Maxwell Moe

EVLA Observations Constrain the Environment and Progenitor System of Type Ia Supernova 2011fe

We report unique Expanded Very Large Array observations of SN 2011fe representing the most sensitive radio study of a Type Ia supernova to date. Our data place direct constraints on the density of the surrounding medium at radii ~1015-1016 cm, implying an upper limit on the mass loss rate from the progenitor system of (assuming a wind speed of 100 km s–1) or expansion into a uniform medium with density n CSM 6 cm–3. Drawing from the observed properties of non-conservative mass transfer among accreting white dwarfs, we use these limits on the density of the immediate environs to exclude a phase space of possible progenitor systems for SN 2011fe. We rule out a symbiotic progenitor system and also a system characterized by high accretion rate onto the white dwarf that is expected to give rise to optically thick accretion winds. Assuming that a small fraction, 1%, of the mass accreted is lost from the progenitor system, we also eliminate much of the potential progenitor parameter space for white dwarfs hosting recurrent novae or undergoing stable nuclear burning. Therefore, we rule out much of the parameter space associated with popular single degenerate progenitor models for SN 2011fe, leaving a limited phase space largely inhabited by some double degenerate systems, as well as exotic single degenerates with a sufficient time delay between mass accretion and SN explosion.

On the α formalism for the common envelope interaction

The α formalism is a common way to parametrize the common envelope interaction between a giant star and a more compact companion. The α parameter describes the fraction of orbital energy released by the companion that is available to eject the giant star’s envelope. By using new, detailed stellar evolutionary calculations, we derive a user-friendly prescription for the λ parameter and an improved approximation for the envelope binding energy, thus revising the α equation. We then determine α both from simulations and from observations in a self-consistent manner. By using our own stellar structure models as well as population considerations to reconstruct the primary’s parameters at the time of the common envelope interaction, we gain a deeper understanding of the uncertainties. We find that systems with very low values of q (the ratio of the companion’s mass to the mass of the primary at the time of the common envelope interaction) have higher values of α. A fit to the data suggests that lower-mass companions are left at comparable or larger orbital separations to more massive companions. We conjecture that lower-mass companions take longer than a stellar dynamical time to spiral into the giant’s core, and that this is key to allowing the giant to use its own thermal energy to help unbind its envelope. As a result, although systems with light companions might not have enough orbital energy to unbind the common envelope, they might stimulate a stellar reaction that results in the common envelope ejection.

The binary fraction of planetary nebula central stars-I. A high-precision, I-band excess search

We still do not know what causes aspherical planetary nebula (PN) morphologies. A plausible hypothesis is that they are due to the presence of a close stellar or substellar companion. So far, only ∼40 binary central stars of PN have been detected, almost all of them with such short periods that their binarity is revealed by photometric variability. Here we have endeavoured to discover binary central stars at any separation, thus determining the unbiased binary fraction of central stars of PN. This number, when compared to the binary fraction of the presumed parent population, can give a first handle on the origin of PN. By detecting the central stars in the I band we have searched for cool companions. We have found that 30 per cent of our sample have an I-band excess detected between 1 and a few σ , possibly denoting companions brighter than M3–4V and with separations smaller than ∼1000 au. By accounting for the undetectable companions, we determine a debiased binary fraction of 67–78 per cent for all companions at all separations. We compare this number to a main-sequence binary fraction of (50 ± 4) per cent determined for spectral types F6V–G2V, appropriate if the progenitors of today’s PN central star population are indeed the F6V–G2V stars. The error on our estimate cannot be constrained tightly, but we determine it to be between 10 and 30 per cent. We conclude that the central star binary fraction may be larger than expected from the putative parent population. However, this result is based on a sample of 27 bona fide central stars and should be considered preliminary. The success of the I-band method rests critically on high-precision photometry and a reasonably large sample. From a similar analysis, using the more sensitive J band of a subset of 11 central stars, the binary fraction is 54 per cent for companions brighter than ∼M5–6V and with separations smaller than about 900 au. Debiasing this number in the same way as was done for the I band we obtain a binary fraction of 100–107 per cent. The two numbers should be the same and the discrepancy is likely due to small-number statistics. Finally, we note how the previously derived short-period PN binary fraction of 15–20 per cent is far larger than expected based on the main-sequence binary fraction and period distribution. As a byproduct of our analysis we present an accurately vetted compilation of observed main-sequence star magnitudes, colours and masses, which can serve as a reference for future studies. We also present synthetic colours of hot stars as a function of temperature (20–170 kK) and gravity (log g = 6–8) for Solar and PG1159 compositions.

PS1-12sk is a Peculiar Supernova from a He-rich Progenitor System in a Brightest Cluster Galaxy Environment

We report on our discovery and observations of the Pan-STARRS1 supernova (SN) PS1-12sk, a transient with properties that indicate atypical star formation in its host galaxy cluster or pose a challenge to popular progenitor system models for this class of explosion. The optical spectra of PS1-12sk classify it as a Type Ibn SN (SN Ibn; cf. SN 2006jc), dominated by intermediate-width (3 × 10 3 km s −1 ) and time variable Hei emission. Our multi-wavelength monitoring establishes the rise time dt ∼ 9–23 days and shows an NUV–NIR spectral energy distribution with temperature17×10 3 K and a peak magnitude of Mz =− 18.88 ± 0.02 mag. SN Ibn spectroscopic properties are commonly interpreted as the signature of a massive star (17–100 M� ) explosion within an He-enriched circumstellar medium. However, unlike previous SNe Ibn, PS1-12sk is associated with an elliptical brightest cluster galaxy, CGCG 208−042 (z = 0.054) in cluster RXC J0844.9+4258. The expected probability of an event like PS1-12sk in such environments is low given the measured infrequency of core-collapse SNe in red-sequence galaxies compounded by the low volumetric rate of SN Ibn. Furthermore, we find no evidence of star formation at

ON THE NATURE OF THE PROGENITOR OF THE Type Ia SN2011fe IN M101

The explosion of a Type Ia supernova, SN2011fe, in the nearby Pinwheel galaxy (M101 at 6.4 Mpc) provides an opportunity to study pre-explosion images and search for the progenitor, which should consist of a white dwarf (WD), possibly surrounded by an accretion disk, in orbit with another star. We report on our use of deep Chandra observations and Hubble Space Telescope observations to limit the luminosity and temperature of the pre-explosion WD. It is found that if the spectrum was a blackbody, then pre-SN WDs with steady nuclear burning of the highest possible temperatures and luminosities are excluded assuming moderate n{sub H} values, but values of kT between roughly 10 eV and 60 eV are permitted even if the WD was emitting at the Eddington luminosity. This allows the progenitor to be an accreting nuclear-burning WD with an expanded photosphere 4-100 times the WD itself, or a super-critically accreting WD blowing off an optically thick strong wind, or possibly a recurrent nova with luminosities an order of magnitude lower than Eddington. The observations are also consistent with a double degenerate scenario, or a spinning down WD that has been spun up by accretion from the donor.

A New Class of Nascent Eclipsing Binaries with Extreme Mass Ratios

Early B-type main-sequence (MS) stars (M

Stellar Multiplicity Meets Stellar Evolution and Metallicity: The APOGEE View

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Early-type Eclipsing Binaries with Intermediate Orbital Periods

= 5-16 M

Finding binaries from phase modulation of pulsating stars with Kepler: V. Orbital parameters, with eccentricity and mass-ratio distributions of 341 new binaries

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Population synthesis of planetary nebulae from binaries

) with closely orbiting low-mass stellar companions (q = M