[论文解读] Evidence from stellar rotation of enhanced disc dispersal

S. Messina, Berto Monard+3 more|arXiv (Cornell University)|Oct 8, 2014

Astrophysics and Star Formation Studies参考文献 3被引用 10

一句话总结

本研究调查了2100万年大的β Pictoris星协中三合星系统BD−21 1074的恒星自转，以检验近距离恒星伴星是否增强原行星盘的消散。通过测量具有相似年龄和质量的恒星A与B的自转周期和活动水平，发现距离第三颗伴星C更近的B组分自转更快且表现出更高的色球层活动，表明其原行星盘锁止阶段更短。结果表明，伴星C的存在可能加速了B周围的原行星盘消散，导致更早的自转加速，这意味着行星形成更可能通过引力不稳定性而非核心吸积机制。

ABSTRACT

Context. The early stage of stellar evolution is characterized by a magnetic coupling between a star and its accretion disc, known as a star-disc locking mechanism. The disc-locking prevents the star to spin its rotation up, and its timescale depends on the disc lifetime, which should not be longer than about 10 Myr. Some mechanisms can significantly shorten this lifetime, allowing a few stars to start spinning up much earlier than other stars and increasing the observed rotation period dispersion among coeval stars. Aims. In the present study, we aim to investigate how the properties of the circumstellar environment can shorten the disc lifetime, more specifically the presence of a close stellar companion. Methods. 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. Results. In the present study, we have collected timeseries photometry of BD−21 1074, a member of the 21 Myr old β Pictoris association, and measured the rotation periods of its brightest components A and B. They differ significantly, and the component B, which has a closer companion C, rotates faster than the more distant and isolated component A. It also displays a slightly higher chromospheric activity level. Conclusions. Since components A and B have similar mass, age, and initial chemical composition, 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. However, a statistically meaningful sample is yet needed to be able to infer which scenario is more likely. In our hypothesis of different disc-locking phase, any planet orbiting this star, if found by future investigations, 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

• 调查在具有相似年龄和质量的恒星中，近距离恒星伴星的存在是否增强原行星盘的消散。
• 确定多星系统中各组分之间自转周期差异是否源于环境效应导致的盘锁止 timescale 差异。
• 评估由于伴星导致的原行星盘消散增强是否可解释在具有相似年龄的恒星中观测到的自转周期分散现象。
• 评估盘寿命缩短对这类系统中行星形成机制的影响。

提出的方法

• 利用光变曲线时序观测测量BD−21 1074三合星系统中组分A和B的自转周期。
• 利用高分辨率光谱测量得到v sin i和色球层谱线流量，以推断恒星的倾角和活动水平。
• 使用ROTFIT代码推导恒星参数，包括有效温度、表面重力和金属量。
• 应用角动量演化模型，估算每个组分的恒星-盘锁止阶段持续时间。
• 对组分A和B的自转周期与活动水平进行对比分析，其中组分B更靠近第三颗伴星C。
• 利用理论模型检验观测到的周期差异是否可由初始自转周期差异解释，或是否更可能由伴星诱导的盘消散导致的更短盘锁止阶段解释。

实验结果

研究问题

• RQ1 近距离恒星伴星的存在是否导致具有相似年龄和质量的恒星中原行星盘的消散增强？
• RQ2 多星系统中各组分之间自转周期的差异是否可归因于盘锁止持续时间的差异，而非初始自转周期的差异？
• RQ3 第三颗伴星C靠近组分B在多大程度上可解释其比组分A自转更快、活动水平更高的现象？
• RQ4 盘寿命缩短对这类系统中行星形成机制有何影响？
• RQ5 BD−21 1074中观测到的自转周期差异最合理的解释是伴星引起的盘消散增强，还是初始自转周期差异较大？

主要发现

• BD−21 1074系统中组分B的自转周期为5.4天，显著快于组分A的9.3天。
• 组分B表现出略高的色球层活动水平，与其中较快的自转一致。
• 组分A的投影自转速度（v sin i）为3.7 km s⁻¹，组分B为4.9 km s⁻¹，表明其倾角分别约为60°和50°，接近。
• 组分B的恒星-盘锁止阶段估计持续3至10 Myr，而组分A为9至10 Myr，表明B的盘锁止持续时间更短。
• 观测到的自转周期差异不太可能仅由初始自转周期差异解释，更倾向于将伴星C引起的增强盘消散作为主要成因。
• 研究结果表明，任何围绕组分B运行的行星可能更可能通过引力不稳定性而非核心吸积形成，这是由于其盘寿命较短。