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Description
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 73000 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 V_606_-I_814_>2 the variability fraction rises to ~100%. Large amplitude variability (>0.3mag) 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.
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