Outer Cloud Research Articles

Gravitational Collapse of an Isothermal Sphere

view Abstract Citations (294) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gravitational Collapse of an Isothermal Sphere Foster, Prudence N. ; Chevalier, Roger A. Abstract We investigate the spherical gravitational collapse of isothermal spheres using numerical hydrodynamics. The initial configuration is close to hydrostatic equilibrium. If the initial density profile has a finite core radius (i.e., it is not singular), supersonic velocities develop during the initial collapse. At the time of central core formation, when the central density diverges, the central inflow velocity approaches -3.3 times the sound speed and the central density approaches an r-2 profile. These conditions are similar to those found in the self-similar solution of Larson and Penston, but occur only at the center and not at all radii as in the selfsimilar solution at core formation. For the marginally stable equilibrium isothermal sphere, with an initial outer cloud radius to core radius ratio of 3.2, the central mass accretion rate steadily declines after core formation. Only if this ratio is ≳20 does the collapse enter a constant mass accretion rate, as occurs in the self-similar solutions developed by Shu. Assuming optical transparency, we calculate line profiles for the computed collapse. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/173236 Bibcode: 1993ApJ...416..303F Keywords: STARS: PRE-MAIN-SEQUENCE; HYDRODYNAMICS; STARS: FORMATION full text sources ADS | data products SIMBAD (3)

Seyfert galaxy ultraviolet emission-line intensities and variability - A self-consistent photoionization analysis applied to broad-line-emitting gas in NGC 3783

Both strong and weak ultraviolet emission lines in SeYfert 1 galaxies have been accurately measured from reextracted short-wavelength camera data in the IUE satellite data archives. Line ratios, equivalent widths, and trends as functions of continuum variability were used to constrain numerical modeling parameters in detailed photoionization analyses for NGC 3783. All the data for He II λ1640, C IV λ1549, and C III] λ1909 can be reasonably well reproduced by two cloud components. One has a source-cloud distance of 24 lt-days, gas density around 3x10^10^ cm^-3^, ionization parameter range (as the continuum varies) of 4x10^-2^ to 2 x 10^-1^, and cloud thickness such that ~50% of the carbon is doubly ionized and ~50% is triply ionized. The other component is located approximately 96 lt-days from the source, is shielded from the source by the inner cloud, has a density around 3x10^9^ cm^-3^, is characterized by an ionization parameter range 10^-3^ to 2x10^-2^, and the cloud thickness is such that ~45% carbon is doubly ionized and ~55% is singly ionized. In going from the inner to outer clouds, the optical depth increases and the gas density decreases approximately as r^-2^. In order to produce satisfactory line intensities or trends for other higher ionization lines (e.g., O VI λ1034, N V λ1240) and lower ionization lines (e.g., Mg II λ2798), still more gas components are necessary with gas density ranging from roughly 10^11^ cm^-3^ to 10^9^ cm^-3^ or less. For the multiicomponent models the effective cloud ionization parameters would cover a large range from at least several times 10^-1^ to several times 10^-4^ (including allowance for continuum The amount of dust obscuration along the line of sight, as required by the models, is consistent with reddening estimates from He II line ratios, and CNO abundance ratios derived from intercombination line intensities suggest abundances of carbon, nitrogen and oxygen lower by a factor of about 2 relative to solar. The He II line rest equivalent widths from the models suggest a gas covering factor of order 0.25.

Dynamics of the Outer Arm high-velocity cloud and the triaxiality of the Milky Way

Among high-velocity clouds of neutral hydrogen (HVCs) a so-called Outer Arm Cloud, often interpreted as a part of the general warp of the Milky Way disc, can be distinguished, which is located at 49° ≤ l ≤ 161° and 4° ≤ b ≤ 31° and has quite smooth velocity variation along its length. Closer inspection of the arm shows that its velocity field can be explained in two ways: (i) gas is moving in a circular orbit around the Galactic Centre, but the Sun has a galactocentric radial velocity component with respect to the Local Standard of Rest (LSR) of 28.5 km/s rather than -9.2 km/s as usually accepted; (ii) gas is moving in an elliptic orbit around the Glactic Centre

Nonperturbative structure of the nucleon from high energy and low energy

Analysis of high energy pp and p elastic scattering indicates that the nucleon has an inner core and an outer cloud. At largemomentum transfer, one nucleon core interacts with the other via ω meson exchange, and ω is found to behave as an elementary spin-1 boson. Such a behavior of ω is in accord with the gauged nonlinear σ-model of low energy, which introduces ω as a gauge boson coupled to the baryonic charge and describes the nucleon as a topological soliton. The nonlinear σ-model, however, does not predict an outer cloud for the soliton, nor does it predict a realistic mass for it. How these problems can he resolved is discussed, and the same nucleon structure appears to emerge from both high energy and low energy considerations.

BUMPER DEBRIS CLOUD STRUCTURE ESTIMATED BY SHOCK CALCULATIONS

BUMPER DEBRIS CLOUD STRUCTURE ESTIMATED BY SHOCK CALCULATIONS

Molecular Clouds and Star Formation at Large R

In the outer Galaxy (defined here as those parts of our system with galactocentric radii R>R0) the HI gas density (Wouterloot et al., 1990), the cosmic ray flux (Bloemen et al, 1984) and the metallicity (Shaver et al., 1983) are lower than in the inner parts. Also, the effect of a spiral density wave is much reduced in the outer parts of the Galaxy due to corotation. This changing environment might be expected to have its influence on the formation of molecular clouds and on star formation within them. In fact, some differences with respect to the inner Galaxy have been found: the ratio of HI to H2 surface density is increasing from about 5 near the Sun to about 100 at R≈20kpc (Wouterloot et al., 1990). Because of the “flaring” of the gaseous disk, the scale height of both the atomic and the molecular gas increases by about a factor of 3 between R0 and 2R0 (Wouterloot et al., 1990), so the mean volume density of both constituents decreases even more rapidly than their surface densities. The size of HII regions decreases significantly with increasing galactocentric distance (Fich and Blitz, 1984), probably due to the fact that outer Galaxy clouds are less massive (see section 3.3), and therefore form fewer O-type stars than their inner Galaxy counter parts. There are indications that the cloud kinetic temperature is lower by a few degrees (Mead and Kutner, 1988), although it is not clear to what extent this is caused by beam dilution.

Molecular clouds in the outer Galaxy. IV - Studies of star formation

view Abstract Citations (35) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecular Clouds in the Outer Galaxy. IV. Studies of Star Formation Mead, Kathryn N. ; Kutner, Marc L. ; Evans, Neal J., II Abstract Star formation has been studied in 17 outer Galaxy molecular clouds using the VLA to perform 6 and 20 cm continuum observations to search for H II regions as evidence of massive star formation. IRAS data are used to measure the far-IR luminosity as an indicator of the total star formation rate. H II regions are found to be associated with each of the clouds. The number of ionizing photons required by radio luminosity ranges from 6 x 10 to the 45th/s to 5.5 x 10 to the 49th/s. Far-IR emission was detected from each of the clouds with the range of luminosities being 500 to one million solar. From the data, it is concluded that there is no strong correlation between star formation activity and cloud mass. Publication: The Astrophysical Journal Pub Date: May 1990 DOI: 10.1086/168710 Bibcode: 1990ApJ...354..492M Keywords: Galactic Structure; Milky Way Galaxy; Molecular Clouds; Star Distribution; Star Formation; Continuous Radiation; Far Infrared Radiation; H Ii Regions; Radio Astronomy; Star Formation Rate; Stellar Luminosity; Astrophysics; GALAXIES: THE GALAXY; INTERSTELLAR: MOLECULES; STARS: FORMATION full text sources ADS | data products SIMBAD (24) Related Materials (3) Part 1: 1987ApJ...312..321M Part 2: 1988ApJS...67..149M Part 3: 1988ApJ...330..399M

Dynamical History of the Oort Cloud

AbstractDynamical studies during the past decade have resulted in an almost explosive increase in our understanding of the Oort cloud of comets, which surrounds the solar system. Cometary orbits in the cloud evolve under the complex interaction of stellar, galactic, and giant molecular cloud perturbations, as well as planetary and nongravitational perturbations when the orbits re-enter the planetary region. Evidence has continued to build for a dense, inner Oort cloud of comets which acts as a reservoir to replenish the outer cloud as comets there are stripped away. A ring of comets beyond the orbit of Neptune, which may be the source of the short-period comets, is also likely. Both the estimated number and mass of comets in the Oort cloud have grown such that the total mass may be comparable to the mass of the planets. Temporal variations in the flux of comets from the Oort cloud into the planetary region by a factor of 50% are typical, and by factors of 20 to 200 are possible. The most intense cometary “showers” may have serious implications for biological extinction events on Earth as well as for the impact history of planets and satellite systems. Comets in the Oort cloud are processed by galactic cosmic rays, heated by nearby supernovae, eroded by interstellar dust impacts, and disrupted by mutual collisions (in the inner cloud). A detailed estimate of the Oort cloud’s dynamical history is not possible because of the inability to reconstruct the Sun’s varying galactic motion over the history of the solar system, and because of uncertainty over where comets actually formed. However, it is likely that a substantial fraction of the original Oort cloud population has been lost to interstellar space. We are approaching the time when Oort clouds around other stars may be detectable, though searches to date have so far been negative.

The role of turbulent convection in the primitive solar nebula: II. Results

Numerical results from a new model of the primordial solar nebula are presented in which convection is assumed to be the sole source of turbulence that causes the nebula to evolve. We introduce a new model of convective turbulence (described in detail in Paper I of this series) and new grain opacities computed from an improved physical model. The nebula is assumed to be in a stage prior to planetesimal formation in which gas and dust grains are mixed homogeneously, but in a stage after significant infall of matter from the outer cloud. Vertical structures for a thin nebular disk are calculated for different radii and accretion rates assuming vertical hydrostatic and thermal equilibrium; radial sequences of vertical solutions are constructed for constant accretion rates to represent quasistatic disk structures. Some aspects of our results differ markedly from those done previously by Lin and co-workers. Our values for the turbulent efficiency α (10 −2 to 10 −4) are much lower and much more sensitive to opacity and surface density. Our low values of α result in (1) small turbulent speeds (≤1% of sound speed), which will alter prior computations of grain coagulation and sedimentation rates; (2) a more massive disk (> 0.1 M⊙) that becomes gravitationally unstable at outer (super-Uranian) orbits; (3) a lower “best value” of the accretion rate (∼10 18.5 g sec −1); and (4) a longer characteristic dispersal time for the disk (> 2 × 10 6years), which may greatly exceed that inferred from young stellar objects. The high sensitivity of α on surface density produces an inverse accretion rate-surface density relationship, which implies that the Lightman-Eardley diffusive instability develops througout a steady disk structure in the radial direction, causing the disk, at least at the onset, to separate into rings. Because radial gradients are neglected in the base disk structure, the manner in which the instability evolves to finite amplitude is unknown, but it could prevent the disk from reaching a quasistatic structure altogether. We conclude that convection may not be the dominant source of turbulence needed to evolve young solar/stellar nebulae, and may in fact be a disruptive mechanism in disk structure.

Isoscalar charge radius of the chiral bag

We calculate the contribution of the fermionic vacuum in the bag to the isoscalar charge radius (ICR) of the baryon. ICR can be evaluated as a function of the bag radiusR, the chiral angle θ at the surface of the bag and the parameters characterizing outer pion cloud, e.g.e andFπ in the Skyrme lagrangian.

The Central Regions of M82 – Scattered Light and Extinction

AbstractPhotographic photometric measurements of 2 arcsec resolution of the central regions of M82 are presented in the three colours U, B, V. The observed light is decomposed into two parts: stellar light and light from the central source scattered by dust. The variation of the reddening of both components is derived.The dust distribution pattern is different near the spatial centre of M82 and farther out. The equator of the dust complex obscuring the central source is inclined to the overall symmetry plane of M82. The outer dust clouds responsible for the obscuration of the visible stellar light are probably arranged in a spiral‐like fashion; the “plane” containing these clouds is strongly warped and distrubed.The light of the bright knots A and C consists to a considerable part of scattered light from the central source nearly behind them. Hα light and scattered continuum light is strongly correlated in M82, pointing to an ionisation of the gas by the source of this continuum in addition to the presence of scattered Hα.

On the masses of giant molecular cloud complexes

A method of mass estimation for molecular clouds is presented which is based on approximate balance in the outer cloud layers between the cloud's gravitation, the galactic tide, and internal pressure. The largest observed clouds, which have greatest linear extents of 100 pc, are found to have masses of at least 200,000 solar masses. The cloud masses cannot exceed this lower limit by more than a factor of 3, or the velocity distributions of disk stars would be more relaxed than is actually observed. This implied upper limit to cloud masses combined with the galactic tide may be related to the absence of clouds at galactocentric radii less than 4 kpc. If Sagittarius B2 is bound, its mass must be more than 50 million solar masses.

Inner Symmetry in Baryon Structures and Leptonic Decays

It may be accepted that the baryon is composed of the outer virtual cloud and the inner subnuclear system. The central problem of the present day physics. is to investigate the structure of the inner subnuclear system. If we take the partial success of the su6 symmetry and the simple addition rules of the mass formula, we may prefer the speculation that the inner subnuclear part of the baryon is a composite system of three urfermions in a nonrelativistic bound state, to the other possibilities. Therefore, it may be worthwhile to pursue the . problem by means of the notions and the tools of the nuclear structure theory. In particular, it would be interesting to look over the inner symmetries of three urfermions in the baryon structure.*) In this note, we see how the inner sym

Die elektromagnetische Struktur des Protons und Neutrons

The structures of protons and neutrons were studied, including the anomalous magnetic moment by electron scattering experiments. The results indicated that the protons and neutrons are made up of nuclei surrounded by clouds of charge. The charge distribution of the proton and neutron possesses a nucleus containing 35% of the charge and with a radius of about 0.2 fermi. In the proton this nucleus is surrounded by two positive clouds, but in the neutron the inner cloud contains a negative charge. The outer cloud leads to a weak positive covering. The anomalous magnetic moment results from a cloud with a radius of 0.8 fermi, which is positive for the proton and negative for the neutron. (M.C.G.)

Antinucleon-Nucleon Interactions

A simple model for the nucleon-antinucleon interactions is presented, in which the nucleon or the antinucleon is regarded as composed of two parts, the pion cloud and the core. The latter is characterized by two properties : i) it acts as a n.early black body for an incoming antiparticle wave, and ii) its characteristic time for ato.nihilation is much shorter than the oscillation period of the outer pion cloud. These simple assumptions lead to natural semi-quantitative explanations of the salient features of the experimental information so far available concerning the nucleon-antinucleon reaction and capture processes. Some arguments are added to look for a justification of such an approach.