The Iris nebula has a complex structure of clouds and filamentary structures. The distribution of these clouds is highly inhomogeneous containing several dense
irradiated structures. Several
photodissociation regions (PDRs) and
dissociation fronts (FDs) have been observed bordering the cavity as bridge illuminated ridges towards the northwest, southwest and eastern directions. These regions are driven by soft radiation fields with effective temperatures of 18,000 Kelvin and G0 of ∼2.6 ×10^3. The outflows from the binary stars near the center of the Iris nebula has carved out a biconical cavity and triggered the formation of a few photodissociation regions (PDR 1 and PDR 2). In general, every filament of H2 has a corresponding
extended red emission (ERE). These ERE filaments are generally 10% wider. The ERE filaments are edited in a two step process. The first step requires a photon with the energy of 10.5
eV to produce a ERE carrier. These carriers are probably PAHs because they match the three criteria: (1) having ionization or dissociation potential that is in excess of 10.5 eV, (2) exhibiting strong absorption in the optical or near-UV spectral region, and (3) is capable of efficient
photoluminescence. This is possibly done through the process of photoionization and photodissociation of a suitable precursor. The second step requires a sufficient amount of pumping of the ERE which is carried by abundant optical and near-UV photons, energizing the ERE. The Iris nebula contains
polycyclic aromatic hydrocarbons (PAHs), large
organic molecules that have their
carbons arranged in
honey-comb like shapes and their
hydrogens attached to the edges.
Central stars , a
binary star system that has a large role in shaping the nebula. Located at the center of the Iris nebula is a
binary system of
intermediate-mass Herbig Ae/Be stars. The first is a +7.4 magnitude
B2Ve-type stars. The second is a B5-type star. The nebula is solely illuminated by these stars and the
photodissociation physics of the nebula is driven by this system. These stars also shape parts of the nebula such as the central cavity and some of the photodissociation regions. The fullerens are located more towards the central star and in the central cavity unlike the
polycyclic aromatic hydrocarbons (PAH) in the nebula. It was generally believed that fullerenes formed due to the buildup of small
carbonaceous compounds in the hot dense envelopes of
evolved stars. However it was revealed that they seem to form efficiently in tenuous and cold environments such as
interstellar clouds that are illuminated by strong
ultraviolet (UV) fields of
radiation. This means that there must be another pathway for the formation of fullerenes in these environments. Laboratory and theoretical studies show that a possible pathway for their formation involves PAHs being converted into
graphene under UV radiation from massive stars such as HD 200775.
Photochemical modeling of
circumovalene (C60H20) by (
Berné et al. 2015) was done to test the possibility that the PAHs can lead to the formation of fullerenes. These models predict that the formation of fullerenes in the nebula first involves the involves full
dehydrogenation of circumovalene. It then fold into a floppy closed cage that shrinks with the loss of C2 units until it reaches the symmetric C60 molecule. Shrinkage of the cage seems to be the main
kinetically limiting step of the entire process of fullerene formation meaning that molecules larger than circumovalene are unlikely to contribute significantly to the formation of fullerenes. Molecules containing fewer than 60
carbon atoms will often be destroyed not providing significant contributions. This makes molecules containing around 60-66 carbon atoms are most stable and likely to form fullerenes. Once they form, fullerenes can be stable on timescales greater than 10 million years at
radiation fields below
G0 = 104.
Young stellar objects The Iris nebula contains several
young stellar objects (YSOs). There are four candidate YSOs located within the northwestern region of the nebula. This nebula contains the most concentrated amount of YSOs in the
Cepheus Flare. There are several scenarios for the formation of YSOs in the Iris nebula. The first is
gravitational instabilities occurring on small scales forming dense shells and chains of YSOs along fronts of
ionization. The sections scenario is gravitational collapsing occurring on larger scales forming regions of local star formation and young
clusters of YSOs. The third scenario is gravitational instabilities that occur on dense thin, finger-like structures that occur in turbulence mediums irradiated by
O-type stars. The fourth and final scenario is gravitational instabilities occurring from pre-existing condensations compressed by hot gas. ==Notes==