A new reduction of the raw Hipparcos data

Abstract

Abstract. We present an outline of a new reduction of the Hipparcos astrometric data, the justifications of which are described in the accompanying paper. The emphasis is on those aspects of the data analysis that are fundamentally different from the ones used for the catalogue published in 1997. The new reduction uses a dynamical modelling of the satellite's attitude. It incorporates provisions for scan-phase discontinuities and hits, most of which have now been identified. Solutions for the final along-scan attitude (the reconstruction of the satellite's scan phase), the abscissa corrections and the instrument model, originally solved simultaneously in the great-circle solution, are now de-coupled. This is made possible by starting the solution iterations with the astrometric data from the published catalogue. The de-coupling removes instabilities that affected great-circle solutions for short data sets in the published data. The modelling-noise reduction implies smaller systematic errors, which is reflected in a reduction of the abscissa-error correlations by about a factor 40. Special care is taken to ensure that measurements from both fields of view contribute significantly to the along-scan attitude solution. This improves the overall connectivity of the data and rigidity of the reconstructed sky, which is of critical importance to the reliability of the astrometric data. The changes in the reduction process and the improved understanding of the dynamics of the satellite result in considerable formal-error reductions for stars brighter than 8th magnitude.

Modeling of the Vela complex including the Vela supernova remnant, the binary systemγ2 Velorum, and the Gum nebula

“…North et al (2007) estimate the distance to γ 2 Velorum to be 336 +8 −7 pc based on the orbital solution for the γ 2 Velorum binary obtained from the interferometric data. Finally, a revision of the analysis of HIPPARCOS data gives a distance of 334 +40 −32 pc van Leeuwen (2007). Therefore, in the following we take the distance to γ 2 Velorum to be D γ 2 Vel 330 pc.…”

Micrometric measures and orbits of southern visual double stars

“…The elements of the new solution are given below. Using the semi-major axis from this orbit in combination with the recently revised HIPPARCOS parallax derived by van Leeuwen (2007), the new value for the mass sum is 3.1 M . Whilst the orbit can only be regarded as preliminary (it is too undefined to give sensible values for the errors) it does produce a more realistic value for the mass sum assuming there are four dwarfs in the system, and it removes the increasingly large residuals in position angle in the visual orbit of Aab-Bab.…”

Building the cosmic distance scale: from Hipparcos to Gaia

“…Part 1 of the curves: for 6 < G < 12: bright star regime with CCD saturation. Part 2 of the curves: for 12 < G < 20, photon noise regime with sky background noise and electronic noise setting in around G ∼ 20 (courtesy J. de Bruijne, ESA) available from trigonometric parallaxes: in the '90s, there were about a thousand stars with a relative precision on parallaxes better than 10%, they are 22 396 in the original Hipparcos Catalogue (Perryman et al 1997a,b) and 30 579 in the re-reduction (van Leeuwen & Fantino 2005;van Leeuwen 2007a,b) which used an improved and very detailed satellite attitude reconstruction, leading to an improved accuracy for stars brighter than 9 th magnitude.…”

“…and still reenforced when the Hipparcos re-reduction is considered (van Leeuwen 2007b). The main sequence is broad, up to two magnitudes, a result of undetected binaries and of a mixture of stars with various metallicities, rotation velocities, or evolutionary states.…”

“…For the three brightest stars out of the four in common between the two sets of observations (Figure 14 from Perryman et al 1998) This strong constraint on the Hyades distance, combined with a much more reliable information on cluster membership and the very accurate Hipparcos proper motions have been used for many further kinematic and dynamical studies showing for example that the internal velocity dispersion is 0.3 kms −1 and that the mass segregation is clearly visible (Perryman et al 1998;van Leeuwen 2007b;Perryman 2009). Moreover, the characteristics of the cluster HR diagram have been extensively compared with stellar models computed for the Hyades metallicity, [Fe/H] = 0.14 ± 0.05, in order to determine the helium content and age of the cluster: Y = 0.26 ± 0.02 and 625 ± 50 Myr for Perryman et al (1998), Y = 0.0.255 ± 0.009 and an upper limit of ∼650 Myr for Lebreton et al (2001).…”