The Ionized Circumstellar Envelopes of Orion Source I and the Becklin
  Neugebauer Object
  The 229 GHz (lambda 1.3mm) 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 2
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 H30alpha recombination line is seen in its spectrum,
although the v_2=1, 5(5,0)-6(4,3) transition of H2O, 3450 K above the ground
state, is prominent. SrcI is elongated at position angle 140 degrees, 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. (2007). By contrast, the radio spectrum of
BN is consistent with p+/e- free-free emission from a dense (n_e ~ 5x10^7
cm^{-3}), but otherwise conventional, hypercompact HII region. The source is
becoming optically thin at 229 GHz, and the H30alpha recombination line, at
VLSR = 23.2 +/- 0.5 km/sec, is prominent in its spectrum. A Lyman continuum
flux of 5x10^{45} photons/sec, consistent with that expected from a B star, is
required to maintain the ionization. Supplementary 90 GHz observations were
made to measure the H41alpha and H42alpha 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.
