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Register a new userMagnetic properties of three cool CP stars with strong fields
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The study of magnetic fields of cool chemically peculiar stars with effective temperatures less than 10 000 K is very important to understand the nature of their magnetism. We present new results of a long-term spectroscopic monitoring of the well-known magnetic star HD 178892. The analysis of spectra taken with the Russian 6-m telescope has revealed a periodic variation of the surface magnetic field from 17 to 23 kG. A revised rotational period of HD 178892 was extracted from the mean longitudinal field: 8.2549 days. We have continued the study of the components of the magnetic binary BD +40^{circ}175 started by V. Elkin at SAO RAS. Our measurements of magnetically splitted lines in the spectra of each component show the presence of strong magnetic fields in both components. The surface field in the case of the component A was about 14 kG at three different epochs. The component B possesses a slightly weaker field: B_{s} varies from 9 to 11 kG. A preliminary analysis of the chemical abundances allows us to make an assumption about the roAp nature of both components of BD +40^{circ}175.
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We present measurements of fundamental astrophysical properties of nearby, low-mass, K- and M-dwarfs from our DISCOS survey (DIameterS of COol Stars). The principal goal of our study is the determination of linear radii and effective temperatures for these stars. We calculate their radii from angular diameter measurements using the CHARA Array and Hipparcos distances. Combined with bolometric flux measurements based on literature photometry, we use our angular diameter results to calculate their effective surface temperatures. We present preliminary results established on an assortment of empirical relations to the stellar effective temperature and radius that are based upon these measurements. We elaborate on the discrepancy seen between theoretical and observed stellar radii, previously claimed to be related to stellar activity and/or metallicity. Our preliminary conclusion, however, is that convection plays a larger role in the determination of radii of these late-type stars. Understanding the source of the radius disagreement is likely to impact other areas of study for low-mass stars, such as the detection and characterization of extrasolar planets in the habitable zones.
Hot cluster Horizontal Branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (T_eff > 11000K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with T_eff < 30000K, whose temperatures overlap with those of the hot HB stars. We conclude that large-scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it.
The recent years have brought great advances in our knowledge of magnetic fields in cool giant and supergiant stars. For example, starspots have been directly imaged on the surface of an active giant star using optical interferometry, and magnetic fields have been detected in numerous slowly rotating giants and even on supergiants. Here, I review what is currently known of the magnetism in cool giant and supergiant stars, and discuss the origin of these fields and what is theoretically known about them.
The paper presents magnetic field measurements for 15 chemically peculiar (CP) stars of subgroup~1b in the OrionOB1 association. We have found that the proportion of stars with strong magnetic fields among these 15 CP stars is almost twice as large as in subgroup 1a. Along with this, the age of subgroup 1b is estimated as 2 Myr, and the age of subgroup~1a is in the order of 10 Myr. The average root-mean-square magnetic field Be for stars in subgroup 1b is 2.3 times higher than that for stars in subgroup 1a. The conclusions obtained fall within the concept of the fossil origin of large-scale magnetic fields in B and A stars, but the rate of field weakening with age appears anomalously high. We present our results as an important observational test for calibrating the theory of stellar magnetic field formation and evolution.
The dynamics of colliding wind binary systems and conditions for efficient particle acceleration therein have attracted multiple numerical studies in the recent years. These numerical models seek an explanation of the thermal and non-thermal emission of these systems as seen by observations. In the non-thermal regime, radio and X-ray emission is observed for several of these colliding-wind binaries, while gamma-ray emission has so far only been found in $eta$ Carinae and possibly in WR 11. Energetic electrons are deemed responsible for a large fraction of the observed high-energy photons in these systems. Only in the gamma-ray regime there might be, depending on the properties of the stars, a significant contribution of emission from neutral pion decay. Thus, studying the emission from colliding-wind binaries requires detailed models of the acceleration and propagation of energetic electrons. This in turn requires a detailed understanding of the magnetic field, which will not only affect the energy losses of the electrons but in case of synchrotron emission also the directional dependence of the emissivity. In this study we investigate magnetohydrodynamic simulations of different colliding wind binary systems with magnetic fields that are strong enough to have a significant effect on the winds. Such strong fields require a detailed treatment of the near-star wind acceleration zone. We show the implementation of such simulations and discuss results that demonstrate the effect of the magnetic field on the structure of the wind collision region.
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