Molecular complexity on disc scales uncovered by ALMA

Abstract

Context. The chemical composition of high-mass protostars reflects the physical evolution associated with different stages of star formation. In addition, the spatial distribution and velocity structure of different molecular species provide valuable information on the physical structure of these embedded objects. Despite an increasing number of interferometric studies, there is still a high demand for high angular resolution data to study chemical compositions and velocity structures for these objects. Aims. The molecular inventory of the forming high-mass star AFGL 4176, located at a distance of ~3.7 kpc, is studied in detail at a high angular resolution of ~0.35′′, equivalent to ~1285 au at the distance of AFGL 4176. This high resolution makes it possible to separate the emission associated with the inner hot envelope and disc around the forming star from that of its cool outer envelope. The composition of AFGL 4176 is compared with other high- and low-mass sources, and placed in the broader context of star formation. Methods. Using the Atacama Large Millimeter/submillimeter Array (ALMA) the chemical inventory of AFGL 4176 has been characterised. The high sensitivity of ALMA made it possible to identify weak and optically thin lines and allowed for many isotopologues to be detected, providing a more complete and accurate inventory of the source. For the detected species, excitation temperatures in the range 120–320 K were determined and column densities were derived assuming local thermodynamic equilibrium and using optically thin lines. The spatial distribution of a number of species was studied. Results. A total of 23 different molecular species and their isotopologues are detected in the spectrum towards AFGL 4176. The most abundant species is methanol (CH3OH) with a column density of 5.5 × 1018 cm−2 in a beam of ~0.3′′, derived from its 13C-isotopologue. The remaining species are present at levels between 0.003 and 15% with respect to methanol. Hints that N-bearing species peak slightly closer to the location of the peak continuum emission than the O-bearing species are seen. A single species, propyne (CH3C2H), displays a double-peaked distribution. Conclusions. AFGL 4176 comprises a rich chemical inventory including many complex species present on disc scales. On average, the derived column density ratios, with respect to methanol, of O-bearing species are higher than those derived for N-bearing species by a factor of three. This may indicate that AFGL 4176 is a relatively young source since nitrogen chemistry generally takes longer to evolve in the gas phase. Taking methanol as a reference, the composition of AFGL 4176 more closely resembles that of the low-mass protostar IRAS 16293–2422B than that of high-mass, star-forming regions located near the Galactic centre. This similarity hints that the chemical composition of complex species is already set in the cold cloud stage and implies that AFGL 4176 is a young source whose chemical composition has not yet been strongly processed by the central protostar.

A Detailed View of the Circumstellar Environment and Disk of the Forming O-star AFGL 4176

“…Not including isotopologues, we detect a total of 25 different molecules, which are listed in Table 5, from a total of 203 lines, summarized in Table A.5. The line identifications are generally in good agreement with Bøgelund et al (2019), who find 23 molecules in their spectra from a total of 354 lines. In addition to the 23 molecules detected by Bøgelund et al (2019), we also find lines of H 2 CCN and NH 2 CN, bringing our total to 25, which is likely due to the larger aperture used to obtain our spectra.…”

“…The line identifications are generally in good agreement with Bøgelund et al (2019), who find 23 molecules in their spectra from a total of 354 lines. In addition to the 23 molecules detected by Bøgelund et al (2019), we also find lines of H 2 CCN and NH 2 CN, bringing our total to 25, which is likely due to the larger aperture used to obtain our spectra. We did not find sufficient evidence to confirm the detection of gGg' ethylene glycol.…”

“…In the same paper, we also presented 12 CO observations that showed the presence of a large-scale outflow perpendicular to the AFGL 4176 mm1 disk. Using the same ALMA data, Bøgelund et al (2019) published an analysis of the chemistry in AFGL 4176, with a particular focus on measuring the temperatures, column densities, and abundances of various species, deriving excitation temperatures between 120 -320 K for the molecules they detected. They found that the column density with respect to methanol is three times higher for O-bearing species than N-bearing species, indicating that AFGL 4176 is likely a young source, with little evidence for significant processing by the central star.…”

The ALMA-PILS survey: propyne (CH3CCH) in IRAS 16293–2422

“…Whilst CH 3 CCH has been detected in a variety of objects throughout the Galaxy, there are not many sources where the abundances have been determined with a high enough angular resolution, to ensure abundances are not heavily affected by beam dilution issues. One such high angular resolution study was performed recently of the high-mass hot core in the source AFGL 4176 by Bøgelund et al (2019). Their study used ALMA observations to explore the continuum emission and the molecular content of AFGL 4176.…”

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

“…In addition, the 'buckyball' molecules C 60 , C + 60 and C 70 have been detected in circumstellar shells and nebulae [164]. Spectral surveys of star-forming regions with ALMA now reveal myriads of lines from organic molecules with ≈10 atoms, which are complex by astrophysical (if not by chemical) standards [165,166]. collisional data are needed to carry out non-LTE analysis for these species, but which data are needed depends on the situation.…”