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HW Virginis
1 August 2009

  HW Virginis is yet another Algol-type eclipsing systems (p = 0.11671956 days = 168 minutes). These are binaries with spherical or slightly ellipsoidal components. It is possible to specify, for their light curves, the moments of the beginning and end of the eclipses. The "standard" says that between eclipses the light remains almost constant or varies insignificantly because of reflection effects, slight ellipsoidality of components, or physical variations. In this case, however, there is a significant reflection effect or other ... Secondary minima may be absent -- in this case it's easily discerned. An extremely wide range of periods is observed, from 0.2 to >= 10,000 days. Light amplitudes are also quite different and may reach several magnitudes. In the case of HW Virginis, this amplitude is less than 1 magnitude. This binary is a well-studied one, with numerous references in the literature. The time of primary minimum is predicted by the equation:

HJD = 2452500.0552 + 0.11671956 x CycleNo.

Finding chart for HW Virginis

HW Vir:
RA(2000) = 12h 44m 20.196s
Dec(2000) = -8 40' 17.00"

HW Virginis
(Chart from Starry Night Pro)


Eclipse curves for HW Virginis

  Observation of HW Virginis was carried out on three nights for a period of about three hours each night. Each data point represents a 50-second integration exposure using an ST-7XME CCD camera cooled to -30C and a "Clear" filter. Each point is separated by about one minute. The ten-second difference represents the download and processing time needed before the next exposure begins. The telescope is continuously autoguided. After observing, all of the CCD images in the series are calibrated in the standard way to remove dust donuts and vignetting.

  Because the period of this variable is so short, I was able to observe more than one complete cycle in three hours on two of these three night.

  The figures below shows the eclipse curve for HW Virginis, along with three reference stars and a check star. The first is data from 30-31 March 2009 EST (cycle no. 20,746-7), and the observed geocentric time of minimum (middle of eclipse minimum) was approximately 31 March 2009 03:09:22 UT, or HJD = 2454895.57498. The image on the right is a single CCD frame near the time of mid-eclipse, #22 from a series of 170 images. The positions of HW Virginis and the reference stars are indicated in the image and on the graphic. HW Virginis is off-centered in order to ensure that a guide star was available on the offset guider chip.

  The second is data from 23-24 April, with the geocentric minimum occurring on 24 April 2009 at 04:10:43 UT, which is HJD = 2454946.67954. The predicted geocentric minimum was for 24 April 2009, 04:11:24.

  The third night is data from 24-25 April, with a predicted geocentric minimum of 25 April 2009 02:35:58, actually occurring at 25 April 2009 02:34:33 UT, or HJD = 2454946.61272.

HW Virginis
(Plotted in MaxIm DL/CCD)


HW Virginis
(Plotted in MaxIm DL/CCD)


HW Virginis
(Plotted in MaxIm DL/CCD)


Things to Note

  These are preliminary results, based only on inspection of the light curves and corresponding CCD images. The magnitudes shown on the graphics are instrumental magnitudes, and they have not been transformed to a standard photometric system. Nevertheless, the "constant" values for the magnitudes of the reference and check stars in the images above demonstrate the high-quality of the night and that the telescope-CCD system and evening conditions were stable.

  The moments of eclipse beginning and ending are well defined. Thus, this light curve could be said to be typical of Algol-type eclipsing binaries, except in this case there is a superimposed oscillation that the eclipses are occurring against. The primary minimum reaches a depth of about 0.75 magnitudes, while the secondary minimum is only about 0.12 magnitude. The background oscillation is on the order of 0.2 magnitudes.

   Lee et. al., using both old and new eclipse timing observations and others' radial velocity measurements, describe the physical properties of this system and postulate that cyclical variations in the times of minimum are due to two circumbinary planets with masses of 19.2 and 8.5 times the mass of Jupiter.

  All available data are shown in the figure below.

HW Virginis eclipse cycle
HW Virginis - phase vs delta magnitude (Ref1 - HW Vir)



References

  SIMBAD basic data

  SIMBAD clickable finding chart

  Aladin preview image

  Reference Literature



Back to the list of eclipsing binaries.


This data and information on this page are Copyright © 2009, Richard A. Berg, Washington, DC