Pupil phase apodization for achromatic imaging of extra-solar planets

Direct imaging of extra-solar planets in the visible and infrared region has generated great interest among scientists and the general public as well. However, this is a challenging problem. Diffculties of detecting a planet (faint source) are caused, mostly, by two factors: sidelobes caused by starlight diffraction from the edge of the pupil and the randomly scattered starlight caused by the phase errors from the imperfections in the optical system. While the latter diffculty can be corrected by high density active deformable mirrors with advanced phase sensing and control technology, the optimized strategy for suppressing the diffraction sidelobes is still an open question. In this thesis, I present a new approach to the sidelobe reduction problem: pupil phase apodization. It is based on a discovery that an anti-symmetric spatial phase modulation pattern imposed over a pupil or a relay plane causes diffracted starlight suppression sufficient for imaging of extra-solar planets. Numerical simulations with specific square pupil (side D) phase functions, such as ... demonstrate annulling in at least one quadrant of the diffraction plane to the contrast level of better than 10^12 with an inner working angle down to 3.5L/D (with a = 3 and e = 10^3). Furthermore, our computer experiments show that phase apodization remains effective throughout a broad spectrum (60% of the central wavelength) covering the entire visible light range. In addition to the specific phase functions that can yield deep sidelobe reduction on one quadrant, we also found that a modified Gerchberg-Saxton algorithm can help to find small sized (101 x 101 element) discrete phase functions if regional sidelobe reduction is desired. Our simulation shows that a 101x101 segmented but gapless active mirror can also generate a dark region with Inner Working Distance about 2.8L/D in one quadrant. Phase-only modulation has the additional appeal of potential implementation via active segmented or deformable mirrors, thereby combining compensation of random phase aberrations and diffraction halo removal in a single optical element.

The HARPS search for southern extra-solar planets

The meter-per-second precision achieved by today’s velocimeters enables us to search for 1−10 M⊕ planets in the habitable zone of cool stars. This paper reports on the detection of three planets orbiting GJ 163 (HIP 19394), a M3 dwarf monitored by our ESO/HARPS search for planets. We made use of the HARPS spectrograph to collect 150 radial velocities of GJ 163 over a period of eight years. We searched the radial-velocity time series for coherent signals and found five distinct periodic variabilities. We investigated the stellar activity and called into question the planetary interpretation for two signals. Before more data can be acquired we concluded that at least three planets are orbiting GJ 163. They have orbital periods of Pb = 8.632 ± 0.002, Pc = 25.63 ± 0.03, and Pd = 604 ± 8 days and minimum masses msini = 10.6 ± 0.6, 6.8 ± 0.9, and 29 ± 3 M⊕, respectively. We hold our interpretations for the two additional signals with periods P(e) = 19.4 and P(f) = 108 days. The inner pair presents an orbital period ratio of 2.97, but a dynamical analysis of the system shows that it lays outside the 3:1 mean motion resonance. The planet GJ 163c, in particular, is a super-Earth with an equilibrium temperature of Teq = (302 ± 10)(1 − A)1/4 K and may lie in the so-called habitable zone for albedo values (A = 0.34 − 0.89) moderately higher than that of Earth (A⊕ = 0.2−0.3).

The HARPS search for southern extra-solar planets

The vast diversity of planetary systems detected to date is defying our capability of understanding their formation and evolution. Well-defined volume-limited surveys are the best tool at our disposal to tackle the problem, via the acquisition of robust statistics of the orbital elements. We are using the HARPS spectrograph to conduct our survey of ≈850 nearby solar-type stars, and in the course of the past nine years we have monitored the radial velocity of HD 103774, HD 109271, and BD-061339. In this work we present the detection of five planets orbiting these stars, with msin   (i) between 0.6 and 7 Neptune masses, four of which are in two multiple systems, comprising one super-Earth and one planet within the habitable zone of a late-type dwarf. Although for strategic reasons we chose efficiency over precision in this survey, we have the capability to detect planets down to the Neptune and super-Earth mass range as well as multiple systems, provided that enough data points are made available.

From stellar nebula to planets: The refractory components

Context. To date, calculations of planet formation have mainly focused on dynamics, and only a few have considered the chemical composition of refractory elements and compounds in the planetary bodies. While many studies have been concentrating on the chemical composition of volatile compounds (such as H2O, CO, CO2) incorporated in planets, only a few have considered the refractory materials as well, although they are of great importance for the formation of rocky planets. Aims. We computed the abundance of refractory elements in planetary bodies formed in stellar systems with a solar chemical composition by combining models of chemical composition and planet formation. We also considered the formation of refractory organic compounds, which have been ignored in previous studies on this topic. Methods. We used the commercial software package HSC Chemistry to compute the condensation sequence and chemical composition of refractory minerals incorporated into planets. The problem of refractory organic material is approached with two distinct model calculations: the first considers that the fraction of atoms used in the formation of organic compounds is removed from the system (i.e., organic compounds are formed in the gas phase and are non-reactive); and the second assumes that organic compounds are formed by the reaction between different compounds that had previously condensed from the gas phase. Results. Results show that refractory material represents more than 50 wt % of the mass of solids accreted by the simulated planets with up to 30 wt % of the total mass composed of refractory organic compounds. Carbide and silicate abundances are consistent with C/O and Mg/Si elemental ratios of 0.5 and 1.02 for the Sun. Less than 1 wt % of carbides are present in the planets, and pyroxene and olivine are formed in similar quantities. The model predicts planets that are similar in composition to those of the solar system. Starting from a common initial nebula composition, it also shows that a wide variety of chemically different planets can form, which means that the differences in planetary compositions are due to differences in the planetary formation process. Conclusions. We show that a model in which refractory organic material is absent from the system is more compatible with observations. The use of a planet formation model is essential to form a wide diversity of planets in a consistent way.

From planetesimals to planets: volatile molecules

Context. Solar and extrasolar planets are the subject of numerous studies aiming to determine their chemical composition and internal structure. In the case of extrasolar planets, the composition is important as it partly governs their potential habitability. Moreover, observational determination of chemical composition of planetary atmospheres are becoming available, especially for transiting planets. Aims. The present works aims at determining the chemical composition of planets formed in stellar systems of solar chemical composition. The main objective of this work is to provide valuable theoretical data for models of planet formation and evolution, and future interpretation of chemical composition of solar and extrasolar planets. Methods. We have developed a model that computes the composition of ices in planets in different stellar systems with the use of models of ice and planetary formation. Results. We provide the chemical composition, ice/rock mass ratio and C:O molar ratio for planets in stellar systems of solar chemical composition. From an initial homogeneous composition of the nebula, we produce a wide variety of planetary chemical compositions as a function of the mass of the disk and distance to the star. The volatile species incorporated in planets are mainly composed of H2O, CO, CO2, CH3OH, and NH3. Icy or ocean planets have systematically higher values of molecular abundances compared to giant and rocky planets. Gas giant planets are depleted in highly volatile molecules such as CH4, CO, and N2 compared to icy or ocean planets. The ice/rock mass ratio in icy or ocean and gas giant planets is, respectively, equal at maximum to 1.01 ± 0.33 and 0.8 ± 0.5, and is different from the usual assumptions made in planet formation models, which suggested this ratio to be 2–3. The C:O molar ratio in the atmosphere of gas giant planets is depleted by at least 30% compared to solar value.