Dark matter subhalos and unidentified sources in the Fermi 3FGL source catalog

If dark matter consists of weakly interacting massive particles (WIMPs), dark matter subhalos in the Milky Way could be detectable as gamma-ray point sources due to WIMP annihilation. In this work, we perform an updated study of the detectability of dark matter subhalos as gamma-ray sources with the Fermi Large Area Telescope (Fermi LAT). We use the results of the Via Lactea II simulation, scaled to the Planck 2015 cosmological parameters, to predict the local dark matter subhalo distribution. Under optimistic assumptions for the WIMP parameters --- a 40 GeV particle annihilating to $b\bar{b}$ with a thermal cross-section, as required to explain the Galactic center GeV excess --- we predict that at most $\sim 10$ subhalos might be present in the third Fermi LAT source catalog (3FGL). This is a smaller number than has been predicted by prior studies, and we discuss the origin of this difference. We also compare our predictions for the detectability of subhalos with the number of subhalo candidate sources in 3FGL, and derive upper limits on the WIMP annihilation cross-section as a function of the particle mass. If a dark matter interpretation could be excluded for all 3FGL sources, our constraints would be competitive with those found by indirect searches using other targets, such as known Milky Way satellite galaxies.Comment: 17 pages, 8 figure

Formation of TRAPPIST-1 and other compact systems

TRAPPIST-1 is a nearby 0.08 M M-star, which was recently found to harbor a planetary system of at least seven Earth-mass planets, all within 0.1 au. The configuration confounds theorists as the planets are not easily explained by either in situ or migration models. In this Paper we present a scenario for the formation and orbital architecture of the TRAPPIST-1 system. In our model, planet formation starts at the H2O iceline, where pebble-size particles -- whose origin is the outer disk -- concentrate to trigger streaming instabilities. After their formation, planetary embryos quickly mature by pebble accretion. Planet growth stalls at Earth masses, where the planet's gravitational feedback on the disk keeps pebbles at bay. Planets are transported by Type I migration to the inner disk, where they stall at the magnetospheric cavity and end up in mean motion resonances. During disk dispersal, the cavity radius expands and the inner-most planets escape resonance. We argue that the model outlined here can also be applied to other compact systems and that the many close-in super-Earth systems are a scaled-up version of TRAPPIST-1. We also hypothesize that few close-in compact systems harbor giant planets at large distances, since they would have stopped the pebble flux from the outer disk.Comment: 8 pages, accepted for publication in A&

A Lagrangian Model for Dust Evolution in Protoplanetary Disks: Formation of Wet and Dry Planetesimals at Different Stellar Masses

We introduce a new Lagrangian smooth-particle method to model the growth and drift of pebbles in protoplanetary disks. The Lagrangian nature of the model makes it especially suited to follow characteristics of individual (groups of) particles, such as their composition. In this work we focus on the water content of solid particles. Planetesimal formation via streaming instability is taken into account, partly based on previous results on streaming instability outside the water snowline that were presented in Schoonenberg & Ormel (2017). We validate our model by reproducing earlier results from the literature and apply our model to steady-state viscous gas disks (with constant gas accretion rate) around stars with different masses. We also present various other models where we explore the effects of pebble accretion, the fragmentation velocity threshold, the global metallicity of the disk, and a time-dependent gas accretion rate. We find that planetesimals preferentially form in a local annulus outside the water snowline, at early times in the lifetime of the disk ($\lesssim$$10^{5} \: \rm{yr}$), when the pebble mass fluxes are high enough to trigger the streaming instability. During this first phase in the planet formation process, the snowline location hardly changes due to slow viscous evolution, and we conclude that assuming a constant gas accretion rate is justified in this first stage. The efficiency of converting the solids reservoir of the disk to planetesimals depends on the location of the water snowline. Cooler disks with a closer-in water snowline are more efficient at producing planetesimals than hotter disks where the water snowline is located further away from the star. Therefore, low-mass stars tend to form planetesimals more efficiently, but any correlation may be overshadowed by the spread in disk properties.Comment: 18 pages, 15 figures, accepted for publication in A&

What pebbles are made of: Interpretation of the V883 Ori disk

Recently, an Atacama Large Millimeter/submillimeter Array (ALMA) observation of the water snow line in the protoplanetary disk around the FU Orionis star V883 Ori was reported. The radial variation of the spectral index at mm-wavelengths around the snow line was interpreted as being due to a pileup of particles interior to the snow line. However, radial transport of solids in the outer disk operates on timescales much longer than the typical timescale of an FU Ori outburst ($10^{1}$--$10^{2}$ yr). Consequently, a steady-state pileup is unlikely. We argue that it is only necessary to consider water evaporation and re-coagulation of silicates to explain the recent ALMA observation of V883 Ori because these processes are short enough to have had their impact since the outburst. Our model requires the inner disk to have already been optically thick before the outburst, and our results suggest that the carbon content of pebbles is low.Comment: Accepted to A&A Letter

De Van Houtenmonumenten. Een reconstructie van de werkwijze van Eelke van Houten (1872-1970)

In the 1930s, as Chief Inspector of the Amsterdam Municipal Building and Housing Department, Eelke van Houten (1872-1970) devoted himself to placing old sculpted gables, salvaged from demolished houses, on newly built houses in Amsterdam’s inner city. He was quite successful in this and continued his efforts even after his retirement, in 1936, with the full support of both the city of Amsterdam and the Planning Authority. Van Houten kept an inventory of all the available gables and made sure that this inventory was distributed among builders, architects and the like, which prompted a lively trade in gables. Van Houten brought together supply and demand in an effort to give all the available gables a new place in the city. Based on certain stylistic qualities that are characteristic for new buildings of the 1930s, the Bureau of Monuments and Archaeology (BMA) has designated 205 inner-city houses as so-called ‘Van Houten Houses’. Recovered documents from Van Houten himself, such as the inventory and a notebook, have revealed though that Van Houten’s actual involvement was limited to only 65 houses. By writing articles in daily newspapers and weekly magazines, in which he advocated his approach, Van Houten stimulated a large-scale following of his method. Some houses with old gables are not on the BMA list because they lack the stylistic characteristics of ‘Van Houten Houses’. The private houses in which Van Houten was personally involved form a heterogeneous group with many differences between the individual houses. There are houses with the stylistic qualities that are characteristic for new buildings of the 1930s, but also traditional houses that are almost indistinguishable from those built in previous centuries and are therefore not known as ‘Van Houten Houses’. The Inspector preferred this last group of traditionally built houses but wasn’t always successful in obtaining the quality he was after. He had to rely on ‘amicable consultations’. Van Houten aimed for the same restorative approach as restoration architect A.A. Kok. He wanted to build in the same way that Kok restored. His method fits in with pre-war views on the preservation of historic buildings, of which A.A. Kok was a prominent representative. Van Houten greatly admired A.A. Kok and closely collaborated with him. Van Houten’s method of placing old gables on newly built houses was an important contribution in the 1930s in preserving Amsterdam’s cityscape, in a time when there was scarcely any protection. In saving hundreds of gables he contributed to the fact that the inner city at first sight looks a lot older than it actually is. A straightforward historiography, characterized by modernist views on ornament, style and ‘progress’, would not do justice to the much more complex reality of Amsterdam