The nature of the recent extreme outburst of the Herbig Be/FU Ori binary Z CMa

Z CMa is a binary system which consists of two young stars: A Herbig AeBe component "Z CMa NW" embedded in a dust cocoon and a less massive component "Z CMa SE", which is classified as a FU Orionis type star. Recently, the system showed the largest outburst reported during the almost 90 years of available observations. During the recent outburst we detect that the Z CMa system is polarized by 2.6% in the continuum and emission line spectrum, with a position angle still perpendicular to the jet. From the high level of polarization we conclude that the outburst is associated with the dust embedded Herbig AeBe NW component. The main result of our studies is that the bolometric luminosity of Z CMa remained surprisingly constant during the recent "outburst". We conclude that either the geometry of the cavity through which the light escapes from the cocoon has opened a new path, or that the screen of dust, which reflects the light toward the observer became more efficient causing the observed increase of the visual brightness by about 2.5 magnitudes.Comment: letter to A&A, accepted 17/12/200

The light variability of the helium strong star HD 37776 as a result of its inhomogeneous elemental surface distribution

We simulate light curves of the helium strong chemically peculiar star HD 37776 assuming that the observed periodic light variations originate as a result of inhomogeneous horizontal distribution of chemical elements on the surface of a rotating star. We show that chemical peculiarity influences the monochromatic radiative flux, mainly due to bound-free processes. Using the model of the distribution of silicon and helium on HD 37776 surface, derived from spectroscopy, we calculate a photometric map of the surface and consequently the uvby light curves of this star. Basically, the predicted light curves agree in shape and amplitude with the observed ones. We conclude that the basic properties of variability of this helium strong chemically peculiar star can be understood in terms of the model of spots with peculiar chemical composition.Comment: 11 pages, accepted for the publication in Astronomy & Astrophysic

Binarity Versus Magnetic Fields in Upper Main-sequence Stars

We review the status of the work on the detection, possible origin, and modelling of magnetic fields in binary systems with intermediate- and higher-mass primaries at different evolutionary stages. This review also includes interesting findings on a few exotic systems.Fil: Hubrig, S.. Leibniz-Institut für Astrophysik Potsdam; AlemaniaFil: Schöller, M.. European Southern Observatory; AlemaniaFil: Ilyin, I.. Leibniz-Institut für Astrophysik Potsdam; AlemaniaFil: Gonzalez, Jorge Federico. Consejo Nacional de Investigaciones Cienti­ficas y Tecnicas. Centro Cientifico Tecnologico San Juan. Instituto de Ciencias Astronomicas de la Tierra y del Espacio; ArgentinaFil: Mikulásek, Z.. Masaryk University. Department of Theoretical Physics and Astrophysics; República Chec

Period study of TW Draconis

Context. TW Draconis is one of the most well known Algol-type eclipsing binaries. There is significant evidence for miscellaneous interacting physical processes in the system, which manifest themselves as for example period and light curve changes. We study time series data for the star accumulated over the past 150 years to detect changes in minima timings. Aims. Using 561 available minima timings, we compile an extended O–C diagram analysis. A detailed description of period changes allows us to construct the true phased light curve at any moment. Methods. By applying weighting nonlinear robust regression the timing residuals were analysed. The weights of individual types of minima were determined iteratively. The ephemeris are expressed in the orthogonal or quasi-orthogonal forms allowing us for example to determine directly uncertainties of photometric phases for any given moments. Results. The timing residuals (according to the linear ephemeris) display two stages of differing behaviour in 1858–2007. The first part ends around 1942. It is characterised by more or less smooth linear/parabolic course of timing residuals. In 1858–1905, the period was almost constant , but, in 1905–1942, the period increased with the rate of 5.69(5)10-8 d year-1. We interpret this as a result of continuous mass exchange between components at the mean rate of 6.910-7  year-1. Since 1942, the system has been showing alternating and shortening period changes with the length of a cycle of about two decades, modulated by short-term periodic variations. The latter small variations with the period 6.5 years are caused by the light-time effect due to the presence of a third low-mass body in the system. Major oscillations can be explained as consequences of quadrupole moment variation in the system

The puzzling binary HD 143418

Context. HD 143418  was discovered recently to be a double-lined spectroscopic binary with a primary designated as a CP star. Its light displays an orbital phase coupled variability with a peak-to-peak amplitude up to 0.04 mag. Aims. The photometry available and new high dispersion spectra were investigated from a point of view of CP characteristics. Methods. A series of high resolution high S/N coudé spectra was acquired from which 25 weak to strong unblended lines of Fe I and II, Ti II, Cr II, Zr II, and Ba II were selected to study spectral line variability. Two Zeeman spectra were obtained to search for a possible magnetic field of the star, and one echelle spectrum in a wide spectral region was analysed for abundance determination by means of synthetic spectra. The photometric observations were subjected to a PCA disentangling of the complex photometric behaviour. Results. We identified spectral lines of the secondary in the yellow region on the echelle as well as on two coudé spectra, whose occurence belongs to an F6V star and the intensity corresponds to the luminosity ratio 0.06. Equivalent widths of the selected spectral lines of the primary component do not change within the errors of measurements. The spectra taken with a Zeeman analyser do not indicate a magnetic field. The abundance pattern does not correspond to characteristics of a variable CP2 star. The only remarkable deviation is a more than 1.1 dex deficit of scandium, one of the properties of non-variable Am stars. The photometric variability is tied to the orbital period and is due to ellipticity of the primary component and not to a putatively structured surface of the primary that is confirmed to rotate subsynchronously. The seasonal component of the light curve changes in amplitude as well as in shape. Conclusions. We conclude that the primary is a normal, mildly evolved A5V main sequence star. The seasonal variability of the orbitally modulated light curves may be related to an expected incidence of circumstellar matter originating in the tidally spinning up primary component. HD 143418 may be a prototype of a rare detached interacting close binary containing a subsynchronously rotating primary passing through its synchronisation stage

The nature of the light variability of the silicon star HR 7224

Context. Although photometric variations of chemically peculiar (CP) stars are frequently used to determine their rotational periods, the detailed mechanism of their light variability remains poorly understood. Aims. We simulate the light variability of the star HR 722

The extremely rapid rotational braking of the magnetic helium-strong star HD 37776

Context. Light and spectrum variations of the magnetic chemically peculiar (mCP) stars are explained by the oblique rigid rotator model with a rotation period usually assumed to be stable on a long time scale. A few exceptions, such as CU Vir or 56 Ari, have been reported as displaying an increase in their rotation period. A possible increase in the period of light and spectrum variations has also been suggested from observations of the helium-strong mCP star HD 37776 (V901 Ori). Aims. In this paper we attempt to confirm the possible period change of HD 37776 and discuss a possible origin of this change as a consequence of i) duplicity; ii) precession; iii) evolutionary changes; and iv) continuous/discrete/transient angular momentum loss. Methods. We analyse all available observations of the star obtained since 1976. These consist of 1707 photometric measurements obtained in uvby(), (U)BV, V, , and , including 550 of our own recent observations obtained in 2006 and 2007, 53 spectrophotometric measurements of the He I 4026 Å line, 66 equivalent width measurements of He I spectral lines from 23 CFHT spectrograms acquired in 1986, and 69 He I equivalent measurements from spectral lines present in 35 SAO Zeeman spectrograms taken between 1994 and 2002. All of these 1895 individual observations obtained by various techniques were processed simultaneously by means of specially developed robust codes. Results. We confirm the previously suspected gradual increase in the period of HD 37776 and find that it has lengthened by a remarkable 17.70.7 s over the past 31 years. We also note that a decrease in the rate of the period change is not excluded by the data. The shapes of light curves in all colours were found to be invariable. Conclusions. After ruling out light-time effects in a binary star, precession of the rotational axis, and evolutionary changes as possible causes for the period change, we interpret this ongoing period increase as a braking of the star's rotation, at least in its surface layers, due to the momentum loss through events or processes in the extended stellar magnetosphere