Infrared Colors and Variability of Evolved Stars fromCOBEDIRBE Data

For a complete 12 micron flux-limited sample of 207 IRAS sources, we haveextracted light curves in seven infrared bands between 1.25 - 60 microns usingthe database of the DIRBE instrument on the COBE satellite. Using previousinfrared surveys, we filtered these light curves to remove datapoints affectedby nearby companions. In the time-averaged DIRBE color-color plots, we findclear segregation of semi-regulars, Mira variables, carbon stars, OH/IR stars,and red giants without circumstellar dust and with little or no visualvariation. The DIRBE 1.25 - 25 micron colors become progressively redder andthe variability in the DIRBE database increases along the oxygen-rich sequencenon-dusty slightly varying red giants -> SRb/Lb -> SRa -> Mira -> OH/IR and thecarbon-rich SRb/Lb -> Mira sequence. This supports previous assertions thatthese are evolutionary sequences involving the continued production andejection of dust. Carbon stars are redder than oxygen-rich stars for the samevariability type, except in the F(12)/F(25) ratio. Of the 28 sources in thesample not previous noted to be variable, 18 are clearly variable in the DIRBEdata, with amplitudes of variation of ~0.9 magnitudes at 4.9 microns and ~0.6magnitudes at 12 microns, consistent with them being very dusty Mira-likevariables. The light curves of the semi-regular variable L2 Pup areparticularly remarkable. The maxima at 1.25, 2.2, and 3.5 microns occur 10 - 20days before those at 4.9 and 12 microns, and, at 4.9 and 12 microns, anothermaximum is seen between the two near-infrared maxima.Comment: Accepted by the Astronomical Journa