The 'red supergiant problem': the upper luminosity boundary of Type II supernova progenitors

By comparing the properties of red supergiant (RSG) supernova (SN) progenitors to those of field RSGs, it has been claimed that there is an absence of progenitors with luminosities L above log (L/L⊙) > 5.2. This is in tension with the empirical upper luminosity limit of RSGs at log (L/L⊙) = 5.5, a result known as the ‘RSG problem’. This has been interpreted as an evidence for an upper mass threshold for the formation of black holes. In this paper, we compare the observed luminosities of RSG SN progenitors with the observed RSG L-distribution in the Magellanic Clouds. Our results indicate that the absence of bright SN II-P/L progenitors in this sample can be explained at least in part by the steepness of the L-distribution and a small sample size, and that the statistical significance of the RSG problem is between 1σ and 2σ . Secondly, we model the luminosity distribution of II-P/L progenitors as a simple power law with an upper and lower cut-off, and find an upper luminosity limit of log(Lhi/L⊙)=5.20+0.17−0.11 (68 per cent confidence), though this increases to ∼5.3 if one fixes the power-law slope to be that expected from theoretical arguments. Again, the results point to the significance of the RSG problem being within ∼2σ. Under the assumption that all progenitors are the result of single-star evolution, this corresponds to an upper mass limit for the parent distribution of Mhi=19.2M⊙⁠, ±1.3M⊙(systematic)⁠, +4.5−2.3M⊙ (random; 68 per cent confidence limits)

The evolution of Red Supergiant mass-loss rates

The fate of massive stars with initial masses >8M$_\odot$ depends largely on the mass-loss rate (\mdot ) in the end stages of their lives. Red supergiants (RSGs) are the direct progenitors to Type II-P core collapse supernovae (SN), but there is uncertainty regarding the scale and impact of any mass-loss during this phase. Here we used near and mid-IR photometry and the radiative transfer code DUSTY to determine luminosity and \mdot\ values for the RSGs in two Galactic clusters (NGC 7419 and $\chi$ Per) where the RSGs are all of similar initial mass ($M_{\rm initial}$$\sim$16M$_\odot$), allowing us to study how \mdot\ changes with time along an evolutionary sequence. We find a clear, tight correlation between luminosity and \mdot\ suggesting the scatter seen in studies of field stars is caused by stars of similar luminosity being of different initial masses. From our results we estimate how much mass a 16M$_\odot$ star would lose during the RSG phase, finding a star of this mass would lose a total of 0.61$^{+0.92}_{-0.31}$M$_\odot$. This is much less than expected for \mdot\ prescriptions currently used in evolutionary models

The evolution of red supergiants to supernovae

With red supergiants (RSGs) predicted to end their lives as Type IIP core collapse supernova (CCSN), their behaviour before explosion needs to be fully understood. Mass loss rates govern RSG evolution towards SN and have strong implications on the appearance of the resulting explosion. To study how the mass-loss rates change with the evolution of the star, we have measured the amount of circumstellar material around 19 RSGs in a coeval cluster. Our study has shown that mass loss rates ramp up throughout the lifetime of an RSG, with more evolved stars having mass loss rates a factor of 40 higher than early stage RSGs. Interestingly, we have also found evidence for an increase in circumstellar extinction throughout the RSG lifetime, meaning the most evolved stars are most severely affected. We find that, were the most evolved RSGs in NGC2100 to go SN, this extra extinction would cause the progenitor's initial mass to be underestimated by up to 9M$_\odot$

The Initial Masses of the Red Supergiant Progenitors to Type-II Supernovae

There are a growing number of nearby SNe for which the progenitor star is detected in archival pre-explosion imaging. From these images it is possible to measure the progenitor's brightness a few years before explosion, and ultimately estimate its initial mass. Previous work has shown that II-P and II-L supernovae (SNe) have Red Supergiant (RSG) progenitors, and that the range of initial masses for these progenitors seems to be limited to $<$17M$_\odot$. This is in contrast with the cutoff of 25-30M$_\odot$ predicted by evolutionary models, a result which is termed the 'Red Supergiant Problem'. Here we investigate one particular source of systematic error present in converting pre-explosion photometry into an initial mass, that of the bolometric correction (BC) used to convert a single-band flux into a bolometric luminosity. We show, using star clusters, that RSGs evolve to later spectral types as they approach SN, which in turn causes the BC to become larger. Failure to account for this results in a systematic underestimate of a star's luminosity, and hence its initial mass. Using our empirically motivated BCs we reappraise the II-P and II-L SNe that have their progenitors detected in pre-explosion imaging. Fitting an initial mass function to these updated masses results in an increased upper mass cutoff of $M_{\rm hi}$=$19.0^{+2.5}_{-1.3}$M$_\odot$, with a 95% upper confidence limit of $<$27M$_\odot$. Accounting for finite sample size effects and systematic uncertainties in the mass-luminosity relationship raises the cutoff to $M_{\rm hi}$=25M$_\odot$ ($<$33M$_\odot$, 95% confidence). We therefore conclude that there is currently no strong evidence for `missing' high mass progenitors to core-collapse SNe

The distances to star-clusters hosting Red Supergiants: χχ Per, NGC 7419, and Westerlund 1

Galactic, young massive star clusters are approximately coeval aggregates of stars, close enough to resolve the individual stars, massive enough to have produced large numbers of massive stars, and young enough for these stars to be in a pre-supernova state. As such these objects represent powerful natural laboratories in which to study the evolution of massive stars. To be used in this way, it is crucial that accurate and precise distances are known, since this affects both the inferred luminosities of the cluster members and the age estimate for the cluster itself. Here we present distance estimates for three star clusters rich in Red Supergiants ($\chi$ Per, NGC 7419 and Westerlund 1) based on their average astrometric parallaxes $\bar{\pi}$ in Gaia Data Release 2, where the measurement of $\bar{\pi}$ is obtained from a proper-motion screened sample of spectroscopically-confirmed cluster members. We determine distances of $d=2.25^{+0.16}_{-0.14}$kpc, $d=3.00^{+0.35}_{-0.29}$kpc, and $d=3.87^{+0.95}_{-0.64}$kpc for the three clusters respectively. We find that the dominant source of error is that in Gaia's zero-point parallax offset $\pi_{\rm ZP}$, and we argue that more precise distances cannot be determined without an improved characterization of this quantity

'On the red supergiant problem': a rebuttal, and a consensus on the upper mass cut-off for II-P progenitors

The red supergiant (RSG) problem describes the claim that the brightest RSG progenitors to Type II-P supernovae (SNe) are significantly fainter than RSGs in the field. This mismatch has been interpreted by several authors as being a manifestation of the mass threshold for the production of black holes (BHs), such that stars with initial masses above a cut-off of Mhi = 17 M and below 25 M will die as RSGs, but with no visible SN explosion as the BH is formed. However, we have previously cautioned that this cut-off is more likely to be higher and has large uncertainties (Mhi = 19+4-2M), meaning that the statistical significance of the RSG problem is less than 2. Recently, Kochanek has claimed that our work is statistically flawed, and with his analysis has argued that the upper mass cut-off is as low as Mhi = 15.7 0.8M, giving the RSG problem a significance of 10. In this letter, we show that Kochanek s low cut-off is caused by a statistical misinterpretation, and the associated fit to the progenitor mass spectrum can be ruled out at the 99.6 per cent confidence level. Once this problem is remedied, Kochanek s best fit becomes Mhi = 19+4-2M, in excellent agreement with our work. Finally, we argue that, in the search for an RSG vanishing as it collapses directly to a BH, any such survey would have to operate for decades before the absence of any such detection became statistically significant

Red supergiants in M31: the Humphreys-Davidson limit at high metallicity

The empirical upper limit to red supergiant (RSG) luminosity, known as the Humphreys–Davidson (HD) limit, has been commonly explained as being caused by the stripping of stellar envelopes by metallicity-dependent line-driven winds. As such, the theoretical expectation is that the HD limit should be higher at lower metallicity, where weaker mass-loss rates mean that higher initial masses are required for an envelope to be stripped. In this paper, we test this prediction by measuring the luminosity function of RSGs in M31 and comparing it to those in the LMC and SMC. We find that log (Lmax/L⊙) = 5.53 ± 0.03 in M31 (Z ≳ Z⊙), consistent with the limit found for both the LMC (Z ∼ 0.5 Z⊙) and SMC (Z ∼ 0.25 Z⊙), while the RSG luminosity distributions in these three galaxies are consistent to within 1σ. We therefore find no evidence for a metallicity dependence on both the HD limit and the RSG luminosity function, and conclude that line-driven winds on the main sequence are not the cause of the HD limit

A new mass-loss rate prescription for red supergiants

Evolutionary models have shown the substantial effect that strong mass-loss rates (⁠M˙s) can have on the fate of massive stars. Red supergiant (RSG) mass-loss is poorly understood theoretically, and so stellar models rely on purely empirical M˙–luminosity relations to calculate evolution. Empirical prescriptions usually scale with luminosity and effective temperature, but M˙ should also depend on the current mass and hence the surface gravity of the star, yielding more than one possible M˙ for the same position on the Hertzsprung–Russell diagram. One can solve this degeneracy by measuring M˙ for RSGs that reside in clusters, where age and initial mass (Minit) are known. In this paper we derive M˙ values and luminosities for RSGs in two clusters, NGC 2004 and RSGC1. Using newly derived Minit measurements, we combine the results with those of clusters with a range of ages and derive an Minit-dependent M˙ prescription. When comparing this new prescription to the treatment of mass-loss currently implemented in evolutionary models, we find models drastically overpredict the total mass-loss, by up to a factor of 20. Importantly, the most massive RSGs experience the largest downward revision in their mass-loss rates, drastically changing the impact of wind mass-loss on their evolution. Our results suggest that for most initial masses of RSG progenitors, quiescent mass-loss during the RSG phase is not effective at removing a significant fraction of the H-envelope prior to core-collapse, and we discuss the implications of this for stellar evolution and observations of SNe and SN progenitors

Discrepancies in the ages of young star clusters; evidence for mergers?

There is growing evidence that star clusters can no longer be considered simple stellar populations (SSPs). Intermediate and old age clusters are often found to have extended main sequence turn-offs (eMSTOs) which are difficult to explain with single age isochrones, an effect attributed to rotation. In this paper, we provide the first characterisation of this effect in young (<20Myr) clusters. We determine ages for 4 young massive clusters (2 LMC, 2 Galactic) by three different methods: using the brightest single turn-off (TO) star; using the luminosity function (LF) of the TO; and by using the lowest $L_{\rm bol}$ red supergiant (RSG). The age found using the cluster TO is consistently younger than the age found using the lowest RSG $L_{\rm bol}$. Under the assumption that the lowest luminosity RSG age is the `true' age, we argue that the eMSTOs of these clusters cannot be explained solely by rotation or unresolved binaries. We speculate that the most luminous stars above the TO are massive blue straggler stars formed via binary interaction, either as mass gainers or merger products. Therefore, using the cluster TO method to infer ages and initial masses of post-main sequence stars such as Wolf-Rayet stars, luminous blue variables and RSGs, will result in ages inferred being too young and masses too high

The Age of Westerland 1 Revisited

The cluster Westerlund 1 (Wd1) is host to a large variety of post-main-sequence (MS) massive stars. The simultaneous presence of these stars can only be explained by stellar models if the cluster has a finely tuned age of 4–5 Myr, with several published studies independently claiming ages within this range. At this age, stellar models predict that the cool supergiants (CSGs) should have luminosities of log(L L☉)» 5.5, close to the empirical luminosity limit. Here, we test that prediction using archival data and new photometry from Stratospheric Observatory for Infrared Astronomy to estimate bolometric luminosities for the CSGs. We find that these stars are on average 0.4 dex too faint to be 5 Myr old, regardless of which stellar evolutionary model is used, and instead are indicative of a much older age of 10.4-+1.21.3 Myr. We argue that neither systematic uncertainties in the extinction law nor stellar variability can explain this discrepancy. In reviewing various independent age estimates of Wd1 in the literature, we first show that those based on stellar diversity are unreliable. Second, we reanalyze Wd1’s pre-MS stars employing the Damineli extinction law, finding an age of 7.2-+2.31.1 Myr; older than that of previous studies, but which is vulnerable to systematic errors that could push the age close to 10 Myr. However, there remains significant tension between the CSG age and that inferred from the eclipsing binary W13. We conclude that stellar evolutionary models cannot explain Wd1 under the single-age paradigm. Instead, we propose that the stars in the Wd1 region formed over a period of several megayears