Spectroscopy of PNV J18040967-1803581

ATel #11528; J. Rupert (MDM Observatory), R. M. Wagner (LBTO and Ohio State), C. E. Woodward (Minnesota), and S. Starrfield (Arizona State)
on 11 Apr 2018; 19:01 UT
Credential Certification: R. Mark Wagner (rmw@as.arizona.edu)

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Recently, T. Kojima and H. Nishimura independently discovered a new optical transient on 2018 April 8.72 - 8.73 UT in Sagittarius with a magnitude of about 11.2. The object was subsequently designated PNV J18040967-1803581 on the CBAT Transient Object Confirmation Page (TOCP; http://www.cbat.eps.harvard.edu/unconf/followups/J18040967-1803581.html ). On 2018 April 9.4477 UT the transient was observed by G. Masa and M. Schwartz using the 0.4 m robotic telescope of Tenagra Observatories where the source had R = 9.5 mag. A low resolution (~400) spectrum was obtained by H. Akazawa using a 0.35 m telescope on 2018 April 09.827 UT and showed a Hα emission line with a P-Cyg type line profile. Measurements of the spectrum by K. Ayani and reported on the TOCP showed that the bottom of the blue-shifted absorption line was displaced by about -1100 km/s with respect to the emission peak.

We obtained a spectrum (range: 394-687 nm; resolution 0.3 nm) of PNV J18040967-1803581 on 2018 April 10.444 UT with the 2.4 m Hiltner telescope (+OSMOS) of the MDM Observatory on Kitt Peak. The spectrum exhibits emission lines with sharp P Cyg type line profiles superposed on a very red continuum. In agreement with Akazawa and Ayani, the strongest emission line in our spectrum arises from Hα. A simple two-Gaussian deconvolution of the line profile gives a FHWM for the emission component of 774 km/s and an equivalent width of -0.37 nm. The FWHM corresponds to an average expansion velocity of 387 km/s. The absorption component is narrow but quite deep with an equivalent width of 0.98 nm and a FWHM of 632 km/s. In our spectrum, the bottom of the blue-shifted absorption component is displaced by -617 km/s relative to the fitted emission peak. The terminal velocity is about 1400 km/s. The rest of the spectrum is dominated by very weak emission lines with sharp P Cyg type line profiles of Hβ, Hγ, Hδ, and numerous Fe II lines arising from multiplets 27, 37, 38, 42, 48, 49, 55, and 74. No broad absorption lines arising from a late-type secondary star are apparent in our spectrum at this time. The interstellar Na I D1 and D2 lines are strong with equivalent widths of 0.092 and 0.120 nm respectively, and when combined with the red continuum, suggests that the interstellar absorption is high along the line of sight.

Our spectrum does not appear to resemble that of a Fe II classical nova early in the outburst. However, if the possible progenitor from Pan-STARRS DR1 reported by P. Schmeer on the TOCP is related to the transient, then the outburst amplitude is about 12 mag and is comparable to some classical novae. However, the low expansion velocities and the numerous sharp absorption lines suggest that this object may be related to the class of luminous red novae such as V4332 Sgr and V838 Mon. Additional photometry and spectroscopy over the next few weeks may well reveal the nature of PNV J18040967-1803581 so continued observations are encouraged.

MDM spectrum of PNV J18040967-1803581