arXiv subjects

Steven N. Shore

Publications and source records attributed to Steven N. Shore.

At least 37 records · Page 2Linked to original sources

arXiv2017

A Systematic Survey of the Effects of Wind Mass Loss Algorithms on the Evolution of Single Massive Stars

Mass loss is a key uncertainty in the evolution of massive stars. Stellar evolution calculations must employ parametric algorithms for mass loss, and usually only include stellar winds. We carry out a parameter study of the effects of wind mass loss on massive star evolution using the open-source stellar evolution code MESA. We provide a systematic comparison of wind mass loss algorithms for solar-metallicity, nonrotating, single stars in the initial mass range of $15-35\,M_\odot$. We consider combinations drawn from two hot phase algorithms, three cool phase algorithms, and two Wolf-Rayet algorithms. We consider linear wind efficiency scale factors of $1$, $0.33$, and $0.1$ to account for reductions in mass loss rates due to wind inhomogeneities. We find that the initial to final mass mapping for each zero-age main-sequence (ZAMS) mass has a $\sim 50\%$ uncertainty if all algorithm combinations and wind efficiencies are considered. The ad-hoc efficiency scale factor dominates this uncertainty. While the final total mass and internal structure of our models vary tremendously with mass loss treatment, final observable parameters are much less sensitive for ZAMS mass $\lesssim 30\,M_\odot$. This indicates that uncertainty in wind mass loss does not negatively affect estimates of the ZAMS mass of most single-star supernova progenitors from pre-explosion observations. Furthermore, we show that the internal structure of presupernova stars is sensitive to variations in both main sequence and post main-sequence mass loss. We find that the compactness parameter $ξ\propto M/R(M)$ varies by as much as $30\%$ for a given ZAMS mass evolved with different wind efficiencies and mass loss algorithm combinations. [abridged]

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arXiv2016

Three-dimensional simulations of turbulent convective mixing in ONe and CO classical nova explosions

Classical novae are thermonuclear explosions that take place in the envelopes of accreting white dwarfs in binary systems. The material piles up under degenerate conditions, driving a thermonuclear runaway. The energy released by the suite of nuclear processes operating at the envelope heats the material up to peak temperatures about 100 - 400 MK. During these events, about 10-3 - 10-7 Msun, enriched in CNO and, sometimes, other intermediate-mass elements (e.g., Ne, Na, Mg, Al) are ejected into the interstellar medium. To account for the gross observational properties of classical novae (in particular, the large concentrations of metals spectroscopically inferred in the ejecta), models require mixing between the (solar-like) material transferred from the secondary and the outermost layers (CO- or ONe-rich) of the underlying white dwarf. Recent multidimensional simulations have demonstrated that Kelvin-Helmholtz instabilities can naturally produce self-enrichment of the accreted envelope with material from the underlying white dwarf at levels that agree with observations. However, the feasibility of this mechanism has been explored in the framework of CO white dwarfs, while mixing with different substrates still needs to be properly addressed. Three-dimensional simulations of mixing at the core-envelope interface during nova outbursts have been performed with the multidimensional code FLASH, for two types of substrates: CO- and ONe-rich. We show that the presence of an ONe-rich substrate, as in "neon novae", yields larger metallicity enhancements in the ejecta, compared to CO,rich substrates (i.e., non-neon novae). A number of requirements and constraints for such 3-D simulations (e.g., minimum resolution, size of the computational domain) are also outlined.

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arXiv2015

The slow decline of the Galactic recurrent novae T Pyxidis, IM Normae, and CI Aquilae

A distinguishing trait of the three known Galactic recurrent novae with the shortest orbital periods, T Pyx, IM Nor, and CI Aql, is that their optical decline time-scales are significantly longer than those of the other recurrent systems. On the other hand, some estimates of the mass of the ejecta, the velocity of the ejecta, and the duration of the soft X-rays emission of these systems are of the order of those of the other recurrent systems and the fast classical novae. We put forth a tentative explanation of this phenomenon. We propose that in these systems part of the material transferred from the companion during the first few days of the eruption remains within the Roche lobe of the white dwarf, preventing the radiation from ionizing the ejecta of the system and increasing the optical decline time-scale. We explain why this phenomenon is more likely in systems with a high mass transfer rate and a short orbital period. Finally, we present a schematic model that shows that the material transferred from the companion is sufficient to absorb the radiation from the white dwarf in these systems, ultimately supporting this scenario as quantitatively realistic.

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arXiv2014

Pan-chromatic observations of the remarkable nova LMC 2012

We present the results of an intensive multiwavelength campaign on nova LMC 2012. This nova evolved very rapidly in all observed wavelengths. The time to fall two magnitudes in the V band was only 2 days. In X-rays the super soft phase began 13$\pm$5 days after discovery and ended around day 50 after discovery. During the super soft phase, the \Swift/XRT and \Chandra\ spectra were consistent with the underlying white dwarf being very hot, $\sim$ 1 MK, and luminous, $\sim$ 10$^{38}$ erg s$^{-1}$. The UV, optical, and near-IR photometry showed a periodic variation after the initial and rapid fading had ended. Timing analysis revealed a consistent 19.24$\pm$0.03 hr period in all UV, optical, and near-IR bands with amplitudes of $\sim$ 0.3 magnitudes which we associate with the orbital period of the central binary. No periods were detected in the corresponding X-ray data sets. A moderately high inclination system, $i$ = 60$\pm$10$^{\arcdeg}$, was inferred from the early optical emission lines. The {\it HST}/STIS UV spectra were highly unusual with only the \ion{N}{5} (1240Å) line present and superposed on a blue continuum. The lack of emission lines and the observed UV and optical continua from four epochs can be fit with a low mass ejection event, $\sim$ 10$^{-6}$ M$_{\odot}$, from a hot and massive white dwarf near the Chandrasekhar limit. The white dwarf, in turn, significantly illuminated its subgiant companion which provided the bulk of the observed UV/optical continuum emission at the later dates. The inferred extreme white dwarf characteristics and low mass ejection event favor nova LMC 2012 being a recurrent nova of the U Sco subclass.

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arXiv2014

On the Raman O VI and related lines in classical novae

We critically examine the recent claimed detection of Raman scattered O VI at around 6830Å in the iron curtain stage spectra of the classical CO nova V339 Del. The observed line variations are compatible in profile and timing of emission line strength with an excited state transition of neutral carbon. Line formation in classical nova ejecta is physically very different from that in symbiotic binaries, in which the O VI emission line is formed within the wind of the companion red giant at low differential velocity. The ejecta velocity and density structure prevent the scattering from producing analogous features. There might , however, be a broadband spectropolarimetric signature of the Raman process and also Rayleigh scattering at some stage in the expansion. We show that the neutral carbon spectrum, hitherto under-exploited for novae, is especially useful as a probe of the structure of the ejecta during the early, optically thick stages of the expansion

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arXiv2013

A unified model for the spectrophotometric development of classical and recurrent novae: the role of asphericity of the ejecta

There is increasing evidence that the geometry, and not only the filling factors, of nova ejecta is important in the interpretation of their spectral and photometric developments. Ensembles of spectra and light curves have provided general typographies. This Letter suggests how these can be unified.The observed spread in the maximum magnitude - rate of decline (MMRD) relation is argued to result from the range of opening angles and inclination of the ejecta, and not only to their masses and velocities. The spectroscopic classes can be similarly explained and linked to the behavior of the light curves. The secondary maximum observed in some dust forming novae is a natural consequence of the asphericity. Neither secondary ejections nor winds are needed to explain the phenomenology. The spectrophotometric development of classical novae can be understood within a single phenomenological model with bipolar, although not jet-like, mass ejecta. High resolution spectropolarimetry will be an essential analytical tool.

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arXiv2012

"But since the affairs of men rest still uncertain, let's reason with the worst that may befall": Probability, risk, and the 2009 L'Aquila Earthquake

This article is a commentary on the verdict of the "L'Aquila Six", the group of bureaucrats and scientists tried by an Italian court as a result of their public statements in advance of the quake of 2009 Apr. 6 that left the city in ruins and cause more than 300 deaths. It was not the worst such catastrophic event in recent Italian history, but it was one of -- if not the -- worst failures of risk assessment and preventive action. The six were found guilty and condemned by a first level of the justice system to substantial prison terms. The outcry provoked by the verdict in the world press and the international scientific community has fueled the already fiery debate over whether the six should have been tried at all. They have been presented as martyrs to science being treated as scapegoats by a scientifically illiterate justice system and inflamed local population for not being able to perform the impossible (predict the event). Petitions of support have been drafted and signed by thousands of working scientists and technical experts in many fields excoriating the court and the country for such an outrage against the scientific community, often accompanied by ominous warnings about the chilling effect this will have on the availability of expert advice in times of need. My purpose in this essay is to explain why this view of the events of the trial is misguided, however well intentioned, and misinformed.

physics.soc-ph↗

arXiv2012

Spectroscopy of Novae -- A User's Manual

The spectroscopic development of classical novae is described as a narrative of the various stages of the outburst. The review highlights the multiwavelength aspects of the phenomenology and the recent developments related to structure, inhomogeneity, and dynamics of the ejecta. Special emphasis is placed on the distinct behavior of the symbiotic-like recurrent novae.

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arXiv2010

Mixing in classical novae: a 2-D sensitivity study

Classical novae are explosive phenomena that take place in stellar binary systems. They are powered by mass transfer from a low-mass, main sequence star onto a white dwarf. The material piles up under degenerate conditions and a thermonuclear runaway ensues. The energy released by the suite of nuclear processes operating at the envelope heats the material up to peak temperatures of ~ (1 - 4) \times 108 K. During these events, about 10-4 - 10-5 M\odot, enriched in CNO and other intermediate-mass elements, are ejected into the interstellar medium. To account for the gross observational properties of classical novae (in particular, a metallicity enhancement in the ejecta above solar values), numerical models assume mixing between the (solar-like) material transferred from the companion and the outermost layers (CO- or ONe-rich) of the underlying white dwarf. The nature of the mixing mechanism that operates at the core-envelope interface has puzzled stellar modelers for about 40 years. Here we investigate the role of Kelvin-Helmholtz instabilities as a natural mechanism for self-enrichment of the accreted envelope with core material. The feasibility of this mechanism is studied by means of the multidimensional code FLASH. Here, we present a series of 9 numerical simulations perfomed in two dimensions aimed at testing the possible influence of the initial perturbation (duration, strength, location, and size), the resolution adopted, or the size of the computational domain on the results. We show that results do not depend substantially on the specific choice of these parameters, demonstrating that Kelvin- Helmholtz instabilities can naturally lead to self-enrichment of the accreted envelope with core material, at levels that agree with observations.

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arXiv2010

On mixing at the core-envelope interface during classical nova outbursts

Classical novae are powered by thermonuclear runaways that occur on the white dwarf component of close binary systems. During these violent stellar events, whose energy release is only exceeded by gamma-ray bursts and supernova explosions, about 10-4 10-5 Msun of material is ejected into the interstellar medium. Because of the high peak temperatures attained during the explosion, Tpeak ~ (1-4)x10+8 K, the ejecta are enriched in nuclear-processed material relative to solar abundances, containing significant amounts of 13C, 15N, and 17O and traces of other isotopes. The origin of these metal enhancements observed in the ejecta is not wellknown and has puzzled theoreticians for about 40 years. In this paper, we present new 2-D simulations of mixing at the core-envelope interface. We show that Kelvin-Helmholtz instabilities can naturally lead to self-enrichment of the solar-like accreted envelopes with material from the outermost layers of the underlying white dwarf core, at levels that agree with observations.

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arXiv2009

Recurrent Novae: Progenitors of SN Ia?

We present 3D hydrodynamical simulations of the separated binary RS Ophiuchi (RS Oph), a recurrent nova and potential progenitor of a SN Ia. RS Oph is composed of a red giant (RG) and a white dwarf (WD) whose mass is close to the Chandrasekhar limit. In an isothermal scenario, the WD accrets about 10% of a 20 km/s RG wind by a non-Keplerian accretion disk with strong spiral shocks, and about 2% of a 60 km/s RG wind by what we term a 'turbulent accretion ball'. A significantly larger impact have the thermodynamics. In an adiabatic scenario only about 0.7% of the 20 km/s RG wind is accreted. The rate of change of the system separation due to mass and angular momentum loss out of the system is negative in all three cases studied, but is ten times smaller for a fast RG wind (60 km/s) than for a slow RG wind (20 km/s). The results demonstrate that existing nova models and observed recurrence times fit well together with 3D wind accretion and that RS Oph is one of the most promising systems to become an SN Ia.

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arXiv2008

3D simulations of RS Oph: from accretion to nova blast

RS Ophiuchi is a recurrent nova with a period of about 22 years, consisting of a wind accreting binary system with a white dwarf (WD) very close to the Chandrasekhar limit and a red giant star (RG). The system is considered a prime candidate to evolve into an SNIa. We present a 3D hydrodynamic simulation of the quiescent accretion and the subsequent explosive phase. The computed circumstellar mass distribution in the quiescent phase is highly structured with a mass enhancement in the orbital plane of about a factor of 2 as compared to the poleward directions. The simulated nova remnant evolves aspherically, propagating faster toward the poles. The shock velocities derived from the simulations are in agreement with those derived from observations. For v_RG = 20 km/s and for nearly isothermal flows, we derive a mass transfer rate to the WD of 10% of the mass loss of the RG. For an RG mass loss of 10^{-7} solar masses per year, we found the orbit of the system to decay by 3% per million years. With the derived mass transfer rate, multi-cycle nova models provide a qualitatively correct recurrence time, amplitude, and fastness of the nova. Our simulations provide, along with the observations and nova models, the third ingredient for a deeper understanding of the recurrent novae of the RS Oph type. In combination with recent multi-cycle nova models, our results suggests that the WD in RS Oph will increase in mass. Several speculative outcomes then seem plausible. The WD may reach the Chandrasekhar limit and explode as an SN Ia. Alternatively, the mass loss of the RG could result in a smaller Roche volume, a common envelope phase, and a narrow WD+WD system. Angular momentum loss due to graviational wave emission could trigger the merger of the two WDs and - perhaps - an SN Ia via the double degenerate scenario.

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arXiv2007

The Neon Abundance in the Ejecta of QU Vul From Late-Epoch IR Spectra

We present ground-based SpectroCam-10 mid-infrared, MMT optical, and Spitzer Space Telescope IRS mid-infrared spectra taken 7.62, 18.75, and 19.38 years respectively after the outburst of the old classical nova QU Vulpeculae (Nova Vul 1984 #2). The spectra of the ejecta are dominated by forbidden line emission from neon and oxygen. Our analysis shows that neon was, at the first and last epochs respectively, more than 76 and 168 times overabundant by number with respect to hydrogen compared to the solar value. These high lower limits to the neon abundance confirm that QU Vul involved a thermonuclear runaway on an ONeMg white dwarf and approach the yields predicted by models of the nucleosynthesis in such events.

astro-ph↗

arXiv2006

Restoring Color-magnitude Diagrams with the Richardson-Lucy Algorithm

We present an application of the Richardson-Lucy algorithm to the analysis of color-magnitude diagrams by converting the CMD into an image and using a restoring point spread function function ({\it psf}) derived from the known, often complex, sources of error. We show numerical experiments that demonstrate good recovery of the original image and establish convergence rates for ideal cases with single gaussian uncertainties and poisson noise using a $χ^2$ statistic. About 30-50 iterations suffice. As an application, we show the results for a particular case, the Hipparcos sample of the solar neighborhood where the uncertainties are mainly due to parallax which we model with a composite weighted gaussian using the observed error distributions. The resulting psf has a slightly narrower core and broader wings than a single gaussian. The reddening and photometric errors are considerably reduced by restricting the sample to within 80 pc and to M_V \leq 3.5. We find that the recovered image, which has a narrower, better defined main sequence and a more clearly defined red giant clump, can be used as input to stellar evolution modeling of the star formation rate in the solar vicinity and, with more contributing uncertainties taken into account, for general Galactic and extragalactic structure and population studies.

astro-ph↗

arXiv2003

Photo-Ionization Induced Rapid Grain Growth in Novae

An elementary model is developed for grain growth during the expansion stage of nova ejecta. Rather than suppressing grain formation, we argue that agglomeration of atoms by dust nuclei proceeds kinetically through induced dipole reactions in a partially ionized medium. Ionization of a cluster increases once the ejecta become transparent in the ultraviolet which triggers runaway grain growth. This mechanism may also be important in other dust-forming hot stars such as Luminous Blue Variables and Wolf-Rayet systems.

astro-ph↗

arXiv2001

Modeling the Galactic Center Nonthermal Filaments as Magnetized Wake

We simulate the Galactic Center nonthermal filaments as magnetized wakes formed dynamically from amplification of a weak (tens of $μ$G) global magnetic field through the interaction of molecular clouds with a Galactic Center wind. One of the key issues in this cometary model is the stability of the filament against dynamical disruption. Here we show 2-dimensional MHD simulations for interstellar conditions that are appropriate for the Galactic Center. The structures eventually disrupt through a shear driven nonlinear instability but maintain coherence for lengths up to 100 times their width as observed. The final instability, which destroys the filament through shredding and plasmoid formation, grows quickly in space (and time) and leads to an abrupt end to the structure, in accord with observations. As a by-product, the simulation shows that emission should peak well downstream from the cloud-wind interaction site.

astro-ph↗

arXiv2001

Evolution of Li, Be and B in the Galaxy

In this paper we study the production of Li, Be and B nuclei by Galactic cosmic ray spallation processes. We include three kinds of processes: (i) spallation by light cosmic rays impinging on interstellar CNO nuclei (direct processes); (ii) spallation by CNO cosmic ray nuclei impinging on interstellar p and 4He (inverse processes); and (iii) alpha-alpha fusion reactions. The latter dominate the production of 6Li and 7Li. We calculate production rates for a closed-box Galactic model, verifying the quadratic dependence of the Be and B abundances for low values of Z. These are quite general results and are known to disagree with observations. We then show that the multi-zone multi-population model we used previously for other aspects of Galactic evolution produces quite good agreement with the linear trend observed at low metallicities without fine tuning. We argue that reported discrepancies between theory and observations do not represent a nucleosynthetic problem, but instead are the consequences of inaccurate treatments of Galactic evolution.

astro-ph↗

arXiv1999

The Galactic Center Isolated Nonthermal Filaments as Analogs of Cometary Plasma Tails

We propose a model for the origin of the isolated nonthermal filaments observed at the Galactic center based on an analogy to cometary plasma tails. We invoke the interaction between a large scale magnetized galactic wind and embedded molecular clouds. As the advected wind magnetic field encounters a dense molecular cloud, it is impeded and drapes around the cloud, ultimately forming a current sheet in the wake. This draped field is further stretched by the wind flow into a long, thin filament whose aspect ratio is determined by the balance between the dynamical wind and amplified magnetic field pressures. The key feature of this cometary model is that the filaments are dynamic configurations, and not static structures. As such, they are local amplifications of an otherwise weak field and not directly connected to any static global field. The derived field strengths for the wind and wake are consistent with observational estimates. Finally, the observed synchrotron emission is naturally explained by the acceleration of electrons to high energy by plasma and MHD turbulence generated in the cloud wake.

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