Gamma-Ray Bursts in the SALT/Swift Era: GRB/SN Connection

I discuss the Gamma Ray Burst research in the era of SALT and Swift, concentrating on the GRB/SN connection.Comment: Invited talk at the First International Workshop on ``Stellar Astrophysics with the World Largest Telescopes'', Torun, Poland, 7-10 September 2004. 12 pages, 6 figures. See http://cfa-www.harvard.edu/cfa/oir/Research/GRB/ for related material

GALEX Catalog of UV Point Sources in M33

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 GALEX's 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.Comment: Version accepted by MNRAS, updated with suggestions from the referee. For a brief video abstract of the paper, please visit http://www.youtube.com/watch?v=TcJ95LNhGt

Variations of the Selective Extinction Across the Galactic Bulge - Implications for the Galactic Bar

We propose a new method to investigate the coefficient of the selective extinction, based on two band photometry. This method uses red clump stars as a means to construct the reddening curve. We apply this method to the OGLE color-magnitude diagrams to investigate the variations of the selective extinction towards various parts of the Galactic bulge. We find that $A_{_V}/E_{_{V-I}}$ coefficient is within the errors the same for $l=\pm 5\deg$ OGLE fields. Therefore, the difference of $\sim 0.37\;mag$ in the extinction adjusted apparent magnitude of the red clump stars in these fields (Stanek et al.~1994, 1995) cannot be assigned to a large-scale gradient of the selective extinction coefficient. This strengthens the implication of this difference as indicator of the presence of the bar in our Galaxy. However using present data we cannot entirely exclude the possibility of $\sim 0.2\;mag/mag$ variations of the selective extinction coefficient on the large scales across the bulge.Comment: submitted to ApJ Letters, 10 pages, gziped PostScript with figures included; also available through WWW at http://www.astro.princeton.edu/~library/prep.htm

Magnitude Offset between Lensed Stars and Observed Stars - a New Probe of the Structure of the Galactic Bar

We propose a new method that can be used to constrain the properties of the Galactic bar (bulge). If the majority of the lensing objects are in the Galactic bar, then we predict a systematic offset in the apparent magnitude between lensed stars and all observed stars. Using OGLE color-magnitude diagram data we model this effect in the region of the diagram dominated by bulge red clump stars and find that for some models of the Galactic bar the expected offset in the apparent magnitude may be as large as $0.2 mag$. About 100 lensed stars in the red clump region of the color-magnitude diagram is needed to unambiguously detect this effect, a number within the reach of current microlensing projects. We find a good correlation between the extent of the bar along the line of sight and the expected magnitude offset. We also obtain a constraint for the extent of the bar along the line of sight using the observed luminosity function for the red clump stars.Comment: 10 pages, 3 figures, revised version accepted for the publication in ApJL, uses aaspp.sty macro, PostScript figures and PostScript version of the paper available through anonymous ftp from astro.princeton.edu, directory stanek/bar_lens, or at http://astro.princeton.edu/~library/prep.htm

Characterizing Supernova Progenitors via the Metallicities of their Host Galaxies, from Poor Dwarfs to Rich Spirals

We investigate how the different types of supernovae are relatively affected by the metallicity of their host galaxy. We match the SAI Supernova Catalog to the SDSS-DR4 catalog of star-forming galaxies with measured metallicities. These supernova host galaxies span a range of oxygen abundance from 12 + log(O/H) = 7.9 to 9.3 (~ 0.1 to 2.7 solar) and a range in absolute magnitude from MB = -15.2 to -22.2. To reduce the various observational biases, we select a subsample of well-characterized supernovae in the redshift range from 0.01 to 0.04, which leaves us with 58 SN II, 19 Ib/c, and 38 Ia. We find strong evidence that SN Ib/c are occurring in higher-metallicity host galaxies than SN II, while we see no effect for SN Ia relative to SN II. We note some extreme and interesting supernova-host pairs, including the metal-poor (~ 1/4 solar) host of the recent SN Ia 2007bk, where the supernova was found well outside of this dwarf galaxy. To extend the luminosity range of supernova hosts to even fainter galaxies, we also match all the historical supernovae with z < 0.3 to the SDSS-DR6 sky images, resulting in 1225 matches. This allows us to identify some even more extreme cases, such as the recent SN Ic 2007bg, where the likely host of this hypernova-like event has an absolute magnitude MB ~ -12, making it one of the least-luminous supernova hosts ever observed. This low-luminosity host is certain to be very metal poor (~ 1/20 solar), and therefore this supernova is an excellent candidate for association with an off-axis GRB. The two catalogs that we have constructed are available online and will be updated regularly. Finally, we discuss various implications of our findings for understanding supernova progenitors and their host galaxies.Comment: ApJ accepted, 26 pages, 5 figures, 1 table. Updated catalogs are available at http://www.astronomy.ohio-state.edu/~prieto/snhosts