SOLAR COUSIN PRESENTS A MYSTERY FOR THE DYNAMO

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Thursday, December 3, 2015

What might happen to the well-known 11-year solar cycle if the Sun were spun up to rotate more than two times faster? To investigate such questions of the dependence of the Sun's internal dynamo on parameters such as rotation we must look to other stars with different physical conditions. A new study by HAO Newkirk Fellow Ricky Egeland and colleagues presents over 48 years of observations of the young solar analog HD 30495 (58 Eri), which is a Sun-like spectral type G1.5 star with a mean rotation of 11 days, 2.3 times faster than the Sun. HD 30495 has a mass very close to that of the Sun, though its estimated age is a much younger 970 million years.

HD 30495 graphic
Figure 1 (a & b): The 48-year time series of "S-index" measurements tracks the magnetic activity of HD 30495 over three and a half stellar cycles — (c & d): Precision photometry in two visible bands taken by the Automated Photometric Telescopes.

Two sets of observations were analyzed. The first is the 48-year time series of "S-index" measurements that tracks the magnetic activity of HD 30495 over three and a half stellar cycles (see Figure 1[a] and 1[b]). For the Sun, the S-index correlates very well with the presence of magnetic spots which define the solar cycle. These synoptic observations come from the long-running Mount Willson Calcium H and K line survey (1967–2003), the HAO-built Solar Stellar Spectrograph (SSS) operating at Lowell Observatory (1993–present), and the SMARTS H and K observing program (2008–2013) initiated by HAO researchers Travis Metcalfe and Phil Judge, as well as smaller sets of observations from the California Planet Search and the HARPS planet-search data. The second set of observations (see Figure 1[c] and 1[d]) are precision photometry in two visible bands taken by the Automated Photometric Telescopes (APTs) operating at Fairborn Observatory since 1994. On the Sun, the total solar irradiance (TSI) is strongly dependent on the coverage of magnetic surface features such as sunspots, faculae, and active network. TSI varies in lockstep with the solar sunspot cycle, rising with sunspot maximum and falling toward sunspot minimum. Since sunspots are dark in the visible, this means that the TSI is "plage dominated", that is, the positive contribution of bright plage and network features outweigh the negative contribution due to dark spots. For HD 30495, Egeland and colleagues found that the variability in the two visible bands is strongly out of phase with the magnetic activity cycle, as is typical for other fast-rotating so-called "spot dominated" stars. The apparent brightness of HD 30495 decreases as a larger fraction of the unresolved surface is presumably covered by dark spots. Figure 1(d) shows the spectral variability between the blue and yellow bands, indicating that this star trends towards the blue as its brightness increases. Such spectral variability information for the Sun remains an area of active research with important consequences for Earth climate modeling.

Figure 1(a) clearly demonstrates that HD 30495 has a magnetic activity cycle like the Sun. In the nearly half-century of observations, the activity rises and falls three times, as shown using a simple cycle model indicated by the red curve in Figure 1(a). The duration of the cycles were found to vary from 9.6 to 15.5 years, with an average duration of 12.2 years. This cycle is very close to the Sun's cycle, where cycle durations are seen to vary from about 9 to 14 years with an average of about 11 years. A shorter variation was also observed with a mean period of 1.6 years, which is highlighted with the blue sine curve in Figure 1(b). The Sun also manifests short-period variability on the order of two years, sometimes referred to as the Sun's quasi-biennial oscillation. The similarity of the variability of HD 30495 and the Sun, despite the large difference in the mean rotation period, raises the question of what is the role of rotation in the stellar magnetic dynamo. Naively, one might expect that faster rotation should produce a faster cycle as magnetic field is wound up by differential rotation (a dynamo process known as the omega-effect), but when comparing HD 30495 to the Sun this is not the case. Observations of Sun-like stars helps us to understand the breadth of possibilities for magnetic dynamos, which offers researchers unique clues on how these dynamos work.

Links:
ApJ Paper: http://iopscience.iop.org/article/10.1088/0004-637X/812/1/12