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Register a new userThe newborn planet population emerging from ring-like structures in discs
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ALMA has observed a plethora of ring-like structures in planet forming discs at distances of 10-100 au from their host star. Although several mechanisms have been invoked to explain the origin of such rings, a common explanation is that they trace new-born planets. Under the planetary hypothesis, a natural question is how to reconcile the apparently high frequency of gap-carving planets at 10-100 au with the paucity of Jupiter mass planets observed around main sequence stars at those separations. Here, we provide an analysis of the new-born planet population emerging from observations of gaps in discs, under the assumption that the observed gaps are due to planets. We use a simple estimate of the planet mass based on the gap morphology, and apply it to a sample of gaps recently obtained by us in a survey of Taurus with ALMA. We also include additional data from recent published surveys, thus analysing the largest gap sample to date, for a total of 48 gaps. The properties of the purported planets occupy a distinctively different region of parameter space with respect to the known exo-planet population, currently not accessible through planet finding methods. Thus, no discrepancy in the mass and radius distribution of the two populations can be claimed at this stage. We show that the mass of the inferred planets conforms to the theoretically expected trend for the minimum planet mass needed to carve a dust gap. Finally, we estimate the separation and mass of the putative planets after accounting for migration and accretion, for a range of evolutionary times, finding a good match with the distribution of cold Jupiters.
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We study the structure of passively heated disks around T Tauri and Herbig Ae stars, and present a vectorized Monte Carlo dust radiative transfer model of protoplanetary disks. The vectorization provides a speed up factor of 100 when compared to a scalar version of the code. Disks are composed of either fluffy carbon and silicate grains of various sizes or dust of the diffuse ISM. The IR emission and the midplane temperature derived by the MC method differ from models where the radiative transfer is solved in slab geometry of small ring segments. In the MC treatment, dusty halos above the disks are considered. Halos lead to an enhanced IR emission and warmer midplane temperature than do pure disks. Under the assumption of hydrostatic equilibrium we find that the disk in the inner rim puffs up, followed by a shadowed region. The shadow reduces the temperature of the midplane and decreases the height of the extinction layer of the disk. It can be seen as a gap in the disk unless the surface is again exposed to direct stellar radiation. There the disk puffs up a second time, a third time and so forth. Therefore several gaps and ring-like structures are present in the disk surface and appear in emission images. They result from shadows in the disks and are present without the need to postulate the existence of any companion or planet. As compared to Herbig Ae stars, such gaps and ring-like structures are more pronounced in regions of terrestrial planets around T Tauri stars.
While studies of gas-phase metallicity gradients in disc galaxies are common, very little has been done in the acquisition of stellar abundance gradients in the same regions. We present here a comparative study of the stellar metallicity and age distributions in a sample of 62 nearly face-on, spiral galaxies with and without bars, using data from the CALIFA survey. We measure the slopes of the gradients and study their relation with other properties of the galaxies. We find that the mean stellar age and metallicity gradients in the disc are shallow and negative. Furthermore, when normalized to the effective radius of the disc, the slope of the stellar population gradients does not correlate with the mass or with the morphological type of the galaxies. Contrary to this, the values of both age and metallicity at $sim$2.5 scale-lengths correlate with the central velocity dispersion in a similar manner to the central values of the bulges, although bulges show, on average, older ages and higher metallicities than the discs. One of the goals of the present paper is to test the theoretical prediction that non-linear coupling between the bar and the spiral arms is an efficient mechanism for producing radial migrations across significant distances within discs. The process of radial migration should flatten the stellar metallicity gradient with time and, therefore, we would expect flatter stellar metallicity gradients in barred galaxies. However, we do not find any difference in the metallicity or age gradients in galaxies with without bars. We discuss possible scenarios that can lead to this absence of difference.
Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar discs and therefore the formation of planets and brown dwarfs. Recent findings suggest that tidal perturbations of typical circumstellar discs due to close encounters may inhibit rather than trigger disc fragmentation and so would seem to rule out planet formation by external tidal stimuli. However, the disc models in these calculations were restricted to disc radii of 40 AU and disc masses below 0.1 M_sun. Here we show that even modest encounters can trigger fragmentation around 100 AU in the sorts of massive (~0.5 M_sun), extended (>=100 AU) discs that are observed around young stars. Tidal perturbation alone can do this, no disc-disc collision is required. We also show that very-low-mass binary systems can form through the interaction of objects in the disc. In our computations, otherwise non-fragmenting massive discs, once perturbed, fragment into several objects between about 0.01 and 0.1 M_sun, i.e., over the whole brown dwarf mass range. Typically these orbit on highly eccentric orbits or are even ejected. While probably not suitable for the formation of Jupiter- or Neptune-type planets, our scenario provides a possible formation mechanism for brown dwarfs and very massive planets which, interestingly, leads to a mass distribution consistent with the canonical substellar IMF. As a minor outcome, a possible explanation for the origin of misaligned extrasolar planetary systems is discussed.
We present the first detailed spatio-kinematical analysis and modelling of the Southern planetary nebula SuWt 2. This object presents a problem for current theories of planetary nebula formation and evolution, as it is not known to contain a central post-main sequence star. Deep narrowband [NII]6584 images reveal the presence of faint bipolar lobes emanating fromthe edges of the nebular ring. Longslit observations of the H-alpha and [NII]6584 emission lines were obtained using EMMI on the 3.6-m ESO-NTT. The spectra reveal the nebular morphology as a bright torus encircling the waist of an extended bipolar structure. By deprojection, the inclination of the ring is found to be 68$degr$ $pm$ 2$degr$ (c.f. ~90$degr$ for the double A-type binary believed to lie at the centre of the nebula), and the ring expansion velocity is found to be 28 km/s. Our findings are discussed with relation to possible formation scenarios for SuWt 2. Through comparison of the nebular heliocentric systemic velocity, found here to be -25 $pm$ 5 km/s, and the heliocentric systemic velocity of the double A-type binary, we conclude that neither component of the binary could have been the nebular progenitor. However, we are unable to rule out the presence of a third component to the system, which would have been the nebula progenitor.
The high rate of planet detection among solar-type stars argues that planet formation is common. It is also generally assumed that planets form in protoplanetary discs like those observed in nearby star forming regions. On what timescale does the transformation from discs to planets occur? Here we show that current inventories of planets and protoplanetary discs are sensitive enough to place basic constraints on the timescale and efficiency of the planet formation process. A comparison of planet detection statistics and the measured solid reservoirs in T Tauri discs suggests that planet formation is likely already underway at the few Myr age of the discs in Taurus-Auriga, with a large fraction of solids having been converted into large objects with low millimeter opacity and/or sequestered at small disc radii where they are difficult to detect at millimeter wavelengths.
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