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

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

Rotation of the pre-stellar core L1689B

The search for the onset of star formation in pre-stellar cores has focussed on the identification of an infall signature in the molecular line profiles of tracer species. The classic infall signature is a double peaked line profile with an asymmetry in the strength of the peaks such that the blue peak is stronger. L1689B is a pre-stellar core and infall candidate but new JCMT HCO+ line profile data, presented here, confirms that both blue and red asymmetric line profiles are present in this source. Moreover, a dividing line can be drawn between the locations where each type of profile is found. It is argued that it is unlikely that the line profiles can be interpreted with simple models of infall or outflow and that rotation of the inner regions is the most likely explanation. A rotational model is developed in detail with a new 3D molecular line transport code and it is found that the best type of model is one in which the rotational velocity profile is in between solid body and Keplerian. It is firstly shown that red and blue asymmetric line profiles can be generated with a rotation model entirely in the absence of any infall motion. The model is then quantitively compared with the JCMT data and an iteration over a range of parameters is performed to minmize the difference between the data and model. The results indicate that rotation can dominate the line profile shape even before the onset of infall.Comment: Accepted by MNRAS, 7 pages, 4 figure

A study of methanol and silicon monoxide production through episodic explosions of grain mantles in the Central Molecular Zone

Methanol (CH$_3$OH) is found to be abundant and widespread towards the Central Molecular Zone, the inner few hundred parsecs of our Galaxy. Its origin is, however, not fully understood. It was proposed that the high cosmic ray ionisation rate in this region could lead to a more efficient non-thermal desorption of this species formed on grain surfaces, but it would also mean that this species is destroyed in a relatively short timescale. In a first step, we run chemical models with a high cosmic ray ionisation rate and find that this scenario can only reproduce the lowest abundances of methanol derived in this region ($\sim$10$^{-9}$-10$^{-8}$). In a second step, we investigate another scenario based on episodic explosions of grain mantles. We find a good agreement between the predicted abundances of methanol and the observations. We find that the dominant route for the formation of methanol is through hydrogenation of CO on the grains followed by the desorption due to the grain mantle explosion. The cyclic aspect of this model can explain the widespread presence of methanol without requiring any additional mechanism. We also model silicon monoxide (SiO), another species detected in several molecular clouds of the Galactic Centre. An agreement is found with observations for a high depletion of Si (Si/H $\sim$ 10$^{-8}$) with respect to the solar abundance.Comment: Accepted in MNRA

Science with the Square Kilometer Array: Motivation, Key Science Projects, Standards and Assumptions

The Square Kilometer Array (SKA) represents the next major, and natural, step in radio astronomical facilities, providing two orders of magnitude increase in collecting area over existing telescopes. In a series of meetings, starting in Groningen, the Netherlands (August 2002) and culminating in a `science retreat' in Leiden (November 2003), the SKA International Science Advisory Committee (ISAC), conceived of, and carried-out, a complete revision of the SKA science case (to appear in New Astronomy Reviews). This preface includes: (i) general introductory material, (ii) summaries of the key science programs, and (iii) a detailed listing of standards and assumptions used in the revised science case.Comment: 6 pages, 3 tables, espcrc2.sty. to appear in Science with the square kilometer array, New Astronomy Reviews, (Elsevier, Amsterdam), eds. C. Carilli & S. Rawling