Stellar and Solar Magnetic Activity Cycles

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The proposed program is a continuation of the Solar-Stellar Spectrograph (SSS) project at Lowell Observatory. Its purpose is to provide a better understanding of activity in cool stars on three essential timescales: activity-cyclic (on the order of years), rotational (months), and short-term (days). The SSS spectrograph covers several important chromospheric diagnostic lines, including Ca II H&K and Ha, as well as most of the optical spectrum from 5000-9000 A. It is a dedicated instrument, associated with Lowell Observatory's 1.1-m telescope. It receives starlight via a fiber optic feed, but is also equipped with a separate fiber feed that sees the Sun as an unresolved disk. Sunlight and starlight are therefore recorded identically. The PI proposes to study SSS spectra in the following ways:[1] Time series observations of the emergent flux in spectral lines responsive to solar and stellar activity reveals the phase, amplitude, and morphology of activity cycles present. The SSS target list includes stars lying near the Sun's location on the main sequence (roughly spectral types F5 - K7), with emphasis on minimally evolved stars very similar to the Sun, as well as the Sun itself.[2] Examination of seasonal variability for the Sun and stars reveals the degree of rotational modulation - the change in emission level arising not from the activity cycle, but from the passage of active regions across the visible hemispheres.[3] The spectrum-to-spectrum variability, examined across a broad ensemble of related spectral lines, reveals changes in solar and stellar activity on both short and long timescales.The proposers are currently observing the Sun and approximately 100 particularly interesting stars (selected from about 300 stars initially surveyed). Their database comprises approximately 3000 observations of the Sun recorded over roughly 1000 days since 1994, as well as 16,000 stellar spectra. The proposers' solar time series runs from the declining phase of Cycle 22 to the declining phase of Cycle 23. Cyclic and long-term activity is apparent in many of their stellar targets, including a large number included in the now inactive Mount Wilson Observatory HK project. Further observations are required to study stellar activity on various timescales over the course of the cycles -- in particular, those stars now identified from Hipparcos astrometry (and from ground-based spectroscopy and photometry) as low-activity, unevolved, "solar twin" candidates.