Philip J. Diamond

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

Original article can be found at: http://www3.interscience.wiley.com/journal/ Copyright Royal Astronomical Society

The 6-GHz multibeam maser survey – I. Techniques

A new 7-beam 6 7 GHz receiver has been built to survey the Galaxy and the Magellanic Clouds for newly forming high-mass stars that are pinpointed by strong methanol maser emission at 6668 MHz. The receiver was jointly constructed by Jodrell Bank Observatory (JBO) and the Australia Telescope National Facility (ATNF) and allows simultaneous coverage at 6668 and 6035 MHz. It was successfully commissioned at Parkes in January 2006 and is now being used to conduct the Parkes-Jodrell multibeam maser survey of the Milky Way. This will be the first systematic survey of the entire Galactic plane for masers of not only 6668-MHz methanol, but also 6035-MHz excited-state hydroxyl. The survey is two orders of magnitude faster than most previous systematic surveys and has an rms noise level of �0.17Jy. This paper describes the observational strategy, techniques and reduction procedures of the Galactic and Magellanic Cloud surveys, together with deeper, pointed, follow-up observations and complementary observations with other instruments. It also includes an estimate of the survey detection efficiency. The 111 days of observationswith the Parkes telescope have so far yielded >800 methanol sources, of which �350 are new discoveries. The whole project will provide the first comprehensive Galaxy-wide catalogue of 6668-MHz and 6035-MHz masers.

A magnetically collimated jet from an evolved star.

Planetary nebulae often have asymmetric shapes, even though their progenitor stars were symmetric; this structure could be the result of collimated jets from the evolved stars before they enter the planetary nebula phase. Theoretical models have shown that magnetic fields could be the dominant source of jet-collimation in evolved stars, just as these fields are thought to collimate outflows in other astrophysical sources, such as active galactic nuclei and proto-stars. But hitherto there have been no direct observations of both the magnetic field direction and strength in any collimated jet. Here we report measurements of the polarization of water vapour masers that trace the precessing jet emanating from the asymptotic giant branch star W43A (at a distance of 2.6 kpc from the Sun), which is undergoing rapid evolution into a planetary nebula. The masers occur in two clusters at opposing tips of the jets, ∼1,000 au from the star. We conclude from the data that the magnetic field is indeed collimating the jet.

VLBI IMAGES OF 49 RADIO SUPERNOVAE IN ARP 220

We have used a very long baseline interferometry (VLBI) array at 18 cm wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ∼1 pc linear resolution, with very high sensitivity. The resulting map has an rms of 5.5 μJy beam to the -1, and careful image analysis results in 49 confirmed point sources ranging in flux density from 1.2 mJy down to ∼60 μJy. Comparison with high-sensitivity data from 12 months earlier reveals at least four new sources. The favored interpretation of these sources is that they are radio supernovae, and if all new supernovae are detectable at this sensitivity, a resulting estimate of the supernova rate in the Arp 220 system is 4 ± 2 per year. The implied star formation rate is sufficient to power the entire observed far-infrared luminosity of the galaxy. The two nuclei of Arp 220 exhibit striking similarities in their radio properties, although the western nucleus is more compact, and appears to be ∼3 times more luminous than the eastern nucleus. There are also some puzzling differences, and differential free-free absorption, synchrotron aging, and expansion losses may all be playing a role. Comparison with the nearby starburst galaxy M82 supports the hypothesis that the activity in Arp 220 is essentially a scaled-up version of that in M82.

A collimated jet of molecular gas from a star on the asymptotic giant branch

Evolved stars of about one solar mass are in general spherically symmetric, yet the planetary nebulae that they produce in the next phase of their evolution tend not to exhibit such symmetry. Collimated ‘jets’ and outflows of material have been observed up to ∼0.3 parsec from the central stars of planetary nebulae, and precession of those jets has been proposed to explain the observed asymmetries. Moreover, it has recently been shown theoretically that magnetic fields could launch and collimate such jets. Here we report the detection of a collimated and precessing jet of molecular gas that is traced by water-vapour maser spots ∼500 astronomical units (au) from the star W43A in Aquila. We conclude that the jet is formed in the immediate vicinity of the star, and infer that elongated planetary nebulae are formed by jets during the short period, of less than 1,000 years, when the star makes its transition through the proto-planetary nebula phase to become a planetary nebula.

The Radio Spectra of the Compact Sources in Arp 220: A Mixed Population of Supernovae and Supernova Remnants

We report the first detection at multiple radio wavelengths (13, 6, and 3.6 cm) of the compact sources within both nuclei of the ultraluminous infrared galaxy Arp 220, presenting radio spectra of the 18 detected sources. In just over half of these, we find that these spectra and other properties are consistent with the standard model of powerful Type IIn supernovae interacting with their preexplosion stellar wind. The rate of appearance of new radio sources identified with these supernova events suggests that an unusually large fraction of core-collapse supernovae in Arp 220 are highly luminous, possibly implying a radically different stellar initial mass function or stellar evolution compared to galactic disks. Another possible explanation invokes very short (~3 × 10^5 yr) intense (~10^3 M_⊙ yr^(-1)) star formation episodes with a duty cycle of ~10%. A second group of our detected sources, consisting of the brightest and longest monitored sources at 18 cm, do not easily fit the radio supernova model. These sources show a range of spectral indexes from -0.2 to -1.9. We propose that these are young supernova remnants that have just begun interacting with a surrounding ISM with a density between 10^4 and 10^5 cm^(-3). One source is probably resolved at 3.6 cm wavelength with a diameter 0.9 pc. In the western nucleus we estimate that the ionized component of the ISM gives rise to foreground free-free absorption with opacity at 18 cm of <0.6 along the majority of lines of sight. Other sources may be affected by absorption with opacity in the range 1-2. These values are consistent with previous models as fitted to the radio recombination lines and the continuum spectrum.

A MOVIE OF A STAR: MULTIEPOCH VERY LONG BASELINE ARRAY IMAGING OF THE SiO MASERS TOWARD THE MIRA VARIABLE TX CAM

We describe an observing campaign using the Very Long Baseline Array (VLBA) to monitor the time evolution of the v = 1, J = 1-0 SiO maser emission toward the Mira variable TX Cam. The data reported here cover the period 1997 May 24 to 1999 February 19, during which the SiO maser emission was imaged at approximately biweekly intervals. The result is an animated movie at an angular resolution of ~500 μas, over a full pulsation period, of the gas motions in the near circumstellar environment of this star, as traced by the SiO maser emission. This paper serves to release the movie and is the first in a series concerning the scientific results from this observing campaign. In this paper, we discuss the global proper motion of the SiO maser emission as a function of pulsation phase. We measure a dominant expansion mode between optical phases ~ 0.7 and 1.5 confirming ballistic deceleration and compare this to predictions from existing pulsation models for late-type, evolved stars. Local infall and outflow motions are superposed on the dominant expansion mode, and nonradial local gas motions are also evident for individual SiO maser components. The overall morphology and evolution of the SiO emission deviates significantly from spherical symmetry, with important implications for models of pulsation kinematics in the near-circumstellar environments of Mira variables.We describe an observing campaign using the Very Long Baseline Array (VLBA) to monitor the time-evolution of the v=1, J=1-0 SiO maser emission towards the Mira variable TX Cam. The data reported here cover the period 1997 May 24 to 1999 February 19, during which the SiO maser emission was imaged at approximately bi-weekly intervals. The result is an animated movie at an angular resolution of ~500 microarcsec, over a full pulsation period, of the gas motions in the near circumstellar environment of this star, as traced by the SiO maser emission. This paper serves to release the movie and is the first in a series concerning the scientific results from this observing campaign. In this paper, we discuss the global proper motion of the SiO maser emission as a function of pulsation phase. We measure a dominant expansion mode between optical phases

VLBA observations of SiO masers towards Mira variable stars

\phi \sim 0.7-1.5

VLBI Imaging of Luminous Infrared Galaxies: Active Galactic Nucleus Cores in Markarian 231, UGC 5101, and NGC 7469*

confirming ballistic deceleration, and compare this to predictions from existing pulsation models for late-type, evolved stars. Local infall and outflow motions are superimposed on the dominant expansion mode, and non-radial local gas motions are also evident for individual SiO maser components. The overall morphology and evolution of the SiO emission deviates significantly from spherical symmetry, with important implications for models of pulsation kinematics in the near-circumstellar environments of Mira variables.

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

We present new total intensity and linear polarization VLBA observations of the ν = 2a ndν = 1 J = 1−0 maser transitions of SiO at 42.8 and 43.1 GHz in a number of Mira variable stars over a substantial fraction of their pulsation periods. These observations were part of an observing program that also includes interferometric measurements at 2.2 and 3.6 micron (Mennesson et al. 2002); comparison of the results from different wavelengths allows studying the envelope independently of the poorly known distances to these stars. Nine stars were observed at from one to four epochs during 2001. The SiO emission is largely confined to rings which are smaller than the inner radius of the dust shells reported by Danchi et al. (1994). Two stars (U Orionis, R Aquarii) have maser rings with diameters corresponding to the size of the hot molecular layer as measured at 3.6 micron; in the other cases, the SiO rings are substantially larger. Variations of ring diameter for most, but not all stars, had an rms amplitude in agreement with the models of Humphreys et al. (2002) although the expected relationship between the diameter and pulsation phase was not seen. The ring diameter in U Orionis shows remarkably small variation. A correlation between the 2.2/3.6 µm diameter ratio with that of the SiO/3.6 µm diameter ratio is likely due to differences in the opacities at 2.2 and 3.6 µm in a molecular layer. A further correlation with the inner size of the dust shell reported by Danchi et al. (1994) suggest some differences in the temperature structure. Clear evidence is seen in R Aquarii for an equatorial disk similar to that reported by Hollis et al. (2001); rotation is possibly also detected in S Coronae Boralis.