The 229 GHz ( \lambda 1.3mm ) radio emission from Orion-KL was mapped with up to 0.14 ^ { \prime \prime } 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 \pm 45 mJy for SrcI and 240 \pm 35 mJy for BN .
SrcI is optically thick even at 229 GHz .
No trace of the H30 \alpha 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 ^ { \circ } , 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 .
( 35 ) .
By contrast , the radio spectrum of BN is consistent with p ^ { + } /e ^ { - } free-free emission from a dense ( n _ { e } \sim 5 \times 10 ^ { 7 } cm ^ { -3 } ) , but otherwise conventional , hypercompact HII region .
The source is becoming optically thin at 229 GHz , and the H30 \alpha recombination line , at V _ { LSR } = 23.2 \pm 0.5 km s ^ { -1 } , is prominent in its spectrum .
A Lyman continuum flux of 5 \times 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 H41 \alpha and H42 \alpha 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 .