Nova Oph 2015 is a FeII and not a He/N nova
ATel #7367; U. Munari, P. Ochner (INAF Padova), A. Siviero (Univ. Padova), S. Dallaporta and P. Valisa (ANS Collaboration)
on 11 Apr 2015; 10:55 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
Nova Oph 2015 was discovered as PNV J17291350-1846120 by Y. Sakurai (CBET #4086)
on March 29.766 UT at unfiltered 12.2 mag. M. Fujii (CBET #4086) on March 30.7 UT
and K. Ayani (CBET #4086) on March 30.8 UT observed
spectroscopically the transient and, upon finding several HeI, NII and NIII
lines in emission, classified it as an He/N-type nova, at a time when the
brightness of the nova was V=12.2 and still on the rise toward maximum, as
evinced from the various sparse measurements listed in CBET #4086. Danilet et
al. (Atel #7339) obtained a low-res spectrum of Nova Oph 2015 on April
1.459 UT when it was at V=12.0, and they too classified it as an He/N-type
nova, noting however that the small FWHM and sharply peaked profile of the
emission lines were unusual for He/N-type novae.
We are intensively monitoring the evolution of Nova Oph 2015 since its
discovery, obtaining UBVRI photometry and Echelle spectroscopy
with ANS Collaboration telescopes, low resolution spectra with the Asiago
1.22m telescope and high resolution Echelle spectra with the Asiago 1.82m
telescope. Our photometric observations show that the He/N-type
classifications reported in CBET #4086 and ATel #7339 where obtained before
Nova Oph 2015 passed through the pre-maximum halt.
The nova is only now passing through
maximum brightness, at the usual ~2 mag brighter than pre-maximum halt.
Our latest measurement for April 10.112 UT gives V=9.942, B-V=0.913
V-R=0.580 V-I=1.154. Our latest spectrum of Nova Oph 2015 (3300-7900 Ang,
2.31 Ang/pix) was obtained with the Asiago 1.22m+B&C+CCD spectrograph on Apr
11.094 UT and it is a textbook example of a FeII-type nova at maximum
brightness. It is dominated by a reddened strong continuum, with intense
emission lines from Hydrogen Balmer, FeII (multiplets 27, 28, 37, 38, 42,
48, 49, 73, 74), CaII H&K and OI 7772 Ang, all showing deep P-Cyg
absorptions (blue shifted by various amounts, from the -850 km/s of FeII-42,
to 1000 km/s of OI, to 1165 km/s of Halpha). The integrated absolute flux of
the Halpha emission component is 3.3x10(-11) erg cm-2 s-1, and its FWHM
is 1050 km/s, a normal value for a FeII-type nova.
During their rise in brightness, the expanding photospheres of FeII-type
novae cool from initial extremely high temperatures to the ~8000 K reached at the time of
maximum, and in doing this they briefly pass through values
characteristic of hotter He/N-type spectra (Seitter 1990, LNP 369, 79). Thus the He/N-type
classification for Nova Oph 2015 reported in CBET #4086 and ATel #7339
seems to reflect the fact that at those times the nova was at least
still 2 mag away from reaching true maximum brightness. The application of FeII or
He/N classification introduced by Williams (1992, AJ 104, 725) is restricted
to only maximum and early decline from it.
