Color‐Luminosity Relations for the Resolved Hot Stellar Populations in the Centers of M31 and M32

Abstract

We present Faint Object Camera (FOC) ultraviolet images of the central14'' x 14'' of Messier 31 and Messier 32. The hot stellar populationsdetected in the composite UV spectra of these nearby galaxies arepartially resolved into individual stars, and their individual colorsand apparent magnitudes are measured. We detect 433 stars in M31 and 138stars in M32, down to detection limits mF275W = 25.5 mag and mF175W =24.5 mag. We investigate the luminosity functions of the sources, theirspatial distribution, their color-magnitude diagrams, and their totalintegrated far-UV flux. Comparison to IUE and Hopkins UltravioletTelescope (HUT) spectrophotometry and WFPC2 stellar photometry indicatesconsistency at the 0.3 mag level, with possible systematic offsets inthe FOC photometry at a level less than this. Further calibrations orobservations with the Space Telescope Imaging Spectrograph (STIS) willbe necessary to resolve the discrepancies. Our interpretation rests onthe assumption that the published FOC on-orbit calibration is correct.Although M32 has a weaker UV upturn than M31, the luminosity functionsand color-magnitude diagrams (CMDs) of M31 and M32 are surprisinglysimilar and are inconsistent with a majority contribution from any ofthe following: post-asymptotic giant branch (PAGB) stars more massivethan 0.56 M{\sun} (with or without associated planetary nebulae), mainsequence stars, and blue stragglers. Both the luminosity functions andcolor-magnitude diagrams are consistent with a dominant population ofstars that have evolved from the extreme horizontal branch (EHB) alongtracks with masses between 0.47 and 0.53 M{\sun}. These stars are wellbelow the detection limits of our images while on the zero-age EHB butbecome detectable while in the more luminous (but shorter) AGB-manqueand post-early asymptotic giant branch (PEAGB) phases. The FOCobservations require that only very small fractions of the main-sequencepopulations (2% in M31 and 0.5% in M32) in these two galaxies evolvethough the EHB and post-EHB phases, with the remainder evolving throughbright PAGB evolution that is so rapid that few if any stars areexpected in the small field of view covered by the FOC. A model with aflat EHB star mass distribution reproduces the HUT and IUE spectra ofthese two galaxies reasonably well, although there is some indicationthat an additional population of very hot (Teff {\gt} 25,000 K) EHB starsmay be needed to reproduce the HUT spectrum of M31 near the Lyman limitand to bring integrated far-UV fluxes of M31 and M32 into agreement withIUE. In addition to the post-EHB population detected in the FOC, we finda minority population (~10%) of brighter stars that populate a region ofthe CMD that cannot be explained by canonical post-HB evolutionarytracks. The nature of these stars remains open to interpretation. Thespatial distributions of the resolved UV-bright stars in both galaxiesare more centrally concentrated than the underlying diffuse emission,implying that stellar populations of different ages and/or metallicitiesmight be responsible for each component.