The Cycles of Alpha Centauri: Double Dipping of AB

Abstract

Abstract In mid-2023, the Sunlike dwarfs of nearby α Centauri (HD 128620 (“A”): G2 V; HD 128621 (“B”): K1 V; hereafter “AB”) coincidentally both entered coronal (T ≈ 1–3 MK) low states in their long-term X-ray cycles, as captured by the Chandra Observatory and its High Resolution Camera (HRC-I). The assessment benefited from a sensitivity degradation model for HRC-I based on a “check star,” the F subgiant Procyon (α Canis Minoris A; HD 61421: F5 IV–V), further validated by Hubble Space Telescope time series of the Fe xii 1241.985 Å and 1349.396 Å coronal forbidden lines (T ≈ 1.6 MK) of all three stars. The AB starspot-cycle periods noted previously, 19 and 8 yr, appear to be holding. These deviate substantially from the 11 yr solar example, despite only modest differences in the stellar properties. The new cycle-minimum L X/L BOL of α Cen A is similar to that of the previous minimum observed by Chandra in 2005–2010, and close to solar-MIN X-ray levels in 2009 and 2019, with implications for the “basal” coronal flux of low-activity early G stars. The recent α Cen B X-ray MIN is similar to that in 2016 but higher than an earlier one in 2007–2009. Significantly, the B X-ray minima (in L X/L BOL) are comparable to the solar Cycle 24 maximum, suggesting plentiful starspots at the B-cycle MIN (contrary to the solar case). Tangentially, the current AB low states favor Doppler-reflex detections of planets, as opposed to the coronal high states, when elevated chromospheric activity can add nuisance signals to radial velocity time series.