RT @MassiveStarGuy
The paper was accepted by the Astrophysical Journal last week, and we've posted it here:
http://arxiv.org/abs/2211.14147 To me, this will always be my "pandemic paper"--I struggled with this for many, MANY more months than I would have under more normal circumstances. But we got there!
The Time-Averaged Mass-Loss Rates of Red Supergiants As Revealed by their Luminosity Functions in M31 and M33
Mass-loss in red supergiants (RSGs) is generally recognized to be episodic,
but mass-loss prescriptions fail to reflect this. Evolutionary models show that
the total amount of mass lost during this phase determines if these stars
evolve to warmer temperatures before undergoing core collapse. The current
Geneva evolutionary models mimic episodic mass loss by enhancing the quiescent
prescription rates whenever the star's outer layers exceed the Eddington
luminosity by a large factor. This results in a 20 solar-mass model undergoing
significantly more mass loss during the RSG phase than it would have otherwise,
but has little effect on models of lower masses. We can test the validity of
this approach observationally by measuring the proportion of high-luminosity
RSGs to that predicted by the models. To do this, we use our recent
luminosity-limited census of RSGs in M31 and M33, making modest improvements to
membership, and adopting extinctions based on the recent panchromatic M31 and
M33 Hubble surveys. We then compare the proportions of the highest luminosity
RSGs found to that predicted by published Geneva models, as well as to a
special set of models computed without the enhanced rates. We find good
agreement with the models which include the supra-Eddington enhanced mass loss.
The models with lower mass-loss rates predict a larger fraction of
high-luminosity RSGs than observed, and thus can be ruled out. We also use
these improved data to confirm that the upper luminosity limit of RSGs is log
L/Lo~5.4, regardless of metallicity, using our improved data on M31 and M33
plus previous results on the Magellanic Clouds.
RT @MassiveStarGuy
Arguably, the greatest uncertainties in modeling the fate of massive stars is what to assume about the mass-loss rates during the red supergiant (RSG) phase, as most of the mass is lost episodically. @KathrynNeugent and I plus other collaborators tried a different approach.
RT
@astrogradyI'm very excited to share my and my collaborator's work analyzing the environments and spectra of a population of super-AGB stars we previously identified. The paper came out on the arXiv Wednesday!
https://arxiv.org/abs/2211.12438 Paper thread below, with a quick recap of Paper I!
Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II: Spectroscopic and Environmental Analysis of Thorne-Żytkow Object and Super-AGB Star Candidates
In previous work we identified a population of 38 cool and luminous variable
stars in the Magellanic Clouds and examined 11 in detail in order to classify
them as either Thorne-Żytkow Objects (TŻOs, red supergiants with a neutron
star cores) or super-AGB stars (the most massive stars that will not undergo
core collapse). This population includes HV\,2112, a peculiar star previously
considered in other works to be either a TŻO or high-mass AGB star. Here we
continue this investigation, using the kinematic and radio environments and
local star formation history of these stars to place constraints on the age of
the progenitor systems and the presence of past supernovae. These stars are not
associated with regions of recent star formation, and we find no evidence of
past supernovae at their locations. Finally, we also assess the presence of
heavy elements and lithium in their spectra compared to red supergiants. We
find strong absorption in Li and s-process elements compared to RSGs in most of
the sample, consistent with super-AGB nucleosynthesis, while HV\,2112 shows
additional strong lines associated with TŻO nucleosynthesis. Coupled with our
previous mass estimates, the results are consistent with the stars being
massive (~4-6.5M$_{\odot}$) or super-AGB (~6.5-12M$_{\odot}$) stars in the
thermally pulsing phase, providing crucial observations of the transition
between low- and high-mass stellar populations. HV\,2112 is more ambiguous; it
could either be a maximally massive sAGB star, or a TŻO if the minimum mass
for stability extends down to <13 M$_\odot$.
RT @NASAHubble
Coming soon to a screen near you! ⭐
Starting on Monday, join us for two weeks of *stellar* star cluster content! New Hubble images, videos, and more are headed your way.
Get ready to explore #StarrySights from Hubble! Get ready here: https://go.nasa.gov/3ALBoDs
Discoveries - Hubble's Star Clusters
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RT @ArcticSaxifrage
This week on the PVL blog, MSc student Madeline Walters recaps the methane debate on Mars so far and discusses some recent developments:
http://york-pvl.blogspot.com/2022/11/whats-going-on-with-methane-on-mars.html
What’s going on with methane on Mars?
This week, Madeline discusses a critical component of her research into how methane is vertically distributed in the martian atmosphere. Rea...
There is asterisk here, one star that is determined to be just a bit more special than it's friends - it's HV2112! HV2112 is consistently 'The Most' in our analyses: most luminous, most variable, most massive, & has a spectra different than the other stars. HV2112's identity remains ambiguous, with observation-theory clashes for either identity. The theoretical minimum mass for TŻOs is 15 solar masses (HV2112 is 9-12), but the minimum can be lowered with reasonable changes to model assumptions.
Conversely HV2112 shows a possible Ca enhancement that is difficult to explain in sAGBs, but there is some speculation mild Ca enhancement could occur at these masses. If HV2112 is a sAGB, it would be a very massive sAGB star, and a possible electron capture supernova progenitor.
The sAGBs in our sample would be the first *population* identified, slotting alongside the first identified sAGBs (SMC
http://ui.adsabs.harvard.edu/abs/2009A%26A...506.1277G%2F, Galactic
http://ui.adsabs.harvard.edu/abs/2021A%26A...646A..98T), to help provide observable constrains on the low-/high-mass divide for models of stellar evolution.
Luminosities and mass-loss rates of SMC and LMC AGB stars and red supergiants
Context: Mass loss is one of the fundamental properties of Asymptotic Giant Branch (AGB) stars, and through the enrichment of the interstellar medium, AGB stars are key players in the life cycle of dust and gas in the universe. However, a quantitative understanding of the mass-loss process is still largely lacking, particularly its dependence on metallicity. <BR />Aims: To investigate the relation between mass loss, luminosity and pulsation period for a large sample of evolved stars in the Small and Large Magellanic Cloud. <BR />Methods: Dust radiative transfer models are presented for 101 carbon stars and 86 oxygen-rich evolved stars in the Magellanic Clouds for which 5-35 μm Spitzer IRS spectra are available. The spectra are complemented with available optical and infrared photometry to construct the spectral energy distribution. A minimisation procedure is used to fit luminosity, mass-loss rate and dust temperature at the inner radius. Different effective temperatures and dust content are also considered. Periods from the literature and from new OGLE-III data are compiled and derived. <BR />Results: We derive (dust) mass-loss rates and luminosities for the entire sample. Based on luminosities, periods and amplitudes and colours, the O-rich stars are classified as foreground objects, AGB stars and Red Super Giants. For the O-rich stars silicates based on laboratory optical constants are compared to “astronomical silicates”. Overall, the grain type by Volk & Kwok (1988, ApJ, 331, 435) fits the data best. However, the fit based on laboratory optical constants for the grains can be improved by abandoning the small-particle limit. The influence of grain size, core-mantle grains and porosity are explored. A computationally convenient method that seems to describe the observed properties in the 10 μm window are a distribution of hollow spheres with a large vacuum fraction (typically 70%), and grain size of about 1 μm. Relations between mass-loss rates and luminosity and pulsation period are presented and compared to the predictions of evolutionary models, those by Vassiliadis & Wood (1993, ApJ, 413, 641) and their adopted mass-loss recipe, and those based on a Reimers mass-loss law with a scaling of a factor of five. The Vassiliadis & Wood models describe the data better, although there are also some deficiencies, in particular to the maximum adopted mass-loss rate. The derived mass-loss rates are compared to predictions by dynamical wind models and appear consistent with them at a level of a factor 2-4. A better understanding requires the determination of the expansion velocity from future observations from ALMA. The OGLE-III data reveal an O-rich star in the SMC with a period of 1749 days. Its absolute magnitude of M_bol= -8.0 makes it a good candidate for a super-AGB star. <P />Complete Figs. 1-3 and 5-7 are only available in electronic form at http://www.aanda.org Complete Tables 1-4 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/506/1277
In short, we see a variety of enhancement patterns. Coupled with the mass estimates from Paper I, the spectra are consistent with some of the population being massive (~5 solar mass) AGB stars, while many (at least 4-5) are likely superAGB stars (>6-7 solar masses).
SPECTRA - Both TŻOs and sAGB stars are expected to show enhancements of Li and s-process elements (Rb, Mo) compared to RSGs. The paper has (S5.1) a detailed explanation of what we expect for either identity; there can be subtle differences between them! We obtained hi-res spectra from the Magellan telescope @LCOAstro which are available FOR PUBLIC USE!
https://zenodo.org/record/7058608. These spectra are complicated! We follow @emsque's method to compare line ratios of our stars to RSGs.
Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II: Spectroscopic Data of Thorne-Zytkow Object and Super-AGB Star Candidates
This dataset contains Magellan MIKE spectroscopy of a population of cool, luminous stars in the Magellanic Clouds, a sample of confirmed Magellanic Cloud red supergiants, and spectrophotometric standard stars. The spectra were analyzed in the paper "Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II: Spectroscopic and Environmental Analysis of Thorne-\.Zytkow Object and Super-AGB Star Candidates" by O'Grady et al. (2022). More details are provided in the README.
