A. J. Walsh

Publications (55)52.38 Total impact

• Article: Spectral imaging of the Central Molecular Zone in multiple 7-mm molecular lines

  P. A. Jones, M. G. Burton, M. R. Cunningham, N. F. H. Tothill, A. J. Walsh
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  ABSTRACT: We have imaged 24 spectral lines in the Central Molecular Zone (CMZ) around the Galactic Centre, in the range 42 to 50 GHz. The lines include emission from the CS, CH3OH, HC3N, SiO, HNCO, HOCO+, NH2CHO, OCS, HCS+, CCS, C34S, 13CS, 29SiO, H13CCCN, HCC13CN and HC5}N molecules, and three hydrogen recombination lines. The area covered is Galactic longitude -0.7 to 1.8 deg. and latitude -0.3 to 0.2 deg., including the bright cores around Sgr A, SgrB2, SgrC and G1.6-0.025. This work used the 22-m Mopra radio telescope in Australia, obtaining ~ 1.8 km/s spectral and ~ 65 arcsec spatial resolution. We present peak images from this study and conduct a principal component analysis on the integrated emission from the brightest 10 lines, to study similarities and differences in the line distribution. We examine the integrated line intensities and line ratios in selected apertures around the bright cores, as well as for the complete mapped region of the CMZ. We compare these 7-mm lines to the corresponding lines in the 3-mm band, for five molecules, to study the excitation. There is a variation in 3-mm to 7-mm line ratio across the CMZ, with relatively higher ratio in the centre around Sgr B2 and Sgr A. We find that the lines are sub-thermally excited, and from modelling with RADEX find that non-LTE conditions apply, with densities of order 10^4 cm^{-3}.
  04/2013;
• Article: Candidate super star cluster progenitor gas clouds possibly triggered by close passage to Sgr A*

  S. N. Longmore, J. M. D. Kruijssen, J. Bally, J. Ott, L. Testi, J. Rathborne, N. Bastian, E. Bressert, S. Molinari, C. Battersby, A. J. Walsh
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  ABSTRACT: Super star clusters are the end product of star formation under the most extreme conditions. As such, studying how their final stellar populations are assembled from their natal progenitor gas clouds can provide strong constraints on star formation theories. An obvious place to look for the initial conditions of such extreme stellar clusters are gas clouds of comparable mass and density, with no star formation activity. We present a method to identify such progenitor gas clouds and demonstrate the technique for the gas in the inner few hundred pc of our Galaxy. The method highlights three clouds in the region with similar global physical properties to the previously identified extreme cloud, G0.253+0.016, as potential young massive cluster (YMC) precursors. The fact that four potential YMC progenitor clouds have been identified in the inner 100 pc of the Galaxy, but no clouds with similar properties have been found in the whole first quadrant despite extensive observational efforts, has implications for cluster formation/destruction rates across the Galaxy. We put forward a scenario to explain how such dense gas clouds can arise in the Galactic centre environment, in which YMC formation is triggered by gas streams passing close to the minimum of the global Galactic gravitational potential at the location of the central supermassive black hole, Sgr A*. If this triggering mechanism can be verified, we can use the known time interval since closest approach to Sgr A* to study the physics of stellar mass assembly in an extreme environment as a function of absolute time.
  04/2013;
• Article: Variations in the Galactic star formation rate and density thresholdsfor star formation

  S. N. Longmore, J. Bally, L. Testi, C. R. Purcell, A. J. Walsh, E. Bressert, M. Pestalozzi, S. Molinari, J. Ott, L. Cortese, C. Battersby, N. Murray, E. Lee, J. M. D. Kruijssen, E. Schisano, D. Elia
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  ABSTRACT: 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.
  Monthly Notices of the Royal Astronomical Society 02/2013; 429(2):987. · 4.90 Impact Factor
• Article: The H2O southern Galactic Plane Survey(HOPS): NH3 (1,1) and (2,2) catalogues

  C. R. Purcell, S. N. Longmore, A. J. Walsh, M. T. Whiting, S. L. Breen, T. Britton, K. J. Brooks, M. G. Burton, M. R. Cunningham, J. A. Green, [......], M. G. Hoare, B. Indermuehle, P. A. Jones, N. Lo, V. Lowe, C. J. Phillips, M. A. Thompson, J. S. Urquhart, M. A. Voronkov, G. L. White
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  ABSTRACT: 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.
  07/2012;
• Article: New class I methanol masers

  M. A. Voronkov, J. L. Caswell, S. P. Ellingsen, S. L. Breen, T. R. Britton, J. A. Green, A. M. Sobolev, A. J. Walsh
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  ABSTRACT: We review properties of all known collisionally pumped (class I) methanol maser series based on observations with the Australia Telescope Compact Array (ATCA) and the Mopra radio telescope. Masers at 36, 84, 44 and 95 GHz are most widespread, while 9.9, 25, 23.4 and 104 GHz masers are much rarer, tracing the most energetic shocks. A survey of many southern masers at 36 and 44 GHz suggests that these two transitions are highly complementary. The 23.4 GHz maser is a new type of rare class I methanol maser, detected only in two high-mass star-forming regions, G357.97-0.16 and G343.12-0.06, and showing a behaviour similar to 9.9, 25 and 104 GHz masers. Interferometric positions suggest that shocks responsible for class I masers could arise from a range of phenomena, not merely an outflow scenario. For example, some masers might be caused by interaction of an expanding HII region with its surrounding molecular cloud. This has implications for evolutionary sequences incorporating class I methanol masers if they appear more than once during the evolution of the star-forming region. We also make predictions for candidate maser transitions at the ALMA frequency range.
  03/2012;
• Source

  Available from: Paul Andrew Jones

  Article: Spectral imaging of the Central Molecular Zone in multiple 3-mm molecular lines

  P A Jones, M. G. Burton, M. R. Cunningham, M. A. Requena-Torres, K. M. Menten, P. Schilke, A. Belloche, S. Leurini, J. Martin-Pintado, J. Ott, A. J. Walsh
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  ABSTRACT: We have mapped 20 molecular lines in the Central Molecular Zone (CMZ) around the Galactic Centre, emitting from 85.3 to 93.3 GHz. This work used the 22-m Mopra radio telescope in Australia, equipped with the 8-GHz bandwidth UNSW-MOPS digital filter bank, obtaining \sim 2 km/s spectral and \sim 40 arcsec spatial resolution. The lines measured include emission from the c-C3H2, CH3CCH, HOCO+, SO, H13CN, H13CO+, SO, H13NC, C2H, HNCO, HCN, HCO+, HNC, HC3N, 13CS and N2H+ molecules. The area covered is Galactic longitude -0.7 to 1.8 deg. and latitude -0.3 to 0.2 deg., including the bright dust cores around Sgr A, Sgr B2, Sgr C and G1.6-0.025. We present images from this study and conduct a principal component analysis on the integrated emission from the brightest 8 lines. This is dominated by the first component, showing that the large-scale distribution of all molecules are very similar. We examine the line ratios and optical depths in selected apertures around the bright dust cores, as well as for the complete mapped region of the CMZ. We highlight the behaviour of the bright HCN, HNC and HCO+ line emission, together with that from the 13C isotopologues of these species, and compare the behaviour with that found in extra-galactic sources where the emission is unresolved spatially. We also find that the isotopologue line ratios (e.g. HCO+/H13CO+) rise significantly with increasing red-shifted velocity in some locations. Line luminosities are also calculated and compared to that of CO, as well as to line luminosities determined for external galaxies.
  10/2011;
• Source

  Available from: Luke Hindson

  Article: The H2O Southern Galactic Plane Survey (HOPS) – I. Techniques and H2O maser data

  A. J. Walsh, S. L. Breen, T. Britton, K. J. Brooks, M. G. Burton, M. R. Cunningham, J. A. Green, L. Harvey-Smith, L. Hindson, M. G. Hoare, [......], N. Lo, S. N. Longmore, V. Lowe, C. J. Phillips, C. R. Purcell, M. A. Thompson, J. S. Urquhart, M. A. Voronkov, G. L. White, M. T. Whiting
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  ABSTRACT: We present first results of the H2O Southern Galactic Plane Survey (HOPS), using the Mopra Radio Telescope with a broad-band backend and a beam size of about 2 arcmin. We have observed 100 deg2 of the southern Galactic plane at 12 mm (19.5–27.5 GHz), including spectral line emission from H2O masers, multiple metastable transitions of ammonia, cyanoacetylene, methanol and radio recombination lines. In this paper, we report on the characteristics of the survey and H2O maser emission. We find 540 H2O masers, of which 334 are new detections. The strongest maser is 3933 Jy and the weakest is 0.7 Jy, with 62 masers over 100 Jy. In 14 maser sites, the spread in the velocity of the H2O maser emission exceeds 100 km s−1. In one region, the H2O maser velocities are separated by 351.3 km s−1. The rms noise levels are typically between 1 and 2 Jy, with 95 per cent of the survey under 2 Jy. We estimate completeness limits of 98 per cent at around 8.4 Jy and 50 per cent at around 5.5 Jy. We estimate that there are between 800 and 1500 H2O masers in the Galaxy that are detectable in a survey with similar completeness limits to HOPS. We report possible masers in NH3 (11,9) and (8,6) emission towards G19.61−0.23 and in the NH3 (3,3) line towards G23.33−0.30.
  Monthly Notices of the Royal Astronomical Society 09/2011; 416(3):1764 - 1821. · 4.90 Impact Factor
• Source

  Available from: ArXiv

  Article: A 7mm line survey of the shocked and disrupted molecular gas towards the W28 field TeV gamma-ray sources

  B. P. Nicholas, G. Rowell, M. G. Burton, A. J. Walsh, Y Fukui, A Kawamura, N. I. Maxted
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  ABSTRACT: We present 7mm Mopra observations of the dense molecular gas towards the W28 supernova remnant (SNR) field, following a previous 12mm line survey of this region. These observations take advantage of the 7mm beam size to probe the dense and disrupted gas in the region at ~1 arcmin scales. Our observations are focused towards the north-eastern (NE) HESS J1801-233 and southern HESS J1800-240B TeV gamma-ray sources, with slightly less observations towards HESS J1800-240A & C. Using the CS (1-0) transition we reveal multiple regions of dense gas, nH2 ~10^5 cm^-3. We report the discovery of dense gas towards HESS J1800-240C, at the site of a 1720MHz OH maser. The NE molecular cloud is known to be disrupted, many 1720MHz OH masers and broad CO line emission are detected at the rim of W28. Here we reveal this shock interaction region contains generally extended clumpy CS, as well as clumpy SiO and CH3OH emission with broad line profiles. The FWHM of the molecular lines extend up to 18 km/s on the W28 side of the NE cloud. The detection of SiO towards maser clumps OH C, D, E & F provide further evidence of the shocked conditions in the NE cloud. Several other lines associated with star formation are also detected towards the southern source, notably the energetic HII complex G5.89-0.39. The spatial match of dense gas with the TeV emission further supports the CR origin for the gamma-rays. We estimate the mass of several extended dense clouds within the field and predict the TeV flux from the dense cloud components. The predicted fluxes are on the order of 10^{-14} - 10^{-13} ph/cm^2/s, which should be detectable and possibly resolved by a future TeV instrument, such as the Cherenkov Telescope Array.
  08/2011;
• Source

  Available from: Luke Hindson

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

  A. J. Walsh, S. L. Breen, T. Britton, K. J. Brooks, M. G. Burton, M. R. Cunningham, J. A. Green, L. Harvey-Smith, L. Hindson, M. G. Hoare, [......], N. Lo, S. N. Longmore, V. Lowe, C J Phillips, C. R. Purcell, M. A. Thompson, J. S. Urquhart, M. A. Voronkov, G. L. White, M. T. Whiting
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  ABSTRACT: 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.
  05/2011;
• Source

  Available from: ArXiv

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

  M. A. Voronkov, A. J. Walsh, J. L. Caswell, S. P. Ellingsen, S. L. Breen, S. N. Longmore, C. R. Purcell, J. S. Urquhart
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  ABSTRACT: 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.
  01/2011;
• Source

  Available from: Gary A. Fuller

  Article: The 6-GHz methanol multibeam maser catalogue I: Galactic Centre region, longitudes 345 to 6

  J. L. Caswell, G. A. Fuller, J. A. Green, A. Avison, S. L. Breen, K. J. Brooks, M. G. Burton, A. Chrysostomou, J Cox, P. J. Diamond, [......], M. R. Pestalozzi, C J Phillips, L. Quinn, M. A. Thompson, M. A. Voronkov, A. J. Walsh, D. Ward-Thompson, D. Wong-McSweeney, J. A. Yates, R. J. Cohen
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  ABSTRACT: We have conducted a Galactic plane survey of methanol masers at 6668 MHz using a 7-beam receiver on the Parkes telescope. Here we present results from the first part, which provides sensitive unbiased coverage of a large region around the Galactic Centre. Details are given for 183 methanol maser sites in the longitude range 345$^{\circ}$ through the Galactic Centre to 6$^{\circ}$. Within 6$^{\circ}$ of the Centre, we found 88 maser sites, of which more than half (48) are new discoveries. The masers are confined to a narrow Galactic latitude range, indicative of many sources at the Galactic Centre distance and beyond, and confined to a thin disk population; there is no high latitude population that might be ascribed to the Galactic Bulge. Within 2$^{\circ}$ of the Galactic Centre the maser velocities all lie between -60 and +77 \kms, a range much smaller than the 540 \kms range observed in CO. Elsewhere, the maser with highest positive velocity (+107 \kms) occurs, surprisingly, near longitude 355$^{\circ}$ and is probably attributable to the Galactic bar. The maser with the most negative velocity (-127 \kms) is near longitude 346$^{\circ}$, within the longitude-velocity locus of the near side of the `3-kpc arm'. It has the most extreme velocity of a clear population of masers associated with the near and far sides of the 3-kpc arm. Closer to the Galactic Centre the maser space density is generally low, except within 0.25 kpc of the Centre itself, the `Galactic Centre Zone', where it is 50 times higher, which is hinted at by the longitude distribution, and confirmed by the unusual velocities. Comment: 35 pages, 5 figures
  02/2010;
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  Available from: ArXiv

  Article: Mopra line survey mapping of NGC6334I and I(N) at 3mm

  A. J. Walsh, S. Thorwirth, H. Beuther, M. G. Burton
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  ABSTRACT: A 5'x5' region encompassing NGC6334I and I(N) has been mapped at a wavelength of 3mm (from 83.5 to 115.5GHz) with the Mopra telescope at an angular resolution between 33 arcsec and 36 arcsec. This investigation has made use of the recently installed 3mm MMIC receiver and the Mopra Spectrometer (MOPS) with broadband capabilities permitting total coverage of the entire frequency range with just five different observations. In total, the spatial distribution of nineteen different molecules, ions and radicals, along with additional selected isotopologues have been studied. Whilst most species trace the sites of star formation, CH_3CN appears to be most closely associated with NGC6334I and I(N). Both CN and C_2H appear to be widespread, tracing gas that is not associated with active star formation. Both N_2H^+ and HC_3N closely resemble dust continuum emission, showing they are reliable tracers of dense material, as well as the youngest stages of high mass star formation. Hot (E_u/k>100K) thermal CH_3OH emission is preferentially found towards NGC6334I, contrasting with I(N), where only cold (E_u/k<22K) thermal CH_3OH emission is found. Comment: Accepted by MNRAS
  01/2010;
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  Available from: M. R. Cunningham

  Article: Multi-generation massive star-formation in NGC3576

  C. R. Purcell, V. Minier, S. N. Longmore, Ph. André, A. J. Walsh, P. Jones, F. Herpin, T. Hill, M. R. Cunningham, M. G. Burton
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  ABSTRACT: Recent 1.2-mm continuum observations have shown the giant HII region NGC3576 to be embedded in the centre of an extended filamentary dust-cloud. The bulk of the filament away from the HII region contains a number of clumps seen only at (sub-)millimetre wavelengths and which may host massive protostellar objects at a very early stage of evolution. We have used the Australia Telescope Compact Array (ATCA) to image the cloud for the NH3(1,1), (2,2) and (4,4) transitions, 22 GHz water masers, and 23 GHz continuum emission. We also utilised the 22-m Mopra antenna to map the region for the molecular lines 13CO (1-0), C18O (1-0), HCO+ (1-0), H13CO+ (1-0), CS (1-0) and N2H+ (1-0).The HII region is observed to be expanding into the molecular cloud, sweeping up a clumpy shell of gas, while the central star cluster is dispersing the molecular gas to the east. Temperatures are highest adjacent to the central HII region, indicating that the embedded cluster of young stars there is heating the gas. Six new water masers were detected in the arms of the filament, all associated with NH3 emission peaks, confirming that star-formation has begun within these cores. Core masses range from 5 to 516 solar masses and most appear to be gravitationally bound. Complementary results by Andr\'e et al. (2008) imply that seven cores will go on to form massive stars between 15 and 50 solar masses. The large scale velocity structure of the filament is smooth, but at least one clump shows the signature of inward gas motions via asymmetries in the NH3 (1,1) line profiles. The same clump exhibits an enhanced abundance of N2H+, which coupled with an absence of CO indicates depletion onto the dust grain surface. The HII region at the heart of NGC3576 is potentially triggering the formation of massive stars in the bulk of the associated cloud. Comment: 22 pages, 16 figures
  09/2009;
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  Available from: ArXiv

  Article: Hot high-mass accretion disk candidates

  H. Beuther, A. J. Walsh, S. N. Longmore
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  ABSTRACT: To better understand the physical properties of accretion disks in high-mass star formation, we present a study of a 12 high-mass accretion disk candidates observed at high spatial resolution with the Australia Telescope Compact Array (ATCA) in the NH3 (4,4) and (5,5) lines. Almost all sources were detected in NH3, directly associated with CH3OH Class II maser emission. From the remaining eleven sources, six show clear signatures of rotation and/or infall motions. These signatures vary from velocity gradients perpendicular to the outflows, to infall signatures in absorption against ultracompact HII regions, to more spherical infall signatures in emission. Although our spatial resolution is ~1000AU, we do not find clear Keplerian signatures in any of the sources. Furthermore, we also do not find flattened structures. In contrast to this, in several of the sources with rotational signatures, the spatial structure is approximately spherical with sizes exceeding 10^4 AU, showing considerable clumpy sub-structure at even smaller scales. This implies that on average typical Keplerian accretion disks -- if they exist as expected -- should be confined to regions usually smaller than 1000AU. It is likely that these disks are fed by the larger-scale rotating envelope structure we observe here. Furthermore, we do detect 1.25cm continuum emission in most fields of view. Comment: 21 pages, 32 figures, accepted for ApJS. A high-resolution version can be found at http://www.mpia.de/homes/beuther/papers.html
  09/2009;
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  Available from: ArXiv

  Article: Too large and overlooked? Extended free-free emission towards massive star formation regions

  S. N. Longmore, M. G. Burton, E. Keto, S. Kurtz, A. J. Walsh
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  ABSTRACT: We present Australia Telescope Compact Array observations towards 6 massive star formation regions which, from their strong 24 GHz continuum emission but no compact 8 GHz continuum emission, appeared good candidates for hyper-compact HII regions. However, the properties of the ionised gas derived from the 19 to 93 GHz continuum emission and H70 alpha + H57 alpha radio recombination line data show the majority of these sources are, in fact, regions of spatially-extended, optically-thin free-free emission. These extended sources were missed in the previous 8 GHz observations due to a combination of spatial-filtering, poor surface brightness sensitivity and primary beam attenuation. We consider the implications that a significant number of these extended HII regions may have been missed by previous surveys of massive star formation regions. If the original sample of 21 sources is representative of the population as a whole, the fact that 6 contain previously undetected extended free-free emission suggests a large number of regions have been mis-classified. Rather than being very young objects prior to UCHII region formation, they are, in fact, associated with extended HII regions and thus significantly older. In addition, inadvertently ignoring a potentially substantial flux contribution (up to ~0.5Jy) from free-free emission has implications for dust masses derived from sub-mm flux densities. The large spatial scales probed by single-dish telescopes, which do not suffer from spatial filtering, are particularly susceptible and dust masses may be overestimated by up to a factor of ~2. Comment: Accepted MNRAS. 22 pages, 13 figures, 5 tables
  06/2009;
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  Available from: kde-redhat.org

  Article: Cyanopolyynes in hot cores: modelling G305.2+0.2

  J. F. Chapman, T. J. Millar, M. Wardle, M. G. Burton, A. J. Walsh
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  ABSTRACT: We present results from a time-dependent gas-phase chemical model of a hot core based on the physical conditions of G305.2+0.2. While the cyanopolyyne HC3N has been observed in hot cores, the longer chained species, HC5N, HC7N and HC9N, have not been considered as the typical hot-core species. We present results which show that these species can be formed under hot core conditions. We discuss the important chemical reactions in this process and, in particular, show that their abundances are linked to the parent species acetylene which is evaporated from icy grain mantles. The cyanopolyynes show promise as ‘chemical clocks’ which may aid future observations in determining the age of hot core sources. The abundance of the larger cyanopolyynes increases and decreases over relatively short time-scales, ∼102.5 yr. We present results from a non-local thermodynamic equilibrium statistical equilibrium excitation model as a series of density, temperature and column density dependent contour plots which show both the line intensities and several line ratios. These aid in the interpretation of spectral-line data, even when there is limited line information available. In particular, non-detections of HC5N and HC7N in Walsh et al. are analysed and discussed.
  Monthly Notices of the Royal Astronomical Society 03/2009; 394(1):221 - 230. · 4.90 Impact Factor
• Article: Physical and chemical conditions in methanol maser selected hot cores and UCH ii regions

  C. R. Purcell, S. N. Longmore, M. G. Burton, A. J. Walsh, V. Minier, M. R. Cunningham, R. Balasubramanyam
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  ABSTRACT: We present the results of a targeted 3-mm spectral line survey towards the eighty-three 6.67 GHz methanol maser selected star-forming clumps observed by Purcell. In addition to the previously reported measurements of HCO+ (1–0), H13CO+ (1–0) and CH3CN (5–4) and (6–5), we used the Mopra antenna to detect emission lines of N2H+ (1–0), HCN (1–0) and HNC (1–0) towards 82/83 clumps (99 per cent), and CH3OH (2–1) towards 78/83 clumps (94 per cent).The molecular line data have been used to derive virial and local thermodynamic equilibrium masses, rotational temperatures and chemical abundances in the clumps, and these properties have been compared between subsamples associated with different indicators of evolution. The greatest differences are found between clumps associated with 8.6 GHz radio emission, indicating the presence of an Ultra-Compact H ii (UCH ii) region, and ‘isolated’ masers (without associated radio emission), and between clumps exhibiting CH3CN emission and those without. In particular, thermal CH3OH is found to be brighter and more abundant in UCH ii regions and in sources with detected CH3CN, and may constitute a crude molecular clock in single dish observations.Clumps associated with 8.6 GHz radio emission tend to be more massive and more luminous than clumps without radio emission. This is likely because the most massive clumps evolve so rapidly that a Hyper-Compact H ii or UCH ii region is the first visible tracer of star formation.The gas mass to submm/infrared luminosity relation for the combined sample was found to be L∝M0.68, considerably shallower than expected for massive main-sequence stars. This implies that the mass of the clumps is comparable to, or greater than, the mass of the stellar content.We also find that the mass of the hot core is correlated with the mass of the clump in which it is embedded.
  Monthly Notices of the Royal Astronomical Society 03/2009; 394(1):323 - 339. · 4.90 Impact Factor
• Article: High‐velocity H2O maser emission from the post‐asymptotic‐giant‐branch star OH 009.1–0.4

  A. J. Walsh, S. L. Breen, I. Bains, W. H. T. Vlemmings
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  ABSTRACT: Observations of H2O masers towards the post-asymptotic-giant-branch star and water fountain source OH 009.1−0.4 were made as part of The H2O southern galactic Plane Survey, with the Mopra radiotelescope. Together with followup observations using the Australia Telescope Compact Array (ATCA), we have identified H2O maser emission over a velocity spread of nearly 400 km s−1 (−109 to +289 km s−1). This velocity spread appears to be the largest of any known maser source in our Galaxy. High-resolution observations with the ATCA indicate that the maser emission is confined to a region 0.3 × 0.3 arcsec and shows weak evidence for a separation of the red- and blueshifted maser spots. We are unable to determine if the water fountain is projected along the line of sight, or is inclined, but either way OH 009.1−0.4 is an interesting source, worthy of followup observations.
  Monthly Notices of the Royal Astronomical Society Letters 02/2009; 394(1):L70 - L73.
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  Available from: ArXiv

  Article: High velocity water maser emission from the Post-AGB star OH 009.1-0.4

  A. J. Walsh, S. L. Breen, I. Bains, W. H. T. Vlemmings
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  ABSTRACT: Observations of H$_2$O masers towards the post-AGB star and water fountain source OH 009.1--0.4 were made as part of HOPS (The H$_2$O southern galactic Plane Survey), with the Mopra radiotelescope. Together with followup observations using the Australia Telescope Compact Array (ATCA), we have identified H$_2$O maser emission over a velocity spread of nearly 400km/s (--109 to +289km/s). This velocity spread appears to be the largest of any known maser source in our Galaxy. High resolution observations with the ATCA indicate the maser emission is confined to a region $0\farcs3 \times 0\farcs3$ and shows weak evidence for a separation of the red- and blueshifted maser spots. We are unable to determine if the water fountain is projected along the line of sight, or is inclined, but either way OH 009.1--0.4 is an interesting source, worthy of followup observations.
  01/2009;
• Source

  Available from: Tom Megeath

  Article: High Spatial Resolution Observations of NH3 and CH3OH toward the Massive Twin Cores NGC 6334I and NGC 6334I(N)

  H. Beuther, S. Thorwirth, Q. Zhang, T. R. Hunter, S. T. Megeath, A. J. Walsh, and K. M. Menten
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  ABSTRACT: Molecular line observations of NH3 (J,K) = (1,1) and (2, 2) and CH3OH at 24.93 GHz taken with the Australian Telescope Compact Array (ATCA) toward the massive twin cores NGC 6334I and NGC 6334I(N) reveal significant variations in the line emission between the two massive cores. The UC H II region/hot core NGC 6334I exhibits strong thermal NH3 and CH3OH emission adjacent to the UC H II region and coincident with two millimeter continuum peaks observed by T. R. Hunter et al. In contrast, we find neither compact NH3 nor thermal CH3OH line emission toward NGC 6334I(N). There the NH3 emission is distributed over a broad region (>1') without a clear peak, and we find Class I CH3OH maser emission with peak brightness temperatures up to 7000 K. The maser emission peaks appear to be spatially associated with the interfaces between the molecular outflows and the ambient dense gas. Peak NH3 (1, 1) line brightness temperatures ≥70 K in both regions indicate gas temperatures on the same order. NH3 emission is also detected toward the outflow in NGC 6334I, resulting in an estimated rotational temperature of Trot ~ 19 K. Furthermore, we observe CH3OH and NH3 absorption toward the UC H II region; the velocity structure is consistent with expanding molecular gas around the UC H II region. Thermal and kinematic effects possibly imposed from the UC H II region on the molecular core are also discussed.
  The Astrophysical Journal 12/2008; 627(2):834. · 6.02 Impact Factor