Hipparcos - Scientific Results

Scientific Results

The Hipparcos results impact a very broad range of astronomical research, which can be classified into three major themes:

(a) the provision of an accurate reference frame: this has allowed the consistent and rigorous re-reduction of historical astrometric measurements, including those from Schmidt plates, meridian circles, the 100-year old Astrographic Catalogue, and 150 years of Earth-orientation measurements. These, in turn, have yielded a dense reference framework with high-accuracy long-term proper motions (the Tycho-2 Catalogue). Reduction of current state-of-the-art survey data has yielded the dense UCAC2 Catalogue of the US Naval Observatory on the same reference system, and improved astrometric data from recent surveys such as the Sloan Digital Sky Survey and 2MASS. Implicit in the high-accuracy reference frame is the measurement of General Relativistic light bending, and the detection and characterisation of double and multiple stars;

(b) constraints on stellar structure and stellar evolution: the accurate distances and luminosities of 100,000 stars has provided the most comprehensive and accurate data set of fundamental stellar parameters to date, placing constraints on internal rotation, element diffusion, convective motions, and asteroseismology. Combined with theoretical models and other data it yields evolutionary masses, radii, and ages for large numbers of stars covering a wide range of evolutionary states;

(c) Galactic kinematics and dynamics: the uniform and accurate distances and proper motions have provided a substantial advance in understanding of stellar kinematics and the dynamical structure of the solar neighbourhood, ranging from the presence and evolution of clusters, associations and moving groups, the presence of resonance motions due to the Galaxy's central bar and spiral arms, determination of the parameters describing Galactic rotation, discrimination of the disk and halo populations, evidence for halo accretion, and the measurement of space motions of runaway stars, globular clusters, and many other types of star.

Associated with these major themes, Hipparcos has provided results in topics as diverse as Solar System science, including mass determinations of asteroids, Earth's rotation and Chandler Wobble, the internal structure of white dwarfs, the masses of brown dwarfs, the characterisation of extra-solar planets and their host stars, the height of the Sun above the Galactic mid-plane, the age of the Universe, the stellar initial mass function and star formation rates, and strategies for the search for extraterrestrial intelligence. The high-precision multi-epoch photometry has been used to measure variability and stellar pulsations in many classes of objects. The Hipparcos and Tycho Catalogues are now routinely used to point ground-based telescopes, navigate space missions, and drive public planetaria.

Since 1997, several thousand scientific papers have been published making use of the Hipparcos and Tycho Catalogues. A detailed review of the Hipparcos scientific literature between 1997–2007 was published in 2009, and a popular account of the project in 2010. Some examples of notable results include (listed chronologically):

• studies of Galactic rotation from Cepheid variables

• the nature of Delta Scuti variables

• studies of local stellar kinematics

• testing the white dwarf mass-radius relation

• the structure and dynamics of the Hyades cluster

• kinematics of Wolf-Rayet stars and O-type runaway stars

• subdwarf parallaxes: metal-rich clusters and the thick disk

• fine structure of the red giant clump and associated distance determinations

• unexpected stellar velocity distribution in the warped Galactic disk

• confirming the Lutz Kelker bias of parallax measurement

• refining the Oort and Galactic constants

• Galactic disk dark matter, terrestrial impact cratering and the law of large numbers

• vertical motion and expansion of the Gould Belt

• the use of gamma ray bursts as direction and time markers in SETI strategies

• evidence of a galaxy merger in the early formation history of the Milky Way

• study of nearby OB associations

• close approaches of stars to the Solar System

• studies of binary star orbits and masses

• the HD 209458 planetary transits

• formation of the stellar Galactic halo and thick disk

• the local density of matter in the Galaxy and the Oort limit

• ice age epochs and the Sun's path through the Galaxy

• local kinematics of K and M giants and the concept of superclusters

• an improved reference frame for long-term Earth rotation studies

• the local stellar velocity field in the Galaxy
• Identification of two possible "siblings" of the Sun, (HIP 87382 and HIP 47399) to be studied for evidence of exoplanets

One controversial result has been the derived proximity, at about 120 parsecs, of the Pleiades cluster, established both from the original catalogue as well as from the revised analysis. This has been contested by various other recent work, placing the mean cluster distance at around 130 parsecs.