We analyze 221 eclipsing binaries (EBs) in the Large Magellanic Cloud with B-type main-sequence (MS) primaries ($M_1$ $approx$ 4 - 14 M$_{odot}$) and orbital periods $P$ = 20 - 50 days that were photometrically monitored by the Optical Gravitational
Lensing Experiment. We utilize our three-stage automated pipeline to (1) classify all 221 EBs, (2) fit physical models to the light curves of 130 detached well-defined EBs from which unique parameters can be determined, and (3) recover the intrinsic binary statistics by correcting for selection effects. We uncover two statistically significant trends with age. First, younger EBs tend to reside in dustier environments with larger photometric extinctions, an empirical relation that can be implemented when modeling stellar populations. Second, younger EBs generally have large eccentricities. This demonstrates that massive binaries at moderate orbital periods are born with a Maxwellian thermal orbital velocity distribution, which indicates they formed via dynamical interactions. In addition, the age-eccentricity anticorrelation provides a direct constraint for tidal evolution in highly eccentric binaries containing hot MS stars with radiative envelopes. The intrinsic fraction of B-type MS stars with stellar companions $q$ = $M_2$/$M_1$ $>$ 0.2 and orbital periods $P$ = 20 - 50 days is (7 $pm$ 2)%. We find early-type binaries at $P$ = 20 - 50 days are weighted significantly toward small mass ratios $q$ $approx$ 0.2 - 0.3, which is different than the results from previous observations of closer binaries with $P$ $<$ 20 days. This indicates that early-type binaries at slightly wider orbital separations have experienced substantially less coevolution and competitive accretion during their formation in the circumbinary disk.
Early B-type main-sequence (MS) stars (M$_1$ = 5-16 M$_{odot}$) with closely orbiting low-mass stellar companions (q = M$_2$/M$_1$ < 0.25) can evolve to produce Type Ia supernovae, low-mass X-ray binaries, and millisecond pulsars. However, the format
ion mechanism and intrinsic frequency of such close extreme mass-ratio binaries have been debated, especially considering none have hitherto been detected. Utilizing observations of the Large Magellanic Cloud galaxy conducted by the Optical Gravitational Lensing Experiment, we have discovered a new class of eclipsing binaries in which a luminous B-type MS star irradiates a closely orbiting low-mass pre-MS companion that has not yet fully formed. The primordial pre-MS companions have large radii and discernibly reflect much of the light they intercept from the B-type MS primaries ($Delta$I$_{rm refl}$ = 0.02-0.14 mag). For the 18 definitive MS + pre-MS eclipsing binaries in our sample with good model fits to the observed light curves, we measure short orbital periods P = 3.0-8.5 days, young ages $tau$ = 0.6-8 Myr, and small secondary masses M$_2$ = 0.8-2.4 M$_{odot}$ (q = 0.07-0.36). The majority of these nascent eclipsing binaries are still associated with stellar nurseries, e.g. the system with the deepest eclipse $Delta$I$_1$ = 2.8 mag and youngest age $tau$ = 0.6$pm$0.4 Myr is embedded in the bright H II region 30 Doradus. After correcting for selection effects, we find that (2.0$pm$0.6)% of B-type MS stars have companions with short orbital periods P = 3.0-8.5 days and extreme mass ratios q = 0.06-0.25. This is $approx$10 times greater than that observed for solar-type MS primaries. We discuss how these new eclipsing binaries provide invaluable insights, diagnostics, and challenges for the formation and evolution of stars, binaries, and H II regions.
In order to understand the rates and properties of Type Ia and Type Ib/c supernovae, X-ray binaries, gravitational wave sources, and gamma ray bursts as a function of galactic environment and cosmic age, it is imperative that we measure how the close
binary properties of O and B-type stars vary with metallicity. We have studied eclipsing binaries with early-B main-sequence primaries in three galaxies with different metallicities: the Large and Small Magellanic Clouds (LMC and SMC, respectively) as well as the Milky Way (MW). The observed fractions of early-B stars which exhibit deep eclipses 0.25 < Delta(m) (mag) < 0.65 and orbital periods 2 < P (days) < 20 in the MW, LMC, and SMC span a narrow range of (0.7-1.0)%, which is a model independent result. After correcting for geometrical selection effects and incompleteness toward low-mass companions, we find for early-B stars in all three environments: (1) a close binary fraction of (22+/-5)% across orbital periods 2 < P (days) < 20 and mass ratios q = M_2/M_1 > 0.1, (2) an intrinsic orbital period distribution slightly skewed toward shorter periods relative to a distribution that is uniform in log P, (3) a mass-ratio distribution weighted toward low-mass companions, and (4) a small, nearly negligible excess fraction of twins with q > 0.9. Our fitted parameters derived for the MW eclipsing binaries match the properties inferred from nearby, early-type spectroscopic binaries, which further validates our results. There are no statistically significant trends with metallicity, demonstrating that the close binary properties of massive stars do not vary across metallicities -0.7 < log(Z/Z_sun) < 0.0 beyond the measured uncertainties.
We report unique EVLA 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 ~10^15-10^16 cm, implying an upper limit o
n the mass loss rate from the progenitor system of Mdot <~ 6 x 10^-10 Msol/yr (assuming a wind speed of 100 km/s), 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 progenitors 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 the most popular single degenerate progenitor models for SN 2011fe, leaving a limited phase space inhabited by some double degenerate systems and exotic progenitor scenarios.
The {alpha}-formalism is a common way to parametrize the common envelope interaction between a giant star and a more compact companion. The {alpha} parameter describes the fraction of orbital energy released by the companion that is available to ejec
t the giant stars envelope. By using new, detailed stellar evolutionary calculations we derive a user-friendly prescription for the {lambda} parameter and an improved approximation for the envelope binding energy, thus revising the {alpha} equation. We then determine {alpha} both from simulations and observations in a self consistent manner. By using our own stellar structure models as well as population considerations to reconstruct the primarys 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 companions mass to the mass of the primary at the time of the common envelope interaction) have higher values of {alpha}. 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 in to the giants 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.
We present a detailed analysis of the Astrophysical Research Consortium 3.5 m telescope spectrum of QSO SDSS J0838+2955. The object shows three broad absorption line (BAL) systems at 22,000, 13,000, and 4900 km s^-1 blueshifted from the systemic reds
hift of z=2.043. Of particular interest is the lowest velocity system that displays absorption from low-ionization species such as Mg II, Al II, Si II, Si II*, Fe II and Fe II*. Accurate column densities were measured for all transitions in this lowest velocity BAL using an inhomogeneous absorber model. The ratio of column densities of Si II* and Fe II* with respect to their ground states gave an electron number density of log n_e (cm^-3) = 3.75 +/- 0.22 for the outflow. Photoionization modeling with careful regards to chemical abundances and the incident spectral energy distribution predicts an ionization parameter of log U_H = -1.93 +/- 0.21 and a hydrogen column density of log N_H (cm^-2) = 20.80 +/- 0.28. This places the outflow at 3.3+1.5-1.0 kpc from the central AGN. Assuming that the fraction of solid angle subtended by the outflow is 0.2, these values yield a kinetic luminosity of (4.5+3.1-1.8) x 10^45 erg s^-1, which is (1.4+1.1-0.6)% the bolometric luminosity of the QSO itself. Such large kinetic luminosity suggests that QSO outflows are a major contributor to AGN feedback mechanisms.
