VES263: an intermediate mass YSO candidate as a new eruptive variable

ATel #11658; T. Giannini, S. Antoniucci, D. Lorenzetti (INAF - Osservatorio Astronomico di Roma, Monte Porzio, Italy), A. Harutyunyan (Telescopio Nazionale Galileo, La Palma, Spain), D. Licchelli (Universita' del Salento, Lecce, Italy), U. Munari (INAF - Osservatorio Astronomico di Padova, Padova, Italy)
on 21 May 2018; 15:31 UT
Credential Certification: Teresa Giannini (teresa.giannini@oa-roma.inaf.it)

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In the framework of our EXor monitoring programme dubbed EXORCISM (EXOR OptiCal and Infrared Systematic Monitoring - Antoniucci et al. 2013 PPVI, Lorenzetti et al. 2007 ApJ 665, 1182; Lorenzetti et al. 2009 ApJ 693, 1056), we observed the object VES263 signalled by a recent GAIA alert (http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia18azl) as "an emission line star undergoing repeat outbursts" having detected an alerting magnitude of 11.66 wrt a historic magnitude of 12.16. VES263 is located in the Cyg OB2 association [R.A.(J2000) = 20:31:48.84 DEC(J2000) = 40:37:59.9], embedded in a region strongly affected by dust obscuration. It is an entry of both the Hα line emission catalog (SpT B1II - Kohoutek & Wehmeyer 1999 AASupp.Ser, 134, 255), and the Cyg OB2 imaging survey (B=14.18 - Comeron & Pasquali 2012 AA,543A, 101). VES263 is identified as 2MASS 203148.845 +403800.10 (J=9.551, H=9.110, K=8.895) and WISE J203148.84+403759.9 (3.4μm=8.581, 4.6μm=8.496, 12.0μm=8.575, 22.0μm>8.148).

Our photometry has been obtained on 2016 April 26 with the DOLORES@TNG Italian Telescope at La Palma (Canary Islands): B=15.09, V=12.96, R=11.69 I=10.22. Fluctuations of about 1 mag wrt previous determinations are clearly recognizable, as also shown by the Spectral Energy Distribution , in which black points are the observational data (bands from B to WISE12 μm), while red points represent a B1 main sequence star (normalized to the V-band). On the same date we obtained a low-resolution optical spectrum, as well. As expected, Hα is seen in emission, while the other HI lines as well as NaI are in absorption. Hα integrated flux 2E-13 erg/cm-2/s) provides a value for the mass accretion rate of (6.0 ± 1)E-8 M_sun/yr, by applying the relationship given by Gullbring et al. 1998, ApJ 492, 323 and adopting M=1M_sun and R=3 R_sun for the stellar mass and radius, respectively. Notably, the accretion rate linearly decreases with the increasing stellar ratio (mass/radius). According to the GAIA DR2, we obtained a distance value of 1680 pc.

The observational results (SpT=B1, Hα emission, obscuration by dust within a star forming region, presence of IR excess) all contribute to speculate VES263 is a young Herbig Be star, although it does not look like a true emission lines star. Further monitoring (at optical and IR frequencies) both photometric and spectroscopic, is in order to ascertain the real nature of VES263. In particular, whether or not its intermittent brightness is driven by a variable mass accretion rate.