Infrared Colors and Variability of Evolved Stars from COBE DIRBE Data

For a complete 12 μm flux-limited sample of 207 IRAS sources (F12 ≥ 150 Jy, |b| ≥ 5), the majority of which are AGB stars (∼87%), we have extracted light curves in seven infrared bands between 1.25 μm − 60 μm using the database of the Diffuse Infrared Background Experiment (DIRBE) instrument on the Cosmic Background Explorer (COBE) satellite. Using previous infrared surveys, we filtered these light curves to remove datapoints affected by nearby companions, and obtained time-averaged flux densities and infrared colors, as well as estimates of their variability at each wavelength. In the time-averaged DIRBE color-color plots, we find clear segregation of semi-regulars, Mira variables, carbon stars, OH/IR stars, and red giants without circumstellar dust (i.e., V − [12] < 5) and with little or no visual variation (∆V < 0.1 magnitudes). The DIRBE 1.25 − 25 μm colors become progressively redder and the variability in the DIRBE database increases along the oxygen-rich sequence non-dusty slightly varying red giants −> SRb/Lb −> SRa −> Mira −> OH/IR and the carbon-rich SRb/Lb −> Mira sequence. This supports previous assertions that these are evolutionary sequences involving the continued production and ejection of dust. The carbon stars are redder than their oxygen-rich counterparts for the same variability type, except in the F12/F25 ratio, where they are bluer. We find significantly larger infrared variations for Mira stars, carbon stars, and OH/IR stars than for the red giants without circumstellar dust, with semi-regulars and irregulars in between, as expected based on their optical variations. Of the 28 sources in the sample not previous noted to be variable, 18 are clearly variable in the DIRBE data, with amplitudes of variation of ∼0.9 magnitudes at 4.9 μm and ∼0.6 magnitudes at 12 μm, consistent with them being very dusty Mira-like variables. We also present individual DIRBE light curves of a few selected stars. The DIRBE light curves of the semi-regular variable L2 Pup are particularly remarkable. The maxima at 1.25, 2.2, and 3.5 μm occur 10 − 20 days before those at 4.9 and 12 μm, and, at 4.9 and 12 μm, another maximum is seen between the two near-infrared maxima. This extra mid-infrared maximum is likely due to the formation of a new layer of dust.