Photochemical Heating by Water in the Terrestrial Planet-Forming Regions of Disks

Photochemical Heating by Water in the Terrestrial Planet-Forming Regions of Disks Máté Ádámkovics University of California Berkeley Joan R. Najita National Optical Astronomical Observatory Alfred E. Glassgold University of California Berkeley Water in the Universe Apr 2016 - Noordwijk REPORTS ular species was modeled independently as ther- mally excited emission from a slab with a given temperature, column density, and emitting area. Organic Molecules and Water Although these quantities are expected to vary with radius in a real disk, we cannot constrain their radial variation without the benefit of velocity- in the Planet Formation Region resolved line profiles. Our strategy allows us to determine the flux-weighted mean gas temper- of Young Circumstellar Disks ature (T ), column density (N), and emitting area (p × R2)foreachmolecule(Table1).Thecom- John S. Carr1 and Joan R. Najita2 bined synthetic spectrum reproduces well the details of the observed spectrum (Fig. 2). Mo- The chemical composition of protoplanetary disks is expected to hold clues to the physical and lecular abundances with respect to CO (Table 1) chemical processes that influence the formation of planetary systems. However, characterizing were then determined by modeling the 5-mm the gas composition in the planet formation region of disks has been a challenge to date. We fundamental CO emission spectrum of AA Tauri, report here that the protoplanetary disk within 3 astronomical units of AA Tauri possesses a rich obtained with the near-infrared echelle spectro- molecular emission spectrum in the mid-infrared, indicating a high abundance of simple organic graph NIRSPEC (15)attheKeckObservatory. molecules (HCN, C2H2, and CO2), water vapor, and OH. These results suggest that water is The equivalent radii for the emitting areas abundant throughout the inner disk and that the disk supports an active organic chemistry. (Table 1) indicate that the observed gas resides at disk radii that correspond to the region of ter- isks of gas and dust orbit very young sitions of H2O(Fig.1).Rotationaltransitionsof restrial planets in the Solar System; that is, within stars and represent the early stages of OH were also observed between 20 and 31 mm. 2to3AUofthestar.Thederivedtemperaturesfor Dplanetary system formation. Studies of From 13 to 15 mm, ro-vibrational emission bands the emission are also roughly consistent with these disks can establish the conditions under of the fundamental bending modes of C2H2,HCN, predicted gas temperatures at radii ~1 AU for which planets form, complementing studies of and CO are prominent (Fig. 2). models that calculate the gas thermal structure in 2 on July 23, 2010 solar system bodies in efforts to reconstruct the This mid-infrared molecular emission almost the temperature inversion region of disk atmo- origin of our Solar System. In the same way that certainly has its origin in a disk. AA Tauri lacks spheres (16, 17). The significantly lower temperature the chemical record preserved in primitive bodies asurroundingmolecularenvelopeormolecular for CO2 suggests that its emission is concentrated (comets and meteorites) is probed for clues to outflow as potential alternate sites for the emitting at larger radii, which is also consistent with pre- the physical and chemical processes that oper- gas. A disk interpretation is also consistent with dictions of chemical models (18)thatCO2 is de- ated in the early solar nebula (1, 2), the chemical high-spectral-resolution studies of CTTSs, which stroyed at temperatures much higher than 300 K. composition of protoplanetary disks is expected demonstrate a disk origin for their near-infrared The higher mean temperature for CO can be un- to hold clues to the processes that are active in molecular emission features. These include emis- derstood by the fact that CO fundamental emis- disks. Although significant advances have been sion from the 5-mmfundamentalro-vibrational sion is insensitive to gas with T <400K,because www.sciencemag.org made from the study of the dust properties of bands of CO, which is very common in CTTSs of the higher energy of the upper levels of the disks (3–5), much less progress has been made (12), and emission from the 2.3-mmCOovertone transitions and the shorter wavelength of emission. in the study of the gaseous component, partic- bands and hot bands of H2OandOH(6, 13, 14), Vertical gradients in the gas temperature and ularly over the range of radii where planet for- which is less frequently observed. All these near- molecular abundances could also contribute to mation is thought to occur (6–8). Measurements infrared emission bands are thought to arise in a some of the differences; for example, H2Omaybe of disk molecular abundances can provide in- temperature inversion in the atmosphere of the abundant in a cooler part of the atmosphere as sights on issues such as the origin and evolution inner [<1 astronomical unit (AU)] disk (6, 12–14). compared to CO (16). of water in the Solar System, the nature of or- To determine the properties of the emitting The abundances of simple organics and H2O Downloaded from ganic chemistry in disks, and the ability of gas, we modeled the emission spectrum and an- for AATauri (Fig. 3) are about one order of mag- disks to synthesize pre-biotic species. Here we alyzed the measured line fluxes using standard nitude higher than both observations and models report that a protoplanetary disk around a young techniques (11). The emission from each molec- (19)ofhotmolecularcores,thedenseandwarm Carr & Najita, 2008 star displays a rich molecular emission spectrumWarm molecular emission Fig. 1. Spitzer IRS spec- in the mid-infrared that provides detailed trum of AA TauriREPORTS from 9.8 thermal and chemical information on gas in the ◆ OH (several hundred degrees kelvin) regions of mo- suggest that substantial molecular synthesis takes surroundingto low-mass38.0 protostarsmm. The (20). Analogous- spectrum planet formation region of the disk. TH2O = 575lecular gas50K that surround luminousT protostars.OH =place within 525 the disk. The50K observed region of the ly, the indication of molecular synthesis in AA The abundances of HCN and C2H2 in AA Tauri, disk lies well inside± the radius where ices would Tauri suggestscombines that more complex all organic orders mol- of We observed AA Tauri, a fairly typical clas- relative± to H2O, are also substantially higher than sublimate, based on the gas temperatures and in- ecules, including those of pre-biotic interest, measurements for cometary volatiles (fig. S1), ferred radii. This situation is similar to that of hot might bethe produced short-high within disks. and long- sical T Tauri star (CTTS, which are approximately whereas the abundance of CO2 is consistent molecular cores, where a warm gas phase follows Similarly high abundances of C2H2,HCN, with or possibly less than its abundances in both the sublimation of molecules from icy grain man- and COhigh2 have also IRS been observed modules (7)inab- (11). comets and interstellar ices. tles. The subsequent gas-phase chemistry (19)may sorption toward the low-mass protostar IRS 46, solar-mass stars with accretion disks and ages of If molecular cores are representative of the produce the complex organic molecules observed where theRotational molecules potentially transitions reside in a disk of 6 composition of material that is incorporated into with millimeter-wave spectroscopy in both mas- viewed at a favorable edge-on inclination or in less than a few 10 years), with the Infrared Spec- disks, then the higher abundances for AA Tauri sive hot cores and the corresponding regions an outflowingOH aredisk wind. marked Such gas absorption with a is dia- rare toward low-mass protostars (7). In contrast, trograph (IRS) on the Spitzer Space Telescope Fig. 2. Comparison of emissionmond, from a disk, and such as the the ubiquitous Q branches CO the observed spectrum of fundamental emission from CTTSs, does not re- (9, 10), covering wavelengths of 9.9 to 37.2 mm AA Tauri to the combined quire aof special C orientationH ,HCN,andCO to be observable. The model spectrum (11). fact that the molecular2 2 features in the mid-infrared 2 at a spectral resolution (l/Dl)of600.Thehigh The observed continuum- spectrumare of AA labeled Tauri are detected along in emission, with subtracted spectrum from combined with the fact that it is a typical CTTS, 13 to 16 mm(above)is suggests the potential to study organic molecules signal-to-noise ratio (~200 to 300) produced by offset from the model atomic [Ne II]. The ma- and water in a large number of CTTS disks. spectrum (below). The er- The high abundances in the AATauri disk are our observational and data reduction techniques ror bar indicates the av- jority of emission features erage uncertainty in the generally consistent with the predicted abundances (11)revealsarichspectrumofmolecularemis- observed spectrum. The at approximatelyare due 1-AU to radii rotational in a disk chemical tran- model is a slab in local model (21)fortheinner5AUofprotoplanetary thermodynamic equilibri- disks (Fig.sitions 3). However, of the H modelO(unmarked results are ver- sion lines that is dominated by rotational tran- tically integrated abundances for2 the disk, whereas um, with an independent on July 23, 2010 temperature, column den- the molecularfeatures). emission lines we observed are sity, and disk area derived likely to probe only the upper disk atmosphere. for each molecular species The high abundances we derived are not expected 1Remote Sensing Division, Naval Research Laboratory, Code (Table 1). All the unla- in recent models that calculate the vertical thermal- beled features are rotational transitions of H2O. chemical structure of the gas in disk atmospheres 2 (16), which predict low abundances for molecules 7210, Washington, DC 20375, USA. National Optical Astron- Table 1. Molecular gas parameters and abundances derived for AA Tauri. such as H2Ointhetemperatureinversionregionof the atmosphere where emission lines form.

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