A Complete Census of Luminous Stellar Variability on Day to Decade Timescales

TL;DR

This study provides a comprehensive census of stellar variability across a wide range of timescales in M51, revealing the prevalence and characteristics of variability among different stellar populations.

Contribution

It offers the first extensive variability survey on day to decade timescales for a large stellar sample in M51, combining new and archival HST data.

Findings

Over 50% of stars brighter than M_I=-7 are variable.

Red supergiants and luminous blue stars show large amplitude variability.

The Cepheid instability strip exhibits a variability fraction up to 10%.

Abstract

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 for 73,000 stars brighter than $M_I$=-5 in the Whirlpool Galaxy (M51). Our Cycle 24 HST program acquired V and I-band images over 34 epochs spanning one 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 $M_I$=-7 show strong evidence for variability within our Cycle 24 data; among stars with V-I>2 the variability fraction rises to ~100%. Large amplitude variability (>0.3 mag) on decade timescales is restricted to red supergiants 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 color magnitude diagram broadly agrees with theoretical sources of variability, including the instability strip, red supergiant 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.