Mapping the magnetic field in the Taurus/B211 filamentary cloud with SOFIA HAWC + and comparing with simulation

Abstract

Optical and infrared polarization mapping and recent Planck observations of the filametary cloud L1495 in Taurus show that the large-scale magnetic field is approximately perpendicular to the long axis of the cloud. We use the HAWC+ polarimeter on SOFIA to probe the complex magnetic field in the B211 part of the cloud. Our results reveal a dispersion of polarization angles of 36○, about five times that measured on a larger scale by Planck. Applying the Davis-Chandrasekhar-Fermi (DCF) method with velocity information obtained from IRAM 30m C18O(1-0) observations, we find two distinct sub-regions with magnetic field strengths differing by more than a factor 3. The quieter sub-region is magnetically critical and sub-Alfv$\acute{\rm e}$nic; the field is comparable to the average field measured in molecular clumps based on Zeeman observations. The more chaotic, super-Alfv$\acute{\rm e}$nic sub-region shows at least three velocity components, indicating interaction among multiple substructures. Its field is much less than the average Zeeman field in molecular clumps, suggesting that the DCF value of the field there may be an underestimate. Numerical simulation of filamentary cloud formation shows that filamentary substructures can strongly perturb the magnetic field. DCF and true field values in the simulation are compared. Pre-stellar cores are observed in B211 and are seen in our simulation. The appendices give a derivation of the standard DCF method that allows for a dispersion in polarization angles that is not small, present an alternate derivation of the structure function version of the DCF method, and treat fragmentation of filaments.

Energy balance and Alfvén Mach numbers in compressible magnetohydrodynamic turbulence with a large-scale magnetic field

“…We hope that our treatment satisfies other authors' concerns about the Skalidis & Tassis (2020) coupling term method. These concerns have taken two forms; one is that the energy contribution from δB • B0 V must = 0 because δB V = 0, as highlighted in the appendix of Li et al (2021b). But as has been extensively discussed in S+2021, and in Section 3 of the current study, δB • B0 V = 0 is not a valid way of understanding the contribution from the coupling term.…”

“…But as has been extensively discussed in S+2021, and in Section 3 of the current study, δB • B0 V = 0 is not a valid way of understanding the contribution from the coupling term. The other concern raised in the appendix of Li et al (2021a), is that energy balance only involves 2 nd order quantities; this approach by definition omits the coupling term contribution, which is first order, in the energetics. Omission of the coupling term leads to significantly underestimating the magnetic energy in sub-Alfvénic turbulence 9 ; this is strongly supported by our numerical results in Figure 5 and 6.…”

Extragalactic magnetism with SOFIA (SALSA Legacy Program) -- III: First data release and on-the-fly polarization mapping characterization

“…In contrast, observations of molecular clouds or the Galactic center may have extended emission much larger than the polarimetry FOV in any given band. We point the reader to Taurus/L1495 observed with HAWC+ at 214 µm, where further pipeline steps were required to recover those large-scales (Li et al 2021). For all the galaxies in our sample, observations with a sky rotation smaller than 3 • were reduced within the same set, which increases the signal-to-noise ratio (SNR) per pixel to optimize the correlation between the astrophysical, sky, and instrumental signals.…”

“…A similar approach was used in Taurus/L1495 by Li et al (2021), where a simpler approach to estimating the background in a C2N observation was performed. For these observations, an adjacent region equal to the FOV of the HAWC+ array close to the molecular cloud was identified using Herschel images.…”

“…The OTFMAP observing mode had previously only been used for total intensity observations (Harper et al 2018). Recent observations of galaxies, Centaurus A (Lopez-Rodriguez 2021) and NGC 1097 (Lopez-Rodriguez et al 2021), and the molecular cloud Taurus/L1495 (Li et al 2021) have successfully used the OTFMAP polarimetric mode with HAWC+. However, a full characterization of this new observing mode for HAWC+ has not been presented until now.…”