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Littlefair S.P.,University of Sheffield | Naylor T.,University of Exeter | Mayne N.J.,University of Exeter | Saunders E.,Las Cumbres Observatory | Jeffries R.D.,Keele University
Monthly Notices of the Royal Astronomical Society: Letters

We present an analysis of the rotation of young stars in the associations Cepheus OB3b, NGC 2264, 2362 and the Orion Nebula Cluster (ONC). We discover a correlation between rotation rate and position in a colour-magnitude diagram (CMD) such that stars which lie above an empirically determined median pre-main sequence rotate more rapidly than stars which lie below this sequence. The same correlation is seen, with a high degree of statistical significance, in each association studied here. If position within the CMD is interpreted as being due to genuine age spreads within a cluster, then the stars above the median pre-main sequence would be the youngest stars. This would in turn imply that the most rapidly rotating stars in an association are the youngest, and hence those with the largest moments of inertia and highest likelihood of ongoing accretion. Such a result does not fit naturally into the existing picture of angular momentum evolution in young stars, where the stars are braked effectively by their accretion discs until the disc disperses. Instead, we argue that, for a given association of young stars, position within the CMD is not primarily a function of age, but of accretion history. We show that this hypothesis could explain the correlation we observe between rotation rate and position within the CMD. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Littlefair S.P.,University of Sheffield | Naylor T.,University of Exeter | Mayne N.J.,University of Exeter | Saunders E.S.,Las Cumbres Observatory | Jeffries R.D.,Keele University
Monthly Notices of the Royal Astronomical Society

We present a photometric study of I-band variability in the young association Cepheus OB3b. The study is sensitive to periodic variability on time-scales of less than a day, to more than 20 d. After rejection of contaminating objects using V, I, R and narrow-band Hα photometry, we find 475 objects with measured rotation periods, which are very likely pre-main-sequence members of the Cep OB3b star-forming region.We revise the distance and age to Cep OB3b, putting it on the self-consistent age and distance ladder of Mayne & Naylor. This yields a distance modulus of 8.8 ± 0.2 mag, corresponding to a distance of 580 ± 60 pc, and an age of 4-5 Myr.The rotation period distribution confirms the general picture of rotational evolution in young stars, exhibiting both the correlation between accretion (determined in this case through narrow-band Hα photometry) and rotation expected from disc locking, and the dependence of rotation upon mass that is seen in other star-forming regions. However, this mass dependence is much weaker in our data than found in other studies. Comparison to the similarly aged NGC 2362 shows that the low-mass stars in Cep OB3b are rotating much more slowly. This points to a possible link between star-forming environment and rotation properties. Such a link would call into question models of stellar angular momentum evolution, which assume that the rotational period distributions of young clusters and associations can be assembled into an evolutionary sequence, thus ignoring environmental effects. © 2010 The Authors. Journal compilation © 2010 RAS. Source

Bramich D.M.,European Southern Observatory | Horne K.,SUPA Physics and Astronomy | Albrow M.D.,University of Canterbury | Tsapras Y.,Las Cumbres Observatory | And 8 more authors.
Monthly Notices of the Royal Astronomical Society

We present a general framework for matching the point-spread function (PSF), photometric scaling and sky background between two images, a subject which is commonly referred to as difference image analysis (DIA). We introduce the new concept of a spatially varying photometric scale factor which will be important for DIA applied to wide-field imaging data in order to adapt to transparency and airmass variations across the field-of-view. Furthermore, we demonstrate how to separately control the degree of spatial variation of each kernel basis function, the photometric scale factor and the differential sky background. We discuss the common choices for kernel basis functions within our framework, and we introduce the mixed-resolution delta basis functions to address the problem of the size of the least-squares problem to be solved when using delta basis functions. We validate and demonstrate our algorithm on simulated and real data. We also describe a number of useful optimizations that may be capitalized on during the construction of the least-squares matrix and which have not been reported previously. We pay special attention to presenting a clear notation for the DIA equations which are set out in a way that will hopefully encourage developers to tackle the implementation of DIA software. © 2012 The Authors. Source

Fossati L.,Open University Milton Keynes | Haswell C.A.,Open University Milton Keynes | Froning C.S.,University of Colorado at Boulder | Hebb L.,Vanderbilt University | And 18 more authors.
Astrophysical Journal Letters

We present near-UV transmission spectroscopy of the highly irradiated transiting exoplanet WASP-12b, obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. The spectra cover three distinct wavelength ranges: NUVA (2539-2580 Å), NUVB (2655-2696 Å), and NUVC (2770-2811 Å). Three independent methods all reveal enhanced transit depths attributable to absorption by resonance lines of metals in the exosphere of WASP-12b. Light curves of total counts in the NUVA and NUVC wavelength ranges show a detection at a 2.5σ level. We detect extra absorption in the Mg II λλ2800 resonance line cores at the 2.8σ level. The NUVA, NUVB, and NUVC light curves imply effective radii of 2.69 ± 0.24 R J, 2.18 ± 0.18 RJ, and 2.66 ± 0.22 R J respectively, suggesting the planet is surrounded by an absorbing cloud which overfills the Roche lobe. We detect enhanced transit depths at the wavelengths of resonance lines of neutral sodium, tin, and manganese, and at singly ionized ytterbium, scandium, manganese, aluminum, vanadium, and magnesium. We also find the statistically expected number of anomalous transit depths at wavelengths not associated with any known resonance line. Our data are limited by photon noise, but taken as a whole the results are strong evidence for an extended absorbing exosphere surrounding the planet. The NUVA data exhibit an early ingress, contrary to model expectations; we speculate this could be due to the presence of a disk of previously stripped material. © 2010 The American Astronomical Society. All rights reserved. Source

Maxted P.F.L.,Keele University | Anderson D.R.,Keele University | Collier Cameron A.,University of St. Andrews | Hellier C.,Keele University | And 12 more authors.
Publications of the Astronomical Society of the Pacific

We report the discovery of a transiting planet with an orbital period of 3.05 days orbiting the star TYC 7247-587-1. The star, WASP-41, is a moderately bright G8 V star (V=11.6) with a metallicity close to solar ([Fe/H]=-0.08 ± 0.09). The star shows evidence of moderate chromospheric activity, both from emission in the cores of the Ca II H and K ines and photometric variability with a period of 18.4 days and an amplitude of about 1%. We use a new method to show quantitatively that this periodic signal has a low false-alarm probability. The rotation period of the star implies a gyrochronological age for WASP-41 of 1.8 Gyr with an error of about 15%. We have used a combined analysis of the available photometric and spectroscopic data to derive the mass and radius of the planet (0.92±0.06 MJup, 1.20±0.06 RJup). Further observations of WASP-41 can be used to explore the connections between the properties of hot Jupiter planets and the level of chromospheric activity in their host stars. © 2011. The Astronomical Society of the Pacific. Source

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