252 research outputs found
The origin of the first neutron star -- neutron star merger
The first neutron star-neutron star (NS-NS) merger was discovered on August
17, 2017 through gravitational waves (GW170817) and followed with
electromagnetic observations. This merger was detected in an old elliptical
galaxy with no recent star formation. We perform a suite of numerical
calculations to understand the formation mechanism of this merger. We probe
three leading formation mechanisms of double compact objects: classical
isolated binary star evolution, dynamical evolution in globular clusters and
nuclear cluster formation to test whether they are likely to produce NS-NS
mergers in old host galaxies. Our simulations with optimistic assumptions show
current NS-NS merger rates at the level of 10^-2 yr^-1 from binary stars, 5 x
10^-5 yr^-1 from globular clusters and 10^-5 yr^-1 from nuclear clusters for
all local elliptical galaxies (within 100 Mpc^3). These models are thus in
tension with the detection of GW170817 with an observed rate 1.5 yr^-1 (per 100
Mpc^3; LIGO/Virgo estimate). Our results imply that either (i) the detection of
GW170817 by LIGO/Virgo at their current sensitivity in an elliptical galaxy is
a statistical coincidence; or that (ii) physics in at least one of our three
models is incomplete in the context of the evolution of stars that can form
NS-NS mergers; or that (iii) another very efficient (unknown) formation channel
with a long delay time between star formation and merger is at play.Comment: A&A: accepte
Masses of Astrometrically-Discovered and Imaged Binaries: G 78-28AB and GJ 231.1BC
The Stellar Planet Survey (STEPS) is an ongoing astrometric search for giant
planets and brown dwarfs around a sample of ~30 M-dwarfs. We have discovered
several low-mass companions by measuring the motion of our target stars
relative to their reference frames. The highest mass discovery thus far is G
78-28B, a companion to the M-dwarf G 78-28A. The orbital period is 4.18 +/-
0.03 y, the system mass is 0.565 +/- 0.055 Msolar, and the semi-major axis is
2.19 +/- 0.10 AU. Imaging observations with the Keck laser guide star adaptive
optics (LGSAO) and the Palomar AO instruments resolved the system and also
yielded JHK-band delta magnitudes. We use the orbital solution, light ratios,
and mass-luminosity relationships to derive component masses of MA = 0.370 +/-
0.034 Msolar and MB = 0.195 +/- 0.021 Msolar. G 78-28B is of type M4 V based
upon its colors and mass. We also discovered GJ 231.1C, a companion to GJ
231.1B, with STEPS and imaged the companion with LGSAO and Palomar AO, but the
orbital period is longer than our observing baseline; thus the system
parameters are less constrained. In GJ 231.1BC the masses are MB = 0.25 +/-
0.06 Msolar and MC =0.12 +/- 0.02 Msolar. The inferred spectral type of GJ
231.1C is M5 V. We demonstrate the results of the current state of mass
estimation techniques with our data.Comment: 25 pages, 8 figures, accepted for Ap
Noninteracting Black Hole Binaries with Gaia and LAMOST
Until recently, black holes (BHs) could be discovered only through accretion
from other stars in X-ray binaries, or in merging double compact objects.
Improvements in astrometric and spectroscopic measurements have made it
possible to detect BHs also in non-interacting BH binaries (nBHB) through a
precise analysis of the companion's motion. In this study, using an updated
version of the Startrack binary-star population modelling code and a detailed
model of the Milky Way (MW) galaxy we calculate the expected number of
detections for Gaia and LAMOST surveys. We develop a formalism to convolve the
binary population synthesis output with a realistic stellar density
distribution, star-formation history (SFH), and chemical evolution for the MW,
which produces a probability distribution function of the predicted
compact-binary population over the MW. This avoids the additional statistical
uncertainty which is introduced by methods which Monte Carlo sample from binary
population synthesis output to produce one potential specific realisation of
the MW compact-binary distribution, and our method is also comparatively fast
to such Monte Carlo realisations. Specifically, we predict - nBHBs
to be observed by Gaia, although the numbers may drop to - if the
recent ( Myr) star formation is low (/yr ). For
LAMOST we predict detectable nBHBs, which is lower partially
because its field-of-view covers just of the Galaxy.Comment: 23 pages, 15 figures, 8 table
The effect of pair-instability mass loss on black-hole mergers
Context. Mergers of two stellar-origin black holes are a prime source of gravitational waves and are under intensive investigation. One crucial ingredient in their modeling has been neglected: pair-instability pulsation supernovae with associated severe mass loss may suppress the formation of massive black holes, decreasing black-hole-merger rates for the highest black-hole masses.
Aims. We demonstrate the effects of pair-instability pulsation supernovae on merger rate and mass using populations of double black-hole binaries formed through the isolated binary classical evolution channel.
Methods. The mass loss from pair-instability pulsation supernova is estimated based on existing hydrodynamical calculations. This mass loss is incorporated into the StarTrack population synthesis code. StarTrack is used to generate double black-hole populations with and without pair-instability pulsation supernova mass loss.
Results. The mass loss associated with pair-instability pulsation supernovae limits the Population I/II stellar-origin black-hole mass to 50 M⊙, in tension with earlier predictions that the maximum black-hole mass could be as high as 100 M⊙. In our model, neutron stars form with mass 1−2 M⊙. We then encounter the first mass gap at 2−5 M⊙ with the compact object absence due to rapid supernova explosions, followed by the formation of black holes with mass 5−50 M⊙, with a second mass gap at 50−135 M⊙ created by pair-instability pulsation supernovae and by pair-instability supernovae. Finally, black holes with masses above 135 M⊙ may potentially form to arbitrarily high mass limited only by the extent of the initial mass function and the strength of stellar winds. Suppression of double black-hole-merger rates by pair-instability pulsation supernovae is negligible for our evolutionary channel. Our standard evolutionary model, with the inclusion of pair-instability pulsation supernovae and pair-instability supernovae, is fully consistent with the Laser Interferometric Gravitational-wave Observatory (LIGO) observations of black-hole mergers: GW150914, GW151226, and LVT151012. The LIGO results are inconsistent with high (≳ 400 km s-1) black hole (BH) natal kicks. We predict the detection of several, and up to as many as ~60, BH-BH mergers with a total mass of 10−150 M⊙ (most likely range: 20−80 M⊙) in the forthcoming ~60 effective days of the LIGO O2 observations, assuming the detectors reach the optimistic target O2 sensitivity
Masses, Luminosities, and Orbital Coplanarities of the mu Orionis Quadruple Star System from PHASES Differential Astrometry
mu Orionis was identified by spectroscopic studies as a quadruple star
system. Seventeen high precision differential astrometry measurements of mu Ori
have been collected by the Palomar High-precision Astrometric Search for
Exoplanet Systems (PHASES). These show both the motion of the long period
binary orbit and short period perturbations superimposed on that caused by each
of the components in the long period system being themselves binaries. The new
measurements enable the orientations of the long period binary and short period
subsystems to be determined. Recent theoretical work predicts the distribution
of relative inclinations between inner and outer orbits of hierarchical systems
to peak near 40 and 140 degrees. The degree of coplanarity of this complex
system is determined, and the angle between the planes of the A-B and Aa-Ab
orbits is found to be 136.7 +/- 8.3 degrees, near the predicted distribution
peak at 140 degrees; this result is discussed in the context of the handful of
systems with established mutual inclinations. The system distance and masses
for each component are obtained from a combined fit of the PHASES astrometry
and archival radial velocity observations. The component masses have relative
precisions of 5% (component Aa), 15% (Ab), and 1.4% (each of Ba and Bb). The
median size of the minor axes of the uncertainty ellipses for the new
measurements is 20 micro-arcseconds. Updated orbits for delta Equulei, kappa
Pegasi, and V819 Herculis are also presented.Comment: 12 Pages, Accepted for publication in A
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