UV photolysis, organic molecules in young disks, and the origin of meteoritic amino acids
aa r X i v : . [ a s t r o - ph . E P ] D ec UV Photolysis, Organic Molecules in Young Disks, and the Originof Meteoritic Amino AcidsSubmitted to
Icarus
Henry B. ThroopSouthwest Research InstituteDepartment of Space Studies1050 Walnut St Ste 300, Boulder, CO 80302 [email protected]
Received ; accepted 2 –
ABSTRACT
The origin of complex organic molecules such as amino acids and their precur-sors found in meteorites and comets is unknown. Previous studies have accountedfor the complex organic inventory of the Solar System by aqueous chemistry onwarm meteoritic parent bodies, or by accretion of organics formed in the inter-stellar medium. This paper proposes a third possibility: that complex organicswere created in situ by ultraviolet light from nearby O/B stars irradiating icesalready in the Sun’s protoplanetary disk. If the Sun was born in a dense clus-ter near UV-bright stars, the flux hitting the disk from external stars could bemany orders of magnitude higher than that from the Sun alone. Such photolysisof ices in the laboratory can rapidly produce amino acid precursors and othercomplex organic molecules. I present a simple model coupling grain growth andUV exposure in a young circumstellar disk. It is shown that the production maybe sufficient to create the Solar System’s entire complex organic inventory within10 yr. Subsequent aqueous alteration on meteoritic parent bodies is not ruledout.
1. Introduction
The early Solar System was replete with complex organic molecules, which can be seentoday in preserved ancient bodies such as meteorites and comets. Upwards of 100 differentamino acids have been detected on chondritic meteorites such as Murchison, Allende andOrguiel. Many of these have no known natural terrestrial occurrence (Ehrenfreund et al. ? Hayatsu and Anders 1981). Laboratory studies have found Murchison to have some10–30 ppm by mass in identified amino acids (Shock and Schulte 1990). The simplestamino acid (glycine, NH CH COOH) has recently been detected in comets, but so far haseluded detection in the ISM (Elsila et al. et al. et al. et al. et al. et al. endogenically , by chemicalsynthesis within the solar system itself. A variety of energy sources for this exist, includinginfall heating, radiogenic heating, lightning, and shocks. For the amino acids present onmeteoritic parent bodies, the best-studied endogenic process is Strecker synthesis. This isa method by which amino acids are formed in aqueous environments, such as the warmsub-surface aquifers that could have been present on asteroids as they were heated by Aland other radionuclides during their first few Myr (Ehrenfreund et al. Al existed to heat parentbodies to liquid water temperatures (McSween et al. enhancements of δD = 600-2000 / , while measurements of the water inthese same bodies show deuterium depletions of roughly 100 / . In situ synthesis of aminosdoes not preferentially change the D/H ratio, so this argues that the water and organicscome from distinct sources (Lerner 1997).Second, the molecules may have been inherited exogenically from the interstellarmedium which formed the solar nebula. The ISM is known to be rich in complexchemistry, with over 150 different gas-phase species detected to date in molecular clouds(Herbst and van Dishoeck 2009). Amino acids have not been detected in the ISM, butsugars, alcohols, polycyclic aromatic hydrocarbons (PAHs), and other complex moleculeshave been. The formation of these compounds in the ISM is thought to be due to acombination of processes, including gas phase, gas-grain, and UV-grain reactions. Oncemolecules are formed, stable compounds can be incorporated into young disks as newYSOs condense out of the ISM. This pathway is supported by measurements showing theorganic composition of comets and the ISM to be quite similar (Bockelee-Morvan et al. et al. et al. et al. et al. <
70 K can preferentially increase D/H because deuterium’s higher mass 5 –allows it to bond more readily than H at low temperatures. D/H enrichments are seenthroughout the ISM, and the D/H enrichments in amino acids suggests that they too have alow-temperature origin compared with the water in meteorite parent bodies (Sandford et al. − –10 − G , where G is the interstellar flux at the Sun today(Prasad and Tarafdar 1983). Second, some organic molecules, such as the CHON particlesin comets like Halley, may be easily destroyed by shocks during infall, although somedebate exists on this point (Visser et al. et al. α -aminoisobutyricacid (AIB) and isovaline, have not yet been produced by irradiation in the laboratory(Hudson et al. et al. et al. O, CO and NH , are exposed to the UV and begin to photolyze into more complex species. Eachindividual grain is exposed only briefly to UV light; they spend most of their time in thedisk’s dark, turbulent interior. Grains continue to grow, gas which has not formed planetsis lost due to photo-evaporation and viscous loss, and within 5 Myr a gas-free debris diskis left with its ices enriched in complex organics. These organics can then be incorporatedinto comets, asteroids, and the planets. Organics produced in this way could complement,and perhaps greatly exceed, those produced by the other two pathways. This method issimilar to the exogenic ISM production in that it relies on UV photochemistry, but at farhigh flux (10 G vs. 10 − G ), at warmer temperatures, for a much shorter time. Themodel allows for subsequent aqueous alteration as seen in the meteoritic record.Work by Robert (2002) proposed that the organic material was created by X-rayirradiation of the disk by the Sun during its T Tauri phase. They did not present a diskmodel, but used high-precision D/H measurements of organic and non-organic materialto show that D/H fractionation varied with heliocentric distance, as would irradiation.Remusat et al. (2006) compared the meteoritic and interstellar D/H values, along with theirC-H bond dissociation energies, and concluded that the solar system’s D/H enrichment wascreated in situ , rather than inherited from the ISM. They proposed that the young Sunmight provide the necessary UV source; their model did not study the timescales or fluxesinvolved.The source of the Earth’s organic inventory (as opposed to the Solar nebula’s)is a parallel question which has received some attention. It is believed that althoughsome organics were probably synthesized on the young Earth in situ by lightning or 7 –spark discharges (Miller and Urey 1959), shock heating in the terrestrial atmosphere(Chyba and Sagan 1992), warm ocean vents (Corliss 1990), or any number of otherterrestrial processes, far more were probably delivered from external sources such as comets,asteroids, and interplanetary dust particles (Chyba and Sagan 1992). This paper doesnot further address the origin of life or the Earth’s inventory, except to acknowledge thatincreasing global abundance of organics in the solar nebula probably results in increaseddelivery to the Earth as well.This paper uses a simple model to describe the production of complex organics inUV-illuminated disks in a variety of cases. The results are necessarily general, and do notexplain the abundances or species in one particular sample or disk, but provides an initialassessment of the problem. External UV photolysis has been ignored in almost all previousmodels, yet it may be one of the most important sources of energy in both the youngSolar System and other proto-planetary disks. The problem is set up in §
2, and the modeldescribed in § § §
5. Conclusions are in §
6, andAppendix A contains a derivation of the simple grain growth model used here.
2. Background2.1. Energy sources in the early solar nebula
Various surveys have shown that the majority of stars are born in dense clusters of300–10 stars, where massive O and/or B stars can form (Adams et al. Fe thatare only produced in supernovae, our Sun is thought to have formed in such an environment(Hester et al. et al. et al. et al. et al. et al. et al. (2007),Irvine et al. (2000), Throop (2000), Fegley (1999), Prinn (1993), and van Dishoeck et al. (1993). Prinn and Fegley (1989) provides a complete review but is now somewhat eclipsedby more newer RT models in the astrophysical literature (discussed below). Their paperdoes not directly address the formation of organics, but examines the energy budget forchemistry from various sources in the young solar nebula. They find that the largestenergy source is the gravitational energy given off by the nebula as it collapses. In additionto the entire bulk disk heating, individual grains are heated and may sublimate ices asthey fall into the nebula (Visser et al. et al. × − that of the total gravitational collapse energy of the disk. Radionuclides ( e.g. , Al) wereestimated to provide a few orders of magnitude again less energy. They also estimatedenergy from photochemistry, based on fluxes from both the Sun and interstellar sources.They argued that photochemistry in the solar nebula was unimportant, especially from from 9 –internal sources. However, more sophisticated recent models show that photochemistry canin fact be the dominant energy source.
Internal UV sources.
Several recent models for disk formation consider UV radiativetransfer from the central star to the inner disk. The model of Woitke et al. (2009) handlesphotochemistry using a Monte Carlo method in a 2D disk of mass 1 MMSN, surroundinga T Tauri star. Over 70 chemical species are considered, including five ices, with a totalof nearly 1000 different chemical reactions. A broadly similar model with fewer species isdescribed by Jonkheid et al. (2007). The model by Gorti and Hollenbach (2004) describesthe UV photochemistry of 73 gas species in a 10 Myr old debris disk, putting it in a muchlower optical depth regime than the other models. All three models are computed for a fixedage and do not evolve. The grain size distribution in all is uniform across the disk. All threemodels present sophisticated pictures of the radiative transfer and photochemistry withinthe disks, allowing predictions to be made for abundances and line strengths. However, themodel does not include ice photolysis, grain growth, differences in grain size across the disk,or external illumination. The UV field reaches ∼ G inward of 10 AU, but this is stillmuch lower flux than the 10 G or more from external illumination.In calculating the energy budget of the nascent solar system, Prinn and Fegley (1989)estimated that the the line-of-sight UV optical depth through the disk to be “7 million to110 million orders of magnitude. ” In their model, this extreme optical depth would preventsolar-driven photochemistry virtually anywhere outside the central 0.35 AU. However, theyassumed a direct line-of-sight from the source was required, which Gladstone (1993) pointsout is not the case because solar Ly α photons can reflect off of interplanetary hydrogen farabove the disk plane, thus illuminating the disk by reflected solar light. Considering thehigher density and reflectivity of the interplanetary medium (IPM) at the time, and theincrease in early solar UV flux at 10 × or so over present values, Gladstone (1993) calculate 10 –that the young solar system was exposed to a solar UV flux roughly 10 times the presentsolar value. Scattering of solar Ly α photons off of winds, jets and infalling material canalso be a large source of UV onto the disk. Similarly, Hollenbach et al. (1994) showed thatyoung stars can create a thin atmosphere above their disk, and this disk provides a sourceof indirect UV illumination which can drive photo-evaporation (and thus photolysis), atrates 10–100 times lower than external illumination would, or 10 –10 G (Matsuyama et al. et al. (2006a,b) also looked at the case of photo-evaporation from thecentral star in flared disks, which avoids the line-of-sight problem in Prinn and Fegley(1989). Combined with loss to inward viscous evolution, Alexander et al. (2006b) calculateddisk loss timescales of several Myr, consistent with observation of disk dispersal in youngT Tauri stars. None of the papers by Gladstone, Hollenbach or Alexander considered thephotochemistry effects of UV, but between these results and the detailed RT models above,it becomes clear that photochemistry from the central star can be important. Table 3 listsapproximate fluxes for the various sources. External UV sources.
While the interstellar UV flux in Taurus-like regions is low( ∼ outside the cloud, and 10 − G inside), stars in dense clusters such as the OrionNebula Cluster (ONC) are subject to fluxes from O and B stars on the order ∼ G . Icesin these clusters’ disks are exposed to high levels of UV irradiation, enhancing the disks’abundance of organic molecules. In contrast to the internal sources, the external sourcesare stronger by 10 − × , and illuminate the entire disk evenly rather than dropping offwith radial distance.This paper examines only the effects of the external flux. Existing models providesophisticated treatments of the internal UV flux and the gas photochemistry; our goalhere is to understand the broad global effects of external flux on solid-state chemistry, inpreparation for a more detailed model. 11 – A quick calculation shows the importance of external irradiation relative to otherenergy sources. First, consider the flux intercepted by a disk from its central star ofluminosity L . In a disk with a radius:half-height ratio of 10:1, the fraction of flux interceptedby the disk is ≃ /
10, and the total energy deposited at all wavelengths in time ∆ t is just E ⊙ ≃ L ∆ t. (1)The total amount of energy available for chemistry through thermal heating, shocks,lightning, and so forth can be no greater than the disk’s total collapse energy from thecloud to disk inner radius R , given a disk mass M d and stellar mass M s , or E c = GM d M s R . (2)Finally, the total UV energy absorbed by the disk from external stars is roughly E UV = p (2) πR d F ext ∆ t. (3)Typical values for solar-mass stars in dense clusters are are M s = 1 M ⊙ , M d = 0 . M ⊙ , L = L ⊙ , R d = 100 AU, R = 5 AU, F ext = 10 G , and ∆ t = 1 Myr. Plugging in, we findthe ratio E sol : E c : E UV is approximately 1 : 100 : 10 . That is, in 1 Myr, the external UVdose received by the disk exceeds by × the entire direct energy input from the centralstar, and the external UV dose exceeds by × the entire collapse energy of the disk . Moreover, although thermal gradients can limit the amount of energy available for chemicalreactions (Prinn and Fegley 1989), the UV energy is deposited directly into ice and dust If the opposite were the case, photo-evaporation could not remove the disk. 12 –grains, where individual photons cause photolysis. Therefore, the disk’s dominant energysource is external flux, and this energy source’s effect on chemistry cannot be ignored.
The experiments of Miller (1953, see also Urey 1952) showed that electric dischargeswithin an atmosphere can rapidly create amino acids. Since this time similar experimentshave been repeated with different initial species, temperatures, phases, and energy sources,finding the same general result that amino acids are relatively easy to produce givensufficient energy. In an astrophysical context, amino acids can be produced with energysources from ion irradiation, to physical shocks, to pyrolysis (heating), to UV irradiation(Bernstein et al. et al. (2008), “one conclusion is thatenergetic processing of almost any organic ice that contains C, H, N, and O probablyresults in the formation of amino acid precursors, which can be hydrolyzed to give the acidsthemselves.”Most interesting in the regions of cold space are laboratory experiments in the lastdecade involving UV irradiation of ice mixtures. For instance, experiments by bothBernstein et al. (2002) and Munoz Caro et al. (2002) started with thin ice mixturesincluding H O, CH OH, and NH , chosen to simulate ISM conditions. The micron-thickice mixtures were formed at 15 K, and then irradiated for 30 minutes at 10 G . Afterwarming, analysis in a gas chromatograph detected six amino acids, including glycine andalanine. Their laboratory irradiation corresponded to 10 yr in the interior of a densecloud (10 − G ), or 12 hours in the Orion nebula (10 G ). The net photolysis efficiencywas around 0.25% (complex organics produced per photon), and by the end of their runsthe majority of their C and N had been converted into new compounds (Bernstein et al.
13 –1995). More recent experiments by Nuevo et al. (2008, 2007) measured upwards of a dozendifferent amino acids formed from irradiation of H O, CO , and NH .The authors of all three experiments hypothesized that long exposures of cold ices toUV light could explain the multitude of organic molecules found in interstellar regions. If thesolar nebula collapsed from such an enriched region of the ISM, subsequent incorporationof these species into preserved primitive bodies such as Murchison would be a naturalconsequence of their birth environment.Although UV can lead to the synthesis of organic molecules, excessive UV flux canbe damaging to these same molecules, as evidenced by terrestrial sales of skin-protectionproducts. Bernstein et al. (2004) looked into the UV destruction of amino acids, and foundthat given enough flux they eventually formed nitriles, which are perhaps an order ofmagnitude more stable against further destruction. Similarly, Ehrenfreund et al. (2001a)found that several hundred years of illumination at 1 G destroyed most of the aminos in icematrices. Hudson et al. (2008, see also Garrod and Herbst 2006) showed that irradiationand hydrolysis of nitrile-containing ices results in amino acids being created again , solong-term irradiation may eventually result in an equilibrium between aminos and nitriles.In Orion, the flux hitting a disk varies with time as a star orbits through the cluster,changing its distance and tilt angle to the external O/B stars. Typical distances fromthe Orion core range from about 1 pc to 0.01 pc, causing the impinging flux to be in therange 10 – 10 G (Fatuzzo and Adams 2008; Throop and Bally 2008; Bally et al. G represents the flux hitting the disk skin. However, young disks are very opticallythick, so the vast majority of the ice grains are nearer the midplane and thus sheltered fromUV. Simple calculations by Throop (2000) based on the disk mass and small grains gavetypical initial optical thicknesses of close to 10 at 10 AU; thus, these individual grains gothrough a “broil-cool, broil-cool” cycle as they circulate through the disk, only occasionally 14 –being exposed to the impinging flux. Disk midplane temperatures can be as warm as 50Kout to 100 AU, so any nitriles produced during the irradiation are likely to go on to formamino acids during when they shaded but still warm (Visser et al. et al. et al.
3. Disk Model
The simple model used here is a modified version of that presented in Throop and Bally(2005). The current model is 1D and considers grain growth and external irradiation asa function of orbital distance R from a 1 M ⊙ star. The disk is azimuthally and verticallyhomogeneous, with initial parameters specified in Table 1. The initial disk mass is0.04 M ⊙ . The disk is made of gas and dust (ice), in an initial mass ratio of 100:1. Thedisk has 40 logarithmically spaced radial bins, and it is evolved for 3 × yr using aself-adjusting time-stepper coupled with a Crank-Nicolson integrator. At the end of thistime, photo-evaporation has largely removed the gas disk. Real disks might continue to beexposed to UV for several Myr, but because of rapid grain growth, most of the irradiationof small grains happens at the beginning of the simulation and the results are not stronglysensitive to the time cutoff. UV photons can create complex organic molecules or their precursors from simple ices.The effect can be roughly parametrized with a ‘photolysis efficiency’ ǫ p , where p ≈ et al. F ( R, t ) = L πd (1 − exp ( − τ ( R, t ))) (4)where the optical thickness τ ( R, t ) is τ ( R, t ) = 34 ρ d Σ d Q sca r ( R, t ) , (5)assuming particles of size r and density ρ . The surface density Σ d is that of ice grains alone,and does not change during the simulation. We assume scattering efficiency Q sca = 1, whichis appropriate for all but the very smallest grains and does not change the results.At each timestep the flux F ( R, t ) is computed, and the total number of photonsintercepted by each bin is recorded. These are then used to compute the total UV exposurein photons per molecule, Φ(
R, t ) = X t F ( R, t )Σ d m m ∆ t, (6)where m m = 18 amu is the typical molecular mass.The present model assumes a fixed external flux of 10 G . This flux is what thewell-studied Orion proplyds at 0.1 pc receive, and is picked for easy comparison withprevious results on those disks. Two additional cases look at higher and lower fluxes. Grain growth is important to the model, because once surface area is locked up inlarge grains, it cannot be exposed to UV radiation. Both observational and theoretical 16 –arguments shows that grains grow rapidly, reaching centimeter or meter sizes on timescalesas short as decades (Dominik et al. et al. et al. e.g.
Alexander and Armitage 2007, and references therein).The physical mechanisms for the early stages of grain growth are not well understood.However, a simple parametrization is useful for the present study. Throop (2000) performedsemi-analytic calculations of accretionary grain growth of a size distribution n ( r, R, t ) ofparticles. For grains up to a few cm, the grains were coupled to the gas with collisionvelocities depending on the gas eddy speeds of Mizuno et al. (1988) and Voelk et al. (1980),while for larger sizes they used particle-in-a-box collision probabilities. In both regimes theyassumed a constant sticking probability ǫ s . After integrating their distributions numerically,they combined an analytic expression for grain growth with numerically-determinedcoefficients from the eddy velocity simulations to describe a the net grain growth. In theirmodel, adopted here and derived in more details in Appendix A, the typical grain size r grows as drdt ( R, t ) = 1 . × e . p R − − p ǫ s Σ d ρ − / d t (7)for time t and orbital distance R . Σ d is the initial dust surface density at 1 AU, and ρ d isthe dust grain solid density. ǫ s is the sticking efficiency, taken to be a constant in the range0.001–1. p is the radial mass exponent, where Σ ≈ R − p . The constant and exponential aredetermined semi-analytically, and have units such that the final expression is correct for cgsinputs. For particles above one meter, processes such as gravitational instability dominateover accretionary growth, and photolysis is unimportant so growth is simply turned off.Although eq. 7 is a simple expression and considers only a single characteristic size, itbroadly agrees with more detailed calculations of grain growth in the micron-to-meter range 17 –( e.g. , Weidenschilling 1997). The gas disk evolves using the standard prescription of Pringle (1981), with α = 0 . L ⊙ is also included; the photo-evaporation model is as laid out inMatsuyama et al. (2003). The photo-evaporation model considers both EUV and FUV fluxfrom the external star as it removes the gas disk in 10 – 10 years. Flux from the disk’sown central star is not included. The nominal stellar distance is 0.1 pc (giving a flux 10 G ); additional cases consider distances of 0.01 pc and 0.5 pc. Photo-evaporation removesgas but not dust, because except at the outer edge, grains grow too quickly to be entrainedin the escaping flow of gas (Throop 2000). In the current model, photo-evaporation sets thetimescale for removal of the gas disk, but does not itself affect photolysis.Dense clusters such as Orion also are rich with outflows, stellar winds, and close stellarencounters. These effects, while easily visible in the nebula, have only minimal effect onindividual disks (Throop and Bally 2008; Allen et al.
4. Results
Simulations are presented here for our nominal case as well as six additional testcases, called
Massive , High Flux , Low Flux , Slow Grow , Delay , and
Debris . The initialconditions for each are listed in Tables 1 and 2.Results for the
Nominal case running for 3 × years are shown in Figs. 1–5. Grainsgrow steadily and reach meter sizes at the inner edge and almost 1 cm at the outer edge. As 18 –a result the opacity drops (Fig. 2), reaching unity at the inner edge in about 10,000 yearsand dropping below this throughout the entire disk by 10 yr. The surface density at theinner edge is highest, but the rapid grain growth more than compensates for this, clearingthis region earliest. At first, the production of organics proceeds at a uniform rate acrossthe disk, because all regions of the disk are optically thick and therefore intercepting everysingle photon (Fig. 3). As the inner-disk opacity drops, photons begin to pass throughthe disk without interacting. This causes organic production to stop in this region, andthis ‘production front’ slowly moves outward in the disk until organic production hasceased throughout, at about 2 × yr. Further integration beyond this point producesno more changes, because the disk’s opacity has dropped below unity and little UV flux isintercepted. Looking at the total UV exposure (Fig. 4), the greatest dose is delivered at theouter edge, which receives ∼ − . The inner region receives a muchsmaller dose of < ∼
100 photons molecule − inward of 5 AU. The outer edge’s dose is greaterbecause of both slower grain growth and lower surface density. The dose of 3000 photonsmolecule − is be sufficient to photolyze raw ices into complex organics about seven timesover. As seen in Fig 4, the peak organic density is at ∼
20 AU. Outward of this the disk’sraw material inventory drops, and inward the grain growth is too fast to allow for muchphotolysis. By the end of the run, total organic production is 4 . × g, or close to halfof the original ice mass of 1 . × g (Fig. 5). Photo-evaporation has removed the entiregas disk in 2 . × yr. Numerical results from this run and all others are in Table 2.Next shown (Figs. 6–8) are results from the Delay case. In this, the disk is the same,but the UV source comes on only after a delay of 7 × yr, allowing for some grain growthto happen before photolysis. As can be seen, the resulting organic surface density and UVexposure are about 10 × lower than that of the Nominal case.Figures 9–11 show the
Debris case, where the grains have been started at a constant 19 –size of r = 1 m. In this case the disk’s initial opacity is so low ( < .
01 at the outer edge)that very little UV is intercepted, and the organic production is about 10 × lower thanthe Nominal case. Both the
Delay and
Debris cases simulate different conditions for olderdisks: that is, disks which have evolved outside the H II region and then enter into it, orones born before the O/B stars turn on. Low-mass stars are generally believed to form overthe course of several Myr, before the first high-mass stars form, so most disks are probablysomewhere between these two and the Nominal case.In the
Slow Grow case (Figs. 12–14), the grain sticking efficiency has been reduced to ǫ s = 0 . Nominal case, andproduction would continue to increase given more time (in contrast to the
Nominal model).Results for three additional cases are shown without figures, which are qualitativelysimilar to those presented. In the
Massive run, the disk mass was increased by 10 × to0.4 M ⊙ . This decreases the net UV dose per molecule in the inner disk, but in the outerdisk where there is already more than sufficient flux in the Nominal case, the result is anet greater production of organics. In the
High Flux case, the UV flux is increased by10 × to 10 G , which is the highest that most disks typically encounter for brief periods.The effect is to increase photolysis, but by less than 10 × because the outer disk is alreadyflux-saturated. Finally, in the Low Flux case, corresponding to a disk which is 0.5 pcdistant from the cluster core, the photolysis rate is scaled downward, roughly in proportionto the reduced flux.Most of the runs share several similarities, including: a) the total raw exposure issufficient to photolyze the entire initial ice abundance of much of the disk; b) the photolysisis greatest at the outer edge; and c) the dose in a typical cluster is high enough that in 20 –10 yr the entire disk outward of 20 AU is photolyzed.The model here is simplistic in several regards. First, the laboratory yields used hereare for photolysis of virgin ice surfaces exposed for short times, and the efficiency ǫ p willdrop with time as the virgin surface becomes irradiated. More processing will also result inthe destruction of some amino acids into nitriles and gas-phase molecules (Bernstein et al. Nominal case is high but not extreme: the highest-dose region at the outer edge receives only 7 × theflux required for complete photolysis, and regions inside of the edge receive far less thanthis ( < et al. Nominal disk. Fig. 1 shows that by this time, grains in this regionremain mm-sized. In addition, grains of this size are likely to be fluffy, cracked, collisionalaggregates which expose new surface regularly. And in all cases, the rapid grain growthoccurs in the inner disk, where the models predict very little photolysis in the first place.The handling of UV deposition here is thus probably accurate enough given the model’sother uncertainties.The model here assumes the disk to be made of ice only, and not silicates. In reality 21 –the ice:silicate ratio is approximately 1:1 outward of the snow line, causing the net flux tobe roughly halved for homogeneously mixed grains. Even within individual ice grains, theformation process may concentrate some reactants toward the center and others toward theoutside, slowing photolysis further (Collings et al. et al. G . However, for both photolysis andsputtering, large bodies will probably build up a crust of silicates which will prevent furthereffects of exposure. It is difficult to estimate this effect without more detailed laboratorymodeling of heterogeneous mixtures of thick, collisionally processed ices and silicates. Theinteraction between sputtering and photolysis should be considered in future models.Our model handles the radial transport of gas but not dust. Dust transport is a poorlyunderstood yet important process ( e.g. Brownlee et al. –10 yr, somewhatshorter than the observed disk lifetimes of 1–5 Myr. The difference between these twotimescales is that our model considers just monotonic grain growth, while real disks continueto release small secondary dust grains during particle collisions. These continually producedsmall grains remain visible even as most of the mass of the disk grows into planetesimals.UV photolysis (and photo-evaporation) can therefore continue for much longer periods thanthe lower-limit timescales predicted here. The assumption of short disk lifetimes here causesour flux numbers to be conservative. 22 –
5. Discussion
The model here leads to one robust result: there was sufficient external UV flux in theyoung solar system to allow for ice photochemistry to play an important role in the solarsystem’s chemical makeup. Complex organics can be rapidly produced in flux environmentstypical of dense star clusters, especially in the outer solar system. Previous models forphotochemistry in our solar system have not considered ice photochemistry from externalUV sources. The current model describes a new pathway for production of complex organicsin the young solar system. This adds to the two existing pathways of aqueous alterationon meteoritic parent bodies and photo-synthesis in the ISM, thus relaxing the conditionsrequired in the early solar nebula. The total external UV energy available can exceed thatof all other energy sources combined.Of the simplifications made here, two are worth discussing. First, photolysis isassumed to be a one-way process, when in reality continued UV can break apart aminoacids that it creates. Continued laboratory work and improved modeling will constrainthe net production rate, allowing improvements from the upper limit yields presentedhere. Second, the interaction between photo-evaporation and photolysis is simplified.Photo-evaporation is driven by UV absorption into dust or ice grains, and if they aretiny enough the photo-evaporative outflow can entrain these grains, as seen in some largeoutflows (Balog et al. –10 yr, grains grow to meter sizes and larger where UV has 23 –no chance of penetrating to cause any more than trivial amounts of photolysis. In densestar clusters, the formation of low-mass stars is thought to continue steadily up until themoment that massive O/B stars form. Once they have turned on and the molecular cloudis ionized, star formation stops. Low-mass stars that form just prior to the O/B stars willthus end up with the largest amount of organics, while the oldest disks (up to several Myrin the case of Orion) will have much less.The results here predict that organic molecules will be abundant in young circumstellardisks, but to date only a few of these molecules have been actually detected. However,help is on the way in the form of the Stratospheric Observatory For Infrared Astronomy(SOFIA) and the Atacama Large Millimeter Array (ALMA). SOFIA, operating in the3–300 µ m range at a spectra resolution of ∼
6. Conclusions • There was sufficient flux from nearby O/B stars in the Sun’s birth environment toconvert the the majority of the ice mass in the young Solar System and other proto-planetarydisks into complex organic molecules within a few 10 yr. Even when considering the disks’high optical depth and rapid grain growth, both of which sequester surface area, enough iceremained exposed for photolysis to continue rapidly. • UV photolysis from external sources is likely to be the source of the meteoritic amino 24 –acids, which have D/H ratios difficult to reconcile with the warm, aqueous environmentsrequired from Strecker synthesis of these species. • Organic production is predicted to be substantially less for disks that have evolvedfor more than 10 –10 years before O/B stars have turned on, because the grain growth inthis time locks up surface area against photolysis. • Externally driven photochemistry is most important in the outer disk, where graingrowth is slow. The inner disk is warmer, more massive, and sees more rapid growth, andthus processes such as Fischer-Tropsch reactions may dominate the production of organicshere. • External UV flux in 1 Myr provides roughly 100 × as much flux as from the centralstar, and 10 × as much energy as gravitational collapse. This energy is easily usable forchemical reactions because it is directly incident on ice grain surfaces. • Photo-evaporation can occur at the same time as photolysis of ices, but these twoprocesses operate largely independently. Photo-evaporation removes only the smallestgrains, and photolysis can continue long after the gas disk has been removed.Dense star clusters have historically been thought of as a hazard to planet formation,because the rapid timescales of disk destruction limit the conditions under which planets canform. Our earlier results showed that under some conditions photo-evaporation may in fact speed the formation of planetesimals (Throop and Bally 2005). Our new results here addto this irony, showing that planetary systems that can form in such ‘hostile’ environmentsmay also be be among the richest in organic molecules and and the pre-cursors of life. 25 –
7. Acknowledgments
This work was supported by NASA Exobiology grant NNG05GN70G. I thank MaxBernstein, John Bally, and Randy Gladstone for useful discussions.
A. Appendix: Grain Growth
Throop (2000) and Throop et al. (2001) developed a simple model for accretionarygrain growth. The details of that model were presented only schematically, so a moredetailed derivation is given here. In this model, particle grow by pure accretion, which isoften considered to be the dominant process for the earliest phase of growth, where particlesrange from microns to ∼ r . The disk is arranged such that its full height H at radius R is given by H = R/
10 (A1)Within this disk, dust grains collide and begin to grow by simple accretion. For simplicity,we assume that the particles can be defined by a single grain size r ( R ) which is a functionof orbital distance. A particle-in-a-box collision model gives the grain growth rate as dm/dt = σ n v m ǫ s (A2)where the collision cross-section is σ = 2 πr , n is the particle number density, v is thecollision velocity, the particle mass is m = πr ρ d , and ǫ s = [0 ,
1] is the sticking efficiencyfor a single collision. Assuming vertical homogeneity and no radial transport, the particlevolume density n can be computed directly from the disk structure as 26 – n = Σ / ( H m ) (A3)and the surface density of the dust component Σ d (normalized to Σ d at 1 AU),Σ d = Σ d (cid:20) R AU (cid:21) − p . (A4)To compute the collision velocity v , Throop (2000) examined the interparticle velocitiespresent in the convecting, turbulent disk. The collision velocity depends on particle sizebecause particles are trapped in eddies of different strengths. Performing semi-analytic fitsto the work of Voelk et al. (1980) and Mizuno et al. (1988), they approximated the collisionvelocity for dust grains in a gas disk surrounding a 1 M ⊙ star as v = 10 r / (2 . − R ) p/ (A5)where all units are cgs. Combining these eqs. A1–A5, integrating, and solving for r yieldsthe result dr/dt = 1 . × e . p R − − p ǫ s Σ d ρ − / d t (A6)which can then be easily used to compute r ( t ) or m ( t ). The constant and exponentialout front are determined semi-empirically and have the proper units such that the resultis correct for cgs inputs. The expression shows a strong exponential dependence on thesurface density exponent p , but this is deceptive: p usually has only a small range of values(1–2), and the dependence is reduced by other terms so that p only has an appropriatelysmall effect on the grain growth.Throop (2000) tested this equation (Fig. 15) and found it to agree well with accretionarygrowth calculations done numerically with full size distributions, using both their own 27 –calculations and those of Mizuno et al. (1988) and Weidenschilling (1997). In the case of ǫ s = 1, it gives an upper limit to the grain size in a young disk growing by coagulation. For astandard 0.05 M ⊙ disk with gas:dust ratio of 100:1 and 0.1 µ m initial grains, eq. A6 predictsgrowth to 1 cm in 10 years and 1 m in 10 years inward of 30 AU. This rapid growth isverified by Weidenschilling (1997), which finds growth of up to several meters within 10 years at 30 AU. Because the model considers only accretionary growth, it is useful onlyfor particles up to a few meters in size, after which gravity and settling begin to becomeimportant. It also ignores radial drift, fragmentation, and possibly faster mechanismssuch as gravitational instability (Johansen et al. REFERENCES
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Parameter Value,
Nominal
Surface Density Σ ∝ R − / Initial grain size r = 0.2 µ mPhotolysis efficiency ǫ p = 0.0025Sticking efficiency ǫ s = 1.0Stellar mass M = 1 M ⊙ Initial disk mass M = 0.04 M ⊙ Disk edges R = 0.4 AU; R = 30 AUIce:Gas mass ratio 1:100UV flux F = 10 G UV start time t UV = 0 yrRun time t run = 3 10 yrTable 1: List of parameters, Nominal case. 38 –
Trial Name Changed Parameter Organic Production
Nominal – 4 . g Massive M → . M ⊙ . g High Flux F → G . g Low Flux F → . × G . g Slow Grow ǫ s → .
001 6 . g Delay t UV → . g Debris r → . gTable 2: List of parameters varied. Source Flux at 10 AU Reference
UV flux inside dense molecular cloud 10 − –10 − G Prasad and Tarafdar (1983)Present day, interstellar 1 G Habing (1968)Present day, Solar 10 G Cox (2000)Young Sun, reflected from IPM 1000 G Gladstone (1993)Central T Tauri onto flared disk 3000 G Alexander and Armitage (2007)External stars, small cluster 10 –10 G Adams et al. (2006)External stars, large cluster 10 –10 G Johnstone et al. (1998)Table 3: UV fluxes in different environments. 39 –Fig. 1.— Grain size,
Nominal . Grains grow to almost cm sizes throughout the disk within3 × yr. Growth is stopped at 5 m (black) as other growth mechanisms begin to takeover.Fig. 2.— Opacity, Nominal.
By the end, the opacity has dropped to below 1 throughoutthe disk as grains grow. In the regions of low opacity, photolysis is inhibited because thedisk intercepts little flux. 40 –Fig. 3.— Organic production,
Nominal . The integrated flux and opacity is converted into amaximum organic photolysis yield. The peak density is reached near the outer edge: inwardof this grains grow rapidly, and outward the disk density drops off. 41 –Fig. 4.— Photons per molecule,
Nominal.
This shows the net exposure of each original icemolecule in the disk to UV flux. Grains at the outer edge receive the highest flux becauseof their slow growth and the low surface density. 42 –Fig. 5.— Mass evolution,
Nominal.
As ices are converted into organics, the solid and dashedlines approach each other. The dotted line plots the mass of the gas disk, as it is dispersedby photo-evaporation and viscous evolution. 43 –Fig. 6.— Photolysis,
Delay.
In this case the disk is given a head-start of 700,000 years toevolve before the UV flux is started. Grains grow, and the total organic production is about5% of that in the
Nominal case.Fig. 7.— Photons per molecule,
Delay.
44 –Fig. 8.— Mass evolution,
Delay.
45 –Fig. 9.— Photolysis,
Debris.
Here the grains start as a uniform 1 m size, with the sameradial distribution as in the
Nominal case. Organic exposure is very low, about 10 − that ofthe Nominal model.Fig. 10.— Photons per molecule,
Debris.
46 –Fig. 11.— Mass evolution,
Debris.
47 –Fig. 12.— Photolysis,
Slow Grow.
The grain sticking coefficient is reduced from 1.0 to 10 − .Grain growth is slowed, and exposure to UV increases as a result because of long-lasting smallgrains.Fig. 13.— Photons per molecule, Slow Grow.
48 –Fig. 14.— Mass evolution,
Slow Grow.
In this case more than 50% of the original ices canbe photolyzed into organic molecules.Fig. 15.— Grain size evolution. Plot show accretionary grain growth according to eq. A6,for disk of p = 3 / M ⊙⊙