Lyman alpha absorption from the only mini Neptune with measured helium outflow

0 Assigned ID: 2108 ----- Attempting Submission 1 (Fri Apr 09 18:51:23 GMT 2021) ----- HST Phase I Proposal 2108 successfully submitted. Receipt: # 2108-1 ----- Attempting Submission 2 (Fri Apr 09 22:48:57 GMT 2021) ----- HST Phase I Proposal 2108 successfully submitted. Receipt: # 2108-2 I might change the phases a bit before the Phase II deadline. Depending on the geocoronal emission in the first visit, I might change the slit size for future visits. If the "Hail Mary" visit proves too hard to schedule, I'll relax the phase requirements. 2095190647 ----- Attempting Submission 1 (Thu Jul 08 23:52:45 GMT 2021) ----- Lyman alpha absorption from the only mini Neptune with measured helium outflow TOI 560.01 is a mini Neptune orbiting a young (400-600 Myr) and nearby (32 pc) K star, the first and only mini Neptune with measured helium absorption from its photoevaporating atmosphere. We propose to observe three transits of the planet with STIS/G140M in order to measure Lyman alpha absorption from the outflow. Photoevaporation is especially interesting to probe in this regime because it predominantly occurs in the active youth of a star's life, creating the ``Fulton gap" in radius which divides super-Earths (1--1.8 REarth) from mini-Neptunes (2--3 REarth). These observations will allow us to measure the size, shape, and kinematic structure of the exosphere, constrain the mass loss rate, and evaluate the magnitude of any visit-to-visit variability in the outflow structure. The combination of Lyman alpha and He I 1083 nm observations will provide stringent constraints on hydrodynamical models. These two wavelengths provide complementary information: Ly alpha probes high-velocity hydrogen in the tenuous outer reaches of the exosphere, while helium probes low-velocity helium in the denser part of the exosphere, much closer to the planet surface. Finally, recent TESS observations indicate that there may be another transiting planet in the TOI 560 system on an exterior orbit, although it has not yet been confirmed. Multi-planet systems are ideal for studying mass loss because the two planets share the same high-energy irradiation history and similar formation histories, making comparative planetology possible. Our program will observe 3 transits of HD 73583b. Each transit observation will consist of 5 consecutive orbits. Between 1 and 2 of these orbits are in-transit, depending on the exact phase of the start of observations. We set the phase requirements to +- 0.5 hours for two visits (Centrist 1/2), but set a much more stringent requirement for one visit (Hail Mary) to maximize the amount of in-transit observation time, at the cost of schedulability. We'll probably adjust the phase requirements as visits are completed to try to maximize the phase coverage. The ephemeris uncertainty is negligible, at less than 2 minutes. We planned our observations so that the first orbit of each visit has an ACQ and ACQ-PEAKUP exposure, followed by a science exposure and wavelength calibration. Subsequent orbits only have a science exposure and a wavelength calibration exposure, with no ACQ or ACQ-PEAKUP. Because the few minutes' difference in orbital visibility due to precession has minimal impact on our science, we change the schedulability parameter from 30 to 90 to make scheduling easier. If possible, we kindly ask the Program Coordinator to expand our science observations to fill up the entire orbital visibility for the orbits that are actually scheduled. Reduced gyro mode impacts our program by limiting the number of scheduling opportunities. Otherwise, we do not forsee major impacts from RGM. Michael Zhang studies exoplanet atmospheres and was the PI of program 16319, which measured Ly alpha outflow from another mini Neptune. He has additionally studied helium mass loss using Keck/NIRSPEC. Heather Knutson has extensive expertise in many areas of exoplanet atmosphere observations, especially with HST and Spitzer. Fei Dai is an expert on numerical modelling of exoplanet mass loss, especially from small planets such as mini Neptunes. Exoplanets and Exoplanet Formation 6 false false false false false false false false SUPPORTED STAR EXTRA-SOLAR PLANET EXTRA-SOLAR PLANETARY SYSTEM K V-IV NO The coordinates and proper motion were obtained from Gaia EDR3. J2000 J2000 STIS/FUV-MAMA Spectroscopic No No G140M 1222 /tmp/OPVisit14981447887012509220.xml 1625788349467 The scheduling data was generated by CASM 29.2. CASM was run in Phase II mode. 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