The Circumstellar Environments of Young Stars at AU ScalesRafael Millan-GabetCalifornia Institute of TechnologyFabien MalbetLaboratoire d’Astrophysique de GrenobleRachel AkesonCalifornia Institute of TechnologyChristoph LeinertMax-Planck-Institut f¨ur AstronomieJohn MonnierUniversity of MichiganRens WatersUniversity of AmsterdamWe review recent advances in our understanding of the inner-most regions of the cir-cumstellar environment around young stars made possible by the technique of long baselineinterferometry at infrared wavelengths. Near-infrared observations directly probe the locationof the hottest dust. The characteristic sizes found are much larger than previously thought, andstrongly correlate with the luminosity of the central young stars. This relation has motivatedin part a new class of models of the inner disk structure. The first mid-infrared observationshave probed disk emission over a larger range of scales, and spectrally resolved interferometryhas for the first time revealed mineralogy gradients in the disk. These new measurementsprovide crucial information on the structure and physical properties of young circumstellardisks, as initial conditions for planet formation. In addition to summarizing these pioneeringobservations, we expose the many open questions that accompany the impressive progressmade, and anticipate the experimental and modelling efforts that promise to help elucidate thediverse phenomena associated with the close circumstellar environment of young stars.1. INTRODUCTIONStars form from collapsing clouds of gas and dust andin their earliest infancies are surrounded by complex en-vironments that obscure our view at optical wavelengths.As evolution proceeds, a stage is revealed with three maincomponents: the young star, a circumstellar disk and an in-falling envelope. Eventually, the envelope dissipates, andthe emission is dominated by the young star-disk system.Later on, the disk also dissipates to very tenuous levels.It is out of the young circumstellar disks that planets areexpected to form, and therefore understanding their phys-ical conditions is necessary before we can understand theformation process. Of particular interest are the inner fewAU (Astronomical Unit), corresponding to formation sitesof Terrestrial type planets, and to migration sites for gasgiants presumably formed further out in the disk (see thechapter by Udry, Fisher and Queloz).A great deal of direct observational support exists for thescenario outlined above, and in particular for the existenceof circumstellar disks around young stars (see the chaptersby Dutrey, Guilloteau & Ho and by Menard et al.). In addi-tion, the spectroscopic and spectro-photometric character-istics of these systems (i.e. spectral energy distributions,emission lines) are also well described by the disk hypoth-esis. However, state of the art optical and millimeter-waveimaging typically probe scales of 100−1000s of AU withresolutions of 10s of AU, and models of unresolved obser-vations are degenerate with respect to the spatial distribu-tion of material. As a result, our understanding of even themost general properties of the circumstellar environment atfew AU or smaller spatial scales is in its infancy.Currently, the only way to achieve sufficient angularresolution to directly reveal emission within the inner AUis through optical interferometry at visible and infraredwavelengths. An interferometer with a baseline length ofB = 100 m (typical of current facilities) operating atnear to mid-infrared wavelengths (typically H to N bands,1λ0= 1.65 − 10 µm) probes 1800 − 300 K material andachieves an angular resolution ∼ λ0/2B ∼ 3 − 26 mil-liarcseconds (mas), or 0.5−4 AU at a distance typical ofthe nearest well known star forming regions (150 pc). Thisobservational discovery space is illustrated in Fig. 1, alongwith the domains corresponding to various young stellar ob-ject (YSO) phenomena and complementary techniques andinstruments. Optical interferometers are ideally suited todirectly probe the innermost regions of the circumstellar en-vironment around young stars, and indeed using this tech-nique surprising and rapid progress has been made, as theresults reviewed in this chapter will show.Optical, as well as radio, interferometers operate by co-herently combining the electromagnetic waves collectedby two or more telescopes. Under conditions that ap-ply to most astrophysical observations, the amplitude andphase of the resulting interference patterns are relatedvia a two-dimensional Fourier transform to the bright-ness distribution of the object being observed. For a de-tailed description of the fundamental principles, variationsin their practical implementation, and science highlightsin a variety of astrophysics areas we refer the reader tothe reviews by Monnier (2003) and Quirrenbach (2001).The reader may also be interested in consulting the pro-ceedings of topical summer schools and workshops suchas Lawson (2000), Perrin and Malbet (2003), Garcia etal. (2003) and Paresce et al. (2006) (online proceedings ofthe Michelson Summer Workshops may also be found athttp://msc.caltech.edu/michelson/).While interferometers are capable of model-independentimaging by combining the light from many telescopes, mostresults to date have been obtained using two-telescopes(single-baseline). The main characteristics of current fa-cilities involved in these studies are summarized in Table 1.Interferometer data, even with sparse spatial frequency cov-erage, provides direct constraints about source geometryand thus contains some of the power of direct imaging.However, with such data alone only the simplest single-component objects can be constrained. Therefore, moretypically, a small number of fringe visibility data points arecombined with a spectral energy distribution (SED) for fit-ting simple physically-motivated geometrical models, suchas a Gaussian profile or ring emission (depending on thecontext). This allows us to determine “characteristic sizes”at the wavelength of observation for many types of YSOs(e.g. T Tauri, Herbig Ae/Be, FU Orioni). In addition, thevisibility amplitudes in a given spectral band can be com-pared to predictions of specific physical models. Realisticmodels models are usually defined by many more parame-ters than can be uniquely constrained by the interferometerdata alone, however in combination with simultaneous SEDfitting, this exercise has allowed to rule out certain
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