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)

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 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 (Lada 1987) αIR = -dlog(νFν)/dlog(ν) = log(25F25/2F2)/log(2/25) 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 Observed Chiang & Goldreich 1997 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 disk The 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 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

Dependence of SED on disk geometry

Dependence of SED on disk geometry

Dependence of SED on disk geometry

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 L = GMMdot/R Annual Reviews 1981 Accretion disk theory L = GMMdot/R 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

Viscous evolution magnetospheric accretion accretion shocks 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. Andrews & Williams 2007

Dust mineralogy observed olivine pyroxene hydrosilicate ISM silicate van Boekel et al. (2004)

Grain growth kn  n b b ~ 2 b ~ 0 submillimeter emission “efficiency” ISM grains pebbles/snowballs b related to size of largest solids in disk e.g. Pollack et al. (1994), Draine (2006)

Grain growth disk ~ 1 ISM ~ 2 Isella et al. 2007

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 <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 Simon et al. (2000)

Millimeter observations: the cold outer reservoir Qi PhD thesis 2000

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 fdisk > 80% at ~1 Myr fdisk ~ 50% at ~3 Myr Haisch et al. 2001 fdisk > 80% at ~1 Myr fdisk ~ 50% at ~3 Myr fdisk ~ few% at >10 Myr Hillenbrand 2005

Inner and outer disks have similar dissipation timescales Disk lifetimes NIR excess  outer disk Inner and outer disks have similar dissipation timescales Andrews & Williams 2005

Disk evolution (at mm) sub-mm emission (disk masses) decreases with IR SED evolution sub-mm SED changes with IR SED evolution (particle growth) Class I disks Class II disks Class III disks Sean Andrews PhD 2007

Transitional disks Viscous 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 “UV-switch” model where stellar wind very rapidly erodes disk (from inside out but in only ~105yr) as accretion rate drops below photoevaporation rate Alexander et al. 2006

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 As for protostellar disks, images are rare (but critically important); many properties inferred from infrared excesses and SED studies alone. See http://astro.berkeley.edu/~kalas/disksite/

Summary Disks are ubiquitous Masses range from 0.001–0.3 M☉ 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