SALT high-resolution spectroscopic follow-up of nova ASASSN-18ds

ATel #11364; 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 27 Feb 2018; 15:21 UT
Credential Certification: David Buckley (dibnob@saao.ac.za)

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We report on SALT high-resolution spectroscopy of nova ASASSN-18ds, which was discovered by Stanek et al. (2018; ATel #11348) on 2018 February 24.36 UT and was reported as a classical nova by Schmidtobreick et al. (2018; ATel #11360) on 2018 February 26.96 UT. Here we present a detailed description of the high-resolution spectroscopy.

We obtained a 1200 s spectrum of this nova under the SALT Large Science Program on transients on 2018 February 27.08 (UTC; HJD 2458176.57), 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 LR mode of HRS, covering a spectral range of 3800-8900 Å at a resolution of R = 14,000. The data were reduced with the SALT HRS MIDAS pipeline (Kniazev et al. 2016, MNRAS 459, 3068).

The spectrum is dominated by narrow emission lines of H I, Fe II, O I, Ca II, Mg I, Mg II, and Si II (see below a full list of the line identification). The lines have P Cygni and rounded profiles, characteristics of optically thick ejecta. We measure a FWHM of ~550 ± 50 km/s for Hα and a FWHM of ~470 ± 50 km/s for Hβ). The absorption components are blue-shifted by ~240 ± 30 km/s. The line profiles and their widths are consistent with the conclusion of Schmidtobreick et al. (2018; ATel #11360) that the nova is still rising to maximum. We expect that future spectroscopic follow-up show narrower lines with weaker or possibly no emission profiles, while the nova eruption approaches the visual maximum.

We identify the interstellar Na I (D2) and (D1) absorption features. Their large equivalent width (EW D2 ~ 1.6Å and EW D1 ~ 1.48Å) indicates that the nova is highly reddened. This is consistent with its low Galactic latitude.

Line identifications:
H I (Balmer): 4102, 4340, 4861 and 6563Å
Ca II: 8498, 9542, and 8662Å
Mg I: 5528Å
Mg II: 4481 and 7896Å
Si II: 6347 and 6371Å
O I: 7773 and 8446Å

Fe II lines:
Multiplet (27): 4233, 4303, 4352, 4385, and 4417Å
Multiplet (37): 4491, 4515, 4556, and 4629Å
Multiplet (38): 4508, 4523, 4549, and 4584Å
Multiplet (42): 4924, 5018 and 5169Å
Multiplet (48): 5265, 5317, 5363,and 5414Å
Multiplet (49): 5198, 5235, 5276, 5317 and 5425Å
Multiplet (55): 5535Å
Multiplet (73): 7462 and 7712Å
Multiplet (74): 6148, 6248, 6417 and 6456Å

Possible lines:
H I (Paschen): 8454, 8598 and 8750Å

Relatively strong unidentified emission features at:
7420, 7440, 7875, 7466, 8627, 8682, 8717, and 8723Å

Relatively weak unidentified emission features at:
4130, 4617, 5039, 5055, 5100, 5127, 5434, and 5490Å

Relatively strong unidentified absorption features at:
5779, 6795, 6283, and 6613Å

Relatively weak unidentified absorption features at:
4725, 4818, 4842, 4962, 5417, 5671, 5702, 6201, 6660, 6670, 6697, 6715Å