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Title:
The chromospheric activity-age relation for M dwarf stars in wide binary systems
Authors:
Silvestri, Nicole Marie
Affiliation:
AA(FLORIDA INSTITUTE OF TECHNOLOGY)
Publication:
Thesis (PhD). FLORIDA INSTITUTE OF TECHNOLOGY, Source DAI-B 63/10, p. 4724, Apr 2003, 261 pages.
Publication Date:
11/2002
Category:
Physics: Astronomy and Astrophysics
Origin:
UMI
Comment:
Publication Number: 3067511; Advisor: Oswalt, Terry D.
Bibliographic Code:
2002PhDT........28S

Abstract

We present new chromospheric activity-age relations for 139 M dwarf stars in wide binary systems. For this study, we selected binaries containing both an M dwarf and a white dwarf star. We determined the cooling age of the white dwarf from the atmospheric and evolutionary models of Bergeron, Saumon, & Wesemael (1995) and Wood (1995), respectively. The cooling age, when added to the main sequence lifetime of the white dwarf progenitor star as determined from the initial → final mass relation of Weidemann (2000), yields the age of the white dwarf from the zero-age main sequence. Assuming the binary is coeval, the age of the white dwarf is the age of the M dwarf. Our results indicate that the activity-age relations for old M dwarf stars in binary systems do not agree with current activity-age relations for young M dwarfs in clusters. The point at which chromospheric activity becomes pervasive (at the “Hα limit”) has been shown to correlate linearly with log(Age) in young M dwarf stars in clusters (Hawley, Tourtellot, & Reid, 1999). We find that M dwarfs in wide binaries older than 4 Gyr depart from this log-linear relation and are found to have activity at magnitudes and colors brighter, bluer, and more massive than M dwarfs in clusters. Our results indicate a change in the magnetic activity of M dwarf stars between M3 M4, suggesting a change in the magnetic heating mechanism in old M dwarfs at these spectral types. In addition to our activity-age relations we present the measured radial velocities and complete space motions for 161 white dwarf stars with M dwarf companions. Twenty pairs have “halo-like” velocities. According to a recent study by Oppenheimer et al. (2001a), all of these high velocity white dwarfs should be considered part of the dark-matter heavy halo of the Galaxy based on their kinematics. Based upon the near-solar abundance levels of the M dwarf companions, we conclude that 19 of our 20 high velocity white dwarfs are actually part of the high velocity tail of the thick disk, in agreement with the results of a study of 514 M dwarfs performed by Reid, Sahu, & Hawley (200I). Our results suggest that the assignment of population membership based solely upon incomplete kinematical information is not definitive and that a more robust examination of suspected halo white dwarfs must be performed.
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