[论文解读] ALMA observations of alpha Centauri: First detection of main-sequence stars at 3mm wavelength

R. Liseau, W. H. T. Vlemmings+20 more|arXiv (Cornell University)|Dec 12, 2014

Stellar, planetary, and galactic studies参考文献 17被引用 23

一句话总结

本研究首次利用ALMA在3毫米波段探测到主序星，成功解析了α Centauri AB双星系统的两个组分。高灵敏度、高分辨率的观测显示，3.1毫米发射的强度显著超过光球层预测值，主要由电离色球层等离子体的光学厚自由-自由辐射主导，3.1毫米波段的通量比为 Sν^B/Sν^A = 0.47 ± 0.006。

ABSTRACT

The precise mechanisms that provide the non-radiative energy for heating the chromosphere and the corona of the Sun and those of other stars constitute an active field of research. By studying stellar chromospheres one aims at identifying the relevant physical processes. Defining the permittable extent of the parameter space can also serve as a template for the Sun-as-a-star. Earlier observations with Herschel and APEX have revealed the temperature minimum of alpha Cen, but these were unable to spatially resolve the binary into individual components. With the data reported here, we aim at remedying this shortcoming. Furthermore, these earlier data were limited to the wavelength region between 100 and 870mu. In the present context, we intend to extend the spectral mapping to longer wavelengths, where the contrast between stellar photospheric and chromospheric emission becomes increasingly evident. ALMA is particularly suited to point sources, such as unresolved stars. ALMA provides the means to achieve our objectives with both its high sensitivity of the collecting area for the detection of weak signals and the high spatial resolving power of its adaptable interferometer for imaging close multiple stars. This is the first detection of main-sequence stars at a wavelength of 3mm. Furthermore, the individual components of the binary alpha CenAB are clearly detected and spatially well resolved at all ALMA wavelengths. The high S/N of these data permit accurate determination of their relative flux ratios. The previously obtained flux ratio of 0.44, which was based on measurements in the optical and at 70mu, is consistent with the present ALMA results, albeit with a large error bar. Given the distinct difference in their cyclic activity, the similarity of their submm SEDs appears surprising.

研究动机与目标

• 在亚毫米波和毫米波段解析α Centauri AB双星系统的各个组分，该波段以色球层发射为主。
• 将光谱能量分布（SED）测量扩展至870微米以上波长范围，以最大化色球层与光球层发射的对比度。
• 测定毫米波段α Centauri A与B的通量比，以评估色球层加热机制及活动性差异。
• 检验两颗恒星（磁活动水平显著不同）的色球层发射是否在子毫米波段表现出相似的亚毫米波SED，尽管其活动水平不同。
• 为恒星色球层模型提供实证约束，并通过与类太阳恒星的比较，增进对太阳作为恒星的理解。

提出的方法

• 利用ALMA的高灵敏度与高角分辨率，在3.1毫米、870微米和440微米波段对α Centauri进行观测，实现了对双星系统的空间解析。
• 采用综合孔径成像技术生成双星的高保真度图像，从而实现对各组分的精确通量测量。
• 在多个ALMA波段测量α Centauri A与B的通量比，以评估相对发射强度与谱指数。
• 从远红外到亚毫米波段分析光谱能量分布（SED），将实测数据与超出40微米的PHOENIX模型大气外推结果进行比较。
• 通过分析谱指数评估光学深度与发射机制，结论为发射为光学厚自由-自由（轫致辐射）辐射。
• 利用高信噪比数据，获得3.1毫米波段（波段3）通量比的精确测量，不确定度低于5%，从而实现与以往观测的稳健比较。

实验结果

研究问题

• RQ1 ALMA能否在3毫米波段（色球层发射占主导的波段）成功解析α Centauri AB双星系统的各个组分？
• RQ2 尽管α Centauri A与B的磁活动水平不同，其在长波长下的亚毫米波SED是否表现出相似的发射特征？
• RQ3 观测到的3.1毫米发射是否与电离色球层等离子体的光学厚自由-自由发射一致，还是需要其他解释？
• RQ4 α Centauri A与B在毫米波段的通量比与以往在较短波长及光学波段的测量结果相比如何？
• RQ5 在长波长下，观测到的SED与光球层预测值偏离程度如何，表明存在色球层加热？

主要发现

• 这是首次成功探测主序星在3毫米波段的发射，ALMA以高信噪比解析了α Centauri A与B在3.1毫米波段的发射。
• 在3.1毫米波段测得的通量比 Sν^B/Sν^A 为 0.47 ± 0.006，与以往估计值一致，但不确定度显著降低。
• 在观测波段范围内，谱指数与光学厚自由-自由发射一致，表明发射源为电离色球层等离子体。
• 3.1毫米发射远超光球层预测值，证实其在温度高于8000 K时具有色球层起源。
• 尽管α Centauri B的活动性显著高于α Centauri A，但其亚毫米波SED却出人意料地相似，挑战了活动性依赖发射的假设。
• 数据证实两颗恒星大气中均存在温度极小值，与早期赫歇尔DUNES观测结果一致，但现具有更高的空间分辨率与测量精度。