Willie Soon

Patterns of Photometric and Chromospheric Variation among Sun-like Stars: A 20 Year Perspective

We examine patterns of variation of 32 primarily main-sequence Sun-like stars [selected at project onset as stars on or near the main sequence and color index 0.42 ≤ (B - V) ≤ 1.4], extending our previous 7-12 yr time series to 13-20 yr by combining Stromgren b, y photometry from Lowell Observatory with similar data from Fairborn Observatory. Parallel chromospheric Ca II H and K emission data from the Mount Wilson Observatory span the entire interval. The extended data strengthen the relationship between chromospheric and brightness variability at visible wavelengths derived previously. We show that the full range of photometric variation has probably now been observed for a majority of the program stars. Twenty-seven stars are deemed variable according to an objective statistical criterion. On a year-to-year timescale, young active stars become fainter when their Ca II emission increases, while older less active stars such as the Sun become brighter when their Ca II emission increases. The Suns total irradiance variation, scaled to the b and y stellar filter photometry, still appears to be somewhat smaller than stars in our limited sample with similar mean chromospheric activity, but we now regard this discrepancy as probably due mainly to our limited stellar sample.

Photometric and Ca II H and K Spectroscopic Variations in Nearby Sun-like Stars with Planets. III.

We present the results of an analysis of time-series photometry, Ca II H and K spectrophotometry, and high-dispersion visible spectra of nine nearby Sun-like stars recently identified as having planets. For the six stars whose presumed planets have orbital periods of less than 4 months (? Boo, 51 Peg, ? And, ?1 Cnc, ? CrB, and 70 Vir), sine-curve fits to the photometric data show no variations with semiamplitude greater than 1 or 2 parts in 104. Photometric variations in 47 UMa are similarly small, although our photometric data of this star are slightly affected by variability of the comparison star. Nonvariability at this level of precision is sufficient to rule out surface magnetic activity as the cause of the observed radial-velocity variations in these seven stars and makes nonradial pulsations unlikely as well. Thus, our photometry provides indirect but strong support for true reflex motions?planets?in these seven stars, but cannot yet so support the planetary hypothesis for the two additional stars, 16 Cyg B and Gl 411. Continued photometric monitoring of the short-period systems may soon result in the direct detection of these planets in reflected light. We have used our photometric fluxes to search for possible transits of the extrasolar planets. Transits definitely do not occur in ? Boo, 51 Peg, ? And, and ?1 Cnc, and probably do not occur in ? CrB and 70 Vir. Our transit-search results are inconclusive for 47 UMa, and we cannot address the issue for 16 Cyg B and Gl 411. The precision of our photometry is sufficient to detect transits of planets even if they are not gas giants, as currently assumed, but much smaller objects with rocky compositions. The chance of finding at least one transit in the six stars is ~40%. We find significant year-to-year photometric variability only in ? Boo, which is not only the youngest star in the sample but also the star with the shallowest convective zone. The interseasonal range in its yearly mean photometric flux is ~0.002 mag, roughly twice the 0.0008 mag decadal variation in the Suns total irradiance. Monitoring of the relative Ca II H and K fluxes began between 1966 and 1968 for 51 Peg, ? Boo, ? CrB, and Gl 411, between 1990 and 1993 for 47 UMa, 70 Vir, 16 Cyg B, and ?1 Cnc, and in 1996 for ? And. The data have been newly recalibrated for improved long-term instrumental stability, resulting in better precision of the Ca II records. Five of the nine stars in this study have little or no detectable year-to-year variation in Ca II flux. The remaining four show moderate or pronounced variability: ? Boo, whose radial-velocity and photometric variations have comparatively high amplitudes; Gl 411, whose planetary companion was inferred astrometrically, not spectroscopically; ?1 Cnc, which may undergo decadal cyclic activity; and ? And, which shows moderate year-to-year variability. Except for 47 UMa, intraseasonal variability consistent with rotation was detected in the Ca II records of all stars. However, the rotation periods determined for ? And, 70 Vir, and 16 Cyg B are of low confidence. An examination of the recalibrated Ca II records for 51 Peg finds a rotation period of 22 days, in contrast to our previous result of 37 days. Ages have been estimated from the mean Ca II flux and, where possible, the rotation period. We find general consistency with the ages determined by others comparing properties determined from high-resolution spectroscopy to evolutionary models, although the uncertainties are, in general, large.

Multiple and changing cycles of active stars II. Results

Aims. We study the time variations in the cycles of 20 active stars based on decade-long photometric or spectroscopic observations. Methods. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data. Results. Fifteen stars definitely show multiple cycles, but the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics and vary in parallel, indicating a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars. Conclusions. Stellar activity cycles are generally multiple and variable.

Magnetic Field and Rotation in Lower Main-Sequence Stars: An Empirical Time-Dependent Magnetic Bode's Relation?

We find a significant correlation between the magnetic and rotational moments for a sample of 112 lower main-sequence stars. The rotational moment is calculated from measurements of the rotation period in most of the stars (not from the projected rotational velocity inferred from Doppler broadening). The magnetic moment is computed from a database of homogeneous measurements of the mean level of Ca II H and K emission fluxes sampled for most of the stars over an interval of 25 yr. The slope connecting the logarithm of the magnetic moment and the logarithm of the rotational moment is about +0.5-0.6, with a Pearson correlation coefficient of about +0.9. The scatter of points from the mean relation has a component that is natural and caused by decade-long surface variability.

Time scales and trends in the central England temperature data (1659–1990): A wavelet analysis

We have applied the standard wavelet and the adaptive wavelet transform algorithms to the record of the Central England Temperature (CET) from 1659–1990. Peaks in the CET spectra include 7.5±1.0 yr, 14.4±1.0 yr, 23.5±2.0 yr, as well as a previously unreported variation at 102±15 yr. Our wavelet analysis of CET agrees with previous results from Singular Spectrum Analysis (SSA) by Plaut et al. [1995] and gives additional results of variability on longer timescales. The interdecadal and century-scale variability in CET is strongly dependent on the interval of analysis. Estimates of a data trend are also shown to be sensitive to the cutoff timescale of the filter. A cooling of ≈ 0.3°C during 1659–1720 is found relative to the temperatures during the 1800s. The complex time dependence of the actual data cautions against using model-derived representations of natural variability on such long timescales.

Properties of Sun-like Stars with Planets: ρ1 Cancri, τ Bootis, and υ Andromedae

Planets have been reported orbiting the Sun-like stars ρ1 Cnc, τ Boo, and υ And based on low-amplitude radial velocity variations. We have derived information on the first two stars from analysis of spectra, as well as parallel records of high-precision Stromgren b and y photometry and Ca II H + K fluxes. In the case of ρ1 Cnc, the upper limit (peak to peak) of nondetection of photometric variability at the orbital period is Δy ~ 0.0004 mag. The possibility of a planetary transit cannot be ruled out completely from the photometric data. Variations of the Ca II fluxes suggest a rotational period of ~42 days, in agreement with the inferred v sin i ~ 2 km s-1. The age of ρ1 Cnc is ~5 Gyr, based on its average Ca II flux and a relation between Ca II flux and age. The star τ Boo, unlike the other reported solar-type stars with planets, is relatively young (~2 Gyr). Despite its young age, it is photometrically nonvariable at the orbital period with an amplitude of Δ(b + y)/2 ~ 0.0004 mag (peak to peak); however, small-amplitude interseasonal variability is seen. No planetary transits were found in the photometry, which limits the inclination of the planets orbital plane to Earths line of sight to less than 83° (where 90° is coplanar). The Ca II record shows a weakly significant rotational period near 3.3 days, coincident with the orbital period of the companion. The Ca II record also shows a period of 116 days that has persisted for 30 years and is not seen in the photometric record. The persistence and timescale of this Ca II variation mean that it has no counterpart in Sun-like magnetic activity. The amplitude of the reflex velocity of the parent star (~450 m s-1) is much larger than the radial velocity perturbations expected from the presence of either surface inhomogeneities or line-bisector variations. Thus the anticipated perturbations from those stellar effects do not refute the inference of reflex velocities. We have few Ca II flux measurements for υ And. Its age and rotational period are estimated to be ~5 Gyr and 12 days, respectively. Our results for ρ1 Cnc and τ Boo are consistent with the explanation of planets as the cause of the velocity variations.

Reconstructing Climatic and Environmental Changes of the Past 1000 Years: A Reappraisal

The 1000-year climatic and environmental history of the Earth contained in various proxy records is examined. As indicators, the proxies duly represent or record aspects of local climate. Questions on the relevance and validity of the locality paradigm for climatological research become sharper as studies of climatic changes on timescales of 50–100 years or longer are pursued. This is because thermal and dynamical constraints imposed by local geography become increasingly important as the air-sea-land interaction and coupling timescales increase. Because the nature of the various proxy climate indicators are so different, the results cannot be combined into a simple hemispheric or global quantitative composite. However, considered as an ensemble of individual observations, an assemblage of the local representations of climate establishes the reality of both the Little Ice Age and the Medieval Warm Period as climatic anomalies with world-wide imprints, extending earlier results by Bryson et al. (1963), Lamb (1965), and numerous other research efforts. Furthermore, these individual proxies are used to determine whether the 20th century is the warmest century of the 2nd Millennium at a variety of globally dispersed locations. Many records reveal that the 20th century is likely not the warmest nor a uniquely extreme climatic period of the last millennium, although it is clear that human activity has significantly impacted some local environments.

The Maunder minimum (1645–1715) was indeed a grand minimum: A reassessment of multiple datasets

Aims. Although the time of the Maunder minimum (1645–1715) is widely known as a period of extremely low solar activity, it is still being debated whether solar activity during that period might have been moderate or even higher than the current solar cycle #24. We have revisited all existing evidence and datasets, both direct and indirect, to assess the level of solar activity during the Maunder minimum. Methods. We discuss the East Asian naked-eye sunspot observations, the telescopic solar observations, the fraction of sunspot active days, the latitudinal extent of sunspot positions, auroral sightings at high latitudes, cosmogenic radionuclide data as well as solar eclipse observations for that period. We also consider peculiar features of the Sun (very strong hemispheric asymmetry of the sunspot location, unusual differential rotation and the lack of the K-corona) that imply a special mode of solar activity during the Maunder minimum. Results. The level of solar activity during the Maunder minimum is reassessed on the basis of all available datasets. Conclusions. We conclude that solar activity was indeed at an exceptionally low level during the Maunder minimum. Although the exact level is still unclear, it was definitely lower than during the Dalton minimum of around 1800 and significantly below that of the current solar cycle #24. Claims of a moderate-to-high level of solar activity during the Maunder minimum are rejected with a high confidence level.

Wavelet Analysis of Stellar Chromospheric Activity Variations

Observations of chromospheric activity variations for some lower main-sequence stars from the Mount Wilson Observatorys HK project reveal a cyclic behavior comparable to the sunspot cycle. Even in the relatively short interval that they have been observed, those stars show stellar cycles and other features, like grand minima. The quasi-periodic nature of such variations is not completely compatible with the standard Fourier analysis, so we applied a wavelet analysis to study the nature of regularities in the data. We computed wavelet transforms and energy spectra for the 25 yr records of surface magnetic activity in four stars: HD 3651, HD 10700, HD 10476, and HD 201091. We present a modified wavelet technique that is suitable for analysis of data with gaps and find that the common aliasing problems due to the finite length of the observations and irregularly spaced gaps between data can be reduced on both large and small scales by applying this algorithm.

Variations of solar coronal hole area and terrestrial lower tropospheric air temperature from 1979 to mid-1998: astronomical forcings of change in earth's climate?

Abstract The temperature anomaly of the terrestrial lower troposphere, inferred from the Microwave Sounding Unit (MSU) radiometers, is found to be inversely correlated with the area of the Sun covered by coronal holes. The correlation between the monthly time series of global tropospheric temperature anomaly and total coronal hole area from January 1979 to April 1998 has a Pearson coefficient of −0.46, which is different from zero at a 95% confidence level. Physical reasonings for the explained and unexplained parts of the correlation are discussed. The coronal hole area is a physical proxy for both the global-scale, 22-yr geometrical and shorter-term, dynamical components of the cosmic ray modulation, as well as the corpuscular emission of the Sun. Other solar parameters that may indicate a solar radiative effect on climate are also evaluated. It is concluded that variable fluxes either of solar charged particles or cosmic rays modulated by the solar wind, or both, may influence the terrestrial tropospheric temperature on timescale of months to years.