Dust in circumstellar disks

This thesis presents observational and theoretical studies of the size and spatial distribution of dust particles in circumstellar disks. Using millimetre interferometric observations of optically thick disks around T Tauri stars, I provide conclusive evidence for the presence of millimetre- to centimetre-sized dust aggregates. These findings demonstrate that dust grain growth to pebble-sized dust particles is completed within less than 1 Myr in the outer disks around low-mass pre-main-sequence stars. The modelling of the infrared spectral energy distributions of several solar-type main-sequence stars and their associated circumstellar debris disks reveals the ubiquity of inner gaps devoid of substantial amounts of dust among Vega-type infrared excess sources. It is argued that the absence of circumstellar material in the inner disks is most likely the result of the gravitational influence of a large planet and/or a lack of dust-producing minor bodies in the dust-free region. Finally, I describe a numerical model to simulate the dynamical evolution of dust particles in debris disks, taking into account the gravitational perturbations by planets, photon radiation pressure, and dissipative drag forces due to the Poynting-Robertson effect and stellar wind. The validity of the code it established by several tests and comparison to semi-analytic approximations. The debris disk model is applied to simulate the main structural features of a ring of circumstellar material around the main-sequence star HD 181327. The best agreement between model and observation is achieved for dust grains a few tens of microns in size locked in the 1:1 resonance with a Jupiter-mass planet (or above) on a circular orbit

A Giant Planet Around a Metal-poor Star of Extragalactic Origin

Stars in their late stage of evolution, such as Horizontal Branch stars, are still largely unexplored for planets. We report the detection of a planetary companion around HIP 13044, a very metal-poor star on the red Horizontal Branch, based on radial velocity observations with a high-resolution spectrograph at the 2.2-m MPG/ESO telescope. The star's periodic radial velocity variation of P=16.2 days caused by the planet can be distinguished from the periods of the stellar activity indicators. The minimum mass of the planet is 1.25 Jupiter masses and its orbital semi-major axis 0.116 AU. Because HIP 13044 belongs to a group of stars that have been accreted from a disrupted satellite galaxy of the Milky Way, the planet most likely has an extragalactic origin.Comment: 32 pages, 9 Figure

The visitor from an ancient galaxy: A planetary companion around an old, metal-poor red horizontal branch star

We report the detection of a planetary companion around HIP 13044, a metal-poor red horizontal branch star belonging to a stellar halo stream that results from the disruption of an ancient Milky Way satellite galaxy. The detection is based on radial velocity observations with FEROS at the 2.2-m MPG/ESO telescope. The periodic radial velocity variation of P=16.2 days can be distinguished from the periods of the stellar activity indicators. We computed a minimum planetary mass of 1.25 Jupiter masses and an orbital semimajor axis of 0.116 AU for the planet. This discovery is unique in three aspects: First, it is the first planet detection around a star with a metallicity much lower than few percent of the solar value; second, the planet host star resides in a stellar evolutionary stage that is still unexplored in the exoplanet surveys; third, the planetary system HIP 13044 most likely has an extragalactic origin in a disrupted former satellite of the Milky Way.Comment: 5 pages, 2 figures, 2 tables, submitted to the Proceedings of the 276th IAU Symposium "The Astrophysics of Planetary Systems

Precise short- and long-term propagation of orbits and uncertainties using the Hermite integration scheme for LEO space debris objects

The Institute of Technical Physics of the German Aerospace Center (DLR) is actively developing laser-based optical tracking methods to determine 3D positions of LEO space debris objects to within 10 metres to support several SSA-related use cases, e.g. conjunction assessment, maneuver detection, or analysis of re-entry events. For the purpose of precise orbit determination and the subsequent propagation of states and their uncertainties, an accurate, fast, and versatile integration method is needed. We will present a numerical integration technique based on the Hermite scheme, a self-starting, implicite predictor-corrector method originally developed and widely used for gravitational N-body systems. The Hermite scheme uses the acceleration and its first time derivative to predict future position and velocity vectors from previous values. The gist of the method relies on the fact that the second and third derivatives can be explicitly calculated from the acceleration and its first derivative alone. These are then used iteratively to correct the object's state vector, yielding an integration method with fourth-order global error. The method can be used with constant or variable timesteps. In the case of constant timesteps, the Hermite integrator is time-symmetric and shows no secular error in the semi-major axis and eccentricity, which makes the integrator extremely useful for long-term dynamical simulations. The code can integrate an ensemble of objects in parallel, with either shared or individual timesteps. It can therefore be applied to propagate a catalogue of space objects and/or to perform realistic uncertainty propagation via Monte-Carlo or sigma-point methods. We will show current results from our orbit propagator development which is geared towards LEO space debris objects and discuss the implementation of the force model

A Planetary Companion around a Metal-Poor Star with Extragalactic Origin

We report the detection of a planetary companion around HIP 13044, a metal-poor star on the red Horizontal Branch. The detection is based on radial velocity observations with FEROS, a high-resolution spectrograph at the 2.2-m MPG/ESO telescope, located at ESO La Silla observatory in Chile. The periodic radial velocity variation of P = 16.2 days can be distinguished from the periods of the stellar activity indicators. We computed a minimum planetary mass of 1.25 MJup and an orbital semi-major axis of 0.116 AU for the planet. This discovery is unique in three aspects: First, it is the first planet detection around a star with a metallicity much lower than few percent of the solar value; second, the planet host star resides in a stellar evolutionary stage that is still unexplored in the exoplanet surveys; third, the star HIP 13044 belongs to one of the most significant stellar halo streams in the solar neighborhood, implying an extragalactic origin of the planetary system HIP 13044 in a disrupted former satellite of the Milky Way.Comment: Part of PlanetsbeyondMS/2010 proceedings http://arxiv.org/html/1011.660

Future improvements in conjunction assessment and collision avoidance using a combined laser tracking/nudging network

The expected significant increase of space launch activities in the next years, both from spacefaring nations and in the private sector, yields an enhanced risk of space debris generation. In this regard, space situational awareness (SSA) is mandatory not only for the protection of active space missions, but as a prerequisite to prevent aggravation of the space debris environment by cascading effects of secondary debris generation due to in-space collisions. High accuracy in laser ranging to space objects (within a meter or better) has already been demonstrated, e.g., by the International Laser Ranging Service (ILRS) network. Thus, laser ranging can be considered as a highly promising sensor technology for space surveillance in the Low Earth Orbit (LEO) which has the potential to complement existing radar facilities in terms of achievable state vector accuracy. Furthermore several laser-based concepts on orbit modification have been proposed in the recent years. In particular, momentum transfer to space debris via photon pressure appears to become feasible, due to advancements in adaptive optics and the commercial availability of high power lasers with an average power output beyond the 10 kW level. This allows for the setup of a network of comparably cost-efficient laser stations for momentum transfer in the near future paving the way for the capability to remotely operate space debris in particular in terms of debris vs. debris collision avoidance maneuvers. In the scope of the conceptual study LARAMOTIONS (SSA P3-SST-XV) funded by the European Space Agency (ESA) simulations of a ground-based laser tracking and momentum transfer network have been carried out in order to estimate the subsequent improvements in conjunction assessment and collision avoidance for operational satellites as well as for debris vs. debris encounters. Therefore, the software generates reference trajectories from a Two-Line-Element (TLE) catalogue for any number of target objects in LEO. From these trajectories station passes as well as random measurement samples are computed and the orbit determination process is simulated yielding the collision rate and false alert rate of the given network. Special emphasis is taken on considering station downtimes due to weather conditions by introducing a station-specific duty cycle based on the analysis of historical weather data. Afterwards a momentum transfer network can be simulated. In order to achieve this, forces induced by photon pressure are computed from tabulated data of target-specific Laser-Matter-Interaction simulations and are applied to the object’s trajectories. A second laser tracking simulation based on the modified orbits eventually shows the advantages of the given system in terms of conjunction analysis and avoidance, in particular considering debris vs. debris collisions for which at present collision avoidance maneuvers are not yet available. The paper will outline the software architecture as well as the results for different network geometries considering the number of stations, their geographical distribution and different duty cycle values. Among the results, the effects of the network geometry and station distribution on the achievable orbit accuracy will be presented. Two operational scenarios will be compared: On-demand tracking in response to conjunction alerts and a laser catalog scenario yielding the maximum number of objects, which can continuously be tracked by a given network independently from radar-based SSA data. Finally, an outlook will be given regarding future simulations and possible enhancements of the simulation environment

Discovery of an 86 AU Radius Debris Ring Around HD 181327

HST/NICMOS PSF-subtracted coronagraphic observations of HD 181327 have revealed the presence of a ring-like disk of circumstellar debris seen in 1.1 micron light scattered by the disk grains, surrounded by a di use outer region of lower surface brightness. The annular disk appears to be inclined by 31.7 +/- 1.6 deg from face on with the disk major axis PA at 107 +/-2 deg . The total 1.1 micron flux density of the light scattered by the disk (at 1.2" < r < 5.0") of 9.6 mJy +/- 0.8 mJy is 0.17% +/- 0.015% of the starlight. Seventy percent of the light from the scattering grains appears to be confined in a 36 AU wide annulus centered on the peak of the radial surface brightness (SB) profile 86.3 +/- 3.9 AU from the star, well beyond the characteristic radius of thermal emission estimated from IRAS and Spitzer flux densities assuming blackbody grains (~ 22 AU). The light scattered by the ring appears bilaterally symmetric, exhibits directionally preferential scattering well represented by a Henyey-Greenstein scattering phase function with g = 0.30 +/- 0.03, and has an azimuthally medianed SB at the 86.3 AU radius of peak SB of 1.00 +/- 0.07 mJy arcsec^-2. No photocentric offset is seen in the ring relative to the position of the central star. A low surface brightness diffuse halo is seen in the NICMOS image to a distance of ~ 4" Deeper 0.6 micron HST/ACS PSF-subtracted coronagraphic observations reveal a faint outer nebulosity, asymmetrically brighter to the North of the star. We discuss models of the disk and properties of its grains, from which we infer a maximum vertical scale height of 4 - 8 AU at the 87.6 AU radius of maximum surface density, and a total maximum dust mass of collisionally replenished grains with minimum grain sizes of ~ 1 micron of ~ 4 M(moon).Comment: 45 pages, 15 figures, accepted for publication in Ap

Formation and Evolution of Planetary Systems: Cold Outer Disks Associated with Sun-like stars

We present the discovery of debris systems around three solar mass stars based upon observations performed with the Spitzer Space Telescope as part of a Legacy Science Program, ``the Formation and Evolution of Planetary Systems'' (FEPS). We also confirm the presence of debris around two other stars. All the stars exhibit infrared emission in excess of the expected photospheres in the 70 micron band, but are consistent with photospheric emission at <= 33 micron. This restricts the maximum temperature of debris in equilibrium with the stellar radiation to T < 70 K. We find that these sources are relatively old in the FEPS sample, in the age range 0.7 - 3 Gyr. Based on models of the spectral energy distributions, we suggest that these debris systems represent materials generated by collisions of planetesimal belts. We speculate on the nature of these systems through comparisons to our own Kuiper Belt, and on the likely planet(s) responsible for stirring the system and ultimately releasing dust through collisions. We further report observations of a nearby star HD 13974 (d =11 pc) that is indistinguishable from a bare photosphere at both 24 micron and 70 micron. The observations place strong upper limits on the presence of any cold dust in this nearby system (L_IR/L_* < 10^{-5.2}).Comment: 31 pages, 9 figures, accepted for publication in Ap

Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer

We summarize the progress to date of our Legacy Science Program entitled "The Formation and Evolution of Planetary Systems" (FEPS) based on observations obtained with the Spitzer Space Telescope during its first year of operation. In addition to results obtained from our ground-based preparatory program and our early validation program, we describe new results from a survey for near-infrared excess emission from the youngest stars in our sample as well as a search for cold debris disks around sun-like stars. We discuss the implications of our findings with respect to current understanding of the formation and evolution of our own solar system.Comment: 8 postscript pages including 3 figures. To appear in "Spitzer New Views of the Cosmos" ASP Conference Series, eds. L. Armus et al. FEPS website at http://feps.as.arizona.ed