Periodic Reporting for period 4 - ASSESS (Episodic Mass Loss in the Most Massive Stars: Key to Understanding the Explosive Early Universe)
Periodo di rendicontazione: 2023-03-01 al 2024-08-31
Multiple lines of evidence from both the massive star and supernova communities are pointing to violent, episodic mass-loss events being responsible for removing a large part of the envelope of massive stars, especially in low-metallicity galaxies. Episodic mass loss, however, is not understood theoretically, neither accounted for in state-of-the-art models of stellar evolution, which has far-reaching consequences for many areas of astronomy. The goal of ASSESS was to determine the role of episodic mass loss in the evolution of the most massive stars by conducting the first extensive, multi-wavelength survey of evolved massive stars in the nearby Universe. The target selection was based on the fact that mass-losing stars form dust and are bright in the mid-infrared. ASSESS aimed to (i) derive physical parameters of a large sample of dusty, evolved massive stars and estimate the amount of ejected mass, (ii) constrain evolutionary models, (iii) quantify the duration and frequency of episodic mass loss as a function of metallicity.
ASSESS developed a photometric classifier, i.e. a machine-learning algorithm, based on existing multi-band photometry and applied it to luminous sources in ~26 nearby galaxies, yielding reliable classifications of over 275,000 dusty sources in 21 nearby galaxies. ASSESS also conducted a large spectroscopic survey of dusty, evolved stars in nearby galaxies spanning a range of metallicity, resulting in the largest catalog of evolved massive stars (185 stars) beyond the Local Group, including 129 low-metallicity red supergiants, 3 new luminous blue variables and 6 new supergiant B[e] stars. We investigated the different temperature scales of RSG, i.e. those based on TiO lines measured in the optical vs. atomic lines measured in the near-IR and derived a scaling relation that translates one to the other. We also found substantial changes in the spectral types (i.e. effective temperatures) of >10% of our RSGs, providing evidence for episodic mass loss and an estimate on the occurrence rate.
ASSESS for the first time measured precise and accurate mass-loss rates using the largest-ever sample of red supergiants in the Magellanic Clouds, finding enhanced mass loss at luminosities above log(L/Lo)~4.5 and lower rates by 2-3 orders of magnitude, compared to the mass-loss relations applied in evolutionary models. Furthermore, we applied four empirical mass-loss rate prescriptions to stellar evolutionary models and found that no prescription can explain all the observational constraints for red supergiants. We highlight the extreme (in luminosity and radius) red supergiant [W60] B90 in the Large Magellanic Cloud (LMC), which is a massive analog of Betelgeuse, exhibiting 3 dimming events, as well as evidence for a bow shock. Finally, we discovered that WOH G64, a red supergiant in the LMC boasting the highest luminosity and mass-loss rate, underwent a never-seen before transition to the blue in 2013-2014, which has implications for the ‘red supergiant problem’ and the Humphreys-Davidson limit.
The ASSESS team has been awarded 44 h on FORS2/VLT to obtain spectra of over 900 stars in ten southern galaxies and 67 h (partially completed) on OSIRIS/GTC to obtain spectra of 90 priority stars in 3 northern galaxies. The catalogs of massive stars and their properties are publically available. We also obtained observations with the Magellan telescopes over 10 hours and with EMIR/GTC to investigate the temperature scale and spectral variability of luminous and dusty red supergiants. The results of this project were presented in 41 scientific publications (36 of which are in peer-reviewed journals and 5 are conference proceedings), 15 conferences, 11 workshops and 6 seminars.
ASSESS for the first time measured precise and accurate mass-loss rates using the largest-ever sample of red supergiants in the Magellanic Clouds, finding enhanced mass loss at luminosities above log(L/Lo)~4.5 and lower rates by 2-3 orders of magnitude, compared to the mass-loss relations applied in evolutionary models. Furthermore, we applied four empirical mass-loss rate prescriptions to stellar evolutionary models and found that no prescription can explain all the observational constraints for red supergiants. We highlight the extreme (in luminosity and radius) red supergiant [W60] B90 in the Large Magellanic Cloud (LMC), which is a massive analog of Betelgeuse, exhibiting 3 dimming events, as well as evidence for a bow shock. Finally, we discovered that WOH G64, a red supergiant in the LMC boasting the highest luminosity and mass-loss rate, underwent a never-seen before transition to the blue in 2013-2014, which has implications for the ‘red supergiant problem’ and the Humphreys-Davidson limit.
The ASSESS team has been awarded 44 h on FORS2/VLT to obtain spectra of over 900 stars in ten southern galaxies and 67 h (partially completed) on OSIRIS/GTC to obtain spectra of 90 priority stars in 3 northern galaxies. The catalogs of massive stars and their properties are publically available. We also obtained observations with the Magellan telescopes over 10 hours and with EMIR/GTC to investigate the temperature scale and spectral variability of luminous and dusty red supergiants. The results of this project were presented in 41 scientific publications (36 of which are in peer-reviewed journals and 5 are conference proceedings), 15 conferences, 11 workshops and 6 seminars.
ASSESS has advanced our knowledge of episodic mass loss beyond the state of the art:
-The machine-learning photometric classifier that we have developed is novel and its application has yielded a dataset that will be very valuable for research within the massive star community and beyond.
-The newly derived mass-loss rates for red supergiants are extremely valuable for producing accurate stellar evolution models. We expect that our new mass-loss prescription for red supergiants will be widely used.
-The detailed study of two extreme red supergiants has provided great insight and advanced our knowledge for these objects, paving a way forward to understanding the physics governing these extreme evolutionary states.
-The catalogs of spectroscopically classified massive stars in both southern and northern galaxies are made available to the community for further use.
-The machine-learning photometric classifier that we have developed is novel and its application has yielded a dataset that will be very valuable for research within the massive star community and beyond.
-The newly derived mass-loss rates for red supergiants are extremely valuable for producing accurate stellar evolution models. We expect that our new mass-loss prescription for red supergiants will be widely used.
-The detailed study of two extreme red supergiants has provided great insight and advanced our knowledge for these objects, paving a way forward to understanding the physics governing these extreme evolutionary states.
-The catalogs of spectroscopically classified massive stars in both southern and northern galaxies are made available to the community for further use.