Ben Zuckerman

Direct Imaging of Multiple Planets Orbiting the Star HR 8799

Direct imaging of exoplanetary systems is a powerful technique that can reveal Jupiter-like planets in wide orbits, can enable detailed characterization of planetary atmospheres, and is a key step toward imaging Earth-like planets. Imaging detections are challenging because of the combined effect of small angular separation and large luminosity contrast between a planet and its host star. High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units. Multi-epoch data show counter clockwise orbital motion for all three imaged planets. The low luminosity of the companions and the estimated age of the system imply planetary masses between 5 and 13 times that of Jupiter. This system resembles a scaled-up version of the outer portion of our solar system.

Images of a fourth planet orbiting HR 8799

High-contrast near-infrared imaging of the nearby star HR 8799 has shown three giant planets. Such images were possible because of the wide orbits (>25 astronomical units, where 1 au is the Earth–Sun distance) and youth (<100 Myr) of the imaged planets, which are still hot and bright as they radiate away gravitational energy acquired during their formation. An important area of contention in the exoplanet community is whether outer planets (>10 au) more massive than Jupiter form by way of one-step gravitational instabilities or, rather, through a two-step process involving accretion of a core followed by accumulation of a massive outer envelope composed primarily of hydrogen and helium. Here we report the presence of a fourth planet, interior to and of about the same mass as the other three. The system, with this additional planet, represents a challenge for current planet formation models as none of them can explain the in situ formation of all four planets. With its four young giant planets and known cold/warm debris belts, the HR 8799 planetary system is a unique laboratory in which to study the formation and evolution of giant planets at wide (>10 au) separations.

Infrared Signatures of Disrupted Minor Planets at White Dwarfs

Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T eff 20, 000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457?086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal-accretion rates dM/dt 3 ? 108 g s?1 display a warm infrared excess from orbiting dust; the likely result of a tidally destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 106 g s?1 in the solar system zodiacal cloud and 1010 g s?1 often inferred for debris disks at main-sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

Giant planet companion to 2MASSW J1207334-393254

We report new VLT/NACO imaging observations of the young, nearby brown dwarf 2MASSW J1207334-393254 and its suggested planetary mass companion (2M1207 b). Three epochs of VLT/NACO measurements obtained over nearly one year show that the planetary mass companion candidate shares the same proper motion and, with a high confidence level, is not a stationary background object. This result confirms the status of 2M1207 b as of planetary mass (5 times the mass of Jupiter) and the first image of a planetary mass companion in a different system than our own. This discovery offers new perspectives for our understanding of chemical and physical properties of planetary mass objects as well as their mechanisms of formation.

The Chemical Composition of an Extrasolar Minor Planet

We report the relative abundances of 17 elements in the atmosphere of the white dwarf star GD 362, material that, very probably, was contained previously in a large asteroid or asteroids with composition similar to the Earth-Moon system. The asteroid may have once been part of a larger parent body not unlike one of the terrestrial planets of our solar system.

ANCIENT PLANETARY SYSTEMS ARE ORBITING A LARGE FRACTION OF WHITE DWARF STARS

Infrared studies have revealed debris likely related to planet formation in orbit around ~30% of youthful, intermediate mass, main-sequence stars. We present evidence, based on atmospheric pollution by various elements heavier than helium, that a comparable fraction of the white dwarf descendants of such main-sequence stars are orbited by planetary systems. These systems have survived, at least in part, through all stages of stellar evolution that precede the white dwarf. During the time interval (~200 million years) that a typical polluted white dwarf in our sample has been cooling it has accreted from its planetary system the mass of one of the largest asteroids in our solar system (e.g., Vesta or Ceres). Usually, this accreted mass will be only a fraction of the total mass of rocky material that orbits these white dwarfs; for plausible planetary system configurations we estimate that this total mass is likely to be at least equal to that of the Suns asteroid belt, and perhaps much larger. We report abundances of a suite of eight elements detected in the little studied star G241-6 that we find to be among the most heavily polluted of all moderately bright white dwarfs.

Externally Polluted White Dwarfs with Dust Disks

We report Spitzer Space Telescope photometry of 11 externally polluted white dwarfs. Of the nine stars for which we have IRAC photometry, we find that GD 40, GD 133, and PG 1015+161 each has an infrared excess that can be understood as arising from a flat, opaque, dusty disk. GD 56 also has an infrared excess characteristic of circumstellar dust, but a flat-disk model cannot reproduce the data unless there are grains as warm as 1700 K and perhaps not even then. Our data support the previous suggestion that the metals in the atmosphere of GD 40 are the result of accretion of a tidally disrupted asteroid with a chondritic composition.

Chemical Abundances in the Externally Polluted White Dwarf GD 40: Evidence of a Rocky Extrasolar Minor Planet

We present Keck/High Resolution Echelle Spectrometer data with model atmosphere analysis of the helium-dominated polluted white dwarf GD 40, in which we measure atmospheric abundances relative to helium of nine elements: H, O, Mg, Si, Ca, Ti, Cr, Mn, and Fe. Apart from hydrogen, whose association with the other contaminants is uncertain, this material most likely accreted from GD 40s circumstellar dust disk whose existence is demonstrated by excess infrared emission. The data are best explained by accretion of rocky planetary material, in which heavy elements are largely contained within oxides, derived from a tidally disrupted minor planet at least the mass of Juno, and probably as massive as Vesta. The relatively low hydrogen abundance sets an upper limit of 10% water by mass in the inferred parent body, and the relatively high abundances of refractory elements, Ca and Ti, may indicate high-temperature processing. While the overall constitution of the parent body is similar to the bulk Earth being over 85% by mass composed of oxygen, magnesium, silicon, and iron, we find n(Si)/n(Mg) = 0.30 ? 0.11, significantly smaller than the ratio near unity for the bulk Earth, chondrites, the Sun, and nearby stars. This result suggests that differentiation occurred within the parent body.

An Improved Optical Spectrum and New Model Fits of the Likely Brown Dwarf GD 165B

Long thought by some researchers to be an oddity, GD 165B has instead proved to be the first example of a class of very cool objects (the L dwarfs) that, because of dust formation in their photospheres, lack the dominant bands of TiO seen in warmer M dwarfs. Here we present an improved optical (6200-10300 A) spectrum of GD 165B and identify its prominent spectral features. Among these are newly identified bands of FeH and the first identification of CrH bands in a dwarf. We use the latest generation of model atmospheres, which include the effects of condensation and dust opacities, to derive values of Teff = 1900 ± 100 K and log g = 5.0 ± 0.5 for GD 165B. We also derive a crude age of 1.2-5.5 Gyr for the GD 165 system. A comparison of the temperature and age of GD 165B with evolutionary models predicts GD 165B to be an object in the transition zone between stars and brown dwarfs. Further observational evidence—the discovery of lithium in other dwarfs spectroscopically similar to GD 165B and the scarcity of GD 165B-like companions found by imaging surveys—favors a substellar interpretation for this object. We argue that the weight of this observational evidence, together with the known shortcomings of the evolutionary models (which do not yet include opacity by grains), indicate that GD 165B is probably a brown dwarf.

Warm Dust in the Terrestrial Planet Zone of a Sun-like Pleiades Star: Collisions between Planetary Embryos?

Only a few solar-type main-sequence stars are known to be orbited by warm dust particles; the most extreme is the G0 field star BD +20 307 that emits ~4% of its energy at mid-infrared wavelengths. We report the identification of a similarly dusty star HD 23514, an F6-type member of the Pleiades. A strong mid-IR silicate emission feature indicates the presence of small warm dust particles, but with the primary flux density peak at the nonstandard wavelength of ~9 μm. The existence of so much dust within an AU or so of these stars is not easily accounted for given the very brief lifetime in orbit of small particles. The apparent absence of very hot (≳1000 K) dust at both stars suggests the possible presence of a planet closer to the stars than the dust. The observed frequency of the BD +20 307/HD 23514 phenomenon indicates that the mass equivalent of Earths Moon must be converted, via collisions of massive bodies, to tiny dust particles that find their way to the terrestrial planet zone during the first few hundred million years of the life of many (most?) Sun-like stars. Identification of these two dusty systems among youthful nearby solar-type stars suggests that terrestrial planet formation is common.