The Study of Dust Formation of Six Tidal Disruption Events

This paper investigates eleven (UV-)optical-infrared (IR) spectral energy distributions (SEDs) of six tidal disruption events (TDEs), which are ASASSN-14li, ASASSN-15lh, ASASSN-18ul, ASASSN-18zj, PS18kh, and ZTF18acaqdaa. We find that all the SEDs show evident IR excesses. We invoke the blackbody plus dust emission model to fit the SEDs, and find that the model can account for the SEDs. The derived masses of the dust surrounding ASASSN-14li, ASASSN-15lh, ASASSN-18ul, ASASSN-18zj, PS18kh, and ZTF18acaqdaa are respectively $\sim0.7-1.0\,(1.5-2.2)\times10^{-4}\,M_\odot$, $\sim0.6-3.1\,(1.4-6.3)\times10^{-2}\,M_\odot$, $\sim1.0\,(2.8)\times10^{-4}\,M_\odot$, $\sim0.1-1.6\,(0.3-3.3)\times10^{-3}\,M_\odot$, $\sim1.0\,(2.0)\times10^{-3}\,M_\odot$, and $\sim 1.1\,(2.9)\times10^{-3}\,M_\odot$, if the dust is graphite (silicate). The temperature of the graphite (silicate) dust of the six TDEs are respectively $\sim1140-1430\,(1210-1520)$\,K, $\sim1030-1380\,(1100-1460)$\,K, $\sim1530\,(1540)$\,K, $\sim960-1380\,(1020-1420)$\,K, $\sim900\,(950)$\,K, and $\sim1600\,(1610)$\,K. By comparing the derived temperatures to the vaporization temperature of graphite ($\sim 1900$\,K) and silicate ($\sim 1100-1500$\,K), we suggest that the IR excesses of PS18kh can be explained by both the graphite and silicate dust, the rest five TDEs favor the graphite dust while the silicate dust model cannot be excluded. Moreover, we demonstrate the lower limits of the radii of the dust shells surrounding the six TDEs are significantly larger than those of the radii of the photospheres at the first epochs of SEDs, indicating that the dust might exist before the the TDEs occurred.