Hobby-Eberly Telescope Witnesses Vaporizing of a Cometlike Body by a Very Young Hot Star

Evidence that a cometlike body 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 appeared in 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 of 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 them as comets.” The observed infall of a cometlike body around LkHa 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, as well as LkHa 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.

Steve Miller