The Lutz–Kelker bias in trigonometric parallaxes

The theoretical prediction that trigonometric parallaxes suffer from a statistical effect has become topical again now that the results of the Hipparcos satellite have become available. This statistical effect, the so-called Lutz–Kelker bias, causes observed parallaxes to be too large. This has the implication that inferred distances, and hence inferred luminosities are too small. Published analytic calculations of the Lutz–Kelker bias indicate that the inferred luminosity of an object is, on average, 30 per cent too small when the error in the parallax is only 17.5 per cent. Yet, this bias has never been determined empirically. In this paper we investigate whether there is such a bias by comparing ground-based measurements with the best Hipparcos parallaxes. We find that there is indeed a large bias with an average and scatter comparable to predictions. We propose a simple method to correct for the LK bias, and apply it successfully to a subsample of our stars. We then analyse the sample of the 26 'best' Cepheids used by Feast & Catchpole to derive the zero-point of the period–luminosity relation. The final result is based on the 20 fundamental mode pulsators and leads to a distance modulus to the Large Magellanic Cloud — based on Cepheid parallaxes — of 18.56 ± 0.08, consistent with previous estimates.     

A search for soft X-ray radiation from pulsars with the Astronomical Netherlands Satellite

We present the results of a search for X-ray emission in the energy range 0.2–0.28 keV and 1–7 keV from a number of radio pulsars, including Crab, Vela and the binary pulsar PSR 1913+16, using the soft X-ray experiment aboard ANS. Except for the Crab no pulsed flux has been found. From the Vela pulsar we have detected continuous flux in agreement with earlier observations. Upper limits are given.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Dynamics of Transition Region `Moss' at high time resolution

Solar Physics - Recent observations of solar active regions made with the Transition Region and Coronal Explorer (TRACE) have revealed finely textured, low-lying extreme ultraviolet (EUV) emission,...