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you

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[Music]

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well thanks to the conference organizers

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and the session chairs for giving me the

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opportunity to talk today so that's some

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of my thesis work so some motivating

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questions for this talk or how to stars

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of different math and composition

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evolved how does this impact the

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location of the habitable zone around

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the star how does the habitable zone

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co-evolved with the star over time and

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how will utilizing this knowledge apply

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in the continued search for potentially

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habitable earth-like planets in the

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future so I know I don't need to you

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know belabor the point that habitable

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zones are complicated concepts but I

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just want to emphasize that for the

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context of this work we're approaching

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habitability or the habitable zone from

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the Astrophysical perspective in terms

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of thinking about how stars of different

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math and temperature how they're

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habitable zones differ and also make the

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point that for different math stars not

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only are the distances of the habitable

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zones variable but they expand outward

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at different rates so the two main

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measurable parameters that I look at our

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math which impacts the rate of hydrogen

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fusion in the star which thus impacts

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the main sequence lifetime of the star

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as well as the composition which affects

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the opacity energy transport within the

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star so something with a star with lower

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opacity for example would have more

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efficient energy escapes and a shorter

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main sequence lifetime than a star of

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equal mass so what I do to model stellar

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evolution is I use Tycho a

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one-dimensional hydrodynamic alcoa which

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outputs stellar surface quantities of

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interest for each time step of the

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evolution so typically temperature and

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luminosity or what we're primarily

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interested in and I've created an online

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database for the evolutionary tracks

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that's available it has an interactive

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interpolation tool as well and primarily

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what we want from these evolutionary

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tracks is to look

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at how the location and lifetime of the

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habitable zone depends on the changes

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over the stars made sequence lifetime as

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it goes so looking at the temperature in

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luminosity you have the HR diagram even

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just for looking at oxygen if we have

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depleted oxygen

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- enriched oxygen compared to the Sun

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and dots you can see that there's quite

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a significant spread in the temperature

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in luminosity outputs for the HR

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diagrams for each mass in our range and

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if we plot the Sun on our tracks you can

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see that this is a good estimate of the

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current Sun temperature and luminosity

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so just believable estimates for what

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we're getting here so the limit

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equations that we use also come from

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Ravi copper Rafi's 2013-2014 papers what

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we're focused on here again the

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temperature and luminosity so we take

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that from Tyco and input this into these

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equations that calculate the habitable

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zone distance at each time step in our

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evolution and this is the program that I

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use that I wrote to do these radii

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calculations are easily upgradeable for

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any particular habitable zone

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prescriptions that you want to use but

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we chose the conservative cases from

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copper abu which are the runaway

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greenhouse and the maximum greenhouse

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cases and that's where total ocean

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evaporation to the point at which

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Raleigh scattering by co2 starts out

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with a greenhouse effect on the outer

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edge so with elemental abundances what

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we're thinking about is that usually

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only iron is measured in stars because

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it's easy there are lots of lines and

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other elements are just assumed to scale

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in the same proportions that are

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observed at the Sun but as in natalie

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Hinkle's 2014 papers especially we see

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that specific chemical abundances can

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vary significantly among stars and also

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previous work by patrick Young has shown

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that oxygen specifically has

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a really strong impact to the stellar

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evolution so my database that I put

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together for the stellar evolution

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tracks in 2015 paper I discussed a grid

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of three hundred and seventy-six models

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that range from 0.5 to 1.2 solar masses

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it has a scaled overall metallicity

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relative to the Sun so just scaling all

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the elements proportionally from point 1

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to 1.5 solar metallicity and then a

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specific oxygen abundance values ranging

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from point four four to two point two

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eight solar oxygen and in my recent

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paper we added additional 528 models so

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that's four carbon magnesium and neon

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and neon is an artificial range here

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just because it's very difficult to

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accurately measure in stars but we want

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to include it even hypothetically

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because it's important for

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considerations into the stellar

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evolution so what we were plotting here

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is how does one distance versus stellar

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age days age and distance and we have

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the inner limits and the outer limits

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here each color represents a different

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elemental abundance so we have oxygen in

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red and purple and you can see that they

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make the most difference versus the

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black line which is solar then orange

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and blue or magnesium and yellow and

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green or carbon and interestingly Carvin

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actually makes less of the difference

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than magnesium even though it's

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relatively more abundant in stars the

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magnesium has a higher opacity per gram

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so it impacts the difference in

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evolution the temperature and luminosity

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outputs and not only do the Hobbit alone

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distances differ with variable

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composition we also see that the

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lifetimes again can be significantly

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truncated for example if we're looking

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at depleted oxygen here versus enriched

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oxygen there's a difference of maybe

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three billion years so it's quite a big

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difference if we plot the earth here you

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can see that Earth is on the inner edge

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of the habitable zone currently however

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for thinking about some sort of

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hypothetical

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X

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planet at the same current time then you

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can see that it's only recently entered

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the habitable zone so up until this

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point it's been what we would maybe

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consider outside and we don't know now

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what implications that would have for

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this planet so generally though we want

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to avoid planets that have only recently

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entered the habitable zone this table is

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showing the fraction of orbital radii

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that do enter after the mid point I want

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to highlight is that up to a third to a

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half of all orbits in the habitable zone

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only become so in the second half of

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additionally a trend that we see is that

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with increasing composition a higher

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percentage of planets are not in the

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habitable zone until the second half of

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the stars main sequence lifetime so this

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prompts us to constrain with a time

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criteria our consideration of the

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habitable zones so I wanted to show this

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it's kind of a complicated figure but

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mostly what we're looking at is distance

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here versus stellar mass and this is for

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all of the 376 grid

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it just basically is meant to illustrate

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that we see a wide range of habitable

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distances based on the stellar

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composition so it can really impact what

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we're seeing in terms of distance based

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on the wide variety of stellar

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okay okay so if we pick out one which is

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just the solar value and we look at what

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we would think of as a continuously

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habitable zone the green shaded region

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here is what we would think of as the CH

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Z 1 which is the where something would

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be continuously habitable for the Stars

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entire main sequence lifetime so you see

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that that's not a very comprehensive way

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to think about it because if we plot

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earth on here that doesn't find earth so

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we we know that Earth is in what we

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would consider the habitable zone so we

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need to be more careful about how we

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define is continuously habitable zone

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instead if we use a 2 billion year

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criteria which is based on obviously

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earth took about 2 billion years to

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produce enough biogenic gases that would

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be detectable with something in the most

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recent decade all survey then we have

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that earth does fall into this range of

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orbits that would be continuously

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habitable for at least two billion years

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so anything within this green range

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would meet that criteria now here's a

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table illustrating that and what I want

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to draw the attention to here is that

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for low mass stars obviously they have a

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very high percent of time that a planet

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would spend in the continuously have 2

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billion year continuously have it alone

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versus the entire main sequence whereas

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something with lower composition at

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higher mass don't have any orbits that

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are continuously habitable for 2 billion

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years just because of the main short

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main sequence lifetime so that could

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help us constrain maybe the types of

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stars we want to focus on as well we can

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provide accurate time evolution of a

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wider array of stars to better evaluate

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individual cases of interest as they

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arise so the current work that I've been

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doing

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is expanding a catalogue to also include

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a grid of M dwarf stars which utilize

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the same range of abundances for oxygen

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carbon magnesium etc except for now we

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have from point one to point four five

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solar masses

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- hopefully incorporate all the interest

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for M dwarf habitability and I'm also

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writing a type of subroutine that will

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be able to estimate the activity versus

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age for each star in our catalog this is

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based on a minimalist couple evolution

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model from Blackmon and O in 2016 and

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I'm also working on keeping the database

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updated it's designed for the

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astrobiology exoplanet communities to

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basically characterize any particular

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system that we're interested in any new

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discoveries that are coming up planetary

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candidates so we can look at the host

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star characterization and try to get an

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idea of what's going on with that so

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with that I will stop I'll take

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questions questions for Amanda so Amanda

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this is really cool and I was curious if

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you had so you said you were going to

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these smaller stars and I was curious if

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you had any intuition as to what the

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essentially certain you start forming

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molecular species in their atmospheres

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and what that might say for your stellar

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effective temperatures and luminosities

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I think that would you mean I mean it's

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just generally lower but are you bring

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like different species could yeah

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differently right so is that going to

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affect your your luminosity of it'll I

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think yeah definitely will but I'm not

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sure I can say anything about specific

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species like how how particular

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molecules might affect but I think

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absolutely that yes

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having having that and at the lower

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effective temperatures will impact the

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stellar evolution but I'm not I'm not

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exactly sure because I don't know how

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each specific species will interact in

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the whole process so

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Thanks I guess the idea that I've heard

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around that is that by increasing the

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amount of absorption towards the red you

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effectively widened a half zone a little

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further out and that especially around M

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dwarfs hey where the light is redder

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right in planetary albedo is smaller at

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those wavelengths I guess that I was

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thinking about you know the minimum age

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or the minimum continuously habitable

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time you know in terms of setting

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constraints on which types of stars

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could absolute habitable planets around

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them yeah we have fun too

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I mean we thought about this you know a

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lot but it would be fun to translate you

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know what if you wanted planets that are

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at least 1 billion years habitable or to

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really into actual mission detection

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parameters because planets in the

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habitable zones around a stars for

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example have a much steeper contrast

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rate rate that you have to reach then to

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detect those than in the half zone

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around a G Star by K star and M star and

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then but then also they're further out

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so it's a little easier in terms of

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interworking angle but it would be fun

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to actually try to map that out in terms

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of contrast interworking angle as well

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as that the actual apparent magnitude of

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the planet you have to get down to four

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specific stars yeah that's really

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interesting I've had discussions about

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kind of doing contour type plots even

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with like different ages or different

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time criteria so like 1 billion years or

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two or five or whatever so that's also

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really interesting at the end one day

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you kind of have to do it star by star

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so right super time-consuming but it has