Miloslav Zejda

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(β), (U)BV, V, BTVT ,a ndHp, including 550 of our own recent observations obtained in 2006 and 2007, 53 spectrophotometric measurements of the Hei λ 4026 A line, 66 equivalent width measurements of Hei spectral lines from 23 CFHT spectrograms acquired in 1986, and 69 Hei 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 1. 5387 period of HD 37776 and find that it has lengthened by a remarkable 17.7 ± 0.7 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.

The Dwarf project: Eclipsing binaries – precise clocks to discover exoplanets

We present a new observational campaign, Dwarf, aimed at detection of circumbinary extrasolar planets using the timing of the minima of low-mass eclipsing binaries. The observations will be performed within an extensive network of relatively small to medium-size telescopes with apertures of similar to 20-200 cm. The starting sample of the objects to be monitored contains (i) low-mass eclipsing binaries with M and K components, (ii) short-period binaries with a sdB or sdO component, and (iii) post-common-envelope systems containing a WD, which enable to determine minima with high precision. Since the amplitude of the timing signal increases with the orbital period of an invisible third component, the timescale of the project is long, at least 5-10 years. The paper gives simple formulas to estimate the suitability of individual eclipsing binaries for the circumbinary planet detection. Intrinsic variability of the binaries (photospheric spots, flares, pulsation etc.) limiting the accuracy of the minima timing is also discussed. The manuscript also describes the best observing strategy and methods to detect cyclic timing variability in the minima times indicating the presence of circumbinary planets. First test observations of the selected targets are presented ((c) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Rotationally modulated variations and the mean longitudinal magnetic field of the Herbig Ae star HD 101412

Despite the importance of magnetic fields to a full understanding of the properties of accreting Herbig Ae/Be stars, these fields have scarcely been studied until now over the rotation cycle. One reason for the paucity of these observations is the lack of knowledge of their rotation periods. The sharp-lined young Herbig Ae star HD 101412 with a strong surface magnetic field has become in the past few years one of the most well-studied targets among the Herbig Ae/Be stars. We present our multi-epoch polarimetric spectra of this star acquired with FORS 2 to search for a rotation period and constrain the geometry of the magnetic field. Methods: We measured longitudinal magnetic fields for 13 different epochs distributed over 62 days. These new measurements and our previous measurements of the magnetic field in this star were combined with available photometric observations to determine the rotation period. Results: We find the rotation period to be P = 42.076+/-0.017 d.

Photometric Study of the Very Short Period Shallow Contact Binary DD Comae Berenices

The first photometric solutions of the very short period (VSP) close binary DD Comae Berenices (P = 0(d).26920811) based on our new complete (IR)(C) light curves are derived by the 2003 version Wilson-Van Hamme code. They show that the system belongs to shallow contact W-type W UMa systems with a degree of overcontact of 8.7%. The observed light curve distortions are explained by employing the spots model due to the late-type nature of both components. We have collected all available photometric data about the system with emphasis on the individual observational data, which we treated simultaneously using our own method based on the usage of computed model light curves as templates. We recalculated published times of light minimum and added new ones of our own to construct an O-C diagram that spans over 70 years. Using a least squares method orthogonal quadratic model function, we found that the orbital period of DD Com is continuously increasing with (P) over dot = 0.00401(22) s yr(-1). The period increase may be caused by the mass transfer from the less-massive component to the more-massive one. With the period increase, the binary is evolving from the present shallow contact phase to the broken stage predicted by the thermal relaxation oscillation (TRO) theory. Compared with other VSP systems, DD Com is a rare system that lies on the expanding phase of the TRO cycle. Until now, only four such systems including DD Com are found in this stage. Thus, this target is another good observational proof of the TRO theory in a very short period region.

Catalogue of variable stars in open cluster fields

Observatory and Planetarium of Johann Palisa, VSB – Technical University, Ostrava, Czech RepublicˇReceived 20 March, 2012; accepted 4 October 2012ABSTRACTContext.We present the first catalogue of known variable stars in open cluster regions and with up to two times the given clusterradius. This gives basic information about the distribution of variable stars in cluster fields for the complete sky.Aims.Knowledge of the variable star contents in open clusters is a significant advantage in their study. Analysing variabilit y of clustermembers and fields stars as well allows us to study the charact eristics of stars and clusters together. This catalogue of variable stars inopen cluster fields is the first step in supporting such studie s.Methods.We took all variable and suspected variable stars into account from the most complete collection, “The AAVSO VariableStar Index”, and did a cross-match of these stars with the most complete catalogue of galactic open clusters named DAML02.Results.Our on-line catalogue presently contains 18065 variable stars. We present the basic statistical distribution according to typesof variability.Key words. Open clusters and associations: general – Catalogues – Star s: variables: general – Proper motions

HR 7355 – another rapidly braking He-strong CP star?

We acquired 114 new BV observations of HR 7355 at observatories in Arizona, U.S.A and Cape Town, South Africa. We performed period analyses of the new observations along with new analyses of 732 archival measurements from the Hipparcos and ASAS projects. We find that the light curves of HR 7355; in various filters are quite similar, double-peaked, with unevenly deep minima. We substantially refine the rotational period to be P = 0.5214410(4)d, indicating that HR 7355gt; is the most rapidly rotating CP star known. Our period analyses reveal a possible lengthening of the rotational period with dP/dt/P = 2.4(8)x10-6 yr-1. We conclude that the shape and amplitude of HR 7355; light curves are typical of magnetic He-strong CP stars, for which light variations are the result of photospheric spots on the surface of a rotating star. We hypothesise that the light variations are caused mainly by an uneven distribution of overabundant helium on the stars surface. We discuss the cause of the rapid rotational braking of the star.

Surprising variations in the rotation of the chemically peculiar stars CU Virginis and V901 Orionis

We aim to study the stability of the periods in CU Vir and V901 Ori using all accessible observational data containing phase information. We found that the shapes of their phase curves were constant, while the periods were changing. Both stars exhibit alternating intervals of rotational braking and acceleration. The rotation period of CU Vir was gradually shortening until the year 1968, when it reached its local minimum of 0.52067198 d. The period then started increasing, reaching its local maximum of 0.5207163 d in the year 2005. Since that time the rotation has begun to accelerate again. We also found much smaller period changes in CU Vir on a timescale of several years. The rotation period of V901 Ori was increasing for the past quarter-century, reaching a maximum of 1.538771 d in the year 2003, when the rotation period began to decrease. A theoretically unexpected alternating variability of rotation periods in these stars would remove the spin-down time paradox and brings a new insight into structure and evolution of magnetic upper-main-sequence stars.

Apsidal motion in southern eccentric eclipsing binaries : V539 ara, GG lup, V526 SGR and AO vel

Several new times of minimum light have been observed photoelectricaly for the early-type double-lined eccentric eclipsing binaries V539 Ara (P = 3. 2, e = 0.05), GG Lup (1. d 9, 0.15), V526 Sgr (1. d 9, 0.22) and AO Vel (1. d 6, 0.07). The O−C diagrams are analysed using all reliable timings found in the literature and improved values for the elements of the apsidal motion are computed. We find more precise, relatively short periods of apsidal motion of about 162, 102, 155 and 54 years for V539 Ara, GG Lup, V526 Sgr and AO Vel, respectively. The corresponding internal structure constants, log k2, are then found to be -2.33, -2.16, -2.36 and -2.24 under the assumption that the component stars rotate pseudosynchronously. The relativistic effects are negligible, being about 6-7% of the total apsidal motion rate in all systems. Using the light-time effect solution, for V539 Ara we have found a third component orbiting with a period of about 42 years, for AO Vel we confirmed the third body with an orbital period of 33 years.

THE PHOTOMETRIC STUDY OF A NEGLECTED NEAR CONTACT BINARY: BS VULPECULAE

We present a detailed study of the close eclipsing binary BS Vulpeculae. Although it is relatively bright (V: 10.9-11.6 mag) and belongs to short-periodic variable stars (P = 0.48 days), it is rather neglected. To perform a thorough period analysis, we collected all available photometric observations that span the time interval of 1898-2010. Observations include archive photographic plate measurements and visually determined eclipse minima timings done in 1979-2003, which were later shown to be biased to accommodate the existing linear ephemeris. Applying our own direct period analysis we found a well-defined shortening of the orbital period of dP/dt = -6.70(17) x 10(-11) = -2.11(6) ms yr(-1), which implies a continual mass flow from the primary to the secondary component. Using the 2003 version of the Wilson-Van Hamme code, our new complete BV(IR)(C) light curves were analyzed and the physical parameters of the system were derived. We found that BS Vul is a near contact binary system with the primary component filling its critical Roche lobe. The luminosity enhancement on the left shoulder of the secondary minimum shown in the light curves can be explained as a result of a persistent hot spot on the secondary due to the mass transfer from the primary component to the secondary one and heating the facing hemisphere of the secondary component, which is consistent with our result of period analysis. With the period decrease, BS Vul will evolve toward the contact phase. It is another good observational example as predicted by the theory of thermal relaxation oscillations.

Orbital period changes of the nova-like cataclysmic variable AC Cancri: evidence of magnetic braking and an unseen companion

Aims. The source AC Cnc is a nova-like cataclysmic variable containing a white-dwarf primary with a mass of 0.76 M-circle dot and a K2-type secondary with a mass of 0.77 M-circle dot. We intend to study its period changes and search for evidence of magnetic braking and unseen third body. Methods. The period changes were investigated based on the analysis of the O-C curve, which is formed by one new eclipse time together with the others compiled from the literature. Results. A cyclic change with a period of 16.2 yr was found to be superimposed on a long-term period decrease at a rate of. P = -1.24(+/- 0.44) x 10(-8) days/year. Conclusions. It is shown that the mechanism of magnetic activity-driven changes in the quadrupole momentum of the secondary star (Applegates mechanism) does not explain it easily. This period oscillation was plausibly interpreted by a light-travel time effect caused by the presence of a cool M-type dwarf companion (M-3 > 0.097 M-circle dot) in a long orbit (16.2 yr) around the binary. Since the masses of both components are nearly the same, the mass transfer from the lobe-filling secondary to the primary is not efficient to cause the continuous period decrease. It may be strong evidence of an enhanced magnetic stellar wind from the K2-type component. If the Alfen radius of the cool secondary is the same as that of the Sun (i.e., R-A = 15 R-circle dot), the mass-loss rate should be. M-2 = -1.65 x 10(-10) M-circle dot/year. By using the enhanced mass loss proposed by Tout & Eggleton (1988), the mass-loss rate should be. M-2 = -1.18 x 10(-9) M-circle dot/year. In this case, the Alfen radius is determined to be R-A = 5.2 R-circle dot. However, the long-term decrease of the period may be only a part of a long-period (> 100 yr) oscillation caused by the presence of an additional body. To check the conclusions, new precise times of light minimum will be required.