Kate Brooks (ATNF)

Now equipped with new 12- and 3-mm receivers, the Australia Telescope Compact Array can measure the flux of an astronomical source over discrete wavebands spanning the frequency range 1 to 100 GHz. In an ongoing project, Brooks and her collaborators are using this valuable capability of the Compact Array to study one of the most massive young stellar objects known to harbour a supersonic jet of ionised gas. The jet is associated with the infrared source IRAS 16547-4247, located at a distance of 2.9 kpc and which has a bolometric luminosity that is equivalent to that of a single O8 zero-age-main-sequence star. This is the first reported case of a radio jet associated with a young O-type star, supporting the notion that the accretion mechanism that produces jets in low-mass star formation also operates in the higher-mass regime.

Observations of IRAS 16547-4247 using the Compact Array were first made in 2000 as part of a larger study of a sample of objects though to be very young massive stars (masses greater than eight times the mass of our Sun). Data were obtained over four radio continuum bands centred on 1.4, 2.5, 4.8 and 8.6 GHz. What set IRAS 16547-4247 apart from the rest of the sources in the sample was that the detected radio emission for IRAS 16547-4247 was confined to three discrete sources all in a line and with the outer sources symmetrically offset from the central source. From his previous work on radio jets, collaborator Garay from the University of Chile immediately recognised this striking morphology. On further inspection of the flux intensities at each frequency band, the central unresolved source was found to have a spectral index consistent with an ionised thermal jet and the two outer sources were found to have spectral indices consistent with non-thermal emission arising from the working surfaces of the jet as it interacts with the surrounding ambient medium.

Subsequent observations at near-infrared wavelengths were carried out in 2002 using the Very Large Telescope (VLT) in Chile. A complex chain of H₂ 2.12 μm emission knots was detected that traces a collimated flow extending over 1.5 pc (see Figure 1). The alignment of the H₂ flow and the central location of the radio jet imply that these phenomena are coupled. It is reasonable to assume that the radio jet is the driving force of the collimated flow. Follow-up observations using the VLA were made in 2003 using continuum bands centred on 8.46 and 14.9 GHz. These data resolved angularly the central thermal radio jet and yielded an estimate of ~25 degrees for the opening angle of the jet, indicating significant collimation.

In 2005 data was taken again with the Compact Array but this time using the new continuum bands centred on 24.9 GHz (12 mm) and 88 GHz (3 mm). At 24.9 GHz, all three radio components were detected plus an additional source offset to the southwest from the central radio jet and corresponding to a bright H₂ emission knot (see Figure 1). Emission at 88 GHz was detected towards the central radio jet. The measured spectral energy distribution of the central radio jet is shown in Figure 2. A model for the thermal emission from a collimated jet has been fit to the data at low frequencies and a model for the thermal dust emission from a circumstellar envelope/disk has been fit to the data at high frequencies. The changeover occurs at 50 GHz.

The findings made with the Australia Telescope Compact Array towards IRAS 16547-4247 are significant because they confirm that collimated jets can be present in young massive stars and provide tantalizing evidence for the presence of circumstellar disks as well. The new receivers operating in the 7-mm wave band (30-50 GHz) to be installed at the Australia Telescope Compact Array in 2007 will open a window of opportunity in the study of young massive stars, particularly in the hunt for more stars with collimated jets and circumstellar disks. As evident in Figure 2, 7-mm marks the turnover between emission from ionised gas and emission from dust. It is in this waveband that the jet will not only be at its brightest but also at its smallest angular size, and therefore closest to the powering star.

Figure 1: Collimated jet source associated with the massive protostar in IRAS 16547-4247. VLT H₂ 2.12 μm emission image overlaid with contours of emission at 24.9 GHz (12 mm) detected with the Australia Telescope Compact Array.

Image credit: K. Brooks (ATNF) and M. Voronkov (ATNF).

Figure 2: Spectral Energy Distribution of IRAS 16547-4247. Data have been obtained over a variety of telescopes including the Australia Telescope Compact Array and the VLA (indicated by blue triangles). Emission less than ~50 GHz corresponds to ionised thermal emission (in this case arising from a thermal jet) whereas emission greater than ~50 GHz is primarily from thermal dust emission from a circumstellar disk or envelope.

Image credit: K. Brooks (ATNF), Guido Garay (UChile) and Luis-Felipe Rodriguez (UNAM).