emamajek on 10-Sep-2021 07:42:57 GMT about * del Eri
SIMBAD currently (9/2021) classifies del Eri as a "Variable of RS CVn type". The classification appears to come from the GCVS, but is demonstrably erroneous.
The original report of del Eri as a candidate RS CVn variable goes back to Fisher+1983 (1983IBVS.2259....1F), which reported del Eri as having ~0.02 mag variability with periodicity ~10 days. Unfortunately all of their observations used the neighboring bright star eps Eri as a standard star - which itself is now known to be a very active periodic variable (at ~0.01-0.03 mag level) with multiple reported rotation periods between about 10 and 12 days (e.g. Frey+1991
1991AJ....102.1813F
, Baliunas+1996
1996ApJ...457L..99B
, Giguere+2016
2016ApJ...824..150G
). Subsequent time series photometry by Eaton & Poe 1985 (1985IBVS.2712....1E) of del Eri did not recover the variability at the levels reported by Fisher+1983, and Frey+1991 pointed out that the original variability for del Eri reported by Fisher+1983 was most likely due to the variability of the comparison star eps Eri itself. Del Eri is a slow rotator (Prot = 71 d; Baliunas+1996
1996ApJ...457L..99B
; vsini = 1.54+-0.23 km/s; Jofre+2015 2015A&A...574A..50J), inconsistent with RS CVns. Interferometry observations by Thevinin+2005 (2005A&A...436..253T) found no evidence of a companion down to the levels of ~2% of del Eri's luminosity on orbits close enough to be responsible for any RS CVn activity. Precision radial velocity data shows no sign of a stellar companion, with RV variations at the ~<5-10 m/s level (Butler+ 2017
2017AJ....153..208B
). Del Eri is extremely magnetically inactive - both chromospherically (logR'HK = -5.184; Baliunas+1996) and coronally (logLx/Lbol = -7.14; Morel+2004 2004A&A...423..677M) - i.e. significantly less active than even the Sun (logLx/Lbol ~ -6; Judge+2003
2003ApJ...593..534J
), and orders of magnitude weaker than typical RS CVn systems (logLx/Lbol ~ -5 to -3; Dempsey+1993
1993ApJS...86..599D
). The weak activity is perfectly consistent with the star's slow rotation. There seems to be no supporting observational evidence that del Eri could be a chromospherically active binary (which typically have orbital and rotation periods of between ~1 and ~14 days; Hall 1976
1976ASSL...60..287H
), and its classification as a "RS CVn" variable should be removed. Del Eri appears to be an otherwise unremarkable inactive single subgiant star.
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