0 Assigned ID: 7059 ----- Attempting Submission 1 (Tue Apr 14 12:33:40 GMT 2026) ----- HST Phase I Proposal 7059 successfully submitted. Receipt: # 7059-1 ----- Attempting Submission 2 (Thu Apr 16 14:54:46 GMT 2026) ----- Initial submission. The Bright Object Tool is producing alerts, but we are unsure of their origin. We checked through the ETC that the counts rates expected for a V=5.61 A1V main sequence star are well below the MAMA brightness limits. -945583934 ----- Attempting Submission 1 (Fri Jul 31 17:36:47 GMT 2026) ----- Characterisation of exocomets in the 49 Cet system 49 Ceti is a young (40 Myr) A1V star hosting a prominent debris disc and standing as one of only three systems where spectroscopic signatures of transiting exocomets have been confirmed. Previous HST observations revealed strong exocomet absorption in carbon lines (C II, C IV), but the absence of simultaneous coverage of refractory species (Mg II, Fe II) prevented any measurement of the volatile-to-refractory content of these objects, leaving their composition and formation history unconstrained. We propose to obtain high-resolution UV spectra of 49 Cet at five distinct epochs with HST/STIS, covering the full 1250-2900 A range at resolving power > 100,000. Each visit consists of 3 sequential orbits using the E140H and E230H gratings, providing simultaneous access to volatile tracers (C I, C II, C IV, O I) and refractory species (such as Fe II and Mg II). These observations will allow us to: (1) characterise the physical properties of 49 Cet exocomets (dynamics, distance to the star, ionisation state) and assess how these properties are shaped by the high stellar luminosity; (2) measure their elemental C/Fe ratios, which will yield valuable insight into their formation location and nature; and (3) test whether exocomet evaporation can explain the replenishment of the observed carbon-rich gaseous disc. This programme will deliver a complete characterisation of exocomets in the 49 Ceti system, opening a new window into the properties of small bodies across the diversity of extrasolar systems and how they can contribute to redistribute volatiles throughout planetary systems. The goal of HST program 18399 is to determine the physical and chemical properties of exocomets in the 49 Cet system. The observing strategy consists of obtaining 5 high-resolution spectra of 49 Cet at different epochs, each separated by at least 10 days. At each epoch, we expect to detect a small number of exocomet gas tails transiting the star, typically 0-3. The absorption signatures of these exocomets will be observed in numerous atomic species, including carbon, oxygen, iron, and magnesium. These observations will provide constraints on the bulk composition of exocomets in the 49 Cet system, as well as on the physical processes governing exocomet evolution around hot stars (Teff = 9000 K). Each visit will consist of 3 consecutive HST orbits using the STIS instrument. To cover the full 1280-2900 wavelength range, the exposures are distributed as follows: - Orbit 1: A single exposure with the FUV/MAMA/E140H grating at the i1380 central wavelength, covering lines of C I, C II, O I, and Si IV. A long exposure time (Texp = 1900s) is required to compensate for the low stellar flux at these wavelengths. This observation is divided into 5 sub-exposures, allowing us to monitor the temporal evolution of the detected exocomet signatures. - Orbit 2: Two exposures: - FUV/MAMA/E140H at the c1598 central wavelength (Texp = 1600s, divided into 4 sub-exposures), covering Si II, C I, IV, and Al II. - NUV/MAMA/E230H at the c1763 central wavelength (Texp = 600s), covering Fe II and Al III. - Orbit 3: Four exposures (Texp = 420s each), all with the NUV/MAMA/E230H grating: - c2013, covering C I, Cr II, and Zn II. - c2263, covering Ni II and Fe II. - c2513, covering Fe I and Fe II. - c2763, covering Fe II, Mg I, and Mg II. The E140H and E230H gratings provide a resolving power > 100,000, enabling the absorption signatures of individual transiting exocomets to be resolved in great detail. All observations will be obtained through the 0.2"x0.2" aperture. The maximum count rates predicted by the ETC are approximately 100,000 cts/s, well below the STIS brightness limit for non-variable targets. The expected S/N per resolution element ranges from 40 to 60 across the observed spectral range. All team members have long term expertise in both HST data analysis (STIS, ACS, COS) and modeling of exocomets transits. They are co-authors of more than one hundred papers on exoplanets atmospheres and observation of exocomets (including many papers in Nature publications, with a recent paper in Nature Scientific Reports on exocomets in Beta Pic : Lecavelier et al. 2022). They are first authors of dozens of papers directly related to HST observations covering both theoretical and observational aspects of exoplanetary science. They have been PIs of more than 10 HST programs on exoplanets transit, circumstellar disks and exocomets observations, including recent programs aimed at studying exocomets in cycle 30 (GO 17113, PI A. Lecavelier) and cycle 32 (GO 17790, PI. T. Vrignaud). 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