Kate J. Brooks

A Triple Radio Continuum Source Associated with IRAS 16547-4247: A Collimated Stellar Wind Emanating from a Massive Protostar

We report the discovery, made using the Australia Telescope Compact Array, of a triple radio source toward IRAS 16547-4247, a luminous infrared source with a bolometric luminosity of 6.2 × 104 L☉. The radio source shows an almost linear structure consisting of a compact central object and two outer lobes separated by about 20, located symmetrically to the central source. The radio emission from the lobes has spectral indices of -0.61 and -0.33, characteristic of nonthermal emission. The emission from the central object has a spectral index of 0.49, consistent with free-free emission from a thermal jet. Also reported are 1.2 mm continuum and molecular line observations made with the Swedish ESO Submillimeter Telescope. The 1.2 mm observations show that the dust emission arises from a region of 33 × 25 (FWHM) with a total flux of 16.4 Jy, implying a mass of 1.3 × 103 M☉. The line observations indicate that IRAS 16547-4247 is associated with a molecular core with a FWHM deconvolved angular size of 27 (diameter of 0.38 pc at the distance of 2.9 kpc), a molecular hydrogen density of 5.2 × 105 cm-3, and a mass of 9.0 × 102 M☉. We propose that this dense massive core hosts a high-mass star in an early stage of evolution in which it is undergoing the ejection of a collimated stellar wind. The radio emission from the lobes arises in shocks resulting from the interaction of this collimated wind with the surrounding medium. Our observations indicate that the jets found in the formation of low-mass stars are also produced in high-mass stars, with IRAS 16547-4247 being the most luminous young stellar object presently known to host a jet.

Photodissociation regions and star formation in the Carina nebula

We have obtained wide-field thermal infrared (IR) images of the Carina nebula, using the SPIREX/Abu telescope at the South Pole. Emission from polycyclic aromatic hydrocarbons (PAHs) at 3.29 μm, a tracer of photodissociation regions (PDRs), reveals many interesting well-defined clumps and diffuse regions throughout the complex. Near-IR images (1-2 μm), along with images from the Midcourse Space Experiment (MSX) satellite (8 - 21 μm) have been incorporated to study the interactions between the young stars and the surrounding molecular cloud in more detail. Two new PAH emission clumps have been identified in the Keyhole nebula, and have been mapped in 1 2 CO(2-1) and (1-0) using the Swedish-ESO Submillimetre Telescope (SEST). Analysis of their physical properties reveals that they are dense molecular clumps, externally heated with PDRs on their surfaces and supported by external pressure in a similar manner to the other clumps in the region. A previously identified externally heated globule containing IRAS 10430-5931 in the southern molecular cloud shows strong 3.29-, 8- and 21-μm emission, the spectral energy distribution (SED) revealing the location of an ultracompact (UC) HΙΙ region. The northern part of the nebula is complicated, with PAH emission intermixed with mid-IR dust continuum emission. Several point sources are located here, and through a two-component blackbody fit to their SEDs we have identified three possible UC HΙΙ regions as well as a young star surrounded by a circumstellar disc. This implies that star formation in this region is ongoing and not halted by the intense radiation from the surrounding young massive stars.

The giant pillars of the Carina Nebula

Results are presented from a multi-wavelength study of the giant pillars within the Carina Nebula. Using near-IR data from 2MASS, mid-IR data from MSX, 843 MHz radio continuum maps from the MOST and molecular line and continuum observations from the SEST, we investigate the nature of the pillars and search for evidence of ongoing star formation within them. Photodissociation regions (PDRs) exist across the whole nebula and trace the giant pillars, as well as many ridges, filaments and condensations (A v > 7 mag). Morphological similarities between emission features at 21 μm and 843MHz adjacent to the PDRs, suggests that the molecular material has been carved by the intense stellar winds and UV radiation from the nearby massive stars. In addition, star forming cores are found at the tips of several of the pillars. Using a stellar density distribution, several candidate embedded clusters are also found. One is clearly seen in the 2MASS images and is located within a dense core (G287.84-0.82). A search for massive young stellar objects and compact H II regions using mid-IR colour criteria, reveal twelve candidates across the complex. Grey-body fits to SEDs for four of these objects are suggestive of OB-stars. We find that massive star formation in the Carina Nebula is occurring across the whole complex and confirm it has been continuous over the past 3 Myrs.

A Parsec-Scale Flow Associated with the IRAS 16547–4247 Radio Jet

IRAS 16547-4247 is the most luminous (6.2 × 104 L☉) embedded young stellar object known to harbor a thermal radio jet. We report the discovery using the Very Large Telescope Infrared Spectrometer and Array Camera of a chain of H2 2.12 μm emission knots that trace a collimated flow extending over 1.5 pc. The alignment of the H2 flow and the central location of the radio jet imply that these phenomena are intimately linked. We have also detected an isolated unresolved 12 μm infrared source toward the radio jet using TIMMI2. Our findings affirm that IRAS 16547-4247 is excited by a single O-type star that is driving a collimated jet. We argue that the accretion mechanism that produces jets in low-mass star formation also operates in the higher mass regime.

Two Massive Star-forming Regions at Early Evolutionary Stages

We report sensitive Australia Telescope Compact Array radio continuum observations toward IRAS 15596-5301 and 16272-4837, two luminous objects (L > 2 × 104 L☉) thought to represent massive star-forming regions in early stages of evolution (due to previously undetected radio emission at the 1 σ level of 2 mJy beam-1). Also reported are 1.2 mm continuum and a series of molecular line observations made with the Swedish ESO Submillimeter Telescope. The radio continuum observations toward IRAS 15596-5301 reveal the presence of three distinct compact sources, with angular sizes of 27-88 (FWHM), all located within a region of 30 in diameter. Assuming that these are regions of ionized gas, we find that they have diameters of 0.06-0.2 pc and electron densities of 8 × 102-2 × 103 cm-3 and that they are excited by early B-type stars. The 1.2 mm observations show that the dust emission arises from a region of 42 × 25 (FWHM) with a total flux of 5.8 Jy, implying a mass of 1.4 × 103 M☉. The line observations indicate that IRAS 15596-5301 is associated with a molecular cloud with a FWHM angular size of 37 (~0.4 pc radius at the distance of 4.6 kpc), a molecular hydrogen density of ~4 × 105 cm-3, and a rotational temperature of ~27 K. We suggest that the massive dense core associated with IRAS 15596-5301 contains a cluster of B stars that are exciting compact H II regions that are in pressure equilibrium with the dense molecular surroundings. No radio continuum emission was detected from IRAS 16272-4837 up to a 3 σ limit of 0.2 mJy. However, the 1.2 mm observations show strong dust emission arising from a region of 41 × 25 (FWHM) with a total flux of 13.8 Jy, implying a mass of 2.0 × 103 M☉. The line observations indicate the presence of an elongated molecular cloud with FWHM major and minor axes of 61 and 42(0.50 × 0.35 pc in radius at the distance of 3.4 kpc), a molecular hydrogen density of ~2 × 105 cm-3, and a rotational temperature of ~27 K. The high luminosity (2.4 × 104 L☉) and lack of radio emission from this massive core suggest that it hosts an embedded young massive protostar that is still undergoing an intense accretion phase. This scenario is supported by the observed characteristics of the line profiles and the presence of a bipolar outflow detected from observations of the SiO emission. We suggest that IRAS 16272-4837 is a bona fide massive star-forming region in a very early evolutionary stage, being the precursor of an ultracompact H II region.

Detection of a Collimated Jet Towards the High-Mass Protostar IRAS 16547-4247

We report the discovery towards IRAS 16547-4247 of a chain of H2 2.12 μm emission knots that trace a collimated flow extending over 1.5 pc. IRAS 16547-4247 is a massive young stellar object with a luminosity of 6.2×104 L⊙. It is associated with a thermal radio jet and two non-thermal radio lobes which correspond to the working surfaces of the jet. The geometry of the H2 flow implies that it is driven by the thermal jet. We have also identified an isolated unresolved mid-infrared object associated with the jet and which is likely to be responsible for the excitation of IRAS 16547-4247.