arXiv subjects

William P. Blair

Publications and source records attributed to William P. Blair.

At least 91 records · Page 5Linked to original sources

arXiv2006

What is the Total Deuterium Abundance in the Local Galactic Disk?

Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, together with spectra from the Copernicus and IMAPS instruments, reveal an unexplained very wide range in the observed deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk beyond the Local Bubble. We argue that spatial variations in the depletion of deuterium onto dust grains can explain these local variations in the observed gas-phase D/H ratios. We present a variable deuterium depletion model that naturally explains the constant measured values of D/H inside the Local Bubble, the wide range of gas-phase D/H ratios observed in the intermediate regime (log N(H I} = 19.2-20.7), and the low gas-phase D/H ratios observed at larger hydrogen column densities. We consider empirical tests of the deuterium depletion hypothesis: (i) correlations of gas-phase D/H ratios with depletions of the refractory metals iron and silicon, and (ii) correlation with the molecular hydrogen rotational temperature. Both of these tests are consistent with deuterium depletion from the gas phase in cold, not recently shocked, regions of the ISM, and high gas-phase D/H ratios in gas that has been shocked or otherwise heated recently. We argue that the most representative value for the total (gas plus dust) D/H ratio within 1 kpc of the Sun is >=23.1 +/- 2.4 (1 sigma) parts per million (ppm). This ratio constrains Galactic chemical evolution models to have a very small deuterium astration factor, the ratio of primordial to total (D/H) ratio in the local region of the Galactic disk, which we estimate to be f_d <= 1.19 +/-0.16 (1 sigma) or <= 1.12 +/- 0.14 (1 sigma) depending on the adopted light element nuclear reaction rates.

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arXiv2006

Far Ultraviolet Spectroscopic Explorer Survey of Magellanic Cloud Supernova Remnants

We report the progress to date from an ongoing unbiased ultraviolet survey of supernova remnants in the Magellanic Clouds using the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. This survey is obtaining spectra of a random large sample of Magellanic Cloud supernova remnants with a broad range of radio, optical, and X-ray properties. To date, 39 objects have been observed in the survey (38 in the LMC and one in the SMC) and 15 have been detected, a detection rate of nearly 40%. Our survey has nearly tripled the number of UV-detected SNRs in the Magellanic Clouds (from 8 to 22). Because of the diffuse source sensitivity of FUSE, upper limits on non-detected objects are quite sensitive in many cases. Estimated total luminosities in O~VI span a broad range from considerably brighter to many times fainter than the inferred soft X-ray luminosities, indicating that O~VI can be an important and largely unrecognized coolant in certain objects. We compare the optical and X-ray properties of the detected and non-detected objects but do not find a simple indicator for ultraviolet detectability. Non-detections may be due to clumpiness of the emission, high foreground extinction, slow shocks whose emission gets attenuated by the Magellanic interstellar medium, or a combination of these effects.

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arXiv2006

Possible Detection of OVI from the LMC Superbubble N70

We present FUSE observations toward four stars in the LMC superbubble N70 and compare these spectra to those of four comparison targets located in nearby field and diffuse regions. The N70 sight lines show OVI 1032 absorption that is consistently stronger than the comparison sight lines by ~60%. We attribute the excess column density (logN_OVI=14.03 cm^-2) to hot gas within N70, potentially the first detection of OVI associated with a superbubble. In a survey of 12 LMC sight lines, Howk et al. (2002a) concluded that there was no correlation between ISM morphology and N_OVI. We present a reanalysis of their measurements combined with our own and find a clear difference between the superbubble and field samples. The five superbubbles probed to date with FUSE show a consistently higher mean N_OVI than the 12 non-superbubble sight lines, though both samples show equivalent scatter from halo variability. Possible ionization mechanisms for N70 are discussed, and we conclude that the observed OVI could be the product of thermal conduction at the interface between the hot, X-ray emitting gas inside the superbubble and the cooler, photoionized material making up the shell seen prominently in Halpha. We calculate the total hydrogen density n_H implied by our OVI measurements and find a value consistent with expectations. Finally, we discuss emission-line observations of OVI from N70.

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arXiv2006

A high resolution UV absorption spectrum of supernova ejecta in SN1006

We report a high resolution, far-ultraviolet, STIS E140M spectrum of the strong, broad Si II, III, and IV features produced by the ejecta of SN1006 seen in absorption against the background Schweizer-Middleditch star. The spectrum confirms the extreme sharpness of the red edge of the redshifted Si II 1260 A feature, supporting the idea that this edge represents the location of the reverse shock moving into the freely expanding ejecta. The expansion velocity of ejecta at the reverse shock is measured to be 7026 +-3(relative) +-10(absolute) km/s. If the shock model is correct, then the expansion velocity should be decreasing at the observable rate of 2.7 +-0.1 km/s per year. The pre-shock velocity, post-shock velocity, and post-shock velocity dispersion are all measured from the Si II 1260 A feature, and consistency of these velocities with the shock jump conditions implies that there is little or no electron heating in this fast (2680 km/s) Si-rich shock.

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arXiv2006

Dust Destruction in Type Ia Supernova Remnants in the Large Magellanic Cloud

We present first results from an extensive survey of Magellanic Clouds supernova remnants (SNRs) with the Spitzer Space Telescope. We describe IRAC and MIPS imaging observations at 3.6, 4.5, 5.8, 8, 24, and 70 microns of four Balmer-dominated Type Ia SNRs in the Large Magellanic Cloud (LMC): DEM L71 (0505-67.9), 0509--67.5, 0519--69.0, and 0548-70.4. None was detected in the four short-wavelength IRAC bands, but all four were clearly imaged at 24 microns, and two at 70 microns. A comparison of these images to Chandra broadband X-ray images shows a clear association with the blast wave, and not with internal X-ray emission associated with ejecta. Our observations are well described by 1-D shock models of collisionally heated dust emission, including grain size distributions appropriate for the LMC, grain heating by collisions with both ions and electrons, and sputtering of small grains. Model parameters are constrained by X-ray, optical, and far-ultraviolet observations. Our models can reproduce observed 70/24 micron flux ratios only by including sputtering, destroying most grains smaller than 0.03-0.04 microns in radius. We infer total dust masses swept up by the SNR blast waves, before sputtering, of order 0.01 solar masses, several times less than those implied by a dust/gas mass ratio of 0.3 percent as often assumed for the LMC. Substantial dust destruction has implications for gas-phase abundances.

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arXiv2005

STIS Spectroscopy of Young Star Clusters in "The Antennae" Galaxies (NGC 4038/4039)

Long-slit spectra of several dozen young star clusters have been obtained at three positions in the Antennae galaxies with the Space Telescope Imaging Spectrograph (STIS) and its 52"x0.2" slit. Based on H-alpha emission-line measurements, the average cluster-to-cluster velocity dispersion in 7 different cluster aggregates ("knots") is <10 \kms. The fact that this upper limit is similar to the velocity dispersion of gas in the disks of typical spiral galaxies suggests that the triggering mechanism for the formation of young massive compact clusters ("super star clusters") is unlikely to be high velocity cloud-cloud collisions. On the other hand, models where preexisting giant molecular clouds in the disks of spiral galaxies are triggered into cluster formation are compatible with the observed low velocity dispersions. These conclusions are consistent with those reached by Zhang et al. (2001) based on comparisons between the positions of the clusters and the velocity and density structure of the nearby interstellar medium. We find evidence for systematically lower values of the line ratios [NII]/H-alpha and [SII]/H-alpha in the bright central regions of some of the knots, relative to their outer regions. This suggests that the harder ionizing photons are used up in the regions nearest the clusters, and the diffuse ionized gas farther out is photoionized by 'leakage' of the leftover low-energy photons. The low values of the [SII]/H-alpha line ratio, typically [SII]/H-alpha<0.4, indicates that the emission regions are photoionized rather than shock heated. The absence of evidence for shock-heated gas is an additional indication that high velocity cloud-cloud collisions are not playing a major role in the formation of the young clusters.

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arXiv2005

FUV Spectroscopy of the Dwarf Novae SS Cygni and WX Hydri in Quiescence

We present time-resolved FUV spectra of the dwarf novae SS Cyg and WX Hyi in quiescence from observations using the Hopkins Ultraviolet Telescope on the Astro-1 and Astro-2 Space Shuttle missions and the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Both dwarf novae are characterized by blue continua that extend to the Lyman limit punctuated by broad emission lines including transitions of O VI, N V, Si IV, and C IV. The continuum of WX Hyi can be fit with a white dwarf model with physically reasonable model parameters, but neither system actually shows unambiguous signatures of white dwarf emission. The shape and flux of the spectrum of SS Cyg cannot be self-consistently reconciled with a white dwarf providing all of the FUV continuum flux. Combination white dwarf/disk or white dwarf/optically thin plasma models improve the fit but still do not give physically reasonable model parameters for a quiescent dwarf nova. Assuming that the UV emission lines arise from the disk, the line shapes indicate that surface fluxes fall roughly as R^{-2} in both systems. Fits to the double-peaked line profiles in SS Cyg indicate that the FUV line forming region is concentrated closer to the white dwarf than that of the optical lines and provide no evidence of a hole in the inner disk. Although the flux from SS Cyg was relatively constant during all of our observations, WX Hyi showed significant variability during the GHRS observations. In WX Hyi, the line and continuum fluxes are (with the exception of He II) highly correlated, indicating a link between the formation mechanisms of the line and continuum regions.

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arXiv2005

Revealing the Supernova Remnant Population of M33 with Chandra

We present results of a search for supernova remnants (SNRs) in archival Chandra images of M33. We have identified X-ray SNRs by comparing the list of Chandra, X-ray sources in M33 with tabulations of SNR candidates identified from (1) elevated [S II]/Halpha ratios in the optical, and (2) radio spectral indices. Of the 98 optically known SNRs in M33, 22 have been detected at > 3-sigma level in the soft band (0.35-1.1 keV). At least four of these SNR candidates are spatially extended based on a comparison of the data to simulated images of point sources. Aside from the optically matching SNRs, we have found one soft X-ray source in M33 which exhibits no optical emission and is coincident with a known radio source. The radio spectral index of this source is consistent with particle acceleration in shocks, leading us to suggest that it is a non-radiative SNR. We have also found new optical counterparts to two soft X-ray SNRs in M33. Pending confirmation from optical spectroscopy, the identification of these two optical counterparts increases the total number of known optically emitting SNRs in M33 to 100. This brings the total number of identified SNRs with X-ray counterparts, including those exclusively detected by the XMM-Newton survey of M33, to 37 SNRs. We find that while there are a similar number of confirmed X-ray SNRs in M33 and the LMC with X-ray luminosities in excess of 1e35 ergs/s, nearly 40% of the LMC SNRs are brighter than 1e36 ergs/s, while only 13% of the M33 sample exceed this luminosity. The differences in luminosity distributions cannot be fully explained by uncertainty in spectral model parameters, and is not fully accounted for by abundance differences between the galaxies (abridged).

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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.

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arXiv2004

FORTIS: Pathfinder to the Lyman Continuum

Shull et al. (1999) have asserted that the contribution of stars, relative to quasars, to the metagalactic background radiation that ionizes most of the baryons in the universe remains almost completely unknown at all epochs. The potential to directly quantify this contribution at low redshift has recently become possible with the identification by GALEX of large numbers of sparsely distributed faint ultraviolet galaxies. Neither STIS nor FUSE nor GALEX have the ability to efficiently survey these sparse fields and directly measure the Lyman continuum radiation that may leak into the low redshift (z < 0.4) intergalactic medium. We present here a design for a new type of far ultraviolet spectrograph, one that is more sensitive, covers wider fields, and can provide spectra and images of a large number of objects simultaneously, called the Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy (FORTIS). We intend to use a sounding rocket flight to validate the new instrument with a simple long-slit observation of the starburst populations in the galaxy M83. If however, the long-slit were replaced with microshutter array, this design could isolate the chains of blue galaxies found by GALEX over an ~30' diameter field-of-view and directly address the Lyman continuum problem in a long duration orbital mission. Thus, our development of the sounding rocket instrument is a pathfinder to a new wide field spectroscopic technology for enabling the potential discovery of the long hypothesized but elusive Lyman continuum radiation that is thought to leak from low redshift galaxies and contribute to the ionization of the universe.

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arXiv2004

Two New Low Galactic D/H Measurements from FUSE

We analyze interstellar absorption observed towards two subdwarf O stars, JL 9 and LSS 1274, using spectra taken by the Far Ultraviolet Spectroscopic Explorer (FUSE). Column densities are measured for many atomic and molecular species (H I, D I, C I, N I, O I, P II, Ar I, Fe II, and H2), but our main focus is on measuring the D/H ratios for these extended lines of sight, as D/H is an important diagnostic for both cosmology and Galactic chemical evolution. We find D/H=(1.00+/-0.37)e-5 towards JL 9, and D/H=(0.76+/-0.36)e-5 towards LSS 1274 (2 sigma uncertainties). With distances of 590+/-160 pc and 580+/-100 pc, respectively, these two lines of sight are currently among the longest Galactic lines of sight with measured D/H. With the addition of these measurements, we see a significant tendency for longer Galactic lines of sight to yield low D/H values, consistent with previous inferences about the deuterium abundance from D/O and D/N measurements. Short lines of sight with H I column densities of log N(H I)<19.2 suggest that the gas-phase D/H value within the Local Bubble is (D/H)_LBg=(1.56+/-0.04)e-5. However, the four longest Galactic lines of sight with measured D/H, which have d>500 pc and log N(H I)>20.5, suggest a significantly lower value for the true local-disk gas-phase D/H value, (D/H)_LDg=(0.85+/-0.09)e-5. One interpretation of these results is that D is preferentially depleted onto dust grains relative to H and that longer lines of sight that extend beyond the Local Bubble sample more depleted material. In this scenario, the higher Local Bubble D/H ratio is actually a better estimate than (D/H)_LDg for the true local-disk D/H, D/H)_LD. However, if (D/H)_LDg is different from (D/H)_LBg simply because of variable astration and incomplete ISM mixing, then (D/H)_LD=(D/H)_LDg.

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arXiv2003

Optical and Far-UV Spectroscopy of Knot D in the Vela Supernova Remnant

We present spectra of optical filaments associated with the X-ray knot D in the Vela supernova remnant. It has been suggested that Knot D is formed by a bullet of supernova ejecta, that it is a break-out of the shock front of the Vela SNR, and also that it is an outflow from the recently discovered remnant RXJ0852.0-4622. We find that Knot D is a bow shock propagating into an interstellar cloud with normal abundances and typical cloud densities (n_H ~ 4-11 cm^-3). Optical longslit spectra show that the [S II] 6716,6731 to Halpha line ratio is greater than unity, proving that the optical filaments are shock excited. The analysis of far-ultraviolet spectra obtained with the Hopkins Ultraviolet Telescope and with the Far Ultraviolet Spectroscopic Explorer (FUSE) LWRS aperture show that slower shocks (~100 km s^-1) produce most of the low ionization lines such as O III] 1662, while faster shocks (~180 km s^-1) produce the O VI 1032,1038 and other high ionization lines. C III and O VI lines are also detected in the FUSE MDRS aperture, which was located on an X-ray bright region away from the optical filaments. The lines have two velocity components consistent with ~150 km s^-1 shocks on the near and far sides of the knot. The driving pressure in the X-ray knot, P/k ~ 1.8E+7 cm^-3 K, is derived from the shock properties. This is over an order of magnitude larger than the characteristic X-ray pressure in the Vela SNR. The velocity distribution of the emission and the overpressure support the idea that Knot D is a bow shock around a bullet or cloud that originated near the center of the Vela remnant.

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arXiv2002

A FUSE Survey of Interstellar Molecular Hydrogen in Translucent Clouds

We report the first ensemble results from the FUSE survey of molecular hydrogen in lines of sight with A_V $\gtrsim$ 1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H2, and thus determining column densities for J = 0 and J = 1, which contain $\gtrsim$99% of the total H2. From these column densities and ancillary data we have derived the total H2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of 10^21 cm^-2, measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN abundance, the steepness of the far-UV extinction rise, and the width of the 2175A bump. Despite the relatively large molecular fractions, we do not see the values greater than 0.8 expected in translucent clouds. With the exception of a few lines of sight, we see little evidence for the presence of individual translucent clouds in our sample. We conclude that most of the lines of sight are actually composed of two or more diffuse clouds similar to those found toward targets like zeta Oph. We suggest a modification in terminology to distinguish between a "translucent line of sight" and a "translucent cloud."

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arXiv2001

An Atlas of FUSE Sight Lines Toward the Magellanic Clouds

We present an atlas of 57 Large Magellanic Cloud (LMC) and 37 Small Magellanic Cloud (SMC) observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. The atlas highlights twelve interstellar absorption line transitions at a resolution of ~15 km/s. These transitions cover a broad range of temperatures, ionization states, and abundances. The species included are OVI, which probes hot (T~3x10^5 K) ionized gas; CIII and FeIII, which probe warm (T~10^4 K) ionized gas; SiII, PII, CII, FeII, and OI, warm neutral gas; and six different molecular hydrogen transitions, which trace cold (T<=500 K) gas. We include Schmidt Halpha CCD images of the region surrounding each sight line showing the morphology of warm ionized gas in the vicinity, along with continuum images near each FUSE aperture position. Finally, we present several initial scientific results derived from this dataset on the interstellar medium of the Magellanic Clouds and Galactic halo.

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arXiv2001

FUSE Observations of the Cygnus Loop: OVI Emission from a Nonradiative Shock

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of a Balmer filament in the northeast region of the Cygnus Loop supernova remnant. The data consist of one spectrum obtained through the 30"x30" (LWRS) aperture and three spectra at adjacent positions obtained through the 4"x20" (MDRS) aperture. The nonradiative shocks in the region giving rise to these faint optical filaments produce strong OVI 1032,1038 emission, which is detected in all the spectra. The OVI emission is resolved by FUSE into a strong component centered at 0 km/s, and weaker components centered at +/- 140 km/s. The MDRS spectra allow us to study the variation of OVI emission in the post-shock structure. We find that the zero velocity emission is associated directly with the Balmer filament shock, while the high velocity emission comes from a more uniformly distributed component elsewhere along the line of sight. We also find that the shocks producing the emission at +/- 140 km/s have velocities between 180 km/s and 220 km/s, if we assume that the ram pressure driving them is the same as for the zero velocity component shock. In the context of the cavity model for the Cygnus Loop, the interaction of the blast wave with the spherical shell that forms most of the cavity wall can naturally give rise to the similar red and blue-shifted components that are observed.

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arXiv2001

Far Ultraviolet Spectroscopic Explorer Observations of a Supernova Remnant in the Line of Sight to HD 5980 in the Small Magellanic Cloud

We report a detection of far ultraviolet absorption from the supernova remnant SNR 0057 - 7226 in the Small Magellanic Cloud (SMC). The absorption is seen in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the LBV/WR star HD 5980. Absorption from O VI 1032 and C III 977 is seen at a velocity of +300 km/s with respect to the Galactic absorption lines, +170 km/s with respect to the SMC absorption. The O VI 1038 line is contaminated by H_2 absorption, but is present. These lines are not seen in the FUSE spectrum of Sk80, only ~1' (~17 pc) away from HD 5980. No blue-shifted O VI 1032 absorption from the SNR is seen in the FUSE spectrum. The O VI 1032 line in the SNR is well described by a Gaussian with FWHM=75 km/s. We find log N(O VI)=14.33-14.43, which is roughly 50% of the rest of the O VI column in the SMC (excluding the SNR) and greater than the O VI column in the Milky Way halo along this sight line. The N(C IV)/N(O VI) ratio for the SNR absorption is in the range of 0.12-0.17, similar to the value seen in the Milky Way disk, and lower than the halo value, supporting models in which SNRs produce the highly ionized gas close to the plane of the Galaxy, while other mechanisms occur in the halo. The N(C IV)/N(O VI) ratio is also lower than the SMC ratio along this sight line, suggesting that other mechanisms contribute to the creation of the global hot ionized medium in the SMC. The O VI, C IV, and Si IV apparent column density profiles suggest the presence of a multi-phase shell followed by a region of higher temperature gas.

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arXiv2001

A Detailed Analysis of a Cygnus Loop Shock-Cloud Interaction

The XA region of the Cygnus Loop is a complex zone of radiative and nonradiative shocks interacting with interstellar clouds. We combine five far ultraviolet spectral observations from the Hopkins Ultraviolet Telescope (HUT), a grid of 24 IUE spectra and a high-resolution longslit Halpha spectrum to study the spatial emission line variations across the region. These spectral data are placed in context using ground-based, optical emission line images of the region and a far-UV image obtained by the Ultraviolet Imaging Telescope (UIT). The presence of high-ionization ions (OVI, NV, CIV) indicates a shock velocity near 170 km/s while other diagnostics indicate v_shock=140 km/s. It is likely that a large range of shock velocities may exist at a spatial scale smaller than we are able to resolve. By comparing CIV 1550, CIII 977 and CIII] 1909, we explore resonance scattering across the region. We find that a significant column depth is present at all positions, including those not near bright optical/UV filaments. Analysis of the OVI doublet ratio suggests an average optical depth of about unity in that ion while flux measurements of [SiVIII] 1443 suggest a hot component in the region at just below 10^6K. Given the brightness of the OVI emission and the age of the interaction, we rule out the mixing layer interpretation of the UV emission. Furthermore, we formulate a picture of the XA region as the encounter of the blast wave with a finger of dense gas protruding inward from the pre-SN cavity.

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arXiv2000

Far Ultraviolet Spectroscopy of a Nonradiative Shock in the Cygnus Loop: Implications for the Postshock Electron-Ion Equilibration

We present far ultraviolet spectra of a fast (~ 300 km/s) nonradiative shock in the Cygnus Loop supernova remnant. Our observations were performed with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, covering the wavelength range 905-1187 Angstroms. The purpose of these observations was to estimate the degree of electron-ion equilibration by (1) examining the variation of O VI 1032, 1038 intensity with position behind the shock, and (2) measuring the widths of the O VI lines. We find significant absorption near the center of the O VI 1032 line, with less absorption in O VI 1038. The absorption equivalent widths imply an O VI column density greatly exceeding that of interstellar O VI along the Cygnus Loop line of sight. We suggest that the absorption may be due to resonant scattering by O VI ions within the shock itself. The widths of the O VI emission lines imply efficient ion-ion equilibration, in agreement with predictions from Balmer-dominated spectra of this shock.

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