Title: The methanol maser flare of S255IR and an outburst from the high-mass YSO S255IR-NIRS3 - more than a coincidence?

ATel #8732; Bringfried Stecklum (TLS Tautenburg), ), Alessio Caratti o Garatti (DIAS Dublin), Maria Concepcion Cardenas (IAA-CSIC Granada, MPIA Heidelberg), Jochen Greiner, Thomas Kruehler (MPE Garching), Sylvio Klose, Jochen Eisloeffel (TLS Tautenburg)
on 25 Feb 2016; 11:25 UT
Credential Certification: Bringfried Stecklum (stecklum@tls-tautenburg.de)

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Fujisawa et al. (ATel # 8286) reported on a flare of the 6.7 GHz class II methanol maser in S255IR, the first one ever recorded from this maser since its discovery by Menten (1991 ApJ, 380, L75). From 2015 July to October the maser flux density rose by a factor of about ten. Since IR radiation from heated dust is thought to be the pumping mechanism of this maser transition (Sobolev & Deguchi 1994, A&A, 291, 569), we performed near-infrared (NIR) imaging of the embedded cluster to identify an object which brightened and warmed up its surrounding dust. To this aim PANIC (Baumeister et al. 2008, Proc. SPIE 7014, 70142) at the Calar Alto 2.2-m telescope as well as GROND (Greiner et al. 2008, PASP 120, 405) at the La Silla 2.2-m telescope were used. PANIC imaging was performed on 2015 Nov 28 (Ks), 2016 Jan 18 (H, Ks) as well as Feb 23 (H, Ks). GROND JHK imaging started on 2016 Feb 18.

A comparison of the first PANIC Ks-band image with the UKIDSS K-band image (epoch 2009 Dec 08) shows a brightening of the point source located at 06h 12m 54.020s, +17d 59' 23.07", known as S255IR-NIRS3. It is a high-mass YSO (HMSYO) with a stellar mass of ~20 M_sun (Simpson et al. 2009, ApJ 700, 1488; Ojha et al. 2011, ApJ 738, 156), driving a bipolar outflow (Wang et al. 2011, A&A, 527, A32; Zinchenko et al. 2015, ApJ 810, 10). Mid-infrared interferometry with MIDI at the ESO VLTI points to the presence of a circumstellar disk seen close to edge-on (Boley et al. 2013, A&A 558, A24). Since S255IR-NIRS3 is within 0.2" of the VLBI position of the 6.7 GHz maser (Minier et al. 2001, A&A, 369, 278), a physical association between the maser flare and the NIR brightening seems plausible. Surprisingly, the GROND imaging led to the first ever detection of this source in the J band.

The UKIDSS K-band image and the first two PANIC Ks-band images, convolved to the same resolution, are shown at
ftp://ftp.tls-tautenburg.de/pub/stecklum/comparison.png
Here, a square-root intensity stretch was applied to enhance the contrast.

Aperture photometry was performed based on zero points derived from 2MASS field stars. The first PANIC Ks-band magnitude amounts to 8.90+/-0.05 mag. Taking the (H-K) color into account, the UKIDSS K-band magnitude corresponds to Ks=10.90+/-0.01 mag. This implies a brightness increase by ~2 mag in the Ks band. The first PANIC H-band magnitude amounts to 14.6+/-0.4 mag. Although UKIDSS does not list an H-band magnitude, the source is detected at 17.1+/-0.1 mag. This implies a brightness increase by ~2.5 mag. The associated reflection nebula which emerges from the outflow lobes brightened considerably as well. Its relative brightness variation compared to UKIDSS hints at spatial changes of the optical depth over recent years.

Further photometric monitoring and spectroscopy will show whether an FU Ori-type outburst from the HMYSO is ongoing which would be the first of its kind. The recent dimming in the K band seen in the latest PANIC image might indicate extinction changes due to disk scale height or accretion column variations.