A high-order nonconservative approach for hyperbolic equations in fluid dynamics

It is well known, thanks to Lax-Wendroff theorem, that the local conservation of a numerical scheme for a conservative hyperbolic system is a simple and systematic way to guarantee that, if stable, a scheme will provide a sequence of solutions that will converge to a weak solution of the continuous problem. In [1], it is shown that a nonconservative scheme will not provide a good solution. The question of using, nevertheless, a nonconservative formulation of the system and getting the correct solution has been a long-standing debate. In this paper, we show how get a relevant weak solution from a pressure-based formulation of the Euler equations of fluid mechanics. This is useful when dealing with nonlinear equations of state because it is easier to compute the internal energy from the pressure than the opposite. This makes it possible to get oscillation free solutions, contrarily to classical conservative methods. An extension to multiphase flows is also discussed, as well as a multidimensional extension

Interstellar H2 toward HD 37903

We present an analysis of interstellar H2 toward HD 37903, which is a hot, B 1.5 V star located in the NGC 2023 reflection nebula. Meyer et al. (2001) have used a rich spectrum of vibrationally excited H2 observed by the HST to calculate a model of the interstellar cloud toward HD 37903. We extend Mayer's analysis by including the v"=0 vibrational level observed by the FUSE satellite. The T01 temperature should not be interpreted as a rotational temperature, but rather as a temperature of thermal equilibrium between the ortho and para H2. The ortho to para H2 ratio is lower for collisionally populated levels than for the levels populated by fluorescence. The PDR model of the cloud located in front of HD 37903 points to a gas temperature Tkin=110-377 K, hydrogen density nH=1874-544 cm^-3 and the star-cloud distance of 0.45 pc

A FUSE survey of high-latitude Galactic molecular hydrogen

Measurements of molecular hydrogen (H_2) column densities are presented for the first six rotational levels (J=0 to 5) for 73 extragalactic targets observed with FUSE. All of these have a final signal-to-noise ratio larger than \snlimit, and are located at galactic latitude |b|>20 deg. The individual observations were calibrated with the FUSE calibration pipeline CalFUSE version 2.1 or higher, and then carefully aligned in velocity. The final velocity shifts for all the FUSE segments are listed. H_2 column densities or limits are determined for the 6 lowest rotational (J) levels for each HI component in the line of sight, using a curve-of-growth approach at low column densities ~16.5), and Voigt-profile fitting at higher column densities. Detections include 73 measurements of low-velocity H_2 in the Galactic Disk and lower Halo. Eight sightlines yield non-detections for Galactic H_2. The measured column densities range from log N(H_2)=14 to log N(H_2)=20. Strong correlations are found between log N(H_2) and T_01, the excitation temperature of the H_2, as well as between log N(H_2) and the level population ratios (log (N(J')/N(J))). The average fraction of nuclei in molecular hydrogen (f(H_2)) in each sightline is calculated; however, because there are many HI clouds in each sightline, the physics of the transition from HI to H_2 can not be studied. Detections also include H2 in 16 intermediate-velocity clouds in the Galactic Halo (out of 35 IVCs). Molecular hydrogen is seen in one high-velocity cloud (the Leading Arm of the Magellanic Stream), although 19 high-velocity clouds are intersected; this strongly suggests that dust is rare or absent in these objects. Finally, there are five detections of H_2 in external galaxies.Comment: Accepted for ApJ Supplement. Note: figs 7 and 8 not included because astro-ph rejects them as too bi

Low Redshift Intergalactic Absorption Lines in the Spectrum of HE0226-4110

We present an analysis of the FUSE and STIS E140M spectra of HE0226-4110 (z=0.495). We detect 56 Lyman absorbers and 5 O VI absorbers. The number of intervening O VI systems per unit redshift with W>50 m\AA is dN(O VI)/dz~ 11. The O VI systems unambiguously trace hot gas only in one case. For the 4 other O VI systems, photoionization and collisional ionization models are viable options to explain the observed column densities of the O VI and the other ions. If the O VI systems are mostly photoionized, only a fraction of the observed O VI will contribute to the baryonic density of the warm-hot ionized medium (WHIM) along this line of sight. Combining our results with previous ones, we show that there is a general increase of N(O VI) with increasing b(O VI). Cooling flow models can reproduce the N-b distribution but fail to reproduce the observed ionic ratios. A comparison of the number of O I, O II, O III, O IV, and O VI systems per unit redshift show that the low-z IGM is more highly ionized than weakly ionized. We confirm that photoionized O VI systems show a decreasing ionization parameter with increasing H I column density. O VI absorbers with collisional ionization/photoionization degeneracy follow this relation, possibly suggesting that they are principally photoionized. We find that the photoionized O VI systems in the low redshift IGM have a median abundance of 0.3 solar. We do not find additional Ne VIII systems other than the one found by Savage et al., although our sensitivity should have allowed the detection of Ne VIII in O VI systems at T~(0.6-1.3)x10^6 K (if CIE applies). Since the bulk of the WHIM is believed to be at temperatures T>10^6 K, the hot part of the WHIM remains to be discovered with FUV--EUV metal-line transitions.Comment: Accepted for publication in the ApJS. Full resolution figures available at http://www.journals.uchicago.edu/ApJ/journal/preprints/ApJS63975.preprint.pd

Physical Properties and Baryonic Content of Low-Redshift Intergalactic Ly-alpha and O VI Absorption Systems: The PG1116+215 Sight Line

We present HST and FUSE observations of the intergalactic absorption toward PG1116+215 in the 900-3000 A spectral region. We detect 25 Ly-alpha absorbers at rest-frame equivalent widths W_r > 30 mA, yielding (dN/dz)_Ly-alpha = 154+/-18 over an unblocked redshift path of 0.162. Two additional weak Ly-alpha absorbers with W_r ~ 15-20 mA are also present. Eight of the Ly-alpha absorbers have large line widths (b > 40 km/sec). The detection of narrow OVI in the broad Ly-alpha absorber at z=0.06244 supports the idea that the Ly-alpha profile is thermally broadened in gas with T > 10^5 K. We find dN/dz ~ 50 for broad Ly-alpha absorbers with W_r > 30 mA and b > 40 km/sec. If the broad Ly-alpha lines are dominated by thermal broadening in hot gas, the amount of baryonic material in these absorbers is enormous, perhaps as much as half the baryonic mass in the low-redshift universe. We detect OVI absorption in several of the Ly-alpha clouds along the sight line. Two detections at z=0.13847 and z=0.16548 are confirmed by the presence of other ions at these redshifts, while the detections at z=0.04125, 0.05895, 0.05928, and 0.06244 are based upon the Ly-alpha and OVI detections alone. The information available for 13 low-redshift OVI absorbers with W_r > 50 mA along 5 sight lines yields (dN/dz)_OVI ~ 14 and Omega_b(OVI) > 0.0027/h_75, assuming a metallicity of 0.1 solar and an OVI ionization fraction < 0.2. The properties and prevalence of low-redshift OVI absorbers suggest that they too may be a substantial baryon repository, perhaps containing as much mass as stars and gas inside galaxies. The redshifts of the OVI absorbers are highly correlated with the redshifts of galaxies along the sight line, though few of the absorbers lie closer than 600/h_75 kpc to any single galaxy. [abbreviated]Comment: 99 pages, 30 figures, aastex format, ApJS in pres

The Radiative Feedback of the First Cosmological Objects

In hierarchical models of structure formation, an early cosmic UV background (UVB) is produced by the small (T_vir < 10^4 K) halos that collapse before reionization. The UVB at energies below 13.6eV suppresses the formation of stars or black holes inside small halos, by photo-dissociating their only cooling agent, molecular H2. We self-consistently compute the buildup of the early UVB in Press-Schechter models, coupled with H2 photo-dissociation both in the intergalactic medium (IGM), and inside virialized halos. We find that the intergalactic H2 has a negligible effect on the UVB, both because its initial optical depth is small (tau<0.1), and because it is photo-dissociated at an early stage. If the UV sources in the first collapsed halos are stars, then their UV flux suppresses further star-formation inside small halos. This results in a pause in the buildup of the UVB, and reionization is delayed until larger halos (T_vir> 10^4 K) collapse. If the small halos host mini-quasars with hard spectra extending to approximately 1 keV, then their X-rays balance the effects of the UVB, the negative feedback does not occur, and reionization can be caused by the small halos.Comment: 16 pages, 16 figures included, uses emulateapj.sty. Submitted to Ap

A new constraint on cosmological variability of the proton-to-electron mass ratio

Exotic cosmologies predict variability of the fundamental physical constants over the cosmic time. Using the VLT/UVES high resolution spectra of the quasar Q0347-3819 and unblended electronic - vibrational - rotational lines of the H2 molecule identified at z = 3.025 we test possible changes in the proton - to - electron mass ratio mu_0 = m_p/m_e over the period of 11 Gyr. We obtained a new constraint on the time - averaged variation rate of mu_0 of |d mu /d t /mu_0| < 5 10^{-15} yr^{-1} (1 sigma c.l.). The estimated 1 sigma uncertainty interval of the |Delta mu/mu_0| ratio of about 0.004% implies that since the time when the H2 spectrum was formed at z = 3.025, mu_0 has not changed by more than a few thousands of a percent.Comment: 5 pages, 3 figures, a revised version accepted by MNRA

Thermal stability of cold clouds in galaxy halos

We consider the thermal properties of cold, dense clouds of molecular hydrogen and atomic helium. For cloud masses below 10^-1.7 Msun, the internal pressure is sufficient to permit the existence of particles of solid or liquid hydrogen at temperatures above the microwave background temperature. Optically thin thermal continuum emission by these particles can balance cosmic-ray heating of the cloud, leading to equilibria which are thermally stable even though the heating rate is independent of cloud temperature. For the Galaxy, the known heating rate in the disk sets a minimum mass of order 10^-6 Msun necessary for survival. Clouds of this type may in principle comprise most of the dark matter in the Galactic halo. However, we caution that the equilibria do not exist at redshifts z > 1 when the temperature of the microwave background was substantially larger than its current value; the formation and survival of such clouds to the present epoch therefore remain open questions.Comment: 5 pp incl 2 figs, LaTeX, emulateapj.sty; ApJ Letters in press. Significant revisions, results qualitatively unchange

Dynamical Expansion of Ionization and Dissociation Front around a Massive Star. II. On the Generality of Triggered Star Formation

We analyze the dynamical expansion of the HII region, photodissociation region, and the swept-up shell, solving the UV- and FUV-radiative transfer, the thermal and chemical processes in the time-dependent hydrodynamics code. Following our previous paper, we investigate the time evolutions with various ambient number densities and central stars. Our calculations show that basic evolution is qualitatively similar among our models with different parameters. The molecular gas is finally accumulated in the shell, and the gravitational fragmentation of the shell is generally expected. The quantitative differences among models are well understood with analytic scaling relations. The detailed physical and chemical structure of the shell is mainly determined by the incident FUV flux and the column density of the shell, which also follow the scaling relations. The time of shell-fragmentation, and the mass of the gathered molecular gas are sensitive tothe ambient number density. In the case of the lower number density, the shell-fragmentation occurs over a longer timescale, and the accumulated molecular gas is more massive. The variations with different central stars are more moderate. The time of the shell-fragmentation differs by a factor of several with the various stars of M_* = 12-101 M_sun. According to our numerical results, we conclude that the expanding HII region should be an efficient trigger for star formation in molecular clouds if the mass of the ambient molecular material is large enough.Comment: 49 pages, including 17 figures ; Accepted for publication in Ap

The Deuterium, Oxygen, and Nitrogen Abundance Toward LSE 44

We present measurements of the column densities of interstellar DI, OI, NI, and H2 made with FUSE, and of HI made with IUE toward the sdO star LSE 44, at a distance of 554+/-66 pc. This target is among the seven most distant Galactic sight lines for which these abundance ratios have been measured. The column densities were estimated by profile fitting and curve of growth analyses. We find D/H = (2.24 +1.39 -1.32)E-5, D/O = (1.99 +1.30 -0.67)E-2, D/N = (2.75 +1.19 -0.89)E-1, and O/H = (1.13 +0.96 -0.71)E-3 (2 sigma). Of the most distant Galactic sight lines for which the deuterium abundance has been measured LSE 44 is one of the few with D/H higher than the Local Bubble value, but D/O toward all these targets is below the Local Bubble value and more uniform than the D/H distribution. (Abstract abridged.)Comment: 20 pages, including 9 figures. Accepted for publication in Ap