Centroid Velocity Research Articles

Intervening O vi Quasar Absorption Systems at Low Redshift: A Significant Baryon Reservoir.

Far-UV echelle spectroscopy of the radio-quiet QSO H1821+643 (zem=0.297), obtained with the Space Telescope Imaging Spectrograph (STIS) at approximately 7 km s-1 resolution, reveals four definite O vi absorption-line systems and one probable O vi absorber at 0.15<zabs<0.27. The four definite O vi absorbers are located near galaxies and are highly displaced from the quasar in redshift; these are likely intervening systems unrelated to the background QSO. In the case of the strong O vi system at zabs=0.22497, multiple components are detected in Si iii and O vi as well as H i Lyman series lines, and the differing component velocity centroids and b-values firmly establish that this is a multiphase absorption system. A weak O vi absorber is detected at zabs=0.22637, i.e., offset by approximately 340 km s-1 from the zabs=0.22497 system. Lyalpha absorption is detected at zabs=0.22613, but no Lyalpha absorption is significantly detected at 0.22637. Other weak O vi absorbers at zabs=0.24531 and 0.26659 and the probable O vi system at 0.21326 have widely diverse O vi/H i column density ratios with N(O vi)/N(H i) ranging from </=0.14+/-0.03 to 5.2+/-1.2. The number density of O vi absorbers with rest equivalent width greater than 30 mÅ in the H1821+643 spectrum is remarkably high, dN&solm0;dz approximately 48, which implies with a high (90%) confidence that it is greater than 17 in the low-redshift intergalactic medium. We conservatively estimate that the cosmological mass density of the O vi systems is Omegab(Ovi&parr0; greater, similar0.0008 h-175. With an assumed metallicity of 1/10 solar and a conservative assumption that the fraction of oxygen in the O vi ionization stage is 0.2, we obtain Omegab(Ovi&parr0; greater, similar0.004 h-175. This is comparable to the combined cosmological mass density of stars and cool gas in galaxies and X-ray-emitting gas in galaxy clusters at low redshift.

Determining the period of the long-period activity of the water-vapor maser in W75N

We present the results of our study of the H2O maser emission from the source W75N, which is associated with a star-forming region, between November 1994 and March 1999. The observations were carried out with the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory (Lebedev Physical Institute). The maser emission in 1994–1999 can be represented as a superposition of flares of separate components with a duration from two to six months, which occurred mainly in the radial-velocity range 8–17.5 km s−1. We detected a regular drift of the velocity centroid from 13 to 9 km s−1 and an abrupt change in its velocity from 9 to 5 km s−1, which took place at the initial stage of maser activity. Based on the variability of the total H2O flux in all years of our observations of W75N (from December 1979 through March 1999), we conclude that the long-period variability of the water-vapor maser emission has a period of ∼11.5 years. We give arguments that this variability is mainly associated with the most compact group of maser spots, whose positions coincide with the position of the continuum source VLA 2.

Fast Electron Production at Intermediate Energies: Evidence for Fermi Shuttle Acceleration and for Deviations from Simple Relativistic Kinematics

We measured fast electron velocity spectra with the multidetector ARGOS in a large angular range, for atomic collisions induced by a $45A\mathrm{MeV}$ ${}^{58}\mathrm{Ni}$ beam on different solid targets. The velocity centroids of binary encounter electrons are shifted towards lower velocities than predicted by simple two-body relativistic kinematics, independently of target material and thickness. With increasing target atomic number, the high energy tails of the spectra exhibit an unexpectedly large number of very fast electrons. A multiple scattering (Fermi-shuttle) mechanism is invoked to explain this effect.

Velocity Field Statistics in Star‐forming Regions. I. Centroid Velocity Observations

The probability density functions (pdfs) of molecular line centroid velocity fluctuations, and of line centroid velocity fluctuation differences at different spatial lags, are estimated for several nearby molecular clouds with active internal star formation. The data consist of over 75,000 13CO line profiles divided among 12 spatially and/or kinematically distinct regions. These regions range in size from less than 1 to more than 40 pc and are all substantially supersonic, with centroid fluctuation Mach numbers ranging from about 1.5 to 7. The centroid pdfs are constructed using three different types of estimators. Although three regions (all in Mon R2) exhibit nearly Gaussian centroid pdfs, the other regions show strong evidence for non-Gaussian pdfs, often nearly exponential, with possible evidence for power-law contributions in the far tails. Evidence for nearly exponential centroid pdfs in the neutral H I component of the interstellar medium is also presented, based on older published data for optical absorption lines and H I emission and absorption lines. These strongly non-Gaussian pdfs disagree with the nearly Gaussian behavior found for incompressible turbulence (except possibly shear flow turbulence) and simulations of decaying mildly supersonic turbulence. Spatial images of the largest magnitude centroid velocity differences for the star-forming regions appear less filamentary than predicted by decay simulations dominated by vortical interactions. No evidence for the scaling of difference pdf kurtosis with Reynolds number, as found in incompressible turbulence experiments and simulations, is found. We conclude that turbulence in both star-forming molecular clouds and diffuse H I regions involves physical processes that are not adequately captured by incompressible turbulence or by mildly supersonic decay simulations. The variation with lag of the variance and kurtosis of the difference pdfs is presented as a constraint on future simulations, and we evaluate and discuss the implications of the large scale and Taylor scale Reynolds numbers for the regions studied here.

New results of the study of H2O masers in star formation regions

Abstract New results of the study of H2O masers in the regions of star formation are presented. They reflect the global character of separated H2O maser variations – the anticorrelation of the fluxes of individual H2O groups of the features; the simultaneous velocity drift of the H2O main spectral features; the non-chaotic character of the variations of the velocity centroid, and certain others.

Path integral centroid molecular dynamics method for Bose and Fermi statistics: formalism and simulation

A method is proposed for path integral centroid molecular dynamics (CMD) extended to Bose/Fermi statistics. It is based on the `pseudo-Boltzmann' canonical partition function of quantum statistical mechanics. An extended technique of path integral molecular dynamics (PIMD) is further presented for the calculation of thermodynamic properties and centroid mean force of Bose/Fermi systems. Bosonic PIMD and CMD simulations have been performed for 4 He and the ideal Bose gas, respectively. The remnant of λ transition is observed for 4 He, while Bose statistics causes a decay of the centroid velocity autocorrelation function of the ideal Bose gas in a nanosecond scale.

A survey of high-frequency SiO masers in supergiants and AGB stars

We report the results of a survey of high-frequency SiO maser emission from a sample of 34 late-type supergiant and AGB stars. Four new sources were detected, including the first detection of 301-GHz emission from an S-type star. Variability of the maser emission with stellar phase is discussed in detail. We find that high-frequency SiO maser emission appears to be exceptionally weak or absent in Mira variables over an optical phase range of approximately 0.4 to 0.7. No significant correlation is found between maser photon luminosity and spectral type for non-supergiant types. Centroid velocities of spectra appear to be redder than the stellar velocity over a phase range that is wider than the band where emission is weak on average. However, the conclusion that only the blue features appearing in spectra after an optical phase of 0.7--0.8 correspond to new masing objects should be viewed with caution.

Destruction of the Environment of the BN‐KL Nebula

We describe observations of the 2.12 μm molecular hydrogen emission in Orion using an IR Fabry-Perot interferometer with a spectral resolution of 24 km s-1 and a 2'' spatial resolution covering a region of 36×36 (0.46×0.46 pc2) that contains the H2 filamentary finger system located to the northwest of the Trapezium. We develop a simple model to explain the observed low-velocity structure as described by its radial moments: intensity, velocity centroid, velocity dispersion, and skewness. We assume a strong wind of 230 km s-1 produced by IRc2 interacting with a set of molecular clumps with density of 5.6×105 cm-3. This simple model provides a good match to the observed moments, gives clues to the development of filaments or fingers and entrainment of the molecular material, and associates the observed high-velocity blueshifted emission to the region. The driving source of the wind requires a high mass-loss rate and thus is likely IRc2. The H2 line emission is produced by a slow J-type shock (20 km s-1) in the clumps with an emissivity proportional to v1.8. Estimates for the total wind mass and clump mass yield 0.5 and 15 M☉ inside a radius of 1' (0.1 pc). The individual clumps have masses and sizes of few ×10−3M☉ and 0.007 pc, respectively. We conclude that the central 0.1 pc region surrounding the BN-KL nebula in front of OMC-1 is in the process of being disrupted by the strong wind of IRc2 on a timescale of 2000 yr.

Neutral hydrogen absorption observations of the central region of NGC 5929

We present 0.15-arcsec (25-pc) resolution MERLIN observations of neutral hydrogen absorption detected towards the nuclear region of the type 2 Seyfert galaxy NGC 5929. Absorption is detected only towards the north-eastern radio component with a column density of (6.5 ± 0.6) × 1021 cm−2. Based on comparison with an HST WFPC2 continuum image, we propose that the absorption is caused by a 1.5-arcsec structure of neutral gas and dust offset 0.3 arcsec south-east of the nucleus and running NE–SW. A separate cloud of dust is apparent 1.5 arcsec to the south-west of the nucleus in the HST image. A comparison of the centroid velocity (2358 ± 5 km s−1) and full width at half-maximum (43 ± 6 km s−1) of the absorbing gas with previous [O III] observations suggests that both the neutral and ionized gas are undergoing galactic rotation towards the observer in the north-east and away from the observer in the south-west. The main structure is consistent with an inclined ring of gas and dust encircling the active galactic nucleus (AGN); alternatively it may be a bar or inner spiral arm. We do not detect neutral hydrogen absorption or dust obscuration against the radio nucleus (column density < 3.1 × 1021 cm−2) expected by a torus of neutral gas and dust in unified models of AGNs for a type 2 Seyfert galaxy.

A Parameterized Study of the Detection of Infall in Protostellar Systems

We report new radiative transfer calculations of submillimeter and millimeter molecular line emission from fully three-dimensional models of protostars. The radiative transfer calculations are performed using the parameterized, semianalytic, rotating collapse solutions of Terebey, Shu, & Cassen. These results are compared with the class of self-consistent, nonspherical, hydrodynamic, rotating collapse solutions (Boss; Walker, Narayanan, & Boss). We explore the sensitivity of the blue-bulge infall signature to rotational rate, infall time, sound speed, and abundance gradients in the molecular tracer of choice. At high angular resolutions, models with moderate to high rotational rates exhibit the polar blue bulge—a centroid velocity signature of underlying Keplerian rotation in an embedded cloud core. Submillimeter transitions of HCO+ and CS are found to be better than millimeter transitions in detecting infall, especially at early collapse times.

Statistical Properties of Line Centroid Velocity Increments in the ρ Ophiuchi Cloud

We present a comparison of histograms of CO (2-1) line centroid velocity increments in the rho Ophiuchi molecular cloud with those computed for spectra synthesized from a three-dimensional, compressible, but non-starforming and non-gravitating hydrodynamic simulation. Histograms of centroid velocity increments in the rho Ophiuchi cloud show clearly non-Gaussian wings, similar to those found in histograms of velocity increments and derivatives in experimental studies of laboratory and atmospheric flows, as well as numerical simulations of turbulence. The magnitude of these wings increases monotonically with decreasing separation, down to the angular resolution of the data. This behavior is consistent with that found in the phase of the simulation which has most of the properties of incompressible turbulence. The time evolution of the magnitude of the non-Gaussian wings in the histograms of centroid velocity increments in the simulation is consistent with the evolution of the vorticity in the flow. However, we cannot exclude the possibility that the wings are associated with the shock interaction regions. Moreover, in an active starforming region like the rho Ophiuchi cloud, the effects of shocks may be more important than in the simulation. However, being able to identify shock interaction regions in the interstellar medium is also important, since numerical simulations show that vorticity is generated in shock interactions.

The Anatomy of the Perseus Spiral Arm:12CO andIRASImaging Observations of the W3‐W4‐W5 Cloud Complex

Panoramic images of 12CO J = 1-0 and thermal dust emissions from the W3-W4-W5 region of the outer Galaxy are presented. These data and recently published H I 21 cm line emission images provide an approximate 1' resolution perspective to the dynamics and thermal energy content of the interstellar gas and dust components contained within a 9 deg. arc of the Perseus spiral arm. We tabulate the molecular properties of 1560 clouds identified as closed surfaces within the l-b-v CO data cube at a threshold of 0.9 K T* (sub R). Relative surface densities of the molecular (28:1) and atomic (2.5:1) gas components determined within the arm and interarm velocity intervals demonstrate that the gas component that enters the spiral arm is predominantly atomic. Molecular clouds must necessarily condense from the compressed atomic material that enters the spiral arm and are likely short lived within the interarm regions. From the distribution of centroid velocities of clouds, we determine a random cloud-to-cloud velocity dispersion of 4 km s (exp. -1) over the width of the spiral arm but find no clear evidence within the molecular gas for streaming motions induced by the spiral potential. The far-infrared images are analyzed with the CO J = 1-0 and H I 21 cm line emission. The enhanced UV (Ultraviolet) radiation field from members of the Cas OB6 association and embedded newborn stars provide a significant source of heating to the extended dust component within the Perseus arm relative to the quiescent cirrus regions. Much of the measured far-infrared flux (69% at 60 micrometers and 47% at 100 micrometers) originates from regions associated with star formation rather than the extended, infrared cirrus component.

High‐resolution Monte Carlo simulation of flow and conservative transport in heterogeneous porous media: 1. Methodology and flow results

In this, the first of two papers concerned with the use of numerical simulation to examine flow and transport parameters in heterogeneous porous media via Monte Carlo methods, various aspects of the modelling effort are examined. In particular, the need to save on core memory causes one to use only specific realizations that have certain initial characteristics; in effect, these transport simulations are conditioned by these characteristics. Also, the need to independently estimate length scales for the generated fields is discussed. The statistical uniformity of the flow field is investigated by plotting the variance of the seepage velocity for vector components in the x, y, and z directions. Finally, specific features of the velocity field itself are illuminated in this first paper. In particular, these data give one the opportunity to investigate the effective hydraulic conductivity in a flow field which is approximately statistically uniform; comparisons are made with first‐ and second‐order perturbation analyses. The mean cloud velocity is examined to ascertain whether it is identical to the mean seepage velocity of the model. Finally, the variance in the cloud centroid velocity is examined for the effect of source size and differing strengths of local transverse dispersion.

The “Blue‐Bulge” Infall Signature toward IRAS 16293−2422

We present the first detection of the infall signature toward a protostellar source. The blue-bulge infall signature can be observed in the centroid velocity maps of protostellar objects when infall dominates over rotation. This infall signature can be detected under a wide variety of source conditions. The detection of the blue-bulge infall signature toward a protobinary system such as IRAS 16293-2422 suggests that detailed studies of gravitational collapse toward a large number of protostellar sources may now be possible. Our CS J = 7 → 6 data appear to be tracing gaseous material in the inner circumbinary core, while the CS J = 5 → 4 and HCO+ J = 4 → 3 appear to trace the outer envelope and static core in addition to the inner circumbinary core. The mass accretion rate through the infall region appears to be consistent with an inside-out collapse model for the source. Using three-dimensional radiative transfer models based on the rotating, collapse solution of Terebey, Shu, & Cassen, we derive the infall parameters of the IRAS 16293-2422 cloud core. Our best-fit model suggests that the infall radius of the IRAS 16293-2422 cloud core is ~39'' (0.03 pc).

Dissecting Capella's Corona: GHRS Spectra of the Fexxiλ1354 and Heiiλ1640 Lines from Each of the Capella Stars

We report on moderate (λ/Δλ = 20,000) and high (λ/Δλ = 90,000) resolution spectra of the 104-day period Capella binary system (HD 34029) obtained with Hubble Space Telescope's Goddard High-Resolution Spectrometer (GHRS) on 1995 September 9 and 1996 April 9. The observations include a long-duration, moderate-resolution spectrum of the coronal Fe XXI λ1354 line and both moderate- and high-resolution spectra of the He II λ1640 multiplet. Our objective in observing the Fe XXI line formed at T = 1 × 107 K is to determine for the first time the line shape parameters and the contribution of each star's corona. This is feasible because the GHRS can resolve the 53 km s-1 radial velocity separation of the stars. Our analysis led to four surprising results: (1) The contribution of the slowly rotating G8 III star to the total Fe XXI λ1354 flux is similar to that of the more active rapidly rotating G1 III star, in contrast to other UV lines formed at lower temperatures. (2) The centroid velocities of the Fe XXI lines from both stars are near their respective photospheric radial velocities. Thus, there is no evidence for downflows or winds, and the hot coronal plasma must be confined, presumably by strong, closed magnetic fields. This is the first direct kinematic evidence for magnetic confinement in the corona of a giant star. (3) The line widths are thermal, indicating very low turbulence (ξ < 23 km s-1) compared with the 54 km s-1 thermal speed. (4) Our analysis of Extreme-Ultraviolet Explorer (EUVE) spectra that include four Fe XXI lines shows that the volume emission measures of these lines are about a factor of 3 smaller than for the λ1354 line, which was observed several months after the closest EUVE observation. We consider possible explanations for this discrepancy and conclude that variability of the high-temperature coronal emission is the most likely explanation. We observed the He II λ1640 line twice with a time separation of 7 months, corresponding to about 2 orbital periods. The profiles are nearly identical, except for a large difference in flux near the expected radial velocity of the G8 III star. We believe that the broad He II emission produced by the G1 star is formed mostly by collisional excitation in its transition region, while the emission from the G8 star is produced predominantly by a photoionization/recombination process driven by EUV radiation from the G8 star's corona. The He II λ1640 flux that we measure from the G8 star is consistent with predictions based on EUVE flux measurements. The decrease in the λ1640 emission from the G8 star between the phase 0.73 and 0.78 measurements indicates that its EUV radiation is variable, as is observed by EUVE.

Analysis of the Spinning Mechanism of a Golfball Using the "s parameter" and Its Experimental Certification.

Understanding the spinning mechanism of an impacted golfball is an important factor in golf science and engineering. The rigid sphere model which considers the Coulomb-type slip friction presented independently by Daish and Kawamura (referred to here as the Daish-Kawamura model) seems to well explain the experimental results of golfball spinning. For a more general interpretation of the spin phenomenon, we attempted to modify this model by introducing a physical quantity called the s parameter, which is defined as the ratio of the circumferential velocity of the ball to its centroid velocity. An experiment performed to verify the appropriateness of the new parameter introduction led to the following conclusions: The spin phenomenon in golf impact can be divided into two phases, in both of which the Daish-Kawamura model is effective. A method is described to evaluate the coefficient of slip friction under high sliding velocities, and the value is estimated as 0.07 for a slippery surface condition.

Evolution of H2O maser emission in the direction of the semiregular variable RT Virginis during 1985-1996

We present a catalog of the 22 GHz H2 Om aser proles of the semiregular variable RT Vir. The observa- tions were made from 1985 to 1996 at the RT-22 radio telescope of the Radio Astronomical Station at Pushchino, Russia. Results obtained after the analysis of the inte- grated fluxes, the average spectra, the centroid velocity and the superposition of the spectra are reported. Anticorrelation of the integrated fluxes of two spectral intervals (VLSR 21 km s 1 ) was found. The characteristic time variation of the maser emission at each of these groups was about 5-6 years. This phe- nomenon may be explained by a variation of the outflow of the matter from the star. The outflow is weakly bipolar. We can suppose that in such a model the enhancement of the outflow occured alternately with a period of about 5-6 years. The anticorrelation of the integrated fluxes of two groups of maser features, located symmetrically relative to the star, may be a result of this alternation.

Time and scale dependent effective retardation factors in heterogeneous aquifers

The time and scale dependence of the retardation factor is examined for transport of a linearly adsorbing solute in an aquifer with spatially variable distribution coefficient ( K d). The variability of 1n K d and the logarithm of the hydraulic conductivity (ln K) are modeled as stationary random fields. Two definitions of the effective retardation factor are introduced: (i) the ratio of the ensemble average centroid velocities of a passive and retarded solute and (ii) the ratio of the ensemble average arrival time of a passive solute to that of a retarded solute. The effective retardation factor based on the first definition is shown to be time-dependent and that based on the second definition is shown to be distance (scale) dependent. Theoretical expressions are developed for computing the effective retardation factors based on a knowledge of statistical parameters of the ln K and ln K d fields. Illustrative computations are presented for one-dimensional and three-dimensional heterogeneous media.

Kinematics of the Ursa Major Molecular Clouds

We present a kinematic analysis of the atomic and molecular gas in the Ursa Major molecular clouds. The analysis is based on a new CO survey of the complex made with linear resolution of 0.05 pc and existing H I observations. The clouds lie in projection on an expanding shell of material known as the North Celestial Pole loop. The molecular structure of the complex is dominated by several long (>5 pc) filaments, some of which are both extremely straight and extremely narrow (<0.2 pc across). These filaments are enclosed in a sheath of neutral atomic hydrogen that has a kinematic signature distinct from the surrounding atomic gas. The tips of the filaments are regions of enhanced IRAS 12 and 25 μm emission. We find an offset of up to 4 km s-1 between the centroid velocities of the CO and H I, a large-scale velocity gradient in the gas of ~0.3 km s-1 pc-1, and a similar large-scale gradient in the H I line width. The CO velocity field follows a trend similar to the H I but is much less organized. A weak line width gradient in the CO may also be present. We present a model in which the clouds lie near the surface of the expanding bubble and, after having interacted with the bubble's wind, are now slowly sliding down the bubble toward the Galactic plane. This model adequately accounts for the atomic-molecular velocity offset, the velocity gradients, the line width gradient, and the IRAS colors.

Monitoring of 6-cm excited OH masers -- II

A search towards 69 star-forming regions has led to the discovery of three new 4765-MHz excited OH masers. No 4750- or 4660-MHz excited OH masers were detected above a level of ∼ 1 Jy. The results of monitoring nine sources of 4765-MHz excited OH masers for periods of up to 7 yr are reported. Many of the linewidths are narrow and have had constant widths and centroid velocities throughout the monitoring period. The maser flux of one source was constant over 7 yr while in the most extreme case of variability the flux doubled in a period of 2 weeks.