B. Zuckerman

A Giant Planet Candidate near a Young Brown Dwarf ? Direct VLT/NACO Observations using IR Wavefront Sensing

We present deep VLT/NACO infrared imaging and spectroscopic observations of the brown dwarf 2MASSWJ 1207334−393254, obtained during our on-going adaptive optics survey of southern young, nearby associations. This ∼25 MJup brown dwarf, located ∼70 pc from Earth, has been recently identified as a member of the TW Hydrae Association (age ∼ 8 Myr). Using adaptive optics infrared wavefront sensing to acquire sharp images of its circumstellar environment, we discovered a very faint and very red object at a close separation of ∼780 mas (∼55 AU). Photometry in the H, Ks and Lbands and upper limit in J-band are compatible with a spectral type L5−L9.5. Near-infrared spectroscopy is consistent with this spec- tral type estimate. Different evolutionary models predict an object within the planetary regime with a mass of M = 5 ± 2 MJup and an effective temperature of Teff = 1250 ± 200 K.

Submillimetre images of dusty debris around nearby stars

Indirect detections of massive — presumably Jupiter-like — planets orbiting nearby Sun-like stars have recently been reported,. Rocky, Earth-like planets are much more difficult to detect, but clues to their possible existence can nevertheless be obtained from observations of the circumstellar debris disks of dust from which they form. The presence of such disks has been inferred from excess far-infrared emission but, with the exception of β Pictoris, it has proved difficult to image these structures directly as starlight dominates the faint light scattered by the dust. A more promising approach is to attempt to image the thermal emission from the dust grains at submillimetre wavelengths,. Here we present images of such emission around Fomalhaut, β Pictoris and Vega. For each star, dust emission is detected from regions comparable in size to the Suns Kuiper belt of comets. The total dust mass surrounding each star is only a few lunar masses, so any Earth-like planets present must already have formed. The presence of the central cavity, approximately the size of Neptunes orbit, that we detect in the emission from Fomalhaut may indeed be the signature of such planets.

The β Pictoris Moving Group

Following the 1983 IRAS detection and subsequent imaging of its extensive dusty circumstellar disk, β Pictoris became the prototypical and most studied example of a potential forming planetary system. Here we report the identification of 17 star systems, each with one or more characteristics indicative of extreme youth, that are moving through space together with β Pic. This diverse set of ~12 million yr old star systems, which includes a ~35 Jupiter mass brown dwarf, and a wide assortment of dusty circumstellar disks, is the comoving, youthful group closest to Earth. Their unique combination of youth and proximity to Earth makes group members—many of which have masses similar to that of the Sun—prime candidates for imaging of warm planets and dusty circumstellar disks with ground- and space-based telescopes.

A Dust Ring around epsilon Eridani: Analog to the Young Solar System

Dust emission around the nearby star Eridani has been imaged using a new submillimeter camera (the Submillimetre Common-User Bolometer Array at the James Clerk Maxwell Telescope). At an 850 μm wavelength, a ring of dust is seen peaking at 60 AU from the star and with much lower emission inside 30 AU. The mass of the ring is at least ~0.01 M⊕ in dust, while an upper limit of 0.4 M⊕ in molecular gas is imposed by CO observations. The total mass is comparable to the estimated amount of material, 0.04-0.3 M⊕, in comets orbiting the solar system. The most probable origin of the ring structure is that it is a young analog to the Kuiper Belt in our solar system and that the central region has been partially cleared by the formation of grains into planetesimals. Dust clearing around Eri is seen within the radius of Neptunes orbit, and the peak emission at 35-75 AU lies within the estimated Kuiper Belt zone of 30-100 AU radius. Eri is a main-sequence star of type K2 V (0.8 M⊙) with an estimated age of 0.5-1.0 Gyr, so this interpretation is consistent with the early history of the solar system where heavy bombardment occurred up to ≈ 0.6 Gyr. An unexpected discovery is the substructure within the ring, and these asymmetries could be due to perturbations by planets.

Characterization of Dusty Debris Disks: the IRAS and Hipparcos Catalogs

Dusty debris disks around main-sequence stars are signposts for the existence of planetesimals and exoplanets. From cross-correlating Hipparcos stars with the IRAS catalogs, we identify 146 stars within 120 pc of Earth that show excess emission at 60 μm. This search took special precautions to avoid false positives. Our sample is reasonably well distributed from late B to early K-type stars, but it contains very few later type stars. Even though IRAS flew more than 20 years ago and many astronomers have cross-correlated its catalogs with stellar catalogs, we were still able to newly identify debris disks at as many as 33 main-sequence stars; of these, 32 are within 100 pc of Earth. The power of an all-sky survey satellite like IRAS is evident when comparing our 33 new debris disks with the total of only 22 dusty debris disk stars first detected with the more sensitive, but pointed, satellite ISO. Our investigation focuses on the mass, dimensions, and evolution of dusty debris disks.

Discovery and spectroscopy of the young jovian planet 51 Eri b with the Gemini Planet Imager

An exoplanet extracted from the bright Direct imaging of Jupiter-like exoplanets around young stars provides a glimpse into how our solar system formed. The brightness of young stars requires the use of next-generation devices such as the Gemini Planet Imager (GPI). Using the GPI, Macintosh et al. discovered a Jupiter-like planet orbiting a young star, 51 Eridani (see the Perspective by Mawet). The planet, 51 Eri b, has a methane signature and is probably the smallest exoplanet that has been directly imaged. These findings open the door to understanding solar system origins and herald the dawn of a new era in next-generation planetary imaging. Science, this issue p. 64; see also p. 39 The Gemini Planet Imager detects a Jupiter-like exoplanet orbiting the young star 51 Eridani. [Also see Perspective by Mawet] Directly detecting thermal emission from young extrasolar planets allows measurement of their atmospheric compositions and luminosities, which are influenced by their formation mechanisms. Using the Gemini Planet Imager, we discovered a planet orbiting the ~20-million-year-old star 51 Eridani at a projected separation of 13 astronomical units. Near-infrared observations show a spectrum with strong methane and water-vapor absorption. Modeling of the spectra and photometry yields a luminosity (normalized by the luminosity of the Sun) of 1.6 to 4.0 × 10−6 and an effective temperature of 600 to 750 kelvin. For this age and luminosity, “hot-start” formation models indicate a mass twice that of Jupiter. This planet also has a sufficiently low luminosity to be consistent with the “cold-start” core-accretion process that may have formed Jupiter.

New Members of the TW Hydrae Association, β Pictoris Moving Group, and Tucana/Horologium Association

We have identified five new members of the TW Hydrae association (TWA), 11 new members of the ? Pic moving group, and 11 new Tucana/Horologium association members. These are the three youngest (30 Myr) known kinematic stellar groups near the Earth. Newly identified ? Pic group members are located mostly in the northern hemisphere, and they have a slightly different U-component of Galactic velocity compared to that of previously known members. Tracing the motion of ? Pic members backward in time for 12 Myr indicates that they might have formed in a small region with an initial velocity dispersion of ~8 km s-1. A couple of mid-M spectral type ? Pic members show emission features [He I ?5876+?6678) and Na D ?5890+?5896)] seen among earlier spectral type stars in the TWA and ? Pic groups. To derive the distances of the non-Hipparcos members of these groups, we have constructed a V-K versus MK color-magnitude diagram that is very useful in separating young K/M stars from older main-sequence counterparts and constraining theoretical pre-main-sequence evolutionary tracks. All newly identified K- and M-type members of the three groups show saturated X-ray activity (LX/Lbol ~ 10-3). One newly identified TWA member, SSS 101727-5354, is estimated to be only 22 pc away from Earth. Its extreme youth, late spectral type (~M5), and proximity to Earth make SSS 101727-5354 perhaps the best target for direct imaging detection of cooling planets.

The Circumstellar Disk of HD 141569 Imaged with NICMOS.

Coronagraphic imaging with the Near-Infrared Camera and Multiobject Spectrometer on the Hubble Space Telescope reveals a large, approximately 400 AU (4&arcsec;) radius, circumstellar disk around the Herbig Ae/Be star HD 141569. A reflected light image at 1.1 µm shows the disk oriented at a position angle of 356&j0;+/-5&j0; and inclined to our line of sight by 51&j0;+/-3&j0;; the intrinsic scattering function of the dust in the disk makes the side inclined toward us, the eastern side, brighter. The disk flux density peaks 185 AU (1&farcs;85) from the star and falls off to both larger and smaller radii. A region of depleted material, or a gap, in the disk is centered 250 AU from the star. The dynamical effect of one or more planets may be necessary to explain this morphology.

DUSTY DEBRIS AROUND SOLAR-TYPE STARS: TEMPORAL DISK EVOLUTION

Using ISO-ISOPHOT, we carried out a survey of almost 150 stars to search for evidence of emission from dust orbiting young main-sequence stars, both in clusters and isolated systems. Over half of the detections are new examples of dusty stellar systems and demonstrate that such dust can be detected around numerous stars older than a few times 106 yr. Fluxes at 60 and either 90 or 100 μm for the new excess sources together with improved fluxes for a number of IRAS-identified sources are presented. Analysis of the excess luminosity relative to the stellar photosphere shows a systematic decline of this excess with stellar age consistent with a power-law index of -2.

Low-Luminosity Companions to White Dwarfs

This paper presents results of a near-infrared imaging survey for low-mass stellar and substellar companions to white dwarfs. A wide-field proper-motion survey of 261 white dwarfs was capable of directly detecting companions at orbital separations between ~100 and 5000 AU with masses as low as 0.05 M?, while a deep near-field search of 86 white dwarfs was capable of directly detecting companions at separations between ~50 and 1100 AU with masses as low as 0.02 M?. Additionally, all white dwarf targets were examined for near-infrared excess emission, a technique capable of detecting companions at arbitrarily close separations down to masses of 0.05 M?. No brown dwarf candidates were detected, which implies a brown dwarf companion fraction of <0.5% for white dwarfs. In contrast, the stellar companion fraction of white dwarfs as measured by this survey is 22%, uncorrected for bias. Moreover, most of the known and suspected stellar companions to white dwarfs are low-mass stars whose masses are only slightly greater than the masses of brown dwarfs. Twenty previously unknown stellar companions were detected, five of which are confirmed or likely white dwarfs themselves, while 15 are confirmed or likely low-mass stars. Similar to the distribution of cool field dwarfs as a function of spectral type, the number of cool unevolved dwarf companions peaks at mid-M type. Based on the present work, relative to this peak, field L dwarfs appear to be roughly 2-3 times more abundant than companion L dwarfs. Additionally, there is no evidence that the initial companion masses have been altered by post-main-sequence binary interactions.