The fraction of solar-type stars having stellar or massive nonstellar companions

The results of the survey for multiplicity by Abt and Levy are used to estimate the fraction of solar-type stars having close companions more massive than 0.01 solar masses. Current knowledge of the multiplicity characteristics of solar-type stars does not require that the fraction be nearly unity.     

High-velocity stars from decay of small stellar systems

In this paper we present numerical results on the decay of small stellar systems under different initial conditions (multiplicity 3 ≤  N  ≤ 10, and various mass spectra, initial velocities and initial configurations). The numerical treatment uses the CHAIN1 code (Mikkola &38; Aarseth). Particular attention is paid to the distribution of high-velocity escapers: we define these as stars with velocity above 30 km s⁻¹. These numerical experiments confirm that small N -body systems are dynamically unstable and produce cascades of escapers in the process of their decay. It is shown that the fraction of stars that escape from small dense stellar systems with an escape velocity greater than 30 km s⁻¹ is ∼1 per cent for all systems treated here. This relatively small fraction must be considered in relation to the rate of star formation in the Galaxy in small groups: this could explain some moderately high-velocity stars observed in the Galactic disc and possibly some young stars with relatively high metallicity in the thick disc.     

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.     

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.     

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.     

Determination of the age of stellar aggregates and flare stars of the galactic field

On the basis of the relationship between the age and the magnitude of the maximal flare amplitude in flare stars found previously by the author we propose a method of determining the age of aggregates. Using new observational data relative to flares in U and B we determine the age of the Cyg T1 association (3.4 · 10 ⁶ years), which differs only slightly from an earlier estimate. We give estimated upper bounds for the ages of some flare stars in the solar neighborhood: UV Ceti, EV Lac, AD Leo, EG Peg, and YZ CMi, and also for seven flare stars of the galactic field. It follows from these results that the ages vary noticeably.Translated fromAstrofizika, Vol. 38, No. 3, 1995.     

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.     

On the generation of the large-scale and turbulent magnetic fields in solar-type stars

It is thought that the large-scale solar-cycle magnetic field is generated in a thin region at the interface of the radiative core (RC) and solar convection zone (SCZ). We show that the bulk of the SCZ virogoursly generates a small-scale turbulent magnetic field. Rotation, while not essential, increases the generation rate of this field.Thus, fully convective stars should have significant turbulent magnetic fields generated in their lower convection zones. In these stars the absence of a radiative core, i.e., the absence of a region of weak buoyancy, precludes the generation of a large-scale magnetic field, and as a consequence the angular momentum loss is reduced. This is, in our opinion, the explanation for the rapid rotation of the M-dwarfs in the Hyades cluster.Adopting the Utrecht's group terminology, we argue that the residual chromospheric emission should have three distinctive components: the basal emission, the emission due to the large-scale field, and the emission due to the turbulent field, with the last component being particularly strong for low mass stars.In the conventional dynamo equations, the dynamo frequencies and the propagation of the dynamo wave towards the equator are based on the highly questionable assumption of a constant . Furthermore, meridional motions, a necessary consequence of the interaction of rotation with convection, are ignored. In this context we discuss Stenflo's results about the global wave pattern decomposition of the solar magnetic field and conclude that it cannot be interpreted in the framework of the conventional dynamo equations.We discuss solar dynamo theories and argue that the surface layers could be essential for the generation of the poloidal field. If this is the case an -effect would not be needed at the RC-SCZ interface (where the toroidal field is generated). The two central problems facing solar dynamo theories may the transport of the surface poloidal field to the RC-SCZ interface and the uncertainty about the contributions to the global magnetic field by the small-scale magnetic features.Visitor, National Solar Observatory, National Optical Astronomy Observatories.The National Optical Astronomy Observatories are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.     

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.     

The precision of radial-velocity determinations of solar-type stars by cross-correlation

The theoretical limit to the precision (defined as the inverse of the square of the random error) of a radial-velocity determination of a solar-type star obtained from the technique of digital cross-correlation is constructed as a function of simple parameters of the recording of a stellar spectrum.The relationship is used to establish conditions for maximizing the precision for a given object, exposure time, and observing instrument. Both fibre-fed and Cassegrain-mounted spectrographs are considered.     

EChO spectra and stellar activity II. The case of dM stars

EChO is a dedicated mission to investigate exoplanetary atmospheres. When extracting the planetary signal, one has to take care of the variability of the hosting star, which introduces spectral distortion that can be mistaken as planetary signal. Magneticvariability has to be taken into account in particular for M stars. To this purpose, assuming a one spot dominant model for the stellar photosphere, we develop a mixed observational-theoretical tool to extract the spot’s parameters from the observed optical spectrum. This method relies on a robust library of spectral M templates, which we derive using the observed spectra of quiet M dwarfs in the SDSS database. Our procedure allows to correct the observed spectra for photospheric activity in most of the analyzed cases, reducing the spectral distortion down to the noise levels. Ongoing refinements of the template library and the algorithm will improve the efficiency of our algorithm.     

Modified methods of stellar magnetic field measurements

The standard methods of the magnetic field measurement, based on an analysis of the relation between the Stokes V‐parameter and the first derivative of the total line profile intensity, were modified by applying a linear integral operator L to both sides of this relation. As the operator L, the operator of the wavelet transform with DOG‐wavelets is used. The key advantage of the proposed method is an effective suppression of the noise contribution to the line profile and the Stokes parameter V. The efficiency of the method has been studied using model line profiles with various noise contributions. To test the proposed method, the spectropolarimetric observations of the A0 star α² CVn, the Of?p star HD 148937, and the A0 supergiant HD 92207 were used. The longitudinal magnetic field strengths calculated by our method appeared tobe in good agreement with those determined by other methods. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The decay of the large-scale solar magnetic field

In the absence of new bipolar sources of flux, the large-scale magnetic field at the solar photosphere decays due to differential rotation, meridional flow, and supergranular diffusion. The rotational shear quickly winds up the nonaxisymmetric components of the field, increasing their latitudinal gradients and thus the rates of diffusive mixing of their flux. This process is particularly effective at mid latitudes, where the rotational shear is largest, so that eventually low- and high-latitude remnants of the initial, nonaxisymmetric field pattern survive. In this paper I solve analytically the transport equation describing the evolution of the large-scale photospheric field, to study its time-asymptotic behavior. The solutions are rigidly rotating, uniformly decaying distributions of flux, wound up by differential rotation and localized near either the equator or the poles. A balance between azimuthal transport of flux by the rotational shear and meridional transport by the diffusion gives rise to the rigidly rotating field patterns. The time-scale on which this balance is achieved, and also on which the nonaxisymmetric flux decays away, is the geometric mean of the short time-scale for shearing by differential rotation and the long time-scale for dispersal by supergranular diffusion. A poleward meridional flow alters this balance on its own, intermediate time-scale, accelerating the decay of the nonaxisymmetric flux at low latitudes. Such a flow also hastens the relaxation of the axisymmetric field to a modified dipolar configuration.     

The decay of the mean solar magnetic field

We have analyzed the effects that differential rotation and a hypothetical meridional flow would have on the evolution of the Sun's mean line-of-sight magnetic field as seen from Earth. By winding the large-scale field into strips of alternating positive and negative polarity, differential rotation causes the mean-field amplitude to decay and the mean-field rotation period to acquire the value corresponding to the latitude of the surviving unwound magnetic flux. For a latitudinally broad two-sector initial field such as a horizontal dipole, the decay is rapid for about 5 rotations and slow with a t ^–1/2 dependence thereafter. If a poleward meridional flow is present, it will accelerate the decay by carrying the residual flux to high latitudes where the line-of-sight components are small. The resulting decay is exponential with an e-folding time of 0.75 yr (10 rotations) for an assumed 15 m s^–1 peak meridional flow speed.E.O. Hulburt Center for Space Research.Laboratory for Computational Physics.     

Variation of stellar coronal activity with age

We investigate the variation in coronal activity of late-type stars with age. We determine the age of the star from the colour-colour diagram (U-B vsB-V). We show that the ratio of X-ray luminosity (L _x) to bolometric luminosity (L _bol) is well correlated with age over several orders of magnitude variations. We find thatL _x/L _bol) decreases with age with a power of –1.55. This is consistant with the expected results from the studies of the variation of the corona with chromosphere and the dependence of chromosophere on age.     

Zeeman-Doppler imaging of solar-type stars: Multi line technique

In this work, a multi-line spectropolarimetric detection using an Echelle spectrograph is described. The polarization of Zeeman effect is detected by the use of more than 200 lines observed in the solar type star, HR1099. Using the statistics analysis in a sample of 200 lines, we found on the average a polarization signal of about 3 × 10^–4.