RS CVn-type Star Research Articles

Flare-related plasma motions in the outer atmosphere of the RS CVn-type star II Peg

Analogous to solar flares, stellar flares are dramatic explosions in the atmosphere, which may be accompanied by prominence eruptions, coronal mass ejections (CMEs), and other forms of plasma motion. Based on time-resolved spectroscopic observations of the RS CVn-type star II Peg, we aim to search for the potential plasma motions associated with flares. In these observations, we detected part of the gradual decay phase of an optical flare, for which we find a lower limit on the energy of the H$ alpha $ line of $6.03 $ erg. Converting this H$ alpha $ energy, we find a bolometric white-light energy of $3.10 $ erg. Moreover, a secondary peak is also observed. After removing a quiescence reference, the H$ alpha $ residual shows an asymmetric behavior, including both a blueshifted and a redshifted emission component. The former component has a bulk velocity of about -180 km s$^ $ and extends its velocity to more than -350 km s$^ $. This phenomenon is likely caused by a prominence eruption event or a chromospheric evaporation process. The latter emission component has a bulk velocity of 130 to 70 km s$^ $ and extends its velocity to nearly 400 km s$^ $. We attribute the redshifted emission component to one or a combination of several possible scenarios: flare-driven coronal rain, chromospheric condensation, backward-directed prominence eruption close to the stellar limb, or falling material in a prominence eruption. The minimum masses of the moving plasmas resulting in the blueshifted and redshifted emission components are estimated to be $0.56 $ g and $1.74 $ g, respectively.

High-velocity Blue-shifted Fe xxv Heα Line during a Superflare of the RS Canum Venaticorum–type Star IM Peg

The Monitor of All-sky X-ray Image (MAXI) detected a superflare, releasing 5 × 1037 erg in 2−10 keV, of the RS CVn-type star IM Peg at 10:41 UT on 2023 July 23 with its Gas Slit Camera (2−30 keV). We conducted X-ray follow-up observations of the superflare with the Neutron Star Interior Composition Explorer (NICER; 0.2−12 keV) starting at 16:52 UT on July 23 until 06:00 UT on August 2. NICER X-ray spectra clearly showed emission lines of the Fe xxv Heα and Fe xxvi Lyα for ∼1.5 days since the MAXI detection. The Fe XXV Heα line was blueshifted with its maximum Doppler velocity reaching −2200 ± 600 km s−1, suggesting an upward-moving plasma during the flare, such as a coronal mass ejection (CME) and/or chromospheric evaporation. This is the first case that the Fe xxv Heα line is blueshifted during a stellar flare, and its velocity overwhelmingly exceeds the escape velocity of the star (−230 km s−1). One hour before the most pronounced blueshift detection, a signature of the reheating of the flare plasma was observed. We discuss the origin of the blueshift, a CME, or high-velocity chromospheric evaporation.

Red Asymmetry of H α Line Profiles during the Flares on the Active RS CVn-type Star II Pegasi

Stellar coronal mass ejections (CMEs) have recently attracted much attention for their impacts on stellar evolution and surrounding exoplanets. RS CVn-type stars could produce large flares, and therefore may have frequent CMEs. Here we report the capture of a possible CME or chromospheric condensation on the RS CVn-type star II Pegasi (II Peg) using high-resolution spectroscopic observation. Two flares were detected during the observation, and the low limits of the flare energies are of the order of 1033 erg and 1034 erg, respectively. Using mean spectrum subtraction, the H α residual shows red asymmetry during the flares, and the redshifted broad emission components are probably caused by chromospheric condensation or coronal rain. Moreover, a far redshifted extra emission component with a high bulk velocity of 429 km s−1 was observed during the second flare and is probably due to a prominence eruption. The velocity greatly exceeds the star’s escape velocity, which means that this eruption can develop into a CME. The CME mass is estimated to be 0.83–1.48 × 1020 g, which is slightly larger than the value expected from solar flare-CME extrapolation. The kinetic energy of CME, derived to be 0.76–1.15 × 1035 erg, is less than the kinetic energy extrapolated from solar events. Additionally, we could not completely rule out the possibility of chromospheric condensation resulting in the far redshifted extra emission. Finally, there is a blueshifted broad component in the subtracted H α profile derived using synthesized spectral subtraction when no flare happened, and its behavior is associated with the H α activity features.

Prominence detection and chromosphere feature on the prototype RS CVn of active binary systems

ABSTRACT We present a study of high-resolution spectra of RS Canum Venaticorum (RS CVn), a prototype of active binary systems. Our data were obtained from 1998 to 2017 using different telescopes. We analyse the chromospheric activity indicators $\rm{Ca~{\small II}}$ IRT, Hα, $\rm{Na~{\small I}}$ D1, D2 doublet, $\rm{He~{\small I}}$ D3, and Hβ using a spectral subtraction technique. The chromospheric emission stems mainly from the K2 IV primary star, while the F5 V secondary star only shows weak emission features in a few of our spectra. We find excess absorption features in the subtracted Hα lines and other activity indicators from spectra taken near primary eclipse, which we ascribe to prominence-like material associated with the primary star. We estimate size limits of these tentative prominences based on the geometry of the binary system, and investigate the physical properties of the strongest prominence. An optical flare, characterized by $\rm{He~{\small I}}$ D3 line emission, together with stronger emission in other activity lines, was detected. The flare energy is roughly comparable to strong flares observed on other RS CVn-type stars. The chromospherically active longitudes of RS CVn most frequently appear near the two quadratures of the system and display changes between observing runs, which indicates an ongoing evolution of its active regions.

Detection of a High-velocity Prominence Eruption Leading to a CME Associated with a Superflare on the RS CVn-type Star V1355 Orionis

Stellar coronal mass ejections (CMEs) have recently received much attention for their impacts on exoplanets and stellar evolution. Detecting prominence eruptions, the initial phase of CMEs, as the blueshifted excess component of Balmer lines is a technique to capture stellar CMEs. However, most of prominence eruptions identified thus far have been slow and less than the surface escape velocity. Therefore, whether these eruptions were developing into CMEs remained unknown. In this study, we conducted simultaneous optical photometric observations with Transiting Exoplanet Survey Satellite and optical spectroscopic observations with the 3.8 m Seimei Telescope for the RS CVn-type star V1355 Orionis that frequently produces large-scale superflares. We detected a superflare releasing 7.0 × 1035 erg. In the early stage of this flare, a blueshifted excess component of Hα extending its velocity up to 760–1690 km s−1 was observed and thought to originate from prominence eruptions. The velocity greatly exceeds the escape velocity (i.e., ∼350 km s−1), which provides important evidence that stellar prominence eruptions can develop into CMEs. Furthermore, we found that the prominence is very massive (9.5 × 1018 g < M < 1.4 × 1021 g). These data will clarify whether such events follow existing theories and scaling laws on solar flares and CMEs even when the energy scale far exceeds solar cases.

Activity cycles in RS CVn-type stars

ABSTRACT We compile a list of 120 RS CVn-type stars from the bibliography in Southern hemisphere, to search for rotation periods and activity cycles, covering a large range of luminosities and effective temperatures for late stars. For each system of the list, we use photometric data from the All Sky Automated Survey (ASAS), and complement it with our own photometry, obtained with the Optical Robotic Observatory (ORO). We analyse these data with the Generalized Lomb–Scargle (GLS) periodogram to determine the rotation period and possible activity cycles for each system. We found rotation periods for 102 systems and activity cycles for 91 systems. From the statistical analysis of the results, we found that giant stars behave differently than subgiants and main-sequence stars, and that there is a good correlation between rotation periods and luminosities.

A PHOTOMETRIC STUDY OF THE ACTIVE EW/RS BINARY STAR SYSTEM: GSC 05586-00371

GSC 05586-00371 is a fast-rotating (Prot ≈ 0.44 days) eclipsing binary. Our study showed that this star system is an overcontact eclipsing binary star that belongs to the W UMa class with characteristics of RS CVn type stars. We studied this binary system by photometric means through the V and I bandpass where data by the ASAS-SN survey have also been utilized. our photometry yielded a more precise orbital period that was obtained on its discovery. ASAS-SN data over a 6-year period revealed a long-term photometric wave that is attributed to star spot activity. We monitored the binary system over a 73-hour monitoring period to detect any possible flares from the system, where two consecutive events were monitored from one of our observing stations. Through the use of the Wilson-Devinney program as applied by Binary Maker 3.0, we have obtained a model for this binary system that reveals the presence of star spots on both components. Our results show that the star spot activity is rather dynamic where their influence on the system’s light output changed on seasonal timescales.

The RS CVn–type Star GT Mus Shows Most Energetic X-Ray Flares Throughout the 2010s

Abstract We report that the RS CVn–type star GT Mus (HR 4492, HD 101379+HD 101380) was the most active star in the X-ray sky in the last decade in terms of the scale of recurrent energetic flares. We detected 11 flares from GT Mus in 8 yr of observations with the Monitor of All-sky X-ray Image (MAXI) from 2009 August to 2017 August. The detected flare peak luminosities were 1–4 × 1033 erg s−1 in the 2.0–20.0 keV band for its distance of 109.6 pc. Our timing analysis showed long durations (τ r + τ d) of 2–6 days with long decay times (τ d) of 1–4 days. The released energies during the decay phases of the flares in the 0.1–100 keV band were in the range of 1–11 × 1038 erg, which are at the upper end of the observed stellar flare. The released energies during the whole duration were in the range of 2–13 × 1038 erg in the same band. We carried out X-ray follow-up observations for one of the 11 flares with the Neutron star Interior Composition Explorer (NICER) on 2017 July 18 and found that the flare cooled quasi-statically. On the basis of a quasi-static cooling model, the flare loop length is derived to be 4 × 1012 cm (or 60 R ☉). The electron density is derived to be 1 × 1010 cm−3, which is consistent with the typical value of solar and stellar flares (1010–13 cm−3). The ratio of the cooling timescales between radiative (τ rad) and conductive (τ cond) cooling is estimated to be τ rad ∼ 0.1 τ cond from the temperature; thus, radiative cooling was dominant in this flare.

Analysis of the Spectroscopic Observations of RS CVn Type Star V837 Tau

We present an analysis of the spectra of the star V837 Tau, which belongs to a class of chromosphere-active RSCVn type stars. The observational material was obtained with the 1.5-m Russian-Turkish telescope RTT-150 and the 2.6-m Shajn telescope of the Crimean astrophysical observatory. To reveal signs of the photosphere spottedness of the primary and to isolate the faint secondary, we performed the deconvolution of the “averaged” spectral profile of the system. By describing the radial velocity curves, we refined the orbital elements of the binary system, and by modeling spectrawe determined the fundamental parameters of the components, as well as the chemical composition of the atmosphere of the bright component. Using the Doppler mapping method we reconstructed the distribution of spots on the surface of the bright star. We show that the spots are concentrated in a belt at medium (about 40) latitudes. The hydrogen line profiles have a variable emission feature in the bright component, and the Hα line exhibits traces of chromospheric activity in the secondary.

Another look at AM Herculis – radio-astrometric campaign with the e-EVN at 6 cm

We conducted radio-interferometric observations of the well known binary cataclysmic system AM Herculis. This particular system is formed by the magnetic white dwarf (primary) and the red dwarf (secondary), and is the prototype of so-called polars. Our observations were conducted with the European VLBI Network (EVN) in the e-EVN mode at 5 GHz. We obtained six astrometric measurements spanning one year, which make it possible to update the annual parallax for this system with the best precision to date ($\pi=11.29\pm0.08$ mas), equivalent to the distance of $88.6\pm0.6$ pc. The system was observed mostly in the quiescent phase (visual magnitude $m_v\sim15.3$), when the radio emission was at the level of about $300~\mu$Jy. Our analysis suggests that the radio flux of AM Herculis is modulated with the orbital motion. Such specific properties of the radiation can be explained using the emission mechanism similar to the scenario proposed for V471 Tauri and, in general, for RS CVn type stars. In this scenario the radio emission arises near the surface of the red dwarf, where the global magnetic field strength may reach a few kG. We argue that the quiescent radio emission distinguishes AM Herculis together with AR Ursae Majoris (the second known persistent radio polar) from other polars, as the systems with a magnetized secondary star.

Orbital and physical parameters of eclipsing binaries from the ASAS catalogue – IX. Spotted pairs with red giants

We present spectroscopic and photometric solutions for three spotted systems with red giant components. Absolute physical and orbital parameters for these double-lined detached eclipsing binary stars are presented for the first time. These were derived from the V-, and I-band ASAS and WASP photometry, and new radial velocities calculated from high quality optical spectra we obtained with a wide range of spectrographs and using the two-dimensional cross-correlation technique (TODCOR). All of the investigated systems (ASAS J184949-1518.7, BQ Aqr, and V1207 Cen) show the differential evolutionary phase of their components consisting of a main sequence star or a subgiant and a red giant, and thus constitute very informative objects in terms of testing stellar evolution models. Additionally, the systems show significant chromospheric activity of both components. They can be also classified as classical RS CVn-type stars. Besides the standard analysis of radial velocities and photometry, we applied spectral disentangling to obtain separate spectra for both components of each analysed system which allowed for a more detailed spectroscopic study. We also compared the properties of red giant stars in binaries that show spots, with those that do not, and found that the activity phenomenon is substantially suppressed for stars with Rossby number higher than $\sim$1 and radii larger than $\sim$20 R$_\odot$.

Chemical Composition of the RS CVN-Type Star 29 Draconis

Abstract Photospheric parameters and chemical composition are determined for the single-lined chromospherically active RS CVn-type star 29 Draconis (HD 160538). From the high resolution spectra obtained on the Nordic Optical Telescope, abundances of 22 chemical elements, including the key elements such as 12C, 13C, N and O, were investigated. The differential line analysis with the MARCS model atmospheres gives Teff = 4720 K, log g = 2:5, [Fe/H] = -0:20, [C/Fe] = -0:14, [N/Fe] = 0:08, [O/Fe] = -0:04, C/N = 2:40, 12C/13C = 16. The low value of the 12C/13C ratio gives a hint that extra mixing processes in low-mass chromospherically active stars may start earlier than the theory of stellar evolution predicts.

Chemical Composition of the RS CVn-Type Star Lambda Andromedae

AbstractPhotospheric parameters and chemical composition are determined for the single-lined chromospherically active RS CVn-type star λ And (HD 222107). From the high resolution spectra obtained on the Nordic Optical Telescope, abundances of 22 chemical elements and isotopes, including such key elements as

Period investigation of two RS CVn-type binary stars: RU Cancri and AW Herculis

Orbital period variations of two RS CVn-type binaries, RU Cnc and AW Her, are presented based on the analysis of all available times of light minima. It is discovered that the orbital period of RU Cnc shows two possible period oscillations with periods of 13.38(±0.23) and 37.6(±3.4) years. The corresponding amplitudes of the oscillations are about 0.0098(±0.0023) and 0.0119(±0.0017) days. For AW Her, it is found that the period shows a cyclic variation with a period of 12.79(±0.34) years and an amplitude of about 0.0327(±0.0063) days. Since RU Cnc and AW Her are two RS CVn-type systems, the cyclic period oscillations are more likely to be caused by the magnetic activity cycles.

Long-term chromospheric activity of non-eclipsing RS CVn-type stars

Context. The IUE database provides several UV high and low-resolution spectra of RS CVn-type stars from 1978 to 1996. In particular, many of these stars were monitored continuously during several seasons by IUE. Aims. Our main purpose is to study the short and long-term chromospheric activity of the RS CVn systems most observed by IUE: HD 22 468 (V711 Tau, HR 1099, K1IV+G5V), HD 21 242 (UX Ari, K0IV+G5V), and HD 224 085 (II Peg, K2IV). Methods. We first obtained the Mount Wilson index S from the IUE high and low-resolution spectra. Secondly, we used the Lomb

EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars

Aims. We observe the slowly-rotating, active, single giant, EK Eri, to study and infer the nature of its magnetic field directly. Methods. We used the spectropolarimeter NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least Square Deconvolution method to create high signal-to-noise ratio Stokes V profiles. We fitted the Stokes V profiles with a model of the large-scale magnetic field. We studied the classical activity indicators, the Ca ii H and K lines, the Ca ii infrared triplet, and Hα line. Results. We detected the Stokes V signal of EK Eri securely and measured the longitudinal magnetic field Bl for seven individual dates spanning 60% of the rotational period. The measured longitudinal magnetic field of EK Eri reached about 100 G and was as strong as fields observed in RSCVn or FK Com type stars: this was found to be extraordinary when compared with the weak fields observed at the surfaces of slowly-rotating MS stars or any single red giant previously observed with NARVAL. From our modeling, we infer that the mean surface magnetic field is about 270 G, and that the large scale magnetic field is dominated by a poloidal component. This is compatible with expectations for the descendant of a strongly magnetic Ap star.

Spectroscopic investigation of stars on the lower main sequence

Aims. The aim of this paper is to provide fundamental parameters and abundances with a high accuracy for a large sample of cool main sequence stars. This study is part of wider project, in which the metallicity distribution of the local thin disc is investigated from a complete sample of G and K dwarfs within 25 pc. Methods. The stars were observed at high resolution and a high signal-to-noise ratio with the ELODIE echelle spectrograph. The V sini were obtained with a calibration of the cross-correlation function. Effective temperatures were estimated by the line depth ratio method. Surface gravities (log g) were determined by two methods: parallaxes and ionization balance of iron. The Mg and Na abundances were derived using a non-LTE approximation. Abundances of other elements were obtained by measuring equivalent widths. Results. Rotational velocities, atmospheric parameters (Teff ,l ogg ,[ Fe/H], Vt ), and Li, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Co, Ni, and Zn abundances are provided for 131 stars. Among them, more than 30 stars are active stars with a fraction of BY Dra and RS CVn type stars for which spectral peculiarities were investigated. We find the mean abundances of the majority of elements in active and nonactive stars to be similar, except for Li, and possibly for Zn and Co. The lithium is reliably detected in 54% of active stars but only in 20% of nonactive stars. No correlation is found between Li abundances and rotational velocities. A possible anticorrelation of log A(Li) with the index of chromospheric activity GrandS is observed. Conclusions. Active and nonactive cool dwarfs show similar dependencies of most elemental ratios vs. [Fe/H]. This allows us to use such abundance ratios to study the chemical and dynamical evolution of the Galaxy. Among active stars, no clear correlation has been found between different indicators of activity for our sample stars.

Butterfly Diagram and Activity Cycles in HR 1099

We analyze photometric data of the active RS CVn–type star HR 1099 for the years 1975–2006 with an inversion technique and reveal the nature of two activity cycles of 15–16 yr and yr duration. The 16 yr cycle is 5.3 0.1 related to variations of the total spot area and is coupled with the differential rotation, while the 5.3 yr cycle is caused by the symmetric redistribution of the spotted area between the opposite stellar hemispheres (flip-flop cycle). We recover long-lived active regions comprising two active longitudes that migrate in the orbital reference frame with a variable rate because of the differential rotation along with changes in the mean spot latitudes. The migration pattern is periodic with the 16 yr cycle. Combining the longitudinal migration of the active regions with a previously measured differential rotation law, we recover the first stellar butterfly diagram without an assumption about spot shapes. We find that mean latitudes of active regions at opposite longitudes change antisymmetrically in the course of the 16 yr cycle: while one active region migrates to the pole, the other approaches the equator. This suggests a precession of the global magnetic field with respect to the stellar rotational axis. Subject headings: binaries: spectroscopic — stars: activity — stars: individual (HR 1099) — stars: rotation — stars: spots — techniques: photometric

Long-Term Starspot Activity of the Eclipsing Variable System CG Cyg

Photometric observations of the eclipsing variable system CG Cyg (G9+K3, P=0d.63), which belongs to a group of short-period, chromospherically active RS CVn type stars with the same type of activity as the sun, have been made over two seasons in 2003 and 2004. The spotting of the star’s surface is modelled using the original observations together with photometric data published since 1965. In every season the spots were concentrated near the equator and covered up to 18% of the star’s entire surface, while the temperature difference between the quiescent photosphere and the spotted regions was 2100 K. Cyclical variations in the total area and average latitude of the spots, as well as flip-flop of the active longitudes, are suspected. Latitudinal drift of the spots during a cycle and differential rotation of the star are discovered. The following orbital elements are obtained: M1 = 0.93 M⊙, M2 = 0.81 M⊙, R1 = 1.01 R⊙, and R2 = 0.82 R⊙.

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 (ΔT=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.