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[Music]

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hi my name is Reyes uh and I'm a zero

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three year grad student at Cal Tech I'll

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be starting there in the fall I did most

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of this stuff at Columbia um and like

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Brandon said I'm going to be talking

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about predicting complex organic

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molecule emission from the TW hydro

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protoplanetary disc which is this eye of

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Sauron like thing in the center of the

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slide um so that's a lot and I'm going

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to try to break this project down into

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little pieces so that hopefully you can

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take something away from it starting

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with this um you know roomful of astro

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chemists or astrobiologists rather uh

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why do you all care about what a

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protoplanetary disk is well I wrote this

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talk before realizing that everyone was

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going to talk about this so bear with me

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while I go through the introduction

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again um but I'm going to try to pay a

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little bit more attention to why they

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might be asked about logically

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significant so as astrobiologists we are

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in some way or another all interested in

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life in the universe right as we know it

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it's a planetary phenomenon we obviously

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know about life on Earth we had a great

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talk yesterday about habitable zones and

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bio signatures on in exoplanetary

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systems so our mindset is sort of a

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planetary one and that brings to light

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an interesting question is there

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anything inherent within the process of

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how these planets form that seeds these

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planetary systems with the materials

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necessary for life and by materials

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necessary for life I mean complex

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organic molecules as has been talked

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about at length by like everyone else um

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so that kind of behooves us to look at

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the process of how stars and planets

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form and see if there are any

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interesting chemical environments that

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we can investigate so yeah you've seen

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this picture before you start off with a

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large cold cloud of gas and dust in the

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interstellar medium parts of this cloud

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can become over dense and get dense or

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denser and hotter and hotter as they

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create more material thus forming a baby

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star or a protostar um but this process

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isn't rotationally static um the

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accreting material has angular momentum

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and so to conserve it what ends up

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forming around the protostar is a disk

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of that same gas and dust called the

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proto planet

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disk and it's so-called because the

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material in that disk is what ultimately

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ends up being the stuff that forms

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planetary systems so in some way you can

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kind of think about um the

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protoplanetary disk is setting the

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chemical inventory for the material

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available in this form and planetary

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system which makes it really interesting

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and important to study so let's do that

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um before I jump into explaining this

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cartoon of a disk I want to reiterate

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something that's been talked at length

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before there's kind of two ways we can

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think about forming complex organic

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molecules one way is in the gas phase

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where you have you know two body gas

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phase collisions or another way is on

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icy grant surfaces right keep that in

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mind and as was mentioned twice before I

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think we can't really detect complex

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organics in the ice phase unless the

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source is backlit so we really can only

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detect things in the gas phase so with

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that in mind let's look at the different

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parts of this disc and see where the

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complex organic species might be

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residing so in this sort of red region

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up here the disc atmosphere is kind of

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hot so there's not a lot of gas phase

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canvas not a lot of ice phase chemistry

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happening um and there's a lot of

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radiation so even if you know complex

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organics do form they're oftentimes

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broken up by radiation into like

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radicals ions other constituent species

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so no dice there right so in this blue

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region the disc mid plane um it is cold

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so you do have the ability to make

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complex organics on ICE's um but in this

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you know cartoon scenario sometimes

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there's not enough radiation that comes

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in and pops those things off of the ices

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for us to observe so this is kind of

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like you know in this situation a

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Goldilocks thing right it's up here it's

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too hot down here it's too cold so

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somewhere in the middle in this yellow

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region there's just the right amount of

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heat and radiation to have these

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molecules form in the ices but come off

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into the gas phase for us to observe

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that's what's called the molecular layer

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of the disk and that's where we expect

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to find molecules like methanol ch3oh

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and methyl cyanide CH dcn now I keep

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saying um observation of these molecules

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I want to take a second and talk about

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how that actually works I'm not a radio

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astronomer but I'll just go through it

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quickly how these molecules emit light

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which are then

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observed by us here on earth so in this

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obviously cartoon situation imagine a

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methanol molecule floating out in space

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we know the rotation of molecules is

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quantized it can be excited to a higher

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rotational state maybe by collision with

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h2 or something it can relax to a lower

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rotational state but only if it's

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accompanied by the release of a photon

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at a very specific wavelength and of

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course if that wavelength is in the

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millimeter/submillimeter regime it can

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be picked up by Alma well not one

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specific photon but you know you can

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imagine if this is an astronomical

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source you can get enough photons to

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produce an observable signal so that's

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kind of how that works but as I said

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before this kind of rotational

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spectroscopy only works for molecules in

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the gas phase right um and I mentioned

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how a lot of these molecules are thought

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to form on ICE's so let's briefly lay

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out how that happens for one example so

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this is kind of a picture of how

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methanol forms um basically you start

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off with the carbon monoxide ice and you

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keep hydrogenating it and hydrogenating

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it on the surface until you end up with

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methanol the different directions of

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arrows suggest that this is not as

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simple process as that but for now we

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can treat it as it is um but even then

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you get methanol on ice right so how do

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we get it off

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one way is thermal desorption if it's

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hot enough the methanol can just come

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off on its own another way is photo

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desorption wherein you have an energetic

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photon that pops it off that way there's

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also reactive desorption which I won't

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get too much into but basically yeah

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this is kind of the status quo right we

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have we feel like we have a good idea of

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how some of these complex organics are

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forming how they come off the grains and

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how they emit light so putting that all

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together if we have all these components

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we should be able to build some kind of

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predictive model for how chemistry

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occurs in these protoplanetary discs and

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that's sort of what i was tasked with

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doing is trying to build such a model

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for one specific protoplanetary disc

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namely the one around the pro to start

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TW Hydra so let me talk about how I

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built that model we started off with a

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physical structure that's sort of been

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pre calculated by another group um if

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you're interested in who did that stuff

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come make friends with me later um but

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what you're looking at is a radial slice

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of the disk so radius on the x-axis

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height on the Y and the color scale

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represents the

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temperature so parameters like gas

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temperature dust temperature density

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ionization field all sorts of stuff have

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basically been pre calculated for us on

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this you know cylindrical isometric grid

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and to this grid we hook on a chemical

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model and if you're unfamiliar with how

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like rate equation based chemical

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modeling works let me like briefly walk

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you through that because I'm sure some

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people don't know what it is so you have

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you know a chemical reaction

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X plus y equals Z you can write down the

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rate for that reaction as sort of an

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ordinary differential equation so the

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rate at which Z is formed is

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proportional to the concentrations of

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the reactants times some kind of rate

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coefficient um so people much smarter

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than I have compiled enormous databases

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of reactions like these in the gas phase

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and on ICE's that occur in these sources

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and so you can imagine that's a like

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ninety four hundred equate equation

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strong system of coupled OD es so you

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need a computer to solve that and once

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you do you get something called like the

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chemical evolution as a function of time

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so in this case you know you could get

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the concentration of Z as a function of

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time um so you pick a suitable time to

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stop at uh and we do that at each grid

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point throughout the disk and doing so

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allows us to kind of see the chemical

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structure of the disk on a large scale

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so what does that look like in practice

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I'm sure people don't really care about

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the methodology um so in practice this

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is what it looks like for methanol again

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you're looking at a radial slice of disk

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so radius on the X height on the Y and

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the color scale is a number density of

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methanol um and I've sort of labeled the

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kind of different phases in which you

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can form gas phase methanol and sort of

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the precursors to the gas phase methanol

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in parentheses so what do we see here we

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see sort of a you know small but

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significant hot gas reservoir in the

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upper disc atmosphere but most of the

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methanol scene is becoming you know that

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same thing that I was talking about with

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things being formed on ice and photo

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desorbing or reactively desorbing off of

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the ice into the gas phase for us to

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observe so great it seems like things

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kind of stack up the way we thought they

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would right so the next step is taking

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the output from this chemical model and

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running it through

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radiative transfer code which I

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definitely don't have time to talk about

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in detail but you can think of it as a

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black body that turns the chemical model

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into a synthetic observation comparable

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to real Alma data so that's what we did

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my adviser was involved in observations

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of methanol on this source like last

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year so she gave me data for that and I

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compared that with my stuff so what

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you're looking at is the disk which is

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actually oriented close to face on

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towards us RA and Dec on the x and y

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axis so you looking at position on the

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sky don't worry about the color scale

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that's like the velocity structure of

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the disk but the black contours are my

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synthetic observations and the green

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contours are effectively the real

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observations that were carried out so

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you might be looking at this image and

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thinking you know for all this talk

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about this like fancy astro chemical

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model that you may like it seems to do a

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pretty bad job and you're right so so

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let's let's try to figure out why what's

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going on here right the first thing you

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00:10:02,730 --> 00:10:00,850
might notice is that the sort of green

275
00:10:05,280 --> 00:10:02,740
contours aren't really super symmetric

276
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about the origin of the disk um and

277
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according to my adviser who is a radio

278
00:10:11,550 --> 00:10:09,340
astronomer not me who is not um this is

279
00:10:13,740 --> 00:10:11,560
due to low signal-to-noise ratio in the

280
00:10:17,520 --> 00:10:13,750
observations so that's not something I

281
00:10:19,050 --> 00:10:17,530
can directly fix the another thing you

282
00:10:21,870 --> 00:10:19,060
might notice is that the black contours

283
00:10:24,450 --> 00:10:21,880
are really extended relative to the

284
00:10:26,160 --> 00:10:24,460
green contours um and you know I'll talk

285
00:10:27,030 --> 00:10:26,170
about that in a second the third thing

286
00:10:29,070 --> 00:10:27,040
which you definitely couldn't have

287
00:10:30,450 --> 00:10:29,080
noticed because I had to artificially

288
00:10:32,670 --> 00:10:30,460
inflate this so that the plot would even

289
00:10:34,560 --> 00:10:32,680
show up is that the black contours are

290
00:10:36,120 --> 00:10:34,570
actually really weak they're about five

291
00:10:37,530 --> 00:10:36,130
times weaker than the observations which

292
00:10:39,960 --> 00:10:37,540
Ron Lewis pointed out yesterday are

293
00:10:41,880 --> 00:10:39,970
already pretty weak to begin with so

294
00:10:43,380 --> 00:10:41,890
what this means in essence is that

295
00:10:45,720 --> 00:10:43,390
there's something missing from the model

296
00:10:47,700 --> 00:10:45,730
that should be producing a more compact

297
00:10:50,280 --> 00:10:47,710
more abundant reservoir of methanol than

298
00:10:53,190 --> 00:10:50,290
we're already than we're seeing um and

299
00:10:55,440 --> 00:10:53,200
we've been looking into mechanisms to

300
00:10:58,020 --> 00:10:55,450
try and figure out why this might be

301
00:10:59,370 --> 00:10:58,030
happening I have if I have time we'll

302
00:11:01,680 --> 00:10:59,380
talk about at the end if not come make

303
00:11:04,070 --> 00:11:01,690
friends with me at lunch um but for

304
00:11:06,210 --> 00:11:04,080
right now this is kind of the status quo

305
00:11:08,250 --> 00:11:06,220
but we can play the same game for other

306
00:11:09,060 --> 00:11:08,260
complex organics right so we did the

307
00:11:10,920 --> 00:11:09,070
same thing for meth

308
00:11:13,920 --> 00:11:10,930
cyanide same physical structure same

309
00:11:15,870 --> 00:11:13,930
chemical model etc and this is what the

310
00:11:16,800 --> 00:11:15,880
chemical model output looks like for

311
00:11:18,630 --> 00:11:16,810
methyl cyanide

312
00:11:20,520 --> 00:11:18,640
and you can see it's like kind of

313
00:11:21,210 --> 00:11:20,530
markedly different than the the methanol

314
00:11:22,830 --> 00:11:21,220
story right

315
00:11:24,920 --> 00:11:22,840
whereas with methanol we had a lot of

316
00:11:27,480 --> 00:11:24,930
stuff forming on Isis and you know for

317
00:11:30,060 --> 00:11:27,490
Devoto desorbing or reactively desorbing

318
00:11:31,890 --> 00:11:30,070
off of the Isis with methyl cyanide you

319
00:11:34,380 --> 00:11:31,900
see that there's a lot of methyl cyanide

320
00:11:35,700 --> 00:11:34,390
gas up here in the disk atmosphere so

321
00:11:38,100 --> 00:11:35,710
there's a lot of methyl cyanide forming

322
00:11:39,540 --> 00:11:38,110
in the gas phase and this is kind of at

323
00:11:40,800 --> 00:11:39,550
odds with that whole like working

324
00:11:43,020 --> 00:11:40,810
hypothesis we had before that

325
00:11:45,240 --> 00:11:43,030
observations of these complex organics

326
00:11:47,490 --> 00:11:45,250
are tracing the molecular layer and the

327
00:11:49,050 --> 00:11:47,500
disk ice structure on because in this

328
00:11:50,670 --> 00:11:49,060
case because methyl cyanide has

329
00:11:54,000 --> 00:11:50,680
increased gas phase pathways to

330
00:11:55,650 --> 00:11:54,010
formation it might according to our

331
00:11:58,560 --> 00:11:55,660
model and might be tracing the disk

332
00:12:02,010 --> 00:11:58,570
atmosphere more so than the you know

333
00:12:04,710 --> 00:12:02,020
cold gas in the disk molecular layer so

334
00:12:06,570 --> 00:12:04,720
the next step from here would be to you

335
00:12:07,500 --> 00:12:06,580
know compare these two observations but

336
00:12:10,290 --> 00:12:07,510
unfortunately we don't have any

337
00:12:13,380 --> 00:12:10,300
published observations of methyl cyanide

338
00:12:17,310 --> 00:12:13,390
and the source yet um but we also did

339
00:12:19,650 --> 00:12:17,320
some some feasibility things studies to

340
00:12:20,370 --> 00:12:19,660
see if if we could do that with Alma in

341
00:12:22,440 --> 00:12:20,380
the next cycle

342
00:12:25,590 --> 00:12:22,450
if you're interested in talking to me

343
00:12:27,840 --> 00:12:25,600
about that later let come find me um I

344
00:12:30,180 --> 00:12:27,850
also have extra slides for that to

345
00:12:32,040 --> 00:12:30,190
5-time but for now I just kind of want

346
00:12:33,810 --> 00:12:32,050
to go back and give a summary what we

347
00:12:35,220 --> 00:12:33,820
talked about so first of all we talked

348
00:12:37,080 --> 00:12:35,230
about how protoplanetary discs are

349
00:12:39,360 --> 00:12:37,090
important environments because they can

350
00:12:42,150 --> 00:12:39,370
set the chemical inventory for forming

351
00:12:43,770 --> 00:12:42,160
planetary systems we then talked about

352
00:12:47,340 --> 00:12:43,780
how complex organic molecules

353
00:12:48,960 --> 00:12:47,350
um can trace uh disk ices in the disk

354
00:12:50,460 --> 00:12:48,970
molecular layer but then we sort of

355
00:12:51,900 --> 00:12:50,470
challenged that by showing the example

356
00:12:54,270 --> 00:12:51,910
of methyl cyanide which might actually

357
00:12:56,430 --> 00:12:54,280
be tracing the disk atmosphere due to

358
00:12:59,300 --> 00:12:56,440
its increased gas phase pathway to

359
00:13:01,650 --> 00:12:59,310
formation that's a mouthful um and

360
00:13:03,540 --> 00:13:01,660
finally we show that even when we do

361
00:13:05,190 --> 00:13:03,550
think complex organics should be tracing

362
00:13:08,190 --> 00:13:05,200
the disk Isis there's an enormous

363
00:13:10,140 --> 00:13:08,200
discrepancy between what our modeling

364
00:13:11,970 --> 00:13:10,150
and what theory says should happen and

365
00:13:14,040 --> 00:13:11,980
what the observations observations

366
00:13:16,320 --> 00:13:14,050
actually show um which is as of yet

367
00:13:19,200 --> 00:13:16,330
unresolved so clearly there's a lot of

368
00:13:20,670 --> 00:13:19,210
work left to be done I have a little bit

369
00:13:22,950 --> 00:13:20,680
of extra extra time actually so I'll

370
00:13:27,570 --> 00:13:22,960
talk a little bit about what we're

371
00:13:31,110 --> 00:13:27,580
what we're looking at for reasons that

372
00:13:32,190 --> 00:13:31,120
this discrepancy might exist so how well

373
00:13:35,850 --> 00:13:32,200
what are ways in which we can produce

374
00:13:37,650 --> 00:13:35,860
compact abundant methanol one way that

375
00:13:39,930 --> 00:13:37,660
we've been considering is called Co

376
00:13:41,610 --> 00:13:39,940
desorption in which you know this

377
00:13:44,820 --> 00:13:41,620
methanol is forming on carbon monoxide

378
00:13:47,010 --> 00:13:44,830
Isis sometimes you know the carbon

379
00:13:48,540 --> 00:13:47,020
monoxide can get the methanol can get

380
00:13:51,480 --> 00:13:48,550
caught up in the carbon monoxide and

381
00:13:52,890 --> 00:13:51,490
thus when the carbon monoxide desorbs it

382
00:13:54,770 --> 00:13:52,900
can carry off a little bit of methanol

383
00:13:59,580 --> 00:13:54,780
which is more volatile on these surfaces

384
00:14:01,500 --> 00:13:59,590
and so we thought that this might give

385
00:14:05,730 --> 00:14:01,510
sort of a boost near the carbon monoxide

386
00:14:07,410 --> 00:14:05,740
snowline um unfortunately we've gotten

387
00:14:08,880 --> 00:14:07,420
laboratory results back about this

388
00:14:12,780 --> 00:14:08,890
process and it doesn't look like it's

389
00:14:14,550 --> 00:14:12,790
actually a thing so um that's that's one

390
00:14:16,320 --> 00:14:14,560
way we were looking at there's other

391
00:14:19,000 --> 00:14:16,330
stuff too but I think I'm running out of

392
00:14:28,700 --> 00:14:19,010
time so I will stop there thank you

393
00:14:34,320 --> 00:14:32,310
hey really nice talk so you said in your

394
00:14:35,850 --> 00:14:34,330
model you're calculating the you're

395
00:14:38,130 --> 00:14:35,860
running the chemical model for every

396
00:14:39,750 --> 00:14:38,140
individual cell in your model disk is

397
00:14:42,120 --> 00:14:39,760
there any crosstalk between the cells

398
00:14:43,350 --> 00:14:42,130
like turbulent mixing or transport or

399
00:14:46,320 --> 00:14:43,360
any of that yeah that's a good question

400
00:14:48,000 --> 00:14:46,330
no there's no okay um and how do you

401
00:14:50,730 --> 00:14:48,010
think that would affect the the results

402
00:14:53,250 --> 00:14:50,740
so yeah there's the the turbulent mixing

403
00:14:57,270 --> 00:14:53,260
thing one thing that we were thinking it

404
00:14:59,700 --> 00:14:57,280
might do is um like there's a the

405
00:15:01,770 --> 00:14:59,710
vertical turbulence settling that in

406
00:15:03,240 --> 00:15:01,780
which like a lot of these species drift

407
00:15:05,070 --> 00:15:03,250
to where it's inwards towards a mid plan

408
00:15:06,360 --> 00:15:05,080
so that might be one way in which

409
00:15:07,920 --> 00:15:06,370
there's also like the inward radial

410
00:15:09,510 --> 00:15:07,930
drift of the dust grains to consider

411
00:15:11,700 --> 00:15:09,520
that's actually one of the other things

412
00:15:14,820 --> 00:15:11,710
we're considering is radial drift but we

413
00:15:24,200 --> 00:15:14,830
haven't done it yet I should say so stay

414
00:15:31,290 --> 00:15:26,820
great talk um if you go back to the

415
00:15:33,030 --> 00:15:31,300
plots of the methanol abundance you plot

416
00:15:36,120 --> 00:15:33,040
that as absolute abundance is that both

417
00:15:38,340 --> 00:15:36,130
gas phase and a surface no it's just gas

418
00:15:40,860 --> 00:15:38,350
it it's just gas phase okay yeah so then

419
00:15:42,360 --> 00:15:40,870
the the hot component up there what is

420
00:15:44,520 --> 00:15:42,370
the pathway that's producing that

421
00:15:46,500 --> 00:15:44,530
because methanol can only form through

422
00:15:49,890 --> 00:15:46,510
grain surface processes right so is that

423
00:15:51,600 --> 00:15:49,900
just through transient sorption and

424
00:15:54,240 --> 00:15:51,610
desorption off of grain surfaces up

425
00:15:57,900 --> 00:15:54,250
there or yeah it's a lot of that and

426
00:15:58,950 --> 00:15:57,910
there's a lot of like so when we were

427
00:16:00,720 --> 00:15:58,960
looking at this there were a lot of

428
00:16:02,850 --> 00:16:00,730
other gas phase processes that will

429
00:16:04,860 --> 00:16:02,860
honestly look kind of suss um and it's

430
00:16:06,600 --> 00:16:04,870
because I don't think the the chemical

431
00:16:08,430 --> 00:16:06,610
Network is super complete at high

432
00:16:12,930 --> 00:16:08,440
temperatures the high temperatures that

433
00:16:18,330 --> 00:16:12,940
are uh sort of in that kind of region so

434
00:16:20,450 --> 00:16:18,340
okay great yeah astro chemists aren't

435
00:16:24,560 --> 00:16:20,460
scary I promise

436
00:16:28,310 --> 00:16:24,570
all right I got one so how how

437
00:16:30,910 --> 00:16:28,320
representative is say the hot core phase

438
00:16:33,770 --> 00:16:30,920
after you run it through the

439
00:16:35,450 --> 00:16:33,780
protoplanetary disk phase how much if I

440
00:16:37,070 --> 00:16:35,460
if I knew the abundance in a hot core

441
00:16:39,830 --> 00:16:37,080
phase and then I ran it through a disk

442
00:16:42,880 --> 00:16:39,840
how badly does the disk scramble the

443
00:16:46,070 --> 00:16:42,890
chemistry that is a really good question

444
00:16:50,210 --> 00:16:46,080
for which I don't have a really good

445
00:16:52,940 --> 00:16:50,220
answer um I think there's evidence that

446
00:16:55,160 --> 00:16:52,950
it does scramble the chemistry some but

447
00:16:56,630 --> 00:16:55,170
not all I think you know there are

448
00:16:58,820 --> 00:16:56,640
people that have shown that a lot of

449
00:17:01,100 --> 00:16:58,830
water is inherited directly from like

450
00:17:02,960 --> 00:17:01,110
the cloud the initial cloud but we

451
00:17:05,990 --> 00:17:02,970
assume for our initial conditions here

452
00:17:08,150 --> 00:17:06,000
is we take a dark cloud model run it to

453
00:17:10,670 --> 00:17:08,160
an appropriate time and then assume that

454
00:17:11,300 --> 00:17:10,680
the disk inherits Isis from the dark

455
00:17:13,280 --> 00:17:11,310
cloud model

456
00:17:15,079 --> 00:17:13,290
um which is obviously not the most

457
00:17:20,960 --> 00:17:15,089
accurate thing to do but if you have a

458
00:17:23,449 --> 00:17:20,970
better solution please let me know ok

459
00:17:24,230 --> 00:17:23,459
any more questions then let's thank the

460
00:17:27,140 --> 00:17:24,240
speaker again

461
00:17:27,640 --> 00:17:27,150
[Applause]