OZ Geminorum (OZ Gem) is a galactic Mira variable in the Milky Way (MW). We measured its annual parallax with VLBI Exploration of Radio Astrometry to be π = 0.806 ± 0.039 mas, corresponding to a distance of D = 1.24 ± 0.06 kpc. Based on multi-epoch infrared observations with the Kagoshima University 1 m telescope, we also derived the mean J-, H-, and K′-band magnitudes of OZ Gem to be 5.75 ± 0.47 mag, 4.00 ± 0.16 mag, and 2.65 ± 0.16 mag, respectively. We derived a pulsation period of OZ Gem as 592 ± 1 d from the K′-band lightcurve. From the period–luminosity (P–L) relation and two-color diagram of the Large Magellanic Cloud (LMC), the property of OZ Gem suggests that OZ Gem is assigned among the carbon-rich Mira variables. However, our optical spectroscopic observational results (with the 1.5 m Kanata telescope) confirmed OZ Gem to be an oxygen-rich Mira star with the detection of multiple titanium oxide transition absorption lines. We suggest that OZ Gem is a low-mass star evolving to an OH/IR star with large mass loss and dust formation. It is predicted that the lower limit to the initial mass of AGB stars for developing the C-rich surface chemistry is larger in the MW than in the LMC because of larger metallicity, and OZ Gem is likely to be the first example to prove this. Our results highlight the necessity of deriving the PL relation of the Milky Way with high accuracy.

中文翻译：

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富含O的Mira变星OZ宝石（IRAS 07308 + 3037）的三角视差：大麦哲伦星云与银河系P–L关系之间差异的确认

OZ Geminorum（OZ宝石）是银河系（MW）中的银河Mira变量。我们用VLBI射电占星术测得的年视差为π= 0.806±0.039 mas，对应于D = 1.24±0.06 kpc的距离。基于鹿儿岛大学1 m望远镜的多周期红外观测，我们还得出OZ宝石的J-，H-和K '波段平均幅度分别为5.75±0.47 mag，4.00±0.16 mag和2.65±分别为0.16 mag 我们从K得出OZ Gem的脉动周期为592±1 d'波段光曲线。从大麦哲伦星云（LMC）的周期-光度（P-L）关系和双色图，OZ Gem的性质表明OZ Gem被分配了富碳的Mira变量。但是，我们的光学光谱观察结果（使用1.5 m Kanata望远镜）证实了OZ Gem是富含氧气的Mira星，并且可以检测到多个氧化钛过渡吸收线。我们建议OZ宝石是一颗低质量的恒星，逐渐演变为质量损失较大且形成粉尘的OH / IR恒星。可以预见的是，由于金属性较大，用于发展富C表面化学的AGB恒星初始质量的下限在MW中要比LMC大，而OZ Gem可能是第一个证明这一点的例子。