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 � 10 6 K) emission from the archetype ‘‘ noncoronal ’’ red giants Arcturus (� Bootis=HD 124897, K1 III) and Aldebaran (� Tauri=HD 29139, K5 III). Our program follows up previous detections of ultraviolet coronal proxies such as C iv � 1548 (T � 1 � 10 5 K) and O vi � 1031 (T � 3 � 10 5 K). The deep (� 19 ks) HRC-I pointings obtained a tentative 3 � detection of Arcturus, with fXð0:2 2 keV Þ¼ 1:0 þ1:8 � 0:8 � 10 � 15 ergs cm � 2 s � 1 (95% confidence limits [CLs]), but failed to record Aldebaran, with an upper limit of d1:5 � 10 � 15 ergs cm � 2 s � 1 (also at 95% CL). The corresponding LX=Lbol 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 � 1393. 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. Subject headings: stars: coronae — stars: individual (Aldebaran, Arcturus) — ultraviolet: stars — X-rays: stars