We present first results of modeling convective photospheres of neutron stars. We show that in photospheres composed of the light elements convection arises only at relatively low effective temperatures (< 3-5*10^4 K), whereas in the case of iron compositon it arises at < 3*10^ K. Convection changes the depth dependence of the photosphere temperature and the shapes of the emergent spectra. Thus, it should be taken into account for the proper interpretation of EUV/soft-X-ray observations of the thermal radiation from neutron stars

Iglesias, C A

Pavlov, G G

Rogers, F J

Shibanov, Yu A

Zavlin, V E

arXiv

We present first results of modeling convective photospheres of neutron stars. We show that in photospheres composed of the light elements convection arises only at relatively low effective temperatures ({le}3 - 5 x 10{sup 4} K), whereas in the case of iron composition it arises at T{sub eff}{le} 3 x 10{sup 5}K. Convection changes the depth dependence of the photosphere temperature and the shapes of the emergent spectra. Thus, it should be taken into account for the proper interpretation of EUV/soft-X-ray observations of the thermal radiation from neutron stars

Zavlin, V. E.

Pavlov, G. G.

Shibanov, Yu. A.

Rogers, F. J.

Iglesias, C. A.

UNT Digital Library

UCRGJC-123091 PREPRINT X-ray spectra from convective photospheres of neutron stars V. E. ZavIin Max-Planck Institute G. G. Pavlov Pennsylvania State University Yu. A. Shibanov Ioffe Institute of Physics and Technology F. J. Rogers C. A. IgIesias Lawrence Livermore National Laboratory This paper was prepared for submittal to the Conference on Roentgenstrahlung from the Universe Wuerzburg, Germany, September 25 - 29,1995 I . .  . .  . - . .. '... . ,... .\ . . . ... L .: . .  . Thisisa preprintof apaperintended forpublication h a  journal orproceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the P authar. DISCLAIMER This document was prepared as a n  account of work sponsored by an  agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees. makes any wnrranty, express or implied, or assumes any legal liability or responsibility for the accuracy. completeness, or useful- ness of any information. apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufadurer, or otherwise. does not necessarily constitute or imply its endorsement. recommendation, or favoring by the United States Government or the  University of Qlifornia. T h e  views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. X-ray spectra from convective photospheres of neutron stars V.E. Zavlid, G.G. P ~ V I O V ~ ~ ~ ,  Yu.A. ShibanoG, F.J. Rogers4 and C.A. Iglesias4 Max-Planck-Institut fiir Extraterrestrische Physik, Giessenbachstrasse, D-85740 Garching, Germany Pennsylvania State University, 525 Davey Lab, PA 16802, USA . ’ Ioffe Institute of Physics and Technology, 194021, St. Petersbwg, Russia ‘ Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Abstract. We present first results of modeling convective photospheres of neutron stars. We show that in photo- spheres composed of the light elements convection arises only at relatively low effective temperatures ( s 3-5x io4 K), whereas in the case of iron compositon it arises at Teff s 3 x lo5 K. Convection changes the depth depen- dence of the photosphere temperature and the shapes of the emergent spectra. Thus, it should be taken into ac- count for the proper interpretation of EUV/soft-X-ray ob- servations of the thermal radiation from neutron stars. 1. Introduction Recent ROSAT observations of pulsars revealed that some of them emit thermal-like radiation in the soft X-ray range (Ogelman 1995). These observations stimulated further in- vestigations of neutron star (NS) photospheres responsible for the properties of the emitted radiation. One of important and hitherto untouched problems associated’with modeling of the NS photospheres is the problem of convective energy transport which can affect the temperature distribution and the emergent spectra. Due to huge gravitational accelerations, - 1014 - 1015 cm s’~, the NS photospheres are much denser, p - 0.001 - 1 g ~ m ’ ~ ,  than those of usual stars. The increased densi- ties shift ionization equilibrium: the nonionized fraction grows with p at moderate densities, p sO.01-0.1 g cme3, and sharply decreases at p 2 0.1 - 1 g cm-3 due to pres- sure ionization. As a result, zones of increased opacity (increased radiative gradient Vmd) and reduced adiabatic gradient Vd develop in the NS photospheres at temper- atures much higher than in photospheres of usual stars, which may cause convective instability at depths where the emergent spectrum is formed. So far the NS photo- spheres have been considered either for stars with very strong surface magnetic fields B - 1 O l 2  - lOI3 G (see, for example, Pavlov et al. 1994) where existing convection theories are not applicable, or for ‘nonmagnetic’ (low-field) photospheres, B < lo8- lo9 G, with high surface effective temperatures Ten > - lo6 K (Romani 1987), where the convection can hardly be expected. Since thermal NS radiation can be detected for Teff 2 2 x lo5 K in the soft X-ray range, and for even lower temeratures, 2 2 x lo4 I<, in the UV/optical range (Pavlov et al. 1995), the study of the convective NS photospheres is important for the proper interpretation of these observations. The convective flow in stellar photospheres is turbu- lent and imposes many complicated problems. A com- mon practice, which we follow here, is to  use the phe- nomenological mixing-length theory with the traditional Schwarzschild criterion for convective instability, Vrad > vad. This theory has been widely implemented and testi- fied. Details of our nimerical calculations will be described elsewhere. Generally, we employ the complete lineariza- tion method for computing the photosphere models and include the convective energy transfer as described by Mi- halas (1978). 2. Results and Discussions We calculated the model of nonmagnetic photospheres for different chemical compositions. Here we present examples for pure hydrogen, helium and iron compositions at the gravitational acceleration g = 2.43 x 1014 cm s - ~  which corresponds to standard NS mass M = 1.4Mo and radius R = 10 km. The raditive opacities and equation of state for the iron composition were taken from the OPAL library (Iglesias et al. 1992). The results can be directly applicable to very old NSs with low magnetic fields (e. g., millisecond pulsars). Our results show that, similar to the case of usual stel- lar photospheres, the convective energy transfer begins to play a role at lower surface temperatures when atoms are not fully ionized and the radiative opacities are strongly increased by contribution of the bound-free and bound- bound transitions. The increased opacities result in high values of V d  - the depth dependence of the tempera- ture becomes steeper in order to transfer the energy flux throughout the photosphere. On the other hand, V,d in the dense partially ionized layers can be much smaller than its limiting value 0.4 for an ideal fully ionized or fully non- ionized gas (see, for example, Cox and Guili 1968). As a result, superficial convection zones in NS photospheres B 1 lM5; I c * 1- The convective transfer affects not only the structure of photosphere but also the spectra of the NS thermal radiation (Fig. 2) because the temperature profiles are changed in the layers where the radiation escapes from. In particular, convection substantially (up to two orders of magnitide) lowers the flux from H and He photospheres at photon energies above the main photoionization edges, so that the high-energy spectral tails become softer. The spectra remain the same at low and very high energies since both shallow and very deep layers are not affected by convection. In the case of Fe composition, the convec- tive zone lies so deep that only a high-energy tail (E 2 0.3 keV) is affected. The effect of convection on the spectra disappears with increasing effective temperature (e. g., at Teff > 3 x lo4 K for H, at Teff > 5 x lo4 K for He and at Teff > 3 x lo5 K for Fe photospheres). The presented results correspond to the convective ef- ficiency Z/H = 1. Acceptable values of this parameter can vary from 0.3 to 2.5. Bergeron et a2. (1992) showed that higher efficiency enhances convection and smoothes the spectra of white dwarfs. The same effect can be expected in the NS photospheres and, consequently, the convection le03 le02 141 let00 EOteV) - Fe - -  . ---, H e  I3 there may develop at higher effective temperatures than . . . '  . - . '  . . ..' . ' . '  . ..' ' . L  in the models presented. Hence, radiation from convective Technical Itformation Departmerzt e Lawrence Livennore National Laboratory University of California Livermore, California 9455 1 I 

X-ray spectra from convective photospheres of neutron stars

X-ray spectra from convective photospheres of neutron stars

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