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On The Presence of Neon in the Spectrum of V5668 Sgr and other CO Novae

ATel #10557; C. E. Woodward (U. Minnesota), R. M. Wagner (LBTO and Ohio State U.), S. Starrfield (Arizona State U.)
on 5 Jul 2017; 03:07 UT
Credential Certification: R. Mark Wagner (rmw@as.arizona.edu)

Subjects: Optical, Nova

V5668 Sgr was discovered by J. Seach on 2015 March 15.634 UT (CBET No. 4030) and confirmed as an Fe II classical nova by Williams, Darnley, and Bode (ATEL #7230). It was also detected by the Fermi-LAT as a VHE γ-ray emitting nova (Cheung et al., ATEL #7283 and #7315). It then exhibited strong CO in the infrared (Banerjee et al., ATEL #7303) and formed dust (Walter, ATEL #7643). This early evolution is that characteristic of a typical dust-forming CO nova. This identification was strengthened by the discovery of 7Be (Molaro et al. MN, 463, L117, 2016; Tajitsu et al. ApJ, 818, 191, 2016) and 7Li (Wagner et al. 2017 in prep.) early in the outburst.

We obtained optical spectroscopy of V5668 Sgr on 2017 May 28.40 UT using the twin 8.4 m Large Binocular Telescope on Mt. Graham, Arizona with the MODS1 & MODS2 two-channel spectrographs covering the spectral range 320.0 to 1000.0 nm at a spectral resolution of ~0.3 nm. Our new observations show that V5668 Sgr is well into the nebular stage and exhibits a diversity of ionization states. Prominent permitted emission lines include Hα, Hβ, and higher order Balmer lines through H10; H Paschen series lines of Pε through P16 at 850.2 nm; He I 447.1, 587.5, 667.8, and 706.5 nm; He II 468.5 and 541.1 nm; C II 426.7 nm; C III 465.0 nm; N III 451.2, 463.4, 464.1; O I 844.6 nm and weaker lines of O VI 381.1 and 383.4 nm. Prominent forbidden lines in the spectrum include [Ne V] 334.6 and 342.6 nm, [Ne III] 386.9 and 396.8 nm (blended with H7); [Ne IV] 471.4 and 472.5 nm are weak or absent; [O I] 630.0 and 636.3 nm; [O II] 732.5 nm; [O III] 436.3, 495.9, and 500.7 nm; [N II] 575.5 nm; [Ca V] 530.9; [Fe VI] 567.7 nm; [Fe VII] 358.6, 572.1 and 608.6 nm; [Ar III] 713.6 and 775.1 nm; [Ar IV] 723.7 nm; and [S III] 631.2, 906.9 and 953.1 nm. The forbidden [Ne V] and [Fe VII] lines imply that a hot underlying white dwarf (WD) is present. Our spectrum is qualitatively similar to that of V443 Sct 1989 presented by Williams, Phillips, and Hamuy (ApJS, 90, 297, 1994) in their Fig. 3 observed on June 16, 1991.

The emission line profiles also show distinct castellated structures suggestive of a bipolar and asymmetric ejection morphology. The [Fe VII] 608.6 nm line exhibits two fully resolved narrow peaks of nearly the same intensity each with an instrumental-corrected FWHM of about 0.85 nm and a peak separation of ±493 km/s from line center. In contrast, the Balmer, [O III], and [Ne V] lines exhibit triple peaks. For Hβ, the separation is (-432, +419) km/s whereas the separation for [O III] 500.7 nm is ±443 km/s both respect to line center. The observed [O III] 500.7 to Hβ intensity ratio is about 31 whereas the observed [Ne V] 342.6 to Hβ ratio is about 1.

The presence of the neon lines suggests that neon may be slightly enriched above Solar. An analysis of the chemical abundances of recent classical novae based on infrared and optical emission line diagnostics (Helton et al., ApJ, 755, 37, 2012) finds that there is a modest enrichment of neon in the ejecta of some CO novae. However, recent observations of V5668 Sgr in near- and mid-IR emission lines by Calvén et al. (2017, AAS 230, 317.01) report that its neon abundance is about solar. New simulations of the outbursts of CO Classical Novae imply that modest enrichments of neon may occur under some conditions (Starrfield et al. 2017, AAS 230, 317.02) in agreement with the observations. Further observations of this nova are encouraged.