Project Leader: Stan Walker

One of the most difficult observing targets in our galaxy is QZ Carinae, a massive EB system. It comprises four stars arranged in two pairs: an eclipsing secondary pair with a period of 5.99875 days and a non-eclipsing primary pair with a period of 20.74 days. Masses are 16.7 and 28.0, and ~40.0 and ~9.0 respectively. Separation of the two pairs is about 50 AU and the absolute luminosity about –7.5. It’s almost naked eye at 2500 parsecs!

Eclipses travel slowly eastward and at present are best observed from Australia and New Zealand. By 2012 they will be out in the Pacific! Conventional eclipse timings are not reliable as they are distorted by the orbital light variations of the non-eclipsing pair. The central eclipse also shows light time variations of at least +/- 7.2 hours over a period of 30 years or more. This accounts for the phase shifts. What we’d like to do is to disentangle the assorted light variations and establish:

• The size in AU and period of the orbit of the two pairs of stars.
• The true period of the eclipsing pair with the light time effects removed
• The period of the non-eclipsing pair and its light curve

How do we do this? The out of eclipse light variations comprise a combination of aspect variations of the eclipsing pair with an amplitude of ~0.1 magnitudes—and similar variations, but eccentric, of the non-eclipsing pair with an amplitude of 0.03—0.05. We need about a hundred good V, B-V, fully transformed, measures outside of eclipses to establish these. By determining the epoch and exact period these ‘distortions’ can then be removed from the light curve of the eclipsing pair to provide us with reliable amplitudes and times of eclipses. The ‘season’ is about late summer to early spring in the Southern Hemisphere.

We need also to establish seasonal epochs of eclipses. To do this we fit all ‘in eclipse’ measures to each other using the expected period at the time. This period varies for reasons explained in Background to QZ Carinae. The epochs for 2010 and 2011 can be added to the O-C diagram of eclipse epochs – this will hopefully show that the system has reached and passed the maximum positive displacement, thus providing an amplitude for the light time effects and a better diameter of the overall system. Both CCD and PEP V measures are wanted, but it’s not a visual target. Comparison stars are shown on the QZ Car Finder Chart.

Background to QZ Carinae

Introduction – A Review of the System

QZ Carinae is a massive eclipsing binary system in the region of eta Carinae and at much the same distance. It comprises four stars with a total mass of ~94 solar masses. Its absolute magnitude is -7.5. Its variability was originally established from 61 UBV measures made at Auckland Observatory over two seasons by Marino and Walker, 1972, as shown in Figure 1.

Figure 1: The original 61 measures of QZ Carinae. These were rather fortuitous in that the eclipses were almost entirely visible from Auckland and that there was little distortion of these by the primary pair.

Since then, it has been measured sporadically by the Auckland group, usually in conjunction with measures of eta Carinae itself. The light curve of the 160 measures to date is shown below.

Figure 2: V measures of QZ Carinae from Auckland during the period 1971 to 1995, phased with the light elements HJD 2441033.084 + 5.99857E

This second plot looks very messy but there are reasons for this. The scatter in the data reflects the problems associated with measures of this system. Our estimate of the period using a relatively short baseline was 6.007 days. However, reference to Figure 3 shows that the cycle-to-cycle periods vary between 5.99821 and 5.99924 days due to light time effects. A mean period of 5.99857 has been assumed and is close to, but not exactly, the true mean period. From the original 61 measures, Morrison and Conti, 1979, and Leung et al, 1979, determined a mean light curve and, using a variety of spectroscopic and other measures, determined masses for the components as shown below. The radii and masses are relative to the Sun. The actual uncertainties are given in the original papers but there is no doubt that the system is massive and bright.

The inclination of the secondary pair in QZ Carinae is given by Leung et al at 85.9o with ~60o for the non-eclipsing primary pair. This pair make up a spectroscopic binary with an orbital period of 20.72 days. This period also varies slightly due to the light time effects shown in Figure 3.

Later Mayer et al, 2001, made additional measures and published an updated O-C diagram of light time distortions of the eclipse timings. Following that, Walker, 2006, matched the random measures made at Auckland, together with some unpublished measures supplied by Mayer, to Morrison & Conti’s, 1979, mean light curve and derived additional epochs of minima. Observations by Hipparcos and ASAS3 were also analysed using this method and the following O-C diagram was derived.

Figure 3: O-C deviations of primary eclipses of QZ Carinae from the adopted mean period of 5.99857 days. These show an amplitude of 0.6 days, or 14.4 hours. The observed period will oscillate around the assumed mean period, dependent upon the phase of the mutual orbits of the two pairs. Based upon masses and measured orbital velocities Leung et al had suggested a period of 25.4 years, or ~9280 days. Mayer et al, using a much longer baseline, suggested 40-50 years and this is supported by the observations at SAAO and ASAS3. The source of measures is shown below.

The scales are both in days, with the HJD requiring the addition of 2400000. This information then allowed corrections to be made to the phased light curve producing the following result. It does not take into account the possible perturbations caused by the orbital motion of the primary pair.

Figure 4: This shows a much neater light curve, with an obvious totality at both primary and secondary eclipse.

From the above figure the elliptical amplitude of the eclipsing pair appears to be ~0.10 magnitudes, with primary eclipse depth of 0.17 and secondary of 0.10.

We considered the possibility of removing any effects caused by the ellipsoidal variations of the primary pair in the system. A search for aspect changes and their effect on the overall light curve involves several assumptions. The first is that the components will be tidally distorted which, given the masses and the orbital period, seems likely. The second is that rotation will be synchronous, which allows a search for periods near the known orbital period.

We selected 93 measures made at Auckland of QZ Carinae outside of eclipses. A sinusoidal curve with an amplitude of 0.10 magnitudes, as determined from the mean light curve of the system, was removed from the data. Combined with the expected smaller tidal distortion of the latter because of the greater separation the light variations caused by ellipticity would be small. The amplitude of 0.04 detected after considerable smoothing of the observations is probably real. A period search produced a period of 20.744 days. In order to make it more visible a running mean was applied to the data. If anything, the result is probably too good, bearing in mind the less than ideal quality of some of the measures!

Figure 5: The 93 outside-of-eclipse measures of QZ Carinae, with the mean light curve subtracted, plotted to the best fit of 20.744 days.

Morrison and Conti, 1979, give a spectroscopic period of 20.72 days but when combined with Mayer et al’s measures this changes to 20.736 days. Their eccentricity of 0.34 is not incompatible with the light curve above. No attempt to check phase has been made. Further measures of the outside eclipse light curves should resolve the slight discrepancy of 0.008 days. From Figure 3 it is seen that Morrison and Conti were measuring near a conjunction—Mayer et al near maximum recession velocity.

The system was observed by Nelan et al, 2004, using the Hubble Space Telescope Fine Guidance Sensor in 2002, when as the individual pairs could not be resolved, a maximum separation of 35 AU was suggested. This measure appears to have been made near conjunction, so this result is not unexpected—the true separation is much greater.

The Observing Project

The duration of totality and the difficult orbital period makes the determination of mid-eclipse difficult. It takes ~11.5 years to make one circuit of the Earth in longitude and it is observable from only a relatively small band of longitudes each season. It would, of course, be an ideal project for an Antarctic telescope. Added to this is the strong indication of aspect variations in the non-eclipsing pair which are probably sufficiently large, compared to the eclipse amplitude, to distort the eclipses sufficiently to be unsure of a determination in the normal manner.

The project is aimed at resolving some of the uncertainties about this system. They are:

• To establish a period, epoch and reliable light curve for the non-eclipsing, primary pair. This will also provide a measure of the amplitude of its variations.
• To use this light curve and the deduced light elements to correct observations of the total system and to provide a true light curve of the eclipsing pair.
• To establish annual times of minima of the eclipsing pair to determine the amplitude and phase of the light time effect and provide information about the separation of the two pairs.

Comparison stars are described under the chart, together with comments about possible variability. Because of the nature of the area it is difficult to observe and almost all stars are suspected of variability, some erroneously. The measures are required in V only, as the colour changes in the system are very small, so either B and V filters, or V and R filters are adequate, but full transformation to the standard system is essential.

Approximate out of eclipse times, ingress, egress and totality are set out below. There is some uncertainty, due to the primary pair’s effect upon the eclipses themselves. Note that each cycle has two sets of ingress, totality, etc.

The most recent ephemeris available is JD 2453918.2467 + 5.9985 and is based upon the assumption that the light time effects, which vary the period by ~0.0005 days, are decreasing slightly at present. The epoch has been adjusted by adding 0.30 days, as shown in Figure 3. Eclipses at 2010 April 30 are centred at longitude ~180E. So the star will be well placed in Australia and New Zealand that year.

Depending on the amplitude of the light time effects, which may be as great as 0.6 days, and the orbital period of the two pairs, the observed period of the eclipsing pair, QZ Carinae, may differ at times by .0005 days, and the primary pair by 0.0018 days. As well, the light time effects may not necessarily be symmetrical.

The initial goal of the project is to acquire sufficient out-of-eclipse measures to determin a light curve for the primary pair which appears to show elliptical light variations. The orbital period of slightly under 21 days suggests a measure every 5 hours or so to provide 100 points on the light curve. More would be preferred. The eclipsing pair needs a measure each 1.5 hours to provide a similar degree of resolution. Measures from a variety of sites would be welcome, although the primary pair’s light variations can be observed from a single site.

Another factor which will cause some problems is the orbital period ratio of 3.458. If the respective phases are aligned in a difficult manner this will distort a number of eclipses. This did not occur during the initial measures from Auckland, but is noticeable in measures by Christie and by the Mayer group from South Africa. For this reason measuring the eclipses continuously is not an ideal approach – a more accurate epoch can probably be derived by fitting a number of single (snapshot) measures to a mean light curve.

The field around QZ Carinae is shown on QZ Carinae Chart. The location of the suitable CCD comparisons is marked. Classical PEP methods have been used previously, although CCD photometry (the telescope may need stopping down) is adequate. The simple sequence, Comp, QZ, QZ, Check, Comp, with Sky thrown in for the pep people, will provide enough accuracy in most cases. Fully transformed measures are essential in V, and either V-R or B-V. QZ is at opposition in late summer each year. It is circumpolar (at reasonable altitude) at 40S. Thus it is well placed in this region for the next two years.

Comparison Stars & Observing Methods

We consider two types of photometry – PEP and CCD. For PEP the comparison stars used are shown below in preferred order. There is some possibility that HD 93131 is variable at about the 1% range, although Hipparcos found no variations >0.005, hence it may be sensible to use either HD 93502 or HD 93943 as a check star. The V, B-V values for these two stars are not as well known as the main two.

For CCD observers we have prepared a chart which is accessed at QZ Carinae Chart and shows suitable nearby comparison stars for ensemble photometry. Values of these will be published once they are available. Stars 1-5 are recommended for this, star 6 is a brighter object HD 93131 from the table above. Star 7 is too red and nothing is known of star 8.

Differential photometry is essential in all PEP cases as the accuracy desired is 0.008 or better.

The papers listed below in the references are worthwhile reading.

References

Leung, K-C, Moffat, A F J, Seggewiss, W 1979 Astrophysical Journal, 231, 741

Mayer, P 2006 Private Communication

Mayer, P, Lorenz, R, Drechsel, H, Abseim, A 2001 Astronomy & Astrophysics, 366, 558

Mayer, P, Niarchos, P G, Lorenz, R, Wolf, M, 1998 Astrony & Astro Supp Ser, 130, 311 Christie, G

Morrison, N D & Conti, P S 1979 IAU Syposium 83, 277

Nelan, E P, Walborn, N R, Wallace D J, 2004 Astronomical Journal., 128, 323

Moffat A F J, Makidon R B, Gies D R, Panagia N

Walker, W S G, Marino, B F 1972 Internat’l Bull.of Variable Stars, 681

Walker, W S G 2006 Royal Astronomical Society of NZ, Annual Conference

Project Target Data

Files and Charts for downloading