Spots,activity cycles,and differential rotation on cool stars

The first results are reported from a search for activity cycles in stars similar to the sun based on modelling their spotting with an algorithm developed at the Crimean Astrophysical Observatory. Of the more than thirty program stars, 10 manifested a cyclical variation in their central latitudes and total starspot area. The observed cycles have durations of 4–15 years, i.e., analogous to the 11 year Schwabe sunspot cycle. Most of the stars have a rough analog of the solar butterfly pattern, with a reduction in the average latitude of the spots as their area increases. A flip-flop effect during the epoch of the maximum average latitude is noted in a number of these objects (e.g., the analog LQ Hya of the young sun or the RS CVn-type variable V711 Tau), as well as a reduction in the photometric rotation period of a star as the spots drift toward the equator, an analog of the differential rotation effect in the sun. Unlike in the sun, the observed spot formation cycles do not correlate uniquely with other indicators of activity— chromospheric emission in the CaII HK lines (Be Cet, EK Dra, Dx Leo), H line emission (LQ Hya, VY Ari, EV Lac), or cyclical flare activity (EV Lac). In V833 Tau, BY Dra, EK Dra, and VY Ari short Schwabe cycles coexist with long cycles that are analogous to the Gleissberg solar cycle, in which the spotted area can approach half the entire area of the star.Translated from Astrofizika, Vol. 48, No. 1, pp. 29–43 (February 2005).     

Rotational Brightness Modulation and Starspots on the RS CVn-type Stars IN Com, IL Com, UX Ari, and V711 Tau

UBVRI photometric observations and models of spotting are presented for four noneclipsing RS CVn systems: IN Com (G5III/IV), IL Com (F8V+F8V), UX Ari (K01V+G5V), and V711 Tau (K1IV+G5V). A low amplitude variability caused by cold (T=1700K)spots which can occupy up to 19% of the star's surface is confirmed for the little-studied star IL Com. Long-term light curves are constructed and the stellar magnitudes and color indices of the unspotted photospheres are estimated for IN com, UX Ari, and V711 Tau. It is shown that UX Ari becomes bluer with decreasing brightness, so its variability cannot be fully explained in terms of cold spots. Models of spotting on In Com and V711 Tau are constructed from the full set of available photometric observations. The spots on both of these variables lie at middle latitudes and occupy up to 22% (In Com) and 33% (V711 Tau) of the stars' surfaces. Both stars manifest a tendency for the width of the spots to decrease as their area increases. This is a crude analog of the Maunder butterflies. These stars experience cyclical spot activity and have a differential rotation of the type found on the sun.     

Stellar irradiance variations due to surface temperature inhomogeneities

Observations of rotational modulation of continuum brightness and photospheric and chromospheric spectral-line profiles of late-type stars indicate the presence of very inhomogeneous surface temperature distributions. We present three stellar examples (VY Ari, HR 7275, HU Vir) where time-series photometry is used to trace the evolution of spotted regions. Simultaneous spectroscopy and Doppler imaging for one of the three stars (HU Virgo, Fig. 1) makes it possible to compute the temperature distribution of the photosphere and the relative intensity distribution of parts of the chromosphere (from CaII K and H line profiles). The combination of time-series spot modeling and Doppler imaging enabled us to determine thesign and amount of differential surface rotation on HU Vir. We found a big, cool polar spot (see figure below) and a differential (surface) rotation law where higher-latitude regions rotate faster than lower-latitude regions (opposite to what we see on the Sun). Currently, this ensemble of techniques - time-series photometry and photospheric and chromospheric Doppler imaging - is only applicable to stars overactive by approximately a factor of 100 as compared to the active Sun, e.g. the evolved components in RS CVn-type binaries and some rapidly-rotating, single, pre-main sequence stars or giant stars. Stellar rotation is a fundamental parameter for (magnetic) activity. Starspots, or any other surface inhomogeneities, allow one to derive very precise stellar rotation rates and, if coupled with seismological observations of solar-type stars, could provide information on the internal angular momentum distribution in overactive late-type stars.To be published in Astronomy & Astrophysics.     

Spots and active regions on the emission-line, red dwarf star v 775 her

Quasi-simultaneous photoelectric and spectroscopic observations of the active spotted star V 775 Her have been made for the first time and showed an increase in the equivalent width of the pure emission in the H_a line with a decrease in the star’s brightness. Such an increase was due to an increase in electron density in the active regions and demonstrates a connection between active regions and cool spots. The system’s photometric variability is fully described within the framework of a zonal model. The spotted regions occupy up to 42% of the star’s total surface if the temperature difference between the quiet photosphere and a spot is about 900 K. The ratio of the masses of the components of V 775 Her is estimated for the first time. Translated from Astrofizika, Vol. 43, No. 3, pp. 339-351, July–September, 2000.     

Butterfly diagrams of strongly spotted late type stars

At present, long-term (over 30 years) multicolor photometric observations give the possibility to determine general properties of spotted areas on late-type stars. Star-spot modeling from broadband photometric data has been carried out by Alekseev and Gershberg since 1996 under the assumption that spots are situated in two latitudinal zones. Here we propose a new analysis of their results for several G and K dwarf stars with high irregular activity. On these stars, EK Dra, VY Ari, V775 Her, and V833 Tau, two spot belts exist separately and do not merge into a single equatorial active region, as occurs on cooler red-dwarf stars. The zonal spottedness models allow us to fit simultaneously both rotational modulation and long-term variability of stellar brightness. These models give evidence for an equatorward drift of the lower latitude boundary of the spotted region, φ₀, during the rising phase of activity, beyond any possible errors concerned with our methodology. In order to evaluate the drift rate we introduce the concept of `effective' spot belt, whose width is independent of longitude. This permits us to construct butterfly diagrams for stellar spots. The equatorward drift rates of the lower boundary of the spotted region D=dφ_low/dt are (− 1)–(− 2) deg year⁻¹ in the years of increasing spottedness. These values are less than the analogous solar one D≈−4 deg year⁻¹ for the rising phase of the cycle. Thus, cyclic activity can be revealed from butterfly diagrams and derived drifts of starspots prior to a possible detection from the spectral analysis of photometric variability. Finally, we briefly discuss a possible explanation of high-latitude activity and surface drifts of starspots in the framework of the current state of dynamo theory.     

Spots and Active Regions on Emission Stars. III. LQ Hya

Quasisimultaneous photoelectric, polarimetric, and spectroscopic observations of the active single red dwarf LQ Hya are presented. The photometric variability of LQ Hya is fully described by a zone model. Spotted regions occupy up to 25% of the entire surface of the star with a temperature difference of 800 K between the spots and the calm photosphere. The spots are localized in the middle and low latitudes. A cyclical variation is observed in the total areas of the spotted regions and in the average latitude of the spots. The most spotted regions, local magnetic fields, and chromospherically active regions tend to concentrate in the same distinct active longitudes.     

A Photometric Investigation of the Eclipsing System OO Aql

The paper is concerning with the light curve analysis of an active binary system of OO Aql. Basic orbital and physical parameters of this system were estimated by analysing photometric observations by Lafta and Grainger (1985) and Essam et al. (1992). Both groups of observations gave quite different light curves with a conspicuous asymmetry arising from unequal height of successive maxima. A slightly changed programme by Djuraevi (1992a) was applied. The programme was based on the Roche model and it included an option of overcontact configuration and active (bright or dark) spotted regions on the system's components. An inverse-problem method by Djuraevi (1992b) was used to estimate the parameters of the system and active regions. The light-curve analysis suggested a significant change in the system. For Lafta and Grainger's light curves our analysis showed the presence of a dark spotted area near the polar region of the primary component. This dark area covered about 22% of the primary's hemisphere. Essam et al.'s light curves had very complex shape. In this case the mechanism of the mass transfer and the exchange of the thermal energy between the components could produce a bright-spot (bs) area on the secondary, near the Lagrange equilibrium point L₁ around the neck region between the stars. To explain a sharp increase of the system's brightness in the B filter immediately after the secondary minimum, we had to introduce also a hot-spot (hs) area of a high temperature contrast on the secondary. With the increase of the brightness in the V-filter being of much lower intensity, this hot spot region could be taken as a local eruption. The behaviour of the B–V colour index with the orbital phase is in favour of this hypothesis also. The obtained results show that the light-curve changes are due to development and moving of active spot regions on the components.     

A simultaneous X-ray and optical study of TT Arietis

Prelimary results of an extended program of coordinated X-ray and optical observations of TT Ari are presented. The object was observed on August 21/22 1985 both in X-rays (EXOSAT) and optical range, about 100 days after the return to the active state. The first detailed simultaneous study of TT Ari in active state indicates the presence of strongly absorbing structure in the system.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

The role of the magnetic field intensity and geometry in the type III burst generation

We study the association of type III bursts related to H flares in different magnetic environments in the period 1970–1981. Special attention is paid to flares which partly cover a major spot umbra (Z-flares). In particular we consider the location of the spots in the active regions and the magnetic field intensities of spots covered by a ribbon. The association rate with type III bursts decreases to 17% when the flare is located inside the bipolar pattern of a large active region, compared with an association rate of 54% when the flare is situated outside it. The association rate increases with the magnetic field intensity of the spot covered by H emission; this is most clearly revealed for the flares occurring outside the bipolar pattern of active regions. Ninety-three percent of the flare-associated type III burst were accompanied by 10 cm radio bursts. For the most general case in which a flare is developing anywhere in an active region, the association with type III bursts generation increases with the increasing magnetic field intensity of the main spot of the group.     

Active regions

A summary of data on the occurrence of flares and the development of active regions, based on cinematographic data is given. It is shown that flare frequency is determined by the orientation of the magnetic axis relative to the direction of solar rotation and the morphology of the magnetic field as seen in H. In particular, flares are most numerous in simple round spots with reversed polarity nearby, although they may also be frequent in complex spots with polarity reversal.Important solar active regions are shown to evolve principally along two lines; typically they appear as bright regions with loops and grow rapidly to stable bipolar magnetic form. Important activity will occur as the result of later growth of following polarity ahead of the main spots, or some other source of reversal. However, some groups appear as reversed polarity regions and grow rapidly to a level of extreme activity.A series of papers giving case histories is promised.     

RR Ari: Not a variable star

NewV standard measures of RR Ari are presented and the star is found not to be variable. The light of the star Ari BCSV 5954 seems also to be constant.     

Near infrared photometry of some RS CVn systems and candidates

Infrared observations in the J, H, and K bands were made on eleven RS CVn systems. Infrared (IR) excess 0.3 mag in the J, H, and K bands was observed from the three systems UX Ari, HR 1099 and CrB. Similar observations were also made on the RS CVn candidates HD 26354, 39937, 42449, 51268, and 86005.The IR excess in the RS CVn systems for which near infrared photometric data are available is found to be correlated with the maximum of optical wave amplitude of these systems.The observed excess cannot be explained on the basis of a single process such as: (i) emission from cool spots on the photosphere; (ii) free-free emission from an optically thin circumstellar plasma; or (iii) dust grain emission from circumstellar material.On leave from TIFR, Bombay, India.     

Spectropolarimetric Observations of the Unique White Dwarf GD 356 on the 6-m BTA

The results of spectropolarimetric observations of a unique magnetic white dwarf with emission lines, GD 356, on the 6-m BTA are presented. The star's magnetic field is estimated and it is shown not to rotate over times of 5 yr. Allowance for magnetic deformation of the star's surface, resulting an a nonuniform temperature distribution over its surface, showed that in the region of the magnetic poles the conditions may be satisfied for a radiative diskon in the form of a plasma jet, originating from the action of cyclotron radiation pressure. It is shown that the effect of nonuniformity of the surface temperature distribution of the white dwarf can produce the observed broad-band and resonance polarization of the stellar emission.     

Plasma motion in umbrae and the surrounding photosphere derived from spectroscopic Doppler measurements and tracer measurements of spots

The rotational velocity of the Sun is determined by sunspot tracings and by spectroscopic measurements of the photospheric plasma using the non-Zeeman-split line Fe i 5576 and absolute iodine reference. Stationary line shifts as limb-effect and longperiodical shifts introduced by supergranulation are discussed. The dependence on solar activity as Ca⁺ emissivity and magnetic fields is investigated including line asymmetries. The results are: (a) The non active photospheric regions rotate with 1995 ± 30 m s^-1. Solar active regions yield a 60 m s^-1 higher value. (b) In quiet regions the absolute limb shift varies between 170 m s^-1 at the line core and 310 m s^-1 at I/I _cont 0.8 (C-shape); thus the limb shift is mainly due to entire line shifts. (c) In solar active regions (close to spots) asymmetries are widely reduced in line cores; this effect cannot be associated with a variation of the limb effect due to a large scatter of Doppler shifts near spots. (d) A reduced limb shift of 50 m s^-1 is found in network boundaries and is mainly due to a small scale downflow. (e) Observations with a smaller influence of stray light yield symmetric profiles in umbrae. (f) Differences between umbral rotation rates from tracer and spectroscopic measurements do not exceed 20 m s^-1, when considering straylight. The rotational velocity from umbrae exceeds that from the photosphere by 30–60 m s^-1. Some individual spots yield nearly the same rotation rate as the photosphere.     

Flows,flares, and formation of umbrae and light bridges in BBSO region No. 1167

We present high-resolution observations of the large active region BBSO No. 1167 (Boulder No. 5060) which cast new light on the structure of sunspot regions. We obtained excellent data, highlighted by videomagnetograms (VMG) obtained with our 65-cm telescope, which give unprecedented spatial resolution, about 0.5' for much of two 11-hr periods. This permitted us to see details of the field evolution and flows in the AR. The H filtergrams and D3 filtergrams permit study of these magnetic changes compared to spots and chromospheric structure.The region was a huge but simple active region (CMP July 2, 1988) in which we observed rapid flux emergence for several days. Because the new flux generally matched the old, there were few large flares. However, there were 14 flares on June 28 and 29, mostly in two sites. The first site was a spot which already existed when the active region appeared on the east limb. This site showed little change of magnetic structure during our observing period. The second site is an area disturbed by new flux emergence, which included a spot which formed and disappeared in two days, and a rapidly moving p spot. Flares ocurring at one site almost always produced footpoints at the other. The delay between flash phases of the same flare at the two sites ranges from 40 to 160 s.The magnetograms show complex fine structure, with some closely interwined regions of opposite polarity. In a region of new flux emergence, positive (leading polarity) flux flows along elongated channels immersed in the negative flux. Moving magnetic features occur around all of the spots.We point out other interesting aspects of this large region: (1) While there is extensive penumbra around the main umbrae, there is also significant penumbra apparently unrelated to any spot. These unusual penumbrae are either due to flux returning to the surface, flux left behind by the moving umbra, or associated with pores that appear and disappear. (2) We observed umbrae to move faster than the accompanying penumbrae, and concluded that penumbrae are not a simple extension of the umbra. (3) We found that combining spots of the same polarity do not completely merge, but are always separated by a thin light bridge. This means that the emerging flux loops are discrete entities.     

Features of the Energy Distribution in the Spectrum of the Symbiotic Star CH Cyg in Different Brightness States

The results of spectrophotometric observations of CH Cyg in an active phase and during emergence from it are given: the spectral energy distribution in the 3225-7500 range with 23 resolution, the absolute monochromatic illuminances at eight selected wavelengths in the 3225-8015 , and the emission fluxes in the H and H lines. The features of the energy distribution in the stellar spectrum and the observed forms of variability, different between the short-wavelength and red parts of the spectrum, are determined by the instability of both of the system's components. In the active phase, CH Cyg's emission in the observed spectral range can be represented as the total emission from a cool M6 III giant and a hot hydrogen gas with a temperature T _e = 10,000-15,000 K. In the low-brightness state, the star's energy distribution fully corresponds to that of a cool giant, the spectrum of which varies from M6 III to M7 III. The photometric variability of the cool component in the 8015 range is 0 ^m .3-0 ^m .4. An increase in the brightness of the cool giant preceded a bright outburst of the hot component.     

Large turbulent elements in supergiant photospheres

During the cool phase of the super-supergiant HR 8752, which happened around 1973, when the star's spectral type was K2...K5 Ia⁺, the most probable vertical extent of the main turbulent elements in the star's photosphere was about 6 times the density scale height, which is about half the stellar radius. In early-type photospheres (class Ia) it is about 10 times the atmospheric density scale height (about 0.25 of the stellar radius), while in less extreme (luminosity class Ib) medium-type supergiants the most probable vertical extent of the elements is approx. 8 times the density scale height (0.05R). Large turbulent elements are apparently a common feature in supergiant photospheres; the more extreme the supergiant the larger the relative size of the eddies.     

Photospheric Magnetic Activity in a Proxy for the Young Sun: HD 134319

We present the results of a long-term photometric monitoring of the young single main-sequence star HD134319. It shows short-term variability of the optical-band continuum flux with a period of 4.448 days. The variability is attributed to dark spots or spots groups unevenly distributed in longitude on the star's photosphere, whose visibility is modulated by the star's rotation. Maps of the photospheric spot pattern have been obtained with light curve inversion techniques based on the Maximum Entropy and the Tikhonov regularization criteria. The overall spot pattern shows evidence for two long-lasting active longitudes located about 180° apart, with a total area of at least 16% of the stellar surface (assuming an inclination of the stellar rotation axis of 90° on the line of sight). The longitude distribution of the spot pattern and its total area do not show any clear evidence for a long-term variation along the five years of observations. A comparison with recent mean field dynamo models is also addressed, suggesting a possible interpretation of such a behaviour. Singularity, spectral type, youth and a high level of photospheric and chromospheric activity make HD134319 a suitable proxy for studying the magnetic activity of the young Sun not far after its arrival on the zero age main sequence.     

Annihilation radiation in cosmic gamma-ray bursts

The emission features observed in the energy spectra of cosmic gamma-ray bursts imply the existence of two radiation components of comparable intensity. The softer component is similar to the continua of featureless bursts. The fast decrease in the intensity of this radiation with increasing photon energy is apparently due to the neutron star's magnetosphere being opaque to hard photons because of the formation of electron-positron pairs in single- (,B) and two-photon (,), processes. The hard component originates from the annihilation of electron-positron pairs, its spectrum representing a broad line with an extended power-law wing. Such a shape of the spectrum is apparently due to either thermal broadening in a source with a spatially inhomogeneous and rapidly time-varying plasma temperature, or nonthermal energy distribution of particles in their motion along the magnetic field lines. It is assumed that the sources of these components are spatially separated, the annihilation radiation escaping from the polar regions of a strongly magnetized neutron star in a collimated beam without appreciable attenuation.     

A statistical study of the spiral spots on the solar disc

After adding the data observed in the years from 1979 to 1982 to those obtained earlier (Ding et al., 1981), we re-examine the previous results and conclude:

1. The longitudinal distribution of spiral spots on the solar disc is generally the same as that of sunspot groups with areas of S _p ≥ 400, but their active longitudes seem to be more concentrated.
2. The distribution of spiral patterns in the southern and northern hemispheres shows that the differential rotation may be a fundamental solar dynamo for the formation of the spiral spots.
3. The statistical directions of the emerging twisted magnetic vectors in the active regions in the southern and northern hemispheres are synchronously inverse with a period of about two years. This period seems to be detected in other solar observations.