Density PDFs of diffuse gas in the Milky Way

The probability distribution functions (PDFs) of the average densities of the diffuse ionized gas (DIG) and the diffuse atomic gas are close to lognormal, especially when lines of sight at |b|5 degree are considered separately. Our results provide strong support for the existence of a lognormal density PDF in the diffuse ISM, consistent with a turbulent origin of density structure in the diffuse gas.Comment: 4 pages, 1 figure. To be published in the proceedings of the August 2008 conference (held in Eapinho, Portugal) "The Role of Disk-Halo Interaction in Galaxy Evolution: Outflow vs Infall?", Ed. M. A. de Avillez, EAS Publications Serie

Instantaneous pressure measurements on a spherical grain under threshold flow conditions

River morphodynamics and sediment transportMechanics of sediment transpor

Incipient rolling of coarse particles in water flows: a dynamical perspective

River morphodynamics and sediment transportMechanics of sediment transpor

The End of the Lines for OX 169: No Binary Broad-Line Region

We show that unusual Balmer emission line profiles of the quasar OX 169, frequently described as either self-absorbed or double peaked, are actually neither. The effect is an illusion resulting from two coincidences. First, the forbidden lines are quite strong and broad. Consequently, the [N II]6583 line and the associated narrow-line component of H-alpha present the appearance of twin H-alpha peaks. Second, the redshift of 0.2110 brings H-beta into coincidence with Na I D at zero redshift, and ISM absorption in Na I D divides the H-beta emission line. In spectra obtained over the past decade, we see no substantial change in the character of the line profiles, and no indication of intrinsic double-peaked structure. The H-gamma, Mg II, and Ly-alpha emission lines are single peaked, and all of the emission-line redshifts are consistent once they are correctly attributed to their permitted and forbidden-line identifications. A systematic shift of up to 700 km/s between broad and narrow lines is seen, but such differences are common, and could be due to gravitational and transverse redshift in a low-inclination disk. Stockton & Farnham (1991) had called attention to an apparent tidal tail in the host galaxy of OX 169, and speculated that a recent merger had supplied the nucleus with a coalescing pair of black holes which was now revealing its existence in the form of two physically distinct broad-line regions. Although there is no longer any evidence for two broad emission-line regions in OX 169, binary black holes should form frequently in galaxy mergers, and it is still worthwhile to monitor the radial velocities of emission lines which could supply evidence of their existence in certain objects.Comment: 19 pages, 5 figures, accepted for publication in Ap.

Density probability distribution functions of diffuse gas in the Milky Way

In a search for the signature of turbulence in the diffuse interstellar medium in gas density distributions, we determined the probability distribution functions (PDFs) of the average volume densities of the diffuse gas. The densities were derived from dispersion measures and HI column densities towards pulsars and stars at known distances. The PDFs of the average densities of the diffuse ionized gas (DIG) and the diffuse atomic gas are close to lognormal, especially when lines of sight at |b|=5 degrees are considered separately. The PDF of at high |b| is twice as wide as that at low |b|. The width of the PDF of the DIG is about 30 per cent smaller than that of the warm HI at the same latitudes. The results reported here provide strong support for the existence of a lognormal density PDF in the diffuse ISM, consistent with a turbulent origin of density structure in the diffuse gas.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter

A 21 Centimeter Absorber Identified with a Spiral Galaxy: Hubble Space Telescope Faint Object Spectrograph and Wide-Field Camera Observations of 3CR 196

We present imaging and spectroscopy of the quasar 3CR 196 (z(sub e) = 0.871), which has 21 cm and optical absorption at z(sub a) = 0.437. We observed the region of Ly alpha absorption in 3CR 196 at z(sub a) = 0.437 with the Faint Object Spectrograph on the Hubble Space Telescope. This region of the spectrum is complicated because of the presence of a Lyman limit and strong lines from a z(sub a) approx. z(sub e) system. We conclude that there is Ly alpha absorption with an H I column density greater than 2.7 x 10(exp 19) cm(exp -2) and most probably 1.5 x 10(exp 20) cm(exp -2). Based on the existence of the high H I column density along both the optical and radio lines of sight, separated by more than 15 kpc, we conclude that the Ly alpha absorption must arise in a system comparable in size to the gaseous disks of spiral galaxies. A barred spiral galaxy, previously reported as a diffuse object in the recent work of Boisse and Boulade, can be seen near the quasar in an image taken at 0.1 resolution with the Wide Field Planetary Camera 2 on the HST. If this galaxy is at the absorption redshift, the luminosity is approximately L(sub *) and any H I disk should extend in front of the optical quasar and radio lobes of 3CR 196, giving rise to both the Ly alpha and 21 cm absorption. In the z(sub a) approx. z(sub e) system we detect Lyman lines and the Lyman limit, as well as high ion absorption lines of C III, N V, S VI, and O VI. This absorption probably only partially covers the emission-line region. The ionization parameter is approximately 0.1. Conditions in this region may be similar to those in broad absorption line QSOs

A detailed X-ray investigation of zeta Puppis I. The dataset and some preliminary results

Aims: zeta Puppis, one of the closest and brightest massive stars, was the first early-type object observed by the current generation of X-ray observatories. These data provided some surprising results, confirming partly the theoretical predictions while simultaneously unveiling some problematic mismatches with expectations. In this series of papers, we perform a thorough study of zeta Puppis in X-rays, using a decade of XMM observations. Methods: zeta Puppis was observed 18 times by XMM, totaling 1Ms in exposure. This provides the highest-quality high-resolution X-ray spectrum of a massive star to date, as well as a perfect dataset for studying X-ray variability in an "archetype" object. Results: This first paper reports on the data reduction of this unique dataset and provides a few preliminary results. On the one hand, the analysis of EPIC low-resolution spectra shows the star to have a remarkably stable X-ray emission from one observation to the next. On the other hand, the fitting by a wind model of individual line profiles recorded by RGS confirms the wavelength dependence of the line morphology.Comment: 9 pages, 3 figures, accepted for publication by A&

Discovery of the first tau Sco analogues: HD 66665 and HD 63425

The B0.2 V magnetic star tau Sco stands out from the larger population of massive OB stars due to its high X-ray activity, peculiar wind diagnostics and highly complex magnetic field. This paper presents the discovery of the first two tau Sco analogues - HD 66665 and HD 63425, identified by the striking similarity of their UV spectra to that of tau Sco. ESPaDOnS spectropolarimetric observations were secured by the Magnetism in Massive Stars CFHT Large Program, in order to characterize the stellar and magnetic properties of these stars. CMFGEN modelling of optical ESPaDOnS spectra and archived IUE UV spectra showed that these stars have stellar parameters similar to those of tau Sco. A magnetic field of similar surface strength is found on both stars, reinforcing the connection between the presence of a magnetic field and wind peculiarities. However, additional phase-resolved observations will be required in order to assess the potential complexity of the magnetic fields, and verify if the wind anomalies are linked to this property.Comment: 6 pages, 2 tables, 3 figures. Accepted for publication in MNRAS. The definitive version will be available at www.blackwel-synergy.co