700 research outputs found
Double Neutron Star Populations and Formation Channels
In the past five years, the number of known double neutron stars (DNS) in the
Milky Way has roughly doubled. We argue that the observed sample can be split
into three distinct sub-populations based on their orbital characteristics: (i)
short-period, low-eccentricity binaries; (ii) wide binaries; and (iii)
short-period, high-eccentricity binaries. These sub-populations also exhibit
distinct spin period and spindown rate properties. We focus on sub-population
(iii), which contains the Hulse-Taylor binary. Contrary to previous analysis,
we demonstrate that, if they are the product of primordial binary evolution,
the and distribution of these systems requires that the
second-born NSs must have been formed with small natal kicks (25 km
s) and have pre-SN masses narrowly distributed around 3.2 M.
These constraints challenge binary evolution theory and further predict closely
aligned spin and orbital axes, inconsistent with the Hulse-Taylor binary's
measured spin-orbit misalignment angle of 20. Motivated by
the similarity of these DNSs to B2127+11C, a DNS residing in the globular
cluster M15, we argue that this sub-population is consistent with being formed
in, and then ejected from, globular clusters. This scenario provides a pathway
for the formation and merger of DNSs in stellar environments without recent
star formation, as observed in the host galaxy population of short gamma ray
bursts and the recent detection by LIGO of a merging DNS in an old stellar
population.Comment: 8 pages, 4 figures, 1 table, accepted for publication in ApJ
Double Neutron Star Formation: Merger Times, Systemic Velocities, and Travel Distances
The formation and evolution of double neutron stars (DNS) have traditionally
been studied using binary population synthesis. In this work, we take an
alternative approach by focusing only on the second supernova (SN) forming the
DNS and the subsequent orbital decay and merger due to gravitational wave
radiation. Using analytic and numerical methods, we explore how different NS
natal kick velocity distributions, pre-SN orbital separations, and progenitor
He-star masses affect the post-SN orbital periods, eccentricities, merger
times, systemic velocities, and distances traveled by the system before
merging. Comparison with the set of 17 known DNSs in the Milky Way shows that
DNSs have pre-SN orbital separations ranging between 1 and 44 .
Those DNSs with pre-SN separations 1 have merger time
distributions that peak 10-100 Myr after formation, regardless of the
kick velocity received by the NS. These DNSs are typically formed with systemic
velocities 10 km s and may travel 1-10 kpc before
merging. Depending on progenitor mass of the second-born NS, the short merger
time can account for the -process enrichment observed in compact stellar
systems such as ultra-faint dwarf galaxies. For Milky Way-mass galaxies only
DNSs with the tightest pre-SN orbits and large kick velocities (10
km s) can escape. However, those DNSs that do escape may travel as far
as Mpc before merging, which as previous studies have pointed out has
implications for identifying the host galaxies to short gamma ray bursts and
gravitational wave events.Comment: 16 pages, 10 figures, accepted for publication in MNRA
Constraining Compact Object Formation with 2M0521
We show that the recently discovered binary 2M05215658+4359220 (2M0521),
comprised of a giant star (GS) orbiting a suspected black hole (BH) in a ~80
day orbit, may be instrumental in shedding light on uncertain BH-formation
physics and can be a test case for studying wind accretion models. Using binary
population synthesis with a realistic prescription for the star formation
history and metallicity evolution of the Milky Way, we analyze the evolution of
binaries containing compact objects (COs) in orbit around GSs with properties
similar to 2M0521. We find ~100-1000 CO-GS binaries in the Milky Way observable
by Gaia, and 0-12 BH-GS and 0-1 neutron star-GS binaries in the Milky Way with
properties similar to 2M0521. We find that all CO-GSs with Porb<5 yr, including
2M0521, go through a common envelope (CE) and hence form a class of higher mass
analogs to white dwarf post-CE binaries. We further show how the component
masses of 2M0521-like binaries depend strongly on the supernova-engine model we
adopt. Thus, an improved measurement of the orbit of 2M0521, imminent with
Gaia's third data release, will strongly constrain its component masses and as
a result inform supernova-engine models widely used in binary population
synthesis studies. These results have widespread implications for the origins
and properties of CO binaries, especially those detectable by LIGO and LISA.
Finally, we show that the reported X-ray non-detection of 2M0521 is a challenge
for wind accretion theory, making 2M0521-like CO-GS binaries a prime target for
further study with accretion models.Comment: 7 pages, 5 figures, Accepted for Publication in ApJ
Did GW170817 harbor a pulsar?
If the progenitor of GW170817 harbored a pulsar, then a Poynting flux
dominated bow-shock cavity would have been expected to form around the
traveling binary. The characteristic size of this evacuated region depends
strongly on the spin-down evolution of the pulsar companion, which in turn
depends on the merging timescale of the system. If this evacuated region is
able to grow to a sufficiently large scale, then the deceleration of the jet,
and thus the onset of the afterglow, would be noticeably delayed. The first
detection of afterglow emission, which was uncovered 9.2 days after the
-ray burst trigger, can thus be used to constrain the size of a
pre-existing pulsar-wind cavity. We use this information, together with a model
of the jet to place limits on the presence of a pulsar in GW170817 and discuss
the derived constraints in the context of the observed double neutron star
binary population. We find that the majority of Galactic systems that are close
enough to merge within a Hubble time would have carved a discernibly large
pulsar-wind cavity, inconsistent with the onset timescale of the X-ray
afterglow of GW170817. Conversely, the recently detected system J1913+1102,
which host a low-luminosity pulsar, provides a congruous Milky Way analog of
GW170817's progenitor model. This study highlights the potential of the
proposed observational test for gaining insight into the origin of double
neutron star binaries, in particular if the properties of Galactic systems are
representative of the overall merging population.Comment: Accepted for publication in ApJL, 6 pages, 5 figure
Evolutionary Channels for the Formation of Double Neutron Stars
We analyze binary population models of double-neutron stars and compare
results to the accurately measured orbital periods and eccentricities of the
eight known such systems in our Galaxy. In contrast to past similar studies, we
especially focus on the dominant evolutionary channels (we identify three); for
the first time, we use a detailed understanding of the evolutionary history of
three double neutron stars as actual constraints on the population models. We
find that the evolutionary constraints derived from the double pulsar are
particularly tight, and less than half of the examined models survive the full
set of constraints. The top-likelihood surviving models yield constraints on
the key binary evolution parameters, but most interestingly reveal (i) the need
for electron-capture supernovae from relatively low-mass degenerate, progenitor
cores, and (ii) the most likely evolutionary paths for the rest of the known
double neutron stars. In particular, we find that J1913+16 likely went through
a phase of Case BB mass transfer, and J1906+0746 and J1756-2251 are consistent
with having been formed in electron-capture supernovae.Comment: 17 pages, 9 figure
Total r-process Yields of Milky Way Neutron Star Mergers
While it is now known that double neutron star binary systems (DNSs) are
copious producers of heavy elements, there remains much speculation about
whether they are the sole or even principal site of rapid neutron-capture
(r-process) nucleosynthesis, one of the primary ways in which heavy elements
are produced. The occurrence rates, delay times, and galactic environments of
DNSs hold sway over estimating their total contribution to the elemental
abundances in the Solar system and the Galaxy. Furthermore, the expected
elemental yield for DNSs may depend on the merger parameters themselves -- such
as their stellar masses and radii -- which is not currently considered in many
galactic chemical evolution models. Using the characteristics of the observed
sample of DNSs in the Milky Way as a guide, we predict the expected
nucleosynthetic yields that a population of DNSs would produce upon merger, and
we compare that nucleosynthetic signature to the heavy-element abundance
pattern of the Solar system elements. We find that with our current models, the
present DNS population favors production of the lighter r-process elements,
while underproducing the heaviest elements relative to the Solar system. This
inconsistency could imply an additional site for the heaviest elements or a
population of DNSs much different from that observed today.Comment: 12 pages, 6 figures, 2 table
r-process enrichment of ultra-faint dwarf galaxies by fast merging double neutron stars
The recent aLIGO/aVirgo discovery of gravitational waves from the neutron
star merger (NSM) GW170817 and the follow up kilonova observations have shown
that NSMs produce copious amount of r-process material. However, it is
difficult to reconcile the large natal kicks and long average merging times of
Double Neutron Stars (DNSs), with the levels of r-process enrichment seen in
ultra faint dwarf (UFD) galaxies such as Reticulum II and Tucana III. Assuming
that such dwarf systems have lost a significant fraction of their stellar mass
through tidal stripping, we conclude that contrary to most current models, it
is the DNSs with rather large natal kicks but very short merging timescales
that can enrich UFD-type galaxies. These binaries are either on highly
eccentric orbits, or form with very short separations due to an additional
mass-transfer between the first-born neutron star and a naked helium star,
progenitor of the second-born neutron star. These DNSs are born with a
frequency that agrees with the statistics of the r-process UFDs, and merge well
within the virial radius of their host halos, therefore contributing
significantly to their r-process enrichment.Comment: Accepted for publication in Ap
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