COSMIC ORIGINS SPECTROGRAPH OBSERVATIONS OF NGC 2024: AN INDIRECT VIEW INTO THE HEART OF THE FLAME

As part of a program to study translucent interstellar clouds, NGC 2024 IRS 1 was observed by the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope. IRS 1 is a heavily reddened B0.5 V star (E(B – V) = 1.69, RV = 4.5, and AV = 7.61) lying outside the core of the NGC 2024 cluster and not considered to be the ionizing source of the nebula. At wavelengths below about 1300 A, the observed spectrum deviates from that expected of a reddened B star, showing an increase in flux to shorter wavelengths and the presence of P-Cygni profiles in both the C III] λ1176 and N V λ1240 lines. Because of the presence of these lines, the absence of any P-Cygni signature in the Si IV λ1394/1403 lines, and the measurement of extended emission on the detector focal plane, we believe this portion of the spectrum to be scattered light from an O6 to B0 supergiant. The COS entrance aperture is 2.5 arcsec in diameter and can let in substantial scattered light from the nebula. NGC 2024 IRS 2b is an O8 star and believed to be the ionizing source of the nebula. It is currently identified as a main-sequence star, but should it be a supergiant the presence of such an evolved star implies an age for the cluster of the order of 3 Myr, in conflict with age determinations based on infrared spectroscopy, and may have implications for the energetics and density structure of the nebula.