MASER EMISSION PROBES FORMATION OF MASSIVE STARS AND MILKY WAY’S STRUCTURE

Astronomers are announcing today that some spiral arms of our Milky Way galaxy may extend farther than is depicted in leading models. This is one of the results from the Arecibo Methanol Maser Galactic Plane Survey that is being presented by graduate student Jagadheep D. Pandian of Cornell University, Ithaca, New York, Dr. Paul F. Goldsmith of Jet Propulsion Laboratory, Pasadena, California and Dr. Avinash A. Deshpande of Raman Research Institute, Bangalore, India to the American Astronomical Society meeting in Washington DC.

The Arecibo Methanol Maser Galactic Plane Survey is a blind search for 6.7 GHz methanol masers (masers are microwave analogs of lasers) in the plane of the Milky Way's disk. The survey is being done using the C-Band High receiver (which functions between 6 and 8 GHz) at the National Science Foundation's Arecibo Observatory which houses the largest single dish radio telescope in the world.

Methanol masers at 6.7 GHz are beacons of massive star formation in our Galaxy. They are extremely bright, and are hence excellent tools for searching and identifying regions in the Milky Way where massive stars are being born. Many of these regions are very distant and are heavily obscured by gas and dust; they are invisible in optical and occasionally even in mid-infrared wavelengths. This often makes these regions difficult to detect through other techniques. The question of where the methanol masers originate is still a topic of current research, but some of the hypotheses include disks and outflows around massive stars in the process of formation, and shock fronts near massive young stellar objects.

Massive stars are known to form preferentially along spiral arms of galaxies. This is because the spiral arms compress the gas in giant interstellar clouds to very high densities that are required for massive stars to form. Since it is not possible to take a picture of the Milky Way from outside, astronomers have been using massive stars and the ionized gas bubbles that they create around them to study the spiral structure of our galaxy. Since 6.7 GHz methanol masers are signposts of massive star formation, they can be used as additional tools to study Galactic spiral structure.

Methanol masers are also very valuable for studying the process of massive star formation itself. The process of how massive stars form is poorly understood compared to their low-mass counterparts. Do massive stars form from accretion disks like low-mass stars, or do they from mergers of low-mass stars in dense clusters, or is a combination of both processes at work? The discovery of more young massive stellar objects that are traced by 6.7 GHz methanol masers will be of immense help to shed better light on massive star formation.

The 6.7 GHz frequency at which methanol masers emits was, until a few years ago, too high for observing with the 305m diameter Arecibo radio telescope. With the completion of the Gregorian upgrade in 1997, and subsequent adjustment of the surface of the telescope, the potential was there to use the instrument's great collecting area for a maser survey of unequalled sensitivity.  The one missing ingredient, a low-noise receiver, was built by a NAIC team led by Jagadheep Pandian. With low noise amplifiers developed at the California Institute of Technology, the new C-Band High receiver started observations in February 2004.  The survey of the portion of the Milky Way visible with the Arecibo telescope will be completed early in 2006.

"The great collecting area of the Arecibo telescope combined with the high frequency capability afforded by the Gregorian upgrade and the sensitive low-noise C-Band High receiver have allowed an unbiased methanol maser survey of unprecedented sensitivity. It has already resulted in the detection of more than 40 new methanol masers including some of the most distant yet detected", says Dr. Paul F. Goldsmith who is a Principal Scientist at the Jet Propulsion Laboratory and Professor Emeritus of Astronomy at Cornell University.

The survey is also providing valuable data for Galactic structure studies. Two of the methanol masers are estimated to be at a distance of over 55,000 light years from the Sun. This puts the masers outside most of the spiral arms depicted in leading models, such as that of Dr. James M. Cordes of the Department of Astronomy at Cornell University and Dr. T. Joseph W. Lazio of the Naval Research Laboratory. "What is intriguing is that an extension of one of the spiral arms will fit these points reasonably well. This would suggest that some of the spiral arms extend further than has been assumed", says Mr. Jagadheep D. Pandian who is a Graduate student in the Department of Astronomy at Cornell University. He cautions that the distances for the sources would have to be verified by future work.

Another intriguing result is the discovery of methanol masers that do not have counterparts in existing catalogs at radio and mid-infrared wavelengths. In particular, counterparts have been searched for in catalogs of sources at 21 cm and 5 cm surveyed by the Very Large Array (taken by teams led by Dr. James J. Condon of National Radio Astronomy Observatory and Dr. Robert H. Becker of the Department of Physics at the University of California, Davis respectively), 100 to 12 mm sources from the Infrared Astronomical Satellite (IRAS) and 21.3 to 8.28 mm sources from the Midcourse Space Experiment (MSX). "We know that very crowded regions in the Galactic plane are omitted in the IRAS catalog. But the lack of counterparts in MSX wavelengths and at centimeter wavelengths is consistent with the idea that these methanol masers are associated with a very early phase of massive star formation. Future work on these sources should help us to get a more complete picture of the process of massive star formation", says Jagadheep.

The Arecibo Observatory is part of the National Astronomy and Ionosphere Center and is operated by Cornell University under a cooperative agreement with the National Science Foundation.

Illustrations:

Figure 1: A schematic diagram showing the origin of 6.7 GHz methanol maser emission. The exact location of the maser emitting regions is still a topic of current research, but hypotheses include disks and outflows around young forming massive stars, and shock fronts near massive young stellar objects.
PHOTO CREDIT: Yvonne Kei-Nam Tang (Cornell University)

 
Figure 2: Selected 6.7 GHz methanol maser positions detected in the Arecibo Methanol Maser Galactic Plane Survey overlaid on spiral arm models of the Milky Way. Crosses mark the positions of the Sun and the Galactic Center (GC). The red curve shows the model of Dr. James M. Cordes of Cornell University and Dr. T. Joseph W. Lazio of the Naval Research Laboratory. The proposed extension of the Crux-Scutum arm is shown in magenta. The blue curve shows the model of Dr. Jacques P. Vallee of the Herzberg Institute of Astrophysics, and the proposed spiral arm extension is shown in cyan. Methanol maser positions are shown as diamonds; the uncertainties in the deduced positions are also shown. At present, distances can be estimated only for a small sample of methanol masers detected in the survey.
PHOTO CREDIT: Jagadheep D. Pandian (Cornell University)

For more information:

Mr. Jagadheep D. Pandian (607-255-0609, jagadheep@astro.cornell.edu)
Dr. Paul F. Goldsmith (818-393-0518, Paul.F.Goldsmith@jpl.nasa.gov)