EDP Sciences
Free Access
Issue
A&A
Volume 600, April 2017
Article Number L8
Number of page(s) 5
Section Letters
DOI https://doi.org/10.1051/0004-6361/201730659
Published online 03 April 2017

© ESO, 2017

1. Introduction

The bright 51−60 A+ CH3OH maser transition at 6668.5 MHz has been recognized for a long time to be a selective signpost for recently formed luminous young stars (Lbol> 5 × 103L). Its excitation requires radiative pumping by 20–30 μm photons through the second torsionally excited state (Sobolev et al. 1997). Intense IR radiation typically permeates the warm dusty environment surrounding massive young stellar objects (YSO). Although the emission of most 6.7 GHz maser sources is stable on timescales of (at least) several years (Sanna et al. 2010; Moscadelli et al. 2011), in some cases, strong flares of up to several 100 Jy have been recorded. In addition, in some sources, selected maser components show a well-defined periodicity (e.g., Goedhart et al. 2004). These variations can be naturally related to an occasional and/or periodic change of the background radiation (amplified by the maser) or the IR pump field.

Recently, we have reported on the first ever detected accretion burst from a massive young star (NIRS 3) in the star-forming region S255 (Caratti O Garatti et al. 2016). This detection was triggered by the serendipitous discovery of a CH3OH maser flare toward S255 NIRS 3 by Fujisawa et al. (2015) in 2015 November. Subsequent subarcsecond near-IR (NIR) observations showed that the K- and H-band fluxes of this source had increased with respect to the pre-burst level by 2.9 and 3.5 mag, respectively, suggesting a relationship with the maser flare (Stecklum et al. 2016). Additional IR observations proved that the integrated luminosity from NIR to millimeter wavelengths had grown from 2.9 × 104  to   1.6 × 105L (Caratti O Garatti et al. 2016).

This letter reports on observations made with the European VLBI Network (EVN) and the Jansky Very Large Array (JVLA) of the 6.7 GHz CH3OH maser emission in S255 NIRS 3  (hereafter NIRS 3) at the time of the outburst. We compare the new observations with previous interferometric and single-dish data, and discuss the change in the maser spatial distribution, structure, and flux.

2. Observations and data analysis

2.1. EVN 6.7 GHz CH3OH maser

We observed the 6.7 GHz CH3OH maser emission toward NIRS 3 with the EVN1 as a Target of Opportunity program on 2016 April 12 (code: RS002). We also reduced archival EVN observations obtained before the flare on 2004 November 6 (code: EL032). Both observations were conducted in phase-referencing mode by fast switching between the maser target (at a Doppler velocity of 5 km s-1) and a strong (C-band flux >0.1 Jy) reference position calibrator, J0613+1708 (for exp. EL032) and J0603+1742 (for exp. RS002). Left and right circular polarizations were observed with two (EL032) and eight (RS002) baseband converters (BBC), each BBC being 2 MHz wide. The EL032 and RS002 experiments were processed at the correlator of the Joint Institute for VLBI in Europe (JIVE) using an averaging time of 0.5 s and 2 s, respectively. Data were analyzed with the NRAO2 Astronomical Image Processing System (AIPS) following the VLBI spectral line procedures. Absolute positions of the CH3OH maser spots are registered with an accuracy of about ± 1 mas at each epoch. Additional information on the EVN observations is summarized in Table 1.

Table 1

EVN and JVLA 6.7 GHz CH3OH maser observations.

2.2. JVLA continuum and 6.7 GHz CH3OH maser

We observed the C-band (4–8 GHz) continuum and 6.7 GHz CH3OH maser emission in NIRS 3 with the JVLA of the NRAO using both the B- and A-Array configurations (code: 16A-424). Table Wly. e3NRAes o87ly. e3NRAes o8egne"> Wlyed hangah",o8egne"8201 angervations of 6phase, e> ssio4d (4an>> a> 1wo (Er These v>OH m1664d sinnel">&3C48s mat>30.1̴an>smass Yngax gocesn>0.1̴yerau2 anJ0534n>1708 (for exp. EL032) and J0603927llari177hasmass r These vmple-mation from a few milliarcseconds to OH maserCommer a_pressrelSoftw> OH maser

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