An XMM—Newton observation of the young open cluster NGC 2547: coronal activity at 30 Myr
Jeffries, R. D. ; Evans, P. A. ; Pye, J. P. ; Briggs, K. R.
In: Monthly Notices of the Royal Astronomical Society, 2006, vol. 367, no. 2, p. 781-800
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- We report on XMM—Newton observations of the young open cluster NGC 2547 which allow us to characterize coronal activity in solar-type stars, and stars of lower mass, at an age of 30 Myr. X-ray emission is seen from stars at all spectral types, peaking among G stars at luminosities (0.3-3 keV) of Lx≃ 1030.5 erg s−1 and declining to Lx≤ 1029.0 erg s−1 among M stars with masses ≥0.2 M⊙. Coronal spectra show evidence for multi-temperature differential emission measures and low coronal metal abundances of Z≃ 0.3. The G- and K-type stars of NGC 2547 follow the same relationship between X-ray activity and Rossby number established in older clusters and field stars, although most of the solar-type stars in NGC 2547 exhibit saturated or even supersaturated X-ray activity levels. The median levels of Lx and Lx/Lbol in the solar-type stars of NGC 2547 are very similar to those in T-Tauri stars of the Orion Nebula cluster (ONC), but an order of magnitude higher than in the older Pleiades. The spread in X-ray activity levels among solar-type stars in NGC 2547 is much smaller than in older or younger clusters. Coronal temperatures increase with Lx, Lx/Lbol and surface X-ray flux. The most active solar-type stars in NGC 2547 have coronal temperatures intermediate between those in the ONC and the most active older zero-age main-sequence (ZAMS) stars. We show that simple scaling arguments predict higher coronal temperature in coronally saturated stars with lower gravities. A number of candidate flares were identified among the low-mass members and a flaring rate [for total flare energies (0.3-3 keV) > 1034 erg] of one every 350+350−120 ks was found for solar-type stars, which is similar to rates found in the ONC and Pleiades. Comparison with ROSAT High Resolution Imager (HRI) data taken 7 yr earlier reveals that only 10-15 per cent of solar-type stars or stars with Lx > 3 × 1029 erg s−1 exhibit X-ray variability by more than a factor of 2. This is comparable with clusters of similar age but less than in both older and younger clusters. The similar median levels of X-ray activity and rate of occurrence for large flares in NGC 2547 and the ONC demonstrate that the X-ray radiation environment around young solar-type stars remains relatively constant over their first 30 Myr