Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995mnras.272..828r&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 272, Issue 4, pp. 828-842.
Astronomy and Astrophysics
Astronomy
55
Stars: Activity, Stars: Coronae, Stars: Late-Type, Stars: Low-Mass, Brown Dwarfs, Open Clusters And Associations: Individual: Hyades, X-Rays: Stars
Scientific paper
We present an extensive survey of the chromospheric and coronal activity among low-mass members of the Hyades open cluster. We have obtained H alpha spectroscopy of 91 probable cluster members, including 23 that lie within three fields for which we have deep ROSAT X-ray observations. 20 of the 23 stars are detected in X-rays; the non-detections are probably not Hyades members. Combining these observations with data from the literature, we find that the mean H alpha to bolometric luminosity ratio log (L_H alpha/L_bol) is constant at a value of ~-3.9 for M_bol>8, with a substantial dispersion and an upper limit of -3.5. The X-ray to bolometric luminosity ratio log (L_x/L_bol) first increases with increasing M_bol, then flattens at a value of ~-3.3 for M_bol>8. Comparing this behaviour with stars in the Pleiades cluster, there are significant differences: first, the log (L_H alpha/L_bol) ratio increases with increasing M_bol while M_bol<10, but appears to turn over sharply at fainter magnitudes, with the lower luminosity stars being nearly an order of magnitude lower in the ratio; secondly, we find no evidence for saturation in the log (L_x/L_bol) ratio; L_x increases monotonically with M_bol, reaching a value of ~-2.5 at M_bol~10. Lower luminosity stars have not yet been detected in X-ray observations. The nearly constant H alpha to H beta ratio in the Hyades stars, coupled with the small range in coronal temperature seen in the X-ray data, suggest that the (presumably magnetically generated) heating rate is nearly constant in the outer atmospheres of these stars. Differences in the relative activity levels are then ascribed to differences in the filling factor of active regions, and in the sizes of the coronal structures.
Hawley Suzanne L.
Mateo Mario
Reid Neill
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