Near-Infrared Spectroscopy of Nova Delphini 2013

ATel #5336; Guy S. Stringfellow (Center for Astrophysics and Space Astronomy, University of Colorado), Leslie Hebb (Hobart and William Smith Colleges), Michele Fumagalli (Carnegie Observatories, Princeton University)
on 29 Aug 2013; 08:55 UT
Credential Certification: Guy S. Stringfellow (Guy.Stringfellow@Colorado.edu)

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We have secured near-IR spectra of Nova Delphini 2013 on August 27.09 UT using the cross-dispersed spectrograph TripleSpec on the Apache Point Observatory 3.5m telescope. A 1.1″ slit was used, providing a spectral resolution of R~3200 that spans 0.91-2.43 µm. The A0V star HD 191082 was observed immediately after the nova at a similar airmass, and used to eliminate telluric features in the spectrum and to perform the flux calibration. The nova is well into the post-fireball stage, with the hydrogen Paschen and Brackett series in strong emission. There is strong C I emission throughout the spectrum, further strengthening identification as an Fe II nova.

In the K-band the strongest lines are Brγ and Brδ. Other prominent spectral features include lines of C I at 1.973 (half the strength of Brγ), 2.103, 2.125 (a blend), 2.216, 2.291, and 2.315 µm. There is distinct broad emission on the blue wing of Brγ which could be Na I (it is nearly as strong as the C I 2.103 µm line), and there is a shoulder on the blue wing of the C I 2.216 µm line that could also arise from the Na I 2.206/8 µm doublet. There is a weak, broad feature at 2.063 µm that could be He I emission on the rise. The spectrum is void of any CO emission, though there is a very weak emission line-blend near 2.345 µm which is not attributed to CO emission.

The strongest line in the H-band spectrum is C I 1.689 µm, with Br-11 blended in the blue wing at about half the peak line intensity. The Brackett series is present and in emission. C I at 1.601 µm and 1.705 µm (very weak) are present, along with a broad blend of numerous C I lines that reside between 1.720-1.790 µm; these latter lines rival the C I 1.689 µm line in strength. There is no indication of Mg I being present in either the H or K band.

At shorter wavelengths, the strongest line is C I at 1.070 µm, which is stronger than Paβ. Other C I blends, with peaks at 1.166 and 1.176 µm, are nearly as strong as the 1.070 µm line; the strength of the 1.176 µm line is equivalent to Paβ. Emission lines of O I at 1.247, 1.258, 1.297, and 1.317 µm are of moderate strength. C I 1.297 is very weak. He I 1.083 is likely present, blended on the blue wing of Paγ, but is presently weak.

There are no P Cyg absorption features associated with any of the emission lines. There are several features (substructure peaks) located in both the blue and red wings of all emission lines. The most prominent feature in strength is the red sub-peak that lies roughly 0.003 µm from the Brγ line center (a shift of about 490 km/s). This feature is identifiable in all emission lines throughout the J, H, and K bands. These features suggest the ejecta is forming complex structure, either becoming highly clumped or possibly forming one or more shells.

The AAVSO V light curve indicates the nova may now be slowing in decline. Between August 16.5 to 27.5 the light curve shows four rather distinct rates of decline (changes in slope). During August 19-23 the decline was faster (the slope being steeper). Around August 23 the rate of decline slows - the nova may now be stalling, entering a plateau phase. The time for the nova to decline 2 magnitudes from peak light, t₂, is roughly 11 days (August 16.5 to 27.5), classifying it as a fast nova. Our IR spectrum, secured when t₂ was reached, closely resembles overall that of V1280 Sco between February 24 and March 3 in 2007, except that at present He I is much weaker or absent, and the C I blend at 1.75 µm is much weaker relative to the C I 1.689 µm line when compared to V1280 Sco (cf, Das et al. 2008, MNRAS, 391, 1874). V1280 Sco had already begun forming dust prior to reaching t₂.