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Spots on stellar surfaces are thought to be stellar analogues of sunspots. Thus, starspots are direct manifestations of strong magnetic fields. Their decay rate is directly related to the magnetic diffusivity, which itself is a key quantity for the deduction of an activity cycle length. So far, no single starspot decay has been observed, and thus no stellar activity cycle was inferred from its corresponding turbulent diffusivity.
We investigate the evolution of starspots on the rapidly-rotating K0 giant XX Triangulum. Continuous high-resolution and phase-resolved spectroscopy was obtained with the robotic 1.2-m STELLA telescope on Tenerife over a timespan of six years. With our line-profile inversion code iMap we reconstruct a total of 36 consecutive Doppler maps. To quantify starspot area decay and growth, we match the observed images with simplified spot models based on a Monte-Carlo approach.
It is shown that the surface of XX Tri is covered with large high-latitude and even polar spots and with occasional small equatorial spots. Just over the course of six years, we see a systematically changing spot distribution with various time scales and morphology such as spot fragmentation and spot merging as well as spot decay and formation.
For the first time, a starspot decay rate on another star than the Sun is determined. From our spot-decay analysis we determine an average linear decay rate of D = -0.067±0.006 Gm^2/day. From this decay rate, we infer a turbulent diffusivity of η_τ = (6.3±0.5) x 10^14 cm^2/s and consequently predict an activity cycle of 26±6 years. The obtained cycle length matches very well with photometric observations.
Our time-series of Doppler maps further enables to investigate the differential rotation of XX Tri. We therefore applied a cross-correlation analysis. We detect a weak solar-like differential rotation with a surface shear of α = 0.016±0.003. This value agrees with similar studies of other RS CVn stars.
Furthermore, we found evidence for active longitudes and flip-flops. Whereas the more active longitude is located in phase towards the (unseen) companion star, the weaker active longitude is located at the opposite stellar hemisphere. From their periodic appearance, we infer a flip-flop cycle of ~2 years. Both activity phenomena are common on late-type binary stars.
Last but not least we redetermine several astrophysical properties of XX Tri and its binary system, as large datasets of photometric and spectroscopic observations are available since its last determination in 1999. Additionally, we compare the rotational spot-modulation from photometric and spectroscopic studies.
The characterization of exoplanets is a young and rapidly expanding field in astronomy.
It includes a method called transmission spectroscopy that searches for planetary spectral
fingerprints in the light received from the host star during the event of a transit. This
techniques allows for conclusions on the atmospheric composition at the terminator region,
the boundary between the day and night side of the planet. Observationally a big
challenge, first attempts in the community have been successful in the detection of several
absorption features in the optical wavelength range. These are for example a Rayleighscattering
slope and absorption by sodium and potassium. However, other objects show
a featureless spectrum indicative for a cloud or haze layer of condensates masking the
probable atmospheric layers.
In this work, we performed transmission spectroscopy by spectrophotometry of three
Hot Jupiter exoplanets. When we began the work on this thesis, optical transmission
spectra have been available for two exoplanets. Our main goal was to advance the current
sample of probed objects to learn by comparative exoplanetology whether certain
absorption features are common. We selected the targets HAT-P-12b, HAT-P-19b and
HAT-P-32b, for which the detection of atmospheric signatures is feasible with current
ground-based instrumentation. In addition, we monitored the host stars of all three objects
photometrically to correct for influences of stellar activity if necessary.
The obtained measurements of the three objects all favor featureless spectra. A variety
of atmospheric compositions can explain the lack of a wavelength dependent absorption.
But the broad trend of featureless spectra in planets of a wide range of temperatures,
found in this work and in similar studies recently published in the literature, favors an
explanation based on the presence of condensates even at very low concentrations in the
atmospheres of these close-in gas giants. This result points towards the general conclusion
that the capability of transmission spectroscopy to determine the atmospheric composition
is limited, at least for measurements at low spectral resolution.
In addition, we refined the transit parameters and ephemerides of HAT-P-12b and HATP-
19b. Our monitoring campaigns allowed for the detection of the stellar rotation period
of HAT-P-19 and a refined age estimate. For HAT-P-12 and HAT-P-32, we derived upper
limits on their potential variability. The calculated upper limits of systematic effects of
starspots on the derived transmission spectra were found to be negligible for all three
targets.
Finally, we discussed the observational challenges in the characterization of exoplanet
atmospheres, the importance of correlated noise in the measurements and formulated
suggestions on how to improve on the robustness of results in future work.
Robotic telescopes & Doppler imaging : measuring differential rotation on long-period active stars
(2004)
The sun shows a wide variety of magnetic-activity related phenomena. The magnetic field responsible for this is generated by a dynamo process which is believed to operate in the tachocline, which is located at the bottom of the convection zone. This dynamo is driven in part by differential rotation and in part by magnetic turbulences in the convection zone. The surface differential rotation, one key ingredient of dynamo theory, can be measured by tracing sunspot positions.To extend the parameter space for dynamo theories, one can extend these measurements to other stars than the sun. The primary obstacle in this endeavor is the lack of resolved surface images on other stars. This can be overcome by the Doppler imaging technique, which uses the rotation-induced Doppler-broadening of spectral lines to compute the surface distribution of a physical parameter like temperature. To obtain the surface image of a star, high-resolution spectroscopic observations, evenly distributed over one stellar rotation period are needed. This turns out to be quite complicated for long period stars. The upcoming robotic observatory STELLA addresses this problem with a dedicated scheduling routine, which is tailored for Doppler imaging targets. This will make observations for Doppler imaging not only easier, but also more efficient.As a preview of what can be done with STELLA, we present results of a Doppler imaging study of seven stars, all of which show evidence for differential rotation, but unfortunately the errors are of the same order of magnitude as the measurements due to unsatisfactory data quality, something that will not happen on STELLA. Both, cross-correlation analysis and the sheared image technique where used to double check the results if possible. For four of these stars, weak anti-solar differential rotation was found in a sense that the pole rotates faster than the equator, for the other three stars weak differential rotation in the same direction as on the sun was found.Finally, these new measurements along with other published measurements of differential rotation using Doppler imaging, were analyzed for correlations with stellar evolution, binarity, and rotation period. The total sample of stars show a significant correlation with rotation period, but if separated into antisolar and solar type behavior, only the subsample showing anti-solar differential rotation shows this correlation. Additionally, there is evidence for binary stars showing less differential rotation as single stars, as is suggested by theory. All other parameter combinations fail to deliver any results due to the still small sample of stars available.