Small-Scale Structure of O VI Interstellar Gas in the Direction of the Globular Cluster NGC 6752

In order to study the small-scale structure of the hot interstellar gas, we obtained Far Ultraviolet Spectroscopic Explorer interstellar O VI interstellar absorption spectra of 4 four post-extreme horizontal branch stars in the globular cluster NGC 6752 [(l,b) = (336.50,-25.63), d = 3.9 kpc, z= -1.7 kpc]. The good quality spectra of these stars allow us to measure both lines of the O VI doublet at 1031.926 \AA and 1037.617 \AA. The close proximity of these stars permits us to probe the hot interstellar gas over angular scale of only 2\farcm2 - 8\farcm9, corresponding to spatial scales \la 2.5-10.1 pc. On these scales we detect no variations in the O VI column density and velocity distribution. The average column density is log = 14.34 \pm 0.02 (log = 13.98). The measured velocity dispersions of the O VI absorption are also indistinguishable. Including the earlier results of Howk et al., this study suggests that interstellar O VI is smooth on scales \Delta \theta \la 12\arcmin, corresponding to a spatial scale of less than 10 pc, and quite patchy at larger scales. Although such small scales are only probed in a few directions, this suggests a characteristic size scale for the regions producing collisionally-ionized O VI in the Galaxy.Comment: Accepted for publication in the PASP (to appear in the October 2004 issue

A Reservoir of Ionized Gas in the Galactic Halo to Sustain Star Formation in the Milky Way

Without a source of new gas, our Galaxy would exhaust its supply of gas through the formation of stars. Ionized gas clouds observed at high velocity may be a reservoir of such gas, but their distances are key for placing them in the Galactic halo and unraveling their role. We have used the Hubble Space Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the foreground of Galactic stars. We show that iHVCs with 90 < |v_LSR| < 170 km/s are within one Galactic radius of the sun and have enough mass to maintain star formation, while iHVCs with |v_LSR|>170 km/s are at larger distances. These may be the next wave of infalling material.Comment: This paper is part of a set of three papers on circumgalactic gas observed with COS and STIS on HST, to be published in Science, together with related papers by Tripp et al. and Tumlinson et al., in the November 18, 2011 edition. This version has not undergone final copyediting. Please see Science online for the final printed versio

Experimental Limits on Primordial Black Hole Dark Matter from the First Two Years of Kepler Data

We present the analysis on our new limits of the dark matter (DM) halo consisting of primordial black holes (PBHs) or massive compact halo objects (MACHOs). We present a search of the first two years of publicly available Kepler mission data for potential signatures of gravitational microlensing caused by these objects, as well as an extensive analysis of the astrophysical sources of background error. These include variable stars, flare events, and comets or asteroids which are moving through the Kepler field. We discuss the potential of detecting comets using the Kepler lightcurves, presenting measurements of two known comets and one unidentified object, most likely an asteroid or comet. After removing the background events with statistical cuts, we find no microlensing candidates. We therefore present our Monte Carlo efficiency calculation in order to constrain the PBH DM with masses in the range of 2 x 10^-9 solar masses to 10^-7 solar masses. We find that PBHs in this mass range cannot make up the entirety of the DM, thus closing a full order of magnitude in the allowed mass range for PBH DM.Comment: 12 pages, 6 figure

Detection of Interstellar C_2 and C_3 in the Small Magellanic Cloud

We report the detection of absorption from interstellar C_2 and C_3 toward the moderately reddened star Sk 143, located in the near 'wing' region of the SMC, in optical spectra obtained with the ESO VLT/UVES. These detections of C_2 (rotational levels J=0-8) and C_3 (J=0-12) absorption in the SMC are the first beyond our Galaxy. The total abundances of C_2 and C_3 (relative to H_2) are similar to those found in diffuse Galactic molecular clouds -- as previously found for CH and CN -- despite the significantly lower average metallicity of the SMC. Analysis of the rotational excitation of C_2 yields an estimated kinetic temperature T_k ~ 25 K and a moderately high total hydrogen density n_H ~ 870 cm^-3 -- compared to the T_01 ~ 45 K and n_H ~ 85-300 cm^-3 obtained from H_2. The populations of the lower rotational levels of C_3 are consistent with an excitation temperature of about 34 K.Comment: accepted to MNRAS; 10 pages, 6 figure