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Formation and Destruction of Silicon Monoxide in SN 1987A

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Formation and Destruction of Silicon Monoxide in SN 1987A Formation and Destruction of Silicon Monoxide in SN 1987A The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Liu, Weihong, and A. Dalgarno. 1996. “Formation and Destruction of Silicon Monoxide in SN 1987A.” The Astrophysical Journal 471 (1): 480–84. https://doi.org/10.1086/177982. Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:41397475 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA THE ASTROPHYSICAL JOURNAL, 471:480È484, 1996 November 1 ( 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A. FORMATION AND DESTRUCTION OF SILICON MONOXIDE IN SN 1987A WEIHONGLIU AND A. DALGARNO Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 Received 1996 March 21; accepted 1996 May 31 ABSTRACT We present a new chemistry model for the silicon monoxide molecules in SN 1987A. It incorporates an enhanced rate of formation by radiative association and an enhanced rate of destruction through charge transfer between SiO and Ar` and Ne`, which is more efficient than the energetic electron impact ionization and dissociation in a previous model. The conclusion is unchanged that the 10~4È10~3 M_ of SiO observed in SN 1987A can be produced by chemistry models in which there is no microscopic mixing of helium into the silicon-oxygen region of the ejecta. Subject headings: molecular processes È supernovae: individual (SN 1987A) 1. INTRODUCTION region. Dynamical instabilities during and after the super- nova explosion cause macroscopic mixing of elements in Infrared emission from vibrationally excited silicon mon- di†erent layers rather than microscopic mixing. We used oxide molecules was observed in the ejecta of SN 1987A as unmixed models with the elemental compositions predicted early as 160 days after the supernova explosion(Aitken et by the explosive nucleosynthesis models ofWoosley, Pinto, al.1988) and could no longer be detected after 578 days & Weaver(1988) and of Thielemann, Hashimoto, & (Rocheet al. 1989). The fundamental emission spectra of Nomoto(1990). For the composition of Woosley et al. SiO have been analyzed byLiu & Dalgarno (1994). By (1988), 0.14M of silicon and 0.02M of oxygen in the accounting for optical depth e†ects on the emission and _ _ silicon-oxygen region are mixed with 0.06M of sulfur, non-LTE e†ects on the vibrational level populations,Liu & _ 0.0093M of argon, and 0.0043M of calcium. The adja- Dalgarno(1994) have inferred 10 È10 M for the mass _ _ ~4 ~3 _ cent region, which contains most of the oxygen mass in the of SiO in the ejecta. supernova, is not expected to be a major site for SiO forma- The chemical processes that form and destroy SiO mol- tion because neon is the dominant constituent in this region ecules in the supernova have been explored byLiu & Dal- and Ne` is e†ective in destroying the SiO molecules even if garno(1994), who suggested that the radiative association there is silicon mixed into the region. For the composition of silicon atoms and oxygen atoms to form SiO is the major ofThielemann et al. (1990), SiO is likely to be abundant in mechanism for the formation of SiO in the supernova. Liu the region where there is 0.01M of silicon and 0.1M of & Dalgarno(1994) assumed a rate coefficient that is one _ _ oxygen together with 0.004M of sulfur, 0.007M of mag- order of magnitude smaller than the rate coefficient for the _ _ nesium, and a small amount of carbon. In other regions radiative association of carbon atoms and oxygen atoms to with signiÐcant amounts of silicon, SiO is not expected to form CO(Dalgarno, Du, & You 1990). However, the rate be abundant, either because of the deÐciency of oxygen or coefficient for the radiative association to form SiO has now because of destruction by Ne` and Ar`. To investigate the been calculated byAndreazza, Singh, & Sanzovo (1995). It sensitivity of the chemistry to possible microscopic mixing, is a factor of 30 larger than that adopted byLiu & Dalgarno we also used the composition in the mixed model of (1994). Thus, more efficient destruction mechanisms for SiO Nomotoet al. (1991) in which 0.085M of silicon and 1.48 must be operative in the supernova ejecta than the ioniza- _ M of oxygen are mixed with 0.114M of carbon, 0.147 tion and dissociation by energetic electron impact that were _ _ M of magnesium,0.229 M of neon,0.0229 M of sulfur, proposed byLiu & Dalgarno (1994). _ _ _ 0.00378 M of argon, 0.00325M of calcium,0.073 M of We point out in this paper chemical reactions that _ _ _ iron, and other minor elements. destroy SiO in the ejecta that can balance the more rapid The chemistry is not sensitive to the temperature, and we formation of SiO. The presence of argon and neon in the assumed that the gas kinetic temperature is the same as the silicon-oxygen region of the ejecta signiÐcantly enhances temperature derived byLiu & Dalgarno (1994) from the the efficiency of destruction of SiO in the supernova due to rotational level populations of SiO, which is about 2000 K. charge transfer reactions with Ar` and Ne`. As a result, the We assumed that the silicon-oxygen region of the ejecta conclusion of our earlier chemistry model(Liu & Dalgarno expanded homologously with the density 1994)remains valid that the 10~4È10~3 M_ of SiO observed in SN 1987A can be produced by chemistry n \ 7 ] 1010(t/100 days)~3 cm~3 , (1) models in which there is no microscopic mixing of helium into the silicon-oxygen region of the supernova ejecta. at time t after the supernova explosion(Liu & Dalgarno 1994). 2. CHEMISTRY OF SiO We have explored the chemical processes that form and 2.1. Formation of SiO destroy silicon monoxide molecules in SN 1987A, in addi- There was no evidence of dust formation in the ejecta of tion to those presented in our previous model(Liu & Dal- SN 1987A until about 530 days after the supernova explo- garno1994). The SiO chemistry depends on the elemental sion(Lucy et al. 1991). In the absence of grains, molecular composition of the silicon-oxygen region of the supernova formation in the gas phase must have been initiated by ejecta and the degree of mixing of other elements in this radiative processes.Liu & Dalgarno (1994) suggested that 480 SILICON MONOXIDE IN SN 1987A 481 the direct radiative association the decay of the radioactive nuclei 56Co and 57Co. The fast electrons can ionize and dissociate SiO, Si ] O ] SiO ] hl (2) e SiO ] e SiO` e , (17) is the most important mechanism for the formation of ] ] ] SiO molecules in SN 1987A. Its rate coefficient has been ] e ] Si ] O , (18) calculated byAndreazza et al. (1995) to be ] e ] Si` ] O ] e , (19) 5.52 ] 10~18T 0.31B0.02 cm3 s~1, where T is the tem- perature in degrees kelvin. The formation of SiO may also ] e ] O` ] Si ] e , (20) be accomplished indirectly by the radiative association (Liu& Dalgarno 1994). The similar processes for carbon Si` ] O ] SiO` ] hl , (3) monoxide have been analyzed in detail(Liu & Victor 1994) and applied to the CO chemistry of SN 1987A(Liu, Dal- with a rate coefficient of 9.7 10 7.4 10 T ] ~19 [ ] ~23 garno, & Lepp1992; Liu & Dalgarno 1995). We assumed 4.4 10 T cm s (Andreazzaet al. 1995), followed ] ] ~27 2 3 ~1 the same rate of destruction by energetic electron impact for by charge transfer reactions with metals such as SiO as for CO. For an average energy deposition rate per SiO` ] Si ] Si` ] SiO , (4) particle in the oxygen core of SiO` ] Ca ] Ca` ] SiO . (5) L \ 60 exp ([t/q)M1 [ exp [[/ (t/t )~2]vN eV day~1 , 0 0 However, this sequence is an unimportant source of SiO (21) because SiO` is destroyed predominantly by dissociation where q 111.26 days and/ 31.1 att 100 days (Liu recombination \ \ \ & Dalgarno1995), and for a0 mean energy0 per ion pair of SiO` ] e ] Si ] O . (6) about 30 eV for SiO, the rate of destruction of SiO by energetic electron impact can be estimated to be Dissociation recombination proceeds rapidly in the super- nova ejecta, which has an ionization fraction of 10~2.An R \ 2 exp ([t/q)M1 [ exp [[/ (t/t )~2]N day~1 . (22) additional source of SiO is provided by the radiative associ- 0 0 Since the radiative association(2) proceeds faster than ation assumed in our previous chemistry model(Liu & Dalgarno O ] O ] O ] hl , (7) 1994) in which energetic electron impact is the dominant 2 mechanism for the destruction of SiO in SN 1987A, more with a rate coefficient of about 8.5 ] 10~21 cm3 s~1 at 2000 efficient destruction mechanisms must exist and will be K(Babb & Dalgarno 1995), followed by the neutral reac- explored below. tion 2.2.2. Photodissociation and Photoionization Si ] O ] SiO ] O , (8) 2 A possible mechanism for the destruction of SiO in SN with a rate coefficient of 2.7 ] 10~10 cm3 s~1 (Husain & 1987A is provided by photodissociation and photoioniza- Norris1978) or 9 ] 10~12 cm3 s~1 (Swearengen, Davies, & tion. The ultraviolet photons are produced by excitation of Niemczyk1978). The sequence initiated by radiative attach- atoms and ions by energetic electron impact and by recom- ment bination of ions following ionization by energetic electron e ] Si ] Si~]hl , (9) impact.
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