标题： SDSS-V DR19中的双白矮星双星：发现一个罕见的DA+DQ白矮星双星，轨道周期为31小时 显示英文标题

标题： Double White Dwarf Binaries in SDSS-V DR19 : The discovery of a rare DA+DQ white dwarf binary with 31 hour orbital period

摘要： 双白矮星（WDs）是一类重要的天体物理对象，理论认为它们会导致Ia型超新星，并且也被用来深入了解恒星双星演化中的复杂过程。 我们报告发现了SDSS~J090618.44+022311.6，这是一个罕见的共包层双星，包含一个氢大气DA WD和一个DQ WD，显示出碳吸收特征，是已知的第四种这样的双星。 我们结合现有的光谱、测光和视向速度数据，为该双星提供了一个自洽的模型，并讨论其作为DA+DQ双星的历史。 该系统的周期为31.17小时，DA WD的质量为0.42 M$_{\odot}$，DQ WD的质量为0.49 M$_{\odot}$。 相应的冷却年龄表明，该系统经历了类似Algol的演化，质量较低的恒星首先演化成DA WD，之后较重的DQ WD形成。 该系统的合并时间尺度为450 Gyrs，并将导致大质量WD的形成。 通过此发现，已知的DA+DQ WD双星数量增加到四个，我们发现它们的恒星特性都位于同一范围内。 对更多此类系统的详细研究对于理解这一稀有双星类别形成过程中涉及的共同过程以及对WD光谱演化的更广泛图景具有重要意义。 显示英文摘要

摘要： Binaries of two white dwarfs (WDs) are an important class of astrophysical objects that are theorized to lead to Type Ia supernovae and are also used to gain insight into complex processes involved in stellar binary evolution. We report the discovery of SDSS~J090618.44+022311.6, a rare post-common envelope binary of a hydrogen atmospheric DA WD and a DQ WD which shows carbon absorption features, and is only the fourth such binary known. We combine the available spectroscopic, photometric, and radial velocity data to provide a self-consistent model for the binary and discuss its history as a binary DA+DQ. The system has a period of 31.17 hours with masses of 0.42 M$_{\odot}$ for DA WD and 0.49 M$_{\odot}$ for DQ WD. The corresponding cooling ages point to an Algol type of evolution with the lower mass star evolving into a DA WD first and later the massive DQ WD is formed. The system has a merger timescale of 450 Gyrs and will lead to the formation of a massive WD. With this, the number of known DA+DQ WD binaries has increased to four, and we find that their stellar properties all lie in the same range. Detailed study of more such systems is vital to understand common processes involved in the formation of this rare class of binaries and give insights towards the broader picture of WD spectral evolution.