Evolution of the spectrum and VLBI structure of W75N during the huge OH maser flare in 2003-2007

Abstract

We present results of single‐dish and very long baseline interferometry (VLBI) observations of a strong 1000‐Jy OH maser flare in the star‐forming region W75N. The flare was first seen in 2003, and persisted for at least four years. Three major spectral features were present during the whole period of observations, with N1 being the strongest initially and N2 becoming stronger later. The large velocity range of 43 km s−1 observed during the flare is not kinematic in origin, but is caused by the Zeeman splitting in the strong magnetic field generated by the central star. The maps and proper motion of the maser spots were monitored over eight epochs, between 1998 and 2006, using the European VLBI Network (EVN) supplemented by published Very Large Baseline Array (VLBA) data. Mapping of the maser spots has shown that all the new spectral features of the flare were located close to the ultracompact H ii region VLA2, while the quiescent maser features were found near VLA1. We suggest that the flare was initiated by a disturbance, probably an magnetohydrodynamic (MHD) shock launched from the massive young star, also responsible for the excitation of the ultracompact H ii region. The proper motion of the majority of the maser spots was not detected at 1665 and 1667 MHz with the upper limit of several km s−1. The only positive detection of the proper motion was made for the two brightest flare features N1 and N2. The spot N2 is moving along the filamentary spot N3, with a tangential velocity of 14 km s−1. This motion is caused by the MHD shock propagating along the filament. The measured low upper limit for the velocity of OH maser spots is consistent with models of a slowly rotating Keplerian disc rather than with outflows, jets or expanding shells.

The role of the magnetic field in the massive star-forming region W75N.

“…This value is in agreement with the value obtained studying the OH flare, which is located between the two sources. During this OH flare a magnetic field strength of 70 mG was measured [11]. Comparing the magnetic field pressure and the dynamics pressure of the gas we can conclude that the magnetic field must play a dominant role in the dynamics of the W75N star-forming region.…”

“…No CH 3 OH masers are associated to VLA 2 (e.g., [10] and references therein). The OH masers are located between VLA 1 and VLA 2 (e.g., [11]) and on a ring structure around the three radio sources [12].…”

Flares and proper motions of ground-state OH masers in W75N

“… a The 1720‐MHz masers have been put into the 1665/1667‐MHz reference frame using the absolute position of the 1720‐MHz reference feature and the coordinates of spot A in Slysh et al (2010) as described in . …”

“…The Galactic star‐forming region W75N, located at a distance of 1.3 kpc (Rygl et al 2010), is known to host bright OH masers with significant variability. Observations of the 1665‐MHz masers have detected flares above 1000 Jy, making W75N temporarily the brightest OH maser source in the sky (Alakoz et al 2005; Slysh, Alakoz & Migenes 2010). The brightest masers, found near the continuum source VLA 2 (as defined in Torrelles et al 1997), also exhibit the greatest variability.…”

Intrinsic Sizes of the W3 (OH) Masers via Short Time Scale Variability

“…Galactic OH masers have been found to be variable on a variety of time scales: from years (Caswell & Vaile 1995) and months (e.g., Slysh et al 2010) to days (Clegg & Cordes 1991). The origin of this long-time scale variability is unknown, although proposed mechanisms have included changes in the physical conditions of the maser column (e.g., Zuckerman et al 1972), sudden onset of pumping (Salem & Middleton 1978), and radiative instabilities in the maser column (Scappaticci & Watson 1992).…”