Vol. 5 No. 4 October 1, 2009

Page 208

Journal of Double Star Observations

SAO 97883 - A New Double Star
Dave HeraldCanberra, Australia
International Occultation Timing Association (IOTA)
Email: drherald@bigpond.net.au
Robert Sandy
Blue Springs, Missouri
IOTA
Email: bobgraze@sbcglobal.net
Abstract: Observations at a lunar grazing occultation show the 8th magnitude star SAO97883
to be a hitherto unknown close double star that is probably increasing in separation.

Background
It has long been known that when there is a lunar
occultation of a close double star, the light level will
drop to an intermediate level when one component isocculted, with the star completely disappearing
shortly afterwards when the other component is occulted. In recent years occultation observers have developed the use of video with GPS time insertion forrecording such events. This allows for a precise determination of relative magnitudes, and potentially theaccurate measurement of separation and position angle.
Here we report the detection of a new double star,detected during a lunar grazing occultation.

The Observations
The star was observed in grazing occultation conditions on 1 May 2009, at a location north of Canberra, Australia. In grazing occultation conditions, thestar is seen to just clip the edge of the moon as it
moves past, with the star disappearing and reappearing behind hills and mountains on the lunar limb. The
graze occurred at a position angle of about 29°. It wasseen visually by three observers, and recorded onvideo by one observer.
Figure 1 shows the video recording, made using a
20 cm SCT with a Watec 120N+ video camera. The
recording shows three distinct light levels, characteristic of a double star, with one or both of the components being occulted. One (but only one) of the visualobservers also noted the light change to the intermedi-ate light level.
The star SAO 97883 = TYC 1387-971-1 = HD
72093 (8 31 41.2, +19 27 39) is not previously identified as a double star. It is not in the WDS, the Fourth
Catalog of Interferometric Measurements of Binary
Stars, or the Tycho Double Star Catalogue (2002).
A review of past occultations showed that the star
had been observed on 17 previous occasions – but onlythe last of those (an observation on 6 June 2000) included any indication that the star might be double(with the observer - Robert Sandy - reporting that ‘thestar was SLOW in disappearing’). By good fortune,that observation was recorded on video, and Robert
Sandy still had the recording. Nine years ago our ability to ‘measure’ a video recording was very limited, so
we measured the recording with modern measurement tools. Figure 2 shows the light curve that wasobtained.
A video of the two observations has been posted onYouTube, at:
http://www.youtube.com/watch?v=_S83iUfkC3Y

Analysis
The PA of the lunar limb at the points corresponding to the first and last events in the light curve of thegrazing occultation differed by almost 12 degrees –due largely to the lunar topography. This enabled thePA and separation to be computed from the observation: 0.121" ± 0.002" in PA 220° ± 6°.
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Vol. 5 No. 4 October 1, 2009

Journal of Double Star Observations

Page 209

SAO 97883 - A New Double Star

Figure 1: Light curve of the lunar grazing occultation of SAO 97883, extending for 90 seconds. The horizontal axis is theframe number in the video recording. The vertical axis is the intensity, in arbitrary units.

Figure 2: Light curve of the lunar occultation of SAO 97883, observed by Bob Sandy on 2000 June 6 with a 15 cm Newtonian. The light curve extends for 5.6 seconds. The step at about 1760 lasts for 4 frames, or 0.12 sec.

Vol. 5 No. 4 October 1, 2009

Journal of Double Star Observations

Page 210

SAO 97883 - A New Double Starservers. The rate of motion of the moon normal to thatFor the June 2000 occultation there was only one direction is computed from the observation.]
3.The height of the companion of a double starobservation, and consequentially a solution for the PAand separation is not possible. The position angle of above the lunar limb is given by:
the event against the moon was 33.9°, which is almost
the same PA as for the grazing occultation (29°). If Height = Separation * Cos(PAstar – PAevent – limb slope)
there was no change in the PA and separation of thestar over the 9-year period, the expected separation where limb slope is the inclination of the local lunar
normal to the lunar limb would be 0.120". However the limb to the mean lunar limb (with a positive value inseparation of the stars derived from the duration of the dicating the local limb is decreasing in height as thestep event was only 0.025". Clearly the separation and/ position angle increases).
or position angle of the pair has changed significantly
4. The equations of condition for the solution ofover the last 9 years. Speckle interferometry should Separation and PAstar are based on the condition thatenable its motion to be determined.
the heights determined under (2) and (3) should be
The Tycho-2 VT magnitude for the star is 7.86. In equal. A minimum of two independent observations are
figure 1 the full light is at 700 units, intermediate light required for a solution, with the PAevent needing to be
is at 400 units, and zero light is at -10 units – giving significantly different. Preferably a least squares solu8.80 and 8.46 as the magnitudes of the two compo- tion is performed, using multiple observations.
nents. In figure 2 the full light is at 1050 units, intermediate light is at 630 units, and zero light is at 50 Appendix 2 – deriving magnitudes fromunits – giving magnitudes 8.80 and 8.45. That is, the V an occultation
magnitudes of the components are 8.45, and 8.80 –An occultation recording is not suited for obtaining
with an uncertainty of a few hundredths of a magni- an absolute measurement of the brightness of a star.
tude.
However it will accurately show the relative brightnessof the component stars. The magnitudes of the twoConclusioncomponents are simply derived from the requirements
SAO 97883 is a hitherto unknown double star, that the total magnitude must correspond with themagnitudes 8.45 and 8.80, with a separation of 0.121" catalogue value for the star, and their relative bright± 0.002" in PA 220° ± 6° at 2009.3. There has been a ness must correspond to the ratio of the step heights inconsiderable change in separation and/or position an- the video recording.
gle since 2000.
The magnitudes determined by the relative brightness will accurately represent the magnitudes of theAppendix 1 – deriving PA and separation
two stars in the magnitude band used for the total
from a lunar occultation
magnitude of the star – provided the component stars
A lunar occultation is analyzed to determine the are not greatly different in color. When the stars are of
PA and separation on the following basis:greatly different color, the magnitude measurement
1. Assume the lunar limb is a straight line at the will be affected by the difference in response of thepoint of occultation. In general, the lunar topography video camera compared to the color band used for theresults in the ‘local’ lunar limb being inclined to the star magnitude. The unfiltered video cameras used formean lunar limb.
the observations have a spectral response somewhere
2. The observed height of the companion of a dou- between the V and R magnitude bands. Consequently,
ble star above the lunar limb is given by duration of a by using magnitudes in the V or R band, the magnistep event multiplied by the rate of motion of the moon tudes derived from the video recording are a reliable
in a direction normal to the lunar limb. [The direction measure of the magnitudes of the components in thatof the lunar limb is derived using past lunar occulta- band, even when the components have a large differtions made at the same part of the moon, or (for a graz- ence in color.
ing occultation) using the observations of the other ob(Continued from page 208)