用VLTI/GRAVITY长基线干涉法研究β-Piccadib的形成历史
Peering into the formation history of beta Pictoris b with VLTI/GRAVITY long baseline interferometry
摘要：
我们的目标是估计β Picasso b大气层中的C/O比，并估计该行星的动力学质量，以及使用高精度天体测量来改进其轨道参数。我们使用重力仪器和甚大望远镜干涉仪的四个8.2米望远镜来获得$β$ Pic b上的K波段光谱干涉数据。我们提取了一个中等分辨率（R=500）的K波段光谱的行星和高精度的天体测量位置。我们估计行星的C/O比使用两种不同的方法（前向建模和免费检索）从两个不同的代码（ExoREM和petitRADTRANS，分别）。最后，我们使用了一个简化的模型，两个形成的情况下（引力坍缩和核心吸积），以确定哪一个可以最好地解释测得的C/O比。我们的新天体测量不赞成$β$ Pic b（$e=0.15^{+0.05}_{-0.04}$）的圆形轨道。结合以前的结果和依巴谷/盖亚的测量，这个天体测量指向一个行星质量为$M = 12.7\pm{}2.2\,M_\mathrm{Jup}$。该值与仅使用光谱数据的免费检索代码petitRADTRANS导出的质量兼容。正演模拟和免费检索的方法产生非常相似的结果，关于β Pic b的大气。特别是，用两个代码导出的C/O比是相同的（$0.43\pm{}0.05$与$0.43^{+0.04}_{-0.03}$）。我们认为，如果恒星的C/O在$β$ Pic是太阳，那么这种组合的一个非常高的质量和低的C/O比的行星表明通过核心吸积形成，与强大的星子富集。
英文摘要：
Our objective is to estimate the C/O ratio in the atmosphere of beta Pictoris b and obtain an estimate of the dynamical mass of the planet, as well as to refine its orbital parameters using high-precision astrometry. We used the GRAVITY instrument with the four 8.2 m telescopes of the Very Large Telescope Interferometer to obtain K-band spectro-interferometric data on $β$ Pic b. We extracted a medium resolution (R=500) K-band spectrum of the planet and a high-precision astrometric position. We estimated the planetary C/O ratio using two different approaches (forward modeling and free retrieval) from two different codes (ExoREM and petitRADTRANS, respectively). Finally, we used a simplified model of two formation scenarios (gravitational collapse and core-accretion) to determine which can best explain the measured C/O ratio. Our new astrometry disfavors a circular orbit for $β$ Pic b ($e=0.15^{+0.05}_{-0.04}$). Combined with previous results and with Hipparcos/GAIA measurements, this astrometry points to a planet mass of $M = 12.7\pm{}2.2\,M_\mathrm{Jup}$. This value is compatible with the mass derived with the free-retrieval code petitRADTRANS using spectral data only. The forward modeling and free-retrieval approches yield very similar results regarding the atmosphere of beta Pic b. In particular, the C/O ratios derived with the two codes are identical ($0.43\pm{}0.05$ vs $0.43^{+0.04}_{-0.03}$). We argue that if the stellar C/O in $β$ Pic is Solar, then this combination of a very high mass and a low C/O ratio for the planet suggests a formation through core-accretion, with strong planetesimal enrichment.
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