Core-collapse supernovae (CCSNe) have very distinct observational properties that depend on the composition of the progenitor star, the dynamics of the explosion mechanism, and the surrounding stellar wind environment. In recent years, due to the uncertainty behind the type of massive star that evolves into different types of core-collapse events, there has been an increase in core-collapse supernova surveys aiding the advancement of numerical supernova simulations that explore the properties of the star before the explosion. Observationally, the unpredictable nature of these events makes it difficult to identify the type of star from which the CCSNe subtype evolves, but the issue from a theoretical standpoint relies on a gap in our current understanding of the explosion mechanism. The general light curve properties of CCSNe (rise, peak, and decay) by subtype are diverse, but appear to be homogeneous within each subtype, with the exception of Type IIn.. Simplified SN models can be processed quickly in order to explore the properties of the progenitor star along with the explosion mechanism and circumstellar medium. Here, we present a suite of SN light curve models presented using a 1-temperature, homologous outflow light curve code. The SN explosion is modeled from shock breakout through the ultimate uncovering of the nickel core. We are able to rapidly explore the diversity of the SN light curves by studying the effects of various explosion and progenitor star parameters, including ejecta mass, explosion energy, shock temperature, and stellar radii using this "simple" calculation technique. Furthermore, we compare UV and optical modeled light curves to Swift UVOT IIn observations to identify the general initial conditions that enable the difference between SN 2009ip and SN 2011ht light curve properties. Our results indicate that the peak light curve is dominated by the shock temperature and explosion energy, whereas the shape depends on the mass of the ejecta and the explosion energy. Based on this modeling approach, the comparison SN light curves are a product of processes occurring after shock breakout, but before Ni-56 decay. Therefore, the energy from nickel decay does not play a major role in the light curves of these explosions. In general, the diversity between SN 2009ip and SN 2011ht can be explained by the differences in the outer ejecta mass and the explosion energy.

Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z(circle dot)/2. Such tests are particularly important for interpreting the surprising high-ionization ultraviolet (UV) line emission detected at z > 6 in recent years. We present HST/COS UV spectra of 10 nearby metal-poor star-forming galaxies selected to show He II emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + logO/H < 8.5) and present uniformly large specific star formation rates (sSFR similar to 10(2) Gyr(-1)). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching CIII] equivalent widths comparable to those seen in systems at z similar to 6-7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + logO/H less than or similar to 8.0 (Z/Z(circle dot) less than or similar to 1/5) presenting minimal stellar wind features and prominent nebular emission in He II and C IV. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He+-ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often-neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C IV and He II powered by these stars may provide useful metallicity constraints in the reionization era.

We present the first detailed 3D hydrodynamic implicit large eddy simulations of turbulent convection of carbon burning in massive stars. Simulations begin with radial profiles mapped from a carbon-burning shell within a 15M circle dot 1D stellar evolution model. We consider models with 128(3), 256(3), 512(3), and 1024(3) zones. The turbulent flow properties of these carbon-burning simulations are very similar to the oxygen-burning case. We performed a mean field analysis of the kinetic energy budgets within the Reynolds-averaged Navier-Stokes framework. For the upper convective boundary region, we find that the numerical dissipation is insensitive to resolution for linear mesh resolutions above 512 grid points. For the stiffer, more stratified lower boundary, our highest resolution model still shows signs of decreasing sub-grid dissipation suggesting it is not yet numerically converged. We find that the widths of the upper and lower boundaries are roughly 30 per cent and 10 per cent of the local pressure scaleheights, respectively. The shape of the boundaries is significantly different from those used in stellar evolution models. As in past oxygen-shell-burning simulations, we observe entrainment at both boundaries in our carbon-shell-burning simulations. In the large Peclet number regime found in the advanced phases, the entrainment rate is roughly inversely proportional to the bulk Richardson number, Ri(B) (alpha Ri(B)(-alpha) a, 0.5 less than or similar to alpha less than or similar to 1.0). We thus suggest the use of Ri(B) as a means to take into account the results of 3D hydrodynamics simulations in new 1D prescriptions of convective boundary mixing.

We present spectral line mapping observations towards four massive star-forming regions Cepheus A, DR21S, S76E and G34.26+0.15 - with the IRAM 30-m telescope at the 2 and 3 mm bands. In total, 396 spectral lines from 51 molecules, one helium recombination line, 10 hydrogen recombination lines and 16 unidentified lines were detected in these four sources. An emission line of nitrosyl cyanide (ONCN, 14(0), 14-13(0), (13)) was detected in G34.26+0.15, as the first detection in massive star-forming regions. We found that c-C3H2 and NH2D show enhancement in shocked regions, as suggested by the evidence of SiO and/or SO emission. The column density and rotational temperature of CH3CN were estimated with the rotational diagram method for all four sources. Isotope abundance ratios of C-12/C-13 were derived using HC3N and its C-13 isotopologue, which were around 40 in all four massive star-forming regions and slightly lower than the local interstellar value (similar to 65). The N-14/N-15 and O-16/O-18 abundance ratios in these sources were also derived using the double isotopic method, which were slightly lower than in the local interstellar medium. Except for Cep A, the S-33/S-34 ratios in the other three targets were derived, which were similar to that in the local interstellar medium. The column density ratios of N(DCN)/N(HCN) and N( DCO+)/N(HCO+) in these sources were more than two orders of magnitude higher than the elemental [D]/[H] ratio, which is 1.5 x 10(-5). Our results show that the later stage sources, G34.26+0.15 in particular, present more molecular species than earlier stage sources. Evidence of shock activity is seen in all stages studied.

The Galactic blue supergiant SBW1 with its circumstellar ring nebula represents the best known analogue of the progenitor of SN 1987A. High-resolution imaging has shown H alpha and infrared structures arising in an ionized flow that partly fills the ring's 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 Hubble Space Telescope Cosmic Origins Spectrograph. 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) M-circle dot yr(-1), although we find that cooling time-scales probably become comparable to (or longer than) the flow time below 10(-8) M-circle dot yr(-1). We therefore adopt this latter value as a conservative upper limit. For the central star, the model yields T-eff = 21 000 +/- 1000 K, log(g(eff)) = 3.0, L similar or equal to 5 x 10(4) L-circle dot, and roughly Solar composition except for enhanced N abundance. SBW1' s very low mass-loss rate may hinder the wind's ability to shape its nebula and to shed angular momentum. The spin-down time-scale for magnetic breaking is more than 500 times longer than the age of the ring. This, combined with the star's slow rotation rate, constrains 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 SBW1' s wind is so weak presents an interesting mystery.

We explore the effects of rotation on convective carbon, oxygen, and silicon shell burning during the late stages of evolution in a 20 M-circle dot star. Using the Modules for Experiments in Stellar Astrophysics we construct one-dimensional (1D) stellar models both with no rotation and with an initial rigid rotation of 50% of critical. At different points during the evolution, we map the 1D models into 2D and follow the multidimensional evolution using the FLASH compressible hydrodynamics code for many convective turnover times until a quasi-steady state is reached. We characterize the strength and scale of convective motions via decomposition of the momentum density into vector spherical harmonics. We find that rotation influences the total power in solenoidal modes, with a slightly larger impact for carbon and oxygen shell burning than for silicon shell burning. Including rotation in 1D stellar evolution models alters the structure of the star in a manner that has a significant impact on the character of multidimensional convection. Adding modest amounts of rotation to a stellar model that ignores rotation during the evolutionary stage, however, has little impact on the character of the resulting convection. Since the spatial scale and strength of convection present at the point of core collapse directly influence the supernova mechanism, our results suggest that rotation could play an important role in setting the stage for massive stellar explosions.

We compile observations of early-type binaries identified via spectroscopy, eclipses, long-baseline interferometry, adaptive optics, common proper motion, etc. Each observational technique is sensitive to companions across a narrow parameter space of orbital periods P and mass ratios q. =. M-comp/M-1. After combining the samples from the various surveys and correcting for their respective selection effects, we find that the properties of companions to O-type and B-type main-sequence (MS) stars differ among three regimes. First, at short orbital periods P less than or similar to 20. days (separations a less than or similar to 0.4 au), the binaries have small eccentricities e... 0.4, favor modest mass ratios < q > less than or similar to 0.5, and exhibit a small excess of twins q. >. 0.95. Second, the companion frequency peaks at intermediate periods log P (days). approximate to. 3.5 (a approximate to 10 au), where the binaries have mass ratios weighted toward small values q. approximate to 0.2-0.3 and follow a Maxwellian " thermal" eccentricity distribution. Finally, companions with long orbital periods log P (days). approximate to 5.5-7.5 (a approximate to 200-5000 au) are outer tertiary components in hierarchical triples and have a mass ratio distribution across q. approximate to 0.1-1.0 that is nearly consistent with random pairings drawn from the initial mass function. We discuss these companion distributions and properties in the context of binary-star formation and evolution. We also reanalyze the binary statistics of solar-type MS primaries, taking into account that 30% +/-. 10% of single-lined spectroscopic binaries likely contain white dwarf companions instead of low-mass stellar secondaries. The mean frequency of stellar companions with q. >. 0.1 and log P (days). <. 8.0 per primary increases from 0.50. +/- 0.04 for solar-type MS primaries to 2.1. +/- 0.3 for O-type MS primaries. We fit joint probability density functions f (M-1, q, P, e) not equal f (M-1) f (q) f (P) f (e) to the corrected distributions, which can be incorporated into binary population synthesis studies.

The MiMeS (Magnetism in Massive Stars) project is a large-scale, high-resolution, sensitive spectropolarimetric investigation of the magnetic properties of O- and early B-type stars. Initiated in 2008 and completed in 2013, the project was supported by three Large Program allocations, as well as various programmes initiated by independent principal investigators, and archival resources. Ultimately, over 4800 circularly polarized spectra of 560 O and B stars were collected with the instruments ESPaDOnS (Echelle SpectroPolarimetric Device for the Observation of Stars) at the Canada–France–Hawaii Telescope, Narval at the Télescope Bernard Lyot and HARPSpol at the European Southern Observatory La Silla 3.6 m telescope, making MiMeS by far the largest systematic investigation of massive star magnetism ever undertaken. In this paper, the first in a series reporting the general results of the survey, we introduce the scientific motivation and goals, describe the sample of targets, review the instrumentation and observational techniques used, explain the exposure time calculation designed to provide sensitivity to surface dipole fields above approximately 100 G, discuss the polarimetric performance, stability and uncertainty of the instrumentation, and summarize the previous and forthcoming publications.

Many young, massive stars are found in close binaries. Using population synthesis simulations. we predict the likelihood of a companion star being present when these massive stars end their lives as core-collapse supernovae (SNe). We focus on stripped-envelope SNe, whose progenitors have lost their outer hydrogen and possibly helium layers before explosion. We use these results to interpret new Hubble Space Telescope observations of the site of the broad-lined Type. Ic SN 2002ap, 14 years post-explosion. For a subsolar metallicity consistent with SN 2002ap, we expect a main-sequence (MS) companion present in about two thirds of all stripped-envelope SNe and a compact companion (likely a stripped helium star or a white dwarf/neutron star/black hole) in about 5% of cases. About a quarter of progenitors are single at explosion (originating from initially single stars, mergers, or disrupted systems). All of the latter scenarios require a massive progenitor, inconsistent with earlier studies of SN 2002ap. Our new, deeper upper limits exclude the presence of an MS companion star > 8-10 M., ruling out about 40% of all stripped-envelope SN channels. The most likely scenario for SN 2002ap includes nonconservative binary interaction of a primary star initially. 23 M.. Although unlikely (< 1% of the scenarios), we also discuss the possibility of an exotic reverse merger channel for broadlined Type. Ic events. Finally, we explore how our results depend on the metallicity and the model assumptions and discuss how additional searches for companions can constrain the physics that govern. the evolution of SN progenitors.

Among Wolf-Rayet stars, those of subtype WN8 are the intrinsically most variable. We have explored the long-term photometric variability of the brightest known WN8 star, WR 40, through four contiguous months of time-resolved, single-passband optical photometry with the BRIght Target Explorer nanosatellite mission. The Fourier transform of the observed light curve reveals that the strong light variability exhibited by WR 40 is dominated by many randomly triggered, transient, low-frequency signals. We establish a model in which the whole wind consists of stochastic clumps following an outflow visibility promptly rising to peak brightness upon clump emergence from the optically thick pseudo-photosphere in the wind, followed by a gradual decay according to the right-half of a Gaussian. Free electrons in each clump scatter continuum light from the star. We explore a scenario where the clump size follows a power-law distribution, and another one with an ensemble of clumps of constant size. Both scenarios yield simulated light curves morphologically resembling the observed light curve remarkably well, indicating that one cannot uniquely constrain the details of clump size distribution with only a photometric light curve. Nevertheless, independent evidence favours a negative-index power law, as seen in many other astrophysical turbulent media.