Sachiko Tsuruta

THERMAL PROPERTIES AND DETECTABILITY OF NEUTRON STARS. II. THERMAL EVOLUTION OF ROTATION-POWERED NEUTRON STARS

Abstract After a brief introduction we present a progress report on the recent works carried out during the last ten years or so, on the thermal evolution of neutron stars and related problems. The effects of various improvements applied in recent years are discussed. Among these the most important may be the effects of magnetic fields and stellar atmospheres. When the problem is treated realistically with the two-dimensional method a magnetic neutron star appears to be kept warmer for a longer period, due to the tangential heat flow from the more insulated equatorial regions to the polar areas where heat escapes more efficiently. Possibly serious implications of this finding in relation to the interpretation of the observational data are discussed. The composition of the stellar atmosphere may be an important factor when we are to correctly interpret the observed temperature data. The current theoretical and observational considerations may support a ‘best buy model’, where some neutron stars with mass close to 1.4M⊙ possess a central core in which a ‘non-standard’ accelerating cooling mechanism, under the influence of significant suppression due to the presence of superfluid particles, is in operation. On the other hand, some less massive stars can still cool with the standard scenario. Some other interesting problems to be reviewed include: the most recent work on various ‘non-standard’ cooling scenarios; the pulsed radiation to be expected from magnetized neutron stars due to the anisotropic nature of conductivity and photon fluxes; the possible importance of the effects of various heating mechanisms and circumstellar electron positron pairs; the detectability of infant neutron stars in very young supernova remnants; and some applications to particle physics and cosmology. The topics to be covered are not meant to be complete, but the emphasis is placed on potentially interesting problems in relation to the recent and future observations of pulsars.

Core-Collapse Very Massive Stars: Evolution, Explosion, and Nucleosynthesis of Population III 500-1000 M☉ Stars

We calculate evolution, collapse, explosion, and nucleosynthesis of Population III very massive stars with 500 and 1000 M☉. Presupernova evolution is calculated in spherical symmetry. Collapse and explosion are calculated by a two-dimensional code, based on the bipolar jet models. We compare the results of nucleosynthesis with the abundance patterns of intracluster matter, hot gases in M82, and extremely metal-poor stars in the Galactic halo. It was found that both 500 and 1000 M☉ models enter the region of pair instability but continue to undergo core collapse. In the presupernova stage, silicon-burning regions occupy a large fraction, more than 20% of the total mass. For moderately aspherical explosions, the patterns of nucleosynthesis match the observational data of both the intracluster medium and M82. Our results suggest that explosions of Population III core-collapse very massive stars contribute significantly to the chemical evolution of gases in clusters of galaxies. For Galactic halo stars our [O/Fe] ratios are smaller than the observational abundances. However, our proposed scenario is naturally consistent with this outcome. The final black hole masses are ~230 and ~500 M☉ for the 500 and 1000 M☉ models, respectively. This result may support the view that Population III very massive stars are responsible for the origin of intermediate-mass black holes, which were recently reported to be discovered.

Heated Polar Caps in PSR 0656+14 and PSR 1055–52

We present new ASCA observations covering the 0.5-10 keV X-ray range of the cooling neutron star candidates PSR 0656+14 and PSR 1055-52. Previous ROSAT observations had shown that two-component models, either two blackbodies or a blackbody plus a power-law, provided the best spectral fits to their X-ray emission. The combined ASCA and ROSAT spectrum of PSR 0656+14 reveals two blackbody components with T ≈ 8 × 105 K and T ≈ 1.5 × 106 K and shows evidence that a power-law component is needed to account for higher energy photons. This three-component fit gives a reduced χ2 that is half the value of a more conventional two component fit (1.3 as compared to 2.4). The fit to the combined spectrum for PSR 1055-52 yields a two-blackbody fit with T ≈ 8 × 105 K and T ≈ 3.7 × 106 K. Our results favor the existence of a hot polar cap in each of these pulsars with the ratio of the polar cap area to the neutron star surface area being 7 × 10-3 and 3 × 10-5 for PSR 0656+14 and PSR 1055-52, respectively. The results are compared to models that make predictions of polar cap heating processes.

EVOLUTION OF VERY MASSIVE POPULATION III STARS WITH MASS ACCRETION FROM PRE-MAIN SEQUENCE TO COLLAPSE

We calculate the evolution of zero-metallicity Population III (Pop III) stars whose mass grows from the initial mass of ~1 M ☉ by accreting the surrounding gases. Our calculations cover whole evolutionary stages from the pre-main sequence, via various nuclear burning stages, through the final core-collapse or pair-creation instability phases. We adopt two different sets of stellar mass accretion rates as our fiducial models. One is derived from a cosmological simulation of the first generation (PopIII.1) stars, and the other is derived from a simulation of the second generation stars that are affected by radiation from PopIII.1 stars. The latter represents one case of PopIII.2 stars. We also adopt additional models that include radiative feedback effects. We show that the final mass of Pop III.1 stars can be as large as ~1000 M ☉, beyond the mass range (140-300 M ☉) for the pair-instability supernovae. Such massive stars undergo core-collapse to form intermediate-mass black holes, which may be the seeds for merger trees to supermassive black holes. On the other hand, Pop III.2 stars become less massive (40-60 M ☉), being in the mass range of ordinary iron core-collapse stars. Such stars explode and eject heavy elements to contribute to chemical enrichment of the early universe as observed in the abundance patterns of extremely metal-poor stars in the Galactic halo. In view of the large range of possible accretion rates, further studies are important to see if these fiducial models are actually the cases.

Thermal properties and detectability of neutron stars - I cooling and heating of neutron stars

Abstract In this report, we first review earlier and recent developments in some of thermodynamic problems of neutron stars, especially those involving cooling mechanisms and theoretical predictions of surface temperatures of neutron stars. Emphasis is placed particularly on: the effect of equations of state and hence that of nuclear and strong interactions; the effect of better treatment of various neutrino cooling mechanisms, especially those involving pion condensates; and implication of these better and more detailed theoretical estimates on the prospect of directly observing thermal radiation from the surface of neutron stars. In connection with the last problem, we briefly review recent developments on the observational side — the HEAO-B and other programs already existing or expected to be planned for near future, which are directly related to the above problem. In connection with the possibilities of observing older neutron stars we briefly summarise various heating mechanisms. From these studies, we see that exciting possibilities exist through the HEAO-B and some other programs which may be realised in the 1980s, that we may observe radiation directly from neutron star surfaces if they are ≳ (3−5) × 10 5 ° K . If such radiation is detected, the observed surface temperatures and further spectral studies may give invaluable insight into various important problems, such as magnetic properties of dense matter, equations of state, pion condensates, and other fundamental problems in nuclear, particle and high energy physics. If the surface temperatures of younger members of these stars (≲ 10 4 years) are observationally found to be less than ≈ (5−10) × 10 5 ° K (depending on the individual objects), we note that at the moment only pion coolings are consistent with observations, and the outcome may be equally far reaching. Among various observed neutron stars (pulsars) and neutron star candidates (e.g. supernova remnants), the Vela pulsar may prove to be the most rewarding one. If regular pulsar-like periodicities are discovered in radiations from any of supernova remnants, we can assume the presence of neutron stars in these objects. In that case, some supernova remnants, such as SN 1006, may also turn out to be promising. If we defect surface radiations from older pulsars (≳ 10 5 years), that may support some of heating theories. At the end, we point out that there may be many point sources of very soft weak thermal X-rays across the sky (as old neutron stars accrete interstellar matter) and some of the closest ones may be detectable through the HEAO-B and similar devices.

Confronting Neutron Star Cooling Theories with New Observations

With the successful launch of Chandra and XMM/Newton X-ray space missions combined with the lower energy band observations, we are now in the position where careful comparison of neutron star cooling theories with observations will make it possible to distinguish among various competing theories. For instance, the latest theoretical and observational developments appear to exclude both nucleon and kaon direct Urca cooling. In this way we can now have realistic hopes for determining various important properties, such as the composition, degree of superfluidity, equation of state, and stellar radius. These developments should help us obtain better insight into the properties of dense matter.

COOLING OF PULSARS.

Cooling rates are calculated for superfluid neutron stars of about one solar mass and 10 km radius, with magnetic fields from zero to about 10 to the 14th power Gauss, when possible internal friction effects are neglected. The results show that most old pulsars are so cold that thermal ionization of surface atoms would be negligible. At an age of a million years and with canonical magnetic fields of 10 to the 12th power Gauss, the estimated stellar surface temperature is several thousand to a hundred thousand degrees. However, if we neglect magnetic fields and superfluid states of nucleons, the same surfaces would be about a million degrees.

Thermal evolution of neutron stars with internal frictional heating

It has been suggested that the frictional interaction of neutron superfluids with normal matter in the inner crust of neutron stars dissipates rotational energy of superfluids and generates heat. Incorporating of general formula of internal heating into the detailed numerical codes of thermal evolution, we examine the effects of the internal heating on thermal evolution of neutron stars. We find that when a very stiff equation of state is used, it takes as long as ∼2×10 4 yr for the interior of a neutron star to reach the isothermal state, even if a strong heat source is placed in a thin layer of the inner crust

Gamma rays, X-rays, and optical light from the cobalt and the neutron star in SN 1987A

Recent developments in modeling the X-ray and gamma-ray emission from SN 1987A are discussed by taking into account both the decaying cobalt and the buried neutron star. The light curve and the spectra evolution of X-rays and gamma-rays are well modeled up to day of about 300 if mixing of Co-56 into hydrogen-rich envelope is assumed. However, the 16-28 keV flux observed by Ginga declines very slowly, whereas the spherical mixing model predicts that the flux should have decreased by a large factor at t greater than 300d. It is shown that this problem can be solved if the photoelectric absorption of X-rays is effectively reduced as a result of the formation of chemically inhomogeneous clumps. Based on the adopted hydrodynamical model and the abundance distribution, predictions are offered for future optical, X-ray, and gamma-ray light curves by taking into account other radioactive sources and various types of the central source, e.g., a buried neutron star accreting the reinfalling material or an isolated pulsar.

Photon opacity in surfaces of magnetic neutron stars

Approximate expressions are derived for free-free, bound-free, and Thomson cross-sections of photons by gaseous matter in the presence of superstrong magnetic fields. For photons in modes whose electric field polarization is perpendicular to this magnetic field, the cross-section is reduced by approximately the squared ratio of the photon frequency to the electron cyclotron frequency if this ratio is small.