Photochemical Heating by 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 ) 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 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- 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 ), with the Infrared Spec- disks, then the higher abundances for AA Tauri sive hot cores and the corresponding regions an outflowingOH diskare 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. If omy Observatory, Tucson, AZ 85719, USA. Molecule T (K) N (1016 cm−2) R* (AU) Abundance to CO these models are correct, our measurements may 2 www.sciencemag.org H2O 575 ± 50 65 ± 24 2.1 ± 0.1 1.3 be a sign of vertical mixing that carries molecular Water in the Universe OH 525 ± 50 8.1 ± 5.2 2.2 ± 0.1 0.18 species from deep in the disk, where they are Apr 2016 - Noordwijk HCN 650 ± 100 6.5 ± 3.3 0.60 ± 0.05 0.13 abundant, up to the surface where they are ob- C2H2 650 ± 150 0.81 ± 0.32 0.60† 0.016 served. Such turbulent mixing is predicted (22)by 1504 14 MARCH 2008 VOL 319 SCIENCECO2 350 ± 100www.sciencemag.org0.2 –13 1.2 ± 0.2 0.004 – 0.26 the magnetorotational instability (23), the process CO 900 ± 100 49 ± 16 0.7 ± 0.1 1.0 that is hypothesized to power disk accretion. *The equivalent radius for the emitting area A (R =[A/p]1/2). †Area was set to that derived for HCN. Our results for AA Tauri demonstrate the po-

tential to measure the distribution of water vapor Downloaded from Fig. 3. Comparison of abun- throughout the terrestrial planet region of disks, dances relative to CO. The abun- 1 information that was previously restricted to dances for each molecule X 5 hot H2Oinsideof0.3AU(6, 13). The abun- relative to CO [N(X)/N(CO)] dance of water vapor in the inner disk is im- derived for AA Tauri (squares) portant in setting the oxidation state of the gas, are compared to the abun- which influences the chemistry and mineralogy. dances for hot molecular cores Knowledge of the distribution of water (gas and (triangles) and to disk chem- ice) is also central to understanding the origin of ical models (21)atradiiof 5 water in inner solar system bodies and the forma- 1and5AU(opencircleswith 1 radius labeled). The abun- tion of giant planets via the core accretion mech- dances for hot cores are based anism. The high abundance of H2OforAATauri, on observations of absorption 1 if typical, would rule out simple models where bands made with the Infrared 5 outward diffusion rapidly depletes water vapor in Space Observatory (26–28). the inner disk (24), leading to large ice density 5 enhancements at larger radii and the rapid growth The organic molecules were 1 studied using the same mid- of giant planet cores. More detailed models (25) infrared bands that were ana- that include a variety of physical processes predict lyzed for AA Tauri. amorecomplicatedevolutionofwatervaporin the inner disk, with both enhancements and deple- tions that vary with time and radius, indicating the importance of measuring the abundance and dis- tribution of water with evolutionary age.

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(several hundred degrees kelvin) regions of mo- suggest that substantial molecular synthesis takes surrounding low-mass protostars (20). Analogous- lecular gas that surround luminous protostars. place within the disk. The 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 suggests that more complex organic mol- 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 be produced within disks. 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 CO2 have also been observed (7)inab- comets and interstellar ices. tles. The subsequent gas-phase chemistry (19)may sorption toward the low-mass protostar IRS 46, If molecular cores are representative of the produce the complex organic molecules observed where the molecules potentially reside in a disk composition of material that is incorporated into with millimeter-wave spectroscopy in both mas- viewed at a favorable edge-on inclination or in disks, then the higher abundances for AA Tauri sive hot cores and the corresponding regions an outflowing disk wind. Such gas absorption is rare toward low-mass protostars (7). In contrast, Fig. 2. Comparison of emission from a disk, such as the ubiquitous CO the observed spectrum of fundamental emission from CTTSs, does not re- AA Tauri to the combined quire a special orientation to be observable. The model spectrum (11). fact that the molecular features in the mid-infrared The observed continuum- spectrum of AA Tauri are detected in emission, 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 REPORTS offset from the model and water in a large number of CTTS disks. spectrum (below). The er- ular species was modeled independently as ther- The high abundances in the AATauri disk are ror bar indicates the av- mally excited emission from a slab with a given temperature, column density, and emitting area. generally consistent with the predicted abundances Organic Molecules anderage Water uncertainty in the Although these quantities are expected to vary with radius in a real disk, we cannot constrain their at approximately 1-AU radii in a disk chemical in the Planet Formationobserved Region spectrum. The radial variation without the benefit of velocity- resolved line profiles. Our strategy allows us to model (21)fortheinner5AUofprotoplanetary model is a slab in local determine the flux-weighted mean gas temper- of Young Circumstellar Disks ature (T ), column density (N), and emitting area thermodynamic equilibri- (p × R2)foreachmolecule(Table1).Thecom- disks (Fig. 3). However, the model results are ver- John S. Carr1 and Joan R. Najita2 bined synthetic spectrum reproduces well the tically integrated abundances for the disk, whereas um, with an independent details of the observed spectrum (Fig. 2). Mo- on July 23, 2010 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 planetarytemperature, systems. However, column characterizing den- were then determined by modeling the 5-mm the molecular emission lines we observed are 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 astronomicalsity, and units of disk AA Tauri area possesses derived a rich obtained with the near-infrared echelle spectro- likely to probe only the upper disk atmosphere. molecular emission spectrum in the mid-infrared, indicatingfor each a high abundance molecular of simple species organic graph NIRSPEC (15)attheKeckObservatory. The high abundances we derived are not expected 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 in recent models that calculate the vertical thermal- (Table 1). All the unla- disk radii that correspond to the region of ter- isks of gas and dust orbit very young sitionsbeled of H2O(Fig.1).Rotationaltransitionsof features are rotationalrestrial planets transitions in the Solar System; of Hthat2 is,O. within chemical structure of the gas in disk atmospheres stars and represent the early stages of OH were also observed between 20 and 31 mm. 2to3AUofthestar.Thederivedtemperaturesfor Carr & Najita, 2008 Dplanetary system formation. Studies of WarmFrom 13 to 15 mm, ro-vibrational molecular emission bands the emission emission are also roughly consistent with (16), which predict low abundances for molecules 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 andTable CO are prominent 1. Molecular (Fig. 2). gasmodels parameters that calculate the and gas thermal abundances structure in derived for AA Tauri. 2 on July 23, 2010 such as H2Ointhetemperatureinversionregionof 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 lower16 temperature−2 the atmosphere where emission lines form. If the chemical record preserved in primitive bodies asurroundingmolecularenvelopeormolecularMolecule T (K)for CO2 suggests that its emissionN (10 is concentratedcm ) R* (AU) Abundance to CO (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- these models are correct, our measurements may the physical and chemical processes that oper- gas. A disk interpretation is also consistent with dictions of chemical models (18)thatCO2 is de- www.sciencemag.org ated in the early solar nebula (1, 2), the chemical high-spectral-resolutionH2O studies of CTTSs,575 which ±stroyed 50 at temperatures much higher65 than ± 300 24 K. 2.1 ± 0.1 1.3 be a sign of vertical mixing that carries molecular 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 molecularOH emission features. These include525 emis- ±derstood 50 by the fact that CO fundamental8.1 ± emis-5.2 2.2 ± 0.1 0.18 species from deep in the disk, where they are 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 bandsHCN of CO, which is very common in650 CTTSs ±of 100 the higher energy of the upper 6.5 levels ± of 3.3 the 0.60 ± 0.05 0.13 abundant, up to the surface where they are ob- 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- bandsC2 andH hot2 bands of H2OandOH(6650, 13, 14), ±Vertical 150 gradients in the gas 0.81 temperature ± 0.32 and 0.60† 0.016 served. Such turbulent mixing is predicted (22)by 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 infraredCO emission2 bands are thought to arise350 in a ±some 100 of the differences; for example,0.2 H2Omaybe–13 1.2 ± 0.2 0.004 – 0.26 the magnetorotational instability (23), the process 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 innerCO [<1 astronomical unit (AU)] disk (6900, 12–14). ±compared 100 to CO (16). 49 ± 16 0.7 ± 0.1 1.0 that is hypothesized to power disk accretion. of water in the Solar System, the nature of or- To determine the properties of the emitting The abundances of simple organics and1/2 H2O Downloaded from ganic chemistry in disks, and the ability of gas,*The we modeled equivalent the emission radius spectrum for and an- thefor emitting AATauri (Fig. area 3) areA about(R =[ one orderA/p] of mag-). †Area was set to that derived for HCN. Our results for AA Tauri demonstrate the po- 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 tential to measure the distribution of water vapor Downloaded from star displays a rich molecular emission spectrum Fig. 1. Spitzer IRS spec- in the mid-infrared that provides detailed Fig. 3. Comparison of abun- throughout the terrestrial planet region of disks, trum of AA Tauri from 9.8 thermal and chemical information on gas in the REPORTS dances relative(several hundredto degreesCO. kelvin) The regions of mo- abun-suggest that substantial molecular◆ synthesis takes surroundingto low-mass 38.0 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 1 information that was previously restricted to 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 Carr et al., 2004 relative to H O, are also substantially higher than sublimate, based on± the gas temperatures and in- ecules, including those of pre-biotic interest, We observed AA Tauri, a fairly typical clas- ± 2 dances formeasurements each for cometary molecule volatiles (fig. S1), ferred radii.X This situation is similar to that of hot might bethe produced short-high within disks. and long- 5 sical (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, hot H2Oinsideof0.3AU(6, 13). The abun- 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). Carr & Najita, 2011 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 relative to COIf molecular [ coresN are( representativeX)/N of the(CO)]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 dance of water vapor in the inner disk is im- 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 diskare wind. marked Such gas absorption witha is dia- rare toward low-mass protostars (7). In contrast, Pascucci et al., 2009 trograph (IRS) on the Spitzer Space Telescope derived for AAFig. 2. Comparison Tauri of (squares) 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 portant in setting the oxidation state of the gas, model spectrum (11). fact that the molecular2 2 features in the mid-infrared 2 at a spectral resolution (l/Dl)of600.Thehigh are comparedThe observed continuum- to the abun- spectrumare of AA labeledTauri are detected along in emission, with Pontoppidan et al., 2010a, 2010b subtracted spectrum from combined with the fact that it is a typical CTTS, 13 to 16 mm(above)is which influences the chemistry and mineralogy. 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- dances for hot molecular cores The high abundances in the AATauri disk are Salyk et al., 2011 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- Knowledge of the distribution of water (gas and model is a slab in local model (21)fortheinner5AUofprotoplanetary sion lines that is dominated by rotational tran- (triangles) andthermodynamicto equilibri- disk chem- disks (Fig.sitions 3). However, of the H model2O(unmarked results are ver- Najita et al., 2013 tically integrated abundances for the disk, whereas um, with an independent on July 23, 2010 temperature, column den- the molecularfeatures). emission lines we observed are ice) is also central to understanding the origin of sity, and disk area derived likely to probe only the upper disk atmosphere. ical modelsfor each ( molecular21 species)atradiiof The high abundances we derived are not expected 5 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 water in inner solar system bodies and the forma- 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. 1and5AU(opencircleswith such as H2Ointhetemperatureinversionregionof the atmosphere where emission lines form. If omy Observatory, Tucson, AZ 85719, USA. Molecule T (K) N (1016 cm−2) R* (AU) Abundance to CO 1 these models are correct, our measurements may www.sciencemag.org tion of giant planets via the core accretion mech- H2O 575 ± 50 65 ± 24 2.1 ± 0.1 1.3 be a sign of vertical mixing that carries molecular radius labeled).OH 525 ± 50The8.1 ±abun- 5.2 2.2 ± 0.1 0.18 species from deep in the disk, where they are HCN 650 ± 100 6.5 ± 3.3 0.60 ± 0.05 0.13 abundant, up to the surface where they are ob- C2H2 650 ± 150 0.81 ± 0.32 0.60† 0.016 served. Such turbulent mixing is predicted (22)by anism. The high abundance of H OforAATauri, 1504 14 MARCH 2008 VOL 319 SCIENCECO2 350 ± 100www.sciencemag.org0.2 –13 1.2 ± 0.2 0.004 – 0.26 the magnetorotational instability (23), the process 2 dances for hotCO cores900 ± 100 are 49 ± based 16 0.7 ± 0.1 1.0 that is hypothesized to power disk accretion. *The equivalent radius for the emitting area A (R =[A/p]1/2). †Area was set to that derived for HCN. Our results for AA Tauri demonstrate the po-

tential to measure the distribution of water vapor Downloaded from Fig. 3. Comparison of abun- if typical, would rule out simple models where throughout the terrestrial planet region of disks, dances relative to CO. The abun- 1 on observations of absorption1 information that was previously restricted to dances for each molecule X 5 hot H2Oinsideof0.3AU(6, 13). The abun- relative to CO [N(X)/N(CO)] dance of water vapor in the inner disk is im- derived for AA Tauri (squares) 5 Water in the Universe portant in setting the oxidation state of the gas, outward diffusion rapidly depletes water vapor in bands madeare comparedwith to the abun- the Infrared which influences the chemistry and mineralogy. dances for hot molecular cores Knowledge of the distribution of water (gas and (triangles) and to disk chem- Apr 2016 - Noordwijk ice) is also central to understanding the origin of ical models (21)atradiiof 5 water in inner solar system bodies and the forma- the inner disk (24), leading to large ice density 1and5AU(opencircleswith Space Observatory (26–28).1 tion of giant planets via the core accretion mech- radius labeled). The abun- dances for hot cores are based anism. The high abundance of H2OforAATauri, 5 3 on observations of absorption 1 if typical, would rule out simple models where bands made with the Infrared 5 outward diffusion rapidly depletes water vapor in enhancements at larger radii and the rapid growth The organic molecules were the inner disk (24), leading to large ice density Space Observatory (26–28). 5 enhancements at larger radii and the rapid growth 1 The organic molecules were 1 studied using the same mid- of giant planet cores. More detailed models (25) infrared bands that were ana- that include a variety of physical processes predict of giant planet cores. More detailed models (25) studied usinglyzed for AAthe Tauri. same mid- amorecomplicatedevolutionofwatervaporin the inner disk, with both enhancements and deple- tions that vary with time and radius, indicating the importance of measuring the abundance and dis- that include a variety of physical processes predict infrared bands that were ana- tribution of water with evolutionary age. lyzed for AA Tauri. www.sciencemag.org SCIENCE VOL 319 14 MARCH 2008 1505 amorecomplicatedevolutionofwatervaporin the inner disk, with both enhancements and deple- tions that vary with time and radius, indicating the importance of measuring the abundance and dis- tribution of water with evolutionary age.

www.sciencemag.org SCIENCE VOL 319 14 MARCH 2008 1505 REPORTS

1) (7, 8), whereas the backward photolytic re- actions are driven by UV photons in the wave- Formation and Survival length range from 110 to 180 nm (5, 6). The chemical processing of the disk material depends primarily on the local density of stellar photons, of Water Vapor in the Terrestrial the gas temperature, and the gas density (9). 8 −3 At the densities of interest (nH ≥ 10 cm ), Planet–Forming Region chemical time scales are on the order of minutes, much shorter than the time scales for transport Thomas Bethell* and Edwin Bergin processes (9, 10). Thus, the chemistry rapidly reaches its steady-state conditions, and the mo- Recent astronomical observations have revealed what may prove to be the ubiquity of lecular fractions f of water and OH may be water vapor during the early stages of planet formation. We present here a simple expressed as mechanism showing how water vapor forms in situ and is capable of shielding itself from molecule-destroying stellar radiation. The absorption of this radiation by water can control the r2K fH O fO and thermodynamics of the terrestrial planet–forming zone. Similar to Earth's ozone layer, which 2 ¼ r2K rK 1 tot shelters the chemistry of life, the water layer protects other water molecules and allows for a þ þ fOH fH O=r 2 rich organic chemistry. The total abundance of water vapor in the natal habitable zone is ¼ 2 ð Þ equal to that of several thousand oceans. The molecular fractions are expressed as a fraction of the total available oxygen fraction he initial steps of planet formation in both coupled to but is actually controlled by the onset fOtot (defined as the abundance of oxygen nuclei the solar nebula and extrasolar planet- of water formation? This idea represents a shift not in CO relative to that of hydrogen, fOtot ~2× Tforming disks are believed to involve the in our assumptions about the survival mecha- 10−4 for solar abundances) (11). The tempera- growth of submicrometer dust grains and their nisms of molecules in dense interstellar gas ture parameter K = kH2O/kOH,wherek indicates a subsequent settling from the upper layers toward bathed by energetic photons, where solid grains two-body rate coefficient, quantifies the relative the dust-rich disk midplane. A key facet of this are generally assumed to provide most of the strengths of the forward reactions O + H kOH OH 2 À! kH2O on November 28, 2012 process is that as grains grow to larger and larger shielding. In fact, in the absence of dust shielding, and O + H2 H2O. As such, it increases rap- À! sizes, the accompanying attenuation of molecule- an exposed circumstellar water molecule would idly with gas temperature T, reflecting the fact destroying energetic radiation is reduced (1). This normally be destroyed in only a few seconds. that the gas-phase production of water proceeds should eventually lead to the increased destruc- Under typical galactic interstellar conditions, mol- vigorously above T ~300K.Thedensitypa- tion of water vapor in the terrestrial planet– ecules are never sufficiently abundant to compete rameter r = kH2OnH/sH2Onph expresses the com- forming region at 1 astronomical unit (AU). with dust opacity. In contrast, we show here that petition between two-body forward reactions and Over the past decade, astronomical observa- Waterduring chemistry the early stages& self-shielding of planet formation,Bethell when & Bergin,the 2009 backward reactions caused by the destruction tions have revealed the presence of water and OH grains have grown and settled sufficiently, the of molecules by ambient UV photons with den- vapor in the disks around young stars [for exam- rapid formation of water and OH allows these sity nph.Inthecontextofaprotoplanetarydisk, www.sciencemag.org ple, (2)]. The observed conditions of these sys- molecules to be the dominant absorbers of UV both nH and nph will vary considerably with tems are consistent with the expectation that water radiation. height above the disk midplane. O resides in the gas, forming via high-temperature The gas-phase production of water in H2- Examining Eq. 2 reveals the following cri- chemical reactions or from the evaporation of icy dominated disks is believed to follow a simple terion determining when water is the dominant planetesimals. These conditions illustrate that water reaction sequence that is characterizedH by2O for- oxygen-bearing species is prevalent in these systems, at least in the warm ward kinetic reactions and backward photolytic 4 upper layers of the circumstellar disk. reactions 2r − 1 ≥ 1 3 rffiffiffiffiffiffiffiffiffiffiffiffiK Recent observations made with the Spitzer þ ð Þ

H2 H2 Downloaded from Space Telescope (3, 4) show that the inferred Figure S1: ComparisonO ↽ of the⇀ verticalOH stratification↽ ⇀ H of2 moleO cules in a hypothetical1 disk modelWhen this criterion is met, then the disk is in which water self-shieldingÀÀÀÀÀÀÀUV has beenÀÀÀ neglectedÀÀÀÀUV (left panel)andadiskinwhichthewaterð Þ water-column density varies by only a factor of self-shielding mechanism has been properly included (right panel). Dust is assumed“wet to have” (that is, H2Oisthedominantcarrierof Waterlargely in the Universe settled to the midplane in this simple model and so this comparison is an extremal one, highlightingApr 2016 - Noordwijk the potentially disastrous results of omitting self-shielding. As the amount of dust 5acrossasampleofthreeprotoplanetarydisks: For the forward reactions to be most efficient, oxygen);4 otherwise, it is “dry” (that is, atomic 17 17 in the upper layers is increased the left and right panels willconverge.Themolecularfractions N(H2O) ≡∫n(H2O)dz ~ 1.6 × 10 to 8 × 10 theof water gas (blue temperature solid line), OH (green should solid line exceed), and O (yellow 300 solid K line (Fig.)areshownalongwithoxygen is the dominant form). The criterion is −8 −2 –2 the photon density nph (white dashed line,multipliedby10 ). The horizontal line marks the cm , where N is the column density (cm ), n bottom of the warm layer. The height above the midplane is given in units of the scaleheight of –3 is the volumetric density (cm ), and z is the the warm gas, Hw.Thecoloredshadingissimplyafurthervisualindicationofwhichspecies Tableis the dominant 1. AsummaryofthesystempropertiesandobservedabundancesofwarmwaterandOHvaporwithin carrier of oxygen nuclei - light blue indicates atomic oxygen and dark blue height above the disk midplane (cm) (Table 1). indicates water. OH almost never dominates the oxygen budget(intermsofcolumndensity), thealthough inner in the ~3 tenuous AU warm of three layer its protoplanetary abundance may be compa disksrable ( to2, that3). of We water. also It is clear show the column densities predicted by our self- Given that the far-ultraviolet (FUV, wavelength that the inclusion of water self-shielding increases the water abundance both in the warm layer shielding(tenfold) and inmodel. the midplane The region. total Note column the accompanying densityreduction of oxygen-bearing in photon density and species is used as a proxy for the hydrogen the cessation of OH production at the onset of water self-shielding. The− model4 presented in the ~91.2to200nm)absorptioncross-sectionof column, NH ≥ (N(CO)+N(H2O)+N(OH))/4 × 10 .Columndensitiesinthetableareintegratedvertically, −18 2 right panel is roughly consistent with the available observational results for AA17 Tau. −2 ˙ H2O(andOH)issH2O ~ sOH ~5× 10 cm corrected for inclination, and must be multiplied by 10 cm . Macc ,stellarmassaccretionrate;Mʘ,solarmass. (5, 6), the FUV optical depth t of both water and OH in these disks is on the order of unity 10 Protoplanetary disks Properties (t = sN ~few).Whent exceeds unity, the par- DR Tau AS 205A AA Tau ticipating medium impedes the propagation of M˙ (10–7 M –1) radiation. Is it possible that the propagation of acc ʘ 0.32 to 79 7.2 0.07 energetic photons in these systems is not simply N(CO) 70 T 2.1 60 T 1.8 1.3 T 0.4 T(K) 1000 T 150 1000 T 150 550 T 75

Observed N(H2O) 8 T 2.4 6 T 1.8 1.6 T 0.6 Department of Astronomy, University of Michigan, 830 Den- Predicted N(H O) ~10 ~10 ~1 nison Building, 500 Church Street, Ann Arbor, MI 48109, USA. 2 Observed N(OH) 2 T 0.6 2 T 0.6 0.2 T 0.1 *To whom correspondence should be addressed. E-mail: Predicted N(OH) ~2 ~2 ~0.1 [email protected] www.sciencemag.org SCIENCE VOL 326 18 DECEMBER 2009 1675 Our thermal-chemical disk model Glassgold, Najita, & Igea, 1997 Glassgold, Najita, & Igea, 2004 Glassgold, Meijerink, & Najita, 2009 Adamkovics, Glassgold, & Meijerink, 2011

• X-ray and FUV irradiated gas in a T Tauri disk

• Dust: H2 formation, FUV opacity, and thermal accommodation • ~120 species, ~1200 reactions • Photo-rates: use local FUV field, molecular cross sections, shielding • Python codebase: - Kinetics “pre-processor” of chemical rate equations - Modules for disk structure, heating, cooling & FUV - Wrapper to C-library for LSODE in ODEPACK

Water in the Universe Apr 2016 - Noordwijk 5 Layered disk gas Ádámkovics, Glassgold, & Najita, 2014

hot atomic layer

hot irradiated molecular layer

warm shielded layer

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 6 The Astrophysical Journal,733:102(18pp),2011June1 Carr & Najita Observations Carr & Najita, 2011; 2014 model and the final HCN + C2H2 model were subtracted from 0.06 2 H2O the data. Example χ contour plots for CO2 are shown in Figure 6. HCN TH2O 650K ⇡ For C2H2,weassumedthatHCNandC2H2 have the same excitation temperature and emitting area, and adjusted the C2H2 0.04 abundance relative to HCN to fit the C2H2 band. The reason for this approach is that T and N were not generally constrained at

CO2 ausefullevelofconfidence,probablybecauseofthelowerflux in the C2H2 Q-branch and greater contamination from H2Oand 0.02 HCN. An exception is RW Aur, which has the largest C2H2-to- HCN flux ratio in the sample (see Section 4.2.1).

4.2. Modeling Results 0 OH T 4000K 4.2.1. Organic Emission OH ⇡ 12 13 14 15 The best-fit (minimum χ 2)syntheticspectraforHCNand C2H2 are shown in Figure 7, overplotted on the observed Figure 4. Synthetic H2O model spectrum for BP Tau (dashed red line) over- spectra with the best H2O + OH model subtracted. In some Water in theplotted Universe on the observed continuum-subtracted spectrum. The model parameters Apr 2016 - Noordwijk cases (e.g., DK Tau), significant residuals remain after fitting the are given in Table 2. The solid bold (blue) line shows the model OH spectrum 7 used for BP Tau. HCN + C2H2 model; many of these residuals coincide with H2O (A color version of this figure is available in the online journal.) features that are not well matched by the best-fit water spectrum. We were able to determine HCN parameters (Table 3)forfiveof the six stars in which HCN was detected. For UY Aur, the HCN After subtraction of the synthetic H2Ospectrum,amodelOH band is only marginally detected (Figure 7), but its measured spectrum was calculated to match the OH spectrum (see Figure 4 strength is sensitive to the choice of the continuum fit. Emission for an example). We subtract this OH synthetic spectrum from from C2H2 in UY Aur can only be considered as an upper limit. the data, though the only feature that has some impact on our The observed HCN Q-branches are a blend of multiple results is the OH feature at 14.06 µm(N 20), which blends = rovibrational bands, with significant contribution to the emission with the red wing of the HCN band (see Figure 3). from vibrational levels up to v 3 or 4 (depending on the = 4.1.2. The Organics: HCN, C H ,andCO temperature) and rotational levels up to J 24–30. In the best 2 2 2 cases (AA Tau and BP Tau in Figure 7),= the R-branch lines In modeling the HCN, C2H2,andCO2 bands, we first subtract can be discerned extending to shorter wavelengths to at least the best H2O+OHmodelforeachobjectfromtheobserved the J 22–21 transition. Hence, HCN is both rotationally and = spectrum. H2Ocancontributesignificantlytothefluxwithinthe vibrationally hot, and the fits in Figure 7 are consistent with Q-branches of HCN and C2H2.Inaddition,bothHCNandC2H2 asimilarrotationalandvibrationalexcitationtemperature.A contribute emission to the Q-branch of the other, which can be possible exception is GI Tau, which shows an excess at 13.85 µm important when the emission from one molecule is significantly relative to the model. This could be excess emission from the greater than the other. BP Tau is a good example where this v 4and5vibrationalbands,butanunidentifiedspectralfeature = blending produces an apparent C2H2 feature. As can be seen in is also a possibility. The definition of the continuum is also an Figure 4,BPTaushowsemissionnear13.7µmthatcouldbe issue at this spectral resolution and wavelength, especially since mistaken for the C2H2 band; however, the model in Figure 4 HCN and C2H2 sit atop the 13.7 µmsolid-statefeaturewhich shows that H2Oemissionaccountsformostofthisflux.Further has an unknown spectral structure. modeling shows HCN emission can account for the remaining The best-fit model for the HCN band is always optically thick residual. or marginally optically thick, with τ 0.4–4 in the Q(10) line The shapes of the Q-branches are determined by both temper- of the v 1–0 band. The HCN column= densities for the five ature and column density. Synthetic spectra were calculated on stars have= a range of about three, with temperatures between agridinT and N, scaling Re to match the model flux to the ob- 550 and 850 K. The contour plots in Figure 5 show that as the served flux integrated over the band. χ 2 was calculated at each temperature is decreased (or increased) a respective increase (or grid point from the difference of the model and observed spec- decrease) in the column density can produce acceptably good fits trum, and contour plots of χ 2 were produced to determine the over a range in parameters. Within the 90% confidence interval, best-fit values and confidence intervals. There is significant de- the lower limit on the temperature corresponds to the upper limit generacy between T and N in fitting each of the three molecular on the column density; these parameters are given in Table 3 as bands, with increasing T offset by decreasing N. the “Optically Thick” solution. The upper limits on the HCN 2 When calculating the grid of HCN models, a synthetic C2H2 column densities are 2–5 times greater than the minimum χ spectrum was subtracted from the observed spectrum to account solution. for C2H2 contribution to the HCN band. This fixed C2H2 model For each object there also exists a warmer, optically thin spectrum was calculated from a combined HCN and C2H2 model solution within the 90% confidence interval. This is where the that was close to the final solution based on a first round of contours in Figure 5 continue to lower column density at roughly fitting. Two examples of contour plots of χ 2 for HCN are shown constant temperature. Hence, while the fits place an upper limit in Figure 5.Theheavycontouristhe90%confidencelimit, on the temperature that corresponds to the optically thin case, which we adopt as the uncertainty range. there is no formal lower limit on the column density, as long as AsimilarprocedurewasfollowedforfittingtheCO2 Q- an arbitrarily large area (but constant number of molecules) can branch. Before fitting the CO2 band, both the best H2O+OH be invoked to produce the total emission flux. A lower limit can

7 Layered disk gas Ádámkovics, Glassgold, & Najita, 2014

hot atomic layer

hot irradiated molecular layer

warm shielded layer

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 8 FUV is important

Water in the Universe Apr 2016 - Noordwijk 9 FUV radiative transfer: scattered Lyα Bethell & Bergin, 2011 The Astrophysical Journal,739:78(11pp),2011October1 Bethell & Bergin

Water in the Universe Apr 2016 - Noordwijk 10

Figure 9. Maps (r–z)ofthephotondensityandanisotropyoftheLyα (left-hand panels) and FUV-continuum (right-hand panels) radiation fields in the ϵ 0.01 3 = modified D’Alessio disk model. The star is located at the origin. Top: photon density (units cm− ). Black arrows indicate the direction of the net flux. Bottom: the anisotropy of the radiation field, γ I(k)kdΩ /4πJ , where k is the unit direction vector, I(k) is the intensity, and J is the mean intensity. The limit γ 1 implies unidirectionality and γ 0 implies≡ | isotropy.| The flared feature of low anisotropy seen in the Lyα panel (lower left) is attributable to (and coincident with)= the resonantly scattering H-layer. Note= also! how the net Lyα flux emerging from the base of the H-layer does so perpendicularly. associated with dust depleted models, due to the proportional field is clearly seen in the left-hand anisotropy panel. The reduction in the H2 formation rate. The precipitous drop in n(H) net flux of transmitted Lyα photons emerges from the H-layer seen in Figure 8 is indicative of the sudden onset of H2 self- perpendicularly, providing a more direct illumination of the disk shielding. The vertical optical depth (for resonant scattering) below. of the H-layer depends upon the wavelength displacement from Vertical profiles of the Lyα/FUV-continuum photon density line center; for the Lyα spectrum shown in Figure 1 the photon- ratio are shown in Figure 10 for ϵ 0.01, 0.1, and 1, at radii z = averaged optical depths are τLyα 0.2, 1, 6forϵ 1, 0.1, 0.01, r 1and100AU.Thesamequalitativebehavioroftheratio respectively. Although these vertical∼ optical depths= are not large, is= seen at all radii, although the quantitative effect is greatest when viewed from the star the slant optical depth of this H-layer in the inner disk. It is clear from Figure 10 that the H-layer is z is typically τLy∗ α 20τLyα 1andthusthe H-layer will inter- FUV-continuum-dominated, whereas the molecular disk (and cept essentially every∼ stellar≫ Lyα photon incident upon it.We therefore the vast majority of disk mass) is Lyα-dominated. are mostly interested in following the portion of Lyα emerging Ultimately, the enhancement of the Lyα photon density (above from the lower surface of the H-layer. The remainder is scat- the intrinsic stellar value) may exceed an order of magnitude. tered into space from the upper surface. It is worth noting that The ratios eventually asymptote to a constant value deep into the H-layer is not so optically thick to these line-wing photons the disk where both fields are approaching the diffusive limit that they become line-trapped and destroyed by dust absorption and therefore behaving similarly. (Neufeld 1990). While these results suggest that resonant scat- tering of Lyα is indeed important, the H-layers computed lack 5. DISCUSSION the self-consistency required to render them entirely realistic. Regardless of the value of ϵ, essentially every stellar Lyα Figure 9 shows spatial maps providing a visual comparison of photon is intercepted by the highly flared H-layer. Upon the the Lyα and FUV-continuum photon densities in the ϵ 0.01 α = first scattering the Ly field is completely isotropized and a modified D’Alessio disk. In addition to the photon density nph fraction (<50%) is transmitted through to the base H-layer. we also show flux arrows (direction only) and the anisotropy This is clearly seen in Figure 9 (top left and bottom left panels). of the field γ (the ratio of net flux to photon density). The ViewedfromwithinthemolecularregiontheLyα would seem to highly flared H-layer and its isotropizing effect on the Lyα originate in a diffuse blanket overlying the disk, analogous to the

7 FUV continuum and Lyα Ádámkovics, Najita, & Glassgold, 2016

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 11 Thermal processes: heating Ádámkovics, Najita, & Glassgold, 2016

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 12 Photochemical heating Glassgold & Najita, 2015

photodissociation of molecules heats gas

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 13 Hot molecular gas Ádámkovics, Najita, & Glassgold, 2016

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 14 The Astrophysical Journal,756:171(17pp),2012September10 France et al. Fluorescent H2 Emisssion France et al., 2012 200 Ardila et al., 2002; Herczeg et al., 2002 AA Tau 140 AA Tau (5) (3) (2) (8) P P P P 6)R(3) 6) 6) 6) 6) − − − − 150 120 − (1 (1 (0 (0 V4046 Sgr / 4 (1 ) 1 ) −

1 100 −

•H2 excitation ~ 2500 K Å 1

Å V4046 Sgr − 1 −

100 s 2 s − 2 80 − x x •Hot H2 pumped by Lyα u u s cm Fl s cm g Fl

g GM Aur

er 60 er

50 15 − 15

− •emitting region < 0.5 AU

(10 GM Aur

(10 40

0 LkCa19 20 α CII SiIV CIV OI HeII Ly 0 −50 1430 1440 1450 1460 1470 1200 1300 1400 1500 1600 Wavelength (Å) Wavelength (Å) Figure 2. The 1430–1470 Å spectral region for the gas-rich targets plotted Figure 1. Examples of COS spectra (1170–1690 Å) for a range of gas and in Figure 1. All of the strong spectral features in this bandpass are emission dust disk parameters. From top to bottom: the primordial disk target AA Tau 15 2 1 1 lines from Lyα-pumped fluorescent H2.Emissionlinesusedinthisanalysis (offset by +150 ( 10− erg cm− s− Å− )), the pre-transitional disk V4046 are marked with blue diamonds and several bright features are labeled. Objects Sgr (flux divided× by 4Water and offsetin the Universe by +75 ( 10 15 erg cm 2 s 1 Å 1)), the Apr 2016 - Noordwijk− − − − are plotted in order of decreasing near-IR dust excesses: AA Tau (primordial, transitional disk system GM Aur, and the gas-poor× WTTS LkCa 19 (offset by 15 2 1 1 n13–31 0.51; Furlan et al. 2009), V4046 Sgr (pre-transitional) has a15 sub-AU 50 ( 10− erg cm− s− Å− )). The spectra have been binned by one spectral scale hole= − in the inner disk dust distribution (Jensen & Mathieu 1997), and GM resolution− × element (7 pixels) for display. Except for the atomic lines identified Aur (transitional, n13–31 1.75; Furlan et al. 2009)hasan 24 AU hole in in the spectrum of LkCa 19, most of the emission lines in the spectra of the the inner dust disk (Calvet= et al. 2005). The spectra are binned∼ to 1/2ofa other three stars are fluorescent H2 emission lines pumped by Lyα. spectral resolution element (3 pixels), and representative error bars≈ are shown (A color version of this figure is available in the online journal.) overplotted. The H2 spectra are qualitatively similar, independent of the inner disk dust properties. (A color version of this figure is available in the online journal.) the 0′′.2 0′′.2slitforexposuretimesbetweentwoandthree orbits per× object. The far-UV STIS spectra were combined using the STIS echelle software developed for the StarCAT catalog (Ayres 2010,T.Ayres2011,privatecommunication). ratios (S/N) per resolution element are typically between 5 and 40 in the brightest fluorescent H2 emission lines for our CTTS 3. ANALYSIS targets. The (1–7)R(3) λlab 1489.57 Å transition is relatively free from spectral contamination= and is displayed for all targets We observe fluorescent H emission from all of the 27 CTTSs 2 in Figures 3(a)–(c). When available, the stellar radial velocities in our sample, and no H emission from the 7 WTTS targets. The 2 are indicated in Figure 3 with green dash-dotted lines. Within number of observed fluorescent progressions varies significantly the 15 km s 1 wavelength solution accuracy of COS, most of across the sample, and we present our measurements of the total − the∼ H progressions are consistent with the stellar velocity. In H fluxes below. We detect strong fluorescent emission in all 2 2 several cases the H lines appear to have line wings extending of the transitional objects in our sample (CS Cha, DM Tau, 2 to negative velocities or statistically significant differences in GM Aur, UX Tau A, LkCa 15, HD 135344B, and TW Hya; the velocity widths of different H progressions. RW Aur is Section 4.3). 2 an extreme example of this behavior. Ardila et al. (2002)and The fluorescent H lines observed in the CTTS sample can be 2 Herczeg et al. (2006)havenotedthepresenceofblueshifted used to determine the relative amount of H in the circumstellar 2 H emission in their sample of TTSs observed with GHRS and environment and to constrain its spatial distribution. For our 2 STIS. The presence of blueshifted emission creates additional line-profile analysis, we focus on the measurement of two uncertainty in the measured line widths at the resolution of progressions ([v ,J ] [1,7] and [1,4]) detected in all of the ′ ′ our spectra, and we present a discussion of outflow signatures CTTS targets.11 These= emission lines are pumped through observed in our sample in Section 4.2.1. the (1–2)R(6) λ 1215.73 Å and (1–2)P(5) λ 1216.07 Å lab lab The high S/NoftheCOSdatameansthatwecanrestrictthe transitions, respectively. The absorbing transitions are within analysis to the brightest lines from the progressions of interest. +15 to +100 km s 1 of Lyα line center. The signal-to-noise − Systematics were minimized by focusing on emission lines with wavelengths 1395 ! λ ! 1640 Å. This choice is optimal be- 11 The quantum numbers v and J denote the vibrational and rotational cause (1) fluorescent transitions cascading to vibrational levels 1 + quantum numbers in the ground electronic state (X Σg), the numbers v′ and J ′ v 5donotsuffersignificantself-absorptionbeforeescaping 1 + ′′ " characterize the H2 in the excited (B Σu) electronic state, and the numbers v′′ the circumstellar environment (e.g., Figure 7 of Herczeg et al. and J ′′ are the rovibrational levels in the electronic ground state following the fluorescent emission. Absorption lines are described by (v′ – v) and emission 2004), enabling more robust flux measurements; (2) the non- lines by (v′ – v′′). Gaussian wings of the COS instrumental line-spread function

4 The Astrophysical Journal,756:171(17pp),2012September10 France et al.

2.0 0.10 V−SU−AUR SZ−102 TWA13A 0.10 TWA13−OFFSET 0.08 1.5 1.5 0.08 0.06 0.06 1.0 1.0 0.04 0.04 0.5 0.5 0.02 0.02 0.0 0.0 0.00 0.00 −100 0 100 −100 0 100 −100 0 100 −100 0 100 Velocity (km s−1) Velocity (km s−1) Velocity (km s−1) Velocity (km s−1) ) 1 − 0.15 100 0.10

Å TWA−7 TWHya NAME−UX−TAU−A HBC−427

1 5 −

s 80 0.08 2 − 0.10 4 60 0.06

s cm s 3 g 40 0.04 er 0.05 2 14 − 20 1 0.02 x (10

u 0.00 0 0 0.00 Fl −100 0 100 −100 0 100 −100 0 100 −100 0 100 Velocity (km s−1) Velocity (km s−1) Velocity (km s−1) Velocity (km s−1)

25 V4046SGR 1.0 V−V836−TAU 20 0.8 15 0.6 10 0.4 5 0.2 0 0.0 −100 0 The100 Astrophysical Journal−,756:171(17pp),2012September10100 0 100 France et al. −1 −1 Velocity (km s ) Velocity (km s ) Table 4 France et al., 2012 H2 Fluorescent EmissionMolecular(c) and Atomic Emission Line Parameters a a b c a d e Target FWHM[1,7] FigureRH2 [1,7] 3. (Continued)Rin FWHMCO n13–31 L(H2) L(Lyα) L(C iv) 1 ⟨ ⟩ 1 29 1 29 1 29 1 (km s− )(AU)(AU)(kms− )(10erg s− )(10erg s− ) (10 erg s− ) AA Tau 62 40.690.08 0.24 92 0.51 46.7 3.1 818.5 146.6 6.1 −1 ± ± broadening due− to bulk motions± and Keplerian± rotation domi- H2 VelocityAK Sco(km s ) 57 35 1.25 0.77 0.43 ...... 8.1 1.3 114.4 11.8 ± ± ± BP Tau 70 60.130.02 0.05nate the 87 observed0.58 line shapes 68.0 7.4 when the 731.7 FWHM129.7 of the 40.3 emission −100 0 100 ± 200 ± − ± 1 ± CS Cha 18 79.004.55 3.11line is greater... than2.89 the 189.9 17 km12.9 s spectral 2076.7 resolution 20.6 of COS. ± ± ± − 40 CV Cha 22 30 4.75 3.88 1.64 ... 0.27 139.1 35.5 1540.7 169.1 ± ± For the case of− H2 in a circumstellar± disk, we define a simple H2 (1−6) P(5) DE Tau 55 60.230.04 0.08 ... 0.12 20.1 3.0 361.2 106.0 7.8 fAA Tau ± ± − ± ± DF Tau A 64 70.160.03 0.06metric to 79 characterize1.09 the 95.7 average4.6 H2 radius, 1064.7RH2 , 23.2 ± ± − ± COS LSF DK Tau AV4046f 55Sgr20.240.02 0.08 ... 0.81 21.3 1.2 276.7⟨ ⟩ 4.6

) ± ± − ± 2 1 30 DM Tau 27 50.800.24 0.28 ... 1.30 9.7 0.7 106.5 11.7 1.9 − GM Aur± ± ± 2sin(i±) DN Tau 71 19 0.09 0.04 0.03 ... 0.43R 210.6GM67.8 2279.5, 290.8 (2) Å ± ± − H2 m ± 1 DR Tau 35 72.090.62 0.72 29 0.40 14117.7 1590.0∗ 149385.1 3986.7 − ± ± −⟨ ⟩ = ± FWHMm

s GM Aur 41 11 1.68 0.65 0.58 47 1.76 18.5 1.8! 286.1 70.4" 7.6 2 ± ± ± ± − HD 104237 94 77 0.10 0.07 0.03 ...... 964.4 315.6 10239.6 91.6 x ± ± where M is the ,± i is the inclination angle, and u 20 HD 135344B 26 1 ...... 47∗ ... 15.1 1.3 212.0 27.4 f ± ± s cm Fl HN Tau A 61 17 0.47 0.18 0.16FWHM...m is the0.44 mean of 19.1 the1.1 Gaussian 307.2 FWHMs60.3 for 2.6 a given g ± ± − ± ± IP Tauf 102 29 0.17 0.07 0.06progression... m.Thisdefinitionoftheaveragemolecularradius0.11 4.0 0.2 94.0 1.2

er ± ± − ± LkCa 15 53 30.620.06 0.21 ... 0.62 24.4 1.3 403.4 66.4 9.2 15 ± ± ± ± − RECX 11 54 30.850.08 0.29is analogous... to ... the UV-CO1.9 radius0.1 used 58.3 by3.9 Schindhelm 1.7 et al. ± ± ± ± 10 RECX 15f 41 40.620.10 0.21(2012a).... When possible, ... 5.4 we use0.3 disk 130.6 inclinations17.8 derived 0.4 from

(10 ± ± ± ± RU Lupi 40 20.300.03 0.10 24 ... 27.2 15.2 635.3 148.3 12.0 ± ± the submillimeter dust continuum± observations± presented by RW Aur Ag ...... 0.54 860.7 228.9 9169.5 58.9 − ± SU Aur 49 62.670.58 0.92Andrews 121 & Williams 0.74 (2007 36.3 4.0)andAndrewsetal.( 811.4 191.4 17.32011). In ± ± ± ± SZ 102 47 7 ...... principle, ... the error ... on196.2 the average13.8 H 2182.4radius should 60.8 include ± ± 2 0 TW Hya 18 2 ...... 17 ... 16.8 2.0 199.6 34.3 17.0 ± uncertainties on the stellar mass± and disk± inclinations; however UX Tau A 29 31.760.33 0.61 21 1.83 12.6 0.3 146.3 12.3 2.4 1445.5 1446.0 1446.5 ±1447.0 ± ± ± V4046 Sgr 45 10.950.06 0.33these uncertainties...... are not19.8 available0.9 for 383.3 all targets.42.1 Therefore, 4.5 the ± ± ± ± WavelenV836gth Tau (Å) 47 20 0.99 0.50 0.34 ... 0.45 80.2 7.3 900.5 17.7 ± ± quoted error on− RH2 only includes± measurement uncertainties h ⟨ ⟩ Figure 4. Expanded view of the H2 (1–6) P(5)HBC (λlab 427 1446.12... Å) emission line, ...... from the ... H2 line ... fitting. !0.5 !57.7 0.2 showing the velocity width beyond the COSLKCa line-spread 19= function... (shown as ... the ...... !1.1 !63.6 0.5 In Table 4 we present RH2 for the [v′, J ′] [1,7] progression gray dash-dotted line) for the three exampleLKCa targets 4 shown in Figure...2.TheAA ...... ⟨ !1.0⟩ !62.6= 1.6 RECX 1 ...... ( RH ...[1,7]). We ... focus on0.3 the [1,7] progression27.7 due0.5 to its Tau and V4046 Sgr spectra have beenWater normalized in the Universe to the peak flux of the GM ≡⟨ 2 ⟩ ! ! AprTWA 2016 13A - Noordwijk...... proximity ... to the ... Lyα line!0.1 center. This! progression9.2 should0.1 be Aur H2 emission and shifted to the centroidTWA velocity 13B of the H...2 emission. The ...... 0.2 10.2 0.1 emission lines are spectrally resolved by COS. one of the most readily observable! in weakly! accreting systems 16 TWA 7 ...... !0.0 !8.6 0.1 (A color version of this figure is available in the online journal.) because as the accretion-powered Lyα flux decreases, the flux in Notes. the wings of Lyα may be insufficient to excite a detectable level a 1 + 1 + Average FWHM and average H2 radius (see Section 3.2) were calculated from four lines of the H2 B Σu–X Σg (1 – v′′) R(6)+P(8) progression. of H2 emission. However, the [1,7] progression should continue The H2 is the sum of the 12 progressions measured in this work (Table 2). b 2 Inner H2 radius, defined as Rin GM (sin(i)/(1.7toHWHM be observable[1,7])) . even in systems with narrow Lyα profiles (e.g., = ∗ × c CO line widths taken from Salyk et al. (2011a)andBastetal.(2011). d H i Lyα with error bars were calculated from the Lyα fluxes presented by Schindhelm et al. (2012b). Values without error bars were extrapolated from the relationship between7F(H2)andF(Lyα). e Measurement error on the C iv flux is taken as 5%. f Targets where molecular outflows may contribute to the observed H2 line width. g Strong molecular outflows in RW Aur contaminate the Gaussian fitting (see Figures 3(b) and 8). h Targets below the double line do not show measurable H2 emission in their far-UV spectra.

likely less than one (Herczeg et al. (2004)foundη 0.25 in The total Lyα flux is the full, unabsorbed Lyα profile as it is model fits to the spectrum of TW Hya), while for sources≈ with a emitted from the immediate stellar environment. Figure 7 shows significant outflow component η may be 1. Yang et al. (2011) that the ratio of the total emitted Lyα flux that is reprocessed by ∼ and France et al. (2012)presentanalysesofH absorption lines H2 is 6.2% 2.1%. A more meaningful measure of the degree 2 ± imposed on the Lyα profiles of the CTTSs V4046 Sgr, DF Tau, of H2 reprocessing is the ratio of incident Lyα that arrives at and AA Tau, systems with both high and low inclinations. This the molecular material. The Lyα profile will experience some implies that at least some portion of the H2 in these systems degree of absorption in the circumstellar environment prior to has an Lyα covering fraction of near unity, suggesting that η reaching the molecules (Wood & Karovska 2004;Herczegetal. 1eveninsomedisk-dominatedsystems.Furthermore,incases∼ 2004;Schindhelmetal.2012a). Using the incident Lyα profile where Lyα has been scattered out of our line of sight or self- observed by the H2, we find that the reprocessing fraction absorption redistributes the fluorescence to the higher v levels (ηF (H2)/F(Lyα)) is 11.5% 1.8%. Therefore, modulo the ′′ ± we use to determine F(H2), η can be >1(Woodetal.2002). In factor of η,weinferthatH2 is capable of reprocessing >10% of the absence of a more sophisticated radiative transfer treatment the incident Lyα flux. This is interesting because the H2 (1) will of each system individually, we assume η 1 for the present isotropically redistribute the Lyα photons and (2) represents a analysis. = means for transferring Lyα photons out of the Lyα line core and

11 Fluorescent H2 Emisssion Ádámkovics, Najita, & Glassgold, 2016

R = 0.25AU R = 1AU

Dissociation of H2O & OH by Lyα heats gas, Hot H2 for fluorescent emission can be thermally pumped

Water in the Universe Apr 2016 - Noordwijk 17 Role of mechanical heating: hot irradiated layer 9 = ↵ ⇢c2⌦ mech 4 h

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 18 Role of mechanical heating: hot irradiated layer 9 = ↵ ⇢c2⌦ mech 4 h

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 19 Role of mechanical heating: warm shielded layer

mechanical heating

warm shielded layer

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 20 Role of mechanical heating: warm shielded layer

no mechanical heating

R = 1AU

Water in the Universe Apr 2016 - Noordwijk 21 Turbulent (mechanical)The Astrophysical Journal heating,769:76(21pp),2013May20 Bai & Stone, 2013 Bai & Stone plasma β (ratio of gas pressure to magnetic pressure) reaches −5 order unity within the turbulent layer. Moreover, we find that 10 Maxwell the magnetic field is predominantly toroidal in the entire disk. Reynolds −6 From the simulation, we find the Shakura–Sunyaev parameter 10 6 using Equation (12)tobeαMax 1.7 10− ,andαRey 6 ≈ × ≈ −7 1.3 10− .Forsuchsmalllevelofstress,wefindthatthe

Stress 10 × 11 1 resulting accretion rate is only about 2.5 10− M yr− , × ⊙ −8 which is three orders of magnitude too small compared with 10 observations. We can compare this result with optimistic predictions of the −9 100 MRI-driven accretion rate using the semianalytical framework 10 of Bai (2011a). Using this framework, we first extract the −1 P 10 gas density profiles and the profiles of Am and ηO .Assuming −2 10 constant magnetic field strength across the MRI-active layer, the −3 MRI-driven accretion rate can be expressed as (Equation (28) 10 P P mag of Bai 2011a) −4 c dz 10 M s B2 , (14) −5 ˙ ≈ 8Ω2 H 10 !active −6 K 10 E where the integral is performed in regions where the MRI is −7 permitted based on the criteria (20) of Bai (2011a). We scan the 10 R = 1AU 10 1 −6 −4 −2 0 2 4 6 field strength to maximize M,whichgives6.6 10− M yr− ˙ × ⊙ z/H as an upper limit of the MRI-driven accretion rate. We see that Figure 4. Same as Figure 3, but for zero net vertical flux run with both Ohmic this value is a factor of more than 20 larger than our simulation resistivity and AD (OA-nobz). Note the different scales. result. The main reason that our simulation yields an accretion (A color version of this figure is available in the online journal.) rate that is significantly smaller than theoretical expectations is Water in the Universe that the field geometry is not optimal: in both the ideal MHD Apr 2016 - Noordwijk (e.g., Hawley et al. 1995;Bai&Stone2013) and non-ideal and the density near the vertical boundaries stays at the value of MHD (e.g., Fleming22 et al. 2000;Bai&Stone2011), one finds ρfloor.Thissituationimpliesthatthegasnearverticalboundaries stronger turbulence in the presence of a net vertical magnetic tends to fall back toward disk midplane, but this is prevented flux, and the optimistic estimate can possibly be achieved only by the outflow boundary condition. Therefore, we gradually when a optimal field geometry is realized. lower the density floor over the course of the simulation and find This simulation has also demonstrated the physical signifi- that the phenomena of artificial magnetic flux accumulation and cance of AD, which drastically reduces the accretion rate com- density cutoff at ρfloor disappears when ρfloor is brought down to pared with the Ohmic-only case. Although such a comparison 10 8,whichisachievedatt 230 Ω 1.Thewholeprocessis − = − may be unfair since our Ohmic-only simulation contains net reflected in Figure 2.Fromthistimeonward,thesystemevolves vertical magnetic flux, Ohmic-only simulations with a similar smoothly and no artifacts are seen from the vertical boundaries. setup and zero net vertical flux generally yield a total stress level 8 Also note that setting such a small density floor of 10− at that is only slightly smaller than ours (e.g., Turner et al. 2007;Il- the beginning would lead to dramatically small simulation time gner & Nelson 2008). Clearly, with AD taken into account, zero step, which is not quite realistic, hence gradual reduction of net vertical magnetic field geometry is far from being capable ρfloor is necessary, although one has to be patient to get rid of of driving rapid accretion in the inner region of PPDs. the artifacts. We see from Figure 2 that in the saturated state, the disk only 3.3. Net Vertical Flux Run with both Ohmic Resistivity and AD has extremely weak level of MRI turbulence in a very thin layer between about z 4and5H .NotethattheFUVionization The inclusion of net vertical magnetic flux in our fiducial run ∼ with both Ohmic resistivity and AD (OA-b5) makes the field front is located at z 4H,beyondwhichwefindAm ! 50 and the gas behaves=± almost as in the ideal MHD regime (since geometry more favorable for the MRI as we originally expect. density profile is still largely Gaussian, one can still use Figure 1 The bottom panel of Figure 2 illustrates the time evolution as a reference for the profile of Am), hence the MRI operates of Maxwell stress. The initial field configuration is unstable in this region. Below z 4H ,thevalueofAm is of order to the MRI, which gives rise to channel-flow-like behaviors unity or smaller, and there= is no evidence of turbulent activity. and initiates some turbulent activities in the surface layer. This is consistent with unstratified simulation results of Bai & However, we find that quite surprisingly, the system then quickly Stone (2011), where it was found that the MRI is suppressed for relaxes into a non-turbulent state in about 10 orbits with the MRI suppressed completely. The laminar configuration is then Am " 1intheabsenceofnetverticalmagneticflux. 9 There is some residual Maxwell stress around the disk maintained for the remaining of the simulation time. midplane as a result of initial conditions that slowly diminishes As the system settles to a completely laminar state, we extract over the course of the simulation. We extract the profiles of the exact vertical profiles of various physical quantities, as various physical quantities and show them in Figure 4,where shown in Figure 5.Themagneticfieldisstrongestwithinabout 2.5H of the disk midplane, where the field is essentially the time average is performed between Ωt 750 and 900. The ± MRI-active region exhibits as bumps in the stress= plot, as well as 9 the bumps in the kinetic energy plot. The magnetic field strength To justify the validity of this result, particularly that it is not due to an unrealistic initial condition, we restart from the end of run O-b5 with AD roughly stays constant through the entire disk, which is likely turned on and find that also in about 10 orbits of time, the system settles to the set by the turbulent activities in the active zone: the value of same laminar state.

8 Summary

Some roles of water in the inner region of disks:

1. FUV photochemical (radiative) heating of hot irradiated layer 2. Diagnostic of mechanical heating of warm molecular layer 3. Shielding mid-plane from Lyα and FUV continuum radiation

Water in the Universe Apr 2016 - Noordwijk 23 Thank you!

This work was supported by NASA XRP NNX15AE24G and was performed in part at the Aspen Center for Physics, which is supported by NSF grant PHY-1066293

Water in the Universe Apr 2016 - Noordwijk 24