Selected W UMa stars of the antipodean sky.
This is the first in a series of reports of observations on anipodean W Uma contact binaries.
W UMa stars
W Ursae Majoris stars are members of a class of eclipsing variable stars in which the components are of spectral type late A to mid K; though a recent discovery of Koen and Ishihara (2006) suggest a unique contact binary of spectral type "later" than M4.
They are named after the prototype W UMa. The variability of the prototype was discovered by Muller and Kempf (1903). The components of W UMa stars are similar in brightness and are in contact. Primary and secondary minima of the light curves are nearly equal. Contact binary W UMa stars, display continuous changing brightness because of idal distortion of the components (Paczynski et al. 2006). An energy transfer from the larger more massive of the two to the smaller less massive one results in an almost equal surface temperature over the entire system. They are therefore cooler/warmer than they would be if solitary. The mass ratio q of the two stars is always different from unity and it is this q value that determines the energy transfer rate (Mochnaki 1981). Liu and Yang (2000) confirmed this result and added that the transfer rate is also dependent on the evolutionary factor (the radio of the present radius to the zero age radius of the primary component). In 2001, Kalimeris and Rovithis-Livaniou found the observed rate of transfer is a function of the luminosity of the secondary.
The two components are low and intermediate mass Main Sequence (MS) stars surrounded by a common envelope. The convective common envelope model was developed by Lucy (1968b) and has the secondary deriving most of its luminosity by "sideways convection". Mochnacki (1971) and Moses (1974) suggest that up to a third of the energy generated by the primary is transferred to the secondary. In a contact binary the similarity between the components means that at some point their envelopes come into direct contact as they evolve at a similar rate.
A contact binary is a close binary star in which both components fill their Roche lobes. The components of such a system rotate very rapidly (v sin i ~ 100-200 km [s.sup.-1]) from the spin-orbit synchronisation due to strong tidal interactions between the stars. They are very common (Eggen, 1967). Duerbeck (1984) and Rucinski (1993) suggest the level of at least one such binary per one thousand stars. Studies of open (Kaluzny and Rucinski 1993; Rucinski and Kaluzny 1994) as well as globular clusters (Hut et al. 1992) are showing a much higher relative frequency of occurrence than in the field.
There were, until 1979, two subclasses of W UMa stars. These were suggested by Binnedijk (1965):
A type: effective temperature of the primary and secondary are not the same (i.e. not in thermal equilibrium).The temperature difference is larger and the secondary appears hotter (Mochnacki 1972, Whelan 1972). In A type the primary is hotter or almost the same temperature as the secondary. Also in the A type the systems are of earlier spectral type (Mochnacki 1980) and appear to be more evolved (Wilson 1978).
W type: have shorter periods, are cooler, are generally closer to the Zero Age Main Sequence (ZAMS), have a smaller total mass and a larger mass radio and are in poorer contact (Rucinski 1973). In 1975 Mullan suggested the primary in W type appear cooler relative to the secondary as a result of magnetic star spots on the surface. Doppler imaging techniques have revealed that both components can be covered in star spots, with the primary to be more active than the secondary (Maceroni et al, 1994; Hendry and Mochnacki, 2000; Barnes et al, 2004) Also, W type systems have thinner necks (Mochnaki 1980) and accordingly have higher energy transfer rates and luminosity ratios. This is counter-intuitive, but has proven to be the case (Csizmadia and Klagyivik 2004).
Lucy and Wilson (1979) introduced a class B type system which are in geometrical but not, in thermal contact. Without thermal contact there is a large difference between the surface temperatures of the components. (B systems are sometimes referred to as Poor Thermal Contact (PTC) systems (Rucinski and Duerbeck 1997)).
A third type of contact binary has recently been introduced by Csizmadia and Klagyivik (2004), the H type, having a large mass ratio (q > 0.72). These systems show very different behaviour in the luminosity ratio-transfer parameter diagram. They suggest that the energy transfer rate is less efficient in these systems than in other types of contact binary stars.
An observation campaign from light polluted skies using personal telescopes (PT) on the outskirts of Johannesburg, South Africa, began in winter 2006. The objective of the campaign was the determination of periods of selected antipodean W UMa stars and the modeling of their components.
[FIGURE 1 OMITTED]
Hoffmeister reported the variability of V839 Cen in 1949.
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Harvard sky photometry discovered the variability of V0637 Cen (2002IBVS.5298 ... .1W).
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Gessner and Meinunger (1974) reported the discovery of this star.
Candidates for the observational campaign were chosen according to:
1. 14 > [m.sub.V] >8. The candidates are all in the [vicinity.sup.~]-60 degrees and have had no previous in depth research undertaken on them. (Exhaustive web searches: NASA ADS site, Simbad etc.)
2. The candidates display "high" [DELTA]magnitude amplitude, which is vital considering the size and sensitivity of the equipment deployed.
4. Observations within the 14>[m.sub.V]>8 band should not be adversely affected by bright moon nights
Equipment and observations
V filter data from 2008 is presented. A Starlight Xpress MX716 self-guiding camera was coupled to a pier mounted Meade LX200GPS 30cm PT at a light polluted site on the northern outskirts of Johannesburg, South Africa. Images including the program star were captured to fits files with a field of view (FOV) of ~660 x 600 [arcsec.sup.2] and a resolution of about 110 arcsec mm-1. Control of the PT and camera was done using MSB Astro-Art. Computer time is set every 4 minutes, automatically via the net from Dimension 4 using a local time server.
Astronomical Image Processing 4 Windows (AIP4Win). http://www.willbell.com/aip/ index.htm was utilized in data reduction. AIP4Win uses two dimensional aperture photometry in the reduction process.
From 5524 observations over 7 nights we derive an epoch of:
HJD 2454582.5136 ([+ or -]0.0003) + [E.sup.*](0.3309d) [+ or -] 0.0001d
From 3016 observations over 6 nights we derive an epoch of: HJD 2454573.4791 ([+ or -]0.0006) + [E.sup.*](0.3783d) [+ or -] 0.0001d
From 4588 observations over 7 nights we derive an epoch of HJD 2454626.4533 ([+ or -]0.0005) + [E.sup.*](0.3007d) [+ or -] 0.0001d
Modeling of the data
Binary modeling was undertaken using two modeling software platforms. These data were initially modeled using Binary Maker 3 (Bradstreet, 2005) to generate rough input parameters for later refined modelling in Phoebe (Prsa 2006). Phoebe has the advantage of modelling stellar atmospheres using Kurucz's (1970) code. Both software platforms are based on the Wilson-Devinney code (1971). These models, whilst producing low residuals from good fits to the light curves are not unique as spectra are required to constrain the component masses.
Of interest here is if the inclination or the temperature parameter is adjusted in small steps the syntheic primary minimum jumps to either the 3.25 or 3.35 magnitude position producing an unrealistic fit. Apart from the narrow eclipse at primary minimum the synthetic fit here is satisfactory (Fig. 4 & 5).
The 3 data points in Figure 7 at approximately -0.3 phase, below the secondary minimum are unexplainable. They are not obvious in the unfolded data. Also of interest is the point following the primary minimum that is well away from both the data and synthetic curve at approximately 0.25 of phase. This point manifests a large residual. Apart from the slightly deep synthetic curve at the secondary minimum the fit here is also satisfactory.
[FIGURE 4 OMITTED]
The [A.sub.1] and [A.sub.2] values here are also inconsistent. We would anticipate a higher surface albedo for a hotter star. These values seem to be reversed. Other values like gravity darkening and limb darkening are input parameters.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Balancing a low [chi square] value and the best fit by "eye" produced the fit in figure 9. Again a small difference between data and synthetic curve is noted around the minima. Overall the fit is satisfactory.
The full parameters of all of the models are tabulated in table 5.
Until spectra of the candidates are taken we are unable to confirm temperatures and hence have limited convergence on accurate models.
The principal parameters of W UMa stars that have the biggest effect on the residual values of the model fits are system inclination to our line of sight i, mass ratio of the two components q and the temperature of the component stars. Small incremental changes to these three parameters make a significant contribution to the system fit.
[chi square] values are a reasonable gauge of parameter fits. However, eye fitting also makes a very important contribution in the overall balance of the fit. Eye estimations should never be ignored in this procedure. Spectra of the candidates would constrain temperatures of the components as well as providing radial velocities.
It is important to re-iterate that binaries remain the only source of absolutely determined stellar masses and close binaries remain the only source of radii. All other methods are model dependant and utilize binary data (Rucinski private communication February 2009). Modeling of W UMa stars therefore plays a significant role in the constraint of stellar models in general. Yakut and Eggleton (2005) emphasize this fact.
The reader is directed to a recent comprehensive publication regarding new opportunities and challenges in close binary work wherein further astrophysical research is motivated in this field (Gimenez, Guinan, Niarchos & Rucinski 2006).
It is worth noting that very little theoretical work has been done on W UMa stars recently and many of the listed references are dated. Recent observations of the systems have been restricted to modelling using old theoretical models.
Rucinski has also indicated his dissatisfaction with the current "Contact Model" of W UMa stars. He believes the model does not adequately explain all of the observed parameters. Researchers eagerly anticipate developments in this regard.
Understanding of the physical processes involved in the sharing of a convective envelope, limb and gravity darkening and surface albedoes all contribute greatly to astrophysical understanding of stellar systems. Without determining and liming parameters Astrophysicists would be unable to improve current models.
The modelling processes used were all developed on the code written in the early 1970s. This clearly indicates that more research is required in this field.
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Department of Physics, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg 2006, South Africa email@example.com
Program star details V&39 Cen STAR V839 Cen SPECTRAL TYPE G5V (Simbad) RA Dec ASAS HIC/ALT SIMBAD (.2000) (2000) B-V V 12 58 48 -36 58 33 125850-3658.5 HIC63347 0.62 9.64 V637 Cen STAR V637 Cen SPECTRAL TYPE N/A RA Dec ASAS HIC/ALT SIMBAD (2000) (2000) B-V B 14 16 35 -40 00 27 141635-40 00.2 SVHV7383 12.5 V653 Ara STAR V653 Ara SPECTRAL TYPE N/A RA Dec ASAS HIC/ALT SIMBAD (2000) (2000) B-V V 16 45 03 -54 59 12 164503-5459.2 SVSON5877 -- N/A Table 1: Months of observation and integration time, V Filter Observations 2008 Star Observing Integration period times V839Cen April/May 25 sec V637 Cen April/May 55 sec V653 Ara June 40 sec Analyses of reductions V839 Cen Table 2: This table presents minima times from V839 Cen V Filter 2008. Cycles with fractional values (1, 3, 5, 7, 8, 9 and 11) are secondary minima. (2450000+). Cycle Calculated Observed [DELTA] Observation (mag) -0.50 4582.3482 4582.3492 0.0009 0.00 4582.5136 4582.5133 0.0012 2.50 4583.3409 4583.3410 0.0012 3.00 4583.5064 4583.5058 0.0013 5.50 4584.3337 4584.3341 0.0009 6.00 4584.4991 4584.4982 0.0011 14.50 4587.3119 4587.3122 0.0010 29.50 4592.2758 4592.2764 0.0009 32.50 4593.2686 4593.2688 0.0009 33.00 4593.4340 4593.4335 0.0009 35.50 4594.2613 4594.2617 0.0011 36.00 4594.4268 4594.4263 0.0010 V637 Cen Table 3: This table presents minima times from V637 Cen V Filter 2008. Cycles with fractional values (1, 2, 5, 8 and 9) are secondary minima. (2450000+). Cycle Calculated Observed [DELTA] Observation (mag) -2.50 4572.533305 4572.5355 0.002630 -0.50 4573.289951 4573.2896 0.001730 0.00 4573.479112 4573.4792 0.001400 2.00 4574.235758 4574.2336 0.002370 2.50 4574.424919 4574.4249 0.001490 3.00 4578.397309 4578.3975 0.001500 18.00 4580.288923 4580.2877 0.002140 18.50 4580.478084 4580.4776 0.002170 20.50 4581.234730 4581.2365 0.002290 21.00 4581.423891 4581.4240 0.001610 V653 Ara Table 4: This table presents minima times from V653 Ara V Filter 2008. Cycles with fractional values (1, 3, 6, 8, 10, 12, 13 and 16) are secondary minima. (2450000+). Cycle Calculated Observed [DELTA] Observation (mag) -0.50 4626.3029 4626.3031 0.0019 0.00 4626.4533 4626.4529 0.0016 9.50 4629.3102 4629.3096 0.0019 10.00 4629.4606 4629.4603 0.0020 16.00 4631.2649 4631.2638 0.0021 16.50 4631.4153 4631.4158 0.0018 17.00 4631.5656 4631.56557 0.0018 19.50 4632.3175 4632.3177 0.0020 20.00 4632.4678 4632.4675 0.0018 22.50 4633.2196 4633.2223 0.0025 23.00 4633.3699 4633.3677 0.0018 23.50 4633.5204 4633.5221 0.0023 52.50 4642.2414 4642.2423 0.0018 53.00 4642.3912 4642.3913 0.0016 56.00 4643.2939 4643.2933 0.0017 56.50 4643.4443 4643.444 0.0019 Table 5: Final Model parameters determined by Phoebe. Passband luminosities are the ratio of flux to the integral of the passband transmission function. Description V0S39 Cen [DELTA][empty set] Phase shift -0.4610 (arbitrary [+ or -] 0.0003 user interface) q Mass ratio 0.3587 [+ or -] 0.0008 i Inclination 76.87 of orbit [+ or -] 0.07 [T.sub.eff1] Effective 5384 temperature [+ or -] 102 [T.sub.eff2] 5653 [+ or -] 240 [[OMEGA].sub.1] Surface 2.533 potentials [+ or -] 0.004 [[OMEGA].sub.2] 2.224 [+ or -] 0.006 [A.sub.1] Surface 0.50 albedoes [+ or -] 0.02 [A.sub.2] 0.50 [+ or -] 0.02 [g.sub.1] Gravity 0.28 darkening [+ or -] 0.01 [g.sub.2] coefficients 0.32 [+ or -] 0.01 [L.sup.j.sub.1] Passband 12.59 luminosities [+ or -] 0.08 [L.sup.j.sub.2] 12.57 [+ or -] 0.10 [x.sup.1d,i.sub.1] Linear limb 0.53 darkening [+ or -] 0.01 [x.sup.1d,i.sub.2] coefficients 0.50 [+ or -] 0.05 Description V0637 Cen [DELTA][empty set] Phase shift 0.1964 (arbitrary [+ or -] 0.0003 user interface) q Mass ratio 0.5348 [+ or -] 0.0016 i Inclination 74.30 of orbit [+ or -] 0.08 [T.sub.eff1] Effective 7870 temperature [+ or -] 77 [T.sub.eff2] 6000 [+ or -] 60 [[OMEGA].sub.1] Surface 2.772 potentials [+ or -] 0.003 [[OMEGA].sub.2] 2.845 [+ or -] 0.003 [A.sub.1] Surface 0.57 albedoes [+ or -] 0.05 [A.sub.2] 0.75 [+ or -] 0.06 [g.sub.1] Gravity 0.34 darkening [+ or -] 0.01 [g.sub.2] coefficients 0.33 +0.01 [L.sup.j.sub.1] Passband 12.51 luminosities +0.01 [L.sup.j.sub.2] 12.57 [+ or -] 0.02 [x.sup.1d,i.sub.1] Linear limb 0.45 darkening [+ or -] 0.02 [x.sup.1d,i.sub.2] coefficients 0.50 [+ or -] 0.03 Description V653 Ara [DELTA][empty set] Phase shift 0.1316 (arbitrary [+ or -] 0.0005 user interface) q Mass ratio 0.3544 [+ or -] 0.0011 i Inclination 73.71 of orbit [+ or -] 0.11 [T.sub.eff1] Effective 6094 temperature [+ or -] 61 [T.sub.eff2] 5600 [+ or -] 56 [[OMEGA].sub.1] Surface 2.537 potentials [+ or -] 0.025 [[OMEGA].sub.2] 2.532 [+ or -] 0.025 [A.sub.1] Surface 0.71 albedoes [+ or -] 0.01 [A.sub.2] 0.50 [+ or -] 0.01 [g.sub.1] Gravity 0.29 darkening [+ or -] 0.01 [g.sub.2] coefficients 0.32 +0.01 [L.sup.j.sub.1] Passband 12.58 luminosities +0.13 [L.sup.j.sub.2] 12.57 [+ or -] 0.13 [x.sup.1d,i.sub.1] Linear limb 0.50 darkening [+ or -] 0.01 [x.sup.1d,i.sub.2] coefficients 0.50 [+ or -] 0.01
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|Publication:||Monthly Notes of the Astronomical Society of Southern Africa|
|Date:||Feb 1, 2012|
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