The ejecta of Nova Del 2013 have turned optically thin

ATel #5533; U. Munari (INAF Padova-Asiago), S. Dallaporta, G. Cherini, G. L. Righetti, P. Valisa, A. Milani (ANS Collaboration)
on 31 Oct 2013; 15:45 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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The ejecta of Nova Del 2013 are turning increasingly optically thin, as indicated by the appearance of high ionization emission lines (e.g. HeII), the appearance and monotonic growth in intensity of nebular lines (e.g. [OIII]), and the flattening of the decline in the optical light-curve.

We have obtained low resolution fluxed spectra of Nova Del 2013 with Asiago 1.22m telescope + B&C spectrograph (dispersion 2.3 Ang/pix, range 3200-7700 Ang) and Varese 0.61m telescope, equipped with the Multi Mode Spectrograph (dispersion 2.1 Ang/pix, range 3900-8600 Ang). The following Table lists the observed [OIII] 5007 / Hbeta ratio for some observing dates:

The table shows how the rise in intensity of the nebular lines has been smooth and gradual, with the first appearance of [OIII] occurring around October 1st, at the same time when HeII 4686 started to become visible in emission. Several of the emission lines currently shows a double peaked profile, with a velocity separation of 1200 km/s, and the bluer peak much stronger than the red one.

We are monitoring the BVRI photometric evolution of Nova Del 2013 with several telescopes operated by ANS Collaboration (identifiers 011, 030, 032 and 157). The smooth decline the nova was following prior to the appearance of nebular lines was characterized by a decline rate of 0.12 mag/day in the V band. This rate has progressively reduced in parallel with the increase in intensity of the nebular emission lines. The average decline over the last week has been a mere 0.015 mag/day in the V band.