0 Assigned ID: 4373 ----- Attempting Submission 1 (Wed May 24 15:51:04 GMT 2023) ----- HST Phase I Proposal 4373 successfully submitted. Receipt: # 4373-1 ----- Attempting Submission 2 (Wed May 24 19:52:27 GMT 2023) ----- HST Phase I Proposal 4373 successfully submitted. Receipt: # 4373-2 ----- Attempting Submission 3 (Wed May 24 19:53:28 GMT 2023) ----- Unschedulable Visit Unschedulable Visit Unschedulable Visit In this new Phase-2, a constraint BETWEEN "16-APR-2024:22:00:00" and "17-APR-2024:00:00:00" for the start of the Visit 01 has been added, such that this visit can be scheduled to be executed from April 16 ~23:00 to April 17 ~04:00. 1288785470 ----- Attempting Submission 1 (Wed Sep 13 19:50:51 GMT 2023) ----- HST Phase II Proposal 17421 successfully submitted. Receipt: # 17421-1 ----- Attempting Submission 2 (Wed Sep 13 22:09:00 GMT 2023) ----- ----- Attempting Submission 3 (Wed Mar 13 09:31:29 GMT 2024) ----- ----- Attempting Submission 4 (Fri Mar 29 12:09:24 GMT 2024) ----- The visit planner put a flag "unschedulable visit" but this is just an update of the Phase 2 recovered from STScI, to which a BETWEEN constraint has been added following the advice given by the STScI support. Exocomets are small icy bodies, which orbit stars other than the Sun and sublimate when close to their star, producing a tail of dust and gas (like solar system comets). During transit, the gas causes spectroscopic absorption signatures that are routinely detected in several systems. The most prolific case is beta Pic, which continuously displays narrow, variable, non-photospheric absorption features superimposed on the much broader stellar lines. The features vary on short timescales (hours) and are seen at optical (e.g. CaII) and UV (e.g. FeII, SiIV) wavelengths. Previous HST observations of beta Pic in the UV showed the presence of AlIII lines in the exocomet gas. The mere existence of AlIII is puzzling as beta Pic is unable to twice photoionise Al. Hydrodynamical models suggest that these highly-ionised species are formed in a shock where compression and thus collisions within the shock surface are sufficient to generate AlIII. This is theorised to occur when the exocomets get sufficiently close to the star (a few stellar radii). We propose to monitor beta Pic to characterise the doppler movement of the AlIII lines and from this directly measure the exocomets' acceleration. This we use to calculate the comet-star distances. The highly ionised species are thought only to be present in the shock fronts of exocomets close to the star. Measuring the distance between the star and the exocomets will allow us to directly test this hypothesis. The observations cover a wavelength range with other species thought to only emerge in shocks. Measuring the transit ingress/egress times of these species will give us the composition of the shock as a function of radial distance. One line summary: We plan to observe exocometary absoprtion events in the beta Pictoris planetary system with STIS and to track these features by monitoring how the Al III absoprtion line varies as a function of time. The AlIII line is the only line capable of unequivocally detecting an exocometary shocks, given that the second most favourable line (SiIV, which we previously analysed) yields results right at the edge of statistical significance. In order to measure the acceleration as accurately as possible and to avoid blended absorption features, we will use the higher resolution and sensitivity of STIS (compared to COS) using the E230H setup giving. Our setup with the central wavelength set at 1763 Ang, will provide a wavelength coverage from 1624 to 1901Ang. This wavelength range includes many very interesting lines, in which GHRS data has shown detections of transiting exocomets. The line list in this wavelength range includes Fe II, Al II, SI, SiII}, CI, and NiII. Due to the brightness of beta Pic (our target), STIS is in addition required to avoid saturating the detector. Due to the brightness of the object we will be observing in ACCUM mode. Justification for visits and orbits: --- To ensure the two main science goals (1. Test the hypothesis that AlII are caused by shocks and 2. Measure the species dependent size of the shock front) are met we deem it necessary to obtain observations on three separate visits. Three visits ensures completely different exocometry absorption signatures and would avoid the possible conclusion that a detections from a single visit is an outlier. We would further avoid the blending of exocometary signatures. We initially wrote two visits, but believe it is more important to have three visits (with less orbits) than 2 visits with more orbits due to the upredictable nature of the exocomet absorption features. This change was also in part motivated by the strong encouragement from STSci to avoid 3+ orbit visits. There will in total be three visits. The two first visits will consist of 3 orbits and the last visit will consist of 4 orbits. The team is led by Paul A. Strom (formely Paul A. Wilson), who has been the PI of 5 HST programs since Cycle 23. Those programs were dedicated to the observation of exocomets and transiting Hill sphere in the Beta Pictoris system. Of the papers published using this data, Paul Strom has taken the lead and published two papers with a third currently being written. The team also includes Alain Lecavelier, Alfred Vidal-Madjar, Grant Kennedy, Isabel Rebollido, Flavien Kiefer and Vincent Bourrier, who have extensive knowledge of HST- UV spectroscopy observation of exocomets (some since 1993). Several dozens of papers have come out from these observations of exocomets, mostly based on HST data. V. Bourrier and A. Lecavelier des Etangs developed a 3D numerical model of evaporating gaseous planets (the EVE code, e.g. Bourrier et al. 2016) that has been used to interpret their UV observations in terms of atmospheric mass loss, dynamical, and density structure. The code can be applied to exocometary tails, subjected to similar physics as the tails of evaporating planets, to interpret their spectroscopic signatures. New modules are being implemented in the EVE code, such as the coupling of dust and gas tails, and the presence of a shock induced by the stellar wind, which will be particularly useful to the present program. 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