[Cii] emission from L1630 in the Orion B molecular cloud

Area of Science:

• Astronomy and Astrophysics
• Interstellar Medium Physics
• Astrochemistry

Background:

• L1630 in Orion B, containing the Horsehead Nebula, is a photodissociation region (PDR) illuminated by σ Ori.
• PDRs exhibit complex chemistry and physics at the interface of stellar radiation and dense molecular clouds.
• L1630 offers a less extreme environment than OMC1 for studying these interactions, with limited star formation.

Purpose of the Study:

• To investigate the interplay between stellar radiation and molecular clouds in L1630 under intermediate conditions.
• To correlate ionized carbon ([Cii]) fine-structure line emission with prevailing physical conditions in L1630.
• To compare findings in L1630 with those from the more active Orion Molecular Cloud 1 (OMC1).

Main Methods:

• Observations of the [Cii] 158 μm line emission in L1630 using the upGREAT instrument on SOFIA.
• Mapping an area of 12' × 17' around the Horsehead Nebula.
• Comparison with existing data, including CO(1-0) line emission, far-infrared (FIR) continuum, and polycyclic aromatic hydrocarbon (PAH) emission.

Main Results:

• 95% of [Cii] emission (13 L⊙) originates from the molecular cloud, with only 5% (1 L⊙) from the adjacent Hii region.
• Inferred molecular cloud gas density of nH ∼ 3 × 10³ cm⁻³, with surface layers reaching nH ∼ 4 × 10⁴ cm⁻³ and gas temperature T ∼ 100 K.
• Average [Cii] cooling efficiency is 1.3 × 10⁻², tracing only 8% of the molecular cloud mass; PDR models reproduce FIR-[Cii] and CO(1-0)-[Cii] correlations.
• Observed gas heating efficiency is lower than theoretically predicted, consistent with other observations.

Conclusions:

• Ionized carbon ([Cii]) emission traces a small fraction of the total gas mass in L1630, primarily originating from PDR surfaces.
• The layered structure and limited spatial resolution complicate the relationship between tracers and physical conditions.
• The observed heating efficiency in L1630 is lower than theoretical predictions, suggesting a more complex relationship between [Cii] emission and physical conditions than commonly assumed.