arXiv subjects

C. R. Purcell

Publications and source records attributed to C. R. Purcell.

At least 19 recordsLinked to original sources

arXiv2018

The Coordinated Radio and Infrared Survey for High-mass Star Formation. IV: A new radio selected sample of compact Galactic Planetary Nebulae

We present a new radio-selected sample of PNe from the CORNISH survey. This is a radio continuum survey of the inner Galactic plane covering Galactic longitude, $10^\circ 1000 K) and located closer than 7 kpc. Within this sample is a water-maser PN with a spectral index of $-0.55\pm 0.08$, which indicates non-thermal radio emission. Such a radio-selected sample, unaffected by extinction, will be particularly useful to compare with population synthesis models and should contribute to the understanding of the formation and evolution of PNe.

astro-ph.SR↗

arXiv2018

The Coordinated Radio and Infrared Survey for High-Mass Star Formation III. A catalogue of northern ultra-compact H II regions

A catalogue of 239 ultra-compact HII regions (UCHIIs) found in the CORNISH survey at 5 GHz and 1.5" resolution in the region $10^{\circ} < l < 65^{\circ}, ~|b| < 1^{\circ}$ is presented. This is the largest complete and well-selected sample of UCHIIs to date and provides the opportunity to explore the global and individual properties of this key state in massive star formation at multiple wavelengths. The nature of the candidates was validated, based on observational properties and calculated spectral indices, and the analysis is presented in this work. The physical sizes, luminosities and other physical properties were computed by utilising literature distances or calculating the distances whenever a value was not available. The near- and mid-infrared extended source fluxes were measured and the extinctions towards the UCHIIs were computed. The new results were combined with available data at longer wavelengths and the spectral energy distributions (SEDs) were reconstructed for 177 UCHIIs. The bolometric luminosities obtained from SED fitting are presented. By comparing the radio flux densities to previous observational epochs, we find about 5% of the sources appear to be time variable. This first high-resolution area survey of the Galactic plane shows that the total number of UCHIIs in the Galaxy is ~ 750 - a factor of 3-4 fewer than found in previous large area radio surveys. It will form the basis for future tests of models of massive star formation.

astro-ph.GA↗

arXiv2018

Revealing the Faraday Depth Structure of Radio Galaxy NGC 612 with Broad-Band Radio Polarimetric Observations

We present full-polarisation, broadband observations of the radio galaxy NGC 612 (PKS B0131-637) from 1.3 to 3.1 GHz using the Australia Telescope Compact Array. The relatively large angular scale of the radio galaxy makes it a good candidate with which to investigate the polarisation mechanisms responsible for the observed Faraday depth structure. By fitting complex polarisation models to the polarised spectrum of each pixel, we find that a single polarisation component can adequately describe the observed signal for the majority of the radio galaxy. While we cannot definitively rule out internal Faraday rotation, we argue that the bulk of the Faraday rotation is taking place in a thin skin that girts the polarised emission. Using minimum energy estimates, we find an implied total magnetic field strength of 4.2 microG.

astro-ph.GA↗

arXiv2017

Broadband, radio spectro-polarimetric study of 100 radiative-mode and jet-mode AGN

We present the results from a broadband (1 to 3 GHz), spectro-polarimetry study of the integrated emission from 100 extragalactic radio sources with the ATCA, selected to be highly linearly polarized at 1.4 GHz. We use a general purpose, polarization model-fitting procedure that describes the Faraday rotation measure (RM) and intrinsic polarization structure of up to three distinct polarized emission regions or 'RM components' of a source. Overall, 37%/52%/11% of sources are best fit by one/two/three RM components. However, these fractions are dependent on the signal-to-noise ratio (S/N) in polarization (more RM components more likely at higher S/N). In general, our analysis shows that sources with high integrated degrees of polarization at 1.4 GHz have low Faraday depolarization, are typically dominated by a single RM component, have a steep spectral index, and a high intrinsic degree of polarization. After classifying our sample into radiative-mode and jet-mode AGN, we find no significant difference between the Faraday rotation or Faraday depolarization properties of jet-mode and radiative-mode AGN. However, there is a statistically significant difference in the intrinsic degree of polarization between the two types, with the jet-mode sources having more intrinsically ordered magnetic field structures than the radiative-mode sources. We also find a preferred perpendicular orientation of the intrinsic magnetic field structure of jet-mode AGN with respect to the jet direction, while no clear preference is found for the radiative-mode sources.

astro-ph.GA↗

arXiv2017

H2O Southern Galactic Plane Survey (HOPS): Paper III - Properties of Dense Molecular Gas across the Inner Milky Way

The H2O Southern Galactic Plane Survey (HOPS) has mapped 100 square degrees of the Galactic plane for water masers and thermal molecular line emission using the 22-m Mopra telescope. We describe the automated spectral-line fitting pipelines used to determine the properties of emission detected in HOPS datacubes, and use these to derive the physical and kinematic properties of gas in the survey. A combination of the angular resolution, sensitivity, velocity resolution and high critical density of lines targeted make the HOPS data cubes ideally suited to finding precursor clouds to the most massive and dense stellar clusters in the Galaxy. We compile a list of the most massive HOPS ammonia regions and investigate whether any may be young massive cluster progenitor gas clouds. HOPS is also ideally suited to trace the flows of dense gas in the Galactic Centre. We find the kinematic structure of gas within the inner 500pc of the Galaxy is consistent with recent predictions for the dynamical evolution of gas flows in the centre of the Milky Way. We confirm a recent finding that the dense gas in the inner 100pc has an oscillatory kinematic structure with characteristic length scale of ~20pc, and also identify similar oscillatory kinematic structure in the gas at radii larger than 100pc. Finally, we make all of the above fits and the remaining HOPS data cubes across the 100 square degrees of the survey available to the community.

astro-ph.GA↗

arXiv2017

Detection of a Coherent Magnetic Field in the Magellanic Bridge through Faraday Rotation

We present an investigation into the magnetism of the Magellanic Bridge, carried out through the observation of Faraday rotation towards 167 polarized extragalactic radio sources spanning the continuous frequency range of 1.3 - 3.1 GHz with the Australia Telescope Compact Array. Comparing measured Faraday depth values of sources 'on' and 'off' the Bridge, we find that the two populations are implicitly different. Assuming that this difference in populations is due to a coherent field in the Magellanic Bridge, the observed Faraday depths indicate a median line-of-sight coherent magnetic-field strength of $B_{\parallel}\,\simeq\,0.3\,μ$G directed uniformly away from us. Motivated by the varying magnitude of Faraday depths of sources on the Bridge, we speculate that the coherent field observed in the Bridge is a consequence of the coherent magnetic fields from the Large and Small Magellanic Clouds being pulled into the tidal feature. This is the first observation of a coherent magnetic field spanning the entirety of the Magellanic Bridge and we argue that this is a direct probe of a 'pan-Magellanic' field.

astro-ph.GA↗

arXiv2016

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 -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 $6.5 \times 10^4 yr$. Typical mass loss rates in the jet are found to be $1.4 \times 10^{-5} M_\odot yr^{-1}$ with associated momentum rates of the order $(1-2) \times 10^{-2} M_\odot km s^{-1} yr^{-1}$.

astro-ph.GA↗

arXiv2015

A radio-polarisation and rotation measure study of the Gum Nebula and its environment

The Gum Nebula is 36 degree wide shell-like emission nebula at a distance of only 450 pc. It has been hypothesised to be an old supernova remnant, fossil HII region, wind-blown bubble, or combination of multiple objects. Here we investigate the magneto-ionic properties of the nebula using data from recent surveys: radio-continuum data from the NRAO VLA and S-band Parkes All Sky Surveys, and H-alpha data from the Southern H-Alpha Sky Survey Atlas. We model the upper part of the nebula as a spherical shell of ionised gas expanding into the ambient medium. We perform a maximum-likelihood Markov chain Monte-Carlo fit to the NVSS rotation measure data, using the H-halpha data to constrain average electron density in the shell $n_e$. Assuming a latitudinal background gradient in RM we find $n_e=1.3^{+0.4}_{-0.4} {\rm cm}^{-3}$, angular radius $ϕ_{\rm outer}=22.7^{+0.1}_{-0.1} {\rm deg}$, shell thickness $dr=18.5^{+1.5}_{-1.4} {\rm pc}$, ambient magnetic field strength $B_0=3.9^{+4.9}_{-2.2} μ{\rm G}$ and warm gas filling factor $f=0.3^{+0.3}_{-0.1}$. We constrain the local, small-scale (~260 pc) pitch-angle of the ordered Galactic magnetic field to $+7^{\circ}\lesssim\wp\lesssim+44^{\circ}$, which represents a significant deviation from the median field orientation on kiloparsec scales (~-7.2$^{\circ}$). The moderate compression factor $X=6.0\,^{+5.1}_{-2.5}$ at the edge of the H-alpha shell implies that the 'old supernova remnant' origin is unlikely. Our results support a model of the nebula as a HII region around a wind-blown bubble. Analysis of depolarisation in 2.3 GHz S-PASS data is consistent with this hypothesis and our best-fitting values agree well with previous studies of interstellar bubbles.

astro-ph.GA↗

arXiv2013

Absolutely calibrated radio polarimetry of the inner Galaxy at 2.3 GHz and 4.8 GHz

We present high sensitivity and absolutely calibrated images of diffuse radio polarisation at a resolution of about 10 arcmin covering the range 10 degr <l<34 degr and |b|<5 degr at 2.3 GHz from the S-band Parkes All Sky Survey and at 4.8 GHz from the Sino-German 6 cm polarisation survey of the Galactic plane. Strong depolarisation near the Galactic plane is seen at 2.3 GHz, which correlates with strong Halpha emission. We ascribe the depolarisation to spatial Faraday rotation measure fluctuations of about 65 rad/m2 on scales smaller than 6-9 pc. We argue that most (about 90%) of the polarised emission seen at 4.8 GHz originates from a distance of 3-4 kpc in the Scutum arm and that the random magnetic field dominates the regular field there. A branch extending from the North Polar Spur towards lower latitudes can be identified from the polarisation image at 4.8 GHz but only partly from the polarised image at 2.3 GHz, implying the branch is at a distance larger than 2-3 kpc. We show that comparison of structure functions of complex polarised intensity with those of polarised intensity can indicate whether the observed polarised structures are intrinsic or caused by Faraday screens. The probability distribution function of gradients from the polarisation images at 2.3 GHz indicates the turbulence in the warm ionised medium is transonic.

astro-ph.GA↗

arXiv2013

MALT90: The Millimetre Astronomy Legacy Team 90 GHz Survey

The Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey aims to characterise the physical and chemical evolution of high-mass star-forming clumps. Exploiting the unique broad frequency range and on-the-fly mapping capabilities of the Australia Telescope National Facility Mopra 22 m single-dish telescope, MALT90 has obtained 3' x 3' maps toward ~2000 dense molecular clumps identified in the ATLASGAL 870 um Galactic plane survey. The clumps were selected to host the early stages of high-mass star formation and to span the complete range in their evolutionary states (from prestellar, to protostellar, and on to HII regions and photodissociation regions). Because MALT90 mapped 16 lines simultaneously with excellent spatial (38") and spectral (0.11 km/s) resolution, the data reveal a wealth of information about the clump's morphologies, chemistry, and kinematics. In this paper we outline the survey strategy, observing mode, data reduction procedure, and highlight some early science results. All MALT90 raw and processed data products are available to the community. With its unprecedented large sample of clumps, MALT90 is the largest survey of its type ever conducted and an excellent resource for identifying interesting candidates for high resolution studies with ALMA.

astro-ph.GA↗

arXiv2013

ATLASGAL --- properties of compact HII regions and their natal clumps

We present a complete sample of molecular clumps containing compact and ultra-compact (UC) HII regions between \ell=10\degr and 60\degr\ and $|b|<1\degr, identified by combining the the ATLASGAL submm and CORNISH radio continuum surveys with visual examination of archival infrared data. Our sample is complete to optically thin, compact and UCHII regions driven by a zero age main sequence star of spectral type B0 or earlier embedded within a 1,000 Msun clump. In total we identify 213 compact and UCHII 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 density 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 UCHII 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 UCHII 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.

astro-ph.GA↗

arXiv2012

The Co-ordinated Radio and Infrared Survey for High-Mass Star Formation - II. Source Catalogue

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 deg < l < 65 deg), 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-squared noise level better than 0.4 mJy/beam. Here we describe the data-processing methods and data characteristics, and present a new, uniform catalogue 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 optimised to detect emission on size scales up to 14" and for unresolved sources the catalogue is more than 90 percent complete at a flux density of 3.9 mJy. We have detected 3,062 sources above a 7-sigma detection limit and present their ensemble properties. The catalogue is highly reliable away from regions containing poorly-sampled extended emission, which comprise less than two percent of the survey area. Imaging problems have been mitigated by down-weighting the shortest spacings and potential artefacts flagged via a rigorous manual inspection with reference to the Spitzer infrared data. We present images of the most common source types found: regions, planetary nebulae and radio-galaxies. The CORNISH data and catalogue are available online at http://cornish.leeds.ac.uk

astro-ph.GA↗

arXiv2012

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 Galaxy's disk 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 deg < l < 3.5 deg, |b| < 0.5 deg contains ~10^7 Msun of dense 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, such as the amplitude of turbulent motions, must be included in the star-formation prescription to predict the star formation rate in a given mass of molecular gas.

astro-ph.GA↗

arXiv2012

The Co-ordinated 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 5 GHz 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 deg < l < 65 deg and |b| < 1 deg 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 HII regions to study this key phase in massive star formation. A sensitivity around 2 mJy 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.

astro-ph.GA↗

arXiv2012

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

The H2O Southern Galactic Plane Survey (HOPS) has mapped a 100 degree strip of the Galactic plane (-70deg > l > 30deg, |b| < 0.5deg) using the 22-m Mopra antenna at 12-mm wavelengths. Observations were conducted in on-the-fly mode using the Mopra spectrometer (MOPS), targeting water masers, thermal molecular emission and radio-recombination lines. Foremost among the thermal lines are the 23 GHz transitions of NH3 J,K = (1,1) and (2,2), which trace the densest parts of molecular clouds (n > 10^4 cm^{-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. The median sensitivity of the NH3 data-cubes is sigma_Tmb = 0.20 +/1 0.06 K. For the (1,1) transition this sensitivity equates to a 3.2 kpc distance limit for detecting a 20 K, 400 Msun cloud at the 5-sigma level. Similar clouds of mass 5,000 Msun would be detected as far as the Galactic centre, while 30,000 Msun clouds would be seen across the Galaxy. We have developed an automatic emission finding procedure based on the ATNF DUCHAMP software and have used it to create a new catalogue of 669 dense molecular clouds. The catalogue is 100 percent complete at the 5-sigma detection limit (Tmb = 1.0 K). A preliminary analysis of the ensemble cloud properties suggest 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 dataset constitutes a key tool for examining Galactic structure and evolution. Data-cubes, spectra and catalogues are available to the community via the HOPS website.

astro-ph.GA↗

arXiv2012

A survey for redshifted molecular and atomic absorption lines - II. Associated HI, OH and millimetre lines in the z >~ 3 Parkes quarter-Jansky flat-spectrum sample

We present the results of a z>2.9 survey for HI 21-cm and molecular absorption in the hosts of radio quasars using the GMRT and the Tidbinbilla 70-m telescope. Previously published searches, which are overwhelmingly at redshifts of z<1, exhibit a 42% detection rate (31 out of 73 sources), and the inclusion of our survey yields a 17% detection rate (2 out of 12 sources) at z>2.5. We therefore believe that our high redshift selection is responsible for our exclusive non-detections, and find that at ultra-violet luminosities of >10e23 W/Hz, 21-cm absorption has never been detected. We also find this to not only apply to our targets, but also those at low redshift exhibiting similar luminosities, giving zero detections out of a total of 16 sources over z=0.24 to 3.8. This is in contrast to the < 10e23 W/Hz sources where there is a near 50% detection rate of 21-cm absorption. The mix of 21-cm detections and non-detections is currently attributed to orientation effects, where according to unified schemes of active galactic nuclei, 21-cm absorption is more likely to occur in sources designated as radio galaxies (type-2 objects, where the nucleus is viewed through dense obscuring circumnuclear gas) than in quasars(type-1 objects, where we have a direct view to the nucleus). However, due to the exclusively high ultra-violet luminosities of our targets it is not clear whether orientation effects alone can wholly account for the distribution, although there exists the possibility that the large luminosities are indicative of a changing demographic of galaxy types. We also find that below luminosities of ~10e23 W/Hz, both type-1 and type-2 objects have a 50% likelihood of exhibiting 21-cm absorption.

astro-ph↗

arXiv2011

The H_2O southern Galactic Plane Survey (HOPS): I. Techniques and H_2O maser data

We present first results of the H_2O Southern Galactic Plane Survey (HOPS), using the Mopra radiotelescope with a broad band backend and a beam size of about 2'. We have observed 100 square degrees of the southern Galactic plane at 12mm (19.5 to 27.5GHz), including spectral line emission from water masers, multiple metastable transitions of ammonia, cyanoacetylene, methanol and radio recombination lines. In this paper, we report on the characteristics of the survey and water maser emission. We find 540 water masers, of which 334 are new detections. The strongest maser is 3933Jy and the weakest is 0.7Jy, with 62 masers over 100Jy. In 14 maser sites, the spread in velocity of the water maser emission exceeds 100km/s. In one region, the water maser velocities are separated by 351.3km/s. The rms noise levels are typically between 1-2Jy, with 95% of the survey under 2Jy. We estimate completeness limits of 98% at around 8.4Jy and 50% at around 5.5Jy. We estimate that there are between 800 and 1500 water masers in the Galaxy that are detectable in a survey with similar completeness limits to HOPS. We report possible masers in NH_3 (11,9) and (8,6) emission towards G19.61-0.23 and in the NH_3 (3,3) line towards G23.33-0.30.

astro-ph.GA↗

arXiv2011

Discovery of the new class I methanol maser transition at 23.4 GHz

We report the first detection of a methanol maser in the 10(1)-9(2)A- transition at 23.4 GHz, discovered during the H2O southern Galactic Plane Survey (HOPS) with the 22-m Mopra radio telescope. In the region covered by HOPS, the 23.4 GHz maser was found at only one location, G357.97-0.16, which was also a prominent source of maser emission in the J(2)-J(1)E series near 25 GHz. The Australia Telescope Compact Array (ATCA) was used to follow up these detections at high angular resolution and prove the maser nature of the observed emission. The analysis shows that the new methanol maser at 23.4 GHz is a class I maser, which has properties similar to the 9.9 and 25 GHz masers (i.e. traces strong shocks with higher than average temperature and density). All class I masers were found to originate at the same spatial location (within the measurement uncertainty of 0.5 arcseconds) in the vicinity of the dominant infrared source, but at a clearly distinct position from nearby OH, H2O and class II methanol masers at 6.7 GHz. All maser species are distributed approximately on a line, but it is not clear at present whether this has any physical significance. We also detected a weak (1.3 mJy) continuum source at 25 GHz near the OH maser (at the most northern site, associated with a class II methanol maser and an H2O maser renowned for its extremely wide spread of velocity components). The continuum source has not been reported at lower frequencies and is therefore a candidate hypercompact HII region. We also used the ATCA to find the strongest and only fifth known 9.9 GHz maser towards G357.97-0.16 and another 23.4 GHz maser towards G343.12-0.06 not seen in HOPS.

astro-ph.GA↗