18184( 3) - 15-Sep-2025 19:38:19 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18184 Version: 3 Check-in Time: 15-Sep-2025 19:38:19 STScI Edit Number: 0 Title A comprehensive diagnostic of tau Ceti's active outer layers ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Carl Melis University of California - San Diego Y CoI: Dr. Hans Moritz Guenther Massachusetts Institute of Technology N CoI: Dr. Steven H. Saar Smithsonian Institution Astrophysical N Observatory ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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. ------------------------------------------------------------------------------------ 18184( 3) - 15-Sep-2025 19:38:19 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 -TAU-CET STAR, G V-IV RA=01H44M2.1732S +/- 0.001", J2000 V = 3.5 DEC=-15D56'1.25" +/- 0.001" Reference Frame: ICRS Extended: NO Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2016 -0.11936797220524992 0.8549629999999999 0.27380970000000004 ------------------------------------------------------------------------------------------------------------------------------------ 18184( 3) - 15-Sep-2025 19:38:19 - [ 3] Visit: 01 (STISacq) Visit Requirements: On Hold Comments: Additional Comments: Acqusition Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 -TAU-CET STIS/CCD ACQ F28X50OII MIRROR ACQTYPE=POINT 1 0.1 S (0.1 I S) ------------------------------------------------------------------------------------------------------------------------------------ 2 -TAU-CET STIS/CCD ACQ/PEA 0.3X0.05N MIRROR 1 0.1 S (0.1 K D S) ------------------------------------------------------------------------------------------------------------------------------------ 3 -TAU-CET STIS/NUV ACCUM 0.2X0.2 E230H 2713 1 323 S (323 -MAMA S) ------------------------------------------------------------------------------------------------------------------------------------ 4 -TAU-CET STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=900 1 842 S (842 -MAMA G S) ------------------------------------------------------------------------------------------------------------------------------------ 5 -TAU-CET STIS/FUV TIME-TA 0.2X0.2 E140M 1425 BUFFER-TIME=900 1 2566 S -MAMA G (2566 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ -TAU-CET 1 STIS/CCD ACQ F28X50OI MIRROR none none none none 1 N/A II -TAU-CET 2 STIS/CCD ACQ/PEA 0.3X0.05 MIRROR none none none none 1 N/A K ND -TAU-CET 3 STIS/NUV ACCUM 0.2X0.2 E230H 2713 none none none none 1 N/A -MAMA -TAU-CET 4 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA G -TAU-CET 5 STIS/FUV TIME-TA 0.2X0.2 E140M 1425 none none none none 2 N/A -MAMA G 18184( 3) - 15-Sep-2025 19:38:19 - [ 4] Summary Form for Proposal 18184 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 0.2X0.2, 0.3X0.05ND, F28X50OIII ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ, ACQ/PEAK, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT, BUFFER-TIME=900 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements E140M, E230H, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names -TAU-CET ------------------------------------------------------------------------------------------------------------------------------------