Aims: We aim to better understand the emission of molecular tracers of the diffuse and dense gas in giant molecular clouds and the influence that metallicity, optical extinction, density, far-UV... Show moreAims: We aim to better understand the emission of molecular tracers of the diffuse and dense gas in giant molecular clouds and the influence that metallicity, optical extinction, density, far-UV field, and star formation rate have on these tracers. Methods: Using the IRAM 30 m telescope, we detected HCN, HCO$^{+}$, $^{12}$CO, and $^{13}$CO in six GMCs along the major axis of M 33 at a resolution of ~{}114 pc and out to a radial distance of 3.4 kpc. Optical, far-infrared, and submillimeter data from Herschel and other observatories complement these observations. To interpret the observed molecular line emission, we created two grids of models of photon-dominated regions, one for solar and one for M 33-type subsolar metallicity. Results: The observed HCO$^{+}$/HCN line ratios range between 1.1 and 2.5. Similarly high ratios have been observed in the Large Magellanic Cloud. The HCN/CO ratio varies between 0.4% and 2.9% in the disk of M 33. The $^{12}$CO/$^{13}$CO line ratio varies between 9 and 15 similar to variations found in the diffuse gas and the centers of GMCs of the Milky Way. Stacking of all spectra allowed HNC and C$_{2}$H to be detected. The resulting HCO$^{+}$/HNC and HCN/HNC ratios of ~{}8 and 6, respectively, lie at the high end of ratios observed in a large set of (ultra-)luminous infrared galaxies. HCN abundances are lower in the subsolar metallicity PDR models, while HCO$^{+}$ abundances are enhanced. For HCN this effect is more pronounced at low optical extinctions. The observed HCO$^{+}$/HCN and HCN/CO line ratios are naturally explained by subsolar PDR models of low optical extinctions between 4 and 10 mag and of moderate densities of n 3 { imes} 10$^{3}$-3 { imes} 10$^{4}$ cm$^{-3}$, while the FUV field strength only has a small effect on the modeled line ratios. The line ratios are almost equally well reproduced by the solar-metallicity models, indicating that variations in metallicity only play a minor role in influencing these line ratios. Based on observations with the IRAM 30m telescope, Herschel, and other observatories. IRAM is supported by CNRS/INSU (France), the MPG (Germany), and the IGN (Spain). Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgFITS files of the presented spectra of the ground-state transitions of HCN, HCO$^{+}$, $^{12}$CO and $^{13}$CO are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/549/A17Show less
Öberg, K.; Boamah, M.; Fayolle, E.C.; Garrod, R.; Cyganowski, C.; Tak, F. van der 2013
Complex molecules have been broadly classified into three generations dependent on the mode of formation and the required formation temperature ({lt}25, 25-100 K, and {gt}100 K). Around massive... Show moreComplex molecules have been broadly classified into three generations dependent on the mode of formation and the required formation temperature ({lt}25, 25-100 K, and {gt}100 K). Around massive young stellar objects (MYSOs), icy grain mantles and gas are exposed to increasingly higher temperatures as material accretes from the outer envelope in toward the central hot region. The combination of this temperature profile and the generational chemistry should result in a changing complex molecular composition with radius around MYSOs. We combine IRAM 30 m and Submillimeter Array observations to explore the spatial distribution of organic molecules around the high-mass young stellar object NGC 7538 IRS9, whose weak complex molecule emission previously escaped detection. We find that emission from N-bearing organics and CH$_{3}$OH present substantial increases in emission around 8000 AU and R {lt} 3000 AU, while unsaturated O-bearing molecules and hydrocarbons do not. The increase in line flux for some complex molecules in the envelope, around 8000 AU or 25 K, is consistent with recent model predictions of an onset of complex ice chemistry at 20-30 K. The emission increase for many of the same molecules at R {lt} 3000 AU suggests the presence of a weak hot core, where thermal ice evaporation and hot gas-phase reactions drive the chemistry. Complex organics thus form at all radii and temperatures around this protostar, but the composition changes dramatically as the temperature increases, which is used together with an adapted gas-grain astrochemical model to constrain the chemical generation(s) to which different classes of molecules belong. Show less
