RS CVn-type Star Research Articles

Plage and flare Activity of the RS CVn-type Star UX Arietis during 2001-2002

The very active RS CVn-type star UX Ari was observed using high-resolution echelle spectrograph attached to the 2.16m telescope of Xinglong station in Nov.-Dec. 2001 and Dec. 2002. By means of synthetic spectral subtraction method, the information about chromospheric activity of the system was obtained through several chromospheric activity indicators HeI D $_3$ , NaI D $_1$ D $_2$ , H $_{\alpha}$ , and CaII IRT lines. Based on the analysis for these activity indicators, we found that the chromospheric activity of UX Ari showed obvious orbital modulation phenomenon, and the favorite active longitudes were around the quadratures of the binary system. During the two observing runs, hot plage and very strong optical flare events were detected, which were always happened around the favorite active longitudes of the system. Moreover, they were linked with the photospheric starspots in spatial structure, and appeared just above the main starspots. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Spectroscopic and photometric observations of the short-period RS CVn-type star WY Cnc

Spectroscopic and photometric observations of the short-period RS CVn-type star WY Cnc

The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun

Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSAT surveys, a sample of 100 stars most luminous in X-rays within or around a distance of 50 pc is culled. The smallest X-ray luminosity in the sample, in units of 1029 ergs s-1, is LX = 9.8; the strongest source in the solar neighborhood is II Peg, a RS CVn star, at LX = 175.8. With respect to the origin of X-ray emission, the sample is divided into partly overlapping classes of pre–main-sequence, post–T Tauri, and very young ZAMS objects (type XY), RS CVn–type binary stars (type RS), other active short-period binaries, including binary BY Dra–type objects (type XO), apparently single or long-period binary active evolved stars (type XG), contact binaries of WU UMa kind (type WU), apparently single or long-period binary variable stars of BY Dra kind (type BY), and objects of unknown nature (type X?). Chromospherically active, short-period binaries (RS and XO) make up 40% of the brightest X-ray emitters, followed by young stars (XY) at 30% and unknown sources (X?) at 15%. The fraction of spectroscopically single evolved X-ray emitters of spectral classes IV and III is quite large (10%). The sources identified as RS CVn–type stars (RS, 23 objects) are considerably stronger in X-ray than the XY-objects and the other active binaries (XO and WU, 20 objects). Seven objects have LX > 100, all RS except one XY, viz., BO Mic. Only five (22%) RS objects have LX 25. Formally, the limit of LX = 25 could serve as a statistical criterion to differentiate RS and XY stars. However, the other short-period binaries (including eclipsing stars of Algol and β Lyr type) have a distribution of LX very similar to the XY objects. The contact binaries (WU) appear to be much weaker in X-rays than their detached counterparts of RS type, but the sample of the former is too small (three objects) to reach a firm conclusion. Sources matched with giants (either single or in binaries) are found to be significantly harder, with only 7% of hardness ratios below 0, than subgiants (66% of HR1 < 0) and dwarfs (59% of HR1 < 0). Almost all objects in the sample are binary or multiple stars; the fraction of components (FC), defined as the total number of components in all binary and multiple systems divided by the sum of the total number of components and single stars, is at least 0.90. The FC for the XY objects reaches 0.81, and for the unknown type 0.89. About 70% of RS objects have also visual or astrometric companions, which makes them hierarchical multiple systems. The RS objects (mostly old, evolved stars) and the XY stars have quite different kinematics. While the RS objects move at considerable velocities in apparently random directions with respect to the local standard of rest, the young stars have smaller and orderly velocities and tend to comprise expanding mini-associations such as the β Pic and the Tucana groups. The majority of the young X-ray active stars belong to the Pleiades stream with the mean heliocentric velocity (U, V, W) = (-9.6, -21.8, -7.7) km s-1.

ROSATView ofHipparcosF Stars

For a set of 11,900 F stars that have uvby photometry and are in the Hipparcos catalog, we have found 1980 X-ray counterparts (~17%) in the ROSAT All-Sky Survey (RASS). In this paper we analyze X-ray properties of these ROSAT F stars and compare that sample with the entire set of our F stars. The comparison has shown that the ROSAT F stars are on average younger than stars in the whole set, consistent with previous studies that indicate lower X-ray emission in older stars. However, for the ROSAT stars only, the age-X-ray activity relation has proved to be more complicated, with a group of strong X-ray emitters being the oldest rather than the youngest stars. That group determines the shape of the overall relationship between the kinematics and X-ray luminosity. The relationship is in the sense that the mean tangential velocity and velocity dispersion are higher for stars that are more luminous at the X-ray wavelengths, implying that these stars are on average the oldest rather than the youngest. The group of strongest X-ray emitters, log LX > 30.4, which constitutes only a small fraction of the sample stars, ~2.8%, has been found to include a variety of stellar types. It is dominated by very young objects, followed by RS CVn-type stars and other active binaries. As to the sample as a whole, it seems to consist of at least three distinct groups of F stars that differ in their X-ray luminosity and evolution of their X-ray emission. The first one is represented by normal zero-age main-sequence (ZAMS) and post-ZAMS stars whose X-ray emission declines with age, just as expected in the conventional picture of stellar X-ray evolution (roughly half of the sample). The second group consists of strong X-ray emitters that appear to be very young, possibly pre-main-sequence stars. These stars are characterized by high metallicity, high temperature, and high overluminosity (the latter indicating that the star's optical luminosity is too high for the star's temperature and surface gravity). Many of them are located in the distance range ~130-200 pc, near the known regions of ongoing star formation or OB associations. The third group also consists of strong X-ray emitters. However, those are very old rather than very young stars, and they happen to be extremely overluminous as well. Their strong X-ray emission implies that many F stars are capable of increasing rather than decreasing their X-ray luminosity as they get older, thus challenging the idea that X-ray emission continuously declines with age. Most of these old X-ray emitters have significantly evolved away from the ZAMS. If they increase their outer convective zone as they evolve, they would probably be able to maintain a high level of coronal activity, and hence enhanced X-ray emission, despite decaying rotation. Other relationships found for the ROSAT F stars include the correlation of X-ray luminosity with hardness ratio, optical absolute magnitude, effective temperature, overluminosity, metallicity, and reddening. There is also a difference between binary and single stars, with X-ray luminosity being on average slightly higher for unresolved binaries. There is a distinct difference between the groups of soft and hard X-ray emitters in our sample: while the soft sources show a significant correlation between the X-ray luminosity and hardness ratio, no such correlation is seen for hard sources. The tendency for X-ray luminosity to be higher for harder sources implies that stars more luminous in the X-rays have a hotter corona. This tendency breaks up at large hardness values, which may be due to coronal saturation.

Spectroscopic and photometric observations of the short-period RS CVn-type star ER Vulpeculae

Spectroscopic observations around the H line and BVR photometry of the eclipsing, short-period, RS CVn-type star ER Vul are presented. The solutions of new radial velocity and light curves yielded the following masses and radii of the components: M1 = 1:16 M, M2 = 1:05 M, R1 = 1:25 R, R2 = 1:12 R. The equatorial velocities corresponding to the measured rotational broadenings of the spectral lines are: V 1 = 90 km s 1 and V 2 eq = 80 km s 1 . Their ratio is just equal to the ratio of the stellar radii. We detected excess emission in the H and CaI 6494 lines that changes during the orbital cycle. Our spectral data show the presence of absorbing matter around the mass center of the system.

Spectroscopic and photometric observations of the short-period RS CVn-type star CG Cyg

High-resolution spectroscopic observations around the H α line and BVRI photometry of the eclipsing short-period RS CVn star WY Cnc are presented. The solutions of our radial velocity curves and light curves yielded the following values for masses and radii of the components: M 1 = 0.84 M ○. , M 2 = 0.46 M ○. , R 1 = 1.06 R ○. , R 2 = 0.65 R ○. . The measured rotational broadenings of the spectral lines correspond to equatorial velocities V 1 = 78 km s -1 and V 2 = 49 km s -1 . The distortions of our multicolor light curve of WY Cnc were reproduced by two cool spots on the primary star. An increase of the out-of-eclipse brightness by 0.1 mag in February 2001 was detected. Our spectra show H α line in emission from the secondary star. The chromospheric activity appeared also in the observed emission cores of the CaII H and K lines as well as the CaI 6494 line.

Dynamics of flares on late type dMe stars

We investigate the spectral signatures of stellar flares in the wavelength range 3600 Å to 4500 Å and in broad band photometry. We study the phenomenology of the spectral signatures and we found that flares are best described by two main phases; an impulsive phase and a gradual phase, for which the physical properties are different. Important spectral differences between flares lead us to distinguish four main classes: (i) solar-like chromospheric or two-ribbon flares, (ii) white-light flares, (iii) combined white-light flares with distinct impulsive and gradual phases, and (iv) non solar-like flares (usually occuring on RS CVn type stars). We show how this classification corresponds to substantial differences in the physical properties of the flare components. We compiled all available spectroscopic data for stellar flares. We found several new empirical correlations between the time lags in the spectral lines (rise and decay times). We found for instance that during the gradual phase, the rise time in the H line is well correlated to the rise time in the Ca ii K line, and that the Ca ii K line is 1.63 times slower to rise than the H line. Similar correlations were found between the rise and decay times in these lines. These correlations are evidence that there is a dominant mechanism commanding the temporal flux evolutions during the gradual phase of flares. This mechanism applies on time scales ranging from one minute to more than a hundred minutes. We found correlations between the time lags and the maximum fluxes in the Johnson U-band and the spectral lines. These correlations show that the larger the flare the longer it takes to evolve. We show that the maximum flux in the Johnson U-band correlates well, but not linearly with the maximum flux in the H line during the impulsive phase, and over five orders of magnitude. We argue how this correlation can provide constraints on the currently available models. The spectral line maximum fluxes during the gradual phase also correlate with the Johnson U-band flux, which demonstrates that somehow the impulsive and gradual phases are physically linked. The correlation between the H and Caii K line fluxes during the gradual phase and the lack of dependence of the flux ratio of those lines on flare magnitude reveal that larger flares are not “hotter” than smaller flares during the gradual phase. We examine the behaviour of the spectral line widths: While the Ca ii K line width shows essentially no detectable variation, the Balmer line widths show a complex dependency on the white light intensity. We distinguish three energy domains where the Balmer line widths exhibit a different behaviour: (i) for small flares, the widths remain rather constant as a function of white light intensity, which suggests that in this energy range the U-band is not a good diagnostic of the total energy release and that dense “kernels” do not dominate the Balmer emission, (ii) for medium size flares, the widths rapidly increase with the U-band flux up to about 15 Å, which indicates that emission in the Balmer lines is then dominated by kernel emission, and (iii) for large flares the widths decrease with increasing U-band flux, which suggests that the Balmer line emission is then increasingly dominated by a radiative pumping process between the white light and the Balmer lines. Finally, we remark that the Balmer line fluxes are well correlated to the white light flux during the impulsive phase of flares, and not only at flare maximum. We obtain a power law correlation between the Balmer line and U-band fluxes that is evidence for either a common source (“kernels”) or strongly related flare components.

Nonaxisymmetric cool spot distributions and dynamo action in close binaries

We investigate the nonaxisymmetric magnetic field distribution in the components of close binary systems, gen- erated by a stellar dynamo with a nonaxisymmetric distribution of the-coecient, proportional to the mean helicity of the convective motions in the stellar convective envelope. The nonaxisymmetry of is assumed to be connected with interaction with the binary companion, i.e. the reflection eect and tidal interaction. The degree of asymmetry is estimated from the known parameters of ER Vulpeculae, an active RS CVn-type star. We demonstrate that the synchronization of rotation and orbital motion of ER Vulpeculae due to tidal interaction is sucient to allow nonaxisymmetric magnetic structures to survive in the presence of smoothing by dierential rotation. The dynamo driven nonaxisymmetric magnetic structures are usually steady in the reference frame connected with the-spot. However, we found a resonant excitation of an oscillatory, strongly nonaxisym- metric, configuration, driven by the joint action of the-eect and dierential rotation. The magnetic structures obtained are predominantly manifested on the stellar surface in the form of spots of radial magnetic field. Their form and size are similar to the form and size of cool spots observed on the surfaces of both components of ER Vulpeculae, and their displacements from the-spots are comparable to that of the observed cool spots from the hot spots connected with the reflection eect. Possible relevant observations are discussed.

Magnetic activity in the young solar analog LQ Hydrae

We present the first evidence that a single active dwarf of solar type can show a long-lived, nonaxisymmetric spot distribution - active longitudes on opposite hemispheres, similar to evolved, rapidly rotating RS CVn-type binary stars. We analyse new as well as published photometric observations of the young active dwarf LQ Hya, spanning almost 20 years. We find that activity of the star has three activity cycles: a 5.2-yr flip-flop cycle, a 7.7-yr period in the amplitude modulation of the brightness and an approximately 15-yr period in variations of the mean brightness. The two shorter cycles are related to the alternating active longitudes and are similar to cycles observed in RS CVn-type stars. The 15-yr cycle reflects periodic changes of the mean spottedness of the star and resembles the solar 11-year cycle. The spot rotation period (about 1.6 days) changes during the 15-yr cycle, indicating the presence of small dierential rotation. The lengths of the three cycles are related as 3:2:1, with the repetition of the spot configuration after 15 years. We discuss the possibility that the observed spot cycles represent two dierent magnetic dynamo modes operating in LQ Hya: an axisymmetric mode, as in the Sun, and a nonaxisymmetric higher order mode with two cycles in spot patterns. Our results suggest that young stars exhibit their cycles in spot distribution, as seen in LQ Hya. This is in contrast to the conclusion based on the analysis of Caii H&K emission from plages. The results suggest also that the Vaughan-Preston gap represents a transition from a multiple-mode dynamo to a single-mode dynamo.

Swing excitation and magnetic activity in close binary systems

Parametric resonance between the perturbation of a stellar convective zone affected by a companion of a close binary system and non-axisymmetric dynamo modes can play an important role in close binary systems. This process in combination with the previously suggested α 2 mechanism is probably responsible for strongly non-axisymmetric magnetic activity observed in several close binaries. The conditions required for swing excitation to occur in such systems are directly related to the status of rotational synchronization between the orbital motion and rotation of the star, which can be used as a tool to discover such objects. One potential candidate ‐ an active RS CVn-type star ER Vulpeculae ‐ shows indications of the swing excitation.

Polarimetry of evolved stars

We present broadband optical polarimetry of 3 RS CVn stars and 3 Mira variables, including the symbiotic star R Aqr, which contains a Mira component. Polarimetric variability has been studied on time- scales from hours to years. Our programme objects at the time of our observations showed dierent forms of P () dependence, P/ 4 for most of the RS CVn-type stars, and P/ 2 for the RS CVn-type star UV Psc and the Mira Ceti-type variables, and a signicant increase of polarization to the red for the Mira R Cet. Combining our data with previously published data, we conclude that most of RS CVn-type and Mira Ceti-type objects show evidence of large polarimetric variability at wavelengths shorter than 0.5m, whereas the level of polarization is more stable in the red. This behaviour is consistent with episodic mass ejection and formation of small dust particles in the circumstellar environment. Although all targets showed polarimetric variability on dierent time scales, only for IM Peg might these variations be possibly linked with the photometric period.

A CCD Photometric Survey of Variable Stars in the Field of NGC 188

The results of a recent CCD photometry survey of variables in a 1° field covering the old open cluster NGC 188 are presented. A total of 16 short-period variables were identified during five nights, among which eight are new variables in the program field. By adopting the method of phase-dispersion minimization, the periods for seven of the eight new variables are determined. The main properties, the classifications, as well as the memberships are discussed for all of these variable stars, based on the behavior of light curves, the locations in the color-magnitude diagram, and the results of proper motions. Among the eight new objects, six are W UMa binaries, in which one definitely is and two probably are members of the cluster. A detached or semidetached eclipsing binary with an orbital period longer than 1 day is found, which is a likely cluster star. In addition, a field star with a peculiar light curve is detected, which can be classified as a RRd–type variable. Moreover, revised basic data for the eight old variables are given according to our observations. V8 can be classified as an RS CVn–type star with light variations larger than 0.2 mag.

The optical gravitational lensing experiment. Variable stars in globular clusters

Three elds covering the central part of the globular cluster! Cen were surveyed in a search for variable stars. We present V -band light curves for 39 periodic variables: 24 SX Phe stars, 7 contact binaries, 5 detached or semi-detached binaries, and 3 likely spotted variables (FK Com or RS CVn type stars). Only 2 of these variables were previously known. All SX Phe stars and all contact binaries from our sample belong to blue stragglers. Observed properties of these stars are consistent with their cluster membership. Of particular interest is detection of two well detached binaries with periods P =1.50 day and P =2.47 day. Further study of these two binaries can provide direct information about properties of turno stars in ! Cen. An incomplete light curve of a Mira variable known as V 2w as obtained. We present V vs. V I color-magnitude diagrams for the monitored part of the cluster.

The optical gravitational lensing experiment. Variable stars in globular clusters. I. Fields 5139A-C in ω Centauri

Three fields covering the central part of the globular cluster ω Cen were surveyed in a search for variable stars. We present V-band light curves for 39 periodic variables: 24 SX Phe stars, 7 contact binaries, 5 detached or semi-detached binaries, and 3 likely spotted variables (FK Com or RS CVn type stars). Only 2 of these variables were previously known. All SX Phe stars and all contact binaries from our sample belong to blue stragglers. Observed properties of these stars are consistent with their cluster membership. Of particular interest is detection of two well detached binaries with periods P=1.50 day and P=2.47 day. Further study of these two binaries can provide direct information about properties of turnoff stars in ω Cen. An incomplete light curve of a Mira variable known as was obtained. We present V vs. color-magnitude diagrams for the monitored part of the cluster.

The Inclination Dependence of the Mean Spectral Line Shapes of RS CVn Stars: Further Evidence for Polar Spots

The mean spectral line shape of the Ca I 6439 Å line profile is examined in four RS CVn-type stars having different stellar inclinations. It is found that the low-inclination stars (HR 1099 and EI Eri) have a calcium line with a more pronounced flat-bottomed shape compared to the two high-inclination stars (AR Lac and V471 Tau) whose line shapes are close to those of rotationally broadened profiles. If the flat-bottomed line shape seen in many RS CVn stars arises purely from the atmosphere of the star and is not due to the surface temperature distribution, then one should expect to see flat-bottomed profiles in high-inclination stars as well, and this is not the case. Four models are considered as an explanation for the line shapes seen in the low-inclination RS CVn stars: (1) gravity darkening, (2) a bright band at the equator of the star, (3) surface differential rotation, and (4) a cool polar spot that is viewed with different projected areas. It is shown that gravity darkening cannot produce a flat-bottomed profile even when taking into consideration the temperature sensitivity of the Ca I 6439 Å line. A bright equatorial band is capable of producing a flat-bottomed profile, but the change in line shape with stellar inclination is not as dramatic as is observed with actual stars. Likewise, differential rotation with the poles of the star rotating faster than the equatorial regions can also produce a box-shaped line profile, but an unreasonable amount of differential rotation is required (the poles must rotate with an angular velocity at least twice that of the equator). The polar spot model is the only one of the four considered that is capable of producing a flat-bottomed profile whose shape changes significantly with stellar inclination. This model is also consistent with the presence of cool spots on RS CVn stars that have been established via photometry and the presence of persistent TiO molecular features in the spectra of these stars and provides further support to the presence of polar spots on RS CVn stars.

Simultaneous detection of a large flare in the X-ray and optical regions on the RS CVn-type star II Peg

Results are presented for a large flare detected simultaneously on the RS CVn star, II Peg, by the X-ray satellite Ginga and from ground-based Johnson U-band measurements. The total U-band flare energy is determined. Using a smooth-burst model, the combined U-band plus X-ray radiative output is estimated.

Hα Emission in the Giant Components of RS CVn Systems

The Hα line of hydrogen is a very powerful tool for the study of the solar chromosphere (plages, flares, prominences). Although it is difficult to derive in a simple way quantitative information, like chromospheric temperature and electron density, because of the very extensive depth of its formation and the very complex mechanism of excitation, Hα has been successfully used to study the chromospheres of active stars (Herbig, 1985). Moreover this diagnostic has proved to be very useful for the study of surface inhomogeneities and transient phenomena like flares in systems of the RS CVn type and other active stars (Bopp, 1981; Bopp et al., 1988; Fraquelli, 1982).With the aim of investigating the behaviour of chromospheric activity, we have undertaken an extensive monitoring of Hα emission in a selected group of RS CVn binaries (Strassmeier et al., 1988). We report here preliminary results of the observations obtained in 1988–1989.

Doppler Images of Spotted Late-Type Stars

Doppler imaging is a technique for deriving resolved images of rapidly rotating stars from a detailed analysis of very high signal-to-noise high resolution spectral line profiles. An improved version of this technique is presented, which now uses principles of maximum entropy image reconstruction to invert the line profile information. The effects that noise, finite resolution, and uncertainties in the assumed stellar physics have on the resultant images were explored through various test simulations. The technique is found to be efficient, accurate, and robust at deriving images of certain classes of stars from realistic quality data. Doppler images are presented of two spotted late-type stars, the RS CVn-type star HR 1099, and the FK Com-type star HD 199178. Both stars show surprisingly similar spot distributions. In each case, there is a single large cool spot straddling the pole, and a number of small cool spots at low latitudes. We expect that the small low latitude spots on each star will migrate poleward to join the polar spot, and suspect that the observed long-lived polar spots are the result of the poleward migration and merging of many active region complexes. If true, the poleward migration of starspots suggests that magnetic activity on very rapidly rotating stars is qualitatively different than that seen on our Sun. We suggest that the observed rotational trigger velocity for the appearance of large spots on late-type stars marks the transition from solar-type boundary layer dynamos to distributed dynamos, which occur only in more rapidly rotating stars. The sizes, locations, and migrations of the spots, however, may be more a result of the convective flow patterns than of any dynamo action, since the spots are quite long-lived.

RELATION BETWEEN THE (V-R) AND (R-I) COLOR INDICES AND ACTIVITY FOR SOLAR-TYPE STARS.

ABSTRACT Bright moderately active stars with known Ca II or Mg II emission fluxes and with measured (V-R) and (R-I) color indices in the Cousins system have been checked for the presence of activity-related near-infrared excesses. With increased levels of activity, both colors show very weakly indicated excesses for subgiants and deficits for dwarfs. While the effect observed for subgiants may be related to large near-infrared excesses observed in RS CVn-type stars or other very active stars, the spectral distributions for dwarfs do not show any abnormality beyond what would be expected from the blanketing in (B-V), which was used as the reference color.

Ultraviolet flare on Lambda Andromedae

view Abstract Citations (33) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ultraviolet flare on lambda Andromedae. Baliunas, S. L. ; Guinan, E. F. ; Dupree, A. K. Abstract On November 5, 6, 1982, a luminous, flarelike brightening of the ultraviolet emissions was observed with IUE from the active RS CVn type star Lambda And during the phase of rotation period corresponding to maximum area coverage of the visible hemisphere by starspots and active regions. Enhancements during the flare in the ultraviolet emission lines as large as factors of several and in the ultraviolet continuum up to 80 percent persisted for over 5 hours. The bulk of the radiative output of the flare occurred in Mg II h and k and H I Ly-alpha. Because of the long duration and extreme luminosity of the event, the energy radiated by the flare alone is in excess of 10 to the 35th ergs just in the ultraviolet region. This is the most energetic stellar flare ever recorded in the ultraviolet. In addition, it is the first ultraviolet flare observed from a giant star. In comparison to the largest solar flares, the flare on Lambda And is at least three orders of magnitude more energetic in similar emission lines. Publication: The Astrophysical Journal Pub Date: July 1984 DOI: 10.1086/162256 Bibcode: 1984ApJ...282..733B Keywords: Binary Stars; Flare Stars; M Stars; Stellar Flares; Stellar Spectra; Ultraviolet Astronomy; Andromeda Constellation; Emission Spectra; Iue; Stellar Luminosity; Stellar Models; Astrophysics full text sources ADS | data products SIMBAD (1) MAST (1) INES (1)