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Register a new userBridging the Gap: From Massive Stars to Supernovae
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Almost since the beginning, massive stars and their resultant supernovae have played a crucial role in the Universe. These objects produce tremendous amounts of energy and new, heavy elements that enrich galaxies, encourage new stars to form and sculpt the shapes of galaxies we see today. The end of millions of years of massive star evolution and the beginning of hundreds or thousands of years of supernova evolution are separated by a matter of a few seconds, in which some of the most extreme physics found in the Universe causes the explosive and terminal disruption of the star. Key questions remain unanswered in both the studies of how massive stars evolve and the behaviour of supernovae, and it appears the solutions may not lie on just one side of the explosion or the other or in just the domain of the stellar evolution or the supernova astrophysics communities. The need to view massive star evolution and supernovae as continuous phases in a single narrative motivated the Theo Murphy international scientific meeting Bridging the gap: from massive stars to supernovae at Chicheley Hall in June 2016, with the specific purpose to simultaneously address the scientific connections between theoretical and observational studies of massive stars and their supernovae, through engaging astronomers from both communities.
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Much difficulty has so far prevented the emergence of a consistent scenario for the origin of Type Ib and Ic supernovae (SNe). Here, we follow a heuristic approach by examining the fate of helium stars in the mass range 4 to 12Msun, which presumably form in interacting binaries. The helium stars are evolved using stellar wind mass loss rates that agree with observations, and which reproduce the observed luminosity range of galactic WR stars, leading to stellar masses at core collapse in the range 3-5.5Msun. We then explode these models adopting an explosion energy proportional to the ejecta mass, roughly consistent with theoretical predictions. We impose a fixed 56Ni mass and strong mixing. The SN radiation from 3 to 100d is computed self-consistently starting from the input stellar models using the time-dependent non-local thermodynamic equilibrium radiative-transfer code CMFGEN. By design, our fiducial models yield similar light curves, with a rise time of ~20d and a peak luminosity of ~10^42.2erg/s, in line with representative SNe Ibc. The less massive progenitors retain a He-rich envelope and reproduce the color, line widths, and line strengths of a representative sample of SNe Ib, while stellar winds remove most of the helium in more massive progenitors, whose spectra match typical SNe Ic in detail. The transition between the predicted Ib-like and Ic-like spectra is continuous, but it is sharp, such that the resulting models essentially form a dichotomy. Further models computed with varying explosion energy, 56Ni mass, and long-term power injection from the remnant show that a moderate variation of these parameters can reproduce much of the diversity of SNe Ibc. We conclude that stars stripped by a binary companion can account for the vast majority of ordinary SNe Ib and Ic, and that stellar wind mass loss is the key to remove the helium envelope in SN Ic progenitors. [abridged]
Magnetically confined winds of early-type stars are expected to be sources of bright and hard X-rays. To clarify the systematics of the observed X-ray properties, we have analyzed a large series of Chandra and XMM observations, corresponding to all available exposures of known massive magnetic stars (over 100 exposures covering ~60% of stars compiled in the catalog of Petit et al. 2013). We show that the X-ray luminosity is strongly correlated with the stellar wind mass-loss-rate, with a power-law form that is slightly steeper than linear for the majority of the less luminous, lower-Mdot B stars and flattens for the more luminous, higher-Mdot O stars. As the winds are radiatively driven, these scalings can be equivalently written as relations with the bolometric luminosity. The observed X-ray luminosities, and their trend with mass-loss rates, are well reproduced by new MHD models, although a few overluminous stars (mostly rapidly rotating objects) exist. No relation is found between other X-ray properties (plasma temperature, absorption) and stellar or magnetic parameters, contrary to expectations (e.g. higher temperature for stronger mass-loss rate). This suggests that the main driver for the plasma properties is different from the main determinant of the X-ray luminosity. Finally, variations of the X-ray hardnesses and luminosities, in phase with the stellar rotation period, are detected for some objects and they suggest some temperature stratification to exist in massive stars magnetospheres.
LBVs are massive evolved stars that suffer sporadic and violent mass-loss events. They have been proposed as the progenitors of some core-collapse SNe, but this idea is still debated due to the lack of direct evidence. Since SNRs can carry in their morphology the fingerprints of the progenitor stars as well as of the inhomogeneous CSM sculpted by the progenitors, the study of SNRs from LBVs could help to place core-collapse SNe in context with the evolution of massive stars. We investigate the physical, chemical and morphological properties of the remnants of SNe originating from LBVs, in order to search for signatures, revealing the nature of the progenitors, in the ejecta distribution and morphology of the remnants. As a template of LBVs, we considered the actual LBV candidate Gal 026.47+0.02. We selected a grid of models, which describe the evolution of a massive star with properties consistent with those of Gal 026.47+0.02 and its final fate as core-collapse SN. We developed a 3D HD model that follows the post-explosion evolution of the ejecta from the breakout of the shock wave at the stellar surface to the interaction of the SNR with a CSM characterized by two dense nested toroidal shells, parametrized in agreement with multi-wavelength observations of Gal 026.47+0.02. Our models show a strong interaction of the blast wave with the CSM which determines an important slowdown of the expansion of the ejecta in the equatorial plane where the two shells lay, determining a high degree of asymmetry in the remnant. After 10000 years of evolution the ejecta show an elongated shape forming a broad jet-like structure caused by the interaction with the shells and oriented along the axis of the toroidal shells.
We study the populations of massive stars in the Carina region and their energetic feedback and ejection of $^{26}$Al. We did a census of the stellar populations in young stellar clusters within a few degrees of the Carina Nebula. For each star we estimated the mass, based on the spectral type and the host cluster age. We used population synthesis to calculate the energetic feedback and ejection of $^{26}$Al from the winds of the massive stars and their supernova explosions. We used 7 years of INTEGRAL observations to measure the $^{26}$Al signal from the region. The INTEGRAL $^{26}$Al signal is not significant with a best-fit value of about 1.5e-5 ph/cm^2/s, approximately half of the published Compton Gamma Ray Observatory (CGRO) result, but in agreement with the latest CGRO estimates. Our analysis of the stellar populations in the young clusters leads to an expected signal of half the observed value, but the results are consistent within 2 sigma. We find that the fraction of $^{26}$Al ejected in Wolf-Rayet winds is high, and the observed signal is unlikely to be caused by $^{26}$Al ejected in supernovae alone, indicating a strong wind ejection of $^{26}$Al. Due to the lack of prominent O stars, regions with ages $gtrsim$10 Myr are often neglected in studies of OB associations. We find that in the Carina region such clusters contribute significantly to the stellar mass and the energetics of the region.
Gravitational-wave detections are now starting to probe the mass distribution of stellar-mass black holes (BHs). Robust predictions from stellar models are needed to interpret these. Theory predicts the existence of a gap in the BH mass distribution because of pair-instability supernova. The maximum BH mass below the gap is the result of pulsational mass loss. We evolve massive helium stars through their late hydrodynamical phases of evolution using the open-source MESA stellar evolution code. We find that the location of the lower edge of the mass gap at 45$M_odot$ is remarkably robust against variations in the metallicity ($approx 3M_odot$), the treatment of internal mixing ($approx 1M_odot$), stellar wind mass loss ($approx 4M_odot$), making it the most robust predictions for the final stages of massive star evolution. The reason is that the onset of the instability is dictated by the near-final core mass, which in turn sets the resulting BH mass. However, varying $^{12}Cleft(alpha,gammaright)^{16}O$ reaction rate within its $1sigma$ uncertainties shifts the location of the gap between $40M_odot$ and $56M_odot$. We provide updated analytic fits for population synthesis simulations. Our results imply that the detection of merging BHs can provide constraints on nuclear astrophysics. Furthermore, the robustness against metallicity suggests that there is a universal maximum for the location of the lower edge of the gap, which is insensitive to the formation environment and redshift for first-generation BHs. This is promising for the possibility to use the location of the gap as a standard siren across the Universe.
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