Monday, March 15, 2021

The Skull and Crossbones Nebula

The Skull and Crossbones Nebula (also known as NGC 2467, Sh2-311, Gum 9, and ESO 493-25) is a star forming region, located at a distance around 16,000 light years in the constellation of Puppis. It was discovered in 1784 by William Herschel.

The nebula is a superimposition of many stellar groups that are located at different distances, along the nearly same line of sight. Thus, it does not represent a single open star cluster. Two of the most interesting groups of stars in the region are known as Puppis OB1 and Puppis OB2.

A young massive O6-type star known as HD 64315 dominates the region. Two star clusters known as Haffner 18 (H18) and Haffner 19 (H19) that are superimposed in the region, have attracted the interest of astronomers. 

H18 contains a very young star known as FM3060a that is still surrounded by its cocoon. The age of the stellar group is not known accurately, but it could be as young as one million years. H19 is a compact cluster and it is around two million years old. Within the cluster there is a Stromgren sphere (i.e., a region of ionized hydrogen surrounding a young hot OB star) that is ionized by a B0V- type star. 

This region has been used as a template for star forming regions in order to answer questions that still remain a mystery. One of them is, if the presence of massive OB stars affects future star formation in a region.

Infrared observations with the Spitzer Space Telescope revealed the presence of 45 protostars. Most of them are located on the edge of the H II region of the cloud. The concentration of these stars is correlated with the ionization front, thus it acts as evidence of triggered star formation.

Image: Composite optical image of Skull and Crossbones Nebula taken with ESO’s 2.2 m telescope. The image was created using both broadband and narrow band filters. Emission due to ionized oxygen gas is shown in green, while ionized hydrogen emission is shown is red. Emission from the V-band and R-band, share the same color channel with the oxygen and hydrogen filters respectively, while the B-band is shown in blue. Haffner 18 is located at the center of the image, while Haffer 19 is on its left (the small pink region). Finally, at the center of the largest pink region towards the bottom of the image HD 64315 can be seen.

Image Credit: ESO

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