Is gaseous iron sulfide FeS detectable toward the massive disk Orion SrcI?

A first search for gaseous FeS in the ISM

Iron sulfide (FeS) is a refractory compound: it has a high sublimation temperature, so releasing it from dust grains into the gas phase requires extreme conditions such as temperatures above 500 K, intense radiation, or strong shocks. FeS is common in meteorites, and evidence for it had previously been reported only in solid form in a protoplanetary environment. No gaseous FeS had been detected in the interstellar medium before this work.

The paper presents millimeter-wave spectroscopic calculations for gaseous FeS and then searches for it toward the massive disk Orion SrcI, a source where several molecules, including the metal-bearing compounds NaCl and KCl, had already been detected at high angular resolution. The authors describe this as the first search for gaseous FeS in the interstellar medium.

Four ground-state FeS transitions fall in Band 6

The search uses a stacked spectrum toward SrcI built from ALMA Band 6 data, with the velocity field derived from the NaCl v=1 J=18-17 line and spectra shifted to 0 km/s and averaged over the NaCl-emitting region. The rms noise level of the stacked spectrum in this band is 1.1 K, with a channel width of about 1.3 km/s.

The observed frequency range covers four ground-state and four v=1 transitions of FeS. Each transition is split into two unresolved components, giving 16 spectroscopic entries. Two J=19-18 ground-state transitions are clearly identified: one is fairly clean, and the other is only weakly blended with an SO2 line and remains distinguishable. The J=18-17 Omega=4 transition might be affected by absorption, and the J=18-17 Omega=3 transition appears heavily contaminated. All v=1 transitions are blended with absorption features or intense identified compounds.

The FeS column density depends on assumed temperature

The FeS lines were modeled with synthetic spectra assuming LTE conditions, Gaussian line profiles, and a linewidth of 11 km/s, which provided the best fit to the observed FeS v=0 transitions and is consistent with values obtained for other refractory species in this source. The velocity offset was set to 0 km/s, although a shift to about 1 km/s would improve the fit.

Assuming a rotational temperature of 1500 K, based on the H2O v2=1 and 29SiO v=0 analysis, the best-fit model corresponds to a FeS column density of 1.7 x 10^15 cm^-2. The authors explored temperatures between 500 and 1500 K and found that the derived column density varies by a factor of 3 over that range. They caution that the data do not constrain the excitation temperature and that non-LTE effects may also play a role. In Table 1, the FeS v=0 entry lists 5.4 x 10^14 cm^-2 at 500 K and 1.7 x 10^15 cm^-2 at 1500 K.

FeS sulfur is far below SO2 and SiS

Comparing sulfur reservoirs in Orion SrcI, the amount of sulfur locked in FeS is more than three orders of magnitude lower than that contained in SO2 and SiS. The paper reports 1.7 x 10^15 cm^-2 for FeS versus 4.1 x 10^18 cm^-2 for SiS v=0 and 2.1 x 10^18 cm^-2 for SO2 v=0.

The authors caution that this comparison should be interpreted carefully, because these species may trace different physical components along the line of sight. Maps of H2O v2=1 and SiO v=0 emission toward Orion SrcI show that these species primarily trace the outflow at high elevations above the disk midplane, whereas NaCl traces the disk surface and shows no coincident emission peaks with H2O. Similarly, SiS v=0 mainly originates in the bipolar outflow, and SO2 is associated with shocked, expanding gas surrounding the outflow. In this scenario, vibrationally excited SiS lines would be expected to be compact and might provide a more accurate comparison to the gas traced by metal salts, and possibly FeS.

Narrow linewidths group FeS with the metal salts

The paper uses linewidths to support a spatial distinction between molecular populations. The metal-bearing compounds and SiS v=3 present relatively narrow linewidths of about 8-11 km/s, whereas water, SiO, SO2, and SiS v=0 lines exhibit broader profiles of about 20 km/s, consistent with part of their emission arising from the bipolar outflow.

The derived FeS column density is only about one order of magnitude lower than those obtained for the salts. Comparing the SiS v=3 column density with FeS leads to a SiS/FeS ratio of about 0.2-0.5, slightly lower than the elemental Si/Fe ratio in dust of about 0.8. Considering the large uncertainties, the authors state this result is consistent with grain destruction playing an important role in the origin of the FeS emission. FeS may either be directly released through grain erosion or efficiently re-formed in the gas phase after refractory elements are liberated from dust grains.

Contaminant checks leave FeS as the surviving option

To test whether the two fairly clean lines could belong to another species, the authors examined the spectral vicinity of each transition, within about plus or minus 4.2 MHz, and searched the CDMS and JPL line catalogs for potential molecular contaminants. For each candidate, a synthetic spectrum was generated with XCLASS assuming LTE conditions, exploring temperatures from 500 to 1500 K, fixing the source size to 0.1 arcsec based on moment-0 maps of resolved NaCl emission, and fixing the linewidth to 11 km/s.

Candidate contaminants were discarded based on line inconsistencies: fitting the observed feature with any of these species would make other expected transitions at different frequencies, not affected by absorption, too bright to reproduce the observations. Reproducing the emission features at 230.3836 GHz and 230.9804 GHz would require an abundance of 10^-8 or more for more than 50% of the considered molecules, assuming an H2 column density of 1.8 x 10^25 cm^-2, which the authors describe as unrealistically high for most COMs and refractory compounds. Two refractory candidates were also rejected: Si33S v=2, a possible contaminant of the FeS Omega=4 19-18 transition, and c-SiC3, in the spectral vicinity of FeS Omega=3 19-18.

A warm inner disk with low sulfur depletion

The paper notes that SiS v=0 and SO2 are the most abundant sulfur-bearing molecules in this setup, and that SO2 is predicted to be among the dominant sulfur-bearing species in hot-core chemical models. Assuming most of the sulfur is contained in these molecules, and considering that H2O is expected to be one of the main oxygen carriers at high temperatures, the authors obtain a first-order estimate of S/O of about 0.02.

This value is remarkably similar to that measured in comet 67P and a factor of two below the solar elemental ratio. The authors interpret such a high sulfur abundance as indicating that sulfur depletion is low in the warm inner disk where ices are sublimated. The similarity with cometary material suggests that some regions of the disk may exhibit comet-like volatile chemistry, potentially reflecting chemical inheritance between the protoplanetary disk and the bodies that eventually form within it.

Confirmation needs better constraints on excitation

The detection remains tentative, and the paper is explicit about what the current observations cannot settle. The data are still insufficient to discern whether FeS is a main sulfur reservoir in molecular clouds or a primary carrier of refractory iron-bearing material. The authors also note that the derived H2 column density carries an uncertainty of a factor of two, and that the FeS column density estimate depends on an assumed rotational temperature that the data do not constrain.

The most decision-relevant comparison in this paper is not FeS against the total sulfur budget, but FeS against the other species that share its narrow linewidths and likely disk-surface origin. The FeS column density is about one order of magnitude below the salts, and the SiS/FeS ratio of about 0.2-0.5 from the compact SiS v=3 line is close enough to the elemental Si/Fe ratio in dust, within large uncertainties, to keep grain destruction as a viable origin. Confirming gaseous FeS, and deciding whether it is a sulfur reservoir or an iron carrier, therefore depends on pinning down the excitation temperature and separating disk-surface emission from outflow contamination rather than on simply adding more sulfur-bearing detections.

Sources used in this answer