Assessing the viability of silicate nanoclusters as carriers of the anomalous microwave emission: a quantum mechanical study

¹ Departament de Ciència de Materials i Química Fisica Institut de Química Teòrica i Computacional, Universitat de Barcelona, C/Martí i Franquès 1, 08028 Barcelona, Spain
² Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain
e-mail: s.bromley@ub.edu

Received: 31 July 2019
Accepted: 10 January 2020

Abstract

Nanosized silicate dust is likely to be abundant in many astronomical environments and it is a prime candidate for being the source of the anomalous microwave emission (AME). To assess the viability of silicate nanoclusters as AME carriers, their detailed properties need to be established. Using quantum chemical calculations, we compute the accurate chemical and electronic structures of three families of nanoclusters with astrophysically relevant compositions: Mg-rich olivine (Mg₂SiO₄)_N, Mg-rich pyroxene (MgSiO₃)_N, and silicon monoxide (SiO)_N, all in the ≤1 nm diameter size regime and for neutral and ± 1 charge states. From these fundamental data, we directly derive the shapes, ionization potentials, electron affinities, and dipole moments of all nanoclusters. The aspect ratio of the nanoclusters fluctuates significantly with N for small sizes, but especially for the olivine and pyroxene nanoclusters, it tends to stabilize towards ~1.3 for the largest sizes considered. These latter two nanocluster families tend to have mass distributions consistent with approximately prolate ellipsoidal shapes. Our calculations reveal that the dipole moment of all our nanoclusters can be substantially affected by changes in chemical structure (i.e. different isomers for a fixed N), ionisation, and substitution of Mg by Fe. Although all these factors are important, the dipole moments of our Mg-rich nanoclusters are always found to be large enough to account for the observed AME. However, (SiO)_N nanoclusters are only likely to be potential AME contributors when they are both charged and their chemical structures are anisotropically segregated. We also model the emissivity per H of a representative (Mg₂SiO₄)₃ nanocluster by directly calculating the quantum mechanical rotational energy levels and assuming a distribution of occupied levels in accordance with equilibrium Boltzmann statistics. We compare our bottom-up results with previously published classical models and show that a population of silicate nanoclusters containing only 1% of the total Si budget can reproduce the AME emissivity.