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| Paper: |
Radial-Velocity Variations from Starspots |
| Volume: |
185, Precise Stellar Radial Velocities, IAU Colloquium 170 |
| Page: |
259 |
| Authors: |
Hatzes, A. P. |
| Abstract: |
Stellar surface structure in the form of cool spots create spectral distortions which can be detected as radial velocity (RV) variations that are modulated with the rotation period of the star. These RV variations act as ''noise'' which can obscure the RV signal due to planets. Here the predicted RV and line profile shape variations using realistic spot models and an RV ''measurement'' technique that mimics current methods are calculated for cool spots spanning a range of parameters pertinent to activity in solar-type stars (slow rotation, small spots). For a stars with the same projected rotational velocity as the Sun, the RV amplitude due to a spot comparable to the largest sunspot is about 3 m/s and is less than 1 m/s for a more modest sized starspot. These simulations also show the '' v sin i '' amplification effect; for a given spot size the RV amplitude increases linearly with v sin i. All of these findings confirm the results of Saar & Donahue who also looked at activity related RV variations. The RV noise due to stellar activity will thus be higher in more rapidly rotating stars because 1) the RV amplitude is higher due to the v sin i amplification effect, and 2) stars with higher rotation tend to have more activity (more and larger spots). Searching for Saturn-like planets is best suited for those stars with v sin i < 3 km/s. The RV amplitude increases linearly with spot filling factor and is about 7 m/s for a filling factor of 0.2% and as high as 20 m/s for a filling factor of 1% ( v sin i = 3 km/s) However, if spots are uniformly distributed about the equator, the RV noise due to sunspot-sized spots is always less than about 1 m/s. For modest rotations rates (v sin i = 15 km/s) temperature sensitive lines have an amplitude 15% lower than temperature insensitive lines. An RV analysis of active stars as a function of temperature sensitive lines may help distinguish between RV signals due to spots and those due to companions. Restricting RV measurements to temperature sensitive lines (line strength increasing in the starspot) should also minimize RV noise due to stellar activity. Combining precise stellar radial velocity measurements with line shape information, one may be able to map spot features on slowly rotating stars for which the Doppler imaging technique is ill-suited. |
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