circumstellar disks - a primer ast622 the interstellar medium partially based on les houches lecture...

Circumstellar disks - a primer

Ast622

The Interstellar Medium

Partially based on Les Houches lecture by Michiel Hogeheijde

(http://www-laog.obs.ujf-grenoble.fr/heberges/Houches08/index.htm)

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Motivation

• The last step in the transport of the ISM to stellar scales

• The first step in the formation of planetary systems

Disks are an inevitable ( ubiquitous?) consequence of angular momentum conservation

Indirect evidence for disks

• Emission line (H) stars above the main sequence accretion

• Infrared-millimeter excess emission reprocessing of starlight by a non-spherical geometry

• Ultraviolet excess and X-ray emission accretion hot spots and star-disk interface

Direct evidence for disks(i.e. imaging)

Smith & Terrile 1984

Direct evidence for disks(i.e. imaging)

SED classification

αIR = -dlog(νFν)/dlog(ν)

= log(25F25/2F2)/log(2/25)

(Lada 1987)

Fig from Andre

SED theory

• Chiang & Goldreich (1997) following pioneering work by Adams, Lada & Shu (1987), Kenyon & Hartmann (1987)

• Also see reviews by Beckwith (1999) and Dullemond et al. (2006)

Flat blackbody disk



Flat blackbody disk

Observe

d

Fig. 1.— SED for the flat blackbody disk, with contributions from star and disk identified. The n = 4/3 law is evident between 30 μm and 1 mm. The turnover near 1 mm is due to our truncation of the disk at ao ≈ 270 AU. Chiang & Goldreich 1997

Flared blackbody diskThe vertical component of gravity will decrease with radius along with the surface density. Hydrostatic equilibrium then implies the disk scale height increases with radius: the disk is flared.

The outer regions of the disk of a flared disk intercept more starlight than a flat disk and the mid-to-far infrared emission is stronger.

Flared blackbody disk

Fig. 2.— SED for the flared blackbody disk. At mid-IR wavelengths, Lν ∝ ν−2/3. At longer wavelengths, Lν ∝ ν3.

Radiative equilibrium disk

Fig. 3.— Radiative transfer in the passive disk. Stellar radiation strikes the surface at an angle α and is absorbed within visible optical depth unity. Dust particles in this first absorption layer are superheated to a temperature Tds. About half of the emission from the superheated layer emerges as dilute blackbody radiation. The remaining half heats the interior to a temperature Ti.

Radiative equilibrium disk

Radiative equilibrium flared disk

Fig. 6.— SED for the hydrostatic, radiative equilibrium disk. At mid-IR wavelengths, the superheated surface radiates approximately 2–3 times more power than the interior. Longward of 300 μm, n gradually steepens from about 3 to 3 + β as the disk becomes increasingly optically thin.

Radiative equilibrium flared disk

Adding in solid state features

Fig. 10.— SED for the hydrostatic, radiative equilibrium disk using a grain emissivity profile motivated by data from Mathis (1990). For wavelengths shorter than 0.3 μm, our assumed emissivity is unity; longward of 0.3 μm, it obeys a (single) power-law relation ∊λ = (0.3 μm/λ)1.4, on which are superposed two Gaussians centered on 10 and 20 μm, having amplitudes that are 3 times their local continuum emissivity and FWHM equal to 3 and 9 μm, respectively.

Flaring + hot inner rim



Dullemond et al. 2006, PPV review

Dependence of SED on disk geometry

SED + spatial modeling

disk mass, radius and temperature and surface density profiles, T ~ R-q, ~ R-p

Andrews & Williams 2007



Accretion disk theory

Same temperature profile (and hence SED) as as passive flat blackbody disk, T R-3/4

Flared disk SEDs dominated by stellar irradiation.

Accretion critical for understanding disk evolution

L = GMMdot/R

Annual Reviews 1981

Viscous evolution

accretion shocks

magnetospheric accretion

spreading

Muzerolle et al. (1998, 2001)

Gullbring et al. (1997)

As disk accretes to star, conservation of momentum implies disk spreads out; mass, accretion, decrease with time, radius increases with time.


Dust mineralogy

van Boekel et al. (2004)

observed

olivine

pyroxene

hydrosilicate

ISM silicate

Grain growth

submillimeter emission “efficiency” ~ 2 ~ 0

ISM grains pebbles/snowballs

related to size of largest solids in diske.g. Pollack et al. (1994), Draine (2006)

Grain growth

Isella et al. 2007



disk ~ 1ISM ~ 2

Grain growth

Andrews PhD thesis 2007

Dust settling

Dullemond et al. 2004

The gaseous disk



Molecular Hydrogen

H2 is difficult to detect• no permanent dipole -> no dipole rotational transitions; only weak quadrupole transition in mid-IR that require hundred K or more to excite• conflicting reports about detection• fluorescent H2 emission in UV (electronic transitions) and near-infrared (vibrational) has been detected but is difficult to analyze quantitatively

Molecular Hydrogen

Lahuis et al. (2007)

Near infrared disk ro-vibrational lines





Boogert et al. 2002

Recent Spitzer IRS results

Watson et al. 2007 Carr & Najita 2008



Atomic fine structure lines in disks:probes of the giant planet forming region

Herschel GASPS Key Program

Atomic fine structure lines in disks:probes of the giant planet forming region



Herschel GASPS Key Program

Millimeter observations:the cold outer reservoir

Simon et al. (2000)

• <1% by mass of gas consists of CO, and smaller quantities of other molecules and atoms

• CO easily detected in mm rotational transitions• shows rotation patterns• inferred masses 10-100 times smaller than from dust:

depletion• CO freezes out on dust grains for T<20 K

Qi PhD thesis 2000

Millimeter observations:the cold outer reservoir

Disk chemistry

• most molecules now understood to be present only in a warm layer at intermediate height and close to the star• frozen out in mid-plane• photo-dissociated in the disk surface

Semenov et al. (2008)

Disk chemistry:resolving the D/H ratio

Qi et al. (2008)





Disk lifetimes

Haisch et al. 2001 Hillenbrand 2005fdisk > 80% at ~1 Myrfdisk ~ 50% at ~3 Myr

fdisk ~ few% at >10 Myr

Disk lifetimes


NIR excess outer diskInner and outer disks have

similar dissipation timescales

sub-mm emission(disk masses)decreases with IR SED evolution

sub-mm SEDchanges with

IR SED evolution(particle growth)

Class I disks

Class II disks

Class III disks

Disk evolution (at mm)

Sean Andrews PhD 2007

Transitional disksViscous evolution is expected to be quicker at small radii but transitional disks, with mid-infrared dips in their SED and cold outer rings of dust and gas are rare (and possibly only seen around binaries?)

Brown et al. 2008

Disk clearing through photoevaporation





Alexander et al. 2006

Alexander “UV-switch” model where stellar wind very rapidly erodes disk (from inside out but in only ~105yr) as accretion rate drops below photoevaporation rate

External photoevaporation

O’Dell, McCaughrean, Bally Williams et al. 2005

Rapid mass loss, 10-5 M☉/yr, at center, but massive disks survive at large distances (Rita Mann PhD)

Debris disks



astro-ph/0511083

Debris disks

Williams et al. 2004



Isella et al. 2007

Debris disks have double peaked SEDs with a stellar photosphere plus generally a single temperature dust component. They have very low (if any) gas and have a much simpler geometry than protostellar disks.

Debris disks

See http://astro.berkeley.edu/~kalas/disksite/

As for protostellar disks, images are rare (but critically important); many properties inferred from infrared excesses and SED studies alone.

Summary• Disks are ubiquitous

– but generally only indirectly inferred from infrared excesses

• Masses range from 0.001–0.3 M☉

• Radii range from tens to hundreds of AU• Grains in disks grow to cm sizes• Gas shows Keplerian motion

– Many molecules (but not H2) frozen out in cold interior

• The fraction of stars with disks decreases with time– from >80% at <1 Myr to <10% at 10 Myr– ‘half-life’ of disks ~3 Myr– inner and outer disk dissappear almost simultaneously

• Debris disks from planetesimal collisions may be visible for >>100 Myr after star formation

circumstellar disks - a primer ast622 the interstellar medium partially based on les houches lecture...

Documents

flat blackbody disk

flat disk

flared blackbody disk

passive disk

stardisk interface slide

imaging slide

andre slide

flared disk intercept