“The Big Picture”. OU student Vincent Deguin gives us an overview of his PhD

Star and planet formation begin in the interior of molecular clouds. Those relatively dense regions shelter even more denser clump, referred as dense core. Star formation trigger when the density in the clump become high enough to overcome the processes that keep it stable. Thus, most of the material initially present in the cloud will fall into the star and her surroundings leading to a protoplanetary disk.

Animation of the 20 ALMA images of protoplanetary disks from the DSHARP project. Credit: ALMA (ESO/NAOJ/NRAO), Andrews et al.; N. Lira.

The interesting part at this point is that molecular clouds hosts a rich variety of molecules both in the gas and solid phase. Despite the high density of the cores, the number of molecules is not high enough to provide enough encounter between atoms and molecules and account for the high reactivity, derived by the observed molecular richness.

Let’s stop for a moment to discuss solids in space; because the universe is ubiquitously filled with gas (mostly H2), but also tiny dust grains at sizes of microns and bellow. They are produced in shocked event like supernovae and star envelopes and are made of Carbone or Silicate.

In Cold regions, such as molecular clouds are, because their high density, shield their core from the ambient radiations, dust grains can grow an ice layer around them (mostly water or CO) and that is a key feature to understand the diversity of the universe and you will see later the formation of planets.

But let’s have a look to the structure of this ice layer, which is way different than what you can find on earth. Typically, when you grow ice on your freezer, you let the water molecule slowly rearrange themselves from an unordered phase (liquid) to an ordered one (solid).

The story is totally different in space. In cold dark clouds, water ice form by chemical vapor deposition at 10K, involving reaction of O, H, 02, H2, OH whereas in shocked regions as protoplanetary disks, this occur by water vapor deposition. Both roads of production leads to Amorphous Solid Water ices (ASW), a very peculiar type of ice that you can’t grow in your freezer (Actually it can’t even naturally form at Earth surface). Are you wondering why? Well, the phase diagram of water (and all the physicals behavior of water in general) is really complicated. For example, regarding at what Pressure and Temperature conditions you are in your “freezer”, you can form 15 different crystalline ices. (i.e. 15 different long-range arrangement of the water molecules within your ice cube).

But, hidden at the extreme ranges of the phase diagram (i.e. very low Temperature and high Pressure), you can find amorphous ices. They are solid material but lacking a long-range order (liquid shape). They are metastable on Earth because as soon as you bring some energy to the system, it will rearrange into the more stable crystalline state. That will make them difficult to form and study, and this is the purpose of my PhD. Here is why it is of great importance.

So, let’s come back to our interstellar dust grains. When water condenses on their very cold surfaces, they stick to the surface but the energy (i.e. the Temperature) is too low to allow them to rearrange in a crystalline structure. That give to this molecular layer some very interesting properties.

It is very porous, and can trap gases allowing the encounter between different molecules thus enhancing the reactivity. It can also play a role in the sticking properties of dust particles in protoplanetary disks which is of great importance for planet formation.

Indeed, going from tiny micrometer size dust grain to planetesimal and then planet involve a lot of different processes that it is hard to observe, model and reproduce in the lab. We know that at the smaller length scale, dust grain sticks together by Van der Walls forces forming mm to cm pebbles. Once you have got kilometer bodies, gravity is running the process, but that let a big gap between the two that Science must bridge.

Unfortunately for now, telescopes don`t have the resolution to observe dust at this size and models doesn’t succeed to fit for the formation rate of planet that we observe (really?).
That is why we need to get some lab data about the sticking properties of protoplanetary dust grains analogs, to put relevant numbers into the current models.

This is the mission our group is trying to achieve, by both producing, and performing collisions of such protoplanetary analogs.

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