A quest for sulfur-bearing refractory species. Identification of CaS in the interstellar medium

TL;DR

This study reports the first convincing detection of calcium sulfide (CaS) and tentative detections of KS and KSH in the interstellar medium, specifically in the disk of the massive young stellar object G351.77-mm1, using high-resolution ALMA Band 5 data. Through LTE spectral modeling with XCLASS, the authors derive $N({\rm CaS}) = 4.7\times10^{14}$ cm$^{-2}$ and $X({\rm CaS}) = 4.7\times10^{-11}$ for $T_{\rm rot}=400$ K, while KS and KSH are constrained to $N({\rm KS}) = 1.9\times10^{14}$ cm$^{-2}$ and $N({\rm KSH}) = 1.3\times10^{15}$ cm$^{-2}$ (tentative, at $T_{\rm rot}=400$ K). The CaS detection is supported by an archival CaS 22→21 feature, and the study concludes that these refractory sulfur species are not major sulfur reservoirs at the probed ~300 au scale, representing only a small fraction of the total sulfur budget when compared to SO$_2$ and CH$_3$SH. The results motivate higher-resolution, multi-band follow-up to robustly confirm KS and KSH and to further elucidate the role of refractory sulfur chemistry in hot, high-temperature disk environments and its relation to the broader sulfur cycle in star-forming regions.

Abstract

The recent detection of refractory molecules in massive star-forming regions provides a means of probing the innermost regions of disks around massive stars. These detections also make it possible to explore the chemical composition of refractories through gas-phase observations. In this regard, identifying refractory compounds containing sulfur could reveal potential connections between sulfur and refractories, as well as help determine the sulfur budget in these extreme environments. We find convincing evidence of a reliable detection of CaS, and tentative detections of KS and KSH in the disk G351.77-mm1. These are the first ever identifications of these species in the interstellar medium. The CaS, KS, and KSH column densities are about 3 orders of magnitude lower than those of the abundant sulfur compounds SO$_2$, CH$_3$SH and SiS, proving that these species are not the major reservoir of sulfur at the spatial scales probed by our observations. Higher angular resolution observations at different wavelengths are required to confirm these detections, which are of paramount importance to gain insights into the formation of gas-phase refractory molecules.

Figures (8)

• Figure 1: ALMA 1.5 mm continuum emission towards the G351.77$-$0.54 star-forming region. Crosses mark the positions of the mm continuum sources identified by Beuther2019. The synthesized beam of the image (beam $=0\farcs21\times0\farcs15$, PA$=-86^\circ$; with an $\mathrm{rms}\simeq0.32$ mJy beam$^{-1}$) is shown in the bottom-left corner. The spectra shown in Figs. \ref{['fig:zooms']} and \ref{['fig:total-spectrum']} have been extracted towards the peak position of G351.77-mm1, located at $\alpha(\mathrm{J2000})=17^\mathrm{h}26^\mathrm{m}42.533^\mathrm{s}$ and $\delta(\mathrm{J2000})=-36^{\circ}9^{\prime}17.37^{\prime\prime}$.
• Figure 2: Zoom in panels around the CaS and KS transitions included in the spectra of G351.77-mm1. The observed spectra are shown with grey-filled histograms. The synthetic spectrum generated without including the S-bearing refractory species is shown with a blue line, while the best fits of CaS and KS are shown in red. The best fits of the KSH and CaSH lines are shown with a green line (see Fig. \ref{['fig:zooms-CaSH']} for additional frequency ranges covering non-detected CaSH transitions).
• Figure 3: Spectra towards G351.77-mm1, with the intensity in brightness temperature. ALMA observational data is shown in black. The synthetic spectrum including all identified molecular species (see Sect. \ref{['sec:results']}) is shown in red, and with the intensity multiplied by a factor $-1$. Each major panel corresponds to a spectral window (see Table \ref{['tab:spw-info']}), with the minor panels showing the residual spectra (observed minus synthetic). Frequencies of CaS and KS transitions are marked with vertical lines. Blue and green colored areas mark the zoomed-in frequency ranges around these transitions shown in Fig. \ref{['fig:zooms']}.
• Figure 4: Zoom in panel around the CaSH transitions included in the spectra of G351.77-mm1. The observed spectrum is shown with grey-filled histograms. The synthetic spectrum generated without including the S-bearing refractory species is shown with a blue line, while the best fit of CaSH is shown in green.
• Figure 5: Spectra around the CaS 22$\rightarrow$21 transition at 232.372 GHz observed towards G351.77-mm1, from ALMA archival data: (left panel) project 2017.1.00237.S Ginsburg2023, (right panel) project 2015.1.00496.S Beuther2019. The observed spectra are shown in black with grey-filled histograms. A Gaussian convolution was applied to match the beam size ($\sim0\farcs16$) of our observations, and the spectra was extracted towards the same position at $\alpha(\mathrm{J2000})=17^\mathrm{h}26^\mathrm{m}42.533^\mathrm{s}$ and $\delta(\mathrm{J2000})=-36^{\circ}9^{\prime}17.37^{\prime\prime}$. The synthetic spectrum generated including all the identified molecules listed in Table \ref{['tab:other-species']} is shown in blue, while the best CaS synthetic spectrum is shown in red. The red-shaded area around the red solid line represents the uncertainty derived from different fits assuming different rotational temperatures (see Table \ref{['tab:parameters-fitting']}). The compound C$_2$H$_3$CHO is suggested as the carrier of the $\sim$232.365 GHz feature. However, this molecule does not exhibit intense lines in our observations, which precludes robust confirmation. The shaded vertical bands cover the molecular lines that appear self-absorbed.