Spectroscopic observations of the first helium nova V445 Puppis

¹ Astronomical Observatory of Padova, Asiago Section, Osservatorio Astrofisico, 36012 Asiago (Vi), Italy e-mail: takashi.iijima@oapd.inaf.it
² Nobeyama Radio Observatory, 384-1305 Minamimaki, Minamisaku (Nagano), Japan e-mail: hnakanis@nro.nao.ac.jp

Received: 19 March 2007
Accepted: 15 January 2008

Abstract

At Asiago Astrophysical Observatory, we observed the peculiar, explosive object Nova (V445) Puppis 2000 during its early decline stage between January and April 2001. We acquired both high and medium dispersion spectra for the optical wavelengths  Å. The most remarkable properties of the spectra were an absence of hydrogen lines and unusually strong carbon-ion, emission lines. We identified the most prominent emission line in the optical region at 6585 Å  to be due to C II 2, 6578 and 6583 Å. The second strongest emission line at 5900 Å  may be due to Na I D1 and D2, blended by C II 5, 5900 and 5902 Å. Prominent emission lines of Fe II, Ti II, Cr II, Si II, Mg II, [Fe II], [Ti II], C I, C II, C III, Ca I, He I, and probably of O II, N II, N III, and S II were in addition detected. Some emission lines of Fe II, Ti II, Cr II, Na I, and Mg II were accompanied by P Cygni type absorption components blue-shifted by about -500 km s^-1. Our first spectrum obtained on January 14, 2001 showed an absorption band of CN (cyanogen) molecule at 4215 Å. The overall spectral features resembled those of classical slow novae, apart from the absence of hydrogen lines. The appearance of some [Ti II] lines, and a rather high intensity of the emission line of Mg II 23, 6545.8 Å, were additional peculiar spectral features of this object. This object may be located in or beyond the Orion arm, because two sets of interstellar absorption components of Na I D1 and D2 were detected. Their radial velocities in the local standard of rest are +16.0 ± 0.4 km s^-1  and +73.5 ± 1.0 km s^-1. The radial velocity of this object is estimated to be +224 ± 8 km s^-1, which suggests that this object belongs to the old disk population. The distance and the interstellar extinction to the object are estimated to be 3.5 ≤ d ≤ 6.5 kpc and  mag, respectively. The derived absolute magnitude at light maximum is -5.8 ≤ M_V ≤ -7.1 mag. Our results support the classification of this object as a helium nova.