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arXiv 1807.06115astro-ph.SR
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北极星近伴的轨道:哈勃太空望远镜成像2007至2014年

The Orbit of the Close Companion of Polaris: Hubble Space Telescope Imaging 2007 to 2014

Nancy Remage Evans, Margarita Karovska, Howard E. Bond, Gail H. Schaefer, Kailash C. Sahu, Jennifer Mack, Edmund P. Nelan, Alexandre Gallenne, Evan D. Tingle

摘要:

作为确定造父变星动力学质量计划的一部分,我们用哈勃太空望远镜宽视场行星照相机2和宽视场照相机3拍摄了最近和最亮的造父变星北极星。在2007年至2014年的三个时期进行了观测。在这些图像中,就像我们在2008年的一篇论文中讨论的2005年和2006年获得的HST帧一样,我们从造父变星北极星Aa中分辨出了近伴星北极星Ab。由于北极星Aa和Ab的距离很小,星等相差很大,我们采用PSF反卷积技术对这对双星进行了天体测量。根据这些新的测量结果,我们更新了29.59年轨道的要素。采用最近的盖亚数据发布2到系统的距离,我们发现造父变星的动力学质量为3.45 +/- 0.75 Msun,虽然这是初步的,并将通过CHARA测量覆盖近星点来改善。与最近确定的造父变星V1334 Cyg的动力学质量一样,北极星的质量明显低于通过拟合HR图中的演化轨道预测的“演化质量”。我们讨论了几个问题和影响,这些测量,包括脉动模式,其中不稳定带穿越恒星,和可能的并发症,如旋转,质量损失,和二进制合并。根据等时线拟合,系统中遥远的第三颗恒星北极星B似乎比造父变星更老。这可能表明造父变星北极星相对较老,是双星合并的结果,而不是一颗年轻的单星。

英文摘要:

As part of a program to determine dynamical masses of Cepheids, we have imaged the nearest and brightest Cepheid, Polaris, with the Hubble Space Telescope Wide Field Planetary Camera 2 and Wide Field Camera 3. Observations were obtained at three epochs between 2007 and 2014. In these images, as in HST frames obtained in 2005 and 2006, which we discussed in a 2008 paper, we resolve the close companion Polaris Ab from the Cepheid Polaris Aa. Because of the small separation and large magnitude difference between Polaris Aa and Ab, we used PSF deconvolution techniques to carry out astrometry of the binary. Based on these new measurements, we have updated the elements for the 29.59 yr orbit. Adopting the distance to the system from the recent Gaia Data Release 2, we find a dynamical mass for the Cepheid of 3.45 +/- 0.75 Msun, although this is preliminary, and will be improved by CHARA measurements covering periastron. As is the case for the recently determined dynamical mass for the Cepheid V1334 Cyg, the mass of Polaris is significantly lower than the "evolutionary mass" predicted by fitting to evolutionary tracks in the HR diagram. We discuss several questions and implications raised by these measurements, including the pulsation mode, which instability-strip crossing the stars are in, and possible complications such as rotation, mass loss, and binary mergers. The distant third star in the system, Polaris B, appears to be older than the Cepheid, based on isochrone fitting. This may indicate that the Cepheid Polaris is relatively old and is the result of a binary merger, rather than being a young single star.

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