Search

We have used the James Clerk Maxwell Telescope in Hawaii to search at submillimeter wavelengths for continuum emission from dust, and spectral line emission from carbon monoxide (CO) gas, in the neighborhood of HR 4796A. This Young star has a dusty disk with a central cavity, where planets may have formed. We detect the dust component at a wavelength of 850 [micro]m, and the inferred mass of solid material is [is greater than or equal to] 0.25 [M.sub.[direct sum]]. An upper limit for the CO J = 3-2 rotational line implies less than 1-7 [M.sub.[direct sum]] ([is less than or equal to] 0.003-0.02 Jupiter masses) of molecular [H.sub.2] gas in the system. Thus, it is no longer possible to form new Jupiter-like gaseous giant planets around HR 4796A. If planet formation explains the observed dust cavity and lack of gas, then it must have occurred before the current stellar age of ~10 Myr. A search was also made for CO J = 3-2 emission around four other stars with dust excesses revealed by infrared measurements with the Infrared Astronomical Satellite (IRAS). Two were detected, both of them young sources with optical emission lines indicative of ongoing accretion of disk material onto the star. The gas mass lower limits are approximately 30 and 200 Earth masses, at least an order of magnitude higher than that for HR 4796A, illustrating the diversity of disk properties at ages of up to 10 Myr. Key Words: planets; formation; extraterrestrial planets.

The presence of dusty debris around main-sequence stars denotes the existence of planetary systems. Such debris discs are often identified by the presence of excess continuum emission at infrared and (sub-)millimetre wavelengths, with measurements at longer wavelengths tracing larger and cooler dust grains. The exponent of the slope of the disc emission at submillimetre wavelengths, 'q', defines the size distribution of dust grains in the disc. This size distribution is a function of the rigid strength of the dust producing parent planetesimals. As a part of the survey 'PLAnetesimals around TYpical Pre-main seqUence Stars', we observed six debris discs at 9 mm using the Australian Telescope Compact Array. We obtain marginal (~3σ) detections of three targets: HD 105, HD 61005 and HD 131835. Upper limits for the three remaining discs, HD 20807, HD 109573 and HD 109085 provide further constraint of the (sub-)millimetre slope of their spectral energy distributions. The values of q (or their limits) derived from our observations are all smaller than the oft-assumed steady-state collisional cascade model (q = 3.5), but lie well within the theoretically expected range for debris discs q ~ 3-4. The measured q values for our targets are all <3.3, consistent with both collisional modelling results and theoretical predictions for parent planetesimal bodies being 'rubble piles' held together loosely by their self-gravity. [ABSTRACT FROM AUTHOR]

ε Eridani is a nearby, young Sun-like star that hosts a ring of cool debris analogous to the Solar system's Edgeworth-Kuiper belt. Early observations at (sub-)mm wavelengths gave tentative evidence of the presence of inhomogeneities in the ring, which have been ascribed to the effect of a putative low eccentricity planet, orbiting close to the ring. The existence of these structures has been recently challenged by high-resolution interferometric millimetre observations. Here, we present the deepest single-dish image of ε Eridani at millimetre wavelengths, obtained with the Large Millimetre Telescope Alfonso Serrano (LMT). The main goal of these LMT observations is to confirm (or refute) the presence of non-axisymmetric structure in the disc. The dusty ring is detected for the first time along its full projected elliptical shape. The radial extent of the ring is not spatially resolved and shows no evidence, to within the uncertainties, of dust density enhancements. Additional features of the 1.1 mm map are: (i) the presence of significant flux in the gap between the ring and the star, probably providing the first exo-solar evidence of Poynting-Robertson drag, (ii) an unambiguous detection of emission at the stellar position with a flux significantly above that expected from ε Eridani's photosphere, and (iii) the identification of numerous unresolved sources which could correspond to background dusty star-forming galaxies. [ABSTRACT FROM AUTHOR]

ABSTRACT Two luminous debris discs around 100 Myr old have been searched for 1 cm dust emission, to a depth three times greater than in any such previous study. Very low limits were set towards both HD 377 and HD 104860 (noise levels of 12-14 Jy), extending the spectral range from 70 m to 3 mm where cool dust has previously been sought. The almost-identical fluxes of the two systems were merged into an average spectral energy distribution, which was then fitted using a distribution of grain sizes. The canonical infinite collisional cascade, with numbers of particles n( D) ∝ D−3.5 for diameter D, overpredicts the 1-cm flux, which should have been detected at over 3σ for the merged system. Preferred solutions have truncated grain populations with largest particles around 1.5-4 mm in diameter, and slightly flatter distributions, up to n( D) ∝ D−3.1. The lack of cm-sized and larger particles is reminiscent of the similar deficit inferred around comets from spacecraft encounters. Such departures from simple power-law distributions of particles have been predicted in recent models where some size regimes are more readily broken up than others. A deficit of cm-scale particles can explain the fits we obtain, and reduces the total masses inferred for the comet belts of these stars. [ABSTRACT FROM AUTHOR]

The mass of solids in a young circumstellar disc may be the key factor in its efficiency in building planetesimals and planetary cores, and dust observed around young T Tauri and Herbig Ae stars can be used as a proxy for this initial solid content. The dust-mass distributions are taken from recent millimetre-wavelength data and fitted using survival analysis to take into account upper limits, and threshold disc masses for building planets and belts of comets are estimated. Amongst A stars, 20 per cent gas giant and 55 per cent debris disc systems are predicted, in good agreement with observations. For M stars, the predicted and observed planet frequencies agree at ∼2-3 per cent, and this low incidence is explained by a lack of massive discs. However, debris is predicted around ≈14 per cent of M stars, while only ∼2 per cent such systems have so far been found. This suggests that deeper searches such as with Herschel and SCUBA-2 may find a cold disc population previously missed around these low-luminosity stars. Also, an estimate of the efficiency of building millimetre-detected dust into planetary cores suggests that about one-third of M stars could host an Earth-mass planet - but as the dust is spread over large disc areas, such planets may orbit far from the star. [ABSTRACT FROM AUTHOR]

The thick disc contains stars formed within the first Gyr of Galactic history, and little is known about their planetary systems. The Spitzer MIPS instrument was used to search 11 of the closest of these old low-metal stars for circumstellar debris, as a signpost that bodies at least as large as planetesimals were formed. A total of 22 thick disc stars has now been observed, after including archival data, but dust is not found in any of the systems. The data rule out a high incidence of debris among star systems from early in the Galaxy's formation. However, some stars of this very old population do host giant planets, at possibly more than the general incidence among low-metal Sun-like stars. As the Solar system contains gas giants but little cometary dust, the thick disc could host analogue systems that formed many Gyr before the Sun. [ABSTRACT FROM AUTHOR]

Solar analogues approximately 100 Myr old may have dusty debris from collisions within evolving cometary belts, and such remnant discs might also be associated with earlier stellar-spin braking. We observed at 1.2 mm wavelength a sample of 17 fast and slow rotators, mostly single K dwarfs, in the 100 Myr Pleiades cluster. No dust was detected for individual stars or the ensemble, so there are no cold massive debris discs nor any discernible relation of such distant material to stellar spin. The net limits from these data and our earlier far-infrared results imply that the typical Pleiades G/K dwarf has a relative disc-to-star luminosity . Collisional evolution models have predicted greater luminosities at the 108 yr epoch, for debris discs evolving out of a proto-solar nebula. This suggests that substantial primordial discs such as that of the Sun are not the norm amongst young solar analogues, or that dynamical interactions with giant planets can remove much of the comet belt by as early as 100 Myr. [ABSTRACT FROM AUTHOR]

We have imaged the disc of the young star HL Tau using the Very Large Array (VLA) at 1.3 cm, with 0.08-arcsec resolution (as small as the orbit of Jupiter). The disc is around half the stellar mass, assuming a canonical gas mass conversion from the measured mass in large dust grains. A simulation shows that such discs are gravitationally unstable, and can fragment at radii of a few tens of au to form planets. The VLA image shows a compact feature in the disc at 65 au radius (confirming the ‘nebulosity’ of Welch et al.), which is interpreted as a localized surface density enhancement representing a candidate protoplanet in its earliest accretion phase. If correct, this is the first image of a low-mass companion object seen together with the parent disc material out of which it is forming. The object has an inferred gas plus dust mass of , similar to the mass of a protoplanet formed in the simulation. The disc instability may have been enhanced by a stellar flyby: the proper motion of the nearby star XZ Tau shows it could have recently passed the HL Tau disc as close as ∼600 au. [ABSTRACT FROM AUTHOR]

Magnetic fields are an important but largely unknown ingredient of planetary nebulae. They have been detected in oxygen-rich asymptotic giant branch (AGB) and post-AGB stars, and may play a role in the shaping of their nebulae. Here we present SCUBA submillimetre polarimetric observations of four bipolar planetary nebulae and post-AGB stars, including two oxygen-rich and two carbon-rich nebulae, to determine the geometry of the magnetic field by dust alignment. Three of the four sources (NGC 7027, 6537 and 6302) present a well-defined toroidal magnetic field oriented along their equatorial torus or disc. NGC 6302 may also show field lines along the bipolar outflow. CRL 2688 shows a complex field structure, where part of the field aligns with the torus, whilst an other part approximately aligns with the polar outflow. It also presents marked asymmetries in its magnetic structure. NGC 7027 shows evidence for a disorganized field in the south-west corner, where the SCUBA shows an indication for an outflow. The findings show a clear correlation between field orientation and nebular structure. [ABSTRACT FROM AUTHOR]

We investigate the populations of main-sequence stars within 25 pc that have debris discs and/or giant planets detected by Doppler shift. The metallicity distribution of the debris sample is a very close match to that of stars in general, but differs with >99 per cent confidence from the giant planet sample, which favours stars of above average metallicity. This result is not due to differences in age of the two samples. The formation of debris-generating planetesimals at tens of au thus appears independent of the metal fraction of the primordial disc, in contrast to the growth and migration history of giant planets within a few au. The data generally fit a core accumulation model, with outer planetesimals forming eventually even from a disc low in solids, while inner planets require fast core growth for gas to still be present to make an atmosphere. [ABSTRACT FROM AUTHOR]

We describe results from a survey for J= 3–212CO emission from visible stars classified as having an infrared excess. The line is clearly detected in 21 objects, and significant molecular gas (≥10−3 Jupiter masses) is found to be common in targets with infrared excesses≥0.01 (≥56 per cent of objects), but rare for those with smaller excesses (∼10 per cent of objects).A simple geometrical argument based on the infrared excess implies that disc opening angles are typically≥12° for objects with detected CO; within this angle, the disc is optically thick to stellar radiation and shields the CO from photodissociation. Two or three CO discs have an unusually low infrared excess (≤0.01), implying the shielding disc is physically very thin (≤1°).Around 50 per cent of the detected line profiles are double-peaked, while many of the rest have significantly broadened lines, attributed to discs in Keplerian rotation. Simple model fits to the line profiles indicate outer radii in the range 30–300 au, larger than found through fitting continuum SEDs, but similar to the sizes of debris discs around main-sequence stars. As many as five have outer radii smaller than the Solar System (50 au), with a further four showing evidence of gas in the disc at radii smaller than 20 au. The outer disc radius is independent of the stellar spectral type (from K through to B9), but there is evidence of a correlation between radius and total dust mass. Also the mean disc size appears to decrease with time: discs around stars of age 3-7 Myr have a mean radius∼210 au, whereas discs of age 7-20 Myr are a factor of three smaller. This shows that a significant mass of gas (at least 2 M⊕) exists beyond the region of planet formation for up to∼7 Myr, and may remain for a further∼10 Myr within this region.The only bona fide debris disc with detected CO is HD9672; this shows a double-peaked CO profile and is the most compact gas disc observed, with a modelled outer radius of 17 au. In the case of HD141569, detailed modelling of the line profile indicates gas may lie in two rings, with radii of 90 and 250 au, similar to the dust structure seen in scattered light and the mid-infrared. In both AB Aur and HD163296 we also find that the sizes of the molecular disc and the dust scattering disc are similar; this suggests that the molecular gas and small dust grains are closely co-located. [ABSTRACT FROM AUTHOR]

ABSTRACT We have observed 22 young stars from the Lindroos sample at 850 μm with SCUBA on the James Clerk Maxwell Telescope to search for evidence of dust discs. Stars in this sample are the less massive companions of B-type primaries and have well-defined ages that are 10– 170 Myr; i.e. they are about to, or have recently arrived on the main sequence. Dust was detected around three of these stars (HD 112412, 74067 and 99803B). The emission around HD 74067 is centrally peaked and is approximately symmetrically distributed out to ∼70 arcsec from the star. This emission arises from either a two-component disc, one circumstellar and the other circumbinary with dust masses of 0.3 and <27 M[sub ⊕], respectively, or an unrelated background object. The other two detections we attribute to circumsecondary discs with masses of 0.04 and 0.3 M[sub ⊕]. We were also able to show that a circumprimary disc is present around HD 112413 with a similar mass to that around the companion HD 112412. Cross-correlation of our sample with the IRAS catalogues only showed evidence for dust emission at 25 and 60 μm toward one star (HD 1438); none of the submillimetre detections were evident in the far-infrared data implying that these discs are cold (>40 K assuming β= 1). Our submillimetre detections are some of the first of dust discs surrounding evolved stars that were not detected by IRAS or ISO and imply that 9–14 per cent of stars could harbour previously undetected dust discs that await discovery in unbiased sub-mm surveys. If these discs are protoplanetary remnants, rather than secondary debris discs, dust lifetime arguments show that they must be devoid of small <0.1 mm grains. Thus it may be possible to determine the origin of these discs from their spectral energy distributions. The low inferred dust masses for this sample support the picture that protoplanetary dust discs are depleted to the levels of the brightest debris discs (∼1 M[sub ⊕]) within 10 Myr, although if the extended emission of HD 74067 is associated with the star, this would indicate that >10 M[sub ⊕] of circumbinary material can persist until ∼60 Myr and would also support the theory that T Tauri discs in binary systems are replenished by circumbinary envelopes. [ABSTRACT FROM AUTHOR]

Extrasolar planetary systems range from hot Jupiters out to icy comet belts more distant than Pluto. We explain this diversity in a model where the mass of solids in the primordial circumstellar disc dictates the outcome. The star retains measures of the initial heavy-element (metal) abundance that can be used to map solid masses on to outcomes, and the frequencies of all classes are correctly predicted. The differing dependences on metallicity for forming massive planets and low-mass cometary bodies are also explained. By extrapolation, around two-thirds of stars have enough solids to form Earth-like planets, and a high rate is supported by the first detections of low-mass exo-planets. [ABSTRACT FROM AUTHOR]