Formation of polar starspots through meridional circulation

To explain the observed intermingling of polarities in the magnetic field distributions of rapidly rotating stars, surface magnetic flux transport models demand the presence of fast meridional flows.We combine simulations of the pre-eruptive and post-eruptive magnetic flux transport in cool stars to investigate the influence of a fast meridional circulation on the latitudinal eruption pattern of magnetic flux tubes and on the polar magnetic field properties. Magnetic flux tubes rising through the convection zone experience an enhanced latitude-dependent poleward deflection through meridional flows, which renders the wings of stellar butterfly diagrams convex. The larger amount of magnetic flux emerging at higher latitudes supports the intermingling of opposite polarities of polar magnetic fields and yields magnetic flux densities in the polar regions about 20% higher than in the case disregarding the pre-eruptive deflection. Taking the pre-eruptive evolution of magnetic flux into account therefore eases the need for the fast meridional flows predicted by previous investigations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Solar cycle according to mean magnetic field data

To investigate the shape of the solar cycle, we have performed a wavelet analysis of the large–scale magnetic field data for 1960–2000 for several latitudinal belts and have isolated the following quasi-periodic components: ∼22, 7 and 2 yr. The main 22-yr oscillation dominates all latitudinal belts except the latitudes of ±30° from the equator. The butterfly diagram for the nominal 22-yr oscillation shows a standing dipole wave in the low-latitude domain  (∣θ∣≤ 30°)  and another wave in the sub-polar domain  (∣θ∣≥ 35°)  , which migrates slowly polewards. The phase shift between these waves is about π. The nominal 7-yr oscillation yields a butterfly diagram with two domains. In the low-latitude domain  (∣θ∣≤ 35°)  , the dipole wave propagates equatorwards and in the sub-polar region, polewards. The nominal 2-yr oscillation is much more chaotic than the other two modes; however the waves propagate polewards whenever they can be isolated.
We conclude that the shape of the solar cycle inferred from the large-scale magnetic field data differs significantly from that inferred from sunspot data. Obviously, the dynamo models for a solar cycle must be generalized to include large-scale magnetic field data. We believe that sunspot data give adequate information concerning the magnetic field configuration deep inside the convection zone (say, in overshoot later), while the large-scale magnetic field is strongly affected by meridional circulation in its upper layer. This interpretation suggests that the poloidal magnetic field is affected by the polewards meridional circulation, whose velocity is comparable with that of the dynamo wave in the overshoot layer. The 7- and 2-yr oscillations could be explained as a contribution of two sub-critical dynamo modes with the corresponding frequencies.     

Magnetized relativistic jets and long-duration GRBs from magnetar spin-down during core-collapse supernovae

We use ideal axisymmetric relativistic magnetohydrodynamic simulations to calculate the spin-down of a newly formed millisecond,   B ∼ 10¹⁵ G  , magnetar and its interaction with the surrounding stellar envelope during a core-collapse supernova (SN) explosion. The mass, angular momentum and rotational energy lost by the neutron star are determined self-consistently given the thermal properties of the cooling neutron star's atmosphere and the wind's interaction with the surrounding star. The magnetar drives a relativistic magnetized wind into a cavity created by the outgoing SN shock. For high spin-down powers  (∼10⁵¹–10⁵² erg s⁻¹)  , the magnetar wind is superfast at almost all latitudes, while for lower spin-down powers  (∼10⁵⁰ erg s⁻¹)  , the wind is subfast but still super-Alfvénic. In all cases, the rates at which the neutron star loses mass, angular momentum and energy are very similar to the corresponding free wind values (≲30 per cent differences), in spite of the causal contact between the neutron star and the stellar envelope. In addition, in all cases that we consider, the magnetar drives a collimated  (∼5–10°)  relativistic jet out along the rotation axis of the star. Nearly all of the spin-down power of the neutron star escapes via this polar jet, rather than being transferred to the more spherical SN explosion. The properties of this relativistic jet and its expected late-time evolution in the magnetar model are broadly consistent with observations of long duration gamma-ray bursts (GRBs) and their associated broad-lined Type Ic SN.     

The global magnetic topology of AB Doradus

We have used Zeeman–Doppler maps of the surface field of the young, rapid rotator AB Dor  ( P _rot=0.514 d)  to extrapolate the coronal field, assuming it to be potential. We find that the topology of the large-scale field is very similar in all three years for which we have images. The corona divides cleanly into regions of open and closed field. The open field originates in two mid-latitude regions of opposite polarity separated by about 180° of longitude. The closed field region forms a torus extending almost over each pole, with an axis that runs through these two longitudes. We have investigated the effect on the global topology of different forms of flux in the unobservable hemisphere and in the dark polar spot where the Zeeman signal is suppressed. The flux distribution in the unobservable hemisphere affects only the low-latitude topology, whereas the imposition of a unidirectional polar field forces the polar cap to be open. This contradicts observations that suggest that the closed field corona extends to high latitudes and leads us to propose that the polar cap may be composed of multipolar regions.     

New constraints on a triaxial model of the Galaxy

We determine the most likely values of the free parameters of an N -body model for the Galaxy developed by Fux via a discrete–discrete comparison with the positions on the sky and line-of-sight velocities of an unbiased, homogeneous sample of OH/IR stars. Via Monte Carlo simulation, we find the plausibility of the best-fitting models, as well as the errors on the determined values. The parameters that are constrained best by these projected data are the total mass of the model and the viewing angle of the central bar, although the distribution of the latter has multiple maxima. The other two free parameters, the size of the bar and the (azimuthal) velocity of the Sun, are less well-constrained. The best model has a viewing angle of ∼ 44°, a semimajor axis of 2.5 kpc (corotation radius 4.5 kpc, pattern speed 46 km s⁻¹ kpc⁻¹), a bar mass of 1.7×10¹⁰ M_⊙ and a tangential velocity of the local standard of rest of 171 km s⁻¹. We argue that the lower values that are commonly found from stellar data for the viewing angle (∼25°) arise when too few coordinates are available, when the longitude range is too narrow or when low latitudes are excluded from the fit. The new constraints on the viewing angle of the Galactic bar from stellar line-of-sight velocities decrease further the ability of the distribution of the bar to account for the observed microlensing optical depth toward Baade's window: our model reproduces only half the observed value. The signal of triaxiality diminishes quickly with increasing latitude, fading within approximately 1 scaleheight (≲3°). This suggests that Baade's window is not a very appropriate region in which to sample bar properties.     

Simulation of turbulent convection in a slowly rotating red giant star

The first 3-D non-linear hydrodynamical simulation of the inner convective envelope of a rotating low mass red giant star is presented. This simulation, computed with the ASH code, aims at understanding the redistribution of angular momentum and heat in extended convection zones. The convection patterns achieved in the simulation consist of few broad and warm upflows surrounded by a network of cool downflows. This asymmetry between up and downflows leads to a strong downward kinetic energy flux, that must be compensated by an overluminous enthalpy flux in order to carry outward the total luminosity of the star. The influence of rotation on turbulent convection results in the establishment of largescale mean flows: a strong radial differential rotation and a single cell poleward meridional circulation per hemisphere. A detailed analysis of angular momentum redistribution reveals that the meridional circulation transports angular momentum outward in the radial direction and poleward in the latitudinal direction, with the Reynolds stresses acting in the opposite direction. This simulation indicates that the classical hypothesis of mixing length theory and solid-body rotation in the envelope of red giants assumed in 1-D stellar evolution models are unlikely to be realized and thus should be reconsidered. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectroscopy of the roAp star α Cir — I. Velocities of Hα and metal lines

We present radial velocity measurements of the rapidly oscillating Ap (roAp) star α Cir, obtained from dual-site observations with medium-dispersion spectrographs. The amplitude and phase of the principal pulsation mode vary significantly, depending on which line is being measured. The amplitude is observed to be as high as 1000 m s⁻¹ in some wavelength bands, despite a previous upper limit of 36 m s⁻¹. Furthermore, some lines are apparently pulsating in anti-phase with others. We suggest this indicates a high-overtone standing wave with a velocity node in the atmosphere of the star.     

V440 Per: the longest-period overtone Cepheid

V440 Per is a Population I Cepheid with a period of 7.57 d and low-amplitude, almost sinusoidal light and radial velocity curves. With no reliable data on the first harmonic, its pulsation mode identification remained controversial. We obtained a radial velocity curve of V440 Per with our new high-precision and high-throughput Poznań Spectroscopic Telescope. Our data reach an accuracy of 130 m s⁻¹ per individual measurement and yield a secure detection of the first harmonic with an amplitude of   A ₂= 140 ± 15 m s⁻¹  . The velocity Fourier phase φ₂₁ of V440 Per is inconsistent at the 7.25σ level with those of fundamental-mode Cepheids, implying that the star must be an overtone Cepheid, as originally proposed by Kienzle et al. Thus, V440 Per becomes the longest-period Cepheid with securely established overtone pulsations. We show that a convective non-linear pulsation hydrocode can reproduce the Fourier parameters of V440 Per very well. The requirement to match the observed properties of V440 Per constrains the free parameters of the dynamical convection model used in the pulsation calculations, in particular the radiative loss parameter.     

Evidence for a poleward meridional flow on the sun

We define for observational study two subsets of all polar zone filaments, which we call polemost filaments and polar filament bands. The behavior of the mean latitude of both the polemost filaments and the polar filament bands is examined and compared with the evolution of the polar magnetic field over an activity cycle as recently distilled by Howard and LaBonte (1981) from the past 13 years of Mt. Wilson full-disk magnetograms. The magnetic data reveal that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions that originate in active region latitudes and subsequently drift to the poles. After leaving the active-region latitudes, these unipolar f-polarity regions do not spread equatorward even though there is less net flux equatorward; this indicates that the f-polarity regions are carried poleward by a meridional flow, rather than by diffusion. The polar zone filaments are an independent tracer which confirms both the episodic polar field formation and the meridional flow. We find:

1. The mean latitude of the polemost filaments tracks the boundary of the polar field cap and undergoes an equatorward dip during each arrival of additional polar field.
2. Polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m s^-1.
3. The Mt. Wilson magnetic data, combined with a simple model calculation, show that the filament drift expected from diffusion alone would be slower than observed, and in some cases would be equatorward rather than poleward.
4. The observation that filaments drift poleward along with the magnetic regions shows that fields of both polarities are carried by the meridional flow, as would be expected, rather than only the f-polarity flux which dominates the strength. This leads to the prediction that in the mid-latitudes during intervals between the passage of f-polarity regions, both polarities are present in nearly equal amounts. This prediction is confirmed by the magnetic data.

     

A coupled model of magnetic flux generation and transport in stars

We present a combined model for magnetic field generation and transport in cool stars with outer convection zones. The mean toroidal magnetic field, which is generated by a cyclic thin-layer α Ω dynamo at the bottom of the convection zone is taken to determine the emergence probability of magnetic flux tubes in the photosphere. Following the nonlinear rise of the unstable thin flux tubes, emergence latitudes and tilt angles of bipolar magnetic regions are determined. These quantities are put into a surface flux transport model, which simulates the surface evolution of magnetic flux under the effects of large-scale flows and turbulent diffusion. First results are discussed for the case of the Sun and for more rapidly rotating solar-type stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

XMM–Newton observations of the eclipsing polar V2301 Oph

We present XMM–Newton observations of the eclipsing polar V2301 Oph which cover nearly 2.5 binary orbital cycles and two eclipses. This polar is believed to have the lowest magnetic field strength (7 MG) of any known polar. We find evidence for structure in the X-ray eclipse profile which shows a 'standstill' feature lasting  26 ± 4  s. This allows us to place an upper limit on the mass of the white dwarf of  ∼1.2 M_⊙  . We find no evidence for quasi-periodic oscillations (QPOs) in the frequency range 0.02–10 Hz. This coupled with the absence of QPOs in RXTE data suggests that, if present, any oscillations in the shock front have a minimal effect on the resultant X-ray flux. We find no evidence for a distinct soft X-ray component in its spectrum – it therefore joins another seven systems which do not show this component. We suggest that those systems which are asynchronous, have low mass-transfer rates or have accretion occurring over a relatively large fraction of the white dwarf are more likely to show this effect. We find that the specific mass-transfer rate has to be close to 0.1 g cm⁻² s⁻¹ to predict masses which are consistent with that derived from our eclipse analysis. This may be due to the fact that the low magnetic field strength allows accretion to take place along a wide range of azimuth.     

Spectroscopic solution of the star QX Cas

High-resolution spectroscopic observations around the Hα line of the binary star QX Cas covering the whole orbital period are presented. Our radial velocity solution, the first ever determined, requires an eccentric orbit with the following orbital parameters: eccentricity,   e = 0.22 ± 0.01  ; longitude of periastron,  ω= 45°± 5°  ; semi-amplitudes of the radial velocity curves of the primary and secondary stars,   K ₁ sin  i = 125.8 ± 0.9 km s⁻¹  and   K ₂ sin  i = 144.8 ± 1.1 km s⁻¹  ; gamma velocity,   V ₀= 65.1 ± 0.5 km s⁻¹  ; and mass ratio,   q = 0.869 ± 0.013  . The corresponding lower limits of the masses of the components and their separation are         , and   a sin  i = 31.34 ± 0.48 R_⊙  .     

Observations of the reappearance of polar coronal holes and the reversal of the polar magnetic field

We examine observations relating to the evolution of the polar magnetic field around sunspot maximum, when the net polar flux reverses polarity and coronal holes redevelop around the poles. Coronal hole observations during the last two solar maxima are examined in detail. Long-term averages of the latitudinal dependence of the photospheric magnetic field and the evolutionary pattern of the polar crown filaments are used to trace the poleward motion of the reversal of the large-scale surface field, and are compared to the redevelopment of the polar holes. The polar holes evolve from small, mid-latitude holes of new-cycle polarity which expand poleward until they join and cover the pole. We find that the appearance of these mid-latitude holes, the peak of flux emergence at low latitudes, and the polar polarity reversal all occur within a few solar rotations. Lagging 6 months to 1 1/2 yr after this time, the polar crown disappears and the polar holes redevelop.These results are examined in the context of phenomenological models of the solar cycle. We believe the following results in particular must be accounted for in successful models of the solar cycle: (1) The process of polarity reversal and redevelopment of the polar holes is discontinuous, occurring in 2 or 3 longitude bands, with surges of flux of old-cycle polarity interrupting the poleward migration of new-cycle flux. There is a persistent asymmetry in these processes between the two hemispheres; the polarity reversal in the two hemispheres is offset by 6 months to 1 1/2 yr. (2) Contrary to the Babcock hypothesis, the polar crown disappears months after the magnetic polar reversal. We suggest one possible scenario to explain this effect. (3) Our observations support suggestions of a poleward meridional flow around solar maximum that cannot be accounted for by Leighton-type diffusion.     

On the magnetic structure and wind parameter profiles of Alfvén wave driven winds in late-type supergiant stars

Cool stars at giant and supergiant evolutionary phases present low-velocity and high-density winds, responsible for the observed high mass-loss rates. Although presenting high luminosities, radiation pressure on dust particles is not sufficient to explain the wind acceleration process. Among the possible solutions to this still unsolved problem, Alfvén waves are, probably, the most interesting for their high efficiency in transfering energy and momentum to the wind. Typically, models of Alfvén wave driven winds result in high-velocity winds if they are not highly damped. In this work, we determine self-consistently the magnetic field geometry and solve the momentum, energy and mass conservation equations, to demonstrate that even a low-damped Alfvén wave flux is able to reproduce the low-velocity wind. We show that the magnetic flux tubes expand with a super-radial factor of S > 30 near the stellar surface, larger than that used in previous semi-empirical models. The rapid expansion results in a strong spatial dilution of the wave flux. We obtained the wind parameter profiles for a typical supergiant star of  16 M_⊙  . The wind is accelerated in a narrow region, coincident with the region of high divergence of the magnetic field lines, up to 100 km s⁻¹. For the temperature, we obtained a slight decrease near the surface for low-damped waves, because the wave heating mechanism is less effective than the radiative losses. The peak temperature occurs at   r ≃ 1.5  r ₀  reaching 6000 K. Propagating outwards, the wind cools down mainly due to adiabatic expansion.     

The enigma of the oldest 'nova': the central star and nebula of CK Vul

CK Vul is classified as, amongst others, the slowest known nova, a hibernating nova or a very late thermal pulse object. Following its eruption in ad 1670, the star remained visible for 2 yr. A 15-arcsec nebula was discovered in the 1980s, but the star itself has not been detected since the eruption. We here present radio images which reveal a 0.1-arcsec radio source with a flux of 1.5 mJy at 5 GHz. Deep Hα images show a bipolar nebula with a longest extension of 70 arcsec, with the previously known compact nebula at its waist. The emission-line ratios show that the gas is shock-ionized, at velocities  >100 km s⁻¹  . Dust emission yields an envelope mass of  ∼5 × 10⁻² M_⊙  . Echelle spectra indicate outflow velocities up to 360 km s⁻¹. From a comparison of images obtained in 1991 and 2004 we find evidence for expansion of the nebula, consistent with an origin in the 1670 explosion; the measured expansion is centred on the radio source. No optical or infrared counterpart is found at the position of the radio source. The radio emission is interpreted as thermal free–free emission from gas with   T _e∼ 10⁴ K  . The radio source may be due to a remnant circumbinary disc, similar to those seen in some binary post-AGB stars. We discuss possible classifications of this unique outburst, including that of a sub-Chandrasekhar mass supernova, a nova eruption on a cool, low-mass white dwarf or a thermal pulse induced by accretion from a circumbinary disc.     

Transport Effects in the Evolution of the Global Solar Magnetic Field

The axisymmetric component of the large-scale solar magnetic fields has a pronounced poleward branch at higher latitudes. In order to clarify the origin of this branch we construct an axisymmetric model of the passive transport of the mean poloidal magnetic field in the convective zone, including meridional circulation, anisotropic diffusivity, turbulent pumping and density pumping. For realistic values of the transport coefficients we find that diffusivity is prevalent, and the latitudinal distribution of the field at the surface simply reflects the conditions at the bottom of the convective zone. Pumping effects concentrate the field to the bottom of the convective zone; a significant part of this pumping occurs in a shallow subsurface layer, normally not resolved in dynamo models. The phase delay of the surface poloidal field relative to the bottom poloidal field is found to be small. These results support the double dynamo wave models, may be compatible with some form of a mixed transport scenario, and exclude the passive transport theory for the origin of the polar branch.     

Near-infrared and optical studies of the fast nova V4643 Sgr (Nova Sagittarii 2001)

It is generally accepted that the presence of a hot magnetic corona provides the source of X-ray emission in cool stars. With this connection one could expect to see the variation of magnetic flux in the activity cycle of a star mirrored by a similar variation in the stars X-ray emission. Using magnetic maps produced from flux emergence and transport simulations and assuming a potential field for the corona, we can extrapolate the coronal magnetic field and hence calculate the variation of the X-ray emission. We consider three types of activity cycle that successfully reproduce the pattern of intermingled magnetic flux at high latitudes, a feature observed with Zeeman–Doppler imaging. The three different cycles take the form of (1) an enhanced butterfly pattern where flux emergence is extended to a latitude of 70°, (2) an extended emergence profile as before but with an overlap of 4 yr in the butterfly diagram and (3) where no butterfly diagram is used. The cyclic variation in the X-ray emission is around two orders of magnitude for cases (1) and (3), but less than one order of magnitude for case (2). For all three cases, the rotational modulation of the X-ray emission is greatest at cycle minimum, but the emission measure weighted density varies little over the cycle. For cases (1) and (2) the fraction of the total flux that is open (along which a wind can escape) varies little over the cycle, but for case (3) this is three times larger at cycle minimum than at maximum. Our results clearly show that although magnetic cycles may exist for stars they are not necessarily observable in the X-ray emission.     

Continuous frequency spectrum of the global hydromagnetic oscillations of a magnetically confined mountain on an accreting neutron star

We compute the continuous part of the ideal-magnetohydrodynamic (ideal-MHD) frequency spectrum of a polar mountain produced by magnetic burial on an accreting neutron star. Applying the formalism developed by Hellsten & Spies, extended to include gravity, we solve the singular eigenvalue problem subject to line-tying boundary conditions. This spectrum divides into an Alfvén part and a cusp part. The eigenfunctions are chirped and anharmonic with an exponential envelope, and the eigenfrequencies cover the whole spectrum above a minimum ω_low. For equilibria with accreted mass  1.2 × 10⁻⁶≲ M _a/M_⊙≲ 1.7 × 10⁻⁴  and surface magnetic fields  10¹¹≲ B _*/G ≲ 10¹³, ω_low  is approximately independent of   B _*  , and increases with M _a. The results are consistent with the Alfvén spectrum excited in numerical simulations with the zeus-mp solver. The spectrum is modified substantially by the Coriolis force in neutron stars spinning faster than ∼100 Hz. The implications for gravitational-wave searches for low-mass X-ray binaries are considered briefly.     

Excitation and kinematics in H2 bow shocks: near-infrared observations of HH 99 and VLA 1623A (HH 313)

We present a comprehensive near-infrared study of two molecular bow shocks in two protostellar outflows, HH 99 in R Coronae Australis and VLA 1623A (HH 313) in Rho Ophiuchi. New, high-resolution, narrow-band images reveal the well-defined bow shock morphologies of both sources. These are compared with two-dimensional MHD modelling of molecular bows from which we infer flow inclination angles, shock speeds and the magnetic field in the pre-shock gas in each system. With combined echelle spectroscopy and low-resolution K -band spectra we further examine the kinematics and excitation of each source. Bow shock models are used to interpret excitation (CDR) diagrams and estimate the extinction and, in the case of VLA 1623, the ortho–para ratio associated with the observed H₂ population. For the first time, morphology, excitation and kinematics are fitted with a single bow shock model.
Specifically, we find that HH 99 is best fitted by a C-type bow shock model (although a J-type cap is probably responsible for the [Fe  ii ] emission). The bow is flowing away from the observer (at an angle to the line of sight of ∼45°) at a speed of roughly 100 km s⁻¹. VLA 1623A is interpreted in terms of a C-type bow moving towards the observer (at an angle to the line of sight of ∼75°) at a speed of ∼80 km s⁻¹. The magnetic field associated with HH 99 is thought to be orientated parallel to the flow axis; in VLA 1623A the field is probably oblique to the flow axis, since this source is clearly asymmetric in our H₂ images.     

Differential rotation and meridional flow of Arcturus

The spectroscopic variability of Arcturus hints at cyclic activity cycle and differential rotation. This could provide a test of current theoretical models of solar and stellar dynamos. To examine the applicability of current models of the flux transport dynamo to Arcturus, we compute a mean‐field model for its internal rotation, meridional flow, and convective heat transport in the convective envelope. We then compare the conditions for dynamo action with those on the Sun. We find solar‐type surface rotation with about 1/10th of the shear found on the solar surface. The rotation rate increases monotonically with depth at all latitudes throughout the whole convection zone. In the lower part of the convection zone the horizontal shear vanishes and there is a strong radial gradient. The surface meridional flow has maximum speed of 170 m/s and is directed towards the equator at high and towards the poles at low latitudes. Turbulent magnetic diffusivity is of the order 10¹⁵–10¹⁶ cm²/s. The conditions on Arcturus are not favorable for a circulation‐dominated dynamo (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)