Astronomy Now Online

Comet observed crashing into distant star
Posted: April 19, 2004


A hot star, LKHa 234, surrounded by hot gas and a planetary disk, as imagined in this artist's concept. A comet-like body more than 100 km wide is vaporizing and producing a tail of sodium gas as it is falling onto the star.

Image credit: Tigran Ghulyan, Penn State University.

Evidence that a comet-like body with a diameter of at least 100 kilometres fell into a massive, very young star has been obtained by a team of astronomers at Penn State using the 9.2-metre 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 comet-like 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 that 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 May 1st 2004 issue of Astrophysical Journal Letters.

The evidence of the infall comes from spectral analysis of the young star's light, which has travelled 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 comet-like 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 comet-like 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."


Location of the star LKHa 234 in the star-forming nebula NGC 7129.

Image credit: Lars Lindberg Christensen, ESA/Hubble.

"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 comet-like 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 comet-like body occurs around these young massive stars.

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