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.
New papers: exoplanets and stellar results from NGTS ?>
The Next Generation Transit Survey has had some new discoveries over the past few months which I was happy to contribute to. We’ve discovered a new exoplanet (an inflated hot Jupiter), a fully-convective eclipsing binary system, and a giant flare with pulsations on a pre-main sequence M star. The papers are:
Today I gave the colloquium talk at my new home institution, the AIP. I chose a quite relatable title with “A field trip to the exoplanet zoo”, and I was blown away by how many people showed up. We actually had to open the second half of the lecture hall, which is usually only done for conferences – quite flattering! It was good fun and my colleagues from the AIP and the university had a bunch of interesting questions. Looking forward to all the science I’ll do here from now on.
Last week I was a guest at the Anton Pannekoek Intitute at the University of Amsterdam for two days and gave an invited colloquium. Also learned a lot about the research going on at Amsterdam, especially in the exoplanet groups of Birkby and Desert. Title and abstract of my talk:
Strange beasts in the exoplanet zoo
Almost all exoplanets known today are orbiting around cool stars. This is caused by certain biases in our planet detection methods, but nevertheless it means that almost all exoplanets we can study today live in a system where the host star displays magnetic activity, which is ubiquitous amongst cool stars. As exoplaneteers, we care about stellar activity due to a variety of reasons: stellar activity can drive mass loss of planets; it likely influences the habitability of a planet; and it can make the detection of planets much more difficult, depending on the stellar properties and the techniques used. I will talk about several interesting projects going on in my group, concerning the field of cool stars and exoplanets – there will be news about magnetic activity in close star-planet systems, the decline of stellar activity in cool stars, and strange transit signatures around young and active stars.
I’m a tenured full professor now! I’ve started my new position, a joint professorship at the Leibniz Institute for Astrophysics Potsdam (AIP) and the University of Potsdam, this autumn. I had a very good time in Belfast, but it’s really nice to be back in my home country and have continued access to European funding, whatever will happen with Brexit. Fortunately my two PhD students in Belfast are both in their final year now, so I can supervise them from afar as they’re pretty independent now.
My new office is in one of the historical observatory buildings at AIP, which is quite fancy. Also, the new position comes with a good chunk of funding, plus I am very happy that I was awarded a large grant by the Leibniz Association (similar to an ERC starting grant). So there will be lots of hiring over the next year! I did my PhD in a very large group, and I was always hoping that I would be able to build such a large research group myself – looks like that is actually happening now 🙂
This summer the Cambridge Workshop on Cool Stars, Stellar Systems and the Sun had its 20th anniversary, and took place in Boston. This is my favourite conference series, and this year I was really excited because I had the honour of being an invited speaker. I gave a talk on “How planets affect cool stars”, and had a lot of good interactions with people about the science of stars and exoplanets. This was the first time I’ve given an invited talk wearing a “My little pony” T-shirt. Also, this was the first time for me to introduce my PhD students to the Cool Stars community, which was a neat thing I hadn’t had the chance to do before.
It was a fantastic meeting, with lots of scientific input, and it was wonderful to see all my friends and colleagues from the Harvard-Smithsonian Center for Astrophysics again.
We investigated the X-ray emission line spectra of cool main-sequence stars in order to construct their emission measure distributions and compare them to the Sun:
“A Chandra/LETGS Survey of Main-sequence Stars”
We analyze the X-ray spectra of 19 main-sequence stars observed by Chandra using its LETGS configuration. Emission measure (EM) distributions are computed based on emission line measurements, an analysis that also yields evaluations of coronal abundances. The use of newer atomic physics data results in significant changes compared to past published analyses. The stellar EM distributions correlate with surface X-ray flux (FX) in a predictable way, regardless of spectral type. Thus, we provide EM distributions as a function of FX, which can be used to estimate the EM distribution of any main-sequence star with a measured broadband X-ray luminosity. Comparisons are made with solar EM distributions, both full-disk distributions and spatially resolved ones from active regions (ARs), flares, and the quiet Sun. For moderately active stars, the slopes and magnitudes of the EM distributions are in excellent agreement with those of solar ARs for logT< 6.6, suggesting that such stars have surfaces completely filled with solar-like ARs. A stellar surface covered with solar X-class flares yields a reasonable approximation for the EM distributions of the most active stars. Unlike the EM distributions, coronal abundances are strongly dependent on spectral type, and we provide relations with surface temperature for both relative and absolute abundances. Finally, the coronal abundances of the exoplanet host star τ Boo A (F7 V) are anomalous, and we propose that this is due to the presence of the exoplanet.