Near-IR observations of the outbursting source HBC 722

ATel #2854; R. Leoni (INAF – Osservatorio Astronomico di Roma, Monte Porzio, Italy), V. M. Larionov (Astronomical Institute of St. Petersburg University, Russia) , M. Centrone, T. Giannini, D. Lorenzetti (INAF – Osservatorio Astronomico di Roma, Monte Porzio, Italy)
on 22 Sep 2010; 12:34 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

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In the framework of a near-IR monitoring program of EXor pre-Main sequence variables (Lorenzetti et al. 2007 ApJ 665, 1182; Lorenzetti et al. 2009 ApJ 693, 1056) we are carrying out at the AZT24 1m IR telescope (Campo Imperatore, Italy), we have observed the recent outburst of HBC 722 (ATel #2801) located in NGC 7000/IC 5070. Low resolution spectroscopy in the optical band (ATel #2808) and subsequent follow-up photometry in the BVRI bands (ATel #2819) have confirmed the nature of this object as to be similar to FU Orionis sources. Our observation on 2010 Sept. 01.5 UT provided the following magnitudes: J(1.25 μm) = 10.05 ± 0.02 mag, H(1.65 μm) = 9.14 ± 0.02 mag, K(2.20 μm) = 8.66 ± 0.02 mag To our knowledge, this represents the first near-IR observation of HBC 722 in outburst, therefore we cannot ascertain whether the object is still brightening or has started its IR declining phase. We can just compare the observed values with those obtained about 10 yrs ago (2MASS catalog): J = 13.25 ± 0.04 mag, H = 12.21 ± 0.04 mag, K = 11.46 ± 0.04 mag. The star brightness increment of 3.2, 3.0, and 2.8 mag, in J,H, K bands, respectively, is well compatible with that expected for an EXor event. During the same night, we obtained also a low resolution (R~250) near IR spectrum (0.8 - 2.5 μm) of HBC 722. Unfortunately, we have been forced to perform just a poorly integrated exposure. The resulting spectrum shows some HI recombination emission lines of the Brackett and Paschen series at a low S/N level (~ 2). The only exception is the Paβ emission line (λvac = 1.288 μm) for which we are able to derive a flux of 1.3 ± 0.1 10^(-13) erg s^(-1) cm^(-2) and an equivalent width of –5 Å. Such a determination is quite important since it could represent the only quantitative spectroscopic feature in the near-IR attributable to the current brightening phase. Our observations are fully compatible with episodic accretion events from the disk, as intermittent outbursts. During this events: (i) the luminosity increases; (ii) the UV radiation produced in the accretion shock region heats the inner parts of the disk itself, that in turn emits more strongly (e.g. CO and other ionic lines); (iii) to self-regulate the momentum, an intermittent mass ejection from the star occurs with a consequent increase of line emission (HI recombination) that could be originated in the wind.