The X-ray eclipse of OY Car resolved with XMM-Newton: X-ray emission from the polar regions of the white dwarf

We present the XMM-Newton X-ray eclipse lightcurve of the dwarf nova OY Car. The eclipse ingress and egress are well resolved for the first time in any dwarf nova placing strong constraints on the size and location of the X-ray emitting region. We find good fits to a simple linear eclipse model, giving ingress/egress durations of 30+/-3 sec. Remarkably this is shorter than the ingress/egress duration of the sharp eclipse in the optical as measured by Wood et al. (1989) and ascribed to the white dwarf (43+/-2 sec). We also find that the X-ray eclipse is narrower than the optical eclipse by 14+/-2 sec, which is precisely the difference required to align the second and third contact points of the X-ray and optical eclipses. We discuss these results and conclude that X-ray emission in OY Car most likely arises from the polar regions of the white dwarf. Our data were originally reported by Ramsay et al (2001), but they did not make a quantitative measurement of eclipse parameters. We have also corrected important timing anomalies present in the data available at that time.Comment: 6 pages, 5 figures; accepted for publication in MNRA

ROSAT observations of V471 Tauri, showing that stellar activity is determined by rotation, not age

I present pointed ROSAT PSPC observations of the pre-cataclysmic binary V471 Tauri. The hard X-ray emission (>0.4keV) is not eclipsed by the K star, demonstrating conclusively that this component cannot be emitted by the white dwarf. Instead I show that its spectrum and luminosity are consistent with coronal emission from the tidally spun-up K star. The star is more active than other K stars in the Hyades, but equally active as K stars in the Pleiades with the same rotation periods, demonstrating that rotation--and not age--is the key parameter in determining the level of stellar activity. The soft X-ray emission (<0.4keV) is emitted predominately by the white dwarf and is modulated on its spin period. I find that the pulse-profile is stable on timescales of hours and years, supporting the idea that it is caused by opacity of accreted material. The profile itself shows that the magnetic field configuration of the white dwarf is dipolar and that the magnetic axis passes through the centre of the star. There is an absorption feature in the lightcurve of the white dwarf, which occurs at a time when our line-of-sight passes within a stellar radius of the K star. The column density and duration of this feature imply a volume and mass for the absorber which are similiar to those of coronal mass ejections of the Sun.Comment: 7 pages, 7 figures, accepted for publication in MNRAS. Also available at http://www.star.le.ac.uk/~pjw

Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1

We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an ultracool dwarf star recently discovered to host three transiting and temperate Earth-sized planets. We find the star is a relatively strong and variable coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun, despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with the total XUV emission in the range L_xuv/L_bol=6-9x10^-4, and XUV irradiation of the planets that is many times stronger than experienced by the present-day Earth. Using a simple energy-limited model we show that the relatively close-in Earth-sized planets, which span the classical habitable zone of the star, are subject to sufficient X-ray and EUV irradiation to significantly alter their primary and any secondary atmospheres. Understanding whether this high-energy irradiation makes the planets more or less habitable is a complex question, but our measured fluxes will be an important input to the necessary models of atmospheric evolution.Comment: 5 pages, published as a letter in MNRAS (accepted 16 September 2016

A precise optical transmission spectrum of the inflated exoplanet WASP-52b

We have measured a precise optical transmission spectrum for WASP-52b, a highly inflated hot Jupiter with an equilibrium temperature of 1300 K. Two transits of the planet were observed spectroscopically at low resolution with the auxiliary-port camera (ACAM) on the William Herschel Telescope (WHT), covering a wide range of 4000-8750 \AA. We use a Gaussian process approach to model the correlated noise in the multi-wavelength light curves, resulting in a high precision relative transmission spectrum with errors on the order of a pressure scale height. We attempted to fit a variety of different representative model atmospheres to the transmission spectrum, but did not find a satisfactory match to the entire spectral range. For the majority of the covered wavelength range (4000-7750 \AA) the spectrum is flat, and can be explained by an optically thick and grey cloud layer at 0.1 mbar, but this is inconsistent with a slightly deeper transit at wavelengths $> 7750$ \AA. We were not able to find an obvious systematic source for this feature, so this opacity may be the result of an additional unknown absorber.Comment: Submitted to MNRAS 17 Jan 2017, revised version after comments from reviewer, 12 pages, 10 figure

Single site observations of \textit{TESS} single transit detections

Context: TESS has been successfully launched and has begin data acquisition. To expedite the science that may be performed with the resulting data it is necessary to gain a good understanding of planetary yields. Given the observing strategy employed by TESS the probability of detecting single transits in long period systems is increased. These systems require careful consideration. Aims: To simulate the number of TESS transit detections during its 2 year mission with a particular emphasis on single transits. Additionally, to determine the feasibility of ground-based follow-up observations from a single site. Methods: A distribution of planets is simulated around the $\sim$ 4 million stars in the TESS Candidate Target List. These planets are tested for detectable transits and characterised. Based on simulated parameters the single transit detections are further analysed to determine which are amenable to ground-based follow-up. Results: TESS will discover an approximate lower bound of 4700 planets with around 460 being single transits. A large fraction of these will be observable from a single ground-based site. This paper finds that, in a single year, approximately 1000 transit events of around 320 unique TESS single transit detections are theoretically observable. Conclusions: As we consider longer period exoplanets the need for exploring single transit detections increases. For periods $\gtrsim45$ days the number of single transit detections outnumber multitransits by a factor of 3 (82$\pm$18 and 25$\pm$7 respectively) a factor which only grows as longer period detections are considered. Therefore, it is worth expending the extra effort required to follow-up these more challenging, but potentially very rewarding, discoveries. Additionally, we conclude that a large fraction of these targets can be theoretically observed from just a single ground-based site.Comment: 12 pages, 19 figures. To be published in Astronomy and Astrophysic

The shared evaporation history of three sub-Neptunes spanning the radius-period valley of a Hyades star

We model the evaporation histories of the three planets around K2-136, a K-dwarf in the Hyades open cluster with an age of 700 Myr. The star hosts three transiting planets, with radii of 1.0, 3.0 and 1.5 Earth radii, where the middle planet lies above the radius-period valley and the inner and outer planets are below. We use an XMM-Newton observation to measure the XUV radiation environment of the planets, finding that the X-ray activity of K2-136 is lower than predicted by models but typical of similar Hyades members. We estimate the internal structure of each planet, and model their evaporation histories using a range of structure and atmospheric escape formulations. While the precise X-ray irradiation history of the system may be uncertain, we exploit the fact that the three planets must have shared the same history. We find that the Earth-sized K2-136b is most likely rocky, with any primordial gaseous envelope being lost within a few Myr. The sub-Neptune, K2-136c, has an envelope contributing 1-1.7% of its mass that is stable against evaporation thanks to the high mass of its rocky core, whilst the super-Earth, K2-136d, must have a mass at the upper end of the allowed range in order to retain any of its envelope. Our results are consistent with all three planets beginning as sub-Neptunes that have since been sculpted by atmospheric evaporation to their current states, stripping the envelope from planet b and removing most from planet d whilst preserving planet c above the radius-period valley.Comment: Accepted for publication on MNRA