Experimental study of the removal of excited state phosphorus atoms by H2O and H-2: implications for the formation of PO in stellar winds

• The reactions of the low-lying metastable states of atomic phosphorus, P(D-2) and P(P-2), with H2O and H-2 were studied by the pulsed laser photolysis at 248 nm of PCl3, combined with laser-induced fluorescence detection of P(D-2), P(P-2), and PO. Rate coefficients between 291 and 740 K were measured, along with a yield for the production of PO from P(D-2 or P-2) + H2O of (35 +/- 15) %. H-2 reacts with both excited P states relatively efficiently; physical (i.e. collisional) quenching, rather than chemical reaction to produced PH + H, is shown to be the more likely pathway. A comprehensive phosphorus chemistry network is then developed using a combination of electronic structure theory calculations and a Master Equation treatment of reactions taking place over complex potential energy surfaces. The resulting model shows that at the high temperatures within two stellar radii of a MIRA variable AGB star in oxygen-rich conditions, collisional excitation of ground-state P(S-4) to P(D-2), followed by reaction with H2O, is a significant pathway for producing PO (in addition to the reaction between P(S-4) and OH). The model also demonstrates that the PN fractional abundance in a steady (non-pulsating) outflow is underpredicted by about 2 orders of magnitude. However, under shocked conditions where sufficient thermal dissociation of N-2 occurs at temperatures above 4000 K, the resulting N atoms convert a substantial fraction of PO into PN.

Ämnesord

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Nyckelord

astrochemistry

molecular data

methods: laboratory

stars: wind

outflows

bearing molecules

forming regions

chemistry

origin

pn

Astronomy & Astrophysics

Publikations- och innehållstyp

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