Multiwavelength Polarimetry of the Filamentary Cloud IC 5146. II. Magnetic Field Structures

Abstract

The IC5146 cloud is a nearby star-forming region in Cygnus, consisting of molecular gas filaments in a variety of evolutionary stages. We used optical and near-infrared polarization data toward the IC5146 cloud, reported in the first paper of this series, to reveal the magnetic fields in this cloud. Using the newly released Gaia data, we found that the IC5146 cloud may contain two separate clouds: a first cloud, including the densest main filament at a distance of ∼600 pc, and a second cloud, associated with the Cocoon Nebula at a distance of ∼800 pc. The spatially averaged H-band polarization map revealed a well-ordered magnetic field morphology, with the polarization segments perpendicular to the main filament but parallel to the nearby sub-filaments, consistent with models assuming that the magnetic field is regulating cloud evolution. We estimated the magnetic field strength using the Davis-Chandrasekhar-Fermi method, and found that the magnetic field strength scales with volume density with a power-law index of ∼ 0.5 in the density range from N H2 ∼ 10 to 3000 cm −3 , which indicates an anisotropic cloud contraction with a preferred direction along the magnetic field. In addition, the mass-to-flux ratio of the cloud gradually changes from subcritical to supercritical from the cloud envelope to the deep regions. These features are consistent with strong magnetic field star-formation models and suggest that the magnetic field is important in regulating the evolution of the IC5146 cloud.

Observational Signatures of Cosmic-Ray Interactions in Molecular Clouds

“…We note that there is some debate over the distance to the system (see, e.g., Lada et al 1999;Harvey et al 2008), and whether it is one system or two clouds along the same line of sight at different distances (Wang et al 2020). Such matters would impact the conversion between angular separations and physical distances in this work.…”

“…As such, we assume this system is oriented at ψ = 0°to the plane of the sky. The magnetic field permeating this cloud is observed to be arranged predominantly perpendicularly to the filament orientations on the sky plane (Wang et al 2017(Wang et al , 2020, and the mean magnetic field strength across the filaments can be well described by a 49)). The filamentary structures are therefore ideal test cases of our model, where the heating and ionization rate profiles can be calculated through each filament by adopting a natural boundary condition at the filament edge.…”

“…However, since such a boundary condition could not yield a self-consistent determination of the CR flux flowing through some boundary, Morfill (1982b) argued that practically each cloud may be considered to have its own sphere of influence from which the boundary condition may be taken. In this case, we take s c = 5 pc to reflect the approximate separation of clouds and/or filaments found in the types of astrophysical setting in which we would expect our model to apply (see, e.g., Arzoumanian et al 2011;Wang et al 2020 from which distances between filaments in the IC 5146 14 Strictly, this is only true for particles of tens of MeVs, with high-energy particles of energies above 10 GeV undergoing free-streaming without experiencing any substantial scattering at all (Chernyshov et al 2018). However, we argue that at GeV energies and below, where the bulk of the energy density lies in our adopted CR spectrum (and where the effects of the CRs are correspondingly strongest) this treatment is sufficiently informative for our first model.…”

“…Our CR propagation model uses the derived diffusion coefficient specific to the IC 5146 region, where we adopt a mean value for the diffusion coefficients calculated in the Rc, i′, H, and K bands, weighted according to the number of data points in each band. The filament ridge volume density, n c , is estimated from the measured column densities N H using n c ≈ N H /(2 R flat ) following Wang et al (2020). This assumes each filament has a cylindrical geometry, and we note that R flat has been adjusted to account for the updated distance to IC 5146 of 813 pc (compared to a distance of 460 pc used in Arzoumanian et al 2011).…”

“…We also predict the corresponding outcome if the irradiating CR intensity changes. We then apply our model to the interstellar cloud IC 5146, a complex known to host a broad variety of environments for which a plethora of polarization measurements are available (Wang et al 2017(Wang et al , 2020.…”