Chemical segregation in hot cores with disk candidates

Abstract

Context. In the study of high-mass star formation, hot cores are empirically defined stages where chemically rich emission is detected toward a massive YSO. It is unknown whether the physical origin of this emission is a disk, inner envelope, or outflow cavity wall and whether the hot core stage is common to all massive stars. Aims. We investigate the chemical make up of several hot molecular cores to determine physical and chemical structure. We use high spectral and spatial resolution sub-millimeter observations to determine how this stage fits into the formation sequence of a high mass star. Methods. The sub-millimeter interferometer ALMA (Atacama Large Millimeter Array) was used to observe the G35.20-0.74N and G35.03+0.35 hot cores at 350 GHz in Cycle 0. We analyzed spectra and maps from four continuum peaks (A, B1, B2 and B3) in G35.20-0.74N, separated by 1000-2000 AU, and one continuum peak in G35.03+0.35. We made all possible line identifications across 8 GHz of spectral windows of molecular emission lines down to a 3σ line flux of 0.5 K and determined column densities and temperatures for as many as 35 species assuming local thermodynamic equilibrium (LTE). Results. In comparing the spectra of the four continuum peaks, we find each has a distinct chemical composition expressed in over 400 different transitions. In G35.20, B1 and B2 contain oxygen-and sulfur-bearing organic and inorganic species but few nitrogenbearing species whereas A and B3 are strong sources of O-, S-, and N-bearing organic and inorganic species (especially those with the CN-bond). Column densities of vibrationally excited states are observed to be equal to or greater than the ground state for a number of species. Deuterated methyl cyanide is clearly detected in A and B3 with D/H ratios of 8 and 13%, respectively, but is much weaker at B1 and undetected at B2. No deuterated species are detected in G35.03, but similar molecular abundances to G35.20 were found in other species. We also find co-spatial emission of isocyanic acid (HNCO) and formamide (NH 2 CHO) in both sources indicating a strong chemical link between the two species. Conclusions. The chemical segregation between N-bearing organic species and others in G35.20 suggests the presence of multiple protostars, surrounded by a disk or torus.

Search for high-mass protostars with ALMA revealed up to kilo-parsec scales (SPARKS)

“…Our results on the overall molecular composition of the gas suggest a similar molecular richness compared to other highmass star forming regions hosting classical, radiatively heated hot cores (Hatchell et al 1998;Bisschop et al 2007;Allen et al 2017;Widicus Weaver et al 2017). The COMs typical of hot cores are detected towards G328.2551-0.5321, however, several species are found to peak at the proposed accretion shocks rather than the radiatively heated core towards the protostar and the accretion disk.…”

“…In the past, due to angular resolution and sensitivity limitations, typically only the brightest hot cores have been studied (e.g. Palau et al 2011Palau et al , 2017Jiménez-Serra et al 2012;Öberg et al 2013;Allen et al 2017). These are frequently found in regions in which confusion due to the clustered nature of (highmass) star formation limits the possibility to reveal the spatial location of COMs within single envelopes.…”

Chemistry of the High-mass Protostellar Molecular Clump IRAS 16562–3959

“…A thermal segregation is also observed, with N-bearing molecules tracing hotter gas compared to the O-bearing species (Crockett et al 2015). N-O segregation has been found toward other high-mass star forming regions as well (Su et al 2005;Öberg et al 2013;Allen et al 2017), but it does not seem to be universal (Fontani et al 2007). Figure 16 compares the methanol-normalized abundance of O-and N-bearing molecules with four atoms or more, detected toward the CC core or C8 (thus excluding CH 3 CHO).…”

“…HMCs are defined observationally as dense (> 10 7 cm −3 ), warm (100-500 K), and compact (< 0.05 pc) molecular cores (Allen et al 2017). The definition is not strict and it has changed at the same time that telescopes resolve more compact and hotter components (van Dishoeck & Blake 1998).…”

Interstellar Formamide (NH2CHO), a Key Prebiotic Precursor

“…Figure 1 shows a plot of NH 2 CHO against HNCO fractional abundances relative to molecular hydrogen, H 2 , for all the sources in the literature with reported values of column densities for these two molecules, as well as total H 2 column density values. 17,28,29,42,43,[45][46][47]62,64,72,73 Single-dish and interferometric ob- Figure 1 shows that there is a tight correlation between the two molecular species, which is almost linear and holds across several orders of magnitude in molecular abundance. This has led some authors to propose that HNCO and NH 2 CHO are chemically linked, with either both of them forming from a common precursor, or one forming from the other.…”