archive: SETI [ASTRO] Standard Star-Birth Theories Challenged By Radio

SETI [ASTRO] Standard Star-Birth Theories Challenged By Radio

Larry Klaes ( )
Mon, 21 Dec 1998 13:59:07 -0500

>X-Authentication-Warning: majordom set sender
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>Date: Mon, 21 Dec 1998 4:17:34 GMT
>From: Ron Baalke <>
>Subject: [ASTRO] Standard Star-Birth Theories Challenged By Radio Images
of Wind Flows from Massive Protostar in Orion
>Reply-To: Ron Baalke <>
>Public Affairs
>Harvard-Smithsonian Center for Astrophysics
>Cambridge, Massachusetts
>For additional information, contact:
>Lincoln Greenhill, Harvard-Smithsonian Center for Astrophysics
>Carl Gwinn, University of California, Santa Barbara
>Phil J. Diamond, National Radio Astronomy Observatory, Socorro, NM
>James Cornell, Harvard-Smithsonian Center for Astrophysics
>David Finley, NRAO
>December 16, 1998
>Release No.: 98-16
>Huffing and Puffing: Standard Star-Birth Theories Challenged By
>Radio Images of Wind Flows from Massive Protostar in Orion
>CAMBRIDGE, MA --In apparent contradiction of expectations based on
>current stellar birth theories, a team of radio astronomers has
>found that a well-known massive protostar in the constellation Orion
>seems to be 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 evidence for there being two oppositely directed cones of
>gas flowing away from the star's polar regions. More remarkable, the
>team also found an apparently separate outflow of gas from a
>doughnut-like ring around the star's equator.
>The images were obtained with two National Science Foundation radio
>telescope facilities, the continent-wide Very Long Baseline Array
>(VLBA) and the Very Large Array (VLA) in New Mexico. The results are
>reported in the December 17 edition of Nature.
>"The observations run counter to our expectations -- and to the
>general consensus among astronomers -- that protostars should be
>surrounded by rotating accretion disks," says Lincoln Greenhill of
>the Harvard-Smithsonian Center for Astrophysics (CfA) and the leader
>of the research group.
>"These equatorial disks are important because they channel infalling
>material onto the surface of `young and growing' stars," says
>Greenhill. "Although the stars accumulate most of the disk material,
>a fraction is ejected as often stunning oppositely-directed gas jets
>flowing away from the stars' poles."
>"Here we see the walls of two outflow cones, but no evidence of a
>disk or other rotating material," adds Greenhill.
>The researchers believe an accretion disk once surrounded the
>protostar, but probably was completely disrupted by wind-driven
>material flowing away from the protostar. "The origin of the strong,
>well-defined polar outflow is uncertain, if there is no longer an
>accretion disk," says James Moran, also of the CfA. "Perhaps we are
>viewing a stellar wind, one that is denser and more asymmetric than
>the solar wind."
>The protostar is in the KL star-forming region of Orion, just to the
>northwest and behind the giant nebula in Orion's Sword that is
>well-known to amateur astronomers. While the nebula is plainly
>visible even to the naked eye, the KL region -- and protostar -- is
>deeply embedded within a molecular cloud and cannot be seen with
>optical telescopes. Fortunately, radio waves are not 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 of
>high-mass star formation, " says Carl Gwinn of the University of
>California Santa Barbara and a member of the team. "So the discovery
>of an equatorial outflow from this type of star not only dashes the
>expectation that disks accompany all early stages of stellar life,
>but also suggests just how poorly we understand high-mass star
>High-mass stars can be as massive as tens of Suns, and as luminous
>as many thousands of Suns; for example, the protostar in Orion-KL is
>almost 100 000 times more luminous than our Sun. Using facilities
>such as the Hubble Space Telescope, astronomers have previously
>obtained images of young stars more similar in mass to the Sun with
>outflows 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 North
>America from the Virgin Islands to Hawaii. The signals from each
>telescope are combined to create what is, in effect, a single
>aperture over 8000 km in diameter. With it, angular resolutions as
>fine as 0.2 milliarcseconds were achieved in the study reported
>here.(This is the equivalent of photographing a dime in San
>Francisco with a camera held in New York City.) The VLA is a
>high-sensitivity array of 27 telescopes, which, in this study, were
>spread over a maximum separation of some 35 km in west-central New
>With the VLBA, Greenhill and his team studied radio emission from
>SiO molecules, while with the VLA they studied emission from water
>molecules, both of which serve to trace the dense gas in the
>vicinity of the protostar.
>Both the VLBA and VLA are instruments of the National Radio
>Astronomy Observatory (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
>member of the research team was Colleen Schwartz, a student at the
>Maria Mitchell Observatory and Colby College.
>Clockwise from the top left:
>a) In the current view of stellar formation, a protostar is
>surrounded by a rotating accretion disk. Gas that is falling inward
>under the influence of gravity accumulates near the star's equator.
>Disk material eventually falls in toward the protostar or is ejected
>in the direction of the star's polar regions. The disk is believed
>to fuel this polar outflow. (From Shu, Adams, and Lizano, Annual
>Reviews of Astronomy and Astrophysics, Vol. 25).
>b) A new map of radio emission from dense gas in the vicinity of
>"Radio Source I, " whose position is indicated by the cross. Color
>coding indicates velocity, either toward (blue) or away (red) from
>the observer. Molecules (SiO) in the very-hot gas (i.e., 1000 K,
>about 3 times room temperature) along the walls of two oppositely
>directed outflow cones generate the observed radio emission. The
>heavy arrows show the direction of gas flowing through the cones.
>c) A map of the radio emission from both the cones (shown circled)
>and from a cooler gas rich in water vapor (i.e., 400 K, about the
>boiling point of water). The water occurs in dense cloudlets being
>pushed away from the protostar (i.e., the small arrows). Remarkably,
>these water-rich cloudlets are confined to the equatorial region of
>the protostellar system, where they move more slowly than the gas
>driven away from the polar regions of the protostar. No accretion
>disk is visible, even though there is a strong polar outflow from
>the protostar.
>d) The context of "Radio Source I:" The distribution of water
>emission is superposed on an infrared and radio image of the region
>just northwest of the Orion nebula. The heavy lines indicate the
>walls of the outflow cones. Several sources are labelled, including
>a very bright newly born star that is still visible only at infrared
>wavelengths (i.e., source n), and a string of dust-laden gas clumps
>(i.e., IRc2). These clumps may be either heated by gas flowing away
>from Radio Source I, or by a direct line of sight to the protostar,
>looking down the cone. Outflows in this region of Orion have been
>known to astronomers for decades, though their origin has not been
>well determined on the scales shown here. The large cross is the
>previously suspected location of the origin of the outflows.
>(Background images from papers Dougados et al. and Menten and Reid,
>Astrophysical Journal, 1993 and 1995.)
>e) The new model for Radio Source I (mesh) with the distributions of
>molecular radio emission superposed. In contrast to expectations
>derived from the current theoretical understanding of stellar birth
>(figure a), this very young star does not appear to be surrounded by
>a disk -- or any other rotating material. This protostar often
>considered a prototype for other very young high-mass stars (i.e.,
>many tens of suns). Other similar objects may also drive conical
>outflows but this is the first such flow to have been observed with
>any clarity.