Oxygen Cores Research Articles

Magnetic field breakout from white dwarf crystallization dynamos

ABSTRACT A convective dynamo operating during the crystallization of white dwarfs is one of the promising channels to produce their observed strong magnetic fields. Although the magnitude of the fields generated by crystallization dynamos is uncertain, their timing may serve as an orthogonal test of this channel’s contribution. The carbon–oxygen cores of $M\approx 0.5-1.0\, {\rm M}_{\odot }$ white dwarfs begin to crystallize at an age tcryst ∝ M−5/3, but the magnetic field is initially trapped in the convection zone – deep inside the CO core. Only once a mass of mcryst has crystallized, the convection zone approaches the white dwarf’s helium layer, such that the magnetic diffusion time through the envelope shortens sufficiently for the field to break out to the surface, where it can be observed. This breakout time is longer than tcryst by a few Gyr, scaling as tbreak ∝ tcrystf−1/2, where f ≡ 1 − mcryst/M depends on the white dwarf’s initial C/O profile before crystallization. The first appearance of strong magnetic fields B ≳ 1 MG in volume-limited samples approximately coincides with our numerically computed tbreak(M) – potentially signalling crystallization dynamos as a dominant magnetization channel. However, some observed magnetic white dwarfs are slightly younger, challenging this scenario. The dependence of the breakout process on the white dwarf’s C/O profile implies that magnetism may probe the CO phase diagram, as well as uncertainties during the core helium burning phase in the white dwarf’s progenitor, such as the 12C(α, γ)16O nuclear reaction.

Evolutionary Origins of Binary Neutron Star Mergers: Effects of Common Envelope Efficiency and Metallicity

The formation histories of compact binary mergers, especially stellar-mass binary black hole mergers, have recently come under increased scrutiny and revision. We revisit the question of the dominant formation channel and efficiency of forming binary neutron star (BNS) mergers. We use the stellar and binary evolution code MESA and implement a detailed method for common envelope and mass transfer. We perform simulations for donor masses between 7 M ⊙ and 20 M ⊙ with a neutron star (NS) companion of 1.4 M ⊙ and 2.0 M ⊙ at two metallicities, using varying common envelope efficiencies and two different prescriptions to determine if the donor undergoes core collapse or electron capture, given their helium and carbon–oxygen cores. In contrast to the case of binary black hole mergers, for an NS companion of 1.4 M ⊙, all BNS mergers are formed following a common envelope phase. For an NS mass of 2.0 M ⊙, we identify a small subset of mergers following only stable mass transfer if the NS receives a natal kick sampled from a Maxwellian distribution with velocity dispersion σ = 265 km s−1. Regardless of the supernova prescription, we find more BNS mergers at subsolar metallicity compared to solar.

A Systematic Study of the Connection Between White Dwarf Period Spectra and Model Structure

To date, pulsational variability has been measured for nearly 70 pulsating helium-atmosphere white dwarfs (DBVs) and 500 pulsating hydrogen-atmosphere white dwarfs (DAVs), with only a fraction of these having been the subject of asteroseismic analysis. One way to approach white dwarf asteroseismology is forward modeling, where one assumes an interior structure and calculates the model’s periods. Many such models are calculated, in the search for the one that best matches the observed period spectrum. It is not computationally manageable, nor necessary, to vary every possible parameter for every object. We engage in a systematic study, based on a sample of 14 hydrogen-atmosphere white dwarfs, chosen to be representative of the types of pulsation spectra we encounter in white dwarf asteroseismology. These white dwarfs are modeled with carbon and oxygen cores. Our goal is to draw a connection between the period spectra and what parameters to which they are most sensitive. We find that the presence of longer-period modes generally muddies the mass and effective temperature determinations, unless continuous sequences of ℓ = 1 and ℓ = 2 modes are present. All period spectra are sensitive to the structure in the helium and hydrogen envelopes and most to at least some features of the oxygen abundance profile. Such sensitivity can be achieved either by the presence of specific low-radial-overtone modes, or by the presence of longer-period modes. Convective efficiency only matters when fitting periods greater than 800 s. The results of this study can be used to inform parameter selection and pave the way to pipeline asteroseismic fitting of white dwarfs.

Effects of redox variability and early diagenesis on marine sedimentary Hg records

Volcanism is a dominant natural source of mercury (Hg) to the atmosphere, biosphere, ocean and sediments. In recent years, sedimentary Hg contents have emerged as a tool to reconstruct volcanic activity, and particularly activity of (subaerially emplaced) large igneous provinces in geological deep time. More specifically, Hg has shown potential as a useful proxy to illuminate the previously elusive impact of such large-scale volcanism on marine and terrestrial paleo-environments. While Hg is now widely applied as volcanism tracer, non-volcanic factors controlling sedimentary Hg content are generally not well constrained. Part of this uncertainty stems from our inability to directly observe a natural unperturbed “steady-state” environment as a baseline, as the modern Hg cycle is heavily influenced by anthropogenic activity. Here we focus on the effects of ambient redox conditions in the water column and shallow sediments (early diagenesis), quantify their influence on the geological Hg record and thereby contribute to constraining their potential impact on the use of Hg as a proxy for deep-time volcanic activity. Constraining these factors is of critical importance for the application of Hg as such a proxy. Many periods in the geological past for which records have been generated, such as the Mesozoic Oceanic Anoxic Events, are marked by a variety of high-amplitude environmental perturbations, including widespread deoxygenation and deposition of organic-rich sediments. We estimate the impact of redox changes and early diagenesis on the geological Hg record using a suite of (sub)recent–Pleistocene and Upper Cretaceous sediments representing oxic to euxinic marine conditions. Our sample set includes a transect through an oxygen minimum zone and cores that record transient shifts in oxygenation state, as well as post-depositional effects – all unrelated to volcanism, to the best of our knowledge. We find substantial alterations to the Hg record and the total organic carbon and total sulfur content, which are typically assumed to be the most common carrier phases of Hg in marine sediments. Moreover, these biases can lead to signal alteration on a par with those interpreted to result from volcanic activity. Geochemical modifications are ubiquitous and their potential magnitude implies that the factors leading to biases in the geological record warrant careful consideration before interpretation. Factors of particular concern to proxy application are (1) the disproportionate loss of organic carbon and sulfur relative to Hg during oxidation that strongly modulates normalized Hg records, (2) the evasion of Hg in anoxic and mildly euxinic sediments and (3) sharp focusing of Hg during post-depositional oxidation of organic matter. We suggest that paired analyses of additional redox-sensitive trace elements such as molybdenum, and organic-matter characteristics, particularly the type of organic matter, could provide first-order constraints on the role that redox and diagenetic changes played in shaping the Hg record as part of checking the attribution of enrichments to volcanic activity.

The implications of large binding energies of massive stripped core collapse supernova progenitors on the explosion mechanism

ABSTRACT We examine the binding energies of massive stripped-envelope core collapse supernova (SECCSN) progenitors with the stellar evolution code mesa, and find that the jittering jets explosion mechanism is preferred for explosions where carbon–oxygen cores with masses of ${\gtrsim} 20 \, \mathrm{M}_\odot$ collapse to leave a neutron star (NS) remnant. We calculate the binding energy at core collapse under the assumption that the remnant is an NS. Namely, stellar gas above mass coordinate of ${\simeq} 1.5\text{{--}}2.5 \, \mathrm{M}_\odot$ is ejected in the explosion. We find that the typical binding energy of the ejecta of stripped-envelope (SE) progenitors with carbon–oxygen core masses of $M_{\rm CO} \gtrsim 20 \, \mathrm{M}_\odot$ is $E_{\rm bind} \gtrsim 2 \times 10^{51} {~\rm erg}$. We claim that jets are most likely to explode such cores as jet-driven explosion mechanisms can supply high energies to the explosion. We apply our results to SN 2020qlb, which is an SECCSN with a claimed core mass of ${\simeq} 30\!-\!50 \, \mathrm{M}_\odot$, and conclude that the jittering jets explosion mechanism best accounts for such an explosion that leaves an NS.

Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?

ABSTRACT Ultra-massive white dwarfs ($1.05\, \rm M_\odot \lesssim M_{WD}$) are particularly interesting objects that allow us to study extreme astrophysical phenomena such as type Ia supernovae explosions and merger events. Traditionally, ultra-massive white dwarfs are thought to harbour oxygen–neon (ONe) cores. However, recent theoretical studies and new observations suggest that some ultra-massive white dwarfs could harbour carbon–oxygen (CO) cores. Although several studies have attempted to elucidate the core composition of ultra-massive white dwarfs, to date, it has not been possible to distinguish them through their observed properties. Here, we present a new method for revealing the core-chemical composition in ultra-massive white dwarfs that is based on the study of magnetic fields generated by convective mixing induced by the crystallization process. ONe white dwarfs crystallize at higher luminosities than their CO counterparts. Therefore, the study of magnetic ultra-massive white dwarfs in the particular domain where ONe cores have reached the crystallization conditions but CO cores have not, may provide valuable support to their ONe core-chemical composition, since ONe white dwarfs would display signs of magnetic fields and CO would not. We apply our method to eight white dwarfs with magnetic field measurements and we suggest that these stars are candidate ONe white dwarfs.

The evolution of ultra-massive carbon–oxygen white dwarfs

ABSTRACT Ultra-massive white dwarfs ($\rm \mathit{M}_{WD} \gtrsim 1.05\, {\rm M}_{\odot }$) are considered powerful tools to study Type Ia supernovae explosions, merger events, the occurrence of physical processes in the superasymptotic giant branch phase, and the existence of high magnetic fields. Traditionally, ultra-massive white dwarfs are expected to harbour oxygen–neon (ONe) cores. However, new observations and recent theoretical studies suggest that the progenitors of some ultra-massive white dwarfs can avoid carbon burning, leading to the formation of ultra-massive white dwarfs harbouring carbon–oxygen (CO) cores. Here, we present a set of ultra-massive white dwarf evolutionary sequences with CO cores for a wide range of metallicity and masses. We take into account the energy released by latent heat and phase separation during the crystallization process and by 22Ne sedimentation. Realistic chemical profiles resulting from the full computation of progenitor evolution are considered. We compare our CO ultra-massive white dwarf models with ONe models. We conclude that CO ultra-massive white dwarfs evolve significantly slower than their ONe counterparts mainly for three reasons: their larger thermal content, the effect of crystallization, and the effect of 22Ne sedimentation. We also provide colours in several photometric bands on the basis of new model atmospheres. These CO ultra-massive white dwarf models, together with the ONe ultra-massive white dwarf models, provide an appropriate theoretical framework to study the ultra-massive white dwarf population in our Galaxy.

On the Energy Source of Ultrastripped Supernovae

Abstract Ultrastripped supernovae (USSNe) with a relatively low ejecta mass of ∼0.1 M ⊙ (e.g., iPTF 14gqr and SN 2019dge) are considered to originate from ultrastripped carbon–oxygen cores in close binary systems and are likely to be progenitors of binary neutron stars. Here we conduct the explosion simulations of ultrastripped progenitors with various masses (1.45 M ⊙ ≤ M CO ≤ 2.0 M ⊙) based on results of neutrino-radiation hydrodynamics simulations, and consistently calculate the nucleosynthesis and the supernova light curves. We find that a USSN from a more massive progenitor has a larger ejecta mass but a smaller 56Ni mass mainly due to the fallback that leads to the light curve being dimmer and slower. By comparing the synthetic light curves with the observed ones, we show that SN 2019dge can be solely powered by 56Ni synthesized during the explosion of a progenitor with M CO ≲ 1.6 M ⊙ while iPTF 14gqr cannot be explained by the 56Ni-powered model; ∼0.05 M ⊙ of 56Ni inferred from the light-curve fitting is argued to be difficult to synthesize for ultrastripped progenitors. We consider fallback accretion onto and rotation-powered relativistic wind from the newborn neutron star (NS) as alternative energy sources and show that iPTF 14gqr could be powered by a newborn NS with a magnetic field of B p ∼ 1015 G and an initial rotation period of P i ∼ 0.1 s.

Calcium-Ion Binding Mediates the Reversible Interconversion of Cis and Trans Peroxido Dicopper Cores.

Coupled dinuclear copper oxygen cores (Cu2 O2 ) featured in type III copper proteins (hemocyanin, tyrosinase, catechol oxidase) are vital for O2 transport and substrate oxidation in many organisms. μ-1,2-cis peroxido dicopper cores (C P) have been proposed as key structures in the early stages of O2 binding in these proteins; their reversible isomerization to other Cu2 O2 cores are directly relevant to enzyme function. Despite the relevance of such species to type III copper proteins and the broader interest in the properties and reactivity of bimetallic C P cores in biological and synthetic systems, the properties and reactivity of C P Cu2 O2 species remain largely unexplored. Herein, we report the reversible interconversion of μ-1,2-trans peroxido (T P) and C P dicopper cores. CaII mediates this process by reversible binding at the Cu2 O2 core, highlighting the unique capability for metal-ion binding events to stabilize novel reactive fragments and control O2 activation in biomimetic systems.

Calcium‐Ion Binding Mediates the Reversible Interconversion of Cis and Trans Peroxido Dicopper Cores

AbstractCoupled dinuclear copper oxygen cores (Cu2O2) featured in type III copper proteins (hemocyanin, tyrosinase, catechol oxidase) are vital for O2 transport and substrate oxidation in many organisms. μ‐1,2‐cis peroxido dicopper cores (CP) have been proposed as key structures in the early stages of O2 binding in these proteins; their reversible isomerization to other Cu2O2 cores are directly relevant to enzyme function. Despite the relevance of such species to type III copper proteins and the broader interest in the properties and reactivity of bimetallic CP cores in biological and synthetic systems, the properties and reactivity of CP Cu2O2 species remain largely unexplored. Herein, we report the reversible interconversion of μ‐1,2‐trans peroxido (TP) and CP dicopper cores. CaII mediates this process by reversible binding at the Cu2O2 core, highlighting the unique capability for metal‐ion binding events to stabilize novel reactive fragments and control O2 activation in biomimetic systems.

Type Ic supernova of a 22 M⊙ progenitor

ABSTRACT Type Ic supernovae (SNe Ic) are a sub-class of core-collapse SNe that exhibit no helium or hydrogen lines in their spectra. Their progenitors are thought to be bare carbon–oxygen cores formed during the evolution of massive stars that are stripped of their hydrogen and helium envelopes sometime before collapse. SNe Ic present a range of luminosities and spectral properties, from luminous GRB-SNe with broad-lined spectra to less luminous events with narrow-line spectra. Modelling SNe Ic reveals a wide range of both kinetic energies, ejecta masses, and 56Ni masses. To explore this diversity and how it comes about, light curves and spectra are computed from the ejecta following the explosion of an initially 22 M⊙ progenitor that was artificially stripped of its hydrogen and helium shells, producing a bare CO core of ∼5 M⊙, resulting in an ejected mass of ∼4 M⊙, which is an average value for SNe Ic. Four different explosion energies are used that cover a range of observed SNe. Finally, 56Ni and other elements are artificially mixed in the ejecta using two approximations to determine how element distribution affects light curves and spectra. The combination of different explosion energy and degree of mixing produces spectra that roughly replicate the distribution of near-peak spectroscopic features of SNe Ic. High explosion energies combined with extensive mixing can produce red, broad-lined spectra, while minimal mixing and a lower explosion energy produce bluer, narrow-lined spectra.

Mixing via Thermocompositional Convection in Hybrid C/O/Ne White Dwarfs

Abstract Convective overshooting in super asymptotic giant branch stars has been suggested to lead to the formation of hybrid white dwarfs with carbon–oxygen cores and oxygen–neon mantles. As the white dwarf cools, this core–mantle configuration becomes convectively unstable and should mix. This mixing has been previously studied using stellar evolution calculations, but these made the approximation that convection did not affect the temperature profile of the mixed region. In this work, we perform direct numerical simulations of an idealized problem representing the core–mantle interface of the hybrid white dwarf. We demonstrate that, while the resulting structure within the convection zone is somewhat different than what is assumed in the stellar evolution calculations, the two approaches yield similar results for the size and growth of the mixed region. These hybrid white dwarfs have been invoked as progenitors of various peculiar thermonuclear supernovae. This lends further support to the idea that if these hybrid white dwarfs form, then they should be fully mixed by the time of explosion. These effects should be included in the progenitor evolution, in order to more accurately characterize the signatures of these events.

On the Rate and on the Gravitational Wave Emission of Short and Long GRBs

Abstract On the ground of the large number of gamma-ray bursts (GRBs) detected with cosmological redshift, we classified GRBs in seven subclasses, all with binary progenitors which emit gravitational waves (GWs). Each binary is composed of combinations of carbon–oxygen cores (COcore), neutron stars (NSs), black holes (BHs), and white dwarfs (WDs). The long bursts, traditionally assumed to originate from a BH with an ultrarelativistic jetted emission, not emitting GWs, have been subclassified as (I) X-ray flashes (XRFs), (II) binary-driven hypernovae (BdHNe), and (III) BH–supernovae (BH–SNe). They are framed within the induced gravitational collapse paradigm with a progenitor COcore–NS/BH binary. The SN explosion of the COcore triggers an accretion process onto the NS/BH. If the accretion does not lead the NS to its critical mass, an XRF occurs, while when the BH is present or formed by accretion, a BdHN occurs. When the binaries are not disrupted, XRFs lead to NS–NS and BdHNe lead to NS–BH. The short bursts, originating in NS–NS, are subclassified as (IV) short gamma-ray flashes (S-GRFs) and (V) short GRBs (S-GRBs), the latter when a BH is formed. There are (VI) ultrashort GRBs (U-GRBs) and (VII) gamma-ray flashes (GRFs) formed in NS–BH and NS–WD, respectively. We use the occurrence rate and GW emission of these subclasses to assess their detectability by Advanced LIGO-Virgo, eLISA, and resonant bars. We discuss the consequences of our results in view of the announcement of the LIGO/Virgo Collaboration of the source GW 170817 as being originated by an NS–NS.

ASTEROSEISMOLOGICAL STUDY OF MASSIVE ZZ CETI STARS WITH FULLY EVOLUTIONARY MODELS

We present the first asteroseismological study for 42 massive ZZ Ceti stars based on a large set of fully evolutionary carbon$-$oxygen core DA white dwarf models characterized by a detailed and consistent chemical inner profile for the core and the envelope. Our sample comprise all the ZZ Ceti stars with spectroscopic stellar masses between 0.72 and $1.05M_{\odot}$ known to date. The asteroseismological analysis of a set of 42 stars gives the possibility to study the ensemble properties of the massive pulsating white dwarf stars with carbon$-$oxygen cores, in particular the thickness of the hydrogen envelope and the stellar mass. A significant fraction of stars in our sample have stellar mass high enough as to crystallize at the effective temperatures of the ZZ Ceti instability strip, which enables us to study the effects of crystallization on the pulsation properties of these stars. Our results show that the phase diagram presented in Horowitz et al. (2010) seems to be a good representation of the crystallization process inside white dwarf stars, in agreement with the results from white dwarf luminosity function in globular clusters.

Multidimensional simulations of pair-instability supernovae

According to theoretical models, massive stars with masses within the 100-250 Mrange should explode as pair-instability supernovae (PISNe). Since the first stars of the Universe are believed to be very massive, these supernovae should play a significant role in the early stages of its history. But these stars represent the last unobserved population, owing to detection limits of current telescopes. In this work we analyze pair-instability supernovae explosions using various numerical codes. We evolve series of the configurations of oxygen cores to establish a range of masses and initial conditions where this type of explosion is possible. We also study the role of possible instabilities in the propagation of shockwaves during the last stage of the explosion. This investigation could help us to predict the observational properties of PISNe for future space and ground telescopes.

Pair-Instability Explosions: observational evidence

AbstractIt has been theoretically predicted many decades ago that extremely massive stars that develop large oxygen cores will become dynamically unstable, due to electron-positron pair production. The collapse of such oxygen cores leads to powerful thermonuclear explosions that unbind the star and can produce, in some cases, many solar masses of radioactive 56Ni. For many years, no examples of this process were observed in nature. Here, I briefly review recent observations of luminous supernovae that likely result from pair-instability explosions, in the nearby and distant Universe.

Copper/alpha-ketocarboxylate chemistry with supporting peralkylated diamines: reactivity of copper(I) complexes and dicopper-oxygen intermediates.

To further understand copper-promoted oxidation reactions, the Cu(I) complexes LCuX (L = N,N'-di-tert-butyl-N,N'-dimethylethylenediamine; X = benzoylformate (BF) or p-nitro-benzoylformate) were synthesized, fully characterized by X-ray crystallography and spectroscopy in solution, and their reactivity with O(2) at -80 degrees C examined. Oxidative decarboxylation of the alpha-ketocarboxylate ligand was observed, but only to a significant extent when cyclohexene, cyclooctene, or acetonitrile was present. Spectroscopic and conductivity data are consistent with mechanistic postulates involving displacement of the alpha-ketocarboxylate by the additives to a small extent, followed by oxygenation of the LCu(I) moiety to yield copper-oxygen species that subsequently induce decarboxylation. To test these hypotheses, spectroscopic and kinetic studies of the reactions of Bu(4)NBF with preformed mu-eta(2):eta(2)-peroxodicopper(II) and/or bis(mu-oxo)dicopper(III) complexes supported by L or N,N,N',N'-tetramethylpropylenediamine were performed. In an illustration of a new mode of reactivity for such dicopper-oxygen cores, decarboxylation of the added alpha-ketocarboxylate was observed and the intermediacy of a carboxylate-bridged mu-eta(2):eta(2)-peroxodicopper(II) complex was implicated.

Different stellar demise

A decades-old theory of stellar evolution — that the most massive stars end their life in a peculiar type of explosion termed a pair-instability supernova — finally seems to have been confirmed by observations. Stars like the Sun end their stellar lives as white dwarfs. Theory predicts a different fate for stars with masses over 140 times that of the Sun (if they exist, which they don't in the Milky Way). When they have evolved to the stage of having oxygen cores the pressure-supporting photons turn into electron–positron pairs, absorbing energy and letting the core collapse to produce a 'pair instability' supernova. Analysis of the spectrum and light curve of supernova 2007bi, a luminous event in a nearby dwarf galaxy, provides evidence of such an explosion. The SN 2007bi progenitor is estimated to have had a core of greater than 100 solar masses. Calculations point to an explosion producing more than three solar masses worth of radioactive nickel-56, in line with what would be expected from a massive oxygen core. The implication is that there are extremely massive stars in the local Universe that could provide astronomers with a close-up of the type of star that may have dominated the early Universe.

Supernova 2007bi as a pair-instability explosion

Stars with initial masses such that 10M[symbol: see text] <or= M(initial) <or= 100M[symbol: see text], where M[symbol: see text] is the solar mass, fuse progressively heavier elements in their centres, until the core is inert iron. The core then gravitationally collapses to a neutron star or a black hole, leading to an explosion-an iron-core-collapse supernova. By contrast, extremely massive stars with M(initial) >or= 140M[symbol: see text] (if such exist) develop oxygen cores with masses, M(core), that exceed 50M[symbol: see text], where high temperatures are reached at relatively low densities. Conversion of energetic, pressure-supporting photons into electron-positron pairs occurs before oxygen ignition and leads to a violent contraction which triggers a nuclear explosion that unbinds the star in a pair-instability supernova. Transitional objects with 100M[symbol: see text] < M(initial) < 140M[symbol: see text] may end up as iron-core-collapse supernovae following violent mass ejections, perhaps as a result of brief episodes of pair instability, and may already have been identified. Here we report observations of supernova SN 2007bi, a luminous, slowly evolving object located within a dwarf galaxy. We estimate the exploding core mass to be M(core) approximately 100M[symbol: see text], in which case theory unambiguously predicts a pair-instability supernova. We show that >3M[symbol: see text] of radioactive (56)Ni was synthesized during the explosion and that our observations are well fitted by models of pair-instability supernovae. This indicates that nearby dwarf galaxies probably host extremely massive stars, above the apparent Galactic stellar mass limit, which perhaps result from processes similar to those that created the first stars in the Universe.

A Simple Model for Amphiprotic Water and Proton Transfer Reactions

Proton transfer is important for chemistry in general and for protein function in particular. Water is often involved as a donor, a receptor, or an element in a chain of concerted transfers. Recently we have shown that the amphiprotic behavior of water can be captured by a simple model that is inspired by the traditional Lewis construct. The model comprises explicit and fully charged oxygen cores, valence electron pairs, and protons, all interacting via pair-wise pseudo-potentials that reflect Heisenberg uncertainty and Pauli exclusion. These independently mobile particles produce stable neutral, protonated and deprotonated water clusters. They also exhibit transport of protons and proton-holes through water chains. A self-consistent extension of the model to nitrogen hydrides provides a description of ammonia that forms hydrogen bonds and transports protons. Further generalization to include carbon allows us to build “Lewis” amino acids. In in vacuo simulations, initially zwitterionic forms of the amino acids evolve to non-ionic forms via an intramolecular proton transfer. The intermediate in this process is a 5-member ring with the migrating hydrogen bridging the amine nitrogen and a carboxyl oxygen (see figure).View Large Image | View Hi-Res Image | Download PowerPoint Slide