The Ionized Circumstellar Envelopes of Orion Source I and the Becklin-Neugebauer Object

Abstract

The 229 GHz (λ1.3 mm) radio emission from Orion-KL was mapped with up to 0. 14 angular resolution with CARMA, allowing measurements of the flux densities of Source I ("SrcI") and the Becklin-Neugebauer Object (BN), the two most massive stars in this region. We find integrated flux densities of 310 ± 45 mJy for SrcI and 240 ± 35 mJy for BN. SrcI is optically thick even at 229 GHz. No trace of the H30α recombination line is seen in its spectrum, although the v 2 = 1, 5(5,0)-6(4,3) transition of H 2 O, 3450 K above the ground state, is prominent. SrcI is elongated at position angle 140 • , as in 43 GHz images. These results are most easily reconciled with models in which the radio emission from SrcI arises via the H − free-free opacity in a T < 4500 K disk, as considered by Reid et al. By contrast, the radio spectrum of BN is consistent with p + /e − free-free emission from a dense (n e ∼ 5 × 10 7 cm −3 ), but otherwise conventional, hypercompact H ii region. The source is becoming optically thin at 229 GHz, and the H30α recombination line, at V LSR = 23.2 ± 0.5 km s −1 , is prominent in its spectrum. A Lyman continuum flux of 5 × 10 45 photons s −1 , consistent with that expected from a B star, is required to maintain the ionization. Supplementary 90 GHz observations were made to measure the H41α and H42α recombination lines toward BN. Published 43 and 86 GHz data suggest that SrcI brightened with respect to BN over the 15 year period from 1994 to 2009.

ALMA OBSERVATIONS OF ORION SOURCE I AT 350 AND 660 GHz

“…SrcI's flux density is proportional to ν 2 from 43 GHz to 229 GHz, suggesting that the emission is optically thick over this frequency range, but the brightness temperature calculated from the high resolution images is 1500 K, much lower than expected for an HII region. Although these results do not rule out the possibility that SrcI is an exceptionally dense HII region that is not fully resolved by the observations, Plambeck et al (2013) concluded that the continuum most likely originated from electron-neutral free-free emission (the H − opacity), as proposed originally by Reid et al (2007). The H − opacity is important at temperatures below 4500 K where electrons, produced plambeck@berkeley.edu by ionization of Na, K, and other metals, scatter off neutral hydrogen atoms or molecules.…”

“…Omitting all the short spacing data greatly reduces the magnitudes of these ripples. (Plambeck et al 2013). Red circles indicate radii of 100, 250, and 850 kλ.…”

“…Testi et al (2010) infer that the NIR emission is produced in a disk around a ∼10 M star, accreting at a very high rate of a few × 10 −3 M yr −1 . The radio continuum from SrcI has been imaged with the VLA at 43 GHz Goddi et al 2011) and with CARMA at 229 GHz (Plambeck et al 2013). The continuum source is elongated, consistent with a 100 AU diameter disk viewed nearly edge-on.…”

A Vla Survey for Faint Compact Radio Sources in the Orion Nebula Cluster

“…Sources 279 and 308 each have radio spectra that are steeper than n 0.6 . Source 279 is the Becklin-Neugebauer Object, and is thought to be a runaway B star, ejected from a system with Source I (our Source 308) in an explosive event 500 years ago (e.g., Plambeck et al 1995;Gómez et al 2008;Plambeck et al 2013). The BN Object has a spectral dependence of n 1.3 below 100 GHz, above which it flattens, and is suggested to be freefree emission from a dense, hypercompact H II region.…”

“…The BN Object has a spectral dependence of n 1.3 below 100 GHz, above which it flattens, and is suggested to be freefree emission from a dense, hypercompact H II region. Source I has a spectral dependence of n 2 and is most easily explained by H − free-free emission in a disk (e.g., Plambeck et al 2013). Both sources have massive stars driving ionizing circumstellar material and driving the free-free emission we detect, explaining their significant fluxes despite their distance from q 1 Ori C.…”