Observations examine connection of supernova remnant with nearby H II region

Chinese astronomers have conducted multi-wavelength observations of a galactic supernova remnant known as HB9. Results of the observation campaign, published on August 9 on the preprint server arXivshed more light on the properties of the remnant and its possible physical connection with a nearby region of ionized atomic hydrogen (H II region).

Supernova remnants (SNRs) are diffuse, expanding structures resulting from a supernova explosion. They contain expanding ejected material from the explosion and other interstellar material that was carried away by the passage of the shock wave from the exploded star.

The study of supernova remnants is important to astronomers because they play a key role in the evolution of galaxies, dispersing heavy elements produced during the supernova explosion and providing the energy needed to heat the interstellar medium. SNRs are also thought to be responsible for the acceleration of galactic cosmic rays.

Observations show that young stars form in giant molecular clouds, ionizing the surrounding gas and forming H II regions. Later, when the stars explode, they leave SNRs in these regions. However, the physical connection between SNRs and the surrounding H II regions has not yet been thoroughly studied.

HB9 is a large, relatively young (about 6,600 years old) Galactic SNR located about 2,000 light-years away, with a small H II region G159.2+3.3 apparently projected nearby. HB9 has a shell-like radio morphology and its angular size is estimated to be about 2 degrees in diameter. As for G159.2+3.3, it appears brighter, more compact than HB9, and is located about 2 degrees north of the southern edge of the SNR shell.

Now, a team of astronomers led by Jiang-Tao Li of the Purple Mountain Observatory in Nanjing, China, has used the 1.3-m McGraw-Hill telescope at the Michigan-Dartmouth-MIT (MDM) Observatory to inspect the possible physical connection between HB9 and G159.2+3.3.

Observations showed that HB9 is bright in gamma rays but its morphology is not clearly associated with the surrounding molecular clouds. However, a weak apparent connection of this residue to the bright infrared shell enveloping G159.2+3.3 was identified in gamma rays.

The obtained data show that the multiwavelength morphology of G159.2+3.3 is typical of galactic H II regions. This suggests that the Balmer line and the radio emissions in its interior are of thermal origin, while the bright infrared shell consists of dust heated by ultraviolet photons from young stars.

The study revealed that the radial velocity of the southeast shell of HB9 is between -30 and +50 km/s. According to the astronomers, this indicates a possible heating of the gas by SNR shock.

Additionally, the researchers calculated a median electron density for the southeastern layer of HB9, which turned out to be at a level of 100 cm^-3What is remarkable is a smaller median value of about 50 cm.^-3was obtained for the brighter G159.2+3.3. This, according to the authors of the paper, could mean that the distance of G159.2+3.3 is indeed greater than that of HB9, so that the two objects are not physically connected to each other.

“Our density estimate suggests that G159.2+3.3, although appearing brighter and more compact, is likely located at a much greater distance than HB9, so the two objects have no physical connections, unless the shock-compressed gas in HB9 has a significantly lower filling factor,” the scientists concluded.

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