The Instability Strip of RRab Stars From LAMOST-Gaia Data

We compile 995 RRab samples from various literature and cross-match them with LAMOST to identify the boundaries of instability strip (IS) at red and blue edges and to associate them with metallicity. We find a significant gap at [Fe/H]=-0.5 dex, which separates the RRab stars into metal-poor and metal-rich groups. For the metal-poor group, the IS moves toward cooler T_eff with declining [Fe/H]. The slopes, dT_eff/d[Fe/H], are 192±19 K/dex and 354±51 K/dex for red and blue edges, respectively. For the metal-rich group, the blue edge of IS moves toward hotter T_eff with declining [Fe/H], while the red edge is nearly independent on [Fe/H]. A similar trend is also confirmed in T_eff-luminosity diagram. By comparing with various theoretical models of RRab stars, we find the correlation between IS and metallicity is not consistent with the theoretical model.