Initial results from the HST STIS spectrum of Nova V5668 Sgr (2015 #2)

ATel #8275; Paul Kuin (MSSL/UCL), Kim Page, Julian Osborne (U. Leicester), Steve Shore (Universita di Pisa), Greg Schwarz (AAS), Fred Walter (Stonybrook)
on 11 Nov 2015; 22:14 UT
Credential Certification: Paul Kuin (npkuin@gmail.com)

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On Friday, November 6th, 2015, from 1:34 to 2:36 UT we obtained a medium resolution spectrum with the STIS instrument on the Hubble Space Telescope of V5668 Sgr, also known as Nova Sgr 2015b and PNV J18365700-2855420.

Earlier this year the nova formed dust (ATEL #7643, #7748, #7862, #7986) with an exceptionally strong photometric signature with the brightness decreasing from 6th magnitude by 7 magnitudes within a week. By the start of September the nova brightness had recovered 4 magnitudes to 9th magnitude and since then has only shown a very minor decrease in brightness.

The nova has been monitored with the low resolution Swift UV grism and filter throughout this process (ATEL #7953, #8054) and therefore therefore the addition of a medium resolution HST UV spectrum that also extends into the far UV is invaluable. We observe emission lines of multiple ionisation stages of, for example, O I 1302, O II 2783, C II 2324, C III 1175, 2838, C III] 1909, C IV 1550, N II] 2143, N III] 1750, N IV] 1486, 1718, N V 1240, O V] 1371 as well as lines of He II 1640, Al III 1854, Mg II 2800. The Si III and Si IV lines appear faint, suggesting a low abundance for these ions. The emission lines show two or three peaks, and the line ratios are different in the peaks which provides evidence of differences in ionisation and/or abundances in the outflow. Contemporaneous optical spectra obtained using the Chiron spectrograph on the SMARTS/CTIO 1.5m at R=28,000 also show lines from C IV, N V, and O VI.

The interstellar lines are clearly defined. The H Ly-alpha line is comprised of a saturated absorption line with two emission peaks offset at about the same velocities where the peaks are in other emission lines. Using the wings of the ISM H Ly-alpha profile a neutral hydrogen column density is derived of N_HI = 1.4x10²¹ cm^-2. The X-ray spectrum allows only a rough estimate of NH ~ 10²¹ cm^-2, consistent with the measurement from H Ly-alpha. We estimate the reddening E(B-V) to be around 0.2-0.3.

We acknowledge support from the AAVSO observers, specifically Costantino Sigismondi and Arne Henden by providing photometry in preparation of the HST observation. We thank all who contributed to the success of this observation, in particular the people at STScI, and the Swift team for continuous observations which allowed us to monitor the nova in the ultraviolet.