A. Z. Bonanos’s research while affiliated with National Observatory of Athens and other places

Red Supergiants (RSGs) are cool, evolved massive stars in the last evolutionary stage before exploding as a supernova. However, the most luminous RSGs may evolve blueward before exploding, given the observational evidence for luminous, warm, post-RSG objects and the lack of supernova progenitors originating from luminous RSGs. In this work, we analyze WOH G64, considered since the 1980s as the most extreme RSG in the Large Magellanic Cloud in terms of its size, luminosity, and mass-loss rate. Time-series photometry over the last 30 years reveals a sudden, yet smooth change from semi-regular to irregular variability in 2014. Multi-epoch optical spectroscopy confirms the transition, as WOH G64 now exhibits properties of a B[e] star in the optical, and warm-star features in the near-infrared. We report that WOH G64 has transitioned from a RSG to a Yellow Hypergiant and, moreover, has a B-star companion. The dramatic transition can be explained by: a) binary interactions partially stripping the envelope, b) the return of WOH G64 to a quiescent state after an outstanding eruption exceeding 30 years, and c) the expulsion of its outer layers due to a pre-SN superwind phase, indicating its imminent explosion. WOH G64 offers a unique opportunity to witness stellar evolution in real-time, providing crucial clues for the late phases of massive stars and their resulting supernovae.

The rate and mechanism of mass loss of red supergiants (RSGs) remain poorly understood, especially at low metallicities. Motivated by the new empirical prescription by Yang et al. 2023, based on the largest and most complete sample in the Small Magellanic Cloud, we investigate the impact of different popular and recent RSG mass-loss prescriptions that span a range of RSG mass-loss rates on the evolution and observable properties of single massive stars. Our results show that higher mass-loss rates result in earlier envelope stripping and shorter RSG lifetimes, particularly for the more luminous stars, leading to a steeper luminosity function and predicting hotter final positions for the SN progenitors. None of the considered mass-loss prescriptions is fully consistent with all observational constraints, highlighting ongoing uncertainties in deriving and modeling RSGs mass loss. The mass-loss rates suggested by Kee et al. predict rapid envelope stripping, inconsistent with the observed population of luminous RSGs and SN progenitor detections, while the models implementing the commonly used de Jager et al. and the recent Beasor et al. prescriptions overestimate the number of luminous RSGs. While the increased mass-loss rates for luminous RSGs predicted by Yang et al. lead to better agreement with the observed RSG luminosity function, naturally reproducing the updated Humphreys-Davidson limit, they also produce luminous yellow supergiant progenitors not detected in nearby supernovae. We also estimate that binary interactions tend to slightly increase the formation of luminous RSGs due to mass accretion or merging. Our study examines the impact of RSG mass loss during the late stages of massive stars, highlighting the significance of using comprehensive observational data, exploring the uncertainties involved, and considering the effects of binary-induced or episodic mass loss.

Mass loss is a key property to understand stellar evolution and in particular for low-metallicity environments. Our knowledge has improved dramatically over the last decades both for single and binary evolutionary models. However, episodic mass loss although definitely present observationally, is not included in the models, while its role is currently undetermined. A major hindrance is the lack of large enough samples of classified stars. We attempted to address this by applying an ensemble machine-learning approach using color indices (from IR/ Spitzer and optical/Pan-STARRS photometry) as features and combining the probabilities from three different algorithms. We trained on M31 and M33 sources with known spectral classification, which we grouped into Blue/Yellow/Red/B[e] Supergiants, Luminous Blue Variables, classical Wolf-Rayet and background galaxies/AGNs. We then applied the classifier to about one million Spitzer point sources from 25 nearby galaxies, spanning a range of metallicites ( ). Equipped with spectral classifications we investigated the occurrence of these populations with metallicity.

Episodic mass loss 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. We introduce the ERC-funded ASSESS project (2018-2024), which aims to determine whether episodic mass loss is a dominant process in the evolution of the most massive stars, by conducting the first extensive, multi-wavelength survey of evolved massive stars in the nearby Universe. It hinges on the fact that mass-losing stars form dust and are bright in the mid-infrared. We aim to derive physical parameters of ∼1000 dusty, evolved massive stars in ∼25 nearby galaxies and estimate the amount of ejected mass, which will constrain evolutionary models, and quantify the duration and frequency of episodic mass loss as a function of metallicity. The approach involves applying machine-learning algorithms to select dusty, luminous targets from existing multi-band photometry of nearby galaxies. We present the first results of the project, including the machine-learning methodology for target selection and results from our spectroscopic observations so far. The emerging trend for the ubiquity of episodic mass loss, if confirmed, will be key to understanding the explosive early Universe and will have profound consequences for low-metallicity stars, reionization, and the chemical evolution of galaxies.

The role of mass loss from massive stars, especially episodic mass loss, is one of the outstanding open questions facing stellar evolution theory. Multiple lines of evidence are pointing to violent, episodic mass-loss events being responsible for removing a large part of the massive stellar envelope, especially in low-metallicity galaxies. The ERC ASSESS project aims to determine whether episodic mass loss is a dominant process 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 project hinges on the fact that mass-losing stars form dust and are bright in the mid-infrared. We aim to investigate the properties of evolved targets in nearby galaxies and estimate the amount of ejected mass, which will constrain evolutionary models. In this work we present some of our first observational results from the galaxies NGC 6822 and IC 10 obtained with OSIRIS (GTC).

There is evidence that some red supergiants (RSGs) experience phases of episodic mass-loss. These episodes yield more extreme mass-loss rates, further stripping the envelope of the RSG, significantly affecting the further evolution towards the final collapse of the star. Mass lost through RSG outbursts/superwinds will flow outwards and form dust further out from the surface and this dust may be detected and modelled. Here, we aim to derive the surface properties and estimate the global properties of Mid-IR bright RSGs in the Magellanic Clouds. These properties will then be compared to evolutionary predictions and used for future spectral energy distribution fitting studies to measure the mass-loss rates from present circumstellar dust.

Context. Despite mounting evidence that extreme red supergiants (RSGs) undergo episodic mass-loss events, their role in RSG evolution remains uncertain. Critical questions remain unanswered, such as whether or not these events can strip the star, and their timescale and frequency. Aims. This study delves into [W60] B90, one of the most luminous and extreme RSGs in the Large Magellanic Cloud (LMC), with our aim being to search for evidence of episodic mass loss. Our discovery of a bar-like nebular structure at 1 pc, which is reminiscent of the bar around Betelgeuse, raised the question of whether [W60] B90 also has a bow shock, motivating the present study. Methods. We collected and analyzed proper motion data from Gaia, as well as new multi-epoch spectroscopic and imaging data, and archival time-series photometry in the optical and mid-infrared (MIR). We used MARCS models to derive the physical properties of the star from the spectra. Results. We find [W60] B90 to be a walkaway star, with a supersonic peculiar velocity in the direction of the bar. We detect shocked emission between the bar and the star, based on the [S II ]/H α > 0.4 criterion, providing strong evidence for a bow shock. The 30 yr optical light curve reveals semi-regular variability, showing three similar dimming events with Δ V ~ 1 mag, a recurrence of ~12 yr, and a rise time of 400 days. We find the MIR light curve to vary by 0.51 mag and 0.37 mag in the WISE1 and WISE2 bands, respectively, and by 0.42 mag and 0.25 mag during the last dimming event. During this event, optical spectroscopy reveals spectral variability (M3 I to M4 I), a correlation between the T eff and the brightness, increased extinction, and, after the minimum, spectral features incompatible with the models. We also find a difference of >300 K between the T eff measured from the TiO bands in the optical and the atomic lines from our J -band spectroscopy. Conclusions. [W60] B90 is a more massive analog of Betelgeuse in the LMC and therefore the first single extragalactic RSG with a suspected bow shock. Its high luminosity of log( L /L ⊙ ) = 5.32 dex, mass-loss rate, and MIR variability compared to other RSGs in the LMC indicate that it is in an unstable evolutionary state, undergoing episodes of mass loss. Investigating other luminous and extreme RSGs in low-metallicity environments using both archival photometry and spectroscopy is crucial to understanding the mechanism driving episodic mass loss in extreme RSGs in light of the Humphreys-Davidson limit and the “RSG problem”.

Mass loss during the red supergiant (RSG) phase plays a crucial role in the evolution of an intermediate-mass star; however, the underlying mechanism remains unknown. We aim to increase the sample of well-characterized RSGs at subsolar metallicity by deriving the physical properties of 127 RSGs in nine nearby southern galaxies. For each RSG, we provide spectral types and used marcs atmospheric models to measure stellar properties from their optical spectra, such as the effective temperature, extinction, and radial velocity. By fitting the spectral energy distribution, we obtained the stellar luminosity and radius for 92 RSGs, finding that 50 of them have log$(L/ odot 5.0$ and six RSGs have $R odot$. We also find a correlation between the stellar luminosity and mid-IR excess of 33 dusty variable sources. Three of these dusty RSGs have luminosities exceeding the revised Humphreys-Davidson limit. We then derived a metallicity-dependent $J-K_s$ color versus temperature relation from synthetic photometry and two new empirical $J-K_s$ color versus temperature relations calibrated on literature TiO and J-band temperatures. To scale our derived cool TiO temperatures to values that are in agreement with the evolutionary tracks, we derived two linear scaling relations calibrated on J-band and i-band temperatures. We find that the TiO temperatures are more discrepant as a function of the mass-loss rate, and discuss future prospects of the TiO bands as a mass-loss probe. Finally, we speculate that three hot dusty RSGs may have experienced a recent mass ejection (12 of the K-type sample) and classify them as candidate Levesque-Massey variables.

This study delves into [W60] B90, one of the most luminous and extreme Red Supergiants (RSGs) in the Large Magellanic Cloud (LMC), aiming to search for evidence of episodic mass loss. Our discovery of a bar-like nebular structure at 1 pc, reminiscent of the bar around Betelgeuse, raised the question of whether [W60] B90 also has a bow shock. We collected and analyzed proper motion data from Gaia, as well as new multi-epoch spectroscopic and imaging data, and archival time-series photometry in the optical and mid-infrared. We found [W60] B90 to be a walkaway star, with a supersonic peculiar velocity in the direction of the bar. We detected shocked emission between the bar and the star, based on the [S II]/H$\alpha$ > 0.4 criterion, providing strong evidence for a bow shock. The 30-year optical light curve revealed semi-regular variability, showing three similar dimming events with $\Delta V \sim 1$ mag, a recurrence of $\sim$12 yr, and a rise time of 400 d. We found the mid-IR light curve to vary by 0.51 mag and 0.37 mag in the WISE1 and WISE2 bands, respectively, and by 0.42 mag and 0.25 mag during the last dimming event. During this event, optical spectroscopy revealed spectral variability (M3I to M4I), a correlation between the $T_{\rm eff}$ and the brightness, increased extinction, and, after the minimum, spectral features incompatible with the models. We also found a difference of >300 K between the $T_{\rm eff}$ measured from the TiO bands in the optical and the atomic lines from our J-band spectroscopy. We inferred that [W60] B90 is a more massive analog of Betelgeuse in the LMC and the first extragalactic single RSG with a suspected bow shock. Its high luminosity $\log(L/L_{\odot})=5.32$ dex, mass-loss rate, and mid-IR variability compared to other RSGs in the LMC, indicate that it is in an unstable evolutionary state undergoing episodes of mass loss.

We present results of the Near-Earth objects Lunar Impacts and Optical TrAnsients (NELIOTA) campaign for lunar impact flashes observed with the 1.2 m Kryoneri telescope. From August 2019 to August 2023, we report 113 validated and 70 suspected flashes. For the validated flashes, we calculate the physical parameters (masses, radii) of the corresponding projectiles, the temperatures developed during the impacts, and the expected crater sizes. For the multiframe flashes, we present light curves and thermal evolution plots. Using the whole sample of NELIOTA that encompasses 192 validated flashes in total from 2017, the statistics of the physical parameters of the meteoroids, the peak temperatures of the impacts, and the expected crater sizes has been updated. Using this large sample, empirical relations correlating the luminous energies per photometric band were derived and used to roughly estimate the parameters of 92 suspected flashes of the NELIOTA archive. For a typical value of the luminous efficiency, we found that the majority ($>75$) of the impacting meteoroids have masses between 1 and 200 g, radii between 0.5 and 3 cm and produced craters up to 3.5 m. 85 of the peak temperatures of the impacts range between 2000 and 4500 K. Statistics regarding the magnitude decline and the cooling rates of the multiframe flashes are also presented. The recalculation of the appearance frequency of meteoroids (lying within the aforementioned ranges of physical parameters) on the Moon yields that the total lunar surface is bombarded with 7.4 sporadic meteoroids per hour and up to 12.6 meteoroids per hour when the Earth-Moon system passes through a strong meteoroid stream. By extrapolating these rates on Earth, the respective rates for various distances from its surface are calculated and used to estimate the probability of an impact of a meteoroid with a hypothetical infrastructure on the Moon, or with a satellite orbiting Earth for various impact surfaces and duration times of the missions.