Horizontal branch (HB) morphology is a complex multiple-parameter problem. Besides the metallicity, two other leading parameters are the mass loss rate (MLR) and the initial He abundance of the HB progenitors. Using the STAREV stellar evolution code, we produce a wide array of Extreme Horizontal Branch (EHB) stars and also examine their post-HB evolution. EHB stars are produced in our calculations by the so called `delayed (late) hot core flash' scenario. The MLR is increased on the red giant branch (RGB) to the extent that, prior to reaching core flash conditions, only a very thin H-rich envelope remains and helium ignition takes place at hotter positions on the HRD. We perform an extensive, self-consistent parameter study, covering populations I and II (Z=0.0001-0.03), for both normal initial helium abundances and He-enriched models (up to Y=0.40). For completeness of the study and in order to point to complete trends, we chose NOT to cut out several combinations (or results of) that may extend beyond realistic limits. We present results and complete evolutionary tracks for the covered parameter space, showing in particular that: a) Increased He abundance ON ITS OWN -- without having a significant-enough MLR on the RGB -- DOES NOT lead to the production of EHB stars; however, b) The bluest (hottest) HB positions do result from the COMBINED effect of He-enhancement and increased MLR; c) The general trend is that the effective temperature on the HB increases with decreasing metallicity, but there is an indication for a halt, or even a reversal of this trend, as Z further decreases below $10^{-3}$; d) EHB stars can serve as major contributors to the UV flux emanating from their host system. Thus, the present comprehensive study both complements and lends support to previous, more restricted studies of the HB phase, and adds results for unexplored regions of the parameter space.
翻译:水平分支( HB) 形态是一个复杂的多参数问题。 除了金属特性外, 另外两个主要参数是质量损耗率( MLR) 和HB子子体的初始丰度。 我们使用STAREV星级进化代码, 生成了广泛的极端水平分支星( EHB), 并检查了 HB 后演化过程。 EHB 恒星是由所谓的“ 延迟( 延迟) 热核心闪烁” 假设在我们计算中产生的。 红巨分支( RGB) 的 MLR 变速率增加, 以至于在达到核心闪烁条件之前, 只有非常薄的 H- 富星体的留存率和 hei 点点点点点点点。 我们进行了广泛的、 自我一致的极端水平分数研究, 覆盖了I 和II 的正常的初始 helium 丰度( 到 Y=0. 40 ) 。 为了研究的完整性, 我们选择不切换出一些 H- NR RR 的混合值值值值值值值值值值值值值值值值值值值,, 的当前主要的磁度结果可能扩展为 H- 。