J. S. Urquhart

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.

The RMS Survey: Ammonia and water maser analysis of massive star forming regions. ⋆

The Red MSX Source (RMS) survey has identified a sample of∼1200 massive young stellar objects (MYSOs), compact and ultra compact Hii regions from a sample of∼2000 MSX and 2MASS colour selected sources. We have used the 100-m Green Bank telescope to search for 22-24 GHz water maser and ammonia (1,1), (2,2) and (3,3) emission towards∼600 RMS sources located within the northern Galactic plane. We have identified 308 H 2O masers which corresponds to an overall detection rate of∼50 per cent. We find no significant di fference in the detection rate for Hii regions and MYSOs which would suggest that the conditions required to produce maser emission are equally likely in both phases. Comparing the detection rates as a function of luminosity we find the H 2O detection rate has a positive dependence on the source luminosity, with the detection rate increasing with increasing luminosity. We detect ammonia emission towards 479 of these massive young stars, which corresponds to∼80 per cent. Ammonia is an excellent probe of high density gas allowing us to measure key parameters such as gas temperatures, opacities, and column densities, as well as providing an insight into the gas kinematics. The average kinetic temperature, FWHM line width and total NH3 column density for the sample are approximately 22 K, 2 km s −1 and 2× 10 15 cm −2 , respectively. We find that the NH 3 (1,1) line width and kinetic temperature are correlated with luminosity and finding no underlying dep endence of these parameters on the evolutionary phase of the embedded sources, we conclude that the observed trends in the derived parameters are more likely to be due to the energy output of the central source and/or the line width-clump mass relationship. The velocities of the peak H2O masers and the NH3 emission are in excellent agreement with each other, which would strongly suggest an association between the dense gas and the maser emission. Moreover, we find the bolometric luminosity of the embedded source and the isotropic luminosity of the H2O maser are also correlated. We conclude from the correlations of the cloud and water maser velocities and the bolometric and maser luminosity that there is a strong dynamical relationship between the embedded young massive star and the H2O maser.

The Red MSX Source survey: ammonia and water maser analysis of massive star-forming regions

The Red MSX Source (RMS) survey has identified a sample of∼1200 massive young stellar objects (MYSOs), compact and ultra compact Hii regions from a sample of∼2000 MSX and 2MASS colour selected sources. We have used the 100-m Green Bank telescope to search for 22-24 GHz water maser and ammonia (1,1), (2,2) and (3,3) emission towards∼600 RMS sources located within the northern Galactic plane. We have identified 308 H 2O masers which corresponds to an overall detection rate of∼50 per cent. We find no significant di fference in the detection rate for Hii regions and MYSOs which would suggest that the conditions required to produce maser emission are equally likely in both phases. Comparing the detection rates as a function of luminosity we find the H 2O detection rate has a positive dependence on the source luminosity, with the detection rate increasing with increasing luminosity. We detect ammonia emission towards 479 of these massive young stars, which corresponds to∼80 per cent. Ammonia is an excellent probe of high density gas allowing us to measure key parameters such as gas temperatures, opacities, and column densities, as well as providing an insight into the gas kinematics. The average kinetic temperature, FWHM line width and total NH3 column density for the sample are approximately 22 K, 2 km s −1 and 2× 10 15 cm −2 , respectively. We find that the NH 3 (1,1) line width and kinetic temperature are correlated with luminosity and finding no underlying dep endence of these parameters on the evolutionary phase of the embedded sources, we conclude that the observed trends in the derived parameters are more likely to be due to the energy output of the central source and/or the line width-clump mass relationship. The velocities of the peak H2O masers and the NH3 emission are in excellent agreement with each other, which would strongly suggest an association between the dense gas and the maser emission. Moreover, we find the bolometric luminosity of the embedded source and the isotropic luminosity of the H2O maser are also correlated. We conclude from the correlations of the cloud and water maser velocities and the bolometric and maser luminosity that there is a strong dynamical relationship between the embedded young massive star and the H2O maser.

A Radio and mid-infrared survey of northern bright-rimmed clouds

We have carried out an archival radio, optical and infrared wavelength imaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA Sky Survey (NVSS) archive, images from the Digitised Sky Survey (DSS) and the Midcourse Space eXperiment (MSX). The data characterise the physical properties of the Ionised Boundary Layer (IBL) of the BRCs. We have classified the radio detections as: that associated with the ionised cloud rims; that associated with possible embedded Young Stellar Objects (YSOs); and that unlikely to be associated with the clouds at all. The stars responsible for ionising each cloud are identified and a comparison of the expected ionising flux to that measured at the cloud rims is presented. A total of 25 clouds display 20 cm radio continuum emission that is associated with their bright optical rims. The ionising photon flux illuminating these clouds, the ionised gas pressure and the electron density of the IBL are determined. We derive internal molecular pressures for 9 clouds using molecular line data from the literature and compare these pressures to the IBL pressures to determine the pressure balance of the clouds. We find three clouds in which the pressure exerted by their IBLs is much greater than that measured in the internal molecular material. A comparison of external pressures around the remaining clouds to a global mean internal pressure shows that the majority of clouds can be expected to be in pressure equilibrium with their IBLs and hence are likely to be currently shocked by photoionisation shocks. We identify one source which shows 20 cm emission consistent with that of an embedded high-mass YSO and confirm its association with a known infrared stellar cluster. This embedded cluster is shown to contain early-type B stars, implying that at least some BRCs are intimately involved in intermediate to high mass star formation.

The RMS survey: resolving kinematic distance ambiguities towards a sample of compact H ii regions using H i absorption

We present high-resolution Hi data obtained using the Australia Telescope Compact Array to resolve the near/far distance ambiguities towards a sample of compact Hii regions from the Red MSX Source (RMS) survey. The high resolution data are complemented with lower resolution archival Hi data extracted from the Southern and VLA Galactic Plane surveys. We resolve the distance ambiguity for nearly all of the 105 sources where the continuum was strong enough to allow analysis of the Hi absorption line structure. This represents another step in the determination of distances to the total RMS sample, which with over 1,000 massive young stellar objects and compact Hii regions, is the largest and most complete sample of its kind. The full sample will allow the distribution of massive star formation in the Galaxy to be examined.

A radio and mid-infrared survey of northern bright-rimmed clouds

We have carried out an archival radio, optical and infrared wavelength imaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA Sky Survey (NVSS) archive, images from the Digitised Sky Survey (DSS) and the Midcourse Space eXperiment (MSX). The data characterise the physical properties of the Ionised Boundary Layer (IBL) of the BRCs. We have classified the radio detections as: that associated with the ionised cloud rims; that associated with possible embedded Young Stellar Objects (YSOs); and that unlikely to be associated with the clouds at all. The stars responsible for ionising each cloud are identified and a comparison of the expected ionising flux to that measured at the cloud rims is presented. A total of 25 clouds display 20 cm radio continuum emission that is associated with their bright optical rims. The ionising photon flux illuminating these clouds, the ionised gas pressure and the electron density of the IBL are determined. We derive internal molecular pressures for 9 clouds using molecular line data from the literature and compare these pressures to the IBL pressures to determine the pressure balance of the clouds. We find three clouds in which the pressure exerted by their IBLs is much greater than that measured in the internal molecular material. A comparison of external pressures around the remaining clouds to a global mean internal pressure shows that the majority of clouds can be expected to be in pressure equilibrium with their IBLs and hence are likely to be currently shocked by photoionisation shocks. We identify one source which shows 20 cm emission consistent with that of an embedded high-mass YSO and confirm its association with a known infrared stellar cluster. This embedded cluster is shown to contain early-type B stars, implying that at least some BRCs are intimately involved in intermediate to high mass star formation.

A Compact Array imaging survey of southern bright-rimmed clouds

We have carried out a radio-wavelength imaging survey of 45 bright-rimmed clouds (BRCs), using the Australia Telescope Compact Array to characterise the physical properties in their ionised boundary layers. We detected radio emission from a total of 25 clouds and using a combination of Digitised Sky Survey and mid-infrared MSX 8 µm images classified the emission into that associated with the ionised cloud rims, that associated with embedded possible massive YSOs and that unlikely to be associated with the clouds at all. A total of 18 clouds display radio emission clearly associated with the cloud rim and we determine the ionising photon flux illuminating these clouds and the electron density and pressure of their ionised boundary layers. Using a global estimate for the interior molecular pressure of these clouds we show that the majority are likely to be in pressure equilibrium and hence are currently being shocked by photoionisation-induced shocks. We identify those clouds where the predicted ionising photon flux is inconsistent with that derived from the observations and show that either the spectral types of the stars illuminating the BRCs are earlier than previously thought or that there must be additional ionising sources within the HII regions. Finally, we identify the radio sources embedded within the clouds with infrared stellar clusters and show that they contain late O and early B-type stars, demonstrating that a number of BRCs are intimately involved with high to intermediate-mass star formation.

A search for ionized jets towards massive young stellar objects

Radio continuum observations using the Australia telescope compact array at 5.5, 9.0, 17.0 and 22.8 GHz have detected free-free emission associated with 45 of 49 massive young stellar objects and HII regions. Of these, 26 sources are classified as ionized jets (12 of which are candidates), 2 as ambiguous jets or disc winds, 1 as a disc-wind, 14 as HII regions and 2 were unable to be categorised. Classification as ionized jets is based upon morphology, radio flux and spectral index, in conjunction with previous observational results at other wavelengths. Radio-luminosity and momentum are found to scale with bolometric luminosity in the same way as low-mass jets, indicating a common mechanism for jet production across all masses. In 13 of the jets, we see associated non-thermal/optically-thin lobes resulting from shocks either internal to the jet and/or at working surfaces. Ten jets display non-thermal (synchrotron emission) spectra in their lobes, with an average spectral index of α = -0.55 consistent with Fermi acceleration in shocks. This shows that magnetic fields are present, in agreement with models of jet formation incorporating magnetic fields. Since the production of collimated radio jets is associated with accretion processes, the results presented in this paper support the picture of disc-mediated accretion for the formation of massive stars with an upper-limit on the jet phase lasting approximately 6.5×10⁴ yr. Typical mass loss rates in the jet are found to be 1.4 × 10⁻⁵ M☉ yr⁻¹ with associated momentum rates of the order 1-2 × 10⁻² M☉ km s⁻¹ yr⁻¹.

High-mass star formation within the bright-rimmed cloud SFO 79

We report Radio Recombination Line (RRL) and continuum observations toward the IRAS point source 16362−4845, embedded within the Bright-Rimmed Cloud (BRC) SFO 79, a small molecular cloud lying at the edge of the HII region RCW 108. High resolution observations of the H92α hydrogen recombination line and of the continuum emission (3.6 and 6 cm) confirm the presence of a resolved Ultra Compact (UC) HII region embedded within the molecular cloud. The integrated radio fluxes suggest the source of the ionisation to be an O9 Zero Age Main Sequence (ZAMS) star. Millimetre observations of 12 CO, 13 CO and C 18 O( J = 1-0) molecular lines reveal the presence of a molecular condensation offset ∼30 �� to the north of the IRAS position on the boundary of the UC HII region. Analysis of 2MASS data has led to the identification of a small IR cluster of Young Stellar Objects (YSOs) that are positionally coincident with the UC HII region, lying to the south east of the peak of the radio emission. Moreover, the UC HII region appears to be extended in the direction of the IR cluster, which suggests that the radio emission and the IR cluster are in some way related to each other. MSX 8.3 µm and 21.3 µ mi mages have been used to trace the large scale structure of the BRC, revealing the presence of a Photo Dominated Region (PDR) and three embedded thermal sources within the molecular cloud. The PDR has a plane parallel morphology which correlates extremely well with the morphology of the ionised gas traced by the optical emission. The three thermal sources (labelled A, B, C) all lie at a similar projected distance from the interface between the HII region and the molecular gas of the cloud. Thermal sources A and C are positionally coincident with the IRAS point sources 16362-4845 and 16362-4841 respectively, both of which have IRAS colours consistent with the presence of UC HII regions. Given that UC HII regions are relatively short lived (∼10 5 yrs) it is reasonable to suggest that these two UC HII regions are of a similar age. The alignment of the three thermal sources along a line parallel to the bright rim suggests that they could have been triggered by the propagation of a plane parallel shock through the cloud.

ATLASGAL -- A Galaxy-wide sample of dense filamentary structures

[Abridged] Aims. We study the properties of filamentary structures from the ATLASGAL survey. Methods. We use the DisPerSE algorithm to identify spatially coherent structures located across the inner-Galaxy (300 < l < 60 and |b| < 1.5). Results. We have determined distances, masses and physical sizes for 241 of the filamentary structures. We find a median distance of 3.8 kpc, a mean mass of a few 10^3 m_sun, a mean length of ~6pc and a mass-to-length ratio of (M/L) ~200-2000M_sun/ pc. We also find that these filamentary structures are tightly correlated with the spiral arms in longitude and velocity, and that their semi-major axis is preferentially aligned parallel to the Galactic mid-plane and therefore with the direction of large-scale Galactic magnetic field. We find many examples where the dense filaments identified in ATLASGAL are associated with larger scale filamentary structures (~100 pc), and argue that this is likely to be common, and as such these may indicate a connection between large-scale Galactic dynamics and star formation. Conclusions. We have produced a large and Galaxy-wide catalogue of dense filamentary structures that are representative of a particular size and mass range not previously well studied in the literature. Analyses of the properties and distribution of these filaments reveals that they are correlated with the spiral arms and make a significant contribution to star formation in the Galaxy. Massive star formation is ongoing within ~20% of the filaments and is strongly correlated with the filaments with the largest mass-to- length ratios. The luminosity of the embedded sources has a similar distribution to the Galactic-wide samples of young massive stars and can therefore be considered to be representative.