arXiv subjects

Nathan Smith

Publications and source records attributed to Nathan Smith.

At least 325 records · Page 18Linked to original sources

arXiv2007

Galactic Twins of the Ring Nebula Around SN1987A and a Possible LBV-like Phase for Sk-69 202

Some core-collapse supernovae show clear signs of interaction with dense circumstellar material that often appears to be non-spherical. Circumstellar nebulae around supernova progenitors provide clues to the origin of that asymmetry in immediate pre-supernova evolution. Here I discuss outstanding questions about the formation of the ring nebula around SN1987A and some implications of similar ring nebulae around Galactic B supergiants. Several clues hint that SN1987A's nebula may have been ejected in an LBV-like event, rather than through interacting winds in a transition from a red supergiant to a blue supergiant.

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arXiv2007

And in the Darkness Bind Them: Equatorial Rings, B[e] Supergiants, and the Waists of Bipolar Nebulae

We report the discovery of two new circumstellar ring nebulae in the western Carina Nebula. The brighter object, SBW1, resembles a lidless staring eye and encircles a B1.5 Iab supergiant. Its size is identical to the inner ring around SN1987A, but SBW1's low N abundance indicates that the star didn't pass through a RSG phase. The fainter object, SBW2, is a more distorted ring, is N-rich, and has a central star that seems to be invisible. We discuss these two new nebulae in context with rings around SN1987A, Sher25, HD168625, RY Scuti, WeBo1, SuWt2, and others. The ring bearers fall into two groups: Five rings surround hot supergiants, and all except for the one known binary are carbon copies of the ring around SN1987A. We propose a link between these rings and B[e] supergiants, where the rings derive from the same material in an earlier B[e] phase. The remaining four rings surround evolved intermediate-mass stars; all members of this ring fellowship are close binaries, hinting that binary interactions govern the forging of such rings. We estimate that there may be several thousand more dark rings in the Galaxy, but we are scarcely aware of their existence due to selection effects. The lower-mass objects might be the equatorial density enhancements often invoked to bind the waists of bipolar PNe.

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arXiv2007

The Structure of the Homunculus. III. Forming a Disk and Bipolar Lobes in a Rotating Surface Explosion

We present a semi-analytic model for shaping the nebula around eta Carinae that accounts for the simultaneous production of bipolar lobes and an equatorial disk through a rotating surface explosion. Material is launched normal to the surface of an oblate rotating star with an initial kick velocity that scales approximately with the local escape speed. Thereafter, ejecta follow ballistic orbital trajectories, feeling only a central force corresponding to a radiatively reduced gravity. Our model is conceptually similar to the wind-compressed disk model of Bjorkman & Cassinelli, but we modify it to an explosion instead of a steady line-driven wind, we include a rotationally-distorted star, and we treat the dynamics somewhat differently. Continuum-driving avoids the disk inhibition that normally operates in line-driven winds. Our model provides a simple method by which rotating hot stars can simultaneously produce intrinsically bipolar and equatorial mass ejections, without an aspherical environment or magnetic fields. Although motivated by eta Carinae, the model may have generic application to other LBVs, B[e] stars, or SN1987A's nebula. When near-Eddington radiative driving is less influential, our model generalizes to produce bipolar morphologies without disks, as seen in many PNe.

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arXiv2007

A Census of the Carina Nebula -- II. Energy Budget and Global Properties of the Nebulosity

The first paper in this series took a direct census of energy input from the known OB stars in the Carina Nebula, and in this paper we study the global properties of the surrounding nebulosity. We find that the total IR luminosity of Carina is about 1.2E7 Lsun, accounting for only about 50-60% of the known stellar luminosity from Paper I. Similarly, the ionizing photon luminosity -- (abridged; many important details omitted). Synchronized star formation around the periphery of Carina provides a strong case that star formation here was indeed triggered by stellar winds and UV radiation. This second generation appears to involve a cascade toward preferentially intermediate- and low-mass stars, but this may soon change when eta Car and its siblings explode. If the current reservoir of atomic and molecular gas can be tapped at that time, massive star formation may be rejuvinated around the periphery of Carina much as if it were a young version of Gould's Belt. Also, when these multiple SNe occur, the triggered second generation will be pelted repeatedly with SN ejecta bearing short-lived radioactive nuclides. Carina may therefore represent the most observable analog to the cradle of our own Solar System.

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arXiv2007

SN 2006jc: A Wolf-Rayet Star Exploding in a Dense He-Rich Circumstellar Medium

We present optical photometry and spectra of the peculiar Type Ib supernova (SN) 2006jc. Strong and relatively narrow He I emission lines indicate the progenitor star exploded inside a dense circumstellar medium (CSM) rich in He. An exceptionally blue apparent continuum persists from our first spectrum obtained 15 days after discovery through our last spectrum ~1 month later. Based on the presence of isolated Fe II emission lines, we interpret the blue "continuum" as blended, perhaps fluorescent, Fe emission. One or two of the reddest He I line profiles in our spectra are double peaked, suggesting that the CSM has an aspherical geometry. The He I lines that are superposed on the blue continuum show P-Cygni profiles, while the redder He I lines do not, implying that the blue continuum also originates from an asymmetric mass distribution. The He-rich CSM, aspherical geometry, and line velocities indicate that the progenitor star was a WNE Wolf-Rayet (WR) star. A recent (2 years before the SN), coincident, luminous outburst similar to those seen in luminous blue variables (LBVs) is the leading candidate for the dense CSM. Such an eruption associated with a WR star has not been seen before, indicating that the progenitor star may have recently transitioned from the LBV phase. We also present unpublished spectral and photometric data on SN 2002ao which, along with SN 1999cq, is very similar to SN 2006jc. We propose that these three objects may represent a new and distinct class of SNe arising from WR progenitors surrounded by a dense CSM.

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arXiv2006

The Structure of the Homunculus. II. Modeling the physical conditions in Eta Car's molecular shell

We present models that reproduce the observed double-shell structure of the Homunculus Nebula around eta Carinae, including the stratification of infrared H2 and [FeII] emission seen in data obtained with the Phoenix spectrograph on Gemini South, as well as the corresponding stratified grain temperature seen in thermal-infrared data. Tuning the model to match the observed shell thickness allows us to determine the threshold density which permits survival of H2. An average hydrogen density of n_H=(0.5-1)x10^7 cm-3 in the outer zone is required to allow H2 to exist at all latitudes in the nebula, and for Fe+ to recombine. This gives independent confirmation of the very large mass of the Homunculus, indicating a total of roughly 15--35 Msun (although we note reasons why the lower end of this range is favored). At the interface between the atomic and molecular zones, we predict a sharp drop in the dust temperature, in agreement with the bimodal dust color temperatures observed in the two zones. In the outer molecular shell, the dust temperature drops to nearly the blackbody temperature, and becomes independent of grain size because of self-shielding at shorter UV wavelengths and increased heating at longer wavelengths. This relaxes constraints on large grain sizes suggested by near-blackbody color temperatures. Finally, from the strength of infrared [FeII] emission in the inner shell we find that the gas-phase Fe abundance is roughly solar. This is astonishing in such a dusty object, where one normally expects gaseous iron to be depleted by two orders of magnitude.

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arXiv2006

Giant Outbursts of the Eta Carinae-P Cygni Type

I discuss the role of short-duration eruptive mass loss in the evolution of very massive stars. Giant eruptions of Luminous Blue Variables (LBVs) like the 19th century event of $η$ Carinae can remove large quantities of mass almost instantaneously, making them significant in stellar evolution. They can potentially remove more mass from the star than line-driven winds, especially if winds are clumped such that O star mass-loss rates need to be revised downward. When seen in other galaxies as ``supernova impostors'', these eruptions typically last for less than a decade, and they can remove of order 10 M$_{\odot}$ as indicated by massive nebulae around LBVs. Such extreme mass-loss rates cannot be driven by radiation pressure on spectral lines. Instead, these outbursts must either be continuum-driven super-Eddington winds or outright hydrodynamic explosions, both of which are insensitive to metallicity. As such, this eruptive mode of mass loss could play a pivotal role for massive metal-poor stars in the early universe.

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arXiv2006

Eruptive Mass Loss in Very Massive Stars and Population III Stars

I discuss the role played by short-duration eruptive mass loss in the evolution of very massive stars. Giant eruptions of Luminous Blue Variables (LBVs) like the 19th century event of eta Carinae can remove large quantities of mass almost instantaneously, making them significant in stellar evolution. They can potentially remove much more mass from the star than line-driven winds, especially if stellar winds are highly clumped such that previous estimates of O star mass-loss rates need to be revised downward. When seen in other galaxies as ``supernova impostors'', these LBV eruptions typically last for less than a decade, and they can remove of order 10 Msun as indicated by massive nebulae around LBVs. Such extreme mass-loss rates cannot be driven by radiation pressure on spectral lines, because the lines will completely saturate during the events. Instead, these outbursts must either be continuum-driven super-Eddington winds or outright hydrodynamic explosions, both of which are insensitive to metallicity. As such, this eruptive mode of mass loss could also play a pivotal role in the evolution and ultimate fate of massive metal-poor stars in the early universe. If they occur in these Population III stars, such eruptions would also profoundly affect the chemical yield and types of remnants from early supernovae and hypernovae thought to be the origin of long gamma ray bursts.

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arXiv2006

Spitzer Space Telescope Infrared Imaging and Spectroscopy of the Crab Nebula

We present 3.6, 4.5, 5.8, 8.0, 24, and 70 micron images of the Crab Nebula obtained with the Spitzer Space Telescope IRAC and MIPS cameras, Low- and High-resolution Spitzer IRS spectra of selected positions within the nebula, and a near-infrared ground-based image made in the light of [Fe II]1.644 micron. The 8.0 micron image, made with a bandpass that includes [Ar II]7.0 micron, resembles the general morphology of visible H-alpha and near-IR [Fe II] line emission, while the 3.6 and 4.5 micron images are dominated by continuum synchrotron emission. The 24 micron and 70 micron images show enhanced emission that may be due to line emission or the presence of a small amount of warm dust in the nebula on the order of less than 1% of a solar mass. The ratio of the 3.6 and 4.5 micron images reveals a spatial variation in the synchrotron power law index ranging from approximately 0.3 to 0.8 across the nebula. Combining this information with optical and X-ray synchrotron images, we derive a broadband spectrum that reflects the superposition of the flatter spectrum jet and torus with the steeper diffuse nebula, and suggestions of the expected pileup of relativistic electrons just before the exponential cutoff in the X-ray. The pulsar, and the associated equatorial toroid and polar jet structures seen in Chandra and HST images (Hester et al. 2002) can be identified in all of the IRAC images. We present the IR photometry of the pulsar. The forbidden lines identified in the high resolution IR spectra are all double due to Doppler shifts from the front and back of the expanding nebula and give an expansion velocity of approximately 1264 km/s.

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arXiv2006

On the role of continuum-driven eruptions in the evolution of very massive stars and Population III stars

We suggest that the mass lost during the evolution of very massive stars may be dominated by optically thick, continuum-driven outbursts or explosions, instead of by steady line-driven winds. In order for a massive star to become a WR star, it must shed its H envelope, but new estimates of the effects of clumping in winds indicate that line driving is vastly insufficient. We discuss massive stars above roughly 40-50 Msun, for which the best alternative is mass loss during brief eruptions of luminous blue variables (LBVs). Our clearest example of this phenomenon is the 19th century outburst of eta Car, when the star shed 12-20 Msun or more in less than a decade. Other examples are circumstellar nebulae of LBVs, extragalactic eta Car analogs (``supernova impostors''), and massive shells around SNe and GRBs. We do not yet fully understand what triggers LBV outbursts, but they occur nonetheless, and present a fundamental mystery in stellar astrophysics. Since line opacity from metals becomes too saturated, the extreme mass loss probably arises from a continuum-driven wind or a hydrodynamic explosion, both of which are insensitive to metallicity. As such, eruptive mass loss could have played a pivotal role in the evolution and fate of massive metal-poor stars in the early universe. If they occur in these Population III stars, such eruptions would profoundly affect the chemical yield and types of remnants from early SNe and hypernovae.

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arXiv2006

Cleaning up Eta Carinae: Detection of Ammonia in the Homunculus

We report the first detection of ammonia in the Homunculus nebula around eta Carinae, which is also the first detection of emission from a polyatomic molecule in this or any other luminous blue variable (LBV) nebula. Observations of the NH3 (J,K)=(3,3) inversion transition made with the Australia Telescope Compact Array reveal emission at locations where infrared H2 emission had been detected previously, near the strongest dust emission in the core of the Homunculus. We also detect ammonia emission from the so-called ``strontium filament'' in the equatorial disk. The presence of NH3 around eta Car hints that molecular shells around some Wolf-Rayet stars could have originated in prior LBV eruptions, rather than in cool red supergiant winds or the ambient interstellar medium. Combined with the lack of any CO detection, NH3 seems to suggest that the Homunculus is nitrogen rich like the ionized ejecta around eta Car. It also indicates that the Homunculus is a unique laboratory in which to study unusual molecule and dust chemistry, as well as their rapid formation in a nitrogen-rich environment around a hot star. We encourage future observations of other transitions like NH3 (1,1) and (2,2), related molecules like N2H+, and renewed attempts to detect CO.

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arXiv2006

Silicate Emission Profiles from Low-Mass Protostellar Disks in the Orion Nebula: Evidence for Growth and Thermal Processing of Grains

We present 8--13 micron low resolution spectra (R~100) of 8 low-mass protostellar objects ("proplyds") in the Orion Nebula using the Long Wavelength Spectrometer (LWS) at the W. M. Keck Observatory. All but one of the sources in our sample show strong circumstellar silicate emission, with profiles that are qualitatively similar to those seen in some T Tauri and Herbig Ae/Be stars. The silicate profile in all cases is significantly flattened compared to the profile for typical interstellar dust, suggesting that the dominant emitting grains are significantly larger than those found in the interstellar medium. The 11.3-to-9.8 micron flux ratio--often used as an indicator of grain growth--is in the 0.8 to 1.0 range for all of our targets, indicating that the typical grain size is around a few microns in the surface layers of the attendant circumstellar disk for each object. Furthermore, the silicate profiles show some evidence of crystalline features, as seen in other young stellar objects. The results of our analysis show that the grains in the photoevaporating protostellar disks of Orion have undergone significant growth and perhaps some annealing, suggesting that grain evolution for these objects is not qualitatively different from other young stellar objects.

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arXiv2006

Evolution of the Reverse Shock Emission from SNR 1987A

We present new (2004 July) G750L and G140L Space Telescope Imaging Spectrograph (STIS) data of the H-alpha and Ly-alpha emission from supernova remnant (SNR) 1987A. With the aid of earlier data, from Oct 1997 to Oct 2002, we track the local evolution of Ly-alpha emission and both the local and global evolution of H-alpha emission. In addition to emission which we can clearly attribute to the surface of the reverse shock, we also measure comparable emission, in both H-alpha and Ly-alpha, which appears to emerge from supernova debris interior to the surface. New observations taken through slits positioned slightly eastward and westward of a central slit show a departure from cylindrical symmetry in the H-alpha surface emission. Using a combination of old and new observations, we construct a light curve of the total H-alpha flux, F, from the reverse shock, which has increased by a factor ~ 4 over about 8 years. However, due to large systematic uncertainties, we are unable to discern between the two limiting behaviours of the flux - F ~ t (self-similar expansion) and F ~ t^5 (halting of the reverse shock). Such a determination is relevant to the question of whether the reverse shock emission will vanish in less than about 7 years (Smith et al. 2005). Future deep, low- or moderate-resolution spectra are essential for accomplishing this task.

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arXiv2006

The Structure of the Homunculus: I. Shape and Latitude Dependence from H2 and [Fe II] Velocity Maps of Eta Carinae

High resolution long-slit spectra obtained with the Phoenix spectrograph on Gemini South provide our most accurate probe of the three dimensional structure of the Homunculus around eta Car. The new near-infrared spectra dramatically confirm the double-shell structure inferred previously from thermal dust emission, resolving the nebula into a very thin outer shell seen in H2 21218, and a warmer, thicker inner layer seen in [Fe II] 16435. The thin H2 skin hints that the most important mass loss during the 19th century eruption had a very short duration of less than 5 yr. H2 emission traces the majority of the mass in the nebula, and has an average density of order 10^6.5 cm-3. This emission, in turn, yields our first definitive picture of the exact shape of the nebula, plus a distance of 2350pm50 pc and an inclination angle of 41deg (the polar axis is tilted 49deg from the plane of the sky). The distribution of the H2 emission provides the first measure of the latitude dependence of the speed, mass loss, and kinetic energy associated with eta Car's 19th century explosion. Almost 75 percent of the total mass and more than 90 percent of the kinetic energy in the ejecta were released at high latitudes. This rules out a model for the bipolar shape wherein an otherwise spherical explosion was pinched at the waist by a circumstellar torus. Also, the ejecta could not have been deflected toward polar trajectories by a companion star, since the kinetic energy of the polar ejecta is greater than the binding energy of the putative binary system. Instead, most of the mass appears to have been directed poleward by the explosion itself. [abridged]

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arXiv2006

The UV Scattering Halo of the Central Source Associated with Eta Carinae

We have made an extensive study of the UV spectrum of Eta Carinae, and find that we do not directly observe the star and its wind in the UV. Because of dust along our line of sight, the UV light that we observe arises from bound-bound scattering at large impact parameters. We obtain a reasonable fit to the UV spectrum by using only the flux that originates outside 0.033". This explains why we can still observe the primary star in the UV despite the large optical extinction -- it is due to the presence of an intrinsic coronagraph in the Eta Carinae system, and to the extension of the UV emitting region. It is not due to peculiar dust properties alone. We have computed the spectrum of the purported companion star, and show that it could only be directly detected in the UV spectrum preferentially in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectral region (912-1175 Ang.). However, we find no direct evidence for a companion star, with the properties indicated by X-ray studies and studies of the Weigelt blobs, in UV spectra. This might be due to reprocessing of the companion's light by the dense stellar wind of the primary. Broad FeII and [FeII] emission lines, which form in the stellar wind, are detected in spectra taken in the SE lobe, 0.2" from the central star. The wind spectrum shows some similarities to the spectra of the B & D Weigelt blobs, but also shows some marked differences in that high excitation lines, and lines pumped by Ly-alpha, are not seen. The detection of the broad lines lends support to our interpretation of the UV spectrum, and to our model for Eta Carinae.

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arXiv2006

A Census of the Carina Nebula. I: Cumulative Energy Input from Massive Stars

(abridged) The Carina Nebula (NGC 3372) is our richest nearby laboratory in which to study feedback through UV radiation and stellar winds from very massive stars during the formation of an OB association before supernova explosions have influenced the environment. This paper takes inventory of the combined effect from all the known massive stars that power the Carina Nebula. The present-day total ionizing photon luminosity produced by the 65 O stars and 3 WNL stars in Carina is about 1e51/s, the total bolometric luminosity of all stars earlier than B2 is 2.5e7 Lsun, and the total mechanical luminosity of stellar winds is about 1e5 Lsun. The total ionizing flux was about 25 per cent higher when eta Car was on the main sequence, before it and its companion were surrounded by its obscuring dust shell, and when the 3 WNL stars were on the main sequence. For the first 3 Myr, the net ionizing flux of the 70 O-type stars in Carina was about 150 times greater than in the Orion Nebula. About 400-500 Msun has been contributed to the region by stellar wind mass loss during the past 3 Myr. The net ionizing flux and mechanical luminosity are also given for the individual clusters Tr14, 15, and 16, and Bo10 and 11, which are relevant on smaller spatial scales.

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arXiv2005

The Limits of Motion Prediction Support for Ad hoc Wireless Network Performance

A fundamental understanding of gain provided by motion prediction in wireless ad hoc routing is currently lacking. This paper examines benefits in routing obtainable via prediction. A theoretical best-case non-predictive routing model is quantified in terms of both message overhead and update time for non-predictive routing. This best- case model of existing routing performance is compared with predictive routing. Several specific instances of predictive improvements in routing are examined. The primary contribution of this paper is quantification of predictive gain for wireless ad hoc routing.

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arXiv2005

Infrared [Fe II] emission from P Cygni's nebula: Atomic data, mass, kinematics, and the 1600 A.D. outburst

We present moderate and high-dispersion 1-2.5 micron spectra of the 10 arcsec nebula around P Cygni, dominated by bright emission lines of [Fe II]. Observed [Fe II] line ratios disagree with theoretical transition rates in the literature, so we use the spectrum of P Cygni's nebula to constrain the atomic data for low-lying levels of [Fe II]. Of particular interest is the ratio [Fe II] 12567/16435, often used as a reddening indicator, for which we empirically derive an intrinsic value of 1.49, which is 10--40 per cent higher than previous estimates. High-dispersion spectra of [Fe II] 16435 constrain the geometry, detailed structure, and kinematics of P Cygni's nebula, which is the major product of P Cygni's outburst in 1600 A.D. We use the [N II]/[N I] line ratio to conclude that the nebula is mostly ionized with a total mass of ~0.1 Msun; more than the mass lost by the stellar wind since the eruption. For this mass, we would expect a larger infrared excess than observed. We propose that the dust which obscured the star after the outburst has since been largely destroyed, releasing Fe into the gas phase to produce the bright [Fe II] emission. The kinetic energy of this shell is ~2e46 ergs, far less than the kinetic energy released during the giant eruption of eta Car in the 1840s. The mass and kinetic energy in the nebulae of eta Car and P Cygni give insight to the range of parameters expected for extragalactic eta Car-like eruptions.

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