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Published March 1, 2013 | Published
Journal Article Open

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


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_2O, 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.

Additional Information

© 2013 American Astronomical Society. Received 2012 October 27; accepted 2013 January 15; published 2013 February 13. Support for CARMA construction was derived from the states of California, Illinois, and Maryland, the James S. McDonnell Foundation, the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the University of Chicago, the Associates of the California Institute of Technology, and the National Science Foundation. Ongoing CARMA development and operations are supported by the National Science Foundation under a cooperative agreement, and by the CARMA partner universities. Facility: CARMA

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