ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: determines the age, metallicity, and dust attenuation of the stellar populations. Consequently, the light-weighted ages and metallicities derived above are driven by spectral features that are less degenerate than the spectral shape alone (see Figure 1 and Table 1 ). Table 1 Prominent Stellar Features Note. The first column gives the feature and wavelength. The second and third columns give the age range that the features are found in, in a 0.4 Z ⊙ bpass and starburst 99 model, respectively. The two main types of FUV stellar features are strong, broad stellar wind P-Cygni features and weak stellar photospheric absorption lines. Both types of features can be contaminated by neighboring ISM absorption and require high S/N and moderate spectral resolutions to resolve. In the following two subsections, we discuss both types of spectral features individually, and we illustrate how the individual features relate to the inferred stellar population ages and metallicities. The purpose of these subsections is not to advocate for determining the stellar ages and metallicities using single features, but rather to demonstrate that the stellar properties inferred from the full spectral fits are entirely consistent with the trends of stellar spectral features. 4.1. Stellar Wind Features The most notable stellar features in the FUV are the broad blueshifted absorption and redshifted emission profiles (called P-Cygni profiles; orange labels in Figure 1 ). These P-Cygni profiles arise from strong winds that are radiatively driven off of stellar photospheres (Castor et al. 1975 ; Lamers Cassinelli 1999 ). The terminal velocity, ionization structure, and mass-outflow rates sensitively depend on the stellar luminosity (Castor et al. 1975 ; Lamers Leitherer 1993 ; Lamers et al. 1995 ; Leitherer et al. 1995 ; Puls et al. 1996 ; Kudritzki Puls 2000 ). The terminal velocity describes the maximal velocity extent of the absorption component, while the mass-loss rate determines the depth of the profile. In turn, these establish the shape of the P-Cygni absorption and emission. Whether a given ion is observed as a P-Cygni profile in the wind depends on the ionization structure of the stellar wind. The peak ionization stages for stellar winds typically are the C v , N iv , O iv , and Si v states (Lamers Cassinelli 1999 ), none of which have resonant transitions in the rest-frame FUV. Alternatively, the presence of adjacent ionization stages with P-Cygni profiles (N v , C iv , or Si iv ) provides information on the stellar temperature of the most luminous stars and, by inference, the stellar population age. The age of a stellar population can be inferred from the strength of the observed P-Cygni transitions: O v and N v have the highest ionization states and are strongest in stars with lifetimes of 2–3 Myr, while C iv and Si iv are lower ionization and peak in stars with lifetimes near 5 Myr. Stellar temperature, or age, is not the sole determinant of the stellar wind