43 research outputs found
The formation of polycyclic aromatic hydrocarbons in evolved circumstellar environments
The formation of Polycyclic Aromatic Hydrocarbons in the circumstellar
outflows of evolved stars is reviewed, with an emphasis on carbon stars on the
Asymptotic Giant Branch. Evidence for PAHs present in their winds is provided
by meteoritic studies and recent observations of the Unidentified Infrared
bands. We detail the chemical processes leading to the closure of the first
aromatic ring as well as the growth mechanisms leading to amorphous carbon
grains. Existing studies on PAH formation in evolved stellar envelopes are
reviewed and new results for the modelling of the inner wind of the archetype
carbon star IRC+10216 are presented. Benzene, C6H6, forms close to the star, as
well as water, H2O, as a result of non-equilibrium chemistry induced by the
periodic passage of shocks. The growth process of aromatic rings may thus
resemble that active in sooting flames due to the presence of radicals like
hydroxyl, OH. Finally, we discuss possible formation processes for PAHs and
aromatic compounds in the hydrogen-rich R CrB star, V854 Cen, and their
implication for the carriers of the Red Emission and the Diffuse Interstellar
Bands.Comment: 13 pages, 4 figures, Invited review at the conference 'PAHs and the
Universe', Toulouse, France, June 201
Molecules and dust in Cas A: I - Synthesis in the supernova phase and processing by the reverse shock in the clumpy remnant
Aims: We study the chemistry of the Type IIb supernova ejecta that led to the
Cas A supernova remnant to assess the chemical type and quantity of dust that
forms and evolves in the remnant phase. We later model a dense oxygen-rich
ejecta knot that is crossed by the reverse shock in Cas A to study the
evolution of the clump gas phase and the possibility to reform dust clusters in
the post-reverse shock gas.
Methods: A chemical network including all processes efficient at high gas
temperatures and densities is considered. The formation of key bimolecular
species (CO, SiO) and dust clusters is described. Stiff, coupled, ordinary,
differential equations are solved for the conditions pertaining to both the SN
ejecta and the post-reverse shock gas.
Results: We find that the ejecta of Type IIb SNe are unable to form large
amounts of molecules and dust clusters as opposed to their Type II-P
counterparts because of their diffuse ejecta. The gas density needs to be
increased by several orders of magnitude to allow the formation of dust
clusters. We show that the chemical composition of the dust clusters changes
drastically and gains in chemical complexity with increasing gas density.
Hence, the ejecta of the Cas A supernova progenitor must have been in the form
of dense clumps to account for the dust chemical composition and masses
inferred from infrared observations of Cas A. We show that the ejecta molecules
in a clump that is processed by the reverse shock reform in the post-reverse
shock gas with lower abundances than those of the initial ejecta clump, except
SiO. These molecules include CO, SiS and O2. Dust clusters are destroyed by the
reverse shock and do not reform in the post-reverse shock gas, even for the
highest gas density. These results indicate that the synthesis of dust grains
from the gas phase in the dense knots of Cas A and in other supernova remnants
is unlikely.Comment: 11 pages, 8 figures, accepted for publication in A&
Dust production in Supernovae
Supernovae have long been proposed to be efficient dust producers in galaxies. Observations in the mid-infrared indicate that dust forms a few hundred days after the stellar explosion. Yet, the chemical type and the amount of dust produced by supernovae are not well quantified. In this review, we summarise the current knowledge of dust formation derived from observations of supernovae, present the various theoretical models on dust synthesis and their predictions, and discuss these results in the context of the most recent observations of dust in supernova remnants
Infrared fluorescence from PAHs in the laboratory
Several celestial objects, including UV rich regions of planetary and reflection nebulae, stars, H II regions, and extragalactic sources, are characterized by the unidentified infrared emission bands (UIR bands). A few years ago, it was proposed that polycyclic aromatic hydrocarbon species (PAHs) are responsible for most of the UIR bands. This hypothesis is based on a spectrum analysis of the observed features. Comparisons of observed IR spectra with lab absorption spectra of PAHs support the PAH hypothesis. An example spectrum is represented, where the Orion Bar 3.3 micron spectrum is compared with the absorption frequencies of the PAHs Chrysene, Pyrene, and Coronene. The laser excited 3.3 micron emission spectrum is presented from a gas phase PAH (azulen). The infrared fluorescence theory (IRF) is briefly explained, followed by a description of the experimental apparatus, a report of the results, and discussion
The Chemistry of Population III Supernova Ejecta. II. The Nucleation of Molecular Clusters as a Diagnostic for Dust in the Early Universe
We study the formation of molecular precursors to dust in the ejecta of Population III supernovae (Pop. III SNe) using a chemical kinetic approach to follow the evolution of small dust cluster abundances from day 100 to day 1000 after explosion. Our work focuses on zero-metallicity 20 M sun and 170 M sun progenitors, and we consider fully macroscopically mixed and unmixed ejecta. The dust precursors comprise molecular chains, rings, and small clusters of chemical composition relevant to the initial elemental composition of the ejecta under study. The nucleation stage for small silica, metal oxides and sulfides, pure metal, and carbon clusters is described with a new chemical reaction network highly relevant to the kinetic description of dust formation in hot circumstellar environments. We consider the effect of the pressure dependence of critical nucleation rates and test the impact of microscopically mixed He + on carbon dust formation. Two cases of metal depletion on silica clusters (full and no depletion) are considered to derive upper limits to the amounts of dust produced in SN ejecta at 1000 days, while the chemical composition of clusters gives a prescription for the type of dust formed in Pop. III SNe. We show that the cluster mass produced in the fully mixed ejecta of a 170 M sun progenitor is ~ 25 M sun whereas its 20 M sun counterpart forms ~ 0.16 M sun of clusters. The unmixed ejecta of a 170 M sun progenitor SN synthesize ~5.6 M sun of small clusters, while its 20 M sun counterpart produces ~0.103 M sun . Our results point to smaller amounts of dust formed in the ejecta of Pop. III SNe by a factor of ~ 5 compared to values derived by previous studies, and to different dust chemical compositions. Such deviations result from some erroneous assumptions made, the inappropriate use of classical nucleation theory to model dust formation, and the omission of the synthesis of molecules in SN ejecta. We also find that the unmixed ejecta of massive Pop. III SNe chiefly form silica and/or silicates, and pure silicon grains whereas their lower mass counterparts form a dust mixture dominated by silica and/or silicates, pure silicon, and iron sulfides. Amorphous carbon can only condense via the nucleation of carbon chains and rings characteristic of the synthesis of fullerenes when the ejecta carbon-rich zone is deprived of He + . The first dust enrichment to the primordial gas in the early universe from Pop. III massive SN comprises primarily pure silicon, silica, and silicates. If carbon dust is present at redshift z > 6, alternative dust sources must be considered
Molecules in Supernova Ejecta
The first molecules detected at infrared wavelengths in the ejecta of a Type II supernova, namely SN1987A, consisted of CO and SiO. Since then, confirmation of the formation of these two species in several other supernovae a few hundred days after explosion has been obtained. However, supernova environments appear to hamper the synthesis of large, complex species due to the lack of microscopically-mixed hydrogen deep in supernova cores. Because these environments also form carbon and silicate dust, it is of importance to understand the role played by molecules in the depletion of elements and how chemical species get incorporated into dust grains. In the present paper, we review our current knowledge of the molecular component of supernova ejecta, and present new trends and results on the synthesis of molecules in these harsh, explosive events
Molecules in Supernova Ejecta
The first molecules detected at infrared wavelengths in the ejecta of a Type II supernova, namely SN1987A, consisted of CO and SiO. Since then, confirmation of the formation of these two species in several other supernovae a few hundred days after explosion has been obtained. However, supernova environments appear to hamper the synthesis of large, complex species due to the lack of microscopically-mixed hydrogen deep in supernova cores. Because these environments also form carbon and silicate dust, it is of importance to understand the role played by molecules in the depletion of elements and how chemical species get incorporated into dust grains. In the present paper, we review our current knowledge of the molecular component of supernova ejecta, and present new trends and results on the synthesis of molecules in these harsh, explosive event
Molecules in nearby and primordial supernovae
We present new chemical models of supernova (SN) ejecta based on a chemical kinetic approach. We focus on the formation of inorganic and organic molecules including gas phase dust precursors, and consider zero-metallicity progenitor, massive supernovae and nearby core-collapse supernovae such as SN1987A. We find that both types are forming large amounts of molecules in their ejecta at times as early as 200 days after explosion. Upper limits on the dust formation budget are derived. Our results on dust precursors do not agree with existing studies on dust condensation in SN ejecta. We conclude that PMSNe could be the first non-primodial molecule providers in the early universe, ejecting up to 34% of their progenitor mass under molecular form to the pristine, local ga
Condensation of dust in the ejecta of type II-P supernovae
Aims: We study the production of dust in Type II-P supernova by coupling the
gas-phase chemistry to the dust nucleation and condensation phases. We consider
two supernova progenitor masses with homogeneous and clumpy ejecta to assess
the chemical type and quantity of dust that forms. Grain size distributions are
derived as a function of post-explosion time. Methods: The chemistry of the gas
phase and the simultaneous formation of dust clusters are described by a
chemical network. The formation of key species (CO, SiO) and dust clusters of
silicates, alumina, silica, metal carbides and sulphides, pure metals, and
amorphous carbon is considered. The master equations describing the chemistry
of the nucleation phase are coupled to a dust condensation formalism based on
Brownian coagulation. Results: Type II-P supernovae produce dust grains of
various chemical compositions and size distributions as a function of time. The
grain size distributions gain in complexity with time, are slewed towards large
grains, and differ from the usual MRN power-law distribution used for
interstellar dust. Gas density enhancements in the form of clumps strongly
affect the dust chemical composition and the grain size distributions.
Silicates and pure metallic grains are highly dependent on clumpiness.
Specifically, clumpy ejecta produce grains over 0.1 micron, and the final dust
mass reaches 0.14 Msun. Conversely, carbon and alumina dust masses are
controlled by the mass yields of alumina and carbon in the zones where the dust
is produced. Several dust components form in the ejecta and the total dust mass
gradually builds up over a time span of 3 to 5 years post-outburst. This
gradual growth provides a possible explanation for the discrepancy between the
small dust masses formed at early post-explosion times and the high dust masses
derived from recent observations of supernova remnants.Comment: 21 pages, 10 figures. Accepted for publication in Astronomy &
Astrophysic