McNeil's Nebula for 2015

McNeil's Nebula suddenly appeared being recognized as a new object by M78 January 23, 2004 when Jay McNeil found it testing a 3" refractor. Searching old images turned it up in an October 1966 image by Evered Kreimer that was published in a book on the Messier objects yet no one at that time recognized it as a new object. But images before and after that failed to show it. True to form it vanished sometime while Orion was lost in the sun. Finally in the fall of 2008 when Orion reappeared it was back. So far it hasn't faded again. It is lit by an FU Orionis star, V1647 Orionis, that appeared and vanished at the same time as the nebula. FU Orionis stars are a type of eruptive proto star.

While it is considered a variable nebula it doesn't act like its more famous cousins such as Hubble's Variable Nebula, Gyulbudaghian's Nebula, etc. They are constantly changing over short time spans, even less than one week. They seem to be lit by cloud enshrouded proto stars. The moving clouds cast shadows on the nebula illuminating various parts as they orbit the star. But for this nebula it is the extreme brightness changes of the star itself that make the entire nebula appear and disappear. At least, I've been taking an annual image of it since its reappearance in 2008 and it just doesn't change. Seeing changes so some images are sharper than other, transparency changes hide or reveal fainter features but there's no change I can see that is real. The star has stayed bright since 2008. I keep imaging it each year hoping to see a change but I never do. It gives no sign that 8 years ago it wasn't visible though hits of it can be seen in deep IR images of the area prior to its reappearance. I'm not sure why I keep taking it since it doesn't change. I wasn't planning to do it this year for that reason but I saw some very low resolution wide field images of M78 that picked it up and it seemed different in those. So I tried yet again but like all prior years came up with no real change.

No one has caught it appearing or vanishing. So we don't know what changes it goes through at those times. Does the bottom near the star light up first and it spread to the top because it really has the 3D structure we see in 2D or could most parts all be the same distance and thus the entire nebula appears at once? Its size, top to bottom is about half a light-year minimum assuming its distance is 1,500 light-years that I see most often quoted. If the bottom to top illumination idea is right it would take at least a half year to appear and disappear. But since it seems to come and go at the same time as we see the star appear and vanish this puts some interesting constraints on the geometry of the system. We have to be seeing it in such a way all illumination arrives here at about the same time. It could mean the closest parts to us are at the top but furthest from the star so that the light travel time from star to nebula to earth is about the same but an observer on the opposite side would see it take many months to go from star visible to top part visible. I've not seen anything trying to work out its geometry. This is further complicated by the fact the last time it vanished and then reappeared each happened when it was lost in the sun's glare for a couple months so it could take those two or three months to come and go.

The field is full of proto stars of all types. Some SIMBAD lists as simply proto stars (pr*). Most were too faint in visible light for me to pick up. More common were T Tauri stars (TT*) a type of protostar that tends to be unstable but not as much so as FU Orionis stars like the one illuminating the nebula and young stellar object (Y*O) which are at a somewhat earlier stage and not yet trying to ignite their Hydrogen fusion reactions. There are also quite a few Herbig Haro objects caused by jets created by these still forming stars. There's a lot of young, still forming stars in this field.

I've included a composite of three prior year for comparison. This image was taken on March 10, 2015. That makes it my first March image so I'm making a bit of progress on my backlog being only 4 months behind with nothing taken for July thanks to thick smoke from Canadian fires as well as a bright moon.

14" LX200R @ f/10, L=4x10' RGB=2x10', STL-11000XM, Paramount ME

Rick