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2012 Annual Science Report

Carnegie Institution of Washington Reporting  |  SEP 2011 – AUG 2012

Project 1: Looking Outward: Studies of the Physical and Chemical Evolution of Planetary Systems

Project Summary

This project integrates the work of Carnegie Institution Astronomers in the 1) the search for extrasolar planets, 2) understanding the flow of matter in protoplanetary disks around young stars, 3) understanding the origin of Near Earth Objects, in particular, their relationship with objects in the asteroid belt, and 4) understanding the composition of disks around young stars and the potential delivery of volatiles to terrestrial planets in other solar systems.

4 Institutions
3 Teams
68 Publications
2 Field Sites
Field Sites

Project Progress

Project 1. Looking Outward: Studies of the physical and chemical evolution of planetary systems

1.1 Radial Velocity Searches for Planetary Systems

Co-I Butler continued his work on radial velocity searches for exoplanets in the Lick-Carnegie survey, Anglo-Australian Planet Search, and Magellan Planet Finding survey. He expects to soon bring on line the dedicated 2.4 m Automated Planet Finder telescope. The goal is to find the smallest mass planets, and potentially habitable mass planets, around the nearest stars.

NAI-supported postdoctoral researcher Guillem Anglada-Escudé developed a new algorithm for obtaining precision radial velocities from the HARPS instrument operated by the European Southern Observatory. This method, named “Template-Enhanced Radial velocity Re-analysis Application” or TERRA provides significant improvement in low amplitude signal detection, especially around M-type stars (Anglada-Escudé and Butler 2012).

1.2 Astrometric Search for Planetary Systems Hospitable for Life

Co-I Boss leads the Carnegie Astrometric Planet Search (CAPS) project at LCO, while
Co-I Weinberger and NAI supported postdoctoral researcher Anglada-Escudé are key members of the CAPS team. The CAPS team is undertaking an astrometric search for gas giant planets and brown dwarfs orbiting nearby low mass dwarf stars with the LCO du Pont telescope. We are following 100 nearby (primarily within about 15 pc) low mass stars, principally late M, L, and T dwarfs, for 10 years or more, in order to detect very low mass companions with orbital periods long enough to permit the existence of habitable, Earth-like planets on shorter-period orbits. These stars are generally too faint and red to be included in ground-based Doppler planet surveys, which are often optimized for FGK dwarfs. The smaller masses of late M dwarfs also yield correspondingly larger astrometric signals for a given mass planet. The CAPS search will also help to determine whether gas giant planets form primarily by core accretion or by disk instability around late M dwarf stars, and help with the census of nearby stars.

Our latest paper (Anglada-Escudé et al. 2012) places an astrometric upper mass limit on the known Doppler exoplanet GJ 317b (see Figure 1.2). GJ 317 is a relatively bright M3.5 M dwarf, necessitating the use of the CAPSCam Guide Window (GW) mode with 0.2 sec exposures. With 18 epochs, the overall accuracy in fitting the parallax of GJ 317 is about 0.15 milliarcsec, and the accuracy should improve for fainter targets. With a similar number of epochs, this accuracy is sufficient to detect a Jupiter-mass companion orbiting 1 AU from a late M dwarf 20 pc away with a signal-to-noise ratio of about 4.

CAPSCam has been operational for 5.5 years, during which about 800 epochs of observations have been taken on the 100 targets on the currently active list. Another 200 epochs have been taken on targets that have since been dropped from the planet search list, but for which kinematic parallaxes have been determined and will be published. The CAPSCam search is just now entering the most exciting phase of this decade-old effort, with a number of targets showing evidence of wobbles that might be caused by unknown companions.

1.3 From Disks to Planets

Co-I Boss is partially supported by the NASA Origins of Solar Systems Program to work on mixing and transport in marginally gravitationally unstable disks. This work is being continued and extended to include an analysis of the time history of a population of individual dust grains, as they traverse high and low temperature regions of the disk. This will allow a determination of the extent to which water is transported as a solid by the dust grains, through a collaborative effort with Prof. Morris Podolak of Tel Aviv University, who has developed a model for silicate dust grains with water ice mantles. By following the extent to which water ice condenses or sublimates on a population of grains being transported around the disk, a better understanding of the distribution of water in the solar nebula will be achieved, with important implications for the delivery of water to the terrestrial planets.

Along with Co-I Conel Alexander, Boss and Podolak published a first paper in EPSL in 2012. The protosun is likely to have experienced multiple FU Orionis outbursts caused by rapid mass accretion from a gravitationally unstable solar nebula. 3D hydrodynamical models of the trajectories of particles in the solar nebula during such a phase show that cm-sized particles can traverse distances of 10 AU or more, both inward and outward, in the midplane of the nebula, in less than 1000 yrs. The particles are subjected to extreme environmental variations during these journeys, such as temperatures ranging from 60 K to 1400 K, pressures from ~10-9 to ~10-7 bar, and oxygen isotope ratios ranging from planetary to solar composition. The models predict that many calcium, aluminum-rich inclusions (CAIs) could have survived FU Orionis phases and been transported across the nebula, from near the protosun, out to the comet-forming regions, and back again, multiple times, acquiring along the way rims with mineral phases indicative of their thermal histories, and oxygen isotope variations derived from the local dispersion of these isotopes.

1.4 Mixing and Migration in the Solar System

The physical and chemical properties of asteroids provide clues about environmental conditions in the solar nebula during their formation over 4 billion years ago, thus they represent the best available analogs to the material from which the terrestrial planets formed. Multi-wavelength observations provide the optimal means for physical and chemical characterization. However, such comprehensive observations are typically limited to only the biggest (10’s to 100’s of km in size) and brightest targets. This has resulted in a gap in our understanding of any differences or similarities (e.g. in surface properties or composition) between the biggest and smallest (<1 km in size) asteroids, of links between large bodies in the Main Belt and meteorites here on Earth whose direct parent bodies are sub-km asteroids in near-Earth space, and of a population of objects that are directly relevant to impact hazard assessment and future spacecraft and human exploration.

Recent efforts by NAI-supported postdoctoral researcher Nick Moskovitz and collaborators have focused on obtaining multi-wavelength observations of near-Earth asteroids during very close approaches to the Earth. Though such observations are challenging due to the rapid motion of the asteroids and the specific timing requirements, the proximity of the targets, and thus their apparent brightness during these events enables observations that would ordinarily be impossible. For example, in November of 2011 a successful multi-observatory campaign was conducted during the near-Earth flyby of asteroid 2005 YU55. This ~350-meter C-type asteroid is an excellent analog for the future targets of NASA’s OSIRIS-Rex, JAXA’s Hayabusa II, and ESA’s Marco Polo missions. Based on these observations it was found that this is a low albedo asteroid with a composition analogous to either carbonaceous chondrite or enstatite chondrite meteorites. A lack of any observable signatures due to organics or hydrated minerals suggests that 2005 YU55 has experienced significant surface alteration during its lifetime, most likely due to the intense radiation environment of the inner Solar System.

1.5 Compositions of circumstellar disks and delivery of volatiles to terrestrial planets

Nearby young stars of age 5 ¬–10 Myr provide our best opportunity to study the late stages of star and planet formation with high sensitivity and spatial resolution. During this time period, the last gas-rich disks dissipate, and the onset of the debris disk phase occurs. To better understand the timescales for delivery of volatiles, Co-I Weinberger has been studying the ages of nearby stars with disks and searching for new young stars that might harbor disks.

The star TW Hydrae sports a massive disk. Because it is the nearest example of a protoplanetary disk, Weinberger has used HST data to study its structure and composition. But to understand its disk in context, TW Hya’s age must be well determined and its disk evolution compared to other stars of similar age and mass. Over the last decade, ~30 members of a putative TW Hydrae Association (TWA) have been identified. Disks in TWA range from the four accreting, gas rich, protoplanetary ones (TWA 1, 3, 27, and 30), to seven transitional or debris disks (TWA 4B, 7, 11A, 26, 28, 31 and 32), to the majority of members that have no detectable disks at all.

The identification of TWA members has largely been based on youth plus similarity to the young star TW Hya in terms of location on the sky, proper motion, and radial velocity. Co-evality was thought to follow under the assumption that these stars with similar motions formed from the same raw material at the same time. Weinberger and collaborators determined parallaxes to fourteen primary stars identified as TWA members, to greatly expanded the knowledge of the kinematics of these young stars. We find that although they do share a common space motion, the stars do not appear to have formed in a concentrated volume with a well-defined expansion velocity. The TWA stars appear to have formed over a larger volume than they presently occupy. The spatial distribution of these nominal TWA stars is filamentary. They also have ages that range from 3–25 Myr as derived from their locations on pre-main sequence tracks. This apparent age spread could be due to a real difference in the times of formation of the stars or it could be due to the lasting effects of episodic accretion (Weinberger et al. 2012, submitted).

Weinberger also collaborated with NAI supported postdoctoral researchers Evgenya Shkolnik and Guillem Anglada-Escudé to identify new low-mass members of nearby young associations (Shkolnik et al. 2012). These will be excellent targets for disk and planet searches.

Radial velocity measurements obtained over 2.5 years by Co-I Butler with the Planet Finder Spectrograph at Carnegie’s Magellan (Clay) Telescope. The top figure shows a star that demonstrates long-term velocity stability at the level of 83 cm/s. The bottom figure shows a star with low amplitude variations that may eventually reveal a planet.

CAPSCam has placed its first constraint on the mass of a gas giant exoplanet, GJ 317b, orbiting outside the habitable zone (blue region) of its 0.4 solar mass host star (Anglada-Escudé et al. 2012).

Time series of near-infrared spectra of asteroid 2005 YU55 taken with the TripleSpec instrument at the Palomar 200-in Hale Telescope on 9 November 2011. The spectra have been normalized and offset for clarity. The grey bars represent regions of low atmospheric transmission where telluric correction is poor. These spectra show a largely featureless, red-sloped reflectance with significant thermal emission longward of ~2.1 μm. These data provide constraints on the composition and thermo-physical properties (e.g. surface temperature and albedo) of the asteroid. At its closest approach, 2005 YU55 passed within 90% of the Earth-Moon separation at a distance of 0.0022 AU.

(Left) Co-I Weinberger determined parallaxes to 14 primary stars in the TW Hydrae Association (TWA), and this plot shows their absolute magnitudes plotted on theoretical pre-main sequence tracks from Baraffe et al. (1998) and Chabrier et al. (2000). Also included are TWA 1 and 9 which had previously determined parallaxes. The stars appear to have a range of ages. (Right) Age histogram of the TWA member stars with parallaxes. The bin size is 3 Myr, the median uncertainty in the individual ages based on the uncertainties in their absolute magnitudes (i.e., parallaxes). The best fit Gaussian to the age distribution is overplotted with the dashed line and has a mean of 9 Myr and standard deviation of 6.8 Myr. The width of the age distribution exceeds what would be expected for a population of stars formed in a single burst. It may not be correct to treat the stars in TWA as co-eval for the purposes of understanding their disk evolution.