613 research outputs found
Eccentricities of Double Neutron Star Binaries
Recent pulsar surveys have increased the number of observed double neutron
stars (DNS) in our galaxy enough so that observable trends in their properties
are starting to emerge. In particular, it has been noted that the majority of
DNS have eccentricities less than 0.3, which are surprisingly low for binaries
that survive a supernova explosion that we believe imparts a significant kick
to the neutron star. To investigate this trend, we generate many different
theoretical distributions of DNS eccentricities using Monte Carlo population
synthesis methods. We determine which eccentricity distributions are most
consistent with the observed sample of DNS binaries. In agreement with
Chaurasia & Bailes (2005), assuming all double neutron stars are equally as
probable to be discovered as binary pulsars, we find that highly eccentric,
coalescing DNS are less likely to be observed because of their accelerated
orbital evolution due to gravitational wave emission and possible early
mergers. Based on our results for coalescing DNS, we also find that models with
vanishingly or moderately small kicks (sigma < about 50 km/s) are inconsistent
with the current observed sample of such DNS. We discuss the implications of
our conclusions for DNS merger rate estimates of interest to ground-based
gravitational-wave interferometers. We find that, although orbital evolution
due to gravitational radiation affects the eccentricity distribution of the
observed sample, the associated upwards correction factor to merger rate
estimates is rather small (typically 10-40%).Comment: 9 pages, 8 figures, accepted by ApJ. Figures reduced and some content
changed, references adde
A New Way to Detect Massive Black Holes in Galaxies: The Stellar Remnants of Tidal Disruption
We point out that the tidal disruption of a giant may leave a luminous
(10^35-10^39 ergs/s), hot (10-100 eV) stellar core. The ``supersoft'' source
detected by Chandra at the center of M31 may be such a core; whether or not it
is, the observations have shown that such a core is detectable, even in the
center of a galaxy. We therefore explore the range of expected observational
signatures and how they may be used to (1) test the hypothesis that the M31
source is a remnant of tidal stripping and (2) discover evidence of black holes
and disruption events in other galaxies.Comment: Four pages with 1 figure. Appeared in ApJL (2001, 551, L37
Evolution of Neutron-Star, Carbon-Oxygen White-Dwarf Binaries
At least one, but more likely two or more, eccentric neutron-star,
carbon-oxygen white-dwarf binaries with an unrecycled pulsar have been
observed. According to the standard scenario for evolving neutron stars which
are recycled in common envelope evolution we expect to observe \gsim 50 such
circular neutron star-carbon oxygen white dwarf binaries, since their formation
rate is roughly equal to that of the eccentric binaries and the time over which
they can be observed is two orders of magnitude longer, as we shall outline. We
observe at most one or two such circular binaries and from that we conclude
that the standard scenario must be revised. Introducing hypercritical accretion
into common envelope evolution removes the discrepancy by converting the
neutron star into a black hole which does not emit radio waves, and therefore
would not be observed.Comment: 25 pages, 1 figure, accepted in Ap
The Search for Stellar Companions to Exoplanet Host Stars Using the CHARA Array
Most exoplanets have been discovered via radial velocity studies, which are
inherently insensitive to orbital inclination. Interferometric observations
will show evidence of a stellar companion if it sufficiently bright, regardless
of the inclination. Using the CHARA Array, we observed 22 exoplanet host stars
to search for stellar companions in low-inclination orbits that may be
masquerading as planetary systems. While no definitive stellar companions were
discovered, it was possible to rule out certain secondary spectral types for
each exoplanet system observed by studying the errors in the diameter fit to
calibrated visibilities and by searching for separated fringe packets.Comment: 26 pages, 5 tables, 8 figure
The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems
With a view to understanding the formation of double neutron-stars (DNS), we
investigate the late stages of evolution of helium stars with masses of 2.8 -
6.4 Msun in binary systems with a 1.4 Msun neutron-star companion. We found
that mass transfer from 2.8 - 3.3 Msun helium stars and from 3.3 - 3.8 Msun in
very close orbits (P_orb > 0.25d) will end up in a common-envelope (CE) and
spiral-in phase due to the development of a convective helium envelope. If the
neutron star has sufficient time to complete the spiraling-in process before
the core collapses, the system will produce very tight DNSs (P_orb ~ 0.01d)
with a merger timescale of the order of 1 Myr or less. These systems would have
important consequences for the detection rate of GWR and for the understanding
of GRB progenitors. On the other hand, if the time left until the explosion is
shorter than the orbital-decay timescale, the system will undergo a SN
explosion during the CE phase. Helium stars with masses 3.3 - 3.8 Msun in wider
orbits (P_orb > 0.25d) and those more massive than 3.8 Msun do not go through
CE evolution. The remnants of these massive helium stars are DNSs with periods
in the range of 0.1 - 1 d. This suggests that this range of mass includes the
progenitors of the galactic DNSs with close orbits (B1913+16 and B1534+12). A
minimum kick velocity of 70 km/s and 0 km/s (for B1913+16 and B1534+12,
respectively) must have been imparted at the birth of the pulsar's companion.
The DNSs with wider orbits (J1518+4904 and probably J1811-1736) are produced
from helium star-neutron star binaries which avoid RLOF, with the helium star
more massive than 2.5 Msun. For these systems the minimum kick velocities are
50 km/s and 10 km/s (for J1518+4904 and J1811-1736, respectively).Comment: 16 pages, latex, 12 figures, accepted for publication in MNRA
Equipotential Surfaces and Lagrangian points in Non-synchronous, Eccentric Binary and Planetary Systems
We investigate the existence and properties of equipotential surfaces and
Lagrangian points in non-synchronous, eccentric binary star and planetary
systems under the assumption of quasi-static equilibrium. We adopt a binary
potential that accounts for non-synchronous rotation and eccentric orbits, and
calculate the positions of the Lagrangian points as functions of the mass
ratio, the degree of asynchronism, the orbital eccentricity, and the position
of the stars or planets in their relative orbit. We find that the geometry of
the equipotential surfaces may facilitate non-conservative mass transfer in
non-synchronous, eccentric binary star and planetary systems, especially if the
component stars or planets are rotating super-synchronously at the periastron
of their relative orbit. We also calculate the volume-equivalent radius of the
Roche lobe as a function of the four parameters mentioned above. Contrary to
common practice, we find that replacing the radius of a circular orbit in the
fitting formula of Eggleton (1983) with the instantaneous distance between the
components of eccentric binary or planetary systems does not always lead to a
good approximation to the volume-equivalent radius of the Roche-lobe. We
therefore provide generalized analytic fitting formulae for the
volume-equivalent Roche lobe radius appropriate for non-synchronous, eccentric
binary star and planetary systems. These formulae are accurate to better than
1% throughout the relevant 2-dimensional parameter space that covers a dynamic
range of 16 and 6 orders of magnitude in the two dimensions.Comment: 12 pages, 10 figures, 2 Tables, Accepted by the Astrophysical Journa
Search for an Near-IR Counterpart to the Cas A X-ray Point Source
We report deep near-infrared and optical observations of the X-ray point
source in the Cassiopeia A supernova remnant, CXO J232327.9+584842. We have
identified a J=21.4 +/- 0.3 mag and Ks=20.5 +/- 0.3 mag source within the
1-sigma error circle, but we believe this source is a foreground Pop II star
with Teff=2600-2800 K at a distance of ~2 kpc, which could not be the X-ray
point source. We do not detect any sources in this direction at the distance of
Cas A, and therefore place 3-sigma limits of R >~ 25 mag, F675W >~ 27.3 mag, J
>~ 22.5 mag and Ks >~ 21.2 mag (and roughly H >~ 20 mag) on emission from the
X-ray point source, corresponding to M_{R} >~ 8.2 mag, M_{F675W} >~ 10.7 mag,
M_{J} >~ 8.5 mag, M_{H} >~ 6.5 mag, and M_{Ks} >~ 8.0 mag, assuming a distance
of 3.4 kpc and an extinction A_{V}=5 mag.Comment: 14 pages, 7 figures. Accepted by Ap
The Role of Helium Stars in the Formation of Double Neutron Stars
We have calculated the evolution of 60 model binary systems consisting of
helium stars in the mass range of M_He= 2.5-6Msun with a 1.4Msun neutron star
companion to investigate the formation of double neutron star systems.Orbital
periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe
overflow starting from the helium main sequence to after the ignition of carbon
burning in the core. We have also examined the evolution into a common envelope
phase via secular instability, delayed dynamical instability, and the
consequence of matter filling the neutron star's Roche lobe. The survival of
some close He-star neutron-star binaries through the last mass transfer episode
(either dynamically stable or unstable mass transfer phase) leads to the
formation of extremely short-period double neutron star systems (with
P<~0.1days). In addition, we find that systems throughout the entire calculated
mass range can evolve into a common envelope phase, depending on the orbital
period at the onset of mass transfer. The critical orbital period below which
common envelope evolution occurs generally increases with M_He. In addition, a
common envelope phase may occur during a short time for systems characterized
by orbital periods of 0.1-0.5 days at low He-star masses (~ 2.6-3.3Msun).
The existence of a short-period population of double neutron stars increases
the predicted detection rate of inspiral events by ground-based
gravitational-wave detectors and impacts their merger location in host galaxies
and their possible role as gamma-ray burst progenitors. We use a set of
population synthesis calculations and investigate the implications of the
mass-transfer results for the orbital properties of DNS populations.Comment: 30 pages, Latex (AASTeX), 1 table, 8 figures. To appear in ApJ, v592
n1 July 20, 200
A New Formation Channel for Double Neutron Stars Without Recycling: Implications for Gravitational Wave Detection
We report on a new evolutionary path leading to the formation of close double
neutron stars (NS), with the unique characteristic that none of the two NS ever
had the chance to be recycled by accretion. The existence of this channel stems
from the evolution of helium-rich stars (cores of massive NS progenitors),
which has been neglected in most previous studies of double compact object
formation. We find that these non-recycled NS-NS binaries are formed from bare
carbon-oxygen cores in tight orbits, with formation rates comparable to or
maybe even higher than those of recycled NS-NS binaries. On the other hand,
their detection probability as binary pulsars is greatly reduced (by about
1000) relative to recycled pulsars, because of their short lifetimes. We
conclude that, in the context of gravitational-wave detection of NS-NS inspiral
events, this new type of binaries calls for an increase of the rate estimates
derived from the observed NS-NS with recycled pulsars, typically by factors of
1.5-3 or even higher.Comment: Accepted for publication in ApJ Letters; 5 pages, 1 figure, 2 tables.
Two new paragraphs and one formula adde
The Galactic Population of Low- and Intermediate-Mass X-ray Binaries
(abridged) We present the first study that combines binary population
synthesis in the Galactic disk and detailed evolutionary calculations of low-
and intermediate-mass X-ray binaries (L/IMXBs). We show that the formation
probability of IMXBs with initial donor masses of 1.5--4 Msun is typically >~5
times higher than that of standard LMXBs, and suggest that the majority of the
observed systems may have descended from IMXBs. Distributions at the current
epoch of the orbital periods, donor masses, and mass accretion rates have been
computed, as have orbital-period distributions of BMPs. Several significant
discrepancies between the theoretical and observed distributions are discussed.
The orbital-period distribution of observed BMPs strongly favors cases where
the envelope of the neutron-star progenitor is more easily ejected during the
common-envelope phase. However, this leads to a >~100-fold overproduction of
the theoretical number of luminous X-ray sources relative to the total observed
number of LMXBs. X-ray irradiation of the donor star may result in a dramatic
reduction in the X-ray active lifetime of L/IMXBs, thus possibly resolving the
overproduction problem, as well as the long-standing BMP/LMXB birthrate
problem.Comment: 12 pages, emulateapj, submitted to Ap
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