Using the United Kingdom Infrared Telescope (UKIRT) astronomers have found the leading mechanism by which most of the massive stars form in our Galaxy. The largest near-infrared survey of massive star forming regions to date has revealed that a major fraction of these massive stars form by collecting matter onto disks around their equatorial regions. This was revealed by the detection of gas outflows and shocked regions associated with massive young stars in formation, located in clouds of gas and dust in our Galaxy. The survey was carried out by a team lead by Dr. Watson Varricatt from the Joint Astronomy Centre and included Dr. Chris Davis (Joint Astronomy Centre), Dr. Suzanne Ramsay (ESO, Germany) and Dr. Stephen Todd (UKATC, Edinburgh, UK).

We know that lower-mass stars like our Sun form by gravitational collapse of material inside clouds of gas and dust in space. The gas and dust spiral down onto the equatorial regions of the young star via a process known as accretion. At the same time these accreting young stars drive high velocity jets of gas outwards at thousands of miles per hour. These “outflows” radiate at infrared wavelengths (this emission is actually produced by hydrogen molecules heated to thousands of degrees). Consequently, observations in the infrared can be used to search for not only the youngest stars, but also evidence of the accretion process.

The big question is, do the massive stars form the same way, or do they form using a different process?

For massive stars, with masses larger than 10 times the mass of our Sun, it has been proposed that the extreme energy output of these young stars, which start nuclear burning in their cores even before they complete their growth through accretion, will prevent further growth by blowing away the accretion disks. Hence, alternate scenarios like mergers of lower mass stars have been suggested as the main mechanism for massive star formation. The presence or absence of outflows from massive young stars will tell us whether accretion or some other methods lead to their formation.

Dr. Watson Varricatt says: “Most of the massive young stars in formation are confined to the galactic plane and are located in giant molecular clouds extending over several or even tens of light-years. They are hidden behind large amounts of gas and dust, which hampers their detection at visible wavelengths. Hence, to understand the formation of massive stars, we need to observe them at wavelengths that allow us to peer through these layers of gas and dust. Obtaining sharp images of these often very distant objects is also crucially important. The high sensitivity of the UKIRT Fast-Track Imager (UFTI) and the infrared capabilities of UKIRT were therefore very well suited to these observations.”

A near-IR imaging survey of 50 bright Young Stellar Objects (YSOs) was performed using UKIRT and UFTI. Observations were done in the narrow-band filter centered at 2.122 microns and in the broad-band filter at 2.2 microns. The former is the wavelength of an emission line of molecular Hydrogen (H2), which is abundant in the molecular clouds where stars form.

Dr. Watson Varricatt says: “76% of the objects surveyed showed molecular line emission; most of these are due to outflows. Within our sample, the outflows are seen to be well-defined irrespective of the energy output of their central young stars and are nearly as well-defined as those from low-mass stars. These outflows detected by us at infrared wavelengths agree with the outflows detected at longer wavelengths such as in the millimeter regime, and appear to be driven by jets like those from low-mass stars. We conclude that massive stars up to at least 30 times the mass of our Sun form through disk accretion.”

Dr. Chris Davis adds that “A conclusive result required observations of a large number of massive young stars. Credit is certainly due to the dedicated team of astronomers, technicians, and software engineers working at UKIRT. Without their help we would not have been able to observe so many stars on so many different nights.”

The observations were performed using UKIRT and UFTI. UFTI employs a 1024 by 1024 infrared imaging detector array and covers a field of view of 1.5 arcminutes by 1.5 arcminutes per image. The high sensitivity and excellent spatial resolution of UFTI on UKIRT (0.091 arcsec/pixel) enables us to obtain high spatial resolution deep images of obscured regions like those in the galactic plane where stars form.

“This is a fantastic and long-awaited result”, says Professor Gary Davis, Director of UKIRT. “This comprehensive survey addresses one of the key questions in modern astrophysics, which is the formation mechanism for high-mass stars. These observations took several years to complete and they took advantage of UKIRT’s unique capabilities. Indeed, this result demonstrates that mature telescopes can still deliver ground-breaking science and it is rewarding to see the observatory used in this way.”

Images: http://outreach.jach.hawaii.edu/pressroom/2010_ukirt_massive_star/

Science Contacts:
Dr. Watson P. Varricatt
Joint Astronomy Centre
+1 808 969 6523
w.varricatt@jach.hawaii.edu

Dr. Christopher J. Davis
Joint Astronomy Centre
+1 808 969 6520
c.davis@jach.hawaii.edu

Dr. Suzanne Ramsay
European Southern Observatory
+49 (0)89 3200 6665
sramsay@eso.org

Dr. Stephen P. Todd
UK Astronomy Technology Centre, ROE
+44 (0)131 668 8246
spt@roe.ac.uk

Dr. Tom Kerr
Joint Astronomy Centre
+1 808 969 6570
t.kerr@jach.hawaii.edu

Prof. Gary Davis
Joint Astronomy Centre
+1 808 969 6504
g.davis@jach.hawaii.edu

Reference: This press release refers to a paper published in the Monthly Notices of the Royal Astronomical Society (MNRAS, 404, 661) “A near-IR imaging survey of intermediate- and high-mass young stellar outflow candidates”; Authors: Watson P. Varricatt, Christopher J. Davis, Suzanne Ramsay, Stephen P. Todd; astro-ph: arXiv: 1001.2708

Notes for Editors

Light-year One light-year is about 10 million million kilometers or 6 million million miles. This is the distance light travels in a year.

Infrared Light Infrared wavelengths are longer wavelengths than visible light waves. They are typically measured in microns, also called micrometers. One micron is one millionth of a meter, one 10000th of a centimeter, or one 25000th of an inch. Visible light has wavelengths around half a micron, while the observations reported here were at wavelengths of about 2 microns. Human eyes are not sensitive to infrared light. We need specially designed cameras with detectors sensitive to infrared radiation to detect them.

Interstellar Extinction Light from distant astronomical objects is absorbed and scattered by dust and gas in our Galaxy between the objects and our telescope. For stars located in the galactic plane, where most of the dust and gas in our Galaxy is located, the interstellar extinction is very high. The level of extinction also depends on the wavelength being observed. It is very high at visible wavelengths and significantly lower in the longer infrared wavelengths. Hence observations at infrared wavelengths lose less light to this phenomenon, and therefore can help us to understand processes happening in highly obscured regions of the galaxy like the galactic plane.

UKIRT One of the world’s largest telescopes dedicated solely to infrared astronomy, the 3.8-meter (12.5-foot) United Kingdom Infrared Telescope (UKIRT) is sited near the summit of Mauna Kea, Hawaii, at an altitude of 4194 meters (13760 feet) above sea level. It is operated by the Joint Astronomy Centre in Hilo, Hawaii, on behalf of the UK Science and Technology Facilities Council. UKIRT’s technical innovation and privileged position on the high, dry Mauna Kea site have placed it at the forefront of infrared astronomy since its opening in 1979. UKIRT is currently engaged in a world-leading infrared sky survey as well as the type of innovative individual program described in this press release. More about the UK Infrared Telescope: http://outreach.jach.hawaii.edu/articles/aboutukirt/

UFTI The UKIRT Fast Track Imager (UFTI) is a near-infrared camera built by Oxford University in the U.K. It houses a 1024×1024 pixel Mercury-Cadmium-Telluride array which is sensitive to light with wavelengths between 0.9 and 2.5 microns. Images can be taken through a variety of filters; for the above project a broad-band “K” filter centered at 2.2 microns in the atmospheric transmission window at these wavelengths was used, along with a narrower filter centered on the brightest near-infrared molecular hydrogen emission line at 2.122 microns. The data were processed in-house using software developed by staff of the Joint Astronomy Centre.

Science and Technology Facilities Council The Science and Technology Facilities Council is an independent, non-departmental public body of the Office of Science and Innovation which itself is part of the Department of Innovation, Universities and Skills. It was formed as a new Research Council on 1 April 2007 through a merger of the Council for the Central Laboratory of the Research Councils (CCLRC) and the Particle Physics and Astronomy Research Council (PPARC) and the transfer of responsibility for nuclear physics from the Engineering and Physical Sciences Research Council (EPSRC). We are one of seven national research councils in the UK. The Science and Technology Facilities Council is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.