CC BY
6ALT'22
B-I-20
BIOMEDICAL PHOTONICS
Broadband dielectric spectroscopy of astrophysical ice analogues: from the most common molecules toward complex organic compounds
Prokhorov General Physics Institute of the Russian Academy of Sciences, Russia, 119991 Moscow, Russia
Estimation of the amount, chemical and physical properties of matter contained in molecular clouds and protoplane-tary disks remains a challenging problem of modern astrophysics [1-3]. The interplay between gas phase and icy mantles forming on the surface of dust grains can significantly affect the physical and chemical properties. Molecular freeze-out, found at the center of pre-stellar cores, and expected in the mid-plane of protoplanetary disks implies that the majority of species heavier than He reside on dust grains in these regions. Then the broadband optical properties of astrophysical ice analogues in the infrared (IR) and terahertz (THz) ranges are required for modelling the dust continuum emission and radiative transfer in dense and cold regions. Such data are still missing from the literature, which can be attributed to the lack of appropriate spectroscopic systems and methods for laboratory studies. Moreover, such data are necessary for answering different key questions of astrophysics, including one about the origin and prevalence of organic compounds in space, and therefore about the possible causes of the organic substances appearance.
In the present research the THz time-domain spectroscopy (TDS) and the Fourier-transform IR spectroscopy (FTIR) are combined to study optical and dielectric properties of CO and CO2 ices in the broad THz-IR spectral range. The measured ices are grown at cryogenic temperatures by gas deposition on a cold silicon window. A method to quantify the broadband THz-IR optical constants of ices is developed. It is based on the direct reconstruction of the complex refractive index of ices in the THz range from the TDS data [4], and the use of the Kramers-Kronig relation in the IR range for the reconstruction from the FTIR data. Uncertainties of the Kramers-Kronig relation are eliminated by merging the THz and IR spectra. The reconstructed THz-IR response is then analyzed using classical models of complex dielectric permittivity.
The complex refractive index of CO and CO2 ices deposited at the temperature of 28 K is obtained in the range of 0.3-12.0 THz, and fitted using the analytical Lorentz model. Based on the measured dielectric constants, opacities of the astrophysical dust with CO and CO2 icy mantles are computed. The developed method can be used for a model-independent reconstruction of optical constants of various astrophysical ice analogs in a broad THz-IR range. Such data can provide important benchmarks to interpret the broadband observations from the existing and future ground-based facilities and space telescopes.
[1] A.C.A. Boogert, P. Gerakines, D. Whittet, Observations of the Icy Universe, Annual Review of Astronomy & Astrophysics, 53, pp. 541 (2015).
[2] A. Dutrey et al., CO study of the GM Aurigae Keplerian disk, Astronomy & Astrophysics, 338, pp. L63 (1998).
[3] P. Caselli et al., CO Depletion in the Starless Cloud Core L1544, The Astrophysical Journal, 523, pp. L165 (1999).
[4] B.M. Giuliano et al., Broadband spectroscopy of astrophysical ice analogues: I. Direct measurement of the complex refractive index of CO ice using terahertz time-domain spectroscopy, Astronomy & Astrophysics, 629, pp. A112 (2019).
A.A. Gavdush
