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Toward a new bright state of the symbiotic nova AS 338 after those of 1983, 1995 and 2010 ?

ATel #11079; U. Munari (INAF Padova), S. Dallaporta, G. L. Righetti, P. Valisa (ANS Collaboration)
on 21 Dec 2017; 15:02 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Subjects: Optical, Cataclysmic Variable, Nova

The symbiotic nova AS 338 (=V1413 Aql) has gone into outburst in 1981 (Schulte-Ladbeck 1985, Msngr 39, 3), and has not yet returned to quiescence after 36 years. The preceding quiescence was characterized by a mean B=15.7 mag brightness, with a large amplitude modulation caused by irradiation of the M5III giant by the WD companion in a 434 day orbit (Munari 1992, A&A 257, 163). The quiescence spectrum was dominated by strong emission lines betraying high ionization conditions, with HeII 4686 at half the intensity of Hbeta, [FeVII], [NeV] and Raman scattered OVI at 6825 Ang (Allen 1984, PASAu 5, 369). In outburst the spectrum of AS 338 lost and never regained the higher ionization emission lines, with a bright hot continuum dominating optical spectra. The light-curve changed from one dominated by irradiation to deep eclipses of the outbursting WD, while preserving the 434.3 day period and its phasing.

The mean brightness level of AS 338 during the outburst is slowly varying, in a way possibly resembling the long term behavior of MWC 560 (=V694 Mon; Munari et al. 2016, NewA 49, 43). After the initial peak reached at V~10.5 in 1983, which was followed by an extremely tedious decline to V~13.2 by 1992, the nova underwent two large re-brightening in 1995 (reaching V~11.0) and 2010 (at V~11.5), declining to a minimum V~13.7 in between. A rise toward a new maximum seems now underway.

We are keeping AS 338 under strict monitoring since 2005, collecting BVRI photometry with various ANS Collaboration telescopes and low- and high-res spectra with the Asiago 1.22m and Varese 0.61m telescopes. After an unusual profile for the last eclipse (centered on 2017 July 7 when we measured B=15.274, V=14.682, R=12.952, I=11.668), the star levelled off and it was measured at B=14.453, V=13.743, R=12.381, I=11.270 on 2017 Dec 6.7, after which it begun a steep rise in brightness bringing it to B=13.175 on Dec 16.7 and B=12.645 V=11.863, R=11.080, I=10.287 on Dec 19.7 (UT), for a mean rise in the B band of 0.18 mag/day. Even larger are the associated spectral changes. High emission lines (in particular HeII 4686 Ang), which have been absent or at most very weak throughout the 36 years of the outburst, including the bright phases of 1995 and 2010, were barely traceable also on our spectrum for 2017 Nov 30, taken just a few days before the onset of the current steep rise in brightness. A new spectrum taken on 2017 Dec 20.7, show now instead a bright 4640 NIII complex and a prominent HeII 4686, about half the intensity of Hbeta, plus stronger than ever HeI lines.

This symbiotic nova is obviously worth a strict monitoring to trace if the current steep rise in brightness will mature into a full flagged third bright phase, following those of 1995 and 2010, and if the spectral evolution will confirm the "hot" nature of the present event compared to the "cool" type of the previous two, an alternance already seen in AG Dra.