发表机构
Manly Astrophysics; Eberhard Karls University（曼利天体物理; 埃尔朗根-纽伦堡大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
提出轨道尘埃气体云掩食模型解释北冕座R型变星的变暗事件，发现固态氢尘埃能产生符合观测的深且不对称掩食，并关联到恒星缺氢特性与中红外超出现象。
AI 中文摘要
北冕座R型变星（RCBs）是缺乏氢的超巨星，会因尘埃的瞬时消光而发生深度变暗事件。该现象首次观测于两个多世纪前，但至今仍未被充分理解。我们的目标是探究变暗事件是否由轨道上的尘埃气体云掩食所致。我们构建了一个简单的物理模型，描述由恒星辐射场从云表面驱动的尘埃风，并计算了风和云产生的消光及散射光。我们考虑了广泛的尘埃成分。对于与数据最匹配的模型，我们概述了其对RCB现象更广泛的启示。我们的计算表明，传统尘埃材料如石墨和硅酸盐产生的掩食，其切入速度过慢且形态过于对称，无法解释RCB的变暗现象。但由固态氢构成的尘埃则能很好地匹配：掩食深且强烈不对称，具有快速开始和缓慢恢复的特点，且当云的近星点位于几十个天文单位（AU）以内时，事件时标大致相符。如此近距离的接近意味着每个云都会受到潮汐剥离/瓦解，缺乏氢的碎片通过吸积盘吸积到恒星上。入射到吸积盘上的星光产生金属离子等离子体，发出强大的轫致辐射连续谱，峰值在中红外波段。我们得出结论：轨道上的尘埃云掩食可能是RCB变暗事件的成因，但前提是尘埃由固态氢构成，在这种情况下，恒星不寻常的缺乏氢特性及其红外超出现象都自然得以解释。然而，解释观测到的高掩食率仍具挑战性；我们提出一种情景，即前身星是位于大量云构成的巨大晕中的宽双星，这暗示了与银河系“缺失质量”问题的联系。
英文摘要
R Coronae Borealis stars (RCBs) are hydrogen-deficient supergiants that undergo deep fading events due to transient extinction by dust. First observed over two centuries ago, that behaviour remains poorly understood. Our aim is to investigate the possibility that the fading events are eclipses by orbiting, dusty gas clouds. We construct a simple physical model of the dust wind that is driven from the cloud surface by the stellar radiation field, and we calculate the extinction and scattered light from wind and cloud. We consider a broad range of dust compositions. For the model that best matches the data, we sketch out implications for the RCB phenomenon more broadly. Our calculations show that conventional dust materials like graphite and silicate produce eclipses whose ingress is too slow and whose morphology too symmetric to account for RCB fadings. But dust made of solid H2 yields a good match: the eclipses are deep and strongly asymmetric, with rapid onset and slow recovery, and event timescales are about right if the clouds' periastra are within a few tens of AU. Such close approaches imply tidal stripping/disruption of each cloud, with hydrogen-deficient debris accreting onto the star via a disk. Starlight incident on the disk creates a metal-ion plasma that emits a powerful bremsstrahlung continuum, peaking in the mid-IR. We conclude that eclipses by orbiting, dusty clouds could be the cause of RCB fading events, but only if the dust is made of solid H2, in which case both the unusual, hydrogen-deficient nature of the stars and their mid-IR excess follow naturally. It is, however, challenging to account for the high eclipse rates that are observed; we propose a scenario in which the progenitor star is a wide binary inside a massive halo of clouds, suggesting a connection to the Galactic "missing mass" problem.
Comments22 pages, 19 figures, Revised manuscript submitted to Astronomy & Astrophysics
