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Disc clearing conventional view: most stars are either rich in circumstellar diagnostics, e.g. UV => M IR => M + disc shape Submm => disc mass Or devoid of same: intermediate states less common ==> RAPID FINAL CLEARING
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 a -a Phenomenological description as due to action of (pseudo) viscosity ~ R => e.g. for a=1 get similarity solution: M = M_in ( 1 + t / t ) ( ~ R ) (Lynden-Bell & Pringle 1974, Hartmann et al 1998) -1.5 in
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: Kenyon & Hartmann 1995 • Incompatible with simple power law decline with time of surface density in the inner disc (Alex Hall)
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: Now there are NIR/MIR surveys of many star forming regions • Recent claims of high incidence of discs with partially cleared inner regions • maybe don’t need a rapid clearing mechanism in general….?? Currie et al 2009
These claims are puzzling because the infared two colour diagram issimilar to Taurus: 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? 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 2008 - 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? e.g. Apai et al 2004 ..but this may mean that disc geometry is less flared: I.e. evidence for dust settling, not dust clearing necessarily…… Discs are claimed to show evidence of clearing because they show less infrared than `Taurus median’ Flat optically thick disc
What about claims of very low disc masses? Not very reliable if lack data longward of 24 m Degeneracy between disc mass and T ( R ): flux @ 24 m may be compatible with flat optically thick discs If fit a flared disc, only mildly optically thin @24m - 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 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 • Weak excess Half these accrete Incidence = 10% Few of these accrete
Incidence by spectral type • Incidence of Classical Tos • Incidence of Classical Tos + weak excess Weak excess objects dominate for cool stars
Working hypotheses: • Transition time short (as before) • 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???