5.4 The far-ultraviolet Sun in time

Guinan et al. (2003Jump To The Next Citation Point) have used the FUSE satellite to probe the transition regions of the “Sun in Time” stars by measuring fluxes of key lines in the 920 – 1180 Å region. An example is shown in Figure 13View Image. The individual flux measurements have been given by Ribas et al. (2005Jump To The Next Citation Point) and are listed below in Table 4 as irradiances for a distance of 1 AU. Measurements pertaining to the intermediately active Sun are also given (see Ribas et al., 2005Jump To The Next Citation Point, for references).

These line fluxes have been used to construct surface flux-rotation relations for solar analogs (Guinan et al., 2003). For essentially the entire rotation range of MS solar analogs, the line fluxes follow power-law relations, F ∝ P −α, with α ≈ 1.8. This power-law decay is steeper than for chromospheric lines (Section 5.3), but shallower than for coronal fluxes or luminosities (recalling that the stellar radii are all close to 1 R⊙ and hence the luminosities are proportional to the surface fluxes).

A very important FUV contribution comes from the H i λ 1216 Lyα line (Ribas et al., 2005Jump To The Next Citation Point). This holds true throughout the entire MS lifetime of a solar analog although the relative contribution of this line with respect to higher-energy emission increases with age (see Table 4).

View Image

Figure 13: Extracts of FUV spectra of solar analogs with different ages. The region of the O vi doublet is shown. All spectral fluxes have been transformed to irradiances at 1 AU from the star. The spectra have been shifted along the ordinate, by multiples of 1 erg s–1 cm–2 Å–1 (from Ribas et al., 2005Jump To The Next Citation Point, reproduced by permission of AAS).

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