Problems in Stellar Atmospheres and Envelopes

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In recent times, binary evolution has been recognized as a fundamental aspect in the lives of massive stars, with observations indicating that more than half of them undergo strong interactions with a companion. Various observations of X-ray binaries, transient events, and the historic detection of gravitational waves from merging binary black holes and neutron stars puts significant pressure on our understanding of binaries. Two important uncertainties that remain concern the stability of mass transfer and the outcome of common envelope events.

In this session, we will investigate the interplay between orbital evolution and mass transfer, studying the different timescales involved in mass transferring systems, their stability, and the outcomes of common envelope evolution. To illustrate these concepts, we will learn how to use MESA to model systems undergoing stable mass transfer, consisting of either two non-degenerate stars, or including a compact object modeled as a point mass. We will then explore how to extend MESA to account for unstable mass transfer leading to common envelope events by implementing a simple prescription commonly used in population synthesis calculations.

We will use MESA to explore nuclear burning in the surface layers of accreting white dwarfs and build models of novae. White dwarfs in close binaries accumulate hydrogen and helium on their surface for thousands of years, in some cases before it rapidly burns, giving a bright transient that we see as a nova. Novae eject significant amounts of mass back into the interstellar medium and so play an important role in nucleosynthesis.

The mass ejection may also prevent the white dwarf mass becoming large enough to trigger carbon burning and a Type Ia supernova. At rapid accretion rates, the burning can be stable, so that the incoming light elements are processed smoothly into a thickening layer of helium that itself can later ignite. In the lectures and labs, we will cover an introduction to novae, and the physics of unstable and stable nuclear burning, including the important nuclear reactions.

We will discuss open issues such as the mechanism of mass loss, the fate of the helium produced by hydrogen burning, and enrichment of the accreting light element layers with carbon from the underlying white dwarf. Currently, the number of very metal-poor stars known is extremely small, and larger samples are critical for progress. This requires large-scale surveys and high-resolution spectroscopic follow-up with optical telescopes up to 30 m in diameter. The deaths of stars give rise to compact stellar remnants—neutron stars, black holes, and white dwarfs—that produce the most exotic and energetic phenomena in the universe, from the brightest known sources of radiation to the steadiest astrophysical clocks.

The properties of compact stellar remnants provide not only unique information about the late stages of stellar evolution, but also a testing. Many fundamental questions about compact stellar remnants center on understanding, both empirically and theoretically, their basic physical properties, including mass, radius, spin, and magnetic field.

Here several key questions in which major progress can be made in the coming decade are highlighted. The basic properties of neutron stars are closely coupled to the physics of their interiors. Because the equation of state for such ultradense matter is still poorly constrained, the basic compositions of neutron star cores are unknown, with exotic new states of matter e.

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This high-density regime is mostly inaccessible to terrestrial laboratories, but its properties determine the mass-radius relation of neutron stars, providing an astrophysical probe Figure 5. In the past decade, the ability to measure masses of binary radio pulsars has dramatically improved, and several X-ray techniques for constraining neutron-star radii have been developed.

Radio pulsars are the most commonly observed neutron stars known, with almost 2, cataloged to date. Pulsars with unusually high or low mass directly constrain the dense matter equation of state, and in fact the measurement of a single neutron star with mass would rule out most forms of exotic material in neutron star cores. Current and future galactic surveys for millisecond radio pulsars are crucial for detecting new systems for neutron-star-mass measurements currently only approximately 1 percent of pulsars are suitable , as well as finding other exotic pulsars for gravitational-wave-detection experiments and strong-field gravity tests.

The past 5 years have seen the discovery of several eccentric binary millisecond radio pulsars that contain neutron stars likely more massive than. Radio surveys with current or soon-to-be-available facilities such as the Green Bank Telescope, Arecibo Observatory, Parkes Observatory, Effelsberg Radio Telescope, and the EVLA should double the number of known radio pulsars in the next decade and because of computational and instrumentation improvements should increase the number of millisecond radio pulsars by an even larger factor.

Measurement of masses and radii for neutron stars in the next decade should suffice to rule out some of the equations of state displayed here as black and green curves in a figure from Lattimer and Prakash, The green region at the lower right is excluded from observations of pulsar Jad. The recent discovery of pulsars with rules out equation-of-state models that do not extend to such high masses. Lattimer and M. Prakash, Neutron star observations: Prognosis for equation of state constraints, Physics Reports , copyright , with permission from Elsevier.

Gamma-ray surveys with facilities such as Fermi Gamma-ray Space Telescope will likely also contribute, albeit with smaller numbers. Current-generation pulsar surveys and timing observations are completely sensitivity-limited, and larger telescope collecting areas are needed. The most extreme outliers from such surveys may be of greatest interest. A recent example is the so-called Double Pulsar J, an exceptionally relativistic pulsar binary discovered in That system has provided the most precise tests of general relativity in the strong-field regime to date, and long-term timing which may require a span of 10 years or more will eventually allow the measurement of spin-orbit effects on periastron advance, which in turn will determine the neutron-star moment of inertia, strongly constraining the neutron-star equation of state since the neutron-star mass is already precisely known in that system.

For the LMXBs, observations with the Rossi X-ray Timing Explorer RXTE have identified both millisecond oscillations during thermonuclear X-ray bursts and longer-lasting accretion-powered millisecond pulsations that encode information about neutron star parameters. Pulse shape and spectral modeling of these phenomena with a larger-area X-ray timing instrument can strongly constrain both neutron-star radius and mass.

Broadband X-ray spectroscopy of Eddington-limited radius-expansion bursts in many of these same systems can also independently constrain neutron-star parameters. Neutron-star radii and masses can also be constrained through pulse shape modeling of the faint thermal pulsations seen from some isolated millisecond pulsars.

Recent observations with X-ray Multi-Mirror Mission XMM -Newton have demonstrated this technique, but a more-sensitive, focusing X-ray telescope is required to obtain strong constraints. In addition, soft X-ray spectroscopic observations of transiently accreting neutron stars in quiescence show thermal X-ray spectra from the cooling neutron star surfaces that yield the neutron star radius when the source distance is known and the atmospheric model is correct. The measurement of these faint targets requires a sensitive focusing X-ray telescope with moderate spectral resolution.

Finally, laboratory nuclear physics experiments are expected to provide complementary constraints on the nuclear-matter equation of state. These measurements will constrain some aspects of the neutron-star equation of state that can serve as input to neutron-star models, allowing one to interpret observations, probe models, and constrain the regimes of the neutron-star equation of state not accessible in the laboratory.

Major advances will be possible in the next decade. Examples include constraints on the nuclear symmetry energy around nuclear density from precision measurements of the neutron skin thicknesses of heavy nuclei, using parity-violating electron scattering, and—for higher densities—from heavy-ion collisions at a range of energies and asymmetries at various advanced rare isotope facilities, including the Facility for Rare Isotope Beams FRIB.

Nuclear-theory work to identify the most useful signatures of equation-of-state properties in heavy-ion collisions and how to interpret them quantitatively is also needed. The physical processes that stop the transfer of angular momentum and thereby establish the maximum spin rates of neutron stars are currently unknown.

One of the primary candidates is the emission of gravitational. Identifying the correct spin frequency distribution of MSPs will help to determine both the maximum spin rates of neutron stars and the limiting physical processes. Current and future radio and X-ray timing surveys have many fewer selection effects toward rapidly rotating pulsars than those in the past.

If such systems are detected, they would directly limit the neutron-star equation of state by determining the maximum radius of the neutron star as a function of its mass for which it does not shed material at its equator. New constraints on neutron-star physics would come from the detection of a neutron star spinning more rapidly than 1, Hz. Astrophysical black holes are completely described by just two quantities, their mass and spin. Although the masses of stellar black holes in X-ray binaries have been measured dynamically for decades, it is only in the past few years that it has become possible to constrain the spins of black holes.

The spin is constrained by determining the inner radius of the accretion disk, either by fitting the thermal disk component of the X-ray continuum spectrum, or through the relativistically broadened shape of the Fe K disk fluorescence line. The radius inferred by these methods is believed to be comparable to that of the last stable orbit in general relativity, but there are systematic uncertainties in this association that limit the precision of current constraints on black hole spin.

Inferences about spin have now been made in 10 systems using a variety of X-ray missions, most recently including Chandra, XMM-Newton, and Suzaku and most are rotating significantly, with a wide variety of black hole spins measured, and several are believed to be spinning near the maximal amount allowed by general relativity. A slowly spinning, disk-accreting black hole must double its mass in order to spin rapidly, which is impossible for a black hole in an X-ray binary; thus, the measured spin distribution is essentially sampling the birth properties of these black holes.

An alternative way of measuring black hole spin is through the spin-orbit coupling of a pulsar with a black hole. This method will require pulsar searches to discover pulsars with black hole companions. A larger sample of black hole spin measurements will provide very strong constraints on models of massive star evolution and core-collapse supernovae. More broadly, an improved understanding of black hole spin can be used to address a number of important issues, including the role of black hole spin in producing jets and in powering GRBs.

In order to make continued progress, soft X-ray continuum spectros-. In addition, further numerical and theoretical work on general-relativistic MHD models of black hole accretion disks is essential for interpreting X-ray continuum observations, and better theoretical models of the X-ray irradiation and fluorescent Fe line emission from the inner accretion disk are needed to interpret Fe line spectra.

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Observational constraints on the initial-final mass relation come primarily from white dwarfs in open clusters and require accurate main-sequence turnoff ages plus white-dwarf cooling ages to infer the initial mass and precise final masses Figure 5. The largest source of error is the variance in theoretical mass-lifetime relationships for stars with convective cores and, by extension, open cluster ages.

Gaia will provide precise distances and membership information for open clusters, and asteroseismology from missions such as CoRoT and Kepler may constrain the sizes of convective cores. The mass of the blanket is a result of the processes that end the AGB evolution of the star. The recent discovery of carbon-atmosphere white dwarfs is as yet unexplained and points to interesting discovery areas in white-dwarf formation. Also crucial to understanding white-dwarf properties is the onset of crystallization, which alters the internal energy structure and causes an abrupt change in effective temperature and luminosity.

Both have been constrained by asteroseismology on a small number of stars, giving a partial picture and great promise for future advances. The mass of a white dwarf originating from a single star such as the Sun is related to the luminosity of the star as it leaves the AGB. This luminosity, and thus the white-dwarf mass, is determined by the mass-loss process. Theoretical and observational studies of the dependence of mass-loss rates on stellar parameters have not reached a consensus, and prescriptions advocated and used differ dramatically from one another.

Empirical and theoretical formulas span a wide range of slopes. However, there is no widely accepted or demonstrably correct mass-loss formula for the mass loss that produces white-dwarf stars, and thus no predictive power for extrapolating to understudied populations such as young, low-metallicity cases. Considerable uncertainty exists for intermediate-mass stars 2 to , including how much initial composition affects the result. With the detection of many more white dwarfs in clusters, enough information to constrain mass-loss models and perhaps enough to extrapolate to unobservable populations low metallicity, high mass—as in the early universe may become available.

Kalirai, B. Hansen, D. Kelson, D. Reitzel, R. Rich, and H. Richer, The initial-final mass relation: Direct constraints at the low-mass end, Astrophysical Journal 1 , , reproduced by permission of the AAS. The modeling of mass-loss processes requires non-local thermodynamic equilibrium hydrodynamics with shocks, non-equilibrium chemistry, and grain nucleation and growth.

Problems in stellar atmospheres and envelopes - Ghent University Library

Molecular lines are ideal for mass-loss studies because 1 they trace the gas, and 2 they carry velocity information. CO has been widely used in our galaxy. Additionally, CO traces mass loss for both C-rich and O-rich giants, which is not the case for tracers currently used to study the highest-mass-loss-rate objects in the Small Magellanic Cloud and LMC.

Most white-dwarf stars are believed to be composed of carbon and oxygen. Observations of novae show that some O-Ne-Mg white dwarfs are formed, presumably from relatively high mass progenitors, with details of the formation channel s as yet unclear. There are also He white dwarfs, including a surprisingly large population of He white dwarfs in very metal-rich clusters and a population of single-field He white dwarfs.

Explaining the origin and evolution of these different classes of objects should be illuminating. The formation of He white dwarfs is understood only in the context of binaries, so further understanding of other ways of forming them potentially by single stars or through disrupted binaries or ejections from dense star clusters is needed. Stars that ignite C in their degenerate cores before losing their envelopes to mass loss may also produce unusual thermonuclear supernovae. Very little is known about this potential channel, but large surveys should yield valuable information.

Astronomical timescales evoke the long stretches of time, reckoned in gigayears, that characterize cosmic expansion and most phases of stellar evolution. For these phenomena, a single comprehensive survey can reveal the essential facts, as in a Hertzsprung-Russell diagram for a cluster. But there are phenomena of rotation and pulsation, of orbiting binaries, of explosions and mass loss, and most spectacularly, of stellar death, for which the physical timescales are measured in seconds, days, or months. For a wide range of stellar events, knowledge has been obtained by observing through narrow windows in time, often set by single-investigator observing strategies or by the technical capabilities of the detectors being used.

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Narrow windows produce limited views. The panel anticipates that in the coming decade the burgeoning technological change due to detector development, fast computers, automated pipelines, and the ability for the entire community to interact with large-volume public databases from a distance, over the Internet will lead to significant scientific progress in revealing and exploring a wide range of stellar phenomena. For these reasons, time-domain surveys represent a significant discovery potential for the study of stars and stellar evolution.

Discovery in the time domain in the next decade will be driven by detectors with large fields of view, which scan the sky with approximately daily-weekly cadence and provide all-sky data sets. In addition to unanticipated discoveries Figure 5. Most explosive events have a rapid rise and a slower decline; this symmetry is anomalous. From its redshift and comparison with other recent supernova discoveries, the object was found to be an unusual, luminous supernova. Surprising, rare events continue to be discovered as time-domain surveys expand in reach and duration and improve in cadence and precision.

Barbary, K. Dawson, K. Tokita, G. Aldering, R. Amanullah, N. Connolly, M. Doi, L. Faccioli, V. Fadeyev, A. Fruchter, G. Goldhaber, A. Goobar, A. Gude, X. Huang, Y. Ihara, K. Konishi, M. Kowalski, C. Lidman, J. Meyers, T. Morokuma, P. Nugent, S. Perlmutter, D. Rubin, D. Schlegel, A. Spadafora, N. Suzuki, H. Swift, N. Takanashi, R. Thomas, and N. Follow-up observations of various types are often essential to carry out the science goals. For example, evolved giants and brown dwarfs observed interferometrically show evidence for time-variable spatial structures, possibly associated with dust-cloud formation and weather-like phenomena; follow-up observations with new interferometric facilities will provide important constraints on the physical mechanisms behind the observed time variations.

Other kinds of time-domain studies not mentioned in Table 5. TABLE 5. A few illustrative examples of additional stellar topics from Table 5. How often does a white dwarf approaching the Chandrasekhar mass in a binary undergo an accretion-induced collapse AIC to form a neutron star, rather than blowing up as an SN Ia? An understanding of this question is essential for understanding the evolution of white dwarfs in binary systems and would dramatically constrain the allowed progenitors of SNe Ia. AIC has also been proposed as one of the most promising sites for third-peak r -process nucleosynthesis.

AICs are predicted to be accompanied by the ejection of up to approximately 0. In the coming decade, it is likely that transient gravitational wave sources will be discovered by experiments such as Advanced LIGO and VIRGO a gravitational wave detector at the European Gravitational Observatory , with lower-frequency gravitational wave sources perhaps becoming detectable toward the end of the decade. To optimize the astrophysics that results from such detections, it is critical to have nearly simultaneous electromagnetic observations.

Wide-field-of-view cameras are essential given the rather poor localizations provided by gravity-wave detectors fractions of a square degree. A unique electromagnetic counterpart temporally and spatially coincident with a gravitational wave source would provide more confidence in the gravitational wave detection.

This information is unique given the gravitational wave constraints on the masses and spins magnitudes and direction of the objects. At the tip of the first-ascent RGB, for stars of , He ignition in the degenerate core leads to the He core flash. Very little is known observationally. The ultimate fate of Earth—into the Sun or backing away—depends on whether the Sun will lose 20 percent of its mass before or during this event.

Yet one cannot point to a single object in the sky that is currently undergoing an He core flash. One probable observational signature will be erratic pulsation with rapid period changes. Large samples, including stars in clusters of various ages, are needed to identify individual objects in this critical stage of stellar evolution.

These data will provide essential input to theoretical models of late-stage stellar evolution. Eclipsing binary stars are powerful diagnostics of stellar structure and evolution, and they are relatively easy to find in time-domain surveys Figure 5. They also provide a secure way to measure masses and radii for stars of all spectral types, metallicities, and ages. Longer-period eclipsing binaries, although geometrically less favorable, should be found in large surveys with long durations.

Such stars will be valuable because their evolution is less likely to be impacted by the presence of the companion synchronous rotation, enhanced activity , which may influence the stellar radius. Large samples will permit tests of the mass ratios and distributions of orbital separations of close binaries, which will in turn inform theories of binary-star formation and evolution. Studies of interacting binaries will also benefit, particularly for unusual and rare systems such as contact binaries and common-envelope systems, probable precursors to stellar mergers leading, for example, to blue stragglers.

Stars of all kinds produce a surprisingly wide variety of nonthermal radio emission from timescales of nanoseconds the giant pulses from the Crab pulsar , to months the radio afterglows of supernovae. Recently, several relatively small-scale radio surveys have uncovered new forms of transients from known sources, such as extremely rare millisecond-duration pulses from rotating neutron stars the so-called RRATs, or rotating radio transients , and bright coherent emission from brown dwarfs.

Other surveys have found unidentified radio transients in extragalactic blank fields and toward the galactic center. Yet these surveys have covered. Time-domain surveys will provide large numbers of new eclipsing binary systems for stars across the Hertzsprung-Russell diagram, allowing for much-improved basic data for unusual as well as common types of stars. Becker, E. Agol, N. Silvestri, J. Bochanski, C. Laws, A. West, G. Basri, V. Belokurov, D.

Bramich, J. Carpenter, P. Challis, et al. As radio fields of view continue to increase and computing capability grows to allow wide-field, rapid-cadence, radio imaging, new surveys will uncover many more transient events of both known and unknown origin. These events have the potential to tell about particle acceleration, stellar magnetic fields and rotation, strong-field gravity, the interstellar and intergalactic media, the violent deaths of stars, and possibly physics beyond the standard model.

In summary, the time domain represents great discovery potential well matched to the timescales that are relevant for stellar phenomena during their lifetimes and their death throes. Astronomers look forward to the next decade as a period of renaissance for stellar astronomy as time information is added to the new advances in three-dimensional spatial resolution and the idealization of a star as a static, spherical object is put to bed.

Every 10 years the National Research Council releases a survey of astronomy and astrophysics outlining priorities for the coming decade. The most recent survey, titled New Worlds, New Horizons in Astronomy and Astrophysics , provides overall priorities and recommendations for the field as a whole based on a broad and comprehensive examination of scientific opportunities, infrastructure, and organization in a national and international context. Panel Reports--New Worlds, New Horizons in Astronomy and Astrophysics is a collection of reports, each of which addresses a key sub-area of the field, prepared by specialists in that subarea, and each of which played an important role in setting overall priorities for the field.

The collection, published in a single volume, includes the reports of the following panels:. The Committee for a Decadal Survey of Astronomy and Astrophysics synthesized these reports in the preparation of its prioritized recommendations for the field as a whole. These reports provide additional depth and detail in each of their respective areas. The book of panel reports will be useful to managers of programs of research in the field of astronomy and astrophysics, the Congressional committees with jurisdiction over the agencies supporting this research, the scientific community, and the public.

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What are the progenitors of Type Ia supernovae and how do they explode? How do the lives of massive stars end? What controls the mass, radius, and spin of compact stellar remnants? The subsections below summarize the main points. Discovery Area: Time-Domain Astronomy. SSE 1. Beyond the Standard Picture.

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How Are Chromospheres and Coronae Formed? SSE 2. The Standard Picture. Opportunities for the Coming Decade. SSE 3. Blasts from the Past. SSE 4. Electromagnetic Counterparts to Gravitational Wave Sources. Eclipsing Binaries and Binary Star Evolution.


This page intentionally left blank. Login or Register to save! Stay Connected! Variable Stars and the Sun. Spectroscopy, photometry, spectropolarimetry. Photometry, spectroscopy, radial velocities. Multiwavelength photometry, spectroscopy; improved models. Pulsating variables—classical, rare. Photometry, spectroscopy; interferometry; improved models. Photometry, spectroscopy; improved models; interferometry. Despite the fact that major efforts have being carried out on both observational and theoretical grounds in recent years, our knowledge of AGB stars is still deficient due to uncertainties related to mass loss, convection, mixing, dredge-up efficiencies, and the role of binary interaction processes in many observed phenomena.

These uncertainties in our understanding of AGB stars directly propagate into the field of extragalactic astronomy, where they affect critically the interpretation of galaxy properties, e. The complexity of the objects also makes it difficult for individual researchers to master all aspects of their role as galaxy inhabitants, a problem that the proposed symposium aims to illuminate and overcome.

Problems in Stellar Atmospheres and Envelopes Problems in Stellar Atmospheres and Envelopes
Problems in Stellar Atmospheres and Envelopes Problems in Stellar Atmospheres and Envelopes
Problems in Stellar Atmospheres and Envelopes Problems in Stellar Atmospheres and Envelopes
Problems in Stellar Atmospheres and Envelopes Problems in Stellar Atmospheres and Envelopes
Problems in Stellar Atmospheres and Envelopes Problems in Stellar Atmospheres and Envelopes

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