Hydroxyl maser disc and outflow in the Orion-BN/KL region

Abstract

Multi Element Radio Linked Interferometer Network (MERLIN) measurements of 1.6‐GHz hydroxyl (OH) masers associated with Orion‐Becklin–Neugebauer/Kleinmann–Low (Orion‐BN/KL) are presented, and the data are compared with data on other masers, molecular lines, compact radio continuum sources and infrared sources in the region. OH masers are detected over an area of 30 arcsec in diameter, with the majority lying along an approximately E–W structure that extends for ∼18 arcsec, encompassing the infrared sources IRc2, IRc6 and IRc7. Radial velocities range from −13 to +42 km s−1. The system of OH masers shows a velocity gradient together with non‐circular motions. The kinematics are modelled in terms of an expanding and rotating disc or torus. The rotation axis is found to be in the same direction as the molecular outflow. There is an inner cavity of radius ∼1300 au with no OH masers. The inner cavity, like the H2O ‘shell’ masers and SiO masers, is centred on radio source I. Some of the OH masers occur in velocity‐coherent strings or arcs that are longer than 5 arcsec (2250 au). One such feature, Stream A, is a linear structure at position angle ∼45°, lying between IRc2 and BN. We suggest that these masers trace shock fronts, and have appeared, like a vapour trail, 200 yr after the passage of the runaway star BN. The radio proper motions of BN, source I and source n project back to a region near the base of Stream A that is largely devoid of OH masers. The 1612‐MHz masers are kinematically distinct from the other OH masers. They are also more widely distributed and appear to be associated with the outflow as traced by H2O masers and by the 2.12‐μm emission from shocked H2. The magnetic field traced by the OH masers ranges from 1.8 to 16.3 mG, with a possible reversal. No OH masers were found associated with even the most prominent proplyds within 10 arcsec of θ1 Ori C.

Orion KL: the hot core that is not a “hot core”

“…Interstellar OH masers exist mostly in the slowly (few km s −1 ) expanding, warm (150 K), dense (10 6−7 cm −3 ) envelopes of ultracompact HII regions (Bloemhof et al 1996;Fish et al 2005), but weak ones have also been found, associated with the H 2 O outflow from the W3OH TW object (Argon et al 2003). We note that the total velocity spread of the Orion KL OH masers (≈55 km s −1 ) is about 10 times larger than that found for a typical interstellar OH maser source (Cohen et al 2006). The latter reference describes the idiosyncratic behavior of the emission in the different OH hyperfine transitions.…”

“…Molecular lines that trace colder gas, such as NH 3 (4, 4), are not present at the locations of the mid-infrared and vibrationally/torsionally excited molecular emission (Fig. 5 of Cohen et al 2006).…”

A FEATURE MOVIE OF SiO EMISSION 20-100 AU FROM THE MASSIVE YOUNG STELLAR OBJECT ORION SOURCE I

“…Assuming n = 10 10 cm −3 (Section 3.1.1) and V = 14.0 km s −1 (Section 3.3), the implied magnetic field strength for Source I is ∼0.3 G. One possible source for this field might be the original interstellar magnetic field threading the molecular cloud out of which Source I was born. Based on measurements of OH 1665 MHz masers across a ∼10 4 AU region surrounding Source I, Cohen et al (2006) derived field strengths of 1.8 to 16.3 mG. Since the OH masers are believed to arise from material with molecular hydrogen density n H 2 ∼ 7 × 10 6 cm −3 (Gray et al 1992), and magnetic field strength is expected to scale as the square root of the gas density, it is plausible that field strengths of order the value predicted by assuming equipartition might now be present within the denser, SiO-emitting gas.…”

Resolving the chemical substructure of Orion-KL

“…Two roughly perpendicular outflows are seen in Orion-KL, traced by the proper motion of H 2 O masers at 22 GHz . One is a high-velocity outflow (30-100 km s −1 , along the north-west-south-east (NW−SE) axis); this outflow is also detected in OH (hydroxyl) masers (Norris 1984;Cohen et al 2006), broad bipolar CO line wings (Zuckerman et al 1976;Kwan & Scoville 1976;Erickson et al 1982;Masson et al 1987;Chernin & Wright 1996;Beuther & Nissen 2008;Zapata et al 2009), and lobes of poorly collimated shock-excited H 2 (Beckwith et al 1978;Scoville et al 1982;Taylor et al 1984;Allen & Burton 1993;Sugai et al 1994). Although the source responsible for driving the outflow is still uncertain, it has been suggested that the outflow is only about 500−700 years old (Lee & Burton 2000;Doi et al 2002;Gómez et al 2005Gómez et al , 2008Nissen et al 2007;Beuther & Nissen 2008;Bally et al 2011;Goddi et al 2011b;Nissen et al 2012).…”