The ultraviolet Sun in time

5.3 The ultraviolet Sun in time

Ultraviolet excess emission from late-type stars originates in magnetic chromospheric and transition-zone regions that have been heated to temperatures of order 10⁴–10⁵ K. Magnetic activity thus makes the UV spectra rich in diagnostics (see Figure 12

; from Ribas et al. 2005

). Dorren and Guinan (1994a ) and Ayres (1997

) have studied the evolution of ultraviolet (UV) line fluxes in detail, based on spectral measurements obtained by IUE. Ribas et al. (2005

) extended these investigations to the “Sun in Time” sample and included spectral information from HST. The bulk of the UV flux in the region shortward of 1700 Å is in emission lines while the continuum is negligible. Emission lines include those of O i $λ$ 1304, C ii $λ$ 1335, Si iv $λ$ 1400, C iv $λ$ 1550, He ii $λ$ 1640, and C i $λ$ 1657.

Figure 12:

Extracts of UV spectra of solar analogs with different ages. 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 0.2 erg s^–1 cm^–2 Å^–1 (from Ribas et al., 2005

, reproduced by permission of AAS).

Dorren et al. (1994 ) studied the activity-rotation relationship for Mg ii and C iv for solar analogs in the “Sun in Time” sample. They found (I also include their X-ray result for comparison, to be discussed further in Section 5.5.1 below)

29 −0.76±0.096 −1 LMg II = (9.1 ± 0.2) × 10 P [erg s ] , (9 ) LC IV = (1.2 ± 0.4) × 1029 P −1.6±0.15 [erg s−1] , (10 ) L = (9.1 ± 4.7) × 1030 P −2.5±0.23 [erg s−1] . (11 ) X

Emission from hotter regions is more strongly dependent on rotation. Because P decreases with age, harder emission decays more rapidly than softer emission. I will return to this point in more detail in Section 5.6.