1,305 research outputs found
Cosmic-ray ionisation in circumstellar discs
Galactic cosmic rays are a ubiquitous source of ionisation of the
interstellar gas, competing with UV and X-ray photons as well as natural
radioactivity in determining the fractional abundance of electrons, ions and
charged dust grains in molecular clouds and circumstellar discs. We model the
propagation of different components of Galactic cosmic rays versus the column
density of the gas. Our study is focussed on the propagation at high densities,
above a few g cm, especially relevant for the inner regions of
collapsing clouds and circumstellar discs. The propagation of primary and
secondary CR particles (protons and heavier nuclei, electrons, positrons, and
photons) is computed in the continuous slowing down approximation, diffusion
approximation, or catastrophic approximation, by adopting a matching procedure
for the different transport regimes. A choice of the proper regime depends on
the nature of the dominant loss process, modelled as continuous or
catastrophic. The CR ionisation rate is determined by CR protons and their
secondary electrons below g cm and by electron/positron
pairs created by photon decay above g cm. We show that a
proper description of the particle transport is essential to compute the
ionisation rate in the latter case, since the electron/positron differential
fluxes depend sensitively on the fluxes of both protons and photons. Our
results show that the CR ionisation rate in high-density environments, like,
e.g., the inner parts of collapsing molecular clouds or the mid-plane of
circumstellar discs, is larger than previously assumed. It does not decline
exponentially with increasing column density, but follows a more complex
behaviour due to the interplay of different processes governing the generation
and propagation of secondary particles.Comment: 19 pages, 11 figures, accepted by A&
Proof Systems for Retracts in Simply Typed Lambda Calculus
Abstract. This paper concerns retracts in simply typed lambda calculus assuming ÎČη-equality. We provide a simple tableau proof system which characterises when a type is a retract of another type and which leads to an exponential decision procedure.
Production of atomic hydrogen by cosmic rays in dark clouds
The presence of small amounts of atomic hydrogen, detected as absorption dips
in the 21 cm line spectrum, is a well-known characteristic of dark clouds. The
abundance of hydrogen atoms measured in the densest regions of molecular clouds
can be only explained by the dissociation of H due to cosmic rays. We want
to assess the role of Galactic cosmic rays in the formation of atomic hydrogen,
by using recent developments in the characterisation of the low-energy spectra
of cosmic rays and advances in the modelling of their propagation in molecular
clouds. We model the attenuation of the interstellar cosmic rays entering a
cloud and compute the dissociation rate of molecular hydrogen due to collisions
with cosmic-ray protons and electrons as well as fast hydrogen atoms. We
compare our results with the available observations. The cosmic-ray
dissociation rate is entirely determined by secondary electrons produced in
primary ionisation collisions. These secondary particles constitute the only
source of atomic hydrogen at column densities above cm. We
also find that the dissociation rate decreases with column density, while the
ratio between the dissociation and ionisation rates varies between about 0.6
and 0.7. From comparison with observations we conclude that a relatively flat
spectrum of interstellar cosmic-ray protons, as the one suggested by the most
recent Voyager 1 data, can only provide a lower bound for the observed atomic
hydrogen fraction. An enhanced spectrum of low-energy protons is needed to
explain most of the observations. Our findings show that a careful description
of molecular hydrogen dissociation by cosmic rays can explain the abundance of
atomic hydrogen in dark clouds. An accurate characterisation of this process at
high densities is crucial for understanding the chemical evolution of
star-forming regions.Comment: 7 pages, 7 figures, accepted by Astronomy and Astrophysic
The VLA Survey of the Chandra Deep Field South. IV. Source Population
We present a detailed analysis of 256 radio sources from our deep (flux
density limit of 42 microJy at the field centre at 1.4 GHz) Chandra Deep Field
South 1.4 and 5 GHz VLA survey. The radio population is studied by using a
wealth of multi-wavelength information in the radio, optical, and X-ray bands.
The availability of redshifts for ~ 80% of the sources in our complete sample
allows us to derive reliable luminosity estimates for the majority of the
objects. X-ray data, including upper limits, for all our sources turn out to be
a key factor in establishing the nature of faint radio sources. Due to the
faint optical levels probed by this study, we have uncovered a population of
distant Active Galactic Nuclei (AGN) systematically missing from many previous
studies of sub-millijansky radio source identifications. We find that, while
the well-known flattening of the radio number counts below 1 mJy is mostly due
to star forming galaxies, these sources and AGN make up an approximately equal
fraction of the sub-millijansky sky, contrary to some previous results. The AGN
include radio galaxies, mostly of the low-power, Fanaroff-Riley I type, and a
significant radio-quiet component, which amounts to approximately one fifth of
the total sample. The ratio of radio to optical luminosity depends more on
radio luminosity, rather than being due to optical absorption.Comment: 13 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Unresolved Unidentified Source Contribution to the Gamma-ray Background
The large majority of EGRET point sources remain without an identified
low-energy counterpart, and a large fraction of these sources are most likely
extragalactic. Whatever the nature of the extragalactic EGRET unidentified
sources, faint unresolved objects of the same class must have a contribution to
the diffuse extragalactic gamma-ray background (EGRB). Understanding this
component of the EGRB, along with other guaranteed contributions from known
sources, is essential if we are to use this emission to constrain exotic
high-energy physics. Here, we follow an empirical approach to estimate whether
a potential contribution of unidentified sources to the EGRB is likely to be
important, and we find that it is. Additionally, we show how upcoming GLAST
observations of EGRET unidentified sources, as well as of their fainter
counterparts, can be combined with GLAST observations of the Galactic and
extragalactic diffuse backgrounds to shed light on the nature of the EGRET
unidentified sources even without any positional association of such sources
with low-energy counterparts.Comment: 11 pages, 5 figures, submitted to Ap
Protonated CO2 in massive star-forming clumps
Interstellar CO2 is an important reservoir of carbon and oxygen, and one of
the major constituents of the icy mantles of dust grains, but it is not
observable directly in the cold gas because has no permanent dipole moment. Its
protonated form, HOCO+, is believed to be a good proxy for gaseous CO2.
However, it has been detected in only a few star-forming regions so far, so
that its interstellar chemistry is not well understood. We present new
detections of HOCO+ lines in 11 high-mass star-forming clumps. Our observations
increase by more than three times the number of detections in star-forming
regions so far. We have derived beam-averaged abundances relative to H2 in
between 0.3 and 3.8 x 10^{-11}. We have compared these values with the
abundances of H13CO+, a possible gas-phase precursor of HOCO+, and CH3OH, a
product of surface chemistry. We have found a positive correlation with H13CO+,
while with CH3OH there is no correlation. We suggest that the gas-phase
formation route starting from HCO+ plays an important role in the formation of
HOCO+, perhaps more relevant than protonation of CO2 (upon evaporation of this
latter from icy dust mantles).Comment: 5 pages, 4 figures, 1 table, accepted for publication in MNRA
Radio faint AGN: a tale of two populations
We study the Extended Chandra Deep Field South (E-CDFS) Very Large Array
sample, which reaches a flux density limit at 1.4 GHz of 32.5 microJy at the
field centre and redshift ~ 4, and covers ~ 0.3 deg^2. Number counts are
presented for the whole sample while the evolutionary properties and luminosity
functions are derived for active galactic nuclei (AGN). The faint radio sky
contains two totally distinct AGN populations, characterised by very different
evolutions, luminosity functions, and Eddington ratios: radio-quiet
(RQ)/radiative-mode, and radio-loud/jet-mode AGN. The radio power of RQ AGN
evolves ~ (1+z)^2.5, similarly to star-forming galaxies, while the number
density of radio-loud ones has a peak at ~ 0.5 and then declines at higher
redshifts. The number density of radio-selected RQ AGN is consistent with that
of X-ray selected AGN, which shows that we are sampling the same population.
The unbiased fraction of radiative-mode RL AGN, derived from our own and
previously published data, is a strong function of radio power, decreasing from
~ 0.5 at P_1.4GHz ~ 10^24 W/Hz to ~ 0.04$ at P_1.4GHz ~ 10^22 W/Hz. Thanks to
our enlarged sample, which now includes ~ 700 radio sources, we also confirm
and strengthen our previous results on the source population of the faint radio
sky: star-forming galaxies start to dominate the radio sky only below ~ 0.1
mJy, which is also where radio-quiet AGN overtake radio-loud ones.Comment: 19 pages, 13 figures, accepted for publication in MNRA
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