2006CBET..775....1M

U. Munari, M. Valentini, and A. Siviero, Istituto Nazionale di Astrofisica, Padova Astronomical Observatory; A. Henden, AAVSO; and P. Valisa, A. Stanzione, S. Dallaporta, G. Cherini, S. Moretti, and S. Tomaselli, ANS ("Asiago Novae and Symbiotic stars") Collaboration, report that, following the conclusion on Nov. 30 (cf. CBET 769) of the long- lasting re-brightening phase that started in July (cf. CBET 671), V2362 Cyg (= N Cyg 2006) has entered a rapid decline that has reached its fastest speed around Dec. 11.5 UT, when the decline rate had peaked at 0.7 mag/day in the B band. Since then, the decline has slowed, and the rate around Dec. 15.0 is only 0.1 mag/day in the B band. Overall, the recent decline phase has taken the nova from B = 10.46, B-V = +0.48, V-R_c = +0.62, and R_c–Ic = +0.54 on Nov. 30.2 to B = 13.75, B-V = +0.41, V-R_c = +1.16, and R_c–Ic = +0.53 on Dec. 15.73. The sudden acceleration in the photometric evolution has been parallelled by dramatic and rapid changes in the spectral appearance, as well. Echelle-mode, high-S/N, absolutely-fluxed spectra (range 350-890 nm), obtained with the Asiago 1.82-m telescope (+ AFOSC) on Dec. 14.7, show that the nova has now entered the nebular stage and that the ionization is increasing. Observed emission lines include He I 706.5-, 667.8-, 587.6-, 501.6-, and 447.1-nm; O I 777.2- and 844.6-nm; [O I] 630.0- and 636.4-nm; [O II] 732.5-nm; [O III] 436.3-, 495.9-, and 500.7-nm; C II 426.7- and 723.4-nm; N I 821.2- and 868.3-nm; N II 500.1- and 567.4-nm; [N II] 575.5-nm; N III 453.4- and 464.4-nm; Ca II 854.2-nm; and various Fe II and [Fe III] at 464.4 and 527.0 nm. The Balmer series is fully developed from H_alpha to H_9, and higher terms merge into the Balmer continuum in emission. The integrated H_beta flux is 9.5**(-12) erg/cm**2/s. Emission lines present saddle-like profiles, with a velocity separation of the blue and red peaks of 860 km/s for He I and [NII], 820 km/s for Balmer and O I, 760 km/s for [O III], and 620 km/s for [O I]. The FWHM of a Gaussian fit to the whole emission line profile is 1130 km/s for [O I], 1250 km/s for He I, 1330 km/s for the Balmer lines, 1400 km/s for [N II], and 1610 km/s for [O II].