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you

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[Music]

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what I'd like to talk to you guys today

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about is some of the work we did in 2015

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looking at some of these small stars

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that may potentially build up a biotic

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oxygen in their atmospheres and I want

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to take a particular I want to take a

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couple of slides and actually talk about

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Proxima Centauri B so just to orient

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everyone with the big picture I'm going

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to bash on M dwarves for a while so not

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an exhaustive list of the pros and cons

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but potentially M dwarfs have very long

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main sequence lifetimes they have very

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stable habitable zones over long periods

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of time which is great for evolving life

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they are also the most abundant star

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type and they are more likely to have

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rocky planets as some of Robbie's work

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pointed out looking at occurrence rates

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for M dwarves on the con side they have

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very long super luminous pre main

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sequence lifetimes which could

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potentially dehydrate planets or turn

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gas giants into terrestrial planets as

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some of those Roderigo Lugar's work

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showed they may start out dry initially

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to start with just no water at all

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depending on where they form the planets

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in their habitable zones could be

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tidally locked which has a number of

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effects and I'm certainly not going to

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touch on that and flares as we've heard

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several times today could have a very

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detrimental effect on a planet even if

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it does have an ozone shield but one of

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the things that funk Ann and myself

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pointed out in 2014-2015 is that some of

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these planets could potentially build up

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oxygen in their atmospheres to the level

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where we might be able to see it and

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detect it as a false positive so what do

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I mean by a false positive if we look at

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the Earth's history and particularly the

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evolution of the atmospheric oxygen

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budget on the earth the Archaean is

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noted to have very low oxygen and then

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at the goe

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you see a rise of oxygen to between one

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and 10 percent classically this estimate

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has been revised downward from some NOAA

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plan Topsy's work so at most it might be

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0.1% of the present atmospheric level of

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oxygen which is not a lot of oxygen and

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if you think about this from a detection

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perspective and I misses again broad

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brushstrokes I'm sure we're going to

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hear more about this likely

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Sarab Homer if you look at this from

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broad brushstrokes you can detect oxygen

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if it's greater than about 1% of modern

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oxygen you can detect ozone for about

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0.1% of modern oxygen levels and that's

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from a 2003 paper from Antigonus agora

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now that means that there could

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potentially be vast portions of our

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geologic history where we might not see

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life but there could be life happening

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on the surface of that planet as

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mentioned earlier by Nikki and the

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corollary here is that a false-positive

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could be any amount of oxygen that

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exceeds the amount of oxygen we expected

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for Earth's history where oxygen was a

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factor so how are we applying this to

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Proxima Centauri P I want to point out

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first that with any modeling effort it

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is important to state explicitly what

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your underlying assumptions are for that

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model now for our model we're

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considering global redox balances one of

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the the gold standards for the

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self-consistent steady-state of

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atmospheric composition and global redox

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balance is essentially the balance

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between reducing power coming into the

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atmosphere either through volcanic gases

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being introduced in the atmosphere or

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oxidative weathering so that's a sink

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fraction or a net source for hydrogen

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essentially balanced by the escape of

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hydrogen into space and the burial of

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reducing constituents now because my

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planet is lifeless a lot of these things

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go away which makes global redox balance

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a much easier equation to solve and for

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us global reacts balance is satisfied

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when we balance volcanic outgassing by

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the escape of hydrogen to space you can

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certainly make very different

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assumptions about your planet for

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example you can choose not to satisfy

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global reax valve but you must state

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explicitly that you are doing so and has

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a debt it has a very different effect on

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your model results so sitting in

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explicitly as an important

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and what this looks like from a

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schematic level at least for M star

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planets well let me back up it looks

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like this for most terrestrial planets

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but it looks very different in

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application for M star plants is in the

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upper part of the atmosphere you can

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photolyze co2 and you can make CO and O

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2 and stoichiometric quantities and then

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that Co no.2 can flow into the lower

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atmosphere where water vapor photolysis

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catalyzes the recombination of co no

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back into CO 2 and part of the 2015

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paper we outlined that this process is

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fairly slower around M dwarfs and so one

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of the big questions is what happens to

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the ce o-- and the O 2 in the solution

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in the ocean global ocean and they could

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potentially recombine but those aqueous

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reaction rates are relatively unknown so

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that's why there's a big question mark

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down here and so I mentioned before that

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this catalysis is relatively slow in M

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dwarf planets and that's a relic of the

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fact that the spectra that you're seeing

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at the top of the atmosphere is very

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different for solar-type stars as

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compared to M dwarf stars here you can

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see that a sun-like star has very bright

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in the near UV and it's fairly dim in

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the fuv but for M stars particularly

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Proxima Centauri it's essentially flat

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across the UV which gives it a very high

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f UV to n UV ratio and I zoomed in here

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just a little bit so that you can see

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and these are 1 au equivalent so these

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are integrated to be 1360 Watts premier

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squared at the top of the atmosphere

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that's why you're seeing relatively low

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contributions of UV and visible

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radiation from process and it's mostly

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in the infrared where you're getting

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most of the radiation but you can see

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that because it's relatively flatted

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it's a very different character f UV to

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n UV ratio which affects the chemistry

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because a lot of the fatalis is of

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important constituents in the atmosphere

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like co2 and o2 and water vapor happen

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in a relatively narrow UV region

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the 2015 paper pointed out that the fuv

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to nuv ratio was the dominant control

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for the abiotic production of oxygen in

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a terrestrial planetary atmosphere so

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these are 5% co2 cases located at about

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1.3 au equivalent so they're relatively

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far out in the habitable zone relatively

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colder and that decrease in radiation

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allows essentially the buildup of co2

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because we assume that there's a

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carbonate silicate feedback cycle but as

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you see for a sun-like star you get

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relatively low levels of abiotic oxygen

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vanishingly small what we think is can

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contend

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commensurate with what we see in the

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Archaean but as you change the fuv to

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any view ratio it's especially 4k

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dwarves and M dwarves you see that the

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amount of oxygen you produce from co2

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photolysis is actually on the order of a

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few percent in some cases and the new

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result here is that Proxima Centauri

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plots off the plot which is never

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encouraging and so you get up to about

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15% oxygen from a co2 dominated

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atmosphere in this case and this is a

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very limited part of parameter space if

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you include more co2 you get more oxygen

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it's just a consequence of the fact that

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your photo lysing co2 but if you have

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less co2 you will see less oxygen that's

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this bottom panel here where we varied

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the amount of co2 in the atmosphere and

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watched as the oxygen scaled with it and

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so I've put this dashed line here which

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is my threshold for a false positive

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oxygen signal and you can see that for

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large amounts of co2 which is

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commensurate with being located in the

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outer regions of the habitable zone it's

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entirely possible that you could build

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up some oxygen above that threshold but

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for relatively small amounts of co2 that

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oxygen false-positive does go away and

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you can include the effect of other

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reducing gases like more hydrogen

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methane ammonia outgassing for example

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which is not really a thing on earth but

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could be on other planets composition is

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a thing it actually decreases the amount

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of oxygen you can accumulate in the

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atmosphere but because these planets are

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and it's never easy the likelihood of Co

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runaway which is a consequence of some

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of the modeling we've done has been

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outlined by Kevin Donnelly in the past

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the Co runaway becomes a more

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problematic issue it occurs more often

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especially when you introduce more

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complex chemistry in these cases and so

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what I want you to take away from this

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talk is that with the same volcanic al

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casting and boundary conditions Proxima

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Centauri B behaves much like the other M

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star planets in that it can accumulate

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oxygen potentially above this threshold

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that we've assumed for a false positive

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from Earth's history but what you should

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note is that it also is dependent on the

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the UV spectrum of the hosts are for so

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for F and G and K stars they don't

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really show the same sort of behavior so

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for much of the planets that we may

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observe in the far future it's entirely

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likely that this mechanism is not going

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to be a large source of uncertainty in

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the amount of oxygen we might detect and

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this result also is a fairly dependent

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on which modelling group has contributed

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these results and so we are currently

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well underway in a model enter

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comparison to try to route out exactly

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what's driving the differences between

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whether or not you generate abiotic

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oxygen and so with that I'd like to take

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we have we have time for two to three

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questions here we are on time our

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speakers are awesome go to micron I was

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precisely on time thank you yeah let me

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repeat on NASA Goddard Space Flight

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Center yeah so the here you discussed

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the effect of the far UV and the near UV

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of photochemistry and - atmospheres but

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the we find that deflect of XUV EUV an

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x-ray are much more important especially

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when it comes to photo ionization

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because the process that you discover

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that they described is only works when

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everything is neutral and nice however

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it's not the case and we found and I

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don't know you didn't mention our paper

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when we will discuss the approxima be

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there atmospheric was escape due to the

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iron escape mechanism which we find very

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very important especially when you have

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a high high fluxes the energy fluxes and

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in EU V which is basically the case for

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a lot of Emsworth

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we're breaking a with exception of

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probably couple of very very weakly

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that's one thing the second is magnetic

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field we know that the Proxima Centauri

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has a magnetic field three hundred times

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stronger than our Sun so that means that

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at the distance of 0.05 the magnetic

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field the BZ component will be

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comparable to the magnetic field of

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Earth which will destroy the magnetic

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field so you

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so the megiddo steric effect come there

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become extremely important if not

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defining in the the whole climate models

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were on the assumption that everything

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is nice and soft so that will change the

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whole dynamics I totally agree with your

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opinion about the x-ray being a large

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component that and magnetic fields are

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very important but we do have some

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evidence that there are M star planets

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like the trampa system that are

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compatible with being a few percent

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water and if you've completely destroyed

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your atmosphere by drag offs and where's

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the water component so it seems like

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there's a disconnect there between what

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we're just seeing in nature and what

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we're modeling which is I agree a big

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issue we will have

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to go back to the next question if maybe

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we can continue it right there again the

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show some of the results of simulations

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for trapeze yeah after much shorter

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question your fuv is it is it dominated

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by Diamond alpha absolutely and then in

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that case the cross-sections of

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different molecules will be different

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right

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so yes the that also disconnect is

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because we're assuming that it's an

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average absorption across a wave band so

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that that is something that we can do

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better in the future and it is also

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pressured a payment yes yeah yeah okay

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we have noble questions all right so

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we'll go to the next speaker