Nebular Mass Research Articles

Sizes and Masses of Chondrules and Metal–Troilite Grains in Ordinary Chondrites: Possible Implications for Nebular Sorting

Intergroup variation in chondrule sizes is commonly attributed to mass or aerodynamic sorting in the solar nebula, a concept that has recently been extended to other chondrite constituents (metal chondrules, metal–troilite grains). Both sorting mechanisms are dependent on grain density and size. Because metal chondrules and metal–troilite grains have smaller average sizes than their coexisting chondrule populations, the assumption of mass equivalence has been made and invoked in support of nebular sorting. We present the results of a quantitative comparison of the sizes, masses, and aerodynamic stopping times of chondrules and metal–troilite grains from three ordinary chondrites: Kelly (LL4), Bjurböle (L/LL4), and Hammond Downs (H4). Chondrule volumes were determined from corrected thin-section measurements, and metal–troilite grain volumes were estimated from X-ray tomographic images. Chondrule and metal–troilite grain populations have similar masses in Hammond Downs but are dissimilar in Bjurböle and Kelly. The difference in the average particle aerodynamic stopping times of the chondrules and metal–troilite grains (15% for Hammond Downs, 16% for Bjurböle, 32% for Kelly) are much smaller than differences in their average masses (28% for Hammond Downs, 73% for Bjurböle, 82% for Kelly). The observed ranges in mass of the two populations are relatively narrow in Hammond Downs and are wider in Bjurböle and Kelly. Furthermore, in all three meteorites the observed range in mass of the chondrules is narrower than that of the corresponding metal–troilite grain populations. It appears that the chondrules were sorted more efficiently than the metal–troilite grains. Our results agree with the idea that aerodynamic stopping times vary with particle size and density and disagree with sorting only by mass. While the average stopping times (expressed as rp ρs) of the two populations correlate better than their average masses, the percent difference in (rp ρs) between the two populations (Hammond Downs, 15%; Bjurböle, 16%; and Kelly, 32%) is greater than that between the chondrules and metal chondrules previously reported for the Acfer 059 carbonaceous chondrite (W. R. Skinner and J. M. Leenhouts 1993, Proc. Lunar Planet. Sci. Conf. 24th, 1315–1316). We attribute this to the small and irregular shapes of the metal–troilite grains, although thermal metamorphism may have also affected the metal–troilite grain data. For these reasons, our results are at variance with the concept of nebular mass sorting but may be in agreement with aerodynamic sorting models. Our results are consistent with the hypothesis that chondrule sorting is related to the phenomena of metal–silicate fractionation. However, these data are only preliminary. Final interpretation should be reserved until more meteorites can be analyzed, the effects of thermal metamorphism on metal grain sizes quantified, and software capable of true three-dimensional analysis developed.

Morphology and Composition of the Helix Nebula

We present new narrowband filter imagery in Hα and [N II] λ6584, along with UV and optical spectrophotometry measurements from 1200 to 9600 A of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large diameter and low surface brightness. Detailed models of the observable ionized nebula support the recent claim that the Helix is actually a flattened disk whose thickness is roughly one-third its diameter, with an inner region containing hot, highly ionized gas that is generally invisible in narrowband images. The outer visible ring structure is of lower ionization and temperature and is brighter because of a thickening in the disk. We also confirm a central star effective temperature and luminosity of 120,000 K and 100 L☉, and we estimate a lower limit to the nebular mass to be 0.30 M☉. Abundance measurements indicate the following values: He/H=0.12 (± 0.017), O/H=4.60×10-4 (± 0.18), C/O=0.87 (± 0.12), N/O=0.54 (± 0.14), Ne/O=0.33 (± 0.04), S/O=3.22 × 10-3 (± 0.26), and Ar/O=6.74 × 10-3 (± 0.76). Our carbon abundance measurements represent the first of their kind for the Helix Nebula. The S/O ratio that we derive is anomalously low; such values are found in only a few other planetary nebulae. The central star properties, the supersolar values of He/H and N/O, and a solar level of C/O are consistent with a 6.5 M☉ progenitor that underwent three phases of dredge-up and hot bottom burning before forming the planetary nebula.

Ring nebulae: what they tell us about Wolf-Rayet stars

The environments of evolved massive stars provide an opportunity of obtaining information on the past, as well as current, condition of the stars themselves. In this review we will look at the incidence of ring nebulae around Wolf-Rayet stars, their differing morphologies at various wavelengths and the existence of multiple, concentric shells. We use this information to show that WRs are indeed evolved stars and that the various phases of evolution for a WR star are evidenced in their environments. Abundance measurements and kinematics show that complex forms of mass ejection are likely to have occurred in the evolution of WR stars providing clumpy structures of dust, and both ionized and neutral gas. Gas kinematics also provide estimates to the time-scales of each of the evolutionary phases of WR stars, which can be combined with estimates of nebular masses to provide the approximate values for such crucial parameters as total mass-loss and historical mass-loss rates. Overall, it is illustrated that studies of the environments of WR stars have the potential to provide important information about the mass-loss history of very massive stars, including estimates of the time period of each mass-loss phase, typical mass loss rates, total mass lost and likely evolutionary path. Some of the remaining problems relating to the use of ring nebulae as probes to the evolutionary history of WR stars are also discussed.

A Circumstellar Dust Disk around T Tauri N: Subarcsecond Imaging at λ = 3 Millimeters

We present high-resolution imaging of the young binary T Tauri in continuum emission at ? = 3 mm. Compact dust emission with integrated flux density 50 ? 6 mJy is resolved in an aperture synthesis map at 05 resolution and is centered at the position of the optically visible component, T Tau N. No emission above a 3 ? level of 9 mJy is detected 07 south of T Tau N at the position of the infrared companion, T Tau S. We interpret the continuum detection as arising from a circumstellar disk around T Tau N, and estimate its properties by fitting a flat-disk model to visibilities at ? = 1 and 3 mm, and to the flux density at ? = 7 mm. Given the data, probability distributions are calculated for values of the free parameters, including the temperature, density, dust opacity, and disk outer radius. The radial variation in temperature and density is not narrowly constrained by the data. The most likely value of the frequency dependence of the dust opacity, ?=0.53 -->?0.17+0.27, is consistent with that of disks around other single T Tauri stars in which grain growth is believed to have taken place. The outer radius, R=41 -->?14+26 AU, is smaller than the projected separation between T Tau N and T Tau S, and may indicate tidal or resonance truncation of the disk by T Tau S. The total mass estimated for the disk, log (MD/M?) = -2.4 -->?0.6+0.7, is similar to masses observed around many single pre-main-sequence sources and, within the uncertainties, is similar to the minimum nebular mass required to form a planetary system like our own. This observation strongly suggests that the presence of a binary companion does not rule out the possibility of formation of a sizable planetary system.

Sakurai's object: the ionized nebula at radio wavelengths

Sakurai's object (V4334 Sgr) is a planetary nebula nucleus which is undergoing its final helium shell flash. This is the first of these rare and important events to be observable with non-optical instruments. We report the first radio detection, using a short (2-h) observation with the Very Large Array (VLA) at 4.86 GHz. The radio emission structure is coincident with the 34-arcsec diameter planetary nebula seen in optical emission lines. We find a statistical distance ∼ 3.8 ± 0.6 kpc, with a range of 1.9 < D < 5.3 kpc, depending on the planetary nebula (PN) mass. While we have no direct evidence for a new (post-flash) stellar wind, we estimate an upper limit to the mass-loss rate due to any such wind of 1.7 × 10−7 M⊙ yr−1. The number of emitting knots in the radio-visible nebula indicates an electron density of ∼ 2 × 108 m−3 in those knots, and a total emitting ionized mass of ∼ 0.15 M⊙, at an assumed distance of 3.8 kpc. The radio flux density indicates an Hβ flux of ∼ 6 × 10−16 W m−2, suggesting an extinction E(B − V) ∼ 1.15, comparable with reddening estimates in the direction of V4334 Sgr.

Mid‐Infrared (8–21 micron) Imaging of Proto–Planetary Nebulae

We present mid-infrared (8-21 μm) images of thermal dust emission from two proto-planetary nebulae (PPNs), IRAS 07134+1005 and IRAS 22272+5435, which show a strong 21 μm emission feature. Both of the sources are well resolved and show evidence for axial symmetry. From our images, we calculate temperature and optical depth maps and estimate the abundance of the 11 μm and 21 μm feature carriers. In both sources, the dust temperatures range from ~160-200 K. The optical depths in IRAS 07134 are about a factor of 3 lower than those in IRAS 22272, but the emission is optically thin in both sources. Our analyses of the feature-to-continuum ratios suggests that 0.5%-5% of the carbon in these objects may be in the form of large PAH molecules. We construct optically thin, axially symmetric cylindrical shell models to simulate the observed mid-IR morphologies and spectra, and calculate nebular masses of 0.26 M☉ for IRAS 07134 and 0.42 M☉ for IRAS 22272. Although the mid-IR emission primarily comes from warm (T ≈ 190 K) dust, our models require a significant cooler dust (T ≈ 80 K) component to fit the observed mid- and far-IR spectral energy distributions.

Models for the fractionation of moderately volatile elements in the solar nebula

Abstract— We test the validity of the idea that the abundances of moderately volatile elements in chondritic meteorites reflect global condensation and coagulation in an evolving solar nebula and explore what constraints these abundances might place on nebular parameters. The abundances of moderately volatile elements were identified as particularly suitable for modeling the nebula because the data represent a simple pattern explicable in terms of a straightforward hypothesis which has implications for global evolution. The models incorporate a correspondingly simple prescription, derived from theory, for the rate at which condensible material is decoupled from the evolving nebular gas to become part of the surviving planetary system. It is concluded from model results that cooling, diminishing nebular mass and coagulation of solids, through their mutual dependences on accretion rate and opacity, lead naturally to the chondritic depletion patterns of moderately volatile elements. In particular, the trends found in CO and CV meteorites are readily and accurately produced; those of the CM meteorites are less so. The CM meteorites and ordinary chondrite patterns may require a more complicated model, but the full range of possible results from even the simplest model has yet to be completely determined.

Distances to Planetary Nebulae BD +30 degrees 3639 and NGC 6572

view Abstract Citations (37) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Distances to Planetary Nebulae BD +30 degrees 3639 and NGC 6572 Kawamura, J. ; Masson, C. Abstract Third-epoch Very Large Array proper-motion measurements on the planetary nebulae BD +30°3639 and NGC 6572 have been made. Combining these measurements with simple models for the nebulae and using improved analysis, it has been determined that the distance to BD +30°3639 is 1.5±0.4 kpc and NGC 6572 is 1.2±0.4 kpc. These measurements are consistent with the distances determined from the previous epoch. However, the errors are significantly smaller and are now dominated not by the measurements but by the modeling and the uncertainty in the flux variation. The distances are significantly higher than those determined by statistical methods, with important consequences for calculation of the nebular masses and luminosities. In addition to their distances, the observations have elucidated the dynamical properties of the objects. First, the expansion of a shell is consistent with the self-similar expansion that is usually assumed for these objects, with the magnitude of the proper motion of a portion of a shell being linearly related to its distance from the center of symmetry. Second, the proper-motion measurements have directly yielded the ages for the nebulae, which are determined to be 900±200 yr for BD +3003639 and 1000±200 yr for NGC 6572. Publication: The Astrophysical Journal Pub Date: April 1996 DOI: 10.1086/177054 Bibcode: 1996ApJ...461..282K Keywords: ASTROMETRY; ISM: GENERAL; ISM: KINEMATICS AND DYNAMICS; ISM: PLANETARY NEBULAE: INDIVIDUAL ALPHANUMERIC: BD +30 DEGREES 3639; RADIO CONTINUUM: ISM full text sources ADS | data products SIMBAD (3)

The features of infra-red emission of the galactic bulge planetary nebulae

Abstract 302 planetary nebulae located within the sector of galactic longitudes from 345° to 15° have been investigated. All nebulae have been subdivided into four mass classes and different distance scales for each class have been used. Planetary nebulae located within the galactic bulge have been selected. The galactic radial gradients of infra-red luminosities and excesses are obtained for each nebula mass class. Possible explanations of the effect are discussed.

The Abell 35 Nebula Inside Out

view Abstract Citations (20) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Abell 35 Nebula Inside Out Hollis, J. M. ; van Buren, D. ; Vogel, S. N. ; Feibelman, W. A. ; Jacoby, G. H. ; Pedelty, J. A. Abstract We present deep 6 cm continuum and narrowband optical emission-line images in Hα λ6563, [O III] λ5007, [O II] λ3727, and [N II] λ6584 of the Abell 35 planetary nebula (PN), showing its unique features. First, surrounding the central binary system, there is a bow shock, best seen predominantly in [O III], possibly weakly in Hα, and not at all in [O II], [N II], or 6 cm continuum. Fabry-Perot [O III] data are also presented which constrain the bow shock kinematical parameters. Second, external to the central bow shock, there are two prominent parallel nebular features (the "pipes"), seen at 6 cm continuum and in Hα, [O II], and [N II], which suggest dynamical interaction between the nebular material and the interstellar medium. Fabry-Perot [N II] data provide the kinematical parameters for the pipes. Archival IUE spectra are also presented which indicate that the temperature due to the hot companion with its accretion disk is on the order of 150,000 K. We derive a mass-loss rate of ≤3.0 × 10-10 Msun yr-1, a systemic wind velocity of ≥530 km s-1, and a total nebular mass of ∼0.23 Msun. We discuss the data presented here in the context of a binary system with a systemic wind moving at Mach 1.3 with respect to the PN medium to give rise to the bow shock morphology. Further, we also discuss the PN-ISM interaction as a Mach 10 shock which gives rise to two prominent linear features as a result of an inhomogeneous PN ejection mechanism. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176685 Bibcode: 1996ApJ...456..644H Keywords: ISM: STRUCTURE; ISM: INDIVIDUAL NAME: ABELL 35; SHOCK WAVES; ULTRAVIOLET: ISM full text sources ADS | data products SIMBAD (3) MAST (1)

Gas Drag and the Orbital Evolution of a Captured Triton

Gas drag is a possible mechanism for capturing Triton. We examine the energetics of gas drag capture, compare the role of gas drag in subsequent orbital evolution with that of tides, and evaluate whether a Triton captured by gas drag could have avoided spiraling into Neptune. On its own, gas drag causes the inclination of an orbit to become more prograde, and Triton's retrograde status potentially places an upper limit on the orbital energy lost due to gas drag and hence a lower limit on the amount of tidal evolution and concomitant heating. A putative Neptune nebula is modeled after a minimum mass Uranus nebula and is assumed to be isothermal and cool. Triton's initial apocenter is set at the radius of Neptune's Hill sphere and its initial pericenter within the radial range of the nebula. Despite strong evolution of both semimajor axis and eccentricity, Triton's inclination only changes by a few degrees due to gas drag, when uncoupled to inclination variations due to solar-torque-induced precession. Results are insensitive to variations in radial surface density distribution or temperature, for fixed nebular mass. Thus, Triton could have evolved to its present retrograde circular orbit by gas drag alone, and massive tidal heating is not an inevitable consequence of capture. Nontrivial tidal heating is still likely, though, because Triton's post-gas-drag eccentricity, i.e., that remaining when Triton's present-day angular momentum was reached, is likely to have exceeded ∼0.2.Despite Triton's mass, single-pass capture is shown to be energetically favorable for our nominal nebula. Capture is also energetically possible for (i) nebulae that are less massive by an order of magnitude, if Triton's heliocentric orbit was energetically close to a temporary gravitational capture, and (ii) the solar gas flowing into Neptune's accretion sphere at an earlier stage of formation. Survival of Triton against gas drag in the last case is unlikely, but the low-mass case may apply to the compact H- and He-depleted nebulae that form around planets such as Uranus and Neptune in various accretion scenarios. Once captured, evolution is rapid in the absence of solar perturbations, with the orbital angular momentum reduced to Triton's present value in under 103 years for the case of the nominal nebula. Solar tides, however, cause Triton's orbit to oscillate in and out of the protasatellite nebula and can extend the gas drag time scale, for certain initial conditions, to ∼104-105 years. Therefore Triton could have survived a short-lived (∼103 years), hot, turbulent nebula that may have formed by a giant impact with Neptune. It might have also survived a cooler, less turbulent, but longer-lived (∼106 years) low-mass nebula: the mass and angular momentum of Triton and that of a low-mass (Hand He-depleted) nebula are comparable, so orbital evolution for such a capture may "self-terminate." In particular, Triton may clear an annular region in the nebula after multiple passes through it, and for sufficiently slow radial redistribution of nebular mass, cease evolving by gas drag entirely.

Hopkins Ultraviolet Telescope observations of H2 toward the planetary nebula NGC 1535

We have observed the far-ultraviolet spectrum (912-1860 A) of the bright high-excitation planetary nebula NGC 1535 with approximately 3 A resolution using the Hopkins Ultraviolet Telescope (HUT) aboard the Astro-1 space shuttle pmission in 1990 December. We see strong continuum emission down to the Lyman limit and strong P Cygni profiles from high-excitation lines such as C IV wavelength 1549, N V wavelength 1240, O V wavelength 1371, and O VI wavelength 1035. Below 1150 A strong absorption bands of H2 are seen, which were unanticipated by us because of the low reddening and high galactic latitude of the object and the absence of detected H2 emission in the infrared. We construct model H2 spectra and convolve them to the HUT resolution for comparison with the NGC 1535 data. We find good agreement with a population distribution characterized by a single temperature (T = 300 K) or a two-temperature model (T = 144/500 K), and determine limits on the H2 column density. While both inter-stellar and circumstellar origins for the observed H2 absorption are plausible, we ascribe the material to the planetary nebula in order to estimate the conditions of excitation and place upper limits on the mass of both H2 and H1 in this system. Because the UV transitions are ground-state connected, we determine a stringent upper limit of 0.03 d(sup 2)(sub 1.6) solar mass on the mass of H2, where d(sub 1.6) is the distance relative to an assumed distance of 1.6 kpc. This value is less model-dependent than IR estimates. Along with the central star and nebular masses, these estimates allow us to limit the main-sequence mass of the progenitor star to less than 1.8 solar mass. This upper limit is consistent with a relatively low-mass extended thick disk or Population II progenitor, as expected for an object approximately 1 kpc off the galactic plane.

Multiple rings around Wolf-Rayet evolution

We present optical narrow-band imaging of multiple rings existing around galactic Wolf-Rayet (WR) stars. The existence of multiple rings of material around Wolf-Rayet stars clearly illustrates the various phases of evolution that massive stars go through. The objects presented here show evidence of a three stage evolution. O stars produce an outer ring with the cavity being partially filled by ejecta from a red supergiant of luminous blue variable phase. A wind from the Wolf-Rayet star then passes into the ejecta materials. A simple model is presented for this three stage evolution. Using observations of the size and dynamics of the rings allows estimates of time scales for each stage of the massive star evolution. These are consistent with recent theoretical evolutionary models. Mass estimates for the ejecta, from the model presented, are consistent with previous ring nebula mass estimates from IRAS data, showing a number of ring nebulae to have large masses, most of which must in be in the form of neutral material. Finally, we illustrate how further observations will allow the determination of many of the parameters of the evolution of massive stars such as total mass loss, average mass loss rates, stellar abundances, and total time spent in each evolutionary phase.

Near-infrared polarimetric imaging of the bipolar nebula OH 231.8+4.2: The death of a beta Pic-like system

The source OH 231.8+4.2 consists of a Mira variable star embedded in a highly collimated bipolar nebula, and thus appears to be representative of a transitional stage between red giant and planetary nebula. We obtained polarimetric near-infrared images of this source to ascertain its density structure. The patterns of polarization revealed in J and K images are characteristic of scattering of light from the central star by dust distributed in bipolar lobes. The signature of a dense circumstellar disk, previously hypothesized to explain the obscuration of the central star at optical and near-infrared wavelengths, is evident in the polarization maps and in a map of J-K color. Combined with the polarization information, the measured color gradients across the bipolar lobes suggest that the lobes are evacuated bubbles. We use the images to derive absorbing and scattering column densities of dust grains as functions of position within the nebula. The results for the total absorbing and scattering dust masses are similar, at 10 <SUP>-2</SUP> Solar mass. This similarity lends further weight to a model in which the bulk of the dust mass of the nebula lies along the lobe walls. Various lines of evidence suggest that only a small fraction of the total nebula mass resides in the circumstellar disk. This finding would appear to set OH 231.8+4.2 apart from other classical evolved bipolar nebulae, in which disks may dominate the circumstellar masses. We consider the possibility that the disk in OH 231.8+4.2 originated as a result of the influence of a binary companion to the Mira variable. Alternatively, we propose that the OH 231.8+4.2 nebula may be the end point of stellar evolution for an early A star that was surrounded throughout its main-sequence lifetime by a particulate disk. Such a disk might have resembled the dusty disk around beta Pic and, like that structure, would have had to be continually replenished in order to survive until the central star's ascent of the red giant branch. This model naturally explains this confinement of bipolar planetary nebulae to low galactic latitudes.

Utilitarian Models of the Solar Nebula

Models of the primitive solar nebula based on a combination of theory, observations of T Tauri stars, and global conservation laws are presented. The models describe the motions of nebular gas, mixing of interstellar material during the formation of the nebula, and evolution of thermal structure in terms of several characteristic parameters. The parameters describe key aspects of the protosolar cloud (its rotation rate and collapse rate) and the nebula (its mass relative to the Sun, decay time, and density distribution). For most applications, the models are heuristic rather than predictive. Their purpose is to provide a realistic context for the interpretation of solar system data, and to distinguish those nebula characteristics that can be specified with confidence, independently of the assumptions of particular models, from those that are poorly constrained. It is demonstrated that nebular gas typically experienced large radial excursions during the evolution of the nebula and that both inward and outward mean radial velocities on the order of meters per second occurred in the terrestrial planet region, with inward velocities predominant for most of the evolution. However, the time history of disk size, surface density, and radial velocities are sensitive to the total angular momentum of the protosolar cloud, which cannot be constrained by purely theoretical considerations. It is shown that a certain amount of "formational" mixing of interstellar material was an inevitable consequence of nebular mass and angular momentum transport during protostellar collapse, regardless of the specific transport mechanisms involved. Even if the protosolar cloud was initially homogeneous, this mixing was important because it had the effect of mingling presolar material that had experienced different degrees of thermal processing during collapse and passage through the accretion shock. Nebular thermal structure is less sensitive to poorly constrained parameters than is dynamical history. A simple criterion is derived for the condition that silicate grains are evaporated at midplane, and it is argued that this condition was probably fulfilled early in nebular history. Cooling of a hot nebula due to coagulation of dust and consequent local reduction of optical depth is examined, and it is shown how such a process leads naturally to an enrichment of rock-forming elements in the gas phase.

Luminous blue variables at quiescence: The zone of avoidance in the Hertzsprung-Russell diagram

view Abstract Citations (31) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Luminous Blue Variables at Quiescence: The Zone of Avoidance in the Hertzsprung-Russell Diagram Stothers, Richard B. ; Chin, Chao-Wen Abstract Two phases of dynamical instability are theoretically predicted to exist during the evolution of supergiants of normal metallicity that are initially more massive than approximately 60 solar mass. One phase occurs briefly in the yellow or red region of the Hertzsprung-Russell diagram for stars in the early stages of core helium burning, and the other phase occurs for a longer time in the blue or blue-white region for stars exhausting their core helium. Probably only the second phase exists in the case of supergiants with initial masses between approximately 60 solar mass and approximately 30 solar mass or with low metallicities The cause of instability is the partial ionization of hydrogen and helium in a quasi-isolated outer region of the stellar envelope, above the layer where the iron opacity attains a large local maximum. Predicted luminosities, effective temperatures, ejected nebular masses, remnant masses, eruption recurrence times, and lifetimes, though very approximate, are generally consistent with available observational data for the important class of unstable supergiants known as luminous blue variables. Publication: The Astrophysical Journal Pub Date: May 1994 DOI: 10.1086/187335 Bibcode: 1994ApJ...426L..43S Keywords: Blue Stars; Hertzsprung-Russell Diagram; Magnetohydrodynamic Stability; Metallicity; Stellar Composition; Stellar Envelopes; Stellar Evolution; Stellar Luminosity; Stellar Mass Ejection; Stellar Models; Stellar Winds; Supergiant Stars; Variable Stars; Mathematical Models; Opacity; Stellar Mass; Stellar Temperature; Astrophysics; STARS: EVOLUTION; STARS: OSCILLATIONS; STARS: VARIABLES: OTHER MISCELLANEOUS full text sources ADS | data products SIMBAD (10)

The features of infra-red emission of planetary nebulae in the inner galaxy

414 galactic PN with measured infra-red luminosities (302 of which are located within the sector of galactic longitudes from 345° to 15°) are investigated. All nebulae are subdivided into four mass classes and different distance scales for each class are used. Planetary nebulae located in the inner Galaxy are selected. The galactic radial gradients of infra-red luminosities and excesses are obtained for each nebula mass class. Possible explanations of the effect are discussed.

Planetary nebula chemical abundances: Z-distribution and connection with the central star masses

Dependencies of galactic planetary nebula chemical abundances and their central star masses on the distance from the galactic plane are discussed.Z-dependencies of He/H, N/H, N/O and Ar/H and dependencies of He/H, N/H, N/O, Ne/H and Ar/H on central star mass are found. Three galactic planetary nebula distance scale samples are used and it is shown that the distance scale system (where distances of each planetary nebula mass class are determined with the separate scale) is the most reliable. The correlations obtained for the Magellanic Cloud planetary nebulae are used for comparison.

Clumpy Disk Accretion and Chondrule Formation

Chondrules are the major constituent of most of the primitive meteorites, yet the mechanism of their formation is largely unknown. Chondrule textures and compositions place tight constraints on the thermal processing necessary to turn precursor dust aggregates into chondrules—temperatures between about 1500 and 2000 K and cooling times of hours are indicated. As suggested by Hood and Horanyi (1991, Icarus 93, 259-269), shock heating within the nebula may be capable of matching the thermal constraints, provided that a suitable source of nebular shocks can be found. We suggest here that the source of the shocks possibly responsible for chondrule formation was episodic accretion onto the solar nebula of low-mass clumps (∼1022 g) of interstellar gas. Three types of astronomical observations are best explained by the existence of opaque clumps that block stellar radiation while moving at high velocities within a few astronomical units of young stellar objects: (a) rapid variations in stellar luminosity, independent of wavelength; (b) rapid changes in surface brightness of nearby reflection nebulae; and (c) rapid variations in spectral lines excited in circumstellar regions. While the origin of these clumps is presently unknown, they might arise from the inhomogeneous infall of the residual molecular cloud core, from the return of matter ejected by the stellar wind, or from a combination of these two effects. Infalling clumps will impact the protoplanetary disk at high velocity, producing a localized, intense source of heat suitable for chondrule formation. As the resulting shock wave propagates into the nebula, preexisting aggregates of dust grains formed in the nebula will be melted by the shock front, producing chondrules. For reasonable estimates of the clump impact rate onto a minimum mass nebula, we find that a nonnegligible fraction of the nebula mass undergoes the required thermal processing. Clumpy disk accretion is a mechanism that is capable of providing a long-lived, late-phase, episodic, and variable intensity source of cyclical thermal processing for chondrule precursor grains.

Evolution of the Solar Nebula. II. Thermal Structure during Nebula Formation

view Abstract Citations (63) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of the Solar Nebula. II. Thermal Structure during Nebula Formation Boss, Alan P. Abstract Models of the thermal structure of protoplanetary disks are required for understanding the physics and chemistry of the earliest phases of planet formation. Numerical hydrodynamical models of the protostellar collapse phase have not been evolved far enough in time to be relevant to planet formation, i.e., to a relatively low-mass disk surrounding a protostar. One simplification is to assume a pre-existing solar-mass protostar, and calculate the structure of just the disk as it forms from the highest angular momentum vestiges of the placental cloud core. A spatially second-order accurate, axisymmetric (two-dimensional), radiative hydrodynamics code has been used to construct three sets of protoplanetary disk models under this assumption. Because compressional heating has been included, but not viscous or other heating sources, the model temperatures obtained should be considered lower bounds. The first set started from a spherically symmetric configuration appropriate for freely falling gas: ρ ∝ r-3/2, υr ∝ r-1/2, but with rotation (Ω ∝ r-1, where r is the spherical coordinate radius). These first models turned out to be unsatisfactory because in order to achieve an acceptable mass accretion rate onto the protostar (Mṡ ≤ 10-5 Msun yr-1 for low-mass star formation), the disk mass became much too small (∼ 0.0002 Msun). The second set improved on the first set by ensuring that the late-arriving, high angular momentum gas did not accrete directly onto the protosun. By starting from a disklike cloud flattened about the equatorial plane and flowing vertically toward the midplane, these models led to Mṡ → 0, as desired. However, because the initial cloud was not chosen to be close to equilibrium, the disk rapidly contracted vertically, producing an effective disk mass accretion rate Mṡd ∼ 10-2 Msun yr-1, again too high. Hence, the third (and most realistic) set started from an approximate equilibrium state for an adiabatic, self-gravitating "fat" Keplerian disk, with surface density σ ∝ r-1/2, surrounded by a much lower density "halo" infalling onto the disk. This initial condition produced Mṡs → 0 and Mṡd ∼ 10-6 to 10-5 Msun yr-1, as desired. The resulting nebula temperature distributions show that midplane temperatures of at least 1000 K inside 2.5 AU, falling to around 100 K outside 5 AU, are to be expected during the formation phase of a minimum mass nebula containing ∼0.02 Msun within 10 AU. This steady state temperature distribution appears to be consistent with cosmochemical evidence which has been interpreted as implying a phase of relatively high temperatures in the inner nebula. The temperature distribution also implies that the nebula would be cool enough outside 5 AU to allow ices to accumulate into planetesimals even at this relatively early phase of nebula evolution. Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173318 Bibcode: 1993ApJ...417..351B Keywords: ACCRETION; ACCRETION DISKS; HYDRODYNAMICS; SOLAR SYSTEM: FORMATION full text sources ADS | Related Materials (8) Part 1: 1989ApJ...345..554B Part 3: 1996ApJ...469..906B Part 4: 1998ApJ...503..923B Part 5: 2002ApJ...576..462B Part 6: 2004ApJ...616.1265B Part 7: 2005ApJ...629..535B Part 8: 2007ApJ...660.1707B Part 9: 2011ApJ...739...61B