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[Music]

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I'm SIA

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Kazak as' and I'm graduate student at

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Cornell University and part of the new

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Carl Sagan Institute it's very exciting

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we're going to talk about it more and oh

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I don't get a timer like everyone else

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okay cool

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so I'm going to be talking to you guys

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about UV environments of earth-like

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planets orbiting white dwarf

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so the warm-up talked was really good

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thank you for that I'm going to still

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review some of it in case you weren't

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listening or you know it's just fun

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alright so the main my main motivation

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and the main motivation of our group is

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to look for life in the universe and we

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have no idea what kinds of forms that

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life can take but we know that we're

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life so we're going to start with that

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and then maybe branch out from there so

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that means I care about habitable zones

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I care about where liquid water could be

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on the surface of a planet and I care

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about possible bio signatures and one of

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the big goals of the Carl Sagan

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Institute is to sort of build up this

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kind of spectral library for future

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reference so in the future when we can

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actually spectrally characterize

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exoplanet atmospheres maybe we'll know

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what a desert world would look like or a

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water world or an earth-like planet

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orbiting a white door what okay so is

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there a laser on this yes okay so this

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was covered briefly in the warm-up talk

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thank you for that

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so most of a star's life time is when

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it's on the main sequence simply put

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that's just when in the core of the star

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it's fusing hydrogen so we're right here

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most of the star's life time is the main

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sequence so the main sequence is what's

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mainly been studied and it's great my

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whole thesis is everything but the main

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sequence so I'm going to be looking at

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white dwarf tiny but Pierce it's very

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exciting

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so why do we care about planets around

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white dwarfs you might be thinking I've

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never heard of a plan around white door

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that's because none have been found yet

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but there's a lot of evidence for it

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which is why it's very exciting

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and so there is a disc picture in this

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talk too so the majority of white dwarfs

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show evidence of recent accretion of

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rocky bodies in their atmospheres which

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there have even been some papers that

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are able to reconstruct exactly what

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would have had to fall on the white

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dwarf to create that pollution so it

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looks like most white dwarfs have rocky

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bodies accreta on them so maybe not all

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of the planets fall in on them

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also it would be very or people believe

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it would be very easy to detect in a

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transit so like mentioned before a white

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door is about the size of Earth so we're

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using the transit method so we're

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looking at the white dwarf waiting for

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something to dim its brightness if Earth

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passes in front of it it's going to look

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like all of the light and that's a

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really strong signal so there's actually

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a bunch of studies going on looking for

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these white dwarf planets particular k2

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is really excited about it so what I

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care about though because I'm me is if

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these white dwarfs do have planets under

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the best conditions are they suitable

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host for life or will the UV just

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radiate them and if there is life there

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would we be able to detect it so the

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atmospheric composition of these planets

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and taking into account how they would

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interact with the UV profile of the

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white dwarf will help us answer these

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questions so as I'm sure a bunch of you

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know atmospheres of planets are affected

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by so many things so I'm going to pay

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attention to two of them this is what

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I'm going to pay attention to I'm going

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to look at the photochemistry which if

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you're a physicist like me and you that

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word scares you it's just UV photons

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they're really high-energy they're just

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going and they're breaking apart

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molecules and they're changing the

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chemistry of the atmosphere and then I

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care about three different types of UV

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which are pictured here so UVC

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Sandra's u-v-a is not so UVA is actually

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pretty useful it helped power complex

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processes UVB if you've ever had sunburn

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or a tan its EVPs fault it's partially

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shielded by ozone and then UVC is a lot

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worse than sunburn it destroys your DNA

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so that's what I care about the most

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here and it's almost completely shielded

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by Earth's ozone layer so for the sake

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of this short talk I'm going to mainly

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talk about this potential bio signature

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even though there are multiples and

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hopefully we'll hear more about them

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later originally people thought if we're

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looking for indications of life in the

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spectrum of an exoplanet atmosphere

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maybe we just have to look for oxygen

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you're like oxygen that's life but we

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later on realize that oxygen can be

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created by non-biological sources so we

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want to know if it's biological or just

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something else we care about the biology

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so we want to see basically oxygen with

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something that's destroying the oxygen

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so we know the oxygen is being

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continuously created by life so methane

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is the example I'm going to use here it

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is a reducing species it's created by

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termites natural gas all of these things

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that life does and the main chemical

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processes reactions of methane destroy

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oxygen say we see strong oxygen signal

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and on also methane we have a pretty

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good idea that the oxygen is being

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continuously produced some people argue

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with this you could come talk to me

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about it afterwards ok so really briefly

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just so that everyone knows what I'm

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going to be caring about in this talk I

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care a lot about the ozone layer because

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like I said before that's going to

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shield the UV and protect surface life

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so the ozone is created by shorter

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wavelengths in the UV so we care about

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that because of UV shielding and also

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important is that ozone fatalis also

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creates hydroxyl Oh H and we care about

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that because

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called the detergent of the atmosphere

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which basically means it reacts with a

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bunch of other species that we care

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about and it depletes them so it does

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fundamentally change the atmosphere like

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I mentioned before I'm going to talk

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mainly about methane if you want to hear

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about all the other species that I care

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about you could come talk to me after or

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you can read my paper get lots of reads

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on 80s

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great so yes lots of methane being

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destroyed by hydroxyl okay so like I

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mentioned before most people have looked

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at the main sequence and I'm looking at

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Wyatt door so how what do I need to take

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into account about a white door the

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spectra are different so there was

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something like this in the warm-up slide

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if you'll notice here so there I have

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the three temperatures I have six

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thousand five thousand four thousand the

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red lines are from main sequence stars

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and the blue ones are from white dwarfs

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and you'll notice that these sort of

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just the white dwarfs are looks like a

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blackbody

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that's because white dwarfs are highly

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differentiated because there's no

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confusion going on so the heavy elements

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sink to the center so if you're looking

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at it you're basically just seeing only

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hydrogen lines and five thousand Kelvin

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and under hydrogen is neutral so you

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don't even have those lines what I

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really care about is the UV so like I

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mentioned before there's the three types

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of UV which I have color-coded here in

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fun traffic light colors for you

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and you'll notice here the blue lines

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are the white door the red main sequence

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for the same temperature so you can see

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that there are definitely differences

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here and also all of these are scaled to

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the solar constant so they're all scaled

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to the one au equivalent to yield

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roughly similar surface temperatures so

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one of the main differences you see here

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is that there's a little extra UV for

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the main sequence there the studio

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chromosphere ik activity so just what's

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going on the atmosphere of the star so

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just looking at all these from the first

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time I looked at it 4,000 Kelvin cases

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most different so I was seeing the

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photochemistry would be most different

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there let's see

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okay so the models that I ran I

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basically put in planets that were the

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same as Earth with the same outgassing

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rates of life and I ran the three

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temperature cases for white dwarfs and

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the main sequence stars

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so why did I pick those temperatures

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there was actually a reason as sunny

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mentioned during the warm-up talk the

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white dwarf is cooling over time so it

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starts off very very hot and cools over

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time and the habitable zone is

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continuously changing so we want to look

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at a time when the habitable zone stays

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in the same place for long enough that

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life can grow in a ball so if you look

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between 6000 and 4000 Kelvin that's

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about 10 billion years right there and

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considering that we are much less than

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10 billion years here on earth I'd say

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that's enough time so that is why pick

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those I didn't pick anything less than

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4000 because white dwarfs that cool

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don't really exist yet because the

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universe isn't old enough so I try to be

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a little realistic speaking of realistic

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here's the 1d climate code we use it's a

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cup coupled climate photochemistry qey

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basically we it calculates the

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temperature pressure profile looking at

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the major greenhouse gases and it goes

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into the photochemistry code and it uses

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the temperature profile to update the

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chemistry you update the chemistry then

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you update the temperature then you

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update the chemistry and you just sort

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of go back and forth until it's reached

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equilibrium and you have the star as an

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input so you get the atmosphere to reach

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equilibrium for that specific star for

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the results I know that different people

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learn differently so I have a graph and

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then on the next slide I also have it in

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words if you hate graphs so here really

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quick just hitting on the highlights we

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have each row is a different temperature

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at just at first glance you can see the

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4000 Kelvin case is the most different

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as we all predict it together

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so there's actually an excess of ozone a

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little bit in each case which actually

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caused more hydroxyl to be created

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and that caused more depletion of

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methane so there was more ozone because

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there's more UV at specific wavelengths

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which caused more hydroxyl which caused

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less methane and then I put in water

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because we all care about water and

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you'll see there is the difference

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mainly in the 4000 Kelvin case because

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at the wavelengths that water is

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breaking a broken apart by photolysis

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there was a lot more of UV flux at that

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wavelength for the white dwarf then the

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main sequence star so in words basically

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a lot of the white dwarf planets

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compared to the main sequence star at

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the wavelengths where ozone is created

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there was more ozone which created more

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hydroxyl in the hydroxyl depleted the

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methane and also I didn't talk about

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these but I put them in for fun more

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because of the increased hydroxyl there

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was more co2 which is very green house

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of gas and then more methyl chloride and

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nitrous oxide which are other biological

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species and real quick here's an idea of

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what the UV to the ground looks like for

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reference we are roughly like this 6,000

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Kelvin main sequence star right here you

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might notice that ozone is most

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efficient shielding at around 260

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nanometers which is why there let's

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shape so you could see that the stars

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with less UV flux the cooler ones are

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actually they have more UV at the

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surface harmful UV and that's because

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there's less ozone so this is morally

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just a general comment for stars but if

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you're going to compare white dwarf

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planets to main-sequence planets the UV

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to the ground is comparable so we don't

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have to worry about surface life being

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irradiated so in summary we have to

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relay with white dwarfs think about the

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changing habitable zone as the white

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dwarf cools the lack of chromis Pyrrhic

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activity causes changes in the

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photochemistry the UVC shielding is

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comparable so we don't have to worry

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about DNA damage and also with the

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hydroxyl there could be depletion of

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potential

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I owe signatures for the white dwarfs so

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you know if there is life there it might

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be more difficult to find it so a paper

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I'm writing right now is creating

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planetary spectra for these models so I

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will be doing an analysis of the bio

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signature deductions oh thank you hi

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that was really neat talk thank you have

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a quick question regarding the white

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dwarfs and proposing that you would have

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a habitable planet how do you what's the

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scenario you can envision in terms of

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having the raw material hydrated

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material that would avaible would be

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able to survive the transition to white

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dwarf

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through the red giant expansion phase

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that's a very good question my adviser

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says officially I'm not supposed to

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comment on this but because we just care

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about the modelling but as I mentioned

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earlier that there are a lot of disks

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that sort of resemble protoplanetary

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discs found their own white dwarfs so

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there could be some sort of second wave

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planet formation within late water

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delivery thank you that was an

339
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absolutely fantastic talk oh thank you

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so my question is and maybe this is

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embedded somewhere in the modeling but

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so you mentioned that obviously the

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epital zone is highly dependent on the

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rate of cooling you also mentioned that

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do do these photochemical effects you're

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getting a lot of carbon dioxide which as

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you mentioned is a greenhouse gas yeah

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therefore would that nuts potentially be

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conducive to having a habitable

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temperature range on our planet for a

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longer period of time just because you

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have all that co2 to keep it warm even

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as the dwarf cools yeah oh that's very

354
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interesting yes it possibly could that

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00:15:05,369 --> 00:15:02,569
was yes thank you totally possible okay

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um be my co-author we'll do it together

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yes I might have missed this but um how

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did you decide what your atmospheric

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composition was and is it just like

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starting with like the moderate Earth

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atmospheric composition or yes I'm

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actually also writing a paper on earth

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throughout time around white doors I'm

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just really into white dwarfs yes we

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were using modern earth for this

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as sort of the best case scenario and

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seeing if we could even detect life and

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have life exists starting off from Earth

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okay

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just very stupid question

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and what's carrot what's going to happen

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there - white white doors at the end of

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their lives can they become a planet

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themselves so Wyatt Doris when they cool

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down completely they become black Dorf

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but those don't exist yet because the

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00:15:56,670 --> 00:15:54,110
universe isn't old enough so it just

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sort of cools down completely and then

379
00:16:03,990 --> 00:16:00,670
just there being really dense and very

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00:16:06,490 --> 00:16:04,000
cold and it's very sad

381
00:16:10,840 --> 00:16:06,500
but don't worry they don't exist don't

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00:16:13,960 --> 00:16:12,000
since you're messing with the

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00:16:15,970 --> 00:16:13,970
atmospheric chemistry do you know what's

384
00:16:18,100 --> 00:16:15,980
going to happen to cloud formation and

385
00:16:22,480 --> 00:16:18,110
aerosol formation because that really

386
00:16:24,460 --> 00:16:22,490
affects the temperature and oh yeah so

387
00:16:27,550 --> 00:16:24,470
I'll put in a plug for my officemate

388
00:16:29,769 --> 00:16:27,560
Jack Madden's poster which is eventually

389
00:16:32,680 --> 00:16:29,779
this week where he will talk about those

390
00:16:35,199 --> 00:16:32,690
sorts of effects so yes these were clear

391
00:16:36,970 --> 00:16:35,209
sky models that I use our code does use

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00:16:39,699 --> 00:16:36,980
clouds but this was just sort of to see

393
00:16:42,699 --> 00:16:39,709
even with everything going perfectly

394
00:16:45,730 --> 00:16:42,709
could we still detect life but yes

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00:16:49,150 --> 00:16:45,740
clouds have a big effect so Jack we'll

396
00:16:52,240 --> 00:16:49,160
explain that later hi I might have

397
00:16:53,829 --> 00:16:52,250
missed this but how different is the

398
00:16:57,120 --> 00:16:53,839
radiation environment around white doors

399
00:17:00,310 --> 00:16:57,130
than like M dwarfs which also high UV

400
00:17:02,380 --> 00:17:00,320
yeah so the white dwarfs don't have

401
00:17:05,020 --> 00:17:02,390
flares like the M dwarfs the main

402
00:17:07,809 --> 00:17:05,030
difference between the UV as I showed

403
00:17:10,439 --> 00:17:07,819
before is that the white dwarfs don't

404
00:17:12,790 --> 00:17:10,449
have chromosphere ik activity which

405
00:17:14,500 --> 00:17:12,800
white dwarfs literally looks like black

406
00:17:16,210 --> 00:17:14,510
bodies whereas if you have a

407
00:17:17,770 --> 00:17:16,220
main-sequence star with chromosphere ik

408
00:17:20,049 --> 00:17:17,780
activity there's a black body but

409
00:17:22,870 --> 00:17:20,059
there's also a bit at the end tacked on

410
00:17:29,320 --> 00:17:22,880
from the chromosphere ik activity so

411
00:17:35,950 --> 00:17:33,850
one last quick question nabis for fast

412
00:17:37,659 --> 00:17:35,960
have you thought about maybe doing some

413
00:17:39,779 --> 00:17:37,669
sort of bench work looking at

414
00:17:42,159 --> 00:17:39,789
polymerization under different UV

415
00:17:45,190 --> 00:17:42,169
radiation environments for example if

416
00:17:48,820 --> 00:17:45,200
you have a more lean starting chemistry

417
00:17:51,220 --> 00:17:48,830
for a white dwarf scenario I haven't but

418
00:17:53,139 --> 00:17:51,230
that's a good idea I'm open to lots of

419
00:17:55,840 --> 00:17:53,149
different things I have like 30 years of

420
00:17:58,330 --> 00:17:55,850
grad school yeah left I figure so I'm

421
00:18:01,390 --> 00:17:58,340
gonna do it all and on that high note