Intermediate-velocity Clouds Research Articles

The Fragmentation and Vaporization of Dust in Grain-Grain Collisions

view Abstract Citations (55) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Fragmentation and Vaporization of Dust in Grain-Grain Collisions Borkowski, Kazimierz J. ; Dwek, Eli Abstract We present a detailed analysis of the evolution of the size distribution of fast-moving interstellar dust particles decelerating in a dusty plasma. The physical processes considered in this paper include sputtering by gas-grain collisions, and partial and complete evaporation, fragmentation, and cratering by grain-grain collisions. We find that the final distribution of grains, initially characterized by a power-law distribution in grain sizes, exhibits an excess of small-to-large particles, compared to the initial distribution. This excess depends on the initial grain velocity and should be detectable by infrared observations of low- and intermediate-velocity clouds with the Infrared Space Observatory scheduled for launch around 1996. Publication: The Astrophysical Journal Pub Date: November 1995 DOI: 10.1086/176480 Bibcode: 1995ApJ...454..254B Keywords: ISM: DUST; EXTINCTION full text sources ADS |

Optical and H I observations of the low-velocity and intermediate-velocity gas toward the globular cluster M13

High-resolution spectra of interstellar lines toward 11 stars in the globular cluster M13 and toward three foreground stars which lie within 3 deg from the cluster are reported. The optical spectra are compared with an H I profile of the gas recorded in the direction of the cluster. The principal interstellar components have LSR radial velocities centered near +10 km/s and -4km/s, but there is a considerable variation in the velocities and the column densities of both components across the face of the cluster. The positive velocity gas has a high Na I/H I column density ratio, and it lies beyond the foreground stars at a distance approximately greater than 200 pc; this gas may be associated with the receding part of the Hercules shell discussed in detail by Lilienthal et al. The negative velocity gas is detected toward the foreground stars, and the observed Na I/Ca II ratios suggest an association with gas at the approaching side of the Hercules shell. Gas at a velocity approximately equal -80 km/s has been previously reported in H I emission in the M13 direction and in UV spectra of the post-AGB cluster star, Barnard 29. This intermediate-velocity clouds in the lower halo. We discuss also evidence for the detection of this gas in Na I and show that it is most likely located at a distance.

Properties of the +70 kilometers per second cloud toward HD 203664

I present high-resolution International Ultraviolet Explorer (IUE) spectra of the ultraviolet absorption in an intermediate-velocity interstellar cloud (nu(sub LSR) approximately equal to +70 km/s) toward HD 203664. The combined, multiple IUE images result in spectra with S/N = 15-40 and resolutions of approximately 20-25 km/s. The intermediate-velocity cloud absorption is present in ultraviolet lines of C II, C II(sup *), C IV, N I, O I, Mg I, Mg II, Al II, Al III, Si II, Si III, Si IV, S II, Cr II, Mn II, Fe II, and Zn II. The relative abundances of low-ionization species suggest an electron density of 0.15-0.34/cu cm and a temperature of 5300-6100 K in the neutral and weakly ionized gas. Given the presence of high-ionization gas tracers such as Si IV and C IV, ionized portions of the cloud probably contribute to the relatively large values of n(sub e) derived from measurements of the lower ionization species. The high-ionization species in the cloud have an abundance ratio, N(C IV)/N(Si IV) approximately equal to 4.5, similar to that inferred for collisionally ionized cloud interfaces at temperatures near 10(exp 5) K along other sight lines. When referenced to sulfur, the abundances of most elements in the cloud are within a factor of 5 of their solar values, which suggests that the +70 km/s gas has a previous origin in the Galactic disk despite a recent determination by Little et al. that the cloud lies at a distance of 200-1500 pc below the Galactic plane. I have checked this result against a model of the ionization for the diffuse ionized gas layer of the Galaxy and find that this conclusion is essentially unchanged as long as the ionization parameter is low as implied by the abundances of adjoining ionization states of aluminum and silicon. The processes responsible for the production of highly ionized gas in the +70 km/s cloud appear to be able to account for the inferred dust grain destruction as well.

A supershell model for the high- and intermediate-velocity neutral hydrogen clouds

view Abstract Citations (24) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Supershell Model for the High- and Intermediate-Velocity Neutral Hydrogen Clouds Verschuur, Gerrit L. Abstract The anomalous negative-velocity H I over the first and second quadrants of the Galactic longitude, especially in the Northern Hemisphere, has yet to be addressed by a model that successfully explains the properties of high-velocity clouds (HVCs) and intermediate-velocity cloud. Taking into account the influence of the radio continuum spur known as Loop III, parameters describing the supershell are derived through comparisons of model predictions with the latitude-velocity distributions of the HVCs in 5-deg longitude intervals. The agreement obtained between model results and Northern Hemisphere data indicates that the brightest HVCs result from column-density enhancements over a large depth in space. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/172657 Bibcode: 1993ApJ...409..205V Keywords: Astronomical Models; Gas Expansion; H I Regions; Molecular Clouds; Neutral Gases; Kinetic Energy; Milky Way Galaxy; Solar Neighborhood; Spatial Distribution; Astrophysics; GALAXY: KINEMATICS AND DYNAMICS; ISM: CLOUDS full text sources ADS | data products SIMBAD (3)

FAR-SIDELOBE CONTAMINATION OF THE BELL LABORATORIES H I SURVEY

ABSTRACT This paper describes an approximate calculation of the far-sidelobe contamination present in the Bell Laboratories H I Survey. The general method is to determine the position of the sidelobe, and to assume that the emission measured at the position of the sidelobe is free of contamination. The maximum possible contamination for a given velocity observed by the main beam is estimated to be the maximum emission within the sidelobe and within a velocity range determined by the motion of the earth, scaled by the antenna response. We have analyzed the errors introduced by saturation in the main beam and found them to be insignificant. The far-sidelobe contamination is shown to be less than 2 X 1016 cm-2 per km s-1 over most of the entire sky. The low level of far-sidelobe contamination suggests that such contamination does not affect results from studies of H I 21 cm radiation at high latitudes using the Bell Laboratories Survey. In particular, the low-column-density features of high velocity and intermediate velocity clouds can be deemed real.

FORMATION OF A GIANT GALACTIC GASEOUS HALO: METAL-ABSORPTION LINES AND HIGH-VELOCITY CLOUDS

FORMATION OF A GIANT GALACTIC GASEOUS HALO: METAL-ABSORPTION LINES AND HIGH-VELOCITY CLOUDS

Metals and Molecules in Halo Clouds

The abundance of the elements in clouds of halo gas, as determined from observations, is an important parameter for the test of the validity of models explaining the existence of, e.g., the high latitutude high-velocity clouds (HVCs) of the Milky Way. Individual HVCs have been detected in absorption only on very few lines of sight so that the distance of the HVCs, another important parameter for the models, stays ill determined as well. We will follow here the more or less established convention by calling HVCs those with /v/ >100 km s−1 and IVCs (intermediate-velocity clouds) those with 50 </v/ <100 km s−1. We will define halo as the space with /z/> 1 kpc, although for /b/> 45 also /z/> 0.5 kpc is used.

Structure of the interstellar medium in the Magellanic Clouds

Using optical and UV observations, the internal gas structure of Magellanic Clouds (MCs) was examined for clues to the recent evolution of the Magellanic System and further signs of this collision event. Results indicate that the intermediate velocity clouds (IVCs) have different origins. Thus, the 60-km/s IVC is clearly of Galactic origin. The 130-km/s IVC is probably also Galactic and is not directly associated with the MCs. The 170 km/s IVC is quite clearly associated with the LMC and seems to be the front of the LMC. The overall gas structure in the MCs indicates that there is a distance-velocity association except in two regions: one to the south-west of the SMC (and is enigmatic), and the other to the south of 30 Doradus and appears to be smoothly linked with the H I bridge, indicating that both features have a common overall cause such as the recent tidal collision. 89 refs.

CO and IRAS detection of an intermediate-velocity cloud

In the course of a radio survey of high-Galactic-latitude clouds, CO emission was detected at the position l = 210.8 deg and b = 63.1 deg with an LSR velocity of -39 km/sec. This molecular cloud constitutes the third one with an unusually large absolute velocity at these latitudes, as compared with the 5.4-km/sec cloud-to-cloud velocity dispersion of the high-latitude molecular clouds. The position is coincident with an H I intermediate-velocity cloud (GHL 11, Verschuur H, OLM 268) and the IR-excess cloud 306 in the list by Desert et al. (1988). This cloud is clearly detected at all four IRAS wavelengths and has warmer colors than the local ISM.

NGC 1058 - Gas motions in an extended, quiescent spiral disk

view Abstract Citations (142) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS NGC 1058: Gas Motions in an Extended, Quiescent Spiral Disk Dickey, John M. ; Hanson, Margaret Murray ; Helou, George Abstract We investigate in detail the motion of gas in the galaxy NGC 1058, using the VLA to map the emission in the 21 cm line. This galaxy is so nearly face-on that the contribution to the line width due to the variation of the rotational velocity across the D-array beam is small compared with the random z-motion of the gas. We confirm results of earlier studies, including the fact that the rotation curve drops faster than Keplerian at the outer edge of the disk, which we interpret as a fortuitous twist of the plane of rotation in the outer disk. Our very high velocity resolution (2.58 km s^-1^ after Hanning smoothing) allows us to measure the line width accurately throughout the disk. This width decreases with optical surface brightness, but in the extended gas disk beyond the Holmberg radius the line width is remarkably constant, with σ_v_ ~ 5.7 km s^-1^. There is no correlation of line width with spiral phase or H I surface density in the outer disk. If the line width is determined by microscopic thermal motions, this implies a surprisingly cool temperature for the H I gas, while if it represents the velocity distribution of cold clouds, then its lack of spatial variation is remarkable. We see evidence for an "intermediate velocity cloud" population whose velocity width is several times broader than that of the bulk of the H I in the optically bright disk. This second Gaussian component is not present in the outer, optically very faint disk. Publication: The Astrophysical Journal Pub Date: April 1990 DOI: 10.1086/168555 Bibcode: 1990ApJ...352..522D Keywords: Gas Dynamics; Radio Sources (Astronomy); Spiral Galaxies; Very Large Array (Vla); Gas Density; H Lines; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1058; GALAXIES: INTERNAL MOTIONS; RADIO SOURCES: 21 CM RADIATION full text sources ADS | data products SIMBAD (1) NED (1)

Galactic Fountains

SummaryThe intercloud medium (ICM) is kept hot and supplied with gas by supernova remnants of Type II. The rate at which mass is added balances the rate at which gas flows away from the ICM into the galactic halo. A fountain is thus formed, since the gas only has enough energy to escape from the disk, but not from the Galaxy as a whole. The upward flow must be followed by the descent of the gas, in the form of a shocked sheet: the return flow seems to be observed in the form of intermediate velocity clouds.

The Intermediate Velocity Cloud IVC86+38.5-45, Related to High Velocity Clouds?

Examining the high velocity cloud complex C and its possible interaction with gas of the galactic disk we became aware of a dust cloud located at galactic longitude l = 86° and latitude b = 38°.5 seen in the l00µm image of the IRAS. This cloud consists of a cucumber shaped core surounded by a horseshoe shaped halo opening towards larger galactic coordinates. At very low 100µm intensities dust lanes starting at the core or the ends of the horseshoe can be traced for about 5° towards high galactic latitude and longitude.

ISM chemical abundances in two intermediate-velocity clouds in the line of sight to SN 1987A

The earliest IUE high-resolution spectra of SN 1987A have been studied and reveal the presence of several clouds in the line of sight to the LMC. In particular, there are two clouds with radial velocities of about 130 km/s and about 180 km/s. These clouds' velocities are between those of Galactic clouds at 0-80 km/s and those of LMC gas at about 270 km/s. Chemical-abundance determinations may help to determine the origin and location of these clouds. Curve-of-growth analysis and 21-cm observations show that they may be underabundant in heavy elements by about a factor of 2 as compared to solar abundances. No depletion indicative of grain formation can be seen.

Abundances in intermediate-velocity clouds toward SN 1987A

Abundances are derived for intermediate-velocity clouds toward SN 1987A, using high-resolution data from the IUE. For the 70-km/s cloud, abundances are close to their values in the sun, and a Galactic-halo origin is proposed for the cloud. The two other features, at 129 and 171 km/s, have mean abundances of -0.59 + or - 0.15 dex and -0.46 + or - 0.15 dex relative to solar values and are similar to LMC abundances derived from ionized gas. Taken along with other evidence, it is argued that the features arise in gas associated with the LMC and not the Galaxy. 24 references.

Starlight polarization in the direction of two H I complexes

view Abstract Citations (4) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Starlight polarization in the direction of two H I complexes. Morras, R. Abstract Upper limits to the gas/dust ratio in the direction of two complexes are derived based on 21-cm line observations and starlight polarization measurements. Elongated H I structures parallel to the galactic magnetic field suggest that the gas and dust comprise a common cloud or cloud system. Dust producing the starlight polarization is mixed with the H I gas, aiding in estimating the distance for the two H I complexes. Distances of 100 pc for the filament at low velocities and of 200 pc for an intermediate-velocity cloud complex (IVC) are established from polarimetric data, and possible origins for the filaments and IVC are suggested. Publication: The Astronomical Journal Pub Date: June 1981 DOI: 10.1086/112960 Bibcode: 1981AJ.....86..875M Keywords: Hydrogen Clouds; Interstellar Magnetic Fields; Interstellar Matter; Polarized Light; Stellar Radiation; Astronomical Maps; Cosmic Dust; Interstellar Gas; Tables (Data); Astronomy full text sources ADS | data products SIMBAD (96)

Copernicus observations of interstellar matter toward the Orion OB1 association. I - Epsilon and Pi-5 Orionis

view Abstract Citations (19) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Copernicus observations of interstellar matter toward the Orion OB1 association. I. epsilon and pi 5 Orionis. Shull, J. M. Abstract Copernicus UV data on interstellar lines toward Epsilon Ori and Pi-5 Ori are analyzed to study abundances and physical conditions in both low- and intermediate-velocity components. Clouds at -8 and +5 km/s (LSR) toward Epsilon Ori show typical depletions of Fe, Ti, Mg, and Si in dense (H number density about 100 per cu cm) gas. Low-column-density intermediate-velocity clouds toward both stars, with low densities (hydrogen number density less than 1 per cu cm) and near-cosmic Si abundances, are consistent with a widespread pattern of high-velocity gas over a 15-deg area surrounding the Orion region. Such activity may be attributed to the repeated action of supernovae in a patchy low-density region of interstellar gas. Publication: The Astrophysical Journal Pub Date: October 1979 DOI: 10.1086/157379 Bibcode: 1979ApJ...233..182S Keywords: Astronomical Spectroscopy; Interstellar Matter; Orion Nebula; Satellite Observation; Spaceborne Astronomy; Abundance; Gas Density; Oao 3; Spectral Line Width; Tables (Data); Astrophysics; Interstellar Lines:UV; Interstellar Matter:Element Abundances; Interstellar Matter:Stellar Associations full text sources ADS | data products SIMBAD (2) MAST (1)

The depletion of interstellar gaseous iron

view Abstract Citations (136) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The depletion of interstellar gaseous iron. Savage, B. D. ; Bohlin, R. C. Abstract The Copernicus UV telescope was used to measure equivalent widths of interstellar Fe II resonance lines toward 55 early-type stars; the measurements permit the determination of Fe II column densities. The depletion of interstellar gaseous iron was obtained by combining these measurements with the results from a previous atomic and molecular hydrogen survey program; the derived depletions refer mostly to matter in H I regions. As an example, the nearly normal gaseous iron abundance in the distant high-latitude intermediate-velocity cloud toward HD 93521 is consistent with the idea that these clouds are produced by galactic supernova explosions. Publication: The Astrophysical Journal Pub Date: April 1979 DOI: 10.1086/156938 Bibcode: 1979ApJ...229..136S Keywords: Astronomical Spectroscopy; Interstellar Gas; Iron; Metallic Stars; Abundance; Chemical Clouds; H I Regions; Supernova Remnants; Astrophysics; Interstellar Matter:Iron full text sources ADS | data products SIMBAD (56)

Refractory grain destruction in low-velocity shocks

Recent abundance determinations in intermediate-velocity clouds near the Orion association have shown that silicon abundances may be nearly cosmic even in clouds with a velocity as low as 40 to 50 km/sec. It is argued that grain destruction in radiative shocks traveling perpendicular to the magnetic field may be much more effective than was previously considered. The destruction mechanism consists of acceleration of the gyromotion of the grains about their drift center owing to magnetic field compression, followed by sputtering of the high-velocity grains by He and (CNO) atoms. It is found that MgSiO3 grains may be almost fully destroyed in shocks of velocity greater than 80 km/sec and graphite grains substantially destroyed at shock velocities of 100 km/sec.

21-cm observations of nonplanar H I associated with the Perseus spiral arm

view Abstract Citations (2) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS 21-cm observations of nonplanar H I associated with the Perseus spiral arm. Grayzeck, E. J. Abstract An intermediate-velocity cloud (-55 km sec-1) centered at 1 = 122 , b = -15 has been mapped at 21 cm to determine its extent and possible connection to gas in the plane. The observations were obtained using the NRAO 140-ft radio telescope and the 413-channel autocorrelation receiver. Grid points spaced every beamwidth (22 arcmin) were observed from 1 =1 l9. 5 to l27. 5 and from b = -10 to -22 and the data were organized into b - V and 1- V contour maps, smoothed to a velocity resolution of 1 km sec '. The extent of the gas is presented in the form of contour maps of the H i column density over the velocity range of -45 to -70 km sec '. This material appears to be a continuation of the H i seen in the plane at velocities between -40 and -60 km . Additional information on young OB stars located near 1 =1 22 , b = -15 particularly HD 4460, which has a distance of 2 kpc and V LSR = -55 km sec ' further substantiate the association of this intermediate-velocity gas with the Perseus spiral arm. Publication: The Astronomical Journal Pub Date: November 1977 DOI: 10.1086/112142 Bibcode: 1977AJ.....82..886G Keywords: Hydrogen Clouds; Interstellar Gas; Nebulae; Spiral Galaxies; Astronomical Maps; Centimeter Waves; Early Stars; Gas Density; Microwave Spectra; Astrophysics full text sources ADS | data products SIMBAD (34)

Grain disruption in interstellar hydromagnetic shocks

view Abstract Citations (63) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Grain disruption in interstellar hydromagnetic shocks. Shull, J. M. Abstract The observed abundance variations of Ca, Fe, Si, and Ti in intermediate-velocity interstellar gas suggest that grains have been disrupted in clouds with velocities as low as 20 to 50 km/s. A simplified hydromagnetic shock model for such clouds is described; the dynamical equations for charged grains in the postshock region are derived, including the collisional drag and 'betatron-acceleration' effect of a magnetic-field gradient; and the fraction of grains destroyed in evaporative collisions with other grains is calculated. It is found that for shocks of 20 to 50 km/s in which the fractional H-ionization remains low, 3-10% of the grain material may be destroyed - sufficient to explain in part the heavy-element depletion pattern in intermediate velocity clouds and the well-known correlation of N(Ca II)/N(Na I) with cloud velocity. Publication: The Astrophysical Journal Pub Date: August 1977 DOI: 10.1086/155417 Bibcode: 1977ApJ...215..805S Keywords: Abundance; Interstellar Gas; Interstellar Matter; Magnetohydrodynamic Waves; Molecular Collisions; Shock Waves; Astronomical Models; Cosmic Plasma; Oao 3; Astrophysics full text sources ADS |