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We present a catalog of 417 luminous infrared variable stars with periods exceeding 250 days. These were identified in 20 nearby galaxies by the ongoing SPitzer InfraRed Intensive Transients Survey survey with the Spitzer Space Telescope. Of these, 359 variables have M[4.5] (phase-weighted mean magnitudes) fainter than −12 and periods and luminosities consistent with previously reported variables in the Large Magellanic Cloud (LMC). However, 58 variables are more luminous than M[4.5] = −12, including 11 that are brighter than M[4.5] = −13, with the brightest having M[4.5] = −15.51. Most of these bright variable sources have quasi-periods longer than 1000 days, including four over 2000 days. We suggest that the fundamental period–luminosity relationship, previously measured for the LMC, extends to much higher luminosities and longer periods in this large galaxy sample. We posit that these variables include massive asymptotic giant branch (AGB) stars (possibly super-AGB stars), red supergiants experiencing exceptionally high mass-loss rates, and interacting binaries. We also present 3.6, 4.5, 5.8, and 8.0 μm photometric catalogs for all sources in these 20 galaxies. [ABSTRACT FROM AUTHOR]

We have derived the Galactic bulge initial mass function (IMF) of the Sagittarius Window Eclipsing Extrasolar Planet Search field in the mass range 0.15 1.0, using deep photometry collected with the Advanced Camera for Surveys on the Hubble Space Telescope. Observations at several epochs, spread over 9 years, allowed us to separate the disk and bulge stars down to very faint magnitudes, 26 mag, with a proper-motion accuracy better than 0.5 mas yr−1 (20 km s−1). This allowed us to determine the IMF of the pure bulge component uncontaminated by disk stars for this low-reddening field in the Sagittarius window. In deriving the mass function, we took into account the presence of unresolved binaries, errors in photometry, distance modulus and reddening, as well as the metallicity dispersion and the uncertainties caused by adopting different theoretical color–temperature relations. We found that the Galactic bulge IMF can be fitted with two power laws with a break at 0.56 , the slope being steeper () for the higher masses, and shallower () for the lower masses. In the high-mass range, our derived mass function agrees well with the mass function derived for other regions of the bulge. In the low-mass range however, our mass function is slightly shallower, which suggests that separating the disk and bulge components is particularly important in the low-mass range. The slope of the bulge mass function is also similar to the slope of the mass function derived for the disk in the high-mass regime, but the bulge mass function is slightly steeper in the low-mass regime. We used our new mass function to derive stellar mass-to-light values for the Galactic bulge and we obtained 2.1 2.4 and 3.1 3.6 according to different assumptions on the slope of the IMF for masses larger than . [ABSTRACT FROM AUTHOR]