Buried Alive in the Coronal Graveyard

Abstract

We have used the High Resolution Camera (HRC-I) of the Chandra X-Ray Observatory to search for coronal (T ~ 10⁶ K) emission from the archetype "noncoronal" red giants Arcturus (α Bootis=HD 124897, K1 III) and Aldebaran (α Tauri=HD 29139, K5 III). Our program follows up previous detections of ultraviolet coronal proxies such as C IV λ1548 (T ~ 1 × 10⁵ K) and O VI λ1031 (T ~ 3 × 10⁵ K). The deep (~19 ks) HRC-I pointings obtained a tentative 3 σ detection of Arcturus, with f_X(0.2-2 keV) = 1.0 × 10^-15 ergs cm^-2 s^-1 (95% confidence limits [CLs]), but failed to record Aldebaran, with an upper limit of 1.5 × 10^-15 ergs cm^-2 s^-1 (also at 95% CL). The corresponding L_X/L_bol ratios are a factor of ten thousand less than the Sun, a low-activity coronal dwarf. At the same time, Hubble Space Telescope Imaging Spectrograph far-ultraviolet spectra suggest the presence of a "cool absorber," probably near the base of the red giant chromosphere, imprinting discrete low-excitation absorptions on top of highly ionized features such as Si IV λ1393. The hot emission zones thus are at least partially buried under a large column of chromospheric material, which would severely attenuate any soft X-rays that might be emitted. The submerged hot structures presumably are magnetic because of their high temperatures and broad C IV profiles (FWHM ~ 130 km s^-1). Perhaps these structures are analogous to small-scale ephemeral bipolar regions seen ubiquitously on the Sun throughout the sunspot cycle and thought to be of direct convective origin. If small-scale magnetic fields indeed are present in the lower atmospheres of red giants such as Arcturus and Aldebaran, they might play a role in initiating the cool winds of such stars, perhaps through a mechanism similar to solar spicules.