Tauri Stage Research Articles

Proper motions of molecular hydrogen outflows in theρOphiuchi molecular cloud

We aim to take a census of molecular hydrogen emission line objects (MHOs) in the Rho Ophiuchi molecular cloud and to make the first systematic proper motion measurements of these objects in this region. Deep H2 near-infrared imaging is performed to search for molecular hydrogen emission line objects. Multi-epoch data are used to derive the proper motions of the features of these objects, and the lengths and opening angles of the molecular hydrogen outflows. Our imaging covers an area of about 0.11 deg2 toward the L1688 core in the Rho Ophiuchi molecular cloud. In total, six new MHOs are discovered, 32 previously known MHOs are detected, and the proper motions for 86 features of the MHOs are measured. The proper motions lie in the range of 14 to 247 mas/yr, corresponding to transversal velocities of 8 to 140 km s-1 with a median velocity of about 35 km s-1. Based on morphology and proper motion measurements, 27 MHOs are ascribed to 21 driving sources. The molecular hydrogen outflows have a median length of about 0.04 pc and random orientations. We find no obvious correlation between H2 jet length, jet opening angle, and the evolutionary stage of the driving sources as defined by their spectral indices. We find that the fraction of protostars (23%) that drive molecular hydrogen outflows is similar to the one for Class II sources (15%). For most molecular hydrogen outflows, no obvious velocity variation along the outflow has been found. In Ophiuchus the frequency of occurrence of molecular hydrogen outflows has no strong dependency on the evolutionary stage of the driving source during the evolution from the protostellar stage to the classical T Tauri stage.

Water transport in protoplanetary disks and the hydrogen isotopic composition of chondrites

The D/H ratios of carbonaceous chondrites, believed to reflect the hydrogen isotopic composition of water in the inner early Solar System, are intermediate between the protosolar value and that of most comets. The isotopic composition of cometary water has been accounted for by several models where the isotopic composition of water vapor evolved by isotopic exchange with hydrogen gas in the protoplanetary disk. However, the position and the large range of variation of the distribution of D/H ratios in carbonaceous chondrites have yet to be explained. In this paper, we assume that the D/H composition of cometary ice was achieved in the disk building phase and model the further isotopic evolution of water in the inner disk in the classical T Tauri stage. Reaction kinetics compel isotopic exchange between water and hydrogen gas to stop at ∼500K, well inside the snow line. However, the equilibrated water can be transported to the snow line (and beyond) via turbulent diffusion and consequently mix with isotopically comet-like water. Thus the competition between outward diffusion and net inward advection established an isotopic gradient, which is at the origin of the large isotopic variations in the carbonaceous chondrites and other water-bearing objects accreted in the protoplanetary disk.Under certain simplifying assumptions, we calculate analytically the probability distribution function of the D/H ratio of ice accreted in planetesimals and compare it with observational data. The distribution is found to essentially depend on two parameters: the radial Schmidt number ScR, which ratios the efficiencies of angular momentum transport and turbulent diffusion, and the range of heliocentric distances over which currently sampled chondrite parent bodies were accreted. The minimum D/H ratio of the distribution corresponds to the composition of water condensed at the snow line, which is a function of both the composition of equilibrated water having diffused from hotter disk regions and the efficiency of this outward transport as measured by ScR. Observations constrain the latter to low values (0.1–0.3), which suggests that turbulence in the planet-forming region was hydrodynamical in nature, as would be expected in a dead zone. Such efficient outward diffusion would also account for the presence of high-temperature minerals in comets.

Tidal truncation of circumplanetary discs

We analyse some properties of circumplanetary discs. Flow through such discs may provide most of the mass to gas giant planets, and such discs are likely sites for the formation of regular satellites. We model these discs as accretion discs subject to the tidal forces of the central star. The tidal torques from the star remove the disc angular momentum near the disc outer edge and permit the accreting disc gas to lose angular momentum at the rate appropriate for steady accretion. Circumplanetary discs are truncated near the radius where periodic ballistic orbits cross, where tidal forces on the disc are strong. This radius occurs at approximately 0.4 r_H for the planet Hill radius r_H. During the T Tauri stage of disc accretion, the disc is fairly thick with aspect ratio H/r > 0.2 and the disc edge tapering occurs over a radial scale ~ H ~ 0.1 r_H. For a circular or slightly eccentric orbit planet, no significant resonances lie within the main body of the disc. Tidally driven waves involving resonances nonetheless play an important role in truncating the disc, especially when it is fairly thick. We model the disc structure using one dimensional time-dependent and steady-state models and also two dimensional SPH simulations. The circumplanetary disc structure depends on the variation of the disc turbulent viscosity with radius and is insensitive to the angular distribution of the accreting gas. Dead zones may occur within the circumplanetary disc and result in density structures. If the disc is turbulent throughout, the predicted disc structure near the location of the regular Jovian and Saturnian satellites is smooth with no obvious feature that would favor formation at their current locations.

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.

The Magnetospheric Gap and the Accumulation of Giant Planets Close to a Star

The bunching of giant planets at a distance of several stellar radii may be explained by the disruption of the inner part of a disk by the magnetosphere of a star during the T Tauri stage of evolution. The rotating magnetic field of the star gives rise to a low-density magnetospheric gap where stellar migration is strongly suppressed. We performed full three-dimensional (3D) magnetohydrodynamic simulations of the disk-magnetosphere interaction and examined conditions for which the magnetospheric gap is "empty," by changing the misalignment angle between the magnetic and rotational axes of the star, Θ, and by lowering the adiabatic index γ, which is a mock-up of the effect of heat conductivity and cooling. Our simulations show that for a wide range of plausible conditions, the gap is essentially empty. However, in the case of large misalignment angles Θ, part of the funnel stream is located in the equatorial plane, and the gap is not empty. Furthermore, if the adiabatic index is small (γ ~ 1.1), and the rotational and magnetic axes are almost aligned, then matter penetrates through the magnetosphere due to 3D instabilities, forming high-density equatorial funnels. For these two limits, there is appreciable matter density in the equatorial plane of the disk so that a planet may migrate into the star.

Mid?Infrared Observations of T Tauri Stars: Probing the Star?Disk Connection in Rotational Evolution

We present mid-IR N-band (λeff = 10.2 μm) photometry of a carefully selected sample of T Tauri stars thought to be single from the Taurus-Auriga molecular cloud. Infrared excesses in these stars are generally attributed to circumstellar dust disks. Combining observations at 2.16 (Ks band) and 10.2 μm (N band) we probe a region in the circumstellar dust disk from a few stellar radii through the terrestrial planet zone (0.02-1.0 AU). By analyzing the distribution of the (Ks - N) color index with respect to previously measured photometric rotation periods we investigate what role circumstellar disks play in the rotational evolution of the central star. The resulting positive correlation between these two variables is consistent with the notion that a star-disk interaction facilitates the regulation of angular momentum during the T Tauri stage. We also demonstrate how including nonsingle stars in such an analysis weakens any correlation in the relation between (Ks - N) color and period. To further understand disk properties we also present spectral energy distributions for a few objects with new ground-based M-band (λeff = 4.8 μm) and Q-band (λeff = 20 μm) data and compare them to a geometrically thin, optically thick disk model.

Magnetohydrodynamic Simulations of Disk–Magnetized Star Interactions in the Quiescent Regime: Funnel Flows and Angular Momentum Transport

Magnetohydrodynamic (MHD) simulations have been used to study disk accretion to a rotating magnetized star with an aligned dipole moment. Quiescent initial conditions were developed in order to avoid the fast initial evolution seen in earlier studies. A set of simulations was performed for different stellar magnetic moments and rotation rates. Simulations have shown that the disk structure is significantly changed inside a radius rbr where magnetic braking is significant. In this region the disk is strongly inhomogeneous. Radial accretion of matter slows as it approaches the area of strong magnetic field, and a dense ring and funnel flow (FF) form at the magnetospheric radius rm, where the magnetic pressure is equal to the total, kinetic plus thermal, pressure of the matter. FFs, where the disk matter moves away from the disk plane and flows along the stellar magnetic field, are found to be stable features during many rotations of the disk. The dominant force driving matter into the FF is the pressure gradient force, while gravitational force accelerates it as it approaches the star. The magnetic force is much smaller than the other forces. The FF is found to be strongly sub-Alfvénic everywhere. The FF is subsonic close to the disk, but it becomes supersonic well above the disk. Matter reaches the star with a velocity close to that of free fall. Angular momentum is transported to the star dominantly by the magnetic field. In the disk the transport of angular momentum is mainly by the matter, but closer to the star the matter transfers its angular momentum to the magnetic field, and the magnetic field is dominant in transporting angular momentum to the surface of the star. For slowly rotating stars we observed that magnetic braking leads to the deceleration of the inner regions of the disk, and the star spins up. For a rapidly rotating star, the inner regions of the disk rotate with a super-Keplerian velocity, and the star spins down. The average torque is found to be zero when the corotation radius rcor ≈ 1.5rm. The evolution of the magnetic field in the corona of the disk depends on the ratio of magnetic to matter energies in the corona and in the disk. Most of the simulations were performed in the regime of a relatively dense corona where the matter energy-density was larger than the magnetic energy-density. In this case the coronal magnetic field gradually opens, but the velocity and density of outflowing matter are small. In a test case where a significant part of the corona was in the field-dominated regime, more dramatic opening of the magnetic field was observed with the formation of magnetocentrifugally driven outflows. Numerical applications of our simulation results are made to T Tauri stars. We conclude that our quasi-stationary simulations correspond to the classical T Tauri stage of evolution. Our results are also relevant to cataclysmic variables and magnetized neutron stars in X-ray binaries.

Evolution of the Fu Orionis Object BBW 76

We have carried out a long-term photometric and spectroscopic monitoring program of the southern FU Orionis–type object BBW 76 spanning the period from 1982 to 1997. BBW 76 has the same radial velocity as the small cloud toward which it is projected, and for which a kinematic distance of about 1.8 kpc has been derived. We have determined a large reddening of E(B-V) ~ 0.7 for BBW 76. Optical and infrared spectra show the change toward later spectral type with increasing wavelength characteristic of FU Orionis stars and indicative of a hot luminous disk. High-resolution echelle spectra of BBW 76 show P Cygni profiles with extended blueshifted absorption troughs at the Hα and sodium lines from a neutral, supersonic wind. Comparison of such spectra obtained at six different epochs between 1985 and 1997 reveals major changes in these Hα and sodium line profiles. For a period of 10 years from 1985, the massive absorption troughs diminished in extent and depth, until by 1994 they had all but disappeared, while at the same time the blueshifted emission peak in the Hα line increased markedly in strength. However, when observed in 1997, the absorption had increased again and the emission had diminished. We interpret this in terms of an extended period during which accretion through a circumstellar disk decreased, with a resulting decrease in wind production. But the increased activity by 1997 shows that this is not a constant decay and that the star was not about to revert to its presumably original T Tauri stage. We monitored the star with optical photometry from 1983 to 1994, during which period it decreased almost monotonically in brightness by 0.2 mag in V. Infrared J, H, and K photometry from 1983 to 1991 shows a period of monotonic fading between 1984 and 1988, followed by more irregular behavior. In a search of the Harvard plate archives we have found a plate from the year 1900 on which BBW 76 is seen at approximately its present brightness, certainly not 2 mag brighter as expected if the optical decline between 1983 and 1994 had persisted during the whole century. Also, a plate taken for the Franklin-Adams charts in 1927 again shows BBW 76 at approximately the same brightness. This historical light curve makes BBW 76 the FU Orionis star with the longest-documented period in a high state. Overall, the observations suggest that BBW 76 is virtually identical to the prototype of its class, FU Orionis itself, in all respects except that BBW 76 has not shown the regular fading that FU Orionis has displayed after its eruption in 1936. This may be due to continued replenishment of the circumstellar accretion disk.

Near-Infrared Coronagraphic Imaging of the Circumstellar Disk around TW Hydrae

We present ground-based near-infrared (H-band) imaging of the circumstellar disk around the nearby classical T Tauri star TW Hydrae. The scattered-light image shows a face-on disk with a radius of 4'' (corresponding to 225 AU) and a morphology that agrees with recent images from the Hubble Space Telescope and the Very Large Array. The best-fit power-law for the disk's radial surface brightness profile obeys the law r-3.3±0.3. We use our image and published continuum flux densities to derive properties of the disk with a simple model of emission from a flat disk. The best-fit values for disk mass and inner radius are 0.03 M☉ and 0.3 AU, respectively; the best-fit values for the temperature, density, and grain opacity power-law exponents (q, p, and β) are 0.7, 1.3, and 0.9, respectively. These properties are similar to those of disks around classical T Tauri stars located in more distant molecular clouds. Because of TW Hydrae's nearby location and pole-on orientation, it is a uniquely favorable object for future studies of radial disk structure at the classical T Tauri stage.

Millimeter Continuum Image of the Circumstellar Disk around the Young Star Haro 6-5B

Aperture synthesis observations of dust continuum emission at 2 mm from the class I object Haro 6-5B, made with the Nobeyama Millimeter Array, are presented. A disklike structure that coincides with the dark lane in the visible and near-infrared images is clearly resolved with a small synthesized beam of 133 × 121 with its position angle (P.A.) of 134°. The beam-deconvolved size of the disk is 233 × 097 at P.A. = 138°, and the total flux density is 36.7 ± 2.6 mJy. The radius and inclination angle of the disk derived from the image are 309 ± 18 AU and 67° ± 5°, respectively. By model fitting of the spectral energy distribution of Haro 6-5B with these values, the power-law index of the dust opacity, β, is derived to be 1.05 ± 0.04 and the disk mass is calculated to be 0.021 ± 0.002 M☉. No extended emission with 1000 AU scale is detected, suggesting that the envelope around Haro 6-5B has been almost dissipated and that the object is a transient source from the protostar stage to the T Tauri stage.

X-Ray Emission from Distant Stellar Clusters

Observations with the Einstein Observatory indicated that stellar X-ray activity diminishes in clusters older than 70 Myr (Pleiades). ROSAT observations of older clusters also support this result (see Caillault 1995 and references therein). The timescales over which young stars diminish in X-ray luminosity depends on spectral type (Randich et al. 1996), leading to the conclusion that X-ray activity in late type PMS depends on age and stellar mass. F and G-stars approach the main sequence much faster and the diminishing rates of X-ray activity from F to M stars start to differ considerably. Kastner et al. (1997) observed that the mean of the ratio Lx/Lbol for K and M dwarf stars increases monotonically for low-mass stars from the very early T Tauri stage through the age of the Pleiades cluster, reflecting the contraction and spin-up of such stars during pre-main sequence evolution. This ratio then decreases towards middle aged stars, as late-type main sequence stars spin down. Here we extend this result by including more distant clusters that are younger overall than those considered by Kastner et al. and also including earlier spectral types.

The Quest for X-Rays from Protostars

The field of low-mass star formation and early evolution has made rapid progress in recent years, thanks in particular to observations in the IR and mm ranges. The current evolutionary scheme calls for two main stages, themselves divided into two substages (e.g., André & Montmerle 1994): (i) protostars, comprizing the newly discovered so-called “Class 0 sources”, detected mostly or only in the mm range, which are young protostars with estimated ages ~ 104 yrs, and “Class I sources”, visible in the near- to mid-IR, which are evolved protostars with estimated ages ~ 105 yrs; (ii) T Tauri stars, which are visible in the IR but also in the optical, the younger being the “classical” T Tauri stars (called “Class II” in the IR), and the “weak-line” T Tauri stars (“Class III” in the IR), with a large age spread of ~ 106 – 107 yrs. According to current models (e.g., Shu et al. 1987), protostars consist of a forming star surrounded by an extended envelope (up to ~ 10,000 AU in radius); the star forms via an accretion disk inside a cavity ~ several 100 AU in radius. The disk probably plays an important role in generating molecular outflows, running through the envelope. Classical T Tauri stars are only surrounded by a disk, which disappears at the weak-line T Tauri stage.

The Dispersing Cloud Core around T Tauri

We have made <SUP>13</SUP>CO (J = 1-0) observations of T Tauri with the Nobeyama Millimeter Array (NMA) and with the Nobeyama 45 m telescope. The <SUP>13</SUP>CO (J = 1-0) emission detected with the NMA shows three different features: (1) a pair of ringlike features surrounding T Tauri with a radius of 30" (corresponding to 4200 AU at the distance of 140 pc to T Tauri) at the velocities blueshifted and redshifted by less than 1 km s<SUP>-1</SUP> from the systemic velocity, (2) a blueshifted compact feature 3" east of T Tauri, and (3) another compact feature 7" southwest of T Tauri at velocities redshifted by more than 1 km s<SUP>-1</SUP>. On the other hand, the <SUP>13 </SUP>CO maps obtained with the 45 m telescope show a smoothly extended feature near the systemic velocity, which is missed in the NMA observations, as well as the above- mentioned three features. The total masses of gas detected with the NMA and the 45 m telescope are estimated to be (0.054-0.23) M<SUB>sun</SUB> and (0.31-1.3) M<SUB>sun</SUB>, respectively. The difference in the estimated mass between the two observations is mainly due to resolving out of the smoothly extended feature in the NMA observations. <P />The <SUP>13</SUP>CO rings are interpreted as biconical outfiowing shells in a nearly pole-on configuration. The high-velocity stellar wind ejected from T Tauri is estimated to be energetic enough to drive these out- flowing shells. We have analyzed the three-dimensional structure of the shells by correcting for the projection effect on the plane of the sky, and we have found that the spatial extent of the shells is nearly equal to or slightly smaller than the typical size of the molecular cloud cores in the Taurus Molecular Cloud. This fact indicates that the outflowing shells are part of the parent cloud core that still remains around T Tauri, which is now dispersing under the influence of the stellar wind. Such an environment around T Tauri is quite different from those around typical T Tauri stars, which are associated only with compact gaseous components. These results suggest strongly that T Tauri is one of the objects in the transitional phase from the protostar stage, in which a central star is deeply embedded in an infalling envelope, to the T Tauri stage, in which a central star is surrounded by a compact circumstellar disk instead of a spatially extended envelope.

Evolution of Carbonaceous Matter in Galactic Growth

Recently astronomical observation revealed that optical properties of interstellar dusts in galaxies nearby our Galaxy shows gradual variation of optical properties closely related to their stage of galactic growth. The average interstellar extinction curves of Small Magellanic Cloud (SMC), Large Magellanic Cloud (LMC), and our Galaxy are summarized with astronomical observations of artificial satellites. An extinction curve of SMC, the youngest of them, showed smooth feature from far UV to near IR. But that of our Galaxy, the oldest of them, showed a large hump peaked at 220 nm. In this paper we discussed evolution of carbonaceous dusts in galactic growth.With high resolution observation of IR spectrum near our solar system, a life of carbonaceous dusts were clarified. The carbonaceous dusts were formed in circumstellar space of carbon rich aged stars and then they were expelled into interstellar space to be interstellar dusts. After the long residence in interstellar space the dusts were condensed in a dark cloud to form a protostar. In aged stage of dusts there are at least three substages such as dark cloud stage, protostellar stage, T Tauri stage and so on. The spectral changes of their carbonaceous dusts in these stages were summarized.

Star formation induced by supersonic turbulence in interstellar molecular cloud

We studied fragmentation process of the interstellar molecular cloud which is predominated by supersonic turbulence with special regard to collisions of turbulent gas elements and formation of a shock-compressed layer by receding shock waves. The propagation of the shock waves and the evolution of the compressed layer are followed by one-dimensional gas dynamical simulation until self-gravity becomes significant, taking account of the effects of thermal properties of the molecular gas and magnetic fields. It is shown that the efficient cooling by CO molecules and its sensitive dependence on gas density make the shock-compressed layer so cold and dense that the layer becomes gravitationally unstable and breaks into fragments even if the gas elements are gravitationally stable prior to the collision. The mass of the unstable fragment is estimated to be about two solar masses or less, irrespective of the presence of the magnetic field. The stars formed by collisions of supersonic turbulent gas elements accelerate the surrounding gas in T Tauri stage and replenish the turbulent energy to maintain the mechanical equilibrium of the molecular cloud.

The evolution of rotation in the early history of the Solar System

Theories that require the co-genetic formation of the Sun and planets have difficulty in explaining the slow rotation of the Sun. An analysis is made of various mechanisms for slowing down the core of an evolving nebula. Two of these involve a high magnetic dipole moment for the early Sun. The first envisages magnetic linkage to an external plasma but requires a dipole moment 10 6 times that of the present Sun. The other is based on the co-rotation of m atter leaving the Sun during a T Tauri stage, and requires a dipole moment 10 4 times the present value. A mechanical process for transferring angular momentum outward involving dissipation in a solar-nebula disc is incapable of giving what is required. Two processes of star formation in a turbulent cloud are discussed. Both are capable of giving a slowly rotating Sun. Various models for producing planets are examined in relation to the spin they would produce. Planets formed from floccules would be spinning quickly but could evolve in such a way as to give observed spins for giant planets and also satellite families. Accretion models are very sensitive to assumptions, and parameters and can be adjusted to explain almost any observation. Protoplanets formed in elliptical orbits would acquire spin angular momentum through solar tidal action and would evolve to give reasonable spin rates and regular satellite families. The various tilts of their spin axes could be explained by interactions between protoplanets in the early Solar System.

Dissipation of the Primordial Terrestrial Atmosphere Due to Irradiation of the Solar Far-UV during T Tauri Stage

Dissipation of the Primordial Terrestrial Atmosphere Due to Irradiation of the Solar Far-UV during T Tauri Stage

Dissipation of the Primordial Terrestrial Atmosphere Due to Irradiation of the Solar EUV

The escape of a primordial Earth's atmosphere due to heating by solar radiation is studied by integrating numerically hydrodynamic equations for steady and spherically­ symmetric outflow of hydrogen molecules and helium atoms. As heating sources, we take account of (1) the solar EUV radiation which is expected to be very strong during the T Tauri stage, (2) the solar visible light and (3) the release of gravitational energy of accreting planetesimals. The effect of solar wind is neglected but the condition of this neglect is estimated. The results show that the primordial atmosphere, having existed in the early stage of the Earth's history, is dissipated within a period of 5 X 108 y, which is the upper limit imposed from the origin of the present terrestrial atmosphere, as far as the solar EUV flux is more than 2 >< 10 2 times as large as the present one.

Pre-main sequence stellar evolution with mass loss

A semi-empirical formulation is given of the rate of stellar mass loss by stellar winds. Evolutionary studies of stars in the pre-main sequence (T Tauri) stage are presented for a variety of rates of mass loss. It has been found that different mass loss rates produce only small changes in the positions of equal evolutionary time lines in HR diagrams. Thus it is concluded that the spread of points in HR diagrams of young clusters results from a spread in their times of formation. This is consistent with the initiation of star formation by violent hydrodynamic compression of a typical interstellar cloud.

The early despinning of the Sun

It is shown that if the Sun passed through a T Tauri stage, then a mass loss of only 15% would be sufficient to despin the Sun to an angular velocity of 0 (10−5 rad/sec) at 107 years without the additional braking effect of an enhanced magnetic field. Thus the present Sun could have a core rotating at most ten times faster than its surface.