Timing by Stellar Pulsations as an Exoplanet Discovery Method

The stable oscillations of pulsating stars can serve as accurate timepieces, which may be monitored for the influence of exoplanets. An external companion gravitationally tugs the host star, causing periodic changes in pulsation arrival times. This method is most sensitive to detecting substellar companions around the hottest pulsating stars, especially compact remnants like white dwarfs and hot subdwarfs, as well as delta Scuti variables (A stars). However, it is applicable to any pulsating star with sufficiently stable oscillations. Care must be taken to ensure that the changes in pulsation arrival times are not caused by intrinsic stellar variability; an external, light-travel-time effect from an exoplanet identically affects all pulsation modes. With more long-baseline photometric campaigns coming online, this method is yielding new detections of substellar companions.Comment: 9 pages, 2 figures: Invited review to appear in 'Handbook of Exoplanets,' Springer Reference Works, edited by Hans J. Deeg and Juan Antonio Belmont

HD 69686: A Mysterious High Velocity B Star

We report on the discovery of a high velocity B star, HD 69686. We estimate its space velocity, distance, surface temperature, gravity, and age. With these data, we are able to reconstruct the trajectory of the star and to trace it back to its birthplace. We use evolutionary tracks for single stars to estimate that HD 69686 was born 73 Myr ago in the outer part of our Galaxy ($r \sim 12$ kpc) at a position well below the Galactic plane ($z \sim -1.8$ kpc), a very unusual birthplace for a B star. Along the star's projected path in the sky, we also find about 12 other stars having similar proper motions, and their photometry data suggest that they are located at the same distance as HD 69686 and probably have the same age. We speculate on the origin of this group by star formation in a high velocity cloud or as a Galactic merger fragment.Comment: 28 pages, 6 figures, accepted for publication in Ap

Asteroseismology and Interferometry

Asteroseismology provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Recent developments, including the first systematic studies of solar-like pulsators, have boosted the impact of this field of research within Astrophysics and have led to a significant increase in the size of the research community. In the present paper we start by reviewing the basic observational and theoretical properties of classical and solar-like pulsators and present results from some of the most recent and outstanding studies of these stars. We centre our review on those classes of pulsators for which interferometric studies are expected to provide a significant input. We discuss current limitations to asteroseismic studies, including difficulties in mode identification and in the accurate determination of global parameters of pulsating stars, and, after a brief review of those aspects of interferometry that are most relevant in this context, anticipate how interferometric observations may contribute to overcome these limitations. Moreover, we present results of recent pilot studies of pulsating stars involving both asteroseismic and interferometric constraints and look into the future, summarizing ongoing efforts concerning the development of future instruments and satellite missions which are expected to have an impact in this field of research.Comment: Version as published in The Astronomy and Astrophysics Review, Volume 14, Issue 3-4, pp. 217-36

Identifying birth places of young isolated neutron stars

Young isolated radio-quiet neutron stars are still hot enough to be detectable at X-ray and optical wavelengths due to their thermal emission and can hence probe cooling curves. An identification of their birth sites can constrain their age. For that reason we try to identify the parent associations for four of the so-called Magnificent Seven neutron stars for which proper motion and distance estimates are available. We are tracing back in time each neutron star and possible birth association centre to find close encounters. The associated time of the encounter expresses the kinematic age of the neutron star which can be compared to its characteristic spin-down age. Owing to observational uncertainties in the input data, we use Monte-Carlo simulations and evaluate the outcome of our calculations statistically. RX J1856.5-3754 most probably originated from the Upper Scorpius association about 0.3 Myr ago. RX 0720.4-3125 was either born in the young local association TWA about 0.4 Myr ago or in Tr 10 0.5 Myr in the past. Also RX J1605.3+3249 and RBS 1223 seem to come from a nearby young association such as the Sco-Cen complex or the extended Corona-Australis association. For RBS 1223 also a birth in Sct OB2 is possible. We also give constraints on the observables as well as on the radial velocity of the neutron star. Given the birth association, its age and the flight time of the neutron star, we estimate the mass of the progenitor star. Some of the potential supernovae were located very nearby (<100pc) and thus should have contributed to the 10Be and 60Fe material found in the Earth's crust. In addition we reinvestigate the previously suggested neutron star/ runaway pair PSR B1929+10/ zeta Ophiuchi and conclude that it is very likely that both objects were ejected during the same supernova event.Comment: 14 figures, 13 table

The Distances of the Magellanic Clouds

The present status of our knowledge of the distances to the Magellanic Clouds is evaluated from a post-Hipparcos perspective. After a brief summary of the effects of structure, reddening, age and metallicity, the primary distance indicators for the Large Magellanic Cloud are reviewed: The SN 1987A ring, Cepheids, RR Lyraes, Mira variables, and Eclipsing Binaries. Distances derived via these methods are weighted and combined to produce final "best" estimates for the Magellanic Clouds distance moduli.Comment: Invited review article to appear in ``Post Hipparcos Cosmic Candles'', F. Caputo & A. Heck (Eds.), Kluwer Academic Publ., Dordrecht, in pres

Interaction Between Convection and Pulsation

This article reviews our current understanding of modelling convection dynamics in stars. Several semi-analytical time-dependent convection models have been proposed for pulsating one-dimensional stellar structures with different formulations for how the convective turbulent velocity field couples with the global stellar oscillations. In this review we put emphasis on two, widely used, time-dependent convection formulations for estimating pulsation properties in one-dimensional stellar models. Applications to pulsating stars are presented with results for oscillation properties, such as the effects of convection dynamics on the oscillation frequencies, or the stability of pulsation modes, in classical pulsators and in stars supporting solar-type oscillations.Comment: Invited review article for Living Reviews in Solar Physics. 88 pages, 14 figure

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

Spectropolarimetry of stars across the H-R diagram

The growing sample of magnetic stars shows a remarkable diversity in the properties of their magnetic fields. The overall goal of current studies is to understand the origin, evolution, and structure of stellar magnetic fields in stars of different mass at different evolutionary stages. In this chapter we discuss recent measurements together with the underlying assumptions in the interpretation of data and the requirements, both observational and theoretical, for obtaining a realistic overview of the role of magnetic fields in various types of stars.Comment: 23 pages, 3 figures, chapter 7 of "Astronomical Polarisation from the Infrared to Gamma Rays", published in Astrophysics and Space Science Library 46