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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

A far-infrared spectral line survey of 23 infrared-bright Galaxy nuclei

We present results from a KAO survey of fine-structure lines observed in 23 infrared-luminous galaxies. One or more of the following lines was observed and/or detected in each galaxy: (S III) 19, 33 microns, (Ne V) 24 microns, (O IV) 26 microns, (Fe II) 26 microns, (Si II) 35 microns, (O III) 52, 88 microns, (0 I) 63, 146 microns, (N III) 57 micro ns, (N II) 122, 205 microns, (C II) 158 microns. The galaxies span a wide range of morphologies (irregular to grand design), have varying metallicities, and include mergers, AGN's, and starburst systems. The observations were made beginning in 1988 using the facility Cryogenic Grating Spectrometer onboard the KAO at a typical resolution of approximately 60-140 km/s and with a 30-44 deg beam. We interpret the (C II) and (O I) fluxes, along with previous measurements of the IR continuum fluxes, in the context of photo dissociation region (PDR) models (Tielens & Hollenbach 1985; Wolfire et al. 1990). With these models, we obtain estimates of the typical interstellar UV fields incident on the line emitting regions (102-104 times the local interstellar radiation field) and the total masses (10(exp 7)-10(exp 8) Solar Mass), densities (10(exp 3)-10(exp 4)/cu cm), and temperatures (100-250 K) of the warm atomic gas. The (O III) (52/88) and (S III) (33/19) line flux ratios constrain the range of electron densities and pressures found within the ionized regions. The (O III) and (S III) lines also provide estimates of the effective temperature of the ionizing stars and elemental abundances within the ionized regions of these galactic nuclei. Our measurements imply typical gas pressures of nT approximately 5 x 10(exp 6)/cu cm K and typical upper mass cutoffs of 25-35 Solar Mass. The low-metallicity systems show high (C II)/CO and (O I)/CO flux ratios, 3-5 times the Milky Way value, indicating that they contain a larger fraction of photodissociated gas relative to the molecular material.

Lord, Steven D.↗
NASA NTRS1994

The interstellar medium in the starburst regions of NGC 253 and NGC 3256

We discuss observations of the (C II) 158 micrometers, (O I) 63 micrometers, (Si II) 35 micrometers, (O III) 52,88 micrometers, and (S III) 33 micrometers fine-structure transitions toward the central 45 seconds of the starburst galaxies NGC 253 and NGC 3256. The (C II) and (O I) emission probably originates in photodissociated gas at the surfaces of molecular clouds, although a small (less than or approximately 30%) contribution to the (C II) flux from H II regions cannot be ruled out. The (O III) and (S III) lines originate in H II regions and the (Si II) flux is best explained as originating in H II regions with some contribution from photodissociation regions (PDRs). The gas phase silicon abundance is nearly solar in NGC 253, which we interpret as evidence for grain destruction in the starburst region. We find that the photodissociated atomic gas has densities approximately 10(exp 4)/cu cm and temperature 200-300 K. About 2% of the gas is in this phase. The thermal gas pressure in the PDRs, P(PDR)/k approximately 1-3 x 10(exp 6) K/cu cm, might represent the 'typical' interstellar gas pressure in starburst systems. The Far Ultraviolet (FUV) radiation fields illuminating the clouds are 10(exp 3)-10(exp 4) stronger than the local Galactic FUV field and come from the contribution of many closely packed O and B stars. For the central 250 pc of NGC 253, we find that the H II gas has an average density n(sub e) is approximately 400/cu cm. This corresponds to a thermal pressure P(H II)/k approximately 7 x 10(exp 6) K/cu cm which is approximately P(PDR)/k, suggesting that the ionized gas is in pressure equilibrium with the photodissociated gas at the surfaces of molecular clouds. The H II gas fills a significant fraction, approximately 0.01-0.3, of the volume between the clouds. The effective temperature of the ionizing stars in NGC 253 is greater than or approximately 34,500 K; 2 x 10(exp 5) O7.5 stars would produce the observed Lyman countinuum photon luminosity. The average separation between the stars is approximately 3 pc. Applying the simple model for the interstellar medium in galactic nuclei of Wolfire, Tielens, & Hollenbach (1990), we find the molecular gas in the central regions of NGC 253 and NGC 3256 to be distributed in a large number (5 x 10(exp 3) to 5 x 10(exp 5)) of small (0.5-2 pc), dense (approximately 10(exp 4)/cu cm) clouds (or alternatively 'thin-flattened' structures) with volume filling factors 10(exp -3) to 10(exp -2), very different from the local Interstellar Medium (ISM) of the Galaxy. We suggest a self-consistent scenario for the ISM in NGC 253 in which clouds and H II gas are in pressure balance with a supernova-shocked, hot 1-3 x 10(exp 6) K, low-density (approximately 10(exp 4)/cu cm), all pervasive medium. A feedback mechanism may be indicated in which the pressure generated by the supernovae compresses the molecular clouds and triggers further massive star formation. The similarity of ISM parameters deduced for NGC 253, NGC 3256, and M82 (Lord et al. 1993) suggests that the ISM properties are independent of the luminosity of the starburst or the triggering mechanism, but are rather endemic to starburst systems. The starburst in NGC 3256 appears to be a scaled-up version of the NGC 253 and M82 starbursts.

Carral, P.↗
NASA NTRS1994

Origin of the O(+) pick-up ions in the hehiosphere

The main sources for the ions in the interplanetary medium are the solar wind and the interstellar gas intruding into the heliosphere. Other sources, i.e. comets and planets can, however, contribute significantly locally. We present approximate expressions for deriving fluxes of pick-up ions originating from local neutral gas sources. Since the Io-torus is thought to be a relatively strong emitter of fast atoms, particular O and S, we used the O(+) data obtained by SWICS-Ulysses as a function of distance from Jupiter, both in and out-off the ecliptic plane in order to recognize a possible contribution of the Ionian source to the measured O(+). We find that - except for a very limited zone near the Jovian magnetosphere - the contribution of the Io-torus to the O(+) fluxes is minor, thus confirming our previous results on the pick-up H(+), He(+), N(+), O(+), and Ne(+) fluxes of interstellar origin.

Geiss, J.↗
NASA NTRS1977

A survey of interstellar molecular hydrogen. I

Data from the Copernicus satellite's ultraviolet telescope were used to survey column densities of atomic and molecular hydrogen from a large sample of early-type stars; these data have bearing on an eventual understanding of diffuse and dense interstellar clouds. Column densities are derived by fitting damping profiles to the observed spectra, most of which exhibit strong damping lines in the lower rotational levels surveyed. Plots of dust column density, fractional abundance of molecular hydrogen, and the logarithm of fractional abundance versus total gas column density are given for many of the stars; stars with abnormally large or small hydrogen column densities, as well as some distant stars at high galactic latitudes, are considered. Equilibrium and nonequilibrium theories accounting for the abundance of interstellar hydrogen are compared, and support is found in the data for an account which balances hydrogen formation on interstellar grains with destruction through photodissociation. Overall averages for atomic and molecular hydrogen levels in the galactic plane are also calculated.

Savage, B. D.↗
NASA NTRS1994

An IUE survey of interstellar H I Ly alpha absorption. 2: Interpretations

We present an analysis of interstellar neutral hydrogen column densities obtained from archival Ly alpha absorption line data toward the complete sample of B2 and hotter stars observed at high spectral resolution with the IUE satellite. The full sample includes 554 stars and more than doubles the number of lines of sight previously observed. Many of the B2 and B1.5 stars exhibit contamination from stellar Ly alpha absorption, and were excluded from the subsequent analysis. The final working sample includes 393 stars. We present statistical averages for a number of quantities derived for the sample. The stars range in distance from 0.12 to 11 kpc with an average distance of 2.1 kpc. Values for average sight-line density range from 0.017 to 8.62 atoms cm(exp -3), with an overall average of 0.23 atoms cm(exp -3). Neutral hydrogen and dust are well correlated, with N(H I)/E(B - V) = 4.93 x 10(exp 21) cm(exp -2) mag(exp -1), and N(H I)/E(Bump) = 2.17 x 10(exp 21) cm(exp -2) mag(exp -1). There is an increase in the H I to dust ratios for the densest sight lines. Evidently, the H I to dust ratios in dense clouds are higher than in the lower density medium between the clouds and in interarm directions. The effect is probably an indication of dust modification in the dense clouds. The H I to dust ratios are approx. 17% smaller for the sight lines to O stars compared to the ratios found toward B stars. We have identified sight lines with anomalously large and small values of N(H I)/E(B - V) and H(H I)/E(Bump). Some of these sight lines are well-known examples of peculiar extinction (i.e., HD 147933 and HD 37061 (NU Ori)). Others are less well known and may provide important new examples of regions with highly modified dust. The number of stars with anomalously low H I to dust ratios is disproportionately larger than the number of stars with anomalously high H I to dust ratios. The distribution of the gas away form the Galactic plane for the sample of objects is complex. For the subset of 375 stars whose lines of sight have a statistically low probablility of intersecting a large cloud, the distribution may be roughly approximated by an exponential density distribution, with a midplane density of 0.366 atoms cm(exp -3), a scale height of 195 pc, and a random logarithmic scattering parameter sigma (sub p) of 0.159 dex. For the same sample of stars, interstellar dust is found to be confined closer to the plane than the H I, with (E(B - V)/r)(sub 0) = 0.257 mag kpc(exp -1), scale height h = 152 pc, and sigma(sub p) = 0.263 dex. The dust distribution is more inhomogeneous than the gas distribution. The observed distribution of H I away from the Galactic plane can also be fitted with a two-component exponential model including the effects of patchiness. Unfortunately, the foreground contamination produced by the cloudy and more confined component of H I makes it difficult to estimate the scale height of the extended component without invoking an independent estimate of its midplane density. Assuming n(sub 2)(0) = 0.16 cm(exp -3) from the Copernicus satellite H I/H(sub 2) survey, we obtain n(sub 1)(0) = 0.247 cm(exp -3), h(sub 1) = 73 PC, and h(sub 2) + 357 pc.

Diplas, Athanassios↗
NASA NTRS1976

Components in interstellar molecular hydrogen

Results are reported for precise spectrophotometric measurements of the profiles of selected Lyman absorption lines produced by hydrogen molecules in various rotational levels along the line of sight to 13 stars which have shown some evidence for an increase in line width with increasing rotational quantum number (J). The line profiles were measured by multiple scans with the Copernicus satellite telescope. Based on analysis of the radial velocities, derivations of the column densities, and line-profile fitting, the following conclusions are made: (1) the increase in interstellar H2 line width with increasing J results from the presence of the most shortward component, which is relatively weak at low J but becomes more important at higher J; (2) the relative column densities found for the different J levels in each component may be fitted by a theoretical model in which rotational excitation is due to absorption of UV photons followed by radiative quadrupole spontaneous transitions or collisionally induced downward transitions between different J levels; (3) the atomic hydrogen density is between 300 and 1000 per cu cm in the most shortward component for each of three stars; (4) the approaching gas which produces each shortward component must be in the form of thin sheets; and (5) the sheets are the compressed gas behind a shock front moving through the interstellar medium.

Spitzer, L., Jr.↗
NASA NTRS2001

Spectroscopy of Cosmic Carbon Analogs in Inert-Gas Matrices and in the Gas-Phase: Comparative Results and Perspectives for Astrophysics

Recent studies of the spectroscopy of large (up to approx. 50 carbon atoms) neutral and Ionized polycyclic aromatic hydrocarbons (PAHs) and Fullerenes isolated in inert gas matrices will be presented. The advantages and the limitations of matrix isolation spectroscopy for the study of the molecular spectroscopy of interstellar dust analogs will be discussed. The laboratory data will be compared to the astronomical spectra (the interstellar extinction, the diffuse interstellar bands). Finally, the spectra of PAH ions isolated in neon/argon matrices will be compared to the spectra obtained for PAH ion seeded in a supersonic expansion. The astrophysical implications and future perspectives will be discussed.

Salama, Farid↗
NASA NTRS1978

Observations of interstellar chlorine and phosphorus

Copernicus observations of interstellar Cl I, Cl II, and P II UV lines toward 10 stars are reported. Column densities are estimated for each species, and upper limits are computed for HCl column densities. Derivation of the gas-phase abundances of chlorine and phosphorus indicates that the averages of both the chlorine and the phosphorus logarithmic abundances relative to hydrogen are between 5.0 and 5.1. It is suggested that interstellar chlorine may be depleted by about a factor of 3 relative to the solar abundance and that interstellar phosphorus is depleted by a factor of 2 to 3. The results are shown to support the prediction that chlorine is ionized in regions containing primarily atomic oxygen and is neutral in regions where there is a significant amount of molecular hydrogen. The photoionization rate of neutral chlorine toward 15 Mon is estimated, and it is concluded that most chlorine is contained within the gas phase.

Jura, M.↗
NASA NTRS2006

Laboratory Spectroscopy of Large Carbon Molecules and Ions in Support of Space Missions

One of the major objectives of Laboratory Astrophysics is the optimization of data return from space missions by measuring spectra of atomic and molecular species in laboratory environments that mimic interstellar conditions (WhitePaper (2002, 2006)). Among interstellar species, PAHs are an important and ubiquitous component of carbon-bearing materials that represents a particularly difficult challenge for gas-phase laboratory studies. We present the absorption spectra of jet-cooled neutral and ionized PAHs and discuss the implications for astrophysics. The harsh physical conditions of the interstellar medium have been simulated in the laboratory. We are now, for the first time, in the position to directly compare laboratory spectra of PAHs and carbon nanoparticles with astronomical observations. This new phase offers tremendous opportunities for the data analysis of current and upcoming space missions geared toward the detection of large aromatic systems (HST/COS, FUSE, JWST, Spitzer).

Salana, Farid↗
NASA NTRS2003

Theoretical Studies of the Extra-terrestrial Chemistry of Biogenic Elements and Compounds

Results are presented on the following:(A) Ab initio quantum chemical studies of reactions in astrophysical ices.Theoretical electronic structure calculations were used to investigate reactions between formaldehyde (H2CO) and both hydrogen cyanide (HCN) and isocyanide (HNC) in search of other favorable reactions such as ammonia-formaldehyde addition, which was found in a recent theoretical study to be strongly enhanced when it occurs within cold ices.The present study examines further reactions between this product and H2CO in ices.(B) Heterogeneous hydrogenation of CO and H2CO on icy grain mantles.Formaldehyde (H2CO) and methanol (CH30H) are thought to be produced in the interstellar medium by the successive hydrogenation of carbon monoxide (CO) on grain surfaces. In the gas phase, the steps in which H adds to CO and H2CO possess modest barriers and are too inefficient to account for the observed abundances. Recent laboratory work has confirmed that formaldehyde and methanol are formed when H atoms are deposited on CO ice at 12 K. The present study employed ab initio quantum chemical calculations to investigate the impact of water ice on the sequential hydrogenation of CO.(C) Ice-bound condensed-phase reactions involving formic acid (HCOOH), methylenimine (CH2NH), hydrogen cyanide (HCN), hydrogen isocyanide (HNC), and ammonia ( 3) were investigated in order to characterize possible pathways to larger organic species that are efficient at the cold temperatures prevalent in cometary nuclei and the interstellar medium. (D) Pathways to glycine and other amino acids in ultraviolet-irradiated ices determined via quantum chemical modeling.(E) Photoionization in ultraviolet processing of astrophysical ice analogs at cryogenic temperatures.

Woon, David E.↗
DOE OSTI2023

Gas-phase formation of the resonantly stabilized 1-indenyl (C 9 H 7 • ) radical in the interstellar medium

The 1-indenyl (C 9 H 7 • ) radical, a prototype aromatic and resonantly stabilized free radical carrying a six- and a five-membered ring, has emerged as a fundamental molecular building block of nonplanar polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanostructures in deep space and combustion systems. However, the underlying formation mechanisms have remained elusive. Here, we reveal an unconventional low-temperature gas-phase formation of 1-indenyl via barrierless ring annulation involving reactions of atomic carbon [C( 3 P)] with styrene (C 6 H 5 C 2 H 3 ) and propargyl (C 3 H 3 • ) with phenyl (C 6 H 5 • ). Macroscopic environments like molecular clouds act as natural low-temperature laboratories, where rapid molecular mass growth to 1-indenyl and subsequently complex PAHs involving vinyl side-chained aromatics and aryl radicals can occur. These reactions may account for the formation of PAHs and their derivatives in the interstellar medium and carbonaceous chondrites and could close the gap of timescales of their production and destruction in our carbonaceous universe.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1997

Discovery of Interstellar Hydrogen Fluoride

We report the first detection of interstellar hydrogen fluoride. Using the Long Wavelength Spectrometer of the Infrared Space Observatory (ISO), we have detected the 121.6973 micron J = 2-1 line of HF in absorption toward the far-infrared continuum source Sagittarius B2. The detection is statistically significant at the 13 sigma level. On the basis of our model for the excitation of HF in Sgr B2, the observed line equivalent width of 1.0 nm implies a hydrogen fluoride abundance of about 3 x 10 (exp -10) relative to H, If the elemental abundance of fluorine in Sgr B2 is the same as that in the solar system, then HF accounts for about 2% of the total number of fluorine nuclei. We expect hydrogen fluoride to be the dominant reservoir of gas-phase fluorine in Sgr B2, because it is formed rapidly in exothermic reactions of atomic fluorine with either water or molecular hydrogen; thus, the measured HF abundance suggests a substantial depletion of fluorine onto dust grains. Similar conclusions regarding depletion have previously been reached for the case of chlorine in dense interstellar clouds. We also find evidence at a lower level of statistical significance (about 5 sigma) for an emission feature at the expected position of the 4(sub 32)-4(sub 23) 121.7219 micron line of water. The emission-line equivalent width of 0.5 mm for the water feature is consistent with the water abundance of 5 x 10(exp -6) relative to H, that has been inferred previously from observations of the hot core of Sgr B2.

Neufeld, David A.↗
NASA NTRS1997

Discovery of Interstellar Hydrogen Fluoride

We report the first detection of interstellar hydrogen fluoride. Using the Long Wavelength Spectrometer of the Infrared Space Observatory (ISO), we have detected the 121.6973 micron J = 2-1 line of HF in absorption toward the far-infrared continuum source Sagittarius B2. The detection is statistically significant at the 13 sigma level. On the basis of our model for the excitation of HF in Sgr B2, the observed line equivalent width of 1.0 nm implies a hydrogen fluoride abundance of approximately 3 x 10(exp -10) relative to H2. If the elemental abundance of fluorine in Sgr B2 is the same as that in the solar system, then HF accounts for approximately 2% of the total number of fluorine nuclei. We expect hydrogen fluoride to be the dominant reservoir of gas-phase fluorine in Sgr B2, because it is formed rapidly in exothermic reactions of atomic fluorine with either water or molecular hydrogen; thus, the measured HF abundance suggests a substantial depletion of fluorine onto dust grains. Similar conclusions regarding depletion have previously been reached for the case of chlorine in dense interstellar clouds. We also find evidence at a lower level of statistical significance (approximately 5 sigma) for an emission feature at the expected position of the 4(sub 32)-4(sub 23) 121.7219 micron line of water. The emission-line equivalent width of 0.5 nm for the water feature is consistent with the water abundance of 5 x 10(exp -6) relative to H2 that has been inferred previously from observations of the hot core of Sgr B2.

Neufeld, David A.↗
NASA NTRS1975

Charge transfer reactions in multiply charged ion-atom collisions

Charge-transfer reactions in collisions between highly charged ions and neutral atoms of hydrogen and/or helium may be rapid at thermal energies. If these reactions are rapid, they will suppress highly charged ions in H I regions and guarantee that the observed absorption features from such ions cannot originate in the interstellar gas. A discussion of such charge-transfer reactions is presented and compared with the available experimental data. The possible implications of these reactions for observations of the interstellar medium, H II regions, and planetary nebulae are outlined.

Steigman, G.↗
NASA NTRS1980

Copernicus observations of C I and CO in diffuse interstellar clouds

Copernicus was used to observe absorption lines of C I in its ground state and excited fine structure levels and CO toward 29 stars. We use the C I data to infer densities and pressures within the observed clouds, and because our results are of higher precision than previous work, much more precise estimates of the physical conditions in clouds are obtained. In agreement with previous work, the interstellar thermal pressure appears to be variable, with most clouds having values of p/k between 1000/cu cm K and 10,000/cu cm K, but there are some clouds with p/k as high as 100,000/cu cm K. Our results are consistent with the view that the interstellar thermal pressure is so variable that the gas undergoes continuous dynamic evolution. Our observations provide useful constraints on the physical processes on the surfaces of grains. In particular, we find that grains are efficient catalysts of interstellar H2 in the sense that at least half of the hydrogen atoms that strike grains come off as part of H2. Results place strong constraints on models for the formation and destruction of interstellar CO. In many clouds, an order of magnitude less CO than predicted in some models was found.

Jenkins, E. B.↗
NASA NTRS2019

The Characterization of Secondary Interstellar Neutral Oxygen Beyond the Heliopause: A Detailed Analysis of the IBEX-Lo Oxygen Observations

In this study, we analyze the directional distribution of the secondary interstellar neutral (ISN) O population observed by the IBEX-Lo neutral atom camera on the Interstellar Boundary EXplorer (IBEX) via the comparison with simulated ISN O intensity maps produced by an analytical model. In the analytical model, we assume that there are primary and secondary ISN populations at the heliopause. We further assume that each population is represented by a Maxwellian velocity distribution function with its own flow parameters. For the viewing directions of IBEX-Lo, we compute the incoming atom speeds at the heliopause with a Keplerian equation of motion in the solar gravity field. Then, we calculate analytically the distribution function to obtain the ISN intensities at Earth’s orbit. We compare the simulated O intensity maps with the IBEX-Lo O sky map to determine the most likely flow parameters of the secondary ISN O population. Using this method, we find the most likely flow parameters of the secondary ISN O population: V(sub SecISNO) = 11 ± 2.2 km s(exp -1), λ(sub SecISNO) = 67° ± 1°.5, β(sub SecISNO) = -12° ± 1°.6, and T(sub SecISNO) = 10,000 ± 1500 K. The results indicate that the secondary ISN O flow direction is deflected toward lower ecliptic longitude and higher negative ecliptic latitude from the ISN gas flow direction at the heliopause. The secondary ISN O flow direction is more deflected from the ISN gas flow direction than the secondary ISN He flow direction.

Interstellar matter clouds↗
NASA NTRS1994

The photoelectric heating mechanism for very small graphitic grains and polycyclic aromatic hydrocarbons

We have theoretically modeled the gas heating associated with the photoelectric ejection of electrons from a size distribution of interstellar carbon grains which extends into the molecular domain. We have considered a wide range of physical conditions for the interstellar gas (1 less than G(sub 0) less than 10(exp 5), with G(sub 0) being the intensity of the incident far-UV field in units of the Habing interstellar radiation field; 2.5 x 10( exp -3) less than n(sub e) less than 75/cu cm, with n(sub e) being the electron density; 10 less than T less than 10,000 K, with T being the gas temperature). The results show that about half of the heating is due to grains less than 1500 C atoms (less than 15 A). The other half originates in somewhat larger grains (1500-4.5 x 10(exp 5) C atoms; 15 less than 100 A). While grains larger than this do absorb about half of the available far-UV photons, they do not contribute appreciably to the gas heating. This strong dependence of gas heating on size results from the decrease in yield and from the increased grain charge (hence larger Coulomb losses) with increasing grain size. We have determined the net photoelectric heating rate and evaluated a simple analytical expression for the heating efficiency, dependent only on G(sub 0), T, and n(sub e). This expression is accurate to 3% over the whole parameter range and is valid up to gas temperatures of 10(exp 4) K, at which point the dominant gas-dust heat exchange mechanism becomes the recombination of electrons with grains rather than photoelectric ejection. The calculated heating efficiency for neutral grains is in good agreement with that derived from observations of the diffuse interstellar clouds. Our results also agree well with the Far Infrared Absolute Spectrometer (FIRAS) observations on the Cosmic Background Explorer Satellite. Finally, our photoelectric heating efficiency is compared to previous studies.

Bakes, E. L. O.↗
NASA NTRS2003

Ring Around the Black Hole

Regardless of size, black holes easily acquire accretion disks. Supermassive black holes can feast on the bountiful interstellar gas in galactic nuclei. Small black holes formed from collapsing stars often belong to binary systems in which a bulging companion star can spill some of its gas into the black hole s reach. In the chaotic mess of the accretion disk, atoms collide with one another. Swirling plasma reaches speeds upward of 10% that of light and glows brightly in many wavebands, particularly in X-rays. Gas gets blown back by a wind of radiation from the inner disk. New material enters the disks from different directions.

Wanjek, Christopher↗