Planetary Nebula – V651 Mon

NGC 2346 – V651 Mon

NGC 2346 was discovered by William Herschel in 1785. NGC 2346 is remarkable because it’s central star V651 Mon is actually known to be a very close pair of stars. The star is a spectroscopic binary with an orbital period of almost 16 days.

The bright component of an A5 V star, that is not the one exciting the nebula, went through a period of “eclipses” in 1982-1985, the nature of which was interpreted as a dust cloud passing in front of it’s orbit. (Costero et al. 1986). The period of the “eclipses”, or better, the occultations, was the same as that of the binary. After 1985 there were no more reports of  occultations in this system and it was predicted that it would not occur again for 100’s of years. However, the second-ever deep fading was reported in August 1996,  when the star began to show similar fading episodes, also periodic, same 16 day period, but this time much faster. The entire duration of the 1996-1997 event was 400 days, remarkably shorter than the 1981-1985 event.

I first observed V651 Mon in May 1997 after being contacted by Rafael Costero, Institute of Astronomy UNAM Mexico, asking if it was possible to obtain observations from the Southern Hemisphere as the object was to low in the evening twilight sky for Northern Hemisphere observers. The first observation I sent to Rafael was <14.4 on May 30, which implied that I clearly observed the exiting of the star behind the cloud. The star re-brightened to 11.3 then I observed another fading to <13.2 on June 15. The two red fainter than points can be seen in the light curve. V651 Mon then brightened to 11.2.

I was able to follow the occultations up to June 22. V651 Mon has been on my list ever since and the light curve below details my visual observations.

As seen from the light curve after the first fading in 1997 two more fading episodes in November 2004 and September 2008 have occurred. The mystery remains in explaining why so many of such episodes have taken place in the last 28 years or so, while none were registered before the fading episode in 1982. Albeit, “predicted” events were said to only repeat in another one million years!!!). V651 Mon also varies around 0.5 mag during it’s normal state.

A detailed light curve of the 2004 fading episodes.

V651 Mon can be a challenge to observe visually  due to the nebula surrounding the star. The image below of V651 Mon is similar to the visual field I see when observed through my 16″ telescope.

CCD Image Kyoto University Japan

An article on the 1996-1997 Fading of V651 Mon.

PASJ: Publ. Astron. Soc. Japan 53, 901-903, 2001 October 25


2 Responses to Planetary Nebula – V651 Mon

  1. Berto Monard says:

    Very interesting indeed! I will add this PN to my monitoring (V and Ic) program of symbiotic stars.

    Great website, great observer! Well done, Rod!

    Berto Monard / Kleinkaroo Observatory, Calitzdorp, South Africa


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