Probing the tidal disruption event iPTF16axa with CLOUDY and disc-wind models

Abstract

We present both a disc-wind model on the optical/UV emission continuum and CLOUDY modelling on the spectral lines of the tidal disruption event (TDE) iPTF16axa to understand the disc-wind emission and the properties of the atmosphere that impacts the line luminosity of the TDE. Assuming the optical/UV emission from the wind due to the disc super-Eddington phase, we use the steady structured disc-wind model with a spherical wind with constant velocity to fit the observations on multiple days. The extracted parameters are stellar-mass $M_{\star} = 6.20 \pm 1.19 M_{\odot}$, disc radiative efficiency $\log_{\rm 10}(\eta) = -1.22 \pm 1.327$, wind inner radius $r_l = (2.013 \pm 0.551) \times 10^{14}~{\rm cm}$ and velocity $v_w = 18999.4 \pm 1785.1 ~{\rm km~s^{-1}}$. The photosphere temperature for wind emission is $ \sim 2 \times 10^4~{\rm K}$ and the disc single blackbody temperature is $\sim 0.995 \times 10^5~{\rm K}$. We also perform CLOUDY modelling to explain the observed He and H line luminosities that estimate a wind inner radius $r_l = 7.07 \times 10^{14}~{\rm cm}$ and velocity $v_w = 1.3 \times 10^4~{\rm km~s^{-1}}$. The independent analyses of iPTF16axa using CLOUDY and disc-wind models show comparable results that agree with observations. The CLOUDY modelling finds that both the super solar abundance of He and a smaller He II line optical depth is responsible for the enhancement of He II line luminosity over the H$\alpha$ line luminosity. The super-solar abundance of He II agrees with a relatively large stellar mass and suggests that the disrupted star might have been a red giant.