oxygen-rich evolved stars in the Magellanic Clouds for which 5-35 \mum\
{\it Spitzer} IRS spectra are available. The spectra are complemented with
available optical and infrared photometry to construct the spectral energy
distribution. A minimisation procedure is used to fit luminosity, mass-loss
rate and dust temperature at the inner radius. Different effective temperatures
and dust content are also considered. Periods from the literature and from new
OGLE-III data are compiled and derived. The O-rich stars are classified in
foreground objects, AGB stars and Red Super Giants.
For the O-rich stars silicates based on laboratory optical constants are
compared to "astronomical silicates". Overall, the grain type by Volk & Kwok
(1988) fit the data best. However, the fit based on laboratory optical
constants for the grains can be improved by abandoning the small-particle
limit. The influence of grain size, core-mantle grains and porosity are
explored.
Relations between mass-loss rates and luminosity and pulsation period are
presented and compared to the predictions of evolutionary models, those by
Vassiliadis & Wood (1993) and their adopted mass-loss recipe, and those based
on a Reimers mass-loss law with a scaling of a factor of five. The Vassiliadis
& Wood models describe the data better, although there are also some
deficiencies, in particular to the maximum adopted mass-loss rate.
The OGLE-III data reveal an O-rich star in the SMC with a period of 1749
days. Its absolute magnitude of makes it a good candidate
for a super-AGB star.Comment: A&A accepte
Extended fine structure and continuum emission from S140/L1204
Grating spectra, covering the wavelength range 45 to 187μm have been taken with the ISO Long Wavelength Spectrometer (LWS) at a series of pointing positions over the S 140 region, centred on the cluster of embedded young stellar objects at the south-west corner of the L1204 molecular cloud. Extended emission from [CII]158μm and [OI]63μm is seen, peaking near the position of the embedded stars. The measurements of the fine structure lines are interpreted in terms of PDR models for the emission, as well as the underlying thermal continuum for the heated gas and dust
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