Centroid shift (xi) during background transits of dim stars in front of bright stars; no such centroid shift occurs during (foreground) transits of exoplanets (from Günther et al. 2017).
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.
Magnetic activity, measured as X-ray luminosity divided by stellar surface, as a function of stellar age. The decline of activity steepens for stars older than a gigayear. From Booth et al. (2017).
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.
Dr. Yamila Miguel giving an invited talk about the internal structure of Jupiter.
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.
Visual light curve of R Coronae Borealis over the past 18 years, from the AAVSO archive.
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 ?>
Different scenarios for what Fomalhaut b might be: planet hypothesis in grey, limiting neutron star model in black (temperature ca. 90000 K, distance ca. 13 pc, i.e. located in the background of the Fomalhaut disk), reflected starlight model from a large planetary ring system in green. Emission from the star Fomalhaut for comparison in blue. Reflected starlight is the most likely of the hypotheses. From Poppenhaeger et al. (2017).
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.
I’m giving a public lecture “Exotic worlds: planets in other solar systems and what they might look like” in the lecture series of the Irish Astronomy Association (IAA) on March 1st 2017. The location is the Bell Lecture Theatre at Queen’s University Belfast, 7pm. There will be biscuits and tea afterwards.
Here’s a synopsis of the talk from the IAA: Dr Poppenhaeger will talk about how astronomers discover planets in other solar systems, and show a few of the most breathtaking scenarios for what those planets may look like. What would life be like on a habitable world around a tiny red sun? Could a moon around a giant planet be habitable? What would happen if an Earth-like planet were just a tiny bit closer to its sun than we are to ours? She will give a glimpse into the science behind these questions, and show which stars out there actually have possibly habitable worlds around them. There will be ample opportunity for asking questions after the talk.
New paper on magnetic cycle simulation of Proxima Centauri ?>
Time evolution of the area-averaged surface magnetic field strength, the longitudinally-averaged longitudinal magnetic field, and the longitudinally-averaged radial magnetic field in a simulation of a Proxima-Centauri-like M dwarf.
The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period <20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here we present an anelastic dynamo simulation for Proxima Centauri, a representative case for slowly rotating fully connective M-stars. The rotating convection spontaneously generates strong differential rotation in the convection zone which drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the `activity’ cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.
Arcus is a NASA/MIDEX mission under development in response to the anticipated 2016 call for proposals. It is a freeflying, soft X-ray grating spectrometer with the highest-ever spectral resolution in the 8-51 Å (0.24 – 1.55 keV) energy range. The Arcus bandpass includes the most sensitive tracers of diffuse million-degree gas: spectral lines from O VII and O VIII, H- and He-like lines of C, N, Ne and Mg, and unique density- and temperature-sensitive lines from Si and Fe ions. These capabilities enable an advance in our understanding of the formation and evolution of baryons in the Universe that is unachievable with any other present or planned observatory. The mission will address multiple key questions posed in the Decadal Survey1 and NASA’s 2013 Roadmap2: How do baryons cycle in and out of galaxies? How do black holes and stars influence their surroundings and the cosmic web via feedback? How do stars, circumstellar disks and exoplanet atmospheres form and evolve? Arcus data will answer these questions by leveraging recent developments in off-plane gratings and silicon pore optics to measure X-ray spectra at high resolution from a wide range of sources within and beyond the Milky Way. CCDs with strong Suzaku heritage combined with electronics based on the Swift mission will detect the dispersed X-rays. Arcus will support a broad astrophysical research program, and its superior resolution and sensitivity in soft X-rays will complement the forthcoming Athena calorimeter, which will have comparably high resolution above 2 keV.
“Arcus: the x-ray grating spectrometer explorer”, Proceedings of the SPIE, Volume 9905, id. 99054M 7 pp. (2016), Smith, R. K.; Abraham, M. H.; Allured, R.; Bautz, M.; Bookbinder, J.; Bregman, J. N.; Brenneman, L.; Brickhouse, N. S.; Burrows, D. N.; Burwitz, V.; Carvalho, R.; Cheimets, P. N.; Costantini, E.; Dawson, S.; DeRoo, C.; Falcone, A.; Foster, A. R.; Grant, C. E.; Heilmann, R. K.; Hertz, E.; Hine, B.; Huenemoerder, D.; Kaastra, J. S.; Madsen, K. K.; McEntaffer, R. L.; Miller, E. D.; Miller, J.; Morse, E.; Mushotzky, R.; Nandra, K.; Nowak, M.; Paerels, F.; Petre, R.; Plice, L.; Poppenhaeger, K.; Ptak, A.; Reid, P.; Sanders, J.; Schattenburg, M. L.; Schulz, N.; Smale, A.; Temi, P.; Valencic, L.; Walker, S.; Willingale, R.; Wilms, J.; Wolk, S. J.
“The evolution of structure and feedback with Arcus”, Proceedings of the SPIE, Volume 9905, id. 99054P 18 pp. (2016), Brenneman, Laura W.; Smith, Randall K.; Bregman, J.; Kaastra, J.; Brickhouse, N.; Allured, R.; Foster, A.; Wolk, S.; Wilms, J.; Valencic, L.; Willingale, R.; Grant, C.; Bautz, M.; Heilmann, R.; Huenemoerder, D.; Miller, E.; Nowak, M.; Schattenburg, M.; Schulz, N.; Burwitz, V.; Nandra, K.; Sanders, J.; Bookbinder, J.; Petre, R.; Ptak, A.; Smale, A.; Burrows, D.; Poppenhaeger, K.; Costantini, E.; DeRoo, C.; McEntaffer, R.; Mushotzky, R.; Miller, J. M.; Temi, P.
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