0 Assigned ID: 2214 ----- Attempting Submission 1 (Fri Apr 09 20:32:04 GMT 2021) ----- HST Phase I Proposal 2214 successfully submitted. Receipt: # 2214-1 ----- Attempting Submission 2 (Fri Apr 09 22:31:13 GMT 2021) ----- There is growing consensus amongst theorists that massive stars above 20Msun should collapse immediately to black holes (BHs) with little or no visible explosion, and while empirical evidence suggests there is a lack of SN progenitors above this mass detected via pre-explosion imaging, the statistical significance of this result is weak. Here, we are proposing to analyse 6 SN that each have both pre-explosion and late-time imaging available in the archive, but which have not had their progenitor masses estimated using image subtraction techniques. By returning to the sites, we will be able to 1) confirm the disappearance of the suspected progenitor and 2) increase the accuracy of progenitor mass estimates by using the image subtraction technique. The masses estimated for each of these objects will be complimented with masses determined for 4 further SN progenitors for which we are submitting a separate GO proposal, each with pre-explosion and but which have not been followed up at late times. If these 10 progenitors all follow the same mass distribution as those already in the sample, the 20 Msun BH threshold mass will reach the statistical significance necessary (4.4 sigma) to be considered a confirmation. If however these progenitors are higher in mass, the statistical significance of the lack of progenitors above 20 Msun is weakened, and could possibly imply a higher threshold for BH formation. Dr Emma Beasor has extensive knowledge of working with mid-IR data, and of modelling stellar SEDs with radiative transfer codes. Dr Beasor has published 12 papers on red supergiants, including 2 specifically addressing the red supergiant problem and 4 including extensive SED/radiative transfer modeling. Dr. Ben Davies is an expert on observations and modelling of Red Supergiants and the RSG Problem. He has a great deal of technical experience in IR observations, having been the PI of several HST, Spitzer and ground based programmes. In addition, he has published dozens of papers on radiative transfer modelling of RSGs. Dr. Nathan Smith is an expert in massive star evolution, eruptive transients, and supernovae, with a particular emphasis on mass loss through winds, eruptions, and binary interaction, and the corresponding circumstellar material around supernovae. His technical expertise is in UV/optical/IR spectroscopy and imaging of radiatively and shock excited circumstellar material, dusty circumstellar material, explosive transients, and massive star forming regions. Stellar Physics and Stellar Types 0 Regular false false false false false SUPPORTED