Break-up Limit Research Articles

Gravitational radiation from thermal mountains on accreting neutron stars: sources of temperature non-axisymmetry

ABSTRACT The spin distribution of accreting neutron stars in low-mass X-ray binary systems shows a concentration of pulsars well below the Keplerian break-up limit. It has been suggested that their spin frequencies may be limited by the emission of gravitational waves, due to the presence of large-scale asymmetries in the internal temperature profile of the star. These temperature asymmetries have been demonstrated to lead to a non-axisymmetric mass distribution, or ‘mountain’, that generates gravitational waves at twice the spin frequency. The presence of a toroidal magnetic field in the interior of accreting neutron stars has been shown to introduce such anisotropies in the star’s thermal conductivity, by restricting the flow of heat orthogonal to the magnetic field and establishing a non-axisymmetric temperature distribution within the star. We revisit this mechanism, extending the computational domain from (only) the crust to the entire star, incorporating more realistic microphysics, and exploring different choices of outer boundary condition. By allowing a magnetic field to permeate the core of the neutron star, we find that the likely level of temperature asymmetry in the inner crust (ρ ∼ 1013 g cm−3) can be up to 3 orders of magnitude greater than the previous estimate, improving prospects for one day detecting continuous gravitational radiation. We also show that temperature asymmetries sufficiently large to be interesting for gravitational wave emission can be generated in strongly accreting neutron stars if crustal magnetic fields can reach ∼1012 G.

A new method for identifying dynamical transitions in rubble-pile asteroid scenarios

Context. Evidence supports the idea that asteroids are rubble piles, that is, gravitational aggregates of loosely consolidated material. This makes their dynamics subject not only to the complex N-body gravitational interactions between its constituents, but also to the laws of granular mechanics, which is one of the main unsolved problems in physics. Aims. We aim to develop a new method to identify dynamical transitions and predict qualitative behavior in the granular N-body problem, in which the dynamics of individual bodies are driven both by mutual gravity, contact and collision interactions. Methods. The method has its foundation in the combination of two elements: a granular N-body simulation code that can resolve the dynamics of granular fragments to particle-scale precision, and a theoretical framework that can decode the nature of particle-scale dynamics and their transitions by means of ad hoc indicators. Results. We present here a proof-of-concept of the method, with application to the spinning rubble-pile asteroid problem. We investigate the density-spin parameter space and demonstrate that the approach can identify the breakup limit and reshape region for spinning rubble-pile aggregates. Conclusions. We provide the performance of several ad hoc indicators and discuss whether they are suitable for identifying and predicting the features of the dynamical problem.

Effects of rotation on the spectra of brown dwarfs

ABSTRACT Measured rotational speeds of giant planets and brown dwarfs frequently constitute appreciable fractions of the breakup limit, resulting in centrifugal expansion of these objects at the equator. According to models of internal energy transport, this expansion ought to make the poles of a rotator significantly hotter than the equator, so that inclination of the rotational axis greatly affects both spectral shape and total flux. In this paper, we explore the dependence of a substellar object’s observables on its rotational speed and axis inclination. To do so, we combine picaso (Planetary Intensity Code for Atmospheric Spectroscopy Observations) with software pars (Paint the Atmospheres of Rotating Stars). The former computer program models radiative transfer within plane-parallel planetary atmospheres, while the latter computes disc-integrated spectra of centrifugally deformed gaseous masses. We find that the specific flux of a typical fast-rotating brown dwarf can increase by as much as a factor of 1.5 with movement from an equator-on to a pole-on view. On the other hand, the distinctive effect of rotation on spectral shape increases toward the equator-on view. The latter effect also increases with lower effective temperature. The bolometric luminosity estimate for a typical fast rotator at extreme inclinations has to be adjusted by as much as ${\sim} 20{{\ \rm per\ cent}}$ due to the anisotropy of the object’s observed flux. We provide a general formula for the calculation of the corresponding adjustment factor in terms of rotational speed and inclination.

The Evolutionary Effects of the Radius and Moment of Inertia of Rapidly Rotating Neutron Stars

Abstract A newly born millisecond magnetar is thought to be the central engine of some gamma-ray bursts (GRBs), especially those that present long-lasting X-ray plateau emissions. By solving the field equations, we find that when the rotational speed of the magnetar is approaching the breakup limit, its radius R and moment of inertia I undergo an obvious evolution as the magnetar spins down. Meanwhile, the values of R and I sensitively depend on the adoption of a neutron star (NS) equation of state (EoS) and the NS baryonic mass. With different EoSs and baryonic mass considered, the magnetic dipole radiation luminosity (L dip) could be variant within one to two orders of magnitude. We thus suggest that when using the X-ray plateau data of GRBs to diagnose the properties of the nascent NSs, EoS and NS mass information should be invoked as simultaneously constrained parameters. On the other hand, due to the evolution of R and I, the temporal behavior of L dip becomes more complicated. For instance, if the spin-down process is dominated by gravitational wave emission due to the NS asymmetry caused by magnetic field distortion ( ϵ ∝ B p 2 ), the segment L dip ∝ t 0 could be followed by L dip ∝ t −γ with γ larger than 3. This case could naturally interpret the so-called internal X-ray plateau feature shown in some GRB afterglows, which means the sharp decay following the plateau unnecessarily corresponds to the collapse of the NS. This may explain why some internal X-ray plateaus are followed by late-time central engine activity, manifested through flares and second shallow plateaus.

Centrifugally driven mass-loss and outbursts of massive stars

ABSTRACT Rotation and mass-loss are crucially interlinked properties of massive stars, strongly affecting their evolution and ultimate fate. Massive stars rotating near their break-up limit shed mass centrifugally, creating Be stars with circumstellar discs and possibly driving outbursts. Using the mesa stellar evolution code, we examine the effects of efficient angular momentum transport on the main-sequence and post-main-sequence rotational evolution of massive stars. In rapid rotators, angular momentum transported from the contracting core to the expanding envelope can spin-up the surface layers past the break-up rate, particularly for stars near (or beyond) the end of the main-sequence and in low-metallicity environments. We also demonstrate that centrifugal instabilities could arise in rapidly rotating massive stars, potentially triggering the S Doradus outbursts observed in luminous blue variable stars. Prior mass accretion from a binary companion increases both the likelihood and the intensity of centrifugal mass-loss. We discuss implications for massive stellar evolution, Be stars, and luminous blue variables.

Active Asteroid (6478) Gault: A Blue Q-type Surface below the Dust?

Abstract We present near-infrared spectroscopy of the sporadically active asteroid (6478) Gault collected on the 3 m NASA/Infrared Telescope Facility observatory in late 2019 March/early April. Long-exposure imaging with the 0.5 m Near Earth Environment Monitoring T05 telescope and previously published data simultaneously monitored the asteroid activity, providing context for our measurements. We confirm that Gault is a silicate-rich (Q- or S-type) object likely linked to the (25) Phocaea collisional family. The asteroid exhibits substantial spectral variability over the 0.75–2.45 μm wavelength range, from unusual blue (s′ = −13.5 ± 1.1% μm−1) to typical red (s′ = +9.1 ± 1.2% μm−1) spectral slope, that does not seem to correlate with activity. Spectral comparisons with samples of ordinary chondrite meteorites suggest that the blue color relates to the partial loss of the asteroid dust regolith, exposing a fresh, dust-free material at its surface. The existence of asteroids rotating close to rotational break-up limit and having similar spectral properties as Gault further supports this interpretation. Future spectroscopic observations of Gault, when the tails dissipate, will help further testing of our proposed hypothesis.

Using electric current to surpass the microstructure breakup limit

The elongated droplets and grains can break up into smaller ones. This process is driven by the interfacial free energy minimization, which gives rise to a breakup limit. We demonstrated in this work that the breakup limit can be overpassed drastically by using electric current to interfere. Electric current free energy is dependent on the microstructure configuration. The breakup causes the electric current free energy to reduce in some cases. This compensates the increment of interfacial free energy during breaking up and enables the processing to achieve finer microstructure. With engineering practical electric current parameters, our calculation revealed a significant increment of the obtainable number of particles, showing electric current a powerful microstructure refinement technology. The calculation is validated by our experiments on the breakup of Fe3C-plates in Fe matrix. Furthermore, there is a parameter range that electric current can drive spherical particles to split into smaller ones.

Leaky-wave-induced disks around Be stars: a pulsational analysis on their formation

AbstractBe stars are B-type stars near the main sequence which undergo episodic mass loss events detected by emission lines, whose line shape and intensity vary with a timescale of the order of decades. Spectroscopic observations show a large rotation velocity such that one of the prevailing scenarios for the formation of the equatorial disk consists in an increasing equatorial rotation velocity to the break-up limit where gravity is challenged by the centrifugal force. We investigate here a new scenario recently suggested by Ishimatsu & Shibahashi (2013), in which the transport of angular momentum through the photosphere would be achieved by leaky waves, keeping the rotation velocity still below the break-up limit.

Fast rotating stars resulting from binary evolution will often appear to be single

AbstractRapidly rotating stars are readily produced in binary systems. An accreting star in a binary system can be spun up by mass accretion and quickly approach the break-up limit. Mergers between two stars in a binary are expected to result in massive, fast rotating stars. These rapid rotators may appear as Be or Oe stars or at low metallicity they may be progenitors of long gamma-ray bursts.Given the high frequency of massive stars in close binaries it seems likely that a large fraction of rapidly rotating stars result from binary interaction. It is not straightforward to distinguish a a fast rotator that was born as a rapidly rotating single star from a fast rotator that resulted from some kind of binary interaction. Rapidly rotating stars resulting from binary interaction will often appear to be single because the companion tends to be a low mass, low luminosity star in a wide orbit. Alternatively, they became single stars after a merger or disruption of the binary system during the supernova explosion of the primary.The absence of evidence for a companion does not guarantee that the system did not experience binary interaction in the past. If binary interaction is one of the main causes of high stellar rotation rates, the binary fraction is expected to be smaller among fast rotators. How this prediction depend on uncertainties in the physics of the binary interactions requires further investigation.

Massive binaries as the source of abundance anomalies in globular clusters

Abundance anomalies observed in globular cluster stars indicate pollution with material processed by hydrogen burning. Two main sources have been suggested: asymptotic giant branch (AGB) stars and massive stars rotating near the break-up limit (spin stars). We propose massive binaries as an alternative source. We compute the evolution of a 20 Msun star in a close binary and find that it sheds about 10 Msun. The ejecta are enriched in He, N, Na, and Al and depleted in C and O, similar to the abundance patterns observed in gobular cluster stars. In contrast to the fast, radiatively driven winds of massive stars, this material is typically ejected with low velocity. We expect that it remains inside the potential well of a globular cluster and becomes available for the formation or pollution of a second generation of stars. We estimate that the amount of processed low-velocity material ejected by massive binaries is greater than the contribution of AGB stars and spin stars combined, assuming that the majority of massive stars in a proto-globular cluster interact. If we take the possible contribution of intermediate mass stars in binaries into account and assume that the ejecta are diluted with an equal amount of unprocessed material, we find that this scenario can provide enough material to form a second generation of low-mass stars, which is as numerous as the first generation of low-mass stars. In this scenario there is no need to make commonly adopted assumptions, such as preferential loss of the first generation of stars, external pollution of the cluster, or an anomalous initial mass function. [Abridged]

Massive binaries and the enrichment of the interstellar medium in globular clusters

AbstractAbundance anomalies observed in globular cluster stars indicate pollution with material processed by hydrogen burning. Two main sources have been suggested: asymptotic giant branch (AGB) stars and massive stars rotating near the break-up limit (spin stars). We discuss the idea that massive binaries may provide an interesting alternative source of processed material. We discuss observational evidence for mass shedding from interacting binaries. In contrast to the fast, radiatively driven winds of massive stars, this material is typically ejected with low velocity. We expect that it remains inside the potential well of a globular cluster and becomes available for the formation or pollution of a second generation of stars. We estimate that the amount of processed low-velocity material that can be ejected by massive binaries is larger than the contribution of the two previously suggested sources combined.

The Thousand Asteroid Light Curve Survey

We present the results of our Thousand Asteroid Light Curve Survey (TALCS) conducted with the Canada–France–Hawaii Telescope in September 2006. Our untargeted survey detected 828 Main Belt asteroids to a limiting magnitude of g ′ ∼ 22.5 corresponding to a diameter range of 0.4 km ⩽ D ⩽ 10 km . Of these, 278 objects had photometry of sufficient quality to perform rotation period fits. We debiased the observations and light curve fitting process to determine the true distribution of rotation periods and light curve amplitudes of Main Belt asteroids. We confirm a previously reported excess in the fraction of fast rotators but find a much larger excess of slow rotating asteroids (∼15% of our sample). A few percent of objects in the TALCS size range have large light curve amplitudes of ∼1 mag. Fits to the debiased distribution of light curve amplitudes indicate that the distribution of triaxial ellipsoid asteroid shapes is proportional to the square of the axis ratio, ( b / a ) 2 , and may be bi-modal. Finally, we find six objects with rotation periods that may be less than 2 h with diameters between 400 m and 1.5 km, well above the break-up limit for a gravitationally-bound aggregate. Our debiased data indicate that this population represents <4% of the Main Belt in the 1–10 km size range.

Detecting gravitational wave emission from the known accreting neutron stars

Detection of gravitational waves from accreting neutron stars (NSs) in our galaxy, due to ellipticity or internal oscillation, would be a breakthrough in our understanding of compact objects and explain the absence of NSs rotating near the break-up limit. Direct detection, however, poses a formidable challenge. Using the current data available on the properties of the accreting NSs in Low Mass X-Ray Binaries (LMXBs), we quantify the detectability for the known accreting NSs, considering various emission scenarios and taking into account the negative impact of parameter uncertainty on the data analysis process. Only a few of the persistently bright NSs accreting at rates near the Eddington limit are detectable by Advanced LIGO if they are emitting gravitational waves at a rate matching the torque from accretion. A larger fraction of the known population is detectable if the spin and orbital parameters are known in advance, especially with the narrow-band Advanced LIGO. We identify the most promising targets, and list specific actions that would lead to significant improvements in detection probability. These include astronomical observations (especially for unknown orbital periods), improvements in data analysis algorithms and capabilities, and further detector development.

Effect of reactive compatibilization on droplet coalescence in shear flow

The evolution of the morphology of blends of polyethylene and polyamide with a droplet–matrix morphology, generated by transient shear flows, has been determined using a method based on quenching following deformation of the samples kept between the parallel plates of a rheometer. Droplet size evolution and transient viscosity of blends compatibilized with different amounts of polyethylene-graft-maleic anhydride were measured as a function of the total strain. The effects of applied shear rates and concentration of the reactive compatibilizer were investigated. The results indicate that initially the droplet size was governed by coalescence, whereas, when the droplet size became comparable to the break-up limit, no further significant increase was observed at large strain values. A significant coalescence inhibition was only observed when a relatively high amount of compatibilizer was used. This corresponded to an important interfacial coverage of the interface by the grafted copolymer chains. In that situation, the intensity of the coalescence became independent of the applied shear rate. On the contrary, the coalescence was favoured by the absence of compatibilizer or the use of a low compatibilizer concentration and increased with increasing shear rate. The rather “complex” effect of the compatibilizer addition on coalescence phenomena has been analyzed in terms of blend microstructure changes induced by the compatibilizer addition in relation to droplet collision frequency. Predictions of the droplet size evolution and viscosity under transient shear flows using a modified version of Lee and Park model is shown to describe the coalescence and break-up phenomena under large deformation shear flow. The predictions of the droplet size evolution are greatly improved when an additional parameter for non-affine deformation (slip parameter), is introduced in the model. On the other hand, the predictions are much less satisfactory for the transient viscosity data.

Pertubations on a liquid curtain near break-up: Wakes and free edges

We report experiments on liquid curtains falling between two vertical wires. The flow is mainly driven by gravity, so that the Weber number (We) (ratio of momentum flux to twice the surface tension) is close to zero at the top of the curtain and increases downstream, with the possible existence of a location where We equals 1 (which turns out to be a singular point in the sheet, in terms of waves propagation). In the present paper, we focus on the curtain response to localized perturbations, i.e., formation of either surface waves or free edges behind a thin needle touching the curtain, with a special emphasis to what happens near the break-up limit. We extract and compare the shapes of two kind of “wakes” left behind the obstacle: classical triangular wake of standing sinuous waves and stationary hole involving two free edges pinned on the needle. It is found that these two wakes are very similar for high enough We, but behave very differently when We reaches 1 from above; the sinuous wake disappears, while the “hole wake” still exists, and its shape becomes rounded. Below We=1, the hole can either stay stable, oscillate or expand and break the curtain. We provide exact analytical expressions for stationary free-edges that compare very well with experiments.

Nuclear-powered millisecond pulsars and the maximum spin frequency of neutron stars.

Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companion. It is not known whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many neutron stars that are accreting mass from a companion star exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces. Millisecond-period brightness oscillations during bursts from ten neutron stars (as distinct from other rapid X-ray variability that is also observed) are thought to measure the stellar spin, but direct proof of a rotational origin has been lacking. Here we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting neutron stars, corroborating earlier evidence. The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most neutron-star models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars.

Magnetars in the Metagalaxy: An Origin for Ultra–High‐Energy Cosmic Rays in the Nearby Universe

I show that the relativistic winds of newly born magnetars (neutron stars with petagauss surface magnetic fields) with initial spin rates close to the centrifugal breakup limit, occurring in all normal galaxies with massive star formation, can provide a source of ultrarelativistic light ions with an E-1 injection spectrum, steepening to E-2 at higher energies, with an upper cutoff at 1021-1022 eV. Interactions with the cosmic microwave background yield a spectrum at the Earth that compares favorably with the spectrum of ultra-high-energy cosmic rays (UHECRs) observed at energies up to a few times 1020 eV. The fit to the observations suggests that ~5%-10% of the magnetars are born with rotation rates and voltages sufficiently high to allow the acceleration of the UHECR. The form the spectrum incident on the Earth takes depends sensitively on the mechanism and the magnitude of gravitational wave losses during the early spin-down of these neutron stars: pure electromagnetic spin-down (the E-1 injection spectrum) yields a GZK feature [a flattening of the E3J(E) spectrum] below 1020 eV, rather than a cutoff, while a moderate GZK cutoff appears if gravitational wave losses are strong enough to steepen the injection spectrum above 1020 eV. The flux above 1020 eV comes from magnetars in relatively nearby galaxies (D 100 EeV air showers, the model predicts gravitational wave strains ~3 × 10-21. Such bursts of gravitational radiation should correlate with bursts of ultra-high-energy particles. The Auger experiment should see bursts of particles with energy above 100 EeV every few years.

Lightcurves of Centaurs 2000 QC$\mathsf{_{243}}$ and 2001 PT$\mathsf{_{13}}$

We present CCD photometric observations of Centaurs 2000 QC243 and 2001 PT13. For 2000 QC243, a large amplitude period is found at , which likely corresponds to half the rotation period of the body, or coincides with the full rotation period, depending on whether the variability is induced by an irregular shape or by albedo features. The apparent double-peaked lightcurve favors the first interpretation. The large amplitude of the oscillations imply a very irregular shape for this object or a large albedo assymmetry between the two hemispheres. For 2001 PT13, a clear period of was detected, with an amplitude of 0.16 mag. Since 4.15 h is close to the breakup limit for typical cometary densities and tensile strengths, 8.3 h appears to be a more likely rotation period.

Tidal Spin‐up of Stars in Dense Stellar Cusps around Massive Black Holes

We show that main-sequence stars in dense stellar cusps around massive black holes are likely to rotate at a significant fraction of the centrifugal breakup velocity as a result of spin-up by hyperbolic tidal encounters. We use realistic stellar structure models to calculate analytically the tidal spin-up in soft encounters and extend these results to close and penetrating collisions using smoothed particle hydrodynamics simulations. We find that the spin-up effect falls off only slowly with distance from the black hole because the increased tidal coupling in slower collisions at larger distances compensates for the decrease in the stellar density. We apply our results to the stars near the massive black hole in the Galactic center. Over their lifetime, ~1 M☉ main-sequence stars in the inner 0.3 pc of the Galactic center are spun-up on average to ~10%-30% of the centrifugal breakup limit. Such rotation is ~20-60 times higher than is usual for such stars and may affect their subsequent evolution and their observed properties.

Advection-dominated accretion: A self-similar solution

view Abstract Citations (1929) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Advection-dominated Accretion: A Self-similar Solution Narayan, Ramesh ; Yi, Insu Abstract We consider viscous rotating accretion flows in which most of the viscously dissipated energy is stored as entropy rather than being radiated. Such advection-dominated flows may occur when the optical depth is either very small or very large. We obtain a family of self-similar solutions where the temperature of the accreting gas is nearly virial and the flow is quasi-spherical. The gas rotates at much less than the Keplerian angular velocity; therefore, the central stars in such flows will cease to spin up long before they reach the break-up limit. Further, the Bernoulli parameter is positive, implying that advection-dominated flows are susceptible to producing outflows. Convection is likely in many of these flows and, if present, will tend to enhance the above effects. We suggest that advection-dominated accretion may provide an explanation for the slow spin rates of accreting stars and the widespread occurrence of outflows and jets in accreting systems. Publication: The Astrophysical Journal Pub Date: June 1994 DOI: 10.1086/187381 arXiv: arXiv:astro-ph/9403052 Bibcode: 1994ApJ...428L..13N Keywords: Accretion Disks; Advection; Astronomical Models; Black Holes (Astronomy); Cooling Flows (Astrophysics); Stellar Mass Accretion; Viscous Flow; Hydrodynamics; Rotating Disks; Two Dimensional Flow; Astrophysics; ACCRETION; ACCRETION DISKS; BLACK HOLE PHYSICS; HYDRODYNAMICS; Astrophysics E-Print: 7 pages of text, 1 postscript figure, plain TeX, Harvard-Smithsonian Center for Astrophysics Preprint Series No. 3809 full text sources arXiv | ADS |