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 12View Image; from Ribas et al. 2005Jump To The Next Citation Point). Dorren and Guinan (1994a) and Ayres (1997Jump To The Next Citation Point) have studied the evolution of ultraviolet (UV) line fluxes in detail, based on spectral measurements obtained by IUE. Ribas et al. (2005Jump To The Next Citation Point) 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.
View Image

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., 2005Jump To The Next Citation Point, 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.

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