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Register a new userUV spectroscopy of the blue supergiant SBW1: the remarkably weak wind of a SN 1987A analog
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The Galactic blue supergiant SBW1 with its circumstellar ring nebula represents the best known analog of the progenitor of SN 1987A. High-resolution imaging has shown H-alpha and IR structures arising in an ionized flow that partly fills the rings interior. To constrain the influence of the stellar wind on this structure, we obtained an ultraviolet (UV) spectrum of the central star of SBW1 with the HST Cosmic Origins Spectrograph (COS). The UV spectrum shows none of the typical wind signatures, indicating a very low mass-loss rate. Radiative transfer models suggest an extremely low rate below 10$^{-10}$ Msun/yr, although we find that cooling timescales probably become comparable to or longer than the flow time below 10$^{-8}$ Msun/yr. We therefore adopt this latter value as a conservative upper limit. For the central star, the model yields $T_{rm eff}$=21,000$pm$1000 K, $Lsimeq$5$times$10$^4$ $L_{odot}$, and roughly Solar composition except for enhanced N abundance. SBW1s very low mass-loss rate may hinder the winds ability to shape the surrounding nebula. The very low mass-loss rate also impairs the winds ability to shed angular momentum; the spin-down timescale for magnetic breaking is more than 500 times longer than the age of the ring. This, combined with the stars slow rotation rate, constrain merger scenarios to form ring nebulae. The mass-loss rate is at least 10 times lower than expected from mass-loss recipes, without any account of clumping. The physical explanation for why SBW1s wind is so weak presents an interesting mystery.
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Based on the work of Menon & Heger (2017), we present the bolometric light curvesand spectra of the explosions of blue supergiant progenitors from binary mergers. We study SN 1987A and two other peculiar Type IIP supernovae: SN 1998A and SN 2006V. The progenitor models were produced using the stellar evolution code Keplerand then exploded using the 1D radiation hydrodynamic code Crab. The explosions of binary merger models exhibit an overall better fit to the light curve of SN 1987A than previous single star models, because of their lower helium-core masses, larger envelope masses, and smaller radii. The merger model that best matches the observational constraints of the progenitor of SN 1987A and the light curve is a model with a radius of 37 solar radii, an ejecta mass of 20.6 solar masses, an explosion energy of 1.7 Bethe, a nickel mass of 0.073 solar masses, and a nickel mixing velocity of 3,000 km/s. This model also works for SN 1998A and is comparable with earlier estimates from semi-analytic models. In the case of SN 2006V, however, a model with a radius of 150 solar radii and ejecta mass of 19.1 solar masses matches the light curve. These parameters are significantly higher than predictions from semi-analytic models for the progenitor of this supernova.
Even after elaborate investigations for 30 years, we still do not know well how the progenitor of SN 1987A has evolved. To explain unusual red-to-blue evolution, previous studies suggest that in a red giant stage either the increase of surface He abundance or the envelope mass was necessary. It is usually supposed that the He enhancement is caused by the rotational mixing, and the mass increase is by a binary merger. Thus, we have investigated these scenarios thoroughly. The obtained findings are that rotating single star models do not satisfy all the observational constraints and that the enhancement of envelope mass alone does not explain observations. Here, we consider a slow merger scenario in which both the He abundance and the envelope mass enhancements are expected to occur. We indeed show that most observational constraints such as the red-to-blue evolution, lifetime, total mass, position in the HR diagram at collapse, and the chemical anomalies are well reproduced by the merger model of 14 and 9 M$_{odot}$ stars. We also discuss the effects of the added envelope spin in the merger scenarios.
We revisit the evidence for the contribution of the long-lived radioactive nuclides 44Ti, 55Fe, 56Co, 57Co, and 60Co to the UVOIR light curve of SN 1987A. We show that the V-band luminosity constitutes a roughly constant fraction of the bolometric luminosity between 900 and 1900 days, and we obtain an approximate bolometric light curve out to 4334 days by scaling the late time V-band data by a constant factor where no bolometric light curve data is available. Considering the five most relevant decay chains starting at 44Ti, 55Co, 56Ni, 57Ni, and 60Co, we perform a least squares fit to the constructed composite bolometric light curve. For the nickel isotopes, we obtain best fit values of M(56Ni) = (7.1 +- 0.3) x 10^{-2} Msun and M(57Ni) = (4.1 +- 1.8) x 10^{-3} Msun. Our best fit 44Ti mass is M(44Ti) = (0.55 +- 0.17) x 10^{-4} Msun, which is in disagreement with the much higher (3.1 +- 0.8) x 10^{-4} Msun recently derived from INTEGRAL observations. The associated uncertainties far exceed the best fit values for 55Co and 60Co and, as a result, we only give upper limits on the production masses of M(55Co) < 7.2 x 10^{-3} Msun and M(60Co) < 1.7 x 10^{-4} Msun. Furthermore, we find that the leptonic channels in the decay of 57Co (internal conversion and Auger electrons) are a significant contribution and constitute up to 15.5% of the total luminosity. Consideration of the kinetic energy of these electrons is essential in lowering our best fit nickel isotope production ratio to [57Ni/56Ni]=2.5+-1.1, which is still somewhat high but is in agreement with gamma-ray observations and model predictions.
We present optical photometric and spectroscopic observations of the 1987A-like supernova (SN) 2009mw. Our $BVRI$ and $griz$ photometry covers 167 days of evolution, including the rise to the light curve maximum, and ends just after the beginning of the linear tail phase. We compare the observational properties of SN 2009mw with those of other SNe belonging to the same subgroup, and find that it shows similarities to several objects. The physical parameters of the progenitor and the SN are estimated via hydrodynamical modelling, yielding an explosion energy of $1$ foe, a pre-SN mass of $19,{rm M_{odot}}$, a progenitor radius as $30,{rm R_{odot}}$ and a $^{56}$Ni mass as $0.062,{rm M_{odot}}$. These values indicate that the progenitor of SN 2009mw was a blue supergiant star, similar to the progenitor of SN 1987A. We examine the host environment of SN 2009mw and find that it emerged from a population with slightly sub-solar metallicty.
We present an 80-d long uninterrupted high-cadence K2 light curve of the B1Iab supergiant rho Leo (HD 91316), deduced with the method of halo photometry. This light curve reveals a dominant frequency of $f_{rmrot}=0.0373$d$^{-1}$ and its harmonics. This dominant frequency corresponds with a rotation period of 26.8d and is subject to amplitude and phase modulation. The K2 photometry additionally reveals multiperiodic low-frequency variability ($<1.5 $d$^{-1}$) and is in full agreement with low-cadence high-resolution spectroscopy assembled during 1800 days. The spectroscopy reveals rotational modulation by a dynamic aspherical wind with an amplitude of about 20km s$^{-1}$ in the H$alpha$ line, as well as photospheric velocity variations of a few km s$^{-1}$ at frequencies in the range 0.2 to 0.6 d$^{-1}$ in the SiIII 4567AA line. Given the large macroturbulence needed to explain the spectral line broadening of the star, we interpret the detected photospheric velocity as due to travelling super-inertial low-degree large-scale gravity waves with dominant tangential amplitudes and discuss why $rho$~Leo is an excellent target to study how the observed photospheric variability propagates into the wind.
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