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The truncated disk of CoKu Tau/4

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  • Physics


THE TRUNCATED DISK OF COKU TAU/4 Paola D’Alessio,1 Lee Hartmann,2 Nuria Calvet,2 Ramiro Franco-Herna´ndez,1 William J. Forrest,3 Ben Sargent,3 Elise Furlan,4 Keven Uchida,4 Joel D. Green,3 Dan M. Watson,3 Christine H. Chen,5 F. Kemper,6 G. C. Sloan,4 and Joan Najita7 Receivved 2004 Auggust 4; accepted 2004 Novvember 16 ABSTRACT We present a model of a dusty disk with an inner hole that accounts for the Spitzer Space Telescope Infrared Spectrograph observations of the low-mass pre–main-sequence star CoKu Tau/4.We have modeled the mid-infrared spectrum (between 8 and 25 �m) as arising from the inner wall of a disk. Our model disk has an evacuated inner zone of radius �10 AU, with a dusty inner ‘‘wall’’ of half-height �2 AU that is illuminated at normal incidence by the central star. The radiative equilibrium temperature decreases from the inner disk edge outward through the optically thick disk; this temperature gradient is responsible for the emission of the silicate bands at 10 and 20 �m. The observed spectrum is consistent with being produced by Fe-Mg amorphous glassy olivine and/or pyroxene, with no evidence of a crystalline component. The mid-infrared spectrum of CoKu Tau/4 is reminiscent of that of the much older star TWHya, where it has been suggested that the significant clearing of its inner disk is due to planet formation. However, no inner disk remains in CoKu Tau/4, consistent with the star being a weak-emission (nonaccreting) T Tauri star. The relative youth of CoKu Tau/4 (�1 Myr) may indicate much more rapid planet formation than typically assumed. Subject headinggs: circumstellar matter — infrared: stars — planetary systems: formation — planetary systems: protoplanetary disks — stars: pre–main-sequence Online material: color figure 1. INTRODUCTION The formation of planets around young stars is thought to involve the eventual clearing of the natal dusty protoplanetary disks, as planets accrete and sweep out material. Coagulation and accretion processes are expecte

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