skip to main content


Title: Mid-infrared Polarization of the Diffuse Interstellar Medium toward CygOB2-12
Abstract

We present the first mid-IR detection of the linear polarization toward the star CygOB2-12, a luminous blue hypergiant that, withAV≈ 10 mag of foreground extinction, is a benchmark in the study of the properties of dust in the diffuse interstellar medium. The 8–13μm spectropolarimetry, obtained with the CanariCam multimode camera at the Gran Telescopio CANARIAS shows clear trends with wavelength characteristic of silicate grains aligned in the interstellar magnetic field. The maximum polarization, detected with 7.8σstatistical significance near 10.2μm, is (1.24 ± 0.28)% with position angle 126° ± 8°. We comment on these measurements in the context of recent models for the dust composition in the diffuse interstellar medium.

 
more » « less
Award ID(s):
1908625
NSF-PAR ID:
10381334
Author(s) / Creator(s):
; ; ; ; ;
Publisher / Repository:
DOI PREFIX: 10.3847
Date Published:
Journal Name:
The Astrophysical Journal Letters
Volume:
940
Issue:
1
ISSN:
2041-8205
Format(s):
Medium: X Size: Article No. L26
Size(s):
["Article No. L26"]
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    We present a new model of interstellar dust in which large grains are a single composite material, “astrodust,” and nanoparticle-sized grains come in distinct varieties including polycyclic aromatic hydrocarbons (PAHs). We argue that a single-composition model for grains larger than ∼0.02μm most naturally explains the lack of frequency dependence in the far-infrared (FIR) polarization fraction and the characteristic ratio of optical to FIR polarization. We derive a size distribution and alignment function for 1.4:1 oblate astrodust grains that, with PAHs, reproduce the mean wavelength dependence and polarization of Galactic extinction and emission from the diffuse interstellar medium while respecting constraints on solid-phase abundances. All model data and Python-based interfaces are made publicly available.

     
    more » « less
  2. Abstract

    Using new large-area maps of the cold neutral medium (CNM) fraction,fCNM, we investigate the relationship between the CNM, the abundance of polycyclic aromatic hydrocarbons (PAHs), and the anomalous microwave emission (AME). We first present ourfCNMmap based on full-sky HI4PI data, using a convolutional neural network to convert the spectroscopic Hidata tofCNM. We demonstrate thatfCNMis strongly correlated with the fraction of dust in PAHs as estimated from mid- and far-infrared dust emission. In contrast, we find no correlation betweenfCNMand the amount of AME per dust emission and no to weakly negative correlation betweenfCNMand the AME peak frequency. These results suggest PAHs preferentially reside in cold, relatively dense gas, perhaps owing to enhanced destruction in more diffuse media. The lack of positive correlation betweenfCNMand AME peak frequency is in tension with expectations from theoretical models positing different spectral energy distributions of AME in the cold versus warm neutral medium. We suggest that different PAH abundances and emission physics in different interstellar environments may explain the weaker-than-expected correlation between 12μm PAH emission and AME even if PAHs are the AME carriers.

     
    more » « less
  3. Abstract

    Dust-induced polarization in the interstellar medium (ISM) is due to asymmetric grains aligned with an external reference direction, usually the magnetic field. For both the leading alignment theories, the alignment of the grain’s angular momentum with one of its principal axes and the coupling with the magnetic field requires the grain to be paramagnetic. Of the two main components of interstellar dust, silicates are paramagnetic, while carbon dust is diamagnetic. Hence, carbon grains are not expected to align in the ISM. To probe the physics of carbon grain alignment, we have acquired Stratospheric Observatory for Infrared Astronomy/Higch-resolution Airborne Wideband Camera-plus far-infrared photometry and polarimetry of the carbon-rich circumstellar envelope (CSE) of the asymptotic giant branch star IRC+10° 216. The dust in such CSEs are fully carbonaceous and thus provide unique laboratories for probing carbon grain alignment. We find a centrosymmetric, radial, polarization pattern, where the polarization fraction is well correlated with the dust temperature. Together with estimates of a low fractional polarization from optical polarization of background stars, we interpret these results to be due to a second-order, direct radiative external alignment of grains without internal alignment. Our results indicate that (pure) carbon dust does not contribute significantly to the observed ISM polarization, consistent with the nondetection of polarization in the 3.4μm feature due to aliphatic CH bonds on the grain surface.

     
    more » « less
  4. Abstract

    With ΣSFR∼ 4200Myr−1kpc−2, SPT 0346–52 (z= 5.7) is the most intensely star-forming galaxy discovered by the South Pole Telescope. In this paper, we expand on previous spatially resolved studies, using ALMA observations of dust continuum, [Nii] 205μm, [Cii] 158μm, [Oi] 146μm, and undetected [Nii] 122μm and [Oi] 63μm emission to study the multiphase interstellar medium (ISM) in SPT 0346–52. We use pixelated, visibility-based lens modeling to reconstruct the source-plane emission. We also model the source-plane emission using the photoionization codecloudyand find a supersolar metallicity system. We calculateTdust= 48.3 K andλpeak= 80μm and see line deficits in all five lines. The ionized gas is less dense than comparable galaxies, withne< 32 cm−3, while ∼20% of the [Cii] 158μm emission originates from the ionized phase of the ISM. We also calculate the masses of several phases of the ISM. We find that molecular gas dominates the mass of the ISM in SPT 0346–52, with the molecular gas mass ∼4× higher than the neutral atomic gas mass and ∼100× higher than the ionized gas mass.

     
    more » « less
  5. Abstract

    To investigate the effects of stellar feedback on the gravitational state of giant molecular clouds (GMCs), we study12CO and13CO Atacama Large Millimeter/submillimeter Array maps of nine GMCs distributed throughout the Large Magellanic Cloud (LMC), the nearest star-forming galaxy to our own. We perform noise and resolution matching on the sample, working at a common resolution of 3.″5 (0.85 pc at the LMC distance of 50 kpc), and we use the Spectral Clustering for Molecular Emission Segmentation clustering algorithm to identify discrete substructure, or “clumps.” We supplement these data with three tracers of recent star formation: 8μm surface brightness, continuum-subtracted Hαflux, and interstellar radiation field energy density inferred from dust emission. The12CO clumps identified cover a range of 3.6 dex in luminosity-based mass and 2.4 dex in average 8μm surface brightness, representative of the wide range of conditions of the interstellar medium in the LMC. Our observations suggest evidence for increased turbulence in these clouds. While the turbulent linewidths are correlated with clump surface density, in agreement with previous observations, we find even better correlation with the three star formation activity tracers considered, suggesting stellar energy injection plays a significant role in the dynamical state of the clumps. The excess linewidths we measure do not appear to result from opacity broadening.12CO clumps are found to be typically less gravitationally bound than13CO clumps, with some evidence of the kinetic-to-gravitational potential energy ratio increasing with star formation tracers. Further multiline analysis may better constrain the assumptions made in these calculations.

     
    more » « less