Star formation in metal-poor gas clouds

Observations of molecular clouds in metal-poor environments typically find that they have much higher star formation rates than one would expect based on their observed CO luminosities and the molecular gas masses that are inferred from them. This finding can be understood if one assumes that the conversion factor between CO luminosity and H2 mass is much larger in these low metallicity systems than in nearby molecular clouds. However, it is unclear whether this is the only factor at work, or whether the star formation rate of the clouds is directly sensitive to the metallicity of the gas. To investigate this, we have performed numerical simulations of the coupled dynamical, chemical and thermal evolution of model clouds with metallicities ranging from 0.01 Z_solar to Z_solar. We find that the star formation rate in our model clouds has little sensitivity to the metallicity. Reducing the metallicity of the gas by two orders of magnitude delays the onset of star formation in the clouds by no more than a cloud free-fall time and reduces the time-averaged star formation rate by at most a factor of two. On the other hand, the chemical state of the clouds is highly sensitive to the metallicity, and at the lowest metallicities, the clouds are completely dominated by atomic gas. Our results confirm that the CO-to-H2 conversion factor in these systems depends strongly on the metallicity, but also show that the precise value is highly time-dependent, as the integrated CO luminosity of the most metal-poor clouds is dominated by emission from short-lived gravitationally collapsing regions. Finally, we find evidence that the star formation rate per unit H2 mass increases with decreasing metallicity, owing to the much smaller H2 fractions present in our low metallicity clouds.Comment: 14 pages, 6 figures. Updated to match version accepted by MNRA

Nitrogen hydrides in interstellar gas II. Analysis of Herschel/HIFI observations towards W49N and G10.6-0.4 (W31C)

We have used the Herschel-HIFI instrument to observe interstellar nitrogen hydrides along the sight-lines towards W49N and G10.6-0.4 in order to elucidate the production pathways leading to nitrogen-bearing species in diffuse gas. All detections show absorption by foreground material over a wide range of velocities, as well as absorption associated directly with the hot-core source itself. As in the previously published observations towards G10.6-0.4, the NH, NH2 and NH3 spectra towards W49N show strikingly similar and non-saturated absorption features. We decompose the absorption of the foreground material towards W49N into different velocity components in order to investigate whether the relative abundances vary among the velocity components, and, in addition, we re-analyse the absorption lines towards G10.6-0.4 in the same manner. Abundances, with respect to molecular hydrogen, in each velocity component are estimated using CH. The analysis points to a co-existence of the nitrogen hydrides in diffuse or translucent interstellar gas with a high molecular fraction. Towards both sources, we find that NH is always at least as abundant as both o-NH2 and o-NH3, in sharp contrast to previous results for dark clouds. We find relatively constant N(NH)/N(o-NH3) and N(o-NH2)/N(o-NH3) ratios with mean values of 3.2 and 1.9 towards W49N, and 5.4 and 2.2 towards G10.6-0.4, respectively. The mean abundance of o-NH3 is ~2x10^-9 towards both sources. The nitrogen hydrides also show linear correlations with CN and HNC towards both sources, and looser correlations with CH. The upper limits on the NH+ abundance indicate column densities < 2 - 14 % of N(NH). Surprisingly low values of the ammonia ortho-to-para ratio are found in both sources, ~0.5 - 0.7 +- 0.1. This result cannot be explained by current models as we had expected to find a value of unity or higher.Comment: 35 pages, 74 figure

Control of star formation by supersonic turbulence

Understanding the formation of stars in galaxies is central to much of modern astrophysics. For several decades it has been thought that stellar birth is primarily controlled by the interplay between gravity and magnetostatic support, modulated by ambipolar diffusion. Recently, however, both observational and numerical work has begun to suggest that support by supersonic turbulence rather than magnetic fields controls star formation. In this review we outline a new theory of star formation relying on the control by turbulence. We demonstrate that although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28 figures, in pres

SPLASH: the Southern Parkes Large-Area Survey in Hydroxyl – first science from the pilot region

The Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) is a sensitive, unbiased, and fully sampled survey of the southern Galactic plane and Galactic Centre in all four ground-state transitions of the hydroxyl (OH) radical. The survey provides a deep census of 1612-, 1665-, 1667-, and 1720-MHz OH absorption and emission from the Galactic interstellar medium, and is also an unbiased search for maser sources in these transitions. We present here first results from the SPLASH pilot region, which covers Galactic longitudes 334° to 344° and latitudes ±2?. Diffuse OH is widely detected in all four transitions, with optical depths that are always small (averaged over the Parkes beam), and with departures from local thermodynamic equilibrium common even in the 1665- and 1667-MHz main lines. To a 3σ sensitivity of ~30 mK, we find no evidence of OH envelopes extending beyond the CO-bright regions of molecular cloud complexes, and conclude that the similarity of the OH excitation temperature and the level of the continuum background is at least partly responsible for this. We detect masers and maser candidates in all four transitions, approximately 50 per cent of which are new detections. This implies that SPLASH will produce a substantial increase in the known population of ground-state OH masers in the southern Galactic plane