Evidence that a cometlike object with a diameter of at least 100
kilometers fell into a massive, very young star has been obtained by
a team of astronomers at Penn State using the 9.2-meter Hobby-Eberly
Telescope at the McDonald Observatory in Texas. “This discovery is
significant because this is the youngest star ever found with this
kind of infall of a cometlike body,” says Jian Ge, assistant
professor of astronomy and astrophysics and the leader of the team.
The other scientists involved in the work are Abhijit Chakraborty, a
postdoctoral researcher in astronomy, and Suvrath Mahadevan, a
graduate student, both at Penn State.

The star, which astronomers identify as LkHalpha 234, is classified
as a Herbig Be star, which has a mass about six times the mass of the
sun and an estimated very young age of about 100,000 years. “This
detection indicates that solid bodies of 100 km in size can form this
early around a star,” Ge explains. A report of the work will appear
in the 1 May 2004 issue of Astrophysical Journal Letters.

The evidence of the infall comes from spectral analysis of the young
star’s light, which has traveled about 3200 years to reach Earth.
Five sets of observations taken at intervals of 5 to 10 days during
October and November 2003 indicated that the stellar light was
absorbed by clouds of hydrogen and helium surrounding the star as
well as by emissions from these clouds. “The spectacular appearances
and disappearances of the neutral-sodium-absorption lines on one
particular observation and the absence of its correlation with the
hydrogen and helium lines suggests a cometlike body,” says
Chakraborty. “We know how hot the star is and how close to the star
the neutral sodium atoms can survive. From that, and from the motion
of the cometlike body during infall onto the star, we calculated how
large the body would have to be to get this close to the
star–one-tenth of the distance between the Sun and the Earth–before
vaporizing.”

“This is a quite extraordinary event,” said Eric Feigelson, Penn
State professor of astronomy and astrophysics, who specializes in the
study of young stars. “Something happened on a time scale of days or
less that created an enormous change in the spectrum of this star
while the astronomers were looking.” According to Feigelson,
evidence for cometary infall has been seen in the spectrum of the
nearby star beta Pictoris, which is older and less massive than
LkHalpha 234, but not with the dramatic spectral variations seen here.

The infall provides new data for understanding planetary formation
and the timescale involved in the evolution of a massive star system.
“The main reason we see comets in our solar system is that large
snowballs in the outer parts of the solar system are disturbed by
Jupiter’s gravity,” says Ge. “Eventually, some of the snowballs fall
towards the inner solar system and we see then as comets.” The
observed infall of a cometlike body around LkH_234 may also point to
disturbances produced by giant planets in this young star system.
The team is now monitoring a number of similar stars and also LkH_234
in order to understand how common and how often this type of
cometlike body occurs around these young massive stars.

This research was funded by the National Aeronautics and Space
Administration and the National Science Foundation.

[ S M / B K K]

CONTACTS:

Jian Ge: jian@astro.psu.edu, (+1)814-863-9553

Abhijit Chakraborty: (+1)814-863-6091

Barbara Kennedy (PIO): science@psu.edu, (+1)814-863-4682

ILLUSTRATIONS:

High-resolution images for use in press reports concerning this
research are available from a link on the web at
http://www.science.psu.edu/alert/Ge4-2004.htm.