New Observational Frontiers in the Multiplicity of Young StarsGaspard DuchˆeneLaboratoire d’Astrophysique de GrenobleEduardo Delgado-DonateStockholm ObservatoryKarl E. Haisch Jr.Utah Valley State CollegeLaurent Loinard and Luis F. Rodr´ıguezUniversidad Nacional Aut´onoma de M´exicoIt has now been known for over a decade that low-mass stars located in star-formingregions are very frequently members of binary and multiple systems, even more so than mainsequence stars in the solar neighborhood. This high multiplicity rate has been interpreted as theconsequence of the fragmentation of small molecular cores into a few seed objects that accreteto their final mass from the remaining material and dynamically evolve into stable multiplesystems, possibly producing a few ejecta in the process. Analyzing the statistical properties ofyoung multiple systems in a variety of environments therefore represents a powerful approachto place stringent constraints on star formation theories. In this contribution, we first review anumber of recent results related to the multiplicity of T Tauri stars. We then present a series ofstudies focusing on the multiplicity and properties of optically-undetected, heavily embeddedprotostars. These objects are much younger than the previously studied pre-main sequencestars, and they therefore offer a closer look at the primordial population of multiple systems.In addition to these observational avenues, we present new results of a series of numericalsimulations that attempt to reproduce the fragmentation of small molecular cores into multiplesystems, and compare these results to the observations.1. INTRODUCTIONThe prevalence of binary and higher-order multiple sys-tems is a long-established observational fact for field low-mass stars (Duquennoy and Mayor, 1991; Fischer andMarcy, 1992). For over a decade, it has been known thatyoung pre-main sequence stars are also often found in mul-tiple systems. The chapter by Mathieu et al. (2000) in theprevious volume in this series has summarized various sta-tistical surveys for visual multiple systems among T Tauristars in star-forming regions as well as of zero-age mainsequence stars in open clusters. These early multiplicitysurveys have shown that multiple systems are ubiquitousamong young stellar objects (YSOs) and further revealedan environment-dependenttrend. The multiplicity rate in allstellar clusters, even those with the youngest ages such asthe Orion Trapezium cluster, is in excellent agreement withthat observed in main sequence field stars. On the otherhand, the least dense T Tauri populations, like the Taurus-Auriga and Ophiuchus clouds, show a factor of ∼2 multi-plicity excess, relative to field low-mass stars. However, itremained impossible to decide whether this behavior wasthe consequence of an intrinsic difference in the fragmen-tation process, or the result of dynamical disruptive inter-actions acting on timescales shorter than 1 Myr in stellarclusters (see Patience and Duchˆene, 2001 for a review).The purpose of this chapter is to review a variety ofobservational results concerning the multiplicity of younglow-mass stars in order to update the view presented byMathieu et al. (2000). In addition, we present some nu-merical results related to the fragmentation and subsequentevolution of low-mass prestellar cores. These models makepredictions that can be readily tested with the observationalresults discussed here. Throughout this chapter, we focuson low-mass stellar objects with masses roughly rangingfrom 0.1 to 2M. The multiplicity of young substellar ob-jects is discussed in detail in the chapters by Burgasser et al.and Luhman et al, whereas the multiplicity of higher-massobjects is addressed in the chapter by Beuther et al. Otherchapters in this volume present complementary insights onthe subject: Goodwin et al. present more numerical resultson the collapse and fragmentation of molecular cores, aswell as on the dynamical evolution of small stellar aggre-gates; Whitworth et al., Ballesteros-Paredes et al. and Kleinet al.) discuss the collapse of larger-scale molecular cores;Mathieu et al. and Monin et al. focus on various proper-1ties (dynamical masses and disks properties, respectively)of known T Tauri binary stars.2. AN UPDATE ON THE MULTIPLICITY OFYOUNG LOW-MASS STARSAs mentioned above, the first efforts to study youngmultiple systems were focused on determining the aver-age number of wide companions per object in well-knownpre-main sequence populations. Attempting to account forthese multiplicity rates has led to various theories that in-volve the fragmentation of molecular cores and the subse-quent dynamical evolution of aggregates of stars embeddedin gaseous clouds. So far, the observed multiplicity rate ofT Tauri populations alone has not proved entirely conclu-sive, and since the review by Mathieu et al. (2000), thefocus of statistical studies of young multiple systems hasshifted to other areas. Before probing much younger, stillembedded multiple systems (Section 3), we review here anumber of studies on T Tauri multiple systems that go be-yond the surveys that were conducted in the 1990s.2.1 Multiplicity In Young Nearby AssociationsThe clear dichotomy between high- and low-multiplicitystar-forming regions has usually been considered evi-dence of an environment-dependent star formation sce-nario. However, since most stars form in stellar clusters,one could also consider that the rare molecular clouds thathost too many companions are exceptions for yet unde-termined reasons. Over the last decade, several groups ofa few tens of stars with ages typically between ∼10 and50 Myr have been identified in the Sun’s vicinity based ontheir common three-dimensional motion and youth indica-tors (Zuckerman and Song, 2004). Most members of theseassociations are low-mass pre-main sequence stars. There-fore, these co-moving groups represent additional, nearbypopulations of young stars whose multiplicity could be ex-pected to resemble that of the Taurus-Auriga populationgiven their low stellar densities.Soon after their discovery, systematic searches for vi-sual companions were conducted in some of these groupsin order to complement the previous surveys. For in-stance, Chauvin et al. (2002, 2003) and Brandeker etal. (2003) conducted surveys for visual companions in theTW Hya, Tucana-Horologium and MBM 12 groups; we in-clude MBM 12 in this discussion despite the continuing de-bate regarding its distance
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