Observational Constraints on Dust DiskLifetimes: Implications for Planet FormationBy Lynne A. Hillenbrand1California Institute of Technology, MS 105-24, Pasadena, CA 91105, USAThus far our impressions regarding the evolutionary time scales for young circumstellar diskshave been based on small number statistics. Over the past decade, however, in addition to preci-sion study of individual star/disk systems, substantial observational effort has been invested inobtaining less detailed data on large numbers of objects in young star clusters. This has resultedin a plethora of information now enabling statistical studies of disk evolutionary diagnostics.Along an ordinate one can measure disk presence or strength through indicators such as ul-traviolet/blue excess or spectroscopic emission lines tracing accretion, infrared excess tracingdust, or millimeter flux measuring mass. Along an abscissa one can track stellar age. Whilebulk trends in disk indicators versus age are evident, observational errors affecting both axes,combined with systematic errors in our understanding of stellar ages, both cloud and bias anysuch trends. Thus detailed understanding of the physical processes involved in disk dissipationand of the relevant time scales remains elusive. Nevertheless, a clear effect in current data thatis unlikely to be altered by data analysis improvements is the dispersion in disk lifetimes. Inneraccretion disks are traced by near-infrared emission. Moderating a generally declining trend innear-infared continuum excess and excess frequency with age over <1to8±4Myr,isthefactthat a substantial fraction of rather young (<1 Myr old) stars apparently have already lost theirinner accretion disks while a significant number of rather old (8-16 Myr) stars apparently stillretain inner accretion disks. The age at which evidence for inner accretion disks ceases to beapparent for the vast majority (∼90%) of stars is in the range 3-8 Myr. Terrestrial zone dust istraced by mid-infrared emission where sufficient sensitivity and uniform data collection are onlynow being realized with data return from the Spitzer Space Telescope. Constraints on mid-diskdissipation and disk clearing trends with radius are forthcoming.1. IntroductionA long standing paradigm for the formation of stars, and subsequently planets, involvesthe rotating collapse of a molecular cloud core to form on a time scale of ∼105yr a centralproto-star surrounded by an infalling envelope and accreting disk. Typical ages of revealedyoung T Tauri and Herbig Ae/Be stars are ∼106yr. Gradual dispersal of the initiallyoptically thick circumstellar material occurs in the early pre-main sequence phase as thesystem evolves through the final stages of disk accretion, which can last ∼107yr or morein at least some well known cases (TW Hya, Hen 3-600, TWA 14 – Muzerolle et al. 2000,2001 and Alencar & Batalha 2002; PDS 66 – Mamajek et al. 2002; ECha J0843.3-7905 –Lawson et al. 2002; St 34 – White & Hillenbrand 2005).Physical processes occurring in younger disks include viscous accretion onto the centralstar, mass loss due to outflow, irradiation by the central star, ablation due to the stellarwind, turbulent mixing of material, stratification, and gradual settling of the dust towardsthe disk mid-plane – this last process a critical and limiting step in the path towardsplanet formation in the standard core accretion model (e.g. Weidenschilling et al. 1997,2000; Pollack et al. 1996). The total disk mass decreases and the dust:gas mass ratio,assumed at least initially to be in the interstellar ratio, changes with time due to acombination of the above effects. Similarly, the dust particules are assumed interstellar-like in their composition and structure. Of particular interest here is the expected lossof dust opacity due to assembly of small particles into larger bodies that might later12 L. A. Hillenbrand: Dust Disk LifetimesFigure 1. Images of disks at various evolutionary stages scaled to a time line showing ourgeneral understanding of the basic phenomena. Data are courtesy of J. Stauffer and B. Patten(left panel, Ori 114-426 optically thick “silhouette disk” as imaged with HST/WFPC), Kalas& Jewitt 1995 (middle panel, β Pic as imaged by a ground-based coronagraph), and P. Kalas(right panel, our own zodiacal dust disk along with a comet, as photographed from Calar Alto).be known as planetesimals. For solar-type stars, the ultimate result in at least 10% andperhaps as many as 50% of cases is a mature solar system (see Marcy, this volume).In parallel with the discovery and study of exo-solar planets and planetary systemsover the last decade (the topic of this conference), we have had dramatic observationalconfirmation in this same time period of the basic paradigm for star formation as brieflyoutlined above. Direct images and interferometric observations which spatially resolveyoung circumstellar disks at optical, near-infrared, and millimeter wavelengths have be-come common, though are far from ubiquitous. When combined with measured spectralenergy distributions, such spatially resolved data are valuable for breaking model degen-eracies and thus improving our understanding of source geometry and dust characteris-tics.Rough correlation of the spatially resolved and SED appearances of a source, whichindicate circumstellar status, with stellar evolutionary state, or age, has long been ad-vocated (e.g. Lada 1987). However, it remains unclear whether the established sequenceof circumstellar evolutionary states corresponds directly with source age. White & Hil-lenbrand (2004) argue for the Class I/II stages that this is not necessarily the case giventhe similarities in the stellar photospheric and accretion properties of Class I and II starsas inferred from high dispersion spectroscopy of a large sample in Taurus-Auriga. Like-wise, Kenyon & Hartmann 1995 discuss the Class II/III distribution in the HR diagram,which is indistinguishably intermingled and therefore suggestive of similar ages. Becauseof uncertainties in age assignments, particularly for the most enshrouded sources whichtypically do not have ages estimated independent of their circumstellar characteristics,the time scales associated with the dispersal of circumstellar material and the formationof planets are only vaguely constrained at best.How, then, do we catalog young circumstellar disks and characterize their evolution?L. A. Hillenbrand: Dust Disk Lifetimes
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