arXiv subjects

H. C. Spruit

Publications and source records attributed to H. C. Spruit.

72 records · Page 4Linked to original sources

arXiv2000

Circumbinary disks and cataclysmic variable evolution

The influence of a circumbinary (CB) disk on the evolution of cataclysmic variable (CV) binary systems is investigated. We show that CB mass surface densities sufficient to influence the evolution rate are plausibly provided by the outflows observed in CVs, if the net effect of these winds is to deliver $10^{-4}$--$10^{-3}$ of the mass transfer rate to the CB disk. The torque exerted by the CB disk provides a positive feedback between mass transfer rate and CB disk mass which can lead to mass transfer rates of $\sim 10^{-8} -10^{-7} \mpy$. This mechanism may be responsible for causing the range of variation of mass transfer rates in CV's. In particular, it may explain rates inferred for the novalike variables and the supersoft X-ray binary systems observed near the upper edge of the period gap ($P \sim 3 - 4$ hr), as well as the spread in mass transfer rates above and below the period gap. Consquences and the possible observability of such disks are discussed.

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arXiv2000

Large scale magnetic fields and their dissipation in GRB fireballs

We consider possible geometries of magnetic fields in GRB outflows, and their evolution with distance from the source. For magnetically driven outflows, with an assumed ratio of magnetic to kinetic energy density of order unity, the field strengths are sufficient for efficient production of gamma-rays by synchrotron emission in the standard internal shock scenario, without the need for local generation of small scale fields. In these conditions, the MHD approximation is valid to large distances (>10^19cm). In outflows driven by nonaxisymmetric magnetic fields, changes of direction of the field cause dissipation of magnetic energy by reconnection. This dissipation takes place outside the photosphere of the outflow, and can convert a significant fraction of the magnetic energy flux into radiation.

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arXiv2000

The sign of temperature inhomogeneities deduced from time-distance helioseismology

Inhomogeneities in wave propagation conditions near and below the solar surface have been detected by means of time-distance helioseismology. Here we calculate the effect of temperature inhomogeneities on the travel times of sound waves. A temperature increase, e.g. in active regions, not only increases the sound speed but also lengthens the path along which the wave travels because the expansion of the heated layers shifts the upper turning of the waves upward. Using a ray tracing approximation we find that in many cases the net effect of a temperature enhancement is an {\it increase} of the travel times. We argue that the reduced travel times that are observed are caused by a combination of magnetic fields in the active region and {\it reduced} subsurface temperatures. Such a reduction may be related to the increased radiative energy loss from small magnetic flux tubes.

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arXiv2000

Radiatively inefficient accretion disks

Radiatively inefficient (or advection dominated) disks are discussed at an introductory level. Ion supported and radiation supported flows are discussed, the different consequences of advection dominated flows onto black holes vs. solid surfaces (neutron stars, white dwarfs), hydrodynamics, the role of the ratio of specific heats, and the possible connection between ADAFs and outflows.

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arXiv2000

Accretion Disks

In this lecture the basic theory of accretion disks is introduced, with emphasis on aspects relevant for X-ray binaries and Cataclysmic Variables.

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arXiv2000

Spirals and the size of the disk in EX Dra

Observations at high spectral and temporal resolution are presented of the dwarf nova EX Dra in outburst. The disk seen in the He I line reconstructed by Doppler tomography shows a clear two-armed spiral pattern pointing to spiral shocks in the disk. The Balmer and He II maps also give evidence for the presence of spirals. The eclipse as seen in the red continuum indicates a disk radius of 0.31 times the orbital separation, which might be large enough to explain the observed spiral shocks through excitation by the tidal field of the secondary. The eclipse in the Balmer line profiles, well resolved in our observations, indicates a somewhat smaller disk size (0.25). We discuss the possibility that this is related to an optical depth effect in the lines.

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arXiv2000

Jets from compact objects

Some topics in the theory of jets are reviewed. These include jet precession, unconfined jets, the origin of knots, the internal shock model as a unifying theme from protostellar jets to Gamma-ray bursts, relations between the Blandford-Znajek and MHD disk-wind models, and jet collimation in magnetic acceleration models.

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arXiv2000

Theory of solar luminosity variations

The theory of stellar structure can be used to identify the most plausible mechanisms for the irradiance variations associated with the solar cycle. Changes in surface emissivity, i.e. the reduced cooling in spots and enhanced emission by small scale magnetic fields, are the most effective mechanisms and account for most of the observed variation. Helioseismology will soon be able to test the consequences of changes in surface emissivity, and distinguish them from other scenarios for irradiance variability.

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arXiv2000

Magnetosphere of Oscillating Neutron Star. Nonvacuum Treatment

We generalize a formula for the Goldreich-Julian charge density (ρ), originally derived for rotating neutron star, for arbitrary oscillations of a neutron star with arbitrary magnetic field configuration under assumption of low current density in the inner parts of the magnetosphere. As an application we consider toroidal oscillation of a neutron star with dipole magnetic field and calculate energy losses. For some oscillation modes the longitudinal electric field can not be canceled by putting charged particles in the magnetosphere without a presence of strong electric current j = (c/(ωr))ρc. It is shown that the energy losses are strongly affected by plasma in the magnetosphere, and cannot be described by vacuum formulas.

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arXiv2000

The X-ray spectrum of a disk illuminated by ions

The X-ray spectrum from a cool disk embedded in an ion supported torus is computed. The interaction of the hot ions with the disk increases the hard X-ray luminosity of the system}. A surface layer of the disk is heated by the protons from the torus. The Comptonized spectrum produced by this layer has a shape that depends only weakly on the incident energy flux and the distance from the accreting compact object. It consists of a `blue bump' of unComptonized soft photons and a flat high energy tail, reminiscent of the observed spectra. The hard tail becomes flatter as the thermalization depth in the cool disk is increased. Further evidence for ion illumination are the Li abundance in the secondaries of low mass X-ray binaries and the 450 keV lines sometimes seen in black-hole transient spectra.

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arXiv1999

Differential rotation and magnetic fields in stellar interiors

The processes contributing to the evolution of an initially weak magnetic field in a differentially rotating star are reviewed. These include rotational smoothing (akin to convective expulsion) and a list of about 5 instabilities, among them magnetorotational instability, byoyancy instability, and pinch-type instabilities. The important effects of thermal and magnetic diffusion on these instabilities are analyzed in some detail. The first instability to set in is a pinch-type instability. It becomes important in modifying the field configuration before magnetic buoyancy-driven instabilities set in. The evolution of an initially strong field remains a more open question, including the old problem whether dynamically stable magnetic equilibria exist in stars.

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arXiv1998

Gamma-ray bursts from X-ray binaries

A weakly magnetized ($\sim 10^7$G) neutron star, slowly spun up by accretion in an X-ray binary, crosses the instability boundary for r-mode instability at P=1--2 msec. The amplitude of the oscillation, which initially increases only at the spinup time scale, is secularly unstable due to the negative temperature dependence of the viscosity in neutron star matter, and diverges after a few hundred years. Angular momentum loss by the gravitational wave causes strong differential rotation, in which the magnetic field is wound up to $10^{17}$G on a time scale of a few months. When this field becomes unstable to buoyancy instability, a surface field strength of a few $10^{16}$G is produced on a time scale of seconds, which then powers a GRB with energies of $\sim 1-8\times 10^{51}$ and duration of 1--100 sec.

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arXiv1998

Fast maximum entropy Doppler mapping

A numerical code is described for constructing Doppler maps from the orbital variation of line profiles of (mass transfering) binaries. It uses an algorithm related to Richardson-Lucy iteration, and is much faster than the standard algorithm used for ME problems. The method has been tested on data of cataclysmic variables (including WZ Sge and SS Cyg), producing maps of up to 300X300 points. It includes an IDL-based set of routines for manipulating and plotting the input and output data, and can be downloaded from http://www.mpa-garching.mpg.de/~henk

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arXiv1998

Origin of the rotation rates of single white dwarfs

I argue that the rotation of white dwarfs is not a remnant of the angular momentum of their main sequence progenitors but a result of the mass loss process on the AGB. Weak magnetic fields, if present in stellar interiors, are likely to maintain approximately uniform rotation in stars, both on the main sequence and on the giant branches. The nearly uniform rotation of the core of the Sun is evidence for the existence of such fields. Exactly axisymmetric mass loss on the AGB from uniformly rotating stars would lead lead to white dwarfs with very long rotation periods ($>$ 10 yr). Small random non-axisymmetries ($\sim 10^{-3}$) in the mass loss process, on the other hand, add sufficient angular momentum to explain the observed rotation periods around one day. The process illustrated with a computation of the probability distribution of the rotation periods under the combined influence of random forcing by weak nonaxisymmetries and angular momentum loss in the AGB superwind. Such asymmetries can in principle be observed by proper motion studies of the clumps in interferometric images of SiO maser emission.

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arXiv1996

Convection in stellar envelopes: a changing paradigm

Progress in the theory of stellar convection over the past decade is reviewed. The similarities and differences between convection in stellar envelopes and laboratory convection at high Rayleigh numbers are discussed. Direct numerical simulation of the solar surface layers, with no other input than atomic physics, the equations of hydrodynamics and radiative transfer is now capable of reproducing the observed heat flux, convection velocities, granulation patterns and line profiles with remarkably accuracy. These results show that convection in stellar envelopes is an essentially non-local process, being driven by cooling at the surface. This differs distinctly from the traditional view of stellar convection in terms of local concepts such as cascades of eddies in a mean superadiabatic gradient. The consequences this has for our physical picture of processes in the convective envelope are illustrated with the problems of sunspot heat flux blocking, the eruption of magnetic flux from the base of the convection zone, and the Lithium depletion problem.

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arXiv1996

Magnetohydrodynamic winds and jets from accretion disks

The theory of magnetically accelerated outflows and jets from accretion disks is reviewed at an introductory level, with special attention to problem areas like the launching conditions of the flow at the disk surface, stability of the magnetic field, and collimation mechanisms. This text will appear in R.A.M.J. Wijers, M.B. Davies and C.A. Tout, eds., Physical processes in Binary Stars, Kluwer Dordrecht, 1996 (NATO ASI series).

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arXiv1995

Interchange instability in an accretion disc with a poloidal magnetic field

We investigate the stability to nonaxisymmetric perturbations of an accretion disc in which a poloidal magnetic field provides part of the radial support against gravity. Interchange instability due to radial gradients in the magnetic field are strongly stabilized by the shear flow in the disc. For smooth field distributions this instability is restricted to discs in which the magnetic energy is comparable to the gravitational energy. An incompressible model for the instability akin to the Boussinesq approximation for convection is given which predicts the behaviour of the instability accurately. Global axisymmetric disturbances are also considered and found to be stable for a certain class of models. The results indicate that accretion discs may be able to support poloidal fields which are strong enough to suppress other forms of magnetic instability. These strong and stable field distributions are likely to be well suited for the magnetic acceleration of jets and winds.

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arXiv1995

Accretion Disks

This is an introduction to accretion disk theory, with emphasis on aspects relevant for X-ray Binaries and Cataclysmic Variables. The text corrects some mistakes in an earlier version, which appeared in 'Lives of Neutron Stars', A. Alpar, Ü. Kizilo\u glu and J. van Paradijs (eds.), Kluwer, Dordrecht (NATO ASI series, 1994).

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