Acquisition notes

This is my longest project to date. I accumulated 83 hours of narrowband data (and one hour of RGB for the stars) over a period of two years . Mostly in moon-poor nights as the subject is weak and the OIII signal is easily polluted by moonlight (The Ha signal is even fainter). The weather leaves much to be desired in autumn and although I had planned to give it more exposure time,  I decided to call it quits. I believe that this data is the most that I can do with my skies and the OSC camera/ dual band filter combination.About half of the data was captured with the Optolong L-Enhance filter. The other part with the IDAS NBZ filter. The IDAS NBZ filter is especially suited to fast optical systems and is also relatively good in halo control. 

The interesting appearance of HFG1

Heckathorn-Fesen-Gull is a large, but very faint planetary nebula with a lot of interesting features. It is unusual in that it has a bow shock structure, but also a cometary trail. As can be seen in the images, the planetary nebula has an inner part and an outer ring. In between these two structures, there are filaments. If these are actually bridge elements or just projection effects because we are seeing a 2d representation of a 3d non uniform object is unclear.


The peculiar star that generates HFG1

The central star of HFG1 is actually a binary system under the name of V664 Cas. One of the stars is a main sequence star of about 1 solar mass and more or less the same composition as our sun. However, the other star and the star that created the planetary nebula has the spectral signature of a very rare type: a subdwarf O star (sdO). A not well understood type of hot dwarf stars. Subdwarf O stars are less massive than our sun (in this case half a sun mass), but are much hotter (T=40-100K)  an a lot more luminous than our sun (10-100 times). Many of the known subdwarf O stars have are moving at high velocity as is the case here. Subdwarf O stars are believed to be products in the ending stages of evolution, where the star evolves from a main sequence star to a red giant and eventually, after all fusion has come to an end, to a white dwarf. The structure of o subdwarf O star is believed to be a carbon and oxygen core surrounded by a helium burning shell, with most of the hydrogen outer shell prematurely stripped off. As a result, we are looking directly at the fusing helium layer.


Explaining the shape of HFG1

Although many mysteries still remain on planetary nebulae and subdwarf O stars, a possible picture of what happens at HFG1 is this *:


- At 1600 light-years distance to earth, the binary system V664 Cas is hurtling through space at supersonic speed through the Milky Way.

- The progenitor star of the sdO dwarf was a three solar mass star that arrived the very end of its evolution, with stellar wind ejecting the outer layer of hydrogen.  As the binary system ploughs through the interstellar gas, this material is being swept away behind the star due to the pressure of the interstellar gas in which it is speeding. This created the collimated tail of HFG1.

- In the later stages of the end-game for this star, there was sudden increase in the stellar wind . Both in the amount of material and in the speed of the wind. This wind now collides with and pushes the incoming (relative to the speeding star) interstellar gas away from the star, creating a bow wave  and a shock front.

- Due to the high radiation level of the sdO dwarf and the temperature rise caused by the shock wave, OIII emission is generated.. This is what forms the main structure of HFG1.

*       This scenario is a much simplified account of research I could find on HFG1, with many uncertainties and nuances left out_ It is based especially upon the hydrodynamic computer simulation in paper # 3


Sources:
1.       The original discovery publication in 1982: https://www.researchgate.net/publication/234470305_Discovery_of_a_large_high-excitation_planetary_nebula_at_L_136_deg_B_5_deg
2.       The discovery of the cometary trail in 2009: https://arxiv.org/abs/0903.2852
3.       Modeling of the cometary trail:  https://arxiv.org/abs/1512.00864