Thermal generation of magnetic fields with radiation pressure in rotating stars

It has been suggested by Biermann that in rotating stars the electron partial pressure could generate a toroidal magnetic field of a considerable strength. However, Mestel and Roxburgh have shown recently that the generation of such a toroidal magnetic field could almost completely be suppressed when a weak primodial poloidal magnetic field exists in the star. In this paper it is shown that a toroidal magnetic field of a moderate strength could be generated even in the presence of a primodial poloidal magnetic field, if the effect of radiation pressure is taken into consideration. This considered mechanism is effective for moderately massive stars, and numerical estimate indicates that in A type stars a toroidal magnetic field of the order of a thousand gauss can be generated near the surface within the time scale of the evolution of the star.Visiting Scientist to the High Altitude Observatory on leave of absence from the Department of Astronomy, University of Tokyo, Japan.     

Rotational evolution of solar-type stars

The aim of the present investigation has been to consider rotational evolution of solar-type stars simulated by a polytropic model that possesses differential rotation of Clement's type. A properly defined reduction factor moderates the effects of such a rotation. The present treatment is based upon the general Eulerian equation, governing nonuniform (i.e., nonrigid-body) rotation, which has been set up in a previous investigation. Nonconservative terms, arising when stellar wind torque is under consideration, are taken into account. Data available for the viscosity of the Sun are used to construct a plausible viscosity model. Certain assumptions are made that remove the mathematical difficulties and simplify the physical ground. The obtained results are compared to corresponding estimates of recent observations.     

The age decay of stellar magnetic activity in MS field solar-type stars

The age decay of two indicators of the stellar magnetic activity (λ 2800 Mgii emission flux and λ 10830 Hei equivalent width) have been studied for field solar-type stars. The Li abundance has been used, in most of the cases, as a stellar age indicator. A calibrated decay law for Mgii has been determined and compared with a similar one, recently published, for the Caii K emission. The greater scatter in the Hei results is atributed to the different rotation rates.     

Filaments and large-scale magnetic fields

We have used Stanford magnetic field maps to construct distributions of longitudinal magnetic field gradients in the neighbourhood of polarity inversion lines. The distributions were constructed with proper account of the type of the polarity inversion lines and of the existence or absence of dark filaments above them. It is shown that for polarity inversion lines that pass inside active regions or on their boundary, grad B_II distributions for portions of the lines with persisting filament are shifted toward lower values of gradient as compared with grad B_II distributions for portions of the lines without filaments. The influence of the spatial resolution of the magnetograms upon polarity inversion line characteristics is discussed.     

Dynamo saturation in rapidly rotating solar-type stars

The magnetic activity of solar-type stars generally increases with stellar rotation rate. The increase, however, saturates for fast rotation. The Babcock-Leighton mechanism of stellar dynamos saturates as well when the mean tilt angle of active regions approaches ninety degrees. Saturation of magnetic activity may be a consequence of this property of the Babcock-Leighton mechanism. Stellar dynamo models with a tilt angle proportional to the rotation rate are constructed to probe this idea.Two versions of the model- treating the tilt angles globally and using Joy's law for its latitude dependence- are considered. Both models show a saturation of dynamogenerated magnetic flux at high rotation rates. The model with latitude-dependent tilt angles also shows a change in dynamo regime in the saturation region. The new regime combines a cyclic dynamo at low latitudes with an(almost) steady polar dynamo.     

Rotation and activity in the solar-type stars of NGC 2547

We present high-resolution spectroscopy of a sample of 24 solar-type stars in the young (15–40 Myr), open cluster NGC 2547. We use our spectra to confirm cluster membership in 23 of these stars, to determine projected equatorial velocities and chromospheric activity, and to search for the presence of accretion discs. We find examples of both fast ( v _e sin  i >50 km s⁻¹) and slow ( v _e sin  i <10 km s⁻¹) rotators, but no evidence for active accretion in any of the sample. The distribution of projected rotation velocities is indistinguishable from the slightly older IC 2391 and IC 2602 clusters, implying similar initial angular momentum distributions and circumstellar disc lifetimes. The presence of very slow rotators indicates either that long (10–40 Myr) disc lifetimes or internal differential rotation are needed, or that NGC 2547 (and IC 2391/2602) were born with more slowly rotating stars than are presently seen in even younger clusters and associations. The solar-type stars in NGC 2547 follow a similar rotation–activity relationship to that seen in older clusters. X-ray activity increases until a saturation level is reached for v _e sin  i >15–20 km s⁻¹. We are unable to explain why this saturation level, of log( L _x L _bol)≃−3.3, is a factor of 2 lower than in other clusters, but rule out anomalously slow rotation rates or uncertainties in X-ray flux calculations.     

Variations in surface activity of the sun and solar-type stars

Twenty-five-year records of relative Caii H and K emission fluxes of lower Main-Sequence stars have been measured at Mount Wilson Observatory and reveal surface activity in most of the older G- and K-type dwarf stars that is similar to the aperiodical activity cycle of the contemporary Sun (i.e., the cyclic and the occasional episode of reduced activity in the past few centuries). We find an inverse relationship between the amplitude of the activity cycle and the length of the cycle for the ensemble of those solar-type stars. We also find a similar relationship using the 250-year sunspot record (Cycles 1 to 21). The similarity between the two inverse relationships for the solar-type stars observed for 25 years and the Sun for a longer interval of time may suggest one common underlying physical mechanism that is responsible for the variations in surface activity ranging from decades to centuries.Also at Center for Excellence in Information Systems at Tennessee State University.     

Fourier analysis of spectra of solar-type stars

Fourier transform techniques were used to determine the macroturbulent velocity under the condition that mictoturbulent and stellar rotation velocities are not known. In order to distinguish the effects of rotation from macroturbulence effects in slowly rotating stars, primarily the main lobe of residual Fourier transforms of the observed lines, which were taken from the solar spectrum and the spectra of two other stars, was used. This case of Fourier analysis of spectral lines is the most complicated one. The end results were in a satisfactory agreement with the data obtained using different methods. The average values of microturbulent, macroturbulent, and rotation velocities were 0.85, 2.22, and 1.75 km/s for the Sun as the star; 0.58, 1.73, and 0.78 km/s for HD 10700; and 1.16, 3.56, and 6.24 km/s for HD 1835. It was found that the macroturbulent velocity decreases with height in the atmosphere of the Sun and HD 1835. In the case of HD 10700, the macroturbulent velocity did not change with height, and the determined rotation velocity was two times lower than the one obtained using other methods. It was concluded that Fourier transform techniques are suitable for determining the velocities in atmospheres of solar-type stars with very slow rotation.     

Comparison of UV spectra from solar-type stars

We compare high-resolution spectra from the Sun and the four solar-type stars 16 Cyg A, 16 Cyg B, HD 32008, HD 34411 obtained with IUE in the wavelength range 2650–2930 Å. The comparison is made for peak intensities between absorption lines. At the level of accuracy of the IUE observations, the stars 16 Cyg A, 16 Cyg B, HD 34411 are indistinguishable from the Sun, in particular the Mgii resonance line profiles are identical. HD 32008 is not a solar analog but is evidently of late G to early K spectral type.Based on observations with the International Ultraviolet Explorer collected at the Villafranca Satellite Tracking Station of the European Space Agency.     

Lithium abundances of 21 solar-type stars near the Sun

We present high-resolution spectroscopic observations for a sample of 21 young, solar-type stars near the Sun recently discovered in the X-ray wavelength range during the ROSAT all-sky survey. Based on these observations, we derive the lithium (Li) abundances of these 21 sample stars. Using the lithium abundances and the X-ray luminosity, we investigated the relationship between the Li abundances and the X-ray activity. We found a clear correlation between the lithium abundances and the X-ray luminosity: as the X-ray luminosity became stronger, the lithium abundance decreases in our sample stars. Our sample results provide further evidence that a correlation appears to exist between Li abundances, X-ray activity and age for a large number of solar-type stars. The results also confirm the presence of very active young stars close to the Sun, in agreement with recent findings from UV and X-ray surveys.     

On the turbulent decay of strong magnetic fields and the development of sunspot areas

This paper deals with the conception that two-dimensional turbulence is present in a sunspot where the magnetic field is strong. This conception is based upon the incapacity of even a strong magnetic field to influence an arbitrary two-dimensional turbulence, if the magnetic field is parallel to a constant direction and the motion occurs in planes orthogonal to it. It is, moreover, shown that such a two-dimensional turbulence provides for a turbulent decay of the magnetic field. The decay rate possesses nearly the same dependence on the scales as for three-dimensional turbulence. Finally, the turbulent decay is studied by investigating a simple model and comparing the results with those deduced by Bumba from the observed decay of sunspot groups areas. By means of our conception even a good quantitative agreement is stated.     

Zeeman-doppler imaging: A new option for magnetic field study of Ap and solar-type stars

In the task of studying stellar magnetic fields, polarimetric methods have been intensively used in Ap stars. But the observational material classically used to reconstruct stellar magnetic structures (average longitudinal magnetic field as a function of rotational phase) is not rich enough in spatial information to derive geometries more complex than centered or decentered dipoles.In solar-type stars, all evidences of activity recently detected on their surfaces (starspots, flares, ...) indicate they are most likely magnetic stars. But polarimetric methods have always failed in these stars, probably due to the complex magnetic topologies encountered which even prevented until now a simple detection (Borra, Edwards, and Mayor, 1984). With the Zeeman broadening measurement technique proposed by Robinson (1980), no reliable results can be derived for rapid rotators, which are otherwise presumed to be the best candidates for magnetic detections. Once more, if temperature inhomogeneity charts are already available for solar-type stars (Vogt, 1987), spatial information on their magnetic distributions has conversely not yet been obtained.The new option, recently proposed by Semel (1989) and qualified by Donati, Semel, and Praderie (1989), is based on the rotational modulation study of a rapid rotator Stokes parameter V(), obtained with both high spectral resolution R, and high signal-to-noise ratio S/N. Since the magnetic information used refers to localized strips on the stellar disc (as a consequence of the star rotation), multipolar structures can thus be resolved.A new instrumentation and observing procedure have been defined for ZDI, in order to obtain very high S/N data. The method has been successfully tested on two bright magnetic Ap stars: a magnetic detection was obtained on UMa and a 15-point phase coverage of ² CVn is available for the reconstruction of complete 2D abundance and magnetic mappings of its photosphere.Concerning solar-type stars, a numerical simulation was carried out in order to determine the observational constraints required for the detection of typical magnetic field similar to those reported in slow rotators with the Robinson method (Saar, 1988). The specifications needed are S/N 400 per 40 mÅ pixel and R - 6 × 10⁴.     

Statistics of magnetic fields for OB stars

Based on an analysis of the catalog of magnetic fields, we have investigated the statistical properties of the mean magnetic fields for OB stars. We show that the mean effective magnetic field B of a star can be used as a statistically significant characteristic of its magnetic field. No correlation has been found between the mean magnetic field strength B and projected rotational velocity of OB stars, which is consistent with the hypothesis about a fossil origin of the magnetic field. We have constructed the magnetic field distribution function for B stars, F(B), that has a power-law dependence on B with an exponent of ≈−1.82. We have found a sharp decrease in the function F(B) for B ⩽ 400 G that may be related to rapid dissipation of weak stellar surface magnetic fields.     

On the first generation of stars

We argue that the first stars may have spanned the conventional mass range rather than be identified with the very massive objects  (∼100–10³ M_⊙)  favoured by numerical simulations. Specifically, we find that magnetic field generation processes acting in the first protostellar systems suffice to produce fields that exceed the threshold for magneto-rotational instability (MRI) to operate, and thereby allow the MRI dynamo to generate equipartition-amplitude magnetic fields on protostellar mass scales below  ∼50 M_⊙  . Such fields allow primordial star formation to occur at essentially any metallicity by regulating angular momentum transfer, fragmentation, accretion and feedback in much the same way as occurs in conventional molecular clouds.     

The evolution of the magnetic fields of neutron stars

Observational evidence, and theoretical models of the magnetic field evolution of neutron stars is discussed. Observational data indicates that the magnetic field of a neutron star decays significantly only if it has been a member of a close interacting binary. Theoretically, the magnetic field evolution has been related to the processing of a neutron star in a binary system through the spin evolution of the neutron star, and also through the accretion of matter on the neutron star surface. I describe two specific models, one in which magnetic flux is expelled from the superconducting core during spin-down, via a copuling between Abrikosov fluxoids and Onsager-Feynman vortices; and another in which the compression and heating of the stellar crust by the accreted mass drastically reduces the ohmic decay time scale of a magnetic field configuration confined entirely to the crust. General remarks about the behaviour of the crustal field under ohmic diffusion are also made.     

On the choice of boundary conditions when studying the turbulent diffusion of magnetic fields

In this paper, we present a discussion and results pertaining to the role of the choice of the boundary conditions in models that seek to address the form of the quenching of the turbulent magnetic diffusivity. We show that, for several natural choices of boundary conditions frequently selected in numerical investigations, 'weak' magnetic fields have a profound effect on the rate of diffusion. We go on to discuss further results that suggest a way by which such a catastrophic quenching may be avoided.     

The physics of strong magnetic fields in neutron stars

In this paper we present a new result, namely that the primal magnetic field of the collapsed core during a supernova explosion will, as a result of the conservation of magnetic flux, receive a massive boost to more than 90 times its original value by the Pauli paramagnetization of the highly degenerate relativistic electron gas just after the formation of the neutron star. Thus, the observed super-strong magnetic field of neutron stars may originate from the induced Pauli paramagnetization of the highly degenerate relativistic electron gas in the interior of the neutron star. We therefore have an apparently natural explanation for the surface magnetic field of a neutron star.