Star-disk Interaction Research Articles

ROSATX‐Ray Spectral Properties of Nearby Young Associations: TW Hydrae, Tucana‐Horologium, and the β Pictoris Moving Group

We present archival ROSAT data for three recently identified, nearby (D < 70 pc), young (~10-40 Myr) stellar associations: the TW Hydrae association, the Tucana-Horologium association, and the β Pictoris moving group. The distributions of ROSAT X-ray hardness ratios (HR1, HR2) for these three groups, whose membership is dominated by low-mass, weak-lined T Tauri stars, are tightly clustered and very similar to one another. The value of HR1 for TW Hya itself—the only bona fide classical T Tauri star in any of the nearby groups—is clearly anomalous among these nearby young stars. We compare the hardness ratio distributions of stars in the three nearby groups with those of T Tauri stars, the Hyades, and main-sequence dwarfs in the field. This comparison demonstrates that the X-ray spectra of F through M stars soften with age and that F and G stars evolve more rapidly in X-ray spectral hardness than do K and M stars. It is as yet unclear whether this trend can be attributed to age-dependent changes in the intrinsic X-ray spectra of stars of type F and later, to a decrease in the column density of circumstellar gas (e.g., in residual protoplanetary disks), or to the diminishing contributions of star-disk interactions to X-ray emission. Regardless, these results demonstrate that analysis of archival ROSAT X-ray spectral data can help both to identify nearby, young associations and to ascertain the X-ray emission properties of members of known associations.

Self-Collimated Jets, Accretion Discs and Young Stars

This paper is focused on the huge dynamical importance of the mass load in magnetized jet models. I will first review some ‘naive’ questions (Why should jets be magnetized? What is the jet driving source?), then show why the jet mass load is so important and why numerical simulations are unable yet to deal with jet formation. I will afterwards briefly describe some results of the only accretion disc model addressing explicitely this question and present a possible star-disc magnetospheric interaction giving rise to time-dependent outbursts.

Magnetic star-disk interaction in classical T Tauri systems

We carry out 2.5D MHD simulations to study the interaction between a dipolar magnetic field of a T Tauri Star, a circumstellar accretion disk, and the halo above the disk. The initial disk is the result of 1D radiation hydrodynamics computations with opacities appropriate for low temperatures. The gas is assumed resistive, and inside the disk accretion is driven by a Shakura–Sunyaev-type eddy viscosity. Magnetocentrifugal forces due to the rotational shear between the star and the Keplerian disk cause the magnetic field to be stretched outwards and part of the field lines are opened. For a solar-mass central star and an accretion rate of 10−8 solar masses per year a field strength of 100 G (measured on the surface of the star) launches a substantial outflow from the inner parts of the disk. For a field strength of 1 kG the inner parts of disk is disrupted. The truncation of the disk turns out to be temporary, but the magnetic field structure remains changed after the disk is rebuilt.

Collimation of a Central Wind by a Disk-Associated Magnetic Field

We present the results of time-dependent, numerical magnetohydrodynamic simulations of a realistic young stellar object outflow model with the addition of a disk-associated magnetic field. The outflow produced by the magnetic star-disk interaction consists of an episodic jet plus a wide-angle wind with an outflow speed comparable to that of the jet (100–200 km s-1). An initially vertical field of ≪ 0.1 Gauss, embedded in the disk, has little effect on the wind launching mechanism, but we show that it collimates the entire flow (jet + wide wind) at large (several AU) distances. The collimation does not depend on the polarity of the vertical field. We also discuss the possible origin of the disk-associated field.

Collimation of a central wind by a disc-associated magnetic field

Studies of jets from young stellar objects (YSOs) suggest that material is launched from a small central region at wide opening angles and collimated by an interaction with the surrounding environment. Using time-dependent, numerical magnetohydrodynamic simulations, we follow the detailed launching of a central wind via the coupling of a stellar dipole field to the inner edge of an accretion disc. Our method employs a series of nested computational grids, which allows the simulations to follow the central wind out to scales of tens of astronomical units (au), where it may interact with its surroundings. The coupling between the stellar magnetosphere and the inner edge of the disc has been known to produce an outflow containing both a highly collimated jet plus a wide-angle flow. The jet and wide-angle wind flow at roughly the same speed (100–200 km s−1), and most of the energy and mass is carried off at relatively wide angles. We show that the addition of a weak disc-associated field (≪ 0.1 G) has little effect on the wind launching, but it collimates the entire flow (jet + wide wind) at a distance of several au. The collimation is inevitable, regardless of the relative polarity of the disc field and stellar dipole, and the result is a more powerful and physically broader collimated flow than from the star–disc interaction alone. Within the collimation region, the morphology of the large-scale flow resembles a pitchfork, in projection. We compare these results with observations of outflows from YSOs and discuss the possible origin of the disc-associated field.

Hard X-ray emission from a young massive star-forming cluster

We report the detection of hard X-ray emission (>2 keV) from a number of point sources associated with the very young massive star-forming region IRAS 19410+2336. The X-ray emission is detected from several sources located around the central and most deeply embedded mm continuum source, which remains undetected in the X-ray regime. All X-ray sources have K-band counterparts, and those likely belonging to the evolving massive cluster show near-infrared colors in the 2MASS data indicative of pre-main-sequence stages. The X-ray luminosities around 10^{31} erg/s are at the upper end of luminosities known for low-mass pre-main-sequence sources, and mass estimates based on the infrared data indicate that at least some of the X-ray detected sources are intermediate-mass objects. Therefore, we conclude that the X-ray emission is due to intermediate-mass pre-main-sequence Herbig Ae/Be stars or their precursors. The emission process is possibly due to magnetic star-disk interaction as proposed for their low-mass counterparts.

The role of the central stellar cluster in active galactic nuclei

The subject of the paper is the role of the massive stellar cluster in the activity phenomenon and in the structure of active galactic nuclei. We introduce a simple model of stellar dynamics in the internal part of the cluster, which allows us to include both the star-disk and the star-star interactions. It is shown that the properties of the distribution of stars in the vicinity of the black hole are determined both by the interaction of the stars with the accretion disk and by the pair gravitational and contact interaction between the stars. We calculate the distribution of stars in the central parts of the cluster and we discuss possible effects of stellar mass-loss due to the star-disk interaction. Finally, we study the implications of the central cluster for active galactic nuclei activity. We model the broad line region assuming that the gaseous wakes, following stars after each disk crossing, play the role of the broad line region clouds, and we calculate the corresponding line profiles. We also analyze the contribution of star-star and star-disk collisions to active galactic nuclei variability.

Heating and Ionization ofX‐Winds

In order to compare the X-wind with observations, one needs to be able to calculate its thermal and ionization properties. We formulate the physical basis for the streamline-by-streamline integration of the ionization and heat equations of the steady X-wind. In addition to the well-known processes associated with the interaction of stellar and accretion funnel hot spot radiation with the wind, we include X-ray heating and ionization, mechanical heating, and a revised calculation of ambipolar diffusion heating. The mechanical heating arises from fluctuations produced by star-disk interactions of the time-dependent X-wind that are carried by the wind to large distances where they are dissipated in shocks, MHD waves, and turbulent cascades. We model the time-averaged heating by the scale-free volumetric heating rate, Γmech = αρv3s-1, where ρ and v are the local mass density and wind speed, respectively, s is the distance from the origin, and α is a phenomenological constant. When we consider a partially revealed but active young stellar object, we find that choosing α ~ 10-3 in our numerical calculations produces temperatures and electron fractions that are high enough for the X-wind jet to radiate in the optical forbidden lines at the level and on the spatial scales that are observed. We also discuss a variety of applications of our thermal-chemical calculations that can lead to further observational checks of X-wind theory.

Disk Locking and the Presence of Slow Rotators among Solar‐Type Stars in Young Star Clusters

The simultaneous presence of both the so-called ultrafast rotators and slowly rotating stars among the solar-type stars (~0.6-1.2 M☉) in the same young star clusters has been a puzzle in the field of stellar rotation. No model to date has been able to explain both by a single mechanism intrinsic to the star, and questions about the appropriate initial conditions for models often complicate the problem. In this paper, using the same starting conditions for the models that we used in examining the origin of the ultrafast rotators in young star clusters, we show that the slowest rotators demand an extrinsic mechanism. Assuming that this mechanism is a disk-star interaction, we determine that a disk-locking timescale of a few Myr must operate for this type of star. If, instead of allowing (radial) differential rotation, we enforce solid-body rotation, the models require timescales about 2-3 times as long.

Accretion disc-stellar magnetosphere interaction: field line inflation and the effect on the spin-down torque

We calculate the structure of a force-free magnetosphere which is assumed to corotate with a central star and which interacts with an embedded differentially rotating accretion disc. The magnetic and rotation axes are aligned, and the stellar field is assumed to be a dipole. We concentrate on the case when the amount of field line twisting through the disc—magnetosphere interaction is large, and consider different outer boundary conditions. In general the field line twisting produces field line inflation (e.g. Bardou < Heyvaerts), and in some cases with large twisting many field lines can become open. We calculate the spin-down torque acting between the star and the disc, and we find that it decreases significantly for cases with large field line twisting. This suggests that the oscillating torques observed for some accreting neutron stars could be caused by the magnetosphere varying between states with low and high field line inflation. Calculations of the spin evolution of T Tauri stars may also have to be revised in the light of the significant effect that field line twisting has on the magnetic torque resulting from star—disc interactions.

Recurrent magnetic reconnection in protostellar magnetospheres

The solar X-ray satellite Yohkoh, has found extensive evidence for magnetic reconnection in the solar corona. Here we extend the magnetic reconnection model of solar flares to hard X-ray flares observed in star forming regions. A new ingredient is a protostellar disk which can inject helicity into the magnetosphere if the disk is threaded by the dipole magnetic field of the protostar. We carried out 2.5-dimensional magnetohydrodynamical (MHD) simulations of the disk-star interaction. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. In the presence of resistivity, magnetic reconnection takes place in the current sheet formed inside the expanding loops. A hot, outgoing plasmoid and hot post flare loops are formed as a result of the reconnection. Numerical results are consistent with the observed plasma temperature (10 7 – 10 8K), the length of the flaring loop (10 11 – 10 12cm), and the velocity of optical jets (200 – 400km/s). We use high-resolution numerical simulations to show that multiple magnetic islands are created in the current sheet due to the growth of the tearing mode instability. The magnetic islands are ejected quasi-periodically. Intermittent flaring activity continues as long as the disk matter twists the dipole magnetic field.

Stellar capture by an accretion disc

The long-term evolution of a stellar orbit captured by a massive galactic centre via successive interactions with an accretion disc has been examined. An analytical solution describing the evolution of the stellar orbital parameters during the initial stage of the capture has been found. Our results are applicable to thin Keplerian discs with an arbitrary radial distribution of density and a rather general prescription for the star—disc interaction. Temporal evolution is given in the form of quadrature which can be carried out numerically.

X-Ray Flares and Outflows Driven by Magnetic Interaction Between a Protostar and its Surrounding Disk

AbstractHere we present a model of hard X-ray flares and hot plasma outflows (optical jets) observed in protostars. Assuming that the dipole magnetic field of a protostar threads the protostellar disk, we carried out 2.5-dimensional magnetohydrodynamic (MHD) simulations of the diskstar interaction. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. In the presence of resistivity, magnetic reconnection takes place in the current sheet formed inside the expanding loops. Hot, outgoing plasmoid and post flare loops are formed as a result of the reconnection. Numerical results are consistent with the observed plasma temperature (107 – 108K), the length of the flaring loop (1011 – 1012cm), and the speed of optical jets (200 – 400 km s−1 ).

X-Ray Flares and Mass Outflows Driven by Magnetic Interaction between a Protostar and Its Surrounding Disk

We propose a model of hard X-ray flares in protostars observed by the ASCA satellite. We assumed that the dipole magnetic field of the protostar threads the protostellar disk and carried out 2.5 dimensional magnetohydrodynamic (MHD) simulations of the disk-star interaction. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. As the twist accumulates, magnetic loops expand and finally approach the open field configuration. A current sheet is formed inside the expanding loops. In the presence of resistivity, magnetic reconnection takes place in the current sheet. Outgoing magnetic island and postflare loops are formed as a result of the reconnection. The timescale of this flare is the order of the rotation period of the disk. The released magnetic energy partly goes into the thermal energy and heats up the flaring plasma up to 108 K. The length of the flaring loop is several times the radius of the central star, consistent with observations. The speed of the hot plasmoid ejected by the reconnection is 200-400 km s-1 when the footpoint of the loop is at 0.03 AU from 1 M☉ protostar. The hot plasma outflow can explain the speed and mass flow rate of optical jets. Dense, cold, magnetically accelerated wind (v ~ 150-250 km s-1) emanates from the surface of the disk along the partially open magnetic field lines threading the disk. This dense, cold wind may correspond to high-velocity neutral winds.

Dynamical evolution of star clusters around a rotating black hole with an accretion disc

The dynamical evolution of relativistic star clusters around a massive Kerr black hole with an accretion disc is examined, in the regime where the black hole dominates the potential and star-disc interactions dominate the evolution. A set of diagrams exhibiting the time development of the energy-dependent distribution function, f (∊), and the distributions of semimajor axes, a, eccentricities, e, and inclinations, i, of the model system are presented; plots of the latter three quantitities over time for a few illustrative orbits are also given. A simple approximation for the final radius of an orbit brought into the disc under star-disc interactions, namely |$a_\text f\approx a_0(1-e^2_0) \enspace\text {cos}^4(i_0/2)$|⁠) (or, in terms of angular momentum, |$L_\text f\approx(L_0+L_{z,0})/2$|⁠), is derived. It is found that the main effect of star-disc interactions on an isotropic cluster, besides the circularization and alignment of orbits, is to steepen an initial density profile ρ* ∝ r–n to the approximate ‘asymptotic’ profile |$\rho_\ast\propto r^{-2.5}$| when n ≲ 2.5 to leave the profile unchanged when n ≳ 2.5, and in both cases to increase the central density (by several hundred very close to the black hole); initially anisotropic clusters are found to exhibit similar patterns. The numerical results can be explained in terms of a simple analytic model. Relativistic effects are found to affect the cluster properties significantly only at very small radii (≲ 10 GM/c2); in particular, the location of the last stable orbit limits the cluster's inner extent. By significantly increasing the central stellar density, star-disc interactions could be self-limiting by causing stellar collisions to become important; the future evolution of the cluster in this case will depend on the relative balance of the collisional, alignment and stellar evolutionary time-scales.

The effect of star-disc interactions on the binary mass-ratio distribution

The effect of star-disc interactions on the binary mass-ratio distribution

Implications of neutrino balls as the source of gamma-ray bursts

(To appear in the Astrophysical Journal) Holdom and Malaney (1994) have suggested a mechanism for gamma-ray bursts which requires that stars be captured by a neutrino ball. Neutrino balls would be, for the most part, denser than main sequence stars, but their density would decrease as their mass increased. We show that small neutrino balls would subject stars to tidal forces sufficient to disrupt them. We thus argue that if neutrino balls existed at the centres of galaxies, only the largest would be able to act as a source of gamma-ray bursts. Such neutrino balls would have a mass of order $10^7\Msun$. Tidal capture of stars by a neutrino ball would not be important, but dynamical friction against the neutrinos or star-disc interactions could both be important capture mechanisms. We find that a gamma-ray burst would occur in a galaxy containing such a neutrino ball roughly every $10^2\y$, and the fraction of all galaxies contributing to the gamma-ray burst flux would be $\sim 10^{-4}$, assuming that this was the mechanism of all gamma-ray bursts. These numbers have implications for neutrino ball models of active galaxies, assuming that all gamma-ray bursts and all AGN come from neutrino balls. Either a small fraction $\sim 10^{-2}$ of the lifetime of such an object could be spent as an AGN, or that the probability of a neutrino ball becoming an AGN would be $10^{-2}$. It is not possible to rule out the possibility that neutrino balls might exist at the centres of galaxies through direct ground-based observation of stellar kinematics.

Star-Disc Interactions and Binary Formation

By the time a binary arrives on the main sequence, its dynamics have essentially become those of a simple two-body system, so that its orbital parameters are constants of the motion thereafter. This statement of course needs to be modified under several circumstances, such as where tidal effects, mass exchange or interactions with field stars come into play. For the majority of binaries, however, orbital evolution is all but over by the main sequence. Therefore, in order to explain the distribution of binary orbital parameters one has to look to earlier times (prior to 107 years) when the more complicated dynamics can drive the orbital evolution that establishes these parameters.

The formation of primordial binaries in globular clusters by star-disk interactions

The formation of primordial binaries in globular clusters is examined using simple numerical models. Clusters of protostars collapse until their velocity dispersion rises sufficiently to reverse the infall and the cluster reaches equilibrium. During the collapse, interactions between stars and protostellar disks lead to stellar capture. It is found that binary fraction of a few percent typically result. Binary formation is terminated when the velocity dispersion rises to a point at which most encounters result in disk destruction rather than capture. As a result, much gas is returned to the cluster ISM, limiting the star formation efficiency to a value significantly below 100 percent.

Star-disc interactions near a massive black hole

We examine the possibility that stars in the nuclei of galaxies could be ground down into short-period orbits, by interaction with a disc around a central massive black hole. A star on a highly eccentric orbit, which passed close to the hole, would lose energy and momentum in passing through the disc. The cumulative effect of many passages would bring the star into a circular orbit corotating with the disc. The star would then either open up a gap in the disc (if the disc is thinner than the star's Roche radius and the viscosity in the disc is sufficiently low), or accrete material from the disc.