Lifetime of Surface Features and Stellar Rotation: A Wavelet Time-Frequency Approach

We explore subtle variations in disk-integrated measurements spanning \lsim 18 years of stellar surface magnetism by using a newly developed time-frequency gapped wavelet algorithm. We present results based on analysis of the Mount Wilson Ca II H and K emission fluxes in four, magnetically-active stars (HD 1835 [G2V], 82885 [G8IV-V], 149661 [K0V] and 190007 [K4V]) and sensitivity tests using artificial data. When the wavelet basis is appropriately modified (i.e., when the time-frequency resolution is optimized), the results are consistent with the existence of spatially localized and long-lived Ca II features (assumed here as activity regions that tend to recur in narrowly-confined latitude bands), especially in HD 1835 and 82885. This interpretation is based on the observed persistence of relatively localized Ca II wavelet power at a narrow range of rotational time scales, enduring as long as \gsim 10 years.Comment: to appear in THE ASTROPHYSICAL JOURNAL LETTER

High-amplitude, long-term X-ray variability in the solar-type star HD 81809: the beginning of an X-ray activity cycle?

We present the initial results from our XMM program aimed at searching for X-ray activity cycles in solar-type stars. HD 81809 is a G2-type star (somewhat more evolved than the Sun, and with a less massive companion) with a pronounced 8.2 yr chromospheric cycle, as evident from from the Mt. Wilson program data. We present here the results from the initial 2.5 years of XMM observations, showing that large amplitude (a factor of approx. 10) modulation is present in the X-ray luminosity, with a clearly defined maximum in mid 2002 and a steady decrease since then. The maximum of the chromospheric cycle took place in 2001; if the observed X-ray variability is the initial part of an X-ray cycle, this could imply a phase shift between chromospheric and coronal activity, although the current descent into chromospheric cycle minimum is well reflected into the star's X-ray luminosity. The observations presented here provide clear evidence for the presence of large amplitude X-ray variability coherent with the activity cycle in the chromosphere in a star other than the Sun.Comment: Accepted for publication in A&

51 Pegasi - a planet-bearing Maunder minimum candidate

We observed 51 Peg, the first detected planet-bearing star, in a 55 ks XMM-Newton pointing and in 5 ks pointings each with Chandra HRC-I and ACIS-S. The star has a very low count rate in the XMM observation, but is clearly visible in the Chandra images due to the detectors' different sensitivity at low X-ray energies. This allows a temperature estimate for 51 Peg's corona of T<1MK; the detected ACIS-S photons can be plausibly explained by emission lines of a very cool plasma near 200eV. The constantly low X-ray surface flux and the flat-activity profile seen in optical CaII data suggest that 51 Peg is a Maunder minimum star; an activity enhancement due to a Hot Jupiter, as proposed by recent studies, seems to be absent. The star's X-ray fluxes in different instruments are consistent with the exception of the HRC Imager, which might have a larger effective area below 200eV than given in the calibration.Comment: accepted by A&

Magnetic cycles at different ages of stars

We study the different patterns of interannual magnetic variability in stars on or near the lower main sequence, approximately solar-type (G-K dwarf) stars in time series of 36 years from the Mount Wilson Observatory Ca\,{\sc ii}\,H\&K survey. Our main aim is to search for correlations between cycles, activity measures and ages. Time-frequency analysis has been used to discern and reveal patterns and morphology of stellar activity cycles, including multiple and changing cycles, in the datasets. Both the results from short-term Fourier transform and its refinement using the Choi-Williams distribution, with better frequency resolution, are presented in this study. Rotational periods of the stars were derived using multi-frequency Fourier analysis.From the studied 29 stars we found at least one activity cycle on 28 of them. Twelve stars, with longer rotational periods ($39.7\pm6.0$ days) have simple, smooth cycles, and the rest of the stars, with on-average much faster rotation ($18.1\pm12.2$ days) show complex and sometimes vigorously changing, multiple cycles. The cycles are longer and quite uniform in the first group ($9.7\pm1.9$ years), while are generally shorter and with greater variety in the second one ($7.6\pm4.9$). There is a clear age division between stars with smooth and complex cycles that follows the known separation between the older and younger stars at around 2 to 3~Gyr of age.Comment: Accepted to Astronomy and Astrophysic

Climate hypersensitivity to solar forcing?

International audienceWe compare the equilibrium climate responses of a quasi-dynamical energy balance model to radiative forcing by equivalent changes in CO2, solar total irradiance (Stot) and solar UV (SUV). The response is largest in the SUV case, in which the imposed UV radiative forcing is preferentially absorbed in the layer above 250 mb, in contrast to the weak response from global-columnar radiative loading by increases in CO2 or Stot. The hypersensitive response of the climate system to solar UV forcing is caused by strongly coupled feedback involving vertical static stability, tropical thick cirrus ice clouds and stratospheric ozone. This mechanism offers a plausible explanation of the apparent hypersensitivity of climate to solar forcing, as suggested by analyses of recent climatic records. The model hypersensitivity strongly depends on climate parameters, especially cloud radiative properties, but is effective for arguably realistic values of these parameters. The proposed solar forcing mechanism should be further confirmed using other models (e.g., general circulation models) that may better capture radiative and dynamical couplings of the troposphere and stratosphere

Combined ultraviolet studies of astronomical sources

Topics addressed include: Cygnus Loop; P Cygni profiles in dwarf novae; YY Gem; nova shells; HZ Herculis; activity cycles in cluster giants; Alpha Ori; metal deficient giant stars; ultraviolet spectra of symbiotic stars detected by the Very Large Array; time variability in symbiotic stars; blue galaxies; and quasistellar objects with X-ray spectra

Dynamo models and differential rotation in late-type rapidly rotating stars

Increasing evidence is becoming available about not only the surface differential rotation of rapidly rotating cool stars but, in a small number of cases, also about temporal variations, which possibly are analogous to the solar torsional oscillations. Given the present difficulties in resolving the precise nature of such variations, due to both the short length and poor resolution of the available data, theoretical input is vital to help assess the modes of behaviour that might be expected, and will facilitate interpretation of the observations. Here we take a first step in this direction by studying the variations in the convection zones of such stars, using a two dimensional axisymmetric mean field dynamo model operating in a spherical shell in which the only nonlinearity is the action of the azimuthal component of the Lorentz force of the dynamo generated magnetic field on the stellar angular velocity. We consider three families of models with different depths of dynamo-active regions. For moderately supercritical dynamo numbers we find torsional oscillations that penetrate all the way down to the bottom of the convection zones, similar to the case of the Sun. For larger dynamo numbers we find fragmentation in some cases and sometimes there are other dynamical modes of behaviour, including quasi-periodicity and chaos. We find that the largest deviations in the angular velocity distribution caused by the Lorentz force are of the order of few percent, implying that the original assumed `background' rotation field is not strongly distorted.Comment: Astronomy and Astrophysics, in pres

Rotation periods of exoplanet host stars

The stellar rotation periods of ten exoplanet host stars have been determined using newly analysed Ca II H & K flux records from Mount Wilson Observatory and Stromgren b, y photometric measurements from Tennessee State University's automatic photometric telescopes (APTs) at Fairborn Observatory. Five of the rotation periods have not previously been reported, with that of HD 130322 very strongly detected at Prot = 26.1 \pm 3.5 d. The rotation periods of five other stars have been updated using new data. We use the rotation periods to derive the line-of-sight inclinations of the stellar rotation axes, which may be used to probe theories of planet formation and evolution when combined with the planetary orbital inclination found from other methods. Finally, we estimate the masses of fourteen exoplanets under the assumption that the stellar rotation axis is aligned with the orbital axis. We calculate the mass of HD 92788 b (28 MJ) to be within the low-mass brown dwarf regime and suggest that this object warrants further investigation to confirm its true nature.Comment: Accepted for publication in MNRAS. 15 pages, 11 figure

Fluxes in H\alpha and Ca II H and K for a sample of Southern stars

The main chromospheric activity indicator is the S index, which is esentially the ratio of the flux in the core of the Ca II H and K lines to the continuum nearby, and is well studied basically for stars from F to K. Another usual chromospheric proxy is the H\alpha line, which is beleived to be tightly correlated with the Ca II index. In this work we characterize both chromospheric activity indicators, one associated with the H and K Ca II lines and the other with H\alpha, for the whole range of late type stars, from F to M. We present periodical medium-resolution echelle observations covering the complete visual range, which were taken at the CASLEO Argentinean Observatory. These observations are distributed along 7 years. We use a total of 917 flux-calibrated spectra for 109 stars which range from F6 to M5. We statistically study these two indicators for stars of different activity levels and spectral types. We directly derive the conversion factor which translate the known S index to flux in the Ca II cores, and extend its calibration to a wider spectral range. We investigate the relation between the activity measurements in the calcium and hydrogen lines, and found that the usual correlation observed is basically the product of the dependence of each flux with stellar colour, and not the product of similar activity phenomena.Comment: 12 pages, including 11 figures and 2 tables. Accepted for publication in Astronomy and Astrophysic