Absorption at 11 μm in the interstellar medium and embedded sources: evidence for crystalline silicates

Abstract

An absorption feature is occasionally reported around 11 µm in astronomical spectra, including those of forming stars. Candidate carriers include water ice, polycyclic aromatic hydrocarbons (PAHs), silicon carbide, crystalline silicates or even carbonates. All are known constituents of cosmic dust in one or more types of environments, though not necessarily together. In this paper we present new ground-based 8-13 µm spectra of one evolved star, several embedded young stellar objects (YSOs) and a background source lying behind a large column of the interstellar medium (ISM) toward the Galactic Centre. Our observations, obtained at a spectral resolution of ∼ 100, are compared with previous lower resolution data, as well as data obtained with the Infrared Space Observatory (ISO) on these and other targets. By presenting a subset of a larger sample our aim is to establish the reality of the feature and subsequently speculate on its carrier. All evidence points toward crystalline silicate. For instance, the 11 µm band profile is well matched with the emissivity of crystalline olivine. Furthermore, the apparent association of the absorption feature with a sharp polarisation signature in the spectrum of two previously reported cases suggests a carrier with a relatively high band strength compared to amorphous silicates. If true, this would either set back the evolutionary stage in which silicates are crystallised, either to the embedded phase or even before within the ISM, or else the silicates ejected from the outflows of evolved stars retain some of their crystalline identity during their long residence in the ISM.

Dust evolution, a global view: III. Core/mantle grains, organic nano-globules, comets and surface chemistry

“…The cycling of dust between dense and diffuse interstellar media during cycles of cloud collapse (a-C:H mantle accretion) and star formation (a-C:H mantle photo-processing to a-C) would probably lead to the accumulation of thicker a-C(:H) mantles, which would nevertheless be progressively eroded during their sojourn in the low-density ISM. Thus, in the same way that silicate dust in the ISM has been processed to an amorphous form [32,33], with some small fraction remaining unprocessed and crystalline [122], carbonaceous dust would be expected to show these same traces, despite perhaps being more fragile [31,66,69]. Thus, carbonaceous core/mantle grains ought to present a range of compositions depending on their exposure to irradiation (by photons, electrons and ions) in the ISM, i.e.…”

Modelling the mid-infrared polarization in dust around young stars

“…Unfortunately, the conventional spectrum of MWC 1080A given by Sakon et al (2007) does not show any such apparent absorption feature around 11.2 µm which contradicts the findings of our model. While most cases of crystalline silicate detection have been reported in emission [Watson et al (2009);Olofsson et al (2009);Juhász et al (2010)], recent studies have shown a considerable number of such features in absorption as well [Poteet et al (2011);Fujiyoshi et al (2015); Wright et al (2016)]. All three objects in our sample of study: MWC 1080A, MWC 297 and HL Tau, show polarization in absorption but only the MWC 1080A polarization seems to have been caused due to SiC mixed with silicates.…”

“…Polarization is represented as the difference between the parallel and perpendicular components of extinction efficiency which are independently related to the dust optical constants [Whittet (2003)]. The polarization profile (peak wavelength, shape and strength) is highly susceptible to the specific shape, composition as well as dust grain size and hence, the study of polarization across an absorption/emission feature can be very efficiently used to constrain these properties [Dullemond & Dominik (2008); Fujiyoshi et al (2015); Wright et al (2016)].…”

“…Separate studies have placed an upper limit on the abundance of silicates found in crystalline form in the ISM: 2.2% by Kemper et al (2004Kemper et al ( , 2005 and 1% by Min et al (2007). More recently, Wright et al (2016) have detected a 2.5-5% crystalline abundance towards the Galactic centre based on three features at 11.1, 11.9 and 23.5 µm, which is consistent with the upper limit of Kemper et al (2005) and within the 3-5% limit set by Li et al (2007). In case of protoplanetary disks, a review by Henning (2010) has stated that amorphous silicates with various compositions are expected to be found in the outer regions while crystalline dust is expected towards the innermost (high temperature ∼ 1300 K) regions of the disk with the existence of annealed amorphous grains in-between the two regions.…”

Cosmic dust VIII

“…However, the DL model may be incompatible with the latest findings of grain materials and sizes in the local interstellar medium, most of which have been presented in a series of Cosmic Dust meetings: The size distribution of interstellar grains is skewed toward heavier masses by more than one order of magnitude and has a shallower powerlaw index than expected from the DL model (Kimura et al, 2003;Sterken et al, 2015); There is no evidence for the presence of carbonaceous materials in interstellar grains measured by Cassini and Stardust, although graphite grains, if exist as in the DL model, should have been identified by Cassini (Kimura et al, 2015b;Westphal, 2015); Interstellar grains extracted from the aerogel collector of Stardust contain forsteritic olivine, contrary to amorphous silicate expected from the DL model (Westphal, 2015;Westphal et al, 2014). Very recently, Wright et al (2016) have found evidence for crystalline silicates in the interstellar medium from their infrared spectral observations toward the Galactic Center, in contrast to the DL model. Furthermore, it turned out that interstellar grains are compositionally homogeneous and seem to contain magnesium-rich silicates and metallic iron, dissimilar from Mg 1.1 Fe 0.9 SiO 4 in the DL model (Altobelli et al, 2016).…”