Showing papers by "Sallie L. Baliunas published in 1992"

The Activity Cycle of sigma Draconis

Abstract: The temperature and granulation of several dwarf stars have been monitored since 1984. The results for σ Dra (HR 7462, HD 185144, K0 V) are discussed in this paper. Temperature is monitored using a line-depth ratio, and temperature variations: 5 K are seen. Intermediate-band photometry and Ca II H and K-line emission mimic the temperature changes: a monotonic decline from the 1984 season, a smooth minimum around 1988, followed by a rise back to the 1984 values at the current time. These types of correlated changes are similar to the solar behavior

Evidence of differential surface rotation in the solar-type star HD 114710

Abstract: Observations of the chromospheric Ca II H and K emission variability of the intermediate-age solar-type star HD 114710 (β Comae Berenices, G0 V) obtained at Mount Wilson Observatory over the past 10 years reveal a secular change in the seasonal rotation period that can be interpreted as surface differential rotation. The dependence of rotation period on chromospheric flux (i.e., activity-cycle phase) suggests that the star may have two latitudinal zones of activity: one in which changes in rotation period appear to follow the starspot activity cycle, and another confined to a narrow range of periods that does not

Periodogram analysis of 240 years of sunspot records

Abstract: We have analyzed the direct records of sunspot number between 1749 and 1990 with the same technique currently used in the study of stellar activity cycles observed with Mount Wilson Observatory's 60-inch telescope. In order to mimic the stellar time series, which span only two decades, we analyzed twenty- and fifty-year intervals of the sunspot data in comparison to the entire record. We also examined the reliability of the oldest (pre-1850) sunspot records. The mean solar cycle period determined from the entire record (1749–1990) is 11.04 yr with a computed precision of ± 0.01 yr, but an overall accuracy of only ±1.1 yr. The large uncertainty is caused by variation of the cycle period with time and not observational uncertainty. The ‘correct’ sunspot period is found slightly more often (82%) in 50-year intervals compared to 20-year (74%). The cause is twofold: first, a more precise period results from the longer sample length, and second, other periodicities exist in the sunspot record, so that a more accurate determination of the dominant 11.0-year period results from the longer time series. As a guideline for cycle periodicities in other stars, the solar results indicate that the 50-year intervals would produce more precise and accurate periods than the 20-year time series. On the other hand, useful statistics concerning long-term activity could be obtained from a less-frequently sampled group of stars that is substantially larger than the group of ∼100 lower Main-Sequence stars currently observed at Mount Wilson, although knowledge of short-term variability would be sacrificed.