Star Formation and the Interstellar Medium



The Astrophysics group has a strong research program studying star formation and the interstellar medium. Combining optical, infrared, millimetre and radio telescopes the group is undertaking a diverse range of projects exploring the environment in which star formation occurs in our Galaxy. The group makes major use of the Mopra millimetre wave telescope, at Siding Spring Observatory and the NANTEN2 telescope at the 5,000m Atacom plateau of Chile.

Research programs centre around both the study of individual sources and the development of new facilities for furthering this work.


Polyaromatic Hydrocarbon (PAH) emission at 3.3µm in the NGC 6334 star forming complex, imaged with the SPIREX telescope at the South Pole


Star Formation

 

Infrared (left, 2MASS) and Optical (right, AAO) images of Eta Carina and the Keyhole Nebula.

The principal research areas are:

  • The evolutionary sequence to massive star formation

  • The environment of star forming complexes

  • Jets and outflows from young stellar objects

The various stages of the star formation process, from initial collapse of a cloud core, through the formation of a disk and outflow, to the emergence of a young star from its natal cloud, are now beginning to be defined from an observational basis. Massive star formation, in particular, is a spectacular event which can be seen across the Galaxy. Through conducting an investigation of 6.6GHz methanol maser emission associated with the ultra-compact HII region phase of massive star formation (when an ionized bubble forms around the young star), our work showed that the masers actually predate the UCHII region phase, and are a signpost to a hitherto unknown stage of the formation process. We have postulated they originate in 'Hot Molecular Cores', and mark the very earliest phase of the massive star forming process. Our work has opened up new avenues of investigation into star formation, including searching for (and discovering!) protostellar embedded sources.

After the massive stars form they have a dramatic effect on their environment, bombarding their natal molecular clouds with winds and intense radiation fields. This both lights up the environment, and feeds back into regulating the rate of star formation in the cloud. We have been studying the evolution of star forming complexes, such as NGC 6334 in its early stages, and the Keyhole Nebula in Carina which has nearly finished shredding its natal cloud.

Star formation also produces collimated outflows. This is particularly evident in low mass star formation, where the confusing effects of winds and intense radiation fields are absent. These flows also stir up the clouds and drive turbulence within them, shock-heating the ambient gas they interact with. We are studying these mechanisms, in particular the interaction of the outflows with the clouds.

Excitation of Molecular Clouds

The main research areas here are:

  • Shock waves

  • UV-Fluorescence

This research involves studying the excitation of the hydrogen molecule by interstellar shock waves and by UV-fluorescence. Such activity takes place in star forming regions, including photodissociation regions (PDRs), the surfaces of molecular clouds heated by UV radiation. This work involves a broad-ranging observing programme applying a combination of imaging, spectroscopic and polarimetric techniques to study the hot gas in molecular clouds. While it has concentrated on the near-IR spectrum from the v=1 vibrational level of H2, it has developed to the study of emission from highly-excited levels of the molecule. Such lines allow us to distinguish between the relative merits of competing models for the emission, which diverge in their predictions for the population of higher energy levels. It involved, for instance, the first measurements of the molecular hydrogen emission in the optical CCD regime. Other aspects of this research include observing the H2 line profiles at high spatial resolution, to probe the structure of bow shocks, and comparing the distribution of different emission lines in a source. An example here includes the spectacular discovery of a plethora of bullets (from an explosion in the core of the Orion Nebula).


Debris from an interstellar explosion, imaged with the IRIS on the AAT

The Interaction of Supernova Remnants with Molecular Clouds

Supernovae must commonly explode within or nearby to their natal molecular clouds. Their expanding blast wave interacts and overruns these clouds. Yet relatively few such examples are known. Such a SNR looks different to 'conventional' remnants, with the bulk of the emission irradiated in the IR rather than the optical, through molecular hydrogen and atomic fine-structure lines. These SNRs provide a laboratory to examine the interaction of a shock wave with molecular gas, uncontaminated by the activity associated with star forming regions where such events are usually observed. Our work has included studying these interactions in several Galactic and LMC remnants, undertaking wide-field near-IR imaging of the molecular hydrogen and ionized iron emission, and measuring their total energy budget. The recent suggestion that 1720 MHz OH maser emission may be a signpost to this activity has lead to a new program to search for (and find!), associated shocked molecular hydrogen line emission.

Infrared image of the ICC443 Supernova Remnant (red-shocked molecular hydrogen, blue-shocked iron; 2MASS).

The Centre of the Galaxy

The hostile environment of the Galactic Centre is an unlikely place to find both molecular clouds and star formation, but it is in fact home to both. Massive star formation is occurring within the central 2 parsecs of the Galaxy, and in a number of clusters within the inner 100 parsecs. These stars form a population I core to our Galaxy, whose (unobserved) lower mass members may provide the bulk of the mass within the central parsec? Or it may lie in a million solar mass black hole? Our work involves, particularly, studying the stellar population in these young clusters, to determine the nature of the massive stars that are forming from their near-IR emission spectrum. A recent development has been imaging the molecular hydrogen emission from the circumnuclear disk and in the Sgr A East supernova remnant to understand its excitation and dynamics. This combines both HST/NICMOS and the AAT/UNSWIRF, the former to provide exquisite spatial resolution, the later to remove the strong continuum emission from the stars and to measure the gas velocities. We are currently undertaking a project to map the Central Molecular Zone, the inner 300pc of our Galaxy, in a variety of organic molecules. The CMZ is the repository of 10% of our Galaxy's molecular gas. The project makes extensive use of both the Mopra and NANTEN2 telescopes.

Infrared (IRIS, AAT) view of the Centre of the Galaxy

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Jon Everett School of Physics - UNSW 2010