Research

Debris discs around main sequence stars are belts of planetesimals – asteroids and comets – formed in the protoplanetary discs around young stars. Planetesimals comprise both the building blocks of planets around young stars and the source of dusty debris around older stars. Imaging observations of dust continuum emission and scattered light reveals the location of these planetesimal belts around their host stars. Analysis of debris discs observed at millimetre wavelengths revealed a trend between the discs' radii and the host star luminosities. This trend was tentatively linked to the preferential formation of dust-producing planetesimals near snow lines (specifically CO) in the protoplanetary discs around the host stars. Here we perform a homogeneous analysis of > 100 spatially resolved debris discs observed at far-infrared wavelengths by the Herschel Space Observatory and fit the obtained distribution of radii and widths for these discs with a power law relation with stellar luminosity. We find no evidence of a trend in disc radius as a function of stellar luminosity similar to that identified at millimetre wavelengths. This is attributed to the large uncertainties on disc radius and width inherent in modelling the marginally spatially resolved data. We do see a trend in disc temperature as a function of stellar temperature, consistent with previous findings from analysis of spatially unresolved debris discs.

The debris disc around HD 172555 was recently imaged in near-infrared polarized scattered light by the Very Large Telescope's Spectro-Polarimetric High-contrast Exoplanet REsearch instrument. Here we present optical aperture polarization measurements of HD 172555 by the HIgh Precision Polarimetric Instrument (HIPPI), and its successor HIPPI-2 on the Anglo-Australian Telescope. We seek to refine constraints on the disc's constituent dust grains by combining our polarimetric measurements with available infrared and millimetre photometry to model the scattered light and continuum emission from the disc. We model the disc using the 3D radiative transfer code HYPERION, assuming the orientation and extent of the disc as obtained from the SPHERE observation. After correction for the interstellar medium contribution, our multiwavelength HIPPI/-2 observations (both magnitude and orientation) are consistent with the recent SPHERE polarization measurement with a fractional polarization p = 62.4 ± 5.2 ppm at 722.3 nm, and a position angle θ = 67° ± 3°. The multi-wavelength polarization can be adequately replicated by compact, spherical dust grains (i.e. from Mie theory) that are around 1.2 μm in size, assuming astronomical silicate composition, or 3.9 μm, assuming a composition derived from radiative transfer modelling of the disc. We were thus able to reproduce both the spatially resolved disc emission and polarization with a single grain composition model and size distribution.

HD 105 is a nearby, pre-main-sequence G0 star hosting a moderately bright debris disk (L dust/L ⋆ ∼ 2.6 × 10-4). The star and its surroundings might therefore be considered an analog of the young solar system. We refine the stellar parameters based on an improved Gaia parallax distance and identify it as a pre-main-sequence star with an age of 50 ± 16 Myr. The circumstellar disk was marginally resolved by Herschel/PACS imaging at far-infrared wavelengths. Here, we present an archival ALMA observation at 1.3 mm, revealing the extent and orientation of the disk. We also present Hubble Space Telescope (HST)/NICMOS and VLT/SPHERE near-infrared images, where we recover the disk in scattered light at the ≥ 5-σ level. This was achieved by employing a novel annular averaging technique and is the first time this has been achieved for a disk in scattered light. Simultaneous modeling of the available photometry, disk architecture, and detection in scattered light allow better determination of the disk’s architecture, and dust grain minimum size, composition, and albedo. We measure the dust albedo to lie between 0.19 and 0.06, the lower value being consistent with Edgeworth-Kuiper Belt objects.

∊ 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 ∊ 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 ∊ Eridani's photosphere, and (III) the identification of numerous unresolved sources which could correspond to background dusty star-forming galaxies.

Stars form surrounded by gas- and dust-rich protoplanetary discs. Generally, these discs dissipate over a few (3-10) Myr, leaving a faint tenuous debris disc composed of second-generation dust produced by the attrition of larger bodies formed in the protoplanetary disc. Giant planets detected in radial velocity and transit surveys of main-sequence stars also form within the protoplanetary disc, whilst super-Earths now detectable may form once the gas has dissipated. Our own solar system, with its eight planets and two debris belts, is a prime example of an end state of this process. The Herschel DEBRIS, DUNES, and GT programmes observed 37 exoplanet host stars within 25 pc at 70, 100, and 160 μm with the sensitivity to detect far-infrared excess emission at flux density levels only an order of magnitude greater than that of the solar system's Edgeworth-Kuiper belt. Here we present an analysis of that sample, using it to more accurately determine the (possible) level of dust emission from these exoplanet host stars and thereafter determine the links between the various components of these exoplanetary systems through statistical analysis. We have fitted the flux densities measured from recent Herschel observations with a simple two parameter (Td, LIR/L⋆) black-body model (or to the 3σ upper limits at 100 μm). From this uniform approach we calculated the fractional luminosity, radial extent and dust temperature. We then plotted the calculated dust luminosity or upper limits against the stellar properties, e.g. effective temperature, metallicity, and age, and identified correlations between these parameters. A total of eleven debris discs are identified around the 37 stars in the sample. An incidence of ten cool debris discs around the Sun-like exoplanet host stars (29 ± 9%) is consistent with the detection rate found by DUNES (20.2 ± 2.0%). For the debris disc systems, the dust temperatures range from 20 to 80 K, and fractional luminosities (LIR/L⋆) between 2.4 ×10-6 and 4.1 ×10-4. In the case of non-detections, we calculated typical 3σ upper limits to the dust fractional luminosities of a few ×10-6. We recover the previously identified correlation between stellar metallicity and hot-Jupiter planets in our data set. We find a correlation between the increased presence of dust, lower planet masses, and lower stellar metallicities. This confirms the recently identified correlation between cold debris discs and low-mass planets in the context of planet formation by core accretion.

Dusty debris discs around main sequence stars are thought to be the result of continuous collisional grinding of planetesimals in the system. The majority of these systems are unresolved and analysis of the dust properties is limited by the lack of information regarding the dust location. The Herschel DUNES key program is observing 133 nearby, Sun-like stars (<20 pc, FGK spectral type) in a volume limited survey to constrain the absolute incidence of cold dust around these stars by detection of far infrared excess emission at flux levels comparable to the Edgeworth-Kuiper belt (EKB). We have observed the Sun-like star HD 207129 with Herschel PACS and SPIRE. In all three PACS bands we resolve a ring-like structure consistent with scattered light observations. Using α Boötis as a reference point spread function (PSF), we deconvolved the images, clearly resolving the inner gap in the disc at both 70 and 100 μm. We have resolved the dust-producing planetesimal belt of a debris disc at 100 μm for the first time. We measure the radial profile and fractional luminosity of the disc, and compare the values to those of discs around stars of similar age and/or spectral type, placing this disc in context of other resolved discs observed by Herschel/DUNES.