The Chemistry of Phosphorus-bearing Molecules under Energetic Phenomena

Abstract

For decades, the detection of phosphorus-bearing molecules in the interstellar medium was restricted to high-mass star-forming regions (e.g., SgrB2 and Orion KL) and the circumstellar envelopes of evolved stars. However, recent higher-sensitivity observations have revealed that molecules such as PN and PO are present not only toward cold massive cores and low-mass star-forming regions with PO/PN ratios 1 but also toward the giant molecular clouds in the Galactic center known to be exposed to highly energetic phenomena such as intense UV radiation fields, shock waves, and cosmic rays. In this paper, we carry out a comprehensive study of the chemistry of phosphorus-bearing molecules across different astrophysical environments that cover a range of physical conditions (cold molecular dark clouds, warm clouds, and hot cores/hot corinos) and are exposed to different physical processes and energetic phenomena (proto-stellar heating, shock waves, intense UV radiation, and cosmic rays). We show how the measured PO/PN ratio (either 1, as in, e.g., hot molecular cores, or 1, as in UV strongly illuminated environments) can provide constraints on the physical conditions and energetic processing of the source. We propose that the reaction P+OH→PO+H, not included in previous works, could be an efficient gas-phase PO formation route in shocks. Our modeling provides a template with which to study the detectability of P-bearing species not only in regions in our own Galaxy but also in extragalactic sources.

ALMA and ROSINA detections of phosphorus-bearing molecules: the interstellar thread between star-forming regions and comets

“…11, PO is formed via PH + O, and also via P + OH. This latter formation route, not yet included in any chemical database, has been recently proposed by Jiménez-Serra et al (2018). Although theoretical calculations and laboratory experiments are needed to confirm the viability of this reaction, we have found observational evidence that supports it.…”

“…As shown in Fig. 11 of Jiménez-Serra et al (2018), the models based on photochemistry with intermediate extinctions (A v ∼7 mag) produce PO/PN>1, as observed in the cavity walls of AFGL 5142. This is further support for the photochemistry scenario Figure 12.…”

“…We have presented the first high-angular resolution maps of Pbearing species (PO and PN) in a star-forming region, which allow us to shed light on the poorly constrained chemistry of interstellar P. The non-detection of strong emission of P-bearing species towards the positions of the central hot core (MM position) and the position of the starless core (SC) seems to challenge the formation theories based on hot gas-phase chemistry triggered by the warm-up of protostars (Charnley & Millar 1994), and on cold gas-phase chemistry during the prestellar collapse phase of the parental core (Rivilla et al 2016). Previous theoretical models (Aota & Aikawa 2012;Jiménez-Serra et al 2018) propose that shocks are the most important agent for the chemistry of P-bearing species. According to this scenario, the ice mantles of the grains are sputtered due to the mechanical Figure 11.…”

“…11, the hydrogenated P-bearing species desorbed by shocks (e.g. PH 3 ) can rapidly (on short timescales of 10 4−5 yr) be converted to PO and PN, according to the gas-phase photochemistry models of Jiménez-Serra et al (2018). The formation of PO and PN is enhanced in the cavity walls, where the volume densities become large enough for formation reactions to proceed rapidly.…”

Detection of Phosphorus-bearing Molecules toward a Solar-type Protostar

“…There have been several recent models that aim to reproduce the observed PO/PN ratios of in different star-forming environments. Jiménez-Serra et al (2018) explore the phosphorus chemistry under a number of different energetic conditions, including heating, shocking, UV irradiation, and cosmic ray irradiation; Lefloch et al (2016) model the shocked outflow L1157-B1; and Rivilla et al (2016) model the collapse and warm-up chemistry in massive cores. In all cases, the models can reproduce a PO/PN ratio of ∼1-3 under certain specific conditions, but also predict significant regions of parameter space in which the PO/PN ratio is much greater or much less than unity.…”