A Spitzer Space Telescope survey of extreme Asymptotic Giant Branch stars in M32

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We investigate the population of cool, evolved stars in the Local Group dwarf elliptical galaxy M32, using Infrared Array Camera observations from the Spitzer Space Telescope. We construct deep mid-infrared colour-magnitude diagrams for the resolved stellar populations within 3.5 arcmin of M32's centre, and identify those stars that exhibit infrared excess. Our data is dominated by a population of luminous, dust-producing stars on the asymptotic giant branch (AGB) and extend to approximately 3 mag below the AGB tip. We detect for the first time a sizeable population of `extreme' AGB stars, highly enshrouded by circumstellar dust and likely completely obscured at optical wavelengths. The total dust-injection rate from the extreme AGB candidates is measured to be 7.5 × 10⁻⁷ M yr⁻¹, corresponding to a gas mass-loss rate of 1.5 × 10⁻⁴ M yr⁻¹. These extreme stars may be indicative of an extended star-formation epoch between 0.2 and 5 Gyr ago.