C. R. Purcell

Variations in the Galactic star formation rate and density thresholds for star formation

The conversion of gas into stars is a fundamental process in astrophysics and cosmology. Stars are known to form from the gravitational collapse of dense clumps in interstellar molecular clouds, and it has been proposed that the resulting star formation rate is proportional to either the amount of mass above a threshold gas surface density, or the gas volume density. These star formation prescriptions appear to hold in nearby molecular clouds in our Milky Way Galaxys disc as well as in distant galaxies where the star formation rates are often much larger. The inner 500 pc of our Galaxy, the Central Molecular Zone (CMZ), contains the largest concentration of dense, high-surface density molecular gas in the Milky Way, providing an environment where the validity of star formation prescriptions can be tested. Here, we show that by several measures, the current star formation rate in the CMZ is an order-of-magnitude lower than the rates predicted by the currently accepted prescriptions. In particular, the region 1 degrees several 10(3) cm(-3)) molecular gas - enough to form 1000 Orion-like clusters - but the present-day star formation rate within this gas is only equivalent to that in Orion. In addition to density, another property of molecular clouds must be included in the star formation prescription to predict the star formation rate in a given mass of molecular gas. We discuss which physical mechanisms might be responsible for suppressing star formation in the CMZ.

Beam Size, Shape and Efficiencies for the ATNF Mopra Radio Telescope at 86–115 GHz

We present data characterising the performance of the Mopra Radio Telescope during the period 2000-2004, including measurements of the beam size and shape, as well as the overall beam efficiency of the telescope. In 2004 the full width half maximum of the beam was measured to be 36 ± 3 �� at 86 GHz, falling to 33 ± 2 �� at 115 GHz. Based on our observations of Jupiter we measured the beam efficiency of the Gaussian main beam to be 0.49 ± 0.03 at 86 GHz and 0.42 ± 0.02 at 115 GHz. Sources with angular sizes of ∼80 �� couple well to the main beam, while sources with angular sizes between ∼80 �� and ∼160 �� couple to the both the main beam and inner error beam. Measurements indicate that the inner error beam contains approximately one-third the power of the main beam. We also compare efficiency corrected spectra to measurements made at similar facilities and present standard spectra taken towards the molecular clouds Orion-KL and M17-SW.

Giant magnetized outflows from the centre of the Milky Way

The nucleus of the Milky Way is known to harbour regions of intense star formation activity as well as a supermassive black hole. Recent observations have revealed regions of γ-ray emission reaching far above and below the Galactic Centre (relative to the Galactic plane), the so-called ‘Fermi bubbles’. It is uncertain whether these were generated by nuclear star formation or by quasar-like outbursts of the central black hole and no information on the structures’ magnetic field has been reported. Here we report observations of two giant, linearly polarized radio lobes, containing three ridge-like substructures, emanating from the Galactic Centre. The lobes each extend about 60 degrees in the Galactic bulge, closely corresponding to the Fermi bubbles, and are permeated by strong magnetic fields of up to 15 microgauss. We conclude that the radio lobes originate in a biconical, star-formation-driven (rather than black-hole-driven) outflow from the Galaxy’s central 200 parsecs that transports a huge amount of magnetic energy, about 1055 ergs, into the Galactic halo. The ridges wind around this outflow and, we suggest, constitute a ‘phonographic’ record of nuclear star formation activity over at least ten million years.

The RMS survey - 6 cm continuum VLA observations towards candidate massive YSOs in the northern hemisphere

Context. The Red MSX Source (RMS) survey is an ongoing multi-wavelength observational programme designed to return a large, well-selected sample of massive young stellar objects (MYSOs). We have identified ∼2000 MYSO candidates located throughout the Galaxy by comparing the colours of MSX and 2MASS point sources to those of known MYSOs. The aim of these follow-up observations is to identify other objects with similar colours such as ultra compact (UC) HII regions, evolved stars and planetary nebulae (PNe) and distinguish between genuine MYSOs and nearby low-mass YSOs. Aims. To identify the populations of UCHII regions and PNe within the sample and examine their Galactic distribution. Methods. We have conducted high resolution radio continuum observations at 6 cm towards 659 MYSO candidates in the northern hemisphere (10 ◦ < l < 250 ◦ ) using the Very Large Array (VLA). These observations have a spatial resolution of ∼1–2 �� and typical image rms noise values of ∼0.22 mJy – sensitive enough to detect a HII region powered by B0.5 star at the far side of the Galaxy. In addition to these targeted observations we present archival data towards a further 315 RMS sources extracted from a previous VLA survey of the inner Galaxy. Results. We present the results of radio continuum observations made towards 974 MYSO candidates, 272 (∼27% of the observed sample) of which are found to be associated with radio emission above a 4σ detection limit (∼1 mJy). Using results from other parts of our multi-wavelength survey we separate these RMS-radio associations into two distinct types of objects, classifying 51 as PNe and a further 208 as either compact or UC HII regions. Including all HII regions and PNe identified either from the literature or from the multi-wavelength data these numbers increase to 391 and 79, respectively. Using this well selected sample of HII regions we estimate their Galactic scale height to be 0.6 ◦ . In addition to the RMS-radio associations we are able to set upper limits on the radio emission of ≤1 mJy for the 702 non-detections, which is below the level expected if they had already begun to ionise their surroundings. Conclusions. Using radio continuum and archival data we have identified 79 PNe and 391 HII regions within the northern RMS catalogue. We estimate the total fraction of contamination by PNe in the RMS sample is of order 10%. The sample of HII regions is probably the best representation to date of the Galactic population of HII regions as a whole.

The Coordinated Radio and Infrared Survey for High-mass Star Formation. II. Source Catalog

The CORNISH project is the highest resolution radio continuum survey of the Galactic plane to date. It is the 5 GHz radio continuum part of a series of multi-wavelength surveys that focus on the northern GLIMPSE region (10° < l < 65°), observed by the Spitzer satellite in the mid-infrared. Observations with the Very Large Array in B and BnA configurations have yielded a 1.″5 resolution Stokes I map with a root mean square noise level better than 0.4 mJy beam -1 . Here we describe the data-processing methods and data characteristics, and present a new, uniform catalog of compact radio emission. This includes an implementation of automatic deconvolution that provides much more reliable imaging than standard CLEANing. A rigorous investigation of the noise characteristics and reliability of source detection has been carried out. We show that the survey is optimized to detect emission on size scales up to 14″ and for unresolved sources the catalog is more than 90% complete at a flux density of 3.9 mJy. We have detected 3062 sources above a 7σ detection limit and present their ensemble properties. The catalog is highly reliable away from regions containing poorly sampled extended emission, which comprise less than 2% of the survey area. Imaging problems have been mitigated by down-weighting the shortest spacings and potential artifacts flagged via a rigorous manual inspection with reference to the Spitzer infrared data. We present images of the most common source types found: H II regions, planetary nebulae, and radio galaxies. The CORNISH data and catalog are available online at http://cornish.leeds.ac.uk.

Star-forming Protoclusters Associated with Methanol Masers

We present a multiwavelength study of five methanol maser sites which are not directly associated with a strong (>100 mJy) radio continuum source: G 31.28+0.06, G 59.78+0.06, G 173.49+2.42 (S231, S233IR), G 188.95+0.89 (S252, AFGL5180) and G 192.60-0.05 (S255IR). These radio-quiet methanol maser sites are often interpreted as precursors of ultra- compact H  regions or massive protostar sites. In this work, the environment of methanol masers is probed from mid-IR to millimetre wavelengths at angular resolutions of 8 �� −34 �� . Spectral energy distribution (SED) diagrams for each site are pre- sented, together with mass and luminosity estimates. Each radio-quiet maser site is always associated with a massive (>50 M� ), deeply embedded (Av > 40 mag) and very luminous (>10 4 L� ) molecular clump, with Ltotal ∝ M 0.75 gas . These physical properties characterise massive star-forming clumps in earlier evolutionary phases than H  regions. In addition, colder gas clumps seen only at mm-wavelengths are also found near the methanol maser sites. These colder clumps may represent an even earlier phase of massive star formation. These results suggest an evolutionary sequence for massive star formation from a cold clump, seen only at mm wavelengths, evolving to a hot molecular core with a two-component SED with peaks at far-IR and mid-IR wave- lengths, to an (ultra-compact) H  region. Alternatively, the cold clumps might be clusters of low-mass YSOs, in formation near the massive star-forming clusters. Finally, the values of the dust grain emissivity index (β) range between 1.6 and 1.9.

The Coordinated Radio and Infrared Survey for High-Mass Star Formation (The CORNISH Survey). I. Survey Design

We describe the motivation, design and implementation of the CORNISH survey, an arcsecond resolution radio continuum survey of the inner Galactic plane at 5GHz using the Karl G. Jansky Very Large Array (VLA). It is a blind survey co-ordinated with the northern Spitzer GLIMPSE I region covering 10 o < l <65 o and |b| <1 o at similar resolution. We discuss in detail the strategy that we employed to control the shape of the synthesised beam across this survey that covers a wide range of fairly low declinations. Two snapshots separated by 4 hours in hour angle kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact H II regions to study this key phase in massive star formation. A sensitivity around 2mJy will allow the automatic distinction between radio loud and quiet mid-IR sources found in the Spitzer surveys. This survey has many legacy applications beyond star formation including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and non-thermal sources as well as atomic and molecular phases to build up a complete picture of the ISM in the Galaxy.

ATLASGAL – properties of compact H ii regions and their natal clumps

We present a complete sample of molecular clumps containing compact and ultracompact (UC) Hii regions betweenl = 10 ◦ and 60 ◦ and|b|< 1 ◦ , identified by combining the the ATLASGAL sub-mm and CORNISH radio continuum surveys wit h visual examination of archival infrared data. Our sample is complete to optically thin, compact and UC Hii regions driven by a zero age main sequence star of spectral type B0 or earlier embedded within a 1,000 M⊙ clump. In total we identify 213 compact and UC Hii regions, associated with 170 clumps. Unambiguous kinematic distances are derived for these clumps and used to estimate their masses and physical sizes, as well as the Lyman continuum fluxes and sizes of their embedded Hii regions. We find a clear lower envelope for the surface densit y of molecular clumps hosting massive star formation of 0.05 g cm −2 , which is consistent with a similar sample of clumps associated with 6.7 GHz masers. The mass of the most massive embedded stars is closely correlated with the mass of their natal clump. Young B stars appear to be significantly more luminous in the ultraviolet than predicted by current stellar atmosphere models. The properties of clumps associated with compact and UC Hii regions are very similar to those associated with 6.7 GHz methanol masers and we speculate that there is little evolution in the structure of the molecular clumps between these two phases. Finally, we identify a significant peak in the surface density of compact and UC Hii regions associated with the W49A star-forming complex, noting that this complex is truly one of the most massive and intense regions of star formation in the Galaxy.

Multiwavelength observations of southern hot molecular cores traced by methanol masers - I. Ammonia and 24-GHz continuum data

We present observations of the (1,1), (2,2), (4,4) and (5,5) inversion transitions of para-ammonia (NH 3 ) and 24-GHz continuum, taken with the Australia Telescope Compact Array towards 21 southern Galactic hot molecular cores traced by 6.7-GHz methanol maser emission. We detect NH 3 (1,1) emission towards all 21 regions and 24-GHz continuum emission towards 12 of the regions, including six with no reported 8-GHz continuum counterparts. In total, we find the 21 regions contain 41 NH 3 (1,1) cores but around half of the regions only contain a single core. We extract characteristic spectra for every core at each of the NH 3 transitions and present both integrated intensity maps and channel maps for each region. NH 3 (2,2) emission was detected towards all NH 3 (1,1) cores. NH 3 (4,4) emission was detected in 13 of the NH 3 (1,1) cores with NH 3 (5,5) emission coincident with 11 of these. The NH 3 (4,4) and (5,5) emission is always unresolved and found at the methanol maser position. An analysis of the NH 3 (1,1) and (2,2) line ratios suggests that the cores with NH 3 (4,4) and (5,5) emission are warmer than the remaining cores rather than simply containing more ammonia. The coincidence of the maser emission with the higher spatial resolution NH 3 (4,4) and (5,5) emission indicates that the methanol masers are found at the warmest part of the core. In all cores detected at NH 3 (4,4) (with the exception of G12.68-0.18 core 4), the measured linewidth increases with transition energy. The NH 3 (1,1) spectra of several cores show an emission and absorption component slightly offset in velocity but it is unclear whether or not this is due to systematic motion of the gas. We observe large asymmetries in the NH 3 (1,1) hyperfine line profiles and conclude that this is due to non-local thermodynamic equilibrium conditions arising from a number of dense, small clumps within the beam, rather than systematic motions of gas in the cores. Assuming that the 24-GHz continuum emission is optically-thin bremsstrahlung, we derive properties of the ionized gas. The rate of Lyman continuum photons required to ionize the gas of 10 45 -10 48 s -1 suggests that the continuum emission is powered by stars of mass >8 M ⊙ . We investigate the nature of the 24-GHz continuum sources which were not detected at 8 GHz and find that these are always coincident with both ammonia and methanol maser emission. This is in contrast to those detected at both 8 and 24 GHz which are generally offset from the methanol maser emission. We investigate the possibility that these may be hypercompact H II regions. Finally, we separate the cores into five groups, based on their association with NH 3 , methanol maser and continuum emission. From the different physical properties of the cores in the groups, we discuss the possibility that these groups may represent cores at different evolutionary stages of the massive star formation process.

The H2O Southern Galactic Plane Survey: NH3 (1,1) and (2,2) catalogues

The H₂O Southern Galactic Plane Survey (HOPS) has mapped a 100 degrees strip of the Galactic plane (-70° > l > 30°, |b| 10⁴cm⁻³). In this paper, we present the NH₃ (1,1) and (2,2) data, which have a resolution of 2 arcmin and cover a velocity range of ± 200 km s⁻¹. The median sensitivity of the NH₃ data cubes is σT(mb) =0.20 ± 0.06 K. For the (1,1) transition, this sensitivity equates to a 3.2 kpc distance limit for detecting a 20 K, 400 M⊙ cloud at the 5σ level. Similar clouds of mass 5000 M⊙ would be detected as far as the Galactic Centre, while 30000 M⊙ clouds would be seen across the Galaxy. We have developed an automatic emission finding procedure based on the Australian Telescope National Facility (ATNF) duchamp software and have used it to create a new catalogue of 669 dense molecular clouds. The catalogue is 100 per cent complete at the 5s detection limit (T(mb) = 1.0 K). A preliminary analysis of the ensemble cloud properties suggests that the near-kinematic distances are favoured. The cloud positions are consistent with current models of the Galaxy containing a long bar. Combined with other Galactic plane surveys this new molecular-line data set constitutes a key tool for examining Galactic structure and evolution. Data cubes, spectra and catalogues are available to the community via the HOPS website.