This chapter covers the use of the Hipparcos data in two loosely connected areas: observations and interpretation related to the Solar System, and observations related to exoplanets.

Hipparcos Solar System objects

Astrometric and photometric measurements of a number of Solar System objects were performed either by the Hipparcos main instrument (and thus appear as the Hipparcos Catalogue Solar System Annex) or by the star mapper instrument (and thus appear in the Tycho Catalogue Solar System Annex). A summary is provided in Table 10.1. The results concern mainly asteroids, but also the planetary satellites Europa, Ganymede, Callisto, Titan and Iapetus, and the major planets Uranus and Neptune. Reductions were accurate to the mas level for the main mission. A detailed description of the resulting catalogue contents is given in ESA (1997), Volume 1, Section 2.7. Volume 3, Chapter 15 gives details of the specific data analysis aspects. Hestroffer et al. (1998a) also summarised the measurements and reductions relevant for Solar System objects, the objects observed, and the presentation of results.

There were two main objectives for the inclusion of Solar System objects within the observing programme. The first was to establish the relationship between the resulting dynamical reference system and the stellar reference frame (ICRF). To date, the theoretical positional precision of asteroids had never been met observationally, and they only entered into the FK5 solution, for example, with relatively modest weight. The second objective was to acquire improved positional data at a series of epochs to enable dynamical and physical studies of these objects. In the case of high-accuracy astrometry of very close encounters, for example, masses of some asteroids can be obtained.

Appendix C Author gallery

Introduction

The use of photography to determine star positions began around 1870, flourished with the immense international cooperation of the Carte du Ciel project to map the entire celestial sphere to about 15 mag, and remained one of the most important astrometric techniques until the last decade or so of the twentieth century. Schmidt telescopes were constructed, from the 1930s onwards, with astrometry as their main objective. Such surveys have only recently been superseded by ground-based digital surveys in terms of classifying large numbers of very faint objects. In parallel, fast and accurate measuring machines and associated reduction software were developed. Kovalevsky (2002) provides details of the underlying techniques, including image formation, atmospheric effects, and plate measurements and reductions.

The development of stellar reference frames during the second half of the twentieth century has comprised both meridian and photographic observational campaigns, the former to provide reference stars with a density of about one star per square degree for the reduction of the plates obtained in the latter.

Hipparcos has allowed a re-calibration of basic meridian circle observations, of photographic plates, and of other classical astrometric instruments (Figure 2.1). Telescopes used may be classical astrographs (typified by the Carte du Ciel refracting astrograph of field ˜2°), Schmidt telescopes (with a larger field of view of ˜6°, using reflectors to minimise chromatic aberration and a correcting optical element to control spherical aberration), and long-focus instruments to obtain a larger image scale: either using refractors equipped with photographic plates, such as the Sproul in Princeton, or using reflectors as in the US Naval Observatory 1.55-m Strand telescope at Flagstaff (Figure 2.15 below).

Introduction

While the chemical composition, temperatures and pressures deep in stellar interiors are out of reach of direct investigation, their observational consequences in terms of stellar luminosity, surface temperature, radii, masses, surface chemical composition, seismological oscillations, Galactic chemical enrichment, etc. are more directly accessible.

Stellar evolution is driven by changes in chemical composition caused by nuclear reactions. The development of numerical codes to calculate models of stellar structure and evolution began some 50 years ago with the pioneering works of Schwarzschild (1958) and Henyey et al. (1959). Matching resulting stellar models to a wide range of astronomical observations has been a hugely extensive and extremely successful field of research over many decades. Continuously improving models combined with advances in many observational areas have led to a progressively deeper understanding of the numerous physical processes that occur during the various stages of stellar formation and evolution.

Currently, the Sun provides the most stringent tests of theories of stellar structure and evolution. Its surface chemical composition is generally considered to be well-determined (Anders & Grevesse, 1989), although a significant decrease in metal content of the convection zone has recently been inferred from 3d hydrodynamical models (Asplund et al, 2005; Grevesse et al., 2007).

Introduction

Overall structure of the Galaxy

Billions of stars, as well as planets, interstellar gas (predominantly atomic and molecular hydrogen), interstellar dust, and dark matter are gravitationally bound to form the Galaxy – a magnificent disk spiral system, supported by rotation. Most of the visible stars in the Galaxy, some 1011 objects of all types, masses and ages, lie in a flattened disk, a roughly axisymmetric structure whose constituent stars have formed at a fairly steady rate throughout the history of the Galaxy. They have a relatively high metal content compared to primordial abundances, inherited from the interstellar gas from which they formed, itself comprising the metal-rich debris of exploding supernovae. The Sun is located at about 7.5–8.5 kpc from the Galactic centre, and moves around it in a roughly circular orbit in a period of about 250 million years. The disk appears to be segregated into ‘thin’ and ‘thick’ components with different scale heights, angular rotation, ages, and metallicity. It shows significant spatial and kinematic structure, notably spiral arms, a central bar, and a warping of the outer plane.

The inner kpc of the disk also contains the bulge, which is less flattened, and consists mostly of fairly old stars. At its centre lies a super-massive black hole of ∼3×106M⊙. The stellar bulge is a major element of galaxy classification schemes, and may be relatively small (like the Milky Way), or large and luminous. It includes an old population traced by bulge RR Lyrae stars, but probably includes stars with a wide range of ages.

Introduction

Trigonometric parallaxes provide a distance measurement (essentially) free from model assumptions, at least at levels of order 1 mas. With Hipparcos parallax accuracies of σpi ˜ 1 mas, distances to individual objects at 10% accuracy are achieved out to ∼100 pc. In principle, the method can be applied to objects even at very large distances: the future ESA astrometric mission Gaia, with accuracies of some 10μas at V ∼ 10 mag, will measure distances of large numbers of sufficiently bright objects to 10% accuracy at 10 kpc. SIM PlanetQuest should exceed this by a factor of 10, reaching ∼1 μas for a reasonable number of bright objects. Parallaxes a further factor of 100 more accurate still, at around 10 nanoarcsec, would still be above the effects of interstellar and interplanetary scintillation in the optical, and above stochastic gravitational wave noise, and would in principle provide direct trigonometric distance measurements out to cosmological distances. Below 1 μas, however, stellar surface structure (Eriksson & Lindegren, 2007), and relativistic modelling (Anglada Escudé, 2007), may introduce significant barriers.

At the most basic level, knowledge of the distance to an astronomical object is necessary to convert its apparent properties such as magnitude or angular radius, to absolute quantities (luminosity and linear size respectively). In the absence of direct individual measurements, distance estimates must make recourse to various creative but less direct methods. Mean distances to groups of objects can be derived with higher formal accuracy using the Hipparcos data alone, notably to the Hyades open cluster at ∼45 pc and to the Pleiades open cluster at ∼120–130 pc, simply by averaging (or suitably weighting) a number of individual parallaxes.

Introduction

This chapter considers the Hipparcos contributions to the study of the Population I open clusters (in contrast with globular clusters, treated in Section 9.11), dynamical streams or moving groups, and associations. It should be recognised at the outset that the terms are potentially confusing, lack unambiguous definitions, and are occasionally used interchangeably. Broadly, clusters are reasonably dense concentrations of older stars (typically ≳ 50–100Myr), while associations are more extended collections of younger and more massive stars (typically ≲ 25–50Myr). More precise definitions are given in the box on page 274.

The literature is even more confusing when it comes to the definition of a ‘moving group’. The term is sometimes simply used to denote any system of stars sharing the same space motion, and thus to implicitly include open clusters, globular clusters, and associations. Even in a more restricted sense, excluding these tighter groupings, it has become clear from the Hipparcos results that the term has been used in the past to describe kinematic groups with different origins: thus ‘old moving groups’ (or large-scale stellar or dynamical streams, or ‘superclusters’) appear to include evaporating halos of open clusters, along with resonant dynamical structures in the solar neighbourhood, and systematic velocity structures perhaps imparted by spiral arm shocks. These are treated in Section 6.9. Young moving groups, with ages ≲ 25–50Myr, appear to represent the sparser and more immediate dissipation products of the more youthful associations, and are treated in Section 6.10.

All stars are considered to have been born in dense gas clouds, and clusters and associations appear to formfrommassive dense cores within molecular clouds.

The context

The fundamental task of measuring stellar positions, and the derived properties of distances and space motions, has preoccupied astronomers for centuries. As one of the oldest branches of astronomy, astrometry is concerned with measurement of the positions and motions of planets and other bodies within the Solar System, of stars within our Galaxy and, at least in principle, of galaxies and clusters of galaxies within the Universe as a whole. Accurate star positions provide a celestial reference frame for representing moving objects, and for relating phenomena at different wavelengths. Determining the systematic displacement of star positions with time gives access to their motions through space. Determining their apparent annual motion as the Earth moves in its orbit around the Sun gives access to their distances through measurement of parallax. All of these quantities, and others, are accessed from high-accuracy measurements of the relative angular separation of stars. Repeated measurements over a period of time provide the pieces of a celestial jigsaw, which yield a stereoscopic map of the stars and their kinematic motions.

What follows, either directly from the observations or indirectly from modelling, are absolute physical stellar characteristics: stellar luminosities, radii, masses, and ages; and their dynamical signatures. The physical parameters are then used to understand their internal composition and structure, to disentangle their space motions and, eventually, to explain in a rigorous and consistent manner how the Galaxy was originally formed, and how it will evolve in the future. Significantly, space motions reflect dynamical perturbations of all other matter, visible or invisible.

Numerical quantities This compilation of numerical quantities includes both fundamental defining quantities, and certain relevant derived and associated reference quantities. The system of astronomical units relevant for astrometry is essentially defined by four numbers: the length of the day, d; the mass of the Sun, M⊙, or in practice GM⊙; the astronomical unit, Am; and the Gaussian constant of gravitation, k (a discussion of the complexities and limitations can be found in Klioner 2008). Many of the quantities here have been compiled for the Gaia mission's parameter database by J. de Bruijne. As a result of changing definitions, some of the physical constants used in the construction of the Hipparcos and Tycho Catalogues (Table 1.3, notably Am, GM⊙, GM⊕ and ∈) have been updated compared to those listed on page 9. Other comments are as follows:

• CODATA06: these parameters are ‘2006 CODATA recommended values’, from the CODATA Task Group on Fundamental Constants (see http://www.codata.org and http://physics.nist.gov/cuu/Constants).

• INPOP06: self-consistent (TCB-compatible) Solar System quantities from the numerical planetary ephemeris developed at the IMCCE–Observatoire de Paris (Fienga et al., 2008). They provide an alternative to the JPL development ephemeris solutions DE405 and the latest version DE414 (Konopliv et al., 2006).

• IAU(1976): the IAU (1976) system of astronomical constants.

1. The astronomical unit (in m) is a defining constant in INPOP06, consistent with the combination of the light travel time and the speed of light used in the JPL DE solutions.

2. The value here is derived from the other defining quantities (CODATA06 gives 5.670 400(40) × 10–8).

Overview

This chapter describes various aspects of the Hipparcos and Tycho Catalogues useful for understanding the scientific exploitation considered in subsequent chapters. It describes some of the satellite measurement principles relevant for an understanding of the catalogue contents; the published intermediate astrometry data; details of the adopted reference frame; basic transformations relevant to catalogue users; details of the Tycho 2 Catalogue construction; error assessment; and details of associated data such as radial velocities and cross-identifications.

The Hipparcos Catalogue contains 118 218 entries, corresponding to an average of some three stars per square degree over the entire sky. Median precision of the five astrometric parameters (Hp < 9 mag) exceeded the original mission goals, and are between 0.6–1.0 mas. Some 20 000 distances were determined to better than 10%, and 50 000 to better than 20%. The inferred ratio of external to standard errors is ˜1. 0–1. 2, and estimated systematic errors are below 0.1 mas. The number of solved or suspected double or multiple stars is 23 882. Photometric observations yielded multi-epoch photometry with a mean number of 110 observations per star, a median photometric precision (Hp < 9 mag) of 0.0015 mag, and 11 597 entries were identified as variable or possibly variable.

The Tycho Catalogue of just over 1 million stars was superseded in 2000 by the Tycho 2 Catalogue of some 2.5 million stars; both included two-colour photometry.

Introduction

Stellar systems composed of two or more stars (binaries or multiples) exist in a wide range of configurations (see Trimble, 2002, for an introduction). They range from tight binaries with orbital periods of hours or days, and separations down to less than 10R⊙ (˜0.1 AU), to marginally-bound wide binaries with orbital periods > 105 yr and separations up to 20 000 AU (0.1 pc) or more. Their scientific importance is extensive, ranging from statistics of their physical properties (frequency, periods, mass ratios, eccentricities) providing constraints on theoretical models of star formation, various aspects of stellar evolution and mass transfer, and their central role for mass determination.

Binaries are extremely common in the Galaxy, with various estimates suggesting that some 50% or more of all stars occur in gravitationally bound groups having a multplicity of 2 or higher. The true binary frequency is hard to establish for any given population due to the different techniques required to probe different separation ranges, and to the inherent difficulties of detecting binaries with very small or very large angular separations. Binary frequency appears to correlate well with stellar ages: for example, low-mass pre-main-sequence stars in the star-forming region Taurus–Auriga have an (inferred) binary frequency as high as 80–100% (Leinert et al., 1993; Köhler & Leinert, 1998), values confirmed by studies of other star-forming regions.

Studies of component mass ratios give different results according to observational technique, sample population, and correction for selection effects. A bimodal distribution is often reported, with one peak arising from equal-mass components, and another at a mass ratio of around 0.2–0.3.

This chapter presents results for certain specific stellar types, divided into pre-main-sequence stars, mainsequence stars, X-ray sources in general, and post-mainsequence evolutionary phases. Additionally, it covers the use of the Hipparcos and Tycho stars as probes of the local interstellar medium.

Pre-main-sequence stars

Introduction

According to the theory of Hayashi (1961), a star begins its pre-main-sequence life with a very large radius, well in excess of the value that it finally assumes when it reaches the main sequence. The large surface area implies a high luminosity, more than can be transported through radiation alone. Thus, all sufficiently massive pre-main-sequence objects were predicted to be highly convective, spanning a broad region of the HR diagram above the main sequence, and evolving to the main sequence over the Kelvin–Helmholtz time scale. Subsequent refinements regarding the ignition of light elements and final approach to the main sequence were added by Iben (1965) and others. Detailed models of pre-main-sequence evolution have subsequently been developed by, e.g. D'Antona & Mazzitelli (1994, 1997), Palla & Stahler (1993, 1999), amongst others.

The starting radius is now considered to follow a certain locus, or ‘birthline’, in the HR diagram, where an object first becomes visible. Subsequently, detailed evolution depends on mass, protostellar mass accretion rate, mass loss due to winds, internal rotation, magnetic field, etc. Typical models, however, consider the pre-main-sequence star to be a non-rotating sphere of constant mass, contracting from the birthline to the ZAMS, with fusion dominated by hydrogen with contributions from deuterium and, at ages close to 107 yr, pre-main-sequence lithium burning (for further details see, e.g. D'Antona & Mazzitelli 1997 and Palla & Stahler 1999).

This chapter reviews the details of the photometric data acquired by the Hipparcos satellite, and the scientific results based on the large body of homogeneous photometric data, including variability, provided in the Hipparcos and Tycho Catalogues.

Hipparcos and Tycho photometric data

A description of the photometric measurements and results from Hipparcos is given by van Leeuwen et al (1997a), from where the following details of the data reduction methods used, the variability analysis, and the data products, are extracted. Further descriptions of all these aspects can be found in the Hipparcos and Tycho Catalogues (ESA, 1997), in particular Volume 1, Section 1.3 (description of the photometric data), Volume 3, Chapters 14 and 21 (description of the data reduction methods and the verification of the results), and Volume 4, Chapters 8 and 9 (the Tycho photometric data analysis).

The main Hipparcos passband, Hp, resulted purely from an attempt to maximise the number of photons gathered in the astrometric measurements, with no consideration of astrophysical features. Photometric information was contained in both the mean intensity and the modulation amplitude of the signal as the star image passed across the main modulating grid (Figure 1.2, left). The Hp passband was defined by the spectral response of the S20 photocathode of the image tube detector, combined with the transmission of the optics. The large width of the Hp passband results in significant systematic differences between Hp and standard V magnitudes, depending on effective temperature (or colour), metallicity and interstellar extinction.

The high-precision Hp magnitudes combined with homogeneous whole-sky coverage make the Hp magnitudes a very important mission product, with the multiepoch measurements providing an important database for variability studies (Table 4.3 below).