It goes without saying that it is generally much easier to study the nearby Sun than it is to study much more distant solar-like stars. Nevertheless, stellar research can address questions about the Sun that observations of the Sun alone cannot answer. The Sun only provides one example of a cool main sequence star, so it cannot tell us by itself how its various properties relate to each other. By observing other solar-like stars, we can see how properties such as stellar activity, rotation, and age are correlated. This can teach us a lot about why the Sun has the properties it does today. It can also tell us what the Sun was like in the past and what it will be like in the future.
Many stellar analogs of solar phenomena are available for study: photospheres, chromospheres, coronae, starspots, magnetic fields, rotation, asteroseismology, etc. (see Skumanich, 1972; Linsky, 1980; Vogt et al., 1987; Gustafsson and Jørgensen, 1994; Johns-Krull and Valenti, 1996; Christensen-Dalsgaard, 2003; Favata and Micela, 2003; Güdel, 2004). Comparing solar properties with those observed for other stars provides a useful context for the solar measurements, improving our understanding of the Sun as well as for stars in general. However, one major solar phenomenon that has proven to be very difficult to study for other stars is the solar wind.
Some types of stellar winds are very easy to detect and study spectroscopically. The massive, radiation-pressure driven winds of hot stars and the cool, massive winds of red giants and supergiants both produce P Cygni emission line profiles that allow the measurement of wind properties with reasonable precision (Harper et al., 1995; Mullan et al., 1998; Kudritzki and Puls, 2000). However, these stars are not solar-like and the winds of these stars are not analogous to the much weaker wind that we see emanating from the Sun. The weak and fully ionized solar wind provides no spectral diagnostics analogous to those used to study more massive stellar winds. Directly detecting a truly solar-like wind around another solar-like star has therefore proven to be a formidable problem.
The first clear detections of winds around other solar-like stars have come from UV spectra of nearby stars from the Hubble Space Telescope (HST). Stellar H I Ly lines at 1216 Å are always contaminated by very broad, saturated H I absorption. For a long time, this absorption was assumed to be entirely from interstellar H I. However, for some of the nearest stars, the interstellar medium (ISM) cannot account for all of the observed absorption. With the assistance of complex hydrodynamic models of the solar wind/ISM interaction, the excess Ly absorption has been convincingly identified as being partly due to heated H I gas within our own heliosphere and partly due to analogous H I gas within the “astrospheres” surrounding the observed stars. Note that the word “astrosphere” is used here as the stellar analog for “heliosphere”, although “asterosphere” has also been used in the past (see Schrijver et al., 2003). “Astrosphere” has a longer history, with published usage in the literature dating at least back to 1978 (Fahr, 1978). The term “heliosphere” itself only dates back to the 1960s (Dessler, 1967).
The detection of astrospheric Ly absorption represents an indirect detection of solar-like stellar winds, since astrospheres do not exist in the absence of a stellar wind. Furthermore, the amount of astrospheric absorption is dependent on the strength of the wind, so the astrospheric absorption has provided the first estimates of mass loss rates for solar-like stars. This article reviews the study of solar-like winds around other stars, especially results using the astrospheric Ly absorption technique.
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