2,607 research outputs found
Evidence for Environmental Changes in the Submillimeter Dust Opacity
The submillimeter opacity of dust in the diffuse interstellar medium (ISM) in the Galactic plane has been quantified using a pixel-by-pixel correlation of images of continuum emission with a proxy for column density. We used multi-wavelength continuum data: three Balloon-borne Large Aperture Submillimeter Telescope bands at 250, 350, and 500 μm and one IRAS band at 100 μm. The proxy is the near-infrared color excess, E(J – K_s), obtained from the Two Micron All Sky Survey. Based on observations of stars, we show how well this color excess is correlated with the total hydrogen column density for regions of moderate extinction. The ratio of emission to column density, the emissivity, is then known from the correlations, as a function of frequency. The spectral distribution of this emissivity can be fit by a modified blackbody, whence the characteristic dust temperature T and the desired opacity σ_e(1200) at 1200 GHz or 250 μm can be obtained. We have analyzed 14 regions near the Galactic plane toward the Vela molecular cloud, mostly selected to avoid regions of high column density (N_H > 10^(22) cm^(–2)) and small enough to ensure a uniform dust temperature. We find σ_e(1200) is typically (2-4) × 10^(–25) cm^2 H^(–1) and thus about 2-4 times larger than the average value in the local high Galactic latitude diffuse atomic ISM. This is strong evidence for grain evolution. There is a range in total power per H nucleon absorbed (and re-radiated) by the dust, reflecting changes in the strength of the interstellar radiation field and/or the dust absorption opacity. These changes in emission opacity and power affect the equilibrium T, which is typically 15 K, colder than at high latitudes. Our analysis extends, to higher opacity and lower temperature, the trend of increasing σ_e(1200) with decreasing T that was found at high latitudes. The recognition of changes in the emission opacity raises a cautionary flag because all column densities deduced from dust emission maps, and the masses of compact structures within them, depend inversely on the value adopted
Wind asymmetry imprint in the UV light curves of the symbiotic binary SY Mus
Context: Light curves (LCs) of some symbiotic stars show a different slope of
the ascending and descending branch of their minimum profile. The origin of
this asymmetry is not understood well.
Aims: We explain this effect in the ultraviolet LCs of the symbiotic binary
SY Mus.
Methods: We model the continuum fluxes in the spectra obtained by the
International Ultraviolet Explorer at 10 wavelengths, from 1280 to 3080 \AA. We
consider that the white dwarf radiation is attenuated by atoms,
ions and free electrons in the red giant wind. Variation in the nebular
component is approximated by a sine wave along the orbit as suggested by
spectral energy distribution models. The model includes asymmetric wind
velocity distribution and the corresponding ionization structure of the binary.
Results: We determined distribution of the and , as well as upper
limits of and column densities in the neutral and ionized region at
the selected wavelengths as functions of the orbital phase. Corresponding
models of the LCs match well the observed continuum fluxes. In this way, we
suggested the main UV continuum absorbing (scattering) processes in the
circumbinary environment of S-type symbiotic stars.
Conclusions: The asymmetric profile of the ultraviolet LCs of SY Mus is
caused by the asymmetric distribution of the circumstellar matter at the
near-orbital-plane area.Comment: 9 pages, 9 figure
On the Relationship Between Molecular Hydrogen and Carbon Monoxide Abundances in Molecular Clouds
The most usual tracer of molecular gas is line emission from CO. However, the
reliability of that tracer has long been questioned in environments different
from the Milky Way. We study the relationship between H2 and CO abundances
using a fully dynamical model of magnetized turbulence coupled to a chemical
network simplified to follow only the dominant pathways for H2 and CO formation
and destruction, and including photodissociation using a six-ray approximation.
We find that the abundance of H2 is primarily determined by the amount of time
available for its formation, which is proportional to the product of the
density and the metallicity, but insensitive to photodissociation.
Photodissociation only becomes important at extinctions under a few tenths of a
visual magnitude, in agreement with both observational and prior theoretical
work. On the other hand, CO forms quickly, within a dynamical time, but its
abundance depends primarily on photodissociation, with only a weak secondary
dependence on H2 abundance. As a result, there is a sharp cutoff in CO
abundance at mean visual extinctions A_V < 3. At lower values of A_V we find
that the ratio of H2 column density to CO emissivity X_CO is proportional to
A_V^(-3.5). This explains the discrepancy observed in low metallicity systems
between cloud masses derived from CO observations and other techniques such as
infrared emission. Our work predicts that CO-bright clouds in low metallicity
systems should be systematically larger or denser than Milky Way clouds, or
both. Our results further explain the narrow range of observed molecular cloud
column densities as a threshold effect, without requiring the assumption of
virial equilibrium.Comment: 16 pages, 11 figures. Updated to match version accepted by MNRA
Star Formation Rate Indicators
What else can be said about star formation rate indicators that has not been
said already many times over? The `coming of age' of large ground-based surveys
and the unprecedented sensitivity, angular resolution and/or field-of-view of
infrared and ultraviolet space missions have provided extensive, homogeneous
data on both nearby and distant galaxies, which have been used to further our
understanding of the strengths and pitfalls of many common star formation rate
indicators. The synergy between these surveys has also enabled the calibration
of indicators for use on scales that are comparable to those of star-forming
regions, thus much smaller than an entire galaxy. These are being used to
investigate star formation processes at the sub-galactic scale. I review
progress in the field over the past decade or so.Comment: 41 pages, 5 figures. Proceedings of the XXIII Canary Islands Winter
School of Astrophysics: `Secular Evolution of Galaxies', edited by J.
Falcon-Barroso and J.H. Knape
The Signatures of Large-scale Temperature and Intensity Fluctuations in the Lyman-alpha Forest
It appears inevitable that reionization processes would have produced
large-scale temperature fluctuations in the intergalactic medium. Using toy
temperature models and detailed heating histories from cosmological simulations
of HeII reionization, we study the consequences of inhomogeneous heating for
the Ly-alpha forest. The impact of temperature fluctuations in physically
well-motivated models can be surprisingly subtle. In fact, we show that
temperature fluctuations at the level predicted by our reionization simulations
do not give rise to detectable signatures in the types of statistics that have
been employed previously. However, because of the aliasing of small-scale
density power to larger scale modes in the line-of-sight Ly-alpha forest power
spectrum, earlier analyses were not sensitive to 3D modes with >~ 30 comoving
Mpc wavelengths -- scales where temperature fluctuations are likely to be
relatively largest. The ongoing Baryon Oscillation Spectroscopic Survey (BOSS)
aims to measure the 3D power spectrum of the Ly-alpha forest, P_F, from a large
sample of quasars in order to avoid this aliasing. We find that physically
motivated temperature models can alter P_F at an order unity level at k <~ 0.1
comoving Mpc^{-1}, a magnitude that should be easily detectable with BOSS.
Fluctuations in the intensity of the ultraviolet background can also alter P_F
significantly. These signatures will make it possible for BOSS to study the
thermal impact of HeII reionization at 2 < z < 3 and to constrain models for
the sources of the ionizing background. Future spectroscopic surveys could
extend this measurement to even higher redshifts, potentially detecting the
thermal imprint of hydrogen reionization.Comment: 14 pages, 17 figures, plus 4 pages of Appendix, matches published
versio
Can Reflection from Grains Diagnose the Albedo?
By radiation transfer models with a realistic power spectra of the projected
density distributions, we show that the optical properties of grains are poorly
constrained by observations of reflection nebulae. The ISM is known to be
hierarchically clumped from a variety of observations (molecules, H I,
far-infrared). Our models assume the albedo and phase parameter of the dust,
the radial optical depth of the sphere averaged over all directions, and random
distributions of the dust within the sphere. The outputs are the stellar
extinction, optical depth, and flux of scattered light as seen from various
viewing angles. Observations provide the extinction and scattered flux from a
particular direction.
Hierarchical geometry has a large effect on the flux of scattered light
emerging from a nebula for a particular extinction of the exciting star. There
is a very large spread in both scattered fluxes and extinctions for any
distribution of dust. Consequently, an observed stellar extinction and
scattered flux can be fitted by a wide range of albedos. With hierarchical
geometry it is not completely safe to determine even relative optical constants
from multiwavelength observations of the same reflection nebula. The geometry
effectively changes with wavelength as the opacity of the clumps varies. Limits
on the implications of observing the same object in various wavelengths are
discussed briefly.
Henry (2002) uses a recipe to determine the scattered flux from a star with a
given extinction. It is claimed to be independent of the geometry. It provides
considerably more scattering than our models, probably leading to an
underestimate of the grain albedos from the UV Diffuse Galactic Light.Comment: 27 pages, including 7 figures. Accepted by Ap
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