2,489 research outputs found
Determining the cosmic ray ionization rate in dynamically evolving clouds
The ionization fraction is an important factor in determining the chemical
and physical evolution of star forming regions. In the dense, dark starless
cores of such objects, the ionization rate is dominated by cosmic rays; it is
therefore possible to use simple analytic estimators, based on the relative
abundances of different molecular tracers, to determine the cosmic ray
ionization rate.
This paper uses a simple model to investigate the accuracy of two well-known
estimators in dynamically evolving molecular clouds. It is found that, although
the analytical formulae based on the abundances of H3+,H2,CO,O,H2O and HCO+
give a reasonably accurate measure of the cosmic ray ionization rate in static,
quiescent clouds, significant discrepancies occur in rapidly evolving
(collapsing) clouds. As recent evidence suggests that molecular clouds may
consist of complex, dynamically evolving sub-structure, we conclude that simple
abundance ratios do not provide reliable estimates of the cosmic ray ionization
rate in dynamically active regions.Comment: Accepted by A&A. 17 pages, 4 figure
Oscillations in the stable starless core Barnard 68
New molecular line observations of the Bok globule Barnard 68 in HCO+
irrefutably confirm the complex pattern of red and blue asymmetric line
profiles seen across the face of the cloud in previous observations of CS. The
new observations thus strengthen the previous interpretation that Barnard 68 is
undergoing peculiar oscillations. Furthermore, the physical chemistry of B68
indicates that the object is much older than the sound crossing time and is
therefore long-lived. A model is presented for the globule in which a modest
external pressure perturbation is shown to lead to oscillations about a stable
equilibrium configuration. Such oscillations may be present in other stable
starless cores as manifested by a similar signature of inward and outward
motions.Comment: Accepted for MNRAS letters, 5 pages, 7 figure
The radio luminosity function of radio-loud quasars from the 7C Redshift Survey
We present a complete sample of 24 radio-loud quasars (RLQs) from the new 7C
Redshift Survey. Every quasar with a low-frequency (151 MHz) radio flux-density
S_151 > 0.5 Jy in two regions of the sky covering 0.013 sr is included; 23 of
these have sufficient extended flux to meet the selection criteria, 18 of these
have steep radio spectra (hereafter denoted as SSQs). The key advantage of this
sample over most samples of RLQs is the lack of an optical magnitude limit. By
combining the 7C and 3CRR samples, we have investigated the properties of RLQs
as a function of redshift z and radio luminosity L_151.
We derive the radio luminosity function (RLF) of RLQs and find that the data
are well fitted by a single power-law with slope alpha_1=1.9. We find that
there must be a break in the RLQ RLF at log_10(L_151 / W Hz^-1 sr^-1) < 27, in
order for the models to be consistent with the 7C and 6C source counts. The
z-dependence of the RLF follows a one-tailed gaussian which peaks at z=1.7. We
find no evidence for a decline in the co-moving space density of RLQs at higher
redshifts.
A positive correlation between the radio and optical luminosities of SSQs is
observed, confirming a result of Serjeant et al. (1998). We are able to rule
out this correlation being due to selection effects or biases in our combined
sample. The radio-optical correlation and best-fit model RLF enable us to
estimate the distribution of optical magnitudes of quasars in samples selected
at low radio frequencies. We conclude that for samples with S_151 < 1 Jy one
must use optical data significantly deeper than the POSS-I limit (R approx 20),
in order to avoid severe incompleteness.Comment: 28 pages with 13 figures. To appear in MNRA
Unified Models of Molecular Emission from Class 0 Protostellar Outflow Sources
Low mass star-forming regions are more complex than the simple spherically
symmetric approximation that is often assumed. We apply a more realistic
infall/outflow physical model to molecular/continuum observations of three late
Class 0 protostellar sources with the aims of (a) proving the applicability of
a single physical model for all three sources, and (b) deriving physical
parameters for the molecular gas component in each of the sources.
We have observed several molecular species in multiple rotational
transitions. The observed line profiles were modelled in the context of a
dynamical model which incorporates infall and bipolar outflows, using a three
dimensional radiative transfer code. This results in constraints on the
physical parameters and chemical abundances in each source.
Self-consistent fits to each source are obtained. We constrain the
characteristics of the molecular gas in the envelopes as well as in the
molecular outflows. We find that the molecular gas abundances in the infalling
envelope are reduced, presumably due to freeze-out, whilst the abundances in
the molecular outflows are enhanced, presumably due to dynamical activity.
Despite the fact that the line profiles show significant source-to-source
variation, which primarily derives from variations in the outflow viewing
angle, the physical parameters of the gas are found to be similar in each core.Comment: MNRAS 12 pages, 16 figure
Low-power radio galaxy environments in the Subaru/XMM-Newton Deep Field at z~0.5
We present multi-object spectroscopy of galaxies in the immediate (Mpc-scale)
environments of four low-power (L_1.4 GHz < 10^25 W/Hz) radio galaxies at
z~0.5, selected from the Subaru/XMM-Newton Deep Field. We use the spectra to
calculate velocity dispersions and central redshifts of the groups the radio
galaxies inhabit, and combined with XMM-Newton (0.3-10 keV) X-ray observations
investigate the L_X--sigma_v and T_X--sigma_v scaling relationships. All the
radio galaxies reside in moderately rich groups -- intermediate environments
between poor groups and rich clusters, with remarkably similar X-ray
properties. We concentrate our discussion on our best statistical example that
we interpret as a low-power (FRI) source triggered within a sub-group, which in
turn is interacting with a nearby group of galaxies, containing the bulk of the
X-ray emission for the system -- a basic scenario which can be compared to more
powerful radio sources at both high (z>4) and low (z<0.1) redshifts. This
suggests that galaxy-galaxy interactions triggered by group mergers may play an
important role in the life-cycle of radio galaxies at all epochs and
luminosities.Comment: 12 pages, 7 figures, accepted for publication in MNRAS. High
resolution version available upon reques
Desorption From Interstellar Ices
The desorption of molecular species from ice mantles back into the gas phase
in molecular clouds results from a variety of very poorly understood processes.
We have investigated three mechanisms; desorption resulting from H_2 formation
on grains, direct cosmic ray heating and cosmic ray induced photodesorption.
Whilst qualitative differences exist between these processes (essentially
deriving from the assumptions concerning the species-selectivity of the
desorption and the assumed threshold adsorption energies, E_t) all three
processes are found to be potentially very significant in dark cloud
conditions. It is therefore important that all three mechanisms should be
considered in studies of molecular clouds in which freeze-out and desorption
are believed to be important.
Employing a chemical model of a typical static molecular core and using
likely estimates for the quantum yields of the three processes we find that
desorption by H_2 formation probably dominates over the other two mechanisms.
However, the physics of the desorption processes and the nature of the dust
grains and ice mantles are very poorly constrained. We therefore conclude that
the best approach is to set empirical constraints on the desorption, based on
observed molecular depletions - rather than try to establish the desorption
efficiencies from purely theoretical considerations. Applying this method to
one such object (L1689B) yields upper limits to the desorption efficiencies
that are consistent with our understanding of these mechanisms.Comment: 11 pages, 5 figures, accepted by MNRAS subject to minor revision
which has been carried ou
A Pre-Protostellar Core in L1551. II. State of Dynamical and Chemical Evolution
Both analytic and numerical radiative transfer models applied to high
spectral resolution CS and N2H+ data give insight into the evolutionary state
of L1551 MC. This recently discovered pre-protostellar core in L1551 appears to
be in the early stages of dynamical evolution. Line-of-sight infall velocities
of >0.1km/s are needed in the outer regions of L1551 MC to adequately fit the
data. This translates to an accretion rate of ~ 1e-6 Msun/yr, uncertain to
within a factor of 5 owing to unknown geometry. The observed dynamics are not
due to spherically symmetric gravitational collapse and are not consistent with
the standard model of low-mass star formation. The widespread, fairly uniform
CS line asymmetries are more consistent with planar infall. There is modest
evidence for chemical depletion in the radial profiles of CS and C18O
suggesting that L1551 MC is also chemically young. The models are not very
sensitive to chemical evolution. L1551 MC lies within a quiescent region of
L1551 and is evidence for continued star formation in this evolved cloud.Comment: 27 pages, 7 figures, ApJ accepte
Molecular gas freeze-out in the pre-stellar core L1689B
C17O (J=2-1) observations have been carried out towards the pre-stellar core
L1689B. By comparing the relative strengths of the hyperfine components of this
line, the emission is shown to be optically thin. This allows accurate CO
column densities to be determined and, for reference, this calculation is
described in detail. The hydrogen column densities that these measurements
imply are substantially smaller than those calculated from SCUBA dust emission
data. Furthermore, the C17O column densities are approximately constant across
L1689B whereas the SCUBA column densities are peaked towards the centre. The
most likely explanation is that CO is depleted from the central regions of
L1689B. Simple models of pre-stellar cores with an inner depleted region are
compared with the results. This enables the magnitude of the CO depletion to be
quantified and also allows the spatial extent of the freeze-out to be firmly
established. We estimate that within about 5000 AU of the centre of L1689B,
over 90% of the CO has frozen onto grains. This level of depletion can only be
achieved after a duration that is at least comparable to the free-fall
timescale.Comment: MNRAS letters. 5 pages, 5 figure
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