MRASG

This program generates absorption spectra under a slab model, which assumes that:

the absorbing gas is located in front of, and may partially cover the background mid-infrared (MIR) continuum;
the absorbing gas is isothermal and dust-free.

You may generate such an absorption spectrum under your desired physical parameters listed below. Three spectral resolutions are provided according to that of JWST/MIRI (3,500), SOFIA/EXES (50,000), and IRTF/iSHELL (88,000). At this moment, only the \(\nu_2\) band of water from 5 to 8 \(\mu\)m is available. More bands will be incorporated into this program in the future.

Please refer to Li et al. (2024) (Section 2) for details of this spectrum generator.

Molecule: Band:

Spectral resolution, R ⓘ Spectral resolution of different MIR spectrometer: ISO-SWS: R ∼ 1,500 JWST/MIRI: R ∼ 3,500 SOFIA/EXES: R ∼ 50,000 IRTF/iSHELL: R ∼ 88,000 :
Excitation temperature, T_ex ⓘ Allowed range: 0 ≤ T_ex ≤ 5000 K. :	K
Column density, N_tot:	cm^-2
Fractional coverage, f_c ⓘ Partial coverage of the foreground absorbing gas relative to the background MIR continuum. Allowed range: 0 < f_c ≤ 1. :
Line width, b ⓘ The Doppler parameter in velocity space, b, is related to the full width at half maximum (FWHM) of an optically thin line by Δv_FWHM = 2b√(ln2). b must be positive. :	km s^-1
Velocity shift, Δv ⓘ Relative velocity between the astronomical objects and the Earth at the time of observation. :	km s^-1

While an isothermal slab model of dust-free gas is often considered as the absorbing component against a MIR continuum background, absorption in the continuum can occur when the gas is mixed with the dust that emits the continuum. For such a thermal continuum, to achieve line intensity lower than the nearby continuum, it is necessary for the temperature to decrease towards the outer regions as the optical depth becomes smaller. This scenario is analogous to what happens in a stellar photosphere, as described by Mihalas (1978, Ch 10).

This mechanism for absorption line origin has been proposed in massive protostellar systems, where the lines are considered to originate from the surface of a disk surrounding the protostar and the disk has an outward-decreasing temperature gradient from the mid-plane to the surface (Barr et al. 2020).

A detailed and quantitative comparison between the slab model and the photosphere model can be found in Section 5.2 of Li et al. (2024).

The program below generates absorption spectra under a photosphere model, which thus is specifically designed for massive protostellar systems. In the photosphere model, the total opacity is contributed by both the continuum and the line. A parameter \(\epsilon\), which is in the range of 0 to 1, is required to be designated in the model (\(\epsilon\)=0 for pure line scattering, and \(\epsilon\)=1 for pure line absorption).

Molecule: Band:

Spectral resolution, R ⓘ Spectral resolution of different MIR spectrometer: ISO-SWS: R ∼ 1,500 JWST/MIRI: R ∼ 3,500 SOFIA/EXES: R ∼ 50,000 IRTF/iSHELL: R ∼ 88,000 :
Excitation temperature, T_ex ⓘ Allowed range: 0 ≤ T_ex ≤ 5000 K. :	K
Abundance (w.r.t. H), X ⓘ Allowed range: 1e-5 ≤ X ≤ 1e-2. :
Line width, b ⓘ The Doppler parameter in velocity space, b, is related to the full width at half maximum (FWHM) of an optically thin line by Δv_FWHM = 2b√(ln2). b must be positive. :	km s^-1
Photon destruction prob, \(\epsilon\) ⓘ \(\epsilon\)=1 corresponds to a pure absorption line and \(\epsilon\)=0 corresponds to a pure scattering line. Allowed range: 0 ≤ \(\epsilon\) ≤ 1. :
Velocity shift, Δv ⓘ Relative velocity between the astronomical objects and the Earth at the time of observation. :	km s^-1

Please send questions or comments to jialu@astro.umd.edu
Website designed with the assistance of Dr. Marc Pound.
Last Updated: 2024-Jun-11

Mid-infrared Ro-vibrational Absorption Spectrum Generator