2018ApJ...864..111C

Stellar photometric variability offers a novel probe of the interior structure and evolutionary state of stars. Here we present a census of stellar variability on day to decade timescales across the color-magnitude diagram (CMD) for 73,000 stars brighter than M_I,814 = -5 in the Whirlpool Galaxy (M51). Our Cycle 24 Hubble Space Telescope (HST) program acquired V₆₀₆- and I₈₁₄-band images over 34 epochs spanning 1 year with pseudo-random cadences enabling sensitivity to periods from days to months. We supplement these data with archival V- and I-band HST data obtained in 1995 and 2005, providing sensitivity to variability on decade timescales. At least 50% of stars brighter than M_I,814 = -7 show strong evidence for variability within our Cycle 24 data; among stars with V₆₀₆-I₈₁₄> 2 the variability fraction rises to ≃100%. Large amplitude variability (>0.3 mag) on decade timescales is restricted to red supergiants (RSGs) and very luminous blue stars. Both populations display fairly smooth variability on month-year timescales. The Cepheid instability strip is clearly visible in our data, although the variability fraction within this region never exceeds ≃10%. The location of variable stars across the CMD broadly agrees with theoretical sources of variability, including the instability strip, RSG pulsational instabilities, long-period fundamental mode pulsations, and radiation-dominated envelopes in massive stars. Our data can be used to place stringent constraints on the precise onset of these various instabilities and their lifetimes and growth rates.