The Taurus Molecular Cloud: Multi-Wavelength Surveys withXMM-Newton, the Spitzer Space Telescope, and CFHT1Manuel G¨udelPaul Scherrer InstitutDeborah L. PadgettCalifornia Institute of TechnologyCatherine DougadosLaboratoire d’Astrophysique de GrenobleThe Taurus Molecular Cloud (TMC) ranks among the nearest and best-studied low-massstar formation regions. It contains numerous prototypical examples of deeply embeddedprotostars with massive disks and outflows, classical and weak-lined T Tauri stars, jets andHerbig-Haro objects, and a growing number of confirmed brown dwarfs. Star formation isongoing, and the cloud covers all stages of pre-main sequence stellar evolution. We haveinitiated comprehensive surveys of the TMC, in particular including: (i) a deep X-ray surveyof about 5 sq. degrees with XMM-Newton; (ii) a near-to-mid-infrared photometric survey of≈30 sq. degrees with the Spitzer Space Telescope, mapping the entire cloud in all availablephotometric bands; and (iii) a deep optical survey using the Canada-France-Hawaii Telescope.Each wavelength regime contributes to the understanding of different aspects of young stellarsystems. XMM-Newton and Spitzer mapping of the central TMC is a real breakthrough in diskcharacterization, offering the most detailed studies of correlations between disk properties andhigh-energy magnetic processes in any low-mass star-forming region, extending also to browndwarfs in which disk physics is largely unexplored. The optical data critically complementsthe other two surveys by allowing clear source identification with 0.800resolution, identifyingsubstellar candidates, and, when combined with NIR data, providing the wavelength baselineto probe NIR excess emission. We report results and correlation studies from these surveys. Inparticular, we address the physical interpretation of our new X-ray data, discuss the entire youngstellar population from embedded protostars to weak-lined T Tau stars and their environment,and present new results on the low-mass population of the TMC, including young brown dwarfs.1. INTRODUCTIONIn a modern picture of star formation, complex feed-back loops regulate mass accretion processes, the ejectionof jets and outflows, and the chemical and physical evo-lution of disk material destined to form planets. Observa-tions in X-rays with Chandra and XMM-Newton penetratedense molecular envelopes, revealing an environment ex-posed to high levels of X-ray radiation. In a complementarymanner, observations in the infrared with the Spitzer SpaceTelescope (Spitzer) now obtain detailed infrared photome-try and spectroscopy with diagnostics for disk structure andchemical composition of the gas and dust in the circumstel-1With significant contributions from: Lori Allen (CfA), Marc Au-dard (Columbia University), Jerˆome Bouvier (Grenoble), Kevin Briggs(PSI), Sean Carey (Caltech), Elena Franciosini (Palermo), Misato Fuk-agawa (Caltech), Nicolas Grosso (Grenoble), Sylvain Guieu (Grenoble),Dean Hines (Space Science Institute), Tracy Huard (CfA), Eugene Mag-nier (IfA Honolulu), Eduardo Mart´ın (IAC, Spain), Franc¸ois M´enard(Grenoble), Jean-Louis Monin (Grenoble), Alberto Noriega-Crespo (Cal-tech), Francesco Palla (Florence), Luisa Rebull (Caltech), Luigi Scelsi(Palermo), Alessandra Telleschi (PSI), Susan Terebey (CalStateLA).lar environment. Furthermore, optical surveys have reacheda sensitivity and area coverage with which a detailed censusof the substellar population has become possible.Near- to far-infrared (IR) emission originates predomi-nantly in the dusty environment of the forming stars, eitherin contracting gaseous envelopes or in circumstellar disks.IR excess (relative to the photospheric contributions) hasbeen successfully used to model disk geometry, the struc-ture of the envelope, and also the composition and structureof dust grains (d’Alessio et al., 1999).X-rays play a crucial role in studies of star formation,both physically and diagnostically. They may be gener-ated at various locations in young stellar systems, such asin a “solar-like” coronal/magnetospheric environment, inshocks forming in accretion funnel flows (e.g., Kastner etal., 2002), or in jets and Herbig-Haro flows (e.g., Pravdo etal., 2001; Bally et al., 2003; G¨udel et al., 2005). By ion-izing circumstellar material, X-rays also determine some ofthe prevalent chemistry (Glassgold et al., 2004) while mak-ing the gas accessible to magnetic fields. Ionization of the1circumstellar-disk surface may further drive accretion in-stabilities (Balbus and Hawley, 1991). Many of these X-rayrelated issues are summarized in the chapter by Feigelsonet al. in this volume, based on recent X-ray observations ofstar-forming regions.1.1 The Taurus Molecular Cloud ComplexThe Taurus Molecular Cloud (TMC henceforth) hasplayed a fundamental role in our understanding of low-mass star formation. At a distance around 140 pc (e.g.,Loinard et al., 2005; Kenyon et al., 1994), it is one of thenearest star formation regions (SFR) and reveals character-istics that make it ideal for detailed physical studies. One ofthe most notable properties of the TMC in this regard is itsstructure in which several loosely associated but otherwiserather isolated molecular cores each produce one or only afew low-mass stars, different from the much denser coresin ρ Oph or in Orion. TMC features a low stellar densityof only 1–10 stars pc−2(e.g., G´omez et al., 1993). Strongmutual influence due to outflows, jets, or gravitational ef-fects are therefore minimized. Further, most stars in TMCare subject to relatively modest extinction, providing ac-cess to a broad spectrum of stars at all evolutionary stagesfrom Class 0 sources to near-zero age main-sequence T Taustars. TMC has also become of central interest for the studyof substellar objects, in particular brown dwarfs (BD), withregard to their evolutionary history and their spatial distri-bution and dispersal (Reipurth and Clarke, 2001; Brice˜noet al., 2002).TMC has figured prominently in star-formation studiesat all wavelengths. It has provided the best-characterizedsample of classical and weak-lined T Tau stars (CTTS andWTTS, respectively, or “Class II” and “Class III” objects- Kenyon and Hartmann, 1995); most of our current pic-ture of low-density star formation is indeed based on IRASstudies of TMC (Beichman et al., 1986; Myers et al., 1987;Strom et al., 1989; Kenyon et al., 1990; Weaver and Jones,1992; and Kenyon and Hartmann,
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