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Smart Deals. Big Savings. Compare Save on Stoke. Match Preview: West Ham v Stoke. West Ham head to the Britannia hopeful of securing Europa League football for their first season in the Olympic Stadium. Bubble-blowing Hammers bouncing, Potters blowing hot and cold. Heady days indeed down at Upton Park: four wins in five, sitting snugly in a Champions League. West Ham must wait to see if they will secure European football next season as Mame Biram Diouf's header earns Stoke victory.
West Ham host Stoke City in the Premier League this weekend as they seek to continue their unbeaten start to the campaign. Stoke City vs West Ham: premier League, match preview, form comparison, predicted line up, head to head stats, statistical betting prediction. West Ham v Stoke Preview. Stoke almost snatched a late winner but Adrian palmed a Charlie Adam free-kick wide. Winston Reid banned. Citeste mai departe. Copertura in diretta di Newcastle vs Stoke City sabato.
West Ham United must put the disappointment of Monday night's home loss to fellow Premier League strugglers Wolverhampton Wanderers. Parker 84 M. Obinna Anteprima. Stoke City is ranked on the 14th place in the standings with 42 points and they have failed to win 26 of the last 37 league matches. West Ham is ranked. Score updates and latest action from the London Stadium Slaven Bilic will hope that the headlines are one the pitch.
In some cases outflows commensurate with stellar escape velocities are present. Our current understanding of cool star winds will be reviewed including the implications of stellar observations for identification of atmospheric heating and acceleration processes.
Solar and stellar atmospheric phenomena and their fundamental physical properties such as gravity, effective temperature and rotation rate, which provides the range in parameter space required to test various theoretical models were investigated. The similarity between solar activity and stellar activity is documented. Some of the topics discussed are: atmospheric structure, magnetic fields, solar and stellar activity, and evolution.
Attention is also given to modeling stellar angular momentum evolution, extended micron emission from nearby Mira variables, the PANDORA atmosphere program, the global properties of active regions, oscillations in a stratified atmosphere, lithium abundances in northern RS CVn binaries, a new catalog of cool dwarf stars , the Far UV Spectrograph Explorer, and development of reflecting coronagraphs.
Definition and empirical structure of the range of stellar chromospheres-coronae across the H-R diagram: Cool stars. This observational evidence is reviewed and the trends that are now becoming apparent are discussed. The evidence for non-radiatively heated outer atmospheric layers chromospheres, transition regions, and coronae in dwarf stars cooler than spectral type A7, in F and G giants, pre-main sequence stars , and close bindary systems is unambiguous, as is the evidence for chromospheres in the K and M giants and supergiants.
The existence of non-radiative heating in the outer layers of the A stars remains undetermined despite repeated searches at all wavelengths. Two important trends in the data are the decrease in plasma emission measure with age on the main sequence and decreasing rotational velocity. Variability and atmospheric inhomogeneity are commonly seen, and there is considerable evidence that magnetic fields define the geometry and control the energy balance in the outer atmospheric layers.
In addition, the microwave observations imply that non-thermal electrons are confined in coronal magnetic flux tubes in at least the cool dwarfs and RS CVn systems. The chromospheres in the K and M giants and supergiants are geometrically extended, as are the coronae in the RS CVn systems and probably also in other stars.
Heavy Elements and Cool Stars. We report on progress in the analysis of high-resolution near-IR spectra of alpha Orionis M2 Iab and other cool , luminous stars. Using synthetic spectrum techniques, we search for atomic absorption lines in the stellar spectra and evaluate the available line parameter data for use in our abundance analyses. Our study concentrates on the post iron-group elements copper through zirconium as a means of investigating the slow neutron-capture process of nucleosynthesis in massive stars and the mechanisms that transport recently processed material up into the photospheric region.
We discuss problems with the atomic data and model atmospheres that need to be addressed before theoretically derived elemental abundances from pre-supernova nucleosynthesis calculations can be tested by comparison with abundances determined from observations of cool , massive stars.
The X-ray emission from flares on cool i. Aims: We present a uniform, large-scale survey of X-ray flare emission. The XMM-Newton Serendipitous Source Catalogue and its associated data products provide an excellent basis for a comprehensive and sensitive survey of stellar flares - both from targeted active stars and from those observed serendipitously in the half-degree diameter field-of-view of each observation.
Methods: The 2XMM Catalogue and the associated time-series "light-curve" data products have been used as the basis for a survey of X-ray flares from cool stars in the Hipparcos-Tycho-2 catalogue. In addition, we have generated and analysed spectrally-resolved i. The hardness-ratio plots clearly illustrate the spectral and hence inferred temperature variations characteristic of many flares, and provide an easily accessible overview of the data.
We present flare frequency distributions from both target and serendipitous observations. The latter provide an unbiased with respect to stellar activity study of flare energetics; in addition, they allow us to predict numbers of stellar flares that may be detected in future X-ray wide-field surveys. The serendipitous sample demonstrates the need for care when calculating flaring rates, especially when normalising the number of flares to a total.
Stellar Evolution and Modelling Stars. In this chapter I give an overall description of the structure and evolution of stars of different masses, and review the main ingredients included in state-of-the-art calculations aiming at reproducing observational features.
I give particular emphasis to processes where large uncertainties still exist as they have strong impact on stellar properties derived from large compilations of tracks and isochrones, and are therefore of fundamental importance in many fields of astrophysics. Cooling of neutron stars. It is at present impossible to predict the interior constitution of neutron stars based on theory and results from laboratory studies.
It has been proposed that it is possible to obtain information on neutron star interiors by studying thermal radiation from their surfaces, because neutrino emission rates, and hence the temperature of the central part of a neutron star , depend on the properties of dense matter. The theory predicts that neutron stars cool relatively slowly if their cores are made up of nucleons, and cool faster if the matter is in an exotic state, such as a pion condensate, a kaon condensate, or quark matter.
This view has recently been questioned by the discovery of a number of other processes that could lead to copious neutrino emission and rapid cooling. Weighing Ultra- Cool Stars. The two stars form a binary system and orbit each other in about 10 years. The team obtained high-resolution near-infrared images; on the ground, they defeated the blurring effect of the terrestrial atmosphere by means of adaptive optics techniques.
By precisely determining the orbit projected on the sky, the astronomers were able to measure the total mass of the stars. Additional data and comparison with stellar models then yield the mass of each of the components. The heavier of the two stars has a mass around 8. Both objects are relatively young with an age of about , million years. These observations represent a decisive step towards the still missing calibration of stellar evolution models for very-low mass stars.
Telephone number star Even though astronomers have found several hundreds of very low mass stars and brown dwarfs, the fundamental properties of these extreme objects, such as masses and surface temperatures, are still not well known. Within the cosmic zoo, these ultra- cool stars represent a class of "intermediate" objects between giant planets - like Jupiter - and "normal" stars less massive than our Sun, and to understand them well is therefore crucial to the field of stellar astrophysics.
The problem with these ultra- cool stars is that contrary to normal stars that burn hydrogen in their central core, no unique relation exists between the luminosity of the. Coronal Structures in Cool Stars. We have extended our study of the structure of coronas in cool stars to very young stars still accreting from their surrounding disks.
In addition we are pursing the connection between coronal X-rays and a powerful diagnostic line in the infrared, the He I Angstrom transition of helium. Highlights of these are summarized below including publications during this reporting period and presentations. Spectra of T Tauri N appear consistent with such a wind. The source of the emission and outflow seems restricted to the stars themselves.
Although the mass accretion rate is an order of magnitude less for TW Hya than for T Tau, the outflow reaches higher velocities at chromospheric temperatures in TW Hya. Winds from young stellar objects may be substantially hotter and faster than previously thought. The ultraviolet emission lines, when corrected for absorption are broad. Emission associated with the accretion flow and shock is likely to show turbulent broadening.
We note that the UV line widths are significantly larger than the X-ray line widths. If the X-rays from TW Hya are generated at the accretion shock, the UV lines may not be directly associated with the shock. On the other hand, studies of X-ray emission in young star clusters, suggest that the strength of the X-ray emission is correlated with stellar rotation, thus casting doubt on an accretion origin for the X-rays. We are beginning to access the infrared spectral region where the He I Angstroms transition.
Traditional stellar structure models of non-rotating neutron stars work under the assumption that these stars are perfect spheres. This assumption of perfect spherical symmetry is not correct if the matter inside neutron stars is described by an anisotropic model for the equation of state. Certain classes of neutron stars such as Magnetars and neutron stars which contain color-superconducting quark matter cores are expected to be deformed making them oblong spheroids.
In this work, we investigate the stellar structure of these deformed neutron stars by deriving stellar structure equations in the framework of general relativity. Using a non-isotropic equation of state model, we solve these structure equations numerically in two dimensions. We calculate stellar properties such as masses and radii along with pressure profiles and investigate changes from standard spherical models.
The early chemical evolution of the Galaxy and the Universe is vital to our understanding of a host of astrophysical phenomena. Since the most metal-poor Galactic stars are relics from the high-redshift Universe, they probe the chemical and dynamical conditions as the Milky Way began to form, the origin and evolution of the elements, and the physics of nucleosynthesis.
They also provide constraints on the nature of the first stars , their associated supernovae and initial mass function, and early star and galaxy formation. I will present exemplary metal-poor stars with which these different topics can be addressed. Their stellar chemical signatures support the concept that small systems, analogous to the surviving dwarf galaxies, were the building blocks of the Milky Way's low-metallicity halo. This opens a new window for studying galaxy formation through stellar chemistry.
Gas expulsion vs gas retention in young stellar clusters II: effects of cooling and mass segregation. Gas expulsion or gas retention is a central issue in most of the models for multiple stellar populations and light element anti-correlations in globular clusters. The success of the residual matter expulsion or its retention within young stellar clusters has also a fundamental importance in order to understand how star formation proceeds in present-day and ancient star -forming galaxies and if proto-globular clusters with multiple stellar populations are formed in the present epoch.
It is usually suggested that either the residual gas is rapidly ejected from star -forming clouds by stellar winds and supernova explosions, or that the enrichment of the residual gas and the formation of the second stellar generation occur so rapidly, that the negative stellar feedback is not significant. Here we continue our study of the early development of star clusters in the extreme environments and discuss the restrictions that strong radiative cooling and stellar mass segregation provide on the gas expulsion from dense star -forming clouds.
A large range of physical initial conditions in star -forming clouds which include the star -forming cloud mass, compactness, gas metallicity, star formation efficiency and effects of massive stars segregation are discussed. It is shown that in sufficiently massive and compact clusters hot shocked winds around individual massive stars may cool before merging with their neighbors.
This dramatically reduces the negative stellar feedback, prevents the development of the global star cluster wind and expulsion of the residual and the processed matter into the ambient interstellar medium. The critical lines which separate the gas expulsion and the gas retention regimes are obtained. Geller, Aaron M. Strong encounters between single stars and binaries play a pivotal role in the evolution of star clusters.
Such encounters can also dramatically modify the orbital parameters of binaries, exchange partners in and out of binaries, and are a primary contributor to the rate of physical stellar collisions in star clusters. Often, these encounters are studied under the approximation that they happen quickly enough and within a small enough volume to be considered isolated from the rest of the cluster. For each encounter we evaluate the encounter duration, and compare this with the expected time until another single or binary star will join the encounter.
Thus, the assumption that strong encounters occur in relative isolation breaks down for certain clusters. The APOKASC effort combines this data with asteroseismology and star spot studies, resulting in more than 7, stellar mass estimates for dwarfs and giants with high quality abundances, temperatures, and surface gravities. We highlight the main results from this effort so far, which include a tight correlation between surface abundances in giants and stellar mass, precise absolute gravity calibrations, and the discovery of unexpected stellar populations, such as young alpha-enhanced stars.
We discuss grid modeling estimates for stellar masses and compare the absolute asteroseismic mass scale to calibrators in star clusters and the halo Directions for future efforts are discussed. Stellar and Binary Evolution in Star Clusters. This paper presents a final report on research activities covered on Stellar and Binary Evolution in Star Clusters.
Substantial progress was made in the development and dissemination of the "Starlab" software environment. Significant improvements were made to "kira," an N-body simulation program tailored to the study of dense stellar systems such as star clusters and galactic nuclei.
Key advances include 1 the inclusion of stellar and binary evolution in a self-consistent manner, 2 proper treatment of the anisotropic Galactic tidal field, 3 numerous technical enhancements in the treatment of binary dynamics and interactions, and 4 full support for the special-purpose GRAPE-4 hardware, boosting the program's performance by a factor of over the accelerated version.
The data-reduction and analysis tools in Starlab were also substantially expanded. The site contains detailed information on the structure and function of the various tools that comprise the package, as well as download information, "how to" tips and examples of common operations, demonstration programs, animations, etc.
All versions of the software are freely distributed to all interested users, along with detailed installation instructions. Chemical Soups Around Cool Stars. This artist's conception shows a young, hypothetical planet around a cool star. A soupy mix of potentially life-forming chemicals can be seen pooling around the base of the jagged rocks. Observations from NASA's Spitzer Space Telescope hint that planets around cool stars the so-called M-dwarfs and brown dwarfs that are widespread throughout our galaxy might possess a different mix of life-forming, or prebiotic, chemicals than our young Earth.
Life on our planet is thought to have arisen out of a pond-scum-like mix of chemicals. Some of these chemicals are thought to have come from a planet-forming disk of gas and dust that swirled around our young sun. Meteorites carrying the chemicals might have crash-landed on Earth. Astronomers don't know if these same life-generating processes are taking place around stars that are cooler than our sun, but the Spitzer observations show their disk chemistry is different.
Spitzer detected a prebiotic molecule, called hydrogen cyanide, in the disks around yellow stars like our sun, but found none around cooler, less massive, reddish stars. Hydrogen cyanide is a carbon-containing, or organic compound. Five hydrogen cyanide molecules can join up to make adenine a chemical element of the DNA molecule found in all living organisms on Earth. Cooling of hypernuclear compact stars. We study the thermal evolution of hypernuclear compact stars constructed from covariant density functional theory of hypernuclear matter and parametrizations which produce sequences of stars containing two-solar-mass objects.
We find that intermediate-mass 1. Multiple Stellar Populations in Star Clusters. For half a century it had been astronomical dogma that a globular cluster GC consists of stars born at the same time out of the same material, and this doctrine has borne rich fruits.
In recent years, high resolution spectroscopy and high precision photometry from space and ground-based observations have shattered this paradigm, and the study of GC populations has acquired a new life that is now moving it in new directions. Evidence of multiple stellar populations have been identified in the color-magnitude diagrams of several Galactic and Magellanic Cloud GCs where they had never been imagined before.
Unbound young stellar systems, the loose ensembles of physically related young bright stars , trace the typical regions of recent star formation in galaxies. Their morphologies vary from small few pc-size associations of newly formed stars to enormous few kpc-size complexes composed of stars few Myr old. These stellar conglomerations are located within the disks and along the spiral arms and rings of star -forming disk galaxies, and they are the active star -forming centers of dwarf and starburst galaxies.
Being associated with star -forming regions of various sizes, these stellar structures trace the regions where stars form at various length- and timescales, from compact clusters to whole galactic disks. Stellar associations, the prototypical unbound young systems, and their larger counterparts, stellar aggregates, and stellar complexes, have been the focus of several studies for quite a few decades, with special interest on their demographics, classification, and structural morphology.
The compiled surveys of these loose young stellar systems demonstrate that the clear distinction of these systems into well-defined classes is not as straightforward as for stellar clusters, due to their low densities, asymmetric shapes and variety in structural parameters. These surveys also illustrate that unbound stellar structures follow a clear hierarchical pattern in the clustering of their stars across various scales. Stellar associations are characterized by significant sub-structure with bound stellar clusters being their most compact parts, while associations themselves are the brighter denser parts of larger stellar aggregates and stellar complexes, which are members of larger super-structures up to the scale of a whole star -forming galaxy.
This structural pattern, which is usually characterized as self-similar or fractal, appears to be identical to that of star -forming giant molecular clouds and interstellar gas, driven mainly by turbulence cascade. In this short. In addition we are exploring the effects of coronas on the He I transition that is observed in the infrared. Ground-based magnetic Doppler imaging of cool stars suggests that active stars have active regions located at high latitudes on their surface.
We have performed similar imaging in X-ray to locate the sites of enhanced activity using Chandra spectra. After careful analysis of the spectrum of each binary, a composite line-profile is constructed by adding the individual spectral lines. In conjunction with X-ray lightcurves, the phase-binned composite line-profiles constrain coronal structures to be small and located at high latitudes. These observations and techniques show the power of the Doppler Imaging Technique applied to X-ray line emission.
Stellar 'Incubators' Seen Cooking up Stars. Data of this same region from the Institute for Radioastronomy millimeter telescope in Spain revealed four dense knots, or cores, of dust outlined by yellow circles , which are 'incubators' for embryonic stars. Astronomers thought these cores were not yet ripe for stars , until Spitzer spotted the warmth of rapidly growing massive embryos tucked inside.
These embryos are indicated with arrows in the false-color Spitzer picture right, figure 1 , taken by the telescope's infrared array camera. The same embryos cannot be seen in the visible-light pictures left, figure 1. Spitzer found clusters of embryos in two of the cores and only single embryos in the other two. This is one of the first times that multiple embryos have been observed in individual cores at this early stage of stellar development. Chromospheric Activity in Cool Luminous Stars.
Spatially unresolved spectra of giant and supergiant stars demonstrate ubiquitous signatures of chromospheric activity, variable outflows, and winds. The advent of imaging techniques and spatially resolved spectra reveal complex structures in these extended stellar atmospheres that we do not understand. The presence and behavior of these atmospheres is wide ranging and impacts stellar activity, magnetic fields, angular momentum loss, abundance determinations, and the understanding of stellar cluster populations.
Here, we present a method to estimate distances to stars with spectroscopically derived stellar parameters. The technique is a Bayesian approach with likelihood estimated via comparison of measured parameters to a grid of stellar isochrones, and returns a posterior probability density function for each star 's absolute magnitude.
This removes biasing assumptions about the underlying populations, both due to predictions of the luminosity function from stellar evolution modeling, and from Galactic models of stellar populations along each line of sight. Mass-loss rates of cool stars. Over much of the initial mass function, stars lose a significant fraction of their mass through a stellar wind during the late stages of their evolution when being a super giant star.
As of today, we can not yet predict the mass-loss rate during the super giant phase for a given star with specific stellar parameters from first principles. This uncertainty directly impacts the accuracy of current stellar evolution and population synthesis models that predict the enrichment of the interstellar medium by these stellar winds.
Efforts to establish the link between the initial physical and chemical conditions at stellar birth and the mass-loss rate during the super giant phase have proceeded on two separate tracks: 1 more detailed studies of the chemical and morpho-kinematical structure of the stellar winds of super giant stars in our own Milky Way by virtue of the proximity, and 2 large scale and statistical studies of a large sample of stars in other galaxies such as the LMC and SMC and globular clusters eliminating the uncertainty on the distance estimate and providing insight into the dependence of the mass-loss rate on the metallicity.
In this review, I will present recent results of both tracks, will show how recent measurements confirm some theoretical predictions, but also how results from the first track admonish of common misconceptions inherent in the often more simplified analysis used to analyse the large samples from track 2.
High temperature structure in cool binary stars. Strong high temperature emission lines in the EUVE spectra of binary stars containing cool components Alpha Aur Capella , 44 iota Boo, Lambda And, and VY Ari provide the basis to define reliably the differential emission measure of hot plasma.
The emission measure distributions for the short-period P less than or equal to 13 d binary systems show a high temperature enhancement over a relatively narrow temperature region similar to that originally found in Capella Dupree et al. The emission measure distributions of rapidly rotating single stars 31 Com and AB Dor also contain a local enhancement of the emission measure although at different temperatures and width from Capella, suggesting that the enhancement in these objects may be characteristic of rapid rotation of a stellar corona.
This feature might be identified with a polar active region, although its density and absolute size are unknown; in the binaries Capella and VY Ari, the feature is narrow and it may arise from an interaction region between the components. One of the great challenges of understanding stars is measuring their masses. The best methods for measuring stellar masses include binary interaction, asteroseismology, and stellar evolution models, but these methods are not ideal for red giant and supergiant stars.
In this work, we propose a novel method for inferring stellar masses of evolved red giant and supergiant stars using interferometric and spectrophotometric observations combined with spherical model stellar atmospheres to measure what we call the stellar mass index, defined as the ratio between the stellar radius and mass.
For a given star , spectrophotometry measures the Rosseland angular diameter while interferometric observations generally probe a larger limb-darkened angular diameter. We show that these correlations are strong and can lead to precise measurements of stellar masses. Mass and energy injection throughout the lifetime of a star cluster contributes to the gas reservoir available for subsequent episodes of star formation and the feedback energy budget responsible for ejecting material from the cluster.
In addition, mass processed in stellar interiors and ejected as winds has the potential to augment the abundance ratios of currently forming stars , or stars which form at a later time from a retained gas reservoir. Here we present hydrodynamical simulations that explore a wide range of cluster masses, compactnesses, metallicities and stellar population age combinations in order to determine the range of parameter space conducive to stellar wind retention or wind powered gas expulsion in star clusters.
Towards new generation spectroscopic models of cool stars. Abstract: Spectroscopy is a unique tool to determine the physical parameters of stars. Knowledge of stellar chemical abundances, masses, and ages is the key to understanding the evolution of their host populations. I will focus on the current outstanding problems in spectroscopy of cool stars , which are the most useful objects in studies of our local Galactic neighborhood but also very distant systems, like faint dwarf Spheroidal galaxies.
Among the most debated issues is to what extent can we trust the techniques, which rely on the classical assumptions of local thermodynamic equilibrium and hydrostatic balance. I will summarise the ongoing efforts to improve the models of cool stars , with the emphasis on NLTE and 3D modelling. I will then discuss how these exciting observations impact our knowledge of abundances in the Milky Way and in dSph systems, and present outlook for the future studies.
The central compact object in the supernova remnant HESS J appears to be the hottest observed isolated cooling neutron star. The cooling theory of neutron stars enables one to explain observations of this star by assuming the presence of strong proton superfluidity in the stellar core and the existence of the surface heat blanketing envelope which almost fully consists of carbon. The cooling model of this star is elaborated to take proper account of the neutrino emission due to neutron-neutron collisions which is not suppressed by proton superfluidity.
Using the results of spectral fits of observed thermal spectra for the distance of 3. Stellar multiplicity has significant implications for the detection and characterization of exoplanets. A stellar companion can mimic the signal of a transiting planet or distort the true planetary radii, leading to improper density estimates and over-predicting the occurrence rates of Earth-sized planets. Determining the fraction of exoplanet host stars that are also binaries allows us to better determine planetary characteristics as well as establish the relationship between binarity and planet formation.
Using high-resolution speckle imaging to obtain diffraction limited images of K2 planet candidate host stars we detect stellar companions within one arcsec and up to six magnitudes fainter than the host star. By comparing our observed companion fraction to TRILEGAL star count simulations, and using the known detection limits of speckle imaging, we find the binary fraction of K2 planet host stars to be similar to that of Kepler host stars and solar-type field stars. Accounting for stellar companions in exoplanet studies is therefore essential for deriving true stellar and planetary properties as well as maximizing the returns for TESS and future exoplanet missions.
For lower Z, denser gas can be heated up to the coronal temperature because of the inefficient radiation cooling. This indicates that metal accretion on low-mass Pop. III stars is negligible. The soft X-ray flux of the Pop. Therefore, a larger fraction is converted to the thermal energy of the corona and the kinetic energy of the stellar wind. Chromospheric activity of cool giant stars. During the seventh year of IUE twenty-six spectra of seventeen cool giant stars ranging in spectral type from K3 thru M6 were obtained.
Together with spectra of fifteen stars observed during the sixth year of IUE, these low-resolution spectra have been used to: 1 examine chromospheric activity in the program stars and late type giants in general, and 2 evaluate the extent to which nonradiative heating affects the upper levels of cool giant photospheres.
The stars observed in this study all have well determined TiO band strengths, angular diameters determined from lunar occulations , bolometric fluxes, and effective temperatures. Chromospheric activity can therefore be related to effective temperatures providing a clearer picture of activity among cool giant stars than previously available.
The stars observed are listed. Effects of stellar evolution and ionizing radiation on the environments of massive stars. We discuss two important effects for the astrospheres of runaway stars : the propagation of ionizing photons far beyond the astropause, and the rapid evolution of massive stars and their winds near the end of their lives.
Hot stars emit ionizing photons with associated photoheating that has a significant dynamical effect on their surroundings. For late O stars this feedback to the interstellar medium is more important than that from stellar winds. Late in life, O stars evolve to cool red supergiants more rapidly than their environment can react, producing transient circumstellar structures such as double bow shocks.
This provides an explanation for the bow shock and linear bar-shaped structure observed around Betelgeuse. Our primary goals were to estimate flare frequency for the largest flares in the active binary systems, and, if the data permitted, to derive a distribution of flare energy vs. However, the combination of the higher than expected and variable background in the ALEXIS detectors, and the lower throughput of the ALEXIS telescopes resulted in no convincing detections of large flares from the active binary systems.
Lithium in halo stars from standard stellar evolution. A grid has been constructed of theoretical evolution sequences of models for low-metallicity stars from the premain-sequence to the giant branch phases. The grid is used to study the history of surface Li abundance during standard stellar evolution. The Li-7 observations of halo stars by Spite and Spite and subsequent observations are synthesized to separate the halo stars by age. The theory of surface Li abundance is illustrated by following the evolution of a reference halo star model from the contracting fully convective premain sequence to the giant branch phase.
The theoretical models are compared with observed Li abundances. The results show that the halo star lithium abundances can be explained in the context of standard stellar evolution theory using completely standard assumptions and physics. Regulation of star formation and outflows by stellar winds and supernovae. We study the impact of stellar winds and supernovae on the multiphase interstellar medium using three-dimensional hydrodynamical simulations carried out with FLASH.
The simulations include an external stellar potential and gas self-gravity, radiative cooling and diffuse heating, sink particles representing star clusters, stellar winds from these clusters that combine the winds from individual massive stars by following their evolution tracks, and subsequent supernova explosions.
Dust and gas self- shielding is followed to compute the chemical state of the gas with a chemical network. We find that stellar winds can regulate star cluster formation. Since the winds suppress the accretion of fresh gas soon after the cluster has formed, they lead to clusters that have lower average masses In particular, we find an anticorrelation of cluster mass and accretion time-scale.
Overall, the most massive stars provide the most wind energy input, while objects beginning their evolution as B-type stars contribute most of the supernova energy input. Stellar winds alone cannot create a hot volume-filling phase. The models that are in best agreement with observed star formation rates drive either no outflows or weak outflows. Gray Star : Web-based pedagogical stellar modeling. Gray Star is a web-based pedagogical stellar model. It is suitable for a wide range of education and public outreach levels depending on which optional plots and print-outs are turned on.
All plots and renderings are pure basic HTML and the plotting module contains original HTML procedures for automatically scaling and graduating x- and y-axes. Thermal structure and cooling of neutron stars with magnetized envelopes.
The thermal structure of neutron stars with magnetized envelopes is studied using modern physics input. The relation between the internal Tint and local surface temperatures is calculated and fitted by analytic expressions for magnetic field strengths B from 0 to G and arbitrary inclination of the field lines to the surface. The luminosity of a neutron star with dipole magnetic field is calculated and fitted as a function of B, Tint, stellar mass and radius.
In addition, we simulate cooling of neutron stars with magnetized envelopes. In particular, we analyse ultramagnetized envelopes of magnetars and also the effects of the magnetic field of the Vela pulsar on the determination of critical temperatures of neutron and proton superfluids in its core. We analyze the optical continuum of star -forming galaxies in the Sloan Digital Sky Survey by fitting stacked spectra with stellar population synthesis models to investigate the relation between stellar mass, stellar metallicity, dust attenuation, and star formation rate.
We fit models calculated with star formation and chemical evolution histories that are derived empirically from multi-epoch observations of the stellar mass— star formation rate and the stellar mass—gas-phase metallicity relations, respectively. We also fit linear combinations of single-burst models with a range of metallicities and ages.
Star formation and chemical evolution histories are unconstrained for these models. These relations are also consistent with the relation obtained from emission-line analysis of gas-phase metallicity after accounting for systematic offsets in the gas-phase metallicity. We measure dust attenuation of the stellar continuum and show that its dependence on stellar mass and star formation rate is consistent with previously reported results derived from nebular emission lines.
However, stellar continuum attenuation is smaller than nebular emission line attenuation. The continuum-to-nebular attenuation ratio depends on stellar mass and is smaller in more massive galaxies. Our consistent analysis of stellar continuum and nebular emission lines paves the way for a comprehensive investigation of stellar metallicities of star -forming and quiescent galaxies.
S stars in the Gaia era: stellar parameters and nucleosynthesis. S stars are s-process and C-enriched 0. Synthetic spectra of dwarf S stars have been computed and will be sought for in spectroscopic survey data, constraining their luminosity thanks to Gaia parallaxes.
In this study, high school and first-year undergraduate students were asked about their understanding of stars. The hypothesis guiding this research posits that high school students who have taken a semester-long astronomy course will have an understanding of stars most related to scientific knowledge, compared with high school students enrolled….
Stellar and wind parameters of massive stars from spectral analysis. The only way to deduce information from stars is to decode the radiation it emits in an appropriate way. Spectroscopy can solve this and derive many properties of stars. In this work we seek to derive simultaneously the stellar and wind characteristics of A and B supergiant stars. Our stellar properties encompass the effective temperature, the surface gravity, the stellar radius, the micro-turbulence velocity, the rotational velocity and, finally, the chemical composition.
To model the data we use the radiative transport code Fastwind considering the newest hydrodynamical solutions derived with Hydwind code, which needs stellar and line-force parameters to obtain a wind solution. A grid of spectral models of massive stars is created and together with the observed spectra their physical properties are determined through spectral line fittings.
These fittings provide an estimation about the line-force parameters, whose theoretical calculations are extremely complex. In this work we seek to derive simultaneously the stellar and wind characteristics of a wide range of massive stars. Our stellar properties encompass the effective temperature, the surface gravity, the stellar radius, the micro-turbulence velocity, the rotational velocity and the Si abundance. Magnetic neutron star cooling and microphysics.
Aims: We study the relative importance of several recent updates of microphysics input to the neutron star cooling theory and the effects brought about by superstrong magnetic fields of magnetars, including the effects of the Landau quantization in their crusts.
Methods: We use a finite-difference code for simulation of neutron- star thermal evolution on timescales from hours to megayears with an updated microphysics input. For the strongly magnetized neutron stars , we take into account the effects of Landau quantization on thermodynamic functions and thermal conductivities. We simulate cooling of ordinary neutron stars and magnetars with non-accreted and accreted crusts and compare the results with observations.
Results: Suppression of radiative and conductive opacities in strongly quantizing magnetic fields and formation of a condensed radiating surface substantially enhance the photon luminosity at early ages, making the life of magnetars brighter but shorter. These effects together with the effect of strong proton superfluidity, which slows down the cooling of kiloyear-aged neutron stars , can explain thermal luminosities of about a half of magnetars without invoking heating mechanisms.
Observed thermal luminosities of other magnetars are still higher than theoretical predictions, which implies heating, but the effects of quantizing magnetic fields and baryon superfluidity help to reduce the discrepancy. Cooling of Accretion-Heated Neutron Stars. We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron- star crust and core that have been obtained from these studies.
Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron- star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i. The surface temperature is then a direct probe for the core temperature.
By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron- star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars. Clustered star formation and the origin of stellar masses.
Star clusters are ubiquitous in galaxies of all types and at all stages of their evolution. We also observe them to be forming in a wide variety of environments, ranging from nearby giant molecular clouds to the supergiant molecular clouds found in starburst and merging galaxies.
The typical star in our galaxy and probably in others formed as a member of a star cluster, so star formation is an intrinsically clustered and not an isolated phenomenon. The greatest challenge regarding clustered star formation is to understand why stars have a mass spectrum that appears to be universal. This review examines the observations and models that have been proposed to explain these fundamental issues in stellar formation.
Radial velocity studies of cool stars. Our current view of exoplanets is one derived primarily from solar-like stars with a strong focus on understanding our Solar System. Our knowledge about the properties of exoplanets around the dominant stellar population by number, the so-called low-mass stars or M dwarfs, is much more cursory.
Based on radial velocity discoveries, we find that the semi-major axis distribution of M dwarf planets appears to be broadly similar to those around more massive stars and thus formation and migration processes might be similar to heavier stars.
However, we find that the mass of M dwarf planets is relatively much lower than the expected mass dependency based on stellar mass and thus infer that planet formation efficiency around low-mass stars is relatively impaired. Spectroscopic observations of cool degenerate star candidates.
Spectroscopic observations are reported for 23 Luyten Half-Second degenerate star candidates and for 13 Luyten-Palomar common proper-motion pairs containing possible degenerate star components. Twenty-five degenerate stars are identified, 20 of which lack previous spectroscopy. Most of these stars are cool - Luyten color class g or later. A second degenerate star , LHS , exhibits apparent strong Swan bands which are blueshifted about 75 A. Further observations, including polarimetry and photometry, are required to appraise the spectroscopic peculiarities of these stars.
Finally, five cool , sharp-lined DA white dwarfs have been observed to detect lines of metals and to determine line strengths. The attempt to detect Ca MI in the fifth star , G, was inconclusive. Gravitational Conundrum?
Upon their formation, dynamically cool collapsing star clusters will, within only a few million years, achieve stellar mass segregation for stars down to a few solar masses, simply because of gravitational two-body encounters. Since binary systems are, on average, more massive than single stars , one would expect them to also rapidly mass segregate dynamically. If confirmed, then this will offer support for the theoretically predicted but thus far unobserved dynamical disruption processes of the significant population of "soft" binary systems—with relatively low binding energies compared to the kinetic energy of their stellar members—in star clusters, which we have access to here by virtue of the cluster's unique combination of youth and high stellar density.
Upon their formation, dynamically cool collapsing star clusters will, within only a few million years, achieve stellar mass segregation for stars down to a few solar masses due to gravitational two-body encounters. Contrary to these expectations and based on high-resolution Hubble Space Telescope observations, we show that the compact, Myr-old Large Magellanic Cloud cluster NGC is characterized by an increasing fraction of F- star binary systems with combined masses of 1.
This offers unprecedented support of the theoretically predicted but thus far unobserved dynamical disruption processes of the significant population of "soft" binary systems with relatively low binding energies compared to the kinetic energy of their stellar members in star clusters, which we could unravel by virtue of the cluster's unique combination of youth and high stellar density. Improvements in MESA star 's ability to model the evolution of giant planets now extends its applicability down to masses as low as one-tenth that of Jupiter.
This also motivates a numerical recasting of the Ledoux criterion that is more easily implemented when many nuclei are present at non-negligible abundances. This impacts the way in which MESA star calculates semi-convective and thermohaline mixing. We implement diffusion of angular momentum and chemical abundances that enable calculations of rotating- star models, which we compare thoroughly with earlier work.
We introduce a new treatment of radiation-dominated envelopes that allows the uninterrupted evolution of massive stars to core collapse. This enables the generation of new sets of supernovae, long gamma-ray burst, and pair-instability progenitor models. We substantially modify the way in which MESA star solves the fully coupled stellar structure and composition equations, and we show how this has improved the scaling of MESA's calculational speed on multi-core processors.
Updates to the modules for equation of state, opacity, nuclear reaction rates, and atmospheric boundary conditions are also provided. We also highlight a few tools developed by the community for rapid visualization of MESA star results. Geminga: A cooling superfluid neutron star. We compare the recent temperature estimate for Geminga with neutron star cooling models. Because of its age approximately 3.
We show that its surface temperature approximately 5. However, for both types of scenarios, agreement with the observed temperature can only be obtained if baryon pairing is present in most, if not all, of the core of the star. Within the slow neutrino cooling scenario, early neutrino cooling is not sufficient to explain the observed low temperature, and extensive pairing in the core is necessary to reduce the specific heat and increase the cooling rate in the present photon cooling era.
Within all the fast neutrino cooling scenarios, pairing is necessary throughout the whole core to control the enormous early neutrino emission which, without pairing suppression, would result in a surface temperature at the present time much lower than observed. If one assumes that all neutron stars undergo fast neutrino cooling , then these two objects also provide evidence for extensive baryon pairing in their core; but observational uncertainties also permit a more conservative interpretation, with slow neutrino emission and no pairing at all.
We argue though that observational evidence for the slow neutrino cooling model the 'standard' model is in fact very dim and that the interpretation of the surface temperature of all neutron stars could be done with a reasonable theoretical a priori within the fast neutrino cooling scenarios only.
Stellar model chromospheres. XIII - M dwarf stars. Single-component, homogeneous model chromospheres that are consistent with high-resolution profiles of the Ca II K line calibrated in surface flux units for three dMe and 2 dM stars observed at quiescent times are constructed. The models reveal several systematic trends.
Large values of the ratio of T min to T eff are derived, indicating a large amount of nonradiative heating present in the upper photospheres of M dwarf stars. It is also found that the lower chromospheric temperature gradient is similar for all the M dwarf stars. Since for the models here the chromospheric K line emission strength is most sensitive to the total amount of chromospheric material present within the approximate temperature range T min K, increasing the emission strength is not simply due to increasing chromospheric temperature gradients.
It is also found that both the electron density and electron temperature at one thermalization length in the K line below the top of the chromospheres are greater in the dMe stars than in the dM stars. Stellar populations in local star -forming galaxies.
The main goal of this thesis work is studying the main properties of the stellar populations embedded in a statistically complete sample of local active star -forming galaxies: the Universidad Complutense de Madrid UCM Survey of emission-line galaxies. This sample contains local star -forming galaxies at an average redshift of 0.
The survey was carried out using an objective-prism technique centered at the wavelength of the Halpha nebular emission-line a common tracer of recent star formation. We are calculating stellar spectra using Kurucz codes, Castelli models, and Kurucz laboratory lines plus guesses; but must first finish adjusting gf values to match stars of solar metallicity and higher. Moreover, weighted coadditions of spectral calculations can be constructed that match M31 globular clusters over this entire wavelength range.
Both stellar and composite grids will be archived on MAST. The age-metallicity degeneracy can be broken, but only with high-quality data, and only if rare stages of stellar evolution are incorporated where necessary. Stellar Incubators Seen Cooking up Stars. This image composite compares visible-light and infrared views from NASA's Spitzer Space Telescope of the glowing Trifid Nebula, a giant star -forming cloud of gas and dust located 5, light-years away in the constellation Sagittarius.
Data of this same region from the Institute for Radioastronomy millimeter telescope in Spain revealed four dense knots, or cores, of dust outlined by yellow circles , which are "incubators" for embryonic stars. The full stellar population of NGC , one of the most spectacular regions of massive star formation in the nearby Galaxy, has not been well sampled in past studies. We analyze here a mosaic of two Chandra X-ray Observatory images of the region using sensitive data analysis methods, giving a list of faint X-ray sources with arcsecond positions and approximate line-of-sight absorption.
Extrapolating to low X-ray levels, the total stellar population is estimated to be 20,, pre-main-sequence stars. The heavily obscured clusters are mostly associated with previously known far-infrared sources and radio H II regions. The lightly obscured clusters are mostly newly identified in the X-ray images.
Dozens of likely OB stars are found, both in clusters and dispersed throughout the region, suggesting that star formation in the complex has proceeded over millions of years. A number of extraordinarily heavily absorbed X-ray sources are associated with the active regions of star formation. We present a 0. The stars have well-established MK spectral classifications and are mostly restricted to near-solar metallicities.
In contrast to some other spectral libraries, the continuum shape of the spectra is measured and preserved in the data reduction process. Potential uses of the library include studying the physics of cool stars , classifying and studying embedded young clusters and optically obscured regions of the Galaxy, evolutionary population synthesis to study unresolved stellar populations in optically obscured regions of galaxies and synthetic photometry.
Hypervelocity stars from young stellar clusters in the Galactic Centre. The enormous velocities of the so-called hypervelocity stars HVSs derive, likely, from close interactions with massive black holes, binary stars encounters or supernova explosions. In this paper, we investigate the origin of HVSs as consequence of the close interaction between the Milky Way central massive black hole and a passing-by young stellar cluster.
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