Disc clearing conventional view: most stars are either rich in circumstellar diagnostics, e.g. Or devoid of same: intermediate states less common ==> RAPID FINAL CLEARING UV => M IR => M + disc shape Submm => disc mass 

Prior to this rapid clearing, it’s likely that viscous evolution plays an important role in disc evolution UV excess => do accrete at rates ~ M_disc/age Phenomenological description as due to action of (pseudo) viscosity ~ R => e.g. for a=1 get similarity solution: M = M_in ( 1 + t / t ) a -1.5 in  (Lynden-Bell & Pringle 1974, Hartmann et al 1998)  (  ~ R ) -a

Not a bad match to M(t) data (Hartmann et al 1998)  (need scatter: Armitage et al 2004)

EVIDENCE FOR SECOND (RAPID) TIMESCALE: GAP IN IR COLOURS IN TAURUS: EVIDENCE FOR SECOND (RAPID) TIMESCALE: GAP IN IR COLOURS IN TAURUS: Incompatible with simple power law decline with time of surface density in the inner disc Incompatible with simple power law decline with time of surface density in the inner disc (Alex Hall) Kenyon & Hartmann 1995

Not a bad match to M(t) data (Hartmann et al 1998)  But if extrapolate would have to wait > 100 Myr before disc was optically thin in the infrared DISC DISPERSAL ISNT JUST A MATTER OF SELF-SIMILAR VISCOUS EVOLUTION: SOMETHING ELSE HAPPENS TO DISPERSE DISC ON SECOND (SHORT) TIMESCALE

Spitzer muddies the waters: Recent claims of high incidence of discs with partially cleared inner regions Recent claims of high incidence of discs with partially cleared inner regions maybe don’t need a rapid clearing mechanism in general….?? maybe don’t need a rapid clearing mechanism in general….?? Now there are NIR/MIR surveys of many star forming regions    Currie et al 2009

These claims are puzzling because the infared two colour diagram is similar to Taurus: Few objects with colours between Few objects with colours between optically thick discs and bare photospheres Two colour diagram for M stars in range of clusters observed by Spitzer Cf Taurus (Alex Hall)

So why is there this claim of a large population of partially cleared discs? i) Uncleared (optically thick) discs around M stars can look as though they contain inner holes when they don’t! And Spitzer surveys have more cool (M) stars than Taurus. Some M star SEDs in the Coronet cluster from Sicilia-Aguilar et al these are compatible with no inner hole in disc even though disc is negligible cf star at < 6 m  Ercolano, Clarke & Robitaille 2009

So why is there this claim of a large population of partially cleared discs? Discs are claimed to show evidence of clearing because they show less infrared than `Taurus median’ Discs are claimed to show evidence of clearing because they show less infrared than `Taurus median’..but this may mean that disc geometry is less flared: I.e. evidence for dust settling, not dust clearing necessarily……  Flat optically thick disc e.g. Apai et al 2004 

What about claims of very low disc masses? Not very reliable if lack data longward of 24  m Not very reliable if lack data longward of 24  m  Degeneracy between disc mass and T ( R ): 24  m may be compatible with flat optically thick discs  Degeneracy between disc mass and T ( R ): 24  m may be compatible with flat optically thick discs  If fit a flared disc, only mildly optically  m - poor constraint on mass  Beware that derived masses can be biased by correlations between parameters in model SED libraries Partially cleared discs are uncommon also in Spitzer samples - still need a mechanism for rapid removal of disc’s mass at end Partially cleared discs are uncommon also in Spitzer samples - still need a mechanism for rapid removal of disc’s mass at end Robitaille et al 2006, 2007

A new classification scheme for transition discs (Muzerolle et al astroph last week) Classical TOs (= inner holes): rising flux 8-24  m Classical TOs (= inner holes): rising flux 8-24  m Weak excess Weak excess Half these accrete Few of these accrete Incidence = 10%

Incidence by spectral type Incidence of Classical Tos Incidence of Classical Tos Incidence of Classical Tos + weak excess Incidence of Classical Tos + weak excess Weak excess objects dominate for cool stars

Working hypotheses: Transition time short (as before) Transition time short (as before) In older clusters find many flat optically thick discs around M stars with low accretion rate In older clusters find many flat optically thick discs around M stars with low accretion rate The transition cuts in later in evolution than in earlier type stars??? The transition cuts in later in evolution than in earlier type stars???