- All right. My computer clock says 2:00 P.M. Eastern Standard Time, and so I'm happy to introduce Ofer Cohen, another member of the ExoPAG EC, and he will be chairing session two. Ofer.

- Okay, welcome everyone for the second session of today's meeting. We will have four speakers in this session. For all the speakers I'll give you two minutes warning before your time is up. Our first speaker is Tyler Richey-Yowell. Are you there?

- [Tyler] Hi.

- Can you share your screen?

- [Tyler] Yeah. Can you see this.

- Yeah, we're hearing you fine. So Tyler will talk about K dwarf and the ultraviolet imperative for assessing the habitability of planets. So two minutes warning.

- Thank you. Yeah, thank you guys so much for having me. My name is Tyler Richey-Yowell and I am a final year grad student at Arizona State working with Dr. Gina Scoggins. I'm really interested in this question of planet habitability from a stellar perspective, and actually my viewpoint on the potential habitability of planets around K stars has kinda changed a lot during my time in grad school. And so I wanna kinda lead you all through that experience in the findings that I've had over the course of my PhD today. So of course we have to start with the age old saying, "Know thy star, know thy planets," because it really has been shown to be true, especially when we consider M stars and planets around those. So like not only do M stars flare more than our sun, but it's also been recently shown that they exhibit super flares or flares with larger energies than the largest solar events ever recorded on the order of every one to 10 days. And so, although the high energy radiation coming from, or all of this high energy radiation coming from these flares may not actually be good for planetary life. So what happens at these wavelengths is that you can drastically alter your atmospheric chemistry. So here we have a spectrum of a late K star, and you could see that most of the fluxes in the black body peaked. And if we zoom in on the left side of this plot where the UV and the x-rays are, and we can see that there is actually flux there even though it's small. And so the UV will photo dissociate molecules that are necessary for the development of life, such as oxygen or water in signatures that would be indicative that maybe life is there, like methane, ammonia, nitrogen oxide, whereas the x-ray and the extreme UV will ionize the atoms and may cause complete erosion for the atmosphere, so potentially not good if you're trying to live there. So even though these features are quite small relative to the black body peak, they are so quite important. And so we've seen that the M stars do have some disadvantages like the super flares, so what about the next step up, the K stars? Well, in fact, K stars might actually be some of the best candidates for hosts of potentially habitable planets. So Cuntz and Guinan in 2016 determined a parameter that essentially estimates the probability of a certain temperature of star to host habitable planets. So you can see here that this kicks in the late G to early K stars and remains quite high throughout the whole K star regime, even remaining above the probability of a GTV star, like our own hosting a habitable earth size planet, which is what we consider super habitable. So while this does actually peak for the late M stars, all of this severe high energy radiation may actually exclude those types of stars from being good hosts. But what does this high-energy radiation environment look like for K stars? So that's what I worked on when I started grad school. So these are the results from my 2019 paper. So this is UV flux as a function of stellar age and the black and red points here are observations and the triangles are limits. And so the blue diamonds here, what she needs to focus on which represent the median flux per age bin. And so you can see here that there is a region of about constant flux out to maybe about 300 million years before each of these starts to decline slowly. So now I'm gonna take these blue diamonds here and put in the FUV. So these are the exact same points, I'm just plotting them on their own. And the Y axis has been converted from flux density to flux easing and distance proxy just since there were at distances at that time, and so that's why the numbers are a little different now. But here's the FUV median flux for K stars as a function of stellar age. And here's what it looks like when we add in early M stars, by which it mean spectral type and zero to M three. They appear to remain at a constant flux for longer before deeply declining, whereas if we add in the late M stars, the M three to M nine to the fully convective ones, then it actually remains at a constant flux for much longer and doesn't decline much at all. This is because these stars rotate much more quickly, and so they remain active much longer. So these might look pretty different, but you can see that the difference is really only an order of magnitude. So now I'm gonna throw in the near UV and x-ray plots as well. And right about now I'm sure you're probably thinking, "This is a disgusting plot, why would she show that?" But that is exactly what I wanna point out to you guys, that these are all right on top of each other. And so the points here is that the intrinsic UV flux from M and K stars is similar as for what we found in this 2019 paper. But we are focusing on exoplanets here, and so how much of this flux is getting into the habitable zone for those planets? And so what to look at that, we need to know where the habitable zone of each type of star is. So this is the plot of habitables in distance from the star as a function of stellar age. And so you could see that the K stars have a much larger and farther out habitable zones than M stars, which are more narrow and closer into the stars. So if the intrinsic UV flux is the same, then there should be less flux in the habitable zone. And unfortunately, because we're using distance approximates at this time, we couldn't really plot that quantitatively, but there should be less flux in the habitable zone qualitatively for K stars. So after this, we wanted to explore this evolution more through spectroscopy and kinda look at those similar lines to what Edina was talking about earlier. And so I led an HST program with my awesome collaborators at Arizona State, University of Arizona and University of Washington and we were awarded 73 orbits to study K stars of different ages. And so here are three examples of spectra and of K stars at three different ages. So the blue is 45 million years old, orange 625, and then field age green stars. So we can see here that compared to M stars from this paper from Perk Lloyd, the K stars don't appear to evolve at all from 45 to 625 million years. And so as well as comparing the broadband characteristics of the spectra, we also wanted to look at specific spectral lines to see that probe the chromosphere transition region to see how those evolve as well. So this is a lot of plots being thrown up at once, but I wanted you to get a sense of overall what is happening, so that's why I decided to include them all. And so you have surface flux for each line as a function of stellar age. We can see that the K stars or these black lines fit into the observations here actually appear to have a prolonged period of saturation or of constant flux as compared to the M stars which are in red. And so the K stars actually radiate more UV flux at these intermediate ages than do M stars. And well, the evolution for M stars is pretty well known, the age at which K stars actually do begin to decline isn't well constrained at all or isn't constrained at all by this. And so another important factor to look at though is how the UV changes with rotation period. So here I plotted the Rossby number, which is the rotation periods divided by the stellar conducted turnover time for those stars in our samples that had rotation periods. Surprisingly the K dwarfs and the M stars show very similar UV evolution with rotation even though the age was so different, just temporarily. And so when I saw this originally I was like, "What is happening here?" And then last year I went to the double S conference and solve this plot. And so I think that this is actually related to this K dwarf rotation stalling effect seen by Jason Curtis et al in his 2019 and 2020 papers. So this plot shows rotation period as a function of stellar temperature. And so these are all for stars in clusters of different ages. So overlaid are current gyrochronological estimates of where we'd expect the star of that age to fall within this rotation period versus temperature plot. And you can see here that these lines match up pretty well for G dwarfs and that there's a clear rotational evolution from 600 to one billion years for these G dwarfs. However, for the K stars, the rotation periods aren't where we would expect them to be and they appear to not progress at all from 670 million years to 1.4 giga years. And so this is an effect that they're calling K dwarf rotation, little stalling. And so if this is what is causing this, then we know that the UV evolution is stalling for K stars. And since we know that rotation is tied to the magnetic field and therefore the magnetic activity of the star, it's all kind of tied together, so we would expect the evolution of activities to also stall. And if because this is a rotational stalling, we wouldn't expect to see any differences, but in the UV evolution with rotation period and that's kind of exactly what we're seeing. And so my 2019 paper showed that K stars have similar UV flux to M stars, but my HSU work showed that it wasn't, that they had a larger flux. And so I was kind of like, okay, these are two different things, so I did actually decide to go back to that original 2019 paper. And so as I mentioned before, this data was all done in forms of proxies for distance, because we didn't have any Gaia data yet at that time. And so with this new Gaia data, we now have all the distances and we can compare those more precise measurements.

- [Ofer] Tyler, you have to mute.

- Great, thank you. And so I writhed that original project, and now these are the exact plots I showed before, so the UV flux as a function of age. And so these are the median values for each of those groups, again, each of those age bins. And you can see that well, for one, we learned that our distance proxy probably wasn't the best for comparing both the K and M stars, but more importantly this medium flux as a function of age, like there is now a clear distinction between K and M stars and that we do see that particularly in the far UV, this period of constant flux as much longer for K stars. And so with these updated distances, we can now look at the fluxes in the habitable zone and we see that it's right on top of each other. And so there really isn't a big difference in incident UV radiation between a K star and an M star in the habitable zone. So what does this mean for planets around K stars? Well, it means that I probably need to change my title to a affirmative K dwarf advantage to K dwarf advantage? Maybe. And so in conclusion, we see this prolonged period of saturation in K stars compared to M stars and the intrinsic UV flex of K stars is larger for M stars intrinsically, especially by field age, and that the UV flux between K and M stars in the habitable zone is similar. So all of this kinda means that maybe K stars aren't as great of hosts as we thought, but what will really be the determining factor is to study the flare evolution and look at K dwarf flares as a function of age. And a lot of the work that I know has done that in terms of relating the rotation to the age, but now we see those things aren't connected. And so looking at an actual age evolution of the flares is going to be really important and hopefully that's what I'm gonna be working on as a post-doc. Yeah, so thank you and I'll take any questions at this time.

- Okay, thank you very much, right on time. Everyone feel free to provide some virtual applause to our speaker and let's go to the questions. The first one is activity depends on the Rossby number and not only in age. Is the stalling seen when observed against Rossby number?

- Yeah. The stalling is not seen really for the most part as a function of the Rossby number. And so this kind of is why we think that it is this rotational stalling, let's see, this rotational selling here that is causing it. And so like the activity does depend on the rotation, but the rotation is much, much, it's remaining at a higher rotation period or I guess a shorter rotation period for much longer than we would expect or much longer for K stars than for G stars or maybe M stars. We don't quite know yet for the M stars over here. And good question.

- Okay. And let's just take one quick one. How does the K dwarf x-ray flux compare to the M dwarfs?

- Yeah, so that's actually really interesting. And so the K dwarfs do actually, they don't have a long period of saturation in the x-ray, whereas the UV, it is quite much longer. So there is really this big distinction between the two wavelength regimes there. And so the K dwarfs, they start to decline in their x-ray activity much faster, so only after about 40 million years for K stars, whereas for M stars is out to about 150 million years. Yeah.

- Okay, thank you. And we have to move on to our next speaker.