Optical Spectroscopy of Nova Ophiuchi 2015 (PNV J17291350-1846120)

ATel #7339; A. B. Danilet and T. W.-S. Holoien (Ohio State), R. M. Wagner (LBTO/OSU), C. E. Woodward (Minnesota), S. Starrfield (ASU), A. Wilber (ASU), F. Walter (SUNY), S. Shore (U. Pisa and INFN-Pisa)
on 2 Apr 2015; 17:21 UT
Credential Certification: R. Mark Wagner (rmw@as.arizona.edu)

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Following the discovery by Y. Sakurai (Ibaraki-ken, Japan) on 2015 Mar. 29.766 UT of a new stellar object of magnitude 12.2 in Ophiuchus (S. Nakano, CBET 4086) and its subsequent confirmation as a likely He/N classical nova (K Ayani, CBET 4086), we obtained a spectrum (range: 398-685 nm; resolution 0.3 nm) of Nova Oph 2015 on 2015 April 1.459 UT with the 2.4 m Hiltner telescope (+OSMOS) of the MDM Observatory on Kitt Peak.

In agreement with the spectroscopic results described by Ayani, our spectrum shows strong emission lines superposed on a flat continuum of the Balmer series of hydrogen (through Hδ with our spectral range), He I (443.8, 447.1, 471.3, 492.2, 501.5, 587.5, and 667.8 nm), C II (426.7 nm), Fe II (423.3, 430.3 nm and others), N I (648.6 nm), N II (500.1, 547.9, 567.9, and 593.8 nm), N III (451.7 and 463.8 nm), and Si II (634.7 and 637.1 nm). The Balmer and He I lines exhibit prominent P Cygni profiles with terminal velocities of about 2500 km/s. Hα, He I 587.5, and 667.8 nm lines show a detached absorption component. However, He I 447.1 nm shows no absorption component. The FWHM of Hα and Hβ emission is about 1000 km/s, while the FWZI of Hα emission is about 4000 km/s. We measure a Hβ emission equivalent width of 0.34 nm and an absorption equivalent width of about 0.3 nm. The line profiles show evidence of multiple components; however, the emission lines are sharply peaked as opposed to rectangular in profile.

Examination of the POSS2/UKSTU red images from the DSS shows at least 2-3 possible progenitors of Nova Oph 2015 with red magnitudes of about 18-19 near the outburst positions reported by Nakano and others (CBET 4086). The nearest of these candidates to the outburst positions is located at α = 17:29:13.47; δ = -18:46:14.50 (J2000). If so, the outburst amplitude was at least ~7 magnitudes.

Our spectrum of Nova Oph 2015 is reminiscent of the He/N class of classical novae in agreement with Ayani (CBET 4086). However, the relatively lower velocities and sharply-peaked line profiles characterizing the outburst of Nova Oph 2015 are not consistent with that normally seen in He/N novae which generally have broad lines with rectangular line profiles and generally few absorption features. Nova Oph 2015 may be similar to a small fraction of classical novae that are "hybrid" in that they show characteristics of both the Fe II and the He/N classes (cf., Williams 2012, AJ, 144, 98) and as they evolve, transition from one class to the other. A recent example includes V5558 Sgr (Tanaka et al. 2011, PASJ, 63, 911 and references therein). The evolution of Nova Oph 2015 is likely to be unusual so additional observations are requested to monitor its behavior.