NH₃(1₀–0₀) in the pre-stellar core L1544

Abstract

Pre-stellar cores represent the initial conditions in the process of star and planet formation, therefore it is important to study their physical and chemical structure. Because of their volatility, nitrogen-bearing molecules are key to study the dense and cold gas present in pre-stellar cores. The NH 3 rotational transition detected with Herschel-HIFI provides a unique combination of sensitivity and spectral resolution to further investigate physical and chemical processes in pre-stellar cores. Here we present the velocityresolved Herschel-HIFI observations of the ortho-NH 3 (1 0 −0 0 ) line at 572 GHz and study the abundance profile of ammonia across the pre-stellar core L1544 to test current theories of its physical and chemical structure. Recently calculated collisional coefficients have been included in our non-LTE radiative transfer code to reproduce Herschel observations. A gas-grain chemical model, including spin-state chemistry and applied to the (static) physical structure of L1544 is also used to infer the abundance profile of ortho-NH 3 . The hyperfine structure of ortho-NH 3 (1 0 −0 0 ) is resolved for the first time in space. All the hyperfine components are strongly selfabsorbed. The profile can be reproduced if the core is contracting in quasi-equilibrium, consistent with previous work, and if the NH 3 abundance is slightly rising toward the core centre, as deduced from previous interferometric observations of para-NH 3 (1, 1). The chemical model overestimates the NH 3 abundance at radii between 4000 and 15 000 AU by about two orders of magnitude and underestimates the abundance toward the core centre by more than one order of magnitude. Our observations show that chemical models applied to static clouds have problems in reproducing NH 3 observations.

Seeds of Life in Space (SOLIS). III. Zooming Into the Methanol Peak of the Prestellar Core L1544*

“…It is also consistent with the kinetic temperature derived with non-LTE modelling for methanol towards the L1544 dust peak by Vastel et al (2014) (7-15 K). This result confirms that CH 3 OH is mainly tracing a shell around the L1544 dust peak, as towards the centre, Crapsi et al (2007) found temperatures of ∼6 K using NH 3 , which does not appear to freeze-out (see also Caselli et al 2017). The opposing directions of the methanol and ammonia temperature gradients further support the notion that the methanol gradient is only an excitation effect caused by changes in the gas density.…”

Why does ammonia not freeze out in the centre of pre-stellar cores?

“…Radiative transfer simulations of the ortho-ammonia (1 0 − 0 0 ) line. The simulated lines correspond to the Herschel observations ofCaselli et al (2017), shown in black. The ammonia abundance profiles are taken from model CM1 (blue; see text) or model CM2 (red).…”

“…Second, we took the physical structure from K10 and mapped the ammonia abundance profile from Crapsi et al (2007) onto this physical model, i.e., both models use the same parametrization for the ammonia abundance. The beam FWHM and spectral resolution were set to 40 ′′ and 64 m s −1 , respectively, corresponding to the Herschel observations of Caselli et al (2017). The results of this comparison are shown in Fig.…”

Chemical modelling of complex organic molecules with peptide-like bonds in star-forming regions

“…However, for the core centre, the modelled abundance is much lower than the observed value. Recently, Caselli et al (2017) also found that their chemical model predicts a much lower abundance in the central part of L1544 and they concluded that several factors might play an important role in this discrepancy, such as the underestimation of the production of gas phase NH 3 . The observed upper limits of NH 2 CHO in L1544 (≤2.4-6.7×10 −13 ; Jiménez-Serra et al 2016) agree well with our predictions for the estimated age of the core.…”