are announcing today that some spiral arms of
our Milky Way galaxy may extend farther than is depicted in leading
This is one of the results from the Arecibo Methanol Maser Galactic
Survey that is being presented by graduate student Jagadheep
D. Pandian of Cornell University, Ithaca,
Dr. Paul F. Goldsmith of Jet Propulsion Laboratory, Pasadena,
Dr. Avinash A. Deshpande
Raman Research Institute, Bangalore, India to the American Astronomical
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.
6.7 GHz frequency at which methanol masers emits was,
until a few years ago, too high for observing with the 305m diameter
"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.
survey is also providing valuable data for Galactic
structure studies. Two of the methanol masers are estimated to be at a
of over 55,000 light years from the Sun. This puts the masers outside
the spiral arms depicted in leading models, such as that of Dr. James
M. Cordes of the Department of Astronomy
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.
Arecibo Observatory is part of the National Astronomy
1: A schematic diagram showing the origin of 6.7 GHz
methanol maser emission. The exact location of the maser emitting
still a topic of current research, but hypotheses include disks and
around young forming massive stars, and shock fronts near massive young
PHOTO CREDIT: Yvonne Kei-Nam Tang (
PHOTO CREDIT: Jagadheep D. Pandian (
For more information:
Jagadheep D. Pandian
Dr. Paul F. Goldsmith (818-393-0518, Paul.F.Goldsmith@jpl.nasa.gov)