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Session 81 - Variable Cool and Late Type Stars.
Display session, Wednesday, January 15
Metropolitan Ballroom,
Using differential photoelectric Strömgren b, y photometry, we monitored the brightness variations of 41 predominantly main sequence F3-K7 program stars and their 73 associated comparison stars from 1984 through 1995. The program stars span a range of temperature and mean chromospheric activity centered on solar values. About 40% of all the stars show measurable variability, typically at levels well below 0.01 mag, on both night-to-night and year-to-year time scales. The variability is correlated with mean chromospheric activity and with advancing spectral type. We present a sampling of light curves and a statistical description of the results. For program stars closely matching the Sun's age and mean chromospheric activity, we found long term variations in visible light typically several times larger than the Sun's 0.1% irradiance variation in cycle 21 (Lockwood et al. 1992, Nature 360, 653).These reinforce the suggestion that the Sun's present low level of variability may be unusual or temporary. When classified according to mean chromospheric activity, we find that stellar brightness variations correlate negatively with chromospheric variation for stars younger than the Sun, and positively for stars older than the Sun. This result further emphasizes the remarkable delicate balance in the Sun's total irradiance between the brightening effects of faculae and the dimming effects of sunspots.
The RISE program of the Atmospheric Sciences Division of the National Science Foundation supports this program under grant ATM-9313667.
