It is interesting to note that during the solar cycle, the Sun’s wind strength is actually anticorrelated with its X-ray flux. The solar wind is weaker at solar maximum than at solar minimum despite coronal X-ray fluxes being much higher (Lazarus and McNutt Jr, 1990). This is presumably due to the fact that winds are more associated with the large scale dipole component of the solar magnetic field instead of the small scale active regions responsible for most of the Sun’s X-ray emission. The dipole field actually weakens at solar maximum along with the wind. However, the interior magnetic dynamo is ultimately responsible for both the small scale and large scale fields, so as a whole both field components should increase with increasing dynamo activity, consistent with the mass loss/activity correlation in Figure 14 (Schrijver et al., 2003).
The evolved stars are clearly inconsistent with the main sequence stars in Figure 14. The very active coronae of And and DK UMa produce surprisingly weak winds, though it should be noted that both of these astrospheric detections are flagged as being questionable in Table 1. There are three main sequence stars with , which have low mass loss measurements that are not consistent with the wind-activity correlation that seems to exist for the low activity main sequence stars. Two of these stars (Proxima Cen and EV Lac) are tiny M dwarf stars. If these were the only discrepant data points one could perhaps argue that the discrepancy is due to these M dwarfs being significantly less solar-like than the G and K dwarfs that make up the rest of the main sequence sample of stars. However, this interpretation is invalidated by the third discrepant measurement, that of Boo. Being a binary with two rather solar-like stars (G8 V+K4 V), there is no easy way to dismiss the Boo measurement, which implies that the power law relation does not extend to high activity levels for any type of star. More mass loss measurements of active stars would clearly be helpful to better define the characteristics of solar-like winds at high coronal activity levels.
Based on the available data, the mass-loss/activity relation appears to change its character at . One possible explanation for this concerns the existence of polar spots for very active stars. Low activity stars presumably have starspot patterns like that of the Sun, where spots are confined to low latitudes. However, for very active stars not only are spots detected at high latitudes, but a majority of these stars show evidence for large polar spots (Strassmeier, 2002). The existence of high latitude and polar spots represents a fundamental change in the stellar magnetic geometry (Schrijver and Title, 2001), and it is possible that this dramatic change in magnetic field structure could affect the winds emanating from these stars. Perhaps stars with polar spots might have a magnetic field with a strong dipolar component that could envelope the entire star and inhibit stellar outflows, thereby explaining why active stars have weaker winds than the mass-loss/activity relation of less active main sequence stars would predict. For Boo A, high latitude spots of some sort have been detected (Toner and Gray, 1988). Petit et al. (2005) have detected a strong global dipole field component for Boo A, consistent with the picture presented above. They also detected a large-scale toroidal field component, which would have no solar analog whatsoever, consistent with the idea that very active solar-like stars have significantly different magnetic field structures from those of the Sun and other low-activity stars.
Figure 14 illustrates how mass loss varies with coronal activity. But what about age? There is a known connection between activity and age, for the following reasons. The gravitational contraction of interstellar clouds that results in star formation leads to rapid rotation for young, newly born stars. This rapid rotation leads to vigorous dynamo activity and therefore high surface magnetic activity and high coronal X-ray emission. However, the magnetic fields of these young, rapidly rotating stars drag against their winds, and this magnetic braking gradually slows the stellar rotation. This in turn leads to lower activity levels and X-ray fluxes. An enormous amount of effort has been expended in the past few decades to observationally establish exactly how rotation relates to stellar age (see Skumanich, 1972; Soderblom et al., 1993) and how rotation relates to stellar activity, which is most easily measured through X-ray emission (see Pallavicini et al., 1981; Walter, 1982, 1983; Caillault and Helfand, 1985; Micela et al., 1985; Fleming et al., 1989; Stauffer et al., 1994). For solar-like stars, Ayres (1997) finds
Equations (1), (2), and (3) can be combined to obtain the following relation between mass loss and age for solar-like stars:et al., 2005a).
The truncation of the power law relation in Figure 14 leads to the truncation of the mass-loss/age relation in Figure 15 at about . The location of Boo is shown in order to infer what the solar wind might have been like at earlier times. Despite the high activity cutoff, the mass loss measurements obtained so far clearly suggest that winds are generally stronger for young solar-like stars, and as a consequence the solar wind was presumably much stronger early in the Sun’s lifetime. This has many important implications, some of which are discussed in Sections 5.2, 5.3, and 5.4.
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 2.0 Germany License.