Ekaterina Ilin, a PhD student in my group, has recently published her work on stellar flares in three young and two middle-aged open clusters.
From the abstract: “Drawing from the complete K2 archive, we searched 3435∼80 day long light curves of 2111 open cluster members for flares using the open-source software packages K2SC to remove instrumental and astrophysical variability from K2 light curves, and AltaiPony to search and characterize the flare candidates. We confirmed a total of 3844 flares on high probability open cluster members with ages from zero age main sequence(Pleiades) to 3.6 Gyr (M67). We extended the mass range probed in the first study of this series to span from Sun-like stars to mid-Mdwarfs. We added the Hyades (690 Myr) to the sample as a comparison cluster to Praesepe (750 Myr), the 2.6 Gyr old Ruprecht 147, and several hundred light curves from the late K2 Campaigns in the remaining clusters. We found that the flare energy distribution was similar in the entire parameter space, following a power law relation with exponent a=1.84−2.39. We confirmed that flaring rates declined with age, and declined faster for higher mass stars. Our results are in good agreement with most previous statistical flare studies. We found evidence that a rapid decline in flaring activity occurred in M1-M2 dwarfs around Hyades/Praesepe age, when these stars spun down to rotation periods of about 10 d, while higher mass stars had already transitioned to lower flaring rates, and lower mass stars still resided in the saturated activity regime. We conclude that somediscrepancies between our results and flare studies that used rotation periods for their age estimates could be explained by sample selection bias toward more active stars, but others may hint at limitations of using rotation as an age indicator without additional constraints from stellar activity.”
Two new group members are starting their research here in fall 2020: Judy Chebly is a new PhD student who works with Julián Alvarado-Gómez and myself on simulations of coronal mass ejections in stars-planet systems, and Dr. Eliana Amazo-Gomez is a new postdoc who works on stellar rotation and activity. We’re very happy to have them on board!
This week the Exoplanets III is taking place – it has been moved from Heidelberg into a virtual format because of the Covid-19 pandemic. I’m really excited about this particular conference, because it looks like a really well thought-out way to do an online conference, with all talks being available as videos for non-synchronous viewing, interactive online posters, and active discussion on Slack. Several of our group members are presenting their work:
Laura Ketzer: Poster “Using PLATYPOS to estimate the atmospheric mass loss of V1298 Tau’s four young planets”
Vada Xanthippi Alexoudi: Poster “On the degeneracy of the planetary spectral slope with orbital parameters”
Engin Keles: poster “Probing the atmosphere of HD189733b with the Na-I and K-I lines”
Matthias Mallonn: poster “Challenging the weather forecast: the observational study of day side clouds”
myself: talk “Connecting the exoplanet radius gap with stellar activity evolution”
plus a few more are attending (Grace Foster, Nikoleta Ilic).
Paper on the atmospheric evaporation of four very young exoplanets ?>
Together with my PhD student Laura Ketzer and postdoc Matthias Mallonn, we have published a new paper on the atmospheric evaporation of the four very young planets around the star V1298 Tau. We measured the star’s X-ray spectrum by combining ROSAT and Chandra observations, and found that the star is highly active with an X-ray luminosity above 10^30 erg/s. Laura developed a numerical code to estimate the planetary evaporation as the star ages and becomes less X-ray bright. Depending on the masses of the planetary cores and the age at which the star will start spinning down, some of the planets may lose their complete atmosphere by the time the star reaches the age of our Sun.
My freshly graduated PhD student, Dr. Rachel Booth, has published the final paper from her PhD thesis together with me and a few coworkers. We have analysed how the magnetic activity of sun-like stars decays as they age, and have used a sample of stars that all have well-determined asteroseismic ages. We find that even at old stellar ages on the main sequence the spin down and therefore the decay of stellar activity continues.
Together with my colleagues at AIP, and led by Engin Keles, a PhD student in my Star-Planet Systems group, we have published a paper on the detection of potassium in the atmosphere of a Hot Jupiter using high-resolution transmission spectroscopy.
The Large Binocular Telescope (LBT) was used with the PEPSI spectrograph in this work; both the LBT and our institute published press releases about the result (LBT press release; AIP press release). This is an exciting result because not all Hot Jupiters have potassium detected in their atmospheres, even when they have detections of the similar element sodium. The data will be used to gain more insight into the atmospheric chemistry of Hot Jupiters.
Monthly Notices of the Royal Astronomical Society: Letters, Volume 489, Issue 1, p.L37-L41 (2019).
Together with many colleagues, I have contributed to a White Paper on a possible X-ray Interferometry mission. This project is led by Phil Uttley from the University of Amsterdam, and we hope to be considered for ESA’s Vision 2050 mission slot. X-ray interferometry can yield amazing spatial resolution for bright X-ray sources, even though there is still quite some technology to be developed. For stars and exoplanets, we could spatially resolve transits in front of the stellar corona – see the conceptual image I made for the White Paper.
This summer I organized a 3-month research program called Exostar19 (https://www.kitp.ucsb.edu/activities/exostar19) at the Kavli Institute for Theoretical Physics (KITP) in Santa Barbara, together with Bekki Dawson, Dan Huber, and Jim Fuller. Victor Silva Aguirre was the one who brought us all together with his idea to come up with a program that focuses on all the new insights that the stellar and planetary field can gain from TESS and Gaia data. It’s now the last week of the program, and it’s been a blast! My office has a view onto a little slice of the ocean, just behind the palm trees:
and I’ve worked on a bunch of cool new projects with new collaborators – stellar rotation and activity, X-ray and UV observations of exoplanets, some work on transits in the infrared helium lines, plus a near wrap-up of a project on an ultrahot Jupiter. We’ve held a conference in the middle of the program, which went really well I think (https://www.kitp.ucsb.edu/activities/exostar-c19). I really liked the KITP policy that all of their conferences have one third of questions time. So every talk was 20 minutes plus 10 minutes questions, and that led a really lively and interesting discussions, and just just by the same few people that always ask questions at other conferences.
This is us organizers:
I brought my family along, and we stayed in the KITP residence, which has very good spaces for people with kids. We had a lot of great barbecues with the other program participants, and also managed to spend a bit of time at the beach.
New paper: a small planet in the temperate zone of K2-133 ?>
Abstract: We present follow-up observations of the K2-133 multiplanet system. Previously, we announced that K2-133 contained three super-Earths orbiting an M1.5V host star – with tentative evidence of a fourth outer-planet orbiting at the edge of the temperate zone. Here, we report on the validation of the presence of the fourth planet, determining a radius of 1.73 R⊕. The four planets span the radius gap of the exoplanet population, meaning further follow-up would be worthwhile to obtain masses and test theories of the origin of the gap. In particular, the trend of increasing planetary radius with decreasing incident flux in the K2-133 system supports the claim that the gap is caused by photo-evaporation of exoplanet atmospheres. Finally, we note that K2-133 e orbits on the edge of the star’s temperate zone, and that our radius measurement allows for the possibility that this is a rocky world. Additional mass measurements are required to confirm or refute this scenario.
Monthly Notices of the Royal Astronomical Society, Volume 487, Issue 2, p.1865-1873, 2019.
Abstract: The architecture of exoplanetary systems is often different from the solar system, with some exoplanets being in close orbits around their host stars and having orbital periods of only a few days. In analogy to interactions between stars in close binary systems, one may expect interactions between the star and the exoplanet as well. From theoretical considerations, effects on the host star through tidal and magnetic interaction with the exoplanet are possible; for the exoplanet, some interesting implications are the evaporation of the planetary atmosphere and potential effects on the planetary magnetism. In this review, several possible interaction pathways and their observational prospects and existing evidence are discussed. A particular emphasis is put on observational opportunities for these kinds of effects in the high-energy regime.
Astronomische Nachrichten, Volume 340, Issue 4, pp. 329-333, 2019.