Observation of aerodynamic instability in the flow of a particle stream in a dilute gas

Forming macroscopic solid bodies in circumstellar discs requires local dust concentration levels significantly higher than the mean. Interactions of the dust particles with the gas must serve to augment local particle densities, and facilitate growth past barriers in the metre size range. Amongst a number of mechanisms that can amplify the local density of solids, aerodynamic streaming instability (SI) is one of the most promising. This work tests the physical assumptions of models that lead to SI in protoplanetary disks (PPDs). We conduct laboratory experiments in which we track the three-dimensional motion of spherical solid particles fluidized in a low-pressure, laminar, incompressible, gas stream. The particle sizes span the Stokes-Epstein drag regime transition and the overall dust-to-gas mass density ratio is close to unity. Lambrechts et al. (2016) established the similarity of the laboratory flow to a simplified PPD model flow. We obtain experimental results suggesting an instability due to particle-gas interaction: i) there exist variations in particle concentration in the direction of the mean drag forces; ii) the particles have a tendency to 'catch up' to one another when they are in proximity; iii) particle clumping occurs on very small scales, which implies local enhancements above the background dust-to-gas mass density ratio by factors of several tens; v) the presence of these density enhancements occurs for a mean dust-to-gas mass density ratio approaching or greater than 1; v) we find evidence for collective particle drag reduction when the local particle number density becomes high and when the background gas pressure is high so that the drag is in the continuum regime. The experiments presented here are precedent-setting for observing SI under controlled conditions and may lead to a deeper understanding of how it operates in nature.Comment: Accepted for publication in A&A; abstract abridged for arXi

Optical and Near Infrared Monitoring of the Black-Hole X-ray Binary GX 339-4 During 2002-2010

We present the optical/infra-red lightcurve (O/IR) of the black hole X-ray binary GX 339-4 collected at the SMARTS 1.3m telescope from 2002 to 2010. During this time the source has undergone numerous state transitions including hard-to-soft state transitions when we see large changes in the near-IR flux accompanied by modest changes in optical flux, and three rebrightening events in 2003, 2005 and 2007 after GX 339-4 transitioned from the soft state to the hard. All but one outburst show similar behavior in the X-ray hardness-intensity diagram. We show that the O/IR colors follow two distinct tracks that reflect either the hard or soft X-ray state of the source. Thus, either of these two X-ray states can be inferred from O/IR observations alone. From these correlations we have constructed spectral energy distributions of the soft and hard states. During the hard state, the near-IR data have the same spectral slope as simultaneous radio data when GX 339-4 was in a bright optical state, implying that the near-IR is dominated by a non-thermal source, most likely originating from jets. Non-thermal emission dominates the near-IR bands during the hard state at all but the faintest optical states, and the fraction of non-thermal emission increases with increasing optical brightness. The spectral slope of the optical bands indicate that a heated thermal source is present during both the soft and hard X-ray states, even when GX 339-4 is at its faintest optical state. We have conducted a timing analysis of the light curve for the hard and soft states and find no evidence of a characteristic timescale within the range of 4-230 days.Comment: Accepted for publication in AJ, Table 3 can be viewed at http://www.astro.yale.edu/buxton/GX339

Studies of gas-particle interaction : Implications for the streaming instability in protoplanetary disks

We present the early results from a novel experiment to study a particle-laden flow, under a parameter regime relevant to the conditions in planet-forming systems. We investigate the gas-particle interactions to identify the presence of and details regarding the streaming instability, which is theoretically predicted to aid the coalescence of small dust grains to form planetesimals - the macroscopic objects that will eventually interact gravitationally and become planets. We vary properties of the system such as dust-to-gas ratio, relative particle-gas velocity and gas pressure, for comparison to numerical simulations of protoplanetary disks. Experimentally calibrated numerical calculations of the particle motion within the instability regions will be used to model the evolution of protoplanetary disks at the scale of small dust grains, representing an unprecedented precision in our understanding of these difficult to study systems

The fate of icy pebbles undergoing sublimation in protoplanetary discs

Icy pebbles may play an important role in planet foation close to the water ice line of protoplanetary discs. There, dust coagulation is more efficient and recondensation of vapour on pebbles may enhance their growth outside the ice line. Previous theoretical studies showed that disruption of icy pebbles due to sublimation increases the growth rate of pebbles inside and outside the ice line, by freeing small silicate particles back in the dust reservoir of the disc. However, since planet accretion is dependent on the Stokes number of the accreting pebbles, the growth of planetesimals could be enhanced downstream of the ice line if pebbles are not disrupting upon sublimation. We developed two experimental models of icy pebbles using different silicate dusts, and we exposed them to low-temperature and low-pressure conditions in a vacuum chamber. Increasing the temperature inside the chamber, we studied the conditions in which pebbles are preserved through sublimation without disrupting. We find that small silicate particles (<50 m) and a small quantity of ice (around 15 per cent pebble mass) are optimal conditions for preserving pebbles through sublimation. Furtheore, pebbles with coarse dust distribution (100-300 m) do not disrupt if a small percentage (10-20 per cent mass) of dust grains are smaller than 50 m. Our findings highlight how sublimation is not necessarily causing disruption, and that pebbles seem to survive fast sublimation processes effectively