Release No.: 98-16
December 16, 1998
Huffing and Puffing: Standard Star-Birth Theories Challenged ByRadio Images of Wind Flows from Massive Protostar in Orion
CAMBRIDGE, MA --In apparent contradiction of expectations based oncurrent stellar birth theories, a team of radio astronomers has foundthat a well-known massive protostar in the constellation Orion seems tobe exhaling-- when it should be inhaling.
Detailed images of the dense gas surrounding the high-mass protostar,known rather prosaically as "Radio Source I," provide graphic evidencefor there being two oppositely directed cones of gas flowing away fromthe star's polar regions. More remarkable, the team also found anapparently separate outflow of gas from a doughnut-like ring around thestar's equator.
The images were obtained with two National Science Foundation radiotelescope facilities, the continent-wide Very Long Baseline Array(VLBA) and the Very Large Array (VLA) in New Mexico. The results arereported in the December 17 edition of Nature.
"The observations run counter to our expectations-- and to the generalconsensus among astronomers-- that protostars should be surrounded byrotating accretion disks," says Lincoln Greenhill of theHarvard-Smithsonian Center for Astrophysics (CfA) and the leader of theresearch group.
"These equatorial disks are important because they channel infallingmaterial onto the surface of `young and growing' stars," says Greenhill."Although the stars accumulate most of the disk material, a fractionis ejected as often stunning oppositely-directed gas jets flowing awayfrom the stars' poles."
"Here we see the walls of two outflow cones, but no evidence of adisk or other rotating material," adds Greenhill.
The researchers believe an accretion disk once surrounded theprotostar, but probably was completely disrupted by wind-driven materialflowing away from the protostar. "The origin of the strong,well-defined polar outflow is uncertain, if there is no longer anaccretion disk," says James Moran, also of the CfA. "Perhaps we areviewing a stellar wind, one that is denser and more asymmetric thanthe solar wind."
The protostar is in the KL star-forming region of Orion, just to thenorthwest and behind the giant nebula in Orion's Sword that iswell-known to amateur astronomers. While the nebula is plainlyvisible even to the naked eye, the KL region--and protostar-- isdeeply embedded within a molecular cloud and cannot be seen withoptical telescopes. Fortunately, radio waves arenot significantly blocked by molecular gas, and so the VLBA and VLA are well suited to investigation of the region.
"The Orion-KL region has been the archetypal site for studies ofhigh-mass star formation, " says Carl Gwinn of the University ofCalifornia Santa Barbara and a member of the team. "So the discoveryof an equatorial outflow from this type of star not only dashes theexpectation that disks accompany all early stages of stellar life, butalso suggests just how poorly we understand high-mass star formation."
High-mass stars can be as massive as tens of Suns, and as luminous asmany thousands of Suns; for example, the protostar in Orion-KL isalmost 100 000 times more luminous than our Sun. Using facilitiessuch as the Hubble Space Telescope, astronomers have previouslyobtained images of young stars more similar in mass to the Sun withoutflows that take the shape of brilliant, narrow jets.
"In contrast, the polar outflow observed with the VLBA is relatively broad," says team member Phil Diamond of the National Radio Astronomy Observatory. " And the well-defined conical shapes are unexpected--and spectacular.".
The VLBA is a system of 10 individual telescopes spread across NorthAmerica from the Virgin Islands to Hawaii. The signals from eachtelescope are combined to create what is, in effect, a single apertureover 8000 km in diameter. With it, angular resolutions as fine as 0.2milliarcseconds were achieved in the study reported here.( This is theequivalent of photographing a dime in San Francisco with a camera heldin New York City.) The VLA is a high-sensitivity array of 27 telescopes,which, inthis study, were spread over a maximum separation of some 35 km inwest-central New Mexico.
With the VLBA, Greenhill and his team studied radio emission fromSiO molecules, while with the VLA they studied emission from watermolecules, both of which serve to trace the dense gas in the vicinityof the protostar.
Both the VLBA and VLA are instruments of the National Radio AstronomyObservatory (NRAO), a facility of the National Science Foundation,operated under cooperative agreement by Associated Universities, Inc.
In addition to Greenhill, Gwinn, Diamond, and Moran, the other memberof the research team was Colleen Schwartz, a student at the MariaMitchell Observatory and Colby College.
Click here for multi-image illustration and captions.
For additional information, contact:
Lincoln Greenhill, Harvard-Smithsonian Center for Astrophysics,617-495-7194, firstname.lastname@example.org
Carl Gwinn, University of California, Santa Barbara; 805-893-2814,email@example.com
Phil J. Diamond, National Radio Astronomy Observatory, Socorro, NM;505-835-7000, firstname.lastname@example.org
James Cornell, Harvard-Smithsonian Center for Astrophysics;617-495-7462, email@example.com
David Finley, NRAO, 505-835-7302, firstname.lastname@example.org