- Okay, well, if there's no further questions. There don't seem to be. I think there's nothing wrong with maybe going ahead a little bit. Our next speaker is John Debes. John, would you be able to go ahead and start? All right, fantastic. So why don't you go ahead and share your slides?

- Okay, can everyone hear me okay?

- Yep, we can hear you. We're seeing your PowerPoint. You can go ahead and share that. And you've got an extra two minutes now, if you want to take advantage of that.

- All right, well, hopefully, I can get everyone out a little bit early. So thank you everyone for your attention and for sticking to the very last session of the day. At least on the East Coast, it's getting a little late, maybe. So I am here to make a pitch for a study analysis group on ExoZodiacal disks. So I'm gonna take a little bit of time to just sort of motivate why we might wanna do that, and then put some draft areas of study out, and then hopefully get some feedback and, at the very least, try to put this forward for acceptance so that we can study this further. So it's actually been almost 10 years since the last time there was a SAG devoted toward this topic. It was actually SAG 1. And there was this great report led by Aki Roberge that talked about how ExoZodis could be a source of confusion or hindrance in direct imaging of exo-Earths. And interestingly enough, even though we've done a lot of studies and learned a lot, both about dust and how to do direct imaging surveys, this is still somewhat of an open question. And that's just because we don't have yet a lot of knowledge about these kinds of dust disks around other stars. And so it's kind of difficult to make predictions when you don't have a lot of ground truth to compare with. And why we think this is a hindrance, and just the two small animations. What I'm showing here on the left is an infinite spatial resolution observatory that is looking at a exozodi ring that's about five times the surface brightness of our own solar system's zodiacal cloud. And it's located about, at 3AU. And there is a 3 times 10 to the -9 contrast ratio companion at 2 1/2 AU. So it's kind of close to the disk. And when the disk is face-on, it's not too much of a problem in your infinite spatial resolution case, but as the forward scattering nature of dust becomes apparent at higher and higher inclinations, it starts to compete with the brightness of the point source, and then it gets even worse when you actually put things behind a telescope with real spatial resolution. And here, I have the CGI hybrid LEO chronograph, PSF, convolved with the scene, and you can see that, one, point sources get impacted much more by a diffraction than an extended source, like a ring. And secondly, you still have the issue that your disk is very, very close to your planet. And those two things mean that, in certain realms of parameter space, it becomes much harder to see a companion with a dust disk present. So we wanna know, how bad is it, and in what cases is it not so bad? And that's, unfortunately, a complex question to answer, which we'd like to answer better with the way things have progressed in the last 10 years. Primarily, the disks that we have detected are, what I would call, cold debris disks. These are disks that are located at 10s of AU. So they're more like scaled up Kuiper belts than zodiacal clouds in our own solar system, and they tend to be much, much more luminous. And so debris disks, people like to use the ratio of infrared luminosity to stellar luminosity as a proxy for optical depth or mass, depending on how you wanna scale things. And so often when we look at the surface brightness of disks as a function of their optical depth or their radius as a function of optical depth, we see that we're only looking at a fraction of parameter space with our existing technology. On the left, I have a sample of STIS-observed, STIS on the Hubble Space Telescope, observed disks and their surface brightnesses as a function of LIR over L-star. You can see there's a broad diversity, and even a diversity at sort of similar optical depths. And they're all much, much brighter than if you just sort of naively scaled up our own solar systems as that zodiacal cloud. And partially, that's because the composition of these disks is probably different, the grain size distribution of these disks is likely different, and the grain shapes are probably different as well. And all of those come into play for determining what kind of visible light surface brightness you might get scattering off these dust grains. And so if we're going to be focusing on directly imaging exo-Earths in visible light, we need to understand how the dust that we see in the infrared scatters light in the visible. And to date, even with cold debris disks, it's very hard to predict what that looks like. And then, if you look on the right, we have a plot of various different disks that have been spatially resolved with Herschel and with ground or space-based high-contrast imaging. And we see there's sort of a wedge of open parameter space where there just haven't been too many detections. And this is an area of opportunity for a variety of techniques, including future direct imaging surveys, ALMA, ground-based, mid-infrared cameras, all these different kinds of things can start to fill things in. So we start to get a better idea of disks that are more like analogs of our own solar system's zodiacal cloud. And what this does is it allows us to, perhaps, build a sample that we can then predict or find some scaling relations from to better understand what we might be facing when we try to directly image exo-Earths. I would say we are on the precipice of a decade where we will actually start to do this much more routinely than instead of having one system here or there being detected. So in the next 10 years, we're gonna see a rapid opening of different techniques and different observatories that will actually finally be capable of getting down to these sort of compact, low-luminosity disks. What I'm plotting here is actually sort of what you could think of as a disk contrast curve. If you ever seen exoplanet contrast curve plots, there's always, for CGI, for example, you can see what's currently being done and how awesome CGI is going to be and then everything in between where you might be able to see something. I've tried to sort of replicate that for disks, where I have some very well-known circumstellar disks that have been spatially resolved in scattered visible light and comparing their surface brightness profiles to sort of the detection limits of STIS and ACS. The two space-based, visible light chronographs that have taken data, either currently or in the past. And I've also plotted the predictions for CGI performance way down as well as sort of a very toy model zodiacal cloud that's 10 times the surface brightness of our own solar system and 50 times the brightness of our own solar system, if it were located at about 10 parsecs. Additionally, I have plotted here estimates of what the scattered visible light could be from the detected HOSTS survey stars. So this is basically just... The way HOSTS detections work, they report a certain number of zodis, and that's literally just scaling up our own solar system's zodiacal cloud's optical depth sort of at a particular radius. And so I've just converted that to the same sort of surface brightness scaled up by the number of zodis. And you can see that some of these detections are quite bright and certainly brighter than what CGI is predicted to do. But you can see why we haven't observed any of these stars with, say, STIS or ground-based extreme adaptive optics. It's because the contrast ratios are pretty severe at small angular radii. But hopefully, in the next 10 years, we can start following these objects up with direct imaging. And I can't say enough that at HOSTS, this NULL interferometry survey for exozodis is really a helpful survey for getting this process started. Okay, so just in the past year or two, there've been a lot of results coming out that sort of hint at future directions that we might wanna go with our study analysis group. And I'll talk about three of them just in the interest of time. The first one is on a paper-

- Hi, my name's John Ziemer. Last name spelled Z-I-E-M-E-R. And I'd like to make an appointment or-

- John, please mute.

- 7 or 9 to get your passports.

- Okay, well, good luck with that. Defrere et al. just put out a paper in 2021 that looked at the star Beta Leo, which is one of the disks that were detected in the HOSTS survey. And they followed that up with L-prime imaging looking for giant planets, and they also did a bit more work trying to characterize what they thought the dust that they had detected with the HOSTS survey might look like. And so they made a prediction that there should be about 55 zodis, plus or minus 10, at around .3 arcseconds. And so this combination of looking for planets, as well as detecting dust in orbit around the stars, is really a preview of what we hope to be doing in the next 10, 20 years. Additionally, people have been doing planet searches in the mid-IR and Alpha Centauri. There are future JWST observations that will be relevant, and certainly ELT mid-infrared imaging will be capable of starting to probe at these kinds of distances and sensitivity levels. Okay, in addition, closer to home, the Juno mission put out a paper, Jorgensen et al., 2020, where they actually directly measured the density of dust impacts on the spacecraft as it was going out to Jupiter and was able to infer a density distribution of dust as a function of radial distance from the sun, which is very exciting. Because we have quite a few solar system probes currently, and some of them are moving around over a large range of radii and they can better inform our detailed understanding of our own zodiacal cloud, which, in turn, helps us to understand what we might be seeing around other planetary architectures. In addition, the Parker Solar Probe has been putting out a lot of papers recently, probing sort of the dust population closer to the sun. New Horizons has also published papers like this as well. So as we put more spacecraft into the solar system, we're going to see more examples of direct detections of interesting features of our own solar system's zodiacal cloud. And finally, Vanessa, earlier, mentioned this paper by Ewan Douglas. I'm a co-author on this as well. We're starting to look forward to these future missions and what they might be capable of. And since we do have at least some understanding of what's possible from the HOSTS survey, we can compare that to what we might be able to do with direct imaging with something like CGI. And so Ewan had this great paper where he looked at the HabEx target list and said, "Okay, if CGI looked at all these stars, what would our average sensitivity to zodis would be? And if zodis have a certain median level, how many systems would we detect? And so doing that for the criterion that we have a signal-to-noise ratio of about five per resolution element, if that's our detection limit, the current best estimates of the CGI performance, assuming that everything goes successfully, we expect to get sort of an average sensitivity or median sensitivity of about 25 zodis for a large number of systems. And if that's sort of the median level of a zodi around a star, given a log-normal distribution, then we can make predictions in this middle plot for how many systems we might detect given the survey. And it ranges. We've had very low zodi levels, to just a handful, all the way out to several dozen. And then, on top of that, the work that Ewan and our coauthors have done, have demonstrated that we can get post-processing of disks at a level that's very similar to what can be done for point sources. So that opens up the possibility that we will not only detect disks, but perhaps characterize them in meaningful ways. So these three little snapshots suggest that we're at a point where it might be good to take a pause, a step back, and really think, what do we need to be focusing on in the next 5 or 10 years until we do the next SAG on this? What we really wanna be focusing on so that we get the most science from these kinds of surveys, but also how do we retire some of these risks related to dust being sort of a contaminant? So this is the basis of the proposal. I would like this SAG to, maybe not do all of these bullet points, but as many as possible, I think will be really helpful for the field. So we'd wanna review the current state of knowledge for zodiacal dust, both in our own solar system and around nearby stars. We'd like to report on the current state of knowledge on the average zodi level of nearby stars. And what we even mean by that, what's a meaningful definition of this so that everyone's sort of on the same page when they make predictions. We want to identify any gaps in knowledge, both for our own solar system's zodiacal cloud, but also in our understanding of the physics of dust and do we understand how dust is scattering light, and what are the best dust grain models to use? We'd also like to identify new methods of detecting disks. What do we mean when we detect a disk at signal-to-noise of five? It's not quite the same as for a point source. There are other considerations to take into account. And at least my take on the community so far is that's not been sort of consistently discussed, or whether there's a common framework that we should be working in when we're talking about the detection of the disk and extended source. And finally, we really wanna get to a point where we can maybe make a prediction or an empirical way of saying, "This is the kind of scattered light that we would expect to see under a certain set of uncertainties or assumptions for an exozodi disk." And finally, we'd like to identify high priority systems that need to be studied now to help us understand exozodi systems in the future, and provide findings for future work to better retire all the risks that I've sort of talked about here. So I hope people will join me who are interested. I hope this is accepted, of course. And so if this moves forward, I'm really hoping to attract a large, diverse group of people to work on this and to report on what we find as a result of the work. So if you want to look at the details of my draft terms of reference, you can download that at this link. And at this point, I am happy to take any questions.

- Great, thank you very much, John. We have one question in the Q & A, which asks, could you include background galaxies as part of the SAG analysis?

- Oh, in terms of how they might cause confusion?

- Presumably, yeah.

- No, I think that's a very interesting point. I know that there's been work done on this already. And so I guess the question would be, if we think that there are some new areas that have not yet been addressed, or if there's still open questions, I think that's a perfectly valid direction to go as well. I think we're primarily focusing on exozodis because we expect that to be a much more tricky problem because that's always gonna be near the star, whereas a bad galaxy, if you find it early enough, you can time things with these. At least some of the just high, proper motion stars, you can avoid some of them or at least figure out what's going on with them pretty quickly.

- Sounds good. There were a couple... Oh, another question. Does it make sense for the SAG to cover the relationship between the structure and the planetary system architecture; that is, to cover the dust sources and transport?

- Yeah, I mean, I think that's a useful area in some respects. I think what we're often limited by is we don't really have a good idea on what the planetary architectures are for most systems, because there are very few systems where we have a directly detected exoplanet and a directly detected disk . But the ones that we do see are very valuable for sort of testing those relationships.

- Right, it'd be wonderful to be able to know the full extent of both the planetary system architecture all the way through the full disk itself, but that's for the future .

- Right.

- Okay, there were a couple of questions in the WebEx chat, but I have the feeling that might've been addressing very specific points you were making during the presentation about what MIRI can detect. So maybe it'd be better to address those on the Slack channel.

- Okay, will do.

- And we also have a new message from Jennifer saying that we've got some votes on the location of the next ExoPAG meeting, but we could always use more. So anyone who might be interested, please go ahead and vote. And I think that's it for this session. So I'm gonna turn over to Michael Meyer.

- Thanks, Josh. Thank you for, again, being an excellent session chair and we'll thank all of the speakers with some virtual, or unmute and clap. And with that, I just wanted to say a couple of quick things. Thanks, again, John, for rolling out this idea for the SAG 23. We will discuss this further during our business meeting on Wednesday. And just a reminder that we will reconvene with a special session tomorrow at 1:00 p.m., where we'll be joined by Bruce Macintosh and Keivan Stassun from the Executive Committee of ASTRO 2020. Our goal tomorrow is to not critique the report. I think we have a broad community consensus to support the process of ASTRO 2020 in which we were all invited to participate in. And tomorrow our goal is to really make sure we understand and ask any clarifying questions, if we have them, from the panel members that we'll have. We're gonna try to moderate the discussion, and some members of the EC have volunteered to help guide us in that. And then, as we said earlier, we will break at 2:30 p.m., Eastern Time, to give a 15-minute break before the NASA Town Hall where Paul Hertz will make his address. And let me also just thank, again, profusely, everyone behind the scenes, particularly Jennifer Gregory, Rebecca Gonzales, Christine Moran, Jennifer Burt, Eric Mamajek. And Jennifer, if I've forgotten someone I should give a shout out to here, please forgive me and feel free to jump in. But everyone did really heroic work to try to get this meeting pulled off this week, given all of the chaos that we've experienced since the canceling of the AAS. So thanks, again, to everyone.

- [Jennifer] I would add Kristy Kawasaki on website updates and Ray Lemus, keeping us on track through WebEx.

- Yes, Kristy and Ray. Thank you very much for filling that out. And with that, I think we're gonna finish right on time and we look forward to seeing many of you back here tomorrow and then again on Wednesday where we'll resume normal sessions, finishing up our ExoPAG 25. Thanks, everyone, and have a good evening, or afternoon.