1. Circumstellar disks - a primer
Ast622
The Interstellar Medium
Partially based on Les Houches lecture by Michiel Hogeheijde (

2. Motivation The last step in the transport of the ISM to stellar scales
The first step in the formation of planetary systems

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

4. 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

5. Direct evidence for disks (i.e. imaging)
Smith & Terrile 1984

6. Direct evidence for disks (i.e. imaging)

7. SED classification (Lada 1987) αIR = -dlog(νFν)/dlog(ν)
= log(25F25/2F2)/log(2/25)
Fig from Andre

8. 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)

9. Flat blackbody disk

10. 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

11. 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.

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

13. 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.

14. Radiative equilibrium disk

15. Radiative equilibrium disk

16. 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.

17. Radiative equilibrium flared disk

18. 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.

19. Flaring + hot inner rim
Dullemond et al. 2006, PPV review

20. Dependence of SED on disk geometry

21. Dependence of SED on disk geometry

22. Dependence of SED on disk geometry

23. Dependence of SED on disk geometry

24. Dependence of SED on disk geometry

25. SED + spatial modeling
 disk mass, radius and temperature and surface density profiles, T ~ R-q,  ~ R-p
Andrews & Williams 2007

26. 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

27. 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

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

29. 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)

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

31. Grain growth
Andrews PhD thesis 2007

32. Dust settling
Dullemond et al. 2004

33. The gaseous disk

34. 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

35. Molecular Hydrogen
Lahuis et al. (2007)

37. Near infrared disk ro-vibrational lines
Boogert et al. 2002

38. Recent Spitzer IRS results
Watson et al. 2007
Carr & Najita 2008

39. Atomic fine structure lines in disks: probes of the giant planet forming region
Herschel GASPS Key Program

40. Atomic fine structure lines in disks: probes of the giant planet forming region
Herschel GASPS Key Program

41. 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 times smaller than from dust: depletion
CO freezes out on dust grains for T<20 K
Simon et al. (2000)

42. Millimeter observations: the cold outer reservoir
Qi PhD thesis 2000

43. 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)

44. Disk chemistry: resolving the D/H ratio
Qi et al. (2008)

45. 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

46. 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

47. 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

48. 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

49. 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

50. 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)

51. Debris disks
astro-ph/

52. 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.

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

54. 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