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Ast 777: Star and Planet Formation Protoplanetary Disks Ast 777: Star and Planet Formation Jonathan Williams, University of Hawaii Protoplanetary disks https://www.eso.org/public/images/eso2008a/ The main phases of star formation https://www.americanscientist.org/sites/americanscientist.org/files/2005223144527_306.pdf Conventional / historical usage: Protoplanetary disks = Class II But many planetesimals and perhaps planetary cores form in the earlier embedded stage https://www.americanscientist.org/sites/americanscientist.org/files/2005223144527_306.pdf First resolved images of disks by HST https://www.spacetelescope.org/images/opo0113b/ More often, a disk is inferred from its SED http://www.spitzer.caltech.edu/images/1179-ssc2004-08c-Spectra-Show-Protoplanetary-Disk-Structures But now we can really image them! https://public.nrao.edu/gallery/hl-tau-birth-of-planets-revealed-in-astonishing-detail-2/ Some theoretical considerations ✓ expect centrifugal radii ~ 100au ✓ viscous accretion disk profile (ok, maybe not so smooth…) (problem set #3) What about the temperature profile? Viscous heating Williams ISM book Radiative heating same r-3/4 dependence as viscous heating! Williams ISM book Modeling the SED flux density dust opacity see problem set #5 Williams ISM book Modeling the SED… sort of Disks are much brighter than expected in the MIR/FIR… Williams ISM book T(r) ⇢ F(λ) Terrestrial planets Gas and Ice Giants Icy planetesimals 0.1AU 1AU 5AU 100AU ~1000 K ~300 K ~100 K ~20 K NIR MIR FIR mm Sean Andrews PhD Flared disks To match the enhanced MIR/FIR fluxes, disks must intercept more starlight at large radii than a flat disk => they flare Naturally explained by the decreasing gravitational force with radius => increased scale height Flared disks hydrostatic equilibrium isothermal eos —> Flared disks This fits the SED much better (but the math gets more complicated - see Chiang & Goldreich) Scattered NIR from the surface of IM Lup https://www.eso.org/public/news/eso1811/ Thermal mm from dust in IM Lup Same disk, different appearance… https://www.eso.org/public/news/eso1811/ Huang et al 2018 How do protoplanetary disks compare to what we thought formed the Solar System? https://www.eso.org/public/news/eso1811/ Ansdell et al 2017 The Minimum Mass Solar Nebula Ruden 2000, adapted from Weidenschilling 1977 & Hayashi 1981 Measuring dust masses from submillimeter continuum flux Disks have similar masses as the MMSN Williams & Cieza 2011 Fitting SED and interferometer visibilities to infer surface density Andrews et al. 2009 AA49CH03-Williams ARI 14 July 2011 19:21 4.3.1. Surface density. A resolved image of a disk at millimeter wavelengths provides not only a measure of its total mass and radius but also the distribution of mass or surface density. Until p recently this was characterized as a pure power law, ! R− , with values of p generally in the ∝ range 0–1 (Mundy et al. 1996; Lay, Carlstrom & Hills 1997; Wilner et al. 2000; Kitamura et al. 2002; Andrews & Williams 2007b). γ The exponential tapered fits of the form in Equation 4 approximate a power law, ! R− ∝ for R R , but the fitted values of R 30 200 AU correspond to 1 arcsec in all but the # c c $ − ! closest disks, and γ is actually determined largely from the steepness of the exponential taper. The Hughes et al. (2008) comparison of pure power law versus exponentially truncated power- law fits shows similar indices but with slightly steeper pure power-law fits due to the soft edge, p 1.2, γ 0.9forfourdisks. % & = % & = From larger samples, Andrews et al. (2009, 2010b) find a tight range consistent with all having the same value γ 0.9. Using a different modeling technique, however, Isella, Carpenter & % & = Sargent (2009) find a very wide range γ 0.8 to 0.8 with mean γ 0.1 in their data. Negative = − % & = values of γ correspond to decreasing surface densities for R < Rc , which may be an important signature of disk evolution (see Section 6), but these were also found in the smaller disks with Rc < 100 AU, which are barely resolved and, thus, the hardest to characterize. In general, all results agree that young protoplanetary disks have flatter central density profiles than the canonical power law p 1.5 MMSN (Weidenschilling 1977). There is even more = uncertainty in the density profile of the MMSN than its mass, however, and an exponential tapered power-law fit by Davis (2005) has γ 0.5. The more relevant comparison is of absolute values = in the planet-forming zone. Andrews et al. (2009, 2010b) infer surface densities, ! 10 100 g 2 ≈ − cm− at 20 AU, in their sample of Ophiuchus disks that are in good agreement with the MMSN (Figure 2). Whereas the disk-to–star mass ratio may be very high during the initial stages of formation (Section 3), the Toomre Q parameter, Q(R) c #/πG!,wherec is the sound speed and # the = orbital angularFitting velocity SED (Toomre and 1964), isinterferometer generally much greater than visibilities unity for these Class II YSO disks, implying that theyto are infer gravitationally surface stable atdensity all radii (Isella, Carpenter & Sargent 3 Figure 2 MMSN densities AS 205 Saturn Radial surface Elias 24 density (gas dust) + Uranus GSS 39 profiles for Class II Neptune AS 209 2 YSO disks in DoAr 25 ) Ophiuchus based on by INSTITUTE FOR ASTRONOMY on 10/17/11. For personal use only. WaOph 6 –2 VSSG 1 fitting an SR 4 exponentially 1 Annu. Rev. Astro. Astrophys. 2011.49:67-117. Downloaded from www.annualreviews.org SR 13 tapered power-law WSB 52 profile to 880-µm DoAr 33 visibilities and IR WL 18 spectral energy 0 distributions (gas + dust) (g cm ∑ Disks have (Andrews et al. 2009, log 2010b). The dark Current similar surface gray rectangular –1 resolution densities as regions mark the limits the MMSN minimum mass solar nebula (MMSN) surface densities for –2 10 100 1,000 Saturn, Uranus, and Radius (AU) Neptune. Andrews et al. 2010 www.annualreviews.org Protoplanetary Disks 77 • Infrared excess ➞ disk lifetime Disks are common at early times, and almost all gone by ~6 Myr A compilation of many Spitzer surveys Infrared excess ➞ disk lifetime fIR ~ e-(t/2.8Myr) ⇒ disk half-life = 2 Myr But... MIR observations only show the existence of warm dust, and not the amount A compilation of many Spitzer surveys Disk dust masses decreases very rapidly (faster even than you might think from the infrared disk lifetime) MJup MMSN The mass is not going away (planets are common) — its just becoming harder to see because its surface area is decreasing MJup MMSN µm mm REPORTS range from 4567.32 T 0.42 My to 4564.71 T thermal history involving multiple heating and substantial radiogenic Pb by the time the last 0.30 My (Fig. 2, B and C, and Table 1). The melting events. The isochron for the oldest melting event occurred at 4564.71 T 0.30 My. oldest chondrule age overlaps with our estimate Allende chondrule (C30) projects back to an ini- Chondrules from the Allende and NWA of CAI formation and thus requires that ag- tial Pb isotopic composition that is less radio- 5697 chondrites define age ranges (Fig. 4) that gregation of the chondrule precursor material genic than the most primitive estimate of the indicate the presence of multiple generations of and its thermal processing occurred within the initial Pb isotopic composition of the solar sys- chondrules in individual chondrite groups. To uncertainty of its Pb-Pb age. Moreover, the age tem (Fig. 3), based on the Nantan iron meteorite explore the spatial significance of this age range, of the oldest chondrule indicates that it was not (14). The low m value of the precursor material we have measured the 54Cr/52Cr ratios of these heated to temperatures above the Pb closure tem- for chondrules in general and the antiquity of chondrules, because 54Cr/52Cr variations within perature after 4567.32 T 0.42 My and therefore this chondrule in particular indicate that the the inner solar system track genetic relationships has a formation and thermal history indistinguish- Pb isotopic composition of the Nantan iron me- between early-formed solids and their respective able from that of CAIs. These data demonstrate teorite does not represent the initial Pb isotopic reservoirs (25). The five chondrules analyzed that chondrule formation started contemporane- composition of the solar system, but instead an here show significant 54Cr variability (Table 1) ously with CAIs (within the uncertainty of our mea- evolved composition inherited after accretion that is not correlated with their ages. Moreover, surements) and continued for at least ~3 My. and differentiation of its parent body before most chondrules have 54Cr/52Cr ratios that are The majority of chondrules are believed to core formation. Similar to chondrule C30, three distinct from those of their host chondrites (26). have formed as dust aggregates of near-solar of the four younger chondrules we analyzed Collectively, these observations indicate that composition subsequently thermally processed define isochron relationships that project back chondrules from individual chondrite groups by a flash heating mechanism creating the ig- to Pb isotopic compositions that are more pri- formed from isotopically diverse precursor ma- neous textures we observe today (10). However, mitive than the current estimate of the solar terial in different regions of the protoplanetary the presence of relict grains, igneous rims, and system initial Pb composition. This implies that disk and were subsequently transported to the ac- compound chondrules suggests that some chon- the precursor material of these chondrules, es- cretion regions of their respective parent bodies. drules may have grown by collisions and re- pecially C3 with its high m value of ~183, were This is consistent with the proposal that radial melting (22, 23).
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