Why do some spotted stars become bluer as they become fainter?

Chromospherically active, spotted stars generally become redder as well as fainter when large starspots rotate into view on the stellar disc. However, the RS CVn system UX Ari (a triple-lined system), becomes bluer as it gets fainter. One possible explanation is that hot, bright facular regions accompany the cool, dark photospheric spots of the active component. The bluer flux of the hotter, inactive component does not appear to be sufficient to explain the observed behaviour. We have begun a search for additional chromospherically active stars with a similar relation between colour and brightness, to investigate whether these relations can be explained in the same way. Our results for V711 Tau are presented here, and we conclude that the faculae explanation holds also in this case.Comment: 3 pages, 1 figure, proceedings from the 13th Cambridge Workshop on cool stars, stellar systems, and the Sun, Hamburg, Germany, July 2004, ESA-SP, in pres

A Survey of IUE Spectra of the Active Binary System UX Arietis

To investigate the ultraviolet (UV) activity of the bright, non-eclipsing, double-lined spectroscopic binary system UX Ari, IUE spectra (194 images) were taken from the IUE archive. The spectra, obtained during the period 1978-1996, show emission lines originating in the chromosphere and transition region. The long-wavelength low dispersion spectra were examined for ultraviolet excess by comparing the UV continuum level of UX Ari with the levels of $\kappa$ Cet and $\eta$ Cep in the spectral range 2100 $\AA$ - 3200 $\AA$ . The individual MgII h and k emission-line fluxes of component stars show that the contributions to the activity of the system for G5 V and K0 IV are about 20% and 80%, respectively. Apart from the flare event observed on 1979 Jan 1, there are some flux enhancements in the years 1987, 1991 and 1994 which may suggest existence of a periodicity of about 7-9 years. Examination of the ultraviolet excess in the system showed that there is some UV excess in UX Ari, which varies from 1% up to 24% with the exception of two images which showed no UV excess. The results revealed that there was an agreement on the source of emission regions which could be attributed to the magnetic activity. The contribution of G5 V and K0 IV components to the MgII activity of the system suggests a need to take into consideration the spot distribution not only on the surface of K0 IV but also on the surface of G5 V component of UX Ari.Comment: 15 pages, 10 figure

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

Context. The IUE database provides a large number of 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 22468 (V711 Tau, HR 1099, K1IV+G5V), HD 21242 (UX Ari, K0IV+G5V) and HD 224085 (II Peg, K2IV). Methods. We first obtain the Mount Wilson index S from the IUE high and low-resolution spectra. Secondly, we analyse with the Lomb-Scargle periodogram the mean annual index S and the amplitude of its rotational modulation. Results. For HD 22468 (V711 Tau, HR 1099), we found a possible chromospheric cycle with a period of 18 years and a shorter cycle with a period of 3 years, which could be associated to a chromospheric "flip-flop" cycle. The data of HD 224085 (II Peg) also suggest a chromospheric cycle of 21 years and a flip-flop cycle of 9 years. Finally, we obtained a possible chromospheric cycle of 7 years for HD 21242 (UX Ari).Comment: accepted for publication in Astronomy and Astrophysic

Limits of ultra-high-precision optical astrometry: Stellar surface structures

We investigate the astrometric effects of stellar surface structures as a practical limitation to ultra-high-precision astrometry, e.g. in the context of exoplanet searches, and to quantify the expected effects in different regions of the HR-diagram. Stellar surface structures are likely to produce fluctuations in the integrated flux and radial velocity of the star, as well as a variation of the observed photocentre, i.e. astrometric jitter, and closure phase. We use theoretical considerations supported by Monte Carlo simulations to derive statistical relations between the corresponding astrometric, photometric, and radial-velocity effects. For most stellar types the astrometric jitter due to stellar surface structures is expected to be of order 10 micro-AU or greater. This is more than the astrometric displacement typically caused by an Earth-size exoplanet in the habitable zone, which is about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with extremely low photometric variability (<0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of order 1 micro-AU, suffcient to allow the astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general negligible impact on the detection of large (Jupiter-size) planets and on the determination of stellar parallax and proper motion. From the starspot model we also conclude that the commonly used spot filling factor is not the most relevant parameter for quantifying the spottiness in terms of the resulting astrometric, photometric and radial-velocity variations.Comment: 12 pages, 4 figures, submitted to A&

Unravelling the nature of HD 81032 - a new RS CVn Binary

BVR photometric and quasi-simultaneous optical spectroscopic observations of the star HD 81032 have been carried out during the years 2000 - 2004. A photometric period of $18.802 \pm 0.07$ d has been detected for this star. A large group of spots with a migration period of $7.43 \pm 0.07$ years is inferred from the first three years of the data. H$\alpha$ and Ca II H and K emissions from the star indicate high chromospheric activity. The available photometry in the BVRIJHK bands is consistent with spectral type of K0 IV previously found for this star. We have also examined the spectral energy distribution of HD 81032 for the presence of an infrared colour excess using the 2MASS JHK and IRAS photometry, but found no significant excess in any band abovethe normal values expected for a star with this spectral type. We have also analyzed the X-ray emission properties of this star using data obtained by the ROSAT X-ray observatory during its All-Sky Survey phase. An X-ray flare of about 12 hours duration was detected during the two days of X-ray coverage obtained for this star. Its X-ray spectrum, while only containing 345 counts, is inconsistent with a single-temperature component solar-abundance coronal plasma model, but implies either the presence of two or more plasma components, non-solar abundances, or a combination of both of these properties. All of the above properties of HD 81032 suggest that it is a newly identified, evolved RS CVn binary.Comment: 18 pages, 10 figures, 3 tables, Accepted for the publication in JAp

Spots, plages, and flares on lambda Andromedae and II Pegasi

We present the results of a contemporaneous photometric and spectroscopic monitoring of lambda And and II Peg aimed at investigating the behavior of surface inhomogeneities in the atmospheres of these active stars which have nearly the same temperature but different gravity. The light curves and the modulation of the surface temperature, as recovered from LDRs, are used to map the photospheric spots, while the H-alpha emission has been used as an indicator of chromospheric inhomogeneities. The spot temperatures and sizes were derived from a spot model applied to the contemporaneous light and temperature curves. We find larger and cooler spots on II Peg (T_sp ~ 3600 K) compared to lambda And (T_sp ~ 3900 K); this could be the result of both the different gravity and the higher activity level of the former. Moreover, we find a clear anti-correlation between the H-alpha emission and the photospheric diagnostics. We have also detected a modulation of the intensity of the HeI D_3 line with the star rotation. A rough reconstruction of the 3D structure of their atmospheres has been also performed by applying a spot/plage model to the light and temperature curves and to the H-alpha flux modulation. A close spatial association of photospheric and chromospheric active regions has been found in both stars. Larger and cooler spots have been found on II Peg, the system with the active component of higher gravity and higher activity level. The area ratio of plages to spots seems to decrease when the spots get bigger. Moreover, with the present and literature data, a correlation between the temperature difference Delta_T = T_ph - T_sp and the surface gravity has been also suggested. In addition, a strong flare affecting the H-alpha, the HeI D_3, and the cores of NaI D_1,2 lines has been observed on II Peg.Comment: Accepted for publication in A&A (7 November 2007) 12 Pages, 7 Figure

Recovering facular areas through Doppler imaging

Similar to the Sun, stellar active regions appear to consist of dark spots in combination with surrounding bright facular regions. The present study uses the Doppler imaging technique to recover the stellar surface structures from simulated spectral observations obtained from a given theoretical stellar surface. The objective of the study is to investigate how the recovered surface structure depends on observing parameters such as rotational phase coverage, choice of spectral lines, signal-to-noise ratio, and projected equatorial rotational velocity. The inversions demonstrate that those stellar surface active regions with high temperature contrast in both longitudinal and latitudinal directions can be satisfactorily reproduced using Doppler imaging. Observations must, however, fulfill such strict conditions that a complete reconstruction is somewhat unrealistic. Nevertheless, in a more realistic example, the hot facular areas result in quite distinct and easily recognizable features on the stellar surface map, even though they are not completely reconstructed

Why does the spotted

We present simplified calculations of UX Ar

Separation of high resolution spectra of the multiple star system

This paper presents a technique for separation of individual components in composite stellar spectra. The technique is based on a comparison of residual depths of the component spectral lines to residual depths of the corresponding spectral lines of a single star of the same spectral class. We apply the technique to high resolution spectra of the RS CV

BV photometry of UX Ari in the period 1987–2002

We present 14.3 years of previously unpublished photometric observations of UX Ari. The observations were obtained between 1987.9 and 2002.2 using the T3 0.4-metre Automatic Photoelectric Telescope at Fairborn Observatory and consist of 1228 B band and 1213 V band measurements. The comparison star was 62 Ari. We have analyzed the new data together with previously published photometric observations. The V magnitude shows variations with dominant periods of about 12 and 25 years, where the longest period seems to correspond to an activity cycle. The previously reported anticorrelation between the colour and V magnitude variations is confirmed