Abstract
Abstract The alignment of dust grains with the ambient magnetic field produces polarization of starlight as well as thermal dust emission. Using the archival SOFIA/HAWC+ polarimetric data observed toward the ρ Ophiuchus (Oph) A cloud hosted by a B star at 89 and 154 μm, we find that the fractional polarization of thermal dust emission first increases with the grain temperature and then decreases once the grain temperature exceeds ≃25–32 K. The latter trend differs from the prediction of the popular RAdiative Torques (RATs) alignment theory, which implies a monotonic increase of the polarization fraction with the grain temperature. We perform numerical modeling of polarized dust emission for the ρ Oph-A cloud and calculate the degree of dust polarization by simultaneously considering the dust grain alignment and rotational disruption by RATs. Our modeling results could successfully reproduce both the rising and declining trends of the observational data. Moreover, we show that the alignment of only silicate grains or a mixture of silicate–carbon grains within a composite structure can reproduce the observational trends, assuming that all dust grains follow a power-law size distribution. Although there are a number of simplifications and limitations to our modeling, our results suggest grains in the ρ Oph-A cloud have a composite structure, and the grain size distribution has a steeper slope than the standard size distribution for the interstellar medium. Combination of SOFIA/HAWC+ data with JCMT observations 450 and 850 μm would be useful to test the proposed scenario based on grain alignment and disruption by RATs.
Highlights
The magnetic field is believed to play an essential role in various astrophysical phenomena, including the formation of stars and planets (Crutcher 2012)
We showed and interpreted the relation between the fractional polarization of thermal dust emission and dust temperature in the ρ Oph-A molecular cloud using the archival
The observed fractional polarization first increases with increasing dust temperature and decreases once the dust temperature exceeds ;25–32 K
Summary
The magnetic field is believed to play an essential role in various astrophysical phenomena, including the formation of stars and planets (Crutcher 2012). Observations have reported an anticorrelation trend of the fractional polarization of thermal dust emission with the column density of the gas in molecular clouds (e.g., Arce et al 1998; Whittet et al 2008; Planck Collaboration et al 2015, 2020; Fissel et al 2016; Santos et al 2017, 2019; Chuss et al 2019) This trend is explained by the loss of grain alignment toward dense cores (Whittet et al 2008; Hoang et al 2020) or by the turbulent structure of magnetic field within the scale of the beam size (see Jones & Whittet 2015; Planck Collaboration et al 2015). Oph-A is one of the best laboratories to understand the multiband dust polarization in the context of highenergy radiation giving us an opportunity to investigate RAT in detail
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