Single Star Evolution Research Articles

Magnetism in Isolated and Binary White Dwarfs

ABSTRACTSince the discovery of the first isolated magnetic white dwarf (MWD) Grw +70°8047 nearly 60 years ago, the number of stars belonging to this class has grown steadily. There are now some 65 isolated white dwarfs classified as magnetic, and a roughly equal number of MWDs are found in the close interacting binaries known as the magnetic cataclysmic variables (MCVs). The isolated MWDs comprise ∼5% of all WDs, while the MCVs comprise ∼25% of all CVs. The magnetic fields range from ∼ 3 × 104–109 G in the former group with a distribution peaking at 1.6 × 107 G, and ∼ 107–3 × 108 G in the latter group. The space density of isolated magnetic white dwarfs with fields in the range ∼ 3 × 104–109 G is estimated to be ∼ 1.5 × 10-4 pc−3. The MCVs have a space density that is about a hundred times smaller.About 80% of the isolated MWDs have almost pure H atmospheres and show only hydrogen lines in their spectra (the magnetic DAs), while the remainder show He i lines (the magnetic DBs) or molecular bands of C2 and CH (magnetic DQs) and have helium as the dominant atmospheric constituent, mirroring the situation in the nonmagnetic white dwarfs. The incidence of stars of mixed composition (H and He) appears to be higher among the MWDs.There is growing evidence based on trigonometric parallaxes, space motions, and spectroscopic analyses that the isolated MWDs tend as a class to have a higher mass than the nonmagnetic white dwarfs. The mean mass for 16 MWDs with well‐constrained masses is ≳ 0.95 M⊙. Magnetic fields may therefore play a significant role in angular momentum and mass loss in the post–main‐sequence phases of single star evolution affecting the initial‐final mass relationship, a view supported by recent work on cluster MWDs. The progenitors of the vast majority of the isolated MWDs are likely to be the magnetic Ap and Bp stars. However, the discovery of two MWDs with masses within a few percent of the Chandrasekhar limit, one of which is also rapidly rotating (Pspin = 12 minutes), has led to the proposal that these may be the result of double‐degenerate (DD) mergers. An intriguing possibility is that magnetism, through its effect on the initial‐final mass relationship, may also favor the formation of more massive double degenerates in close binary evolution. The magnetic DDs may therefore be more likely progenitors of Type Ia supernovae.A subclass of the isolated MWDs appear to rotate slowly with no evidence of spectral or polarimetric variability over periods of tens of years, while others exhibit rapid rotation with coherent periods in the range of tens of minutes to hours or days. There is a strong suggestion of a bimodal period distribution. The “rapidly” rotating isolated MWDs may include as a subclass stars which have been spun up during a DD merger or a previous phase of mass transfer from a companion star.Zeeman spectroscopy and polarimetry, and cyclotron spectroscopy, have variously been used to estimate magnetic fields of the isolated MWDs and the MWDs in MCVs and to place strong constraints on the field structure. The surface field distributions tend in general to be strongly nondipolar and to a first approximation can be modeled by dipoles that are offset from the center by ∼10%–30% of the stellar radius along the dipole axis. Other stars show extreme spectral variations with rotational phase which cannot be modeled by off‐centered dipoles. More exotic field structures with spot‐type field enhancements appear to be necessary. These field structures are even more intriguing and suggest that some of the basic assumptions inherent in most calculations of field evolution, such as force‐free fields and free ohmic decay, may be oversimplistic.

Extrasolar planets and the rotation and axisymmetric mass-loss of evolved stars

I examine the implications of the recently found extrasolar planets on the planet-induced axisymmetrical mass loss model for the formation of elliptical planetary nebulae (PNs). This model, which was developed in several earlier papers by the author and a few collaborators, attributes low departure from spherical mass loss of upper asymptotic giant branch (AGB) stars to envelope rotation which results from deposition of planet's orbital angular momentum. It was predicted that about 50 percent of all sun-like stars have Jupiter-like planets around them. In light of the new finding that only 5 percent of sun-like stars do have such planets I revise this prediction. I now predict that indeed about 50 percent of PNs progenitors do have close planets around them, but the planets can have much lower masses, as low as 0.01 times Jupiter's mass. To support this claim I follow the angular momentum evolution of single stars as they evolve to the post-AGB phase. The prediction that on average several such planets orbit each star, as in the solar system, still holds.

Star cluster ecology -- IV. Dissection of an open star cluster: photometry

The evolution of star clusters is studied using N-body simulations in which the evolution of single stars and binaries are taken self-consistently into account. Initial conditions are chosen to represent relatively young Galactic open clusters, such as the Pleiades, Praesepe and the Hyades. The calculations include a realistic mass function, primordial binaries and the external potential of the parent Galaxy. Our model clusters are generally significantly flattened in the Galactic tidal field, and dissolve before deep core collapse occurs. The binary fraction decreases initially due to the destruction of soft binaries, but increases later because lower mass single stars escape more easily than the more massive binaries. At late times, the cluster core is quite rich in giants and white dwarfs. There is no evidence for preferential evaporation of old white dwarfs, on the contrary the formed white dwarfs are likely to remain in the cluster. Stars tend to escape from the cluster through the first and second Lagrange points, in the direction of and away from the Galactic center. Mass segregation manifests itself in our models well within an initial relaxation time. As expected, giants and white dwarfs are much more strongly affected by mass segregation than main-sequence stars. Open clusters are dynamically rather inactive. However, the combined effect of stellar mass loss and evaporation of stars from the cluster potential drives its dissolution on a much shorter timescale than if these effects are neglected. The often-used argument that a star cluster is barely older than its relaxation time and therefore cannot be dynamically evolved is clearly in error for the majority of star clusters.

Asteroseismology of the Multiply Periodic δ Scuti StarθTucanae

We present an asteroseismological analysis of the δ Scuti component of the binary star system θ Tucanae, using the 10 pulsational frequencies obtained photometrically by Paparo et al. and the mode identifications of Sterken. We have tested theoretical models with masses between 1.8 and 2.8 M☉ that have solar metal abundances, and we find that based upon mode stability arguments, the photometrically derived mass estimate of 2.1 ± 0.1 M☉ is probably accurate. The models with the best frequency match to θ Tucanae have masses of 1.9-2.1 M☉, luminosities between 20 and 25 L☉, effective temperatures between 7500 and 7685 K, log g values between 3.82 and 3.92, and rotational velocities between 70 and 90 km s-1. The luminosities of our models are more than a factor of 2 less than the Hipparcos-derived luminosity of 54 L☉, which suggests that the secondary star must be of comparable luminosity. We cannot determine whether tidal distortion is causing nonspherical perturbations in the pulsating component of this system with our current models, but the frequency spacing of observed pulsation modes suggests that rotation has a strong effect on the observed spectrum. Our models are consistent with the observed pulsation spectrum if rotational splitting is taken into account. Recently, De Mey, Daems, & Sterken determined spectroscopically that the θ Tucanae system is a spectroscopic binary with an anomalous mass ratio of 0.0896. In their model of this system, the δ Scuti component of the system is probably the beneficiary of mass transfer from the secondary, and the system was likely once an Algol-type system. The secondary is probably a 0.2 M☉ post-red giant branch object at ~7000 K (log g ~ 3.0), which has lost most of its mass via mass transfer and winds. This scenario raises the possibility that the interior of the δ Scuti star may be radically different from what single-star evolution models predict, although our results do not show any obvious differences. However, the age and envelope abundances should be different from single-star model predictions. We believe that this object provides an excellent opportunity to study the interiors of post-mass transfer objects using asteroseismology, and we suggest future observational and theoretical work that will help us understand this system.

Optical spectroscopy of the candidate luminous white dwarf in the young Large Magellanic Cloud cluster NGC 1818

An optical spectrum of the Elson et al. (1998) candidate luminous white dwarf in the young LMC cluster NGC1818 shows conclusively that it is not a degenerate star. A model atmosphere fit gives Teff�31,500K and log g�4.4, typical of a garden-variety main sequence B star. However, if it is a true LMC member then the star is underluminous by almost three magnitudes. Its position in the cluster colour-magnitude diagram also rules out the possibility that this is an ordinary B star. The luminosity is, however, consistent with a �0.5M⊙ post-AGB or post-EHB object, although if it has evolved via single star evolution from a high mass (7.6 9.0M⊙) progenitor then we might expect it to have a much higher mass, �0.9M⊙. Alternatively, it has evolved in a close binary. In this case the object offers no implications for the maximum mass for white dwarf progenitors, or the initial-final mass relation. Finally, we suggest that it could in fact be an evolved member of the LMC disk, and merely projected by chance onto NGC1818. Spectroscopically, though, we cannot distinguish between these evolutionary states without higher resolution (echelle) data.

The effect of close binary evolution on photoionization models for evolving starbursts

Using a population number synthesis code and detailed massive star evolutionary calculations, Van Bever & Vanbeveren (1998) investigated the effect of close binary evolution on the WR and O-type star population of starburst regions. It was concluded that the age determination based on O-type features of a starburst which is older than 4 million years could be erroneous if observations are interpreted from the point of view of single star evolution only. Most important is the formation of rejuvenated OB-type Blue Stragglers when significant matter lost by the binary primary during the Roche lobe overflow (RLOF), is accreted by the companion. The present paper illustrates the latter statement. We consider the most frequently used age determinators, i.e., the strength of the H α and H β emission lines of starburst regions. We distinguish the effects of the OB-type Blue Stagglers, of WR single stars and of hydrogen deficient core helium burning binary components in the post-RLOF phase of evolution (including the WR binary components) and the effects of X-radiation produced in standard high mass X-ray binaries.

On the Nature of the Peculiar Hot Star in the Young Large Magellanic CloudCluster NGC 1818

The blue star reported in the field of the young LMC cluster NGC1818 by Elson et al. (1998) has the wrong luminosity and radius to be a "luminous white dwarf" member of the cluster. In addition, unless the effective temperature quoted by the authors is a drastic underestimate, the luminosity is much too low for it to be a cluster member in the post-AGB phase. Other possibilities, including that of binary evolution, are briefly discussed. However, the implication that the massive main sequence turnoff stars in this cluster can produce white dwarfs (instead of neutron stars) from single-star evolution needs to be reconsidered.

Evolutionary population synthesis: the effect of binary systems

We present in this contribution our set of multi-wavelength synthesis models including the evolution of single and binary stars. The main results we have obtained can be summarized as follows: (a) massive close-binary systems will start to experience mass transfer episodes after the first 4Myr of the starburst evolution; (b) as a result of these mass transfer processes, stars of relatively low initial mass can lose completely their envelope and become a Wolf-Rayet star. In this way, the formation of WR stars is extended over longer than 15 Myr, and does not stop at 6Myr as predicted by models including only single stars; (c) WR stars can thus be coeval with red supergiants, which peak at around 10 Myr for solar metallicities; (d) the accretion of mass will originate relatively massive stars at ages for which they should have already disappeared; these stars, together with the WR stars formed in rather evolved clusters, increase the production of ionizing photons, so that the Hβ equivalent width will not drop as rapidly as predicted by models considering only individual stars; and (e) the mass transfer to compact companions will produce an additional source of high-energy radiation in the form of high-mass X-ray binaries, not predicted either by standard synthesis models.

Expected Differences between AGB Stars in the LMC and the SMC Due to Differences in Chemical Composition

Certain aspects of the AGB population, such as the relative number of M and N stars, the mass loss rates, and the initial masses of carbon-oxygen cores, depend on the initial heavy element abundance Z. I have calculated synthetic populations of AGB stars for different initial Z values taking into consideration the evolution of single and close binary stars. I present the results of population syntheses of AGB stars in clusters as a function of different initial chemical compositions. The relation for the tip luminosity of AGB stars versus cluster age as a function of Z is presented and is used to determine the ages for a number of clusters in the LMC and the SMC, including clusters with no previous age determinations.

Evolution of massive close binaries, compared to observations

Since binaries can not be considered separately from single stars, we first present a brief summary of massive single star evolution. Next, we highlight massive close binary evolutionary facts that are important when massive close binaries are included in population (number) synthesis. Finally, we consider a few interesting test cases.

Are the Bulge C-Stars Related to the Sagittarius Dwarf Galaxy?

When the Sagittarius dwarf galaxy (SDG) crosses the galactic plane star formation might occur in its tidal tail. Carbon stars, due to their brightness, are ideal tracers for such an event. Ng & Schultheis (1997) suggested that the Bulge carbon stars (Azzopardi et al. 1991; ALRW) might be at SDG's distance. In the galactic Bulge these stars are completely different from carbon stars found in the LMC, SMC and dwarf spheroidal galaxies. If one keeps these stars in the Bulge and tries to explain their properties through a binary evolution scenario, one has to explain why they are not found up to the red giant branch tip: there is a discrepancy. Ng (1997) argues that part of the mystery concerning the luminosity of the ALRW stars would be solved if they are related to the SDG. It is demonstrated that the carbon star sequence is in that case similar to the observed SMC sequence. The carbon stars are in that case not metal-rich as previously thought, but they have a metallicity comparable to the LMC, with an age between 0.1–1 Gyr. A significant fraction of the carbon stars still have luminosities fainter than the lower LMC limit of . A similar trend is present among carbon stars found in other dwarf spheroidals. A scenario through binary evolution is assumed for these stars, because TP-AGB models cannot explain their origin through a single-star evolution scenario, even if they form immediately after entering the TP-AGB phase.

Projected rotational velocities of WD 1614+136 and WD 1353+409: implications for the rate of Galactic Type Ia supernovae

The white dwarf stars WD 1614+136 and WD 1353+409 are not sufficiently massive to have formed through single-star evolution. However, observations to date have not yet found any evidence for binarity. It has therefore been suggested that these stars are the result of a merger. In this paper we place an upper limit of ≈ 50 km s−1 on the projected rotational velocities of both stars. This suggests that, if these stars are the results of a merger, efficient angular momentum loss with accompanying mass loss must have occurred. If the same process occurs following the merging of more massive white dwarf stars, the predicted rate of Type Ia supernovae due to merging white dwarfs may have been greatly overestimated. Further observations to determine binarity in WD 1614+136 and WD 1353+409 are therefore encouraged.

Testing the White Dwarf Mass‐Radius Relation withHipparcos

We present the Hipparcos parallaxes and resulting radii for 10 white dwarfs in visual binaries or common proper-motion systems and 11 field white dwarfs. For bright stars, Hipparcos parallaxes have uncertainties approaching 1 mas and are thus considerably more accurate than earlier ground-based parallaxes. Overall, our results support the predictions of the white dwarf mass-radius relation and our understanding of stellar degeneracy. Our most important finding for an individual object is the position of 40 Eri B, a well-known puzzle, now consistent with single-star evolution. In addition, we present evidence supporting the existence of a range of atmosphere thicknesses for hydrogen (DA) white dwarfs.

Cataclysmic Variable Stars

The evolution of single stars on and away from the main sequence is well understood. A degenerate core is formed in a star as the star leaves the main sequence and expands to a giant with a radius typically 50 - 500 Ro . Observationally it is known that most stars are members of binary systems, and among these many have orbital periods less than 100 y. It can happen, therefore, that the expanding envelope of the primary of a binary system can reach to the secondary. As this happens, the primary fills its Roche tidal lobe and transfers matter to the secondary; if the primary has a radiative envelope the rate at which this occurs exceeds the Eddington limit of the secondary, which therefore repels the incoming gas, forming a common envelope around the two stars. Friction within the envelope causes the stars to spiral towards each other until the energy and angular momentum extracted from the binary orbit and transferred to the envelope are sufficient to eject the common envelope as a planetary nebula, leaving a short period binary comprising a white dwarf and a main sequence star.This mechanism of producing short period binaries containing white dwarfs, proposed by Ostriker and by Paczynski (1976), is the probable origin of the class of objects known as Cataclysmic Variable Stars (CVs), which encompass the classical novae, dwarf novae, novalike variables and a variety of related objects. Evidence has been accumulating for forty years (Crawford & Kraft 1956, Warner 1995a) that every CV consists of a secondary star (usually a dwarf, but a few systems contain giants) filling its Roche lobe and transferring mass to a white dwarf primary. In systems of normal chemical composition the orbital periods lie between 75 mins and ~250 d, with the majority having . A few hydrogen-free systems are known for which 17 mins < Porb < 50 mins. It should be noted that CVs are very compact binary systems: for h such a binary would fit inside the Sun.

Rapid binary star evolution for N-body simulations and population synthesis

We present an algorithm for the rapid evolution of binary stars. It is based on simple fitting formulae for the evolution of single stars. Mass loss, mass transfer and common-envelope evolution are followed with the changes to both the mass donor and the accretor where applicable. We describe a procedure for integrating our formulae and give examples of how we can follow the evolution of Algols and cataclysmic variables. The algorithm can be used for simple binary population synthesis. We also explain how we incorporate it into N-body simulations where we introduce a new scheme that deals with colliding stars.

Ultraviolet Color-Magnitude Diagram Studies of Intermediate Age Large Magellanic Cloud Star Clusters.I.NGC 1783

We have imaged the billion-year-old Large Magellanic Cloud star cluster NGC 1783 with the Wide Field/Planetary Camera 2 (WFPC2) in order to obtain broadband photometry at 1600 and 3300 Angstroms. We have identified for the first time the individual stars responsible for the unexpectedly strong ultraviolet light of intermediate-age stellar systems. The behavior of the hot component of aging stellar systems is not well known, and has important implications for the interpretation of light from high-redshift galaxies. Contrary to our expectations, there are far more hot stars present in this cool cluster than can be accounted for by the classical post-asymptotic giant branch. Of particular interest is a group of stars that is strongly concentrated in the core of the cluster. By constructing a relation between the WFPC2 UV colors and stellar effective temperatures, we have found that these stars do not lie in an area of the color-magnitude diagram that is crossed by any theoretical single-star stellar evolution tracks of the cluster's age. These data are insufficient to unambiguously identify the astrophysical nature of the anomalous UV-bright stars; we discuss possibilities for their origin.

Precise stellar mass and mass-luminosity data

Recent progress in observing and data reduction methods for precise mass and mass-luminosity determinations in binary systems are briefly reviewed. The foundations appear to have been laid for a new burst of accurate data. Detailed model simulations of the individual systems are the best way to use these data to critically test the theoretical models and advance our understanding of the evolution of single and binary stars.

Structure and evolution of single WR stars

New computations of massive stars follow the evolution up to advanced stages and include: -A large and flexible nuclear network consisting of 174 nuclear species that are linked by 1742 nuclear reactions. -Semiconvection, overshooting and mass loss. -Modern rates for both strong and weak interaction processes as well as the latest rates for the neutrino processes. -Improved grid distribution and a large number of grid points.

Low Mass White Dwarfs in Binaries

The lowest mass white dwarfs that can have been produced by single star evolution in the lifetime of the Galaxy have masses of about 0.53 M⊙. There are however several white dwarfs known with significantly lower masses. Evolution in a binary provides a straightforward explanation as the star can lose its envelope before ever burning helium. The products are expected to be short-period binary stars. To test this we have looked at 15 such stars and have found that 8 of them are close binaries. Thus binary evolution does have a major role in the formation of low-mass white dwarfs. We discuss whether the non-detections could also be binary stars.

Ultraviolet radiation from evolved stellar populations. 2: The ultraviolet upturn phenomenon in elliptical galaxies

We discuss the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. We derive the broad-band UV colors 1500-V and 2500-V for globular clusters and elliptical galaxies from the available satellite data and investigate color-color and color-line strength correlation. Clusters can be bluer than any galaxy in 15-V and 25-V, implying larger hot star populations, but galaxies are significantly bluer than clusters in 15-25 at a given 15-V. We attribute this primarily to the effect of metal abundance on the mid-UV (2500 A) light. These redder colors of the galaxies also imply that the UVX in galaxies is not produced by metal-poor subpopulations similar to the clusters. We devlop a simple spectral synthesis formulation for all phases of single star evolution from the zero-age main sequence (ZAMS) to the white dwarf cooling track that requires only one or two parameters for each choice of age and abundance. We provide the ingredients necessary for constructing models with arbitrary horizontal branch (HB) morphologies in the age range 2 less than t less than 20 Gyr and for six metallicities in the range -2.26 less than (Fe/H) less than 0.58; we also consider the efect of enhanced Y in metal-rich models. The maximum lifetime UV output is produced by EHB stars with (M(sub env))(sup 0) approximately 0.02 solar mass and can be up to 30 times higher than for post-asymptotic giant branch (P-AGB) stars. The ultraviolet output of old populations is governed primarily by the distribution of (M(sub env))(sup 0)P(M(sub env))(sup 0), on the ZAHB. The UV output is not very sensitive to (Fe/H) or to Y, but it can change very rapidly with (M(sub env))(sup 0). Thus it is extremely sensitive to the precise nature of giant-branch mass loss. Our models use simple descriptions of P(M(sub env))(sup 0) to bracket the colors produced from any real distribution of stars. Our models accurately predict the range of UV colors observed for the globular clusters, given known constraints on their age, abundances, and HB morphologies. We find that models with (Fe/H) greater than or = 0 that do not contain EHB stars cannot reproduce the colors of most of the galaxies. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is approximately 70% and approximately 20% for moderate UVX and strong UVX systems, respectively. We find that 25-V, but not 15-V, is sensitive to the age and abundance, though these cannot always be cleanly distinguished. The galaxy colors place limits of (Fe/H) greater than -0.5 and less than 15% on the contribution of globular cluster-type populations to the V light. Galaxy colors are consistent with solar-abundance models with ages in the range 6-14 Gyr. We discuss several implications of the observations and the models, including the question of light metal versus iron peak enhancements in galaxies, whether the UV color-Mg(sub 2) correlation is continuous or discrete, effects of helium abundnace on the UVX, and the key question of whether red giant branch mass loss can be large enough to produce the necessary EHB population in the strong UVX galaxies.