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Astron. Astrophys. 333, 882-892 (1998)

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1. Introduction

The study of parallaxes of stars in the solar neighborhood is important for several reasons. The main one is obviously that the determination of the distance scale is based on direct distance measurements. Clearly this goal is fulfilled by HIPPARCOS. But there are questions which will not be answered by HIPPARCOS  : for example the knowledge of the bottom of the main sequence. The DENIS project (Epchtein 1994, Reid 1994) will provide some very interesting statistics especially concerning the red dwarfs regime. One way to better know those red dwarfs and eventually the brown dwarfs is to observe them in the solar vicinity where they are luminous enough. A great advantage of nearby objects is that one can access to all the parameters that describe them : position, tangential and radial velocities, distance and luminosity. Another type of object of interest which can be found in the solar neighborhood is Halo stars which kinematic helps to better understand the Halo extension. Several of them can be observed around the Sun. In any case the distance determination is the critical point of the characterization of these objects.

In the General Catalogue of Trigonometric Stellar Parallaxes (van Altena, Lee & Hoffleit 1995), beyond apparent visual magnitude 15, there are very little objects with known trigonometric parallax. On the other hand, theoretical models of stellar density foresee the existence of numerous faint objects (M type). In the same way, the density of stars with known parallax decreases very rapidly when going outward the Sun (Gliese, Jahreiss & Upgren 1986). This suggests strongly the existence of faint stars near the Sun, still unobserved. So the problem is to define an efficient strategy to find these objects.

Twenty years ago, a survey of parallaxes and proper motions of stars in the solar neighborhood has been performed by Murray & Corben (1979) and Murray, Argile & Corben (1986) demonstrating that the necessary astrometric accuracy could be reached by their method. Using [FORMULA] plates from the UK Schmidt telescope they made a systematic search for nearby stars in a selected area near the south Galactic pole by measuring the proper motions and the parallaxes of the brighter objects (6125 stars with B [FORMULA], V [FORMULA]). The advantage of such a method of searching nearby stars is to avoid the bias toward high velocity stars.

We started in 1991 at the Bordeaux observatory our project of wide field search for nearby faint stars using Schmidt plates and CCD observations. Basically our method is the same as the one pioneered by Murray et al. (1979) but we planed first to derive, from the measurements, proper motion and parallax for each object detected on the plates and not only for the brighter ones and to accurately measure the trigonometric parallax with CCD observations for the candidates (with probable detectable parallax) selected with the Schmidt plates.

We used in our project plates taken with CERGA and Tautenburg Schmidt telescopes in order to determine proper motion and parallax for 52 500 objects detected in the field. We observed 20 of the good nearby candidates with the 1.23m telescope of Calar Alto equiped with a [FORMULA] CCD chip. We could confirm for 6 of the observed targets that they were indeed nearby stars.

In Sect.  2, we describe the method of systematic detection of parallax star candidates from Schmidt plates, the astrometric reduction method and the results. In Sect.  3, we describe the CCD observations, their reduction and the astrometric results. In Sect.  4we present V photometry of the confirmed nearby stars and indication of their R and i magnitudes. In Sect.  5we discuss our results and we present our conclusions.

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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