Target TDE-FEATURELESS-1: There is no need or capability to test the scheduling of observations of generic targets in Phase I. This warning will go away the next time the proposal is loaded. 0 This target is not used in this proposal. This target is not used in this proposal. This target is not used in this proposal. This target is not used in this proposal. This target is not used in this proposal. Sensitive exposures should have an ETC run number provided. Sensitive exposures should have an ETC run number provided. Sensitive exposures should have an ETC run number provided. Sensitive exposures should have an ETC run number provided. Enter fixed target AT2026hir for activation 1; exposure time adjusted based on source coordinate; ETC provided assuming g=19 at the time of observation. -287063012 ----- Attempting Submission 1 (Mon Aug 11 19:46:38 GMT 2025) ----- HST Phase II Proposal 18132 successfully submitted. Receipt: # 18132-1 ----- Attempting Submission 2 (Mon Aug 11 21:16:45 GMT 2025) ----- HST Phase II Proposal 18132 successfully submitted. Receipt: # 18132-2 ----- Attempting Submission 3 (Fri Apr 03 20:48:14 GMT 2026) ----- Please ignore fixed targets not used in the proposal. They were entered for planning purposes. Visits 03 and 04 are not activated in this submission and therefore ETC is not provided there. The first UV spectra of optically overluminous and featureless tidal disruption events We propose 8 orbits of HST STIS ultraviolet spectroscopic ToO observations to study a rare subclass of tidal disruption events (TDEs) characterized by featureless optical spectra. Only ~10 such events have been identified to date, and none have UV spectra. Compared to typical TDEs, these events exhibit higher UV and optical luminosities and are hosted by more massive black holes M_BH ~ 10^8 Msun), where general relativistic effects are more pronounced. They may arise from the disruption of massive stars and/or involve rapidly spinning black holes. Owing to their high luminosities, these events are expected to dominate the high-redshift TDE population to be uncovered by LSST, JWST, and Roman. This program will deliver the first UV spectroscopic observations of this elusive subclass, providing critical insights into their origin and enabling the first exploration of their rest-frame UV properties. These data will illuminate the physics of black hole accretion in extreme relativistic regimes and guide the identification of high-z TDE candidates in forthcoming surveys. This proposal consists of non-disruptive ToO observations (up to 25 days response time) of 2 optically featureless tidal disruption events (TDEs), each for 4 orbits of STIS spectroscopic observations. Each trigger will be for a TDE that has (1) a nuclear location, (2) consistent blue colors, (3) an optical spectrum that is featureless, and (4) an apparent magnitude of g<19.5 at the time of triggering. Each TDE trigger will have two visits: - first visit: 2 orbits with STIS/FUV-MAMA using G140L, with the 52x0.2 aperture. - second visit: 2 orbits with STIS/NUV-MAMA using G230L, with the 52x0.2 aperture. Each visit requires an ACQ in the first orbit. As these are triggered observations, we do not yet know the exact brightness of the TDE or the host. We expect that the TDE will be a UV-bright point source on top of a (much redder) galaxy nucleus. The ACQ process uses the STIS/CCD with the F28X50LP filter, but the host galaxy may dominate in the optical. I have entered placeholder parameters for a diffuse acquisition. The exposure times and checkbox size may be adjusted depending on the actual properties of the target, so we have not provided ETC output until the actual fluxes are known. A ToO Activation on TDE-FEATURELESS1 should activate Visits 01 and 02 A ToO Activation on TDE-FEATURELESS2 should activate Visits 03 and 04 Dr. Yuhan Yao is a Miller postdoctoral fellow at UC Berkeley, specialized in using time domain surveys such as the Zwicky Transient Facility to select TDEs and multi-wavelength characterization of transients. She is the co-chair of the LS4 MBH science working group. Prof. Ryan Chornock is an expert on optical/UV observations of TDEs and other optical transients. He is the co-chair of the ZTF MBH Science Working Group. Prof. Raffaella Margutti is an associate professor at UC Berkeley. She is an expert of high-energy observations of transients. Prof. Wenbin Lu is an assistant professor at UC Berkeley. He is an expert in modeling the hydrodynamics of and radiation from TDEs. Supermassive Black Holes and Active Galaxies Stellar Physics and Stellar Types 6 false true false false false false false SUPPORTED UNIDENTIFIED ULTRAVIOLET EMITTER J2000 UNIDENTIFIED ULTRAVIOLET EMITTER J2000 UNIDENTIFIED ULTRAVIOLET EMITTER J2000 UNIDENTIFIED ULTRAVIOLET EMITTER X-RAY EMITTER NO J2000 UNIDENTIFIED ULTRAVIOLET EMITTER J2000 GALAXY NUCLEUS (1) nuclear (2) blue color (3) optically featureless (4) g<19.5 mag GALAXY NUCLEUS (1) nuclear (2) blue color (3) optically featureless (4) g<19.5 mag STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 /Users/yuhanyao/Desktop/proposals/HST/HST_Cycle33/featureless/OP-cache/18132_202603-v01.xml The scheduling data was generated by CASM 34.0. CASM was run in Phase II mode. 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