0 Assigned ID: 899 ----- Attempting Submission 1 (Fri Apr 05 22:54:41 GMT 2019) ----- ----- Attempting Submission 2 (Sun Apr 07 15:26:33 GMT 2019) ----- 0 ----- Attempting Submission 1 (Mon Jul 08 06:54:07 GMT 2019) ----- HST Phase II Proposal 15899 (phaseii.apt) successfully submitted. Receipt: # 15899-1 ----- Attempting Submission 2 (Wed Jul 10 01:10:34 GMT 2019) ----- ----- Attempting Submission 4 (Tue Oct 08 18:14:29 GMT 2019) ----- Late-time UV spectroscopy of Tidal Disruption Events with HST Once every 10^5 years, an accretion-powered flare may temporarily illuminate a supermassive black hole (SMBH) following a tidal disruption event (TDE), in which a star is shredded by tidal stresses close to the SMBH. TDEs provide a new window to study these otherwise dormant SMBHs and probe different accretion modes (super-Eddington to sub-Eddington) on a monthly timescale. Recent studies provide evidence for significant state changes in the late-time TDE emission properties (X-ray re-brightening, light curve flattening) that may be attributed to delayed disk formation or transitions in the accretion processes. These findings highlight our lack of understanding of how and when circularization occurs in TDEs. In light of this, we propose late-time follow-up UV spectroscopic observations of two TDEs that have been observed by HST in their early phases, in order to study the evolution of debris streams on the timescale of a year. Since the UV band captures the strongest atomic transitions (N, C, Si) expected from TDEs, UV spectroscopy will allow us to measure these lines and infer the kinematics, density, and abundance in this late-time TDE environment. Past HST STIS observations of these two targets have identified cold, fast outflows (0.05c) initiated by super-Eddington accretion. One of our targets, AT2018zr, has undergone a remarkable UV spectral hardening that may be related to an increased contribution from the disk. With the proposed HST observations, we wish to ascertain whether such disk exists by searching for high-ionization emission lines similar to those seen in QSOs. We propose to observe two TDE targets, AT2018zr and ZTF18abxftqm, in Cycle 27 in the wavelength range of 1200 - 3000 Angstrom. Extrapolating from existing data, we averages the results from a linear-fit and a t^-5/3 power-law fit to estimate the UVW2 mag of these two targets to be 21.4 and 20.4 mag in October 2019. For the NUV spectrum, we will use STIS NUV-MAMA with the G230L grating instead of COS because we require a broad wavelength coverage to capture NUV features in 1700 - 3000A (where the rest wavelengths of Fe and MgII transitions fall). We compared the FUV spectrum output from both COS and STIS with the G140L grating with HST ETC and found the required exposure time to reach the desired S/N to be comparable. Therefore, we choose to obtain the FUV spectrum with HST STIS with the G140L. To estimate the required exposure time, we assume the SED is a blackbody with a temperature of 20,000 K, as typical of TDEs. We assume an orbital visibility of 54 min, an overhead of 6 min for guide star aquisition at the beginning of a visit, 4 min for guide star reaquisition at the beginning of each subsequent orbits, 6 min for target acquisition, 8 min spectroscopy overhead for each grating, and 4 min for each exposure time interval that exceeds 2300 seconds at the same grating position. This amounts to an on-source time of 34 min in the rst orbit, and 44 min in the subsequent orbits. A 5 orbits observation with STIS FUV-MAMA would result in a S/N >5 per resolution element from the continuum emission alone with the G140L grating for AT2018zr. A 3 HST orbits observation would result in a S/N >5 per resolution element from the continuum emission alone with the G230L grating with STIS NUV-MAMA. For ZTF18abxftqm, we require 2 HST orbits for the FUV spectrum, and 1 HSTorbit for the NUV spectrum with the same configuration to achieve the same level of S/N. The assumed S/N will allow us to detect strong narow (Wr > 1A) absorption lines in the spectrum. In summary, we request for (5 + 3) orbits for AT2018zr and (2 + 1) orbits for ZTF18abxftqm, totaling 11 orbits of HST time in Cycle 27. Dr. Hung, Dr. Gezari, Dr. Cenko, Dr. van Velzen, and Dr. Blagorodnova are experienced observers with expertise in analyzing and interpreting TDE spectroscopic data (15+ papers analyzing multi- wavelength observations of TDEs). Dr. Cenko has led two previously approve HST programs (13853, 15331) to obtain UV spectra of TDEs and has led the analysis and the interpretation of the HST STIS spectra of TDE ASASSN-14li. Dr. Hung has been approved a HST program (15679) to observe TDE ZTF18abxftqm and has led the analysis and the interpretation of the HST STIS spectra of TDE AT2018zr (with Dr. Foley). Dr. Blagorodnova has led the analysis of HST STIS spectrum of TDE iPTF15af as a part of a multi-wavelength investigation. Dr. Roth is an expert in theoretical modeling of TDE spectra (2 first-author publication in the past 3 years) and has led the theoretical interpretation of the HST STIS spectra of TDEs AT2018zr and ASASSN-14li. 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