Spectroscopy of the new bright Galactic transient ASASSN-18fv: A 12 mag outburst from a young stellar object?

ATel #11456; J Strader, L Chomiuk (MSU), T. W.-S. Holoien (Carnegie), J. L. Prieto (Diego Portales; MAS), K. Z. Stanek (OSU), B. J. Shappee (IfA-Hawaii), Subo Dong (KIAA-PKU)
on 21 Mar 2018; 15:45 UT
Credential Certification: Jay Strader (strader@pa.msu.edu)

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We obtained a spectrum of ASASSN-18fv (ATel #11454), the new bright Galactic transient, with the Du Pont 2.5-m telescope at Las Campanas Observatory on UT 2018 Mar 21. The underlying spectrum appears to be that of an early K star, but we also observe P Cygni profiles in many lines: H-alpha, the Ca triplet, O I 7773, Mg I 5173, and several Fe II lines. The emission and absorption profiles are both unresolved at our ~ 400 km/s FWHM resolution. The higher order Balmer lines are seen only in absorption, and Paschen absorption is also present. There is an overall increase in the continuum emission between ~ 8300 and 9300 A that appears as a "bump" in the spectrum, but no obvious broad molecular features are apparent.

We also corrected the astrometry of our acquisition image using a large number of Gaia DR1 stars. After doing so, we find that the transient matches to within 0.1" a star in the Gaia DR1 catalog, which has a J2000 position of 10:36:15.4138 -59:35:53.648. This star is also present in VPHAS+ DR2 catalog (Drew et al 2014, MNRAS, 440, 2036), with g~20.1 and r ~ 19.5. From spectrophotometry of the fluxed spectrum and a first-order slit correction, we find magnitudes of g=7.80, r=7.27, i=7.19, B=8.07, V=7.45. The absolute fluxing is likely uncertain at the 10-20% level, but the colors (g-r = 0.53; B-V = 0.62) should be more reliable. The outburst g magnitude of 7.8 implies that the object has brightened by at least -12.3 mag in g.

The strong, stellar-like underlying continuum and the narrow nature of the emission lines suggest that this transient is not likely to be a classical nova, though this cannot be definitively exluded. Instead, noting that this source is < 10 pc from the main nebulosity of the Carina Nebula, the transient may be a large outburst of a young stellar object. Other exotic explanations that show low expansion velocities, such as stellar mergers, may also be plausible. High-resolution optical/IR spectroscopy of this unusual transient is urgently required.