A detailed spectroscopic study of Tidal Disruption Events

Spectroscopically, tidal disruption events (TDEs) are characterized by broad ( ∼ 10 4 kms − 1 ) emission lines and show a large diversity as well as di ff erent line profiles. After carefully and consistently performing a series of data reduction tasks including host galaxy light subtraction, we present here the first detailed, spectroscopic population study of 16 optical and UV TDEs. We study a number of emission lines prominent among TDEs including Hydrogen, Helium, and Bowen lines and we quantify their evolution with time in terms of line luminosities, velocity widths, and velocity o ff sets. We report a time lag between the peaks of the optical light curves and the peak luminosity of H α spanning between ∼ 7 and 45days. If interpreted as light echoes, these lags correspond to distances of ∼ 2 − 12 × 10 16 cm, which are one to two orders of magnitudes larger than the estimated blackbody radii ( R BB ) of the same TDEs and we discuss the possible origin of this surprisingly large discrepancy. We also report time lags for the peak luminosity of the He i 5876Å line, which are smaller than the ones of H α for H TDEs and similar or larger for N iii Bowen TDEs. We report that N iii Bowen TDEs have lower H α velocity widths compared to the rest of the TDEs in our sample and we also find that a strong X-ray to optical ratio might imply weakening of the line widths. Furthermore, we study the evolution of line luminosities and ratios with respect to their radii ( R BB ) and temperatures ( T BB ). We find a linear relationship between H α luminosity and the R BB ( L line ∝ R BB ) and potentially an inverse power-law relation with T BB ( L line ∝ T − β BB ), leading to weaker H α emission for T BB ≥ 25000K. The He ii / He i ratio becomes large at the same temperatures, possibly pointing to an ionization e ff ect. The He ii / H α ratio becomes larger as the photospheric radius recedes, implying a stratified photosphere where Helium lies deeper than Hydrogen. We suggest that the large diversity of the spectroscopic features seen in TDEs along with their X-ray properties can potentially be attributed to viewing angle e ff ects.