The Far-Ultraviolet Ups and Downs of Alpha Centauri

TL;DR

This study presents a four-year time series of far-ultraviolet emissions from Alpha Centauri A and B, revealing stable emission features and subtle activity cycle effects in their stellar atmospheres.

Contribution

It provides detailed, long-term FUV spectral observations of Alpha Centauri A and B, highlighting the stability of certain emission components despite stellar activity cycles.

Findings

FUV fluxes of Alpha Centauri A remained mostly steady over four years.

Alpha Centauri B's hot lines tracked its starspot cycle, showing rise and fall.

Both stars' hot line profiles consist of narrow and broad components with little variation over time.

Abstract

Four years (2010-2014) of semiannual pointings by Hubble Space Telescope Imaging Spectrograph (STIS) on nearby Alpha Centauri have yielded a detailed time history of far-ultraviolet emissions of the solar-like primary (A: G2V) and the cooler, but more active, secondary (B: K1V). This period saw A climbing out of a prolonged coronal X-ray minimum, as documented by Chandra, while B was rising to, then falling from, a peak of its long-term (8 yr) starspot cycle. The FUV fluxes of the primary were steady over most of the STIS period, although the [Fe XII] 124 nm coronal forbidden line (T= 1.5 MK) partly mirrored the slowly rising X-ray fluxes. The FUV emissions of the secondary more closely tracked the rise and fall of its coronal luminosities, especially the "hot lines" like Si IV, C IV, and N V (T= 80,000-200,000 K), and coronal [Fe XII] itself. The hot lines of both stars were systematically redshifted, relative to narrow chromospheric emissions, by several km/s, showing little change in amplitude over the 4-year period; especially for Alpha Cen B, despite the significant evolution of its coronal activity. Further, the hot line profiles of both stars, individually and epoch-averaged, could be decomposed into two nearly equal components, one narrow (FWHM~ 25-45 km/s), the other broad (60-80 km/s). Not much variation of the component properties was seen over the 4-year period, even over the major cycle changes of B. This suggests that there is a dominant "quantum" of FUV surface activity that is relatively unchanged during the cycle, aside from the fractional area covered.