My PhD student Rachel Booth has been working on X-ray data from several space telescopes and has published our findings in MNRAS recently: X-ray emission from stars quiets down with age much more dramatically than thought before (see here for more details about the paper). Now NASA has published a press release on Rachel’s research, here is the link: https://www.nasa.gov/mission_pages/chandra/news/x-rays-reveal-temperament-of-possible-planet-hosting-stars.html. Some really nice results, and hopefully we’ll be able to collect more data soon and study this in even more detail!
The detection of thousands of extrasolar planets by the transit method naturally raises the question of whether potential extrasolar observers could detect the transits of the Solar System planets. We present a comprehensive analysis of the regions in the sky from where transit events of the Solar System planets can be detected. We specify how many different Solar System planets can be observed from any given point in the sky, and find the maximum number to be three. We report the probabilities of a randomly positioned external observer to be able to observe single and multiple Solar System planet transits; specifically, we find a probability of 2.518% to be able to observe at least one transiting planet, 0.229% for at least two transiting planets, and 0.027% for three transiting planets. We identify 68 known exoplanets that have a favourable geometric perspective to allow transit detections in the Solar System and we show how the ongoing K2 mission will extend this list. We use occurrence rates of exoplanets to estimate that there are ca. 3 and 7 temperate Earth-sized planets orbiting GK and M dwarf stars brighter than $V=13$ and $V=16$ respectively, that are located in the Earth’s transit zone.
Very good news: the Arcus mission – a high-resolution X-ray spectrograph onboard a small space telescope – has received funding from NASA for a concept study! I am part of the proposal team as an international collaborator, our PI is Randall Smith from the Harvard-Smithsonian Center for Astrophysics. We just received 2 million US$ funding through NASA’s MIDEX mission call for phase A, and we will be working frantically on Arcus for the next year to demonstrate it can actually do the things we want it to do.
I’m happy to be part of the Next Generation Transit Survey (NGTS) collaboration, a ground-based array of telescopes in Chile searching for exoplanets transiting their host stars. We have just published a paper on detecting false positives with NGTS, namely background transits of dim stars across brighter stars, through centroid vetting. The paper is accepted for publication by Monthly Notices of the Royal Astronomical Society.
“Centroid vetting of transiting planet candidates from the Next Generation Transit Survey”, accepted by MNRAS (2017); Günther, Maximilian N.; Queloz, Didier; Gillen, Edward; McCormac, James; Bayliss, Daniel; Bouchy, Francois; Walker, Simon. R.; West, Richard G.; Eigmüller, Philipp; Smith, Alexis M. S.; Armstrong, David J.; Burleigh, Matthew; Casewell, Sarah L.; Chaushev, Alexander P.; Goad, Michael R.; Grange, Andrew; Jackman, James; Jenkins, James S.; Louden, Tom; Moyano, Maximiliano; Pollacco, Don; Poppenhaeger, Katja; Rauer, Heike; Raynard, Liam; Thompson, Andrew P. G.; Udry, Stéphane; Watson, Christopher A.; Wheatley, Peter J.
I will teach some new lecture courses this fall, and therefore I think a short review of my teaching over the past two years is in order. I’ve taught the module “Computational modelling in physics” twice in a row now, it’s a lecture and computer lab course for the first-year physics students. For some students, it’s the first time they actually program something, while others already have quite some experience when they start university. I introduced the students to Python via some interesting physics problems in both years (Python 2.7 in the first year and 3.1 in the second year, actually). I think it went well both times, and the student evaluations were positive both times with 4.44 out of 5 the first time around and 4.69 the second time. I don’t know what the standard deviation of teaching evaluations actually is in my department; but since my evaluations went up, not down, it is of course extremely tempting to think that the reason for that is my improved teaching… Things I did differently the second time: I made the course more challenging for the students; I asked more “why” questions in the Assignments the students had to hand in; I didn’t strictly follow the lecture vs. computer lab schedule, and rather showed some live programming during the lectures as well. Interestingly, the average grades I gave were lower the second time around, and still the students seemed to enjoy the course more. On the other hand, maybe it’s all just statistical fluctuations…
In the upcoming year I’ll teach computational physics for the third-year students, which should be quite interesting as well; especially since they are the students I taught in their first year back in 2015/16! So I know exactly what they *should* already know. Also, I’ll teach some astronomy (finally), also to the third-year students. It’s gonna be stellar structure and evolution, a.k.a. the basics of what I actually do in my research, so that should be fun as well.
Happy to report that my PhD student Rachel Booth has successfully published her first paper! It’s a very interesting analysis of the magnetic activity of old cool stars, with a surprising find about the decline of activity at old stellar ages. Our paper has also been featured on the Astrobites blog: https://astrobites.org/2017/07/03/adventures-in-watchmaking-for-cool-stars/.
Here’s the abstract of the paper:
Stars with convective envelopes display magnetic activity, which decreases over time due to the magnetic braking of the star. This age-dependence of magnetic activity is well-studied for younger stars, but the nature of this dependence for older stars is not well understood. This is mainly because absolute stellar ages for older stars are hard to measure. However, relatively accurate stellar ages have recently come into reach through asteroseismology. In this work we present X-ray luminosities, which are a measure for magnetic activity displayed by the stellar coronae, for 24 stars with well-determined ages older than a gigayear. We find 14 stars with detectable X-ray luminosities and use these to calibrate the age-activity relationship. We find a relationship between stellar X-ray luminosity, normalized by stellar surface area, and age that is steeper than the relationships found for younger stars, with an exponent of -2.80 +- 0.72. Previous studies have found values for the exponent of the age-activity relationship ranging between -1.09 to -1.40, dependent on spectral type, for younger stars. Given that there are recent reports of a flattening relationship between age and rotational period for old cool stars, one possible explanation is that we witness a strong steepening of the relationship between activity and rotation.
I just spent a really exciting week at castle Ringberg in south Germany, where the Max-Planck Institute for Astronomy held a conference on exoplanet formation and atmospheric composition. Lots of interesting discussions and new results. I was invited to give a talk on the topic of “Stellar activity and planet characterisation” – one of my favourite topics to talk about. My personal highlight of the conference was Yamila Miguel’s presentation about the latest results from the Juno mission: we finally know now what the interior of Jupiter is made of! Looking forward to coming back to Ringberg for other conferences.
Today Geoff Clayton from Louisiana State University gave a talk about R Coronae Borealis stars at our astrophysics seminar. These kinds of stars show erratic drops of several magnitudes in brightness over hundreds of years (R Coronae Borealis itself was discovered to be variable in 1795), and it’s still a somewhat open question what these things actually *are*. It’s not at all my field of study, but it was a really interesting talk and exactly the kind of talk I hope to see when I go to our seminar series. The talk had some whimsical bits and pieces about the discovery history, introduced the main hypotheses about the nature of these objects early on, then went through several examples of light curves and spectra, how dust is probably causing the brightness drops, some nucleosynthesis background, and then explained how one scenario (white dwarf mergers) is likely winning over the other scenario (final flash stars). It ended with some neat art comparison (Klimt’s “The kiss”, which shows Ariadne and Dionysos; according to Greek mythology, after Ariadne’s death the flowers in her hair became the constellation Corona Borealis). Another thing I really enjoyed was the part of the talk which was about dust formation, because this is actually something I know from my work on young stellar objects and their disks. I find it really rewarding when I go to a talk on something I know very little about, and then all of a sudden some connection to my own work pops up. Anyway, it was really enjoyable and if you ever wonder if you should go to your institute’s seminar or not: when in doubt, the answer is yes.
New paper: Testing if Fomalhaut b is a neutron star ?>
Fomalhaut b is a directly imaged object in the debris disk of the star Fomalhaut. It has been hypothesized to be a planet, however there are issues with the observed colours of the object that do not fit planetary models. An alternative hypothesis is that the object is a neutron star in the near fore- or background of Fomalhaut’s disk. We test if Fomalhaut b could be a neutron star using X-ray observations with Chandra’s HRC-I instrument in the energy range of 0.08-10 keV. We do not detect X-ray emission from either Fomalhaut b or the star Fomalhaut itself. Our nondetection corresponds to an upper limit on the X-ray flux of Fomalhaut b of FX < 1.3e-14 erg/cm/s^2 in the energy range 0.08-10 keV. For the A-type central star Fomalhaut, we derive an X-ray upper limit of LX < 2e25 erg/s in the energy range 0.08-10 keV. Fomalhaut b’s X-ray non-detection constrains the parameter space for a possible neutron star significantly, implying surface temperatures lower than 91000 K and distances closer than 13.3 pc to the solar system. In addition we find that reflected starlight from the central star fits the available optical detections of Fomalhaut b; a smaller planet with a large ring system might explain such a scenario.
Unfortunately I have to cancel several research trips and conferences this spring and summer. I’ve recently come down with pneumonia (quite unexpectedly! with hospital stay and all) and full recovery is expected to take several months. I had planned to go to the Radio Habitability Conference in California and The X-ray Universe in Rome, for both of which I’m on the scientific organizing committee, but I won’t be able to go to those. I’ll also have to cancel several invited colloquium and seminar talks that I had been looking forward to. But health comes first.