Alfven Radius Research Articles

Catastrophic rotational braking among Sun-like stars

Context. Observations of young open clusters show a bimodal distribution of stellar rotation. In those clusters, Sun-like stars group into two main populations of fast and slow rotators. Beyond an age of approximately 600 Myr, the two populations converge towards a single sequence of slow rotators.Aims. The present study addresses the origin of this bimodal distribution and the cause of its observed evolution.Methods. New prescriptions of mass-loss rate and Alfven radius dependences on Rossby number suggested by observations are implemented in a phenomenological model of angular-momentum loss and redistribution. The obtained model is used to calculate the time evolution of a rotation-period distribution of solar-mass stars similar to that observed in the 5 Myr-old NGC 2362 open cluster. The simulated distributions at subsequent ages are compared with those of h Per, the Pleiades, M 50, M 35, and M 37.Results. The model is able to reproduce the appearance and disappearance of a bimodal rotation-period distribution in open clusters providing that a brief episode of large-angular-momentum loss is included in the early evolution of Sun-like stars.Conclusions. I argue that a transitory episode of large-angular-momentum loss occurs on Sun-like stars with Rossby numbers between 0.13 and 0.3. This phenomenon of enhanced magnetic braking by stellar wind would be mainly driven by a rapid increase of mass loss at a critical rotation rate. This scenario accounts for the bimodal distribution of stellar rotation in open clusters with ages between 20−30 Myr and approximately 600 Myr. The mass-loss rate increase could account for a significant fraction of the X-ray luminosity decay of Sun-like stars in the 0.13−0.3 Rossby number range where a transition from the saturated to the non-saturated regime of X-ray emission is observed. Observed correlations between Li abundance and rotation sequences in the Pleiades and M 34 clusters support this scenario.

The inner disc radius in the propeller phase and accretion-propeller transition of neutron stars

We have investigated the critical conditions required for a steady propeller effect for magnetized neutron stars with optically thick, geometrically thin accretion discs. We have shown through simple analytical calculations that a steady-state propeller mechanism cannot be sustained at an inner disc radius where the viscous and magnetic stresses are balanced. The radius calculated by equating these stresses is usually found to be close to the conventional Alfven radius for spherical accretion, r(A). Our results show that: (1) a steady propeller phase can be established with a maximum inner disc radius that is at least similar to 15 times smaller than rA depending on the mass-flow rate of the disc, rotational period and strength of the magnetic dipole field of the star, (2) the critical accretion rate corresponding to the accretion-propeller transition is orders of magnitude lower than the rate estimated by equating rA to the co-rotation radius. Our results are consistent with the properties of the transitional millisecond pulsars that show transitions between the accretion powered X-ray pulsar and the rotational powered radio-pulsar states.

A multi-model approach to X-ray pulsars

The emission characteristics of X-ray pulsars are governed by magnetospheric accretion within the Alfven radius, leading to a direct coupling of accretion column properties and interactions at the magnetosphere. The complexity of the physical processes governing the formation of radiation within the accreted, strongly magnetized plasma has led to several sophisticated theoretical modelling efforts over the last decade, dedicated to either the formation of the broad band continuum, the formation of cyclotron resonance scattering features (CRSFs) or the formation of pulse profiles. While these individual approaches are powerful in themselves, they quickly reach their limits when aiming at a quantitative comparison to observational data. Too many fundamental parameters, describing the formation of the accretion columns and the systems’ overall geometry are unconstrained and different models are often based on different fundamental assumptions, while everything is intertwined in the observed, highly phase-dependent spectra and energy-dependent pulse profiles. To name just one example: the (phase variable) line width of the CRSFs is highly dependent on the plasma temperature, the existence of B-field gradients (geometry) and observation angle, parameters which, in turn, drive the continuum radiation and are driven by the overall two-pole geometry for the light bending model respectively. This renders a parallel assessment of all available spectral and timing information by a compatible across-models-approach indispensable. In a collaboration of theoreticians and observers, we have been working on a model unification project over the last years, bringing together theoretical calculations of the Comptonized continuum, Monte Carlo simulations and Radiation Transfer calculations of CRSFs as well as a General Relativity (GR) light bending model for ray tracing of the incident emission pattern from both magnetic poles. The ultimate goal is to implement a unified fitting model for phase-resolved spectral and timing data analysis. We present the current status of this project.

HD 96446: a puzzle for current models of magnetospheres?

Context. Oblique magnetic dipole fields have been detected in Bp stars for several decades, and more recently also in normal massive stars. In the past decade, it has been established that stellar magnetospheres form through the channelling and confinement of an outflowing stellar wind by the stellar magnetic field. This explains specific properties of magnetic massive stars, such as their rotationally modulated photometric light curve, Hα emission, UV spectra, and X-ray emission. Aims. In the framework of the MiMeS (Magnetism in Massive Stars) project, four HARPSpol observations of the magnetic Bp star HD 96446 have been obtained. HD 96446 is very similar to σ Ori E, the prototype of centrifugally supported rigidly rotating magnetospheres (CM) and is therefore a perfect target to study the validity of this model. Methods. We first updated the basic parameters of HD 96446 and studied its spectral variability. We then analysed the HARPSpol spectropolarimetric observations using the LSD (Least-Squares Deconvolution) technique to derive the longitudinal magnetic field and Zeeman signatures in various types of lines. With LTE spectrum modelling, we derived constraints on the field modulus, the rotational velocity, and the inclination angle, and measured non-solar abundances of several elements which we checked with NLTE modelling. Finally, we calculated the magnetic confinement and Alfven and Kepler radii from the stellar magnetic field and rotation properties, and we examined the various types of magnetospheres that may be present around HD 96446. Results. We find radial velocity variations with a period around 2.23 h, that we attribute to β Cep-type p-mode pulsations. We detect clear direct magnetic Stokes V signatures with slightly varying values of the longitudinal magnetic field, typical of an oblique dipole rotator, and show that these signatures are not much perturbed by the radial velocity variations. The magnetic confinement parameter and Alfven radius in the centrifugally supported, rigidly-rotating magnetosphere (CM) model points towards the presence of confined material in the magnetosphere. However, HD 96446 does not present signatures of the presence of such confined material, such as Hα emission. Conclusions. We conclude that, even though HD 96446 fulfills all criteria to host a CM with confined material, it does not. The rotation period must be significantly revised, or another model of magnetosphere with a leakage mechanism will need to be developed to explain the magnetic environment of this star.

NGC 1624-2: a slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic field

This paper presents a first observational investigation of the faint Of?cp star NGC 1624-2, yielding important new constraints on its spectral and physical characteristics, rotation, magnetic field strength, X-ray emission and magnetospheric properties. Modeling the spectrum and spectral energy distribution, we conclude that NGC 1624-2 is a main sequence star of mass M {\simeq} 30 M{\odot}, and infer an effective temperature of 35 {\pm} 2 kK and log g = 4.0 {\pm} 0.2. Based on an extensive time series of optical spectral observations we report significant variability of a large number of spectral lines, and infer a unique period of 157.99 {\pm} 0.94 d which we interpret as the rotational period of the star. We report the detection of a very strong - 5.35 {\pm} 0.5 kG - longitudinal magnetic field <Bz>, coupled with probable Zeeman splitting of Stokes I profiles of metal lines confirming a surface field modulus <B> of 14 {\pm} 1 kG, consistent with a surface dipole of polar strength >~ 20 kG. This is the largest magnetic field ever detected in an O-type star, and the first report of Zeeman splitting of Stokes I profiles in such an object. We also report the detection of reversed Stokes V profiles associated with weak, high-excitation emission lines of O iii, which we propose may form in the close magnetosphere of the star. We analyze archival Chandra ACIS-I X-ray data, inferring a very hard spectrum with an X-ray efficiency log Lx/Lbol = -6.4, a factor of 4 larger than the canonical value for O-type stars and comparable to that of the young magnetic O-type star {\theta}1 Ori C and other Of?p stars. Finally, we examine the probable magnetospheric properties of the star, reporting in particular very strong magnetic confinement of the stellar wind, with {\eta}* {\simeq} 1.5 {\times} 10^4, and a very large Alfven radius, RAlf = 11.4 R*.

On kHz oscillations and characteristic frequencies of accreting magnetospheres

When an accreting star is close to rotational equilibrium between the dipole component of the stellar magnetic field and the accretion disk, the star's rotation rate is roughly of the order of the Keplerean rotation rate at the inner boundary of the disk, estimated as the conventional Alfven radius. A range of frequencies higher than this equilibrium rotation frequency can naturally arise if the accretion flow is channeled by higher multipoles of the star's magnetic field. The higher multipole components of the magnetic field will balance the material stresses of the accretion flow at radii closer to the star. The Kepler frequencies associated with these generalized Alfven radii increase with the order of the multipole. Other frequency bands, like the epicyclic frequencies associated with the accretion flow, may in turn be higher than the Kepler frequencies. We present expressions for the spectrum of higher frequencies arising due to these effects. Kilohertz quasi-periodic oscillation frequencies that are much higher than the rotation rate of the neutron star, as observed from the recently discovered 11 Hz (P = 90 ms) X-ray pulsar IGR J17480-2446 in the globular cluster Terzan 5, may be due to modulation of the accretion rate by the excitation of these modes in the accretion flow. The very high QPO frequencies observed from the soft gamma repeaters SGR 1806-20 (P = 5.2 s) and SGR 1900+14 (P = 7.5 s) may also correspond to these characteristic frequencies if SGRs accrete from fallback disks around them.

ACCRETION AND PLASMA OUTFLOW FROM DISSIPATIONLESS DISCS

We consider the specific case of disc accretion for negligibly low viscosity and infinitely high electric conductivity. The key component in this model is the outflowing magnetized wind from the accretion disc, since this wind effectively carries away angular momentum of the accreting matter. Assuming magnetic field has variable polarity in the disc (to avoid magnetic flux and energy accumulation at the gravitational center), this leads to radiatively inefficient accretion of the disc matter onto the gravitational center. In such a case, the wind forms an outflow, which carries away all the energy and angular momentum of the accreted matter. Interestingly, in this framework, the basic properties of the outflow (as well as angular momentum and energy flux per particle in the outflow) do not depend on the structure of accretion disc. The self-similar solutions obtained prove the existence of such an accreting regime. In the self-similar case, the disc accretion rate (Ṁ) depends on the distance to the gravitational center, r, as [Formula: see text], where λ is the dimensionless Alfvenic radius. Thus, the outflow predominantly occurs from the very central part of the disc provided that λ ≫ 1 (it follows from the conservation of matter). The accretion/outflow mechanism provides transformation of the gravitational energy from the accreted matter into the energy of the outflowing wind with efficiency close to 100%. The flow velocity can essentially exceed the Kepler velocity at the site of the wind launch.

ATMOSPHERIC ESCAPE FROM HOT JUPITERS

Photoionization heating from ultraviolet (UV) radiation incidents on the atmospheres of hot Jupiters may drive planetary mass loss. Observations of stellar Lyman-α (Lyα) absorption have suggested that the hot Jupiter HD 209458b is losing atomic hydrogen. We construct a model of escape that includes realistic heating and cooling, ionization balance, tidal gravity, and pressure confinement by the host star wind. We show that mass loss takes the form of a hydrodynamic (Parker) wind, emitted from the planet's dayside during lulls in the stellar wind. When dayside winds are suppressed by the confining action of the stellar wind, nightside winds might pick up if there is sufficient horizontal transport of heat. A hot Jupiter loses mass at maximum rates of ~2 × 1012 g s–1 during its host star's pre-main-sequence phase and ~2 × 1010 g s–1 during the star's main-sequence lifetime, for total maximum losses of ~0.06% and ~0.6% of the planet's mass, respectively. For UV fluxes F UV 104 erg cm–2 s–1, the mass-loss rate is approximately energy limited and scales as . For larger UV fluxes, such as those typical of T Tauri stars, radiative losses and plasma recombination force to increase more slowly as F 0.6 UV. Dayside winds are quenched during the T Tauri phase because of confinement by overwhelming stellar wind pressure. During this early stage, nightside winds can still blow if the planet resides outside the stellar Alfven radius; otherwise, even nightside winds are stifled by stellar magnetic pressure, and mass loss is restricted to polar regions. We conclude that while UV radiation can indeed drive winds from hot Jupiters, such winds cannot significantly alter planetary masses during any evolutionary stage. They can, however, produce observable signatures. Candidates for explaining why the Lyman-α photons of HD 209458 are absorbed at Doppler-shifted velocities of ±100 km s–1 include charge-exchange in the shock between the planetary and stellar winds.

The weak magnetic field of the O9.7 supergiant ζ Orionis A★

We report here the detection of a weak magnetic field of 50 - 100 G on the O9.7 supergiant zeta Ori A, using spectropolarimetric observations obtained with NARVAL at the 2m Telescope Bernard Lyot atop Pic du Midi (France). zeta Ori A is the third O star known to host a magnetic field (along with theta^1 Ori C and HD 191612), and the first detection on a 'normal' rapidly-rotating O star. The magnetic field of zeta Ori A is the weakest magnetic field ever detected on a massive star. The measured field is lower than the thermal equipartition limit (about 100 G). By fitting NLTE model atmospheres to our spectra, we determined that zeta Ori A is a 40 Msun star with a radius of 25 Rsun and an age of about 5 - 6 Myr, showing no surface nitrogen enhancement and losing mass at a rate of about 2x10^(-6) Msol/yr. The magnetic topology of zeta Ori A is apparently more complex than a dipole and involves two main magnetic polarities located on both sides of the same hemisphere; our data also suggest that zeta Ori A rotates in about 7.0 d and is about 40 degrees away from pole-on to an Earth-based observer. Despite its weakness, the detected magnetic field significantly affects the wind structure; the corresponding Alfven radius is however very close to the surface, thus generating a different rotational modulation in wind lines than that reported on the two other known magnetic O stars. The rapid rotation of zeta Ori A with respect to theta^1 Ori C appears as a surprise, both stars having similar unsigned magnetic fluxes (once rescaled to the same radius); it may suggest that the sub-equipartition field detected on zeta Ori A is not a fossil remnant (as opposed to that of theta^1 Ori C and HD 191612), but the result of an exotic dynamo action produced through MHD instabilities.

Magnetic field upper limits for jet formation

Context: Very high magnetic fields at the surface of neutron stars or in the accretion disk of black holes inhibit the production of jets. Aims: We quantify here the magnetic field strength for jet formation. Methods: By using the Alfven Radius, R_A, we study what we call {\it the basic condition}, R_A/R_*=1 or R_A/R_{LSO}=1 (LSO, last stable orbit), in its dependency on the magnetic field strength and the mass accretion rate, and we analyse these results in 3-D and 2-D plots in the case of neutron star and black hole accretor systems, respectively. For this purpose, we did a systematic search of all available observational data for magnetic field strength and the mass accretion rate. Results: The association of a classical X-ray pulsar (i.e. B ~10^{12} G) with jets is excluded even if accreting at the Eddington critical rate. Z-sources may develop jets for B \simlt 10^{8.2} G, whereas Atoll-sources are potential sources of jets if B \simlt 10^{7.7} G. It is not ruled out that a millisecond X-ray pulsar could develop jets, at least for those sources where B \simlt 10^{7.5} G. In this case the millisecond X-ray pulsar could switch to a microquasar phase during its maximum accretion rate. For stellar-mass black hole X-ray binaries, the condition is that B \simlt 1.35 x 10^8 G and B \simlt 5 x 10^8 G at the last stable orbit for a Schwarzschild and a Kerr black hole, respectively. For active galactic nuclei (AGNs), it reaches B \simlt 10^{5.9} G for each kind of black hole. These theoretical results are in complete agreement with available observational data.

Application of a MHD hybrid solar wind model with latitudinal dependences to ULYSSES data at minimum

In a previous work, Ulysses data was analyzed to build a complete axisymmetric MHD solution for the solar wind at minimum including rotation and the initial flaring of the solar wind in the low corona. This model has some problems in reproducing the values of magnetic field at 1 AU despite the correct values of the velocity. Here, we intend to extend the previous analysis to another type of solutions and to improve our modelling of the wind from the solar surface to 1 AU. We compare the previous results to those obtained with a fully helicoidal model and construct a hybrid model combining both previous solutions, keeping the flexibility of the parent models in the appropriate domain. From the solar surface to the Alfven, point, a three component solution for velocity and magnetic field is used, reproducing the complex wind geometry and the well-known flaring of the field lines observed in coronal holes. From the Alfven radius to 1 AU and further, the hybrid model keeps the latitudinal dependences as flexible as possible, in order to deal with the sharp variations near the equator and we use the helicoidal solution, turning the poloidal streamlines into radial ones. Despite the absence of the initial flaring, the helicoidal model and the first hybrid solution suffer from the same low values of the magnetic field at 1 AU. However, by adjusting the parameters with a second hybrid solution, we are able to reproduce both the velocity and magnetic profiles observed by Ulysses and a reasonable description of the low corona, provided that a certain amount of energy deposit exists along the flow. The present paper shows that analytical axisymmetric solutions can be constructed to reproduce the solar structure and dynamics from 1 solar radius up to 1 AU.

Large-scale $\vec \alpha\mathsf{^2}$-dynamo in low-mass stars and brown dwarfs

We develop a model based on 3D mean-field MHD for the generation of large scale magnetic fields in fully convective objects like low-mass stars, brown dwarfs and possibly gaseous planets. The dynamo process is of alpha^2 type and thus differs from the shell-dynamo at work in more massive stars.The alpha^2 dynamo becomes supercritical for Rossby numbers Ro\la 10. It generates a large-scale, non-axisymmetric, steady field that is symmetric with respect to the equatorial plane. Saturation of the alpha^2-generated field at the equipartition yields strengths of several kGauss, in agreement with observations of active M dwarfs, and provides a qualitative explanation for the observed activity saturation in late M stars. For brown dwarfs with a conductive core, as occurs at the center of the most massive and oldest of these objects, we have also studied an alpha^2-Omega dynamo, i.e. the effect of differential rotation. In this case the field is predominantly toroidal, axisymmetric and oscillatory, like the solar field. The topology of the field in the fully convective objects exhibits a high order multipole character that differs from the aligned dipole field generated by the alpha-Omega dynamo. The strong reduction of the dipolar component due to the field non-axisymmetry should considerably reduce the Alfven radius and thus the efficiency of magnetic braking, providing an appealing explanation for the decreasing angular momentum loss rate observed in low-mass stars and brown dwarfs. This may have also implications for cataclysmic variables below the period gap. In spite of this large-scale field, the decreasing conductivity in the dominantly neutral atmosphere of these objects prevents the current generation necessary to support a chromosphere and thus activity. (Abridged)

Hot Jupiters and Hot Spots: The Short‐ and Long‐Term Chromospheric Activity on Stars with Giant Planets

We monitored the chromospheric activity in the Ca II H and K lines of 13 solar-type stars (including the Sun): 8 of them over 3 years at the Canada-France-Hawaii Telescope (CFHT) and 5 in a single run at the Very Large Telescope (VLT). A total of 10 of the 13 targets have close planetary companions. All of the stars observed at the CFHT show long-term (months to years) changes in H and K intensity levels. Four stars display short-term (days) cyclical activity. For two, HD 73256 and κ1 Cet, the activity is likely associated with an active region rotating with the star; however, the flaring in excess of the rotational modulation may be associated with a hot Jupiter. A planetary companion remains a possibility for κ1 Cet. For the other two, HD 179949 and υ And, the cyclic variation is synchronized to the hot Jupiter's orbit. For both stars this synchronicity with the orbit is clearly seen in two out of three epochs. The effect is only marginal in the third epoch at which the seasonal level of chromospheric activity had changed for both stars. Short-term chromospheric activity appears weakly dependent on the mean K line reversal intensities for the sample of 13 stars. In addition, a suggestive correlation exists between this activity and the Mp sin i of the star's hot Jupiter. Because of their small separation (≤0.1 AU), many of the hot Jupiters lie within the Alfven radius of their host stars, which allows a direct magnetic interaction with the stellar surface. We discuss the conditions under which a planet's magnetic field might induce activity on the stellar surface and why no such effect was seen for the prime candidate, τ Boo. This work opens up the possibility of characterizing planet-star interactions, with implications for extrasolar planet magnetic fields and the energy contribution to stellar atmospheres.

Disks Surviving the Radiation Pressure of Radio Pulsars

The radiation pressure of a radio pulsar does not necessarily disrupt a surrounding disk. The position of the inner radius of a thin disk around a neutron star, determined by the balance of stresses, can be estimated by comparing the electromagnetic energy density generated by the neutron star as a rotating magnetic dipole in vacuum with the kinetic energy density of the disk. Inside the light cylinder, the near zone electromagnetic field is essentially the dipole magnetic field, and the inner radius is the conventional Alfven radius. Far outside the light cylinder, in the radiation zone, = , and the electromagnetic energy density is S/c ∝ 1/r2, where S is the Poynting vector. Shvartsman argued that a stable equilibrium cannot be found in the radiative zone because the electromagnetic energy density dominates over the kinetic energy density, with the relative strength of the electromagnetic stresses increasing with radius. In order to check whether this is also true near the light cylinder, we employ the Deutsch global electromagnetic field solutions for rotating oblique magnetic dipoles. Near the light cylinder the electromagnetic energy density increases steeply enough with decreasing r to balance the kinetic energy density at a stable equilibrium. The transition from the near zone to the radiation zone is broad. The radiation pressure of the pulsar cannot disrupt the disk for values of the inner radius up to about twice the light cylinder radius if the rotation axis and the magnetic axis are orthogonal. This allowed range beyond the light cylinder extends much farther for small inclination angles. The mass flow rate in quiescent phases of accretion-driven millisecond pulsars can occasionally drop to values low enough that the inner radius of the disk goes beyond the light cylinder. The possibilities considered here may be relevant for the evolution of spun-up X-ray binaries into millisecond pulsars, for some transients, and for the evolution of young neutron stars if there is a fallback disk surrounding the neutron star.

A Revised Geometry for the Magnetic Wind of θ1 Orionis C

ABSTRACTThe star θ1 Orionis C (O6–7 V) is often cited as a hot analog of Bp variables, because its optical and UV line and X‐ray continuum fluxes modulate over the magnetic/rotational period. In this circumstance, one expects emission and absorption components of the UV resonance lines to vary as a flattened magnetosphere corotates with the star. In this paper, we reexamine the detailed velocity behavior of several strong UV lines. Whereas past work has focused on variations of the full profiles, we find that the blue and red wings of the C iv and N v resonance lines exhibit anticorrelated modulations. These appear as absorption excesses at large blueshifts and as flux elevations at moderate redshifts at the edge‐on phase ϕ = 0.5. No rest‐frame absorption features, which are the typical signatures of cool, static disks surrounding Bp stars, can be detected at any phase. We suggest that this behavior is caused by two geometrically distinct components of the wind, which are defined by the relationship between the extent of a magnetic loop and the local Alfvén radius. Streams on field lines opening outside this radius are first channeled toward the magnetic equator, but after reaching the Alfven radius, they are forced outward by radiative forces, eventually to become an expanding radial outflow. This wind component causes blueshifted absorption as the corotating magnetic equatorial plane crosses the observer’s line of sight (ϕ = 0.5). The geometry of the inner component requires a more complicated interpretation. Wind streams first follow closed loops and collide at the magnetic equator with counterpart streams from the opposite pole. There they coalesce and fall back to the star along their original field lines. The high temperatures in these falling condensations cause the redshifted emission. The rapid circulation of these flows is likely the reason for the absence of signatures of a cool disk (e.g., zero‐velocity absorptions at ϕ∼0.5) in the strong UV lines.

Torque bistability in the interaction between a neutron star magnetosphere and a thin accretion disc

We present a time-dependent model of the interaction between a neutron star magnetosphere and a thin (Shakura-Sunyaev) accretion disc, where the extent of the magnetosphere is determined by balancing outward diffusion and inward advection of the stellar magnetic field at the inner edge of the disc. The nature of the equilibria available to the system is governed by the magnetic Prandtl number Pm and the ratio \xi of the corotation radius to the Alfven radius. For \xi > Pm^0.3, the system can occupy one of two stable states, where the torques are of opposite signs. If the star is spinning up initially, in the absence of extraneous perturbations, \xi decreases until the spin-up equilibrium vanishes, the star subsequently spins down, and the torque asymptotes to zero. Vortex-in-cell simulations of the Kelvin-Helmholtz instability suggest that the transport speed across the mixing layer between the disc and magnetosphere is less than the shear speed when the layer is thin, unlike in previous models.

Do weak magnetic fields prevent hydrogen from accreting onto metal-line white dwarf stars

The widely accepted assumption is that metals detected in the spectra of a few cool helium-rich white dwarfs cannot be of primordial origin and therefore must be accreted from the interstellar medium. However, the observed abundances of are much too low to be compatible with the high accretion rates inferred from metal accretion. Hydrogen accretion is therefore suppressed compared to metal accretion. The hypothesis most widely discussed as cause for this hydrogen screening is the propeller mechanism: Metals are accreted in the form of grains onto a slowly rotating, weakly magnetized white dwarf, whereas ionized is repelled at the Alfven radius. We have obtained circular polarization spectra of the helium-rich white dwarfs GD 40 (WD0300-013) and L745-46A (WD0738-172) - which both show strong metal lines as well as - in order to search for signatures of a weak magnetic field. The magnetic field strengths necessary for the propeller mechanism to work in these stars are at least 144 000 G and 3000 G, respectively. Whereas L745-46A might have a magnetic field of about −6900 G no magnetic field could be found with an upper limit for the field strength of 4000 G (with 99% confidence) for GD 40.

Asterospheric Magnetic Fields and Winds of Cool Stars

This study addresses the winds and magnetic fields in the inner asterospheres of Sun-like magnetically active stars by combining empirical relationships between rotation rate and mass loss, angular-momentum loss, and radiative losses with models of the magnetic fields at the surfaces of cool stars and in their inner asterospheres based on the solar example. Our models, for mean magnetic flux densities up to 10 times solar, suggest that the asterospheric fields of such stars are dominated by the large-scale dipole component of the surface field, as is the case for the Sun. Hence, most of the time a single current sheet is expected to separate domains of opposite magnetic polarity; the current sheets of more active stars generally have smaller latitudinal ripples. Magnetic braking requires that the total unsigned asterospheric magnetic flux increase linearly with the stellar angular velocity, which is a very much weaker increase than seen for the flux at the stellar surface. We show that this can be achieved by an increase in the radial distance at which the coronal field is forced open as surface activity increases. Combined with measured mass-loss rates and the assumption that the wind velocity is largely independent of activity, this requires the wind's Alfven radius to be nearly constant, decreasing with surface activity with a power of only -0.16 ± 0.13. We point out that the surface flux density of energy needed to drive a cool-star wind scales linearly with the unsigned surface magnetic flux density, as does that needed to heat the corona.

Propagation of Magnetized Neutron Stars through the Interstellar Medium

This work investigates the propagation of magnetized, isolated old neutron stars through the interstellar medium (ISM). We performed axisymmetric, nonrelativistic magnetohydrodynamic (MHD) simulations of the propagation of a nonrotating star with a dipole field aligned with its velocity through the ISM. Effects of rotation will be discussed in a subsequent work. We consider two cases: (1) where the accretion radius Racc is comparable to the standoff distance or Alfven radius RA and gravitational focusing is important and (2) where Racc RA and the magnetized star interacts with the ISM as a magnetic plow, without significant gravitational focusing. For the first case, simulations were done at a low Mach number = 3 for a range of values of the field B*. For the second case, simulations were done for higher Mach numbers, = 10, 30, and 50. In both cases, the magnetosphere of the star represents an obstacle for the flow, and a shock wave stands in front of the star. Magnetic field lines are stretched downwind from the star and form a hollow elongated magnetotail. Reconnection of the field is observed in the tail, which may lead to the acceleration of particles. Similar powers are estimated to be released in the bow shock wave and in the magnetotail. The estimated powers are, however, below present detection limits. Results of our simulations may be applied to other strongly magnetized stars, for example, white dwarfs and Ap stars. Future more sensitive observations may reveal bow shocks and long magnetotails of magnetized stars moving through the ISM.

Monte Carlo Simulations of Thermal‐Nonthermal Radiation from a Neutron Star Magnetospheric Accretion Shell

We discuss the space- and time-dependent Monte Carlo code we have developed to simulate the relativistic radiation output from compact astrophysical objects, coupled to a Fokker-Planck code to determine the self-consistent lepton populations. We have applied this code to model the emission from a magnetized neutron star accretion shell near the Alfven radius, reprocessing the radiation from the neutron star surface. We explore the parameter space defined by the accretion rate, stellar surface field, and level of wave turbulence in the shell. Our results are relevant to the emission from atoll sources, soft X-ray-transients containing weakly magnetized neutron stars, and recently suggested models of accretion-powered emission from anomalous X-ray pulsars.