arXiv subjects

R. L. Shelton

Publications and source records attributed to R. L. Shelton.

17 recordsLinked to original sources

arXiv2022

The Long Tails of the Pegasus-Pisces Arch Intermediate Velocity Cloud

We present hydrodynamic simulations of the Pegasus-Pisces (PP Arch), an intermediate velocity cloud in our Galaxy. The PP Arch, also known as IVC 86-36, is unique among intermediate and high velocity clouds, because its twin tails are unusually long and narrow. Its -50 km/s line-of-sight velocity qualifies it as an intermediate velocity cloud, but the tails' orientations indicate that the cloud's total three-dimensional speed is at least ~100 km/s. This speed is supersonic in the Reynold's Layer and thick disk. We simulated the cloud as it travels supersonically through the Galactic thick and thin disks at an oblique angle relative to the midplane. Our simulated clouds grow long double tails and reasonably reproduce the H I 21~cm intensity and velocity of the head of the PP Arch. A bow shock protects each simulated cloud from excessive shear and lowers its Reynolds number. These factors may similarly protect the PP Arch and enable the survival of its unusually long tails. The simulations predict the future hydrodynamic behavior of the cloud when it collides with denser gas nearer to the Galactic midplane. It appears that the PP Arch's fate is to deform, dissipate, and merge with the Galactic disk.

astro-ph.GA↗

arXiv2017

Line Ratios for Solar Wind Charge Exchange with Comets

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.

astro-ph.HE↗

arXiv2011

An X-ray Investigation of Three Supernova Remnants in the Large Magellanic Cloud

We have investigated three SNRs in the LMC using multi-wavelength data. These SNRs are generally fainter than the known sample and may represent a previously missed population. One of our SNRs is the second LMC remnant analyzed which is larger than any Galactic remnant for which a definite size has been established. The analysis of such a large remnant contributes to the understanding of the population of highly evolved SNRs. We have obtained X-ray images and spectra of three of these recently identified SNRs using the XMM-Newton observatory. These data, in conjunction with pre-existing optical emission-line images and spectra, were used to determine the physical conditions of the optical- and X-ray-emitting gas in the SNRs. We have compared the morphologies of the SNRs in the different wavebands. The physical properties of the warm ionized shell were determined from the H-alpha surface brightness and the SNR expansion velocity. The X-ray spectra were fit with a thermal plasma model and the physical conditions of the hot gas were derived from the model fits. Finally, we have compared our observations with simulations of SNR evolution.

astro-ph.CO↗

arXiv2010

Hot Gas in the Galactic Thick Disk and Halo Near the Draco Cloud

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.

astro-ph.GA↗

arXiv2007

A Study of the Vertical Motion of Supernova Remnant Bubbles in the Interstellar Medium Drawn from the Results of Three-Dimensional MHD Simulations

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.

astro-ph↗

arXiv2005

Simulations of Supernova Remnants in Diffuse Media III. The Population of Buoyant Remnants Above the Milky Way's Disk

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.

astro-ph↗

arXiv2004

A Survey of OVI Absorption in the Local Interstellar Medium

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).

astro-ph↗

arXiv2004

G65.2+5.7: A Thermal Composite Supernova Remnant With a Cool Shell

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.

astro-ph↗

arXiv2004

Chandra Observations and Models of the Mixed-Morphology Supernova Remnant W44: Global Trends

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.

astro-ph↗

arXiv2003

Surprisingly Little O VI Emission Arises in the Local Bubble

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.

astro-ph↗

arXiv2002

The X-ray Structure and Spectrum of the Pulsar Wind Nebula Surrounding PSR B1853+01 in W44

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.

astro-ph↗

arXiv2001

Upper limits on O VI Emission From Voyager

We have examined 426 {\it Voyager} fields distributed across the sky for \ion{O}{6} ($λλ$ 1032/1038 Å) 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.

astro-ph↗

arXiv2001

Observations of O VI Emission from the Diffuse Interstellar Medium

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.

astro-ph↗

arXiv2000

FUSE Observations of an X-Ray Bright Region in the Vela Supernova Remnant

We present the results of a FUSE observation of an X-ray selected knot in the Vela supernova remnant. Spectra were obtained through the 30"x30" low resolution aperture and the 4"x20" medium resolution aperture. O VI 1032,1038 and C III 977 are detected strongly in both spectra, and S VI 933,944 is detected weakly only in the larger aperture spectrum. We also report the first detection of C II 1037 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 and another with only low excitation emission centered at a velocity of +100 km/s. We associate the -50 km/s component with the observed X-ray knot, and find a dynamical pressure of 3.7x10^{-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 which can be associated with these two components whose ultraviolet emission differs dramatically. These observations demonstrate the importance of high spectral resolution in understanding the proper physical relationships between the various emitting components in supernova remnants.

astro-ph↗

arXiv2000

The Ionization of the Local Interstellar Medium, as Revealed by FUSE Observations of N, O and Ar toward White Dwarf Stars

FUSE 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 EUV flux from nearby stars and hot gases, rather than an incomplete recovery from a past, more highly ionized condition.

astro-ph↗

arXiv1999

Simulations of Supernova Remnants in Diffuse Media. II. Three Remnants and Their X-Ray Emission

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.

astro-ph↗

arXiv1998

Modeling W44 as a Supernova Remnant in a Density Gradient, with a Partially Formed Dense Shell and Thermal Conduction in the Hot Interior

(shortened version) 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 years, consistent with the age of the associated pulsar, and a radius of 11 to 13 pc. Over most of the outer surface, radiative cooling has become important in the post shock 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_sun, 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. 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 demonstrate a simple technique for estimating the optical emission expected. These tools are employed to choose parameters of models which we then explore with our 1d and 2d hydrocodes, providing, respectively, the detailed x-ray spectra and dynamical characteristics.

astro-ph↗