A NEW OBSERVING PROJECT TO DETECT ECLIPSING DWARF NOVAE

INTRODUCTION

This is to introduce a new Pro-Am project run by the Variable Star Section to check for the occurrence of eclipses in dwarf novae not previously known to exhibit eclipses. Both visual and CCD amateur observations are needed. Prof. Tim Naylor (formerly at Keele University, now at Exeter University) has suggested that the results of this project should be publishable whether positive or negative, and we anticipate that if a new eclipsing dwarf nova is detected, professional assistance in confirming its nature will be readily forthcoming.

A simple check shows that out of 400+ known dwarf novae (DNe), no more than a dozen or so are known to undergo eclipses due to the secondary star blocking out light from the accretion disk (and sometimes the primary star) during orbital motion. Eclipses should occur when the angle of inclination of the system (the angle between the orbital axis and the line of sight to the observer) exceeds about 70 degrees. Theoretically about 34% of DNe should show eclipses, implying that eclipsing systems are under-represented by a factor of 7 amongst known DNe!

Recent discussions with professional astronomers strongly support the view that this shortfall is due largely to under-observation rather than to any selection effect. This in turn suggests that a systematic survey of DNe aimed at detecting eclipses should reveal a considerable number of hitherto unknown eclipsing systems, INCLUDING A GOOD NUMBER (~60!) OF DEEPLY ECLIPSING SYSTEMS, i.e. where the angle of inclination is above 80 degrees. Eclipsing systems are highly valued by professionals because eclipse light curves can help to understand the mechanisms underlying the dwarf nova outburst cycle. Thus for example the eclipsing systems U Gem and Z Cha have been particularly important in establishing the present understanding of DNe.

OBJECTIVES

The idea of setting up this Pro-Am observing project is therefore to search known dwarf novae, especially systems which are poorly observed, for orbital light variations which might indicate eclipse behaviour. By-products of this activity might include:

(i) setting upper limits on angles of inclination for systems found to be non-eclipsing;

(ii) detecting orbital light variations due to effects other than eclipses, e.g. an orbital hump due to a hot spot on the accretion disk;

(iii) occasionally, measuring the orbital period where this is not previously known – orbital period is the single most important parameter in characterising cataclysmic variable stars.

Information gained under all of these headings is in principle publishable, as well as both positive and negative results for the occurrence of eclipses. If far fewer eclipsing dwarf novae are eventually found than expected, this would be scientifically interesting because it would indicate that our understanding of dwarf novae is lacking in some key respect.

INSTRUMENTATION AND OBSERVING TECHNIQUES

The CCD technology now being used by amateurs is ideal for the necessary time-resolved photometric observations. Although many of the candidate target stars are probably too faint in quiescence, observations during outburst will in most cases show eclipses if they occur in the system at all. Furthermore IT IS NOT NECESSARY TO USE FILTERS in order to detect eclipses. The most important role for visual observers in this project is to detect outbursts and report as quickly as possible as is currently done in the Recurrent Objects Programme. However in some cases eclipses may be detectable visually, just as superhumps in SU UMa stars have on occasions been detected visually.

Whatever the observing technique, eclipse detection needs observing runs at least as long as the orbital period of the dwarf nova, which is generally in the range 80 minutes to 2 hours for SU UMa-type stars and upwards of 3 hours for U Gem and Z Cam stars. For practical reasons the upper limit is 10 hours. The more demanding long-period systems should provide ideal targets for users of automatic telescopes! Prof F. A. Ringwald (Florida Institute of Technology) has suggested that once 3 orbits of a star have been covered, it should be clear whether eclipses are present or not, and the star can be removed from the programme. A rolling programme is therefore envisaged in which the target list is updated regularly.

The sample of DNe studied in this project will be flux-limited, i.e. only stars brighter than magnitude 15 are to be observed. In most cases this will apply to outburst magnitudes, since relatively few (e.g. TZ Per) are bright enough to observe in quiescence at a sufficient data rate and with adequate signal-to-noise ratio to obtain reliable results. A useful rule-of-thumb for the necessary data rate might be about 100 data points per orbit. Thus for SU UMa stars probably 1 estimate or CCD frame per minute is needed, while for U Gem and Z Cam stars a rate of 1 every 2 or 3 minutes is probably adequate to detect eclipses.

REPORTING OBSERVATIONS

Visual observations of stars on the Eclipsing Dwarf Novae Programme should be reported monthly to the VSS Computer Secretary John Saxton in the usual way. If an outburst of any Programme star is detected, it should be reported immediately to Gary Poyner (especially if it is also on the Recurrent Objects Programme), or to myself so that CCD observers can be alerted as soon as possible. Any Programme star in outburst should be regarded as a high-priority object if it is fainter than visual magnitude 15 at minimum light. Its priority will not be so high towards the beginning and end of a seasonal period of visibility when long observing runs are not possible. When there is more than one programme star in outburst at the same time, I will suggest an order of priority for observing.

CCD observations can be reported either to the Computer Secretary or to myself as soon as possible after reduction of the raw data. It is recommended that the brightness of two other stars in the same field as the variable should be recorded simultaneously, if possible one of them being about the same brightness (the check star) and one rather brighter than the variable (the comparison star). The basic data required is reduced count rate for all 3 stars against time. The comparison should be as bright as possible (without causing saturation) in order to optimise the signal-to-noise ratio of the final results. Note that COMPARISON STARS SIGNIFICANTLY (~1 mag or more) FAINTER THAN THE VARIABLE SHOULD BE AVOIDED since their use will degrade the accuracy of the results. Running means of data points or other forms of post-processing should not be submitted since these can introduce spurious features and obscure the scatter in the data which is important to record. Note also that there is no advantage in reporting or using photometric data on more than the 3 stars suggested since this carries an overhead in terms of data processing and storage requirements and is very unlikely to improve the quality of the final results.

Observers should report;-

(i) instrument details (telescope and camera);

(ii) the frame exposure time

(iii) the frame interval

(iv) whether times refer to the beginning, middle or end of exposures.

Also for each variable, a reference frame should be kept or sent to me carrying some form of labelling of the variable and comparison stars to ensure correct identification when the data is analysed and/or published.

The best way to report observations electronically is by means of simple ASCII text files. THESE ARE STRONGLY PREFERRED against application-based files such as Excel because (i) text files require much less disk space for storage; (ii) text files can be read by any computer using any operating system; (iii) text files can readily be imported to produce light curves in a standard format by the Computer Secretary or myself.

PROGRAMME STARS

The target list below consists of dwarf novae which either have a fairly high but somewhat uncertain orbital inclination, or are rather poorly observed but not thought to have outburst intervals longer than about 1 year. In particular, they are not listed as targets under the Recurrent Objects Programme. Note that ANY DWARF NOVAE ON THE RECURRENT OBJECTS PROGRAMME SHOULD ALSO BE REGARDED AS HIGH-PRIORITY TARGETS UNDER THE ECLIPSING DWARF NOVAE PROGRAMME.

In principle any dwarf nova not already known to be eclipsing is a potential target for this programme, including bright, familiar DNe such as SS Cyg, RU Peg, Z Cam, RX And, SU UMa and so on. These are low-priority targets for the programme, and can be observed whenever no high-priority target is currently in outburst. Observers should contact me for suggestions whenever there is no current high-priority object available.

Stars which have been adequately observed (i.e. for at least 3 orbital periods as noted earlier) will be listed on the BAAVSS web pages

FURTHER INFORMATION

Interested observers should contact me to register their interest and to obtain further information. The BAAVSS web pages <http://www.britastro.org/vss> will carry updated information on the project and regularly updated lists of programme and ex-programme stars.

Dr. W. J. Worraker

65 Wantage Road

Didcot

Oxon. OX11 0AE

U.K.

Tel 01235 211315 (up to 10:30 pm daily)

e-mail: bill.worraker@hyprotech.com or w.worraker@ntlworld.com