Jeffrey C. Hall

The Chromospheric Activity and Variability of Cycling and Flat Activity Solar-Analog Stars

We present an analysis of more than 3700 observations of the Ca II H and K lines in 57 Sun-like stars and over 3000 analogous observations of the Sun. Ten of the 57 stars under consideration are observed in flat states, but these stars do not always exhibit overall Ca II H and K core brightness below that of solar minimum. Solar activity minimum lies near the lowest level observed for stars with cyclic or irregular variability, but many flat stars have HK activity levels comparable to or exceeding that of solar minimum. While flat activity stars may be in periods of extended activity minima analogous to the solar Maunder minimum, a significant reduction in magnetic activity during such periods is not implied (although it is also not rejected) by the data.

A Survey of Chromospheric Activity in the Solar-Type Stars in the Open Cluster M67*

We present the results of a spectroscopic survey of the Ca II H and K core strengths in a sample of 60 solar-type stars that are members of the solar-age and solar-metallicity open cluster M67. We adopt the HK index, defined as the summed H+K core strengths in 1 A bandpasses centered on the H and K lines, respectively, as a measure of the chromospheric activity that is present. We compare the distribution of mean HK index values for the M67 solar-type stars with the variation of this index as measured for the Sun during the contemporary solar cycle. We find that the stellar distribution in our HK index is broader than that for the solar cycle. Approximately 17% of the M67 Sun-like stars exhibit average HK indices that are less than solar minimum. About 7%-12% are characterized by relatively high activity in excess of solar maximum values, while 72%-80% of the solar analogs exhibit Ca II H+K strengths within the range of the modern solar cycle. The ranges given reflect uncertainties in the most representative value of the maximum in the HK index to adopt for the solar cycle variations observed during the period AD 1976-2004. Thus, ~20%-30% of our homogeneous sample of Sun-like stars have mean chromospheric H+K strengths that are outside the range of the contemporary solar cycle. Any cycle-like variability that is present in the M67 solar-type stars appears to be characterized by periods greater than ~6 yr. Finally, we estimate a mean chromospheric age for M67 in the range of 3.8-4.3 Gyr.

ON THE DETERMINATION OF EMPIRICAL STELLAR FLUX SCALES

Several studies have appeared in the literature describing methods for deriving stellar continuum fluxes as simple functions of Johnson color indices. In previous papers, we have used similar relations to derive Ca II H and K line core fluxes for dwarf stars on Lowell Observatorys Solar-Stellar Spectrograph (SSS) program. Our desire is to present our data, and complementary data from the Mt. Wilson and National Solar Observatory stellar and solar programs, in terms of physical fluxes. However, our existing relations break down for mid-K and later stars and for giant stars, which form an important element of our target list. In this work I have used a broader range of spectrophotometry and angular diamters than in our previous work to derive empirical flux scales for spectral types A to early M, luminosity classes I to V, and for four color indices, Johnson B-V, V-R, R-I, and Stromgren b-y. In this paper, I describe the methods I used and present the desired flux scales. I compare the results with those from earlier studies and present examples of the use of the flux scales. The data and results are available on the World Wide Web.

Eclipse observations of RS CVn binaries. I : A survey for extended matter

We survey a number of RS CVn systems to determine the frequency and nature of circumstellar matter in these chromospherically active binaries. A growing body of data suggests that large extended regions of cool, prominence-like matter may be a common phenomenon in these systems. Our targets are eclipsing systems as the established geometries and occultations are useful probes for such extended material. In this paper we discuss our methods and present the results of our survey and our spectroscopic phase monitoring of these systems. We find extended material in eight of the ten systems on our present survey

The Sun-like Activity of the Solar Twin 18 Scorpii

We present the results of 10 yr of complementary spectroscopic and photometric observations of the solar twin 18 Scorpii. We show that over the course of its ~7 yr chromospheric activity cycle, 18 Scos brightness varies in the same manner as the Suns and with a likely total brightness variation of 0.09%, similar to the 0.1% decadal variation in the total solar irradiance.

SUN-LIKE MAGNETIC CYCLES IN THE RAPIDLY ROTATING YOUNG SOLAR ANALOG HD 30495

A growing body of evidence suggests that multiple dynamo mechanisms can drive magnetic variability on different timescales, not only in the Sun but also in other stars. Many solar activity proxies exhibit a quasi-biennial (

The reduction of fiber-fed echelle spectrograph data: Methods and an IDL-based solution procedure

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Evidence of a Pronounced Activity Cycle in the Solar Twin 18 Scorpii

2 year) variation, which is superimposed upon the dominant 11 year cycle. A well-characterized stellar sample suggests at least two different relationships between rotation period and cycle period, with some stars exhibiting long and short cycles simultaneously. Within this sample, the solar cycle periods are typical of a more rapidly rotating star, implying that the Sun might be in a transitional state or that it has an unusual evolutionary history. In this work, we present new and archival observations of dual magnetic cycles in the young solar analog HD 30495, an

The Solar Activity Cycle. I. Observations of the End of Cycle 22, 1993 September-1997 February

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Composite Spectral Indices: A New Method for the Interpretation of Solar and Stellar Activity

1 Gyr-old G1.5V star with a rotation period near 11 days. This star falls squarely on the relationships established by the broader stellar sample, with short-period variations at