We review correlations of spectral accretion diagnostics in young binary star systems. Hydrogen emission lines at visual (Hα) and near-infrared (Brγ) wavelengths indicate the presence of active accretion flows onto young stars. We examine the simultaneity of this process in both components of close binaries (separations 100–1500 AU) and find that active accretion is not a random process in these systems: the two stars are typically both active or both inactive. Only 19% of our sample consists of mixed pairs in which just one component displays evidence of accretion, usually the primary. In systems with two active stars, the equivalent width and in particular the line flux of the primary star is generally dominant, indicating stronger accretion activity. We discuss accretion and disk dissipation processes in close binaries and propose that a circumbinary envelope model accounts for the correlated evolution of circumstellar disks around young binaries.

We describe preliminary results of an HST survey of pre-main sequence binary stars in the Taurus-Auriga cloud. We use STIS medium and low resolution spectra to derive the reddening, spectral type, and veiling for each component of close pairs with angular separations ≲ 1″. This sample yields a frequency of mixed pairs, containing one classical and one weak emission T Tauri star, similar to that of wide pre-main sequence binaries. The components of most pairs are coeval, but the secondaries in four pairs are significantly younger than the primaries for the three sets of evolutionary tracks we considered.

We present the results of a high spatial resolution ultraviolet, optical and near-infrared survey of 44 young binary stars in Taurus-Auriga with separations of 10–1000 AU. The observations were carried out using the Hubble Space Telescope and NASA's IRTF. The binary star properties corroborate our previous work that suggests fragmentation is the dominant binary star formation mechanism. Of particular interest, we find that the components of binary systems are more coeval than randomly paired single T Tauri stars. Several important conclusions are drawn regarding the evolution of circumstellar material in binary systems. The mass accretion rates for primary stars are similar to single stars, which suggests that a companion as close as 10 AU has little effect on the mass accretion rate. These accretion rates, if constant, require replenishment of the inner circumstellar disks for at least the closest (≲ 100 AU) binary systems. On average, circumprimary disks appear to survive longer and accrete at a higher rate than circumsecondary disks do. This suggests that circumprimary disks are being preferentially replenished, possibly from a circumbinary reservoir with low angular momentum relative to the binary. The relative T Tauri types and the binary mass ratios tentatively suggest that systems with separations ≲ 200 AU share a common circumbinary reservoir. The higher mass accretion rates of primary stars relative to secondary stars is most likely due to their larger relative mass.

The pre–main-sequence spectroscopic binary AK Sco is surrounded by a circumbinary disk. Yet while this disk must be somewhat disturbed by the binary at its center, there is no evidence from current data that it has a cleared central hole like those seen in some young binaries, nor that accretion from the outer disk has been cut off. Indeed, most of its spectral and photometric diagnostics are indistinguishable from those of young single stars. Why doesn't this system show any evidence of disturbance of its disk? It may be that the center of the disk is indeed largely cleared, but that a small amount of dust in this region supplies the observed near-infrared excess.

In this paper I discuss three areas in which observational data is likely to have a strong impact on theoretical ideas about binary star formation: the collation of binary statistics for different primary masses, the acquisition of spectral energy distributions for individual components within pre-main sequence binaries and the use of binary statistics to constrain the mix of star forming regions that must combine to form the field. Theoretical and observational work is ongoing in each of these areas. It is suggested that for observers an important future task is to remeasure the wide binary population among local field stars, whilst for theorists a problem yet to be addressed is how the mass of a molecular cloud core affects its binary fragmentation properties.

We examine the conditions under which binary and multiple stars may form out of turbulent molecular cloud cores using high resolution 3-D, adaptive mesh refinement (AMR) hydrodynamics (Truelove et al., 1997, 1998; Klein, 1999). We argue that previous conclusions on the conditions for cloud fragmentation have limited applicability, since they did not allow for the nonlinear density and velocity perturbations that are ubiquitous in molecular cloud cores. Over the past year, we have begun to simulate the evolution of marginally stable, turbulent cores. These models have radii, masses, density contrasts, turbulent linewidths, and projected velocity gradients consistent with observations of low-mass molecular cloud cores. Our models are evolved in time under self-gravitational hydrodynamics with AMR using a barotropic equation of state that models the transition from an isothermal to an adiabatic equation of state. We examine several properties of the resulting proto-stellar fragments and discuss the qualitative nature of the fragmentation process in realistic cloud cores: the transition from single to binary and multiple stars; the formation of misaligned binary systems; and the role played by filament formation in the formation of stars.

Fragmentation is the leading explanation for the formation of binary and multiple stars. However, nearly all three dimensional calculations of the collapse and fragmentation of dense molecular cloud cores have ignored the effects of magnetic fields, whereas magnetic fields are generally regarded to be a dominant force in molecular clouds. Three dimensional models of the collapse of clouds with frozen-in magnetic fields have shown that such clouds cannot fragment for a range of initial conditions. However, calculations that allow for magnetic field loss by am-bipolar diffusion have shown that fragmentation is possible for initially prolate or oblate, rotating, magnetically-supported cloud cores. The latter calculations rely on approximations that should be verified by more detailed, traditional magnetohydrodynamical codes. The most obvious effect of magnetic fields is to delay the onset of the collapse phase, but once collapse begins in earnest, fragmentation proceeds in much the same manner as in nonmagnetic clouds, with initially prolate clouds tending to form binary protostars, and with initially oblate clouds tending to form multiple protostars.

The nonlinear outcome of gravitational instabilities in disks depends critically on the thermal physics of the gas. Under conditions where thermal energy is lost efficiently, disks disrupt into dense arms, arclets, and clumps. However, the evidence about whether clumps can ever become permanent bound objects is currently inconclusive. Under conditions where cooling is less efficient or where a balance between heating and cooling is achieved, the amplitudes reached by gravitational instabilities are relatively modest. The result is disk heating and transport of mass and angular momentum rather than condensation of bound companions. Future numerical simulations need to resolve the disk vertical structure and include more realistic equations of state and energy transport.

The formation and evolution processes of magnetized filamentary molecular clouds are investigated in detail by linear stability analyses and non-linear numerical calculations. A one-dimensionally compressed self-gravitating sheet-like cloud breaks up into filamentary clouds. The directions of the longitudinal axes of the resulting filaments are perpendicular to the directions of magnetic field lines unless the column density of the sheet is very small. These magnetized filaments tend to collapse radially without characteristic density, length, and mass scale for the further fragmentation during the isothermal phase. The characteristic minimum mass for the final fragmentation is obtained by the investigation of thermal processes. The essential points of the above processes are analytically explained in terms of the basic physics. A theory for the expected mass function of dense molecular cloud cores is obtained. The expected mean surface density of companions of dense cores is also discussed.

Dynamical contraction of a slowly-rotating magnetized cloud is studied using 2D magnetohydrodynamical (MHD) simulations. In the isothermal stage (n ≲ nA ∼ 1010cm−3), the cloud evolves similarly to that expected from the Larson-Penston self-similar solution and experiences a run-away collapse. However, after the central density exceeds ∼ nA, an accretion disk is formed around an adiabatic core. Just outside the core, an outflow is ejected by the effect of magnetic torque (magneto-centrifugal wind). Since ∼ 10% of the mass is ejected with almost all the angular momentum, the specific angular momentum of the protostellar core reduces to that observed in pre-main-sequence stars.

We discuss the transport of angular momentum induced by tidal effects in a disk surrounding a star in a pre–main sequence binary system. We consider the effect of both density and bending waves. Although tidal effects are important for truncating protostellar disks and for determining their size, it is unlikely that tidally–induced angular momentum transport plays a dominant role in the evolution of protostellar disks. Where the disk is magnetized, transport of angular momentum is probably governed by MHD turbulence. In a non self–gravitating laminar disk, the amount of transport provided by tidal waves is probably too small to account for the lifetime of protostellar disks. In addition, tidal effects tend to be localized in the disk outer regions.

Properties have been demonstrated of the magneto-rotational instability for two different applications, i.e. for a global spherical model and a box simulation with Keplerian background shear flow. In both nonlinear cases a dynamo operates with a negative (positive) α-effect in the northern (southern) disk hemisphere and in both cases the angular momentum transport is outwards. Keplerian accretion disks should therefore exhibit large-scale magnetic fields with a dipolar geometry of the poloidal components favoring jet formation.

We review the salient observational properties of DQ Tau, with an emphasis on the photometric and spectroscopic time-variability of the system. We summarize the evidence for this variability being linked to the orbital dynamics of the underlying close, eccentric, binary.

Observations show that mass accretion can occur at the surfaces of stars in young binaries of all separations. Given that resonant torques have been expected to clear gaps and dam the inward flow of material from circumbinary disks, this result begs the question of how mass accretion is supplied in binary systems. In this paper we address this question in the context of close binaries. Spectral energy distributions indicate that dynamical clearing is not complete for close binaries having massive circumbinary disks. High-resolution infrared spectra of CO rotational and ro-vibrational lines have also detected small amounts of gas which can be located in the region of dynamical clearing. Thus we have signatures of matter from the circumbinary environment through the predicted gap region to the stellar surfaces. We suggest that accretion streams may supply both dynamically cleared regions and circumstellar disks.

We discuss the evolution of the properties of a protobinary system which forms via fragmentation within a collapsing molecular cloud core and grows to its final mass by accreting from the gaseous envelope. We predict how the mass ratio distributions and the masses of the circumstellar and circumbinary discs depend on the binary's separation, the primary's mass, and the properties of the pre-collapse molecular cloud core. We also discuss how the properties of binaries may depend on whether they were formed in isolated molecular cloud cores or in stellar clusters.

The orbital evolution of a young binary star is strongly affected by the circumstellar disks orbiting each of the stars, and/or the circumbinary disk orbiting about the entire binary. Calculation of the evolution requires the knowledge of disk structure (resonantly emitted density and bending waves, disk gap sizes, possible secular nonaxisymmetry), as well as of any gas streamers falling onto the binary. While the diagnostic signatures of the binary-disk interaction are observable (and have recently been observed), the orbital evolution cannot directly be observed so far, because of long associated time scales. Hence, theory is crucial for understanding the evolutionary stage and the future of the observed systems. We briefly review the status of the theory of orbital evolution and discuss several topics requiring further theoretical work.

The absolute dimensions of the eclipsing pair in the triple system TY CrA provide constraints on the evolutionary models for both main sequence and pre-main sequence phases.

This paper presents the masses of young stars measured by the rotation of their circumstellar disks (Simon, Dutrey, and Guilloteau 2000). Their precision is good enough to enable meaningful tests of theoretical models of pre-main sequence evolution. The tests are however limited by the imprecision with which the distances to the stars are known. The astrometric instruments now being developed have the potential to remove this limitation.

The components of the young hierarchical quadruple GG Tau, which span a wide range in spectral type (K7 – M7.5), are used to test both evolutionary models and the temperature scale for very young, low mass stars under the assumption of coeval formation. Of the evolutionary models tested which extend into the substellar regime, those of Baraffe et al. yield the most consistent ages when combined with a temperature scale intermediate between that of dwarfs and giants. The Palla & Stahler and Siess et al. models are also capable of yielding a coeval age down to their lowest mass (0.1 M⊙). These latter two models, which extend to much higher masses than the Baraffe et al. models, agree reasonably well with the Baraffe et al. models at 1.0 M⊙ and thus could be combined to construct a set evolutionary models that extends from Substellar to several solar masses.

We report the discovery of the first low–mass pre–main sequence eclipsing binary among a sample of double-lined spectroscopic binaries in the Orion star forming region found in a previous high-resolution spectroscopic investigation on ROSAT–discovered weak-T Tauri stars. Here we present the preliminary results from the combined analysis of the spectroscopic orbit and B and V light–curves, using data available till spring 2000. We then compare the fundamental stellar parameters derived from the orbital solution with those inferred from some widely used theoretical evolutionary models.