Phosphorus in the Young Supernova Remnant Cassiopeia A

Abstract

Phosphorus (31 P), which is essential for life, is thought to be synthesized in massive stars and dispersed into interstellar space when these stars explode as supernovae (SNe). Here we report on near-infrared spectroscopic observations of the young SN remnant Cassiopeia A, which show that the abundance ratio of phosphorus to the major nucleosynthetic product iron ( 56 Fe) in SN material is up to 100 times the average ratio of the Milky Way, confirming that phosphorus is produced in SNe. The observed range is compatible with predictions from SN nucleosynthetic models but not with the scenario in which the chemical elements in the inner SN layers are completely mixed by hydrodynamic instabilities during the explosion.1 Phosphorus (P) is an indispensable ingredient for life together with carbon, hydrogen, nitrogen, oxygen, and sulfur (S). In our solar system, its abundance relative to hydrogen is 2.8×10 −7by number, which is 50 to 1900 times less than those of the other life-keeping α elements (1).The abundance of P in the diffuse interstellar medium and stars in our galaxy's disk is comparable with that of the solar system or the cosmic abundance, with some dependence on metallicity (2, 3). P is believed to be mainly formed in massive [ > ∼ 8 M ⊙ ] stars by neutron capture on silicon (Si) in hydrostatic neon-burning shells in the pre-SN stage and also in explosive carbon and neon burning layers during SN explosion (4, 5).Freshly synthesized P should thus be found in young core-collapse SN remnants (SNRs)resulting from the explosion of massive stars. Cassiopeia A (Cas A) is the youngest confirmed core-collapse SNR in our galaxy; it has been extensively studied in all wavebands (Fig. 1).It is located at a distance of 3.4 kpc (6) and thought to be the remnant of a SN event in C.E.1681 ± 19 (7). The spectrum of the light echo from the SN event indicated a SN of Type IIb that had a small hydrogen envelope at the time of explosion (8). The total estimated mass of the SN ejecta is 2 to 4 M ⊙ , and the inferred initial mass of the progenitor star ranges from 15 to 25 M ⊙ (9, 10). Emission from P II was previously detected from Cas A (11), but no analysis of the emission line has been done.We conducted near-infrared (NIR) spectroscopic observations of the main SN ejecta shell using the TripleSpec spectrograph mounted on the Palomar 5-m Hale telescope in 2008 ( The knots show distinct spectroscopic and kinematic properties depending on their origins (Fig. 2) Their properties match those of "quasi-stationary flocculi", the material lost from the hydrogen envelope of the progenitor during its red-supergiant phase as slow wind (13,14). 3) imply that the P-to-Fe abundance ratio by number, X(P/Fe), of the SN ejecta knots is up to 100 times higher than the cosmic abundance X ⊙ (P/Fe) = 8.1 × 10 −3 (1), whereas the knots of the circumstellar medium have ratios close to the cosmic abundance. The enhanced P abundance over Fe in these SN ejecta knots is direct evidence for the in situ production of P in Cas A.The observed range of X...

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