0 Assigned ID: 5601 ----- Attempting Submission 1 (Tue Mar 26 20:01:15 GMT 2024) ----- HST Phase I Proposal 5601 successfully submitted. Receipt: # 5601-1 ----- Attempting Submission 2 (Tue Mar 26 20:41:47 GMT 2024) ----- Change G750L's exposures to the last exposures in the science sequences. Short CCD exposures changed to CR-SPLIT=2. 2086534231 ----- Attempting Submission 1 (Wed Aug 14 13:11:11 GMT 2024) ----- HST Phase II Proposal 17701 successfully submitted. Receipt: # 17701-1 ----- Attempting Submission 2 (Fri Jan 10 19:50:32 GMT 2025) ----- HST Phase II Proposal 17701 successfully submitted. Receipt: # 17701-2 ----- Attempting Submission 3 (Wed Jan 15 08:34:14 GMT 2025) ----- Confirming Onion-Shell Accretion in T Tauri Stars Accretion from disks onto low-mass, pre-main sequence stars (T Tauri stars) is an important process in star and planet formation. Close-in, rocky planets form in an environment affected by magnetospheric accretion in T Tauri disks. Numerical simulations have been used to study this dynamic process, confirming key observable results. In recent years, advances in numerical methods and increases in computational power have enabled detailed studies of accretion by simulating the interaction between the star, its magnetosphere, and an MRI-active inner disk. New simulations suggest the existence of inner filaments, where mass proceeds horizontally and accretes onto the star in an onion-shell-like structure. Observational evidence of such a structure exists as a low-velocity redshifted absorption feature in the H-alpha profiles of T Tauri stars accreting at very low rates, but an independent verification is required. Here, we propose to confirm onion-shell accretion in five low accretors with joint HST/STIS and Gemini/GHOST observations. Modeling the accretion shock emission can help to distinguish between models with inner filaments and models where accretion occurs strictly through high-latitude flows. Our results will provide insight into young stars' immediate environment -- a region crucial for close-in planet formation. We will use 9 HST orbits to obtain NUV-optical spectra of 5 T Tauri stars with very low accretion rates (low accretors) to search for evidence of Onion-shell accretion using multi-column accretion shock models. For each star, we will obtain the NUV spectrum with STIS/NUV-MAMA+G230L and the optical spectra with STIS/CCD+G430L+G750L. We will use the 52x2 slit for all configurations. The NUV spectra are needed to measure the accretion luminosity, while the optical spectra will be used for scaling with photospheric templates. We aim for SNR greater than 3 redward of 2300 Angstrom for the NUV spectra and SNR~50 in the optical. To estimate the NUV exposure times, we scaled the spectrum of a low accretor RECX-11 by distance, and we used the Kurucz/Bruzual synthetic spectra for SNR estimation in the optical. To complement the HST observations, we also request coordinated observations with NOIRLab to use the GHOST instrument on Gemini-South within a few days of the HST observations. The GHOST spectra will provide additional accretion diagnostics for accretion flow modeling to confirm the Onion-shell geometry. PI Thanawuth Thanathibodee is an expert in accretion modeling and spectroscopic observation and analysis of T Tauri stars. He specializes in the detection and characterization of low accretors and has used accretion flow models to determine the accretion properties of T Tauri stars and protoplanets. Co-I Catherine Espaillat is an expert in the observation and modeling of T Tauri stars and protoplanetary disks, particularly to study disk evolution and accretion variability. She has led several joint observational programs with HST/JWST/Chandra to perform multi-wavelength studies of T Tauri stars and disks. Co-I Nuria Calvet is an expert in accretion modeling and UV observation of T Tauri stars. She developed the accretion shock model used in this program and has led many UV observational programs with HST. Co-I Zhaohuan Zhu is an expert in numerical simulations of T Tauri stars and protoplanetary disks. He has authored several papers studying T Tauri accretion, disk simulation, and planet accretion. Co-I Caeley Pittman is an expert in multi-column accretion modeling using STIS/NUV and optical data. She is leading a large-scale accretion analysis of T Tauri stars in the ULLYSES sample and has produced one publication from that project (Pittman et al. 2022). Stellar Physics and Stellar Types Exoplanets and Exoplanet Formation 6 false false false false false false true PRIMARY 0.47 true SUPPORTED STAR T TAURI STAR This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. J2000 J2000 STAR T TAURI STAR This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. J2000 J2000 STAR T TAURI STAR This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. J2000 J2000 STAR T TAURI STAR This object was generated by the targetselector and retrieved from the SIMBAD database. The SIMBAD-resolvable name is [HHM2007] 682. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. J2000 J2000 STAR T TAURI STAR This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. J2000 J2000 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G750L 7751 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G750L 7751 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G750L 7751 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G750L 7751 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/CCD Spectroscopic No No G430L 4300 STIS/CCD Spectroscopic No No G750L 7751 The scheduling data was generated by CASM 33.0. CASM was run in Phase II mode. 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