important for spectral classification and effective temperature determination.
|This section does not cite any references or sources. (September 2012)|
Classical astrometry concerns only motions in the plane of the sky and ignores the star's radial velocity, i.e. its space motion along the line-of-sight. Whilst critical for an understanding of stellar kinematics, and hence population dynamics, its effect is generally imperceptible to astrometric measurements (in the plane of the sky), and therefore it is generally ignored in large-scale astrometric surveys. In practice, it can be measured as a Doppler shift of the spectral lines. More strictly, however, the radial velocity does enter a rigorous astrometric formulation. Specifically, a space velocity along the line-of-sight means that the transformation from tangential linear velocity to (angular) proper motion is a function of time. The resulting effect of secular or perspective acceleration is the interpretation of a transverse acceleration actually arising from a purely linear space velocity with a significant radial component, with the positional effect proportional to the product of the parallax, the proper motion, and the radial velocity. At the accuracy levels of Hipparcos it is of (marginal) importance only for the nearest stars with the largest radial velocities and proper motions, but was accounted for in the 21 cases for which the accumulated positional effect over two years exceeds 0.1 milliarc-sec. Radial velocities for Hipparcos Catalogue stars, to the extent that they are presently known from independent ground-based surveys, can be found from the astronomical database of the Centre de données astronomiques de Strasbourg.
The absence of reliable distances for the majority of stars means that the angular measurements made, astrometrically, in the plane of the sky, cannot generally be converted into true space velocities in the plan of the sky. For this reason, astrometry characterises the transverse motions of stars in angular measure (e.g. arcsec per year) rather than in km/sec or equivalent. Similarly, the typical absence of reliable radial velocities means that the transverse space motion (when known) is, in any case, only a component of the complete, three-dimensional, space velocity.
|• coincidence with ICRS (3 axes)||±0.6 mas|
|• deviation from inertial (3 axes)||±0.25 mas/yr|
|Number of entries||118,218|
|• with associated astrometry||117,955|
|• with associated photometry||118,204|
|Mean sky density||≈3 per sq deg|
|Limiting magnitude||V≈12.4 mag|
|Number of entries||1,058,332|
|• based on Tycho data||1,052,031|
|• with only Hipparcos data||6301|
|Mean sky density||25 per sq deg|
|Limiting magnitude||V≈11.5 mag|
|Completeness to 90 per cent||V≈10.5 mag|
|Completeness to 99.9 per cent||V≈10.0 mag|
|Tycho 2 Catalogue:|
|Number of entries||2,539,913|
|Mean sky density:|
|• at b=0°||≈150 per sq deg|
|• at b=±30°||≈50 per sq deg|
|• at b=±90°||≈25 per sq deg|
|Completeness to 90 per cent||V≈11.5 mag|
|Completeness to 99 per cent||V≈11.0 mag|
The final Hipparcos Catalogue was the result of the critical comparison and merging of the two (NDAC and FAST consortia) analyses, and contains 118,218 entries (stars or multiple stars), 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, and a median photometric precision (Hp<9 mag) of 0.0015 mag, with 11,597 entries were identified as variable or possibly-variable.
For the star mapper results, the data analysis was carried out by the Tycho Data Analysis Consortium (TDAC). The Tycho Ca