We present self-consistent models of gas in optically-thick dusty disks and calculate its thermal, density and chemical structure. The models focus on an accurate treatment of the upper layers where line emission originates, and at radii $\gtrsim 0.7$ AU.

1.Introduction

Circumstellardiskshavebeenobservedaroundstarswithawiderangeofmasses,frombrowndwarfstomassiveBstars(e.g.,Schreyeretal.2006,Luhmanetal.2007).Disksfacilitateaccretionandangularmomentumtransportduringthestarformationprocessandprovideamassreservoirofgasanddustfortheformationofplanetarysystems.Planetfor-mationindisksappearstobefairlycommon,withobservedextrasolarplanetsnowtotallingover250objects(e.g.,Udryetal.2007).Thebulkoftheinitialgasanddustmassindisksaroundsolarmassstarshasa nitelifetimeoforder～1 10Myr(e.g.,Haischetal.2001,Sicilia-Aguilaretal.2006),manyordersofmagnitudesmallerthanstellarlifetimes,andthissetsanupperlimitontheformationtimeofgiantplanets.

Diskstudiesarelargelybasedonmeasurementsofthedustemission.Inprimordialdisks,althoughthedustmassismuchsmallerthanthegasmass,thedustisastrongemitter(seereviewbyWatsonetal.2007).Diskstransitionfromayoungeropticallythickduststagecharacterisedbystronginfraredcontinuumexcesses,toanopticallythinduststagewithweakinfraredemission(e.g.,Najitaetal.2007).Thecensusofopticallythickyoungdisksanddiskswithscantinfraredexcessesliketheweak-linedTTauristars(WTTS)indicatesthattheinitialopticallythickepochlasts～3Myr(e.g.,Haischetal.2001),thetransitionphaseisratherabrupt( 0.1Myr,seeHillenbrand2007),andthattheWTTSphasealsolastsfor～afewMyr.ThemissingsmalldustparticlesinWTTSand/ortransitiondisksmayhavecoalescedtobuildplanetaryobjects(e.g.,Quillenetal2004).However,observationalstudiesofdisksinferinitialmasses(～0.1M⊙or100MJ)muchgreaterthanthatofthemassofplanetsaroundstars( 10MJ),suggestingthatmuchofthegasanddustisremovedfromthediskeitherbyaccretionontothecentralstarorbydiskdispersalprocesses.

Thelimitedobservationsofgasindisksseemtobroadlyagreewiththeevolutionarysequencetracedbydustemission,butdispersaltimescalesarelessconstrainedat 10Myr(e.g.,Zuckermanetal.1995,Hollenbachetal.2005,Pascuccietal.2006).Gasgiantplanetsmustnecessarilyformonthesetimescalesbeforethegasdiskdisperses.Asgasdominatesthemassofgiantplanetsanddeterminesthedynamicsofsolidparticles(untilgasmassbecomessmallerthanparticulatemass),studiesofthediskgasdistributionandevolutionarecriticaltounderstandingbothplanetformationanddiskdestructionprocesses.

Gasdiskobservationsaredi cultbecauseoftheinherentlyweaklineemissionoftheprimaryconstituent,H2.Asaresult,lowabundancetracersaregenerallyusedtoinferthepresenceofgas.Gasemissionlineshavebeenobservedfromnumerousnearbydisks,mainlymillimeterandsub-millimeterwavelengthrotationaltransitionsof12CO,13CO,HCO+,DCO,HCN,CN,near-infraredvibrationallinesofH2andCO,andmorerecentlymid-infraredH2purerotationallines,aswellas[FeI]and[NeII]emission(e.g.,reviewbyDutreyetal.2007,