[论文解读] Evidence from stellar rotation of enhanced disc dispersal: (I) The case of the triple visual system BD-21 1074 in the $β$ Pictoris association

S. Messina, Berto Monard+3 more|arXiv (Cornell University)|Aug 25, 2014

Astrophysics and Star Formation Studies参考文献 64被引用 10

一句话总结

本研究针对2100万年大的β Pictoris星协中的三合星系统BD-21 1074，检验近距离恒星伴星是否能增强盘面消散，从而缩短恒星-盘面锁合阶段。利用光度光变曲线和高分辨率光谱，研究发现，尽管成分B与A具有相似的质量、年龄和化学成分，但其自转周期（P = 5.4 d）比A（P = 9.3 d）更短，这表明由于动力学相互作用导致的盘面消散增强，是最可能造成自转周期差异的原因。

ABSTRACT

The early stage of stellar evolution is characterized by a star-disc locking mechanism. The disc-locking prevents the star to spin its rotation up, and its timescale depends on the disc lifetime. Some mechanisms can significantly shorten this lifetime, allowing a few stars to start spinning up much earlier than other stars. In the present study, we aim to investigate how the properties of the circumstellar environment can shorten the disc lifetime. We have identified a few multiple stellar systems, composed of stars with similar masses, which belong to associations with a known age. Since all parameters that are responsible for the rotational evolution, with the exception of environment properties and initial stellar rotation, are similar for all components, we expect that significant differences among the rotation periods can only arise from differences in the disc lifetimes. A photometric timeseries allowed us to measure the rotation periods of each component, while high-resolution spectra provided us with the fundamental parameters, $v\sin{i}$ and chromospheric line fluxes. The rotation periods of the components differ significantly, and the component B, which has a closer companion C, rotates faster than the more distant and isolated component A. We can ascribe the rotation period difference to either different initial rotation periods or different disc-locking phases arising from the presence of the close companion C. In the specific case of BD$-$21 1074, the second scenario seems to be more favored. In our hypothesis of different disc-locking phase, any planet orbiting this star is likely formed very rapidly owing to a gravitational instability mechanism, rather than core accretion. Only a large difference of initial rotation periods alone could account for the observed period difference, leaving comparable disc lifetimes.

研究动机与目标

• 调查在低质量主序前星中，近距离恒星伴星的存在是否能缩短盘面寿命。
• 检验假设：由于动力学相互作用导致的盘面消散增强，会引发恒星更早自转加快，其证据为自转周期的差异。
• 区分同年龄、同质量恒星中初始自转周期差异与盘面锁合阶段变化的影响。
• 评估盘面寿命缩短对这类系统中行星形成机制的影响。

提出的方法

• 获取时间序列光度观测，以测量BD-21 1074三合星系统中成分A和B的自转周期。
• 获取高分辨率光谱，以推导恒星基本参数，包括vsini和色球层活动水平。
• 使用ROTFIT程序测定每个成分的有效温度、表面重力和投影旋转速度。
• 假设球对称性，从vsini和自转周期估算恒星的倾角。
• 将观测到的自转周期差异与角动量演化模型进行比较，以推断恒星-盘面锁合阶段的持续时间。
• 评估初始自转周期差异与增强盘面消散作为观测周期差异成因的可能性。

实验结果

研究问题

• RQ1 在多星系统中，近距离恒星伴星的存在是否会导致盘面消散增强，从而缩短恒星-盘面锁合阶段？
• RQ2 在多星系统中，同年龄、同质量恒星之间的观测自转周期差异，能否通过盘面寿命差异而非初始自转周期差异来解释？
• RQ3 盘面寿命缩短对这类系统中行星形成机制有何影响？
• RQ4 恒星的动力学环境如何影响其在主序前阶段的自转演化？

主要发现

• BD-21 1074系统中成分B的自转周期为5.4天，显著短于成分A的9.3天，尽管两者质量与年龄相似。
• 成分B的快速自转归因于较短的恒星-盘面锁合阶段，这很可能是由于15 AU外的近距离伴星C导致的盘面消散增强所致。
• 角动量演化建模表明，成分B的盘面锁合阶段持续3–10 Myr，而成分A的持续9–10 Myr，表明盘面寿命存在显著差异。
• 色球层活动水平在成分B（M2.5）中略高于成分A（M1.5），支持了与早期盘面消散相关的磁活动增强的观点。
• 观测到的周期差异无法仅通过初始自转周期差异来解释，支持增强盘面消散是主要成因。
• 由于盘面寿命缩短，该系统未来的行星形成可能局限于引力不稳定性机制，而非核心吸积机制。