Stellar Envelope Research Articles

THE PROPAGATION OF NEUTRINO-DRIVEN JETS IN WOLF-RAYET STARS

We numerically investigate the jet propagation through a rotating collapsing Wolf-Rayet star with detailed central engine physics constructed based on the neutrino-driven collapsar model. The collapsing star determines the evolution of mass accretion rate, black hole mass and spin, all of which are important ingredients for determining the jet luminosity. We reveal that neutrino-driven jets in rapidly spinning Wolf-Rayet stars are capable of breaking out from the stellar envelope, while those propagating in slower rotating progenitors fail to jet breakout due to insufficient kinetic power. For progenitor models with successful jet breakouts, the kinetic energy accumulated in the cocoon could be as large as ~1051erg and might significantly contribute to the luminosity of the afterglow emission or to the kinetic energy of the accompanying supernova if nickel production takes place. We further analyze the post breakout phase using a simple analytical prescription and conclude that the relativistic jet component could produce events with an isotropic-luminosity Lp(iso) ~1052erg/s and isotropic-energy Ej(iso) ~1054erg. Our findings support the idea of rapidly rotating Wolf-Rayet stars as plausible progenitors of GRBs, while slowly rotational ones could be responsible for low luminosity or failed GRBs.

X-ray emission from the ultramassive black hole candidate NGC 1277: implications and speculations on its origin

Abstract We study the X-ray emission from NGC 1277, a galaxy in the core of the Perseus cluster, for which van den Bosch et al. have recently claimed the presence of an ultramassive black hole (UMBH) of mass $1.7\times 10^{10}{\,\mathrm{M}_{{\odot }}}$, unless the initial mass function of the stars in the stellar bulge is extremely bottom heavy. The X-rays originate in a power-law component of luminosity $1.3\times 10^{40}{\,{\rm erg}\,{\rm s}^{-1}\,}$ embedded in a 1 keV thermal minicorona which has a half-light radius of about 1.3 kpc, typical of many early-type galaxies in rich clusters of galaxies. If Bondi accretion operated on to the UMBH from the minicorona with a radiative efficiency of 10 per cent, then the object would appear as a quasar with luminosity $10^{46}{\,{\rm erg}\,{\rm s}^{-1}\,}$, a factor of almost 106 times higher than observed. The accretion flow must be highly radiatively inefficient, similar to past results on M87 and NGC 3115. The UMBH in NGC 1277 is definitely not undergoing any significant growth at the present epoch. We note that there are three UMBH candidates in the Perseus cluster and that the inferred present mean mass density in UMBHs could be $10^5{\,\mathrm{M}_{{\odot }}}\,{\rm Mpc^{-3}}$, which is 20–30 per cent of the estimated mean mass density of all BHs. We speculate on the implied growth of UMBHs and their hosts, and discuss the possibility that extreme active galactic nucleus feedback could make all UMBH host galaxies have low stellar masses at redshifts around 3. Only those which end up at the centres of groups and clusters later accrete large stellar envelopes and become brightest cluster galaxies. NGC 1277 and the other Perseus core UMBH, NGC 1270, have not, however, been able to gather more stars or gas owing to their rapid orbital motion in the cluster core.

On the origins of GEMS grains: A reply

The Comment by Bradley challenges our chemical and isotopic measurements of GEMS grains, arguing that pervasive infiltration of silicone oil and sample heating undermine our comparisons with equally uncertain interstellar grain compositions. However, we have already shown that such effects are negligible and cannot account for the extraordinary chemical variability of GEMS grains. Our data are also in excellent agreement with previous GEMS grain chemical analyses, and with GEMS grains in micrometeorites and within IDPs recently collected without the use of silicone oil. The order of magnitude variations in element abundances displayed by GEMS grains rule out a common origin by the extensive chemical and isotopic homogenization of circumstellar grains, as proposed by Bradley. The vast majority of GEMS grains also do not have radial compositional gradients, crystalline “relict” cores, or rims of condensed materials indicative of radiation exposure. The average element abundances of GEMS grains differ significantly and systematically from solar abundances and from those inferred for interstellar silicates based on element depletion patterns. A few GEMS grains have highly anomalous O isotopic compositions consistent with a condensation origin in evolved O-rich stellar envelopes and supernovae. Yet, on average, GEMS grains have O isotopic compositions very near to terrestrial and meteoritic values. These and other observations discussed in our paper place a strict upper limit on the number of GEMS grains that have preserved presolar origins.

Asteroseismology of pulsating DA white dwarfs with fully evolutionary models

We present a new approach for asteroseismology of DA white dwarfs that consists in the employment of a large set of non-static, physically sound, fully evolutionary models representative of these stars. We already have applied this approach with success to pulsating PG1159 stars (GW Vir variables). Our white dwarf models, which cover a wide range of stellar masses, effective temperatures, and envelope thicknesses, are the result of fully evolutionary computations that take into account the complete history of the progenitor stars from the ZAMS. In particular, the models are characterized by self-consistent chemical structures from the centre to the surface, a crucial aspect of white dwarf asteroseismology. We apply this approach to an ensemble of 44 bright DAV (ZZ Ceti) stars.

Influence of the number of atomic levels on the spectral opacity of low temperature nickel and iron in the spectral range 50–300 eV

Opacity is a fundamental ingredient for the secular evolution of stars. The calculation of the stellar plasma absorption coefficients is complex due to the composition of these plasmas, generally an H /He dominated mixture with a low concentration of partially ionized heavy ions (the iron group). The international collaboration OPAC recently presented extensive comparisons of spectral opacities of iron and nickel for temperatures between 15 and 40 eV and for densities of ∼ 3m g/cm 3 , relevant to the stellar envelope conditions (1, 2). The role of Configuration Interaction (CI) and the influence of the number of atomic levels on the opacity using the recently improved version of HULLAC atomic code (3, 4 )a re illustrated in this article. Comparisons with theoretical predictions already presented in (1) are discussed. 1. HULLAC AND HULLAC-V9 ATOMIC CODES

Iron and Nickel spectral opacity calculations in conditions relevant for pulsating stellar envelopes and experiments

Seismology of stars is strongly developing. To address this question we have formed an international collaboration, OPAC, to perform specific experimental measurements, compare opacity calculations, and improve the opacity calculations in stellar codes [1]. We consider the following opacity codes: SCO, CASSANDRA, STA, OPAS, LEDCOP, OP, SCO-RCG. Their comparison has shown large differences for Fe and Ni in equivalent conditions of envelopes of type II supernova precursors, temperatures between 15 and 40 eV and densities of a few mg/cm3 [2–4]. LEDCOP, OPAS, SCO-RCG structure codes and STA give similar results and differ from OP ones for the lower temperatures and for spectral interval values [3]. In this work we discuss the role of Configuration Interaction (CI) and the influence of the number of used configurations. We present and include in the opacity code comparisons new HULLAC-v9 calculations [5, 6] that include full CI. To illustrate the importance of this effect we compare different CI approximations (modes) available in HULLAC-v9 [7]. These results are compared to previous predictions and to experimental data. Differences with OP results are discussed.

Virtual Laboratory Astrophysics: the STARK-B database for spectral line broadening by collisions with charged particles and its link to the European project VAMDC

Atomic physics in plasmas has been an essential tool for many years. Accurate spectroscopic diagnostics and modeling require the knowledge of numerous collisional line profiles. "Stark broadening" theories and calculations have been extensively developed for about 50 years. Nowadays, the access to such data via an on line database becomes essential. The aim of STARK-B is to reply to this need. It is a collaborative project between the Astronomical Observatory of Belgrade (AOB) and the "Laboratoire d'Étude du Rayonnement et de la matière en Astrophysique" (LERMA) of the Paris Observatory and CNRS. It is a database of widths and shifts of isolated lines of atoms and ions due to electron and ion impacts that we have calculated and published in international refereed journals (more than 150 papers by Dimitrijević & Sahal-Bréchot, and colleagues). It is devoted to modeling and spectroscopic diagnostics of stellar atmospheres and envelopes, laboratory plasmas, magnetic fusion plasmas, laser equipments and technological plasmas. Hence, the domain of temperatures and densities covered by the tables is wide and depends on the ionization degree of the considered ion. The STARK-B database is in free access and is is a part of VAMDC: the Virtual Atomic and Molecular Data Centre is an European Union funded collaboration between groups involved in the generation and use of atomic and molecular data. VAMDC aims to build a secure, documented, flexible and interoperable e-science environment-based interface to existing atomic and molecular data. STARK-B and its VAMDC context are presented in this paper.

FORETELLINGS OF RAGNARÖK: WORLD-ENGULFING ASYMPTOTIC GIANTS AND THE INHERITANCE OF WHITE DWARFS

The search for planets around White Dwarf stars, and evidence for dynamical instability around them in the form of atmospheric pollution and circumstellar discs, raises questions about the nature of planetary systems that can survive the vicissitudes of the Asymptotic Giant Branch (AGB). We study the competing effects, on planets at several AU from the star, of strong tidal forces arising from the star's large convective envelope, and of the planets' orbital expansion due to stellar mass loss. We, for the first time, study the evolution of planets while following each thermal pulse on the AGB. For Jovian planets, tidal forces are strong, and can pull into the envelope planets initially at ~3 AU for a 1M_Sol star and ~5 AU for a 5M_Sol star. Lower-mass planets feel weaker tidal forces, and Terrestrial planets initially within 1.5-3 AU enter the stellar envelope. Thus, low-mass planets that begin inside the maximum stellar radius can survive, as their orbits expand due to mass loss. The inclusion of a moderate planetary eccentricity slightly strengthens the tidal forces experienced by Jovian planets. Eccentric Terrestrial planets are more at risk, since their eccentricity does not decay and their small pericentre takes them inside the stellar envelope. We also find the closest radii at which planets will be found around White Dwarfs, assuming that any planet entering the stellar envelope is destroyed. Planets are in that case unlikely to be found inside ~1.5 AU of a White Dwarf with a 1M_Sol progenitor and ~10 AU of a White Dwarf with a 5M_Sol progenitor.

Stellar feedback and bulge formation in clumpy discs

We use numerical simulations of isolated galaxies to study the effects of stellar feedback on the formation and evolution of giant star-forming gas 'clumps' in high-redshift, gas-rich galaxies. Such galactic disks are unstable to the formation of bound gas-rich clumps whose properties initially depend only on global disk properties, not the microphysics of feedback. In simulations without stellar feedback, clumps turn an order-unity fraction of their mass into stars and sink to the center, forming a large bulge and kicking most of the stars out into a much more extended stellar envelope. By contrast, strong radiative stellar feedback disrupts even the most massive clumps after they turn ~10-20% of their mass into stars, in a timescale of ~10-100 Myr, ejecting some material into a super-wind and recycling the rest of the gas into the diffuse ISM. This suppresses the bulge formation rate by direct 'clump coalescence' by a factor of several. However, the galactic disks do undergo significant internal evolution in the absence of mergers: clumps form and disrupt continuously and torque gas to the galactic center. The resulting evolution is qualitatively similar to bar/spiral evolution in simulations with a more homogeneous ISM.

Testing the opacity and equation of state of LTE and non-LTE model atmospheres with OPAL and OP data for early-type stars

Context. Complex investigations of stars are made by studying their atmospheres, evolutionary states and oscillation properties, but the opacities and equation of state are still uncertain. Aims. Compatibility of model atmospheres with internal structure of early-type stars are investigated at photospheric and subphotospheric regions. Methods. The problem was studied quantitatively by means of diagrams that involve the Rosseland-mean opacities, temperatures, densities, and gradients of radiation pressure as functions of depth using stellar atmosphere and envelope models. Results. Two new grids of radiative-equilibrium models of atmospheres were calculated assuming the local thermodynamic equilibrium (LTE). The first one is based on Kurucz’s ATLAS12 computer code implemented with the occupation probability formalism, which accounts for the destruction of loosely bound states of atoms by interactions with particles in the plasma. The second grid of models is based on monochromatic opacities and equations of state taken from the well-known international opacity project (OP). Non-LTE model atmospheres are also calculated and some aspects of the modeling are pointed out to improve agreement with ATLAS12, OP data, as well as with results of the Rogers & Iglesias OPAL opacity computing code (OPAL). The model atmospheres are also discussed by analyzing emerging fluxes of radiation. Conclusions. Commonly used LTE and non-LTE models of stellar atmospheres of early-type stars differ markedly from each other and do not fit OPAL or OP stellar envelope models at great optical depths. The results of the OP project distributed as the OPCD v 3.3 base are useful for calculating not only stellar envelopes but also fully line-blanketed LTE model atmospheres. These models have diagnostic values for studying the atomic physics used for modeling of photospheric and sub-photospheric regions of stars. The OPAL and OP opacities are markedly underestimated in comparison with the Rosseland-mean opacities taken from the Castelli & Kurucz (2003, IAU Symp., 210, 10) atmosphere models in which a new opacity bump appears at lgT ≈ 5.06. This additional opacity bump affects the OPAL- and OP-driving zone for stellar pulsations and therefore the new envelope models may markedly change predicted spectra of unstable oscillation modes. Differences between current non-LTE and LTE models based on the TLUSTY200 and ATLAS codes, respectively, cannot be assigned entirely to non-LTE effects, but for B stars they reflect mainly opacity effects.

SPIDER - VII. Revealing the stellar population content of massive early-type galaxies out to 8Re

Radial trends of stellar populations in galaxies provide a valuable tool to understand the mechanisms of galaxy growth. In this paper, we present the first comprehensive analysis of optical–optical and optical–NIR colours, as a function of galaxy mass, out to the halo region (8Re) of early-type galaxies (ETGs). We select a sample of 674 massive ETGs (M★ ≳ 3 × 1010 M⊙) from the Sloan Digital Sky Survey (SDSS)-based SPIDER survey. By comparing with a large range of population synthesis models, we derive robust constraints on the radial trends in age and metallicity. Metallicity is unambiguously found to decrease outwards, with a measurable steepening of the slope in the outer regions (Re < R < 8Re). The gradients in stellar age are found to be more sensitive to the models used, but in general, the outer regions of ETGs feature older populations compared to the cores. This trend is strongest for the most massive galaxies in our sample (M★ ≳ 1011 M⊙). Furthermore, when segregating with respect to large-scale environment, the age gradient is more significant in ETGs residing in higher density regions. These results shed light on the processes leading from the formation of the central core to the growth of the stellar envelope of massive galaxies. The fact that the populations in the outer regions are older and more metal-poor than in the core suggests a process whereby the envelope of massive galaxies is made up of accreted small satellites (i.e. minor mergers) whose stars were born during the first stages of galaxy formation.

SUPERLUMINOUS LIGHT CURVES FROM SUPERNOVAE EXPLODING IN A DENSE WIND

Observations from the last decade have indicated the existence of a general class of superluminous supernovae (SLSNe), in which the peak luminosity exceeds 10^{44} erg/s. Here we focus on a subclass of these events, where the light curve is also tens of days wide, so the total radiated energy is order 10^{51} erg. If the origin of these SLSNe is a core-collapse-driven explosion of a massive star, then the mechanism which converts the explosion energy into radiation must be very efficient (much more than in typical core collapse SNe, where this efficiency is of order one percent). We examine the scenario where the radiated luminosity is due to efficient conversion of kinetic energy of the ejected stellar envelope into radiation by interaction with an optically thick, pre-existing circumstellar material (CSM), presumably the product of a steady wind from the progenitor. We base the analysis on a simple, numerically solved, hydrodynamic diffusion model, which allows us to identify the qualitative behavior of the observable light curves, and to relate them to the parameters of the wind. We specifically show that a wide and superluminous supernova requires the mass of the relevant wind material to be comparable to that of ejected material from the exploding progenitor. We find the wind parameters which explain the peak luminosity and width of the bolometric light curves of three particular SLSNe, namely, SN 2005ap, SN 2006gy, and SN 2010gx, and show that they are best fitted with a wind that extends to a radius of order 10^{15} cm. These results serve as an additional indication that at least some SLSNe are powered by interaction of the ejected material with a steady wind of similar mass.

OBSERVABLE SIGNATURES OF PLANET ACCRETION IN RED GIANT STARS. I. RAPID ROTATION AND LIGHT ELEMENT REPLENISHMENT

The orbital angular momentum of a close-orbiting giant planet can be sufficiently large that, if transferred to the envelope of the host star during the red giant branch (RGB) evolution, it can spin-up the star's rotation to unusually large speeds. This spin-up mechanism is one possible explanation for the rapid rotators detected among the population of generally slow-rotating red giant stars. These rapid rotators thus comprise a unique stellar sample suitable for searching for signatures of planet accretion in the form of unusual stellar abundances due to the dissemination of the accreted planet in the stellar envelope. In this study, we look for signatures of replenishment in the Li abundances and (to a lesser extent) 12C/13C, which are both normally lowered during RGB evolution. Accurate abundances were measured from high signal-to-noise echelle spectra for samples of both slow and rapid rotator red giant stars. We find that the rapid rotators are on average enriched in lithium compared to the slow rotators, but both groups of stars have identical distributions of 12C/13C within our measurement precision. Both of these abundance results are consistent with the accretion of planets of only a few Jupiter masses. We also explore alternative scenarios for understanding the most Li-rich stars in our sample---particularly Li regeneration during various stages of stellar evolution. Finally, we find that our stellar samples show non-standard abundances even at early RGB stages, suggesting that initial protostellar Li abundances and 12C/13C may be more variable than originally thought.

TIDAL NOVAE IN COMPACT BINARY WHITE DWARFS

Compact binary white dwarfs (WDs) undergoing orbital decay due to gravitational radiation can experience significant tidal heating prior to merger. In these WDs, the dominant tidal effect involves the excitation of outgoing gravity waves in the inner stellar envelope and the dissipation of these waves in the outer envelope. As the binary orbit decays, the WDs are synchronized from outside in (with the envelope synchronized first, followed by the core). We examine the deposition of tidal heat in the envelope of a Carbon-Oxygen WD and study how such tidal heating affects the structure and evolution of the WD. We show that significant tidal heating can occur in the star's degenerate hydrogen layer. This layer heats up faster than it cools, triggering runaway nuclear fusion. Such "tidal novae" may occur in all WD binaries containing a CO WD, at orbital periods between 5 min and 20 min, and precede the final merger by 10^5-10^6 years.

Flavor oscillations of supernova neutrinos

Neutrinos emitted by core-collapse supernovae (SNe) represent an important laboratory for both particle physics and astrophysics. While propagating in the dense SN environment, they can feel not only the presence of background matter (via ordinary Mikheev-Smirnov-Wolfenstein effects) but also of the gas of neutrinos and antineutrinos (via neutrino-neutrino interaction effects). The MSW effect would imprint on SN neutrinos a track of the shock-wave propagation and of the matter turbulences in the stellar envelope. Moreover, the neutrino-neutrino interactions appear to modify the flavor evolution of SN neutrinos in a collective way, completely different from the ordinary matter effects. In these conditions, the flavor evolution equations become highly nonlinear, sometimes resulting in surprising phenomena when the entire neutrino system oscillates coherently as a single collective mode. In this talk, I will present the recent results on supernova neutrino flavor conversions and I will discuss about the sensitivity of these effects to the ordering of the neutrino mass spectrum.

Investigation of atmosphere nonstationarity in the supergiant 55Cyg. I. Temporal line profile variability

The CCD spectra taken with echelle spectrographs of the 2-m telescope of the Shemakha Astrophysical Observatory of the National Academy of Sciences of Azerbaijan and the 1-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences are used to study the line profile variations in the spectrum of the hot supergiant 55Cyg. The variability of the radial velocity and profiles of the lines of heavy elements is shown to be due to radial pulsation type motions. The corresponding variations for He I lines are due to nonradial pulsations. In the case of the Hβ and Hα lines the pattern and behavior of variations differ for different observing periods. The variability of these lines is mostly due to the photometric and positional variability of the absorption and emission components of their profiles. The profiles of these lines show additional emission components, which move from the blue toward the red line wing. Such a behavior is indicative of the clumpy structure of the stellar envelope.

STAR HOPPERS: PLANET INSTABILITY AND CAPTURE IN EVOLVING BINARY SYSTEMS

Many planets are observed in stellar binary systems, and their frequency may be comparable to that of planetary systems around single stars. Binary stellar evolution in such systems influences the dynamical evolution of the resident planets. Here we study the evolution of a single planet orbiting one star in an evolving binary system. We find that stellar evolution can trigger dynamical instabilities that drive planets into chaotic orbits. This instability leads to planet-star collisions, exchange of the planet between the binary stars ("star-hoppers"), and ejection of the planet from the system. The means by which planets can be recaptured is similar to the pull-down capture mechanism for irregular solar system satellites. Because planets often suffer close encounters with the primary on the asymptotic giant branch, captures during a collision with the stellar envelope are also possible. Such capture could populate the habitable zone around white dwarfs.

Wave-driven mass loss in the last year of stellar evolution: setting the stage for the most luminous core-collapse supernovae

ABSTRACT During the late stages of stellar evolution in massive stars (C fusion and later), the fusion luminosity in the core of the star exceeds the star's Eddington luminosity. This can drive vigorous convective motions which in turn excite internal gravity waves. The local wave energy flux excited by convection is itself well above Eddington during the last few years in the life of the star. We suggest that an interesting fraction of the energy in gravity waves can, in some cases, convert into sound waves as the gravity waves propagate (tunnel) towards the stellar surface. The subsequent dissipation of the sound waves can unbind up to several M⊙ of the stellar envelope. This wave-driven mass loss can explain the existence of extremely large stellar mass-loss rates just prior to core collapse, which are inferred via circumstellar interaction in some core-collapse supernovae (e.g. SNe 2006gy and PTF 09uj, and even Type IIn supernovae more generally). An outstanding question is understanding what stellar parameters (mass, rotation, metallicity and age) are the most susceptible to wave-driven mass loss. This depends on the precise internal structure of massive stars and the power spectrum of internal gravity waves excited by stellar convection.

SIMULATIONS OF ACCRETION POWERED SUPERNOVAE IN THE PROGENITORS OF GAMMA-RAY BURSTS

Observational evidence suggests a link between long duration gamma ray bursts (LGRBs) and Type Ic supernovae. Here, we propose a potential mechanism for Type Ic supernovae in LGRB progenitors powered solely by accretion energy. We present spherically-symmetric hydrodynamic simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in one spatial dimension and with rotation, an explicit shear viscosity, and convection in the mixing length theory approximation. Once the accretion flow becomes rotationally supported outside of the black hole, an accretion shock forms and traverses the stellar envelope. Energy is carried from the central geometrically thick accretion disk to the stellar envelope by convection. Energy losses through neutrino emission and nuclear photodisintegration are calculated but do not seem important following the rapid early drop of the accretion rate following circularization. We find that the shock velocity, energy, and unbound mass are sensitive to convective efficiency, effective viscosity, and initial stellar angular momentum. Our simulations show that given the appropriate combinations of stellar and physical parameters, explosions with energies ~5x10^50 ergs, velocities 3000 km/s, and unbound material masses >6 solar masses are possible in a rapidly rotating 16 solar mass main sequence progenitor star. Further work is needed to constrain the values of these parameters, to identify the likely outcomes in more plausible and massive LRGB progenitors, and to explore nucleosynthetic implications.

AN OBSERVATIONAL IMPRINT OF THE COLLAPSAR MODEL OF LONG GAMMA-RAY BURSTS

The Collapsar model provides a theoretical framework for the well known association between long gamma-ray bursts (GRBs) and collapsing massive stars. A bipolar relativistic jet, launched at the core of a collapsing star, drills its way through the stellar envelope and breaks out of the surface before producing the observed gamma-rays. While a wealth of observations associate GRBs with the death of massive stars, as yet there is no direct evidence for the Collapsar model itself. Here we show that a distinct signature of the Collapsar model is the appearance of a plateau in the duration distribution of the prompt GRB emission at times much shorter than the typical breakout time of the jet. This plateau is evident in the data of all three major satellites. These findings provide an evidence that directly supports the Collapsar model. Additionally, it suggests the existence of a large population of choked (failed) GRBs and that the 2 s duration commonly used to separate Collapsars and non-Collapasars is inconsistent with the duration distributions of {\it Swift} and Fermi GRBs and only holds for BATSE GRBs.