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After a post-maximum plateau Nova Del 2013 has begun a normal decline

ATel #5304; U. Munari (INAF Padova-Asiago), S. Dallaporta, G. Cherini, P. Valisa, G. Cetrulo, A. Milani, L. Ghirotto (ANS Collaboration)
on 20 Aug 2013; 22:41 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Subjects: Optical, Nova

Referred to by ATel #: 5310, 5312, 5376, 5409

The rise toward maximum of Nova Del 2013 was extremely fast. On unfiltered pre-discovery images for Aug 13.998 UT by Denisenko et al. (CBET 3628), the nova was still at 17.1 mag, close to its quiescence value (the average magnitude of the progenitor on Palomar I and II plates is B=17.3 and R=17.6 according to USNO-B1 catalog data, and V=16.9 following GSC ver 2.3.2). When the nova was discovered by K. Itakagi (CBET 3628) on Aug. 14.584 UT it was already at unfiltered 6.8 mag, a jump of 10.3 mag in 14 hours (or less). During the 30 hours following the optical discovery, the rate of rise progressively slowed, mimicking a sort of pre-halt, until the nova reached V=5.3 on Aug 15.85 when a fast and final rise toward maximum begun (at a 2.5 mag/day rate). We estimate that maximum was reached on Aug 16.25 at V=4.3. The nova immediately bounced back from maximum and initiated the decline. The decline proceeded linearly at a rate of 0.6 mag/day for about 22 hours, until Aug 17.15 when it suddenly halted and the nova remained stable at V=4.85 until Aug 19.0 UT. The nova entered this 1.85 day long V-band plateau with a blue color, B-Ic=+0.70, and left it appreciably redder at B-Ic=+1.17. Following the plateau, the nova resumed the decline, this time at a slower 0.28 mag/day rate in the V band. Our latest measurement on Aug 20.817 UT reads B=5.78, V=5.37, Ic=4.52. Spectroscopically the plateau phase was characterized by a large reduction in the intensity of emission lines, with FeII and Hbeta almost vanished, and CaII H and K lines rivalling Halpha in terms of integrated flux. At the same time the absorption spectrum strongly increased in overall intensity, up to a point when the absorption lines where so strong and blended to make impossible to recognize the level of the continuum. The end of the plateau phase has been marked by the rapid vanishing of the absorption spectrum and a large increase in intensity of the emission lines (cf. ATel #5300). Absorption lines are now mainly visible only as weaker, sharper and lower velocity features superimposed on the blue wing of the very wide emission components. Our latest spectra obtained on Aug 20.80-20.85 with the Asiago 1.82m, Varese 0.61m and Polse di Cougnes 0.70m now looks like those of a normal nova during the early optically thick phase of the decline. The strongest emission lines are those of the Balmer series, FeII, OI 7772 and 8446, and CaII far red triplet, with a FWHM of 1950 km/s for Balmer lines and 1100 for FeII.