Showing papers by "Maxim Voronkov published in 2012"

The 6-GHz methanol multibeam maser catalogue – IV. Galactic longitudes 186°–330° including the Orion–Monoceros region

Abstract: We present the fourth portion of a Galactic plane survey of methanol masers at 6668 MHz, spanning the longitude range 186°–330°. We report 207 maser detections, 89 new to the survey. This completes the southern sky part of the methanol multibeam survey and includes a large proportion of new sources, 43 per cent. We also include results from blind observations of the Orion–Monoceros star-forming region, formally outside the latitude range of the methanol multibeam survey; only the four previously known methanol emitting sites were detected, of which we present new positions and spectra for masers at Orion A (south) and Orion B, obtained with the Multi-Element Radio Linked Interferometer Network (MERLIN) array.

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

Abstract: 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.

Search for Ionized Jets toward High-mass Young Stellar Objects

Abstract: We are carrying out multi-frequency radio continuum observations, using the Australia Telescope Compact Array, to systematically search for collimated ionized jets toward high-mass young stellar objects (HMYSOs). Here we report observations at 1.4, 2.4, 4.8, and 8.6 GHz, made with angular resolutions of about 7'', 4'', 2'', and 1'', respectively, toward six objects of a sample of 33 southern HMYSOs thought to be in very early stages of evolution. The objects in the sample were selected from radio and infrared catalogs by having positive radio spectral indices and being luminous (L bol > 2 × 104 L ☉), but underluminous in radio emission compared with that expected from its bolometric luminosity. This criterion makes the radio sources good candidates for being ionized jets. As part of this systematic search, two ionized jets have been discovered: one previously published and the other reported here. The rest of the observed candidates correspond to three hypercompact H II regions and two ultracompact H II regions. The two jets discovered are associated with two of the most luminous (7 × 104 and 1.0 × 105 L ☉) HMYSOs known to harbor this type of object, showing that the phenomena of collimated ionized winds appear in the formation process of stars at least up to masses of ~20 M ☉ and provide strong evidence for a disk-mediated accretion scenario for the formation of high-mass stars. From the incidence of jets in our sample, we estimate that the jet phase in high-mass protostars lasts for ~4 × 104 yr.

12.2-GHz methanol maser MMB follow-up catalogue – I. Longitude range 330° to 10°

Abstract: We present a catalogue of 12.2-GHz methanol masers detected towards 6.7-GHz methanol masers observed in the unbiased Methanol Multibeam (MMB) survey in the longitude range 330° (through 360°) to 10°. This is the first portion of the catalogue which, when complete, will encompass all of the MMB detections. We report the detection of 185 12.2-GHz sources towards 400 6.7-GHz methanol maser targets, equating to a detection rate of 46 per cent. Of the 185 12.2-GHz detections, 118 are reported here for the first time. We draw attention to a number of ‘special’ sources, particularly those with emission at 12.2-GHz stronger than their 6.7-GHz counterpart, and conclude that these unusual sources are not associated with a specific evolutionary stage.

Search for ionized jets towards high-mass young stellar objects

Abstract: We are carrying out multi-frequency radio continuum observations, using the Australia Telescope Compact Array, to systematically search for collimated ionized jets towards high-mass young stellar objects (HMYSOs). Here we report observations at 1.4, 2.4, 4.8 and 8.6 GHz, made with angular resolutions of about 7, 4, 2, and 1 arcsec, respectively, towards six objects of a sample of 33 southern HMYSOs thought to be in very early stages of evolution. The objects in the sample were selected from radio and infrared catalogs by having positive radio spectral indices and being luminous (L_bol > 20,000 L_sun), but underluminous in radio emission compared to that expected from its bolometric luminosity. This criteria makes the radio sources good candidates for being ionized jets. As part of this systematic search, two ionized jets have been discovered: one previously published and the other reported here. The rest of the observed candidates correspond to three hypercompact hii regions and two ultracompact hii regions. The two jets discovered are associated with two of the most luminous (70,000 and 100,000 Lsun) HMYSOs known to harbor this type of objects, showing that the phenomena of collimated ionized winds appears in the formation process of stars at least up to masses of ~ 20 M_sun and provides strong evidence for a disk-mediated accretion scenario for the formation of high-mass stars. From the incidence of jets in our sample, we estimate that the jet phase in high-mass protostars lasts for 40,000 yr.

Wide field imaging for the Square Kilometre Array

Abstract: Wide-field radio interferometric telescopes such as the Square Kilometre Array now being designed are subject to a number of aberrations. One particularly pernicious aberration is that due to non-coplanar baselines whereby long baselines incur a quadratic image-plane phase error. There are numerous algorithms for dealing with the non-coplanar baselines effect. As a result of our experience with developing processing software for the Australian Square Kilometre Array Pathfinder, we advocate the use of a hybrid algorithm, called w snapshots, based on a combination of w projection and snapshot imaging. This hybrid overcomes some of the deficiencies of each and has advantages from both. Compared to pure w projection, w snapshots uses less memory and execution time, and compared to pure snapshot imaging, w snapshots uses less memory and is more accurate. At the asymptotes, w snapshots devolves to w projection and to snapshots.

12.2-GHz methanol maser Methanol Multibeam follow-up catalogue – II. Longitude range 186°–330°

Abstract: We present the second portion of a catalogue of 12.2-GHz methanol masers detected towards 6.7-GHz methanol masers observed in the unbiased Methanol Multibeam (MMB) survey. Using the Parkes radio telescope we have targeted all 207 6.7-GHz methanol masers in the longitude range 186°-330° for 12.2-GHz counterparts. We report the detection of 83 12.2-GHz methanol masers, and one additional source which we suspect is thermal emission, equating to a detection rate of 40 per cent. Of the 83 maser detections, 39 are reported here for the first time. We discuss source properties, including variability and highlight a number of unusual sources. We present a list of 45 candidates that are likely to harbour methanol masers in the 107.0-GHz transition.

Wide field imaging for the Square Kilometre Array

Abstract: Wide- eld radio interferometric telescopes such as the Square Kilometre Array now being designed are subject toa number of aberrations. One particularly pernicious aberration is that due to non-coplanar baselines wherebylong baselines incur a quadratic image-plane phase error. There are numerous algorithms for dealing with thenon-coplanar baselines e ect. As a result of our experience with developing processing software for the AustralianSquare Kilometre Array Path nder, we advocate the use of a hybrid algorithm, called w snapshots, based ona combination of w projection and snapshot imaging. This hybrid overcomes some of the de ciencies of eachand has advantages from both. Compared to pure w projection, w snapshots uses less memory and executiontime, and compared to pure snapshot imaging, w snapshots uses less memory and is more accurate. At theasymptotes, w snapshots devolves to w projection and to snapshots.Keywords: SKA, wide eld, radio interferometry 1. THE SQUARE KILOMETRE ARRAY

Variability monitoring of the hydroxyl maser emission in G12.889+0.489

Abstract: Through a series of observations with the Australia Telescope Compact Array we have monitored the variability of the ground-state hydroxyl maser emission from G12.889+0.489 in all four Stokes polarization products. These observations were motivated by the known periodicity in the associated 6.7-GHz methanol maser emission. A total of 27 epochs of observations were made over 16 months. No emission was seen from either the 1612- or 1720-MHz satellite line transitions (to a typical five sigma upper limit of 0.2 Jy). The peak flux densities of the 1665- and 1667-MHz emission were observed to vary at a level of ∼20 per cent (with the exception of one epoch which dropped by ≤40 per cent). There was no distinct flaring activity at any epoch, but there was a weak indication of periodic variability, with a period and phase of minimum emission similar to that of methanol. There is no significant variation in the polarized properties of the hydroxyl, with Stokes Q and U flux densities varying in accord with the Stokes I intensity (linear polarization, P, varying by ≤20 per cent) and the right and left circularly polarized components varying by ≤33 per cent at 1665 MHz and ≤38 per cent at 1667 MHz. These observations are the first monitoring observations of the hydroxyl maser emission from G12.889+0.489.

New class I methanol masers

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.

Tracing major structures of the inner Galaxy with 6.7-GHz methanol masers

Abstract: Through analysis of correlations within the longitude-velocity distribution of 6.7-GHz methanol masers, we identify density enhancements indicative of large-scale regions of enhanced star formation. In the context of the inner structure of our Galaxy these are interpreted as the starting points of the spiral arms and the interaction of the long Galactic bar with the 3-kpc arms. Signatures of a continuous 3-kpc arm structure are seen including a prominent tangent at -22 ◦ Galactic longitude.

New class i methanol masers

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 in the ALMA frequency range.

25 GHz methanol masers in regions of massive star formation

Abstract: Abstract The bright 25 GHz series of methanol masers is formed in highly energetic regions of massive star formation and provides a natural signpost of shocked gas surrounding newly forming stars. A systematic survey for the 25 GHz masers has only recently been carried out. We present the preliminary results from the interferometric follow up of 51 masers at 25 GHz in the southern sky.

An evolutionary timeline for high-mass star formation

Abstract: The details of the physical process through which high-mass stars form remains nearly as much of a mystery now as it was when the Parkes radio telescope commenced operation. The energy output from high-mass stars influence, or directly drive many important processes in the evolution of galaxies and so understanding in detail when and how they form is important for a broad range of fields of astrophysics. Interstellar masers are one of the most readily observed signposts of regions where young high-mass stars have formed. We have recently made great progress towards using the different ma ser species and transitions to construct a maserbased evolutionary timeline for high-mass star formation. Here we give an overview of this work, highlighting the particular contribution that past and on-going observa tions with the Parkes 64m radio telescope have made to this area. Subject headings:ISM:molecules ‐ masers ‐ radio lines: ISM ‐ stars: formation

Search for ionized jets towards high-mass young stellar objects

Abstract: We are carrying out multi-frequency radio continuum observations, using the Australia Telescope Compact Array, to systematically search for collimated ionized jets towards high-mass young stellar objects (HMYSOs). Here we report observations at 1.4, 2.4, 4.8 and 8.6 GHz, made with angular resolutions of about 7, 4, 2, and 1 arcsec, respectively, towards six objects of a sample of 33 southern HMYSOs thought to be in very early stages of evolution. The objects in the sample were selected from radio and infrared catalogs by having positive radio spectral indices and being luminous (L_bol > 20,000 L_sun), but underluminous in radio emission compared to that expected from its bolometric luminosity. This criteria makes the radio sources good candidates for being ionized jets. As part of this systematic search, two ionized jets have been discovered: one previously published and the other reported here. The rest of the observed candidates correspond to three hypercompact hii regions and two ultracompact hii regions. The two jets discovered are associated with two of the most luminous (70,000 and 100,000 Lsun) HMYSOs known to harbor this type of objects, showing that the phenomena of collimated ionized winds appears in the formation process of stars at least up to masses of ~ 20 M_sun and provides strong evidence for a disk-mediated accretion scenario for the formation of high-mass stars. From the incidence of jets in our sample, we estimate that the jet phase in high-mass protostars lasts for 40,000 yr.

What is happening in G357.96-0.16?

Abstract: In order to answer this question, we examine the relationship between the two sites of maser activity in G357.96-0.16. We also propose future observations for examining the dust properties of this interesting region of massive star formation.

An Evolutionary Timeline for High-mass Star Formation

Abstract: The details of the physical process through which high-mass stars form remains nearly as much of a mystery now as it was when the Parkes radio telescope commenced operation. The energy output from high-mass stars influence, or directly drive many important processes in the evolution of galaxies and so understanding in detail when and how they form is important for a broad range of fields of astrophysics. Interstellar masers are one of the most readily observed signposts of regions where young high-mass stars have formed. We have recently made great progress towards using the different maser species and transitions to construct a maser-based evolutionary timeline for high-mass star formation. Here we give an overview of this work, highlighting the particular contribution that past and on-going observations with the Parkes 64m radio telescope have made to this area.

Water maser follow-up of the Methanol Multi-Beam Survey.

Abstract: The Australia Telescope Compact Array has been used to search for 22-GHz water masers towards the 323 6.7-GHz methanol masers detected in the Methanol Multibeam survey between Galactic longitudes 341◦ to 20◦, through the Galactic centre, and we find water masers associated with 156 (∼48 %). Methanol masers with associated water masers have a higher mean integrated luminosity than those without. In sources where both maser species are observed, the luminosities of the methanol and water masers are weakly correlated even after accounting for the partial correlation due to distance. We have inspected the GLIMPSE three colour images of the regions surrounding the masers and cross-matched the maser positions with existing catalogues of Extended Green Objects and Infrared Dark Clouds. We find more Extended Green Objects at sites where both methanol and water masers are present than at sites with only methanol masers, but no significant difference in the fraction embedded within Infrared Dark Clouds. Studying the mid-infrared colours from GLIMPSE, we found no differences between the colours of those sources associated with both methanol and water masers and those associated with just methanol. Analysis of the 1.1-mm thermal dust emission shows dust clumps associated with masers have greater 1.1-mm flux densities and higher column densities than those without. Dust clumps associated with both water and 6.7-GHz methanol masers are generally the most compact clumps followed by those associated with only methanol then the clumps without associated maser emission. Comparing the column density and dust mass calculated from the thermal dust emission at 870 μm, we found no differences between those sources associated with both water and methanol masers and those with methanol only. At 870 μm we found dust clumps at the very highest column densities (above ∼ 1025 cm−2) almost all had an associated methanol maser. We conclude that there is some evidence that protostars with both methanol and water masers are often older than those with only methanol, however, we suggest that the evolutionary phase traced by water masers is not as well defined as for 6.7-GHz methanol masers. Since water masers are collisionally pumped and often show emission further away from their accompanying YSO than the radiatively pumped 6.7-GHz methanol masers, it is likely that water maser properties are not as tightly correlated to the evolution of the parent YSO.