Disc clearing conventional Disc clearing conventional view: most stars are either view: most stars are either

rich in circumstellar rich in circumstellar diagnostics, e.g. diagnostics, e.g.

Or devoid of same: intermediate states Or devoid of same: intermediate states less common ==> RAPID FINAL less common ==> RAPID FINAL

CLEARINGCLEARING

UV => M

IR => M + disc shape

Submm => disc mass

Prior to this rapid clearing, it’s Prior to this rapid clearing, it’s likely that viscous evolution likely that viscous evolution

plays an important role in disc plays an important role in disc evolutionevolution

UV excess => do UV excess => do accrete at rates ~ accrete at rates ~

M_disc/ageM_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 EVIDENCE FOR SECOND (RAPID) TIMESCALE: GAP IN IR (RAPID) TIMESCALE: GAP IN IR

COLOURS IN TAURUS:COLOURS IN TAURUS:

• Incompatible with simple power law decline Incompatible with simple power law decline with time of surface density in the inner discwith 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:Spitzer muddies the waters:

• Recent claims of high incidence of discs Recent claims of high incidence of discs with partially cleared inner regions with partially cleared inner regions

• maybe don’t need a rapid clearing maybe don’t need a rapid clearing mechanism in general….??mechanism in general….??

Now there are NIR/MIR surveys of many star forming regions

Currie et al 2009

These claims are puzzling These claims are puzzling because the infared two colour because the infared two colour

diagram isdiagram issimilar to Taurus:similar to Taurus: Few objects with colours between Few objects with colours between

optically thick discs and bare photospheresoptically thick discs and bare photospheresTwo colour diagram for M stars in range of clusters observed by Spitzer

Cf Taurus

(Alex Hall)

So why is there this claim So why is there this claim of a large population of of a large population of partially cleared discs?partially cleared discs?

i)i) Uncleared (optically thick) discs Uncleared (optically thick) discs around M stars can look as though around M stars can look as though they contain inner holes when they they contain inner holes when they

don’t! And Spitzer surveys have more don’t! And Spitzer surveys have more cool (M) stars than Taurus. 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 So why is there this claim of a large population of partially a large population of partially

cleared discs?cleared discs? • Discs are claimed to show evidence Discs are claimed to show evidence

of clearing because they show less of clearing because they show less infrared than `Taurus median’ 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 What about claims of very low disc masses?disc masses?

Not very reliable if lack data longward of 24 Not very reliable if lack data longward of 24 mm

Degeneracy between disc mass and T ( R ): Degeneracy between disc mass and T ( R ): flux @ 24 flux @ 24 m may be compatible with flat m may be compatible with flat

optically thick discs optically thick discs

If fit a flared disc, only mildly optically thin If fit a flared disc, only mildly optically thin @24@24mm

- poor constraint on mass- poor constraint on mass

Beware that derived masses can be biased Beware that derived masses can be biased by correlations between parameters in by correlations between parameters in

model model

SED librariesSED libraries

• Partially cleared discs are uncommon also Partially cleared discs are uncommon also in Spitzer samples - still need a in Spitzer samples - still need a

mechanism for rapid removal of disc’s mechanism for rapid removal of disc’s mass at end mass at end

Robitaille et al 2006, 2007

A new classification scheme A new classification scheme for transition discs (Muzerolle for transition discs (Muzerolle

et al astroph last week)et al astroph last week)• Classical TOs (= inner holes): rising Classical TOs (= inner holes): rising

flux 8-24 flux 8-24 mm

• Weak excessWeak excess

Half these accrete

Few of these accrete

Incidence = 10%

Incidence by spectral typeIncidence by spectral type

• Incidence of Incidence of Classical TosClassical Tos

• Incidence of Incidence of Classical Tos Classical Tos + weak + weak excessexcess

Weak excess objects dominate for cool stars

Working hypotheses:Working hypotheses:

• Transition time short (as before)Transition time short (as before)• In older clusters find many flat In older clusters find many flat

optically thick discs around M stars optically thick discs around M stars with low accretion ratewith low accretion rate

• The transition cuts in later in The transition cuts in later in evolution than in earlier type evolution than in earlier type stars???stars???