An official website of the United States government
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 μ m, 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 μ m would be useful to test the proposed scenario based on grain alignment and disruption by RATs.
more »
« less
This content will become publicly available on November 1, 2026
Grain alignment and dust evolution physics with polarisation (GRADE-POL): I. Dust polarisation modelling for isolated starless cores
The polarisation of light induced by aligned interstellar dust serves as a significant tool in investigating cosmic magnetic fields and dust properties, while posing a challenge in characterising the polarisation of the cosmic microwave background and other sources. To establish dust polarisation as a reliable tool, the physics of the grain alignment process must be studied thoroughly. The magnetically enhanced radiative torque (MRAT) alignment is the only mechanism that can induce highly efficient alignment of grains with magnetic fields required by polarisation observations of the diffuse interstellar medium. Here, we aim to test the MRAT mechanism in starless cores using the multi-wavelength polarisation from optical to submillimetre. Our numerical modelling of dust polarisation using the MRAT theory demonstrates that the alignment efficiency of starlight polarisation (pext/AV) and the degree of thermal dust polarisation (pem) first decrease slowly with increasing visual extinction (AV) and then fall steeply as ∝Av-1at largeAVdue to the loss of grain alignment, which explains the phenomenon known as polarisation holes. Visual extinction at the transition from shallow to steep slope (AVloss) increases with maximum grain size. By applying physical profiles suitable for a starless core, 109 in the Pipe nebula (Pipe-109), our model successfully reproduces the existing observations of starlight polarisation in the R band (0.65 μm) and the H band (1.65 μm), as well as emission polarisation in the submillimetre (870 μm). Successful modelling of observational data requires perfect alignment of large grains, which serves as evidence for the MRAT mechanism, and an increased maximum grain size with higher elongation at higherAV. The latter reveals the first evidence for a new model of anisotropic grain growth induced by magnetic grain alignment. This paper introduces the framework for probing the fundamental physics of grain alignment and dust evolution using multi-wavelength dust polarisation (GRADE-POL), and it is the first of our GRADE-POL series.
more »
« less
- PAR ID:
- 10656011
- Publisher / Repository:
- Astronomy and Astrophysics
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 703
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A192
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Circumstellar disk dust polarization in the (sub)millimeter is, for the most part, not from dust grain alignment with magnetic fields but rather indicative of a combination of dust self-scattering with a yet unknown alignment mechanism that is consistent with mechanical alignment. While the observational evidence for scattering has been well established, that for mechanical alignment is less so. Circum-multiple dust structures in protostellar systems provide a unique environment to probe different polarization alignment mechanisms. We present ALMA Band 4 and Band 7 polarization observations toward the multiple young system L1448 IRS3B. The polarization in the two bands are consistent with each other, presenting multiple polarization morphologies. On the size scale of the inner envelope surrounding the circum-multiple disk, the polarization is consistent with magnetic field dust grain alignment. On the very small scale of compact circumstellar regions, we see polarization that is consistent with scattering around sourceaandc, which are likely the most optically thick components. Finally, we see polarization that is consistent with mechanical alignment of dust grains along the spiral dust structures, which would suggest that the dust is tracing the relative gas flow along the spiral arms. If the gas-flow dust grain alignment mechanism is dominant in these cases, disk dust polarization may provide a direct probe of the small-scale kinematics of the gas flow relative to the dust grains.more » « less
-
Abstract Continuum polarization over the UV-to-microwave range is due to dichroic extinction (or emission) by asymmetric, aligned dust grains. Scattering can also be an important source of polarization, especially at short wavelengths. Because of both grain alignment and scattering physics, the wavelength dependence of the polarization, generally, traces the size of the aligned grains. Similarly because of the differing wavelength dependencies of dichroic extinction and scattering polarization, the two can generally be reliably separated. Ultraviolet (UV) polarimetry therefore provides a unique probe of the smallest dust grains (diameter $$< 0.09~\upmu \text{m}$$ < 0.09 μm ), their mineralogy and interaction with the environment. However, the current observational status of interstellar UV polarization is very poor with less than 30 lines of sight probed. With the modern, quantitative and well-tested, theory of interstellar grain alignment now available, we have the opportunity to advance the understanding of the interstellar medium (ISM) by executing a systematic study of the UV polarization in the ISM of the Milky Way and near-by galaxies. The Polstar mission will provide the sensitivity and observing time needed to carry out such a program (probing hundreds of stars in the Milky Way and dozens of stars in the LMC/SMC), addressing questions of dust composition as a function of size and location, radiation- and magnetic-field characteristics as well as unveiling the carrier of the 2175 Å extinction feature. In addition, using high-resolution UV line spectroscopy Polstar will search for and probe the alignment of, and polarization from, aligned atoms and ions - so called “Ground State Alignment”, a potentially powerful new probe of magnetic fields in the diffuse ISM.more » « less
-
Abstract Radiative torques (RATs) or mechanical torques acting on irregular grains can induce the alignment of dust grains in respect to the alignment axis (AA), which can be either the direction of the magnetic field or the direction of the radiation. We show that carbonaceous grains can be aligned with their axes both parallel and perpendicular to the AA, and we explore the conditions where the particular mode of alignment takes place. We identify a new process of alignment of charged carbonaceous grains in a turbulent, magnetized interstellar medium with respect to an electric field. This field acts on grains accelerated in a turbulent medium and gyrorotating about a magnetic field. The electric field can also arise from the temporal variations of the magnetic field strength in turbulent, compressible media. The direction of the electric field is perpendicular to the magnetic field, and the carbonaceous grains precess in the electric field because of their electric moments. If this precession is faster than Larmor precession in the magnetic field, the alignment of such grains is with their long axes parallel to the magnetic field. We explore the parameter space for which the new mechanism aligns grains with long axes parallel to the magnetic field. We compare this mechanism with another process that provides the same type of alignment, namely, the RAT alignment of grains with insufficiently fast internal relaxation. We describe the conditions for which the particular mode of carbonaceous grain alignment is realized and discuss what information can be obtained by measuring the resulting polarization.more » « less
-
Abstract The dust grain size distribution (GSD) likely varies significantly across star-forming environments in the Universe, but its impact on star formation remains unclear. This ambiguity arises because the GSD interacts nonlinearly with processes like heating, cooling, radiation, and chemistry, which have competing effects and varying environmental dependencies. Processes such as grain coagulation, expected to be efficient in dense star-forming regions, reduce the abundance of small grains and increase that of larger grains. Motivated by this, we investigate the effects of similar GSD variations on the thermochemistry and evolution of giant molecular clouds (GMCs) using magnetohydrodynamic simulations spanning a range of cloud masses and grain sizes, which explicitly incorporate the dynamics of dust grains within the full-physics framework of the STARFORGE project. We find that grain size variations significantly alter GMC thermochemistry: the leading-order effect is that larger grains, under fixed dust mass, GSD dynamic range, and dust-to-gas ratio, result in lower dust opacities. This reduced opacity permits interstellar radiation field and internal radiation photons to penetrate more deeply. This leads to rapid gas heating and inhibited star formation. Star formation efficiency is highly sensitive to grain size, with an order-of-magnitude reduction when grain size dynamic range increases from 10−3–0.1μm to 0.1–10μm. Additionally, warmer gas suppresses low-mass star formation, and decreased opacities result in a greater proportion of gas in diffuse ionized structures.more » « less
