Strong UV Neon lines in the spectrum of Nova Herculis 2021

ATel #14805; P. Ochner, V. Andreoli, A. Farina (UniPd), A. Reguitti (UNAB)
on 23 Jul 2021; 08:55 UT
Credential Certification: Andrea Reguitti (andreareguitti@gmail.com)

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We report additional spectroscopic follow-up observations of Nova Herculis 2021 (V1674 Her, TCP J18573095+1653396). We obtained a low-resolution spectrum with Asiago 1.22-m 'Galileo' telescope + B&C spectrograph (330-780 nm, resolution 0.7 nm) on 20 July 2021.

The spectrum shows extremely intense emission lines from [Ne V] 342.6 nm and 334.6 nm. We measured the fluxes from the [Ne V] 342.6 nm and Halpha lines, and calculated a ratio of about 4.5. The [Ne V] 334.6 line is also prominent, with a flux that is nearly double that of Halpha. We carefully inspected the sensitivity function and the data reduction steps to ensure the flux calibration in the UV part of the spectrum was correct. Together with the lines from the Balmer series (up to Hdelta), we also identify strong lines from [Ne III] 386.9 nm and 396.7 nm (probably blended with Hepsilon), He II 468.6 nm and a broad [O III] 500.7 nm feature. The detection of numerous Neon lines is consistent with the classification of Nova Herculis 2021 as a Neon nova (Wagner et al. 2021, ATel #14746). Numerous lines still present a boxy profile (see Albanese et al. 2021, ATel #14718). The Halpha line has a FWHM velocity of 4000 km/s, while from the Neon lines we derive a slightly higher velocity of 5000 km/s. The recent announcement of soft X-ray emission from Nova Herculis 2021 (Page et al. 2021, ATel #14747, Pei et al. 2021, ATel #14798) with a temperature between 700,000 K and a million K may explain the high ionization features and the strength of the Neon forbidden lines in the UV.

Ultraviolet spectroscopy and photometry of Nova Herculis 2021 is encouraged.

The observations were performed in the framework of the ASYAGO Summer School. The spectrum can be visualized at the link below.

Nova Herculis 2021