Variational phase-field methods have been shown powerful for the modeling of complex crack propagation without a priori knowledge of the crack path or ad hoc criteria. However, phase-field models suffer from their energy functional being non-linear and non-convex, while requiring a very fine mesh to capture the damage gradient. This implies a high computational cost, limiting concrete engineering applications of the method. In this work, we propose an efficient and robust fully monolithic solver for phase-field fracture using a modified Newton method with inertia correction and an energy line-search. To illustrate the gains in efficiency obtained with our approach, we compare it to two popular methods for phase-field fracture, namely the alternating minimization and the quasi-monolithic schemes. To facilitate the evaluation of the time step dependent quasi-monolithic scheme, we couple the latter with an extrapolation correction loop controlled by a damage-based criteria. Finally, we show through four benchmark tests that the modified Newton method we propose is straightforward, robust, and leads to identical solutions, while offering a reduction in computation time by factors of up to 12 and 6 when compared to the alternating minimization and quasi-monolithic schemes.
We identify red supergiants (RSGs) in our spiral neighbors M31 and M33 using near-IR (NIR) photometry complete to a luminosity limit of log L/Lo=4.0. Our archival survey data cover 5 deg^2 of M31, and 3 deg^2 for M33, and are likely spatially complete for these massive stars. Gaia is used to remove foreground stars, after which the RSGs can be separated from asymptotic giant branch (AGB) stars in the color-magnitude diagram. The photometry is used to derive effective temperatures and bolometric luminosities via MARCS stellar atmosphere models. The resulting H-R diagrams show superb agreement with the evolutionary tracks of the Geneva evolutionary group. Our census includes 6400 RSGs in M31 and 2850 RSGs in M33 within their Holmberg radii; by contrast, only a few hundred RSGs are known so far in the Milky Way. Our catalog serves as the basis for a study of the RSG binary frequency being published separately, as well as future studies relating to the evolution of massive stars. Here we use the matches between the NIR-selected RSGs and their optical counterparts to show that the apparent similarity in the reddening of OB stars in M31 and M33 is the result of Malmquist bias; the average extinction in M31 is likely higher than that of M33. As expected, the distribution of RSGs follows that of the spiral arms, while the much older AGB population is more uniformly spread across each galaxys disk.
Massive stars briefly pass through the yellow supergiant (YSG) phase as they evolve redward across the HR diagram and expand into red supergiants (RSGs). Higher-mass stars pass through the YSG phase again as they evolve blueward after experiencing significant RSG mass loss. These post-RSG objects offer us a tantalizing glimpse into which stars end their lives as RSGs, and why. One telltale sign of a post-RSG object may be an instability to pulsations, depending on the stars interior structure. Here we report the discovery of five YSGs with pulsation periods faster than 1 day, found in a sample of 76 cool supergiants observed by tess at two-minute cadence. These pulsating YSGs are concentrated in a HR diagram region not previously associated with pulsations; we conclude that this is a genuine new class of pulsating star, Fast Yellow Pulsating Supergiants (FYPS). For each FYPS, we extract frequencies via iterative prewhitening and conduct a time-frequency analysis. One FYPS has an extracted frequency that is split into a triplet, and the amplitude of that peak is modulated on the same timescale as the frequency spacing of the triplet; neither rotation nor binary effects are likely culprits. We discuss the evolutionary status of FYPS and conclude that they are candidate post-RSGs. All stars in our sample also show the same stochastic low-frequency variability (SLFV) found in hot OB stars and attributed to internal gravity waves. Finally, we find four $alpha$ Cygni variables in our sample, of which three are newly discovered.
The binary fraction of unevolved massive stars is thought to be 70-100% but there are few observational constraints on the binary fraction of the evolved version of a subset of these stars, the red supergiants (RSGs). Here we identify a complete sample of RSGs in the Large Magellanic Cloud (LMC) using new spectroscopic observations and archival UV, IR and broadband optical photometry. We find 4090 RSGs with log L/Lo > 3.5 with 1820 of them having log L/Lo > 4, which we believe is our completeness limit. We additionally spectroscopically confirmed 38 new RSG+B star binaries in the LMC, bringing the total known up to 55. We then estimated the binary fraction using a k-nearest neighbors algorithm that classifies stars as single or binary based on photometry with a spectroscopic sample as a training set. We take into account observational biases such as line-of-sight stars and binaries in eclipse while also calculating model-dependent corrections for RSGs with companions that our observations were not designed to detect. Based on our data, we find an initial result of 13.5 +7.56/-6.67% for RSGs with O or B-type companions. Using the Binary Population and Spectral Synthesis (BPASS) models to correct for unobserved systems, this corresponds to a total RSG binary fraction of 19.5 +7.6/-6.7%. This number is in broad agreement with what we would expect given an initial OB binary distribution of 70%, a predicted merger fraction of 20-30% and a binary interaction fraction of 40-50%.
Both rotation and interactions with binary companions can significantly affect massive star evolution, altering interior and surface abundances, mass loss rates and mechanisms, observed temperatures and luminosities, and their ultimate core-collapse fates. The Geneva and BPASS stellar evolution codes include detailed treatments of rotation and binary evolutionary effects, respectively, and can illustrate the impact of these phenomena on massive stars and stellar populations. However, a direct comparison of these two widely-used codes is vital if we hope to use their predictions for interpreting observations. In particular, rotating and binary models will predict different young stellar populations, impacting the outputs of stellar population synthesis (SPS) and the resulting interpretation of large massive star samples based on commonly-used tools such as star count ratios. Here we compare the Geneva and BPASS evolutionary models, using an interpolated SPS scheme introduced in our previous work and a novel Bayesian framework to present the first in-depth direct comparison of massive stellar populations produced from single, rotating, and binary non-rotating evolution models. We calculate both models predicted values of star count ratios and compare the results to observations of massive stars in Westerlund 1, $h + chi$ Persei, and both Magellanic Clouds. We also consider the limitations of both the observations and the models, and how to quantitatively include observational completeness limits in SPS models. We demonstrate that the methods presented here, when combined with robust stellar evolutionary models, offer a potential means of estimating the physical properties of massive stars in large stellar populations.
We present optical spectrophotometry of the red supergiant Betelgeuse from 2020 February 15, during its recent unprecedented dimming episode. By comparing this spectrum to stellar atmosphere models for cool supergiants, as well as spectrophotometry of other Milky Way red supergiants, we conclude that Betelgeuse has a current effective temperature of 3600 +/- 25 K. While this is slightly cooler than previous measurements taken prior to Betelgeuses recent lightcurve evolution, this drop in effective temperature is insufficient to explain Betelgeuses recent optical dimming. We propose that episodic mass loss and an increase in the amount of large-grain circumstellar dust along our sightline to Betelgeuse is the most likely explanation for its recent photometric evolution.
The American Astronomical Society (AAS) Journals are a vital asset of our professional society. With the push towards open access, page charges are a viable and sustainable option for continuing to effectively fund and publish the AAS Journals. However, the existing page charge model, which requires individual authors to pay page charges out of their grants or even out of pocket, is already challenging to some researchers and could be exacerbated in the Open Access (OA) era if charges increase. A discussion of alternative models for funding page charges and publishing costs should be part of the Astro2020 decadal survey if we wish to continue supporting the sustainable and accessible publication of US research in AAS journals in the rapidly-shifting publication landscape. The AAS Publications Committee recommends that the National Academy of Sciences form a task force to develop solutions and recommendations with respect to the urgent concerns and considerations highlighted in this White Paper.
We present the detection of the first candidate colliding-wind binary (CWB) in M33, located in the giant H II region NGC 604. The source was first identified in archival {it Chandra} imaging as a relatively soft X-ray point source, with the likely primary star determined from precise astrometric alignment between archival {it Hubble Space Telescope} and {it Chandra} imaging. The candidate primary star in the CWB is classified for the first time in this work as a carbon-rich Wolf-Rayet star with a likely O star companion based on spectroscopy obtained from Gemini-North. We model the X-ray spectrum using {it Chandra} and {it XMM-Newton} observations, and find the CWB is well-fit as a $sim$ 1 keV thermal plasma with a median unabsorbed luminosity in the 0.5--2.0 keV band of $L_{rm X}$ $sim$ 3 $times$ 10$^{35}$ erg s$^{-1}$, making this source among the brightest of CWBs observed to date. We present a long term light curve for the candidate CWB from archival {it Chandra} and {it XMM-Newton} observations, and discuss the constraints placed on the binary by this light curve, as well as the X-ray luminosity at maximum. Finally, we compare this candidate CWB in M33 to other well-studied, bright CWBs in the Galaxy and Magellanic Clouds, such as $eta$ Car.
The percentage of massive main sequence OB stars in binary systems is thought to be as high as 100%. However, very few Galactic binary red supergiants (RSGs) have been identified, despite the fact that these stars are the evolved descendants of OB stars. As shown in our recent paper, binary RSGs will likely have B-type companions, as dictated by stellar evolution considerations. Such a system will have a very unique photometric signature due to the shape of the spectral energy distribution. Using photometric cutoffs it should therefore be possible to detect candidate RSG+B star binary systems. Here we present our spectoscopic follow-up observations of such candidates. Out of our initial list of 280 candidates in M31 and M33, we observed 149 and confirmed 63 as newly discovered RSG+B star binary systems. Additional spectra of four candidate systems in the Small Magellanic Cloud confirmed all of them as new RSG+B star binaries including the first known RSG+Be star system. By fitting BSTAR06 and MARCS model atmospheres to the newly-obtained spectra we place estimates on the temperatures and subtypes of both the B stars and RSGs. Overall, we have found 87 new RSG+B star binary systems in M31, M33 and the Small and Large Magellanic Clouds. Our future studies are aimed at determining the binary fraction of RSGs.
We present the first results from a study of TESS Sector 1 and 2 light curves for eight evolved massive stars in the LMC: six yellow supergiants (YSGs) and two luminous blue variables (LBVs), including S Doradus. We use an iterative prewhitening procedure to characterize the short-timescale variability in all eight stars. The periodogram of one of the YSGs, HD 269953, displays multiple strong peaks at higher frequencies than its fellows. While the field surrounding HD 269953 is quite crowded, it is the brightest star in the region, and has infrared colors indicating it is dusty. We suggest HD 269953 may be in a post-red supergiant evolutionary phase. We find a signal with a period of $sim5$ days for the LBV HD 269582. The periodogram of S Doradus shows a complicated structure, with peaks below frequencies of 1.5 cycles per day. We fit the shape of the background noise of all eight light curves, and find a red noise component in all of them. However, the power law slope of the red noise and the timescale over which coherent structures arise changes from star to star. Our results highlight the potential for studying evolved massive stars with TESS.
