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 ?>
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.
New paper on the disappearing disk of TYC 8241 2652 1 ?>
TYC 8241 2652 1 is a young star that showed a strong mid-infrared (mid-IR, 8-25 mu) excess in all observations before 2008 consistent with a dusty disk. Between 2008 and 2010 the mid-IR luminosity of this system dropped dramatically by at least a factor of 30 suggesting a loss of dust mass of an order of magnitude or more. We aim to constrain possible models including removal of disk material by stellar activity processes, the presence of a binary companion, or other explanations suggested in the literature. We present new X-ray observations, optical spectroscopy, near-IR interferometry, and mid-IR photometry of this system to constrain its parameters and further explore the cause of the dust mass loss. In X-rays TYC 8241 2652 1 has all properties expected from a young star: Its luminosity is in the saturation regime and the abundance pattern shows enhancement of O/Fe. The photospheric Ha line is filled with a weak emission feature, indicating chromospheric activity consistent with the observed level of coronal emission. Interferometry does not detect a companion and sets upper limits on the companion mass of 0.2, 0.35, 0.1 and 0.05 M_sun at projected physical separations of 0.1-4 AU,4-5 AU, 5-10 AU, and 10-30 AU, respectively (assuming a distance of 120.9 pc). Our mid-IR measurements, the first of the system since 2012, are consistent with the depleted dust level seen after 2009. The new data confirms that stellar activity is unlikely to destroy the dust in the disk and shows that scenarios where either TYC 8241 2652 1 heats the disk of a binary companion or a potential companion heats the disk of TYC 8241 2652 1 are unlikely.
This week I am in Bonn, Germany, to give an invited talk entitled “Interactions between exoplanets and their host stars” at the conference Exoplanets – Bridging the gap between theory and observations. The conference takes place at the beautiful historical Physikzentrum in Bad Honnef. Lots of interesting talks and posters, from planet definitions (and how we should change them) to upcoming observational missions and planet formation theory. Plenty of time for discussion with all participants!
Earth sustains its magnetic field by a dynamo process driven by convection in the liquid outer core. Geodynamo simulations have been successful in reproducing many observed properties of the geomagnetic field. However, while theoretical considerations suggest that flow in the core is governed by a balance between Lorentz force, rotational force and buoyancy (called MAC balance for Magnetic, Archimedean, Coriolis) with only minute roles for viscous and inertial forces, dynamo simulations must employ viscosity values that are many orders of magnitude larger than in the core due to computational constraints. In typical geodynamo models viscous and inertial forces are not much smaller than the Coriolis force and the Lorentz force plays a sub-dominant role. This has led to conclusions that these simulations are viscously controlled and do not represent the physics of the geodynamo. Here we show by a direct analysis of the relevant forces that a MAC balance can be achieved when the viscosity is reduced to values close to the current practical limit. Lorentz force, buoyancy and the uncompensated (by pressure) part of the Coriolis force are of very similar strength, whereas viscous and inertia are smaller by a factor of at least 20 in the bulk of the fluid volume. Compared to non-magnetic convection at otherwise identical parameters, the dynamo flow is of larger scale, less invariant parallel to the rotation axis (less geostrophic) and convection transports twice as much heat, all of which is expected when the Lorentz force strongly influences the convection properties.
Last Saturday I was honored to be an invited speaker at the TEDx conference in Klagenfurt, Austria. I spoke about exoplanets in habitable zones, and how those may compare to what we know from our own solar system. It was an extremely interesting conference – other speakers included Mateja Jamnik from the University of Cambridge who talked about her work in artificial intelligence, Guy Standing from the University of London who discussed unconditional basic income and how it can change society, and Natalie Haas from the University of Salzburg who spoke about her initiatives to help refugees in local communities. Really exciting weekend!
One of my favourite conferences, the Cool Stars Workshop, took place in Uppsala last week. My PhD student Rachel Booth presented her work in a talk at the splinter session on Stellar Magnetic Activity – her first talk at a conference! And she did great! A lot of interesting stuff is going on at the moment with the spin-down and decrease in magnetic activity of old cool stars, and it seems like there may be different trends of rotation with age, activity with age, and rotation versus activity than we thought before. Exciting times!
Another highlight of the conference was the plenary talk of my friend Cecilia Garraffo, who talked about her latest results on angular momentum loss in combination with magnetic field geometries of cool stars. This can potentially explain why we see such a rapid transition from fast-spinning young stars to more slowly rotating stars at older ages.
And of course, as always at the conference dinner, the big announcement where the next Cool Stars Workshop will be held in two years from now. Quite unsurprisingly, for its 20th anniversary Cool Stars will return to Cambridge, MA, where it all started. My former boss and ongoing collaborator Scott Wolk will be one of the organizers, together with Andrea Dupree. Really looking forward to the next workshop!
Great news: my former postdoc and ongoing collaborator Dr. Rakesh Yadav was awarded the Otto-Hahn-Medal of the Max Planck Society. This is a prize the Max Planck Society awards to young scientists for outstanding achievements. Rakesh works on theoretical models of stellar and (exo-)planetary magnetic fields, with a focus on the magnetic dynamo processes in quickly rotating objects.
This week the XMM-Newton workshop is taking place in Madrid. XMM is still going strong 17 years after its launch to space! Although I couldn’t attend the meeting myself, some of my group’s work got showcased in the invited talks, specifically Jürgen Schmitt’s talk on “Exoplanets and their Host Stars”, where some of my tidal spin-up work was shown, and Beate Stelzer’s talk on “Cool Star X-ray Variability”, where the work of my PhD student Rachel Booth on age-activity relationships for old main-sequence stars was mentioned. Go XMM!