FUSE Observations of the Magellanic Bridge Gas toward Two Early-Type Stars: Molecules, Physical Conditions, and Relative Abundance

We discuss FUSE observations of two early-type stars, DI1388 and DGIK975, in the low density and low metallicity gas of Magellanic Bridge (MB). Toward DI1388, the FUSE observations show molecular hydrogen, O VI, and numerous other atomic or ionic transitions in absorption, implying the presence of multiple gas phases in a complex arrangement. The relative abundance pattern in the MB is attributed to varying degrees of depletion onto dust similar to that of halo clouds. The N/O ratio is near solar, much higher than N/O in damped Ly-alpha systems, implying subsequent stellar processing to explain the origin of nitrogen in the MB. The diffuse molecular cloud in this direction has a low column density and low molecular fraction. H2 is observed in both the Magellanic Stream and the MB, yet massive stars form only in the MB, implying significantly different physical processes between them. In the MB some of the H2 could have been pulled out from the SMC via tidal interaction, but some also could have formed in situ in dense clouds where star formation might have taken place. Toward DGIK975, the presence of neutral, weakly and highly ionized species suggest that this sight line has also several complex gas phases. The highly ionized species of O VI, C IV, and Si IV toward both stars have very broad features, indicating that multiple components of hot gas at different velocities are present. Several sources (a combination of turbulent mixing layer, conductive heating, and cooling flows) may be contributing to the production of the highly ionized gas in the MB. Finally, this study has confirmed previous results that the high-velocity cloud HVC 291.5-41.2+80 is mainly ionized composed of weakly and highly ions. The high ion ratios are consistent with a radiatively cooling gas in a fountain flow model.Comment: Accepted for publication in the ApJ (October 10, 2002). Added reference (Gibson et al. 2000

Low Redshift Intergalactic Absorption Lines in the Spectrum of HE0226-4110

We present an analysis of the FUSE and STIS E140M spectra of HE0226-4110 (z=0.495). We detect 56 Lyman absorbers and 5 O VI absorbers. The number of intervening O VI systems per unit redshift with W>50 m\AA is dN(O VI)/dz~ 11. The O VI systems unambiguously trace hot gas only in one case. For the 4 other O VI systems, photoionization and collisional ionization models are viable options to explain the observed column densities of the O VI and the other ions. If the O VI systems are mostly photoionized, only a fraction of the observed O VI will contribute to the baryonic density of the warm-hot ionized medium (WHIM) along this line of sight. Combining our results with previous ones, we show that there is a general increase of N(O VI) with increasing b(O VI). Cooling flow models can reproduce the N-b distribution but fail to reproduce the observed ionic ratios. A comparison of the number of O I, O II, O III, O IV, and O VI systems per unit redshift show that the low-z IGM is more highly ionized than weakly ionized. We confirm that photoionized O VI systems show a decreasing ionization parameter with increasing H I column density. O VI absorbers with collisional ionization/photoionization degeneracy follow this relation, possibly suggesting that they are principally photoionized. We find that the photoionized O VI systems in the low redshift IGM have a median abundance of 0.3 solar. We do not find additional Ne VIII systems other than the one found by Savage et al., although our sensitivity should have allowed the detection of Ne VIII in O VI systems at T~(0.6-1.3)x10^6 K (if CIE applies). Since the bulk of the WHIM is believed to be at temperatures T>10^6 K, the hot part of the WHIM remains to be discovered with FUV--EUV metal-line transitions.Comment: Accepted for publication in the ApJS. Full resolution figures available at http://www.journals.uchicago.edu/ApJ/journal/preprints/ApJS63975.preprint.pd

Microlensing of Kepler Stars as a Method of Detecting Primordial Black Hole Dark Matter

If the Dark Matter consists of primordial black holes (PBHs), we show that gravitational lensing of stars being monitored by NASA's Kepler search for extra-solar planets can cause significant numbers of detectable microlensing events. A search through the roughly 150,000 lightcurves would result in large numbers of detectable events for PBHs in the mass range 5 \ten{-10}\msun to \aten{-4}\msun. Non-detection of these events would close almost two orders of magnitude of the mass window for PBH dark matter. The microlensing rate is higher than previously noticed due to a combination of the exceptional photometric precision of the Kepler mission and the increase in cross section due to the large angular sizes of the relatively nearby Kepler field stars. We also present a new formalism for calculating optical depth and microlensing rates in the presence of large finite-source effects.Comment: 5 pages, 1 figur

Deuterium toward the WD0621-376 sight line: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission

Far Ultraviolet Spectroscopic Explorer observations are presented for WD0621-376, a DA white dwarf star in the local interstellar medium (LISM) at a distance of about 78 pc. The data have a signal-to-noise ratio of about 20-40 per 20 km/s resolution element and cover the wavelength range 905-1187 \AA. LISM absorption is detected in the lines of D I, C II, C II*, C III, N I, N II, N III, O I, Ar I, and Fe II. This sight line is partially ionized, with an ionized nitrogen fraction of > 0.23. We determine the ratio $D/O = (3.9 \pm ^{1.3}_{1.0})\times 10^{-2}$ (2$\sigma$). Assuming a standard interstellar oxygen abundance, we derive ${\rm D/H} \approx 1. 3 \times 10^{-5}$. Using the value of N(H I) derived from EUVE data gives a similar D/H ratio. The D I/N I ratio is $(3.3 \pm ^{1.0}_{0.8})\times 10^{-1}$ (2$\sigma$).Comment: accepted for publication in the ApJ

Head-on collisions of boson stars

We study head-on collisions of boson stars in three dimensions. We consider evolutions of two boson stars which may differ in their phase or have opposite frequencies but are otherwise identical. Our studies show that these phase differences result in different late time behavior and gravitational wave output

Hydroxymethylated Cytosines Are Associated with Elevated C to G Transversion Rates

It has long been known that methylated cytosines deaminate at higher rates than unmodified cytosines and constitute mutational hotspots in mammalian genomes. The repertoire of naturally occurring cytosine modifications, however, extends beyond 5-methylcytosine to include its oxidation derivatives, notably 5-hydroxymethylcytosine. The effects of these modifications on sequence evolution are unknown. Here, we combine base-resolution maps of methyl- and hydroxymethylcytosine in human and mouse with population genomic, divergence and somatic mutation data to show that hydroxymethylated and methylated cytosines show distinct patterns of variation and evolution. Surprisingly, hydroxymethylated sites are consistently associated with elevated C to G transversion rates at the level of segregating polymorphisms, fixed substitutions, and somatic mutations in tumors. Controlling for multiple potential confounders, we find derived C to G SNPs to be 1.43-fold (1.22-fold) more common at hydroxymethylated sites compared to methylated sites in human (mouse). Increased C to G rates are evident across diverse functional and sequence contexts and, in cancer genomes, correlate with the expression of Tet enzymes and specific components of the mismatch repair pathway (MSH2, MSH6, and MBD4). Based on these and other observations we suggest that hydroxymethylation is associated with a distinct mutational burden and that the mismatch repair pathway is implicated in causing elevated transversion rates at hydroxymethylated cytosines

Numerical Simulations of Hyperfine Transitions of Antihydrogen

One of the ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration's goals is the measurement of the ground state hyperfine transition frequency in antihydrogen, the antimatter counterpart of one of the best known systems in physics. This high precision experiment yields a sensitive test of the fundamental symmetry of CPT. Numerical simulations of hyperfine transitions of antihydrogen atoms have been performed providing information on the required antihydrogen events and the achievable precision