Daniel Apai

A Giant Planet Imaged in the Disk of the Young Star β Pictoris

Planet Is Born The 10-million-year-old star β Pictoris, has long been suspected to host a planet. Through images obtained with the Very Large Telescope, an array of four telescopes located in Chile, Lagrange et al. (p. 57, published online 10 June) now confirm the presence of a young, giant planet, β Pictoris b, orbiting within the dusty disk that surrounds the star. β Pictoris b orbits closer to its star than Uranus and Neptune do to the Sun in our solar system. This orbital separation is consistent with the in situ formation of the planet via a core accretion mechanism. Thus, giant planets can form within a stellar dust disk in only a few million years. The Very Large Telescope reveals that a huge planet formed within a star’s dusty disk in a few million years. Here, we show that the ~10-million-year-old β Pictoris system hosts a massive giant planet, β Pictoris b, located 8 to 15 astronomical units from the star. This result confirms that gas giant planets form rapidly within disks and validates the use of disk structures as fingerprints of embedded planets. Among the few planets already imaged, β Pictoris b is the closest to its parent star. Its short period could allow for recording of the full orbit within 17 years.

NEARBY DEBRIS DISK SYSTEMS WITH HIGH FRACTIONAL LUMINOSITY RECONSIDERED

By searching the IRAS and ISO databases, we compiled a list of 60 debris disks that exhibit the highest fractional luminosity values ( fd > 10 � 4 ) in the vicinity of the Sun (d 5 ; 10 � 4 are younger than 100 Myr.Thedistribution of the disks inthe fractional luminosityversus age diagram indicates that (1) the number of old systems with high fd is lower than was claimed before, (2) there exist many relatively young disks of moderate fractional luminosity, and (3) comparing the observations with a current theoretical model of debris disk evolution, a general good agreement could be found. Subject headings: circumstellar matter — infrared: stars — stars: kinematics

The Onset of Planet Formation in Brown Dwarf Disks

The onset of planet formation in protoplanetary disks is marked by the growth and crystallization of sub–micrometer-sized dust grains accompanied by dust settling toward the disk mid-plane. Here, we present infrared spectra of disks around brown dwarfs and brown dwarf candidates. We show that all three processes occur in such cool disks in a way similar or identical to that in disks around low- and intermediate-mass stars. These results indicate that the onset of planet formation extends to disks around brown dwarfs, suggesting that planet formation is a robust process occurring in most young circumstellar disks.

The different evolution of gas and dust in disks around Sun-like and cool stars

Planet formation is profoundly impacted by the properties of protoplanetary disks and their central star. However, how disk properties vary with stellar parameters remains poorly known. Here, we present the first comprehensive, comparative Spitzer/IRS study of the dust and gas properties of disks around young Sun-like stars (K1-M5) and cool stars/brown dwarfs (M5-M9). The comparison of these two large samples of over 60 sources reveal major differences in the evolution of both the dust and gas components. We report the first detection of organic molecules in disks around brown dwarfs. The detection rate statistics and the line flux ratios of HCN and C2H2 show a striking difference between the two samples, demonstrating a significant underabundance of HCN relative to C2H2 in the disk surface of cool stars. We propose this to originate from the large difference in the UV irradiation around the two types of sources. The statistical comparison of the 10 μm silicate emission features also reveals a difference between the two samples. Cool stars and brown dwarfs show weaker features arising from more processed silicate grains in the disk atmosphere. These findings complement previous indications of flatter disk structures and longer disk lifetimes around cool stars. Our results highlight important differences in the chemical and physical evolution of protoplanetary disks as a function of stellar mass, temperature, and radiation field which should be taken into account in planet formation models. We note that the different chemistry of preplanetary materials in the disk may also influence the bulk composition and volatile content of the forming planets. In particular, if exogenous HCN has played a key role in the synthesis of prebiotic molecules on Earth as proposed, then prebiotic chemistry may unfold differently on planets around cool stars.

HST SPECTRAL MAPPING OF L/T TRANSITION BROWN DWARFS REVEALS CLOUD THICKNESS VARIATIONS

Most directly imaged giant exoplanets are fainter than brown dwarfs with similar spectra. To explain their relative underluminosity, unusually cloudy atmospheres have been proposed. However, with multiple parameters varying between any two objects, it remained difficult to observationally test this idea. We present a new method, sensitive time-resolved Hubble Space Telescope near-infrared spectroscopy, to study two rotating L/T transition brown dwarfs (2M2139 and SIMP0136). The observations provide spatially and spectrally resolved mapping of the cloud decks of the brown dwarfs. The data allow the study of cloud structure variations while other parameters are unchanged. We find that both brown dwarfs display variations of identical nature: J- and H-band brightness variations with minimal color and spectral changes. Our light curve models show that even the simplest surface brightness distributions require at least three elliptical spots. We show that for each source the spectral changes can be reproduced with a linear combination of only two different spectra, i.e., the entire surface is covered by two distinct types of regions. Modeling the color changes and spectral variations together reveal patchy cloud covers consisting of a spatially heterogeneous mix of low-brightness, low-temperature thick clouds and brighter, thin, and warm clouds. We show that the same thick cloud patches seen in our varying brown dwarf targets, if extended to the entire photosphere, predict near-infrared colors/magnitudes matching the range occupied by the directly imaged exoplanets that are cooler and less luminous than brown dwarfs with similar spectral types. This supports the models in which thick clouds are responsible for the near-infrared properties of these underluminous exoplanets.

A novel L-band imaging search for giant planets in the Tucana and β Pictoris moving groups

Context. Direct imaging using various techniques for suppressing the stellar halo nowadays can achieve the contrast levels required to detect and characterize the light of substellar companions at orbital distances greater than a few astronomical units from their host stars. The method nicely complements the radial velocity surveys that provide evidence that giant extrasolar planets in close-in orbits are relatively common. Aims. The paper presents results from a small survey of 22 young, nearby stars that was designed to detect substellar companions and ultimately giant extrasolar planets down to Jupiter masses. The targets are members of the Tucana and β Pictoris moving groups, apart from the somewhat older star HIP 71395, which has a radial velocity trend suggesting a massive planet in large orbit. Methods. The survey was carried out in the L-band using adaptive optics-assisted imaging with NAOS-CONICA (NACO) at the VLT. The chosen observation wavelength is well-suited to searching for close companions around young stars and it delivers unprecedented detection limits. The presented technique reaches some of the best sensitivities as of today and is currently the most sensitive method for the contrast-limited detection of substellar companions that are cooler than about 1000 K. Results. The companion to 51 Eri, GJ 3305, was found to be a very close binary on an eccentric orbit. No substellar companions were found around the target stars, although the method permitted companions to be detected down to a few Jupiter masses at orbital distances typically of 5 astronomical units. A planet with a mass ≥1 M Jup at distances >5 AU around AU Mic can be excluded at the time of our observations. The absence of detected planets sets constraints on the frequency distribution and maximum orbital distance of giant exoplanets. For example, a radial distribution power law index of 0.2 in combination with a maximum orbital radius exceeding 30 AU can be rejected at a 90% confidence level. Conclusions.

VERTICAL ATMOSPHERIC STRUCTURE IN A VARIABLE BROWN DWARF: PRESSURE-DEPENDENT PHASE SHIFTS IN SIMULTANEOUS HUBBLE SPACE TELESCOPE-SPITZER LIGHT CURVES

Heterogeneous clouds or temperature perturbations in rotating brown dwarfs produce variability in the observed flux. We report time-resolved simultaneous observations of the variable T6.5 brown dwarf 2MASSJ22282889 431026 over the wavelength ranges 1.1 1.7 µm and broadband 4.5 µm. Spectroscopic observations were taken with Wide Field Camera 3 on board the Hubble Space Telescope and photometry with the Spitzer Space Telescope. The object shows sinusoidal infrared variability with a period of 1.4 hr at most wavelengths with peak-to-peak amplitudes between 1.45% and 5.3% of the mean flux. While the light curve shapes are similar at all wavelengths, their phases differ from wavelength to wavelength with a maximum difference of more than half of a rotational period. We compare the spectra with atmospheric models of different cloud prescriptions, from which we determine the pressure levels probed at different wavelengths. We find that the phase lag increases with decreasing pressure level, or higher altitude. We discuss a number of plausible scenarios that could cause this trend of light curve phase with probed pressure level. These observations are the first to probe heterogeneity in an ultracool atmosphere in both horizontal and vertical directions, and thus are an ideal test case for realistic three dimensional simulations of the atmospheric structure with clouds in brown dwarfs and extrasolar planets. Subject headings: brown dwarfs — stars: atmospheres — stars: individual (2MASSJ22282889 4310262) — stars: variables: general

First Detection of Millimeter Dust Emission from Brown Dwarf Disks

We report results from the first deep millimeter continuum survey targeting brown dwarfs (BDs). The survey led to the first detection of cold dust in the disks around two young BDs (CFHT-BD-Tau 4 and IC 348 613), with deep JCMT and IRAM observations reaching flux levels of a few microjanskys. The dust masses are estimated to be a few Earth masses, assuming the same dust opacities as are usually applied to T Tauri stars.

The position of β Pictoris b position relative to the debris disk

Context. We detected in 2009 a giant, close-by planet orbiting β Pic, a young star surrounded by a disk that has been extensively studied for more than 20 years. We showed that if the planet were located on an inclined orbit, this could account for several peculiarities of the β Pictoris system. However, the available data did not permit us to measure the inclination of β Pic b with respect to the disk, and in particular to establish in which component of the disk – either the main, extended disk or the inner inclined component/disk – the planet was located. Comparison between the observed planet position and the disk orientation measured using previous imaging data was not an option because of potential biases in the measurements. Aims. Our aim is to measure precisely the planet location with respect to the dust disk using a single high-resolution image, and correcting for systematics or errors that degrade the precision of the disk and planet relative-position measurements. Methods. We gathered new NaCo data in the Ks band, with a set-up optimized to derive simultaneously the orientation(s) of the disk(s) and the planet projected position. Results. We show that the projected position of β Pic b is above the midplane of the main disk. With the current data and knowledge of the system, this implies that β Pic b cannot be located in the main disk. The data instead suggest that the planet is located in the inclined component.

A STELLAR-MASS-DEPENDENT DROP IN PLANET OCCURRENCE RATES

The Kepler spacecraft has discovered a large number of planets with up to one-year periods and down to terrestrial sizes. While the majority of the target stars are main-sequence dwarfs of spectral type F, G, and K, Kepler covers stars with effective temperatures as low as 2500 K, which corresponds to M stars. These cooler stars allow characterization of small planets near the habitable zone, yet it is not clear if this population is representative of that around FGK stars. In this paper, we calculate the occurrence of planets around stars of different spectral types as a function of planet radius and distance from the star and show that they are significantly different from each other. We further identify two trends. First, the occurrence of Earth- to Neptune-sized planets (1-4 R ⊕) is successively higher toward later spectral types at all orbital periods probed by Kepler; planets around M stars occur twice as frequently as around G stars, and thrice as frequently as around F stars. Second, a drop in planet occurrence is evident at all spectral types inward of a ~10 day orbital period, with a plateau further out. By assigning to each spectral type a median stellar mass, we show that the distance from the star where this drop occurs is stellar mass dependent, and scales with semi-major axis as the cube root of stellar mass. By comparing different mechanisms of planet formation, trapping, and destruction, we find that this scaling best matches the location of the pre-main-sequence co-rotation radius, indicating efficient trapping of migrating planets or planetary building blocks close to the star. These results demonstrate the stellar-mass dependence of the planet population, both in terms of occurrence rate and of orbital distribution. The prominent stellar-mass dependence of the inner boundary of the planet population shows that the formation or migration of planets is sensitive to the stellar parameters.