Flickering Red Giants a Surprising Find

11 January 2005

A significant number of red giant stars whose output varies on timescales as short as 10 minutes has been found in the nearby Ursa Minor dwarf galaxy by an astronomer at the National Optical Astronomy Observatory (NOAO) and a summer undergraduate student. This surprising variability occurs on a much shorter timescale than previously thought possible for such mature, giant stars.

For some of the larger stars in the sample of nine red giants, the variation in absolute brightness may approach a full solar luminosity, meaning that the star appears to grow or fade in brightness by an amount equal to the entire light output of the Sun. This research is the subject of poster paper [54.18] today in San Diego at the 205th meeting of the American Astronomical Society.

“These flickering red giants are part of a family of some of the oldest stars in the Universe. Until now, astronomers believed that most of their individual variations could be attributed generally to differences in their ages or in the amount of metals that they contain,” said NOAO astronomer Ken Mighell, lead author of the new study. “This discovery could complicate what had been a fairly placid family picture.”

All of the newly detected variable stars exhibit low-amplitude brightness fluctuations of less than one percent in considerably less than an hour. This discovery was made by carefully comparing individual frames of observations of the Ursa Minor dwarf galaxy obtained with the Hubble Space Telescope’s Wide Field and Planetary Camera 2 instrument in 1995 and 1999. These unrelated observations (originally done for James Westphal and Rodrigo Ibata) were retrieved from the HST archives and compared by Mighell and Ian Roederer from Indiana University during this past summer.

The exact nature of the underlying cause of these rapid luminosity variations is not yet known, but sources could include sunspots (i.e., starspots), stellar flares, or a partner star that is tearing mass off of the red giants as part of an interacting binary system. Follow-up observations are being planned with the WIYN 3.5-meter telescope at Kitt Peak National Observatory.

If this new phenomenon is verified and the timescale of the underlying source of variability is found to be a few years or less, then the flickering of red giants could possibly be seen to turn on and off during a long-term, high-precision, space-based photometric survey like NASA’s Kepler mission, or the next generation of ground-based digital sky survey projects like the Large Synoptic Survey Telescope.

Conversely, red giant stars are used as calibration reference points for many kinds of photometric (brightness) and astrometric (star position) observations in astronomy, so it is extremely important to understand any errors that the flickering red giants might induce into other research. For example, a flickering red giant could possibly appear to change its location or size in the precision astrometric measurements to be made by NASA’s planned Space Interferometry Mission (SIM).

Roederer is an undergraduate student at Indiana University in Bloomington who worked with Mighell last summer at the headquarters of Kitt Peak National Observatory, as part of the National Science Foundation-funded Research Experiences for Undergraduates (REU) program.

Spending the summer in Tucson, AZ, learning about the duties and appeal of being a professional astronomer “was a fantastic experience in a great research environment,” Roederer said. “The result of our planned search for new classes of variable stars was really not quite what we expected. It was exciting to me to see how Dr. Mighell excitement built gradually over the summer.”

“I am now certain that I want to go on to graduate school in astronomy,” he added. “I hope to see many of my fellow Kitt Peak REU students at this AAS meeting in San Diego, and at many more for the next 40 years.”

The lower-limit estimate of 10 minutes on the variability of the flickering red giants derives from the time between the imaging frames in the HST archive data sets used in the study. Studies of this sort show the powerful potential of archival research, according to Mighell, which will only expand with the emerging volumes of large data sets and virtual observatories coming online.

Ursa Minor is a dwarf spheroidal galaxy located about 220,000 light-years from Earth in the direction of the constellation Ursa Minor (The Little Bear).

More information

More details about this research may be found in the December 10, 2004, edition of the Astrophysical Journal Letters (Mighell & Roederer 2004, ApJL, 617, L41-L44).

Kitt Peak National Observatory is part of NOAO, which is operated by the Association of Universities for Research in Astronomy (AURA) Inc., under a cooperative agreement with the National Science Foundation.

Contacts

Douglas Isbell
Public Information Officer National Optical Astronomy Observatory
Tel: 520/318-8230
Email: disbell@noao.edu