Caught In The Act (Forwarded)

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For Release: EMBARGOED until 10:00 a.m. MDT, Wednesday, June 2, 2004

Release No.: 04-23

Caught In The Act

Denver, CO -- How old is too old? Pro football players tend to peak in
their late 20s, and few continue their careers beyond the age of 35. For
young stars, the peak age for planet formation is around 1 to 3 million
years. By 10 million years old, their resources are exhausted and they
retire to a life on the stellar "main sequence."

Using telescopes on the ground and in space, a team of astronomers led by
Lee W. Hartmann and Aurora Sicilia-Aguilar (Harvard-Smithsonian
Center for Astrophysics) is studying Sun-like stars in their waning
formative years, within clusters older than previously explored. They seek
to refine our understanding of planet formation by studying dusty
protoplanetary disks around such stars. Their results, presented today at
the 204th meeting of the American Astronomical Society in Denver,
Colorado, better define the time span during which planets might form.

"While the planets that may be forming cannot be detected directly," said
Sicilia-Aguilar, "we can see changes in the circumstellar dusty accretion
disks caused as the planets sweep up and accumulate mass."

"The data also has shown dramatic differences between stars of 3 and 10
million years of age: the younger stars frequently have dusty disks capable
of forming planets, while such disks are essentially absent in the older
population," she continued.

The team used data from the Smithsonian Institution's Whipple
Observatory telescopes, the WIYN telescope at Kitt Peak National
Observatory, and from the Spitzer Space Telescope (the latter made
available as part of the Guaranteed Time Program of Infrared Array
Camera PI Giovanni Fazio), to make these findings.

"We are trying to understand the evolution of protoplanetary disks around
stars not too different from the Sun," said team leader Lee W. Hartmann.
"Many stars about 1 million years old have disks, but by 10 million years,
almost none have disks. We are trying to find stars at an in-between age
and 'catch them in the act' of forming planets."

Circumstellar dust disks enshroud young stars, and astronomers
understand this to be a common feature of stellar evolution and of possible
planetary system formation. The initial protoplanetary disks contain the
gas and dust that provide the raw materials for the formation of later
planetary systems.

"After stars form planets in their disks and clear out most of the material-
either by accretion onto the star, accretion onto planets, or ejection-small
amounts of dust can remain in so-called 'debris disks.' Most or all of this
debris dust is thought to be continuously generated by the collision of
small bodies, much like the zodiacal light in our solar system," said
Hartmann.

The team is presenting the first identification of low mass stars in the
young clusters Trumpler 37 and NGC 7160. (These clusters are loose
associations of stars that have formed together in the comparatively recent
past.) "The cluster members confirm the age estimates of 1 to 5 million
years for Tr37 and 10 million years for NGC 7160," said Sicilia-Aguilar.

"We do find active accretion in some of the stars in Tr37. The average
accretion rate is equivalent to swallowing up 10 Jupiter masses in a
million years," said Sicilia-Aguilar. "This is consistent with models of
viscous disk evolution."

"In comparison, we have detected no signs of active accretion so far in the
older cluster NGC 7160, suggesting that disk accretion ends within 10
million years. This probably coincides with the major phase of giant planet
formation."

Trumpler 37 is of more immediate interest, said Hartmann, because we
hope to find stars with Jupiter-size planets that are still accumulating
material from the disks, so the disks are not completely cleared out yet.
However, there may be a few objects in the 10 million-year-old cluster
NGC 7160 that are also still forming their giant planets. Not all disks
evolve at the same rate.

"Thus we expect eventually to find out more about the frequency of debris
disks, and the rate at which the dust in such disks is removed, by studying
the 10-million-year-old cluster NGC 7160 and comparing it to Trumpler
37," said Hartmann.

In addition to Sicilia-Aguilar and Hartmann, team members include Cesar
Briceno (Centro de Investigaciones de Astronomia), James Muzerolle
(University of Arizona), and Nuria Calvet (Smithsonian Astrophysical
Observatory). This work was supported by NASA grant NAG5-9670.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin, evolution
and ultimate fate of the universe.

Note to editors: A high-resolution image to accompany this release is
available online at:
http://www.cfa.harvard.edu/press/pr0423image.html