Direct Wind Detection Techniques

3 Direct Wind Detection Techniques

The astrospheric Ly $α$ absorption diagnostic described in detail in Section 4 represents only an indirect detection of stellar winds, since the H I that produces the absorption is essentially LISM rather than wind material. The H I is nevertheless heated by its interaction with the stellar wind, and the astrospheric absorption has therefore proven to be very useful for successfully detecting and measuring solar-like winds around other stars. However, there have been attempts to detect these winds more directly, and these mostly unsuccessful efforts are briefly summarized here.

One can try to look for free-free radio emission from solar-like winds, since they are fully ionized and should therefore produce emission at some level. However, current radio telescopes can only detect winds if they are much stronger than that of the Sun. There have been some claims of very high mass loss rates for a few very active stars using observations at millimeter wavelengths (Mullan et al., 1992 ), but these interpretations of the data are highly controversial (Lim and White, 1996 ; van den Oord and Doyle, 1997 ). The problem is that the coronae of these active stars are also sources of radio emission, which makes problematic the identification of a wind as the source of the emission. Furthermore, it has been argued that massive winds around active stars should absorb the flaring coronal emission that is often observed from these stars, suggesting that massive winds cannot be present (Lim and White, 1996 ). Nondetections of radio emission have been used to derive upper limits to the mass loss rates of various stars, but these upper limits are typically 2 – 3 orders of magnitude higher than the solar mass loss rate, so these are not very stringent constraints (Brown et al., 1990 ; Drake et al., 1993 ; Lim et al., 1996b ; Gaidos et al., 2000 ).

Variable ultraviolet absorption features observed from the close, eclipsing binary V471 Tau (K2 V+DA) have been interpreted as being due to a wind from the K2 V star (Mullan et al., 1989 ; Bond et al., 2001 ). Even if this interpretation is correct, it is questionable whether the wind produced by this star can be considered to be truly “solar-like” given the close presence of the white dwarf companion. In addition, instead of a spherically symmetric wind it has been proposed that the UV absorption could instead be indicative of coronal material being funneled directly from the K2 V star to the white dwarf through the magnetospheric interaction of the two stars (Lim et al., 1996a ).

One final wind detection technique that has been proposed is to look for X-ray emission surrounding nearby stars, caused by charge exchange between highly ionized heavy atoms in the stellar wind and inflowing LISM neutrals. This is very analogous to the process by which comets produce X-rays (see Lisse et al., 2001 ; Cravens, 2002 ). In the heliosphere, this charge exchange X-ray emission may be responsible for a significant fraction of the observed soft X-ray background (Cravens, 2000 ). Wargelin and Drake (2002 ) searched for circumstellar X-ray emission in Chandra observations of the nearest star, Proxima Cen, but they failed to detect any. Based on this nondetection, they quote an upper limit for Proxima Cen’s mass loss rate of $M˙ < 14M˙⊙$ . This can be compared with the upper limit of $M˙ < 350M˙ ⊙$ derived from a nondetection of radio emission from Proxima Cen (Lim et al., 1996b ), and the upper limit of $˙ ˙ M < 0.2M ⊙$ derived from the nondetection of astrospheric Ly $α$ absorption (Wood et al., 2001 ). The astrospheric Ly $α$ absorption diagnostic (see Section 4) is roughly two orders of magnitude more sensitive than the X-ray diagnostic and roughly three orders of magnitude more sensitive than the radio measurement.