The Red Supergiant Problem: Circumstellar dust as a solution
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Abstract We investigate the red supergiant problem: the apparent dearth of Type IIP supernova progenitors with masses between 16 and 30 M ⊙ . Although red supergiants with masses in this range have been observed, none have been identified as progenitors in pre–explosion images. We show that, by failing to take into account the additional extinction resulting from the dust produced in the red supergiant winds, the luminosity of the most massive red supergiants at the end of their lives is underestimated. We re–estimate the initial masses of all Type IIP progenitors for which observations exist and analyse the resulting population. We find that the most likely maximum mass for a Type IIP progenitor is 21 +2 −1 M ⊙ . This is in closer agreement with the limit predicted from single star evolution models.Keywords:
Extinction (optical mineralogy)
Circumstellar dust
We use the Binary Population and Spectral Synthesis (BPASS) models to test the recent suggestion that red supergiants can provide an accurate age estimate of a co-eval stellar population that is unaffected by interacting binary stars. Ages are estimated by using both the minimum luminosity red supergiant and the mean luminosity of red supergiants in a cluster. We test these methods on a number of observed star clusters and find our results in agreement with previous estimates. Importantly we find the difference between the ages derived from stellar population models with and without a realistic population of interacting binary stars is only a few 100,000 years at most. We find that the mean luminosity of red supergiants in a cluster is the best method to determine the age of a cluster because it is based o the entire red supergiant population rather than using only the least luminous red supergiant.
Blue supergiant
Blue straggler
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We present a new catalogue of cool supergiants in a section of the Perseus arm, most of which had not been previously identified. To generate it, we have used a set of well-defined photometric criteria to select a large number of candidates (637) that were later observed at intermediate resolution in the infrared calcium triplet spectral range, using a long-slit spectrograph. To separate red supergiants from luminous red giants, we used a statistical method, developed in previous works and improved in the present paper. We present a method to assign probabilities of being a red supergiant to a given spectrum and use the properties of a population to generate clean samples, without contamination from lower luminosity stars. We compare our identification with a classification done using classical criteria and discuss their respective efficiencies and contaminations as identification methods. We confirm that our method is as efficient at finding supergiants as the best classical methods, but with a far lower contamination by red giants than any other method. The result is a catalogue with 197 cool supergiants, 191 of which did not appear in previous lists of red supergiants. This is the largest coherent catalogue of cool supergiants in the Galaxy.
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Massive stars end their lives in spectacular supernova explosions. Identifying the progenitor star is a test of stellar evolution and explosion models. Here we show that the progenitor star of the supernova SN 2008bk has now disappeared, which provides conclusive evidence that this was the death of a red supergiant star.
Star (game theory)
Progenitor
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A massive star can enter the blue supergiant region either evolving directly from the main-sequence, or evolving from a previous red supergiant stage. The fractions of the blue supergiants having different histories depend on the internal mixing and mass-loss during the red supergiant stage. We study the possibility to use diagnostics based on stellar pulsation to discriminate blue supergiants having different evolution histories. For this purpose we have studied the pulsation property of massive star models calculated with the Geneva stellar evolution code for initial masses ranging from 8 to 50 M$_\odot$ with a solar metallicity of $Z=0.014$. We have found that radial pulsations are excited in the blue-supergiant region only in the models that had been red-supergiants before. This would provide us with a useful mean to diagnose the history of evolution of each blue-supergiant. At a given effective temperature, much more nonradial pulsations are excited in the model after the red-supergiant stage than in the model evolving towards the red-supergiant. The properties of radial and nonradial pulsations in blue supergiants are discussed. Predicted periods are compared with period ranges observed in some \alpha-Cygni variables in the Galaxy and NGC 300. We have found that blue supergiant models after the red-supergiant stage roughly agree with observed period ranges in most cases. However, we are left with the puzzle that the predicted surface N/C and N/O ratios seem to be too high compared with those of Deneb and Rigel.
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Abstract We investigate the red supergiant problem: the apparent dearth of Type IIP supernova progenitors with masses between 16 and 30 M ⊙ . Although red supergiants with masses in this range have been observed, none have been identified as progenitors in pre–explosion images. We show that, by failing to take into account the additional extinction resulting from the dust produced in the red supergiant winds, the luminosity of the most massive red supergiants at the end of their lives is underestimated. We re–estimate the initial masses of all Type IIP progenitors for which observations exist and analyse the resulting population. We find that the most likely maximum mass for a Type IIP progenitor is 21 +2 −1 M ⊙ . This is in closer agreement with the limit predicted from single star evolution models.
Extinction (optical mineralogy)
Circumstellar dust
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Massive stars becoming red supergiants lose a significant amount of their mass during that brief evolutionary phase. They then either explode as a hydrogen-rich supernova (SN Type II), or continue to evolve as a hotter supergiant (before exploding). The slow, dusty ejecta of the red supergiant will be over-run by the hot star wind and/or SN ejecta. I will present estimates of the conditions for this interaction and discuss some of the implications.
Blue supergiant
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CHARA/MIRC interferometry of red supergiants: diameters, effective temperatures and surface features
We have obtained H-band interferometric observations of three galactic red supergiant stars using the MIRC instrument on the CHARA array. The targets include AZ Cyg, a field RSG and two members of the Per OB1 association, RS Per and T Per. We find evidence of departures from circular symmetry in all cases, which can be modelled with the presence of hotspots. This is the first detection of these features in the $H$-band. The measured mean diameters and the spectral energy distributions were combined to determine effective temperatures. The results give further support to the recently derived hotter temperature scale of red supergiant stars by Levesque et al. (2005), which has been evoked to reconcile the empirically determined physical parameters and stellar evolutionary theories. We see a possible correlation between spottedness and mid-IR emission of the circumstellar dust, suggesting a connection between mass-loss and the mechanism that generates the spots.
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Abstract We present optical spectrophotometry of the red supergiant (RSG) 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 RSGs, we conclude that Betelgeuse has a current effective temperature of 3600 ± 25 K. While this is slightly cooler than previous measurements taken prior to Betelgeuse’s recent lightcurve evolution, this drop in effective temperature is insufficient to explain Betelgeuse’s 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.
Blue supergiant
Spectral Properties
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The spectra of M-supergiants in the 7–100μm region are considered as part of a study of late-type stars based on the IRAS Low Resolution Spectrometer (LRS) Catalogue. The circumstellar silicate dust features at 9.7 and 18μm are observed to be of variable width. Most stars exhibit the classical circumstellar silicate dust features, typified by those of α Ori, modelled in a previous paper (Skinner & Whitmore). Some stars exhibit much broader features, VX Sgr being the brightest example. The latter stars can be fitted with a similar model, where the continuum is shown to be dominated by free–free emission. Dust condensing within outflows from various oxygen-rich stars generally produce similar emission characteristics, suggesting that the dust grains have a similar composition. This study shows that the strength of the spectral emission features produced by circumstellar dust in the infrared are correlated with surface temperature and luminosity, but not with pulsation characteristics of the stars. Radial pulsations are probably not linked directly to the mass-loss driving mechanism for the M-supergiants. We present mass-loss rates for 31 M-supergiants, which range from |$6.5\times10^{-7} \enspace \text {to}\enspace 4.3\times10^{-4} M_\odot\text {yr}^{-1}$|. The stars AD Per and MZ Cas are M-supergiants which show the 11.5μm SiC feature, a characteristic of carbon stars.
Circumstellar dust
Carbon star
Asymptotic giant branch
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Spectra at 8–13 μm are presented of five Large Magellanic Cloud supergiant stars with circumstellar dust emission. The brightest two objects were discovered in the IRAS survey and have thick dust shells with deep silicate absorption features; they are probably late-type M supergiants undergoing very heavy mass loss. Comparison between the current data and previous mid-infrared photometry suggests that the apparent depth of the silicate feature in the OH/IR star IRAS 04553–6825 has varied over a 5-yr interval. The peculiar B[e] hypergiant R 126 displays a strong circumstellar dust shell, but with a very muted silicate emission feature. The G supergiant R 150 and the luminous blue variable star R 71 display prominent silicate emission bands, but in both objects there is evidence that the dust distribution increases away from the star, suggesting that the bulk of the dust was produced at a previous evolutionary stage wherein the mass-loss rates were much higher. The silicate features indicate that the material lost from the stars is oxygen-rich, in agreement with stellar evolution models.
Circumstellar dust
Large Magellanic Cloud
Blue supergiant
Giant star
Spectral energy distribution
Carbon star
Small Magellanic Cloud
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