Showing papers on "White dwarf published in 2010"
TL;DR: Modules for Experiments in Stellar Astrophysics (MESA) as mentioned in this paper is a suite of open source libraries for a wide range of applications in computational stellar astrophysics, including advanced evolutionary phases.
Abstract: Stellar physics and evolution calculations enable a broad range of research in astrophysics. Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open source libraries for a wide range of applications in computational stellar astrophysics. A newly designed 1-D stellar evolution module, MESA star, combines many of the numerical and physics modules for simulations of a wide range of stellar evolution scenarios ranging from very-low mass to massive stars, including advanced evolutionary phases. MESA star solves the fully coupled structure and composition equations simultaneously. It uses adaptive mesh refinement and sophisticated timestep controls, and supports shared memory parallelism based on OpenMP. Independently usable modules provide equation of state, opacity, nuclear reaction rates, and atmosphere boundary conditions. Each module is constructed as a separate Fortran 95 library with its own public interface. Examples include comparisons to other codes and show evolutionary tracks of very low mass stars, brown dwarfs, and gas giant planets; the complete evolution of a 1 Msun star from the pre-main sequence to a cooling white dwarf; the Solar sound speed profile; the evolution of intermediate mass stars through the thermal pulses on the He-shell burning AGB phase; the interior structure of slowly pulsating B Stars and Beta Cepheids; evolutionary tracks of massive stars from the pre-main sequence to the onset of core collapse; stars undergoing Roche lobe overflow; and accretion onto a neutron star. Instructions for downloading and installing MESA can be found on the project web site (this http URL).
633 citations
TL;DR: In this article, the luminosity functions and number ratios between AGB and red giant branch (RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy Survey Treasury were examined.
Abstract: In an attempt to constrain evolutionary models of the asymptotic giant branch (AGB) phase at the limit of low masses and low metallicities, we have examined the luminosity functions and number ratios between AGB and red giant branch (RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy Survey Treasury. This database provides Hubble Space Telescope optical photometry together with maps of completeness, photometric errors, and star formation histories for dozens of galaxies within 4 Mpc. We select 12 galaxies characterized by predominantly metal-poor populations as indicated by a very steep and blue RGB, and which do not present any indication of recent star formation in their color-magnitude diagrams. Thousands of AGB stars brighter than the tip of the RGB (TRGB) are present in the sample (between 60 and 400 per galaxy), hence, the Poisson noise has little impact in our measurements of the AGB/RGB ratio. We model the photometric data with a few sets of thermally pulsing AGB (TP-AGB) evolutionary models with different prescriptions for the mass loss. This technique allows us to set stringent constraints on the TP-AGB models of low-mass, metal-poor stars (with M < 1.5 M_⊙, [Fe/H] ≾ -1.0). Indeed, those which satisfactorily reproduce the observed AGB/RGB ratios have TP-AGB lifetimes between 1.2 and 1.8 Myr, and finish their nuclear burning lives with masses between 0.51 and 0.55 M_⊙. This is also in good agreement with recent observations of white dwarf masses in the M4 old globular cluster. These constraints can be added to those already derived from Magellanic Cloud star clusters as important mileposts in the arduous process of calibrating AGB evolutionary models.
414 citations
TL;DR: A simulation of the merger of two equal-mass white dwarfs that leads to a sub-luminous explosion, although at the expense of requiring a single common-envelope phase, and component masses of ∼0.9M⊙.
Abstract: Type Ia supernovae are potentially invaluable as cosmological distance indicators, but if they are to provide a reliable measure of the expansion history of the Universe and nature of dark energy, more evidence that they are a largely homogeneous population will be required. A subclass of type Ia supernova, the 'sub-luminous 1991bg-like' objects, has proved problematic, as current models fail to predict their formation from the presumed supernova precursors, white dwarf stars. It had long been speculated that mergers of two white dwarfs should trigger such events, and now a new set of numerical simulations adds support to this idea. A merger of two equal-mass white dwarfs leads to a sub-luminous explosion if a single common-envelope phase is involved, and if the component stars are each about 0.9 solar masses in size. Existing models of type Ia supernovae generally explain their observed properties, with the exception of the sub-luminous 1991bg-like supernovae. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event, but simulations so far have failed to produce an explosion. Here, a simulation of the merger of two equal-mass white dwarfs is presented that leads to a sub-luminous explosion; it requires a single common-envelope phase and component masses of about 0.9 solar masses. Type Ia supernovae are thought to result from thermonuclear explosions of carbon–oxygen white dwarf stars1. Existing models2 generally explain the observed properties, with the exception of the sub-luminous 1991bg-like supernovae3. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event4,5, but hitherto simulations have failed to produce an explosion6,7. Here we report a simulation of the merger of two equal-mass white dwarfs that leads to a sub-luminous explosion, although at the expense of requiring a single common-envelope phase, and component masses of ∼0.9M⊙. The light curve is too broad, but the synthesized spectra, red colour and low expansion velocities are all close to what is observed for sub-luminous 1991bg-like events. Although the mass ratios can be slightly less than one and still produce a sub-luminous event, the masses have to be in the range 0.83M⊙ to 0.9M⊙.
398 citations
TL;DR: In this paper, the authors performed one-dimensional hydrodynamical simulations, associated post-processing nucleosynthesis, and multi-wavelength radiation transport calculations for pure detonations of carbon-oxygen white dwarfs (WDs).
Abstract: Explosions of sub-Chandrasekhar-mass white dwarfs (WDs) are one alternative to the standard Chandrasekhar-mass model of Type Ia supernovae (SNe Ia). They are interesting since binary systems with sub-Chandrasekhar-mass primary WDs should be common and this scenario would suggest a simple physical parameter which determines the explosion brightness, namely the mass of the exploding WD. Here we perform one-dimensional hydrodynamical simulations, associated post-processing nucleosynthesis, and multi-wavelength radiation transport calculations for pure detonations of carbon-oxygen WDs. The light curves and spectra we obtain from these simulations are in good agreement with observed properties of SNe Ia. In particular, for WD masses from 0.97 to 1.15 M ☉ we obtain 56Ni masses between 0.3 and 0.8 M ☉, sufficient to capture almost the complete range of SN Ia brightnesses. Our optical light curve rise times, peak colors, and decline timescales display trends which are generally consistent with observed characteristics although the range of B-band decline timescales displayed by our current set of models is somewhat too narrow. In agreement with observations, the maximum light spectra of the models show clear features associated with intermediate-mass elements and reproduce the sense of the observed correlation between explosion luminosity and the ratio of the Si II lines at λ6355 and λ5972. We therefore suggest that sub-Chandrasekhar-mass explosions are a viable model for SNe Ia for any binary evolution scenario leading to explosions in which the optical display is dominated by the material produced in a detonation of the primary WD.
396 citations
TL;DR: In this article, the authors investigated whether a surface detonation in a helium layer initiates a subsequent core explosion in the underlying carbon/oxygen core leading to an explosion and found that secondary core detonations are triggered for all of the simulated models.
Abstract: The explosion of sub-Chandrasekhar mass white dwarfs via the double detonation scenario is a potential explanation for type Ia supernovae. In this scenario, a surface detonation in a helium layer initiates a detonation in the underlying carbon/oxygen core leading to an explosion. For a given core mass, a lower bound has been determined on the mass of the helium shell required for dynamical burning during a helium flash, which is a necessary prerequisite for detonation. For a range of core and corresponding minimum helium shell masses, we investigate whether an assumed surface helium detonation is capable of triggering a subsequent detonation in the core even for this limiting case. We carried out hydrodynamic simulations on a co-expanding Eulerian grid in two dimensions assuming rotational symmetry. The detonations are propagated using the level-set approach and a simplified scheme for nuclear reactions that has been calibrated with a large nuclear network. The same network is used to determine detailed nucleosynthetic abundances in a post-processing step. Based on approximate detonation initiation criteria in the literature, we find that secondary core detonations are triggered for all of the simulated models, ranging in core mass from 0.810 up to 1.385 M_solar with corresponding shell masses from 0.126 down to 0.0035 M_solar. This implies that, as soon as a detonation triggers in a helium shell covering a carbon/oxygen white dwarf, a subsequent core detonation is virtually inevitable.
392 citations
Weizmann Institute of Science1, Harvard University2, Max Planck Society3, University of Arizona4, University of Texas at Austin5, University of California, Berkeley6, Pennsylvania State University7, San Diego State University8, University of Toronto9, Las Cumbres Observatory Global Telescope Network10, Liverpool John Moores University11, University of Chile12, University of California, Santa Barbara13, Radboud University Nijmegen14, Lawrence Berkeley National Laboratory15
TL;DR: A faint type Ib supernova is reported in the halo of the nearby isolated galaxy, NGC 1032, and it is concluded that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary.
Abstract: Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The ‘old’ environment near the supernova location, and the very low derived ejected mass (~0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous, or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive ^(44)Ti.
370 citations
TL;DR: In this paper, photometric and spectroscopic observations of SN 2007if, an overluminous (M_V = −20.4), red (B-V = 0.16 at B-band maximum), slow-rising (t_(rise) = 24 days) type Ia supernova (SN Ia) in a very faint (m_g = −14.10) host galaxy.
Abstract: We present photometric and spectroscopic observations of SN 2007if, an overluminous (M_V = –20.4), red (B – V = 0.16 at B-band maximum), slow-rising (t_(rise) = 24 days) type Ia supernova (SN Ia) in a very faint (M_g = –14.10) host galaxy. A spectrum at 5 days past B-band maximum light is a direct match to the super-Chandrasekhar-mass candidate SN Ia 2003fg, showing Si II and C II at ~9000 km s^(–1). A high signal-to-noise co-addition of the SN spectral time series reveals no Na I D absorption, suggesting negligible reddening in the host galaxy, and the late-time color evolution has the same slope as the Lira relation for normal SNe Ia. The ejecta appear to be well mixed, with no strong maximum in I band and a diversity of iron-peak lines appearing in near-maximum-light spectra. SN 2007if also displays a plateau in the Si II velocity extending as late as +10 days, which we interpret as evidence for an overdense shell in the SN ejecta. We calculate the bolometric light curve of the SN and use it and the Si II velocity evolution to constrain the mass of the shell and the underlying SN ejecta, and demonstrate that SN 2007if is strongly inconsistent with a Chandrasekhar-mass scenario. Within the context of a "tamped detonation" model appropriate for double-degenerate mergers, and assuming no host extinction, we estimate the total mass of the system to be 2.4 ± 0.2 M_☉, with 1.6 ± 0.1 M_☉ of ^(56)Ni and with 0.3-0.5 M_☉ in the form of an envelope of unburned carbon/oxygen. Our modeling demonstrates that the kinematics of shell entrainment provide a more efficient mechanism than incomplete nuclear burning for producing the low velocities typical of super-Chandrasekhar-mass SNe Ia.
355 citations
TL;DR: A detailed review of the physics of white dwarfs can be found in this paper, with a focus on the physical processes that lead to the formation of the stars and the different energy sources responsible for chemical abundance changes that occur along their evolution.
Abstract: White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs has potential applications in different fields of astrophysics. In particular, white dwarfs can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, such as our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow these stars to be used as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities, the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos and to study problems related to the variation of fundamental constants. These potential applications of white dwarfs have led to renewed interest in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers, and we describe why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with a unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows one to measure stellar masses with unprecedented precision and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.
354 citations
TL;DR: In this paper, the authors performed one-dimensional hydrodynamical simulations, associated post-processing nucleosynthesis and multi-wavelength radiation transport calculations for pure detonations of carbon-oxygen white dwarfs.
Abstract: Explosions of sub-Chandrasekhar-mass white dwarfs are one alternative to the standard Chandrasekhar-mass model of Type Ia supernovae. They are interesting since binary systems with sub-Chandrasekhar-mass primary white dwarfs should be common and this scenario would suggest a simple physical parameter which determines the explosion brightness, namely the mass of the exploding white dwarf. Here we perform one-dimensional hydrodynamical simulations, associated post-processing nucleosynthesis and multi-wavelength radiation transport calculations for pure detonations of carbon-oxygen white dwarfs. The light curves and spectra we obtain from these simulations are in good agreement with observed properties of Type Ia supernovae. In particular, for white dwarf masses from 0.97 - 1.15 Msun we obtain 56Ni masses between 0.3 and 0.8 Msun, sufficient to capture almost the complete range of Type Ia supernova brightnesses. Our optical light curve rise times, peak colours and decline timescales display trends which are generally consistent with observed characteristics although the range of B-band decline timescales displayed by our current set of models is somewhat too narrow. In agreement with observations, the maximum light spectra of the models show clear features associated with intermediate mass elements and reproduce the sense of the observed correlation between explosion luminosity and the ratio of the Si II lines at 6355 and 5972 Angstroms. We therefore suggest that sub-Chandrasekhar mass explosions are a viable model for Type Ia supernovae for any binary evolution scenario leading to explosions in which the optical display is dominated by the material produced in a detonation of the primary white dwarf.
336 citations
Abstract: The first light from a supernova (SN) emerges once the SN shock breaks out of the stellar surface. The first light, typically a UV or X-ray flash, is followed by a broken power-law decay of the luminosity generated by radiation that leaks out of the expanding gas sphere. Motivated by recent detection of emission from very early stages of several SNe, we revisit the theory of shock breakout and the following emission, paying special attention to the photon-gas coupling and deviations from thermal equilibrium. We derive simple analytic light curves of SNe from various progenitors at early times. We find that for more compact progenitors, white dwarfs, Wolf-Rayet stars (WRs), and possibly more energetic blue-supergiant explosions, the observed radiation is out of thermal equilibrium at the breakout, during the planar phase (i.e., before the expanding gas doubles its radius), and during the early spherical phase. Therefore, during these phases we predict significantly higher temperatures than previous analysis that assumed equilibrium. When thermal equilibrium prevails, we find the location of the thermalization depth and its temporal evolution. Our results are useful for interpretation of early SN light curves. Some examples are (1) red supergiant SNe have an early bright peak in optical and UV flux, less than an hour after breakout. It is followed by a minimum at the end of the planar phase (about 10 hr), before it peaks again once the temperature drops to the observed frequency range. In contrast, WRs show only the latter peak in optical and UV. (2) Bright X-ray flares are expected from all core-collapse SNe types. (3) The light curve and spectrum of the initial breakout pulse hold information on the explosion geometry and progenitor wind opacity. Its spectrum in more compact progenitors shows a (nonthermal) power law and its light curve may reveal both the breakout diffusion time and the progenitor radius.
329 citations
TL;DR: In this article, atmospheric pollution by various elements heavier than helium has been used to infer that a comparable fraction of the white dwarf descendants of such main-sequence stars are orbited by planetary systems, and for plausible planetary system configurations, this total mass is likely to be at least equal to that of the Sun's asteroid belt.
Abstract: Infrared studies have revealed debris likely related to planet formation in orbit around ~30% of youthful, intermediate mass, main-sequence stars. We present evidence, based on atmospheric pollution by various elements heavier than helium, that a comparable fraction of the white dwarf descendants of such main-sequence stars are orbited by planetary systems. These systems have survived, at least in part, through all stages of stellar evolution that precede the white dwarf. During the time interval (~200 million years) that a typical polluted white dwarf in our sample has been cooling it has accreted from its planetary system the mass of one of the largest asteroids in our solar system (e.g., Vesta or Ceres). Usually, this accreted mass will be only a fraction of the total mass of rocky material that orbits these white dwarfs; for plausible planetary system configurations we estimate that this total mass is likely to be at least equal to that of the Sun's asteroid belt, and perhaps much larger. We report abundances of a suite of eight elements detected in the little studied star G241-6 that we find to be among the most heavily polluted of all moderately bright white dwarfs.
TL;DR: For carbon-oxygen white dwarfs accreting hydrogen or helium at rates in the range ~1-10 x 10^(-8) Msun/y, a variety of explosive outcomes are possible well before the star reaches the Chandrasekhar mass.
Abstract: For carbon-oxygen white dwarfs accreting hydrogen or helium at rates in the range ~1-10 x 10^(-8) Msun/y, a variety of explosive outcomes is possible well before the star reaches the Chandrasekhar mass. These outcomes are surveyed for a range of white dwarf masses (0.7 - 1.1 Msun), accretion rates (1 - 7 x 10^(-8) Msun/y), and initial white dwarf temperatures (0.01 and 1 Lsun). The results are particularly sensitive to the convection that goes on during the last few minutes before the explosion. Unless this convection maintains a shallow temperature gradient, and unless the density is sufficiently high, the accreted helium does not detonate. Below a critical helium ignition density, which we estimate to be 5 - 10 x 10^5 g cm^(-3), either helium novae or helium deflagrations result. The hydrodynamics, nucleosynthesis, light curves, and spectra of a representative sample of detonating and deflagrating models are explored. Some can be quite faint indeed, powered at peak for a few days by the decay of 48Cr and 48V. Only the hottest, most massive white dwarfs considered with the smallest helium layers, show reasonable agreement with the light curves and spectra of common Type Ia supernovae. For the other models, especially those involving lighter white dwarfs, the helium shell mass exceeds 0.05 Msun and the mass of the 56Ni that is synthesized exceeds 0.01 Msun. These explosions do not look like ordinary Type Ia supernovae, or any other frequently observed transient.
TL;DR: In this article, the authors presented time-dependent multi-wavelength radiative transfer calculations for models with minimum helium shell mass and derived synthetic observables for both the optical and γ-ray spectral regions.
Abstract: In the double-detonation scenario for Type Ia supernovae, it is suggested that a detonation initiates in a shell of helium-rich material accreted from a companion star by a sub-Chandrasekhar-mass white dwarf. This shell detonation drives a shock front into the carbon-oxygen white dwarf that triggers a secondary detonation in the core. The core detonation results in a complete disruption of the white dwarf. Earlier studies concluded that this scenario has difficulties in accounting for the observed properties of Type Ia supernovae since the explosion ejecta are surrounded by the products of explosive helium burning in the shell. Recently, however, it was proposed that detonations might be possible for much less massive helium shells than previously assumed (Bildsten et al.). Moreover, it was shown that even detonations of these minimum helium shell masses robustly trigger detonations of the carbon-oxygen core (Fink et al.). Therefore, it is possible that the impact of the helium layer on observables is less than previously thought. Here, we present time-dependent multi-wavelength radiative transfer calculations for models with minimum helium shell mass and derive synthetic observables for both the optical and γ-ray spectral regions. These differ strongly from those found in earlier simulations of sub-Chandrasekhar-mass explosions in which more massive helium shells were considered. Our models predict light curves that cover both the range of brightnesses and the rise and decline times of observed Type Ia supernovae. However, their colors and spectra do not match the observations. In particular, their B – V colors are generally too red. We show that this discrepancy is mainly due to the composition of the burning products of the helium shell of the Fink et al. models which contain significant amounts of titanium and chromium. Using a toy model, we also show that the burning products of the helium shell depend crucially on its initial composition. This leads us to conclude that good agreement between sub-Chandrasekhar-mass explosions and observed Type Ia supernovae may still be feasible but further study of the shell properties is required.
TL;DR: It is reported that the spectral diversity is a consequence of random directions from which an asymmetric explosion is viewed, and indicates that ignition at an offset from the centre is a generic feature of type Ia supernovae.
Abstract: Type Ia supernovae form an observationally uniform class of stellar explosions, in that more luminous objects have smaller decline-rates. This one-parameter behaviour allows type Ia supernovae to be calibrated as cosmological ‘standard candles’, and led to the discovery of an accelerating Universe. Recent investigations, however, have revealed that the true nature of type Ia supernovae is more complicated. Theoretically, it has been suggested that the initial thermonuclear sparks are ignited at an offset from the centre of the white-dwarf progenitor, possibly as a result of convection before the explosion. Observationally, the diversity seen in the spectral evolution of type Ia supernovae beyond the luminosity–decline-rate relation is an unresolved issue. Here we report that the spectral diversity is a consequence of random directions from which an asymmetric explosion is viewed. Our findings suggest that the spectral evolution diversity is no longer a concern when using type Ia supernovae as cosmological standard candles. Furthermore, this indicates that ignition at an offset from the centre is a generic feature of type Ia supernovae.
TL;DR: In this article, the authors presented full evolutionary calculations appropriate for the study of hydrogen-rich DA white dwarfs for a wide range of stellarmasses and for two different metallicities.
Abstract: We present full evolutionary calculations appropriate for the study of hydrogen-rich DA white dwarfs. This is
done by evolving white dwarf progenitors from the zero-age main sequence, through the core hydrogen-burning
phase, the helium-burning phase, and the thermally pulsing asymptotic giant branch phase to the white dwarf stage.
Complete evolutionary sequences are computed for a wide range of stellarmasses and for two different metallicities,
Z = 0.01, which is representative of the solar neighborhood, and Z = 0.001, which is appropriate for the study of old
stellar systems, like globular clusters. During the white dwarf cooling stage, we self-consistently compute the phase
in which nuclear reactions are still important, the diffusive evolution of the elements in the outer layers and, finally,
we also take into account all the relevant energy sources in the deep interior of the white dwarf, such as the release
of latent heat and the release of gravitational energy due to carbon–oxygen phase separation upon crystallization.
We also provide colors and magnitudes for these sequences, based on a new set of improved non-gray white dwarf
model atmospheres, which include the most up-to-date physical inputs like the Lyα quasi-molecular opacity. The
calculations are extended down to an effective temperature of 2500 K. Our calculations provide a homogeneous set
of evolutionary cooling tracks appropriate for mass and age determinations of old DA white dwarfs and for white
dwarf cosmochronology of the different Galactic populations.
TL;DR: It is reported that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, resolving the age discrepancy for NGC 6791.
Abstract: Fil: Garcia Berro, Enrique. Instituto de Estudios Espaciales de Cataluna; . Universidad Politecnica de Catalunya; Espana
TL;DR: In this paper, the authors derived constraints on one of the most important parameters in the field of close compact binary formation, i.e., the CE efficiency α, based on fits to stellar evolution calculations as well as a parametrized energy equation for common-envelope (CE) evolution.
Abstract: Context. Reconstructing the evolution of post-common-envelope binaries (PCEBs) consisting of a white dwarf and a main-sequence star can constrain current prescriptions of common-envelope (CE) evolution. This potential could so far not be fully exploited due to the small number of known systems and the inhomogeneity of the sample. Recent extensive follow-up observations of white dwarf/main-sequence binaries identified by the Sloan Digital Sky Survey (SDSS) paved the way for a better understanding of CE evolution.
Aims. Analyzing the new sample of PCEBs we derive constraints on one of the most important parameters in the field of close compact binary formation, i.e. the CE efficiency α.
Methods. After reconstructing the post-CE evolution and based on fits to stellar evolution calculations as well as a parametrized energy equation for CE evolution, we determine the possible evolutionary histories of the observed PCEBs. In contrast to most previous attempts we incorporate realistic approximations of the binding energy parameter λ. Each reconstructed CE history corresponds to a certain value of the mass of the white dwarf progenitor and – more importantly – the CE efficiency α. We also reconstruct CE evolution replacing the classical energy equation with a scaled angular momentum equation and compare the results obtained with both algorithms.
Results. We find that all PCEBs in our sample can be reconstructed with the energy equation if the internal energy of the envelope is included. Although most individual systems have solutions for a broad range of values for α, only for α = 0.2–0.3 do we find simultaneous solutions for all PCEBs in our sample. If we adjust α to this range of values, the values of the angular momentum parameter γ cluster in a small range of values. In contrast if we fix γ to a small range of values that allows us to reconstruct all our systems, the possible ranges of values for α remains broad for individual systems.
Conclusions. The classical parametrized energy equation seems to be an appropriate prescription of CE evolution and turns out to constrain the outcome of the CE evolution much more than the alternative angular momentum equation. If there is a universal value of the CE efficiency, it should be in the range of α = 0.2–0.3. We do not find any indications for a dependence of α on the mass of the secondary star or the final orbital period.
TL;DR: In this paper, the authors presented time-dependent multi-wavelength radiative transfer calculations for models with minimum He shell mass and derived synthetic observables for both the optical and gamma-ray spectral regions.
Abstract: Abridged. In the double detonation scenario for Type Ia supernovae (SNe Ia) a detonation initiates in a shell of He-rich material accreted from a companion star by a sub-Chandrasekhar-mass White Dwarf (WD). This shell detonation drives a shock front into the carbon-oxygen (C/O) WD that triggers a secondary detonation in the core. The core detonation results in a complete disruption of the WD. Earlier studies concluded that this scenario has difficulties in accounting for the observed properties of SNe Ia since the explosion ejecta are surrounded by the products of explosive He burning in the shell. Recently, it was proposed that detonations might be possible for much less massive He shells than previously assumed. Moreover, it was shown that even detonations of these minimum He shell masses robustly trigger detonations of the C/O core. Here we present time-dependent multi-wavelength radiative transfer calculations for models with minimum He shell mass and derive synthetic observables for both the optical and {\gamma}-ray spectral regions. These differ strongly from those found in earlier simulations of sub-Chandrasekhar-mass explosions in which more massive He shells were considered. Our models predict light curves which cover both the range of brightnesses and the rise and decline times of observed SNe Ia. However, their colours and spectra do not match the observations. In particular, their B-V colours are generally too red. We show that this discrepancy is mainly due to the composition of the burning products of the He shell of our models which contain significant amounts of Ti and Cr. Using a toy model, we also show that the burning products of the He shell depend crucially on its initial composition. This leads us to conclude that good agreement between sub-Chandrasekhar-mass explosions and observed SNe Ia may still be feasible but further study of the shell properties is required.
TL;DR: This work reports the Fermi Large Area Telescope detection of variable gamma-ray emission from the recently detected optical nova of the symbiotic star V407 Cygni and proposes that the material of the nova shell interacts with the dense ambient medium of the red giant primary and that particles can be accelerated effectively to produce pi(0) decay gamma-rays from proton-proton interactions.
Abstract: Novae are thermonuclear explosions on a white dwarf surface fueled by mass accreted from a companion star. Current physical models posit that shocked expanding gas from the nova shell can produce x-ray emission, but emission at higher energies has not been widely expected. Here, we report the Fermi Large Area Telescope detection of variable γ-ray emission (0.1 to 10 billion electron volts) from the recently detected optical nova of the symbiotic star V407 Cygni. We propose that the material of the nova shell interacts with the dense ambient medium of the red giant primary and that particles can be accelerated effectively to produce π0 decay γ-rays from proton-proton interactions. Emission involving inverse Compton scattering of the red giant radiation is also considered and is not ruled out.
TL;DR: It is concluded that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekharwhite dwarfs make a significant contribution to the supernova rate.
Abstract: Type Ia supernovae are used as standard candles to determine the cosmological distance scale, yet the exact nature of their progenitors is still not known. Two models compete with each other with alternating success. Type Ia supernovae are thought to result from thermonuclear explosions of white dwarf stars, triggered either by the merger of two white dwarfs in a binary system or when a white dwarf reaches a critical size due to gradual accretion of material from a companion. On the accretion scenario, the expanding star is predicted to emit strongly at X-ray wavelengths. Now, based on Chandra satellite X-ray observations of nearby galaxies, Marat Gilfanov and Akos Bogdan estimate that the accretion scenario can account for no more than about 5% of type Ia supernovae in young galaxies — though the situation may be different in late-type galaxies. Type Ia supernovae are thought to be associated with the thermonuclear explosions of white dwarf stars, but the nuclear runaway that leads to the explosion could occur through two different pathways with different X-ray signatures. The X-ray flux from six nearby elliptical galaxies and galaxy bulges is now observed to reveal that it is a factor of about 30–50 less than predicted by the accretion scenario, where a white dwarf accumulates material from a companion star. There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars1,2. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit3, or could be triggered by the merger of two white dwarfs in a compact binary system4,5. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 107 years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of ∼30–50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate.
TL;DR: In this article, the authors suggest that Type Ia supernovae (SNe Ia) instead result from mergers of roughly equal-mass CO WDs, including those that produce sub-Chandrasekhar mass remnants.
Abstract: Type Ia supernovae (SNe Ia) are generally thought to be due to the thermonuclear explosions of carbon-oxygen white dwarfs (CO WDs) with masses near the Chandrasekhar mass. This scenario, however, has two long-standing problems. First, the explosions do not naturally produce the correct mix of elements, but have to be finely tuned to proceed from subsonic deflagration to supersonic detonation. Second, population models and observations give formation rates of near-Chandrasekhar WDs that are far too small. Here, we suggest that SNe Ia instead result from mergers of roughly equal-mass CO WDs, including those that produce sub-Chandrasekhar mass remnants. Numerical studies of such mergers have shown that the remnants consist of rapidly rotating cores that contain most of the mass and are hottest in the center, surrounded by dense, small disks. We argue that the disks accrete quickly, and that the resulting compressional heating likely leads to central carbon ignition. This ignition occurs at densities for which pure detonations lead to events similar to SNe Ia. With this merger scenario, we can understand the type Ia rates and have plausible reasons for the observed range in luminosity and for the bias of more luminous supernovae toward younger populations. We speculate that explosions of WDs slowly brought to the Chandrasekhar limit—which should also occur—are responsible for some of the "atypical" SNe Ia.
TL;DR: In this article, the authors present hydrodynamic explosion models and observable outcomes of these He shell detonations for a range of initial core and envelope masses, and the peak UVOIR bolometric luminosities range by a factor of 10 (from 5×10 41 −5×10 42 erg s −1 ), and the R-band peak varies from MR,peak = −15 to −18.
Abstract: During the early evolution of an AM CVn system, helium is accreted onto the surface of a white dwarf under conditions suitable for unstable thermonuclear ignition. The turbulent motions induced by the convective burning phase in the He envelope become strong enough to influence the propagation of burning fronts and may result in the onset of a detonation. Such an outcome would yield radioactive isotopes and a faint rapidly rising thermonuclear “.Ia” supernova. In this paper, we present hydrodynamic explosion models and observable outcomes of these He shell detonations for a range of initial core and envelope masses. The peak UVOIR bolometric luminosities range by a factor of 10 (from 5×10 41 −5×10 42 erg s −1 ), and the R-band peak varies from MR,peak = −15 to −18. The rise times in all bands are very rapid (< 10 d), but the decline rate is slower in the red than the blue due to a secondary near-IR brightening. The nucleosynthesis primarily yields heavy α-chain elements ( 40 Ca through 56 Ni) and unburnt He. Thus, the spectra around peak light lack signs of intermediate mass elements and are dominated by Ca ii and Ti ii features, with the caveat that our radiative transfer code does not include the non-thermal effects necessary to produce He features. Subject headings: binaries: close— novae, cataclysmic variables— nuclear reactions, nucleosynthesis, abundances— supernovae: general— white dwarfs
TL;DR: In this paper, the authors present three-dimensional simulations on a new mechanism for the detonation of a sub-Chandrasekhar CO white dwarf in a dynamically unstable system where the secondary is either a pure He white dwarf or an He/CO hybrid.
Abstract: We present three-dimensional simulations on a new mechanism for the detonation of a sub-Chandrasekhar CO white dwarf in a dynamically unstable system where the secondary is either a pure He white dwarf or an He/CO hybrid. For dynamically unstable systems where the accretion stream directly impacts the surface of the primary, the final tens of orbits can have mass accretion rates that range from 10–5 to 10–3 M ☉ s–1, leading to the rapid accumulation of helium on the surface of the primary. After ~10–2 M ☉ of helium has been accreted, the ram pressure of the hot helium torus can deflect the accretion stream such that the stream no longer directly impacts the surface. The velocity difference between the stream and the torus produces shearing which seeds large-scale Kelvin-Helmholtz instabilities along the interface between the two regions. These instabilities eventually grow into dense knots of material that periodically strike the surface of the primary, adiabatically compressing the underlying helium torus. If the temperature of the compressed material is raised above a critical temperature, the timescale for triple-α reactions becomes comparable to the dynamical timescale, leading to the detonation of the primary's helium envelope. This detonation drives shock waves into the primary which tend to concentrate at one or more focal points within the primary's CO core. If a relatively small amount of mass is raised above a critical temperature and density at these focal points, the CO core may itself be detonated.
TL;DR: This paper measured atmospheric abundances relative to helium of nine elements: H, O, Mg, Si, Ca, Ti, Cr, Mn, and Fe and found that heavy elements are largely contained within oxides, derived from a tidally disrupted minor planet at least the mass of Juno, and probably as massive as Vesta.
Abstract: We present Keck/High Resolution Echelle Spectrometer data with model atmosphere analysis of the helium-dominated polluted white dwarf GD 40, in which we measure atmospheric abundances relative to helium of nine elements: H, O, Mg, Si, Ca, Ti, Cr, Mn, and Fe. Apart from hydrogen, whose association with the other contaminants is uncertain, this material most likely accreted from GD 40's circumstellar dust disk whose existence is demonstrated by excess infrared emission. The data are best explained by accretion of rocky planetary material, in which heavy elements are largely contained within oxides, derived from a tidally disrupted minor planet at least the mass of Juno, and probably as massive as Vesta. The relatively low hydrogen abundance sets an upper limit of 10% water by mass in the inferred parent body, and the relatively high abundances of refractory elements, Ca and Ti, may indicate high-temperature processing. While the overall constitution of the parent body is similar to the bulk Earth being over 85% by mass composed of oxygen, magnesium, silicon, and iron, we find n(Si)/n(Mg) = 0.30 ? 0.11, significantly smaller than the ratio near unity for the bulk Earth, chondrites, the Sun, and nearby stars. This result suggests that differentiation occurred within the parent body.
TL;DR: In this article, the authors analyzed radial velocity observations of the 12 extremely low-mass (ELM) white dwarfs (WDs) in the MMT Hypervelocity Star Survey.
Abstract: We analyze radial velocity observations of the 12 extremely low-mass (ELM), with {<=}0.25 M{sub sun}, white dwarfs (WDs) in the MMT Hypervelocity Star Survey. Eleven of the twelve WDs are binaries with orbital periods shorter than 14 hr; the one non-variable WD is possibly a pole-on system among our non-kinematically selected targets. Our sample is unique: it is complete in a well-defined range of apparent magnitude and color. The orbital mass functions imply that the unseen companions are most likely other WDs, although neutron star companions cannot be excluded. Six of the eleven systems with orbital solutions will merge within a Hubble time due to the loss of angular momentum through gravitational wave radiation. The quickest merger is J0923+3028, a g = 15.7 ELM WD binary with a 1.08 hr orbital period and a {<=}130 Myr merger time. The chance of a supernova Ia event among our ELM WDs is only 1%-7%, however. Three binary systems (J0755+4906, J1233+1602, and J2119-0018) have extreme mass ratios and will most likely form stable mass-transfer AM CVn systems. Two of these objects, SDSS J1233+1602 and J2119-0018, are the lowest surface gravity WDs ever found; both show Ca II absorption likely from accretion of circumbinarymore » material. We predict that at least one of our WDs is an eclipsing detached double WD system, important for constraining helium core WD models.« less
TL;DR: A review of the research on AM CVn stars covering the last 15 yr is given, and the outlook for future research is discussed in this article, where the relative importance of the three proposed birth channels: a low-mass white dwarf donor, a helium-star donor, or a highly evolved cataclysmic variable (CV) as a donor.
Abstract: AM CVn stars are the outcome of a fine-tuned binary star evolution pathway. They are helium-rich and their binary orbital periods are less than 65 minutes. They evolve through one or two common envelope (CE) events, which are difficult to model. Observations of AM CVn stars are important to understand the CE phase. Thanks to intensive observing campaigns, the number of AM CVn stars has increased from 5 to 25 during the last 15 yr. We have witnessed long photometric campaigns, time-resolved spectroscopy, UV and X-ray observations, and progress in modeling of the internal structure of donor and accretor stars, disk structure, disk atmosphere, and their evolution. Two possible new members of the AM CVn family have orbital periods of less than 10 minutes. For these, four different models have been proposed, including one without mass transfer, driven by electricity generated by the secondary star moving in the magnetic field of the primary. Short-period AM CVn stars are among the first possible detectable sources of low-frequency gravitational wave (GW) radiation. They are also possible progenitors of a Type Ia supernova (SN Ia) and subluminous explosions, and they can produce helium novae during their evolution. From systematic searches in the Sloan Digital Sky Survey, it has been possible to estimate population densities that can be tested against population synthesis models. One important question to investigate is the relative importance of the three proposed birth channels: a low-mass white dwarf donor, a helium-star donor, or a highly evolved cataclysmic variable (CV) as a donor. A review of the research on AM CVn stars covering the last 15 yr is given, and the outlook for future research is discussed.
TL;DR: In this paper, the authors search for planets in NN Ser ab, an eclipsing short-period binary that shows long-term eclipse time variations and find excellent agreement with the light-travel-time effect produced by two additional bodies superposed on the linear ephemeris of the binary.
Abstract: Planets orbiting post-common envelope binaries provide fundamental information on planet formation and evolution. We searched for such planets in NN Ser ab, an eclipsing short-period binary that shows long-term eclipse time variations. Using published, reanalysed, and new mid-eclipse times of NN Ser ab obtained between 1988 and 2010, we find excellent agreement with the light-travel-time effect produced by two additional bodies superposed on the linear ephemeris of the binary. Our multi-parameter fits accompanied by N-body simulations yield a best fit for the objects NN Ser (ab)c and d locked in the 2:1 mean motion resonance, with orbital periods P-c similar or equal to 15.5 yrs and P-d similar or equal to 7.7 yrs, masses M-c sin i(c) similar or equal to 6.9 M-Jup and M-d sin i(d) similar or equal to 2.2 M-Jup, and eccentricities e(c) similar or equal to 0 and e(d) similar or equal to 0.20. A secondary chi(2) minimum corresponds to an alternative solution with a period ratio of 5:2. We estimate that the progenitor binary consisted of an A star with similar or equal to 2 M-circle dot and the present M dwarf secondary at an orbital separation of similar to 1.5 AU. The survival of two planets through the common-envelope phase that created the present white dwarf requires fine tuning between the gravitational force and the drag force experienced by them in the expanding envelope. The alternative is a second-generation origin in a circumbinary disk created at the end of this phase. In that case, the planets would be extremely young with ages not exceeding the cooling age of the white dwarf of 10(6) yrs.
TL;DR: In this article, the authors investigated the situation when nucleosynthetically unprocessed, H-rich material is convectively mixed with a He-burning zone, for example in convectically unstable shell on top of electron-degenerate cores in AGB stars, young white dwarfs or X-ray bursting neutron stars.
Abstract: Depending on mass and metallicity as well as evolutionary phase, stars occasionally experience convective-reactive nucleosynthesis episodes. We specifically investigate the situation when nucleosynthetically unprocessed, H-rich material is convectively mixed with a He-burning zone, for example in convectively unstable shell on top of electron-degenerate cores in AGB stars, young white dwarfs or X-ray bursting neutron stars. Such episodes are frequently encountered in stellar evolution models of stars of extremely low or zero metal content [...] We focus on the convective-reactive episode in the very-late thermal pulse star Sakurai's object (V4334 Sagittarii). Asplund etal. (1999) determined the abundances of 28 elements, many of which are highly non-solar, ranging from H, He and Li all the way to Ba and La, plus the C isotopic ratio. Our simulations show that the mixing evolution according to standard, one-dimensional stellar evolution models implies neutron densities in the He that are too low to obtain a significant neutron capture nucleosynthesis on the heavy elements. We have carried out 3D hydrodynamic He-shell flash convection [...] we assume that the ingestion process of H into the He-shell convection zone leads only after some delay time to a sufficient entropy barrier that splits the convection zone [...] we obtain significantly higher neutron densities (~few 10^15 1/cm^3) and reproduce the key observed abundance trends found in Sakurai's object. These include an overproduction of Rb, Sr and Y by about 2 orders of magnitude higher than the overproduction of Ba and La. Such a peculiar nucleosynthesis signature is impossible to obtain with the mixing predictions in our one-dimensional stellar evolution models. [...] We determine how our results depend on uncertainties of nuclear reaction rates, for example for the C13(\alpha, n)O16 reaction.
TL;DR: In this article, the authors re-analyze the Kepler data and demonstrate that both companions are likely to be white dwarfs, and they use their estimates to consider the likely evolutionary paths and mass transfer episodes of these binary systems.
Abstract: Among the initial results from Kepler were two striking light curves, for KOI 74 and KOI 81, in which the relative depths of the primary and secondary eclipses showed that the more compact, less luminous object was hotter than its stellar host. That result became particularly intriguing because a substellar mass had been derived for the secondary in KOI 74, which would make the high temperature challenging to explain; in KOI 81, the mass range for the companion was also reported to be consistent with a substellar object. We re-analyze the Kepler data and demonstrate that both companions are likely to be white dwarfs. We also find that the photometric data for KOI 74 show a modulation in brightness as the more luminous star orbits, due to Doppler boosting. The magnitude of the effect is sufficiently large that we can use it to infer a radial velocity amplitude accurate to 1 km s(-1). As far as we are aware, this is the first time a radial-velocity curve has been measured photometrically. Combining our velocity amplitude with the inclination and primary mass derived from the eclipses and primary spectral type, we infer a secondary mass of 0.22 +/- 0.03 M(circle dot). We use our estimates to consider the likely evolutionary paths and mass-transfer episodes of these binary systems.
TL;DR: In this paper, the surface abundances of carbon enhanced metal-poor stars enriched in s-process elements (CEMP-s) have been observed in the Galactic halo.
Abstract: A large sample of carbon enhanced metal-poor stars enriched in s-process elements (CEMP-s) have been observed in the Galactic halo. These stars of low mass (M � 0.9 M� ) are located on the main-sequence or the red giant phase, and do not undergo third dredge-up (TDU) episodes. The s-process enhancement is most plausibly due to accretion in a binary system from a more massive companion when on the asymptotic giant branch (AGB) phase (now a white dwarf). In order to interpret the spectroscopic observations, updated AGB models are needed to follow in detail the sprocess nucleosynthesis. We present nucleosynthesis calculations based on AGB stellar models obtained with FRANEC (Frascati Raphson-Newton Evolutionary Code) for low initial stellar masses and low metallicities. For a given metallicity, a wide spread in the abundances of the s-process elements is obtained by varying the amount of 13 C and its profile in the pocket, where the 13 C(�, n) 16 O reaction is the major neutron source, releasing neutrons in radiative conditions during the interpulse phase. We account also for the second neutron source 22 Ne(�, n) 25 Mg, partially activated during convective thermal pulses. We discuss the surface abundance of elements from carbon to bismuth, for AGB models of initial masses M = 1.3 – 2 M� , low metallicities ([Fe/H] from 1 down to 3.6) and for different 13 C-pockets efficiencies. In particular we analyse the relative behaviour of the three s-process peaks: light-s (ls at magic neutron number N = 50), heavy-s (hs at N = 82) and lead (N = 126). Two s-process indicators, [hs/ls] and [Pb/hs], are needed in order to characterise the s-process distribution. In the online material, we provide a set of data tables with surface predictions. Our final goal is to provide a full set of theoretical models of low mass low metallicity s-process enhanced stars. In a forthcoming paper, we will test our results through a comparison with observations of CEMP-s stars.