chap15 - DISK DISPERSAL AROUND YOUNG STARS DAVID J....

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& * 65 1 I. INTRODUCTION . oo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o ooo o o o We review the evidence pertaining t the lifetimes f planet-f rming disks and dis- cuss p ssible disk dispersal mechanisms: (1) visc us accreti n f material nt the central s urce, (2) cl se stellar enc unters, (3) stellar winds, and (4) ph t evap ra- ti n by ultravi let radiati n. We f cus n (3) and (4) and describe the quasi-steady- state appearance and the verall ev luti n f disks under the inFuence f winds and radiati n fr m the central star and f radiati n fr m external OB stars. Visc us accreti n likely d minates disk dispersal in the inner disk ( 10 AU), whereas ph t evap rati n is the principal pr cess f disk dispersal utside f 10 AU. Disk dispersal timescales are c mpared and discussed in relati n t the retical es- timates f r planet f rmati n timescales. Ph t evap rati n may explain the large differences in the hydr gen c ntent f the giant planets in the s lar system. The c mm nly held belief that ur early sun’s stellar wind dispersed the s lar nebula is called int questi n. [401] ,, NASA-Ames Research Center Jet Propulsion Laboratory Canadian Institute for Theoretical Astrophysics M M .M M r r ( DISK DISPERSAL AROUND YOUNG STARS , , 22 2 o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o oo o o o DAVID J. HOLLENBACH HAROLD W. YORKE and DOUG JOHNSTONE The questi n f planet f rmati n is intimately related t the f rmati n and ev luti n f disks ar und c ndensing pr t stars. When c nsidering the ev luti n f dust grains thr ugh the planetesimal stage up t the creati n f (pr t )planets, it is imp rtant t realize that the disks themselves are c ntinually ev lving. Indeed, very sh rt-lived disks may n t have suf±- cient time t pr duce planets. In the standard the ry f l w-mass star f rmati n and disk ev luti n, a r tating, c llapsing m lecular c re accretes at a rate 10 –10 M yr , initially directly nt the pr t star but s n mainly nt an rbiting accreti n disk ar und the pr t star (Terebey et al. 1984). This rapid ac- creti n quickly builds up the disk mass until gravitati nal instabilities set in, when the disk mass is r ughly 0 3 , where is the mass f c d
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& 81 7 6 7 t 402 D. J. HOLLENBACH ET AL. , , , M. M . .. .M M (( ( ( , , , , , 22 oo o o o ooo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o the pr t star (Laughlin and B denheimer 1994; Y rke et al. 1995). The gravitati nal instabilities pr duce spiral density waves, which all w an- gular m mentum t be transp rted utwards. The disk material can then be transferred nt the pr t star at nearly the same rate as the disk ac- cretes material fr m the m lecular c re; alth ugh this pr cess may be quite epis dic, the disk might r ughly “h ver” at the critical gravitati n- ally unstable limit, 0 3 , as the star gr ws. The result at the end f the accreti n phase c uld be, f r example, a 1-M star with a 0 3-M disk.
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chap15 - DISK DISPERSAL AROUND YOUNG STARS DAVID J....

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