Disk Evolution in Young Binaries: from Observations to TheoryJ.-L. MoninLaboratoire d’Astrophysique de Grenoble, FranceC. J. ClarkeInstitute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UKL. PratoLowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USAC. McCabeJPL, 4800 Oak Grove Drive, Pasadena, CA 91109 USAThe formation of a binary system surrounded by disks is the most common outcome ofstellar formation. Hence studying and understanding the formation and the evolution of binarysystems and associated disks is a cornerstone of star formation science. Moreover, since thecomponents within binary systems are coeval and the sizes of their disks are fixed by the tidaltruncation of their companion, binary systems provide an ideal ”laboratory” in which to studydisk evolution under well defined boundary conditions.Since the previous edition of Protostars and Planets, large diameter (8−10m) telescopes havebeen optimized and equipped with adaptive optics systems, providing diffraction-limited obser-vations in the near-infrared where most of the emission of the disks can be traced. These cuttingedge facilities provide observations of the inner parts of circumstellar and circumbinary disks inbinary systems with unprecedented detail. It is therefore a timely exercise to review the obser-vational results of the last five years and to attempt to interpret them in a theoretical framework.In this paper, we review observations of several inner disk diagnostics in multiple systems,including hydrogen emission lines (indicative of ongoing accretion), K − L and K − Ncolor excesses (evidence of warm inner disks), and polarization (indicative of the relativeorientations of the disks around each component). We examine to what degree these propertiesare correlated within binary systems and how this degree of correlation depends on parameterssuch as separation and binary mass ratio. These findings will be interpreted both in terms ofmodels that treat each disk as an isolated reservoir and those in which the disks are subject tore-supply from some form of circumbinary reservoir, the observational evidence for which wewill also critically review. The planet forming potential of multiple star systems is discussedin terms of the relative lifetimes of disks around single stars, binary primaries and binarysecondaries. Finally, we summarize several potentially revealing observational problems andfuture projects that could provide further insight into disk evolution in the coming decade.1. INTRODUCTIONIt is now a matter of common knowledge that the major-ity of stars in star forming regions are in binary or higher or-der multiple systems (Ghez et al., 1993; Leinert et al., 1993;Simon et al., 1995). Likewise, it is undisputed that many ofthe younger stars in these regions exhibit evidence for cir-cumstellar disks and/or accretion. Putting these two factstogether, an inescapable conclusion is that disks typicallyform and evolvein the environment of a binary/multiplestarsystem.This prompts a number of obvious questions. Can thedistribution of dust and gas in young binaries provide a“smoking gun” for the binary formation process? Is diskevolution, and perhaps the possible formation of planets,radically affected by the binary environment and, if so, howdoes this depend on binary separation and mass ratio? Al-ternatively, if the influence of binarity on disk evolution israther mild, we can at least use binary systems as well con-trolled laboratories, constituting coeval stars with disk outerradii set by tidal truncation criteria, to study disk evolutionas a function of stellar mass.However, it is not possible to address any of these issuesunless we can disentangle the disk/accretion signatures pro-duced by each component in the binary. Given that the sep-aration distribution for binaries in the nearest populous starforming regions, such as Taurus-Aurigae, peaks at ∼ 0.300(≡ 40 AU; e.g., Mathieu, 1994), this necessitates the use1of high resolution photometry and spectroscopy. Such anenterprise has only become possible in the past decade.We review what has been learned in recent years aboutthe distribution of dust and gas within young binary sys-tems. We mainly highlight observational developmentssince PP IV, for example, the discovery of a population ofso called passive disks (McCabe et al., 2006) in low masssecondaries and the use of polarimetry to constrain the ori-entations of disks in young binaries (e.g., Jensen et al.,2004; Monin et al., 2005). We also discuss circumbinarydisks and profile in detail a few systems that have been thesubject of intense observational scrutiny. In addition, it istimely to examine the statistical properties of resolved bi-naries that have been accumulating in the literature over thepast decade. We have therefore combined the results from anumber of relatively small scale studies in order to assem-ble around 60 resolved pairs and use this dataset to examinethe relationship between binarity and disk evolution.In this Chapter we progress through a description ofdisk/accretion diagnostics (and their application to resolvedbinary star studies; Section 2) to highlighting some recentresults on disk structure in binaries (Section 3) to a statis-tical analysis of the relationship between binarity and diskevolution (Section 4). In Section 4.5 we briefly considerhow the insights of the preceding section can be applied tothe question of planet formation in binaries. Section 5 ex-amines future prospects and potential projects to advanceour understanding of disk structure and evolution in youngbinaries.2. INNER DISK DIAGNOSTICS IN YOUNG BINA-RIESHow do we know when either circumstellar or circumbi-nary disks are present in a young multiple star system? Ittook over a decade of observations to confirm the existenceof simple circumstellar disk structures after the original ob-servational and theoretical introduction of the concept inthe early 1970s (e.g., Strom et al., 1971, 1972; Lynden-Bell and Pringle, 1974). The paradigm is yet more compli-cated for a binary systems with multiple disks. Over the lasttwo decades, direct means of imaging circumstellar diskshave become available to astronomers, beginning with mil-limeter observations in the mid-1980s and ending with therecent development of high angular resolution laser guidestar adaptive optics. Most critically to this chapter, the lastdecade has witnessed unprecedented improvements in ourability not only to
View Full Document
Unlocking...