Mysterious "Disappearance" of Delta Velorum.

Recently, Paul Fieseler of NASA  fieseler@mail1.jpl.nasa.gov  wrote to David Dunham of IOTA reporting that Delta Velorum, one of the stars used to orient the Galileo Spacecraft, had unexpectedly vanished from the ken of the spacecraft's star scanner.

He writes as follows:

"Yes, we have clean data that shows the other stars in the field of view remaining steady during the time in question. There were no other spacecraft anomalies at the same time and Galileo took the star's apparent disappearance in stride. In the past when this has happened it is because we selected a known variable star - this looks like the same thing except Delta Velorum is not known to be variable - although an eclipsing binary is a possibility. A new and previously undiscovered operational mode of the star scanner is also a distinct possibility. The outage (where the star appeared to fall at least 0.2 in magnitude and thus "disappear") was from GMT 06:30 to 14:30 on June 19. At 14:30 the star re-appeared and took several more hours to regain full intensity. We were using this star for 20 days prior without incident. There is also an indication in our records that this same star may have disappeared in late 1989. We have not used it in between then and now so I have no more data.

Concerning your request to the occultation organizers and other folks in the Southern Hemisphere, I appreciate the effort and you may proceed. However, let me emphasize to all that my request for information springs from curiosity about the star itself. . .

Paul"

So here is an opportunity for amateurs who do not use telescopes or binoculars to do their bit for science. Delta is at the right hand end of the crosspiece of the False Cross and can be compared with the other three stars of the False Cross. Proceding clockwise from Delta, they are: Epsilon Carinae, mag 2.0, Iota Carinae, mag 2.3 and kappa velorum, mag 2.5 Have a good critical look at every opportunity. Don't be fooled by atmospheric extinction when the False Cross is near the horizon. Report any anomaly to Brian Fraser, Jan Hers, Paul or myself. Even if all seems normal you should make a note of the estimated magnitude, date and time of the observation and report these to Jan Hers   janhers@pixie.co.za  at the end of the month. Those observers who eschew visual observations can use monitoring devices ranging from the cheap and simple to the most expensive state of the art systems to do the monitoring for them. Contact me for ideas. If we all make an effort then sooner or later somebody should catch Delta Velorum doing its thing!

Danie Overbeek

Footnote to "Variable of the Month" Article.

After the above was written, it transpired that Sebastian Otero of Argentina has suspected since 1997 that the star is an eclipsing binary and has been monitoring it without arriving at a final solution. The current idea is that the period is 5.88 days. Yesterday morning, when an eclipse was predicted, Raol Salvo of Montevideo and I made some observations under difficult conditions without reaching finality. We need as many people as possible to monitor the star as often as possible.

Convenient comparison stars are Gamma Velorum Mag 1.82, Epsilon Carinae Mag 1.95 and Kappa Velorum, Mag 2.48

Danie Overbeek
2000 10 16