0 Assigned ID: 6912 ----- Attempting Submission 1 (Sat Aug 23 00:02:35 GMT 2025) ----- HST Phase I Proposal 6912 successfully submitted. Receipt: # 6912-1 0 ----- Attempting Submission 1 (Wed Sep 10 20:47:27 GMT 2025) ----- HST Phase II Proposal 18184 successfully submitted. Receipt: # 18184-1 ----- Attempting Submission 2 (Mon Sep 15 15:24:36 GMT 2025) ----- tau Ceti is the closest single Sun-like star to Earth and, despite being expected to have a similar age as the Sun or alpha Centauri trio, is often described as being significantly less active. We propose a focused observational campaign targeting this inactive star's coronal, EUV, and chromospheric emissions, the first of which shows dramatic variability. This will yield unprecedented insight into the connection between active layers from the lower chromosphere, through the transition region, and into the corona and help elucidate why such different flux levels are seen between the ROSAT and Chandra/XMM-Newton epochs. This exhaustive observational effort will also have legacy value for future Habitable Worlds Observatory characterization of the tau Ceti planetary system. This observation sequence will obtain NUV and FUV spectra for tau Ceti. The experimental design borrows from past successful observations of this star in the same setups. NUV will use the E230H grating centered on the MgII h+k lines (emission from these transitions is the primary NUV objective). FUV will use the E140M grating and cover the full FUV range of STIS; the stellar continuum will not be detected in the FUV but several strong chromospheric emission lines will be present and are the primary objective. Acquisition will initially use the narrowband OIII filter and then will peakup in a smaller aperture (0.3x0.05ND) than the science aperture (0.2x0.2). This should provide the best possible wavelength solution and ensure robust absolute flux calibration. We have carefully checked that tau Ceti will not saturate in either the NUV or FUV for the chosen aperture and exposure times and will provide signal-to-noise>~100 (without saturating) for acqusition sequences. NUV will use ACCUM mode as the target star is too bright for TIME-TAG. We will use TIME-TAG for the FUV where we do not expect to detect stellar continuum; only emission lines will be detected and we have confirmed that the brightest emission line (Lyman-alpha) will not overwhelm the buffer for the chosen dump time. PI Dr. Carl Melis (UC San Diego) will lead this project and be responsible for ensuring work is being completed in a timely fashion and take the lead on publications resulting from this work. Dr. Melis completed his Ph.D. in astronomy at UC Los Angeles in 2009 where he conducted studies relevant to the formation and evolution of stars and their planetary systems. He has since been a post-doctoral researcher and now member of the research faculty at UC San Diego conducting a variety of projects related to stellar astrophysics, including using X-ray data from Chandra, XMM-Newton, and other facilities to assess stellar and planetary system properties. He will contribute to designing the observations, helping with data reduction and analysis, interpreting results, and leading publications. Co-I Dr. Hans Moritz Guenther (MIT/CXC) will bring X-ray data analysis and modeling expertise necessary to do this work. Dr. Guenther is an expert in X-ray astronomy and modeling stellar X-ray emission. His major focus will be on modeling the X-ray data products obtained from the proposed observations and delivering fitted parameters or constraints for interpretation and publication. He will contribute to the interpretation of results and the preparation of journal articles. Co-I Dr. Steven Saar (Harvard/CfA) is an expert in stellar activity and coronal emissions. He conducted the 2020 tau Ceti HRC-S pilot observations that this proposal is leveraging. His expertise in this unique observational mode for Chandra will be critical to the project's success. He will otherwise bring essential expertise in stellar activity needed for analyzing, modeling, and interpreting results. Stellar Physics and Stellar Types 6 false false false false false false true SECONDARY CHANDRA 27200138 false SUPPORTED STAR G V-IV NO HD10700 J2000 J2000 STIS/NUV-MAMA Spectroscopic No No E230H 2713 STIS/FUV-MAMA Spectroscopic No No E140M 1425 1757949020896 /private/var/folders/9x/3p0nlt8j63v_p32x8vx2dzf00000gp/T/OPVisit16529524272795640110.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 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