标题： 明亮贫金属变星的合成特性。 II. BL Her 星 显示英文标题

标题： Synthetic properties of bright metal-poor variables. II. BL Her stars

摘要： 我们研究了所谓的BL Her星，即周期短于8天的第二星族造父变星，使用更新的脉动模型和演化轨迹，这些模型和轨迹采用金属丰度在Z=0.0001到Z=0.004范围内的计算方法。 我们推导了在不同光度测量波段下的预测周期-绝对星等（PM）和周期-韦森海特（PW）关系，并表明这些关系的斜率与银河系球状星团中观测变量确定的斜率有良好的一致性，无论采用哪种M_V(RR)-[Fe/H]关系来获得基于RR Lyrae的星团距离。 此外，我们还表明，由预测的PM和PW关系为BL Her星提供的距离在误差范围内与基于RR Lyrae的值一致。 使用与W Vir星（即周期长于8天的第二星族造父变星）相关的预测关系，无法明确证明或反驳在P~10天附近PM和PW斜率的变化。 显示英文摘要

摘要： We investigate the properties of the so-called BL Her stars, i.e., Population II Cepheids with periods shorter than 8 days, using updated pulsation models and evolutionary tracks computed adopting a metal abundance in the range of Z=0.0001 to Z=0.004. We derive the predicted Period-Magnitude (PM) and Period-Wesenheit (PW) relations at the various photometric bands and we show that the slopes of these relations are in good agreement with the slopes determined by observed variables in Galactic globular clusters, independently of the adopted M_V(RR)-[Fe/H] relation to get the cluster RR Lyrae-based distance. Moreover, we show that also the distances provided by the predicted PM and PW relations for BL Her stars agree within the errors with the RR Lyrae based values. The use of the predicted relations with W Vir stars, which are Population II Cepheids with periods longer than 8 days, provides no clear evidence for or against a change in the PM and PW slopes around P~10 days.