The properties of subdwarfs

Abstract

The photometric programmes at the Cape Observatory and the parallel spectroscopic programmes undertaken by Cape staff at the Radcliffe Observatory, Pretoria, have enabled us to produce considerable quantities of southern star data. Some of this has been published in the seriesFundamental Data for Southern Starsof which No. 5 has just appeared. Included among these stars are numbers of subdwarfs and Dr M. E. Dixon and Mr P. A. Fairall of the University of Cape Town and I have recently attempted an analysis of subdwarf properties. We wanted to consider subdwarfs from all over the sky and our first list included 162 objects. However, we have reduced this number to about a half by throwing out all known or suspected double stars, and all stars for which there was not a full and accurate set of observations—that is, position, spectral type,UBVphotometry radial velocity and proper motion. All the stars should be at distances less than a little over 200 pc. The practical definition of a subdwarf in the northern sky is that somebody has said so in the literature. In the south a star is a subdwarf if Przybylski, Deeming or I say so. We are in the hands of northern astronomers for half the sky. For the south it is our own responsiblity. Now subdwarfs are liable to severe observational selection. Any star which has been measured for parallax and found to give a near zero value, coupled with a large proper motion, has a good chance of being a subdwarf. These circumstances will usually give it a high-space velocity. It is elementary that if you take the mean of high velocities you will get a high mean velocity. We have been particularly on the look-out for subdwarfs which are not likely to be so biased. The kind of programme we do, by its very rather pedestrian nature, helps to avoid this sort of observational selection. My part is to classify the spectra of stars, often never previously observed, and not to make mistakes—if I can help it. I am now thinking particularly of subdwarfs which might be given spectral types from about F0 to about G5 at latest. They are picked out from their dwarf characteristics and the weakness of metallic lines. We have attempted to add verbal classifications such asIncipient, Moderate, Marked, ExtremeandVery Extremeand these we have shown correspond to increasing ultraviolet excess. What one must watch for is the clue that the excitation is relatively low—otherwise extreme subdwarfs will be classified as much too early, simply because H and K and the Balmer lines are the only lines of any strength left. For reasons obvious enough from curve-of-growth arguments there will be relatively a much greater reduction in line strength of the remaining metallic spectrum. This will appear as a kind of ghostly or miniaturized version of the kind of metallic spectrum appropriate to the spectral type. It is not much use trying to be more specific and to talk about line ratios because in extreme cases the lines are barely visible. I think that the fact that it is so easy to write off subdwarfs to the wrong spectral class goes far to explain some of the colour anomalies claimed. So, to summarize, you must see the spectrum to identify a star as a subdwarf in this range, but your spectral type will be inaccurate and you must turn to photometric data for any kind of precise discussion. You will usually find an ultraviolet excess. This alone is not sufficient to identify a subdwarf. We are going to push the photometric data pretty hard and photometric anomalies can arise in all kinds of ways—duplicity is one example. We have plotted all our subdwarfs on an ultraviolet excess (B–V) diagram. Ultraviolet excesses are in terms of refractor (U–B); we have a good conversion for the range we need. For analyses we have taken stars in groups. Our first box encloses a natural clustering of 34 points and is bounded by (B–V) = 0·37, (B–V) = 0·52,e= 0·17 ande= 0·10 with the bottom left-hand corner of the box chopped off. We get the solar motion from radial velocities only. Then we take in all the data and assume that a subdwarf of given colour and colour excess is a standard object. We adopt various values of absolute magnitude and compute space motions. Alternatively we put all the stars the same distance below the main sequence, and compute space motions. The means have to match the solar motion as well as possible and we also keep an eye on the trigonometrical parallaxes, such as they are.