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Thesis (Ph.D.) - University of St Andrews, May 2008.

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (p. 101-105).

Exoplanets are now known to be ubiquitous throughout the Galaxy. From the Kepler survey, we expect nearly every main-sequence star to form planetary systems during its formation phase. However, the detection limits of Kepler are confined to planets with short orbital periods, comparable to those in the inner solar system. Thanks to the long observational time baseline of the McDonald Observatory Radial Velocity (RV) Survey, we can identify gas giant planets in the outer regions of extrasolar planetary systems. The statistics of such planets are not well known, and are important for understanding the physics behind planet formation and migration. In this dissertation, I detail the discovery of five giant exoplanets on long-period orbits–so-called “Jupiter analogs.” For two systems of giant planets discovered through our survey, pairs of planets follow closely-packed orbits, creating the possibility for dynamical instability. I therefore examine the orbital resonances that allow these planets to avoid gravitational disruption. Because we see an abundance of small, potentially habitable exoplanets in the Kepler data set, current and upcoming exoplanet surveys concentrate on finding Earth-mass planets orbiting stars near enough to facilitate detailed follow-up observations. Particularly attractive targets are cool, low-mass “M dwarf” stars. Their low masses (and thus higher RV amplitudes from exoplanets) and close-in habitable zones allow for relatively quick detection of low-mass planets in the habitable zone. However, the RV signals of such planets will be obscured by stellar magnetic activity, which is poorly understood for M stars. In an effort to improve the planet detection capabilities of our M dwarf planet survey, I have conducted a detailed investigation of the magnetic behavior of our target stars. I show that, while stellar activity does not appear to systematically influence RV measurements above a precision level of ∼ 5 m/s, activity cycles can occasionally produce RV signals in excess of 10 m/s. Additionally, I show that long-term, solar-type stellar activity cycles are common amongst our M dwarf targets, although they are significantly less frequent than for FGK stars. In the case of GJ 328, I have discovered a magnetic activity cycle that appears in the RV data, causing the giant planet around the star to appear to be on a more circular orbit than indicated by the activity-corrected data. Such corrections are essential for the discovery of Earthlike exoplanets. / text

Wie seit schon einigen Jahrzehnten bekannt ist, werden akustische Oszillationen durch stellare Aktivität beeinflusst. Die globalen akustischen Moden in der Sonne weisen eine Variation mit dem 11-jährigen Sonnenzyklus auf. Ein ähnliches Phänomen konnte auch in anderen Sternen mit Hilfe von Asteroseismologie nachgewiesen werden. In dieser Arbeit erforsche ich den Einfluss von großen Sternflecken auf globale Oszillationen mit niedrigem Grad ℓ. Als wichtigstes Werkzeug benutze ich hierfür den GLASS Kode, der die Ausbreitung von linearen akustischen Wellen im Sterninneren in 3D simuliert. Zunächst habe ich das Problem der konvektiven Stabilisierung betrachtet, welches bei jedem linearen Oszillationskode im Zeitbereich auftritt. Ich präsentiere eine allgemeine Methode um konvektiv stabile Hintergrundsmodelle für ein vorgegebenes Sternmodell zu erzeugen. Dabei werden wichtige Eigenschaften des ursprünglichen Modells beibehalten, beispielsweise das hydrostatische Gleichgewicht. Ich schlage einen störungstheoretischen Ansatz vor, um das akustische Wellenfeld in dem ursprünglichen instabilen Sternmodell näherungsweise zu erlangen. Tests zeigen, dass für Moden mit niedrigem Grad ℓ und einer Frequenz um 3 mHz die korrigierten Frequenzen mit einer Genauigkeit von 1 μHz mit den exakten Werten übereinstimmen. Zweitens habe ich mit Hilfe des GLASS Kodes den Einfluss einer am Nordpol des Sterns lokalisierten Störung der Schallgeschwindigkeit auf radiale, dipolare und quadrupolare Oszillationsmoden untersucht. Diese Studie zeigt auf, dass die axialsymmetrischen Moden dadurch am stärksten beeinflusst werden und im Falle von großen Sternflecken können ihre Frequenzen nicht mit der linearen Theorie berechnet werden. Die Form der Eigenfunktionen der Moden weicht von reinen Kugelflächenfunktionen ab und werden mit Kugelflächenfunktionen mit unterschiedlichem Grad ℓ vermischt. Dies könnte die korrekte Identifikation der Moden in der spektralen Leistungsdichte beeinflussen. Drittens habe ich den beobachtbaren Einfluss eines großen Sternflecks auf Moden mit Grad ℓ betrachtet. Im Falle einer aktiven Region, die mit dem Stern rotiert (und sich nicht am Pol befindet), ist die Störung nicht stabil, wenn sie in einem Inertialsystem betrachtet wird. Der kombinierte Einfluss von Rotation und Sternfleck veranlasst jede Mode, in der beobachteten spektralen Leistungsdichte als (2ℓ + 1)² Peaks aufzutreten. Die Einhüllende der spektralen Leistungsdichte eines Multipletts ist also komplex und hängt von dem Breitengrad ab, wo sich die aktive Region befindet, und vom Inklinationswinkel des Sterns. Ich berechne die spektrale Leistungsdichte für einige Beispiele sowohl mit Störungstheorie als auch mit Hilfe von GLASS. Diese Arbeit soll dazu beitragen, die spektrale Leistungsdichte von oszillierenden Sternen, die Sternflecken aufweisen, zu interpretieren.

In this dissertation, we conduct a census and assessment of the nearest young Sun-like stars and investigate the potential for finding giant planets orbiting spotted stars using the radial velocity (RV) method at optical and near-infrared wavelengths. Based in part on new spectroscopic measurements conducted here, we have assembled a complete list of 129 young (<150 >Myr), nearby Sun-like stars and their fundamental parameters, including rotational and multiplicity information. We also provide a statistical analysis of their stellar parameters, including projected rotational velocity and inclination. Sixteen of these stars have no close companions and have low projected rotational velocities (vsini/s) that are ideal for precision RV planet searches. Seven of these rotate nearly edge-on and are ideal targets for upcoming transiting planet searches, assuming low obliquity. We conduct precision RV planet search of 7 young Sun-like stars using the TRES spectrograph, mounted on the 1.5-m Tillinghast Reflector at the Fred L. Whipple Observatory, and with the SOPHIE spectrograph, mounted on the 1.93-m Telescope at the Observatoire de Haute Provence; we achieve a precision of 10 m/s for both. Four stars are identified as having larger RV variations that are periodic, possibly caused by an orbiting companion. However, the RV variations are correlated with asymmetries in the spectral absorption features, which instead suggests that the variations are caused by spots. Nevertheless our observations provide new independent measures of the rotation periods of these stars. Through this analysis we tentatively confirm the planetary companion around BD+20 1790 in the presence of activity. We additionally investigate the use of comparing red orders of the optical spectrum to the blue orders in order to distinguish spots from planets; we find that this method can be effective for observations that span the full wavelength range of the optical. We also investigate our detection limits at optical wavelengths and find that we are sensitive to over 90% of short period giant planets. Next, we assemble the stellar jitter measurements of our stars with previous studies of all Sun-like stars younger than 1 Gyr to investigate how stellar jitter declines with stellar age. We find that stellar jitter decreases with stellar age as t^(0.53±0.13), similar to the relationship between stellar rotation period and stellar age. The implication is that it will be diffcult to find planets orbiting stars younger than 100 Myr without using techniques that mitigate star spot noise. Furthermore, we present a near-infrared RV search for giant planets orbiting 8 stars observed with CSHELL at the NASA Infrared Telescope Facility (IRTF). Because of the limited wavelength coverage (29 ̊A) and older (1980s) detector technology, the achieved precision of 200 m/s inhibits finding the majority of exoplanets, but is nevertheless sufficient to identify short-period brown dwarfs for these stars. We also analyze our detection limits at IR wavelengths and find that we are only sensitive to roughly 50% of short period giant planets. Finally, we present a new orbital solution for V835 Her, a spectroscopic binary with a 3 day orbital period.

Abstract not available

The discovery of exoplanets represents one of the greatest scientific revolutions in history, and exoplanetary science has rapidly become uniquely positioned to address profound questions about the origins of life, and about humanity's place (and future) in the cosmos. Since the discovery of the first exoplanet over two decades ago, the radial velocity (RV) method has been one of the most productive techniques for discovering new planets. It has also become indispensable for characterising exoplanets detected via other techniques, notably transit photometry. Unfortunately, signals intrinsic to stars themselves - especially magnetic activity signals - can induce RV variations that can drown out or even mimic planetary signals. Modelling and thus mitigating these signals is notoriously difficult, which represents a major obstacle to using next-generation instruments to detect lower mass planets, planets with longer periods, and planets around more magnetically-active stars. Enter Gaussian processes (GPs), which have a number of features that make them very well suited to the joint modelling of stochastic activity processes and dynamical (e.g. planetary) signals. In this thesis, I leverage GPs to enable the study of smaller planets around a wider variety of stars than has previously been possible. In particular, I develop a principled and sophisticated Bayesian framework, based on GPs, for modelling RV time series jointly with ancillary activity-sensitive proxies, thus allowing activity signals to be constrained and disentangled from genuine planetary signals. I show that my framework succeeds even in cases where existing techniques would fail to detect planets, e.g. the case of a weak planetary signal with period identical to its host star's rotation period. In a first application of the framework, I demonstrate that Alpha Centauri Bb - until 2016, thought to be the closest exoplanet to Earth, and also the lowest minimum-mass exoplanet around a Sun-like star - was, in fact, an astrophysical false positive. Next, I use the framework to re-characterise the well-studied Kepler-10 system, thereby resolving a mystery surrounding the mass of planet Kepler-10c. I also use the framework to help discover or characterise various exoplanets. Finally, the activity modelling framework aside, I also present in outline form a few promising applications of GPs in the context of modelling stellar signals and studying exoplanets, viz. GPs for (i) enhanced characterisation of stellar rotation; (ii) generating realistic synthetic observations, and modelling in a systematic way the effects of an observing window function; and (iii) ultra-precise extraction of RV shifts directly from observed spectra, without requiring template cross-correlation.

The study of magnetic fields on low-mass stars is important due to their ubiquity. They are responsible for phenomena spanning a wide range of spatial and temporal scales. Over the last two decades, the Zeeman-Doppler imaging (ZDI) technique has been used to study the topologies of stellar magnetic fields. A great deal has been learnt about how the magnetic characteristics of cool dwarfs vary as a function of parameters such as mass, rotation or age. In this thesis, I assemble a sample of stars with Zeeman-Doppler maps. I study their poloidal and toroidal components as a function of fundamental parameters and also in relation to activity cycles. I find that the relationship between poloidal and toroidal fields is different for stars above and below the fully convective boundary, in line with previous ZDI studies. I also find that the fields of strongly toroidal stars must be generated axisymmetrically. With regards to activity cycles, I find that so called “inactive branch" stars appear to remain poloidal throughout their activity cycle while so called “active branch" stars appear to be able to generate strong toroidal fields. Magnetic activity can also interact with exoplanets that may be orbiting a star. In this thesis, I consider two such interactions. The first is the compression of planetary magnetospheres by stellar winds. Sufficiently powerful winds can strip a planet of its atmosphere and render it uninhabitable. However magnetospheric shielding can provide some protection. I show that planets around 0.6 M⊙ - 0.8 M⊙ stars are the most likely to be able to protect their atmospheres. The second interaction I consider is exoplanetary radio emission. I present a wind model and show that exoplanetary radio emissions will depend strongly on the structure of the magnetic field structure of the central star.

In this thesis I study the magnetic activity of young stars via observations of stellar prominences on single stars and by applying the Zeeman Doppler imaging (ZDI) technique to map the magnetic fields and measure differential rotation of a young binary system. Stellar prominences can be observed as absorption transients in the rotationally broadened chromospheric lines of rapidly rotating stars. Observations of Speedy Mic(K3V) reveal a densely packed prominence system at heights far above the stellar co-rotation radius. Further observations were used to estimate prominence column densities and masses. From very high signal-to-noise observations, loops of emission are found that trace the path of prominences seen transiting the stellar disc. I also present what appears to be the first observation of an erupting stellar prominence on AB Doradus (K0V). I modify an existing ZDI code so that it can recover the magnetic maps of a binary system. The new code is applied to observations of the pre-main sequence binary system HD 155555 (G5IV+K0IV). The radial magnetic maps reveal a complex surface magnetic topology with mixed polarities at all latitudes and rings of azimuthal field present on both stars. The evolution of the relative field strengths between observations in 2004 and 2007 could be indicative of a magnetic activity cycle. I adapt the sheared image technique for measuring differential rotation parameters to the binary case. Both stellar components of HD 155555 are found to have rates of differential rotation similar to those of the same spectral type main sequence single stars. This is in apparent conflict with previous work on evolved binary systems where low rates of differential rotation were found, leading to the suggestion of suppression by binary tidal forces. I find that the depth of convection zone alone can likely explain the differential rotation results without invoking tidal forces.