Reed M.D.,Missouri State University |
Foster H.,Missouri State University |
Telting J.H.,Nordic Optical Telescope |
Ostensen R.H.,Catholic University of Leuven |
And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014
We analyse 2.75 yr of Kepler spacecraft observations of the pulsating subdwarf B star KIC 10670103. These 1.4 million measurements have an impressive duty cycle of 93.8 per cent, a frequency resolution of 0.017 μHz, and a 5σ detection limit of 0.1 parts-per-thousand (ppt). We detect 278 periodicities, making KIC 10670103 the richest pulsating subdwarf B star to date. Frequencies range from 23 to 673 μHz (0.4 and 11.8 h), with amplitudes from the detection limit up to 14 ppt. Follow-up spectroscopic data were obtained from which it was determined that KIC 10670103 does not show significant radial velocity variations. Updated atmospheric model fits determined Teff = 21 485 ± 540 K, log g = 5.14 ± 0.05, and log N(He)/N(H) = 2.60 ± 0.04. We identify pulsation modes using asymptotic period spacings and frequency multiplets. The frequency multiplets indicate a spin period of 88 ± 8 d. Of the 278 periodicities detected in KIC 10670103, 163 (59 per cent) have been associated with low-degree (l ≤ 2) pulsation modes, providing tight constraints for model fitting. While the data are exquisite, amplitudes (and some frequencies) are not stable over the course of the observations, requiring tools which are non-standard for compact pulsators such as sliding Fourier transforms and Lorentzian fitting. Using the 163 identified pulsation modes, it is possible to make detailed examinations of the pulsation structure; including where the pulsation power is concentrated in radial order, over what frequency range mode trapping is inefficient, and how power switches between multiplet members. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
Neiner C.,University Paris Diderot |
Floquet M.,University Paris Diderot |
Samadi R.,University Paris Diderot |
Espinosa Lara F.,French National Center for Scientific Research |
And 27 more authors.
Astronomy and Astrophysics | Year: 2012
Context. Be stars are rapidly rotating stars with a circumstellar decretion disk. They usually undergo pressure and/or gravity pulsation modes excited by the κ-mechanism, i.e. an effect of the opacity of iron-peak elements in the envelope of the star. In the Milky Way, p-modes are observed in stars that are hotter than or equal to the B3 spectral type, while g-modes are observed at the B2 spectral type and cooler. Aims. We observed a B0IVe star, HD 51452, with the high-precision, high-cadence photometric CoRoT satellite and high-resolution, ground-based HARPS and SOPHIE spectrographs to study its pulsations in great detail. We also used the lower resolution spectra available in the BeSS database. Methods. We analyzed the CoRoT and spectroscopic data with several methods: Clean-NG, FreqFind, and a sliding window method. We also analyzed spectral quantities, such as the violet over red (V/R) emission variations, to obtain information about the variation in the circumstellar environment. We calculated a stellar structure model with the ESTER code to test the various interpretation of the results. Results. We detect 189 frequencies of variations in the CoRoT light curve in the range between 0 and 4.5 c d -1. The main frequencies are also recovered in the spectroscopic data. In particular we find that HD 51452 undergoes gravito-inertial modes that are not in the domain of those excited by the κ-mechanism. We propose that these are stochastic modes excited in the convective zones and that at least some of them are a multiplet of r-modes (i.e. subinertial modes mainly driven by the Coriolis acceleration). Stochastically excited gravito-inertial modes had never been observed in any star, and theory predicted that their very low amplitudes would be undetectable even with CoRoT. We suggest that the amplitudes are enhanced in HD 51452 because of the very rapid stellar rotation. In addition, we find that the amplitude variations of these modes are related to the occurrence of minor outbursts. Conclusions. Thanks to CoRoT data, we have detected a new kind of pulsations in HD 51452, which are stochastically excited gravito-inertial modes, probably due to its very rapid rotation. These modes are probably also present in other rapidly rotating hot Be stars. © ESO, 2012.
Camero-Arranz A.,Universities Space Research Association |
Camero-Arranz A.,NASA |
Finger M.H.,Universities Space Research Association |
Finger M.H.,NASA |
And 15 more authors.
Astrophysical Journal | Year: 2012
We present recent contemporaneous X-ray and optical observations of the Be/X-ray binary system A0535+26 with the Fermi/Gamma-ray Burst Monitor (GBM) and several ground-based observatories. These new observations are put into the context of the rich historical data (since ∼1978) and discussed in terms of the neutron-star-Be-disk interaction. The Be circumstellar disk was exceptionally large just before the 2009 December giant outburst, which may explain the origin of the unusual recent X-ray activity of this source. We found a peculiar evolution of the pulse profile during this giant outburst, with the two main components evolving in opposite ways with energy. A hard 30-70mHz X-ray quasi-periodic oscillation was detected with GBM during this 2009 December giant outburst. It becomes stronger with increasing energy and disappears at energies below 25keV. In the long term a strong optical/X-ray correlation was found for this system, however in the medium term the Hα equivalent width and the V-band brightness showed an anti-correlation after ∼2002 August. Each giant X-ray outburst occurred during a decline phase of the optical brightness, while the Hα showed a strong emission. In late 2010 and before the 2011 February outburst, rapid V/R variations are observed in the strength of the two peaks of the Hα line. These had a period of ∼25days and we suggest the presence of a global one-armed oscillation to explain this scenario. A general pattern might be inferred, where the disk becomes weaker and shows V/R variability beginning ∼6 months following a giant outburst. © 2012. The American Astronomical Society. All rights reserved.
Reig P.,Foundation for Research and Technology Hellas |
Reig P.,University of Crete |
Nespoli E.,University of Valencia |
Nespoli E.,Valencian International University |
And 2 more authors.
Astronomy and Astrophysics | Year: 2011
Context. Swift J1626.6-5156 is an X-ray pulsar that was discovered in December 2005 during an X-ray outburst. Although the X-ray data suggest that the system is a high-mass X-ray binary, very little information exists on the nature of the optical counterpart. Aims. We investigate the emission properties of the optical counterpart in the optical and near-IR bands and the long-term X-ray variability of the system to determine unambiguously the nature of this X-ray pulsar. Methods. We have performed an X/optical/IR analysis of Swift J1626.6-5156. We have analysed all RXTE observations since its discovery, archived optical spectroscopic and photometric data and obtained near-IR spectra for the first time. X-ray energy spectra were fitted with models composed of a combination of photoelectric absorption, a power law with high-energy exponential cutoff and a Gaussian line profile at 6.5 keV and an absorption edge at around 9 keV. X-ray power spectra were fitted with Lorentzian profiles. We identified and measured the equivalent width and relative intensity of the spectral features in the optical and infrared spectra to determine the spectral type of the optical counterpart. Results. The K-band spectrum shows He I λ20581Å and H I λ21660 Ã.. (Brackett-gamma) in emission, which confines the spectral type of the companion to be earlier than B2.5. The H-band spectrum exhibits the HI Br-18-11 recombination series in emission. The most prominent feature of the optical band spectrum is the strong emission of the Balmer line Hα. The 4000-5000 Åspectrum contains He II and numerous He I lines in absorption, indicating an early B-type star. The source shows three consecutive stages characterised by different types of variability in the X-ray band: a smooth decay after the peak of a major outburst, high-amplitude flaring variability (reminiscent of type I oytbursts) and quiescence. We observed that the spectrum becomes softer as the flux decreases and that this is a common characteristic of the X-ray emission for all observing epochs. An emission line feature at ~6.5 keV is also always present. Conclusions. The X-ray/optical/IR continuum and spectral features are typical of an accreting X-ray pulsar with an early-type donor. The long-term X-ray variability with its major and minor outbursts and quiescent emission is also characteristic of hard X-ray transients. We conclude that Swift J1626.6-5156 is a Be/X-ray binary with a B0Ve companion located at a distance of ∼10 kpc. © 2011 ESO.
Neiner C.,University Paris Diderot |
Grunhut J.H.,Queen's University |
Grunhut J.H.,Royal Military College of Canada |
Petit V.,West Chester University |
And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012
One hundred and twenty-five new high-precision spectropolarimetric observations have been obtained with ESPaDOnS (Eschelle Spectro-Polarimetric Device for the Observation of Stars) at the Canada-France-Hawaii Telescope and Narval at Télescope Bernard Lyot to investigate the magnetic properties of the classical Be star ω Ori. No Stokes V signatures are detected in our polarimetric data. Measurements of the longitudinal magnetic field, with a median error bar of 30G, and direct modelling of the mean least-squares deconvolved Stokes V profiles yield no evidence for a dipole magnetic field with polar surface strength greater than ∼80G. We are therefore unable to confirm the presence of the magnetic field previously reported by Neiner et al. However, our spectroscopic data reveal the presence of periodic emission variability in H and He lines analogous to that reported by Neiner et al., considered as evidence of magnetically confined circumstellar plasma clouds. We revisit this hypothesis in light of the new magnetic analysis. Calculation of the magnetospheric Kepler radius R K and confinement parameter η * indicates that a surface dipole magnetic field with a polar strength larger than 63G is sufficient to form of a centrifugally supported magnetosphere around ω Ori. Our data are not sufficiently sensitive to detect fields of this magnitude; we are therefore unable to confirm or falsify the magnetic cloud hypothesis. Based on our results, we examine three possible scenarios that could potentially explain the behaviour of ω Ori: (1) that no significant magnetic field is (or was) present in ω Ori, and that the observed phenomena have their origin in another mechanism or mechanisms than corotating clouds. We are, however, unable to identify one; (2) that ω Ori hosts an intermittent magnetic field produced by dynamo processes; however, no such process has been found so far to work in massive stars and especially to produce a dipolar field; and (3) that ω Ori hosts a stable, organized (fossil) magnetic field that is responsible for the observed phenomena, but with a strength that is below our current detection threshold. Of these three scenarios, we consider the second one (dynamo process) as highly unlikely, whereas the other two should be falsifiable with intense monitoring. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.