151 research outputs found
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Looking at the bright side of the ρ Ophiuchi dark cloud. Far infrared spectrophotometric observations of the ρ Oph cloud with the ISO-LWS
We present far infrared (45-195 μm) spectrophotometric observations with the ISO-LWS of the active star forming ρOph main cloud (L 1688). The [CII] 158 μm and [OI] 63 μm lines were detected at each of the 33 positions observed, whereas the [OI] 145 μm line was clearly seen toward twelve. The principal observational result is that the [CII] 158 μm line fluxes exhibit a clear correlation with projected distance from the dominant stellar source in the field (HD 147889). We interpret this in terms of PDR-type emission from the surface layers of the ρOph cloud. The observed [CII] 158 μm/[OI] 63 μm flux ratios are larger than unity everywhere. A comparison of the [CII] 158 μm line emission and the FIR dust continuum fluxes yields estimates of the efficiency at which the gas in the cloud converts stellar to [CII] 158 μm photons (χCII≳0.5%).
We first develop an empirical model, which provides us with a three dimensional view of the far and bright side of the dark ρOph cloud, showing that the cloud surface towards the putative energy source is concave. This model also yields quantitative estimates of the incident flux of ultraviolet radiation (G0 ~ 101 - 102) and of the degree of clumpiness/texture of the cloud surface (filling of the 80" beam ~0.2).
Subsequently, we use theoretical models of PDRS to derive the particle density, n(H), and the temperature structures, for Tgas and Tdust, in the surface layers of the ρOph cloud. Tgas is relatively low, ~60 K, but higher than Tdust ( ~30 K), and densities are generally found within the interval (1-3) 104 cm-3 . These PDR models are moderately successful in explaining the LWS observations. They correctly predict the [OI] 63 μm and [CII] 158 μm line intensities and the observed absence of any molecular line emission. The models do fail, however, to reproduce the observed small [OI] 63 μm/[OI] 145 μm ratios. We examine several possible explanations, but are unable to uniquely identify (or to disentangle) the cause(s) of this discrepancy.
From pressure equilibrium arguments we infer that the total mass of the ρOph main cloud (2pc2) is ~2 500 M⊙, which implies that the star formation efficiency to date is ≲4%, significantly lower than previous estimates
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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
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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
Calibration of the Herschel SPIRE Fourier Transform Spectrometer
The Herschel SPIRE instrument consists of an imaging photometric camera and
an imaging Fourier Transform Spectrometer (FTS), both operating over a
frequency range of 450-1550 GHz. In this paper, we briefly review the FTS
design, operation, and data reduction, and describe in detail the approach
taken to relative calibration (removal of instrument signatures) and absolute
calibration against standard astronomical sources. The calibration scheme
assumes a spatially extended source and uses the Herschel telescope as primary
calibrator. Conversion from extended to point-source calibration is carried out
using observations of the planet Uranus. The model of the telescope emission is
shown to be accurate to within 6% and repeatable to better than 0.06% and, by
comparison with models of Mars and Neptune, the Uranus model is shown to be
accurate to within 3%. Multiple observations of a number of point-like sources
show that the repeatability of the calibration is better than 1%, if the
effects of the satellite absolute pointing error (APE) are corrected. The
satellite APE leads to a decrement in the derived flux, which can be up to ~10%
(1 sigma) at the high-frequency end of the SPIRE range in the first part of the
mission, and ~4% after Herschel operational day 1011. The lower frequency range
of the SPIRE band is unaffected by this pointing error due to the larger beam
size. Overall, for well-pointed, point-like sources, the absolute flux
calibration is better than 6%, and for extended sources where mapping is
required it is better than 7%.Comment: 20 pages, 18 figures, accepted for publication in MNRA
X-ray Spectroscopy of the Virgo Cluster out to the Virial Radius
We present results from the analysis of a mosaic of thirteen XMM-Newton
pointings covering the Virgo Cluster from its center northwards out to a radius
r~1.2 Mpc (~4.5 degrees), reaching the virial radius and beyond. This is the
first time that the properties of a modestly sized (M_vir~1.4e14 M_sun, kT~2.3
keV), dynamically young cluster have been studied out to the virial radius. The
density profile of the cluster can be described by a surprisingly shallow
power-law with index 1.21+/-0.12. In the radial range of 0.3r_vir<r<r_vir, the
best fit temperature drops by roughly 60 per cent. Within a radius r<450 kpc,
the entropy profile has an approximate power-law form with index 1.1, as
expected for gravitationally collapsed gas in hydrostatic equilibrium. Beyond
r~450 kpc, however, the temperature and metallicity drop abruptly, and the
entropy profile becomes flatter, staying consistently below the expected value
by a factor of 2-2.5. The most likely explanation for the unusually shallow
density profile and the flattening of entropy at large radius is clumping in
the ICM. Our data provide direct observational evidence that the ICM is
enriched by metals all the way to r_200 to at least Z=0.1 Solar.Comment: Accepted for publication in MNRA
Constraints on a Local Group X-ray Halo
A simple model for a hot Local Group halo is constructed, using the standard
beta-model for the halo density and by choosing model parameters based on all
available observations of X-ray emission in other groups of galaxies and on
optical data on Local Group morphology. From the predicted X-ray intensities,
total Local Group mass, and central cooling time of the halo, we derive very
conservative upper limits on the central halo density N_0 and global
temperature T of N_0 = 5e-4 cm-3 and kT = 0.5 keV, irrespective of realistic
values of the density profile parameters r_c and beta. A typical poor group
value of beta = 0.5 requires kT < 0.15 keV and N_0 < 1e-4 cm-3, from which it
is concluded that the Local Group is very unlikely to possess a significant
X-ray halo. The prospects for further constraining of halo parameters from UV
absorption line observations are considered. We explicitly calculate the
ability of the halo to distort the cosmic microwave background (CMB) in terms
of the resulting CMB temperature variations and multipole anisotropies.Comment: 24 pages, 16 figures. Accepted for publication in the Astrophysical
Journa
The Hot Galactic Corona and the Soft X-ray Background
I characterize the global distribution of the 3/4 keV band background with a
simple model of the hot Galactic corona, plus an isotropic extragalactic
background. The corona is assumed to be approximately polytropic (index = 5/3)
and hydrostatic in the gravitational potential of the Galaxy. The model
accounts for X-ray absorption, and is constrained iteratively with the ROSAT
all-sky X-ray survey data. Regions where the data deviate significantly from
the model represent predominantly the Galactic disk and individual nearby hot
superbubbles. The global distribution of the background, outside these regions,
is well characterized by the model; the 1 sigma relative dispersion of the data
from the model is about 15%. The electron density and temperature of the corona
near the Sun are about 1.1 x 10^{-3} cm^{-3} and about 1.7 x 10^6 K. The same
model also explains well the 1.5 keV band background. The model prediction in
the 1/4 keV band, though largely uncertain, qualitatively shows large intensity
and spectral variations of the corona contribution across the sky.Comment: An invited talk at IAU Colloquium No. 166: The Local Bubble and
Beyond. 10 pages (including b/w figures). Color versions of Figs. 1 and 4 are
provided separately and may also be found at
www.astro.nwu.edu/astro/wqd/paper/hal
High-redshift X-ray properties of the haloes of simulated disc galaxies
X-ray luminosities and surface brightness profiles of the hot gas haloes of
simulated disc galaxies at redshifts z=0-2 are presented. The galaxies are
extracted from fully cosmological simulations and correspond in mass to the
Milky Way. We find that the bolometric X-ray luminosities of the haloes
decrease by a factor 4-10 from z~1 to z~0, reflecting the decrease in the rate
at which hot halo gas cools out on to the disc. At all redshifts, most of the
emission is found to originate within 10-15 kpc of the disc. When combined with
models in which the evolution of disc X-ray luminosity is dominated by X-ray
binaries, the predicted halo luminosities at z~1 show good agreement with
constraints from spiral galaxies in Chandra Deep Field data. There is an
indication that haloes with a metal abundance of 0.3 solar overpredict observed
X-ray luminosities at z~1, suggesting that halo metallicities are lower than
this value. Prospects for direct detection of the haloes of Milky Way-sized
galaxies with current and future X-ray instrumentation are discussed. It is
found that XEUS should be able to single out the halo emission of highly
inclined Milky Way-sized disc galaxies out to approximately z=0.3. For such
galaxies in this redshift interval, we estimate a lower limit to the surface
density of detectable haloes on the sky of ~10 deg^(-2). More generally, owing
to their luminosity evolution, the optimum redshifts at which to observe such
haloes could be 0.5<z<1, depending on their assembly history.Comment: 9 pages, 4 figures, minor corrections to conform to version accepted
to MNRA
First results on Martian carbon monoxide from Herschel/HIFI observations
We report on the initial analysis of Herschel/HIFI carbon monoxide (CO)
observations of the Martian atmosphere performed between 11 and 16 April 2010.
We selected the (7-6) rotational transitions of the isotopes ^{13}CO at 771 GHz
and C^{18}O at 768 GHz in order to retrieve the mean vertical profile of
temperature and the mean volume mixing ratio of carbon monoxide. The derived
temperature profile agrees within less than 5 K with general circulation model
(GCM) predictions up to an altitude of 45 km, however, show about 12-15 K lower
values at 60 km. The CO mixing ratio was determined as 980 \pm 150 ppm, in
agreement with the 900 ppm derived from Herschel/SPIRE observations in November
2009.Comment: Accepted for publication in Astronomy and Astrophysics (special issue
on HIFI first results); minor changes to match published versio
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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