Does Processing or Formation of Water Ice Mantles Affect the Capacity of Nanosilicates to Be the Source of Anomalous Microwave Emission?

Abstract

Anomalous microwave emission (AME) is detected in many astrophysical environments as a foreground feature typically peaking between 20–30 GHz and extending over a 10–60 GHz range. One of the leading candidates for the source of AME is small spinning dust grains. Such grains should be very small (approx. ≤1 nm diameter) in order for the rotational emission to fall within the observed frequency range. In addition, these nanosized grains should possess a significant dipole moment to account for the observed emissivities. These constraints have been shown to be compatible with spinning bare nanosilicate clusters, assuming that ∼1% of the total Si mass budget is held in these ultrasmall grains. Silicate dust can be hydroxylated by processing in the interstellar medium and is generally known to provide seeds for molecular water ice nucleation in denser regions. Herein, we use quantum chemical calculations to investigate how the dipole moment of Mg-rich pyroxenic (MgSiO3) nanoclusters is affected by both accretion of molecular water and dissociative hydration. Our work thus provides an indication of how the formation of water ice mantles is likely to affect the capacity of nanosilicates to generate AME.

Spinning nano-carbon grains: Viable origin for anomalous microwave emission

“…Figure 7 indicates that (1) if both nano-grain populations co-exist in the diffuse interstellar medium, then the AME must come predominantly from nano-carbon dust emission, and (2) if both populations have non-zero permanent electric dipole moments, then the abundance of nano-silicates cannot exceed ∼1%. This upper limit agrees with the findings of Désert et al (1986), Macià Escatllar & Bromley (2020), and Mariñoso Guiu et al (2021. All three studies derive an upper limit of Y Si = 1%.…”

“…This implies that even if it is possible to reproduce the intensity level at a single given frequency, for instance at 30 GHz, the spectral shape will be incorrect and accordingly, the required intensities at the two other frequencies cannot be retrieved. We note that the quantum mechanical calculations performed by Macià Escatllar & Bromley (2020) and Mariñoso Guiu et al (2021) show that Mg-rich nano-silicates will always have m > 1 D, even when they are covered by an ice layer. However, high dipole moments like this for nano-silicates appear to be inconsistent with the observations (see also Hensley & Draine 2017).…”

Spinning nano-carbon grains: Viable origin for anomalous microwave emission

“…7 indicates that (1) if both nano-grain populations coexist in the diffuse interstellar medium, then the AME must come predominantly from nano-carbon dust emission, and (2) if both populations have non-zero permanent electric dipole moments, then the abundance of nano-silicates cannot exceed ∼ 1%. This upper limit agrees with the findings of Désert et al (1986), Macià Escatllar & Bromley (2020), and Mariñoso Guiu et al (2021. All three studies derive an upper limit of Y Si = 1%.…”