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We found that an intermediate velocity cloud (IVC) IVC 86-36 in HI 21 cm emission shows a head-tail distribution toward the Galactic plane with marked parallel filamentary streamers, which is extended over 40 degrees in the sky. The distance of IVC 86-36 is constrained to be less than ~3 kpc from absorption of a background star as determined from opticalspectroscopy. There is a bridge feature in velocity between the IVC and the local ISM with velocity separation of ~50 km s-1, which may indicate dynamical interaction of the IVC with the disk. If the interaction is correct, the distance estimate d of the IVC ranges from 200 pc to 3 kpc, and the mass of the IVC head is estimated to be 7X103(d/1kpc)2Msol. The IVC shares similar properties to the Smith cloud located at 12 kpc, including the head-tail distribution, streamers, and bridge feature, while the mass of the IVC is less than ~0.1 of the Smith cloud. A comparison between the Hi and the Planck/IRAS dust emission indicates that the dust emission of IVC 86-36 is not detectable in spite of its HI column density of 2X10^20 cm-2, indicating low metalicity of IVC 86-36 by a factor of ~< 0.2 as compared with the solar neighbor. We conclude that IVC 86-38 is an infalling cloud which likely originated in the low-metallicity environment of the Galactic halo or the Magellanic system., Accepted for publication in PASJ

Measurements of high-velocity clouds' metallicities provide important clues about their origins, and hence on whether they play a role in fueling ongoing star formation in the Galaxy. However, accurate interpretation of these measurements requires compensating for the galactic material that has been mixed into the clouds. In order to determine how much the metallicity changes as a result of this mixing, we have carried out three-dimensional wind-tunnel-like hydrodynamical simulations of an example cloud. Our model cloud is patterned after the Smith Cloud, a particularly well-studied cloud of mass $\sim 5 \times 10^6~M_\odot$. We calculated the fraction of the high-velocity material that had originated in the galactic halo, $F_\mathrm{h}$, for various sight lines passing through our model cloud. We find that $F_\mathrm{h}$ generally increases with distance from the head of the cloud, reaching $\sim$0.5 in the tail of the cloud. Models in which the metallicities (relative to solar) of the original cloud, $Z_\mathrm{cl}$, and of the halo, $Z_\mathrm{h}$, are in the approximate ranges $0.1 \lesssim Z_\mathrm{cl} \lesssim 0.3$ and $0.7 \lesssim Z_\mathrm{h} \lesssim 1.0$, respectively, are in rough agreement with the observations. Models with $Z_\mathrm{h} \sim 0.1$ and $Z_\mathrm{cl} \gtrsim 0.5$ are also in rough agreement with the observations, but such a low halo metallicity is inconsistent with recent independent measurements. We conclude that the Smith Cloud's observed metallicity may not be a true reflection of its original metallicity and that the cloud's ultimate origin remains uncertain., Comment: Accepted for publication in the Astrophysical Journal. 8 pages, 8 figures, 2 tables

We present measurements of the soft X-ray background (SXRB) O VII and O VIII intensity between $l = 120\degr$ and $l = 240\degr$, the first results of a survey of the SXRB using archival XMM-Newton observations. We do not restrict ourselves to blank-sky observations, but instead use as many observations as possible, removing bright or extended sources by hand if necessary. The oxygen intensities are typically ~0.5-10 photons/cm^2/s/sr (line units, L.U.) for O VII and ~0-5 L.U. for O VIII. Our dataset includes 69 directions with multiple observations, whose oxygen intensity variations can be used to constrain SWCX models. One observation exhibits an O VII enhancement of ~25 L.U. over 2 other observations of the same direction, although most SWCX enhancements are $\la 4$ L.U. for O VII and $\la 2$ L.U. for O VIII. We find no clear tendency for the O VII centroid to shift toward the forbidden line energy in observations with bright SWCX enhancements. There is also no universal association between enhanced SWCX emission and increased solar wind flux or the closeness of the sightline to the sub-solar region of the magnetosheath. After removing observations likely to be contaminated by heliospheric SWCX emission, we use our results to examine the Galactic halo. There is some scatter in the halo intensity about the predictions of a simple plane-parallel model, indicating a patchiness to the halo emission. The O VII/O VIII intensity ratio implies a halo temperature of ~2.0-2.5 MK, in good agreement with previous studies. (Abridged), Comment: 43 pages, 19 figures. Accepted for publication in the Astrophysical Journal Supplement Series

This report summarizes the discussions in the Session 1 and Session 3 groups which met to discuss the questions: ``What Physical Processes Drive the Multiphase Interstellar Medium in the Local Bubble?'', and ``What are the Energy and Pressure Balances in the Local Bubble?'' Most of our understanding of the Local Bubble has come from soft X-ray observations, but recent appreciation of the importance of solar wind charge exchange (SWCX) reactions has shown that the heliosphere produces some fraction of the soft X-rays that were previously ascribed to the Local Bubble. Some astronomers suggest that the SWCX X-rays rather than Local Bubble emission could explain most of the locally produced X-rays. Our discussions, therefore, also included a debate concerning the Local Bubble's existence.

The hot Local Bubble surrounding the solar neighborhood has been primarily studied through observations of its soft X-ray emission. The measurements were obtained by attributing all of the observed local soft X-rays to the bubble. However, mounting evidence shows that the heliosphere also produces diffuse X-rays. The source is solar wind ions that have received an electron from another atom. The presence of this alternate explanation for locally produced diffuse X-rays calls into question the existence and character of the Local Bubble. This article addresses these questions. It reviews the literature on solar wind charge exchange (SWCX) X-ray production, finding that SWCX accounts for roughly half of the observed local 1/4 keV X-rays found at low latitudes. This article also makes predictions for the heliospheric O VI column density and intensity, finding them to be smaller than the observational error bars. Evidence for the continued belief that the Local Bubble contains hot gas includes the remaining local 1/4 keV intensity, the observed local O VI column density, and the need to fill the local region with some sort of plasma. If the true Local Bubble is half as bright as previously thought, then its electron density and thermal pressure are 1/square-root(2) as great as previously thought, and its energy requirements and emission measure are 1/2 as great as previously thought. These adjustments can be accommodated easily, and, in fact, bring the Local Bubble's pressure more in line with that of the adjacent material. Suggestions for future work are made., Comment: 9 pages, refereed, accepted for publication in the proceedings of the "From the Outer Heliosphere to the Local Bubble: Comparisons of New Observations with Theory" conference and in Space Science Reviews

In order to determine the circumstances under which isolated SNRs are capable of rising into and enriching the thick disk and galactic halo, simulations of supernova remnants are performed with the FLASH magnetohydrodynamic code. We performed simulations in which the interstellar magnetic field is parallel to or perpendicular to the galactic plane as well as a simulation without a magnetic field. The ambient gas density distribution and gravitational potential are based on observations of our galaxy. We evolve the remnants to ages of roughly 10,000,000 years. For our simulation without a magnetic field, we compare the evolution of the hot bubble's velocity with the velocity evolution calculated from the buoyant and drag accelerations. We found surprisingly small vertical velocities of the hot gas, from which we estimated the drag coefficient to be ten for the non-magnetic simulation. Although we found little buoyant motion of the hot gas during the remnant's lifetime, we found rapid vertical motion of the associated cool dense gas near the end of the remnants life. This motion deformed the remnant into a mushroom cloud structure similar to those found in previous simulations. The simulation in which we have a 4 micro-Gauss magnetic field parallel to the galactic mid-plane shows a dramatically elongated bubble parallel to the magnetic field. The magnetic field pins the supernova remnant preventing it from rising. In the simulation with the 4 micro-Gauss magnetic field perpendicular to the midplane the hot bubble rises more, indicating that having the magnetic field in the same direction as the gravitational force enhances the rise of the bubble., Comment: 16 pages (using emulateapj, onecolumn mode), 15 figure panels, published in ApJ

We used FUSE to observe ultraviolet emission from diffuse O VI in the hot gas in the Galactic halo. By comparing our result with another, nearby observation blocked by an opaque cloud at a distance of 230 pc, we could subtract off the contribution from the Local Bubble, leading to an apparent halo intensity of I_{OVI} = 4680^{+570}_{-660} photons/cm^2/s/sr. A correction for foreground extinction leads to an intrinsic intensity that could be as much as twice this value. Assuming T ~ 3 x 10^5 K, we conclude that the electron density, n_e, is 0.01-0.02 /cm^3, the thermal pressure, p/k, is 7000-10,000 K/cm^3, and that the hot gas is spread over a length of 50-70 pc, implying a small filling factor for O VI-rich gas. ROSAT observations of emission at 1/4 keV in the same direction indicate that the X-rays are weaker by a factor of 1.1 to 4.7, depending on the foreground extinction. Simulated supernova remnants evolving in low density gas have similar O VI to X-ray ratios when the remnant plasma is approaching collisional ioinizational equilibrium and the physical structures are approaching dynamical ``middle age''. Alternatively, the plasma can be described by a temperature power-law. Assuming that the material is approximately isobaric and the length scales according to T^(beta) d(ln T), we find beta = 1.5+/-0.6 and an upper temperature cutoff of 10^{6.6(+0.3,-0.2)} K. The radiative cooling rate for the hot gas, including that which is too hot to hold O VI, is 6 x 10^{38} erg/s/kpc^2. This rate implies that ~70% of the energy produced in the disk and halo by SN and pre-SN winds is radiated by the hot gas in the halo., Accepted by the Astrophysical Journal, AASTEX, 39 pages, including 6 figures and 4 tables (manuscript replaced in order to re-typeset Table 3 and correct submission entry)

The Galaxy's population of High Velocity Clouds (HVCs) may include a subpopulation that is confined by dark matter minihalos and falling toward the Galactic disk. We present the first magnetohydrodynamic simulational study of dark matter-dominated HVCs colliding with a weakly magnetized galactic disk. Our HVCs have baryonic masses of $5 \times 10^6\,$M$_{\odot}$ and dark matter minihalo masses of 0, $3 \times 10^8$, or $1 \times 10^9\,$M$_{\odot}$. They are modeled on the Smith Cloud, which is said to have collided with the disk 70 Myr ago. We find that, in all cases, the cloud's collision with the galactic disk creates a hole in the disk, completely disperses the cloud, and forms a bubble-shaped structure on the far side of the disk. In contrast, when present, the dark matter minihalo continues unimpeded along its trajectory. Later, as the minihalo passes through the bubble structure and galactic halo, it accretes up to $6.0 \times 10^5\,$M$_{\odot}$ in baryonic material, depending on the strengths of the magnetic field and minihalo gravity. These simulations suggest that if the Smith Cloud is associated with a dark matter minihalo and collided with the Galactic disk, the minihalo has accreted the observed gas. However, if the Smith Cloud is dark matter-free, it is on its first approach toward the disk. These simulations also suggest that the dark matter is most concentrated either at the head of the cloud or near the cloud, depending upon the strength of the magnetic field, a point that could inform indirect dark matter searches., Comment: 5 pages, 4 figures, 1 table

We present predictions of the Si iv ions in turbulent mixing layers (TMLs) between hot and cool gas and in cool high-velocity clouds (HVCs) that travel through a hot halo, complementing the C iv, N v, and O vi predictions in Kwak & Shelton, Kwak et al., and Henley et al. We find that the Si iv ions are most abundant in regions where the hot and cool gases first begin to mix or where the mixed gas has cooled significantly. The predicted column densities of high velocity Si iv and the predicted ratios of Si iv to C iv and O vi found on individual sightlines in our HVC simulations are in good agreement with observations of high velocity gas. Low velocity Si iv is also seen in the simulations, as a result of decelerated gas in the case of the HVC simulations and when looking along directions that pass perpendicular to the direction of motion in the TML simulations. The ratios of low velocity Si iv to C iv and O vi in the TML simulations are in good agreement with those recorded for Milky Way halo gas, while the ratio of Si iv to O vi from the decelerated gas in the HVC simulations is lower than that observed at normal velocity in the Milky Way halo. We attribute the shortfall of normal velocity Si iv to not having modeled the effects of photoionization and, following Henley et al., consider a composite model that includes decelerated HVC gas, supernova remnants, galactic fountain gas, and the effect of photoionization., 40 pages, 12 figures, and 6 tables

We report the results of a survey of OVI 1032 absorption along the lines of sight to 25 white dwarfs in the local interstellar medium (LISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). We find that interstellar OVI absorption along all sightlines is generally weak, and in a number of cases, completely absent. No OVI absorption was detected with significance greater than 2 sigma for 12 of the 25 stars, where the 1 sigma uncertainty is 4 mA, equivalent to an OVI column density of ~3 x 10^12 cm^-2. Of the remaining stars, most have column densities N(OVI) < 10^13 cm^-2 and no column densities exceed 1.7 x 10^13 cm^-2. For lines of sight to hot (T_eff > 40,000 K) white dwarfs, there is some evidence that the OVI absorption may be at least partially photospheric or circumstellar in origin. We interpret the "patchy" distribution of OVI absorption in terms of a model where OVI is formed in evaporative interfaces between cool clouds and the hot, diffuse gas in the Local Bubble (LB). If the clouds contain tangled or tangential magnetic fields, then thermal conduction will be quenched over most of the cloud surface, and OVI will be formed only in local "patches" where conduction is allowed to operate. We find an average OVI space density in the LISM of 2.4 x 10^-8 cm^-3, which is similar to, or slightly larger than, the value in the Galactic disk over kpc scales. This local density implies an average OVI column density of ~7 x 10^12 cm^-2 over a path length of 100 pc within the LB. The OVI data presented here appears to be inconsistent with the model proposed by Breitschwerdt & Schmutzler (1994), in which highly ionized gas at low kinetic temperature (~50,000 K) permeates the LB. Our survey results are consistent with the supernova-driven cavity picture of Cox & Smith (1974)., 40 pages, 9 figures, AASTeX, submitted to ApJ

This paper presents archival ROSAT PSPC observations of the G65.2+5.7 supernova remnant (also known as G65.3+5.7). Little material obscures this remnant and so it was well observed, even at the softest end of ROSAT's bandpass (~0.11 to 0.28 keV). These soft X-ray images reveal the remnant's centrally-filled morphology which, in combination with existing radio frequency observations, places G65.2+5.7 in the thermal composite (mixed morphology) class of supernova remnants. Not only might G65.2+5.7 be the oldest known thermal composite supernova remnant, but owing to its optically revealed cool, dense shell, this remnant supports the proposal that thermal composite supernova remnants lack X-ray bright shells because they have evolved beyond the adiabatic phase. These observations also reveal a slightly extended point source centered on RA = 19h 36m 46s, dec = 30deg 40' 07'' and extending 6.5 arcmin in radius in the band 67 map. The source of this emission has yet to be discovered, as there is no known pulsar at this location., Comment: In AASTEX preprint form, document is 12 pages long and includes 3 figure files

We present the first X-ray detection of SNR 0450-70.9 the largest known supernova remnant (SNR) in the Large Magellanic Cloud. To study the physical conditions of this SNR, we have obtained XMM-Newton X-ray observations, optical images and high-dispersion spectra, and radio continuum maps. Optical images of SNR 0450-70.9 show a large, irregular elliptical shell with bright filaments along the eastern and western rims and within the shell interior. The interior filaments have higher [S II]/Halpha ratios and form an apparent inner shell morphology. The X-ray emission region is smaller than the full extent of the optical shell, with the brightest X-ray emission found within the small interior shell and on the western rim of the large shell. The expansion velocity of the small shell is ~220 km/s, while the large shell is ~120 km/s. The radio image shows central brightening and a fairly flat radio spectral index over the SNR. However, no point X-ray or radio source corresponding to a pulsar is detected and the X-ray emission is predominantly thermal. Therefore, these phenomena can be most reasonably explained in terms of the advanced age of the large SNR. Using hydrodynamic models combined with a nonequilibrium ionization model for thermal X-ray emission, we derived a lower limit on the SNR age of about 45,000 yr, well into the later stages of SNR evolution. Despite this, the temperature and density derived from spectral fits to the X-ray emission indicate that the remnant is still overpressured, and thus that the development is largely driven by hot gas in the SNR interior.

We present Far Ultraviolet Spectroscopic Explorer and ROSAT X-ray observations toward the Southern Coalsack. An almost complete X-ray halo can be seen around the cloud in the 0.75 and 1.5 keV images, and most of the observed stars show O VI absorption. Both the cloud and the stars have highly accurate distance determinations, allowing us to reliably place the stars and the cloud relative to each other. Using these distance determinations, we find no O VI-bearing gas in the foreground of the Coalsack, while for stars in the background of the cloud, O VI absorption is the norm. The column density of O VI correlates with the 0.75 and 1.5 keV intensities. These results suggest that the X-ray-emitting hot plasma is associated with the dense cloud. We propose that the heating of the Coalsack envelope is due to the hot gas in the interior of the Upper Cen-Lup superbubble. The Coalsack interaction region provides a nearby example of the hot-cold gas interfaces thought to be responsible for the O VI absorptions seen on many sight lines throughout the Galaxy.

This paper reports the first study of the O VI resonance line emission (1032, 1038 Angstroms) originating in the Local Bubble (or Local Hot Bubble) surrounding the solar neighborhood. In spite of the fact that O VI absorption within the Local Bubble has been observed, no resonance line emission was detected during our 230 ksec Far Ultraviolet Spectroscopic Explorer observation toward a ``shadowing'' filament in the southern Galactic hemisphere. As a result, tight 2 sigma upper limits are set on the intensities in the 1032 and 1038 Angstrom emission lines: 500 and 530 photons cm^{-2} s^{-1} sr^{-1}, respectively. These values place strict constraints on models and simulations. They suggest that the O VI-bearing plasma and the X-ray emissive plasma reside in distinct regions of the Local Bubble and are not mixed in a single plasma, whether in equilibrium with T ~ 10^6 K or highly overionized with T ~ 4 to 6 x 10^4 K. If the line of sight intersects multiple cool clouds within the Local Bubble, then the results also suggest that hot/cool transition zones differ from those in current simulations. With these intensity upper limits, we establish limits on the electron density, thermal pressure, pathlength, and cooling timescale of the O VI-bearing plasma in the Local Bubble. Furthermore, the intensity of O VI resonance line doublet photons originating in the Galactic thick disk and halo is determined (3500 to 4300 photons cm^{-2} s^{-1} sr^{-1}), and the electron density, thermal pressure, pathlength, and cooling timescale of its O VI-bearing plasma are calculated. The pressure in the Galactic halo's O VI-bearing plasma (3100 to 3800 K cm^{-3}) agrees with model predictions for the total pressure in the thick disk/lower halo. We also report the results of searches for other emission lines., Comment: accepted by ApJ, scheduled for May 2003, replacement astro-ph submission corrects typos and grammatical errors in original version

We present the result of a Chandra ACIS observation of the pulsar PSR B1853+01 and its associated pulsar wind nebula (PWN), embedded within the supernova remnant W44. A hard band ACIS map cleanly distinguishes the PWN from the thermal emission of W44. The nebula is extended in the north-south direction, with an extent about half that of the radio emission. Morphological differences between the X-ray and radio images are apparent. Spectral fitting reveals a clear difference in spectral index between the hard emission from PSR B1853+01 (Gamma ~ 1.4) and the extended nebula (Gamma ~ 2.2). The more accurate values for the X-ray flux and spectral index are used refine estimates for PWN parameters, including magnetic field strength, the average Lorentz factor, gamma, of the particles in the wind, the magnetization parameter, sigma, and the ratio k of electrons to other particles., Comment: 10 pages, 3 figures, accepted by ApJ

We present Far Ultraviolet Spectroscopic Explorer ($\it FUSE$) satellite measurements of the absorption and emission characteristics of interstellar gas associated with the Local Interstellar Chimney, which is an extension of the rarefied Local Bubble cavity that extends outward from the galactic disk towards the lower galactic halo. Far ultraviolet (FUV) diffuse background emission has been detected in the high ionization line of O VI ( λ 1032 A) for two lines-of-sight ($l = 162.7^{\circ}$, $b = +57.0^{\circ}$) and ($l = 156.3^{\circ}$, $b = +57.8^{\circ}$) at emission levels of $2500\pm700$ photons cm -2 s -1 sr -1 (LU) and $3300\pm1100$ LU respectively. These levels of O VI emission are very similar to those found for four other lines-of-sight sampled thus far by the $\it FUSE$ satellite, implying a fairly constant level of average O VI surface brightness emission at high galactic latitudes of about 2700 LU with a standard deviation of 450 LU. These emission-line data are supplemented by FUV interstellar absorption line measurements taken towards the hot DA white dwarf star, REJ 1032+532 ($l = 157.5^{\circ}$, $ b = +53.2^{\circ}$), whose distance of 116 pc places it within the Local Bubble region. No high ionization interstellar O VI λ 1032 A absorption has been detected $(N({\rm O~VI}) -2 ), which is consistent with the non-detections of interstellar C IV and Si IV absorption reported towards this star by Holberg et al. ([CITE]). Taken together, our FUV absorption and emission data may be explained by a scenario in which the O VI emission and absorption lines are $\it both$ formed at the conductive interface of the neutral boundary to the Local Bubble. For the presently sampled sight-lines we have found no correlation between the OVI emission line intensity and the associated 0.25 keV soft X-ray background flux as measured in the R1 and R2 bands by the $\it ROSAT$ satellite. The OVI line intensities also show no correlation with the soft X-ray background flux attributable to emission from the million degree K gas of the Local Hot Bubble as modeled by Kuntz & Snowden ([CITE]). Any (new) model of the Local Bubble must now be able to explain (i) the low levels of variability in both the O VI emission-line intensity and the associated soft X-ray background flux for galactic sight-lines > $|40|$°, (ii) the observed pressure of $P/k\sim 10\,000$ cm -3 K for the local hot interstellar gas, and (iii) the paucity of high ionization absorption lines observed within the local ISM and the sudden increase in their measured column density for distances beyond the Local Bubble neutral boundary.

Emission from 0 VI (lambda lambda 1032, 1038) in the diffuse interstellar medium was observed in two, long Fur Ultra-violet Spectroscopic Explorer (FUSE) observations at l = approximately equals 1l3 degrees .0 and b approximately equals 70 degrees .7 (near the quasar HS 1307+4617). The observed intensities are 2580 +/- 380 (random) +/- 360 (systematic) and 1100 +/- 160 (random) +/- 160 (systematic) photons cm(exp -2)/s s/r in the 1032 and 1038 A emission lines, respectively. The electron density, thermal pressure, and depth of the emitting gas are calculated from the observed intensities. The velocity of the emitting material is about +10 km/s relative to the local standard of rest and less than + 30 km/s relative to the H I along the line of sight. The similarity suggests that the highly ionized gas is quiescent and not recently shock heated. Emission from the C 11 3 s(exp 2) S(sub 1/2) to 2p(exp 2) P(sub 3/2) transition at 1037 A is also observed, and upper limits are placed on the intensities of ultraviolet line emission from C I, C 111, N I, N 11, Mg 11, Si 11, S 11, S 111. S IV, S VI. Fe 11, and Fe 111.

The fundamental collisional process of charge exchange (CX) has been established as a primary source of X-ray emission from the heliosphere, planetary exospheres, and supernova remnants. In this process, X-ray emission results from the capture of an electron by a highly charged ion from a neutral atom or molecule, to form a highly excited, high-charge state ion. As the captured electron cascades down to the lowest energy level, photons are emitted, including X-rays. To provide reliable CX-induced X-ray spectral models to realistically simulate these environments, line ratios and spectra are computed using theoretical CX cross sections obtained with the multi-channel Landau-Zener, atomic-orbital close-coupling, molecular-orbital close-coupling, and classical trajectory Monte Carlo methods for various collisional velocities relevant to astrophysics. X-ray spectra were computed for collisions of bare and H-like C to Al ions with H, He, and H2 with results compared to available experimental data. Using these line ratios, XSPEC models of CX emission in the northeast rim of the Cygnus Loop supernova remnant and the heliosphere are shown as examples with ion velocity dependence.

Charge exchange (CX) has emerged in X-ray emission modeling as a significant process that must be considered in many astrophysical environment--particularly comets. Comets host an interaction between solar wind ions and cometary neutrals to promote solar wind charge exchange (SWCX). X-ray observatories provide astronomers and astrophysicists with data for many X-ray emitting comets that are impossible to accurately model without reliable CX data. Here, we utilize a streamlined set of computer programs that incorporate the multi-channel Landau-Zener theory and a cascade model for X-ray emission to generate cross sections and X-ray line ratios for a variety of bare and non-bare ion single electron capture (SEC) collisions. Namely, we consider collisions between the solar wind constituent bare and H-like ions of C, N, O, Ne, Na, Mg, Al, and Si and the cometary neutrals H2O, CO, CO2, OH, and O. To exemplify the application of this data, we model the X-ray emission of Comet C/2000 WM1 (linear) using the CX package in SPEX and find excellent agreement with observations made with the XMM-Newton RGS detector. Our analyses show that the X-ray intensity is dominated by SWCX with H, while H2O plays a secondary role. This is the first time, to our knowledge, that CX cross sections have been implemented into a X-ray spectral fitting package to determine the H to H2O ratio in cometary atmospheres. The CX data sets are incorporated into the modeling packages SPEX and Kronos.

High-velocity clouds (HVCs) hydrodynamically interact with their surroundings. In scenarios with small clouds, the net result is erosion of the HVC, but in scenarios with large, massive clouds, it is capture and cooling of environmental gas by the cloud. In order to examine these effects over long periods of time, we made detailed three-dimensional hydrodynamic FLASH simulations of massive HVCs (1.35 × 105 M ☉ to 1.35 × 108 M ☉) traveling through hot, low-density media like that in the extended Galactic halo, circumgalactic media, and intergalactic space for at least 200 Myr. By setting the metallicity of the clouds to differ from that of the ambient gas, we were able to track the transfer of material between the two media. We found that massive clouds condense substantial amounts of ambient gas, up to 100% of their initial mass in certain cases. This gas cools to temperatures below 1.0 × 104 K, but retains some high ions. The amount of condensed gas depends on the cloud mass, the ambient density, and the cloud density and temperature, but apparently not on the velocity. We discuss the ramifications for cloud survivability and for the transport of halo, circumgalactic, and intergalactic gas to the disk of the Galaxy aboard massive HVCs.

In the Galactic halo, metal-bearing Galactic halo material mixes into high velocity clouds (HVCs) as they hydrodynamically interact. This interaction begins long before the clouds completely dissipate and long before they slow to the velocity of the Galactic material. In order to make quantitative estimates of the mixing efficiency and resulting metal enrichment of HVCs, we made detailed two- and three-dimensional simulations of cloud-interstellar medium interactions. Our simulations track the hydrodynamics and time-dependent ionization levels. They assume that the cloud originally has a warm temperature and extremely low metallicity while the surrounding medium has a high temperature, low density, and substantial metallicity, but our simulations can be generalized to other choices of initial metallicities. In our simulations, mixing between cloud and halo gas noticeably raises the metallicity of the high velocity material. We present plots of the mixing efficiency and metal enrichment as a function of time.

A soft X-ray enhancement has recently been reported toward the high-velocity cloud MS30.7-81.4-118 (MS30.7), a constituent of the Magellanic Stream. In order to investigate the origin of this enhancement, we have analyzed two overlapping XMM-Newton observations of this cloud. We find that the X-ray enhancement is $\sim$6' or $\sim$100 pc across, and is concentrated to the north and west of the densest part of the cloud. We modeled the X-ray enhancement with a variety of spectral models. A single-temperature equilibrium plasma model yields a temperature of $(3.69^{+0.47}_{-0.44}) \times 10^6$ K and a 0.4-2.0 keV luminosity of $7.9 \times 10^{33}$ erg s$^{-1}$. However, this model underpredicts the on-enhancement emission around 1 keV, which may indicate the additional presence of hotter plasma ($T \gtrsim 10^7$ K), or that recombination emission is important. We examined several different physical models for the origin of the X-ray enhancement. We find that turbulent mixing of cold cloud material with hot ambient material, compression or shock heating of a hot ambient medium, and charge exchange reactions between cloud atoms and ions in a hot ambient medium all lead to emission that is too faint. In addition, shock heating in a cool or warm medium leads to emission that is too soft (for reasonable cloud speeds). We find that magnetic reconnection could plausibly power the observed X-ray emission, but resistive magnetohydrodynamical simulations are needed to test this hypothesis. If magnetic reconnection is responsible for the X-ray enhancement, the observed spectral properties could potentially constrain the magnetic field in the vicinity of the Magellanic Stream., Comment: 17 pages, 6 figures. Accepted for publication in the Astrophysical Journal

We investigate the convective stability of two popular types of model of the gas distribution in the hot Galactic halo. We first consider models in which the halo density and temperature decrease exponentially with height above the disk. These halo models were created to account for the fact that, on some sight lines, the halo's X-ray emission lines and absorption lines yield different temperatures, implying that the halo is non-isothermal. We show that the hot gas in these exponential models is convectively unstable if $\gamma, Comment: 8 pages, 3 figures. Accepted for publication in the Astrophysical Journal

We have examined 426 {\it Voyager} fields distributed across the sky for \ion{O}{6} ($\lambda \lambda$ 1032/1038 \AA) emission from the Galactic diffuse interstellar medium. No such emission was detected in any of our observed fields. Our most constraining limit was a 90% confidence upper limit of 2600 \phunit on the doublet emission in the direction (l, b) = (117\fdegree3, 50\fdegree6). Combining this with an absorption line measurement in nearly the same direction allows us to place an upper limit of 0.01 cm$^{-3}$ on the electron density of the hot gas in this direction. We have placed 90% confidence upper limits of less than or equal to 10,000 \phunit on the \ion{O}{6} emission in 16 of our 426 observations.

We present the results of a Far Ultraviolet Spectroscopic Explorer observation of an X-ray-selected knot in the Vela supernova remnant. Spectra were obtained through the 30'' × 30'' low-resolution aperture and the 4'' × 20'' medium-resolution aperture. O VI λλ1032, 1038 and C III λ977 are detected strongly in both spectra, and S VI λλ933, 944 is detected weakly only in the larger aperture spectrum. We also report the first detection of C II λ1037 emission in a supernova remnant. The spectra show the presence of two kinematic components along the line of sight—one with both low- and high-excitation emission centered at a velocity of -50 km s-1 and another with only low-excitation emission centered at a velocity of +100 km s-1. We associate the -50 km s-1 component with the observed X-ray knot and find a dynamical pressure of 3.7 × 10-10 dyne cm-2 driving the shock. We compare our results with data obtained using the Hopkins Ultraviolet Telescope at nearby locations and find that differences in the spectra imply the existence of two emitting components in the X-ray knot. Based on the X-ray morphology seen in a ROSAT HRI image, we identify two distinct regions that can be associated with these two components which have dramatically different ultraviolet emission. These observations demonstrate the importance of high spectral resolution in understanding the proper physical relationships between the various emitting components in supernova remnants.

Far Ultraviolet Spectroscopic Explorer spectra of the white dwarf stars G191-B2B, GD 394, WD 2211-495, and WD 2331-475 cover the absorption features out of the ground electronic states of N I, N II, N III, O I, and Ar I in the far-ultraviolet, providing new insights on the origin of the partial ionization of the local interstellar medium (LISM) and, for the case of G191-B2B, the interstellar cloud that immediately surrounds the solar system. Toward these targets the interstellar abundances of Ar I, and sometimes N I, are significantly below their cosmic abundances relative to H I. In the diffuse interstellar medium, these elements are not likely to be depleted onto dust grains. Generally, we expect that Ar should be more strongly ionized than H (and also O and N, whose ionizations are coupled to that of H via charge-exchange reactions) because the cross section for the photoionization of Ar I is very high. Our finding that Ar I/H I is low may help to explain the surprisingly high ionization of He in the LISM found by other investigators. Our result favors the interpretation that the ionization of the local medium is maintained by a strong extreme-ultraviolet flux from nearby stars and hot gases, rather than an incomplete recovery from a past, more highly ionized condition.

Launch of the Far Ultraviolet Spectroscopic Explorer (FUSE) has been followed by an extensive period of calibration and characterization as part of the preparation for normal satellite operations. Major tasks carried out during this period include initial coalignment, focusing and characterization of the four instrument channels, and a preliminary measurement of the resolution and throughput performance of the instrument. We describe the results from this test program, and present preliminary estimates of the on-orbit performance of the FUSE satellite based on a combination of this data and prelaunch laboratory measurements.

We show that many observations of W44, a supernova remnant in the Galactic plane at a distance of about 2500 pc, are remarkably consistent with the simplest realistic model. The model remnant is evolving in a smooth ambient medium of fairly high density, about 6 cm-3 on average, with a substantial density gradient. At the observed time it has an age of about 20,000 yr, consistent with the age of the associated pulsar, and a radius of 11-13 pc. Over most of the outer surface, radiative cooling has become important in the postshock gas; on the denser end there has been sufficient compression of the cooled gas to develop a very thin dense half-shell of about 450 M☉, supported against further compression by nonthermal pressure. The half-shell has an expansion velocity of about 150 km s-1 and is bounded on the outer surface by a radiative shock with that speed. The deep interior of the remnant has a substantial and fairly uniform pressure, as expected from even highly idealized adiabatic models; our model, however, is never adiabatic. Thermal conduction, while the remnant is young and hot, reduces the need for expansion cooling and prevents formation of the intensely vacuous cavity characteristic of adiabatic evolution. It radically alters the interior structure from what one might expect from familiarity with the Sedov solution. At the time of observation, the temperature in the center is about 6 × 106 K, the density about 1 cm-3. The temperature decreases gradually away from the center, while the density rises. Farther out, where cooling is becoming important, the pressure drops precipitously, and the temperature in the denser gas there is quite low. We provide several analytic tools for the assembly of models of this type. We review the early evolution and shell formation analyses and their generalizations to evolution in a density gradient. We also calculate the density and temperature that should be present in the hot interior of a remnant with thermal conduction. We supply the van der Laan mechanism in a particularly useful form for the calculation of radio continuum from radiative remnants. Finally, we estimate the optical emission that should be present from fluorescence of UV light, emitted by the forming shell and the radiative shock and absorbed in the cold shell and the ambient medium, and the associated 63 μm [O I] emission. Both are in agreement with the intensity and spatial structures found in recent observations. Neither requires interaction with a dense molecular cloud for its generation. We calculate the gamma rays that should be emitted by cosmic-ray electrons and ions in the shell, interacting with the cold material, and find each capable of generating about 25% of the flux reported by EGRET for the vicinity.

We present measurements of the Galactic halo's X-ray emission for 110 XMM-Newton sight lines, selected to minimize contamination from solar wind charge exchange emission. We detect emission from few million degree gas on ~4/5 of our sight lines. The temperature is fairly uniform (median = 2.22e6 K, interquartile range = 0.63e6 K), while the emission measure and intrinsic 0.5--2.0 keV surface brightness vary by over an order of magnitude (~(0.4-7)e-3 cm^-6 pc and ~(0.5-7)e-12 erg cm^-2 s^-1 deg^-2, respectively, with median detections of 1.9e-3 cm^-6 pc and 1.5e-12 erg cm^-2 s^-1 deg^-2, respectively). The high-latitude sky contains a patchy distribution of few million degree gas. This gas exhibits a general increase in emission measure toward the inner Galaxy in the southern Galactic hemisphere. However, there is no tendency for our observed emission measures to decrease with increasing Galactic latitude, contrary to what is expected for a disk-like halo morphology. The measured temperatures, brightnesses, and spatial distributions of the gas can be used to place constraints on models for the dominant heating sources of the halo. We provide some discussion of such heating sources, but defer comparisons between the observations and detailed models to a later paper., 22 pages, 8 figures. Accepted for publication in the Astrophysical Journal

We present hydrodynamic simulations of high-velocity clouds (HVCs) traveling through the hot, tenuous medium in the Galactic halo. A suite of models was created using the FLASH hydrodynamics code, sampling various cloud sizes, densities, and velocities. In all cases, the cloud-halo interaction ablates material from the clouds. The ablated material falls behind the clouds, where it mixes with the ambient medium to produce intermediate-temperature gas, some of which radiatively cools to less than 10,000 K. Using a non-equilibrium ionization (NEI) algorithm, we track the ionization levels of carbon, nitrogen, and oxygen in the gas throughout the simulation period. We present observation-related predictions, including the expected H I and high ion (C IV, N V, and O VI) column densities on sight lines through the clouds as functions of evolutionary time and off-center distance. The predicted column densities overlap those observed for Complex C. The observations are best matched by clouds that have interacted with the Galactic environment for tens to hundreds of megayears. Given the large distances across which the clouds would travel during such time, our results are consistent with Complex C having an extragalactic origin. The destruction of HVCs is also of interest; the smallest cloud (initial mass \approx 120 Msun) lost most of its mass during the simulation period (60 Myr), while the largest cloud (initial mass \approx 4e5 Msun) remained largely intact, although deformed, during its simulation period (240 Myr)., 20 pages, 13 figures. Accepted for publication in the Astrophysical Journal

We compare the predictions of three physical models for the origin of the hot halo gas with the observed halo X-ray emission, derived from 26 high-latitude XMM-Newton observations of the soft X-ray background between $l=120\degr$ and $l=240\degr$. These observations were chosen from a much larger set of observations as they are expected to be the least contaminated by solar wind charge exchange emission. We characterize the halo emission in the XMM-Newton band with a single-temperature plasma model. We find that the observed halo temperature is fairly constant across the sky (~1.8e6-2.3e6 K), whereas the halo emission measure varies by an order of magnitude (~0.0005-0.006 cm^-6 pc). When we compare our observations with the model predictions, we find that most of the hot gas observed with XMM-Newton does not reside in isolated extraplanar supernova remnants -- this model predicts emission an order of magnitude too faint. A model of a supernova-driven interstellar medium, including the flow of hot gas from the disk into the halo in a galactic fountain, gives good agreement with the observed 0.4-2.0 keV surface brightness. This model overpredicts the halo X-ray temperature by a factor of ~2, but there are a several possible explanations for this discrepancy. We therefore conclude that a major (possibly dominant) contributor to the halo X-ray emission observed with XMM-Newton is a fountain of hot gas driven into the halo by disk supernovae. However, we cannot rule out the possibility that the extended hot halo of accreted material predicted by disk galaxy formation models also contributes to the emission., 20 pages, 14 figures. New version accepted for publication in ApJ. Changes include new section discussing systematic errors (Section 3.2), improved method for characterizing our model spectra (4.2.2), changes to discussion of other observations (5.1). Note that we can no longer rule out possibility that extended hot halo of accreted material contributes to observed halo emission (see 5.2.1)

Highly ionized species such as C IV, N V, and O VI, are commonly observed in diffuse gas in various places in the universe, such as in our Galaxy's disk and halo, high velocity clouds (HVCs), external galaxies, and the intergalactic medium. One possible mechanism for producing high ions is turbulent mixing of cool gas with hotter gas in locations where these gases slide past each other. By using hydrodynamic simulations with radiative cooling and non-equilibrium ionization (NEI) calculations, we investigate the physical properties of turbulent mixing layers and the production of high ions. We find that most of the mixing occurs on the hot side of the hot/cool interface and that the mixed region separates into a tepid zone containing radiatively cooled, C IV-rich gas and a hotter zone which is rich in C IV, N V, and O VI. Mixing occurs faster than ionization or recombination, making the mixed gas a better source of C IV, N V, and O VI in our NEI simulations than in our collisional ionization equilibrium (CIE) simulations. In addition, the gas radiatively cools faster than the ions recombine, which also allows large numbers of high ions to linger in the NEI simulations. For these reasons, our NEI calculations predict more high ions than our CIE calculations predict. We also simulate various initial configurations of turbulent mixing layers and discuss their results. We compare the results of our simulations with observations and other models, including other turbulent mixing calculations. The ratios of C IV to N V and N V to O VI are in reasonable agreement with the averages calculated from observations of the halo. There is a great deal of variation from sightline to sightline and with time in our simulations. Such spatial and temporal variation may explain some of the variation seen among observations., Comment: 19 pages, 7 figures, 8 tables, Accepted for publication by ApJ

This paper examines the ultraviolet and X-ray photons generated by hot gas in the Galactic thick disk or halo in the Draco region of the northern hemisphere. Our analysis uses the intensities from four ions, C IV, O VI, O VII, and O VIII, sampling temperatures of ~100,000 to ~3,000,000 K. We measured the O VI, O VII and O VIII intensities from FUSE and XMM-Newton data and subtracted off the local contributions in order to deduce the thick disk/halo contributions. These were supplemented with published C IV intensity and O VI column density measurements. Our estimate of the thermal pressure in the O VI-rich thick disk/halo gas, p_{th}/k = 6500^{+2500}_{-2600} K cm^{-3}, suggests that the thick disk/halo is more highly pressurized than would be expected from theoretical analyses. The ratios of C IV to O VI to O VII to O VIII, intensities were compared with those predicted by theoretical models. Gas which was heated to 3,000,000 K then allowed to cool radiatively cannot produce enough C IV or O VI-generated photons per O VII or O VIII-generated photon. Producing enough C IV and O VI emission requires heating additional gas to 100,000 < T < 1,000,000 K. However, shock heating, which provides heating across this temperature range, overproduces O VI relative to the others. Obtaining the observed mix may require a combination of several processes, including some amount of shock heating, heat conduction, and mixing, as well as radiative cooling of very hot gas., 10 pages, 2 figures. Accepted for publication in the Astrophysical Journal

The model of a Local Hot Bubble has been widely accepted as providing a framework that can explain the ubiquitous presence of the soft X-ray background diffuse emission. We summarize the current knowledge on this local interstellar region, paying particular reference to observations that sample emission from the presumed local million degree K hot plasma. However, we have listed numerous observations that are seemingly in conflict with the concept of a hot Local Bubble. In particular, the discovery of solar wind charge exchange that can generate an appreciable soft X-ray background signal within the heliosphere, has led to a reassessment of the generally accepted model that requires a hot local plasma. In order to explain the majority of observations of the local plasma, we forward two new speculative models that describe the physical state of the local interstellar gas. One possible scenario is similar to the present widely accepted model of the Local Hot Bubble, except that it accounts for only 50% of the soft X-ray emission currently detected in the galactic plane, has a lower thermal pressure than previously thought, and its hot plasma is not as hot as previously believed. Although such a model can solve several difficulties with the traditional hot Local Bubble model, a heating mechanism for the dimmer and cooler gas remains to be found. The second possible explanation is that of the Hot Top model, in which the Local Cavity is an old supernova remnant in which no (or very little) million degree local plasma is presently required. Instead, the cavity is now thought to be filled with partially ionized cloudlets of temperature 7000 K that are surrounded by lower density envelopes of photoionized gas of temperature 20,000 K., Comment: Astrophysics and Space Science

We analyze a pair of Suzaku shadowing observations in order to determine the X-ray spectrum of the Galaxy's gaseous halo. We simultaneously fit the spectra with models having halo, local, and extragalactic components. The intrinsic intensities of the halo OVII triplet and OVIII Lyman alpha emission lines are 9.98^{+1.10}_{-1.99} LU (line unit; photons cm^-2 s^-1 Sr^-1) and 2.66^{+0.37}_{-0.30} LU, respectively. Meanwhile, FUSE OVI observations for the same directions and SPEAR CIV observations for a nearby direction indicate the existence of hot halo gas at temperatures of ~10^{5.0} K to ~10^{6.0} K. This collection of data implies that the hot gas in the Galactic halo is not isothermal, but its temperature spans a relatively wide range from ~10^{5.0} K to ~10^{7.0} K. We therefore construct a differential emission measure (DEM) model for the halo's hot gas, consisting of two components. In each, dEM/dlog T is assumed to follow a power-law function of the temperature and the gas is assumed to be in collisional ionizational equilibrium. The low-temperature component (LTC) of the broken power-law DEM model covers the temperature range of 10^{4.80}-10^{6.02} K with a slope of 0.30 and the high-temperature component (HTC) covers the temperature range of 10^{6.02}-10^{7.02} K with a slope of -2.21. We find that a simple model in which hot gas accretes onto the Galactic halo and cools radiatively cannot explain both the observed UV and X-ray portions of our broken power-law model. It can, however, explain the intensity in the Suzaku bandpass if the mass infall rate is 1.35*10^{-3} Msun yr^-1 kpc^-2. The UV and X-ray intensities and our broken power-law model can be well explained by hot gas produced by supernova explosions or by supernova remnants supplemented by a smooth source of X-rays., 35 pages, 4 figures, 8 tables. Accepted for publication in ApJ

We present the analysis of three Suzaku observations of a bright arc in the ROSAT All-Sky Survey 1/4 keV maps at $l \approx 247\degr$, $b \approx -64\degr$. In particular, we have tested the hypothesis that the arc is the edge of a bubble blown by an extraplanar supernova. One pointing direction is near the brightest part of the arc, one is toward the interior of the hypothesized bubble, and one is toward the bubble exterior. We fit spectral models generated from 1-D hydrodynamical simulations of extraplanar supernova remnants (SNRs) to the spectra. The spectra and the size of the arc ($\mathrm{radius} \approx 5\degr$) are reasonably well explained by a model in which the arc is the bright edge of a $\sim$100,000-yr old SNR located $\sim$1--2 kpc above the disk. The agreement between the model and the observations can be improved if the metallicity of the X-ray--emitting gas is $\sim$1/3 solar, which is plausible, as the dust which sequesters some metals is unlikely to have been destroyed in the lifetime of the SNR. The width of the arc is larger than that predicted by our SNR model; this discrepancy is also seen with the Vela SNR, and may be due to the 1-D nature of our simulations. If the arc is indeed the edge of an extraplanar SNR, this work supports the idea that extraplanar supernovae contribute to the heating of the $\sim$million-degree gas in the halo., Comment: 16 pages, 11 figures. Accepted for publication in the Astrophysical Journal

In the Galactic fountain scenario, supernovae and/or stellar winds propel material into the Galactic halo. As the material cools, it condenses into clouds. By using FLASH three-dimensional magnetohydrodynamic simulations, we model and study the dynamical evolution of these gas clouds after they form and begin to fall toward the Galactic plane. In our simulations, we assume that the gas clouds form at a height of z=5 kpc above the Galactic midplane, then begin to fall from rest. We investigate how the cloud's evolution, dynamics, and interaction with the interstellar medium (ISM) are affected by the initial mass of the cloud. We find that clouds with sufficiently large initial densities (> 0.1 hydrogen atoms per cc) accelerate sufficiently and maintain sufficiently large column densities as to be observed and identified as high-velocity clouds (HVCs) even if the ISM is weakly magnetized (1.3 micro Gauss). We also investigate the effects of various possible magnetic field configurations. As expected, the ISM's resistance is greatest when the magnetic field is strong and perpendicular to the motion of the cloud. The trajectory of the cloud is guided by the magnetic field lines in cases where the magnetic field is oriented diagonal to the Galactic plane. The model cloud simulations show that the interactions between the cloud and the ISM can be understood via analogy to the shock tube problem which involves shock and rarefaction waves. We also discuss accelerated ambient gas, streamers of material ablated from the clouds, and the cloud's evolution from a sphere-shaped to a disk- or cigar-shaped object., 46 pages, 16 figures, 3 tables. Accepted for publication in ApJ

We present an analysis of the X-ray spectrum of the Local Bubble, obtained by simultaneously analyzing spectra from two XMM-Newton pointings on and off an absorbing filament in the Southern galactic hemisphere (b ~ -45 deg). We use the difference in the Galactic column density in these two directions to deduce the contributions of the unabsorbed foreground emission due to the Local Bubble, and the absorbed emission from the Galactic halo and the extragalactic background. We find the Local Bubble emission is consistent with emission from a plasma in collisional ionization equilibrium with a temperature $\log T_{LB} = 6.06^{+0.02}_{-0.04}$ and an emission measure of 0.018 cm^{-6} pc. Our measured temperature is in good agreement with values obtained from ROSAT All-Sky Survey data, but is lower than that measured by other recent XMM-Newton observations of the Local Bubble, which find $\log T_{LB} \approx 6.2$ (although for some of these observations it is possible that the foreground emission is contaminated by non-Local Bubble emission from Loop I). The higher temperature observed towards other directions is inconsistent with our data, when combined with a FUSE measurement of the Galactic halo O VI intensity. This therefore suggests that the Local Bubble is thermally anisotropic. Our data are unable to rule out a non-equilibrium model in which the plasma is underionized. However, an overionized recombining plasma model, while observationally acceptable for certain densities and temperatures, generally gives an implausibly young age for the Local Bubble ($\la 6 \times 10^5$ yr)., Comment: Accepted for publication in the Astrophysical Journal. 16 pages, 9 figures

We test the X-ray emission predictions of galactic fountain models against XMM-Newton measurements of the emission from the Milky Way's hot halo. These measurements are from 110 sight lines, spanning the full range of Galactic longitudes. We find that a magnetohydrodynamical simulation of a supernova-driven interstellar medium, which features a flow of hot gas from the disk to the halo, reproduces the temperature but significantly underpredicts the 0.5-2.0 keV surface brightness of the halo (by two orders of magnitude, if we compare the median predicted and observed values). This is true for versions of the model with and without an interstellar magnetic field. We consider different reasons for the discrepancy between the model predictions and the observations. We find taking into account overionization in cooled halo plasma, which could in principle boost the predicted X-ray emission, is unlikely in practice to bring the predictions in line with the observations. We also find that including thermal conduction, which would tend to increase the surface brightnesses of interfaces between hot and cold gas, would not overcome the surface brightness shortfall. However, charge exchange emission from such interfaces, not included in the current model, may be significant. The faintness of the model may also be due to the lack of cosmic ray driving, meaning that the model may underestimate the amount of material transported from the disk to halo. In addition, an extended hot halo of accreted material may be important, by supplying hot electrons that could boost the emission of the material driven out from the disk. Additional model predictions are needed to test the relative importance of these processes in explaining the observed halo emission., 11 pages, 3 figures. Accepted for publication in the Astrophysical Journal

Observations of interstellar clouds that cast shadows in the soft X-ray background can be used to separate the background Galactic halo emission from the local emission due to solar wind charge exchange (SWCX) and/or the Local Bubble (LB). We present an XMM-Newton observation of a shadowing cloud, G225.60-66.40, that is sufficiently compact that the on- and off-shadow spectra can be extracted from a single field of view (unlike previous shadowing observations of the halo with CCD-resolution spectrometers, which consisted of separate on- and off-shadow pointings). We analyzed the spectra using a variety of foreground models: one representing LB emission, and two representing SWCX emission. We found that the resulting halo model parameters (temperature $T_h \approx 2 \times 10^6$ K, emission measure $E_h \approx 4 \times 10^{-3}$ cm$^{-6}$ pc) were not sensitive to the foreground model used. This is likely due to the relative faintness of the foreground emission in this observation. However, the data do favor the existence of a foreground. The halo parameters derived from this observation are in good agreement with those from previous shadowing observations, and from an XMM-Newton survey of the Galactic halo emission. This supports the conclusion that the latter results are not subject to systematic errors, and can confidently be used to test models of the halo emission., 10 pages, 6 figures. Accepted for publication in the Astrophysical Journal

We model SNRs at a variety of heights above the disk. Our detailed numerical simulations include non-equilibrium ionization and recombination and follow the remnants' evolution until their hot bubbles have cooled. We analytically calculate the bubbles' buoyant acceleration. From the results, we estimate the time and space average O VI, N V, and C IV column densities and emission intensities, 1/4 keV soft X-ray surface brightness, area coverage, and volume occupation of the population of isolated SNRs above the Galaxy's HI layer. Irrespective of assumed supernova explosion energy, ambient nonthermal pressure, or frictional drag coefficient, the predicted O VI column density matches the observed distribution between 130 pc and 2000 pc. The SNRs' O VI intensity is a significant fraction of the average observed intensity. Within the range of uncertainty in the SN rate, such SNRs can explain all of the observed 1/4 keV surface brightness attributed to the extraplanar gas beyond the H I layer in the southern hemisphere (~400 x 10^-6 counts/s/arcmin^2). Thus, extraplanar SNRs could be the most important source of hot gas between the Local Bubble and z ~ 2000 pc in the relatively quiescent southern hemisphere. These results stand whether the remnants are assumed to be buoyant or not. The population of old extraplanar SNRs should cover most of the high latitude sky, but bright young extraplanar SNRs should cover less than 1% of the sky. Perhaps the l=247, b=-64 crescent in the 1/4 keV X-ray maps could be a young remnant., Accepted by ApJ for publication on February 10, 2006 (Volume 638). 44 pages, including 6 figures, 10 tables (and 1 blank page)

As with other mixed morphology remnants, W44's projected center is bright in thermal X-rays. It has an obvious radio shell, but no discernable X-ray shell. X-ray bright knots dot W44's image. The Chandra data show that the remnant's hot, bright projected center is metal-rich and that the bright knots are regions of comparatively elevated elemental abundances. The neon abundance is elevated, suggesting that the center is rich in ejecta. Furthermore, some of the emitting iron atoms appear to be underionized with respect to the other ions, providing the first X-ray evidence for dust destruction in a supernova remnant. We use the Chandra data to test the following explanations for W44's X-ray bright center: 1.) entropy mixing from thermal conduction or bulk mixing, 2.) cloud evaporation, and 3.) a metallicity gradient, possibly due to dust destruction and ejecta enrichment. In these tests, we assume that the remnant has evolved beyond the adiabatic evolutionary stage, which explains the X-ray dimness of the shell. The entropy mixed model spectrum was found to be a good match to the Chandra spectrum. The bright knots have similar levels of ionization as the surrounding regions, challenging the evaporating clouds model. While both of these models are known to predict centrally bright X-ray morphologies, their predictions fall short of the observed brightness gradient. The resulting brightness gap can be largely filled in by emission from the extra metals in and near the remnant's projected center. The preponderance of evidence suggests that W44's remarkable morphology can be attributed to dust destruction and ejecta enrichment within an entropy mixed, adiabatic phase supernova remnant., Accepted for publication in the August 20, 2004 issue of ApJ. 22 pages (in well spaced layout), references, captions, and figures

Recent X-ray studies have shown that supernova shock models are unable to satisfactorily explain X-ray emission in the rim of the Cygnus Loop. In an attempt to account for this anomalously enhanced X-ray flux, we fit the region with a model including theoretical charge exchange (CX) data along with shock and background X-ray models. The model includes the CX collisions of $O^{8+}$, $O{7+}$, $N^{7+}$, $N^{6+}$, $C^{6+}$, and $C^{5+}$ with H with an energy of 1 keV/u (438 km/s). The observations reveal a strong emission feature near 0.7 keV that cannot fully be accounted for by a shock model, nor the current CX data. Inclusion of CX, specifically $O^{7+} + H$, does provide for a statistically significant improvement over a pure shock model., Accepted for publication in ApJL, 5 pages, 4 figures, 1 table

We report the first Far Ultraviolet Spectroscopic Explorer (FUSE) measurements of diffuse O VI (lambda,lambda 1032,1038) emission from the general diffuse interstellar medium outside of supernova remnants or superbubbles. We observed a 30arcsec x 30arcsec region of the sky centered at l = 315 and b = -41. From the observed intensities (2930+/-290(random)+/-410(systematic) and 1790+/-260(random)+/-250(systematic) photons/cm/cm/s/sr in the 1032 and 1038 Angstrom emission lines, respectively), derived equations, and assumptions about the source location, we calculate the intrinsic intensity, electron density, thermal pressure, and emitting depth. The intensities are too large for the emission to originate solely in the Local Bubble. Thus, we conclude that the Galactic thick disk and lower halo also contribute. High velocity clouds are ruled out because there are none near the pointing direction. The calculated emitting depth is small, indicating that the O VI-bearing gas fills a small volume. The observations can also be used to estimate the cooling rate of the hot interstellar medium and constrain models. The data also yield the first intensity measurement of the C II 3s2 S1/2 to 2p2 P3/2 emission line at 1037 Angstroms and place upper limits on the intensities of ultraviolet line emission from C I, C III, Si II, S III, S IV, S VI, and Fe III., 30 pages including 5 figures and 5 tables; submitted to ApJ, October 2000; accepted by ApJ, May 2001

We report a Far Ultraviolet Spectroscopic Explorer satellite observation of the supernova remnant N49 in the Large Magellanic Cloud, covering the 905 -- 1187 A spectral region. A 30'' square aperture was used, resulting in a velocity resolution of ~100 km/s. The purpose of the observation was to examine several bright emission lines expected from earlier work and to demonstrate diffuse source sensitivity by searching for faint lines never seen previously in extragalactic supernova remnant UV spectra. Both goals were accomplished. Strong emission lines of O VI 1031.9 A, 1037.6 A and C III 977.0 A were seen, Doppler broadened to +/- 225 km/s and with centroids red-shifted to 350 km/s, consistent with the LMC. Superimposed on the emission lines are absorptions by C III and O VI 1031.9 at +260 km/s, which are attributed to warm and hot gas (respectively) in the LMC. The O VI 1037.6 A line is more severely affected by overlying interstellar and H2 absorption from both the LMC and our galaxy. N III 989.8 A is not seen, but models indicate overlying absorption severely attenuates this line. A number of faint lines from hot gas have also been detected, many of which have never been seen in an extragalactic supernova remnant spectrum., Comment: Part of FUSE Special Issue of ApJ Letters; 8 pages, including 4 figures and one table

This paper describes the X-ray emission from supernova remnants evolving in warm, low density, nonthermal pressure dominated regions. Non-equilibrium ionization hydrocode simulations are used to predict the high resolution spectra, ROSAT PSPC countrates, spatial appearance, color temperatures, and the O VII and O VIII emission line fluxes as a function of time. For comparisons with observations, this paper also applies the standard observational analyses for determining the color temperature, electron density, and thermal pressure to ROSAT ``observations'' of one of the simulated remnants, thus providing a map between observational results and physical conditions. The simulated remnants' C IV, N V, and O VI column densities are also reported. The simulations are applied to studies of the Galactic halo and Local Bubble and additionally may be of interest to studies of external galaxies and Galactic interarm regions., scheduled for the August 10, 1999 issue of ApJ, 29 pages and 26 figures

We present an all-sky catalog of diffuse O VII and O VIII line intensities, extracted from archival XMM observations. The O VII and O VIII intensities are typically ~2-11 and 10 LU were observed. We compared our measurements with SWCX models. The heliospheric SWCX intensity is expected to vary with ecliptic latitude and solar cycle. We found that the observed oxygen intensities generally decrease from solar maximum to solar minimum, both at high ecliptic latitudes (as expected) and at low ecliptic latitudes (not as expected). The geocoronal SWCX intensity is expected to depend on the solar wind proton flux and on the sightline's path through the magnetosheath. The intensity variations seen in directions that have been observed multiple times are in poor agreement with the predictions of a geocoronal SWCX model. The oxygen lines account for ~40-50% of the 3/4 keV X-ray background that is not due to unresolved AGN, in good agreement with a previous measurement. However, this fraction is not easily explained by a combination of SWCX emission and emission from hot plasma in the halo. The line intensities tend to increase with longitude toward the inner Galaxy, possibly due to an increase in the supernova rate in that direction or the presence of a halo of accreted material centered on the Galactic Center. The variation of intensity with Galactic latitude differs in different octants of the sky, and cannot be explained by a single simple plane-parallel or constant-intensity halo model. (Abridged), Comment: Accepted for publication in the Astrophysical Journal Supplement Series. 29 pages (main body of paper) plus 85 pages (full versions of Tables 1, 2, and 4 - these tables will be published as machine-readable tables in the journal, and appear in abbreviated form in the main body of the paper). 12 figures. v2: Minor corrections, conclusions unaltered

In order to determine if the material ablated from high-velocity clouds (HVCs) is a significant source of low-velocity high ions (C IV, N V, and O VI) such as those found in the Galactic halo, we simulate the hydrodynamics of the gas and the time-dependent ionization evolution of its carbon, nitrogen, and oxygen ions. Our suite of simulations examines the ablation of warm material from clouds of various sizes, densities, and velocities as they pass through the hot Galactic halo. The ablated material mixes with the environmental gas, producing an intermediate-temperature mixture that is rich in high ions and that slows to the speed of the surrounding gas. We find that the slow mixed material is a significant source of the low-velocity O VI that is observed in the halo, as it can account for at least ~1/3 of the observed O VI column density. Hence, any complete model of the high ions in the halo should include the contribution to the O VI from ablated HVC material. However, such material is unlikely to be a major source of the observed C IV, presumably because the observed C IV is affected by photoionization, which our models do not include. We discuss a composite model that includes contributions from HVCs, supernova remnants, a cooling Galactic fountain, and photoionization by an external radiation field. By design, this model matches the observed O VI column density. This model can also account for most or all of the observed C IV, but only half of the observed N V., Comment: 17 pages, 8 figures. Accepted for publication in the Astrophysical Journal

With the goal of understanding why X-rays have been reported near some high velocity clouds, we perform detailed 3 dimensional hydrodynamic and magnetohydrodynamic simulations of clouds interacting with environmental gas like that in the Galaxy's thick disk/halo or the Magellanic Stream. We examine 2 scenarios. In the first, clouds travel fast enough to shock-heat warm environmental gas. In this scenario, the X-ray productivity depends strongly on the speed of the cloud and the radiative cooling rate. In order to shock-heat environmental gas to temperatures of > or = 10^6 K, cloud speeds of > or = 300 km/s are required. If cooling is quenched, then the shock-heated ambient gas is X-ray emissive, producing bright X-rays in the 1/4 keV band and some X-rays in the 3/4 keV band due to O VII and other ions. If, in contrast, the radiative cooling rate is similar to that of collisional ionizational equilibrium plasma with solar abundances, then the shocked gas is only mildly bright and for only about 1 Myr. The predicted count rates for the non-radiative case are bright enough to explain the count rate observed with XMM-Newton toward a Magellanic Stream cloud and some enhancement in the ROSAT 1/4 keV count rate toward Complex C, while the predicted count rates for the fully radiative case are not. In the second scenario, the clouds travel through and mix with hot ambient gas. The mixed zone can contain hot gas, but the hot portion of the mixed gas is not as bright as those from the shock-heating scenario., 15 pages, 9 figures, 1 table. Accepted for publication in the Astrophysical Journal