Measuring mass loss via metal lines from the very young planet AU Mic b

0 Assigned ID: 402 ----- Attempting Submission 1 (Fri Mar 06 05:29:48 GMT 2020) ----- HST Phase I Proposal 402 successfully submitted. Receipt: # 402-1 SCHEDULABILITY has been set to 100 for all visits and all over/under-run times have been adjusted. 22713731 ----- Attempting Submission 1 (Wed Jun 24 16:27:25 GMT 2020) ----- HST Phase II Proposal 16164 successfully submitted. Receipt: # 16164-1 ----- Attempting Submission 2 (Fri Jun 26 22:03:41 GMT 2020) ----- HST Phase II Proposal 16164 successfully submitted. Receipt: # 16164-2 ----- Attempting Submission 3 (Mon Jun 29 20:06:40 GMT 2020) ----- ----- Attempting Submission 4 (Wed Mar 10 22:42:25 GMT 2021) ----- ----- Attempting Submission 5 (Thu Apr 01 22:18:07 GMT 2021) ----- Measuring mass loss via metal lines from the very young planet AU Mic b The known populations of super-Earths and sub-Neptunes with orbital periods less than ~100 days have been shaped by atmospheric evaporation, leading to the observed Radius Valley near 1.7 Earth radii. However, it is currently unclear what the dominant mass loss mechanism is for these planets. The two likely culprits are photoevaporation by the host star's high energy radiation and core-powered evaporation, where the latent heat of formation in the planet's core causes the atmosphere to expand and escape. Distinguishing between these two mass loss mechanisms will require much more precise exoplanet radii statistics or, alternatively, measurements of mass loss for planets of varying ages and incident high energy flux. We propose to address this critical question by using COS to observe the transit of the ~20 million year old planet AU Mic b and measure its mass loss rate in UV metal lines of carbon, silicon, and oxygen. AU Mic is the brightest pre-main sequence star known to host a transiting planet and offers a unique opportunity to detect the planet's atmosphere at very high signal-to-noise in a number of UV mass loss indicators. Furthermore, information from the UV metal lines is complementary to absorption in the hydrogen Lyman-alpha line, which is heavily attenuated by the ISM. The metal lines trace material from the base of the thermosphere, whereas Lyman-alpha samples the highly extended unbound exosphere. Measurements of young planet evaporation rates are vital benchmarks for atmospheric evolution models and AU Mic b is currently the best known exoplanet to perform such an experiment. We will observe three transits of AU Mic b in order to measure its extended atmosphere in the metal lines O I 1302.2,1304.9,1306.0, C II 1334.5,1336.7, and Si III 1206.5 angstroms. We will use the COS G130M setting with CENWAVE=1222 angstroms. This provides coverage from approximately 1060 - 1360 angstroms with a detector gap from 1210-1225 angstroms. This setting includes all of the spectral lines of interest, as well as the C III multiplet near 1175 angstroms which we will use to monitor the star's activity level. The same COS setting will be used for all observations. AU Mic is visible for 3000 seconds during an HST orbit. Assuming a total exposure time of 3000 seconds, we expect to achieve a S/N of 65 - 85 in the O I lines and 75 - 100 in the C II lines. Based on our simulated transits, this results in 68% uncertainties of 1% for the individual transit flux measurements. AU Mic b has an orbital period of 8.46321 days and a transit duration of 3.5 hours. According to the APT visit planner, there are 20 possible blocks of time available in during Cycle 28 in which to schedule our transit observations. Thus there should be no obstacle to scheduling the transits. The ephemeris was determined jointly using TESS and Spitzer data collected in August 2018 and February and September 2019, respectively. The uncertainty on the period is 0.00004 days and the uncertainty on the transit midpoint is 0.0007 days. Propagating the period and midpoint errors predicts that the timing of any transit scheduled in Cycle 28 should be accurate to 2 minutes, which is well within the acceptable timing uncertainty for our science case. In order to obtain the longest out-of-transit baseline for each transit, we will stagger the out-of-transit observations for each visit in the following way: for 2 out of 3 transits we will obtain 2 pre-transit exposures and for 1 out of 3 transits we will obtain 2 post-transit exposures. Each transit will have 3 in-transit exposures. We believe this strategy is optimal so that the out-of-transit baseline extends as far as possible from ingress or egress, minimizing the possibility of a highly extended atmosphere creating pre- or post-transit absorption that could skew the in-transit absorption depths. With this strategy in mind, each visit must be scheduled no earlier than planetary orbital phase 0.97 and no later than 0.99. AU Mic is a pre-main sequence M-star with effective temperature T=3700+-100 K and must be considered under the Bright Object Protection Considerations for M Dwarf Flare events (Osten 2017). It exhibits flares with energy ~10^30 ergs less than once every ~16 days (Loyd et al. 2018). This can be compared to the active flare star AD Leo which experiences a flare with the same energy 5 times per day. Thus AU Mic is highly unlikely to show a strong flare during our observations. Our chosen COS conguration also does not contain any of the strongest FUV emission lines (i.e., Ly-alpha, C IV) that are most likely to cause detector damage. We have checked the emission line fluxes in the G130M/1222 bandpass and they fall within the safe range for active M dwarfs. P.I. P. Wilson Cauley is an expert in transmission spectrsocopy of close-in planets and also has expertise in the accretion and outflow processes of young stars. He is experienced working with HST UV data (e.g., Cauley & Johns-Krull 2016, ApJ, 825,147). He is also experienced at modeling atomic absorption in various geometries around short-period planets. Co-I Kevin France is an expert in UV spectroscopy and has been part of or led a number of successful HST campaigns, including the MUSCLES treasury survey (France et al. 2016, ApJ, 820, 89). He is highly knowledgable about the UV emission from young and low-mass stars. Co-I Elisabeth Newton is an expert on M-dwarf spin evolution and is experienced at working with UV HST data, specifically Lyman-alpha transit observations of exoplanets. Co-I Allison Youngblood is an expert in stellar UV spectroscopy and is experienced at re-creating stellar Lyman-alpha profiles. Co-I Peter Plavchan is the author of the AU Mic b discovery paper and is an expert on the system. He is also highly experienced at discovering and modeling exoplanet transits and RV data. Co-I Eric Gaidos has expertise in multiple areas of exoplanet observations and planet detection, formation, and evolution. Co-I Peter Gao is an expert in the chemistry of planetary atmospheres and also the modeling of exoplanet atmospheres, including the effects of clouds and hazes. Co-I Hilke Schlichting is an expert in planetary science and applying theoretical models of planetary formation and evolution to exoplanet populations. Co-I Akash Gupta has expertise in theoretical modeling of planetary formation and evolution. Co-I Sivan Ginzburg is an expert at exoplanet formation and evolution theory. Exoplanets and Exoplanet Formation Stellar Physics and Stellar Types 6 false false true false false false false SUPPORTED STAR M III-I NO This object was generated by the target selector and retrieved from the SIMBAD database. J2000 J2000 COS/FUV Spectroscopic No No G130M 1222 1617315170948 The scheduling data was generated by CASM 29.0. CASM was run in Phase II mode. 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