The peculiar circumstellar environment of NGC 2024 IRS2

¹ Astronomical Institute “Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 3-72, 58090 Morelia, Mich. México
³ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
⁴ Department of Physics and Astronomy, The University of Western Ontario, London, Ontario, Canada N6A 3K7
⁵ Gemini Observatory, 670 N. A'ohoku Place, Hilo, HI 96720, USA

Corresponding author: A. Lenorzer, lenorzer@science.uva.nl

Received: 20 March 2003
Accepted: 2 October 2003

Abstract

We re-examine the nature of NGC 2024 IRS2 in light of the recent discovery of the late O-type star, IRS2b, located 5´´ from IRS2. Using L-band spectroscopy, we set a lower limit of A_V = 27.0 mag on the visual extinction towards IRS2. Arguments based on the nature of the circumstellar material, favor an A_V of 31.5 mag. IRS2 is associated with the UCHii region G206.543–16.347 and the infrared source IRAS 05393–0156. We show that much of the mid-infrared emission towards IRS2, as well as the far infrared emission peaking at ~100 , do not originate in the direct surroundings of IRS2, but instead from an extended molecular cloud. Using new K-, L- and L'-band spectroscopy and a comprehensive set of infrared and radio continuum measurements from the literature, we apply diagnostics based on the radio slope, the strength of the infrared hydrogen recombination lines, and the presence of CO band-heads to constrain the nature and spatial distribution of the circumstellar material of IRS2. Using simple gaseous and/or dust models of prescribed geometry, we find strong indications that the infrared flux originating in the circumstellar material of IRS2 is dominated by emission from a dense gaseous disk with a radius of about 0.6 AU. At radio wavelengths the flux density distribution is best described by a stellar wind recombining at a radius of about 100 AU. Although NGC 2024 IRS2 shares many similarities with BN-like objects, we do not find evidence for the presence of a dust shell surrounding this object. Therefore, IRS2 is likely more evolved.