Astronomical constraints on nebular temperatures: Implications for planetesimal formation

Abstract— Motivated by recent observations of T-Tauri stars and the interpretation of these observations in terms of the properties of circumstellar disks, we derive internal (midplane) temperatures for disks around mature (age ～1 Ma) T-Tauri stars. The estimates are obtained by combining published results for disk masses, sizes, accretion rates, and surface temperatures. For 26 stars (for which adequate data are available), we derive midplane temperatures at 1 AU primarily in the range 200–800 K, and 100–400 K at 2.5 AU. It is likely that the solar nebula, at the same stage of evolution, contained planetesimals and objects destined to become meteorite parent bodies. Observations of young stellar objects at earlier stages of evolution (age ～0.1 Ma) imply that accretion rates were, on the average, at least two orders of magnitude greater than the 10^?8 M_⊙/year rates typical for mature T-Tauri stars. Such high values would result in midplane temperatures at or near the silicate vaporization temperature in the terrestrial planet region. If cooling of the solar nebula from such a hot epoch was responsible for establishing the pervasive elemental fractionation patterns found in chondritic meteorites, then objects in the asteroid belt must have grown rapidly (within 0.1 Ma) to sizes of ～1 km, a conclusion consistent with current theories of planetesimal formation. However, the fact that primitive meteorite parent bodies escaped being melted by the decay of ²⁶Al then implies that further growth of at least some objects was essentially delayed for 2 Ma or more. Such a diminished growth rate appears to be consistent with simulations of the dynamics of solid bodies in the asteroid belt. Other hypotheses seem less attractive. One might assume that the final cooling occurred only after the decay of ²⁶Al (i.e., more than a million years after calcium-aluminum rich inclusion formation), or that ²⁶Al was not ubiquitous in the early solar system. But the first of these conjectures is incompatible with astronomical observations of T-Tauri systems, and the second appears to be contradicted by the evidence for ²⁶Al in diverse meteoritic components. The remaining alternative would then appear to be that, despite a lack of supporting evidence, chondritic fractionation patterns reflect the net effect of many local heating and cooling events and have nothing to do with global nebular cooling. We conclude that the most plausible hypothesis is that both nebular cooling and coagulation of solids to kilometer-sized objects occurred rapidly and that a substantial number of planetesimals in the asteroid belt remained smaller than a few kilometers in radius for at least 2 Ma.     

On the Dynamics of Dust Grains in a Hierarchical Environment

Most main sequence stars are binaries or higher multiplicity Systems and it appears that at birth most stars have circumstellar disks. It is commonly accepted that planetary systems arise from the material of these disks; consequently, binary and multiple systems may have a main role in planet formation. In this paper, we study the stage of planetary formation during which the particulate material is still dispersed as centimetre-to-metre sized primordial aggregates. We investigate the response of the particles, in a protoplanetary disk with radius R_D = 100 AU around a solar-like star, to the gravitational field of bound perturbing companions in a moderately wide (300–1600 AU) orbit. For this purpose, we have carried out a series of simulations of coplanar hierarchical configurations using a direct integration code that models gravitational and viscous forces. The massive protoplanetary disk is around one of the components of the binary. The evolution in time of the dust sub-disk depends mainly on the nature (prograde or retrograde) of the relative revolution of the stellar companion, and on the temperature and mass of the circumstellar disk. Our results show that for binary companions near the limit of tidal truncation of the disk, the perturbation leads to an enhanced accretion rate onto the primary, decreasing the lifetime of the particles in the protoplanetary disk with respect to the case of a single star. As a consequence of an enhanced accretion rate the mass of the disk decreases faster, which leads to a longer resultant lifetime for particles in the disk. On the other hand, binary companions may induce tidal arms in the dust phase of protoplanetary disks. Spiral perturbations with m = 1 may increase in a factor 10 or more the dust surface density in the neighbourhood of the arm, facilitating the growth of the particles. Moreover, in a massive disk (0.01M⊙) the survival time of particles is significantly shorter than in a less massive nebula (0.001M⊙) and the temperature of the disk severely influences the spiral-in time of particles. The rapid evolution of the dust component found in post T Tauri stars can be explained as a result of their binary nature. Binarity may also influence the evolution of circumpulsar disks. This revised version was published online in July 2006 with corrections to the Cover Date.     

Planetary accretion,oxygen isotopes,and the central limit theorem

Abstract— The accumulation of presolar dust into increasingly larger aggregates such as calcium‐aluminum‐rich inclusions (CAIs) and chondrules, asteroids, and planets should result in a drastic reduction in the numerical spread in oxygen isotopic composition between bodies of similar size, in accord with the central limit theorem. Observed variations in oxygen isotopic composition are many orders of magnitude larger than would be predicted by a simple, random accumulation model that begins in a well‐mixed nebula, no matter what size objects are used as the beginning or end points of the calculation. This discrepancy implies either that some as yet unspecified but relatively long‐lived process acted on the solids in the solar nebula to increase the spread in oxygen isotopic composition during each and every stage of accumulation, or that the nebula was heterogeneous (at least in oxygen) and maintained this heterogeneity throughout most of its nebular history. Depending on its origin, large‐scale nebular heterogeneity could have significant consequences for many areas of cosmochemistry, including the application of well‐known isotopic systems to the dating of nebular events and the prediction of bulk compositions of planetary bodies on the basis of a uniform cosmic abundance. The evidence supports a scenario wherein the oxygen isotopic composition of nebular solids becomes progressively depleted in ¹⁶O with time due to chemical processing within the nebula, rather than a scenario where ¹⁶O‐rich dust and other materials are injected into the nebula, possibly causing its initial collapse.     

原行星盘的研究进展

原行星盘是环绕在年轻星天体(如T Tauri型星,HAe/Be星)周围的气体尘埃盘,是具有初始角动量的分子云核在塌缩形成恒星过程中的自然结果,是行星系统的起源地。原行星盘研究不仅是恒星形成理论的重要组成部分,而且是行星形成理论的基础。首先介绍了盘的形成与演化规律;然后介绍了年轻星天体的能谱分布,盘的模型和参数(质量吸积率、质量、尺度、温度、寿命);随后讨论了尘埃颗粒在盘中生长的观测证据以及行星在盘中形成的大致过程;最后对原行星盘研究的现状和未来做了总结与展望。     

The role of massive AGB stars in the early solar system composition

Abstract— We demonstrate that a massive asymptotic giant branch (AGB) star is a good candidate as the main source of short‐lived radionuclides in the early solar system. Recent identification of massive (4–8 M⊙) AGB stars in the galaxy, which are both lithium‐ and rubidium‐rich, demonstrates that these stars experience proton captures at the base of the convective envelope (hot bottom burning), together with high‐neutron density nucleosynthesis with ²²Ne as a neutron source in the He shell and efficient dredge‐up of the processed material. A model of a 6.5 M⊙ star of solar metallicity can simultaneously match the abundances of ²⁶Al, ⁴¹Ca, ⁶⁰Fe, and ¹⁰⁷Pd inferred to have been present in the solar nebula by using a dilution factor of 1 part of AGB material per 300 parts of original solar nebula material, and taking into account a time interval between injection of the short‐lived nuclides and consolidation of the first meteorites equal to 0.53 Myr. Such a polluting source does not overproduce ⁵³Mn, as supernova models do, and only marginally affects isotopic ratios of stable elements. It is usually argued that it is unlikely that the short‐lived radionuclides in the early solar system came from an AGB star because these stars are rarely found in star forming regions, however, we think that further interdisciplinary studies are needed to address the fundamental problem of the birth of our solar system.     

Accumulation processes in the primitive solar nebula

Particle accumulation processes are discussed for a variety of physical environments, ranging from the collapse phase of an interstellar cloud to the different parts of the models of the primitive solar nebula constructed by Cameron and Pine. Because of turbulence in the collapsing interstellar gas, it is concluded that interstellar grains accumulate into bodies with radii of a few tens of centimeters before the outer parts of the solar nebula are formed. These bodies can descend quite rapidly through the gas toward midplane of the nebula, and accumulation to planetary size can occur in a few thousand years. Substantial modifications of these processes take place in the outer convection zone of the solar nebula, but again it is concluded that bodies in that zone can grow to planetary size in a few thousand years.From the discussion of the interstellar collapse phase it is concluded that the angular momentum of the primitive solar nebula was predominantly of random turbulent origin, and that it is plausible that the primitive solar nebula should have possessed satellite nebulae in highly elliptical orbits. It is proposed that the comets were formed in these satellite nebulae.A number of other detailed conclusions are drawn from the analysis. It is shown to be plausible that an iron-rich planet should be formed in the inner part of the outer nebular convection zone. Discussions are given of the processes of planetary gas accretion, the formation of satellites, the T Tauri solar wind, and the dissipation of excess condensed material after the nebular gases have been removed by the T Tauri solar wind. It is shown that the present radial distances of the planets (but not Bode's Law) should be predicted reasonably well by a solar nebula model intermediate between the uniform and linear cases of Cameron and Pine.     

M4–18: the planetary nebula and its WC10 central star

We present a detailed analysis of the planetary nebula M4–18 (G146.7+07.6) and its WC10-type Wolf–Rayet (WR) central star, based on high‐quality optical spectroscopy (WHT/UES, INT/IDS, WIYN/DensPak) and imaging ( HST /WFPC2). From a non-LTE model atmosphere analysis of the stellar spectrum, we derive T _eff=31 kK,     v _∞=160 km s⁻¹ and abundance number ratios of H/He<0.5, C/He=0.60 and O/He=0.10. These parameters are remarkably similar to those of He 2–113 ([WC10]). Assuming an identical stellar mass to that determined by De Marco et al. for He 2–113, we obtain a distance of 6.8 kpc to M4–18 [ E ( B−V )=0.55 mag from nebular and stellar techniques]. This implies that the planetary nebula of M4–18 has a dynamical age of ∼3100 yr, in contrast to ≥270 yr for He 2–113. This is supported by the much higher electron density of the latter. These observations may be reconciled with evolutionary predictions only if [WC]-type stars exhibit a range in stellar masses.
Photoionization modelling of M4–18 is carried out using our stellar WR flux distribution, together with blackbody and Kurucz energy distributions obtained from Zanstra analyses. We conclude that the ionizing energy distribution from the WR model provides the best consistency with the observed nebular properties, although discrepancies remain.     

Can lightning produce significant levels of mass‐independent oxygen isotopic fractionation in nebular dust?

Based on recent evidence that oxide grains condensed from a plasma will contain oxygen that is mass‐independently fractionated compared to the initial composition of the vapor, we present a first attempt to evaluate the potential magnitude of this effect on dust in the primitive solar nebula. This assessment relies on previous studies of nebular lightning to provide reasonable ranges of physical parameters to form a very simple model to evaluate the plausibility that lightning could affect a significant fraction of nebular dust and that such effects could cause a significant change in the oxygen isotopic composition of solids in the solar nebula over time. If only a small fraction of the accretion energy is dissipated as lightning over the volume of the inner solar nebula, then a large fraction of nebular dust will be exposed to lightning. If the temperature of such bolts is a few percent of the temperatures measured in terrestrial discharges, then dust will vaporize and recondense in an ionized environment. Finally, if only a small average decrease is assumed in the ¹⁶O content of freshly condensed dust, then over the last 5 Myr of nebular accretion the average Δ¹⁷O of the dust could increase by more than 30 per mil. We conclude that it is possible that the measured “slope 1” oxygen isotope line measured in meteorites and their components represents a time‐evolution sequence of nebular dust over the last several million years of nebular evolution where ¹⁶O‐rich materials formed first, then escaped further processing as the average isotopic composition of the dust gradually became increasingly depleted in ¹⁶O.     

The Nobeyama Millimeter Array Survey for protoplanetary disks around protostar candidates and T Tauri stars in Taurus

The Nobeyama Millimeter Array Survey for protoplanetary disks has been made for 19 protostellar IRAS sources in Taurus; 13 of them were optically invisible protostars and 6 were young T Tauri stars. We observed 98-GHz continuum and CS(J = 2 – 1) line emissions simultaneously with spatial resolutions of 2 _. 8-8 _. 8 (360-1,200 AU). The continuum emission was detected from 5 out of 6 T Tauri stars and 2 out of 13 protostar candidates: the emission was not spatially resolved and was consistent with being originated from compact circumstellar disks. Extended CS emission was detected around 2 T Tauri stars and 11 protostar candidates. There is a remarkable tendency for the detectability of the 98-GHz continuum emission to be small for protostar candidates. This tendency is explained if the mass of protoplanetary disks around protostars is not as large as that around T Tauri stars; the disk mass may increase with the increase of central stellar mass by dynamical accretion in the course of evolution from protostars to T Tauri stars.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Photodissociation of CO isotopologues: Models of laboratory experiments and implications for the solar nebula

CO photodissociation in the solar nebula and/or parent cloud has been proposed to be the mechanism responsible for forming the ¹⁶O‐poor reservoir of the calcium‐aluminum‐rich inclusion (CAI) mixing line. However, laboratory experiments on CO photolysis found a wavelength dependence in the oxygen isotope ratios of the product O atoms, which was interpreted as proof that CO photolysis was not a viable mechanism. Here, I report photochemical simulations of these experiments using line‐by‐line CO spectra to identify the origin of the wavelength dependence. At long wavelengths (>105 nm), the line‐by‐line spectra for isotopic CO can explain the experimental data with a combination of C¹⁶O self‐shielding and reduced dissociation probabilities for C¹⁸O. At short wavelengths, the greater number of diffuse bands increases the importance of mass‐dependent fractionation, lowering the slope to below unity. The line‐by‐line isotopic spectra are then applied to CO photodissociation in a model solar nebula. Three FUV sources are considered (1) HD 303308, an O4 star in Carina; (2) HD 36981, a B5 star in Orion; and (3) TW Hydrae, a T Tauri star of 10 Myr age. Using reduced dissociation probabilities for C¹⁸O based on the photolysis experiments yields nebular water slopes approximately 0.95–1.0 for HD 303308 and TW Hya, and approximately 0.8–1.5 for HD 36981. For the central protostar case (TW Hya) with a simplified treatment of the 2‐D radiative transfer, slopes approximately 0.95–1.0 are obtained, independent of the C¹⁸O dissociation probability. Greatly improved measurements of the C¹⁷O and C¹⁸O cross sections and dissociation probabilities are in progress.     

Filaments,disks, and jets for of and WR stars

It is noted that the spectra of Wolf-Rayet stars can be well described in terms of emission-line formation in a moderate density wind, a ring-like disk, and magnetically supported filaments which link the disk to the central star. The emission-line spectra of Of, O(f), and O((f)) stars likewise can be understood in terms of jets, filaments, and disks. Some examples of Of spectra are described and it is noted that expanding-spherical-wind models do not appear to be able to describe accurately all which is observed. When interpreting the spectra of Wolf-Rayet and O stars, it is useful to think in terms of the type of model which has proved useful for interpreting the emission-line spectra of Herbig Ae/Be and T Tauri stars.     

Radial transport in the solar nebula: Implications for moderately volatile element depletions in chondritic meteorites

Abstract— In this paper, we explore the possibility that the moderately volatile element depletions observed in chondritic meteorites are the result of planetesimals accreting in a solar nebula that cooled from an initially hot state (temperatures > 1350 K out to ?2–4 AU). A model is developed to track the chemical inventory of planetesimals that accrete in a viscously evolving protoplanetary disk, accounting for the redistribution of solids and vapor by advection, diffusion, and gas drag. It is found that depletion trends similar to those observed in the chondritic meteorites can be reproduced for a small range of model parameters. However, the necessary range of parameters is inconsistent with observations of disks around young stars and other constraints on meteorite parent body formation. Thus, counter to previous work, it is concluded that the global scale evolution of the solar nebula is not the cause for the observed depletion trends. Instead, it appears that localized processing must be considered.     

Planet formation around M-dwarfs: the moving snow line and super-Earths

Planets result from a series of processes within a circumstellar disk. Evidence comes from the near planar orbits in the Solar System and other planetary systems, observations of newly formed disks around young stars, and debris disks around main-sequence stars. As planet-hunting techniques improve, we approach the ability to detect systems like the Solar System, and place ourselves in context with planetary systems in general. Along the way, new classes of planets with unexpected characteristics are discovered. One of the most recent classes contains super Earth-mass planets orbiting a few AU from low-mass stars. In this contribution, we outline a semi-analytic model for planet formation during the pre-main sequence contraction phase of a low-mass star. As the star contracts, the “snow line”, which separates regions of rocky planet formation from regions of icy planet formation, moves inward. This process enables rapid formation of icy protoplanets that collide and merge into super-Earths before the star reaches the main sequence. The masses and orbits of these super-Earths are consistent with super-Earths detected in recent microlensing experiments.     

Processing of chondritic and planetary material in spiral density waves in the nebula

Abstract— A widely held view of nebular evolution is that during the ～0.5 Ma while interstellar material was collapsing onto the disk, the latter grew in mass to the point of gravitational instability. It responded to this by losing axial symmetry, growing spiral arms that had the capacity to tidally redistribute disk mass (inward) and angular momentum (outward) and prevent further increase in the disk/protosun mass ratio. The spiral arms (density waves) rotated differently than the substance of the nebula, and in some parts of the disk, nebular material may have encountered the arms at supersonic velocities. The disk gas, and solid particles entrained in it, would have been heated to some degree when they passed through shock fronts at the leading edges of the spiral arms. The present paper proposes this was the energetic nebular setting or environment that has long been sought, in which the material now in the planets and chondritic meteorites was thermally processed.     

Oxygen isotopic compositions and origins of calcium‐aluminum‐rich inclusions and chondrules

Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ¹⁸O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the ¹⁶O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted ¹⁶O‐rich presolar dust that exchanged oxygen with ¹⁶O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with ¹⁶O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in ¹⁶O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated ¹⁶O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ～10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed.     

A unique Galactic planetary nebula with a [WN] central star

We report the discovery of the first probable Galactic [WN] central star of a planetary nebula (CSPN). The planetary nebula candidate was found during our systematic scans of the AAO/UKST Hα Survey of the Milky Way. Subsequent confirmatory spectroscopy of the nebula and central star reveals the remarkable nature of this object. The nebular spectrum shows emission lines with large expansion velocities exceeding 150 km s⁻¹, suggesting that perhaps the object is not a conventional planetary nebula. The central star itself is very red and is identified as being of the [WN] class, which makes it unique in the Galaxy. A large body of supplementary observational data supports the hypothesis that this object is indeed a planetary nebula and not a Population I Wolf–Rayet star with a ring nebula.     

Photoionization of cool MHD disk winds

Recent ultraviolet observations point out that there is hot, dense plasma associated with the optical jet in some T Tauri stars. In this contribution, cool MHD disk wind physics is reviewed by means of a self-similar analytical model to analyze whether hot (Te ? 80,000 K) and dense (n_e ? 10⁹ cm^-3) plasma can be produced in disk winds. It is shown that these high densities can only be achieved at the base of the wind where the stellar X-rays radiation field is strong. The propagation of the X-rays radiation through the disk wind is analyzed: a cocoon of photoionized gas is generated around the star. However, it is difficult to foresee how temperatures as high as ～ 5 × 10⁴ can be reached unless a significant fraction of the X-rays radiation is produced by magnetic reconnection at the boundary between the stellar magnetosphere and the accretion disk.     

A model for a structure in the galactic nebula S206

The galactic nebula S206 contains a half shell of high excitation nebulosity which is centred on the associated exciting star. The suggestion has been made that this structure is caused by the interaction of stellar mass loss from the star with nebular gas. A steady state model of such an interaction is investigated quantitatively. The required mass loss rate from the star is about 10^–7 M yr^–1 which is compatible with the observationally derived mass-loss rates from early-type stars.     

Temperature distribution in the solar nebula at successive stages of its evolution

We have constructed a model of the solar nebula that allows for the temperature and pressure distributions at various stages of its evolution to be calculated. The mass flux from the accretion envelope to the disk and from the disk to the Sun, the turbulent viscosity parameter α, the opacity of the disk material, and the initial angular momentum of the protosun are the input model parameters that are varied. We also take into account the changes in the luminosity and radius of the young Sun. The input model parameters are based mostly on data obtained from observations of young solar-type stars with disks. To correct the input parameters, we use the mass and chemical composition of Jupiter, as well as models of its internal structure and formation that allow constraints to be imposed on the temperature and surface density of the protoplanetary disk in Jupiter’s formation zone. Given the derived constraints on the input parameters, we have calculated models of the solar nebula at successive stages of its evolution: the formation inside the accretion envelope, the evolution around the young Sun going through the T Tauri stage, and the formation and compaction of a thin dust layer (subdisk) in the disk midplane. We have found the following evolutionary trend: an increase in the temperature of the disk at the stage of its formation, cooling at the T Tauri stage, and the subsequent internal heating of the dust subdisk by turbulence dissipation that causes a temperature rise in the formation zone of the terrestrial planets at the high subdisk density and the opacity in this zone. We have obtained the probable ranges of temperatures in the disk midplane, i.e., the temperatures of the protoplanetary material in the formation region of the terrestrial planets at the initial stage of their formation.     

Terrestrial planet formation surrounding close binary stars

Most stars reside in binary/multiple star systems; however, previous models of planet formation have studied growth of bodies orbiting an isolated single star. Disk material has been observed around both components of some young close binary star systems. Additionally, it has been shown that if planets form at the right places within such disks, they can remain dynamically stable for very long times. Herein, we numerically simulate the late stages of terrestrial planet growth in circumbinary disks around ‘close’ binary star systems with stellar separations 0.05 AU?aB?0.4 AU and binary eccentricities 0?eB?0.8. In each simulation, the sum of the masses of the two stars is 1 M_⊙, and giant planets are included. The initial disk of planetary embryos is the same as that used for simulating the late stages of terrestrial planet formation within our Solar System by Chambers [Chambers, J.E., 2001. Icarus 152, 205-224], and around each individual component of the α Centauri AB binary star system by Quintana et al. [Quintana, E.V., Lissauer, J.J., Chambers, J.E., Duncan, M.J., 2002. Astrophys. J. 576, 982-996]. Multiple simulations are performed for each binary star system under study, and our results are statistically compared to a set of planet formation simulations in the Sun-Jupiter-Saturn system that begin with essentially the same initial disk of protoplanets. The planetary systems formed around binaries with apastron distances QB≡aB(1+eB)?0.2 AU are very similar to those around single stars, whereas those with larger maximum separations tend to be sparcer, with fewer planets, especially interior to 1 AU. We also provide formulae that can be used to scale results of planetary accretion simulations to various systems with different total stellar mass, disk sizes, and planetesimal masses and densities.