Infrared CO emission and disks around young stars

High-resolution near-infrared spectroscopy of CO overtone emission bands has provided some of the best kinematic evidence for the existence of circumstellar disks around young stars. The CO emission flux and the detailed shape of the overtone bands are well matched by simple Keplerian disk models. A brief overview of the use of infrared CO emission as a diagnostic of the kinematics and conditions of gas in the inner disks of young stars is presented.     

Synthetic images of proto-planetary disks around young stars

We present synthetic images of accretion disks around young stars computed from a model where the disk's vertical structure is solved assuming hydrostatic equilibrium. The disk's brightness results from three emission processes: (1) the reprocessing of stellar photons in the optically thick disk's regions; (2) the scattering of stellar photons in the optically thin parts of the disk; and (3) the thermal emission of the disk due to viscous energy dissipation during the accretion process.We discuss the relative importance of these emission processes at wavelengths ranging from 1.2 to 20m.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Debris disks: seeing dust, thinking of planetesimals and planets

Debris disks are optically thin, almost gas-free dusty disks observed arounda significant fraction of main-sequence stars older than about 10 Myr. Since the circumstellar dust is short-lived, the very existence of these disks is considered as evi-dence that dust-producing planetesimals are still present in mature systems, in whichplanets have formed – or failed to form – a long time ago. It is inferred that theseplanetesimals orbit their host stars at asteroid to Kuiper-belt distances and continuallysupply ...     

Growth of large,late-stage planetesimals

The late stage of terrestrial planets' growth determined many of their fundamental properties, including their thermal properties and petrology, their impact records, and possibly the existence of the Moon. A critical result of late-stage models, which bears on observable properties, is the size of the largest planetesimals that grew near, and later impacted,those that became full-size planets. There has been considerable misinterpretation of previous models regarding the relation between the size of planetesimals and their relative velocities. Furthermore, some models neglect the possible decrease in relative velocity as control is transferred from the largest to the second-largest body in an accreation zone. Evidence that Venus helped stir Earth-zone planetismals is not copelling. When models are evaluated, the results are found to depend strongly on uncertain initial conditions. The size of the second-largest planetesimal in the Earth's zone might range from ～300 to ～2500 km, with corresponding accretion times of ～7 × 10⁶ and ～10⁸ years, respectively. Both extremes are generated from plausible initial conditions and both seem consistent with observed planetary properties.     

Circumstellar envelopes and disks of pre-main sequence stars

The current state of knowledge about circumstellar matter of young stellar objects is briefly reviewed. It appears that some very young stars yet to accrete substantial amounts of mass may be seen through their dusty infalling envelopes even at optical wavelengths, because of the presence of holes or large departures from spherical symmetry in the envelopes. The evidence for this picture is summarized in the context of one wellstudied young star, HL Tau, indicating that much of the large-scale structure originally identified as a rotating disk is probably a flattened infalling envelope. Departures from spherical symmetry in protostellar clouds are likely to lead to quite flattened structures once collapse gets under way, further suggesting that infall in large-scale toroids may be a general feature of low-mass star formation. The best kinematic evidence for Keplerian disk rotation comes from optical and near-infrared high-resolution spectroscopy of the innermost regions of circumstellar disks. Disk masses are uncertain but are likely to be at least the order of minimum mass solar nebula models, if not much larger.     

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.     

Non-LTE models of the accretion disks of UX Ori stars

The results of a non-LTE analysis of a number of spectral lines formed in the accreting envelopes of UX Ori stars are given. The accretion rate is estimated from an analysis of the first three lines of the Balmer series: M _a = 10^?8 ?10^?9 M _⊙ The gas temperature in this region is about 10,000 K. In the immediate vicinity of the star there is a hotter region, with T > 15,000 K, in which the 5876 Å line of neutral helium, observed in the spectra of these stars, is formed. The region of formation of this line has a small geometrical thickness, covers a small fraction of the star’s visible disk, and evidently consists of the site of contact of the accreting gas with the stellar surface. The low gas rotation rates in this region (150–200 km/sec) may mean that rapid rotation of the accreting gas is damped by the star’s magnetic field, which is strong enough to affect the gas stream. We estimate the magnetic field strength in this region to be about 150 G.     

Plasma disks around compact objects

Considering the azimuthal velocity fields for different radial dependence we obtain the pressure profiles for the thin disk using the general formalism obtained earlier and further look at the profiles of the luminosity flux function using the approach as given recently by Hanawa (1988). It appears that the profile of this function is not very sensitive to change in ther-dependence of the velocity fields.     

Disks around the nearest stars and substars

We present a review of a publication concerning the problem of the existence of disks around stars and substars within 10 pc from the Solar System; outline the present-day concepts of the astrophysical properties of circumstellar disks and problems connected with and results of their search and detection, on the basis of the IR-excesses in the spectrum of the nearest stellar/substellar systems; discuss some data on the nearest stellar and substellar population; give a list of circumstellar discs discovered within 10 pc from the Sun and their main astrophysical properties; and briefly discuss disk structure yielded by images taken in different spectral bands.     

Manganese content in stars of the galactic thick and thin disks

The manganese content was determined in the atmospheres of 50 F, G, and K dwarfs (?1.0 < [Fe/H] < 0.2) that belong to the galactic thick and thin disks. The observation data were obtained with ELODIE and SOPHIE echelle spectrometers with resolutions of R = 42000 and R = 75000, respectively, using the 1.93-meter telescope of the Haute Provence Observatory. The Mn content was determined under the LTE approximation by the synthetic spectrum approach with a detailed consideration of the superfine structure. The behavior of the Mn content with [Fe/H] in the galactic substructures was analyzed.     

Fossil magnetic field of accretion disks of young stars

We elaborate the model of accretion disks of young stars with the fossil large-scale magnetic field in the frame of Shakura and Sunyaev approximation. Equations of the MHD model include Shakura and Sunyaev equations, induction equation and equations of ionization balance. Magnetic field is determined taking into account ohmic diffusion, magnetic ambipolar diffusion and buoyancy. Ionization fraction is calculated considering ionization by cosmic rays and X-rays, thermal ionization, radiative recombinations and recombinations on the dust grains. Analytical solution and numerical investigations show that the magnetic field is coupled to the gas in the case of radiative recombinations. Magnetic field is quasi-azimuthal close to accretion disk inner boundary and quasi-radial in the outer regions. Magnetic field is quasi-poloidal in the dusty “dead” zones with low ionization degree, where ohmic diffusion is efficient. Magnetic ambipolar diffusion reduces vertical magnetic field in 10 times comparing to the frozen-in field in this region. Magnetic field is quasi-azimuthal close to the outer boundary of accretion disks for standard ionization rates and dust grain size a _d=0.1 μm. In the case of large dust grains (a _d>0.1 μm) or enhanced ionization rates, the magnetic field is quasi-radial in the outer regions. It is shown that the inner boundary of dusty “dead” zone is placed at r=(0.1–0.6) AU for accretion disks of stars with M=(0.5–2)?M _⊙. Outer boundary of “dead” zone is placed at r=(3–21) AU and it is determined by magnetic ambipolar diffusion. Mass of solid material in the “dead” zone is more than 3?M _⊕ for stars with M≥1?M _⊙.     

Determination of effective temperatures of the accretion disks in symbiotic stars

The conditions of the excitation of ultraviolet emission lines in symbiotic stars-triple systems with white dwarf and nebular clouds are examined. A new method is suggested for the determination of effective temperatureT _* of the accretion disk around of white dwarf, which is based on the balance between the summary intensitiesE _e =E _i of ultraviolet emission lines, escaped by nebular cloud, and a definite part of the energy of black-body radiation of accretion disk in the region shorter from the some wavelength ₀. This condition brings us to the formula (9) which is used for the determination of the temperatureT _* and radiusr _n of nebular cloud around the symbiotic system. It is shown that practically in all cases ₀=180 Å=const. The results of the application of this method in relation to the five symbiotic triple-star systems are presented in Tables III and IV.     

Cold dust around chamaeleon stars

Here we present the results of 1.3 millimetre continuum measurements for intermediatemass stars in the Chamaeleon system of dark clouds. The detected millimetre radiation is most probably thermal emission from cold circumstellar dust grains. The measured millimetre fluxes are combined with infrared observations to model the broad-band spectral energy distribution (SED). In this way the parameters of the emission regions are determined.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Amorphous carbon around carbon stars

In this paper we present the results of a simplified model to determine the flux emerging from dust envelopes around cool stars. The model proposed holds under the hypotheses of negligible scattering effects and spherical geometry of the dust cloud.The aim of this work is to compare the effects of a graphitic or amorphous composition of the carbon grains in the envelopes. To do this we have used, for the first time, experimental extinction data obtained in the laboratory for submicron amorphous carbon particles.The model has been used to fit the FIR spectral trend of 78 optically thin sources and to reproduce the full spectra of two of the most IR luminous optically-thick sources: CIT 6 and IRC+10216.Our calculations indicate clearly that solid carbon particles around these sources may be amorphous rather than crystalline.     

Periodic motions around pulsating stars

The periodic motion of a test particle (dust, grain, or a larger body) around a pulsating star with a luminosity oscillation of small amplitude (featured by a small parameterB) is being studied. The perturbations of all orbital elements are determined to first order inB, by using Delaunay-type canonical variables and a method whose bases were put forth by von Zeipel. According to the value of the ratio oscillation frequency/dynamic frequency, three possible situations are pointed out: nonresonant (NR), quasi-resonant (QR), and resonant (R). The solution of motion equations shows that only in the (QR) and (R) cases there are orbital parameters (argument of periastron and mean anomaly) affected by secular perturbations. These solutions (which indicate a secularly stable motion in a first approximation) are valid over prediction times of orderB ^–1 in the (NR) case andB ^–1/2 in the (QR) and (R) cases. The theory may be applied to various astronomical situations.     

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.     

Integrability of motion around galactic razor-thin disks

We consider the three-dimensional bounded motion of a test particle around razor-thin disk configurations, by focusing on the adiabatic invariance of the vertical action associated with disk-crossing orbits. We find that it leads to an approximate third integral of motion predicting envelopes of the form \(Z(R)\propto [\varSigma (R)]^{-1/3}\), where R is the radial galactocentric coordinate, Z is the z-amplitude (vertical amplitude) of the orbit and \(\varSigma \) represents the surface mass density of the thin disk. This third integral, which was previously formulated for the case of flattened 3D configurations, is tested for a variety of trajectories in different thin-disk models.     

Toward planets around neutron stars

The two Earth-like mass objects orbiting a 6.2-ms pulsar, PSR1257+25, have survived more than one year of close scrutiny aimed at verifying their existence and remain the most serious candidates to become the first planets detected beyond the Solar System. The analysis of systematic timing measurements of the pulsar made over a 2.5-year period continues to require the presence of two planets with the minimum masses of 3.4M and 2.8M and the corresponding distances from PSR1257+12 of 0.36 AU and 0.47 AU to correctly predict the pulse arrival times. The presently available 3µs rms accuracy of this procedure leaves little room for significant contributions to the pulsar's timing from any mechanisms other than the Keplerian motion. A detection of the effect of planetary perturbations on pulse arrival times which is commonly accepted as the most convincing way to furnish a 100% proof of the reality of pulsar planets is already possible at a 2 level. Intensive searches for millisecond pulsars now under way at various observatories are expected to address a very intriguing question of the frequency of occurrence of neutron star planetary systems.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.