ACTINIUM ABUNDANCES IN STELLAR ATMOSPHERES

This paper presents a study of radioactive actinium in the atmospheres of stars located in galaxies with different chemical evolution history – namely, Przybylski's Star (HD 101065) in the Milky Way and the red supergiant PMMR27 in the Small Magellanic Cloud; it also reports the findings of the previous research of the red supergiant RM 1-667 in the Large Magellanic Cloud and the red giant BL138 in the Fornax dwarf spheroidal galaxy. The actinium abundance is close to that of uranium in the atmospheres of certain stars in the Milky Way’s halo and in the atmosphere of Arcturus. The following actinium abundances have been obtained (in a scale of lg N(H) = 12): for the red supergiants PMMR27 and RM 1- 667 lg N(Ac) = -1.7 and lg N(Ac) = -1.3, respectively, and for the red giant BL138 lg N(Ac) = -1.6. The actinium abundance in the atmosphere of Przybylski's Star (HD 101065) is lg N(Ac) = `0.94±0.09, which is more than two orders of magnitude higher than those in the atmospheres of the other studied stars.

The Sample of Red Supergiants in 12 Low-mass Galaxies of the Local Group

This work establishes the most complete sample of red supergiants (RSGs) in 12 low-mass galaxies (WLM, IC 10, NGC 147, NGC 185, IC 1613, Leo A, Sextans B, Sextans A, NGC 6822, Pegasus Dwarf, SMC, and LMC) of the Local Group, which forms a solid basis to study the properties of RSGs as well as the star formation rate and initial mass function of the galaxies. After removing the foreground dwarf stars by their obvious branch in the near-infrared color–color diagram ( J − H 0 / H − K 0 ) with the UKIRT/WFCAM and 2MASS photometry as well as the Gaia/EDR3 measurements of proper motion and parallax, RSGs are identified from their location in the color–magnitude diagram J − K 0 / K 0 of the member stars of the specific galaxy. A total of 2190 RSGs are found in 10 dwarf galaxies, and additionally, 4823 and 2138 RSGs are found in LMC and SMC, respectively. The locations of the tip of the red giant branch in the J − K 0 / K 0 diagram are determined to serve as an indicator of the metallicity and distance modulus of the galaxies.

Testing Evolutionary Models with Red Supergiant and Wolf–Rayet Populations

Despite the many successes that modern massive star evolutionary theory has enjoyed, reproducing the apparent trend in the relative number of red supergiants (RSGs) and Wolf–Rayet (WR) stars has remained elusive. Previous estimates show the RSG/WR ratio decreasing strongly with increasing metallicity. However, the evolutionary models have always predicted a relatively flat distribution for the RSG/WR ratio. In this paper we reexamine this issue, drawing on recent surveys for RSGs and WRs in the Magellanic Clouds, M31, and M33. The RSG surveys have used Gaia astrometry to eliminate foreground contamination and have separated RSGs from asymptotic giant branch stars using near-infrared colors. The surveys for WRs have utilized interference-filter imaging, photometry, and image subtraction techniques to identify candidates, which have then been confirmed spectroscopically. After carefully matching the observational criteria to the models, we now find good agreement in both the single-star Geneva and binary BPASS models with the new observations. The agreement is better when we shift the RSG effective temperatures derived from J − Ks photometry downwards by 200 K in order to agree with the Levesque TiO effective temperature scale. In an appendix we also present a source list of RSGs for the SMC which includes effective temperatures and luminosities derived from near-infrared 2MASS photometry, in the same manner as used for the other galaxies.

The impact of winds on the spectral appearance of red supergiants

The rate at which mass is lost during the Red Supergiant evolutionary stage may strongly influence how the star appears. Though there have been many studies discussing how RSGs appear in the mid and far-infrared (IR) as a function of their mass-loss rate, to date there have been no such investigations at optical and near-IR wavelengths. In a preliminary study we construct model atmospheres for RSGs which include a wind, and use these models to compute synthetic spectra from the optical to the mid-infrared. The inclusion of a wind has two important effects. Firstly, higher mass-loss rates result in stronger absorption in the TiO bands, causing the star to appear as a later spectral type despite its effective temperature remaining constant. This explains the observed relation between spectral type, evolutionary stage and mid-IR excess, as well as the mismatch between temperatures derived from the optical and infrared. Secondly, the wind mimics many observed characteristics of a ‘MOLsphere’, potentially providing an explanation for the extended molecular zone inferred to exist around nearby RSGs. Thirdly, we show that wind fluctuations can explain the spectral variability of Betelgeuse during its recent dimming, without the need for dust.

Constraining red supergiant mass-loss prescriptions through supernova radio properties

ABSTRACT Supernova (SN) properties in radio strongly depend on their circumstellar environment and they are an important probe to investigate the mass-loss of SN progenitors. Recently, core-collapse SN observations in radio have been assembled and the rise time and peak luminosity distribution of core-collapse SNe at 8.4 GHz has been estimated. In this paper, we constrain the mass-loss prescriptions for red supergiants (RSGs) by using the rise time and peak luminosity distribution of Type II SNe in radio. We take the de Jager and van Loon mass-loss rates for RSGs, calculate the rise time and peak luminosity distribution based on them, and compare the results with the observed distribution. We found that the de Jager mass-loss rate explains the widely spread radio rise time and peak luminosity distribution of Type II SNe well, while the van Loon mass-loss rate predicts a relatively narrow range for the rise time and peak luminosity. We conclude that the mass-loss prescriptions of RSGs should have strong dependence on the luminosity as in the de Jager mass-loss rate to reproduce the widely spread distribution of the rise time and peak luminosity in radio observed in Type II SNe.

Partial Stellar Explosions - Ejected Mass and Minimal Energy

Many massive stars appear to undergo enhanced mass loss during late stages of their evolution. In some cases, the ejected mass likely originates from non-terminal explosive outbursts, rather than continuous winds. Here we study the dependence of the ejecta mass, mej, on the energy budget E of an explosion deep within the star, using both analytical arguments and numerical hydrodynamics simulations. Focusing on polytropic stellar models, we find that for explosion energies smaller than the stellar binding energy, the ejected mass scales as $m_{\rm ej} \propto E^{\varepsilon _{m}}$, where ϵm = 2.4 − 3.0 depending on the polytropic index. The loss of energy due to shock breakout emission near the stellar edge leads to the existence of a minimal mass-shedding explosion energy, corresponding to a minimal ejecta mass. For a wide range of progenitors, from Wolf-Rayet stars to red supergiants, we find a similar limiting energy of $E_{\rm min} \approx 10^{46}-10^{47} \rm \, erg$, almost independent of the stellar radius. The corresponding minimal ejecta mass varies considerably across different progenitors, ranging from $\sim \! 10^{-8} \, \rm M_\odot$ in compact stars, up to $\sim \! 10^{-2} \, \rm M_\odot$ in red supergiants. We discuss implications of our results for pre-supernova outbursts driven by wave heating, and complications caused by the non-constant opacity and adiabatic index of realistic stars.

The historical record of massive star formation in Cygnus

Context. The Cygnus region, which dominates the local spiral arm of the Galaxy, is one of the nearest complexes of massive star formation, extending over several hundred parsecs. Its massive stellar content, regions of ongoing star formation, and molecular gas have been studied in detail at virtually all wavelengths. However, little is known of the history of the region beyond the past 10 Myr. Aims. We use the correlations between age, mass and luminosity of red supergiants to explore the history of star formation in Cygnus previous to the formation of the present-day associations. The brightness and spectroscopic characteristics of red supergiants make it easy to identify them and build up a virtually complete sample of such stars at the distance of the Cygnus region, thus providing a record of massive star formation extending several tens of Myr into the past, a period inaccessible through the O and early B stars observable at present. Methods. We have made a selection based on the 2MASS colors of a sample of bright, red stars in an area of 84 square degrees covering the whole present extension of the Cygnus association in the Local Arm. We have obtained spectroscopy in the red visible range allowing an accurate, homogeneous spectral classification as well as a reliable separation between supergiants and other cool stars. Our data are complemented with Gaia Data Release 2 astrometric data. Results. We have identified 29 red supergiants in the area, 17 of which had not been previously classified as supergiants. Twenty-four of the 29 most likely belong to the Cygnus region and four of the remaining to the Perseus arm. We have used their derived luminosities and masses to infer the star formation history of the region. Intense massive star formation activity is found to have started approximately 15 Myr ago, and we find evidence for two other episodes, one taking place between 20 and 30 Myr ago and another one having ended approximately 40 Myr ago. There are small but significant differences between the kinematic properties of red supergiants younger or older then 20 Myr, hinting that stars of the older group were formed outside the precursor of the present Cygnus complex, possibly in the Sagittarius-Carina arm.