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Chromospheric Variability in Early F- type Stars Dillon R. Foight Dr. Brian Rachford Embry-Riddle Aeronautical University April 18, 2009

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Objectives Determine level of activity in early F-type stars. The D3 absorption line (Helium) is used as an activity indicator. Search for significant variations in the strength of the D3 line, and thus variations in chromospheric activity. These variations may be able to explain the range of activity levels observed in F- type stars.

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Why the D3 Line? The D3 line has been shown to be a reliable chromospheric activity indicator for early F-type stars. It is a indicator of convective activity. Activity such as observed on the Sun, sunspots D3 measurements provide activity level measurements comparable to those found by space based observations.

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Target Selection The sample group was 13 early F-type stars and 2 late F-type. Many spectra were taken in non-optimal observation conditions, so stars had to be brighter than 5 th magnitude (V). The D3 line becomes difficult to detect with large rotational broadening, so only stars with v sini < 100 km/s were chosen. Goal was S/N ratios of 200-300

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Target Group

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Approach

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Determining Variability The approach is very important. Telluric subtraction should not affect the equivalent width measurements. Basic approach is to compare standard deviation of equivalent widths and average measurement uncertainties.

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Determining Variability, Cont. A more formal way is to use a Χ 2 test. This measures the probability that the sample does not represent a constant variable with normally distributed deviations from the mean value. The Z-statistic was used for testing some of the stars long-term variability.

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Results Out of the 15 stars, 2 (μ Vir and 18 Boo) displayed statistically significant short term variability, and 2 (β Cas and σ Boo) display statistically significant long term variability. Other stars exhibit variability, which can be explained by pure rotational modulation or active longitudes (α CMi and θ Boo).

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Conclusions Determined the activity level of early F-type stars, and used observations to search for variations. Found evidence for variability on both short (hours to days) and long time scales. Some variability can be explained by rotational modulation. The general lack of large variability (larger than a factor of 2) implies that variability alone cannot explain the wide range of activity levels observed in early F-type stars.

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Acknowledgements Dr. Brian Rachford – Mentor of project. Dr. Ron Madler – Space Grant Coordinator The Arizona Space Grant Consortium for financing this project.

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