Katja Poppenhaeger

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 (F_X) in a predictable way, regardless of spectral type. Thus, we provide EM distributions as a function of F_X, 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.

A Chandra/LETGS Survey of Main-sequence Stars, Wood, Brian E.; Laming, J. Martin; Warren, Harry P.; Poppenhaeger, Katja, The Astrophysical Journal, Volume 862, Issue 1, article id. 66, 24 pp. (2018).

The NGTS planet-search project in which I take part has detected a new planet in an interesting system:

“Unmasking the hidden NGTS-3Ab: a hot Jupiter in an unresolved binary system”

We present the discovery of NGTS-3Ab, a hot Jupiter found transiting the primary star of an unresolved binary system. We develop a joint analysis of multi-colour photometry, centroids, radial velocity (RV) cross-correlation function (CCF) profiles and their bisector inverse slopes (BIS) to disentangle this three-body system. Data from the Next Generation Transit Survey (NGTS), SPECULOOS and HARPS are analysed and modelled with our new BLENDFITTER software. We find that the binary consists of NGTS-3A (G6V-dwarf) and NGTS-3B (K1V-dwarf) at <1" separation. NGTS-3Ab orbits every 1.675 days. The planet radius and mass are Rplanet=1.48 ± 0.37 RJand Mplanet=2.38 ± 0.26 MJ, suggesting it is potentially inflated. We emphasise that only combining all the information from multi-colour photometry, centroids and RV CCF profiles can resolve systems like NGTS-3. Such systems cannot be disentangled from single-colour photometry and RV measurements alone. Importantly, the presence of a BIS correlation indicates a blend scenario, but is not sufficient to determine which star is orbited by the third body. Moreover, even if no BIS correlation is detected, a blend scenario cannot be ruled out without further information. The choice of methodology for calculating the BIS can influence the measured significance of its correlation. The presented findings are crucial to consider for wide-field transit surveys, which require wide CCD pixels (>5″) and are prone to contamination by blended objects. With TESS on the horizon, it is pivotal for the candidate vetting to incorporate all available follow-up information from multi-colour photometry and RV CCF profiles.

Unmasking the hidden NGTS-3Ab: a hot Jupiter in an unresolved binary system, Günther, Maximilian N.; Queloz, Didier; Gillen, Edward; Delrez, Laetitia; Bouchy, François; McCormac, James; Smalley, Barry; Almleaky, Yaseen; Armstrong, David J.; Bayliss, Daniel; Burdanov, Artem; Burleigh, Matthew; Cabrera, Juan; Casewell, Sarah L.; Cooke, Benjamin F.; Csizmadia, Szilárd; Ducrot, Elsa; Eigmüller, Philipp; Erikson, Anders; Gänsicke, Boris T.; Gibson, Neale P.; Gillon, Michaël; Goad, Michael R.; Jehin, Emmanuël; Jenkins, James S.; Louden, Tom; Moyano, Maximiliano; Murray, Catriona; Pollacco, Don; Poppenhaeger, Katja; Rauer, Heike; Raynard, Liam; Smith, Alexis M. S.; Sohy, Sandrine; Thompson, Samantha J.; Udry, Stéphane; Watson, Christopher A.; West, Richard G.; Wheatley, Peter J., accepted for publication by Monthly Notices of the Royal Astronomical Society, 2018.

I am happy to be part of the Kepler Object of Interest Network, a network of ground-based telescopes to observe transits of exoplanets discovered by the Kepler space telescope in order to figure out their masses through Transit Timing Variations.
The first paper about this project is now accepted:

“Kepler Object of Interest Network I. First results combining ground and space-based observations of Kepler systems with transit timing variations”

During its four years of photometric observations, the Kepler space telescope detected thousands of exoplanets and exoplanet candidates. One of Kepler’s greatest heritages has been the confirmation and characterization of hundreds of multi-planet systems via Transit Timing Variations (TTVs). However, there are many interesting candidate systems displaying TTVs on such long time scales that the existing Kepler observations are of insufficient length to confirm and characterize them by means of this technique. To continue with Kepler’s unique work we have organized the “Kepler Object of Interest Network” (KOINet). The goals of KOINet are, among others, to complete the TTV curves of systems where Kepler did not cover the interaction timescales well. KOINet has been operational since March, 2014. Here we show some promising first results obtained from analyzing seven primary transits of KOI-0410.01, KOI-0525.01, KOI-0760.01, and KOI-0902.01 in addition to Kepler data, acquired during the first and second observing seasons of KOINet. While carefully choosing the targets we set demanding constraints about timing precision (at least 1 minute) and photometric precision (as good as 1 part per thousand) that were achieved by means of our observing strategies and data analysis techniques. For KOI-0410.01, new transit data revealed a turn-over of its TTVs. We carried out an in-depth study of the system, that is identified in the NASA’s Data Validation Report as false positive. Among others, we investigated a gravitationally-bound hierarchical triple star system, and a planet-star system. While the simultaneous transit fitting of ground and space-based data allowed for a planet solution, we could not fully reject the three-star scenario. New data, already scheduled in the upcoming 2018 observing season, will set tighter constraints on the nature of the system.

“Kepler Object of Interest Network I. First results combining ground and space-based observations of Kepler systems with transit timing variations”, von Essen, C.; Ofir, A.; Dreizler, S.; Agol, E.; Freudenthal, J.; Hernandez, J.; Wedemeyer, S.; Parkash, V.; Deeg, H. J.; Hoyer, S.; Morris, B. M.; Becker, A. C.; Sun, L.; Gu, S. H.; Herrero, E.; Tal-Or, L.; Poppenhaeger, K.; Mallonn, M.; Albrecht, S.; Khalafinejad, S.; Boumis, P.; Delgado-Correal, C.; Fabrycky, D. C.; Janulis, R.; Lalitha, S.; Liakos, A.; Mikolaitis, S.; Moyano D’Angelo, M. L.; Sokov, E.; Pakstiene, E.; Popov, A.; Krushinsky, V.; Ribas, I.; Rodriguez S., M. M.; Rusov, S.; Sokova, I.; Tautvaisiene, G.; Wang, X., accepted by Astronomy & Astrophysics, 2018.

As part of the Young Stellar Object VARiability project (YSOVAR), I am happy to report that we have published a new paper investigating the mid-infrared variability of young stellar objects in the star-forming region Serpens South. Here’s the title and abstract:

“YSOVAR: Mid-infrared Variability among YSOs in the Star Formation Region Serpens South”

We present a time-variability study of young stellar objects (YSOs) in the Serpens South cluster performed at 3.6 and 4.5 μm with the Spitzer Space Telescope; this study is part of the Young Stellar Object VARiability project. We have collected light curves for more than 1500 sources, including 85 cluster members, over 38 days. This includes 44 class I sources, 19 sources with flat spectral energy distributions (SEDs), 17 class II sources, and five diskless YSO candidates. We find a high variability fraction among embedded cluster members of ∼70%, whereas young stars without a detectable disk display no variability. We detect periodic variability for 32 sources with periods primarily in the range of 0.2–14 days and a subset of fast rotators thought to be field binaries. The timescale for brightness changes are shortest for stars with the most photospheric SEDs and longest for those with flat or rising SEDs. While most variable YSOs become redder when fainter, as would be expected from variable extinction, about 10% get bluer as they get fainter. One source, SSTYSV J183006.13‑020108.0, exhibits “cyclical” color changes.

“YSOVAR: Mid-infrared Variability among YSOs in the Star Formation Region Serpens South”, Wolk, Scott J.; Günther, H. Moritz; Poppenhaeger, Katja; Winston, E.; Rebull, L. M.; Stauffer, J. R.; Gutermuth, R. A.; Cody, A. M.; Hillenbrand, L. A.; Plavchan, P.; Covey, K. R.; Song, Inseok, The Astronomical Journal, Volume 155, Issue 2, article id. 99, 20 pp. (2018).

This is somewhat unusual research for me, but it was good fun – some recent work about identifying an emission line in the X-ray regime which is excited by photons from a different atomic species which just happens to have the correct wavelength for that. This can be used as an independent density diagnostics for plasma in flaring loops, for example. The paper has just been published, here is the title and abstract:

“X-ray line coincidence photopumping in a solar flare”

Line coincidence photopumping is a process where the electrons of an atomic or molecular species are radiatively excited through the absorption of line emission from another species at a coincident wavelength. There are many instances of line coincidence photopumping in astrophysical sources at optical and ultraviolet wavelengths, with the most famous example being Bowen fluorescence (pumping of O III 303.80 Å by He II), but none to our knowledge in X-rays. However, here we report on a scheme where a He-like line of Ne IX at 11.000 Å is photopumped by He-like Na X at 11.003 Å, which predicts significant intensity enhancement in the Ne IX 82.76 Å transition under physical conditions found in solar flare plasmas. A comparison of our theoretical models with published X-ray observations of a solar flare obtained during a rocket flight provides evidence for line enhancement, with the measured degree of enhancement being consistent with that expected from theory, a truly surprising result. Observations of this enhancement during flares on stars other than the Sun would provide a powerful new diagnostic tool for determining the sizes of flare loops in these distant, spatially unresolved, astronomical sources.

“X-ray line coincidence photopumping in a solar flare”, Keenan, F. P.; Poppenhaeger, K.; Mathioudakis, M.; Rose, S. J.; Flowerdew, J.; Hynes, D.; Christian, D. J.; Nilsen, J.; Johnson, W. R., Monthly Notices of the Royal Astronomical Society, Volume 474, Issue 3, p.3782-3786, 2018.

The mission paper about the Next Generation Transit Survey (NGTS), a ground-based telescope network to search for transiting exoplanets, has just been published. I’m happy to be part of this project, which is a collaboration between several universities in the UK, Germany, Chile, and Switzerland – lots of exciting discoveries to come soon!

Here’s some more info about the paper: We describe the Next Generation Transit Survey (NGTS), which is a ground-based project searching for transiting exoplanets orbiting bright stars. NGTS builds on the legacy of previous surveys, most notably WASP, and is designed to achieve higher photometric precision and hence find smaller planets than have previously been detected from the ground. It also operates in red light, maximising sensitivity to late K and early M dwarf stars. The survey specifications call for photometric precision of 0.1 per cent in red light over an instantaneous field of view of 100 square degrees, enabling the detection of Neptune-sized exoplanets around Sun-like stars and super-Earths around M dwarfs. The survey is carried out with a purpose-built facility at Cerro Paranal, Chile, which is the premier site of the European Southern Observatory (ESO). An array of twelve 20cm f/2.8 telescopes fitted with back-illuminated deep-depletion CCD cameras are used to survey fields intensively at intermediate Galactic latitudes. The instrument is also ideally suited to ground-based photometric follow-up of exoplanet candidates from space telescopes such as TESS, Gaia and PLATO. We present observations that combine precise autoguiding and the superb observing conditions at Paranal to provide routine photometric precision of 0.1 per cent in 1 hour for stars with I-band magnitudes brighter than 13. We describe the instrument and data analysis methods as well as the status of the survey, which achieved first light in 2015 and began full survey operations in 2016. NGTS data will be made publicly available through the ESO archive.

“The Next Generation Transit Survey (NGTS)”, Wheatley, Peter J.; West, Richard G.; Goad, Michael R.; Jenkins, James S.; Pollacco, Don L.; Queloz, Didier; Rauer, Heike; Udry, Stephane; Watson, Christopher A.; Chazelas, Bruno; Eigmuller, Philipp; Lambert, Gregory; Genolet, Ludovic; McCormac, James; Walker, Simon; Armstrong, David J.; Bayliss, Daniel; Bento, Joao; Bouchy, Francois; Burleigh, Matthew R.; Cabrera, Juan; Casewell, Sarah L.; Chaushev, Alexander; Chote, Paul; Csizmadia, Szilard; Erikson, Anders; Faedi, Francesca; Foxell, Emma; Gansicke, Boris T.; Gillen, Edward; Grange, Andrew; Gunther, Maximilian N.; Hodgkin, Simon T.; Jackman, James; Jordan, Andres; Louden, Tom; Metrailler, Lionel; Moyano, Maximiliano; Nielsen, Louise D.; Osborn, Hugh P.; Poppenhaeger, Katja; Raddi, Roberto; Raynard, Liam; Smith, Alexis M. S.; Soto, Maritza; Titz-Weider, Ruth; accepted for publication by Monthly Notices of the Royal Astronomical Society (2017).

Happy to report that we have discovered three small planets, plus one additional candidate planet, around a small cool star. This is some very nice work by my PhD student Rob Wells. The planets were discovered using data from the Kepler K2 space telescope. The planets are in fairly close orbits around their host star; the candidate planet, for which we need some more data to be certain it is really a planet, might be in the habitable zone of the star. Here’s some more info about the paper:

We report on the detection of three transiting small planets around the low-mass star LP 358-499 (K2-133), using photometric data from the Kepler-K2 mission. Using multiband photometry, we determine the host star to be an early M dwarf with an age likely older than a Gigayear. The three detected planets K2-133 b, c, and d have orbital periods of ca. 3, 4.9 and 11 days and transit depths of ca. 700, 1000 and 2000 ppm, respectively. We also report a planetary candidate in the system (EPIC 247887989.01) with a period of 26.6 days and a depth of ca. 1000 ppm, which may be at the inner edge of the stellar habitable zone, depending on the specific host star properties. Using the transit parameters and the stellar properties, we estimate that the innermost planet may be rocky. The system is suited for follow-up observations to measure planetary masses and JWST transmission spectra of planetary atmospheres.

“Three small transiting planets around the M dwarf host star LP 358-499”, Wells, R.; Poppenhaeger, K.; Watson, C. A., accepted for publication by MNRAS Letters (2017).