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2020-01-08 – Live at AAS - Page 1 of 20 NASA’s Universe of Learning - 2020-01-08 – Live at AAS - Page 1 of 20 Dr. Edward Guinan 0:00 [Slide 7] This gives you a distribution, stellar distribution, of stars with spectral type within 10 parsecs, about 33 light years. And we see that M dwarfs dominate with 73% of all stars. K stars are at 44, not 44%, but 13%. And G stars, main sequence G stars are only 4%. So, G stars are rather rare, when it comes to stars in general. Dr. Edward Guinan 0:33 [Slide 8] Next, please. This plays a role. This is a plot of evolution. It's a luminosity of the star in log scale versus stellar age in billions of years. And we see like an F star, a star like 1.4 solar masses, doesn't live very long. A couple billion years. The sun has a longer time to live, up to about 10 billion years. And its luminosity changes, it gets brighter as the nuclear reaction rates increase, about six and a half to 7% per billion years. What I want you to look at is that the Sun changes. We're now in the inner edge of what's called the habitable zone of the Sun. In a half a billion to 2 billion years, we're no longer in the habitable zone. The habitable Zone is where you can have liquid water; we lose that. So, at the bottom there is K stars [which] evolve at 2% increase in luminosity per billion years. And M stars don't change at all. They live a trillion years; that's pointed out here. What should be pointed out is that by the time the Sun ends its life, and becomes a red giant in 10 billion years. A K star has only changed 20% in its luminosity. So, this is like leaning toward what our premise is, is that K stars make better long-term hosts for life. Dr. Edward Guinan 2:06 [Slide 9] Next slide. These are our scientific slides. We'll just slew through these pretty fast. This is by our student, Casey Purcell, you see her poster here. It shows that the period of rotation of stars, K stars in this case, decrease or increase with age; they spin down. So, when they're young, which is shown at near the 0 to 1 billion years, they're spinning two days, three days per - rotation period of two or three days. At the Sun's age, they're up to 37. And as time goes on, they get into like 50 days. That has an impact. We use this diagram that any star you find in the sky, like a star that's discovered with a planet, and is isolated. You can put it into this diagram, you put the rotation in this diagram, and You can get the approximate age of the star. So, this is called gyro-chronology. And this diagram is used for that. We show the Sun in there, the Sun is sticking right in that area, we're 40 Eridani is. It's 25 days. So, this is the anglular momentum loss over time. Dr. Edward Guinan 3:19 [Slide 10] Next slide. This gets more complex, but what I want to take quick takeaway here is that when the stars spin fast, they have strong dynamos, magnetic dynamos. So, they have large X-ray emissions and ultraviolet missions shown in the right slide. And as time goes on, these emissions slowly die out. What's to be taken here is that the Sun, for example, if you take the Sun, it was when it was young, its X-ray emissions were 500 times stronger than today. Its ultraviolet emissions were approximately five times stronger than today. This is for K stars, but solar stars have similar things. NASA’s Universe of Learning - 2020-01-08 – Live at AAS - Page 2 of 20 Dr. Edward Guinan 4:03 [Slide 11] Next slide. This is the winds of the star. Winds because they're driven by magnetic phenomena. So, you have young stars that are spinning fast, they have strong winds. In the case of K stars, it's of the order of the 30 times higher than the present day. Dr. Edward Guinan 4:21 [Slide 12] Next please. This shows you why we're doing this. It turns out that ultraviolet and X- ray radiation and winds that the planet receives has a major effect on the atmosphere of the hosted planet. Far ultraviolet radiation breaks up, dissociates, molecules. I gave an example of water being broken up into hydrogen and oxygen. The X-ray photoionize; they kick out the electrons of these atoms, making ions. And the solar wind, if this planet is not protected by a strong magnetic field, which this planet we show here isn't, the solar winds come in and they pick up the plasmas and drag them off into space. In other words, the atmosphere's evaporated. So, the thing that happens here is that the planet needs a robust geomagnetic field, magnetic field, to prevent the loss of its water inventories and atmosphere. In our own solar system. The only planet, terrestrial planet, with a significant magnetosphere, magnetic field, is the Earth. Mars doesn't have one now, it had one earlier, but it's gone. Venus had one earlier, gone. Mercury had one earlier, gone. The only planet of those planets with life is Earth because it was protected from the Sun's X-ray, UV radiation winds. So that's why we're here. So, we've survived. And I think we're going to turn this over to Scott who can Know the last part of this. So here we go. Next slide. I guess. Dr. Scott Engle 6:03 [Slide 13] Thank you very much. On this slide, now, as Ed had previously mentioned, we started off studying G type, or solar stars, because obviously, we are orbiting the Sun, so it's proven itself to host habitable planets. But what does all this mean? So, starting off studying G type stars, and moved on to M dwarfs because there are so numerous. As we eventually realiZed, M stars are so much less luminous than the Sun, the planet has to be close to them in order to receive enough light to be warm enough. But they are almost equal X-ray luminosity and ultraviolet luminosity to the Sun. So now we move on to some actual examples. If you're that close to a K star and to an M star, as you are to a solar type G star, well what are the implications for the X-ray radiation, for the ultraviolet radiation, that you're receiving? So, in this plot, you see some concrete examples of different stars. You see the Sun, you see a planet in orbit around Tau Ceti, Alpha Centauri B, 61 Cygni A, and so on so on. So, the example X-ray irradiances that they would receive relative to Earth equivalent distances. So, if you were receiving the same bolometric radiation, bolometric irradiance that you would receive at the Earth around the Sun, now what's the X-ray radiation you're receiving? If you move on from G type into the K's, everything is pretty much fine. You go up to a few times up to 10 times the X- ray radiation. As you go into the early M dwarfs now, you're up to almost 80 times the X-ray radiation. The planet orbiting Proxima Centauri is receiving 350 or so times the X-ray radiation. The planet orbiting within the "habitable Zone," around Trappist 1. As you can see in the text NASA’s Universe of Learning - 2020-01-08 – Live at AAS - Page 3 of 20 there, it's receiving around 1630 times the X-ray radiation that the Earth would receive around the Sun. So it's receiving a pretty intense dosage of X-ray radiation. Dr. Scott Engle 8:01 [Slide 14] Can we move on to the next slide, please. So, to sum things up within this slide here, the M stars become tricky. They are incredibly attractive targets for planet searches because they are so numerous. You can see in the column saying their relative abundance. And as previously mentioned, they are the most numerous stars around within all the universe, making up roughly 73% of all stars that we know of. And in the last column, their longevity, they have extremely long lifetimes. But within the X-ray irradiance, because you have to be so close to them, because of their luminosities, you are receiving a large X-ray irradiance, a large ultraviolet irradiance. The big X factor here is the planet itself, because there are so many of them. And because we're finding so many planets around them. Yes, it's very possible that with all the different planetary properties out there, that there will be planets that will have magnetic fields and atmospheres with the correct properties that they'll be able to shield themselves. So, they are still very attractive targets for planetary searches. It all comes down to the planet, and whether or not it can shield itself. But we realized that when we went straight from studying the G type stars at the bottom of this figure to the M dwarfs at the top, that we had simply skipped over the K dwarfs within the middle.
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