The hottest stars ($>$10,000 K), and by extension typically the most massive ones, are those that will be prevalent in the ultraviolet (UV) portion of the electromagnetic spectrum, and we expect numerous B, O, and Wolf-Rayet stars to be bright in UV data. In this paper, we update the previous UV catalog of M33, created using the Ultraviolet Imaging Telescope (UIT), using data from the Galaxy Evolution Explorer (GALEX). We utilize PSF photometry to better handle the crowded regions in the galaxy, and benefit from GALEXs increased sensitivity compared to UIT. We match our detections with data from the Local Group Galaxies Survey (LGGS) to create a catalog with photometry spanning from the far-UV through the optical for a final list of 24738 sources. All of these sources have far-UV (FUV; 1516A), near-UV (NUV; 2267A), and V data, and a significant fraction also have U, B, R, and I data as well. We compare these sources to a catalog of known Wolf-Rayet stars in M33 and find that we recover 114 of 206 stars with spatially-coincident UV objects. Additionally, we highlight and investigate those sources with unique colors as well as a selection of other well-studied sources in M33.
SN 1961V, one of Zwickys defining Type V supernovae (SN), was a peculiar transient in NGC 1058 that has variously been categorized as either a true core collapse SN leaving a black hole (BH) or neutron star (NS) remnant, or an eruption of a luminous blue variable (LBV) star. The former case is suggested by its association with a decaying non-thermal radio source, while the latter is suggested by its peculiar transient light curve and its low initial expansion velocities. The crucial difference is that the star survives a transient eruption but not an SN. All stars identified as possible survivors are significantly fainter, L_opt ~ 10^5 Lsun, than the L_opt ~ 3 10^6 Lsun progenitor star at optical wavelengths. While this can be explained by dust absorption in a shell of material ejected during the transient, the survivor must then be present as a L_IR ~ 3 10^6 Lsun mid-infrared source. Using archival Spitzer observations of the region, we show that such a luminous mid-IR source is not present. The brightest source of dust emission is only L_IR ~ 10^5 Lsun and does not correspond to the previously identified candidates for the surviving star. The dust cannot be made sufficiently distant and cold to avoid detection unless the ejection energy, mass and velocity scales are those of a SN or greater. We conclude that SN 1961V was a peculiar, but real, supernova. Its peculiarities are probably due to enhanced mass loss just prior to the SN, followed by the interactions of the SN blast wave with this ejecta. This adds to the evidence that there is a population of SN progenitors that have major mass loss episodes shortly before core collapse. The progenitor is a low metallicity, ~1/3 solar, high mass, M_ZAMS > 80 Msun, star, which means either that BH formation can be accompanied by an SN or that surprisingly high mass stars can form a NS.
We report on photometric I-band observations of 147 bright (8<I<13) periodic variables toward the Galactic bulge including 76 new discoveries. We used one of the HATnet telescopes to obtain 151 exposures spanning 88 nights in 2005 of an 8.4 x 8.4 deg^2 field of view (FOV) approximately centered on (l,b) = (1.73, -4.68). We observed the galactic bulge in 2005 as part of a microlensing feasibility study (Nataf et al. 2009), and here we discuss the periodic variables we found in our data. Among our discoveries we count 52 new eclipsing binaries and 24 other periodic variables.
We have started a survey of M 33 in order to find variable stars and Cepheids in particular. We have obtained more than 30 epochs of gri data with the CFHT and the one-square-degree camera MegaCam. We present first results from this survey, including the search for variable objects and a basic characterization of the various groups of variable stars.
We investigate the effect of including a significant ``binary twin population (binaries with almost equal mass stars, q = M2/M1 > 0.95) for the production of double compact objects and some resulting consequences, including LIGO inspiral rate and some properties of short-hard gamma-ray bursts. We employ very optimistic assumptions on the twin fraction (50%) among all binaries, and therefore our calculations place an upper limits on the influence of twins on double compact object populations. We show that for LIGO the effect of including twins is relatively minor: although the merger rates does indeed increase when twins are considered, the rate increase is fairly small (1.5). Also, chirp mass distribution for double compact objects formed with or without twins are almost indistinguishable. If double compact object are short-hard GRB progenitors, including twins in population synthesis calculations does not alter significantly the earlier rate predictions for the event rate. However, for one channel of binary evolution, introducing twins more than doubles the rate of ``very prompt NS-NS mergers (time to merger less than 1 Myr) compared to models with the ``flat q distribution. In that case, 70% of all NS-NS binaries merge within 100 Myr after their formation, indicating a possibility of a very significant population of ``prompt short-hard gamma-ray bursts, associated with star forming galaxies. We also point out that, independent of assumptions, fraction of such prompt neutron star mergers is always high, 35--70%. We note that recent observations (e.g., Berger et al.) indicate that fraction of short-hard GRBs found in young hosts is at least 40% and possibly even 80%.
