arXiv subjects

E. Christodoulou

Publications and source records attributed to E. Christodoulou.

7 recordsLinked to original sources

arXiv2026

A machine-learning photometric classifier for massive stars in nearby galaxies II. The catalog

Mass loss is a key aspect of stellar evolution, particularly in evolved massive stars, yet episodic mass loss remains poorly understood. To investigate this, we need evolved massive stellar populations across various galactic environments. However, spectral classifications are challenging to obtain in large numbers, especially for distant galaxies. We addressed this by leveraging machine-learning techniques. We combined Spitzer photometry and Pan-STARRS1 optical data to classify point sources in 26 galaxies within 5 Mpc, and a metallicity range 0.07-1.36 Z$_\odot$. Gaia data release 3 (DR3) astrometry was used to remove foreground sources. Classifications are derived using a machine-learning model developed in our previous work. We report classifications for 1,147,650 sources, with 276,657 sources (~24%) being robust. Among these are 120,479 red supergiants (RSGs; ~11%). The classifier performs well even at low metallicities (~0.1 Z$_\odot$) and distances under 1.5 Mpc, with a slight decrease in accuracy beyond ~3 Mpc due to Spitzer's resolution limits. We also identified 21 luminous RSGs (log($L/L_\odot)\ge5.5$), 159 dusty yellow hypergiants in M31 and M33, as well as 6 extreme RSGs (log($L/L_\odot)\ge6$) in M31, challenging observed luminosity limits. Class trends with metallicity align with expectations, although biases exist. This catalog serves as a valuable resource for individual-object studies and James Webb Space Telescope target selection. It enables the follow-up on luminous RSGs and yellow hypergiants to refine our understanding of their evolutionary pathways. Additionally, we provide the largest spectroscopically confirmed catalog of extragalactic massive stars and candidates to date, beyond the Clouds, comprising 5,273 sources (including ~330 other objects).

astro-ph.GA↗

arXiv2026

The dramatic transition of the extreme Red Supergiant WOH G64 to a Yellow Hypergiant

Red Supergiants (RSGs) are cool, evolved massive stars in their final evolutionary stage before exploding as a supernova. However, the evolution and fate of the most luminous RSGs remain uncertain. Observational evidence for luminous warm, post-RSG objects and the apparent lack of luminous RSGs as supernova progenitors suggest a blueward evolution. Since the 1980s, WOH G64 has been considered the most extreme RSG in the Large Magellanic Cloud, given its large obscuration, outstanding size, luminosity, and mass-loss rate. Here we report a sudden, yet smooth change in its apparent nature. Time-series photometry and subsequent spectroscopy revealed the most extreme transition ever seen in the optical spectral features of a RSG. We discovered that WOH G64 is a rare, massive symbiotic binary system where the RSG transitioned to a Yellow Hypergiant. The dramatic transition can be explained either by the partial ejection of the pseudo-atmosphere during a common-envelope phase, or the return to a quiescent state after an outstanding eruption exceeding 30 years. WOH G64 offers a unique opportunity to witness stellar evolution in real-time and assess the role of binarity on the final phases of massive stars and their resulting supernovae.

astro-ph.SR↗

arXiv2025

Investigating episodic mass loss in evolved massive stars IV. Comprehensive analysis of dusty red supergiants in NGC 6822, IC 10, and WLM

Mass loss shapes the fate of massive stars; however, the physical mechanism causing it remains uncertain. We present a comprehensive analysis of seven red supergiants, for which we searched evidence of episodic mass loss, in three low-metallicity galaxies: NGC~6822, IC~10, and WLM. Initially, the spectral classification of their optical spectra was refined and compared to previous reported classifications, finding four sources that display spectral variability. We derived the physical properties of five of them using the \textsc{marcs} atmospheric models corrected for nonlocal thermal equilibrium effects to measure stellar properties from our new near-infrared spectra, such as the effective temperature, surface gravity, metallicity, and microturbulent velocity. Additional empirical and theoretical methods were employed to calculate effective temperatures, finding consistent results. We constructed optical and infrared light curves, discovering two targets in NGC~6822 with photometric variability between 1 and 2.5 mag in amplitude in r and ~ 0.5 mag in the mid-infrared. Furthermore, we discovered a candidate-dimming event in one of these sources. Periods for three red supergiants were determined using epoch photometry, which were consistent with the empirical estimations from literature period-luminosity relations. Our comprehensive analysis of all the available data for each target provides evidence for episodic mass loss in four red supergiants.

astro-ph.SR↗

arXiv2025

Episodic mass loss in the very luminous red supergiant [W60] B90 in the Large Magellanic Cloud

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$α$ > 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 $ΔV \sim 1$ 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 $\log(L/L_{\odot})=5.32$, 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.

astro-ph.SR↗

arXiv2024

Investigating episodic mass loss in evolved massive stars: II. Physical properties of red supergiants at subsolar metallicity

Mass loss during the red supergiant (RSG) phase plays a crucial role in the evolution of an intermediate massive 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 presented by Bonanos et al. For each RSG, we provide spectral types and used \textsc{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 97 RSGs, finding $\sim 50\%$ with log$(L/ \rm L_{\odot}) \geq 5.0$ and 6 RSGs with $R \gtrsim 1400 \,\ \rm R_{\odot}$. 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 derive 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 in agreement with the evolutionary tracks, we derive 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 3 hot, dusty RSGs may have experienced a recent mass ejection ($12\%$ of the K-type sample) and indicate them as candidate Levesque-Massey variables.

astro-ph.SR↗

arXiv2023

Investigating episodic mass loss in evolved massive stars: I. Spectroscopy of dusty massive stars in ten southern galaxies

The ASSESS project aims to determine the role of episodic mass-loss in the evolution of massive stars. As a first step, we construct a catalog of spectroscopically identified dusty, evolved massive stars in ten southern galaxies for which Spitzer point-source catalogs are available. We conducted multi-object spectroscopy of dusty massive star candidates in these galaxies (spanning Z = 0.06-1.6 Zo) using the VLT. We obtained 763 spectra in WLM, NGC 55, NGC 247, NGC 253, NGC 300, NGC 1313, NGC 3109, Sextans A, M83 and NGC 7793. The targets were selected using their Spitzer photometry, by prioritizing targets with a strong infrared excess. We determined a spectral classification for each target. Additionally, we used archival images from the HST to provide a visual classification for 80 targets, as a star, cluster, or galaxy. We provide a catalog of 541 spectroscopically classified sources including 185 massive stars, of which 154 are newly classified massive stars. The catalog contains 129 red supergiants, 27 blue supergiants, 10 yellow supergiants, four luminous blue variable candidates, seven supergiant B[e] stars and eight emission line objects. Evidence for circumstellar dust is found in 24% of these massive stars, based on their infrared colors. We report a success rate of 28% for identifying massive stars among our observed spectra, while the average success rate of our priority system in selecting evolved massive stars was 36%. Additionally, the catalog contains 21 background galaxies (including AGN and quasars), 10 carbon stars and 99 HII regions. We measured the line ratios [NII]/Ha and [SII]/Ha for 76 HII regions and 36 other spectra with nebular emission-lines, thereby identifying eight sources with shocked emission. We present the largest catalog of evolved massive stars and in particular of red supergiants in nearby galaxies at low Z beyond the Local Group.

astro-ph.GA↗

arXiv2023

Introducing the ASSESS project: Episodic Mass Loss in Evolved Massive Stars -- Key to Understanding the Explosive Early Universe

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 $\sim$1000 dusty, evolved massive stars in $\sim$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.

astro-ph.SR↗