Browsing by Subject "m-dwarfs"
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Item The Factory And The Beehive. II. Activity And Rotation In Praesepe And The Hyades(2014-11) Douglas, S. T.; Agueros, M. A.; Covey, Kevin R.; Bowsher, E. C.; Bochanski, J. J.; Cargile, P. A.; Kraus, Adam; Law, N. M.; Lemonias, J. J.; Arce, Hector G.; Fierroz, D. F.; Kundert, A.; Kraus, AdamOpen clusters are collections of stars with a single, well-determined age, and can be used to investigate the connections between angular-momentum evolution and magnetic activity over a star's lifetime. We present the results of a comparative study of the relationship between stellar rotation and activity in two benchmark open clusters: Praesepe and the Hyades. As they have the same age and roughly solar metallicity, these clusters serve as an ideal laboratory for testing the agreement between theoretical and empirical rotation-activity relations at approximate to 600 Myr. We have compiled a sample of 720 spectra-more than half of which are new observations-for 516 high-confidence members of Praesepe; we have also obtained 139 new spectra for 130 high-confidence Hyads. We have also collected rotation periods (P-rot) for 135 Praesepe members and 87 Hyads. To compare H alpha emission, an indicator of chromospheric activity, as a function of color, mass, and Rossby number R-o, we first calculate an expanded set of chi values, with which we can obtain the H alpha to bolometric luminosity ratio, L-H alpha/L-bol, even when spectra are not flux-calibrated and/or stars lack reliable distances. Our. values cover a broader range of stellar masses and colors (roughly equivalent to spectral types from K0 to M9), and exhibit better agreement between independent calculations, than existing values. Unlike previous authors, we find no difference between the two clusters in their H alpha equivalent width or L-H alpha/L-bol distributions, and therefore take the merged H alpha and Prot data to be representative of 600 Myr old stars. Our analysis shows that H alpha activity in these stars is saturated for R-o <= 0.11(-0.03)(+0.02) Above that value activity declines as a power-law with slope beta = -0.73(-0.12,)(+0.06) before dropping off rapidly at Ro approximate to 0.4. These data provide a useful anchor for calibrating the age-activity-rotation relation beyond 600 Myr.Item Secretly Eccentric: The Giant Planet And Activity Cycle Of GJ 328(2013-09) Robertson, Paul; Endl, Michael; Cochran, William D.; MacQueen, Phillip J.; Boss, Alan P.; Robertson, Paul; Endl, Michael; Cochran, William D.; MacQueen, Phillip J.We announce the discovery of a similar to 2 Jupiter-mass planet in an eccentric 11 yr orbit around the K7/M0 dwarf GJ 328. Our result is based on 10 years of radial velocity (RV) data from the Hobby-Eberly and Harlan J. Smith telescopes at McDonald Observatory, and from the Keck Telescope at Mauna Kea. Our analysis of GJ 328's magnetic activity via the Na I D features reveals a long-period stellar activity cycle, which creates an additional signal in the star's RV curve with amplitude 6-10 m s(-1). After correcting for this stellar RV contribution, we see that the orbit of the planet is more eccentric than suggested by the raw RV data. GJ 328b is currently the most massive, longest-period planet discovered around a low-mass dwarf.