Protostellar/planetary disk observations (and what they might imply)

Lee HartmannUniversity of Michigan

What do we want to know?

• What are disk masses?

• How is the mass distributed?

• Is there “turbulence”? What is it like? where does it occur?

• What transport processes are operating?

I’ll talk about observations instead...• dust mass estimates• disk structure• time-dependence

disk masses ≈ dust masses

measure here, “optically thin”

star disk

Disk masses from dust emission 850m fluxes (Taurus)

Andrews & Williams 2005

Protostars accreting

median MINIMUM mass (100x dust) ≈ 10 M(J)

Caveat: other regions (e.g., Orion Nebula Cluster) may show systematically smaller “disk” masses

Eisner et al. 2008

(outer disk...)

However:

• The dust opacity problem

• maybe – the “where” problem

The dust opacity problem

Observed spectral slopes imply that dust must grow from ISM sizes;

if growth is does not stop at ~ few cm, opacities are LOWER than typically adopted – disk masses are then larger than usually estimated

Mie calculation for power-law size distribution to a(max); D’Alessio et al. 2001

usual value

X

spectral index

The dust opacity problem

“Clint Eastwood question”:

do we feel lucky?

(especially in outer disk)

D’Alessio et al. 2001

usual value

Dominik & Dullemond 05

Where is the mass?

Andrews et al. 2009

Conventional models (MMSN) yield ∝R –p , p ~ 1.5 - 0.4, <p> ~ 0.8:

⇒ most mass at large R

Best we can do: however, (1) no (R) (2) can’t resolve and/or limit R< 10 AU because of optical depth

Disk accretion: statistical measure of gas

Calvet et al. 2004,Muzerolle et al. 2003,2005, White & Ghez 2001,White & Basri 2003, Natta et al 2004

dM/dt x 106 yr = 0.1M*

submm <Md> / 106 yr

⇒ masses from dust emission may be underestimates

Protostellar/planetary disks (~ few Myr)

optically thick to stellar radiation“large” dust (≥1mm); H = ??

flared disk surface,

“small” (~ 1μm) dust, ~3-5H

as expectednot expected; turbulence??

Grain growth for mm-wave emission but not at 10 m upper layers have small dust⇒

D’Alessio et al. 2001

big grains

“ISM”

Stapelfeldt et al

Scattered light images – must be some growth/settling, otherwise disks are too “fat”

D’Alessio et al. 2001

Dust evolution

(depletion of small dust =10.10.010.001

Models for:

Depletion < 0.1% in inner disk upper layers after 5 Myr(Hernandez & IRAC disk team, 2007)

Disks flatten with age

Sicilia-Aguilar et al. 2009

some correlation of disappearance of silicate feature with less “flared” disk; grain growth/settling;

depletions of small dust ≈ 10-1 – 10-3 (good for MRI?)changes in crystallinity (Bouwman, Sargent et al.)

Furlan et al. 2006

less flared

Watson, IRS disk team, 2009

Disk “frequency” (small dust < 10 AU) decreases over few Myr

disk clearing timescales range over an order of magnitude

⇒ initial conditions

⇒ angular momentum

Hernandez et al. 2007

Disk frequencies decrease rapidly above 1 M

Lada et al. 2006

Disk evolution timescales much faster at higher masses (consistent with dM/dt increasing with M* )

not much known about gas content; inner disk gas not detected (warm CO ro-vib transitions) in disks without near-IR dust emission

Najita, Carr, Mathieu 2003

IR excess

no CO 2m emission

However accretion stops when the near-IR excess disappears

Mass accretion rate decreases with time

Hartmann et al. (1998), Muzerolle et al. (2001), Calvet et al. (2005)

Viscous evolution model

Fraction of accreting objects decreases with time

.50 .23 .12

Why do T Tauri stars accrete? turbulence?

Inner disk (< 0.1 AU) – dust evaporated, ionized, MRI

beyond? MRI active layers (Gammie)?

• why the dM/dt vs. M* dependence? may work...

• if dust settling needed to maintain ionization... why not more variable? why not any apparent dependence on SED?

• GI until dust evaporation? (e.g. Rice & Armitage)

X-ray or EUV heating?... (ionization)

Pascucci et al. 2007 Espaillat et al. 2007

CO J=6-5 in TW Hya; may also need X-ray heating (Qi et al. 2006)

Calvet 1998

Magnetic fields in disks? Cold jets driven by accretion energy

280 AU

Burrows et al.

280 AU

Burrows et al.

Coffey et al. 2007; high-v jet from 0.2-0.5 AUlow-v flow from < 2 AU... but indirect argument

high-velocity wind

accr

etio

n ra

te →

low-velocity wind; photoevaporation?

Hartigan et al. 1995

T Tauri outflows...

Most of the stellar mass is accreted in the protostellar phase - from disks! - in outbursts?

Ibrahimov

FU Ori objects: ~ 0.01 M(sun) accreted in ~ 100 years; unlikely to be accreted from 100 AU in this time

⇒ large lump of material at ≈ few AU, at least in protostellar phase

Zhu et al. 2008, 2009; dead zone + active layer; outbursts during infall to disk

(also Armitage et al. 01, Vorobyov & Basu 05,6,7,8)

Mdisk

M*

Model vs. observation: ridiculous comparison or important suggestion?

model for FU Ori outbursts @ 1 Myr

“Dead zone” (Gammie 1996)

Difficult to explain FU Ori outburst without something like a massive dead zone at ~ 1 AU

Zhu et al. 2009 model w/dead zone

Comparison with Desch reconstruction of solar nebula from “Nice”

model

MRI?

Inner disk holes: consequence of very rapid inner disk accretion?

TW Hya

Calvet et al. 2005 Hughes et al. 2009

D’Alessio et al. 2005

Pre-Transitional Disk LkCa 15:Gap?

large excess, ~optically thick disk

median Taurus SED = optically thick full disk

photosphere

Increasing flux/ optically thick disk

Espaillat & IRS team, 2007

outer radius ≈ 40 AU?

“Transition/evolved disk” timescale? ≈ 15% of “primordial” disks in Taurus < 1 Myr ⇒

Luhman et al. 2009 (inconsistent with Currie et al. 2009)

→

“Transition” disks; difficult to detect if the gap/hole is not large (~ 3x in radius)

We are probably missing many gaps

F

→

LkCa 15; CO not double-peaked; distributed in radius

V836 Tau: CO double-peaked; outer truncation (?)

Najita, Crockett, & Carr 2008

Irresponsible speculations

• Disks must generally be massive at early times. Unless MRI is much more effective than we now think, pileup of mass, ⇒especially in inner disk

• Pileup (aka “dead zone”) is attractive!

- explains FU Ori outbursts

- helps explain “luminosity problem” of protostars (accretion rate onto protostar < infall rate; Kenyon et al 1990,94; Enoch et al. 2009)

- dM/dt(infall) > dM/dt(accretion) helps to make disk evolution more strongly dependent upon initial angular momentum variation of disk evolutionary lifetimes⇒- more mass to make super Jupiters in the inner disk

- more mass to throw away or accrete

- potentially useful effects on migration

• Minus; direct detection in dust emission not currently feasible, but does not contradict current observations... ALMA

summary of disk observations

•Disk frequencies (dust emission) not very different from 3m 24⇒ m evolution similar from 0.1 to ~ 10 AU

• decay time ≈ 3 Myr (but varies by 10x)

• Gas accretion ceases as IR excess disappears- clearing of inner disk

• T Tauri stars accrete ~ MMSN (gas) during their lifetimes; why?

• Small dust in upper disk layers: turbulent support?

• Evidence for dust settling/growth, increasing with age (depletions ~ 0.1-0.001); also X-ray and/or EUV heating in uppermost disk layers

•“Transitional disks (holes, gaps)” ~10% @ 1-2 Myr

• Who knows what is happening at 1 AU @ 1 Myr (optically-thick, not spatially-resolved)

• Disk masses may be systematically underestimated room for mass loss (migration, ejection)

• Massive inner disks? needed to explain FU Ori outbursts...