W 48 A core H-2a is one of the young massive protostellar objects in the W 48 region. We conducted multi-epoch astrometric observations of the water (H₂O) masers associated with the W 48 A core H-2a with VLBI Exploration of Radio Astrometry (VERA). The trigonometric annual parallax of W 48 A core H-2a was measured to be 0.433 ± 0.026 mas, corresponding to a distance of |$2.31^{+0.15}_{-0.13}$| kpc. This agrees with the revised parallax of 0.412 ± 0.014 mas by Wu et al. (2019, ApJ, 874, 94). We obtained the systemic proper motion and local standard of rest velocity to be (μ_αcos δ, μ_δ) = (0.26 ± 0.73, −3.87 ± 0.33) mas yr⁻¹ and v_LSR = 41.9 ± 0.9 km s⁻¹, respectively. The distribution of the H₂O masers covers an area of 70 mas × 80 mas, corresponding to 160 au × 180 au at the distance of 2.31 kpc. The internal proper motions of the H₂O masers trace an east–west bipolar outflow. With the recent absolute position measurement of the 6.7 GHz methanol (CH₃OH) masers and their elliptical distribution, whose major axis is perpendicular to the axis of the bipolar outflow, we suggest the presence of a disk outflow system in core H-2a. The spectral energy distribution (SED) of the driving source of core H-2a was previously reported to yield a luminosity and envelope mass of 8000 ± 1000 |$W_{\odot}$| and 170 ± 30 |$M_{\odot}$|⁠, respectively. Refitting the SED with the new distance, we obtained the luminosity to be 3100 ± 388 |$L_{\odot}$| and derived the zero age main sequence (ZAMS) stellar mass to be 9 ± 1 |$M_{\odot}$|⁠. Using our distance measurement, we derived the peculiar motion of W 48 A to be (U_s, V_s, W_s) = (1 ± 4, 5 ± 6, −15 ± 5) km s⁻¹.

中文翻译：

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带有VERA的W 48 A（G35.20-01.74）H ii中H ₂ O的占星术 ：紧致的磁盘流出H-2a内核

W 48核心H-2a是W 48区域中年轻的大型星状天体之一。我们使用VLBI射电占星术（VERA）对与W 48 A核心H-2a相关的水（H ₂ O）物质进行了多时期的天文观测。W 48 A核心H-2a的三角年视差测得为0.433±0.026 mas，对应于| $ 2.31 ^ {+ 0.15} _ {-0.13} $ |的距离。 kpc。这与Wu等人修正后的视差0.412±0.014 mas是一致的。（2019，ApJ，874，94）。我们得到的全身固有运动和静止速度本地标准为（μ _α COSδ，μ _δ）=（0.26±0.73，-3.87±0.33）MAS年^-1和v _LSR = 41.9±0.9公里S ^-1， 分别。H ₂ O masers的分布覆盖70 mas×80 mas的面积，对应于2.31 kpc距离处的160 au×180 au。H ₂ O脉石的内部固有运动追踪东西向双极流出。借助最近对6.7 GHz甲醇（CH ₃ OH）脉搏的绝对位置测量及其椭圆形分布（其主轴线垂直于双极流出的轴），我们建议在H-2a岩心中存在盘状流出系统。以前曾报道过，核心H-2a的驱动源的光谱能量分布（SED）产生的光度和包络质量为8000±1000  | $ W _ {\ odot} $ | 和170±30  | $ M _ {\ odot} $ |⁠， 分别。用新的距离重新调整SED，我们得到的光度为3100±388  | $ L _ {\ odot} $ | 并将零年龄主序列（ZAMS）恒星质量推导为9±1  | $ M _ {\ odot} $ |⁠。使用我们的距离测量，我们得出W 48 A的奇异运动为（U _s，V _s，W _s）=（1±4、5±6，−15±5）km s ^-1。