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the onset of chromospheric activity among the a and f starsResults are reported from a search for an upper boundary for the onset of main-sequence star activity based on a quest for high-temperature UV line emission in a large collection of IUE spectra. It is shown that strong chromospheric emission is common among early F dwarf and subgiant stars. At its brightest, the emission is equal to that of the most active solar-type stars and is exceeded only by that of the spotted RS CVn and BY Dra variables. It is suggested that the emission from the main-sequence stars reaches a peak near B-V = 0.28, in the vicinity of spectral type F0 V, before it declines to lower flux levels among the late A stars. Emission is seen in some dwarf stars as early as B-V = 0.25. It is demonstrated that the C II emission of stars earlier than the spectral type F5 is uncorrelated with rotation. Previous findings that the coronal X-ray:chromospheric UV flux ratio is lower for stars earlier than spectral type F5 than for those later than F5 are confirmed.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Simon, Theodore
(Hawaii, University Honolulu, United States)
Landsman, Wayne
(NASA Goddard Space Flight Center; ST Systems Corp. Greenbelt, MD, United States)
Date Acquired
August 14, 2013
Publication Date
October 10, 1991
Publication Information
Publication: Astrophysical Journal, Part 1
Volume: 380
ISSN: 0004-637X
Subject Category
Funding Number(s)
Distribution Limits