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GU Orionis
(23 January 2009)

  The General Catalog of Variable Stars (GCVS) reports GU Orionis to be a short-period Algol-type eclipsing systems (p = 0.4706769 days = 678 minutes). Algol-type systems are binaries with spherical or slightly ellipsoidal components. In these systems it should be possible to observe from their light curves the moments of the beginning and end of the eclipses. Between eclipses the light remains almost constant or varies insignificantly because of reflection effects, slight ellipsoidality of components, or physical variations. Secondary minima may be absent. 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 GU Orionis, this amplitude is 0.6 magnitudes, but the light curve does not conform to the shape specified for an Algol-type system. It meets all the criteria of a W Uma star - short period, continuously variable light curve, and nearly equal minima. It should be classified as a "EW" system. This binary is a well-studied one, with numerous references in the literature. The time of primary minimum is predicted by the equation:

HJD = 24525000.4408 + 0.4706769 x CycleNo.

Finding chart for GU Orionis

GU Ori:
RA(2000) = 06h 10m 04.6s
Dec(2000) = 12° 49' 47"

GU Orionis
(Chart from Starry Night Pro)

Eclipse curves for GU Orionis

  Observations of GU Orionis were carried out over several 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 -30°C 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.

  The figure below shows the eclipse curve for GU Orionis, along with three reference stars and one check star. This is data from 01-02 January 2009 EST (cycle no. 4957), and the observed geocentric time of minimum (middle of eclipse primary minimum) was approximately 02 Jan 2008 02:01:42 UT, or HJD = 2454834.09001. The image on the right is a single CCD frame near the time of mid-eclipse, #25 from a series of 110 images. The positions of GU Orionis, the three reference stars and the check star are indicated in the image and on the graphic. The graphic is generated by MaxIm DL/CCD, using the photometry tool.

GU Orionis
(Plotted in MaxIm DL/CCD)

  The figure below shows the eclipse curve for GU Orionis for the second night's observations, along with the same reference and check stars. This is data from 16-17 January 2009 EST (cycle no. 4989), and the observed geocentric time of minimum (middle of eclipse primary minimum) was approximately 17 Jan 2008 03:24:54 UT, or HJD = 2454849.15060. The image on the right is a single CCD frame near the time of mid-eclipse, #119 from a series of 165 images. The identification of GU Orionis, the three reference stars and the check star are indicated on the graphic.

GU Orionis
(Plotted in MaxIm DL/CCD)

  A third and fourth night's observing on GU Orionis took place on 21-22 January and 22-23 January, with a run of 115 observations on the third night (during mid-cycle 4991) and 190 on the fourth night (during mid-cycle 5001). These were both secondary eclipse minima. Posted below is a picture of the fourth night images at the time of secondary minimum (image #36), and below that a table indicating the predicted and observed times of the four minima. Additional observations have been made on subsequent nights.

GU Orionis
(Plotted in MaxIm DL/CCD)

GU Orionis Times of Minima
(Plotted in Excel)

Things to Note

  These preliminary results are based only on inspection of the light curves and corresponding CCD images. The equations used to generate heliocentric predictions for primary minimum were corrected to get a geocentric UT time for the predicted minima. The observed geocentric UT time of primary minimum is determined from the light curve, and corrected again to determine the observed heliocentric Julian date of mid-eclipse.

  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 demonstrate that the telescope-CCD system and evening conditions were stable. As suggested above, this deep light curve is not typical of Algol-type eclipsing binaries.

  Several night's observations have been added. The second night of observing was quite clear, very cold (-14C), and apparently (to the eye) quite steady, but clearly the observational conditions changed during the three hour run, as evidenced by the gentle dimming of the Ref1, Ref3 and Chk1 stars' brightness. Since these stars were rising during the run, one would expect the objects to get brighter, as shown by their instrumental magnitudes. This was not the case.

  The table above suggests that the secondary eclipses occur slightly later than at phase 0.5, compared to the primary eclipses. The difference is too small (about 45 seconds), though, and not supported by the accuracy of the observations which are probably not better than ±60 seconds. The secondary minimum is slightly shallower than the primary minimum.

  Also, surprisingly, it would appear that the Ref2 star is actually a variable star itself. It's identified in catalogs as ZN075:0600/510 in the USNO catalog, 36254508 in the UCAC2, and 06102945+1251226 in the 2MASS catalog. John Farrell points out that it is also identified in the GCVS as "EF Orionis," where it is called an "EA" (Algol-type) system. The eclipse phase of EF Orionis is given by HJD = 2452501.2008 + 1.61944600 x E.

  In the same field of view of GU Orionis, to the south east, is a faint ionized hydrogen region and IR star cluster identified as Sharpless 2-270.


GU Orionis eclipse cycle
GU Orionis - phase vs delta magnitude (Ref1 - GU Ori)


  ●  SIMBAD basic data

  ●  SIMBAD clickable finding chart

  ●  Aladin preview image

  ●  Reference Literature

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This data and information on this page are Copyright © 2008-2009, Richard A. Berg, Washington, DC