Bondi-Hoyle-Lyttleton Accretion onto Star Clusters

An isolated star moving supersonically through a uniform gas accretes material from its gravitationally-induced wake. The rate of accretion is set by the accretion radius of the star and is well-described by classical Bondi-Hoyle-Lyttleton theory. Stars, however, are not born in isolation. They form in clusters where they accrete material that is influenced by all the stars in the cluster. We perform three-dimensional hydrodynamic simulations of clusters of individual accretors embedded in a uniform-density wind in order to study how the accretion rates experienced by individual cluster members are altered by the properties of the ambient gas and the cluster itself. We study accretion as a function of number of cluster members, mean separation between them, and size of their individual accretion radii. We determine the effect of these key parameters on the aggregate and individual accretion rates, which we compare to analytic predictions. We show that when the accretion radii of the individual objects in the cluster substantially overlap, the surrounding gas is effectively accreted into the collective potential of the cluster prior to being accreted onto the individual stars. We find that individual cluster members can accrete drastically more than they would in isolation, in particular when the flow is able to cool efficiently. This effect could potentially modify the luminosity of accreting compact objects in star clusters and could lead to the rejuvenation of young star clusters as well as globular clusters with low-inclination and low-eccentricity.Comment: 15 pages, 12 figures. Accepted to Ap

Numerical simulations of the random angular momentum in convection II: delayed explosions of red supergiants following "failed'' supernovae

When collapse of the iron core in a massive red or yellow supergiant does not lead to a neutrino-driven explosion, a significant fraction of the convective hydrogen envelope will fall in towards the black hole formed from the collapsing core. The random velocity field in the convective envelope results in finite specific angular momentum in each infalling shell. Using 3D hydrodynamical simulations, we follow the infall of this material to small radii, resolving the circularization radii of the flow. We show that infall of the convective envelope leads to nearly complete envelope ejection in a $\gtrsim$ 10$^{48}$ erg explosion with outflow speeds of $\gtrsim$ 200 km/s. The light curve of such an explosion would show a characteristic, red plateau as the ejecta cools and a hydrogen recombination front recedes through the expanding ejecta. Adopting supernova IIp scalings, the event would have a plateau luminosity of $\gtrsim$ 10$^{40}$ erg/s and a duration of several hundreds of days. These events would appear quite similar to luminous red novae with red or yellow supergiant progenitors; some luminous red novae may, in fact, be signposts of black hole formation. The mechanism studied here produces more energetic explosions than the weak shock generated from the radiation of neutrino energy during the proto-neutron star phase. Because we cannot simulate all the way to the horizon, our results are likely lower limits on the energy and luminosity of transients produced during the collapse of a red or yellow supergiant to form a black hole.Comment: 18 pages, 13 figures. Submitted to MNRAS. Comments welcom

Accretion Disk Assembly During Common Envelope Evolution: Implications for Feedback and LIGO Binary Black Hole Formation

During a common envelope episode in a binary system, the engulfed companion spirals to tighter orbital separations under the influence of drag from the surrounding envelope material. As this object sweeps through material with a steep radial gradient of density, net angular momentum is introduced into the flow, potentially leading to the formation of an accretion disk. The presence of a disk would have dramatic consequences for the outcome of the interaction because accretion might be accompanied by strong, polar outflows with enough energy to unbind the entire envelope. Without a detailed understanding of the necessary conditions for disk formation during common envelope, therefore, it is difficult to accurately predict the population of merging compact binaries. This paper examines the conditions for disk formation around objects embedded within common envelopes using the `wind tunnel' formalism developed by MacLeod et al. (2017). We find that the formation of disks is highly dependent on the compressibility of the envelope material. Disks form only in the most compressible of stellar envelope gas, found in envelopes' outer layers in zones of partial ionization. These zones are largest in low-mass stellar envelopes, but comprise small portions of the envelope mass and radius in all cases. We conclude that disk formation and associated accretion feedback in common envelope is rare, and if it occurs, transitory. The implication for LIGO black hole binary assembly is that by avoiding strong accretion feedback, common envelope interactions should still result in the substantial orbital tightening needed to produce merging binaries.Comment: 12 pages, 10 figures, submitted to Ap

Impact of scene decorrelation on geosynchronous SAR data focusing

We discuss the effects of the clutter on geosynchronous SAR systems exploiting long integration times (from minutes to hours) to counteract for two-way propagation losses and increase azimuth resolution. Only stable targets will be correctly focused whereas unstable targets will spread their energy along azimuth direction. We derive here a generic model for the spreading of the clutter energy based on the power spectral density of the clutter itself. We then assume the Billingsley Intrinsic Clutter Motion model, representing the clutter power spectrum as an exponential decay, and derive the expected GEOSAR signal-to-clutter ratio. We also provide some results from a Ground Based RADAR experiment aimed at assessing the long-term clutter statistics for different scenarios to complement the Internal Clutter Motion model, mainly derived for windblown trees. Finally, we discuss the expected performances of two GEOSAR systems with different acquisition geometries.Peer ReviewedPostprint (published version

Canonical solution of a system of long-range interacting rotators on a lattice

The canonical partition function of a system of rotators (classical X-Y spins) on a lattice, coupled by terms decaying as the inverse of their distance to the power alpha, is analytically computed. It is also shown how to compute a rescaling function that allows to reduce the model, for any d-dimensional lattice and for any alpha<d, to the mean field (alpha=0) model.Comment: Initially submitted to Physical Review Letters: following referees' Comments it has been transferred to Phys. Rev. E, because of supposed no general interest. Divided into sections, corrections in (5) and (20), reference 5 updated. 8 pages 1 figur

A territorial scale-based model for Euroregions and its implications for Cross-Border Cooperation in maritime contexts

This article attempts to advance the theoretical debate on Euroregions by focusing on their governance structures and the territorial scale of their activities. It develops around a sample of 60 Euroregions and as many corresponding INTERREG projects provided by previous research on Euroregional practices (Durà et al., 2018). Firstly, it elaborates a Euroregional model for the classification of these cross-border governance structures by focusing on i) a territorial scale-based typology of the cross-border cooperation activities performed and ii) the institutional density and level of self-government of the Euroregional actors involved. Secondly, theoretical contributions are applied in the case of territories mainly focused around maritime borders. By exploiting the results of the research, the article strongly suggests the exploitation of supralocal and regional partnerships alongside employing multilevel Euroregions in cross-sea cooperation.Aquest article pretén avançar en el debat teòric sobre euroregions a través de l'anàlisi de les seves estructures de governança i de l'escala territorial de les seves activitats. L'argumentació es desenvolupa amb un estudi de seixanta euroregions i d'un nombre corresponent de projectes Interreg seleccionats en investigacions anteriors (Durà et al., 2018). Primer, s'intenta elaborar un model euroregional per a la classificació d'aquestes estructures de governança transfronterera a través d'una tipologia de classificació d'activitats de cooperació transfronterera a escala territorial, i a través de l'anàlisi de la densitat institucional i dels nivells administratius de govern dels actors involucrats. Segon, les contribucions teòriques d'aquest article s'apliquen en el context de territoris separats per una frontera marítima. A partir de l'explotació de resultats, l'article argumenta a favor d'activitats d'escala supralocal i regional al costat d'euroregions de tipus multinivell per a la cooperació transfronterera en contextos marítims.Este artículo pretende avanzar en el debate teórico sobre eurorregiones a través del análisis de sus estructuras de gobernanza y de la escala territorial de sus actividades. La argumentación se desarrolla mediante un estudio de sesenta eurorregiones y de un número correspondiente de proyectos Interreg seleccionados en investigaciones anteriores (Durà et al., 2018). Primero, se intenta elaborar un modelo eurorregional para la clasificación de estas estructuras de gobernanza transfronteriza a través de una tipología de clasificación de actividades de cooperación transfronteriza a escala territorial, y a través del análisis de la densidad institucional y de los niveles administrativos de gobierno de los actores involucrados. Segundo, las contribuciones teóricas de este artículo se aplican en el contexto de territorios separados por una frontera marítima. A partir de la explotación de resultados, el artículo argumenta en favor de actividades de escala supralocal y regional, además de adoptar eurorregiones de tipo multinivel para la cooperación transfronteriza en contextos marítimos.Cet article se donne pour objectif d'avancer dans le débat théorique sur les Eurorégions en mettant l'accent sur leurs structures de gouvernance et sur l'ampleur territoriale de leurs activités. Il se développe autour d'une étude portant sur un échantillon de 60 Eurorégions et autant de projets INTERREG correspondants, fournis par des recherches antérieures sur les pratiques eurorégionales (Durà et al., 2018). En premier lieu, l'article propose un modèle eurorégional pour la classification de ces structures de gouvernance transfrontalières en se concentrant sur : a. une typologie à l'échelle territoriale des activités de coopération transfrontalière réalisées, et b. la densité institutionnelle et le niveau d'autonomie des acteurs eurorégionaux impliqués. En second lieu, les contributions théoriques sont appliquées au cas de territoires principalement centrés sur les frontières maritimes. En exploitant les résultats de la recherche, l'article suggère fortement l'emploi d'activités à une échelle supralocale et régionale, et suggère d'employer des Eurorégions de type multiniveaux dans la coopération transfrontalière maritime

IIP or Not IIP: Theoretical Models of Delayed Explosions of Red Supergiants Following "Failed" Supernovae

A red supergiant (RSG) is the most common manifestation of a massive star at the end of its life.When collapse of the iron core of the star leads to a successful supernova (SN) explosion, the observational signature is a Type IIP SN, which is the most common type of SN. The successful explosion of a RSG usually leaves behind a neutron star while the rest of the enriched stellar matter is returned to the surrounding environment. The SN mechanism can fail in a reasonable fraction of stellar deaths. In that case, the core unavoidably forms a black hole (BH) that consumes the inner, metal-rich layers of the star and a large fraction of the extended, convective hydrogen envelope remains bound to the BH. This thesis considers the subsequent evolution of the BH and envelope following a “failed” SN including the critical questions of whether there is still an explosion or observable transient and how much mass the BH is ultimately able to accrete. In Chs. 2 and 3, I use three-dimensional simulations to show that the convective envelope of a RSG has too much turbulent angular momentum to accrete and that infall of the turbulent material leads to a ∼1e48 erg explosion of star and associated transient resembling a luminous red nova. Magnetic fields are important in the envelopes of RSGs but we do not know how they will affect the accretion flow following a failed SN. In Ch. 4, I show that the magnetic fields in the envelope are tangled with strengths of the order of hundreds of Gauss, but grow more shallowly during collapse than r^−2 such that angular momentum and magnetic fields became dynamically important at similar radii. In the final chapter, I discuss my work modeling the light curves of failed supernovae. This work will apply to a wider range of scenarios in which mass is stripped from the envelopes of evolved massive stars. An important application of this work is to differentiate between luminous red novae of binary-merger origin and those arising from BH birth in failed SN. Overall, this work is important for contextualizing the upcoming LSST survey on the Rubin Observatory, which will observe 100s–1000 times more luminous red novae than the Zwicky Transient Facility. In addition, these cooler explosions self-enshroud as they form dust; detailed modeling of these events will help leverage the infrared transient sky made accessible by WINTER, DREAMS, JWST, and Roman