Spectroscopic confirmation of a white dwarf companion to the B star 16 Dra

Using an Extreme Ultraviolet Explorer (EUVE) spectrum, we confirm the identification of a white dwarf companion to the B9.5V star 16 Dra (HD150100), and constrain its surface temperature to lie between 29,000K and 35,000K. This is the third B star + white dwarf non-interacting Sirius-type binary to be confirmed, after y Pup (HR2875, HD59635) and theta Hya (HR3665, HD79469). 16 Dra and its white dwarf companion are members of a larger resolved proper motion system including the B9V star 17 Dra A (HD150117). The white dwarf must have evolved from a progenitor more massive than this star, i.e. ~3.7 solar masses. White dwarf companions to B stars are important since they set an observational limit on the maximum mass for white dwarf progenitors, and can potentially be used to investigate the high mass ends of the initial-final mass relation and the white dwarf mass-radius relation.Comment: Accepted for publication in A&A, 22nd May 200

HR2875: Spectroscopic discovery of the first B star + white dwarf binary

We report the discovery, in an Extreme Ultraviolet Explorer (EUVE) short wavelength spectrum, of an unresolved hot white dwarf companion to the 5th-magnitude B5Vp star HR2875. This is the first time that a non-interacting white dwarf$+$ B star binary has been discovered; previously, the the earliest type star known with a white dwarf companion was Sirius (A1V). Since the white dwarf must have evolved from a main sequence progenitor with a mass greater than that of a B5V star ($\geq$6.0M$_\odot$), this places a lower limit on the maximum mass for white dwarf progenitors, with important implications for our knowledge of the initial-final mass relation. Assuming a pure-hydrogen atmospheric composition, we constrain the temperature of the white dwarf to be between 39,000K and 49,000K. We also argue that this degenerate star is likely to have mass significantly greater than the mean mass for white dwarf stars ($\approx$0.55M$_\odot$). Finally, we suggest that other bright B stars (e.g.\ $\theta$ Hya) detected in the extreme ultraviolet surveys of the ROSAT Wide Field Camera and EUVE may also be hiding hot white dwarf companions.Comment: Accepted for MNRAS pink pages; 4 pages, 1 figur

Evolved solar systems in Praesepe

"Copyright 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics." Original paper can be found at: http://scitation.aip.org/"We have obtained near-IR photometry for the 11 Praesepe white dwarfs, to search for an excess indicative of a dusty debris disk. All the white dwarfs are in the DAZ temperature regime, however we find no indications of a disk around any white dwarf. We have, however determined that the radial velocity variable white dwarf WD0837+185 could have an unresolved T8 dwarf companion that would not be seen as a near-IR excess.Final Accepted Versio

A search for hidden white dwarfs in the ROSAT EUV survey

The ROSAT WFC survey has provided us with evidence for the existence of a previously unidentified sample of hot white dwarfs (WD) in non-interacting binary systems, through the detection of EUV and soft X-ray emission. These stars are hidden at optical wavelengths due to their close proximity to much more luminous main sequence (MS) companions (spectral type K or earlier). However, for companions of spectral type A5 or later the white dwarfs are easily visible at far-UV wavelengths, and can be identified in spectra taken by IUE. Eleven white dwarf binary systems have previously been found in this way from ROSAT, EUVE and IUE observations (e.g. Barstow et al. 1994). In this paper we report the discovery of three more such systems through our programmes in recent episodes of IUE. The new binaries are HD2133, RE J0357+283 (whose existence was predicted by Jeffries, Burleigh and Robb 1996), and BD+27 1888. In addition, we have independently identified a fourth new WD+MS binary, RE J1027+322, which has also been reported in the literature by Genova et al. (1995), bringing the total number of such systems discovered as a result of the EUV surveys to fifteen. We also discuss here six stars which were observed as part of the programme, but where no white dwarf companion was found. Four of these are coronally active. Finally, we present an analysis of the WD+K0IV binary HD18131 (Vennes et al. 1995), which includes the ROSAT PSPC X-ray data.Comment: One Latex file containing text, One Tex file containing tables, 15 figures, MNRAS, in pres

Stellar archaeology with Gaia: the Galactic white dwarf population

Gaia will identify several 1e5 white dwarfs, most of which will be in the solar neighborhood at distances of a few hundred parsecs. Ground-based optical follow-up spectroscopy of this sample of stellar remnants is essential to unlock the enormous scientific potential it holds for our understanding of stellar evolution, and the Galactic formation history of both stars and planets.Comment: Summary of a talk at the 'Multi-Object Spectroscopy in the Next Decade' conference in La Palma, March 2015, to be published in ASP Conference Series (editors Ian Skillen & Scott Trager

Examining the Auroral Ionosphere in Three Dimensions Using Reconstructed 2D Maps of Auroral Data to Drive the 3D GEMINI Model

We use the Geospace Environment Model of Ion-Neutral Interactions (GEMINI) to create three-dimensional, time-dependent simulations of auroral ionospheric parameters in the localized, several 100 km region surrounding auroral arcs observed during a winter 2017 sounding rocket campaign, resolving three-dimensional features of fine-scale (km) flow structures in the vicinity of an auroral arc. The three-dimensional calculations of GEMINI allow (with sufficient driving data) auroral current closure to be investigated without idealizing assumptions of sheet-like structures or height integrated ionospheres. Datamaps for two nearly sheet-like arcs are reconstructed from replications of the Isinglass sounding rocket campaign data, and combined with camera-based particle inversions into a set of driving inputs to run the 3D time-dependent model. Comparisons of model results to radar density profiles and to in situ magnetometry observations are presented. Slices of volumetric current, flow, and conductance structures from model outputs are used to interpret closure currents in an auroral arc region, and are compared to original in situ measurements for verification. The predominant source of return current region field aligned current closure for these slow time variation events is seen to be from the conductance gradients, including the Hall. The importance of the versus terms in the determination of the current structure provides a more complicated picture than a previous GEMINI study, which relied predominantly on the divergence of the electric field to determine current structure. Sensitivity of data-driven model results to details of replication and reconstruction processes are discussed, with improvements outlined for future work

A chemical survey of exoplanets with ARIEL

Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25–7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10–100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed – using conservative estimates of mission performance and a full model of all significant noise sources in the measurement – using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL – in line with the stated mission objectives – will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio

Brown dwarf companions to white dwarfs

"Copyright 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics."Brown dwarf companions to white dwarfs are rare, but recent infra-red surveys are slowly revealing examples. We present new observations of the post-common envelope binary WD0137-349, which reveals the effects of irradiation on the approximate to 0.05M(circle dot) secondary, and new observations of GD 1400 which show that it too is a close, post-common envelope system. We also present the latest results in a near-infrared photometric search for unresolved ultra-cool companions and to white dwarfs with UKIDSS. Twenty five DA white dwarfs were identified as having photometric excesses indicative of a low mass companion, with 8-10 of these having a predicted mass in the range associated with brown dwarfs. The results of this survey show that the unresolved (< 2 '') brown dwarf companion fraction to DA white dwarfs is 0.3 <= f(WD+BD) <= 1.3%.Final Accepted Versio