Spectro-astrometry of the pre-transitional star LkCa 15 does not reveal an accreting planet but extended H a emission
Spectro-astrometry of the pre-transitional star LkCa 15 does not reveal an accreting planet but extended H a emission
复制标题
对过渡前恒星 LkCa 15 的光谱天体测量并未揭示出一颗吸积行星，但显示出延长的 H a 发射
DOI：
10.1051/0004-6361/201834233
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发表时间：
2018
影响因子：
6.5
通讯作者：
Mendigutía I
中科院分区：
文献类型：
--
作者：
Mendigutía I
上下文。在年轻恒星周围的原行星盘中发现正在形成的行星仍然难以捉摸，最先进的观测技术提供了一些模棱两可的结果。过渡前的T-Tauri明星LkCa15就是一个很好的例子。据报道，它可能拥有三颗行星；从它的Hα排放推断，候选行星b正在形成过程中。然而，最近的一项工作让人们对之前探测到的行星性质产生了怀疑。我们以LkCa15为参考案例，测试了Hα光谱天体测量作为探测年轻恒星周围正在形成的行星的另一种观测技术的潜力。用4.2mWilliam HerschelTelescope(WHT)的ISIS光谱仪观察LkCa15。狭缝指向最后报告的位置LkCa15b(平行方向)，与LkCa15b成90°(垂直)。测量了符合H-α和相邻连续谱空间分布的高斯分布的光中心和全宽度半高。使用著名的双星(Gu CMA)作为定标器，对ISIS/WHT的光谱天文测量性能进行了测试。对于GU CMA，恢复了一致的光谱-天体测量特征。然而，对于LkCa15b预测的光中心移动并未被探测到，但对于两个狭缝位置，Hα中的半高宽比连续谱中的要宽。我们的模拟表明，光中心和半高宽的观测不能同时用吸积行星来解释，但是光中心的缺乏本身仍然可以与相邻连续体中对比度为≳5.5mag in Hα或≲6mag的发射行星相一致。反过来，这两个光谱-天体测量观测都是从一个大致对称的Hα发射区自然复制出来的，发射区以恒星为中心，范围与最初归因于该行星的轨道在几个月时相当。LkCa15附近的扩展Hα发射可能与变盘风场有关，但需要额外的多历期数据和详细的模拟才能了解其物理性质。使用中型望远镜进行的光学光谱天体测量能够探测相对较暗的年轻恒星中的小尺度结构，而使用安装在较大望远镜上的最先进的仪器不容易到达这些结构。因此，Hα中的光谱天体测量能够测试吸积行星的存在，并可以作为一种补充技术来探测恒星周盘中的行星形成。
Context. The detection of forming planets in protoplanetary disks around young stars remains elusive, and state-of-the-art observational techniques provide somewhat ambiguous results. The pre-transitional T Tauri star LkCa 15 is an excellent example. It has been reported that it could host three planets; candidate planet b is in the process of formation, as inferred from its Hαemission. However, a more recent work casts doubts on the planetary nature of the previous detections.Aims. We test the potential of spectro-astrometry in Hαas an alternative observational technique to detect forming planets around young stars, taking LkCa 15 as a reference caseMethods. LkCa 15 was observed with the ISIS spectrograph at the 4.2 mWilliam HerschelTelescope (WHT). The slit was oriented towards the last reported position of LkCa 15 b (parallel direction) and 90° from that (perpendicular). The photocenter and full width half maximum (FWHM) of the Gaussians fitting the spatial distribution at Hαand the adjacent continuum were measured. A wellknown binary (GU CMa) was used as a calibrator to test the spectro-astrometric performance of ISIS/WHT.Results. A consistent spectro-astrometric signature is recovered for GU CMa. However, the photocenter shift predicted for LkCa 15 b is not detected, but the FWHM in Hαis broader than in the continuum for both slit positions. Our simulations show that the photocenter and FWHM observations cannot be explained simultaneously by an accreting planet, but the lack of photocenter shift alone could still be consistent with an emitting planet with contrast ≳5.5 mag in Hαor ≲6 mag in the adjacent continuum. In turn, both spectro-astrometric observations are naturally reproduced from a roughly symmetric Hαemitting region centered on the star and extent comparable to the orbit originally attributed to the planet at several au.Conclusions. The extended Hαemission around LkCa 15 could be related to a variable disk wind, but additional multi-epoch data and detailed modeling are necessary to understand its physical nature. Optical spectro-astrometry carried out with mid-size telescopes is capable of probing small-scale structures in relatively faint young stars that are not easily accessible with state-of-the-art instrumentation mounted on larger telescopes. Therefore, spectro-astrometry in Hαis able to test the presence of accreting planets and can be used as a complementary technique to survey planet formation in circumstellar disks.