arXiv subjects

Z. Mikulasek

Publications and source records attributed to Z. Mikulasek.

32 records · Page 2Linked to original sources

arXiv2011

Characterising the magnetic fields of the Herbig Ae/Be stars HD97048, HD150193, HD176386, and MWC480

Our knowledge of the presence and the role of magnetic fields in intermediate-mass pre-main-sequence stars remains very poor. We present the magnetic properties of four Herbig Ae/Be stars that have not been previously studied in detail. Our results for the three Herbig Ae/Be stars HD97048, HD150193, and HD176386 are based on multi-epoch low-resolution spectra obtained in spectropolarimetric mode with FORS2 mounted on the VLT. New high-resolution polarimetric spectra were obtained for MWC480 with the SOFIN spectrograph installed at the Nordic Optical Telescope. We discuss 41 FORS2 low-resolution observations of the Herbig Ae/Be stars HD97048, HD150193, and HD176386 and determine their rotational periods. Using stellar fundamental parameters and the longitudinal magnetic field phase curves, we place constraints on the magnetic field geometry. Three high-resolution circularly polarised SOFIN spectra obtained for MWC480 were measured using the moment technique where wavelength shifts between right- and left-hand side circularly polarised spectra are interpreted in terms of a longitudinal magnetic field . Our search for periodicities resulted in P=0.693d for HD97048, P=1.317d for HD150193, and P=0.899d for HD176386. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength B_d of several hundred Gauss. The longitudinal magnetic-field measurements of MWC480 reveal the presence of a strong kG field, which was undetected in our previous low-resolution polarimetric observations with FORS1. A weak magnetic field was detected in the circumstellar components of the Ca II H&K lines and the Na I D lines, indicating a complex interaction between the stellar magnetic field and the circumstellar environment.

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arXiv2011

Study of Eclipsing Binary and Multiple Systems in OB Associations: I. Ori OB1a - IM Mon

All available photometric and spectroscopic observations were collected and used as the basis of a detailed analysis of the close binary IM Mon. The orbital period of the binary was refined to 1.19024249(0.00000014) days. The Roche equipotentials, fractional luminosities (in (B, V) and H_p bands) and fractional radii for the component stars in addition to mass ratio q, inclination i of the orbit and the effective temperature T_eff of the secondary cooler less massive component were obtained by the analysis of light curves. IM Mon is classified to be a detached binary system in contrast to the contact configuration estimations in the literature. The absolute parameters of IM Mon were derived by the simultaneous solutions of light and radial velocity curves as M_1,2=5.50(0.24)M_o and 3.32(0.16)M_o, R1,2=3.15(0.04)R_o and 2.36(0.03)R_o, T_eff1,2=17500(350) K and 14500(550) K implying spectral types of B4 and B6.5 ZAMS stars for the primary and secondary components respectively. The modelling of the high resolution spectrum revealed the rotational velocities of the component stars as V_rot1=147(15) km/s and V_rot2=90(25) km/s. The photometric distance of 353(59) pc was found more precise and reliable than Hipparcos distance of 341(85) pc. An evolutionary age of 11.5(1.5) Myr was obtained for IM Mon. Kinematical and dynamical analysis support the membership of the young thin-disk population system IM Mon to the Ori OB1a association dynamically. Finally, we derived the distance, age and metallicity information of Ori OB1a sub-group using the information of IM Mon parameters.

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arXiv2011

Photometric and spectroscopic investigation of TW Draconis

Context. TW Draconis is one of the best studied Algol-type eclipsing binaries. There is significant evidence for miscellaneous physical processes between interacting binary components manifesting themselves by period and light curve changes. Aims. Obtaining new set of photometric and spectroscopic observations, we analysed them together with the older spectroscopic and photometric data to build model of this eclipsing system with respect to observed changes of O-C diagram and light curve. Methods. Reduction of new spectra was carried out in the IRAF and SPEFO programs. Radial velocities were determined manually using SPEFO, by CCF using the Zverko's code and from the program KOREL. Orbital elements were derived with the FOTEL program and via disentangling with KOREL. The final combined solution was obtained with the programs PHOEBE and FOTEL. Results. Photometry shows small irregularities in light curves as a results of pulsating of one component and spot activity. Using net of KOREL outputs we found the mass ratio q=0.405(3).We confirm the presence of stellar matter around the primary. Even after subtraction of ADS 9706B influence, light curve solutions show third light in the system. Models in FOTEL and Phoebe are presented. Conclusions. TW Dra is an astrophysically very interesting eclipsing binary. Future combination of interferometry, spectroscopy, and photometry is promising. It could definitely confirm the hypothesis of quadruple system for TW Dra and explain behaviour of this system in complex.

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arXiv2010

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

Despite of the importance of magnetic fields for the full understanding of the properties of accreting Herbig Ae/Be stars, these fields have scarcely been studied over the rotation cycle until now. One reason for the paucity of such observations is the lack of knowledge of their rotation periods. The sharp-lined young Herbig Ae star HD101412 with a strong surface magnetic field became in the last years one of the most studied targets among the Herbig Ae/Be stars. A few months ago we obtained multi-epoch polarimetric spectra of this star with FORS2 to search for a rotation period and to constrain the geometry of the magnetic field. We measured longitudinal magnetic fields on 13 different epochs distributed over 62 days. These new measurements together with our previous measurements of the magnetic field in this star were combined with available photometric observations to determine the rotation period. The search of the rotation period resulted in P=42.076+-0.01d. According to near-infrared imaging studies the star is observed nearly edge-on. The star exhibits a single-wave variation of the longitudinal magnetic field during the stellar rotation cycle. These observations are usually considered as evidence for a dominant dipolar contribution to the magnetic field topology.

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arXiv2010

Modelling the light variability of the Ap star epsilon Ursae Majoris

We simulate the light variability of the Ap star epsUMa using the observed surface distributions of Fe, Cr, Ca, Mn, Mg, Sr and Ti obtained with the help of Doppler Imaging technique. Using all photometric data available we specified light variations of epsUMa modulated by its rotation from far UV to IR. We employed the LLmodels stellar model atmosphere code to predict the light variability in different photometric systems. The rotational period of epsUMa is refined to 5d088631(18). It is shown that the observed light variability can be explained as a result of the redistribution of radiative flux from the UV spectral region to the visual caused by the inhomogeneous surface distribution of chemical elements. Among seven mapped elements, only Fe and Cr significantly contribute to the amplitude of the observed light variability. In general, we find a very good agreement between theory and observations. We confirm the important role of Fe and Cr to the magnitude of the well-known depression around 5200 Å through the analysis of the peculiar $a$-parameter. Finally, we show that the abundance spots of considered elements cannot explain the observed variability in near UV and $β$ index which are likely due to some other causes. The inhomogeneous surface distribution of chemical elements can explain most of the observed light variability of the A-type CP star epsUMa.

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arXiv2010

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

Strong meridional mixing induced by rapid rotation is one reason why all hot main-sequence stars are not chemically peculiar. However, the finding that the He-strong CP star HR 7355 is a rapid rotator complicates this concept. Our goal is to explain the observed behaviour of HR 7355 based on period analysis of all available photometry. Over two years, 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, with amplitudes 0.035(4), 0.036(4), and 0.038(3) mag in B, Hp and V, respectively. The light curves are double-peaked, with unevenly deep minima. We substantially refine the rotational period to be P=0.5214410(4) d, indicating that HR 7355 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)x 10^-6/yr. 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 star's surface. We briefly describe and discuss the cause of the rapid rotational braking of the star.

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arXiv2009

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

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. We simulate the light variability of the star HR 7224 using the observed surface distribution of silicon and iron. We used the TLUSTY model atmospheres calculated for the appropriate silicon and iron abundances to obtain the emergent flux and to predict the rotationally modulated light curve of the star. We also obtained additional photometric measurements and employed our own regression procedure to derive a more precise estimate of the light elements. We show that the light variation of the star can be explained as a result of i) the uneven surface distribution of the elements, ii) the flux redistribution from the ultraviolet to the visible part of the spectrum, and iii) rotation of the star. We show that the silicon bound-free transitions and iron bound-bound transitions provide the main contribution to the flux redistribution, although an additional source of opacity is needed. We confirm that numerous iron lines significantly contribute to the well-known depression at 5200 A and discuss the connection between iron abundance and the value of peculiarity index a. The uneven surface distribution of silicon and iron is able to explain most of the rotationally modulated light variation in the star HR 7224.

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arXiv2009

The X-ray emission from Z CMa during an FUor-like outburst and the detection of its X-ray jet

Accretion shocks have been recognized as important X-ray emission mechanism for pre-main sequence stars. Yet the X-ray properties of FUor outbursts, events that are caused by violent accretion, have been given little attention. We have observed the FUor object Z CMa during optical outburst and quiescence with Chandra. No significant changes in X-ray brightness and spectral shape are found, suggesting that the X-ray emission is of coronal nature. Due to the binary nature of Z CMa the origin of the X-ray source is ambiguous. However, the moderate hydrogen column density derived from our data makes it unlikely that the embedded primary star is the X-ray source. The secondary star, which is the FUor object, is thus responsible for both the X-ray emission and the presently ongoing accretion outburst, which seem however to be unrelated phenomena. The secondary is also known to drive a large outflow and jet, that we detect here for the first time in X-rays. The distance of the X-ray emitting outflow source to the central star is higher than in jets of low-mass stars.

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arXiv2008

Transient jets in the symbiotic prototype Z Andromedae

We present development of the collimated bipolar jets from the symbiotic prototype Z And that appeared and disappeared during its 2006 outburst. In 2006 July Z And reached its historical maximum at U ~ 8.0. During this period, rapid photometric variations with Dm ~ 0.06 mag on the timescale of hours developed. Simultaneously, high-velocity satellite components appeared on both sides of the H-alpha and H-beta emission line profiles. They were launched asymmetrically with the red/blue velocity ratio of 1.2 - 1.3. From about mid-August they became symmetric. Their spectral properties indicated ejection of bipolar jets collimated within an average opening angle of 6.1 degrees. We estimated average outflow rate via jets to dM(jet)/dt ~ 2xE10-6(R(jet)/1AU)**(1/2) M(Sun)/year, during their August - September maximum, which corresponds to the emitting mass in jets, M(jet, emitting) ~ 6xE-10(Rjet)/1AU)^{3/2} M(Sun). During their lifetime, the jets released the total mass of M(jet, total) approx 7.4x1E-7 M(Sun). Evolution in the rapid photometric variability and asymmetric ejection of jets around the optical maximum can be explained by a disruption of the inner parts of the disk caused by radiation-induced warping of the disk.

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arXiv2007

The record-breaking rotational braking of the He strong CP star HD 37776

We study the long-term light and spectral variations in the He-strong magnetic chemically peculiar star HD 37776 (V901 Ori) to search for changes of its 1.5387 d period in 1976-2007. We analyze all published photometric observations and spectrophotometry in the HeI 4026 A line. The data were supplmented with 506 new (U)VB observations obtained during the last 2 observing seasons, 66 estimates of HeI equivalent widths on 23 CFHT spectrograms and 35 of the 6-m Zeeman spectrograms. All the 1895 particular observations heve been processed simultaneously. We confirm the previously suspected increase of the period in HD 37776 which is a record-breaking among CP stars. The mean rate of the period increase during the last 31 years is 0.541+-0.020 s per year. We interpret this ongoing period increase as the slowing down of the star's surface rotation due to momentum loss through events and processes in its magnetosphere.

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arXiv2007

Photometrically simply behaving mCP stars

We analyzed uvby and Hp light curves of 19 well observed magnetic CP stars selected from the "On-line database of photometric observations of mCP stars" of which light curves in all the five colours were similar. We assumed that among these photometrically simply behaving (PSB) stars could be found such ones which have a single photometric spot. The insight into such simple situations would help us to comprehend more complicated cases. Light curves of the 19 PSB mCP stars proved to be generally nearly symmetric but surprisingly diverse. The analysis shows that only in the case of HD 110956B, HD 188041, and perhaps HD 193722 we are able to explain their photometric behaviour by a simple one-spot model. Consequently, occurrence of more than one photometric spot on an mCP star is typical.

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arXiv2007

The benefits of the orthogonal LSM models

In the last few decades both the volume of high-quality observing data on variable stars and common access to them have boomed; however the standard used methods of data processing and interpretation have lagged behind this progress. The most popular method of data treatment remains for many decades Linear Regression (LR) based on the principles of Least Squares Method (LSM) or linearized LSM. Unfortunately, we have to state that the method of linear regression is not as a rule used accordingly namely in the evaluation of uncertainties of the LR parameters and estimates of the uncertainty of the LR predictions. We present the matrix version of basic relations of LR and the true estimate of the uncertainty of the LR predictions. We define properties of the orthogonal LR models and show how to transform general LR models into orthogonal ones. We give relations for orthogonal models for common polynomial series.

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arXiv2007

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.

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arXiv2007

The ZOO of uvby and Hp light curves of magnetic chemically peculiar stars

We present preliminary results of the generalized Principal Component Analysis (PCA) of light curves of 82 magnetic chemically peculiar (further mCP) stars applied to 54 thousand individual photometric observations in the uvby and Hp colours taken from the "On-line database of photometric observations of mCP stars". We show that every of the observed light curves (LCs) can be, with a sufficient accuracy, represented using five parameters of a harmonic polynomial of the second order, and that the third order and higher harmonics reflect only a noise. We found that a prevailing majority of the uvbyHp LCs can be satisfactorily well represented by a linear combination of a constant term and one or two (extraordinarily three) basic, mutually orthonormal functions, which reduces the number of parameters necessarily needed to describe the set of the LCs of a particular star. While the shape of an individual LC depends on the individual distribution of photometric spots on the surface of a rotating star, its amplitude depends on the mechanism of variability. To describe the amplitude of LCs we introduce a robust quantity effective amplitude. Applying the PCA to the set of the effective amplitudes of the LCs in all five colours we revealed at least three different sources of the light variability showing different wavelength dependence. The amplitudes of all the LCs harbor a component monotonously decreasing with increasing wavelengths. The second component reaches a remarkable extremum in the $v$ colour, and is in antiphase relative to the remaining four colours. It occurs particularly at cooler mCP-s having a high degree of the chemical peculiarity. The third component reflects a remarkable diversity of the light curves in the $u$ colour found in a few mCP stars.

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