Because M32 has the weakest known UV upturn (Burstein et al., 1988), its IUE spectrum could have been explained entirely by relatively short-lived post-AGB stars instead of EHB stars. Early UV images of M32 and the M31 bulge, with the Faint Object Camera (FOC) on the Hubble Space Telescope (HST), resolved some of this UV emission into bright stars (Brown et al., 1998b), but later near-UV images with the Space Telescope Imaging Spectrograph (STIS) resolved the EHB stars in M32, demonstrating that they are responsible for nearly all of its UV emission (Brown et al., 2000b). Although no color information was available in those data, the near-UV luminosity function showed a sharp peak at the level of the EHB. Recently, we obtained far-UV observations of the same field with the HST/STIS, in order to construct a deep UV color-magnitude diagram (CMD) of M32 (figure 3). The CMD shows a well-populated EHB, confirming that the UV emission comes from a minority population of EHB stars (~ 2% of the total HB). However, the images are missing many of the post-HB stars expected from canonical stellar evolution theory. Most of the HB stars in M32 lie on the red HB, and those should produce hundreds of UV-bright post-AGB stars, yet the HST/STIS images have only a handful. Furthermore, there should be approximately 1-2 post-EHB stars (also known as AGB-Manque stars) for every 10 EHB stars, yet these post-EHB stars are also under-represented. This is shown by the simulation in figure 3, which assumes a bimodal HB morphology that best reproduces the distribution of stars in the observed CMD. Although the number of EHB stars can be matched, no mass distribution reproduces the number of AGB-Manque and post-AGB stars. Because the missing stars should be the brightest ones in the image, they cannot be missing due to instrumental effects or incompleteness. Instead, these post-HB stars likely evolve on much more rapid timescales than predicted by standard evolution theory.
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Figure 3. Top panel: The CMD constructed from the near-UV and far-UV images of M32, as observed by the HST/STIS (black points), compared to the predicted location of the zero-age HB (grey curve). Although the EHB is well-populated, the UV-bright stars above the EHB are under-represented. Bottom panel: The simulation that best reproduces the observed CMD, which assumes a bimodal HB morphology having a minority EHB population. Note the large numbers of UV-bright stars, and the clear gap between the EHB and AGB-Manque stars, which are not seen in the observed CMD. |