N-bearing complex organics toward high-mass protostars

Abstract

Context. Complex organic species are known to be abundant toward low-and high-mass protostars. No statistical study of these species toward a large sample of high-mass protostars with the Atacama Large Millimeter/submillimeter Array (ALMA) has been carried out so far. Aims. We aim to study six N-bearing species: methyl cyanide (CH 3 CN), isocyanic acid (HNCO), formamide (NH 2 CHO), ethyl cyanide (C 2 H 5 CN), vinyl cyanide (C 2 H 3 CN) and methylamine (CH 3 NH 2 ) in a large sample of line-rich high-mass protostars. Methods. From the ALMA Evolutionary study of High Mass Protocluster Formation in the Galaxy survey, 37 of the most line-rich hot molecular cores with ∼1 angular resolution are selected. Next, we fit their spectra and find column densities and excitation temperatures of the N-bearing species mentioned above, in addition to methanol (CH 3 OH) to be used as a reference species. Finally, we compare our column densities with those in other low-and high-mass protostars. Results. CH 3 OH, CH 3 CN and HNCO are detected in all sources in our sample, whereas C 2 H 3 CN and CH 3 NH 2 are (tentatively) detected in ∼78% and ∼32% of the sources. We find three groups of species when comparing their excitation temperatures: hot (NH 2 CHO; T ex 250 K), warm (C 2 H 3 CN, HN 13 CO and CH 13 3 CN; 100 K T ex 250 K) and cold species (CH 3 OH and CH 3 NH 2 ; T ex 100 K). This temperature segregation reflects the trend seen in the sublimation temperature of these molecules and validates the idea that complex organic emission shows an onion-like structure around protostars. Moreover, the molecules studied here show constant column density ratios across low-and high-mass protostars with scatter less than a factor ∼3 around the mean. Conclusions. The constant column density ratios point to a common formation environment of complex organics or their precursors, most likely in the pre-stellar ices. The scatter around the mean of the ratios, although small, varies depending on the species considered. This spread can either have a physical origin (source structure, line or dust optical depth) or a chemical one. Formamide is most prone to the physical effects as it is tracing the closest regions to the protostars, whereas such effects are small for other species. Assuming that all molecules form in the pre-stellar ices, the scatter variations could be explained by differences in lifetimes or physical conditions of the pre-stellar clouds. If the pre-stellar lifetimes are the main factor, they should be similar for low-and high-mass protostars (within factors ∼2 − 3).

Evidence for Ubiquitous Carbon Grain Destruction in Hot Protostellar Envelopes

“…In this work we use the publicly available ALMA data from the ALMAGAL large program (project ID: 2019.1.00195.L). We consider the (high-luminosity and potentially high-mass) sources that were analyzed by Nazari et al (2022b;see van Gelder et al 2022a andvan Gelder et al 2022b for results on methanol). The works mentioned above present the full details of the data; here we only give a summary of the observational parameters.…”

“…The spectral resolution is around 0.7 km s −1 , which results in spectrally resolved molecular lines toward the protostellar systems. The spectra are the same as those used in Nazari et al (2022b), which were extracted from the peak pixel of the CH 3 CN 12 4 -11 4 moment zero map. The line rms is ∼0.2 K for the spectra.…”

“…In this work we consider CH 3 CN, CH 3 OH, HNCO, and C 2 H 3 CN in the data described in Section 2.1 and analyze them with a two-component temperature model. The warm column densities of CH 3 CN, CH 3 OH, and HNCO are measured from their 13 C or 18 O isotopologues in Nazari et al (2022b). The isotope ratios in that work were calculated from the relations given in Wilson & Rood (1994) and Milam et al (2005) and the distances of the objects from the Galactic center.…”

“…We use the detected lines of these molecules with E up > 400-500 K (i.e., probing T  300 K) for the hot components. We take the warm column densities for the above species from Nazari et al (2022b), who measure them from the 13 C or 18 O isotopologues of the targeted molecules, which only have detected lines with E up  300-400 K (i.e., probing T ∼ 150 K). We then compare the ratios of the hot number of molecules to warm number of molecules for species with and without oxygen in a sample of 37 protostellar systems.…”

Digging into the Interior of Hot Cores with the ALMA (DIHCA). III. The Chemical Link between NH2CHO, HNCO, and H2CO

“…If the findings of Gorai et al (2020) are applicable to the general picture, we expect to find strong positive correlations between NH 2 CHO and HNCO and between NH 2 CHO and H 2 CO in other starforming regions. Nazari et al (2022) investigated the correlation between HNCO and NH 2 CHO using ALMA Band 6 data toward 37 high-mass sources with typical spatial resolutions of 0 5-1 5, corresponding to ∼1000-5000 au. Although they found a correlation between them (ρ = 0.73), a caveat is that they used column densities that may also depend on the gas mass.…”