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2010 Annual Science Report

Carnegie Institution of Washington Reporting  |  SEP 2009 – AUG 2010

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

Project Summary

We study the origin of life through a wide variety of approaches, beginning here with theoretical investigations of protoplanetary disks, the environments in which simple organic molecules first appeared and were concentrated in planetary bodies. We also study the survival of this organic matter during subsequent evolution through observations of circumstellar disks around both young and mature stars, extrasolar planetary systems, and small bodies in our Solar System, and through detailed models of planetary system formation.

4 Institutions
3 Teams
108 Publications
3 Field Sites
Field Sites

Project Progress

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

Project Co-Investigators: Alan Boss, R. Paul Butler, Scott Sheppard, Alycia Weinberger

Postdoctoral Fellows: Guillem Anglada-Escude, Nick Moskovitz, Evgenya Shkolnik,

1.1 Radial Velocity Extrasolar Planet Searches

Over the past fifteen years Co-I Butler’s planet search programs have clarified the statistical outlines of the planetary census for giant planets with velocity semi-amplitudes K > 10 m/s and periods P < 10 yr. This work has had a profound impact on astronomy and science as a whole, including helping to motivate the new field of astrobiology. The study of extrasolar planets is cited as one of the three pillars of modern astrophysics in the most recent Astro2010 Decadal Survey.

Butler and his colleagues now endeavor to push the planetary census into the regime involving semi-amplitudes K < 10 m/s and periods P < 1 yr. Recent detections have revealed an unexpected rich new category of exoplanets midway between Earth-mass rocky planets and Neptune-mass ice giants. These so-called super-Earths are potentially rocky planets with water-rich oceans and deep atmospheres. At least 30% of G and K dwarfs host Neptune or lower mass planets orbiting with periods of 50 days or less.

The Astro2010 Decadal Survey sets the discovery of potentially habitable planets orbiting the nearest stars by precision Doppler surveys as the primary goal of extrasolar planet studies for the next decade. The nearest stars are the most important for several reasons. They have the most accurate distances and precisely determined stellar parameters. These are the only stars that can be followed up with astrometry and direct imaging. For M dwarfs, the habitable (liquid water) zone is typically at 0.1 to 0.2 AU, with corresponding 20 to 50 day orbits, a region for which precision Doppler surveys are optimally sensitive.

Discovery of the lowest amplitude, lowest mass planets requires high cadence observations with spectroscopic systems that achieve precision of ~1 m/s. Of the three systems in the world that produce results at this level (Keck, AAT, HARPS), Butler and his colleagues constructed two. The Magellan PFS (Planet Finding Spectrometer) at Carnegie’s Las Campanas Observatory (LCO) is the first system Butler has built specifically to achieve world-class precision. Figure 1 shows the results of the primary PFS test case, the known stable star HD 53706. This star was observed on 7 nights over three Magellan PFS runs spanning 5 months. The velocity root mean square (RMS) is 66 centimeters per second, a major breakthrough in precision.

All of the lowest mass planets from Keck, HARPS, and AAT have emerged from high cadence runs. Butler’s first long observing run on Magellan PFS was August 2010. Figure 2 shows the results for four stars, including a stable star, as well as three M dwarfs that show variations above the measurement uncertainty. With PFS the ability to discover rocky planets around nearby dwarfs, and potentially habitable planets around M dwarfs, is limited only by the telescope time allocated to the search.

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 Fellow Anglada-Escude 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. The plan is to follow about 100 nearby (primarily within about 10 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, where habitable planets are now known to exist (e.g., Gl 581g — Vogt et al. 2010).

Our first paper (Boss et al. 2009) showed that our analysis of the target star NLTT 48256 yields an astrometric accuracy better than 0.4 milliarcsec over a time scale of several years with CAPSCam-S on the du Pont 2.5-m. We expect the astrometric accuracy to improve to below 0.3 milliarcsec once full color corrections are included in the data analysis, using the several years of Tek5 color photometry taken while building CAPSCam-S. Such a precision is sufficient to detect a Jupiter-mass companion orbiting 1 AU from a late M dwarf 10 pc away with a signal-to-noise ratio of about 4. Given this level of performance, we have proposed for another 30 nights of du Pont bright time in 2011 to continue our astrometric planet search program.

CAPSCam-S has been in operation since March 2007, though only in the last two years with all the necessary improvements in place. We have now gathered (at least) first epoch CAPSCam data for 107 of the 111 targets on the current target list. 17 targets have been dropped in the last year as a result of CAPSCam-S determinations of distances greater then 15 pc, and suitable new targets are added as they are discovered. 55 targets have at least 4 epochs of data, and about a dozen have enough epochs (as many as 15) to confirm known low mass binary companions (Figure 3) or to derive preliminary false alarm probabilities for planetary-mass companions. The CAPSCam effort is on the verge of yielding a steady stream of astrometric measurements of the dynamics of late M dwarfs and any very low mass 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. Boss visited Prof. Podolak in Tel Aviv in April 2010, and continued their plans for accomplishing this interaction. A large suite of new 3D disk models are now underway, studying the evolution of dust grains with sizes of 0.1 cm, 1 cm. 10 cm, and 1 m, evolving in disks with radii ranging from 10 AU to 40 AU. 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

1.4 Late Mixing and Migration in the Solar System

The dynamical and physical properties of small bodies in our Solar System offer one of the few constraints on the formation, evolution and migration of the planets. The strong dynamical connection of the asteroids to the planets make determining their population and orbital properties valuable for gaining insight into Solar System formation and planet evolution. Co-I Sheppard and his colleagues have been conducting wide-field observations in order to discover faint Neptune Trojans and objects beyond the Kuiper Belt edge at 50 AU.

Sheppard and his team have now finished the Neptune Trojan survey. In this survey they found the first Neptune Trojan in the trailing Lagrangian region of gravitational equilibrium within Neptune’s orbit. They estimate that the leading and trailing Neptune Trojan regions have similarly sized populations and dynamics, with both regions dominated by high-inclination objects. Similar populations and dynamics at both Neptune Lagrangian regions indicate that the Trojans were likely captured by Neptune migrating on an eccentric orbit through a dynamically excited planetesimal population. Sheppard and his colleagues have also determined that there is a paucity of smaller Neptune Trojans compared to larger ones. All the observed stable regions in the solar system show evidence for these missing intermediate-sized planetesimals, which indicates a primordial, and not collisional, origin for this dearth of smaller objects compared to larger ones. This suggests that planetesimal formation proceeded directly from small to large objects. The scarcity of intermediate and smaller Neptune Trojans also may limit them as being a likely source for the short period comets.

1.5 Compositions of Circumstellar Disks and Delivery of Volatiles to Comets

Co-I Weinberger studied young stars and the disks around them. She completed an observing program at the Magellan telescope to make high spectral resolution optical spectra of all 130 F-type and G-type Hipparcos identified members of the Sco-Cen OB Association. An unusual accreting binary star HD 101088 was discovered in this set (Bitner et al. 2010). Accretion is rare among such relatively old (12 Myr) stars. Furthermore, this binary had only a weak outer disk with very few small grains that could accompany any remaining gas. This system may be seen in its last gasps of accretion and gas-giant planet formation. The full sample of stars is now under analysis for determining stellar properties, in order to measure activity and metallicity, and to search for correlations with the presence or absence of a circumstellar disk. Weinberger is also looking for any rare stars with circumstellar gas along the line-of-sight, in order to find cases where she can measure planet-forming material directly.

Weinberger also completed observing for the first of her parallax studies of young stellar associations, in this case the TW Hydrae association. Summer undergraduate intern Rebecca Rattray worked with the data to understand the systematic uncertainties and to find distances to seven stars, already doubling the number of TW Hydrae ssociation stars with parallactic distances. If a time of closest approach for the full set of stars can be found, that time would be a measurement of their ages, completely independent of pre-main sequence stellar models.

Figure 1: The known stable star HD 53706 observed over the first three science runs with the Magellan PFS. The velocity RMS is 66 centimeters per second.

Figure 2: Four PFS stars with high cadence observations from August 2010. The stable star Tau Ceti is shown first. The other stars show variation above measurement uncertainty. The provisional Keplerian fit shown at the bottom includes two observations from the first PFS science run in January 2010 in addition to data from August 2010.

Figure 3: Right Ascension (left) and Declination (right) as a function of orbital phase for field 218, in milliarcseonds (mas). Field 218 is the Eps Indi B binary system, with a large astrometric wobble, composed of the T1 (Ba) primary and a T6 (Bb) brown dwarf companion with an 11 yr period. The CAPSCam data spans just 3 three years, but already confirms the companion’s orbit. The residuals (not shown) hint at the possibility of a second companion to Eps Indi Ba with a shorter orbital period than Eps Indi Bb.