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The MRS mode of the JWST-MIRI instrument has been shown to be a powerful tool to characterise the molecular gas emission of the inner region of planet-forming disks. Investigating their spectra allows us to infer the composition of the gas in these regions and, subsequently, the potential atmospheric composition of the forming planets. We present the JWST-MIRI observations of the compact T-Tauri disk, DR Tau, which are complemented by ground-based, high spectral resolution ($R CO ro-vibrational observations. The aim of this work is to investigate the power of extending the JWST-MIRI CO observations with complementary, high-resolution, ground-based observations acquired through the SpExoDisks database, as JWST-MIRI's spectral resolution ($R is not sufficient to resolve complex CO line profiles. In addition, we aim to infer the excitation conditions of other molecular features present in the JWST-MIRI spectrum of DR Tau and link those with CO The archival complementary, high-resolution CO ro-vibrational observations were analysed with rotational diagrams. We extended these diagrams to the JWST-MIRI observations by binning and convolution with JWST-MIRI's pseudo-Voigt line profile. In parallel, local thermal equilibrium (LTE) 0D slab models were used to infer the excitation conditions of the detected molecular species. Various molecular species, including CO CO_2 HCN and C_2H_2 are detected in the JWST-MIRI spectrum of DR Tau, with H_2O being discussed in a subsequent paper. The high-resolution observations show evidence for two CO components: a broad component (full width at half maximum of FWHMsim 33.5 km s$^ $) tracing the Keplerian disk and a narrow component (FWHMsim 11.6 km s$^ $) tracing a slow disk wind. The rotational diagrams yield CO excitation temperatures of $T K. Consistently lower excitation temperatures are found for the narrow component, suggesting that the slow disk wind is launched from a larger radial distance. In contrast to the ground-based observations, much higher excitation temperatures are found if only the high-$J$ transitions probed by JWST-MIRI are considered in the rotational diagrams. Additional analysis of the CO line wings suggests a larger emitting area than inferred from the slab models, hinting at a misalignment between the inner ($i and the outer disk ($i Compared to CO we retrieved lower excitation temperatures of $T K for CO_2 HCN and C_2H_2 We show that complementary, high-resolution CO ro-vibrational observations are necessary to properly investigate the excitation conditions of the gas in the inner disk and they are required to interpret the spectrally unresolved JWST-MIRI CO observations. These additional observations, covering the lower-$J$ transitions, are needed to put better constraints on the gas physical conditions and they allow for a proper treatment of the complex line profiles. A comparison with JWST-MIRI requires the use of pseudo-Voigt line profiles in the convolution rather than simple binning. The combined high-resolution CO and JWST-MIRI observations can then be used to characterise the emission, in addition to the physical and chemical conditions of the other molecules with respect to CO . The inferred excitation temperatures suggest that CO originates from the highest atmospheric layers close to the host star, followed by HCN and C_2H_2 which emit, together with CO from slightly deeper layers, whereas the CO_2 emission originates from even deeper inside or further out of the disk.
The MRS mode of the JWST-MIRI instrument has been shown to be a powerful tool to characterise the molecular gas emission of the inner region of planet-forming disks. Investigating their spectra allows us to infer the composition of the gas in these regions and, subsequently, the potential atmospheric composition of the forming planets. We present the JWST-MIRI observations of the compact T-Tauri disk, DR Tau, which are complemented by ground-based, high spectral resolution ($R CO ro-vibrational observations. The aim of this work is to investigate the power of extending the JWST-MIRI CO observations with complementary, high-resolution, ground-based observations acquired through the SpExoDisks database, as JWST-MIRI's spectral resolution ($R is not sufficient to resolve complex CO line profiles. In addition, we aim to infer the excitation conditions of other molecular features present in the JWST-MIRI spectrum of DR Tau and link those with CO The archival complementary, high-resolution CO ro-vibrational observations were analysed with rotational diagrams. We extended these diagrams to the JWST-MIRI observations by binning and convolution with JWST-MIRI's pseudo-Voigt line profile. In parallel, local thermal equilibrium (LTE) 0D slab models were used to infer the excitation conditions of the detected molecular species. Various molecular species, including CO CO_2 HCN and C_2H_2 are detected in the JWST-MIRI spectrum of DR Tau, with H_2O being discussed in a subsequent paper. The high-resolution observations show evidence for two CO components: a broad component (full width at half maximum of FWHMsim 33.5 km s$^ $) tracing the Keplerian disk and a narrow component (FWHMsim 11.6 km s$^ $) tracing a slow disk wind. The rotational diagrams yield CO excitation temperatures of $T K. Consistently lower excitation temperatures are found for the narrow component, suggesting that the slow disk wind is launched from a larger radial distance. In contrast to the ground-based observations, much higher excitation temperatures are found if only the high-$J$ transitions probed by JWST-MIRI are considered in the rotational diagrams. Additional analysis of the CO line wings suggests a larger emitting area than inferred from the slab models, hinting at a misalignment between the inner ($i and the outer disk ($i Compared to CO we retrieved lower excitation temperatures of $T K for CO_2 HCN and C_2H_2 We show that complementary, high-resolution CO ro-vibrational observations are necessary to properly investigate the excitation conditions of the gas in the inner disk and they are required to interpret the spectrally unresolved JWST-MIRI CO observations. These additional observations, covering the lower-$J$ transitions, are needed to put better constraints on the gas physical conditions and they allow for a proper treatment of the complex line profiles. A comparison with JWST-MIRI requires the use of pseudo-Voigt line profiles in the convolution rather than simple binning. The combined high-resolution CO and JWST-MIRI observations can then be used to characterise the emission, in addition to the physical and chemical conditions of the other molecules with respect to CO . The inferred excitation temperatures suggest that CO originates from the highest atmospheric layers close to the host star, followed by HCN and C_2H_2 which emit, together with CO from slightly deeper layers, whereas the CO_2 emission originates from even deeper inside or further out of the disk.
OCS and SO2 are both major carriers of gaseous sulfur and are the only sulfurated molecules detected in interstellar ices to date. They are thus the ideal candidates for exploring the evolution of the volatile sulfur content throughout the different stages of star formation. We aim to investigate the chemical history of interstellar OCS and SO2 by deriving a statistically significant sample of gas-phase column densities toward massive protostars and comparing them to observations of gas and ices toward other sources, from dark clouds to comets. We analyzed a subset of 26 line-rich massive protostars observed by ALMA in Band 6 as part of the High Mass Protocluster Formation in the Galaxy (ALMAGAL) survey. Column densities were derived for OCS and SO2 from their rare isotopologs O^ CS and SO2 toward the compact gas around the hot cores. We compared the abundance ratios of gaseous OCS SO2 and CH3OH with ice detections toward both high- and low-mass sources as well as dark clouds and comets. We find that gas-phase column density ratios of OCS and SO2 with respect to methanol remain fairly constant as a function of luminosity between low- and high-mass sources, despite their very different physical conditions. In our dataset OCS and SO2 are weakly correlated. The derived gaseous OCS and SO2 abundances relative to CH3OH are overall similar to protostellar ice values, with a significantly larger scatter for SO2 than for OCS . Cometary and dark-cloud ice values agree well with protostellar gas-phase ratios for OCS whereas higher abundances of SO2 are generally seen in comets compared to the other sources. Gaseous SO2 OCS ratios are consistent with ices toward dark clouds, protostars, and comets, albeit with some scatter. The constant gas-phase column density ratios throughout low- and high-mass sources indicate an early-stage formation before intense environmental differentiation begins. Icy protostellar values are similar to the gas-phase medians and are compatible with an icy origin for these species followed by thermal sublimation. The larger spread in SO2 compared to OCS ratios with respect to CH3OH is likely due to a more water-rich chemical environment associated with the former, as opposed to a CO -rich origin for the latter. Post-sublimation gas-phase processing of SO2 can also contribute to the large spread. Comparisons to ices in dark clouds and comets point to a significant inheritance of OCS from earlier to later evolutionary stages.
Carbonyl sulfide (OCS) is widely observed in the gas phase toward star-forming regions and was the first of the only two sulfur-bearing species to be detected in interstellar ices so far. However, the chemical network governing its formation is still not fully understood. While the sulfurization of CO and the oxidation of CS are often invoked to form OCS other mechanisms could have a significant contribution. In particular, the multistep reaction involving CO and SH is a good candidate for forming a significant portion of the OCS in dense cloud environments. We aim to constrain the viability of the $ CO SH $ route for forming solid OCS in the interstellar medium, in a similar manner as $ CO OH $ is known to produce CO2 ice. This is achieved by conducting a systematic laboratory investigation of the targeted reactions on interstellar ice analogs under dense cloud conditions. We used an ultrahigh vacuum chamber to simultaneously deposit CO H2S and atomic H at 10 K. SH radicals produced in situ via hydrogen abstraction from H2S reacted with CO to form OCS . We monitored the ice composition during deposition and subsequent warm-up by means of Fourier-transform reflection absorption infrared spectroscopy (RAIRS). Complementarily, desorbed species were recorded with a quadrupole mass spectrometer (QMS) during temperature-programmed desorption (TPD) experiments. Control experiments were performed to secure the product identification. We also explored the effect of different H2S CO mixing ratios---with decreasing H2S concentrations---on the OCS formation yield. OCS is efficiently formed through surface reactions involving CO H2S and H atoms. The suggested underlying mechanism behind OCS formation is $ CO SH HSCO $ followed by $ HSCO H OCS H2 $. The OCS yield reduces slowly, but remains significant with increasing CO H2S mixing ratios ( CO H2S = 1:1, 5:1, 10:1, and 20:1). Our experiments provide unambiguous evidence that OCS can be formed from $ CO SH $ in the presence of H atoms. This route remains efficient for large H2S dilutions ($5<!PCT!>$ with respect to CO ), suggesting that it is a viable mechanism in interstellar ices. Given that SH radicals can be created in clouds over a wide evolutionary timescale, this mechanism could make a non-negligible contribution to the formation of interstellar OCS ice.
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