David A. Williams's research while affiliated with University College London and other places

... the lower state (Thomas and David, 1999). In the region of the interstellar space with little dust, the number of excited states of atoms is negligible. ...

... Both the radiative properties and the H2 formation rate depend on the specific dust-grain size distribution (GSD) (e.g., Takeuchi et al. 2003). Different prescriptions exist for the GSD of interstellar dust, derived from extinction curves and other observa-E-mail: [email protected] † Postdoctoral Fellow of the Fund for Scientific Research (FWO), Flanders, Belgium tions, such as polarisation and spectroscopy (see e.g., Mathis et al. 1977;Li & Draine 2001b;Cecchi-Pestellini et al. 2010;Jones et al. 2013;Williams & Cecchi-Pestellini 2016). The GSD is the result of the evolution of dust in the ISM, where it undergoes accretion, coagulation, shattering, sputtering and thermal processing, which are all time-and space-dependent processes (e.g., Draine & Salpeter 1979;Dwek & Scalo 1980;Jones et al. 1996;Dwek 1998;Yan et al. 2004;Ormel et al. 2009;Asano et al. 2013;Hirashita 2015). ...

... In the present paper, we first present the results of an experimental study of the following reactions over the temperature range of 291 ✁ 740 K involving the two low-lying excited (metastable) states of atomic phosphorus, P( 2 D) and P( 2 P), with H2O and H2 (H2O is, after H2 and CO, the third most abundant molecule in oxygen-rich conditions, see e.g. Williams and Hartquist (2013)): ...

... Under such circumstances, pure methanol ices can be used as a powerful tracer for isolating the highest density regions in the CMZ, where massive star formation is imminent or likely underway. Methanol ices form efficiently in dense (10 5 cm −3 ) and cold (15 K) environments (Watanabe et al. 2003;Cuppen et al. 2009;Coutens et al. 2017) and serve as a key molecular species in the formation of more complex organic molecules observed in the CMZ (e.g., Requena-Torres et al. 2006. In the Galactic disk, CH 3 OH ices are found in various environments, including YSOs (Allamandola et al. 1992;Skinner et al. 1992;Brooke et al. 1999;Dartois et al. 1999b;Pontoppidan et al. 2003Pontoppidan et al. , 2004Whittet et al. 2011), isolated dense cores (Boogert et al. 2011(Boogert et al. , 2015 and quiescent clouds (Chiar et al. 1996(Chiar et al. , 2011. ...

... Generally, cosmic dust is thought to be hugely important for providing surface sites for astrochemical reactions, especially, but not only, when grains arrive in relatively denser and protected regions of the ISM. 3,4 In the diffuse ISM, the nascent grains are subject to sporadic energetic processing by supernovae shockwaves, high-energy cosmic rays, and strong UV radiation, which lead to sputtering, shattering, and ionization before cooling again. 5,6 Dust grain processing in the ISM is likely to create a huge population of nanosized silicate grains, 7 which could be astrochemically relevant for formation/dissociation of H 2 8,9 and water molecules, 10,11 and are a likely candidate for the source of the ubiquitous anomalous microwave emission. ...

... We attempt to simulate the chemistry of hydrocarbons in a typical diffuse cloud (nH = 100 cm −3 and T = 100 K) using Av (mag) 1 HAC † 20% n(C) † adapted for this study (see text). References: (a) Asplund et al. 2009 , (b) Awad et al. 2016 a real-time-dependent gas-grain chemical model; uclchem 1 (Holdship et al. 2017). The code models a "parcel" of gas of homogeneous density, at a specific Av (determined by the user-defined density and the radius of the parcel). ...

... Owing to the lack of UV photons, the temperatures of the central protostars in the HMCs rise because of the increase in the temperatures of the dust and gaseous material. In ISM, HMCs have a high gas density (n H2 ≥10 6 cm −3 ), a warm temperature (≥100 K), and a compact and small source size (≤0.1 pc)(van Dishoeck & Blake 1998;Williams & Viti 2014). Previous molecular line surveys at millimeter and sub-millimeter wavelengths show that several types of COMs like CH 3 OH, CH 3 OCHO, CH 3 OCH 3 , CH 3 CN, and C 2 H 5 CN are frequently found in different HMCs because of the high temperature and evaporation of H 2 O and organic ice mantles ...

... A young NW-SE outflow (∼400 yr) has been traced with SiO and CO emission (Rao et al. 2009;Girart et al. 2014) using the SMA interferometer. Rawlings et al. (2000) and Rollins et al. (2014) have shown that young outflows can lead to an enhancement of the HCO + abundance in a short period of time. Briefly, the interaction between the jet and/or the outflowing material and the surrounding quiescent (or infalling) gas is eroding the icy mantle of dust grains, desorbing the molecular materials in the gas phase (e.g. ...

... Typical regions within the ISM and range estimates of their associated temperatures T in K and densities n H in cm −3 . Adapted from Ferrière (2001), Williams and Viti (2013), van Dishoeck et al. (2013a) and Williams et al. (2017) the apparent violation of this equilibrium through the increasing temperature T within Table 1.2 is reflective of the complex heating mechanisms that star forming gas is subject to throughout its lifetime. Nevertheless, it is the genesis described by Table 1.2 that forms the complex, multistage process that is star formation. ...

... The existing chemical and physical models focus solely on diffuse clouds (e.g., Dalgarno 1988;Le Petit et al. 2004;Cecchi-Pestellini et al. 2012;Godard et al. 2014). For example, in Godard et al. (2014), a model including dissipation of turbulence was applied to reproduce the observed molecular abundances in the diffuse interstellar medium (ISM). ...