SALT high resolution optical spectroscopy of Nova Cir 2018 = PNV J13532700-6725110

ATel #11287; E. Aydi (South African Astronomical Observatory, University of Cape Town), D. A. H. Buckley (SAAO), S. Mohamed (SAAO, UCT), P. A. Whitelock (SAAO, UCT)
on 10 Feb 2018; 17:12 UT
Credential Certification: David Buckley (dibnob@saao.ac.za)

Subjects: Optical, Nova

We report on high-resolution spectroscopy of nova Cir 2018 obtained on 2018 January 29.45, 30.44, and February 2.47 (UTC) using the High Resolution Spectrograph (HRS; Crause et al. 2014, Proc. SPIE, 91476) mounted on the Southern African Large Telescope (SALT). Observations were taken in the HR mode of HRS, covering a spectral range of 3800-8900 Å at a resolution of R = 65000-67000. The data were reduced with the SALT HRS MIDAS pipeline (Kniazev et al. 2016, MNRAS 459, 3068). The nova has been classified as an Fe II-class nova (ATel #11209), showing emission features of H I and Fe II.

The first HRS spectrum obtained was dominated by P Cygni features of H I, Fe II (42, 46, 55, 74), O I, Ca II, Na I, Al II, Mg I, N I, and N II (see below a full list of the line identification), with very faint Balmer emission features. The P Cygni absorption components were at ~ -500 ± 50 km/s with a FWHM of ~ 300 km/s. The very faint emission features were slightly red shifted (~ +30 km/s) and characterized by a red emission wing extending to more than 500 km/s. Also present are relatively broad Ca II H & K at -500 km/s with a FWHM ~ 1050 km/s. The near-absence of emission lines in the spectrum indicates that, at this stage, the nova is around maximum light. This is consistent with the AAVSO photometric measurements.

In the second spectrum the absorption features have weakened, with some having disappeared, while the emission features have strengthened. These emission features, except for the Balmer lines, are now double-peaked, with a main peak at ~ +30 km/s and another at ~ +400 km/s.

The third spectrum was dominated by strong emission lines while the P Cygni absorption features have faded considerably. Both the emergence of strong emission lines and the fading of the absorption features indicate that the nova has started the decline from maximum, which is also consistent with photometric data. In addition to the pre-existed lines, more lines also appear in emission such as N II and forbidden O I lines.

Besides the Na I (D) blue-shifted absorption features, which are intrinsic to the nova, we can also identify the interstellar Na I (D1) and (D2) absorption features. These lines have EW(D2) = 0.58 ± 0.01 Å and EW(D1) = 0.46 ± 0.01 Å. Using the empirical relations of Poznaski et al. 2012 (MNRAS, 426, 1465), we derive E(B-V) = 0.23 ± 0.05 and Av = 0.71 ± 0.05 towards the nova.

Line identifications:
H I (Balmer): 3970, 4102, 4340, 4861 and 6563Å
H I: (Paschen) 8598, 8665 and 8750Å
Ca II: 3934 and 3968Å
Na I: 5686 6159, 5890 and 5896Å
O I: 7773 and 8446Å
Al II: 6237 and 6830Å
Mg I: 5528 and 8807Å
[O I]: 5577, 6300 and 6364Å
N II: 5479, 5679, and 6347Å
Si II 5958 and 5979Å

Fe II lines:
Multiplet (42): 4924, 5018 and 5169Å
Multiplet (46): 5991 and 6084Å
Multiplet (55): 5535Å
Multiplet (74): 6148, 6248, 6417 and 6456Å

Weak unidentified absorption features at:
5660, 5623, 5651, 5980, 6037 6229, 6361, 6423, 6474, 6483 and 6506Å