Stellar Center Research Articles

THE REMARKABLE DEATHS OF 9–11 SOLAR MASS STARS

The post-helium burning evolution of stars from 7 to 11 solar masses is complicated by the lingering effects of degeneracy and off-center ignition. Here stars in this mass range are studied using a standard set of stellar physics. Two important aspects of the study are the direct coupling of a reaction network of roughly 220 nuclei to the structure calculation at all stages and the use of a sub grid model to describe the convective bounded flame that develops during neon and oxygen burning. Below 9.0 solar masses, degenerate oxygen-neon cores form that may become either white dwarfs or electron-capture supernovae. Above 10.3 solar masses the evolution proceeds "normally" to iron-core collapse, without composition inversions or degenerate flashes. Emphasis here is upon the stars in between which typically ignite oxygen burning off center. After oxygen burns in a convectively bounded flame, silicon burning ignites in a degenerate flash that commences closer to the stellar center and with increasing violence for stars of larger mass. In some cases the silicon flash is so violent that it could lead to the early ejection of the hydrogen envelope. This might have interesting observable consequences. For example, the death of a 10.0 solar mass star could produce two supernova-like displays, a faint low energy event due to the silicon flash, and an unusually bright supernova many months later as the low energy ejecta from core collapse collides with the previously ejected envelope. The potential relation to the Crab supernova is discussed.

A calculator for Roche lobe properties

Abstract Background The Roche lobe geometry is important to understand and study the properties of the mass-losing component in a semi-detached binary system. However it is not easy to calculate accurately, and existing tables usually do not include the parameters of the binary system under study, nor do they allow for non-synchronous rotation. Results A calculator for properties of the Roche lobe is presented in two formats. An easy-to-use Java version has a graphic interface, and a Fortran 90 version has a command line interface. The Fortran version allows for easy modifications by the user. Both versions have two basic output options: one provides values of a set of various quantities (such as the Lagrange points along the binary axis, and area and volume of the Roche lobe); the second provides $R(\theta,\phi)$ R ( θ , ϕ ) , the distance from the stellar center to the stellar surface for any specified polar angle. A single set of input parameters can be entered directly or a large set of input parameters can be specified in a text file. The calculator includes the options to have non-synchronous rotation of the star, or to have the star underfill its Roche lobe. It can be used to calculate Roche lobe properties for the case of elliptical orbits, with some restrictions. Conclusions We present a convenient software tool for quickly and accurately calculating Roche lobe properties for mass ratio in the range 0.01 to 100, for Roche lobe fill-out factor in the range 0.1 to 1.0, and for dimensionless rotation rate of the star in the range 0.1 to 2.0. This will allow anyone working with a binary star system to obtain the Roche lobe or stellar surface geometry for their system.

AN OFF-CENTERED ACTIVE GALACTIC NUCLEUS IN NGC 3115

NGC 3115 is an S0 galaxy that has always been considered to have a pure absorption-line spectrum. Some recent studies have detected a compact radio-emitting nucleus in this object, coinciding with the photometric center and with a candidate for the X-ray nucleus. This is evidence of the existence of a low-luminosity active galactic nucleus (AGN) in the galaxy, although no emission line has ever been observed. We report the detection of an emission-line spectrum of a type 1 AGN in NGC 3115, with an H$\alpha$ luminosity of $L_{H\alpha} = (4.2 \pm 0.4) \times 10^{37}$ erg s$^{-1}$. Our analysis revealed that this AGN is located at a projected distance of $\sim 0.29" \pm 0.05"$ (corresponding to $\sim 14.3 \pm 2.5$ pc) from the stellar bulge center, which is coincident with the kinematic center of this object's stellar velocity map. The black hole corresponding to the observed off-centered AGN may form a binary system with a black hole located at the stellar bulge center. However, it is also possible that the displaced black hole is the merged remnant of the binary system coalescence, after the "kick" caused by the asymmetric emission of gravitational waves. We propose that certain features in the stellar velocity dispersion map are the result of perturbations caused by the off-centered AGN.

Pre-main-sequence accretion and the formation of multiple populations in globular clusters

We investigate the viability of a model in which the chemical anomalies among Globular Cluster stars are due to accretion of gas onto the protostellar discs of low mass stars. This model has been suggested as a way to reduce the large initial cluster masses required by other models for the formation of multiple stellar generations. We numerically follow the evolution of the accreting stars, and we show that the structure of the seed star does not remain fully convective for the whole duration of the accretion phase. Stellar populations showing discrete abundances of helium in the core, that seem to be present in some clusters, might be formed with this mechanism only if accretion occurs before the core of the stars become radiative (within 2-3Myr) or if a thermohaline instability is triggered, to achieve full mixing after the accretion phase ends. We also show that the lithium abundances in accreted structures may vary by orders of magnitude in equal masses obtained by accreting different masses. In addition, the same thermohaline mixing which could provide a homogeneous helium distribution down to the stellar center, would destroy any lithium surviving in the envelope, so that both helium homogeneity and lithium survival require that the accretion phase be limited to the first couple of million years of the cluster evolution. Such a short accretion phase strongly reduces the amount of processed matter available, and reintroduces the requirement of an extremely large initial mass for the protocluster.

Ejection of heavy elements from the stellar core to the periphery of the cloud of ejecta during a supernova explosion: A possible model of the processes

The possibility of simulating the processes during supernova explosions in laboratory conditions using powerful lasers (laboratory astrophysics) is investigated. The Chandra observations of ejecta in the Cassiopeia A supernova remnant are analyzed. Based on the DIANA and NUTCY numerical codes, we have performed 1D and 2D hydrodynamic simulations of the ejecta expansion dynamics for a supernova with a mass of ∼5–15 solar masses within several hundred seconds after its explosion, including an initial asymmetry. We propose a model for the explosion and expansion of ejecta that illustrates strong inhomogeneities in the distribution of material to the extent that the Fe, Si, and S material from the stellar center turns out to be ejected to the periphery, the “star turns inside out,” in agreement with observations. Based on hydrodynamic similarity criteria, we consider possible supernova-simulating laser targets that will allow one to reproduce the physical processes that take place during the explosion of an astrophysical object, such as the shock propagation through the material, the growth of hydrodynamic instabilities at the boundaries of envelopes with different densities, etc.

THIRD-EPOCH MAGELLANIC CLOUD PROPER MOTIONS. II. THE LARGE MAGELLANIC CLOUD ROTATION FIELD IN THREE DIMENSIONS

We present the first detailed assessment of the large-scale rotation of any galaxy based on full three-dimensional velocity measurements. We do this for the LMC by combining our HST average proper motion (PM) measurements for stars in 22 fields, with existing line-of-sight (LOS) velocity measurements for 6790 individual stars. We interpret these data with a model of circular rotation in a flat disk. The PM and LOS data paint a consistent picture of the LMC rotation and their combination yields several new insights. The PM data imply a stellar dynamical center that coincides with the HI dynamical center, and a rotation curve amplitude consistent with that inferred from LOS velocity studies. The implied disk viewing angles agree with the range of values found in the literature, but continue to indicate variations with stellar population and/or radius. Young (RSG) stars rotate faster than old (RGB/AGB) stars due to asymmetric drift. Outside the central region, the circular velocity is approximately flat at Vcirc = 91.7 +/- 18.8 km/s. This is consistent with the baryonic Tully-Fisher relation, and implies an enclosed mass M(8.7 kpc) = (1.7 +/- 0.7) x 10^10 solar masses. The virial mass is larger and depends on the full extent of the dark halo. The tidal radius is 22.3 +/- 5.2 kpc (24.0 +/- 5.6 degrees). Combination of the PM and LOS data yields kinematic distance estimates for the LMC, but these are not yet competitive with other methods.

THE NEAREST MILLISECOND PULSAR REVISITED WITHXMM-NEWTON: IMPROVED MASS-RADIUS CONSTRAINTS FOR PSR J0437-4715

I present an analysis of the deepest X-ray exposure of a radio millisecond pulsar (MSP) to date, an X-ray Multi Mirror-Newton European Photon Imaging Camera spectroscopic and timing observation of the nearest known MSP, PSR J0437--4715. The timing data clearly reveal a secondary broad X-ray pulse offset from the main pulse by $\sim$0.55 in rotational phase. In the context of a model of surface thermal emission from the hot polar caps of the neutron star, this can be plausibly explained by a magnetic dipole field that is significantly displaced from the stellar center. Such an offset, if commonplace in MSPs, has important implications for studies of the pulsar population, high energy pulsed emission, and the pulsar contribution to cosmic ray positrons. The continuum emission shows evidence for at least three thermal components, with the hottest radiation most likely originating from the hot magnetic polar caps and the cooler emission from the bulk of the surface. I present pulse phase-resolved X-ray spectroscopy of PSR J0437--4715, which for the first time, properly accounts for the system geometry of a radio pulsar. Such an approach is essential for unbiased measurements of the temperatures and emission areas of polar cap radiation from pulsars. Detailed modelling of the thermal pulses, including relativistic and atmospheric effects, provides a constraint on the redshift-corrected neutron star radius of R>11.1 km (at 3 sigma conf.) for the current radio timing mass measurement of 1.76 M_sun. This limit favors "stiff'" equations of state.

Proposal for an Observational Test of the Vainshtein Mechanism

Modified gravity theories capable of genuine self-acceleration typically invoke a Galileon scalar which mediates a long-range force but is screened by the Vainshtein mechanism on small scales. In such theories, nonrelativistic stars carry the full scalar charge (proportional to their mass), while black holes carry none. Thus, for a galaxy free falling in some external gravitational field, its central massive black hole is expected to lag behind the stars. To look for this effect, and to distinguish it from other astrophysical effects, one can correlate the gravitational pull from the surrounding structure with the offset between the stellar center and the black hole. The expected offset depends on the central density of the galaxy and ranges up to ∼0.1 kpc for small galaxies. The observed offset in M87 cannot be explained by this effect unless the scalar force is significantly stronger than gravity. We also discuss the systematic offset of compact objects from the galactic plane as another possible signature.

A finite temperature chandrasekhar model: Determining the parameters and calculation of the characteristics of degenerate dwarfs

We suggest a generalization of the standard Chandrasekhar model for degenerate dwarfs. We apply an equation of state for a degenerate ideal electron gas in the form of a Sommerfeld expansion in the parameter k B T(r)/m 0 c 2. The radial temperature distribution T(r) is modeled taking into account the presence of the isothermal core. The model has four dimensionless parameters, two microscopic (the relativistic parameter at the stellar center x 0 and the chemical-composition parameter µ e = A/Z) and two macroscopic (the dimensionless temperature T 0 * = k B T c /m 0 c 2 and dimensionless radius ξ 0 = R c /R of the core, where R c and R are the radii of the core and dwarf). We found x 0, µ e , and T 0 * for about 3000 DA white dwarfs, based on their masses, radii, and effective temperatures from the Sloan Digital Sky Survey Data Release 4; ξ 0 was treated as a free parameter. The influence of temperature effects on the macroscopic characteristics is analyzed, in particular, the minimum mass and maximum radii of the stars. Based on our computed energy-radius dependence, we suggest an interpretation of the observed radius distribution for these dwarfs.

Semi-analytic stellar structure in scalar-tensor gravity

Precision tests of gravity can be used to constrain theproperties of hypothetical very light scalar fields, but thesetests depend crucially on how macroscopic astrophysical objectscouple to the new scalar field. We study the equations of stellarstructure using scalar-tensor gravity, with the goal of seeing howstellar properties depend on assumptions made about the scalarcoupling at a microscopic level. In order to make the studyrelatively easy for different assumptions about microscopiccouplings, we develop quasi-analytic approximate methods forsolving the stellar-structure equations rather than simplyintegrating them numerically. (The approximation involved assumesthe dimensionless scalar coupling at the stellar center is weak,and we compare our results with numerical integration in order toestablish its domain of validity.) We illustrate these methods byapplying them to Brans-Dicke scalars, and their generalization inwhich the scalar-matter coupling slowly runs — or `walks' — as afunction of the scalar field: a(ϕ) ≃ as+bsϕ.(Such couplings can arise in extra-dimensional applications, forinstance.) The four observable parameters that characterize thefields external to a spherically symmetric star are thestellar radius, R, mass,M, scalar `charge', Q, and the scalar's asymptotic value,ϕ∞. These are subject to two relations because of thematching to the interior solution, generalizing the usualmass-radius, M(R), relation of General Relativity. Sinceϕ∞ is common to different stars in a given region (suchas a binary pulsar), all quantities can be computed locally interms of the stellar masses. We identify how these relationsdepend on the microscopic scalar couplings, agreeing with earlierworkers when comparisons are possible. Explicit analyticalsolutions are obtained for the instructive toy model ofconstant-density stars, whose properties we compare to morerealistic equations of state for neutron star models.

The inverse problem of the theory of degenerate dwarfs

Based on the radii and masses of degenerate dwarfs derived from HIPPARCOS and other observations, we estimate the microscopic parameters of a Chandrasekhar model (the relativistic parameter at the stellar center x0, and the chemical-composition parameter µe = A/Z, where A is the mass number and Z is the nuclear charge). We have obtained analytical expressions for the macroscopic characteristics (mass, radius, energy) as functions of x0 and µe. From the calculated dependence of the energy on these parameters, we have found constraints on the range of variability of x0, which are in good agreement with the observed radius distribution of dwarfs. The critical value of x0 at which stability breaks down due to general-relativisitc effects is found more accurately than previously. We propose a generalized model with an inhomogeneous (coordinate-dependent) chemical composition, with µe = µe(r).

THE STELLAR KINEMATIC CENTER AND THE TRUE GALACTIC NUCLEUS OF NGC 253

We present the first sub-arcsecond resolution two-dimensional stellar kinematics and X-ray observations of the prototypical starburst galaxy NGC253 which define the position and nature of the galactic nucleus. We get an estimate of the stellar kinematic center location corresponding to an area of r~1.2" centered 0.7" southwest from the radio core (TH2). Newly processed Chandra data reveal a central hard X-ray source (X-1) lying 0.4" southwest from the kinematic center. Very accurate alignment between radio, infrared and X-ray sources shows that TH2, the IR photometric center and X-1 are not associated with each other. As the kinematic center is consistent with TH2 and X-1, we consider the two as possible galactic nucleus candidates. Although TH2 is the strongest radio source in the nuclear region, it does not have any infrared, optical or X-ray counterparts. If the kinematic center is associated with this source, by analogy we suggest that the nucleus of NGC253 resembles our Galactic Center SgrA*. On the other hand, X-1 is a heavily absorbed object only detected at energies >2 keV. If X-1 is instead associated with the kinematic center, the nucleus of NGC253 is compatible with an obscured low luminosity active galactic nucleus (AGN) or a spatially resolved super star cluster (SSC) brightening up in X-rays most probably due to young supernovae or supernova remnants. If no SSC is associated with the kinematic center, we conclude that NGC253 is a galaxy in which a strong starburst and a weak AGN (either TH2 or X-1) coexist. Results from few other high resolution studies of nearby starburst galaxies indicate that the AGN in these systems, if present, is always in the low luminosity regime. This may indicate that the onset of nuclear activity in galaxies is closely related with the occurrence of star formation, and that we are witnessing the emergence or disappearance of an AGN.

Stellar center is dynamical in Hořava-Lifshitz gravity

In Horava-Lifshitz gravity, regularity of a solution requires smoothness of not only the spacetime geometry but also the foliation. As a result, the regularity condition at the center of a star is more restrictive than in general relativity. Assuming that the energy density is a piecewise-continuous, non-negative function of the pressure and that the pressure at the center is positive, we prove that the momentum conservation law is incompatible with the regularity at the center for any spherically-symmetric, static configurations. The proof is totally insensitive to the structure of higher spatial curvature terms and, thus, holds for any values of the dynamical critical exponent $z$. Therefore, we conclude that a spherically-symmetric star should include a time-dependent region near the center. We also comment on the condition under which linear instability of the scalar graviton does not show up.

Existence of relativistic stars inf(R)gravity

We refute recent claims in the literature that stars with relativistically deep potentials cannot exist in $f(R)$ gravity. Numerical examples of stable stars, including relativistic ($G{M}_{\ensuremath{\star}}/{r}_{\ensuremath{\star}}\ensuremath{\sim}0.1$), constant density stars, are studied. As a star is made larger, nonlinear ``chameleon'' effects screen much of the star's mass, stabilizing gravity at the stellar center. Furthermore, we show that the onset of this chameleon screening is unrelated to strong gravity. At large central pressures $P>\ensuremath{\rho}/3$, $f(R)$ gravity, like general relativity, does have a maximum gravitational potential, but at a slightly smaller value: $G{M}_{\ensuremath{\star}}/{r}_{\ensuremath{\star}}{|}_{\mathrm{max}}=0.345<4/9$ for constant density and one choice of parameters. This difference is associated with negative central curvature $R$ under general relativity not being accessed in the $f(R)$ model, but does not apply to any known astrophysical object.

Constraints on Neutron Star Properties from X‐Ray Observations of Millisecond Pulsars

We present a model of thermal X-ray emission from hot spots on the surface of a rotating compact star with an unmagnetized light-element atmosphere. An application to ROSAT, Chandra, and XMMNewton X-ray observations of the nearest known rotation-powered millisecond pulsar (MSP) PSR J0437–4715 reveals that the thermal emission from this pulsar is fully consistent with such a model, enabling constraints on important properties of the underlying neutron star. We confirm that the observed thermal X-ray pulsations from J0437–4715 are incompatible with blackbody emission and require the presence of an optically thick, light element (most likely hydrogen) atmosphere on the neutron star surface. The morphology of the X-ray pulse profile is consistent with a global dipole configuration of the pulsar magnetic field but suggests an off-center magnetic axis, with a displacement of 0.8 −3 km from the stellar center. For an assumed mass of 1.4 M⊙, the model restricts the allowed stellar radii to R = 6.8 − 13.8 km (90% confidence) and R > 6.7 km (99.9% confidence), which is consistent with standard NS equations of state and rules out an ultracompact star smaller than its photon sphere. Deeper spectroscopic and timing observations of this and other nearby radio MSPs with current and future X-ray facilities (Constellation-X and XEUS) can provide further insight into the fundamental properties of neutron stars. Subject headings: pulsars: general — pulsars: individual (PSR J0437–4715) — stars: neutron — X-rays: stars — gravitation — relativity

Hyades Morphology and Star Formation

Perryman and collaborators found that for the Hyades cluster the fraction of multiple stars increases from G stars to early-A stars. We discuss here whether this may be a general property of star formation or whether collisions in the cluster environment change the fractions of binary stars. A star ring of mainly F and G stars is seen around the Hyades cluster core, supposedly created by a shock wave due to a supernova explosion. These ring stars show the same fraction of binary stars as observed for the F and G stars in the core of the cluster. This suggests that collisions in the high stellar density cluster center did not measurably change the multiple-star fractions. There is so far only evidence of this one supernova explosion in the Hyades. If this is indeed the only one, then only one massive star was born in the cluster. There are also fewer than 10 white dwarf descendants of B stars found in the cluster. Unless many white dwarfs and neutron stars evaporated from the cluster, the initial mass function for stellar masses >2 M⊙ must have been very steep, corresponding to a Salpeter function with an exponent of less than -3.3.

Detonating Failed Deflagration Model of Thermonuclear Supernovae. I. Explosion Dynamics

We present a detonating failed deflagration model of Type Ia supernovae. In this model, the thermonuclear explosion of a massive white dwarf follows an off-center deflagration. We conduct a survey of asymmetric ignition configurations initiated at various distances from the stellar center. In all cases studied, we find that only a small amount of stellar fuel is consumed during deflagration phase, no explosion is obtained, and the released energy is mostly wasted on expanding the progenitor. Products of the failed deflagration quickly reach the stellar surface, polluting and strongly disturbing it. These disturbances eventually evolve into small and isolated shock-dominated regions which are rich in fuel. We consider these regions as seeds capable of forming self-sustained detonations that, ultimately, result in the thermonuclear supernova explosion. Preliminary nucleosynthesis results indicate the model supernova ejecta are typically composed of about 0.1-0.25 Msun of silicon group elements, 0.9-1.2 Msun of iron group elements, and are essentially carbon-free. The ejecta have a composite morphology, are chemically stratified, and display a modest amount of intrinsic asymmetry. The innermost layers are slightly egg-shaped with the axis ratio ~1.2-1.3 and dominated by the products of silicon burning. This central region is surrounded by a shell of silicon-group elements. The outermost layers of ejecta are highly inhomogeneous and contain products of incomplete oxygen burning with only small admixture of unburned stellar material. The explosion energies are ~1.3-1.5 10^51 erg.

Asymmetrical structure of ionization and kinematics in the Seyfert galaxy NGC 5033

We present integral field spectroscopy of NGC 5033, a low luminosity Seyfert galaxy. The observations were made with INTEGRAL, a fiber based system operating at the WHT. The intensity map of the H emission line represents a spiral or ring-like pattern of HII regions. On the contrary, the [OIII] intensity map morphology is markedly anisotropic. The strong morphological differences imply that the [ OIII] emitters represent highly ionized gas illuminated by the central source. The [ OIII] map morphology is compatible with a biconical structure of ionization induced by strong extinction in the galaxy disc that also obscures half of the spheroidal stellar bulge. We identify the spectrum corresponding to the Seyfert 1 nucleus from the presence of H broad emission lines. This spectrum is located in a region where strong extinction is expected but exhibits the bluest spectral energy distribution. The Seyfert 1 nucleus seems to be off center with respect to the stellar rotation center. This result has been also found in other Seyfert galaxies and interpreted in terms of a past merger. The off centering could indicate the presence of nonsymmetric departures in the gravitational potential which could be fueling the active nucleus. The kinematics of the [ OIII] emitters show important deviations at a kpc scale with respect to the stellar velocity field and show features related to the asymmetrical morphology of the high ionization region.

Near infrared coronagraph images of IRC +10216

We present J, H and K band coronagraph images of the circumstellar envelope around IRC +10216 (CW Leo) obtained with a near infrared camera, CIAO and the 8.2 m Subaru telescope. A circular occulting mask of 2″ in diameter was used to block out the light from the bright central object. The images show 2 collimated radial structures to the NNW and WNW, 2 fan-like structures to the S and NE, respectively, and 3 arc-like structures at a radius of 4 to 5 arcsec from the stellar center. We compare this intermediate size-scale structure to that seen on larger and smaller scales and find evidence for a deviation from spherically symmetric outflow beginning ~150 years ago. Previous near infrared speckle imaging has revealed a complex clumpy structure on a scale of less than 200 mas, and it is likely that at least some of the radial features seen in our images could be due to shadowing by dust clumps close to the star.

Reconstruction of the brightness distribution across a stellar disk from gravitational lensing caustic passage observations

This paper describes a method for analyzing observations of the passage of a star through a gravitational lensing caustic that enables the derivation of information about the brightness distribution across the stellar disk without knowledge of the lens parameters. The ill-posed inverse problem of reconstructing a one-dimensional strip brightness distribution across the stellar disk is solved using a regularization method, taking into account the non-negative nature of this function. Assuming the source is circularly symmetric, the search for the radial brightness distribution is carried out over compact sets of upward-convex, non-negative functions or functions that do not grow with distance from the stellar center. The method is used to analyze the gravitational microlensing event MACHO 98-SMC-1, of a star in the Small Magellanic Cloud, obtained by the PLANET international group. The resulting brightness distribution is compared with computations of limb darkening for model stellar atmospheres.