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Search Results: 1 - 10 of 13572 matches for " Nathan Smith "
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Nathan Smith
Revista mexicana de astronomía y astrofísica , 2008,
Abstract: Algunas supernovas de colapso de n′ucleo muestran se nales evidentes de interacci′on asim′etrica con el material circunestelar denso. Las nebulosas circunestelares alrededor de progenitores de supernova proveen informaci′on acerca del origen de dicha falta de simetr′ a en la evoluci′on inmediata de la pre-supernova. En esta contribuci′on discutir′e las preguntas sobresalientes respecto a la formaci′on de la nebulosa anular alrededor de SN1987A y las implicaciones de la presencia de nebulosas anulares similares alrededor de supergigantes B de nuestra Galaxia. Varios indicios sugieren que la nebulosa de SN1987A puede haber sido eyectada en un evento de tipo LBV en vez de a trav′es de vientos interactuantes en una transici′on de supergigante roja a supergigante azul
All Things Homunculus
Nathan Smith
Physics , 2009,
Abstract: The ``Homunculus'' nebula around Eta Carinae is one of our most valuable tools for understanding the extreme nature of episodic pre-supernova mass loss in the most massive stars, perhaps even more valuable than the historical light curve of eta Car. As a young nebula that is still in free expansion, it bears the imprint of its ejection physics, making it a prototype for understanding the bipolar mass loss that is so common in astrophysics. The high mass and kinetic energy of the nebula provide a sobering example of the extreme nature of stellar eruptions in massive stars near the Eddington limit. The historical ejection event was observed, and current parameters are easily measured due to its impressive flux at all wavelengths, so the Homunculus is also a unique laboratory for studying rapid dust formation and molecular chemistry, unusual ISM abundances, and spectroscopy of dense gas. Since it is relatively nearby and bright and is expanding rapidly, its 3-D geometry, kinematics, and detailed structure can be measured accurately, providing unusually good quantitative constraints on the physics that created these structures. In this chapter I review the considerable recent history of observational and theoretical study of the Homunculus nebula, and I provide an up-to-date summary of our current understanding, as well as areas that need work.
Dissecting the Homunculus nebula around Eta Carinae with spatially resolved near-infrared spectroscopy
Nathan Smith
Physics , 2002, DOI: 10.1046/j.1365-8711.2002.05966.x
Abstract: Near-infrared emission lines provide unique diagnostics of the geometry, structure, kinematics, and excitation of eta Carinae's circumstellar ejecta, and give clues to the nature of its wind. The infrared spectrum is a strong function of position in eta Car's nebula, with a mix of intrinsic and reflected emission. Molecular hydrogen traces cool gas and dust in the polar lobes, while [Fe II] blankets their inner surfaces. These lines reveal the back wall of the SE polar lobe for the first time, and give the clearest picture yet of the 3-D geometry. Additionally, collisionally-excited [Fe II] reveals the kinematic structure of a recently discovered `Little Homunculus' expanding inside the larger one. Equatorial gas in the `Fan', on the other hand, shows a spectrum indicating recombination and fluorescent pumping. Some equatorial ejecta glow in the He I 10830 line, showing evidence for material ejected in the 1890 outburst of eta Car. Closer to the star, the compact `Weigelt blobs' are marginally resolved, allowing their infrared spectrum to be separated from the star for the first time. In general, infrared spectra reveal a coherent, directional dependence of excitation in the Homunculus: polar ejecta are collisionally excited, whereas equatorial ejecta are dominated by fluorescence and normal photoexcitation. These are important clues to the geometry of the central star's UV radiation field. Reflected near-infrared emission lines also reveal interesting latitudinal dependence in the stellar wind.
Mass Loss: Its Effect on the Evolution and Fate of High-Mass Stars
Nathan Smith
Physics , 2014, DOI: 10.1146/annurev-astro-081913-040025
Abstract: Our understanding of massive star evolution is in flux, due to recent upheavals in our view of mass loss, and observations of a high binary fraction among O-type stars. Mass-loss rates for standard metallicity-dependent winds of hot stars are now thought to be lower by a factor of 2-3 compared to rates adopted in modern stellar evolution models, due to the influence of clumping. Weaker line-driven winds shift the burden of H-envelope removal elsewhere, so that the dominant modes of mass loss are the winds, pulsations, and eruptions of evolved supergiants, as well as binary mass transfer. Studies of stripped-envelope supernovae, in particular, require binary mass transfer. Dramatic examples of eruptive mass loss are seen in Type IIn supernovae, which have massive shells ejected just a few years before core collapse. The shifting emphasis from steady winds to episodic mass loss is a major change for low-metallicity regions, since eruptions and binary mass transfer are less sensitive to metallicity. We encounter the predicament that the most important modes of mass loss are also the most uncertain, undermining the predictive power of single-star evolution models beyond core H burning. Moreover, the influence of winds and rotation in models has been evaluated by testing single-star models against observed statistics that, as it turns out, are heavily influenced by binary evolution. This alters our view about the most basic outcomes of massive-star mass loss --- are WR stars and SNe Ibc the products of single-star winds, or are they mostly the result of binary evolution and eruptive mass loss? This paradigm shift has far-reaching impact on a number of other areas of astronomy. (abridged)
Episodic Post-Shock Dust Formation in the Colliding Winds of Eta Carinae
Nathan Smith
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15901.x
Abstract: Eta Carinae shows broad peaks in near-infrared (IR) JHKL photometry, roughly correlated with times of periastron passage in the eccentric binary system. After correcting for secular changes attributed to reduced extinction from the thinning Homunculus Nebula, these peaks have IR spectral energy distributions (SEDs) consistent with emission from hot dust at 1400-1700 K. The excess SEDs are clearly inconsistent, however, with the excess being entirely due to free-free wind or photospheric emission. One must conclude, therefore, that the broad near-IR peaks associated with Eta Carinae's 5.5 yr variability are due to thermal emission from hot dust. I propose that this transient hot dust results from episodic formation of grains within compressed post-shock zones of the colliding winds, analogous to the episodic dust formation in Wolf-Rayet binary systems like WR140 or the post-shock dust formation seen in some supernovae like SN2006jc. This dust formation in Eta Carinae seems to occur preferentially near and after periastron passage; near-IR excess emission then fades as the new dust disperses and cools. With the high grain temperatures and Eta Car's C-poor abundances, the grains are probably composed of corundum or similar species that condense at high temperatures, rather than silicates or graphite. Episodic dust formation in Eta Car's colliding winds significantly impacts our understanding of the system, and several observable consequences are discussed.
Explosions Triggered by Violent Binary-Star Collisions: Application to Eta Carinae and other Eruptive Transients
Nathan Smith
Physics , 2010, DOI: 10.1111/j.1365-2966.2011.18607.x
Abstract: This paper discusses a model where a violent periastron collision of stars in an eccentric binary system induces an eruption or explosion seen as a brief transient source, attributed to LBVs, SN impostors, or other transients. The key ingredient is that an evolved primary increases its photospheric radius on relatively short timescales, to a point where the radius is comparable to or larger than the periastron separation in an eccentric binary. In such a configuration, a violent and sudden collision would ensue, possibly leading to substantial mass ejection instead of a binary merger. Repeated periastral grazings in an eccentric system could quickly escalate to a catastrophic encounter, wherein the companion star actually plunges deep inside the photosphere of a bloated primary during periastron, as a result of the primary star increasing its own radius. This is motivated by the case of $\eta$ Carinae, where such a collision must have occured if conventional estimates of the present-day orbit are correct, and where brief peaks in the light curve coincide with periastron. Stellar collisions may explain brief recurring LBV outbursts like SN 2000ch and SN 2009ip, and perhaps outbursts from relatively low-mass progenitor stars (collisons are not necessarily the exclusive domain of very luminous stars). Finally, mass ejections induced repeatedly at periastron cause orbital evolution; this may explain the origin of very eccentric colliding-wind Wolf-Rayet binaries such as WR140.
Eruptive Outflow Phases of Massive Stars
Nathan Smith
Physics , 2010, DOI: 10.1017/S1743921311011458
Abstract: I review recent progress on understanding eruptions of unstable massive stars, with particular attention to the diversity of observed behavior in extragalatic optical transient sources that are generally associated with giant eruptions of luminous blue variables (LBVs). These eruptions are thought to represent key mass loss episodes in the lives of massive stars. I discuss the possibility of dormant LBVs and implications for the duration of the greater LBV phase and its role in stellar evolution. These eruptive variables show a wide range of peak luminosity, decay time, expansion speeds, and progenitor luminosity, and in some cases they have been observed to suffer multiple eruptions. This broadens our view of massive star eruptions compared to prototypical sources like Eta Carinae, and provides important clues for the nature of the outbursts. I also review and discuss some implications about the possible physical mechanisms involved, although the cause of the eruptions is not yet understood.
Circumstellar Material Around Evolved Massive Stars
Nathan Smith
Physics , 2010,
Abstract: I review multiwavelength observations of material seen around different types of evolved massive stars (i.e. red supergiants, yellow hypergiants, luminous blue variables, B[e] supergiants, and Wolf-Rayet stars), concentrating on diagnostics of mass, composition, and kinetic energy in both local and distant examples. Circumstellar material has significant implications for the evolutionary state of the star, the role of episodic mass loss in stellar evolution, and the roles of binarity and rotation in shaping the ejecta. This mass loss determines the type of supernova that results via the stripping of the star's outer layers, but the circumstellar gas can also profoundly influence the immediate pre-supernova environment. Dense circumstellar material can actually change the type of supernova that is seen when it is illuminated by the supernova or heated by the blast wave. As such, unresolved circumstellar material illuminated by distant supernovae can provide a way to study mass loss in massive stars in distant environments.
A Model for the 19th Century Eruption of Eta Carinae: CSM Interaction Like a Scaled-Down Type IIn Supernova
Nathan Smith
Physics , 2012, DOI: 10.1093/mnras/sts508
Abstract: This paper proposes a simple model for the 19th century eruption of Eta Carinae that consists of two components: (1) a strong wind (MdotM=0.33 Msun/yr; v=200 km/s), blowing for 30 years, followed by (2) a 1e50 erg explosion in 1844. The ensuing collision between the fast ejecta and the CSM causes an increase in brightness observed at the end of 1844, followed by a sustained high-luminosity phase lasting for 10-15 years that matches the historical light curve. The emergent luminosity is powered by CSM interaction, analogous to the process in luminous Type IIn supernovae, except with 10 times lower explosion energy and at slower speeds (causing a longer duration and lower emergent luminosity). Such an explosive event provides a natural explanation for the light curve evolution, but also accounts for a number of puzzling attributes of the Homunculus nebula: (1) rough equipartition of total radiated and kinetic energy, (2) the double-shell structure of the Homunculus, (3) the apparent single age and Hubble-like flow resulting from the thin swept-up shell, (4) the complex mottled appearance of the polar lobes in HST images, arising from Raleigh-Taylor or Vishniac instabilities, (5) efficient and rapid dust formation, as seen in Type IIn supernovae, and (6) the fast (5000 km/s) material outside the Homunculus, arising from the acceleration of the forward shock upon exiting the dense CSM. In principle, the bipolar shape has already been explained in earlier studies of interacting winds, except that here the CSM interaction occurs over only 10 years, producing a thin shell with the resulting structures then frozen-in to the expanding bipolar nebula. This self-consistent picture has a number of implications for other eruptive transients, many of which may also be powered by CSM interaction.
Observed Consequences of Presupernova Instability in Very Massive Stars
Nathan Smith
Physics , 2014, DOI: 10.1007/978-3-319-09596-7_8
Abstract: This chapter concentrates on the deaths of very massive stars, the events leading up to their deaths, and how mass loss affects the resulting death. The previous three chapters emphasized the theory of wind mass loss, eruptions, and core collapse physics, but here we emphasize mainly the observational properties of the resulting death throes. Mass loss through winds, eruptions, and interacting binaries largely determines the wide variety of different types of supernovae that are observed, as well as the circumstellar environments into which the supernova blast waves expand. Connecting these observed properties of the explosions to the initial masses of their progenitor stars is, however, an enduring challenge and is especially difficult for very massive stars. Superluminous supernovae, pair instability supernovae, gamma ray bursts, and "failed" supernovae are all end fates that have been proposed for very massive stars, but the range of initial masses or other conditions leading to each of these (if they actually occur) are still very certain. Extrapolating to infer the role of very massive stars in the early universe is essentially unencumbered by observational constraints and still quite dicey.
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