Multi-wavelength centimeter continuum observations of non-dusty, non-pulsating K spectral-type red giants directly sample their chromospheres and wind acceleration zones. Such stars are feeble emitters at these wavelengths, however, and previous observations have provided only a small number of modest signal-to-noise measurements slowly accumulated over three decades. We present multi-wavelength Karl G. Jansky Very Large Array thermal continuum observations of the wind acceleration zones of two dust-free red giants, Arcturus (α Boo: K2 III) and Aldebaran (α Tau: K5 III). Importantly, most of our observations of each star were carried out over just a few days, so that we obtained a snapshot of the different stellar atmospheric layers sampled at different wavelengths, independent of any long-term variability. We report the first detections at several wavelengths for each star including a detection at 10?cm (3.0?GHz: S band) for both stars and a 20?cm (1.5?GHz: L band) detection for α Boo. This is the first time single (non-binary) luminosity class III red giants have been detected at these continuum wavelengths. Our long-wavelength data sample the outer layers of α Boo's atmosphere where its wind velocity is approaching (or possibly has reached) its terminal value and the ionization balance is becoming frozen-in. For α Tau, however, our long-wavelength data are still sampling its inner atmosphere, where the wind is still accelerating probably due to its lower mass-loss rate. We compare our data with published semi-empirical models based on ultraviolet data, and the marked deviations highlight the need for new atmospheric models to be developed. Spectral indices are used to discuss the possible properties of the stellar atmospheres, and we find evidence for a rapidly cooling wind in the case of α Boo. Finally, we develop a simple analytical wind model for α Boo based on our new long-wavelength flux measurements.
展开▼
机译：非尘埃，非脉动K光谱型红色巨人的多波长厘米连续体观测直接采样其色球和风加速区。这样的恒星在这些波长下是微弱的发射器，但是，以前的观察仅提供了少量的适度的信噪比测量值，这些测量值在三十年中逐渐积累。我们介绍了两个无尘红色巨人Arcturus（αBoo：K2 III）和Aldebaran（αTau：K5 III）的多波长Karl G. Jansky超大阵列热连续体观测的风加速区。重要的是，我们对每颗恒星的大多数观测都是在短短几天内完成的，因此我们获得了以不同波长采样的不同恒星大气层的快照，而与任何长期变化无关。我们报告了对每颗恒星在几个波长处的首次探测，包括对两颗恒星在10？cm（3.0？GHz：S波段）的探测和对αBoo的20？cm（1.5？GHz：L波段）的探测。这是首次在这些连续波长下检测到单个（非二进制）发光度III类红色巨人。我们的长波数据采样了αBoo大气的外层，那里的风速接近（或可能已经达到）其终端值，并且电离平衡逐渐冻结。但是，对于αTau，我们的长波数据仍在采样其内部大气，由于质量损失率较低，风在加速。我们将我们的数据与基于紫外线数据的已发布的半经验模型进行比较，并且明显的偏差突出表明需要开发新的大气模型。光谱指数用于讨论恒星大气的可能性质，我们发现在αBoo情况下风迅速冷却的证据。最后，我们基于新的长波通量测量结果，为αBoo开发了一个简单的分析风模型。
展开▼
