RS CVn Stars Research Articles

Seven classes of rotational variables from a study of 50 000 spotted stars with ASAS-SN, Gaia, and APOGEE

ABSTRACT We examine the properties of ∼50 000 rotational variables from the ASAS-SN survey using distances, stellar properties, and probes of binarity from Gaia DR3 and the SDSS APOGEE survey. They have higher amplitudes and span a broader period range than previously studied Kepler rotators. We find they divide into three groups of main sequence stars (MS1, MS2s, MS2b) and four of giants (G1/3, G2, G4s, and G4b). MS1 stars are slowly rotating (10–30 d), likely single stars with a limited range of temperatures. MS2s stars are more rapidly rotating (days) single stars spanning the lower main sequence up to the Kraft break. There is a clear period gap (or minimum) between MS1 and MS2s, similar to that seen for lower temperatures in the Kepler samples. MS2b stars are tidally locked binaries with periods of days. G1/3 stars are heavily spotted, tidally locked RS CVn stars with periods of 10s of days. G2 stars are less luminous, heavily spotted, tidally locked sub-subgiants with periods of ∼10 d. G4s stars have intermediate luminosities to G1/3 and G2, slow rotation periods (approaching 100 d), and are almost certainly all merger remnants. G4b stars have similar rotation periods and luminosities to G4s, but consist of sub-synchronously rotating binaries. We see no difference in indicators for the presence of very wide binary companions between any of these groups and control samples of photometric twin stars built for each group.

The Nearby Dwarf Irregular Galaxy IC 1613 and Its Complex Bubble Region: Chandra and VLA Observations

Abstract We observed the nearby irregular galaxy IC 1613 with the Chandra X-ray Observatory, for 49.9 ks using the ACIS S3 chip, and with the VLA at 6 cm for 0.7 hr. The Chandra observation targeted the extensive bubble region in the northeast quadrant. We detect 31 sources within the D 25 diameter above a significance of 3σ corresponding to a point-source luminosity limit L X of ≈2 × 1035 erg s−1 (0.3–8 keV band). The VLA observation covered the entire galaxy with an angular resolution of 5″. We detect 11 sources: five do not fall on the ACIS chips, four fall on the S3 CCD, and two sit on the edge of the S2 CCD. We also describe corresponding Spitzer IRAC and MIPS images. Our detected source list matches the lists of two previous global point-source analyses except at the faint end. With the exception of one bright supernova remnant, the sources appear to be low-luminosity stellar sources (e.g., RS CVn stars) or X-ray binaries, or background objects. We identify one candidate supersoft source. All but one of the sources have at least one optical or infrared counterpart; three sources have radio counterparts. We describe our search for diffuse emission from the bubble region. We assign an upper limit of ≈5 × 1035 erg s−1 on the integrated flux and ≈1.4 × 1030 erg s−1 pc−2 on the surface brightness emission in the 0.5–2 keV band from that region.

Dynamical Surface Imaging of λ Andromedae

Abstract We present temperature maps of RS CVn star λ Andromedae, reconstructed from interferometric data acquired in 2010 and 2011 by the Michigan Infra-Red Combiner instrument at the Center for High Angular Resolution Astronomy Array. To constrain the stellar parameters required for this imaging task, we first modeled the star using our GPU-accelerated code SImulation and Modeling Tool for Optical Interferometry. The stellar surface was then imaged using our open source interferometric imaging code ROTational Image Reconstruction, in the process further refining the estimation of stellar parameters. We report that the measured angular diameter is 2.742 ± 0.010 mas with a limb-darkening coefficient of 0.231 ± 0.024. While our images are consistent with those of prior works, we provide updated physical parameters for λ Andromedae (R ⋆ = 7.78 ± 0.05 R ⊙, M ⋆ = 1.24 ± 0.72 M ⊙, and log L/L ⊙ = 1.46 ± 0.04).

A Classification Algorithm for Time-domain Novelties in Preparation for LSST Alerts. Application to Variable Stars and Transients Detected with DECam in the Galactic Bulge

Abstract With the advent of the Legacy Survey of Space and Time, time-domain astronomy will be faced with an unprecedented volume and rate of data. Real-time processing of variables and transients detected by such large-scale surveys is critical to identifying the more unusual events and allocating scarce follow-up resources efficiently. We develop an algorithm to identify these novel events within a given population of variable sources. We determine the distributions of magnitude changes (dm) over time intervals (dt) for a given passband f, , and use these distributions to compute the likelihood of a test source being consistent with the population or being an outlier. We demonstrate our algorithm by applying it to the DECam multiband time-series data of more than 2000 variable stars identified by Saha et al. in the Galactic Bulge that are largely dominated by long-period variables and pulsating stars. Our algorithm discovers 18 outlier sources in the sample, including a microlensing event, a dwarf nova, and two chromospherically active RS CVn stars, as well as sources in the blue horizontal branch region of the color–magnitude diagram without any known counterparts. We compare the performance of our algorithm for novelty detection with the multivariate Kernel Density Estimator and Isolation Forest on the simulated PLAsTiCC data set. We find that our algorithm yields comparable results despite its simplicity. Our method provides an efficient way for flagging the most unusual events in a real-time alert-broker system.

Contribution of GT-Mus to the 6.7 keV Emission Line from the Galactic Ridge

We performed the spectral analysis of Suzaku data on GT-Mus which was observed to emit 6.7 keV during its flaring period. GT-Mus was observed by the Suzaku team on December 12, 2007, with observation identity (402095010) for 96 kilo seconds. We downloaded GT-Mus data from the high energy astrophysics Suzaku archive. Our data reduction and analysis were done using XSELECT version 6.9 and XSPEC version 12.8. Errors reported in this work were done using the XPEC error command. We generated the spectrum and deduced a strong He-like (6.70 keV) emission from the source with Equivalent width of 282 ± 0.02 eV. This observed 6.7 keV emission line has an equivalent width which compares favorably with the equivalent width of 6.7 keV emission line from the galactic ridge (300-980 eV) depending on the Galactic position. From our analysis, we generated the light curve of the source which showed strong evidence of stellar flare. We therefore conclude that this observed stellar flare might be responsible for the observed 6.7 keV emission line. We however suggest that GT-Mus (HD101379) and other RS CVn stars that emit in 6.7 keV line could contribute to the 6.7 kev emission line from the galactic ridge during their flaring periods, since they exhibit the same level of chromospheric activities.

Photometry and spectroscopy of II Peg, IM Peg and UX Ari

V band photometry of three RS CVn stars, II Peg, IM Peg and UX Ari, is carried out to study the physical properties of these variables. We verified the significant and regular optical photometric variability to be present in all three stars. The strong photometric variability and emission of Hα and Ca II H & K using high resolution optical spectroscopy with the Hanle Echelle Spectrograph (HESP), which operates in conjunction with the Himalayan Chandra Telescope, verify the strong chromospheric activity which is present in RS CVn stars. The photometric studies of II Peg, UX Ari and IM Peg were subjected to light curve analysis for spot parameters using a two-starspot model.

The Catalina Surveys Southern periodic variable star catalogue

U.S. National Science Foundation [AST-1313422, AST-1413600]; National Aeronautics and Space Administration [NNG05GF22G]; CONICYT's PCI program [DPI20140066]; Ministry for the Economy, Development, and Tourism's Iniciativa Cientifica Milenio [IC 120009]; Proyecto Fondecyt [1141141]; Proyecto Basal [PFB-06/2007]; National Aeronautics and Space Administration; National Science Foundation [1138766]

Optical flare events on the RS Canum Venaticorum star UX Arietis

Based on long-term high-resolution spectroscopic observations obtained during five observing runs from 2001 to 2004, we study optical flare events and chromospheric activity variability of the very active RS CVn star UX Ari. By means of the spectral subtraction technique, several optical chromospheric activity indicators (including the He i D3, Na i D1, D2 doublet, Hα and Ca ii IRT lines) covered in our echelle spectra were analyzed. Four large optical flare events were detected on UX Ari during our observations, which show prominent He i D3 line emission together with great enhancement in emission of the Hα and Ca ii IRT lines and strong filled-in or emission reversal features in the Na i D1, D2 doublet lines. The newly detected flares are much more energetic than previous discoveries, especially for the flare identified during the 2002 December observing run. Optical flare events on UX Ari are more likely to be observed around two quadratures of the system, except for our optical flares detected during the 2004 November observing run. Moreover, we have found rotational modulation of chromospheric activity in the Hα and Ca ii IRT lines, which suggests the presence of chromospherically active longitudes over the surface of UX Ari. The change in chromospherically active longitudes among our observing runs, as well as the variation in chromospheric activity level from 2001 to 2004, indicates a long-term evolution of active regions.

KOI-1003: A NEW SPOTTED, ECLIPSING RS CVN BINARY IN THE KEPLER FIELD

ABSTRACT Using the high-precision photometry from the Kepler space telescope, thousands of stars with stellar and planetary companions have been observed. The characterization of stars with companions is not always straightforward and can be contaminated by systematic and stellar influences on the light curves. Here, through a detailed analysis of starspots and eclipses, we identify KOI-1003 as a new, active RS CVn star—the first identified with data from Kepler. The Kepler light curve of this close binary system exhibits the system’s primary transit, secondary eclipse, and starspot evolution of two persistent active longitudes. The near equality of the system’s orbital and rotation periods indicates the orbit and primary star’s rotation are nearly synchronized ( ; ). By assuming the secondary star is on the main sequence, we suggest the system consists of a subgiant primary and a main-sequence companion. Our work gives a distance of 4400 ± 600 pc and an age of , parameters which are discrepant with previous studies that included the star as a member of the open cluster NGC 6791.

A possible long-term activity cycle for ι Horologii: First results from SPI-HKα project

To detect stellar activity cycles and study the possible star-planet interactions (SPI's), we have developed both HK$\alpha$ and SPI-HK$\alpha$ projects since 1999 and 2012 respectively. In this work, we present preliminary results of possible SPI's studying the chromospheric activity and look for possible correlations between stellar activity and stellar/planetary parameters. We find that for stars with similar T$_{eff}$, stellar activity increases with the mass of the planet, similar to previous works. However, stellar ages can also play a role and a larger stellar sample is needed to verify these trends. We also note that some of these stars present a remarkably high level of chromospheric activity, even comparable with RSCvn or BY Dra active stars. In addition, we do not observe any correlation between stellar activity and semi-major axis. We present the first long-term activity study of the star $\iota$ Horologii, a young solar-type star which hosts a non-transiting Jovian planet and presents a high activity level. We analyze our own spectra, obtained between 2002 and 2015, combined with public HARPS observations. We calculate the Ca II indexes derived from the 987 CASLEO and HARPS spectra and convert them to the Mount-Wilson scale. We found a long-term activity cycle of $\sim$ 5 years which fits the active sequence of Bohm-Vitense. The amplitude of this longer cycle is irregular, as was also observed for the shorter one. This fact could be attributed to an antisymmetric distribution of active regions on the stellar surface.

FIRST ZEEMAN DOPPLER IMAGING OF A COOL STAR USING ALL FOUR STOKES PARAMETERS

Magnetic fields are ubiquitous in active cool stars but they are in general complex and weak. Current Zeeman Doppler imaging (ZDI) studies of cool star magnetic fields chiefly employ circular polarization observations because linear polarization is difficult to detect and requires a more sophisticated radiative transfer modeling to interpret. But it has been shown in previous theoretical studies, and in the observational analyses of magnetic Ap stars, that including linear polarization in the magnetic inversion process makes it possible to correctly recover many otherwise lost or misinterpreted magnetic features. We have obtained phase-resolved observations in all four Stokes parameters of the RS CVn star II Peg at two separate epochs. Here we present temperature and magnetic field maps reconstructed for this star using all four Stokes parameters. This is the very first such ZDI study of a cool active star. Our magnetic inversions reveal a highly structured magnetic field topology for both epochs. The strength of some surface features is doubled or even quadrupled when linear polarization is taken into account. The total magnetic energy of the reconstructed field map also becomes about 2.1-3.5 times higher. The overall complexity is also increased as the field energy is shifted towards higher harmonic modes when four Stokes parameters are used. As a consequence, the potential field extrapolation of the four Stokes parameter ZDI results indicates that magnetic field becomes weaker at a distance of several stellar radii due to a decrease of the large-scale field component.

Evolutionary status of the active star PZ Mon

We use original spectra and available photometric data to recover parameters of the stellar atmosphere of PZ Mon, formerly referred as an active red dwarf. The derived effective temperature Teff=4700K and gravity lgg=2.8 suggest that PZ Mon is a K2III giant. Stellar atmosphere parameters (Teff and lgg) alongside with the evolutionary tracks are used to estimate the stellar mass of ~1.5Mo and the radius of ~7.7Ro. The angular radius derived by the infrared flux method when combined with the linear radius suggests the distance of 250+/-70 pc, a factor 2.5 smaller than that suggested by the Hipparcos parallax. The red giant status of PZ Mon is confirmed by the carbon and nitrogen abundance. The spectrum reveals pronounced HeI 5876 A absorption and Halpha emission indicating the robust chromosphere. The IUE spectrum is found to contain transition layer emission line of CIV 1550 A. The CIV and X-ray luminosities turn out typical of RS CVn stars. The extended set of available photometric data confirms the period of 34.14 days presumably related to the stellar rotation. We found variations of the radial velocity with the amplitude of ~8 kms which could be caused by the orbital motion.

Spots on the surfaces of late-type stars

The spottedness of two stars characterized by significant photometric variability is studied using published data: the recently discovered variable ASAS 063656-0521.0, whose V variability reaches 0.8m, and XXTri (HD12545), which is among the most active RSCVn stars (in 1997–1998, the amplitude of its V variability was 0.63m). The spots cover up to 44% of the total visible surface S of ASAS 063656-0521.0. The mean estimated spottedness of XX Tri was 32%, and varied from29% to 36%. An analysis of the dependence of the spottedness on the properties of spotted stars, primarily their effective temperatures, is also presented. A modification of a simplifiedmethod for estimating the spottedness S, i.e., the fractional surface area of the spots, is applied to a sample of 48 late-type stars. The dependences of the spottedness on the effective temperature of the stars and the rotational velocity projected onto the line of sight are derived. Two groups of objects can be distinguished. The first contains stars displaying the typical dependence of S on the effective temperature (their maximum value of S is 20–25% for stars with temperatures 4500–5000 K, and S decreases for solar-type stars and cool M dwarfs). The second group is formed of the most active stars, which have temperatures of 3700–5200 K and S values from 25% to 50%. Our preliminary conclusion is that spottedness is not related to the period of the stellar rotation. The previously studied variable V410 Tau is used to consider the shortcomings of the method applied compared to the results of light-curve modeling.

Spot activity of the RS Canum Venaticorum starσGeminorum

We model the photometry of RS CVn star $\sigma$ Geminorum to obtain new information on the changes of the surface starspot distribution, i.e., activity cycles, differential rotation and active longitudes. We use the previously published Continuous Periods Search-method (CPS) to analyse V-band differential photometry obtained between the years 1987 and 2010 with the T3 0.4 m Automated Telescope at the Fairborn Observatory. The CPS-method divides data into short subsets and then models the light curves with Fourier-models of variable orders and provides estimates of the mean magnitude, amplitude, period and light curve minima. These light curve parameters are then analysed for signs of activity cycles, differential rotation and active longitudes. We confirm the presence of two previously found stable active longitudes, synchronised with the orbital period $P_{\rm{orb}}=19.60$d and find eight events where the active longitudes are disrupted. The epochs of the primary light curve minima rotate with a shorter period $P_{\rm{min,1}}=19.47$d than the orbital motion. If the variations in the photometric rotation period were to be caused by differential rotation, this would give a differential rotation coefficient of $\alpha \ge 0.103$. The presence of two slightly different periods of active regions may indicate a superposition of two dynamo modes, one stationary in the orbital frame and the other one propagating in the azimuthal direction. Our estimate of the differential rotation is much higher than previous results. However, simulations show that this can be caused by insufficient sampling in our data.

Magnetic field topology of the RS CVn star II Pegasi

The dynamo processes in cool active stars generate complex magnetic fields responsible for prominent surface stellar activity and variability at different time scales. For a small number of cool stars magnetic field topologies were reconstructed from the time series of spectropolarimetric observations using the Zeeman Doppler imaging (ZDI) method. In this study we follow a long-term evolution of the magnetic field topology of the RS CVn binary star II Peg. We collected high-resolution circular polarisation observations of II Peg using the SOFIN spectropolarimeter at the Nordic Optical Telescope. These data cover 12 epochs spread over 7 years. A multi-line diagnostic technique in combination with a new ZDI code is applied to interpret these observations. Magnetic inversions using these data reveals evolving magnetic fields with typical local strengths of 0.5-1.0 kG and complex topologies. Despite using a self-consistent magnetic and temperature mapping technique, we do not find a clear correlation between magnetic and temperature features in the ZDI maps. Neither do we confirm the presence of persistent azimuthal field rings found in other RS CVn stars. Reconstruction of the magnetic field topology of II Peg reveals significant evolution of both the surface magnetic field structure and the extended magnetospheric field geometry. From 2004 to 2010 the total field energy drastically declined and the field became less axisymmetric. This also coincided with the transition from predominantly poloidal to mainly toroidal field topology. A qualitative comparison of the ZDI maps of II Peg with the prediction of dynamo theory suggests that the magnetic field in this star is produced mainly by the turbulent alpha^2 dynamo rather than the solar alphaOmega dynamo. Our results do not show a clear active longitude system, nor is there an evidence of the presence of an azimuthal dynamo wave.

MOA-2010-BLG-523: “FAILED PLANET” = RS CVn STAR

The Galactic bulge source MOA-2010-BLG-523S exhibited short-term deviations from a standard microlensing lightcurve near the peak of an Amax ~ 265 high-magnification microlensing event. The deviations originally seemed consistent with expectations for a planetary companion to the principal lens. We combine long-term photometric monitoring with a previously published high-resolution spectrum taken near peak to demonstrate that this is an RS CVn variable, so that planetary microlensing is not required to explain the lightcurve deviations. This is the first spectroscopically confirmed RS CVn star discovered in the Galactic bulge.

Suzaku Observations of Unidentified X-Ray Sources toward the Galactic Bulge

Abstract We present Suzaku observations of two unidentified sources towards the Galactic bulge, 1RXS J165256.3$-$264503 and 1RXS J174459.5$-$172640, both of which were discovered with the ROSAT All-Sky Survey. We obtained wide-band spectra of these sources above 2 keV for the first time. The X-ray spectrum of 1RXS J165256.3$-$264503 consists of two absorbed thin thermal plasma components with temperatures of 0.7 and 2.4 keV. An RS CVn star, HD 152178, was found within a 90% error circle of the source position. Due to this position coincidence, and an X-ray spectrum analogous to those of other RS CVn binaries, we concluded that 1RXS J165256.3$-$264503 is an RS CVn-type active binary. The X-ray spectrum of 1RXS J174459.5$-$172640 was dominated by continuum emission extending up to $\sim$ 20 keV, which can be explained by an absorbed cutoff power-law or a combination of absorbed multicolor disk-blackbody and Comptonized emission. We inferred that the source is a new Galactic-bulge low-mass X-ray binary (LMXB) with a luminosity of 2 $\times$ 10$^{35}$ erg s$^{-1}$, based on its X-ray spectrum and lightcurve. Hence, 1RXS J174459.5$-$172640 may be an intriguing example of an LMXB in an intermediate state in which X-ray emission mechanisms can be readily explored.

Chemical composition of photospheres in RS CVn stars

AbstractIn order to investigate how magnetic activity influences a stellar evolution, we undertake a detailed study of photospheric abundances in a sample of RS CVn stars. Using the Nordic Optical Telescope we investigate abundances of more than 20 chemical elements, including carbon isotopes, nitrogen and other mixing-sensitive species. In this presentation we report on current results of the analysis, among which there are first evidences that extra-mixing processes may start acting in low-mass chromospherically active stars below a bump of the luminosity function of red giants.

Matching microlensing events with X-ray sources

The detection of old neutron stars and black holes in isolation is one of the cornerstones of compact object astrophysics. Microlensing surveys may help on this purpose since the lensing mechanism is independent of the emission properties of the lens. Indeed, several black hole candidates deriving through microlensing observations have been reported in the literature. The identification of counterparts, especially in the X-rays, would be a strong argument in favor of the compact nature of these lenses. We perform a cross-correlation between the catalogs of microlensing events by the OGLE, MACHO and MOA teams, and those of X-rays sources from XMM-Newton and Chandra satellites. Based on our previous work, we select only microlensing events longer than 100 days, which should contain a large fraction of lenses as compact objects. Our matching criterion takes into account the positional coincidence in the sky. We find a single match between a microlensing event OGLE 2004-BLG-81 and the X-ray source 2XMM J180540.5-273427. The angular separation is \sim 0.5 arc-seconds, well inside the 90% error box of the X-ray source. The hardness ratios reported in the 2XMM catalog suggest a hard spectrum with a peak between 2 and 4.5 keV or a softer but highly absorbed source. Moreover the microlensing event is not fully constrained, and possible association with a flaring CV or a RS CVn star have been suggested as well. The small angular separation is a strong indicator that 2XMM J180540.5-273427 is the X-ray counterpart of the OGLE event. However, the uncertainties on both the nature of lensed system and the lens itself challenge the interpretation of 2XMM J180540.5-273427 as the first confirmed isolated black hole identified so far. In any case, it probes the diagnostic capacity of microlensing surveys and open the path for further identifications of black hole or neutron star candidates.

Spot activity of II Peg

AbstractWe have studied the long‐term spot activity of the RS CVn star II Peg by means of Doppler imaging based on spectroscopy and time series analysis of photometry. We present 28 Doppler imaging temperature maps spanning the years 1994–2010, of which 14 were calculated for the present study. The longitudinal spot distribution, derived from the surface temperature maps, is compared with epochs of the light curve minima, derived from photometric observations. We detect a longitudinal drift in the major spot structure during 1995–2003. After this there is a clear decrease in the activity level and no clear drift can be seen. We conclude that the variations could be caused by a cyclic behaviour of the underlying magnetic dynamo (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)