Revealing the Spectroscopic Variations of FU Orionis Object V960 Mon with High-resolution Spectroscopy

We present the results of the high-resolution spectroscopy of the FU Orionis-type star V960 Mon. The brightness of V960 Mon decreased continuously after the outburst was detected in 2014 November. During this dimming event, we carried out medium-resolution spectroscopic monitoring observations and found that the equivalent width of the absorption features showed variations. To further investigate the spectroscopic variations, we conducted a high-resolution spectroscopic observation of V960 Mon with the Subaru Telescope and the High Dispersion Spectrograph on 2018 January 8 and 2020 February 1. By comparing this spectrum with the archival data of the Keck Observatory and the High Resolution Echelle Spectrometer taken between 2014 and 2017, we found that the absorption profiles changed as V960 Mon faded. The line profile of absorption lines such as Fe i and Ca i can be explained by a sum of the spectra of the disk atmosphere and the central star. The model spectrum created to explain the variations of the line profiles suggests that the effective temperature of the central star is ∼5500 K, which is comparable to that of the pre-outburst phase with a distance of 1.6 kpc with Gaia. The spectrum also shows that the effective temperature of the disk atmosphere decreased as V960 Mon faded. The variations of the Hα and Ca ii lines (8498.0, 8542.1 Å) also show that the V960 Mon spectrum became central-star dominant.