Open AccessX‐rays from the open cluster NGC 6633
Author(s) -
Harmer S.,
Jeffries R. D.,
Totten E. J.,
Pye J. P.
Publication year - 2001
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2001.04342.x
Subject(s) - physics , astrophysics , rosat , open cluster , luminosity , stars , astronomy , metallicity , hubble sequence , galaxy , spiral galaxy
NGC 6633 is a young, open cluster with a similar age to the Hyades and Praesepe, but probably a lower metallicity. We present the results of ROSAT High Resolution Imager observations of an optically selected catalogue of likely members of NGC 6633. 8 out of 51 NGC 6633 members have been detected, with main‐sequence spectral types A to G, above a threshold X‐ray luminosity of ≈6–12×10 28 erg s −1 . We find that NGC 6633 does not contain cool stars that are as X‐ray luminous as the most active objects in the Hyades and that the median X‐ray luminosity of F‐G stars in NGC 6633 is less than that in the Hyades, but probably greater than in Praesepe. However, when X‐ray activity is expressed as the X‐ray to bolometric flux ratio we find that NGC 6633 and the Hyades are very similar and display similar peak levels of coronal activity. We attribute this discrepancy to a number of possible wide binary systems with higher X‐ray (and bolometric) luminosities in the Hyades sample and either a low metallicity in NGC 6633, which makes its cool stars both X‐ray and bolometrically less luminous at the same colour, or a distance to NGC 6633 that has been underestimated, which would decrease stellar X‐ray luminosities without changing X‐ray to bolometric flux ratios.