Southern Eclipsing Binaries – EA Project

Please note that this project is now closed.  For up to date information about southern eclipsing binaries, refer to www.eclipsingbinaries.prettyhill.org.

Programme Coordinator: Tom Richards [@tom-richards]
Project Leaders: Margaret Streamer [@margaret-streamer] & David Moriarty [@david-moriarty]

Project Description

Overview – A Project for beginner to expert

The Southern EAs Project of Variable Stars South is a multi-purpose and ongoing campaign to observe and analyse bright eclipsing binary stars of the EA-type (Algol-like) accessible to Southern Hemisphere observers.  The Project is designed to enable anyone with a tracking telescope and a camera, or familiarity with Excel, to take part. Full guidelines on observing and analysing are available.

Despite their importance and easy observability, large numbers of them have had little or no observational work done on them since discovery, and many more require follow-up work to extend and check existing studies. Very bright EBs such as R Ara which used to be the province of visual observers now need monitoring with DSLRs, which are 10 to 100 times more accurate in magnitude estimation and provide well-timed rapid time-series data through a night. For a summary of the importance of EB research, especially by amateurs, including many valuable suggestions for projects, see (Guinan, Engle & Devinney, 2012).

Tom Richards is the Programme Coordinator for this field of study within VSS. If you’re new to CCD photometry and wondering how to get going, contact Tom. If DSLRs are your instruments of choice, contact Tom Richards.

The Project has a number of research goals, listed below, that require varying levels of equipment for adequate observation and varying levels of sophistication in analysis. At the simplest level, basic to all research goals, nothing more is needed by the observer than a telescope of any size that can automatically track a target star for many hours, a CCD or DSLR camera for imaging, and photometry software. The Project provides target selection and observing advice.

The Project stresses data analysis and astrophysical research as much as observation. All observational data will be studied within the Project to achieve our research goals, rather than archiving them in an online variable star database. Analysts sitting at computers are as central to the Programme as observers at their telescopes. The primary output of the Project is refereed articles in research journals, with everyone in the Project who has made a significant contribution appearing in the author list.

An important aim of the Project is to encourage as many people as possible with suitable equipment or interests in analysis, to learn-by-doing in the Project. To achieve this the Project supports many levels of sophistication in observing, starting with the most basic which is fundamental to all work – obtaining an hours-long time-series with a CCD camera or DSLR, and carrying out photometry on the images. Similarly, analysis work starts by obtaining the simplest and most fundamental result from the photometry – measurement of time of minimum. In all of this work, the Programme provides mentoring and collaboration to all its members, from the starting points just described and moving up to the more advanced levels of observing and analysis.

The Project’s Research Goals

The eight main research goals of the Project are graded in terms of observational and analysis requirements, beginning with the simplest and most basic.

Goal 1: Obtain Times of Minima

Having a number of carefully measured eclipse times of minima (ToM) for each target EB is most important in its own right, and provides the basis for all further research. Our goal is normally to obtain at least three well-timed primary or secondary minima for each target in each observing season.

Observing

To do this task you need a telescope that can track accurately, with a CCD or DSLR camera. You should also synchronise your computer clock to a local NTS time server. Photometric filters are not needed, but data obtained using them can be used in more advanced research. For this reason, we recommend always using a Johnson V filter if you have it. We have a list of target EB systems in the Programme and the comparison and check stars to use for each, which you can view on the Project Targets tab. To select a suitable target on a given night, download David Motl’s freeware Ephemerides program ( http://www.motl.cz/dmotl/predpovedi/) and use our ephemeris file. This will list eclipses visible from your site. When choosing a night’s target it is a good idea to observe again any target you have previously observed – this helps make some of the data more consistent.

You obtain images of the target variable for several hours around the predicted ToM; then calibrate them in the usual way, and use an aperture photometry package that comes with most imaging and image-processing software to obtain a light curve of the target. You then send that data to me on our SEB Observation Report Form for checking and forwarding to the analyst who coordinates work on that target system. At this stage you will also be given mentoring comments on your work and how to improve it (you don’t want to see my early photometry work – nor do I!).  For full information on all this, see the SEB Observing Guide.

Analysis

Each target in the Project is assigned to an analyst, who receives the completed Observation Report Forms for the target. As an analyst you enter the <JD, mag, [error]> data from the ORFs into a PERANSO file for the target (see www.peranso.com – we can supply the program to you if necessary) and use its facilities to measure minima. On completion you have three files – the PERANSO file, an HJD data file it outputs, and an SEB Results Form. These are returned to me and placed on our website, accessible by all participants of the Project at SEB Observations. As more ORFs arrive, you update these files. The whole process is described in detail in the SEB Basic Analysis Procedure, and you will of course receive mentoring as you start up.

As the analyst for a particular target, you have the responsibility to guide the research on it. For example you might ask observers to obtain some secondary minima in this observing season, or advise you have all the primary minima data you need for the season, or request some more sophisticated type of observation for one of the more advanced Programme goals. You can email individual observers, post to the SEB Forum, or use the VSS egroup,  http://groups.google.com/group/variablestarssouth. Of course, you may not wish to take your analysis work beyond the basic Goal 1 level, but if you do there is a wealth of research to carry out. Your comfort zone is up to you.

Goal 2: Deriving Light Elements

This is a job just for the analyst. When you have three or more well-timed minima in a season for a particular target, you can use linear regression to derive accurate linear light elements (i.e. period P plus one well-timed minimum to set as the zero epoch, E0). From these any other minimum can be predicted. Whilst LEs exist in the literature for all the Programme’s targets (and you should find them for your targets), they are often very poor or badly out of date. Even if they’re good, it is only by continually updating ToMs and linear LEs that we can see if a target system’s period is changing – a most interesting phenomenon. The Results file, referred to above, contains an automatic linear regression routine to use on the ToM data.

Goal 3: Obtaining a Full Phased Light Curve

The light curve of an eclipse is only part of a binary’s entire light curve. And for many lines of research a full light curve is needed, covering all phases of the orbit of the stars around each other, from primary eclipse through to secondary eclipse and back again to primary.

Observing

The analyst for a target will advise you if a full light curve is wanted, and will ask for observations at various phases and bandpasses.

For this you will need photometric filters – especially Johnson B and V. Cousins Rc and Ic may also be useful. At this stage, unfiltered work is useless, so only filtered eclipse data obtained for Goal 1 is any use now. That’s why it’s always recommended to do Goal 1 work with a filter – preferably V. It is also very desirable at this stage to have your transformation coefficients for your filters, as transformed data in two bandpasses is the desideratum. However untransformed V data can be used in further research if handled carefully. Your analyst will advise.

Analysis

If you as analyst consider one of your targets is of sufficient interest to pursue beyond Goals 1 and 2, you need to send out requests for further time series data over other phases. You then cobble these together in PERANSO for each bandpass separately – and this can be tricky for untransformed data from different observers. (If possible, get all data from one observer/instrument.) It can take a long time, even years, to get data covering all phases. The result is a complete light curve in two or more bandpasses; of great interest in its own right but also the basis for further research.

Goal 4: Obtaining Spectra

For many purposes, as well as their intrinsic interest, it is valuable to obtain spectra of eclipsing binaries. Obtaining and analysing spectra of eclipsing binaries is a rather specialized task, though many amateurs now use sophisticated spectrographs and are skilled at spectral analysis. Quite amazingly, the bulk of southern EBs have little or no spectral data, and when they do it is just a spectral classification often derived indirectly by statistical means. Deriving a spectral classification from study of a spectrum is crucial for a lot of the more advanced analysis work described below. Where equipment and the target permit, it is useful to compare spectra at primary and secondary minimum and out of eclipse — differences can provide specific information about the component stars. Radial velocity data, though crucial to a full astrophysical modelling of the binary system (Goal 5) is beyond amateur equipment. If we can make a sufficiently important case for a particular system, we will seek time on a large professional telescope.

Observing

When the analyst for a particular EB needs spectral data, he or she will send a request to spectroscopists in the programme. As a spectroscopist, you will likely be asked to supply a spectrum image, assessment of spectral type and class, and possibly to compare spectra in and out of eclipse to try to get classifications of the individual stars. Other issues can arise, such as looking for the signature of circumstellar dust or unusual chemical compositions.

Goal 5: Light Curve Analysis and System Modelling

This is primarily a job for the analyst, where complete phased light curves in two or more bandpasses have been obtained (Goal 3), though with suitable assumptions it is possible to carry out light curve analysis on an un-transformed light curve in one bandpass — see e.g. (Zasche 2009).

The aim here is to infer a range of astrophysical parameters about the system from analysis of the light curve and using the spectral classification (Goal 4). Some of these are the orbital inclination to our line of sight, the radii of the stars relative to the orbital radius, their shapes, the ratios of their masses, luminosities and temperatures. The way to do this is to feed assumed values for these parameters into a program that can generate the light curve of an EB using those parameter values, then to adjust until the generated light curve matches the observed one. Two commonly used programs here are PHOEBE (PHysics Of Eclipsing BinariEs, see  http://phoebe-project.org/) and BinaryMaker 3 (www.binarymaker.com). Both are highly recommended.

Goal 6: Investigate O-C

This also is a job just for the analyst, and includes searching the literature. An O-C diagram shows the extent to which Observed ToM data points (HJDs of primary minima) diverge from those Calculated from some set of linear LEs. To do this properly, all recorded ToMs in the literature from the discovery date onwards are needed, as well as those recorded in this Programme – perhaps over some years. Inspection of a regression along the plotted points can show if the assumed period is right, wrong, or is changing over time. If it is changing in any way, the acceleration may be constant or the O-C diagram may reveal a sudden change in period. Such changes can indicate mass transfer between the two stars or mass loss from the system or some sudden adjustment in one star. Along with light curve analysis (Goal 5) this provides important astrophysical information about the system.

Goal 7: Search for a Third Body

Light curve analysis (Goal 5) can reveal the presence of a “third light” contaminating the light received from the eclipsing pair. Where this is not a faint background star inside the photometry measurement aperture, it can be attributed to a faint star orbiting in the binary system.

More significantly, an O-C diagram can show oscillations with a fixed period, indicative of the presence of a third, invisible body in the system such as a star orbiting the eclipsing pair in the same plane. This third body is shifting the visible pair periodically closer to us then further away, and so advancing or delaying the time the light of an eclipse takes to reach us (the so-called Light Time Effect). If the physical parameters of the eclipsing pair are known (Goal 5), it is possible to derive orbital and mass information about the third body. It may turn out to be a star, a brown dwarf, or even a planet, though detecting the low-amplitude oscillations of a planet requires high quality ToM measurements in the first place – taking us right back to Goal 1. The SPADES Project (Search for Planets Around Detached Eclipsing Stars) within the SEB Programme is aimed at detecting such planets, which would be very difficult to find using other planet-finding techniques such as radial velocity, transits, or microlensing. For more about the SPADES project see here.

Goal 8: Unexpected Opportunities

The Project is particularly interested in targets of opportunity that can arise from time to time, as well pro-am collaborative campaigns and the study of unusual or particularly important EBs when the occasion arises. Some EBs such as V685 Cen and R Ara are evolving rapidly yet are poorly monitored. Others such as Z Cha and VZ Scl are cataclysmic or novalike variables or otherwise given to irregular or eruptive behaviour. These are frequent campaign targets and anyway should be monitored closely. Even the routine monitoring of poorly observed EBs can yield important surprises, such as the discovery of a pulsating component or a significant change in the light curve. Even Algol has not had its light curve checked for several years!

These sorts of systems and campaigns will usually require specialised observing and analysis methods, and may require close collaboration with a professional team. Any suggestions for a campaign on such targets of opportunity should be sent to me.

 

How Can I Take Part?

This is a research project open to anyone. If you have a tracking telescope with a CCD or DSLR camera, you can take part as an observer. If you have reasonable computer skills including Microsoft Excel, you can join in as an analyst. Of course, the best way to learn variable star research is to take on both roles.

At the elementary stages such as Goal 1 – finding minima – only elementary skills are needed, but these underlie all more advanced work, to which you can progress as your equipment and research skills permit. For example, your observing equipment may limit you just to the basic Goal 1 photometry, but your interest in research and analysis may take you all the way to the fascinating business of light curve modelling – Goal 5 – and writing a paper about it.

To join in, send Tom Richards an email telling me about your equipment and what you’d like to do in the project. You’ll be assigned a mentor and given pointers to get going. But first, I suggest you read all the project documentation on our website – most of which are linked from this article.

Or if you can’t wait to get observing, surprise Tom Richards by sending me a completed SEB Observation Report Form with your email – but first you’ll need to read carefully the SEB Observing Guide and do all it says – plus of course selecting your target as described in Goal 1. If DSLR observing is your trade, do contact Mark Blackford [@mark-blackford] first.

Wishing you clear skies and a high bandwidth!

References

Guinan, E.F., Engle, S.G. & Devinney, E.J., 2012. JAAVSO 40, 467-480.

Zasche, P., 2009. New Astronomy 14, 129-132.

What is the SEB-DSLR component about?

The SEB-DSLR component aims to provide observational data in support of Ed Budding’s The Southern Binaries Programme of CONZ and CAAM. The targets will be bright, relatively under-observed, or newly discovered, southern binaries. They will mostly be eclipsing binaries but others may be close binaries with possible photometric effects of binarity but not eclipses.

A secondary aim of the SEB-DSLR component is to establish DSLR photometry as a legitimate technique for scientific studies of variable stars. Due to their brightness our targets will generally be unsuited to the narrow field of view of CCD imaging through a telescope. DSLR cameras with standard lenses provide fields of view wide enough to include suitable comparison stars.

Specific measurements may vary from target to target and will be described in detail as targets are added to the Project. Initially we will concentrate on determining orbital period and well-determined times of minimum of primary (and secondary) eclipses of eclipsing targets. For many of our targets the available epoch and period data are quite old and cannot be relied upon for predictions of eclipses.

Complete light curves may be required for some targets. Combined with radial velocity and spectral data, these light curves will allow determination of many parameters of the orbits and stars themselves.

All significant results are to be published in suitable journals, with observers’ data and summary reports on progress published on this website.

This project is suitable for observers new to DSLR photometry as well as for experienced observers. Feedback and advice on observing and results will be provided. This is a good opportunity to learn and improve technique, as well as making publishable scientific contributions.

Participants in the SEB-DSLR component will need to develop skills in recording and measuring images as well as transforming their measurements to the standard magnitude system. Whilst time of minimum can be determined from untransformed magnitudes, the magnitudes cannot be compared and combined with other observers data. A tutorial on DSLR photometry is available, including determining and applying transformation coefficients.

Lists of targets will be published here from time to time, together with observational requirements. VSS participants will be notified by email of new targets and issues arising about current targets.

Observational requirements may differ somewhat from target to target, but in general they are:

  1. DSLR observation with standard camera lens (not through telescope).
  2. Use the epoch and period data given for each target to predict eclipses using Tom Richards’ Times of minimum spreadsheet. These predictions may be out by many minutes or even hours due to the length of time since the epoch and period were determined. Therefore it may take several attempts to record an eclipse.
  3. Time series are needed, preferably through an entire night or as much of it as possible. Therefore the camera must track the target, e.g. piggy backed on a motorised telescope.
  4. Record time series images with as fast a cadence as you can whilst keeping SNR > 100.
  5. Photometric filters are not required, we will use the three colour channels of the camera.
  6. Exposure time and defocus should be optimised to achieve Green channel peak ADU values of ~3/4 of your camera’s saturation value for the brightest comparison star (or target). This will ensure all three colour channels will be below saturation for the target and comparison stars.
  7. Observers will be advised what data to submit and in what format.

To join this dynamic group of people, contact Tom Richards [@tom-richards].

 

Project News

  • SPADES Project Launched October 17, 2010 We are pleased to announce that the SPADES pro-am project (Search for Planets Around Detached Eclipsing Systems) is now up and running. We seek observers immediately to join the team. Basic requirements are a telescope of about 30 cm aperture or more, an astronomical CCD camera with a Johnson V filter, and experience in CCD photometry. Targets are all south of +10 deg declination.

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