Chromospheric Variability in Sloan Digital Sky Survey M Dwarfs. Ii. Short-timescale Hα Variability

Abstract

We present the first comprehensive study of short-timescale chromospheric H alpha variability in M dwarfs using the individual 15 minute spectroscopic exposures for 52,392 objects from the Sloan Digital Sky Survey. Our sample contains about 10(3)-10(4) objects per spectral type bin in the range M0-M9, with a typical number of three exposures per object (ranging up to a maximum of 30 exposures). Using this extensive data set, we find that about 16% of the sources exhibit H alpha emission in at least one exposure, and of those about 45% exhibit H alpha emission in all of the available exposures. As in previous studies of H alpha activity (L(H alpha)/L(bol)), we find a rapid increase in the fraction of active objects from M0-M6. However, we find a subsequent decline in later spectral types that we attribute to our use of the individual spectra. Similarly, we find saturated activity at a level of L(H alpha)/L(bol) approximate to 10(-3.6) for spectral types M0-M5 followed by a decline to about 10(-4.3) in the range M7-M9. Within the sample of objects with H alpha emission, only 26% are consistent with non-variable emission, independent of spectral type. The H alpha variability, quantified in terms of the ratio of maximum to minimum H alpha equivalent width (R(EW)), exhibits a rapid rise from M0 to M5, followed by a plateau and a possible decline in M9 objects. In particular, variability with R(EW) greater than or similar to 10 is only observed in objects later than M5, and survival analysis indicates a probability of less than or similar to 0.1% that the R(EW) values for M0-M4 and M5-M9 are drawn from the same distribution. We further find that for an exponential distribution, the R(EW) values follow N(R(EW)). exp[-(R(EW) - 1)/2.3] for M0-M4 and alpha exp[-(R(EW) - 1)/2.9] for M5-M9. Finally, comparing objects with persistent and intermittent H alpha emission, we find that the latter exhibit greater variability. Based on these results, we conclude that H alpha variability in M dwarfs on timescales of 15 minutes to 1 hr increases with later spectral type, and that the variability is larger for intermittent sources.