1,721,086 research outputs found
The discovery of X-ray binaries in the sculptor dwarf spheroidal galaxy
No full textWe report the results of a deep Chandra survey of the Sculptor dwarf spheroidal galaxy. We find five X-ray sources with LX of at least 6 × 1033 ergs-1 with optical counterparts establishing them as members of Sculptor. These X-ray luminosities indicate that these sources are X-ray binaries, as no other known class of Galactic point sources can reach 0.5-8 keV luminosities this high. Finding these systems proves definitively that such objects can exist in an old stellar population without stellar collisions. Three of these objects have highly evolved optical counterparts (giants or horizontal branch stars), as do three other sources whose X-ray luminosities are in the range which includes both quiescent low-mass X-ray binaries and the brightest magnetic cataclysmic variables. We predict that large area surveys of the Milky Way should also turn up large numbers of quiescent X-ray binaries
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Double White Dwarf Binaries, Their Explosions and Their Survivors
No full textCompact double white dwarf (WD) binaries with orbital periods below an hour inspiral via emission of gravitational waves, and eventually undergo mass transfer. In this dissertation, I explore the outcomes of the interaction between a helium WD donor and a carbon-oxygen WD accretor. Systems undergoing unstable mass transfer can merge to form a helium shell-burning giant star, and I show that their pulsation properties are consistent with their hydrogen-rich brethren where surface convection drives the pulsations. I also show that for systems undergoing stable mass transfer, the orbital evolution is driven by the thermal properties of the donor. The accretor is initially reheated due to the high mass transfer rate, and subsequently cools as the orbit widens and the mass transfer rate decreases. I explore the potential occurrence of a thermonuclear runaway during the reheating that leads to a type Ia supernova. As the binary unbinds due to the explosion, the helium WD companion leaves at its orbital velocity ≈ 1000 − 2000 km s−1 as a hypervelocity star. I study the mass loss and shock-heating of the surviving donor during its interaction with supernova ejecta. The subsequent evolution of the inflated donor agrees well with several observed hypervelocity stars
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Element Diffusion and Other Mixing in White Dwarf Stars
Full text linkWhite dwarf stars reveal signatures of material accreted from their surroundings. Making quantitative inferences about the processes that supply this material requires theoretical models of white dwarf surface structure. In this dissertation, I examine methods for building evolutionary white dwarf models that include element diffusion, convection, and thermohaline instability. Each of these mixing processes that occur at white dwarf surfaces has important implications for observable signatures of accreted material. Models that account for all types of surface mixing allow for inferences about accretion rates and composition of bodies that supply the material. The picture that emerges from models presented in this work is one of planetary systems supplying rocky debris at higher rates and from larger mass reservoirs than previously thought
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Understanding Type II-Plateau Supernovae and the Red Supergiants that Cause Them
Full text linkObservations of the transient, explosive deaths of massive stars are well-poised to provide insight into stellar physics when combined with theoretical understanding. From spherically symmetric stellar evolution models, we confirm and sharpen early analytic calculations for the Supernova (SN) plateau luminosity and duration as a function of the red supergiant (RSG) progenitor properties. When the RSG radius at the time of the explosion is known, we show how the explosion energy and ejecta mass can be directly inferred; otherwise, we show that a family of explosions could produce the same plateau luminosity, duration, and photospheric velocity. We also explore the impact of large-scale radial stellar pulsations on these predictions.
Then, motivated in part as an effort to understand the turbulent outer envelope responsible for early-time SN emission, we complete global 3D radiation-hydrodynamics (RHD) simulations of RSG envelopes with Athena++. These simulations reveal an extended density structure with
large-scale convective plumes spanning large fractions of the stellar surface.
These computations also provide insights to guide evolutionary modeling efforts, such as a physically-motivated calibration of the convective mixing length which helps determine the envelope density structure.
Driving a strong shock through these 3D simulations, we then show novel results on how the inhomogeneous 3D convective structure leads to a longer-duration, fainter shock breakout (SBO) signal compared to predictions from semi-analytic and spherically-symmetric models
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Understanding Turbulence in Massive Star Envelopes: Impacts of Near-Surface Convection Zones on Stellar Envelope Structure and Observables
Full text linkModeling of massive star (M > 10 solar masses) outer envelopes has remained a challenge for decades. Due to the complex physics involved, 1D models of stellar envelopes can only be evolved under many approximations that attempt to incorporate (or alleviate) the intricate interactions between matter and radiation. To reveal the multi-dimensional nature of massive star envelopes, we performed 3D radiation hydrodynamic simulations of the main sequence and post-main sequence evolution of massive stars using Athena++. These 3D models capture the detailed structures and interactions of the gas and radiation fields, in particular the time-dependent, vigorous turbulence excited by iron and helium opacity peaks in the near-surface convection zones. This turbulence becomes trans-sonic and creates large density fluctuations that propagate to the surface, eliminating the common notion of a spatially confined convection zone and a constant-radius photosphere. Strong anti-correlations between radiation flux and density decrease the radiation pressure force by up to 80%, rendering the dynamical pressure of the turbulence essential in maintaining force balance. As predicted by Henyey et. al. (1965), we show that this turbulent pressure support impacts the adiabatic temperature gradient and significantly reduces the superadiabaticity of these convection zones. Turbulent motions propagating to the surface from the Fe convection zone have significant observational impacts. The dynamic surface topography generates stochastic low-frequency brightness variability that is consistent with that observed in similar stars by recent photometric surveys (e.g. TESS). Additionally, we used the frequency-dependent Monte Carlo radiation transport code Sedona to self-consistently synthesize the spectral features of these turbulent stellar envelopes, revealing that the time-dependent surface velocities generate spectral line broadening and variability. Our work proposes future improvements to 1D stellar evolution models and suggests the need for a novel understanding of how turbulent surface velocities affect spectral line profiles
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Explosive Outcomes from Rapid Accretion of Helium onto White Dwarfs in Tight Binaries
Full text linkAccretion of helium onto white dwarfs at various rates can lead to several different types of explosive outcomes. Slow accretion from low mass He stars or He WDs in binaries with orbital periods less than an hour can build up helium shells of that ignite and detonate, generating faint and fast transients.In this dissertation, I explore this well known channel for generating transients from He accretion onto WDs, as well as with less explored channels that accrete He onto WD at rates that allow for steady helium shell burning.Stable helium burning significantly grows the degenerate core, which can result in several different outcomes, including type Ia supernovae and accretion induced collapse.I also explore deep convection in C/O/Ne hybrid WDs and found effective mixing to a level that will strongly affect their later explosions.Some binaries are unstable to mass transfer and merge the component stars.Previous studies explored explosive outcomes during the merger, whereas I studied a channel that leads to a unique type of explosion years after the merger event.Additionally, I made some predictions about observed compact binaries with He stars and He WDs
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Predicting Microlensing Rates and Properties in Wide-Field Surveys
Full text linkThe rates and properties of out-of-plane microlensing events have been understudied in the past. We seek to remedy this by building a simulation of galactic stars and lensing events, drawing upon numerous up-to-date sources. The resulting code is a well-verified software tool which can be adapted to simulate a wide range of potential survey strategies and parameters. It will be a useful tool for the community to optimize the design of the deep, wide-angle surveys coming on line in the next decades, such as ATLAS, Evryscope, and LSST. In the text, we provide baseline all-sky lensing properties for a deep survey
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Probing Late-Stage Stellar Evolution Through Robotic Follow-Up of Nearby Supernovae
Full text linkMany of the remaining uncertainties in stellar evolution can be addressed through immediate and long-term photometry and spectroscopy of supernovae. The early light curves of thermonuclear supernovae can contain information about the nature of the binary companion to the exploding white dwarf. Spectra of core-collapse supernovae can reveal material lost by massive stars in their final months to years. Thanks to a revolution in technology—robotic telescopes, high-speed internet, machine learning—we can now routinely discover supernovae within days of explosion and obtain well-sampled follow-up data for months and years. Here I present three major results from the Global Supernova Project at Las Cumbres Observatory that take advantage of these technological advances. (1) SN 2017cbv is a Type Ia supernova discovered within a day of explosion. Early photometry shows a bump in the U-band relative to previously observed Type Ia light curves, possibly indicating the presence of a nondegenerate binary companion. (2) SN 2016bkv is a low-luminosity Type IIP supernova also caught very young. Narrow emission lines in the earliest spectra indicate interaction between the ejecta and a dense shell of circumstellar material, previously observed only in the brightest Type IIP supernovae. (3) Type Ibn supernovae are a rare class that interact with hydrogen-free circumstellar material. An analysis of the largest-yet sample of this class has found that their light curves are much more homogeneous and faster-evolving than their hydrogen-rich counterparts, Type IIn supernovae, but that their maximum-light spectra are more diverse
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Supersoft Emission from Thermonuclear Burning on Hydrogen-Accreting White Dwarfs
Full text linkThermonuclear burning of hydrogen on white dwarfs (WDs) is an inevitable occurence for accreting WDs in binary systems. After the onset of thermally-unstable nuclear burning, the WD rapidly expands and ejects much of its hydrogen-rich matter. Once it regains thermal equilibrium, it contracts and becomes a luminous source of supersoft X-rays. This supersoft phase can last anywhere from days (for novae on massive WDs) to millions of years (for persistent supersoft sources). In this dissertation, we explore how the supersoft phase of accreting WDs proceeds and what observations of it reveal about the underlying WD. We present stellar models of persistent supersoft sources and novae. We then use these models to explain the isothermal nature of the ejecta as observed in the radio. We also use these models to test the efficacy of g-modes excited in the burning layer as an explanation of observed oscillations in the supersoft phase of novae. While we find excited modes, their periods are too short to account for the observed oscillations
Revealing the Progenitor Systems of Type Ia Supernovae with Early High-cadence Multiwavelength Data
Full text linkOver 10,000 astronomical transients are now discovered every year. Pairing this wealth of objects with rapid followup facilities such as Las Cumbres Observatory (LCO) allows for high-cadence multiwavelength characterization of supernovae (SNe) within days or even hours of their explosion. Although Type Ia SNe (SNe Ia) are a relatively homogeneous population around peak brightness, notably used as standardizable candles to measure cosmological parameters, at early times their lightcurves show a dramatic range of behavior. One effect sometimes visible in their early lightcurves is a UV excess, likely indicative of the exploding white dwarf having a nondegenerate companion which shocks the SN ejecta as the two collide. Studying their varied early lightcurves can thus reveal information about their progenitor systems, which remain poorly understood beyond the fact that the explosion originates from a white dwarf. Here I present three advancements in SNe Ia research: (1) SN 2019yvq is a SN Ia which displayed the strongest early UV excess ever observed in SNe Ia. This SN shared some characteristics with a rare subclass of SNe Ia called 02es-likes, which for some reason seem to display these excesses more frequently than their predicted rarity. (2) In a sample of 9 SNe Ia with excellent early data from LCO, the distribution of early excess strengths and best-fit viewing angles are consistent with the progenitor systems of SNe Ia predominantly containing a nondegenerate companion. (3) In a sample of 127 SNe Ia observed by the ZTF survey, the rate of early excesses is again consistent with the single-degenerate progenitor scenario
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