The far-ultraviolet Sun in time

5.4 The far-ultraviolet Sun in time

Guinan et al. (2003

) 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 13

. The individual flux measurements have been given by Ribas et al. (2005

) 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., 2005

, 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 Ly $α$ line (Ribas et al., 2005 ). 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).

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., 2005

, reproduced by permission of AAS).