Oddball Ancient Stars Helping Astronomers map Milky Way's Halo
CAMBRIDGE, MA -- A rare class of carbon-rich red giant stars, among the oldest stellar objects in our galaxy, are providing astronomers witha tool for tracing the outer regions of the Milky Way.
Paul Green of the Harvard-Smithsonian Center for Astrophysics reported today at a meeting of the American Astronomical Society that a preliminary survey of faint red stars in the galaxy's halo is revealing intriguing information about the shape and extent of the galaxy itself.
Current theories of our galaxy's early history and evolution suggest two extremely different scenarios: either the Milky Way formed from a single enormous gas cloud, or it is a "melting pot" composed of perhaps dozens of smaller galaxies that merged eons ago. These competing theories can be tested only by probing the galaxy's oldest and largest part--the outer halo.
Good probes require a large sample of objects visible at distancescomparable to the size of the Galaxy, which has a diameter of at least100,000 light years. The classic objects used for such probes have been globular star clusters.
However, although they are bright and easy to find, clusters have complicated histories, and may interact so strongly with the galaxy that, over time, their dynamic history is lost.
By contrast, carbon-rich red giant stars are not onlyintrinsically bright and thus visible to large distances but are unaffected by the type of forces felt by star clusters.
Moreover, "carbon stars" are easily recognizable by the strong effects of carbon molecules visible in their spectra. Equally important, because giant stars can dredge up heavier elements like carbon from their cores to the surface where they can be observed, virtually all carbon stars must be bright giants.
By implication, those carbon giants that appear to be faint must actually be very remote, in other words, ideal probes for measuring the motion and structure of our Galaxy's outer halo.
Green, a Hubble Fellow now at the Center, first undertook a search for distant red giant stars while at the University of Washington, Seattle. He and his collaborators, Bruce Margon and Scott Anderson in Seattle and Kem Cook at Lawrence Livermore, compiled a large sample of faint carbon stars, mostly from published surveys, and were surprised to discover that at least 10% of the them were not giants, but rather nearby dwarf stars whose surfaces nevertheless showed the effects of carbon molecules.
These unexpected objects are probably normal stars like the Sun that were "polluted" by nearby carbon giants that have since faded from view. Although fascinating in their own right, carbon dwarfs had to be weeded out of the faint star sample before the structure of the galactic halo was revealed.
The resulting analysis suggests that the galactic halo may be larger than expected, since the number of faint carbon starsin the Milky Way appears to drop off more slowly with distance thanthe number of globular clusters. This may be because the percentageof giants that wind up as carbon stars increases outward in thegalaxy, an interesting result that would confirm trends seen in othergalaxies.
But, if carbon stars in the outer halo of the Milky Way trace the distribution of other less brilliant stars, then the distributions of globular clusters and stars have evolved differently over the age of the galaxy. This would mean clusters could no longer be trusted as tracers of galactic stars or galactic structural evolution. Indeed, only expanded star samples such as the red giants may offer keys to understanding the history of the Milky Way.
Support for this work was provided in part by NASA through grants awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc.
Poster Presentation, Session 58.02, "Galactic Structure from Faint Halo Carbon Stars" by Paul J. Green, Harvard-Smithsonian Center for Astrophysics, at the 184th meeting of the American Astronoimical Society in Minneapolis, MN.