Abstract
This paper presents a study of circumstellar debris around Sun-like stars using data from the Herschel DEBRIS Key Programme. DEBRIS is an unbiased survey comprising the nearest ∼90 stars of ...each spectral type A-M. Analysis of the 275 F-K stars shows that excess emission from a debris disc was detected around 47 stars, giving a detection rate of 17.1$^{+2.6}_{-2.3}$ per cent, with lower rates for later spectral types. For each target a blackbody spectrum was fitted to the dust emission to determine its fractional luminosity and temperature. The derived underlying distribution of fractional luminosity versus blackbody radius in the population showed that most detected discs are concentrated at f ∼ 10−5 and at temperatures corresponding to blackbody radii 7–40 au, which scales to ∼40 au for realistic dust properties (similar to the current Kuiper belt). Two outlying populations are also evident; five stars have exceptionally bright emission ( f > 5 × 10−5), and one has unusually hot dust <4 au. The excess emission distributions at all wavelengths were fitted with a steady-state evolution model, showing that these are compatible with all stars being born with a narrow belt that then undergoes collisional grinding. However, the model cannot explain the hot dust systems – likely originating in transient events – and bright emission systems – arising potentially from atypically massive discs or recent stirring. The emission from the present-day Kuiper belt is predicted to be close to the median of the population, suggesting that half of stars have either depleted their Kuiper belts (similar to the Solar system) or had a lower planetesimal formation efficiency.
The Herschel DEBRIS (Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimetre) survey brings us a unique perspective on the study of debris discs around main-sequence A-type stars. ...Bias-free by design, the survey offers a remarkable data set with which to investigate the cold disc properties. The statistical analysis of the 100 and 160 μm data for 86 main-sequence A stars yields a lower than previously found debris disc rate. Considering better than 3σ excess sources, we find a detection rate ≥24 ± 5 per cent at 100 μm which is similar to the debris disc rate around main-sequence F/G/K-spectral type stars. While the 100 and 160 μm excesses slowly decline with time, debris discs with large excesses are found around some of the oldest A stars in our sample, evidence that the debris phenomenon can survive throughout the length of the main sequence (∼1 Gyr). Debris discs are predominantly detected around the youngest and hottest stars in our sample. Stellar properties such as metallicity are found to have no effect on the debris disc incidence. Debris discs are found around A stars in single systems and multiple systems at similar rates. While tight and wide binaries (<1 and >100 au, respectively) host debris discs with a similar frequency and global properties, no intermediate separation debris systems were detected in our sample.
Discs of dusty debris around main-sequence stars indicate fragmentation of orbiting planetesimals, and for a few A-type stars, a gas component is also seen that may come from collisionally released ...volatiles. Here we find the sixth example of a CO-hosting disc, around the ∼30 Myr-old A0-star HD 32997. Two more of these CO-hosting stars, HD 21997 and 49 Cet, have also been imaged in dust with SCUBA-2 within the SCUBA-2 Survey of Nearby Stars project. A census of 27 A-type debris hosts within 125 pc now shows 7/16 detections of carbon-bearing gas within the 5–50 Myr epoch, with no detections in 11 older systems. Such a prolonged period of high fragmentation rates corresponds quite well to the epoch when most of the Earth was assembled from planetesimal collisions. Recent models propose that collisional products can be spatially asymmetric if they originate at one location in the disc, with CO particularly exhibiting this behaviour as it can photodissociate in less than an orbital period. Of the six CO-hosting systems, only β Pic is in clear support of this hypothesis. However, radiative transfer modelling with the ProDiMo code shows that the CO is also hard to explain in a proto-planetary disc context.
This paper confronts a simple analytical model for the steady state evolution of debris disks due to collisions with Spitzer observations of dust around main-sequence A stars. It is assumed that ...every star has a planetesimal belt, the initial mass and radius of which are drawn from distributions. In the model disk mass is constant until the largest planetesimals reach collisional equilibrium, whereupon mass falls unk. We find that the detection statistics and trends seen at 24 and 70 mu m can be fitted well by the model. While there is no need to invoke stochastic evolution or delayed stirring to explain the statistics, a moderate rate of stochastic events is not ruled out. Potentially anomalous systems are identified by a high dust luminosity compared with the maximum permissible in the model (HD 3003, HD 38678, HD 115892, HD 172555); their planetesimals may have unusual properties (high strength or low eccentricity), or this dust could be transient. The overall success of our model, which assumes planetesimals in all belts have the same strength, eccentricity, and maximum size, suggests the outcome of planet formation is reasonably uniform. The distribution of planetesimal belt radii, once corrected for detection bias, follows N(r) proportional to r- super(0.8 plus or minus 0.3) for 3-120 AU. Since belt boundaries may be attributed to unseen planets, this provides a unique constraint on A star planetary systems. It is also shown that P-R drag may sculpt the inner edges of A star disks close to the Spitzer detection threshold (HD 2262, HD 19356, HD 106591, HD 115892). This model can be readily applied to the interpretation of future surveys, and predictions for the upcoming SCUBA-2 survey include that 17% of A star disks should be detectable at 850 mu m.
We present resolved Herschel images of a circumbinary debris disc in the 99 Herculis system. The primary is a late F-type star. The binary orbit is well characterized and we conclude that the disc is ...misaligned with the binary plane. Two different models can explain the observed structure. The first model is a ring of polar orbits that move in a plane perpendicular to the binary pericentre direction. We favour this interpretation because it includes the effect of secular perturbations and the disc can survive for Gyr time-scales. The second model is a misaligned ring. Because there is an ambiguity in the orientation of the ring, which could be reflected in the sky plane, this ring either has near-polar orbits similar to the first model or has a 30° misalignment. The misaligned ring, interpreted as the result of a recent collision, is shown to be implausible from constraints on the collisional and dynamical evolution. Because disc+star systems with separations similar to 99 Herculis should form coplanar, possible formation scenarios involve either a close stellar encounter or binary exchange in the presence of circumstellar and/or circumbinary discs. Discovery and characterization of systems like 99 Herculis will help understand processes that result in planetary system misalignment around both single and multiple stars.
ALMA Cycle 0 and Herschel
1
PACS observations are reported for the prototype, nearest, and brightest example of a dusty and polluted white dwarf, G29-38. These long-wavelength programmes attempted to ...detect an outlying, parent population of bodies at 1–100 au, from which originates the disrupted planetesimal debris that is observed within 0.01 au and which exhibits L
IR/L
* = 0.039. No associated emission sources were detected in any of the data down to L
IR/L
* ∼ 10−4, generally ruling out cold dust masses greater than 1024–1025 g for reasonable grain sizes and properties in orbital regions corresponding to evolved versions of both asteroid and Kuiper belt analogues. Overall, these null detections are consistent with models of long-term collisional evolution in planetesimal discs, and the source regions for the disrupted parent bodies at stars like G29-38 may only be salient in exceptional circumstances, such as a recent instability. A larger sample of polluted white dwarfs, targeted with the full ALMA array, has the potential to unambiguously identify the parent source(s) of their planetary debris.
Context. For over a decade, the structure of the inner cavity in the transition disk of TW Hydrae has been a subject of debate. Modeling the disk with data obtained at different wavelengths has led ...to a variety of proposed disk structures. Rather than being inconsistent, the individual models might point to the different faces of physical processes going on in disks, such as dust growth and planet formation. Aims. Our aim is to investigate the structure of the transition disk again and to find to what extent we can reconcile apparent model differences. Methods. A large set of high-angular-resolution data was collected from near-infrared to centimeter wavelengths. We investigated the existing disk models and established a new self-consistent radiative-transfer model. A genetic fitting algorithm was used to automatize the parameter fitting, and uncertainties were investigated in a Bayesian framework. Results. Simple disk models with a vertical inner rim and a radially homogeneous dust composition from small to large grains cannot reproduce the combined data set. Two modifications are applied to this simple disk model: (1) the inner rim is smoothed by exponentially decreasing the surface density in the inner ~3 AU, and (2) the largest grains (>100 μm) are concentrated towards the inner disk region. Both properties can be linked to fundamental processes that determine the evolution of protoplanetary disks: the shaping by a possible companion and the different regimes of dust-grain growth, respectively. Conclusions. The full interferometric data set from near-infrared to centimeter wavelengths requires a revision of existing models for the TW Hya disk. We present a new model that incorporates the characteristic structures of previous models but deviates in two key aspects: it does not have a sharp edge at 4 AU, and the surface density of large grains differs from that of smaller grains. This is the first successful radiative-transfer-based model for a full set of interferometric data.
Have protoplanetary discs formed planets? Greaves, J. S.; Rice, W. K. M.
Monthly notices of the Royal Astronomical Society,
09/2010, Volume:
407, Issue:
3
Journal Article
Peer reviewed
Open access
It has recently been noted that many discs around T Tauri stars appear to comprise only a few Jupiter masses of gas and dust. Using millimetre surveys of discs within six local star formation ...regions, we confirm this result, and find that only a few per cent of young stars have enough circumstellar material to build gas giant planets, in standard core accretion models. Since the frequency of observed exoplanets is greater than this, there is a ‘missing-mass’ problem. As alternatives to simply adjusting the conversion of dust flux to disc mass, we investigate three other classes of solution. Migration of planets could hypothetically sweep up the disc mass reservoir more efficiently, but trends in multiplanet systems do not support such a model, and theoretical models suggest that the gas accretion time-scale is too short for migration to sweep the disc. Enhanced inner-disc mass reservoirs are possible, agreeing with predictions of disc evolution through self-gravity, but not adding to millimetre dust flux as the inner disc is optically thick. Finally, the incidence of massive discs is shown to be higher at the protostellar stages, Classes 0 and I, where discs substantial enough to form planets via core accretion are abundant enough to match the frequency of exoplanets. Gravitational instability may also operate in the Class 0 epoch, where half the objects have potentially unstable discs of ≳30 per cent of the stellar mass. However, recent calculations indicate that forming gas giants inside 50 au by instability is unlikely, even in such massive discs. Overall, the results presented suggest that the canonically ‘protoplanetary’ discs of Class II T Tauri stars have globally low masses in dust observable at millimetre wavelengths, and conversion to larger bodies (anywhere from small rocks up to planetary cores) must already have occurred.
Ion irradiation has been observed to induce a macroscopic flattening and in-plane shrinkage of graphene sheets without a complete loss of crystallinity. Electron diffraction studies performed during ...simultaneous in-situ ion irradiation have allowed identification of the fluence at which the graphene sheet loses long-range order. This approach has facilitated complementary ex-situ investigations, allowing the first atomic resolution scanning transmission electron microscopy images of ion-irradiation induced graphene defect structures together with quantitative analysis of defect densities using Raman spectroscopy.
We describe results from a survey for J = 3–2 12CO 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.
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