Multi-Wavelength Radio Continuum Emission Studies of Dust-Free Red Giants

Abstract

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.

Temporal evolution of the size and temperature of Betelgeuse’s extended atmosphere

“…5 yields an empirically derived temperature profile of the form T gas (r) ∝ r −0.6±0.1 , in good agreement with the power-law fit of T gas (r) ∝ r −0.7 to the semi-empirical temperature profile of . This temperature profile is less steep than the r −1.65 falloff derived for the red giant Arcturus (α Boo: K2 III) based on model dependent multi-wavelength unresolved centimeter studies (O'Gorman et al 2013). …”

Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars*

“…The exception is the well-studied K2 III star α Boo, which shows deep wind absorption despite being very close to the coronal dividing line, implying a high mass loss rate oḟ M = 2 × 10 −10 M ⊙ yr −1 (O'Gorman et al 2013). For the stars that have deep wind absorption, we can estimate the terminal wind speed (V w ) empirically, based on the following prescription.…”

Detection of thermal radio emission from a single coronal giant

“…High S/N detections at 3 GHz and lower could trace emission from ionized stellar winds for coronal giants similar to Pollux. Indeed, detections of thermal continuum emission at 1.5 and 3 GHz from non-cornal giants have already been used to constrain their partially ionized winds (O'Gorman et al 2013). Furthermore, observations of nearby coronal giants at lower frequencies than those presented here could potentially be also used to search for exoplanetary radio emission.…”