Astrochemistry in hot molecular cores
Hot molecular cores (HMCs), the cradles of OB stars, are the most chemically rich sources in the Galaxy (e.g. Beuther 2007). The typical masses of these cores (>100 M☉) together with their high densities (n 107 cm-3) and temperatures (T > 100K) make them the most important reservoirs of complex organic molecules (COMs), including prebiotic ones, such as glycolaldehyde, CH2OHCHO, or ethylene glycol, (CH2OH)2, (Beltrán et al. 2009; Rivilla et al. 2017a). This rich chemistry is the result of thermal evaporation of dust grain mantles by the heating from the deeply embedded early type star and/or of the presence of powerful shocks. Besides being rich in COMs, HMCs spectra at (sub)millimeter exhibit a forest of molecular lines that include hydrogenated molecules, such as H2S, radicals, such as HCO (Rivilla et al. in prep), oxygen-bearing molecules, such as SiO, SO, SO2, and deuterated species such as N2D+, DCN, CH2DCN (Fontani et al. 2014; Belloche et al. 2016). Studying the emission of these species, especially of COMs, is important not only because it allows us to understand how chemistry may have developed in these cores, but also because it allows us to study the physical properties and kinematics of the material surrounding the massive protostars, making it possible, in some cases, to distinguish infall, outflow, and rotation signatures. With the increase of sensitivity of (sub)millimeter instruments, the number of molecular transitions detected in HMCs has stunningly increased, becoming in some cases almost impossible to distinguish line-free channels in the spectra. The richness of the (sub)millimeter spectra of HMCs has started to become a problem for line identification, especially of new and complex species, because the lines can be severely affected by blending. In many cases, different species can contribute to the emission at a certain portion of the spectra, and the only way to distinguish them is by properly fitting all the different transitions of each species. This is especially important for species with many atoms (> 6) that have an increasing number of transitions. In particular, despite the increase in sensitivity of telescopes, so far only two species with more than 10 atoms have been clearly detected in star-forming regions (e.g. Belloche et al. 2009; Tercero et al. 2013; Rivilla et al. 2017a,b).
Need for spectral line surveys
The way to tackle this problem is by carrying out molecular line surveys covering a broad range of frequencies. This allows to observe many transitions of different species and, by fitting them, it is possible to confirm or discard the detections. The proper identification of the species is important not only for the detection of new ones but also for the derivation of important physical parameters, such as abundances, needed to constrain the chemical models (the more transitions of a species, the better the excitation temperature and column density is determined). In addition, we need large spectral surveys because to properly estimate the physical parameters and structure of HMCs and study their kinematics one needs to observe a large number of transitions with different excitation conditions, i.e. a broad range of observed frequencies.
Previous spectral surveys toward hot molecular cores
Spectral line surveys in HMCs have been carried out mainly towards Sgr B2 (see Sánchez-Monge et al. 2017; Belloche et al. 2016, and references therein). This complex located in the central molecular zone has been observed at different wavelengths and as a result, the first detections of heavy COMs in the Galaxy, such as ethylene glycol or ethyl formate (10 and 11 atoms, respectively), have been obtained (e.g., Hollis et al. 2000, 2002). However, the cores in Sgr B2 are not typical HMCs. The current star formation rate of Sgr B2 qualifies it as a mini-starburst region and its location close to the Galactic Center makes the physical conditions in that environment quite extreme, which could have consequences on the chemistry. Therefore, the chemistry in Sgr B2 might not be representative of that typical of HMCs in the disk of the Galaxy. In addition, the presence of several velocity components along the line of sight makes line identification more difficult.
With this in mind, we propose to carry out an unbiased spectral line survey in Band 3 with ALMA of the HMC G31.41+0.31, one of the most chemically rich HMCs in the Galaxy, thus an excellent target to search for even more complex species, and a template for future studies of chemistry in HMCs.
