Stellar Activity and Radial Velocities in M Dwarfs
von Erik N. Johnson
Datum der mündl. Prüfung:2024-06-28
Erschienen:2024-07-17
Betreuer:Prof. Dr. Ansgar Reiners
Gutachter:Prof. Dr. Stefan Dreizler
Gutachter:Prof. Dr. Hardi Peter
Gutachter:Prof. Dr. Artie Hatzes
Zum Verlinken/Zitieren:
http://dx.doi.org/10.53846/goediss-10619
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Zusammenfassung
Englisch
Stellar activity is one of the principal obstacles to achieving sub 1 m/s radial velocity precision in exoplanet detection surveys. Understanding how stellar activity affects the stellar environment is crucial to mitigating or eliminating unexpected radial velocity excursions that mask planetary signals. We look at the two extreme ends of stellar activity in M dwarfs. First, we characterize over 14,000 spectra from 345 M dwarfs of the CARMENES1 Guaranteed Time Observations (GTO) sample and quantify a quiet sample within that set of stars. We were able to reduce the interfering effects of molecular line continuum contamination on measurement methods by normalizing the effect out. In so doing we developed a new measurement method, the Molecular Normalized Index (MNI). This allowed us to detail the behavior of Ca II IRT (InfraRed Triplet), Hα, Na I D1&2, and He I D3 from the quietest stars to the point where emission in these profiles becomes dominant. We found that Hα profiles can be well explained by the combination of an emission and absorption profile. From this we also noticed a significant gap separating out the low activity stars, with Hα profiles in absorption or with low wing emission, from emission profile stars. This gap may be related to the rapid spin down hypothesis. We observed that, for the quietest stars, the Hα absorption strongly correlates with stellar effective temperature. Given the prevailing thought that Hα in M dwarfs should respond to stellar activity induced chromospheric heating by first becoming more absorptive before filling in and going into emission, this strong correlation with effective temperature for the maximally absorptive stars should not occur. We also find no evidence for this initial increase in Hα absorption. We therefore present a modified model in which the Hα absorption that correlates with the effective temperature is due to a basal amount of chromospheric heating that is unlikely to be related to the heating by stellar activity that causes Hα emission. The result of this is that as a quiet star gains active chromospheric regions the wider Hα emission profile of these regions will combine with the Hα absorption from what remains of the quiet chromosphere. Second, we examine the extreme other end of stellar activity in M dwarfs by examining the extremely active, young, rapidly rotating star, GJ 3270. We simultaneously observed this star for a total of 7.7 h with photometric and spectroscopic instruments. We combined our data with TESS observations conducted roughly one month prior to our observation period. We found two large flares during our observation period, one of which released 3.6 × 10^32 erg of energy and had a post-flare co-rotating feature. We tracked this feature for 90 minutes through the asymmetries it caused in the chromospheric activity indicators. We concluded that this feature was likely similar to post-flare arcadal loops on the Sun. To sum up our results we found that the chromosphere of M dwarfs is likely heated by a mechanism that is not related to stellar activity and on active M dwarfs that individual active regions can be tracked as they rotate across the stellar disk.
Keywords: stars: activity; stars: flare; stars: chromospheres; stars: late-type; stars: rotation; star: GJ3270; stars: solar neighborhood; stars: low mass; techniques: radial velocities; techniques: spectroscopic; techniques: photometric
