R. A. Webb

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.

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.

Identification of a Nearby Stellar Association in theHipparcos Catalog: Implications for Recent, Local Star Formation

The TW Hydrae Association (D55 pc from Earth) is the nearest known region of recent star forma- tion. Based primarily on the Hipparcos catalog, we have identi—ed a group of nine or 10 comoving star systems at a common distance (D45 pc) from Earth that appear to comprise another, somewhat older association (ii the Tucanae Association ˇˇ). Together with ages and motions recently determined for some nearby —eld stars, the existence of the Tucanae and TW Hydrae Associations suggests that the Sun is now close to a region that was the site of substantial star formation only 10¨50 Myr ago. The TW Hydrae Association represents a —nal chapter in the local star formation history. Subject headings: open clusters and associations: general ¨ open clusters and associations: individual (TW Hydrae) ¨ stars: formation ¨ stars: premain-sequence

The TW Hydrae Association: Discovery of T Tauri Star Members Near HR 4796

We have identified six T Tauri stars, at least three of which are double, that are members of the nearest region of recent star formation, the TW Hydrae association. The newly discovered systems are mostly located south and east of and farther from Earth than most previously known members; thus, the full extent of the association is not yet established. Including secondaries in multiple star systems, the association is now known to include more than 30 T Tauri stars and a brown dwarf. We compare the TW Hydrae association to four other young nearby associations.

HDE 233517: Lithium and Excess Infrared Emission in Giant Stars

Recent studies have identified a small class of moderately rapidly rotating, chromospherically active, single giants, some of which are lithium rich. We present evidence suggesting the peculiar K-type star HDE 233517 is one such object. Previously, HDE 233517 has been suggested to be a young star, consistent with its large far-infrared excess and our log e(Li) ~ 3.3. However, our high-resolution spectroscopic observations show it is likely a single, post-main-sequence K2 giant with v sin i = 15 km s-1 and modest Ca II H and K emission. The giant status of HDE 233517 is determined directly from luminosity-sensitive line ratios and a lack of significant line wings, and is further supported by a large radial velocity (46.5 km s-1), small proper motion, and the presence of interstellar absorption features. Interpretation of the data in the context of a recent mass outflow model for giant stars proposed by de la Reza and coworkers indicates that HDE 233517 has the largest mass-loss rate, ~3 × 10-7 M☉ yr-1, of any known luminosity class III giant. We suggest that the processes causing rapid rotation, large lithium abundance, and infrared excess are triggered at the base of the giant branch when the convection zone reaches the rapidly rotating core of low-mass stars.