On the [O I] lambda 6300 Line Emission from the Photoevaporating Circumstellar Disks in the Orion Nebula

Details On the [O I] lambda 6300 Line Emission from the Photoevaporating Circumstellar Disks in the Orion Nebula On the [O I] lambda 6300 Line Emission from the Photoevaporating Circumstellar Disks in the Orion Nebula

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...502l..71s&link_type=abstract

Astrophysical Journal Letters v.502, p.L71

Astronomy and Astrophysics

Astronomy

20

Stars: Formation, Line: Formation, Ism: Clouds, Ism: Individual Name: Orion Nebula

Scientific paper

Several of the young stellar objects observed with the Hubble Space Telescope in the Orion Nebula near theta 1C Ori show disklike structures with sizes r~100 AU, similar to our own planetary system. These disklike shapes appear as dark silhouettes in [O III], [S II], [N II], H alpha , and the continuum but are seen in emission in the [O I] lambda 6300 line. We propose in this Letter that the [O I] emission is emerging from a H/H2 photodissociation front that lies close to the disk surface. The H/H2 front lies inside a photodissociation region between the disk surface and an ionization front that typically has a standoff distance of several disk radii from the disk surface. OH is produced efficiently at the warm H/H2 front by the endothermic chemical reaction {O}+{H}2-->{OH}+{H} . However, OH is also efficiently destroyed by photodissociation caused by FUV photons. Approximately 50% of the photodissociated OH produces electronically excited atomic oxygen in the upper level of the 6300 A transition, which radiatively decays as intense [O I] lambda 6300 emission. Essentially, broadband FUV photons are absorbed by OH and efficiently converted to lambda 6300 line photons. The theoretically predicted [O I] lambda 6300 emission agrees well with that observed in 182-413 (HST -10), the best-studied object with a clearly resolved disk. The H/H2 photodissociation front is close to the disk surface of 182-413, and the [O I] line, which peaks at the photodissociation front, thus traces the disk surface. The [O I] emission provides additional evidence in a number of proplyds for the existence of an extended PDR between the disk surface and the ionization front, and the penetration of OH-dissociating FUV photons from the ionization front to the disk surface.

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