Erik Høg

Testing the White Dwarf Mass-Radius Relation with Hipparcos*

We present the Hipparcos parallaxes and resulting radii for 10 white dwarfs in visual binaries or common proper-motion systems and 11 field white dwarfs. For bright stars, Hipparcos parallaxes have uncertainties approaching 1 mas and are thus considerably more accurate than earlier ground-based parallaxes. Overall, our results support the predictions of the white dwarf mass-radius relation and our understanding of stellar degeneracy. Our most important finding for an individual object is the position of 40 Eri B, a well-known puzzle, now consistent with single-star evolution. In addition, we present evidence supporting the existence of a range of atmosphere thicknesses for hydrogen (DA) white dwarfs.

The Tycho double star catalogue

We report the discovery of 13 251 visual double stars, mostly with separations between 0.3 and 1 arcsec, from a dedicated re-reduction of the Tycho data from the star mapper of the ESA Hipparcos satellite. The new doublesare combined with 18 160 WDS systems identified in the Tycho-2 Catalogue, and 1220 new Tycho-2 doubles, to form the Tycho Double Star Catalogue, TDSC, a catalogue of absolute astrometry and B T , V T photometry for 66219 components of 32631 double and multiple star systems. We also include results for 32 263 single components for systems unresolved in TDSC, and a supplement gives Hipparcos and Tycho-1 data for 4777 additional components. The TDSC thus contains a total of 103 259 entries. Cross identifications are given to WDS, HD, Hipparcos and Tycho-2.

The Gaia mission : Science, organization and present status

The ESA space astrometry mission Gaia will measure the positions, parallaxes and proper motions of the 1 billion brightest stars on the sky. Expected accuracies are in the 725 as range down to 15 mag and sub-mas accuracies at the faint limit (20 mag). The astrometric data are complemented by low-resolution spectrophotometric data in the 3301000 nm wavelength range and, for the brighter stars, radial velocity measurements. The scientific case covers an extremely wide range of topics in galactic and stellar astrophysics, solar system and exoplanet science, as well as the establishment of a very accurate, dense and faint optical reference frame. With a planned launch around 2012 and an (extended) operational lifetime of 6 years, final results are expected around 2021. We give a brief overview of the science goals of Gaia, the overall project organisation, expected performance, and some key technical features and challenges.

Gaia Data Release 1 - The photometric data

Context. This paper presents an overview of the photometric data that are part of the first Gaia data release. Aims. The principles of the processing and the main characteristics of the Gaia photometric data are presented. Methods. The calibration strategy is outlined briefly and the main properties of the resulting photometry are presented. Results. Relations with other broadband photometric systems are provided. The overall precision for the Gaia photometry is shown to be at the milli-magnitude level and has a clear potential to improve further in future releases.

The design and performance of the Gaia photometric system

The European Gaia astrometry mission is due for launch in 2011. Gaia will rely on the proven principles of the ESA Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20 mag. Through its massive measurement of stellar distances, motions and multicolour photometry, it will provide fundamental data necessary for unravelling the structure, formation and evolution of the Galaxy. This paper presents the design and performance of the broad- and medium-band set of photometric filters adopted as the baseline for Gaia. The 19 selected passbands (extending from the UV to the far-red), the criteria and the methodology on which this choice has been based are discussed in detail. We analyse the photometric capabilities for characterizing the luminosity, temperature, gravity and chemical composition of stars. We also discuss the automatic determination of these physical parameters for the large number of observations involved, for objects located throughout the entire Hertzsprung-Russell diagram. Finally, the capability of the photometric system (PS) to deal with the main Gaia science case is outlined.

The Mass and Radius of 40 Eridani B from HIPPARCOS: An Accurate Test of Stellar Interior Theory

The astrometric satellite HIPPARCOS has determined the distance to the important white dwarf 40 Eridani B with an accuracy surpassing 1%. We use this datum to redetermine the mass and radius of this star and find that M = 0.501 ± 0.011 M☉ and R = 0.0136 ± 0.00024 R☉. These values, the most accurately determined masses and radii for any white dwarf star, place 40 Eri B securely on the zero-temperature carbon core mass-radius relation, considerably improving observational confirmation of the theory of stellar degeneracy. The mass is now sufficiently high that binary star evolutionary channels are no longer required to account for it. The data exclude the possibility that the surface hydrogen layer is as thick as 10-4 M☉.

Comparing Tycho-2 Astrometry with UCAC1

The Tycho-2 Catalogue, released in February 2000, is based on the ESA Hipparcos space mission data and various ground-based catalogs for proper motions. An external comparison of the Tycho-2 astrometry is presented here using the first US Naval Observatory CCD Astrograph Catalog (UCAC1). The UCAC1 data were obtained from observations performed at Cerro Tololo Inter-American Observatory (CTIO) between 1998 February and 1999 November, using the 206 mm–aperture five-element lens astrograph and a 4K × 4K CCD. Only small systematic differences in position between Tycho-2 and UCAC1, up to 15 mas, are found, mainly as a function of magnitude. The standard deviations of the distributions of the position differences are in the 35 to 140 mas range, depending on magnitude. The observed scatter in the position differences is about 30% larger than that expected from the combined formal, internal errors, also depending on magnitude. The Tycho-2 Catalogue has the more precise positions for bright stars (V ≤ 10 mag, while the UCAC1 positions are significantly better at the faint end (11 mag ≤ V ≤ 12.5 mag) of the magnitude range in common. UCAC1 data go much fainter (to R ≈ 16 mag) than Tycho-2 data; however, complete sky coverage is not expected before mid-2003.

Hipparcos absolute magnitudes for metal‐rich K giants and the calibration of DDO photometry

ABSTRACT Parallaxes for 581 bright K giants have been determined using the Hipparcossatellite. We combine the trigonometric parallaxes with ground based photometricdata to determine the K giant absolute magnitudes. For all these giants, absolutemagnitude estimates can also be made using the intermediate band photometric DDOsystem (Janes 1975, 1979). We compare the DDO absolute magnitudes with the veryaccurateHipparcosabsolute magnitudes, finding various systematicoffsets in the DDOsystem. These systematic effects can be corrected, and we provide a new calibration ofthe DDO system allowing absolute magnitude to be determined with an accuracy of0.35 mag in the range 2 > M V > −1. The new calibration performs well when testedon K giants with DDO photometry in a selection of low reddening open-clusters withwell-measured distance moduli. 1 INTRODUCTIONK giants are bright and ubiquitious, occuring in a wide rangeof Galactic populations, and are convenient tracer objectsfor examining the structure and kinematics of the Galaxy.The chief difficulty with these objects has been the uncer-tainty in their absolute magnitudes, which arises from thefact that stars on the giant branch form from a wide rangeof mass and age, as well as the giant branch being rathersteep as a function of colour. K giants span a broad rangeof absolute magnitude M

GAIA: global astrometric interferometer for astrophysics

We describe a concept for an interferometric space mission dedicated to global (wide-angle) astrometry. The GAIA satellite contains two small (baseline APEQ 3 m) optical interferometers of the Fizeau type, mechanically set at a large and fixed angle to each other. Each interferometer has a field of view of about one degree. Continuous rotation of the whole satellite provides angular connections between the stars passing through the two fields of view. Positions, absolute parallaxes and annual proper motions can be determined with accuracies on the 20 micro-arcsec level. The observing programme may consist of all objects to a limiting magnitude around V = 15-16, including 50 million stars. The GAIA concept, which has been proposed for a Cornerstone Mission within the European Space Agencys long-term science programme, is based on the same general principles as the very successful ESA Hipparcos mission, but takes advantage of the much higher resolution and efficiency permitted by interferometry and modern detector techniques.

Astrometry and Photometry of 400 Million Stars Brighter than 18 Mag

A satellite mission for accurate astrometry and multi-colour photometry is discussed, similar in principle to the ESA Hipparcos mission and here called the Roemer mission. The limiting magnitude will be about V = 18 mag while 13 mag is the limit of the present Hipparcos mission. Luminosities of stars up to 2 kpc away can be obtained, corresponding to a volume 10000 times larger than with Hipparcos. A mission of 5 years will provide an accuracy of 0.1 milli-arcsec at 12th magnitude for positions and parallaxes and 0.05 milli-arcsec for annual proper motions. This is achieved by a satellite using a mosaic of CCD detectors in the focal planes of two beam-combiner telescopes of 0.29 m aperture. The instrument is described and the expected performance with an input catalog of 400 million program stars is given.