arXiv subjects

Kenneth Sembach

Publications and source records attributed to Kenneth Sembach.

7 recordsLinked to original sources

arXiv2015

Elemental depletions in the Magellanic Clouds and the evolution of depletions with metallicity

We present a study of the composition of gas and dust in the Large and Small Magellanic Clouds (LMC and SMC, together -- the MCs) as measured by UV absorption spectroscopy. We have measured P II and Fe II along 85 sightlines toward the MCs using archival FUSE observations. For 16 of those sightlines, we have measured Si II, Cr II, and Zn II from new HST COS observations. We have combined these measurements with H I and H$_2$ column densities and reference stellar abundances from the literature to derive gas-phase abundances, depletions, and gas-to-dust ratios (GDRs). 80 of our 84 P measurements and 13 of our 16 Zn measurements are depleted by more than 0.1 decades, suggesting that P and Zn abundances are not accurate metallicity indicators at and above the metallicity of the SMC. The maximum P and Zn depletions are the same in the MW, LMC, and SMC. Si, Cr, and Fe are systematically less depleted in the SMC than in the MW or LMC. The minimum Si depletion in the SMC is consistent with zero. Our depletion-derived GDRs broadly agree with GDRs from the literature. The GDR varies from location to location within a galaxy by a factor of up to 2 in the LMC and up to 5 in the SMC. This variation is evidence of dust destruction and/or growth in the diffuse neutral phase of the interstellar medium.

astro-ph.GA↗

arXiv2011

The Cosmic Origins Spectrograph

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in May 2009, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F_lambda ~ 1.0E10-14 ergs/s/cm2/Angstrom, COS can achieve comparable signal to noise (when compared to STIS echelle modes) in 1-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (September 2009 - June 2011) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is 9 times that sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of June 2011. COS has measured, for the first time with high reliability, broad Lya absorbers and Ne VIII in the intergalactic medium, and observed the HeII reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.

astro-ph.IM↗

arXiv2010

Science drivers and requirements for an Advanced Technology Large Aperture Space Telescope (ATLAST): Implications for technology development and synergies with other future facilities

The Advanced Technology Large-Aperture Space Telescope (ATLAST) is a concept for an 8-meter to 16-meter UVOIR space observatory for launch in the 2025-2030 era. ATLAST will allow astronomers to answer fundamental questions at the forefront of modern astronphysics, including "Is there life elsewhere in the Galaxy?" We present a range of science drivers that define the main performance requirements for ATLAST (8 to 16 milliarcsec angular resolution, diffraction limited imaging at 0.5 μm wavelength, minimum collecting area of 45 square meters, high sensitivity to light wavelengths from 0.1 μm to 2.4 μm, high stability in wavefront sensing and control). We will also discuss the synergy between ATLAST and other anticipated future facilities (e.g., TMT, EELT, ALMA) and the priorities for technology development that will enable the construction for a cost that is comparable to current generation observatory-class space missions.

astro-ph.IM↗

arXiv2008

Correlations between O VI Absorbers and Galaxies at Low Redshift

We investigate the relationship between galaxies and metal-line absorption systems in a large-scale cosmological simulation with galaxy formation. Our detailed treatment of metal enrichment and non-equilibrium calculation of oxygen species allow us, for the first time, to carry out quantitative calculations of the cross-correlations between galaxies and O VI absorbers. We find the following: (1) The cross-correlation strength depends weakly on the absorption strength but strongly on the luminosity of the galaxy. (2) The correlation distance increases monotonically with luminosity from ~0.5-1h^-1 Mpc for 0.1L* galaxies to ~3-5h^-1 Mpc for L* galaxies. (3) The correlation distance has a complicated dependence on absorber strength, with a luminosity-dependent peak. (4) Only 15% of O VI absorbers lie near >=Lz* galaxies. The remaining 85%, then, must arise ``near'' lower-luminosity galaxies, though, the positions of those galaxies is not well-correlated with the absorbers. This may point to pollution of intergalactic gas predominantly by smaller galaxies. (5) There is a subtle trend that for >~0.5Lz* galaxies, there is a positive correlation between absorber strength and galaxy luminosity in the sense that stronger absorbers have a slightly higher probability of finding such a large galaxy at a given projection distance. For less luminous galaxies, there seems to be a negative correlation between luminosity and absorber strength.

astro-ph↗

arXiv2007

Molecular and Atomic Excitation Stratification in the Outflow of the Planetary Nebula M27

High resolution spectroscopy with FUSE and STIS of atomic and molecular velocity stratification in the nebular outflow of M27 challenge models for the abundance kinematics in planetary nebulae. The simple picture of a very high speed (~ 1000 km/s), high ionization, radiation driven stellar wind surrounded by a slower (~ 10 km/s) mostly molecular outflow, with low ionization and neutral atomic species residing at the wind interaction interface, is not supported... We find ...there is a fast (33 -- 65 km/s) low ionization zone, surrounding a slower (<~ 33 km/s) high ionization zone and, at the transition velocity (33 km/s), vibrationally excited H_2 is intermixed with a predominately neutral atomic medium... Far-UV continuum fluorescence of H_2 is not detected, but Lyman alpha (Lya) fluorescence is present. The diffuse nebular medium is inhospitable to molecules and dust. Maintaining the modest equilibrium abundance of H_2 (N(H_2)/N(HI) << 1) in the diffuse nebular medium requires a source of H_2, mostly likely the clumpy nebular medium. The stellar SED shows no signs of reddening (E(B-V) < 0.01), but paradoxically measurements of Ha/Hb ... indicate E(B-V) ~ 0.1. ...the apparent enhancement of Ha/Hb in the absence of dust may result from a two step process of H_2 ionization by Lyman continuum (Lyc) photons followed by dissociative recombination (H_2 + gamma -> H_2^+ + e -> H(1s) + H (nl)), which ultimately produces fluorescence of Ha and Lya. In the optically thin limit at the inferred radius of the velocity transition we find dissociation of H_2 by stellar Lyc photons is an order of magnitude more efficient than spontaneous dissociation by far-UV photons. We suggest that the importance of this H_2 destruction process in HII regions has been overlooked.

astro-ph↗

arXiv2007

The Abundance of Deuterium in the Warm Neutral Medium of the Lower Galactic Halo

We use high resolution spectra obtained with FUSE and STIS to obtain interstellar column densities of H I, D I, O I, S II, Fe II and P II toward the QSO HE 0226-4110 ( l = 253.4 deg , b = -65.77 deg). We obtain D/H = 21(+8, -6) ppm. Correcting for a small amount of contamination from D and H in the Local Bubble, we obtain D/H = 22 (+8, -6) for the warm neutral medium of the lower Galactic halo. The medium has [O/H] = 0.12(+0.41, -0.20) and [Fe/H] = -1.01 (+0.10, -0.09). This suggests the abundances in the halo toward HE 0226-4110 are not affected by the infall of low metallicity gas and that the gas originates in the disk and is elevated into the halo by energetic processes which erode but do not totally destroy the dust grains. We compare our result to measured values of D/H in other astrophysical sites. The value we measure in the halo gas is consistent with the hypothesis that for many Galactic disk lines of sight D is incorporated into dust. The high average value of D/H = 23.1+/-2.4 ppm measured along 5 sight lines through disk gas in the Solar neighborhood is similar to D/H in the lower Galactic halo. These observations imply the abundance of deuterium in the Galaxy has only been reduced by a factor of 1.12+/-0.13 since its formation.

astro-ph↗

arXiv2004

An Unbiased Far Ultraviolet Survey of Magellanic Cloud Supernova Remnants

We have undertaken a FUSE survey of Magellanic Cloud supernova remnants, looking primarily for O VI and C III emission lines. Work in earlier cycles with FUSE indicates that optical and/or X-ray characteristics of supernova remnants are not always good predictors of the objects that will be bright and detectable in the UV. The goal of our survey is to test this concept by obtaining spectra of a random sample of Magellanic Cloud remnants with a broad range of radio, optical, and X-ray properties. Previously observed objects and remnants with known high extinction (or known high column densities) are the only objects eliminated from consideration. To date, we have clearly detected O VI emission from 12 of the 33 objects observed, with weak or marginal detections possible in a handful more.

astro-ph↗