Abstract
Stellar activity signatures such as spots and plage can significantly limit the search for extrasolar planets. Current models of activity-induced radial velocity (RV) signals focused on the impact of temperature contrast in spots predicting the signal to diminish toward longer wavelengths. On the other hand, the relative importance of the Zeeman effect on RV measurements should grow with wavelength, because the Zeeman displacement itself grows with \lambda, and because a magnetic and cool spot contributes more to the total flux at longer wavelengths. We model the impact of active regions on stellar RV measurements including both temperature contrast in spots and Zeeman line broadening. We calculate stellar line profiles using polarized radiative transfer models including atomic and molecular Zeeman splitting from 0.5 to 2.3\mum. Our results show that the amplitude of the RV signal caused by the Zeeman effect alone can be comparable to that caused by temperature contrast. Furthermore, the RV signal caused by cool and magnetic spots increases with wavelength. We also calculate the RV signal due to variations in average magnetic field strength from one observation to the next, but find it unlikely that this can significantly influence the search for extrasolar planets. As an example, we derive the RV amplitude of the active M dwarf AD Leo as a function of wavelength using data from the HARPS spectrograph. The RV signal does not diminish at longer wavelengths but shows evidence for the opposite behavior. We conclude that the RV signal of active stars does not vanish at longer wavelength but sensitively depends on the combination of spot temperature and magnetic field; in active low-mass stars, it is even likely to grow with wavelength.
Highlights
The precise determination of radial velocities (RV) and their temporal variations is a key data analysis method in stellar astrophysics
The Zeeman effect on radial velocity (RV) measurements counteracts this: the relative importance of the Zeeman effect on RV measurements should grow with wavelength because the Zeeman displacement itself grows with λ, and because a magnetic and cool spot contributes more to the total flux at longer wavelengths
Our results show that the amplitude of the RV signal caused by the Zeeman effect alone can be comparable to that caused by temperature contrast; a spot magnetic field of ∼1000 G can produce a similar RV amplitude as a spot temperature contrast of ∼1000 K
Summary
The precise determination of radial velocities (RV) and their temporal variations is a key data analysis method in stellar astrophysics. For the detection of planets, the assumption is that the shape of a spectral lines does not vary with time so that its centroid position provides information about the projected velocity of the star. The relative RV shift between two epochs is measured either by searching for the best agreement between two spectra with RV as a free parameter, or by locating the centroid position in a cross-correlation profile calculated from the spectrum and some template. Both methods assume that the spectral line shape is identical. That variations in the shape of stellar line profiles on timescales similar to planetary orbits can be caused by several mechanisms, e.g., by the transit of a planet (Rossiter 1924; McLaughlin 1924) or stellar activity
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
