Constraining Models of Binary Black Hole Formation with Gravitational-Wave Observations [HEAP]

Gravitational waves from binary black hole mergers provide a new probe of massive-star evolution and the formation channels of binary compact objects. By coupling the growing sample of binary black hole systems with realistic population synthesis models, we can begin to constrain the parameters of such models and glean unprecedented knowledge of the inherent physical processes that underpin binary stellar evolution. In this study, we apply a hierarchical Bayesian model on mass measurements from a synthetic gravitational-wave sample to constrain the physical prescriptions in population models and the relative fractions of systems generated from various channels. We employ population models of two canonical formation scenarios in our analysis — isolated binary evolution involving common-envelope evolution and dynamical formation within globular clusters — with model variations for different black-hole natal kick magnitude prescriptions. We show that solely with chirp mass measurements, it is possible to constrain natal kick prescriptions and the relative fraction of systems detected from each formation channel with $\mathcal{O}(100)$ of confident detections. This framework can be extended to include additional formation scenarios, model parameters, and measured properties of the compact binary.

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M. Zevin, C. Pankow, C. Rodriguez, et. al.
Wed, 26 Apr 17

Comments: 11 pages, 8 figures. Submitted to ApJ

Detection of intrinsic variability in the eclipsing massive main sequence O+B binary HD 165246 [SSA]

We present the analysis of 29.77 days of K2 space photometry of the well-detached massive 4.6 d O+B binary HD 165246 (V=7.8) obtained during Campaign 9b. This analysis reveals intrinsic variability in the residual lightcurve after subtraction of the binary model, in the frequency range [0,10] $d^{-1}$. This makes HD 165246 only the second O+B eclipsing binary with asteroseismic potential. While some of the frequencies are connected with the rotation of the primary, others are interpreted as due to oscillations with periodicities of order days. The frequency resolution of the current dataset does not allow to distinguish between frequencies due to standing coherent oscillation modes or travelling waves. Further time-resolved high-precision spectroscopy covering several binary orbits will reveal whether HD 165246 is a Rosetta stone for synergistic binary and seismic modelling of an O-type star.

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C. Johnston, B. Buysschaert, A. Tkachenko, et. al.
Wed, 26 Apr 17

Comments: Accepted for publication in Monthly Notices of the Royal Astronomical Society Letters, 5 pages, 3 figures, 2 tables

Radiative rates and electron impact excitation rates for transitions in He II [SSA]

We report calculations of energy levels, radiative rates, collision strengths, and effective collision strengths for transitions among the lowest 25 levels of the n <= 5 configurations of He~II. The general-purpose relativistic atomic structure package (GRASP) and Dirac atomic R-matrix code (DARC) are adopted for the calculations. Radiative rates, oscillator strengths, and line strengths are reported for all electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among the 25 levels. Furthermore, collision strengths and effective collision strengths are listed for all 300 transitions among the above 25 levels over a wide energy (temperature) range up to 9 Ryd (10**5.4 K). Comparisons are made with earlier available results and the accuracy of the data is assessed.

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K. Aggarwal, A. Igarashi, F. Keenan, et. al.
Tue, 25 Apr 17

Comments: 30 pages of text including 12 figures and 5 Tables will appear in ATOMS 5 (2017)

Mass Loss Rates from Coronal Mass Ejections: A Predictive Theoretical Model for Solar-Type Stars [SSA]

Coronal mass ejections (CMEs) are eruptive events that cause a solar-type star to shed mass and magnetic flux. CMEs tend to occur together with flares, radio storms, and bursts of energetic particles. On the Sun, CME-related mass loss is roughly an order of magnitude less intense than that of the background solar wind. However, on other types of stars, CMEs have been proposed to carry away much more mass and energy than the time-steady wind. Earlier papers have used observed correlations between solar CMEs and flare energies, in combination with stellar flare observations, to estimate stellar CME rates. This paper sidesteps flares and attempts to calibrate a more fundamental correlation between surface-averaged magnetic fluxes and CME properties. For the Sun, there exists a power-law relationship between the magnetic filling factor and the CME kinetic energy flux, and it is generalized for use on other stars. An example prediction of the time evolution of wind/CME mass-loss rates for a solar-mass star is given. A key result is that for ages younger than about 1 Gyr (i.e., activity levels only slightly higher than the present-day Sun), the CME mass loss exceeds that of the time-steady wind. At younger ages, CMEs carry 10 to 100 times more mass than the wind, and such high rates may be powerful enough to dispel circumstellar disks and affect the habitability of nearby planets. The cumulative CME mass lost by the young Sun may have been as much as 1% of a solar mass.

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S. Cranmer
Tue, 25 Apr 17

Comments: Accepted for publication in the Astrophysical Journal. 10 pages, 5 figures

An opening criterion for dust gaps in protoplanetary discs [SSA]

We aim to understand under which conditions a low mass planet can open a gap in viscous dusty protoplanetary discs. For this purpose, we extend the theory of dust radial drift to include the contribution from the tides of an embedded planet and from the gas viscous forces. From this formalism, we derive i) a grain size-dependent criterion for dust gap opening in discs, ii) an estimate of the location of the outer edge of the dust gap and iii) an estimate of the minimum Stokes number above which low-mass planets are able to carve gaps which appear only in the dust disc. These analytical estimates are particularly helpful to appraise the minimum mass of an hypothetical planet carving gaps in discs observed at long wavelengths and high resolution. We validate the theory against 3D SPH simulations of planet-disc interaction in a broad range of dusty protoplanetary discs. We find a remarkable agreement between the theoretical model and the numerical experiments.

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G. Dipierro and G. Laibe
Tue, 25 Apr 17

Comments: 17 pages, 13 figures. Accepted for publication in MNRAS

Siberian Radioheliograph: First Results [SSA]

Regular observations of active processes in the solar atmosphere have been started using the first stage of the multiwave Siberian Radioheliograph (SRH), a T-shaped 48-antenna array with a 4-8 GHz operating frequency range and a 10 MHz instantaneous receiving band. Antennas are mounted on the central antenna posts of the Siberian Solar Radio Telescope. The maximum baseline is 107.4 m, and the angular resolution is up to 70″. We present examples of observations of the solar disk at different frequencies, “negative” bursts, and solar flares. The sensitivity to compact sources reaches 0.01 solar flux units ($\approx 10^{-4}$ of the total solar flux) with an accumulation time of about 0.3 s. The high sensitivity of SRH enables monitoring of solar activity and allows studying active processes from characteristics of their microwave emission, including faint events, which could not be detected previously.

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S. Lesovoi, A. Altyntsev, A. Kochanov, et. al.
Tue, 25 Apr 17

Comments: 15 pages, 12 figures

The VLT-FLAMES Tarantula Survey XXVII. Physical parameters of B-type main-sequence binary systems in the Tarantula nebula [SSA]

A spectroscopic analysis has been undertaken for the B-type multiple systems (excluding those with supergiant primaries) in the VLT-FLAMES Tarantula Survey (VFTS). Projected rotational velocities, $v$sin$i$, for the primaries have been estimated using a Fourier Transform technique and confirmed by fitting rotationally broadened profiles. A subset of 33 systems with $v$sin$i$$\leq$ 80 km/s have been analysed using a TLUSTY grid of model atmospheres to estimate stellar parameters and surface abundances for the primaries. The effects of a potential flux contribution from an unseen secondary have also been considered. For 20 targets it was possible to reliably estimate their effective temperatures (T${eff}$) but for the other 13 objects it was only possible to provide a constraint of 20,000$\leq$T${eff}$$\leq$26,000 K — the other parameters estimated for these targets will be consequently less reliable. The estimated stellar properties are compared with evolutionary models and are generally consistent with their membership of 30 Doradus, while the nature of the secondaries of 3 SB2 system is discussed. A comparison with a sample of single stars with $v$sin$i$$\leq$ 80 km/s obtained from the VFTS and analysed with the same techniques implies that the atmospheric parameters and nitrogen abundances of the two samples are similar. However, the binary sample may have a lack of primaries with significant nitrogen enhancements, which would be consistent with them having low rotational velocities and having effectively evolved as single stars without significant rotational mixing. This result, which may be actually a consequence of the limitations of the pathfinder investigation presented in this paper, should be considered as a motivation for spectroscopic abundance analysis of large samples of binary stars, with high quality observational data.

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R. Garland, P. Dufton, C. Evans, et. al.
Tue, 25 Apr 17

Comments: 14 pages, 5 figures