Revealing Physical Properties of a Tidal Disruption Event: iPTF16fnl

Tidal disruption event (TDE) iPTF16fnl shows a relatively low optical flare with observationally very weak X-ray emission and the spectroscopic property that the helium emission line from the source dominates over the hydrogen emission line at early times. We explore these observed signatures by calculating spectral emission lines with the publicly available code, CLOUDY. We estimate five physical parameters by fitting the observed optical UV spectra on multiple days to a theoretical model of a steady-state, slim disk with a spherical outflow. The resultant key parameters among them are black hole mass $M_{\bullet} = (6.73 \pm 0.44) \times 10^5 M_{\odot}$, stellar mass $M_{\star} = (2.59 \pm 0.17) M_{\odot}$, and wind velocity $v_{\rm w} = 7447.43 \pm 183.9~{\rm km~s^{-1}}$. The disk-wind model also estimates the radiative efficiency to be $0.01\lesssim\eta\lesssim0.02$ over the observational time, resulting in the disk being radiatively inefficient, and the disk X-ray luminosity is consistent with the observed low luminosity. In our CLOUDY model, the filling factor of the wind is also estimated to be 0.8, suggesting that the wind is moderately clumpy. We reveal that the helium-to-hydrogen number density ratio of the wind lies between 0.1 and 0.15, which is nearly the same as the solar case, suggesting the tidally disrupted star is originally a main sequence star. Because the optical depth of the helium line is lower than the hydrogen line by two orders of magnitude, the helium line is significantly optically thinner than the hydrogen line. Consequently, our results indicate that the helium line luminosity dominates the hydrogen line luminosity due to the optical depth effect despite a small helium-to-hydrogen number density ratio value.