Near-IR spectroscopy of the eruptive variable source V2493 Cyg (HBC 722)

ATel #4123; D. Lorenzetti (INAF - Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy), N. Efimova, V. Larionov (Astronomical Institute of St. Petersburg University, Russia; Pulkovo Observatory, St. Petersburg, Russia), A. Arkharov, D. Gorshanov (Pulkovo Observatory, St. Petersburg, Russia), T. Giannini, S. Antoniucci, A. Di Paola (INAF - Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy)
on 23 May 2012; 14:18 UT
Credential Certification: Simone Antoniucci (antoniucci@oa-roma.inaf.it)

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As part of our near-IR spectroscopic monitoring program of EXor pre-Main sequence variables (Lorenzetti et al. 2009 ApJ 693, 1056) on-going at the AZT24 1m IR telescope (Campo Imperatore, Italy), we have recently (2012 May 11) obtained a low resolution (R~250) near-IR spectrum (0.8 - 2.5 μm) of the outbursting source V2493 Cyg (otherwise called HBC 722). Since its outburst in August 2009 (Miller et al. 2011 ApJ 730, 80; ATel #2801, ATel #2854), this source has been widely monitored (Kospal et al. 2011 A&A 527, A133, Lorenzetti et al. 2012 ApJ 749 188; Semkov et al. 2012 arXiv1205.2532S) both in optical and near-IR bands. After the outburst (of about 4 mag in V), V2493 Cyg underwent a slow decline of 1.5 mag (from Oct. 2010) with small brightness fluctuations around this level. Since October 2011, another luminosity increase occurred, so that the source has reached the same brightness level of the previous outburst, which is still maintained now. Therefore our recent near-IR spectrum refers to the highest state of the source (shown in the figure as the red spectrum in boldface together with other spectra of the same source taken in different occasions). To our knowledge, this represents the first near-IR spectrum of V2493 Cyg taken after the outburst level was reached again. It displays few HI absorption lines and second overtone (2-0) and (3-1) CO absorption features, whose fluxes are given in the Table below.

Line(μm)	EW(Å)	F(erg/s/cm²) e-13
Paβ(1.28)	4.7	-1.8±0.2
Br11(1.68)	10.3	-3.5±1.0
CO2-0(2.29)	8.1	-1.5±0.2
CO3-1(2.32)	6.3	-1.1±0.2

Near-IR lines represent a strong diagnostic tool for investigating accretion mechanisms (e.g. Antoniucci et al. 2011 A&A 534, A32); in particular, CO may appear in absorption because of two different reasons: (i) it is of photospheric origin, or (ii) it is located in the external portion of the disk whose internal heating dominates the radiation transfer. The second hypothesis appears as the most plausible since the CO bands are not stable, but change according to the fluctuations in the mass accretion rate. Debate exists on the proper classification of this source as a FUor, characterized by accretion bursts of long duration (> tens of years), or as an EXor, showing accretion events lasting one year or less with a recurrent time of months to years. Moreover, sources of the former class display spectra dominated by absorption lines, while EXor objects usually presents emission features. Given the observational scenario, a classification of V2493 Cyg as a FUor seems more appropriate, although CO overtone absorption is very commonly detected in EXor objects as well (Lorenzetti et al. 2009). Further monitoring is needed to ascertain its true nature.