arXiv subjects

J. C. Howk

Publications and source records attributed to J. C. Howk.

At least 37 records · Page 2Linked to original sources

arXiv2007

Simulating Anisotropic Thermal Conduction in Supernova Remnants I : Numerics and the Evolution of Remnants

Anisotropic thermal conduction plays an important role in various astrophysical systems. One of the most stringent tests of thermal conduction can be found in supernova remnants. In this paper we study anisotropic thermal conduction and examine the physical nature of the flux of thermal conduction in the classical and saturated limits. We also present a temporally second-order accurate implicit-explicit scheme for the time-update of thermal conduction terms within a numerical MHD scheme. Several simulations of supernova remnants are presented for a range of ISM parameters. The role of thermal conduction in such remnants has been studied. We find that thermal conduction produces cooler temperatures and higher densities in the hot gas bubbles that form in the remnants. The effect of thermal conduction in changing the thermal characteristics of the hot gas bubble increases as the remnant propagates through denser ISMs. Remnants evolving in denser ISMs are shown to make a faster transition to a centre-bright x-ray morphology, with the trend emerging earlier in hard x-rays than in the soft x-rays.

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arXiv2007

Simulating Anisotropic Thermal Conduction in Supernova Remnants, Implications for the Interstellar Medium

We present a large number of two and a half dimensional simulations of supernova remnants expanding into interstellar media having a range of densities, temperatures and magnetic field strengths. The volume of hot gas produced is strongly dependent on the inclusion of thermal conduction and magnetic fields. The four-volumes and three-areas of hot gas have been catalogued and their dependence on interstellar parameters documented. Simulated line widths of radioactive species ejected by supernovae have also been catalogued.

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arXiv2007

Distances to Galactic high-velocity clouds. Complex C

We report the first determination of a distance bracket for the high-velocity cloud (HVC) complex C. Combined with previous measurements showing that this cloud has a metallicity of 0.15 times solar, these results provide ample evidence that complex C traces the continuing accretion of intergalactic gas falling onto the Milky Way. Accounting for both neutral and ionized hydrogen as well as He, the distance bracket implies a mass of 3-14x10^6 M_sun, and the complex represents a mass inflow of 0.1-0.25 M_sun/yr. We base our distance bracket on the detection of CaII absorption in the spectrum of the blue horizontal branch star SDSS J120404.78+623345.6, in combination with a significant non-detection toward the BHB star BS 16034-0114. These results set a strong distance bracket of 3.7-11.2 kpc on the distance to complex C. A more weakly supported lower limit of 6.7 kpc may be derived from the spectrum of the BHB star BS 16079-0017.

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arXiv2007

Distances to Galactic high-velocity clouds. I. Cohen Stream, complex GCP, cloud g1

The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are tracers of energetic processes in and around the Milky Way. Clouds with near-solar metallicity about one kpc above the disk trace the circulation of material between disk and halo (the Galactic Fountain). The Magellanic Stream consists of gas tidally extracted from the SMC, tracing the dark matter potential of the Milky Way. Several other HVCs have low-metallicity and appear to trace the continuing accretion of infalling intergalactic gas. These assertions are supported by the metallicities (0.1 to 1 solar) measured for about ten clouds in the past decade. Direct measurements of distances to HVCs have remained elusive, however. In this paper we present four new distance brackets, using VLT observations of interstellar \CaII H and K absorption toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7 kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8 to 15.1 kpc for complex GCP (also known as the Smith Cloud or HVC40-15+100 and with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated with the return flow of the Fountain in the Perseus Arm, and 1.8 to 3.8 kpc for cloud g1, which appears to be in the outflow phase of the Fountain. Our measurements further demonstrate that the Milky Way is accreting substantial amounts of gaseous material, which influences the Galaxy's current and future dynamical and chemical evolution.

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arXiv2007

Strongly Variable z=1.48 FeII and MgII Absorption in the Spectra of z=4.05 GRB 060206

We report on the discovery of strongly variable FeII and MgII absorption lines seen at z=1.48 in the spectra of the z=4.05 GRB 060206 obtained between 4.13 to 7.63 hours (observer frame) after the burst. In particular, the FeII line equivalent width (EW) decayed rapidly from 1.72+-0.25 AA to 0.28+-0.21 AA, only to increase to 0.96+-0.21 AA in a later date spectrum. The MgII doublet shows even more complicated evolution: the weaker line of the doublet drops from 2.05+-0.25 AA to 0.92+-0.32 AA, but then more than doubles to 2.47+-0.41 AA in later data. The ratio of the EWs for the MgII doublet is also variable, being closer to 1:1 (saturated regime) when the lines are stronger and becoming closer to 2:1 (unsaturated regime) when the lines are weaker, consistent with expectations based on atomic physics. We have investigated and rejected the possibility of any instrumental or atmospheric effects causing the observed strong variations. Our discovery of clearly variable intervening FeII and MgII lines lends very strong support to their scenario, in which the characteristic size of intervening patches of MgII ``clouds'' is comparable to the GRB beam size, i.e, about 10^16 cm. We discuss various implications of this discovery, including the nature of the MgII absorbers, the physics of GRBs, and measurements of chemical abundances from GRB and quasar absorption lines.

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arXiv2007

Highly ionised plasma in the Large Magellanic Cloud: Evidence for outflows and a possible galactic wind

Based on an analysis of the interstellar highly ionised species C IV, Si IV, N V, and O VI observed in the FUSE and HST/STIS E140M spectra of four hot stars in the Large Magellanic Cloud (LMC), we find evidence for a hot LMC halo fed by energetic outflows from the LMC disk and even possibly an LMC galactic wind. Signatures for such outflows are the intermediate and high-velocity components (v_LSR>100 km/s) relative to the LMC disk observed in the high- and low-ion absorption profiles. The stellar environments produce strong, narrow (T<20,000 K) components of C IV and Si IV associated with the LMC disk; in particular they are likely signatures of H II regions and expanding shells. Broad components are observed in the profiles of C IV, Si IV, and O VI with their widths implying hot, collisionally ionised gas at temperatures of a few times 100,000 K. There is a striking similarity in the O VI/C IV ratios for the broad LMC and high-velocity components, suggesting much of the material at v_LSR>100 km/s is associated with the LMC. The velocity of the high-velocity component is large enough to escape altogether the LMC, polluting the intergalactic space between the LMC and the Milky Way. The observed high-ion ratios of the broad LMC and high-velocity components are consistent with those produced in conductive interfaces; such models are also favored by the apparent kinematically coupling between the high and the weakly ionised species.

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arXiv2004

Small-Scale Structure of O VI Interstellar Gas in the Direction of the Globular Cluster NGC 6752

In order to study the small-scale structure of the hot interstellar gas, we obtained Far Ultraviolet Spectroscopic Explorer interstellar O VI interstellar absorption spectra of 4 four post-extreme horizontal branch stars in the globular cluster NGC 6752 [(l,b) = (336.50,-25.63), d = 3.9 kpc, z= -1.7 kpc]. The good quality spectra of these stars allow us to measure both lines of the O VI doublet at 1031.926 Åand 1037.617 Å. The close proximity of these stars permits us to probe the hot interstellar gas over angular scale of only $2\farcm2 - 8\farcm9$, corresponding to spatial scales $\la 2.5-10.1$ pc. On these scales we detect no variations in the O VI column density and velocity distribution. The average column density is log = 14.34 \pm 0.02 (log = 13.98). The measured velocity dispersions of the O VI absorption are also indistinguishable. Including the earlier results of Howk et al., this study suggests that interstellar O VI is smooth on scales $Δθ\la 12\arcmin$, corresponding to a spatial scale of less than 10 pc, and quite patchy at larger scales. Although such small scales are only probed in a few directions, this suggests a characteristic size scale for the regions producing collisionally-ionized O VI in the Galaxy.

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arXiv2003

H_2 absorption in a dense interstellar filament in the Milky Way halo

We investigate interstellar absorption from molecular hydrogen (H_2) and metals in an intermediate-velocity cloud (IVC) in the direction of the LMC star Sk -68 80 (HD 36521), based on data from the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. H_2 absorption from the Lyman- and Werner bands is detected in 30 lines at radial velocities v=+50 km/s in this IVC that is presumably located in the Milky Way halo. We obtain a total logarithmic H_2 column density of log N(H_2)=16.6 (0.5) along with a very low Doppler parameter of b=1.5 km/s. The presence of molecular material in this cloud is suprising, given the fact that the OI column density (log N(OI)=14.8 (0.1)) implies a very low neutral gas column density of ~10^{18} cm^-2 (assuming a solar oxygen abundance). If the H_2 column density represents its abundance in a formation-dissociation equilibrium, the data imply that the molecular gas resides in a small, dense filament at a volume density of ~800 cm^-3 and a thickness of only 41 Astronomical Units (AU). The molecular filament possibly corresponds to the tiny-scale atomic structures (TSAS) in the diffuse interstellar medium observed in high-resolution optical data, HI 21cm absorption, and in CO emission.

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arXiv2003

OVI Emission in the Halos of Edge-on Spiral Galaxies

We have used the Far Ultraviolet Spectroscopic Explorer to search for OVI 1031.926, 1037.617 A emission in the halos of the edge-on spiral galaxies NGC4631 and NGC891. In NGC4631, we detected OVI in emission toward a soft X-ray bubble above a region containing numerous Halpha arcs and filaments. The line-of-sight component of the motion of the OVI gas appears to match the underlying disk rotation. The observed OVI luminosities can account for 0.2-2% of the total energy input from supernovae (assuming a full OVI emitting halo) and yield mass flux cooling rates between 0.48 and 2.8 M_sun/yr depending on the model used in the derivations. Based on these findings, we believe it is likely that we are seeing cooling, galactic fountain gas. No emission was detected from the halo of NGC891, a galaxy in a direction with considerably high foreground Galactic extinction.

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arXiv2003

Oxygen Gas Phase Abundance Revisited

We present new measurements of the interstellar gas-phase oxygen abundance along the sight lines towards 19 early-type galactic stars at an average distance of 2.6 kpc. We derive O {\small I} column densities from {\it HST}/STIS observations of the weak 1355 Åintersystem transition. We derive total hydrogen column densities [N(H {\small I})+2N(H$_2$)] using {\it HST}/STIS observations of \lya and {\it FUSE} observations of molecular hydrogen. The molecular hydrogen content of these sight lines ranges from f(H$_2$) = 2N(H$_2$)/[N(H {\small I})+2N(H$_2$)] = 0.03 to 0.47. The average $ $ of 6.3$\times10^{21}$ cm$^{-2}$ mag$^{-1}$ with a standard deviation of 15% is consistent with previous surveys. The mean oxygen abundance along these sight lines, which probe a wide range of galactic environments in the distant ISM, is 10$^6$ \oh = $408 \pm 13$ (1 $σ$ in the mean). %$({\rm O/H})_{gas} = 408 \pm 14$(1 $σ$). We see no evidence for decreasing gas-phase oxygen abundance with increasing molecular hydrogen fraction and the relative constancy of \oh suggests that the component of dust containing the oxygen is not readily destroyed. We estimate that, if 60% of the dust grains are resilient against destruction by shocks, the distant interstellar total oxygen abundance can be reconciliated with the solar value derived from the most recent measurements %by Holweger and by Allende Prieto, Lambert & Asplund: of 10$^6$ \oh$_\odot$ = 517 $\pm$ 58 (1 $σ$). We note that the smaller oxygen abundances derived for the interstellar gas within 500 pc %by Meyer, Cardelli & Jura or from nearby B star surveys are consistent with a local elemental deficit.

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arXiv2002

Highly-Ionized Gas in the Galactic Halo: A FUSE Survey of O VI Absorption toward 22 Halo Stars

Far Ultraviolet Spectroscopic Explorer spectra of 22 Galactic halo stars are studied to determine the amount of O VI in the Galactic halo between ~0.3 and \~10 kpc from the Galactic mid-plane. Strong O VI 1031.93 A absorption was detected toward 21 stars, and a reliable 3 sigma upper limit was obtained toward HD 97991. The weaker member of the O VI doublet at 1037.62 A could be studied toward only six stars. The observed columns are reasonably consistent with a patchy exponential O VI distribution with a mid-plane density of 1.7x10^(-8) cm^(-3) and scale height between 2.3 and 4 kpc. We do not see clear signs of strong high-velocity components in O VI absorption along the Galactic sight lines, which indicates the general absence of high velocity O VI within 2-5 kpc of the Galactic mid-plane. The correlation between the H I and O VI intermediate velocity absorption is also poor. The O VI velocity dispersions are much larger than the value of ~18 km/s expected from thermal broadening for gas at T~300,000 K, the temperature at which O VI is expected to reach its peak abundance in collisional ionization equilibrium. Turbulence, inflow, and outflow must have an effect on the shape of the O VI profiles. Kinematical comparisons of O VI with Ar I suggest the presence of two different types of O VI-bearing environments toward the Galactic sight lines. Comparison of O VI with other highly-ionized species suggests that the high ions are produced primarily by cooling hot gas in the Galactic fountain flow, and that turbulent mixing also has a significant contribution. The role of turbulent mixing is most important toward sight lines that sample supernova remnants like Loop I and IV.

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arXiv2002

An Atlas of Galactic Spectra Observed with the Far Ultraviolet Spectroscopic Explorer

An atlas of far-ultraviolet spectra of 45 Galactic OB stars observed with the Far Ultraviolet Spectroscopic Explorer is presented. The atlas covers the wavelength region between 912 and 1185A with an effective spectral resolution of 0.12A. Systematic trends in the morphology and strength of stellar features are discussed. Particular attention is drawn to the variations of the CIII$λ$1176, SIV$λλ$1063, 1073, and PV$λλ$1118, 1128 line profiles as a function of temperature and luminosity class; and the lack of a luminosity dependence associated with OVI$λλ$1032, 1038. Numerous interstellar lines are also identified.

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arXiv2002

Abundances of Deuterium, Nitrogen, and Oxygen toward HZ 43A: Results from the FUSE Mission

We present an analysis of interstellar absorption along the line of sight to the nearby white dwarf star HZ43A. The distance to this star is 68+/-13 pc, and the line of sight extends toward the north Galactic pole. Column densities of OI, NI, and NII were derived from spectra obtained by the Far Ultraviolet Spectroscopic Explorer (FUSE), the column density of DI was derived from a combination of our FUSE spectra and an archival HST GHRS spectrum, and the column density of HI was derived from a combination of the GHRS spectrum and values derived from EUVE data obtained from the literature. We find the following abundance ratios (with 2-sigma uncertainties): DI/HI = (1.66 +/- 0.28) x 10^-5, OI/HI = (3.63 +/- 0.84) x 10^-4, and NI/HI = (3.80 +/- 0.74) x 10^-5. The NII column density was slightly greater than that of NI, indicating that ionization corrections are important when deriving nitrogen abundances. Other interstellar species detected along the line of sight were CII, CIII, OVI, SiII, ArI, MgII, and FeII; an upper limit was determined for NIII. No elements other than HI were detected in the stellar photosphere.

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arXiv2002

Interstellar Deuterium, Nitrogen, and Oxygen Abundances Toward BD +28 4211: Results from the Far Ultraviolet Spectroscopic Explorer

High resolution far-ultraviolet spectra of the O-type subdwarf BD +28 4211 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to measure the interstellar deuterium, nitrogen, and oxygen abundances in this direction. The interstellar D I transitions are analyzed down to Lyman iota at 920.7 A. The star was observed several times at different target offsets in the direction of spectral dispersion. The aligned and coadded spectra have high signal-to-noise ratios (S/N =50-100). D I, N I, and O I transitions were analyzed with curve-of-growth and profile fitting techniques. A model of interstellar molecular hydrogen on the line of sight was derived from H2 lines in the FUSE spectra and used to help analyze some features where blending with H2 was significant. The H I column density was determined from high resolution HST/STIS spectra of Lyman alpha to be log(N HI) = 19.846+/-0.035 (2 sigma), which is higher than is typical for sight lines in the local ISM studied for D/H. We found that D/H =(1.39+/-0.21) E-5 (2 sigma) and O/H = (2.37+/-0.55) E-4 (2 sigma). O/H toward BD +28 4211 appears to be significantly below the mean O/H ratio for the ISM and the Local Bubble.

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arXiv2001

Abundances of Deuterium, Oxygen, and Nitrogen in the Local Interstellar Medium: Overview of First Results from the Far Ultraviolet Spectroscopic Explorer Mission

Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D I, O I, and N I along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 pc to 179 pc. Five of the sight lines are within the Local Bubble and two penetrate the surrounding H I wall. Reliable values of N(H I) were determined for five of the sight lines from HST data, IUE data, and published EUVE measurements. The weighted mean of D I/H I for these five sight lines is (1.52 +/- 0.08) x10-5 (1 sigma uncertainty in the mean). It is likely that the D I/H I ratio in the Local Bubble has a single value. The D I/O I ratio for the five sight lines within the Local Bubble is (3.76 +/- 0.20) x10-2. It is likely that the O I column densities can serve as a proxy for H I in the Local Bubble. The weighted mean for O I/H I for the seven FUSE sight lines is (3.03 +/-0.21) x10-4, comparable to the weighted mean (3.43 +/- 0.15) x10-4 reported for 13 sight lines probing larger distances and higher column densities (Meyer et al. 1998, Meyer 2001). The FUSE weighted mean of N I/H I for the five sight lines is half that reported by Meyer et al. (1997) for seven sight lines with larger distances and higher column densities. This result combined with the variability of O I/N I (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O I, N I cannot be used as a proxy for H I or as a metallicity indicator in the LISM. Subject Headings: cosmology: observations- ISM: abundances- ISM: evolution - Galaxy:abundances-Ultraviolet:ISM

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arXiv2001

Deuterium abundance toward G191-B2B: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission

High-resolution spectra of the hot white dwarf G191-B2B, covering the wavelength region 905-1187A, were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). This data was used in conjunction with existing high-resolution Hubble Space Telescope STIS observations to evaluate the total HI, DI, OI and NI column densities along the line of sight. Previous determinations of N(DI) based upon GHRS and STIS observations were controversial due to the saturated strength of the DI Lyman-alpha line. In the present analysis the column density of DI has been measured using only the unsaturated Lyman-beta and Lyman-gamma lines observed by FUSE. A careful inspection of possible systematic uncertainties tied to the modeling of the stellar continuum or to the uncertainties in the FUSE instrumental characteristics has been performed. The column densities derived are: log N(DI) = 13.40 +/-0.07, log N(OI) = 14.86 +/-0.07, and log N(NI) = 13.87 +/-0.07 quoted with 2-sigma uncertainties. The measurement of the HI column density by profile fitting of the Lyman-alpha line has been found to be unsecure. If additional weak hot interstellar components are added to the three detected clouds along the line of sight, the HI column density can be reduced quite significantly, even though the signal-to-noise ratio and spectral resolution at Lyman-alpha are excellent. The new estimate of N(HI) toward G191-B2B reads: log N(HI) = 18.18 +/-0.18 (2-sigma uncertainty), so that the average (D/H) ratio on the line of sight is: (D/H) = 1.66 (+0.9/-0.6) *10^-5 (2-sigma uncertainty).

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arXiv2001

Deuterium and Oxygen Toward Feige 110: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission

We present measurements of the column densities of interstellar DI and OI made with the Far Ultraviolet Spectroscopic Explorer (FUSE), and of HI made with the International Ultraviolet Explorer (IUE) toward the sdOB star Feige 110 [(l,b) = (74.09, -59.07); d = 179 +265 -67 pc; z = -154 +57 -227 pc]. Our determination of the DI column density made use of curve of growth fitting and profile fitting analyses, while our OI column density determination used only curve of growth techniques. The HI column density was estimated by fitting the damping wings of the interstellar Ly alpha profile. We find log N(DI) = 15.47 +/- 0.06, log N(OI) = 16.73 +/- 0.10, and log N(HI) = 20.14 +0.13 -0.20 (all errors 2 sigma). This implies D/H = (2.14 +/- 0.82) x 10^-5, D/O = (5.50 +1.64 -1.33) x 10^-2, and O/H = (3.89 +/- 1.67) x 10^-4. Taken with the FUSE results reported in companion papers (Moos et al. 2001) and previous measurements of the local interstellar medium, this suggests the possibility of spatial variability in D/H for sight lines exceeding ~100 pc. This result may constrain models which characterize the mixing time and length scales of material in the local interstellar medium.

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arXiv2001

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

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

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