535 research outputs found
Alignment Timescale of the Microquasar GRO J1655-40
The microquasar GRO J1655-40 has a black hole with spin angular momentum
apparently misaligned to the orbital plane of its companion star. We
analytically model the system with a steady state disc warped by Lense-Thirring
precession and find the timescale for the alignment of the black hole with the
binary orbit. We make detailed stellar evolution models so as to estimate the
accretion rate and the lifetime of the system in this state. The secondary can
be evolving at the end of the main sequence or across the Hertzsprung gap. The
mass-transfer rate is typically fifty times higher in the latter case but we
find that, in both cases, the lifetime of the mass transfer state is at most a
few times the alignment timescale. The fact that the black hole has not yet
aligned with the orbital plane is therefore consistent with either model. We
conclude that the system may or may not have been counter-aligned after its
supernova kick but that it is most likely to be close to alignment rather than
counteralignment now.Comment: Accepted for publication in MNRA
Binary coalescence from case A evolution -- mergers and blue stragglers
We constructed some main-sequence mergers from case A binary evolution and
studied their characteristics via Eggleton's stellar evolution code. Both total
mass and orbital angular momentum are conservative in our binary evolutions.
Some mergers might be on the left of the ZAMS as defined by normal surface
composition on a CMD because of enhanced surface helium content. The study also
shows that central hydrogen content of the mergers is independent of mass. As a
consequence, we fit the formula of magnitude and B-V of the mergers when they
return back to thermal equilibrium with maximum error 0.29 and 0.037,
respectively. Employing the consequences above, we performed Monte Carlo
simulations to examine our models in NGC 2682 and NGC 2660. In NGC 2682, binary
mergers from our models cover the region with high luminosity, but its
importance is much less than that of AML. Our results are well-matched to the
observations of NGC2660 if there is about 0.5Mo of mass loss in the merger
process.Comment: 14 pages, 12 figures. accepted by MNRA
Planet Consumption and Stellar Metallicity Enhancements
The evolution of a giant planet within the stellar envelope of a
main-sequence star is investigated as a possible mechanism for enhancing the
stellar metallicities of the parent stars of extrasolar planetary systems.
Three-dimensional hydrodynamical simulations of a planet subject to impacting
stellar matter indicate that the envelope of a Jupiter-like giant planet can be
completely stripped in the outer stellar convection zone of a solar-mass star.
In contrast, Jupiter-like and less massive Saturn-like giant planets are able
to survive through the base of the convection zone of a 1.22 solar-mass star.
Although strongly dependent on details of planetary interior models, partial or
total dissolution of giant planets can result in significant enhancements in
the metallicity of host stars with masses between about 1.0 and 1.3 solar
masses. The implications of these results with regard to planetary orbital
migration are briefly discussed.Comment: 11 pages, 2 figures, accepted for ApJ Letter
The Cepheid Distance to NGC 1637: A Direct Test of the EPM Distance to SN 1999em
Type II-plateau supernovae (SNe II-P) are the classic variety of
core-collapse events that result from isolated, massive stars with thick
hydrogen envelopes intact at the time of explosion. Their distances are now
routinely estimated through two techniques: the expanding photosphere method
(EPM), a primary distance-determining method, and the recently developed
standard-candle method (SCM), a promising secondary technique. Using Cycle 10
HST observations, we identify 41 Cepheid variable stars in NGC 1637, the host
galaxy of the most thoroughly studied SN II-P to date, SN 1999em. Remarkably,
the Cepheid distance that we derive to NGC 1637, D = 11.7 +/- 1.0 Mpc, is
nearly 50% larger than earlier EPM distance estimates to SN 1999em. This is the
first direct comparison between these two primary distance determining methods
for a galaxy hosting a well-observed, spectroscopically and photometrically
normal, SN II-P. Extensive consistency checks show strong evidence to support
the Cepheid distance scale, so we are led to believe that either SN 1999em is
in some heretofore unsuspected way an unusual SN II-P, or that the SN II-P
distance scale must be revised. Assuming the latter, this one calibration
yields H_0(EPM) = 57 +/- 15 km/s/Mpc and H_0(SCM) = 59 +/- 11 km/s/Mpc;
additional calibrating galaxies are clearly desirable in order to test the
robustness of both determinations of H_0. The HST observations of NGC 1637 also
captured the fading SN 1999em two years after explosion, providing the latest
photometry ever obtained for an SN II-P. Through comparison with photometry of
SN 1987A at similar epochs, we conclude that a slightly greater amount of
radioactive Ni-56, ~0.09 M_sun, was ejected by SN 1999em than was derived for
SN 1987A (0.075 M_sun).Comment: Accepted for publication in The Astrophysical Journal; Version with
full figures available at http://astron.berkeley.edu/~leonard/papers
Evolution in Binary and Triple Stars, with an application to SS Lac
We present equations governing the way in which both the orbit and the
intrinsic spins of stars in a close binary should evolve subject to a number of
perturbing forces, including the effect of a third body in a possibly inclined
wider orbit. We illustrate the solutions in some binary-star and triple-star
situations: tidal friction in a wide but eccentric orbit of a radio pulsar
about a B star, the Darwin and eccentricity instabilities in a more massive but
shorter-period massive X-ray binary, and the interaction of tidal friction with
Kozai cycles in a triple such as Algol (beta-Per), at an early stage in that
star's life when all 3 components were ZAMS stars. We also attempt to model in
some detail the interesting triple system SS Lac, which stopped eclipsing in
about 1950. We find that our model of SS Lac is quite constrained by the
relatively good observational data of this system, and leads to a specific
inclination (29 deg) of the outer orbit relative to the inner orbit at epoch
zero (1912). Although the intrinsic spins of the stars have little effect on
the orbit, the converse is not true: the spin axes can vary their orientation
relative to the close binary by up to 120 deg on a timescale of about a
century.Comment: 30 pages, 6 figure
Observational Studies of Early-type Overcontact Binaries: TU Muscae
We present new spectroscopic and photometric data on the early-type
overcontact binary TU Muscae. The analysis of the spectroscopic data shows that
the line of sight to the system crosses three kinematically sharp and
well-separated interstellar reddening sources and that the stars rotate
synchronously. We present new radial velocities that are in good agreement with
earlier optical velocities and, thus, do not confirm the systematically smaller
velocities obtained from IUE spectra. The optical velocities are analyzed
simultaneously with the photometric data to derive accurate absolute dimesions
for the binary components.The results show that TU Mus consists of an O7.5
primary with M_1=23.5 +/- 0.8 M_sun, R_1=7.48 +/- 0.08 R_sun and an O9.5
secondary with M_2=15.3 +/- 0.4 M_sun, R_2=6.15 +/- 0.07 R_sun in an
overcontact configuration and that the orbital period has remained constant
over the three decades covered by the observations. These results might imply
that the mass transfer seen in late-type overcontact binaries does not occur in
their early-type counterparts.Comment: 11 pages, 10 figures, accepted for Dce 2003 issue of A
Dispersion engineered As2S3 planar waveguides for broadband four-wave mixing based wavelength conversion of 40 Gb/s signals
We demonstrate broadband wavelength conversion of a 40 Gb/s
return-to-zero signal using four-wave-mixing (FWM) in a dispersion
engineered chalcogenide glass waveguide. The 6 cm long planar rib
waveguide 2 μm wide was fabricated in a 0.87 μm thick film etched 350nm
deep to correspond to a design where waveguide dispersion offsets the
material leading to near-zero dispersion in the C-band and broadband phase
matched FWM. The reduced dimensions also enhance the nonlinear
coefficient to 9800 W-1km-1 at 1550 nm enabling broadband conversion in a
shorter device. In this work, we demonstrate 80 nm wavelength
conversions with 1.65 dB of power penalty at a bit-error rate of 10-9.
Spectral measurements and simulations indicate extended broadband
operation is possible
Donor Stars in Black-Hole X-Ray Binaries
We study theoretically the formation of black-hole (BH) X-ray binaries.
Consistency of the models with the observed relative numbers of systems with
low-mass (<2 M_sun) and intermediate-mass (~2 M_sun - M_{BH}) donors leads to
severe constraints on the evolutionary parameters of the progenitors. In
particular, we find that (i) BH progenitor masses cannot exceed about 2 M_{BH};
(ii) high values of the common-envelope efficiency parameter (alpha_{CE} > 1)
are required, implying that energy sources other than orbital contraction must
be invoked to eject the envelope; (iii) the mass-loss fraction in helium-star
winds is limited to be <50%. Outside of this limited parameter space for
progenitors we find that either BH X-ray binary formation cannot occur at all
or donors do not have the full range of observed masses. We discuss the
implications of these results for the structure of massive hydrogen-rich stars,
the evolution of helium-stars, and BH formation. We also consider the possible
importance of asymmetric kicks.Comment: 29 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Birthrates and delay times of Type Ia supernovae
Type Ia supernovae (SNe Ia) play an important role in diverse areas of
astrophysics, from the chemical evolution of galaxies to observational
cosmology. However, the nature of the progenitors of SNe Ia is still unclear.
In this paper, according to a detailed binary population synthesis study, we
obtained SN Ia birthrates and delay times from different progenitor models, and
compared them with observations. We find that the Galactic SN Ia birthrate from
the double-degenerate (DD) model is close to those inferred from observations,
while the birthrate from the single-degenerate (SD) model accounts for only
about 1/2-2/3 of the observations. If a single starburst is assumed, the
distribution of the delay times of SNe Ia from the SD model is a weak
bimodality, where the WD + He channel contributes to the SNe Ia with delay
times shorter than 100Myr, and the WD + MS and WD + RG channels to those with
age longer than 1Gyr.Comment: 11 pages, 2 figures, accepted by Science in China Series G (Dec.30,
2009
Turbulence-driven Polar Winds from T Tauri Stars Energized by Magnetospheric Accretion
Pre-main-sequence stars are observed to be surrounded by both accretion flows
and some kind of wind or jet-like outflow. Recent work by Matt and Pudritz has
suggested that if classical T Tauri stars exhibit stellar winds with mass loss
rates about 0.1 times their accretion rates, the wind can carry away enough
angular momentum to keep the stars from being spun up unrealistically by
accretion. This paper presents a preliminary set of theoretical models of
accretion-driven winds from the polar regions of T Tauri stars. These models
are based on recently published self-consistent simulations of the Sun's
coronal heating and wind acceleration. In addition to the convection-driven MHD
turbulence (which dominates in the solar case), we add another source of wave
energy at the photosphere that is driven by the impact of plasma in neighboring
flux tubes undergoing magnetospheric accretion. This added energy, determined
quantitatively from the far-field theory of MHD wave generation, is sufficient
to produce T Tauri-like mass loss rates of at least 0.01 times the accretion
rate. While still about an order of magnitude below the level required for
efficient angular momentum removal, these are the first self-consistent models
of T Tauri winds that agree reasonably well with a range of observational mass
loss constraints. The youngest modeled stellar winds are supported by Alfven
wave pressure, they have low temperatures ("extended chromospheres"), and they
are likely to be unstable to the formation of counterpropagating shocks and
clumps far from the star.Comment: 19 pages (emulateapj style), 13 figures, ApJ, in press (v. 689,
December 10, 2008
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