A long-term study of the magnetic field and activity in the M giant RZ Ari: Magnetism and planet engulfment in a fairly evolved star?
A long-term study of the magnetic field and activity in the M giant RZ Ari: Magnetism and planet engulfment in a fairly evolved star?
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对 M 巨星 RZ Ari 的磁场和活动的长期研究：相当演化的恒星中的磁性和行星吞噬？
DOI：
10.1051/0004-6361/202346949
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发表时间：
2024
影响因子：
6.5
通讯作者：
Drake, N. A.
中科院分区：
文献类型：
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作者：
Konstantinova-Antova, R.;Georgiev, S.;Lèbre, A.;Palacios, A.;Morin, J.;Bogdanovski, R.;Abbott, C.;Baron, F.;Aurière, M.;Drake, N. A.
目标我们对单个 M6 III 巨星 RZ Ari 进行了详细的长期研究，以获得磁场、活动指标和径向速度的直接同步测量，从而推断其活动起源。我们在恒星演化的背景下研究了它的磁性活动，为此，我们还完善了它的演化状态和锂丰度。一般来说，对于M巨星的磁活动特性及其原因知之甚少。 RZ Ari 拥有已知的塞曼探测到的 M 巨星中最强的表面磁场，并且足够明亮，可以对其表面磁结构进行深入研究。研究结果有望揭示这些恒星的活动机制。方法我们使用Bernard Lyot望远镜（法国南皮克天文台）的分光偏振计Narval获得了RZ Ari的一系列StokesI和StokesV剖面。使用最小二乘反卷积技术，我们能够检测磁场的塞曼特征。我们通过平均 StokesV 和 StokesI 剖面测量了其纵向分量。此外，我们还应用塞曼多普勒成像（ZDI）来搜索恒星的自转周期，并构建了初步的磁图。这是第一张在红巨星分支（RGB）甚至渐近巨星分支（AGB）顶端演化的恒星磁力图。这些光谱还使我们能够监测色球发射线，这是众所周知的恒星磁活动指标。根据2010年9月至2019年8月期间获得的观测结果，我们研究了RZ Ari磁场的变化。我们还根据目前的恒星演化轨迹和CHARA干涉测量的角直径重新确定了这颗恒星的初始质量和演化状态。结果我们的结果表明，这颗巨星的初始质量为1.5M⊙，因此这颗巨星更有可能是一颗早期AGB恒星，但也不完全排除RGB尖端的引力作用。当 avsiniof 6.0 ±0.5 km s−1 时，自转周期的上限为 909 天。根据我们的数据集和 AAVSO 光度数据，我们确定了磁场和光度变异性的周期超过 1100 天，谱线活动指标的周期超过 704 天。根据 StokesVprofiles 变异性确定的轮换周期为 530 天。光度数据也有 544 天的类似周期。当我们考虑到这个旋转周期和对流周转时间时，α-ω型发电机的有效作用似乎不太可能，但其他类型的发电机可能在那里运行。这颗恒星似乎位于巨型分支上的两个磁条之外，α-ω型发电机预计将在此处有效运行，并且它的锂含量也比进化模型预测的要高得多。这些事实表明，行星的吞噬可能会加速其自转并引发发电机驱动的磁活动。另一方面，超过1100天的周期不能用旋转调制来解释，而可以用大型对流结构的寿命来解释。然而，在检测到磁场时不存在线性极化，这表明局部发电机可能不会对磁场产生显着贡献，至少在该时间间隔内是如此。
AimsWe present a detailed long-term study of the single M6 III giant RZ Ari to obtain direct and simultaneous measurements of the magnetic field, activity indicators, and radial velocity in order to infer the origin of its activity. We study its magnetic activity in the context of stellar evolution, and for this purpose, we also refined its evolutionary status and Li abundance. In general, for the M giants, little is known about the properties of the magnetic activity and its causes. RZ Ari possess the strongest surface magnetic field of the known Zeeman-detected M giants and is bright enough to allow a deep study of its surface magnetic structure. The results are expected to shed light on the activity mechanism in these stars.MethodsWe used the spectropolarimeter Narval at the TélescopeBernard Lyot(Observatoire du Pic du Midi, France) to obtain a series of StokesIand StokesVprofiles for RZ Ari. Using the least-squares deconvolution technique, we were able to detect the Zeeman signature of the magnetic field. We measured its longitudinal component by means of the averaged StokesVand StokesIprofiles. In addition, we also applied Zeeman-Doppler imaging (ZDI) to search for the rotation period of the star, and we constructed a tentative magnetic map. It is the first magnetic map for a star that evolved at the tip of red giant branch (RGB) or even on the asymptotic giant branch (AGB). The spectra also allowed us to monitor chromospheric emission lines, which are well-known indicators of stellar magnetic activity. From the observations obtained between September 2010 and August 2019, we studied the variability of the magnetic field of RZ Ari. We also redetermined the initial mass and evolutionary status of this star based on current stellar evolutionary tracks and on the angular diameter measured from CHARA interferometry.ResultsOur results point to an initial mass of 1.5M⊙so that this giant is more likely an early-AGB star, but a lotaction at the tip of the RGB is not completely excluded. With avsiniof 6.0 ±0.5 km s−1, the upper limit for the rotation period is found to be 909 days. On the basis of our dataset and AAVSO photometric data, we determined periods longer than 1100 days for the magnetic field and photometric variability, and 704 days for the spectral line activity indicators. The rotation period determined on the basis of the StokesVprofiles variability is 530 days. A similar period of 544 days is also found for the photometric data. When we take this rotation period and the convective turnover time into account, an effective action of anα-ω-type dynamo seems to be unlikely, but other types of dynamo could be operating there. The star appears to lie outside the two magnetic strips on the giant branches, where theα-ω-type dynamo is expected to operate effectively, and it also has a much higher lithium content than the evolutionary model predicts. These facts suggest that a planet engulfment could speed up its rotation and trigger dynamo-driven magnetic activity. On the other hand, the period of more than 1100 days cannot be explained by rotational modulation and could be explained by the lifetime of large convective structures. The absence of linear polarization at the time the magnetic field was detected, however, suggests that a local dynamo probably does not contribute significantly to the magnetic field, at least for that time interval.