mCP Stars Research Articles

The (non-)variability of magnetic chemically peculiar candidates in the Large Magellanic Cloud

The galactic magnetic chemically peculiar (mCP) stars of the upper main sequence are well known as periodic spectral and light variables. The observed variability is obviously caused by the uneven distribution of overabundant chemical elements on the surfaces of rigidly rotating stars. The mechanism causing the clustering of some chemical elements into disparate structures on mCP stars has not been fully understood up to now. The observations of light changes of mCP candidates recently revealed in the nearby Large Magellanic Cloud (LMC) should provide us with information about their rotational periods and about the distribution of optically active elements on mCP stars born in other galaxies. We queried for photometry at the Optical Gravitational Lensing Experiment (OGLE)-III survey of published mCP candidates selected because of the presence of the characteristic 5200A flux depression. In total, the intersection of both sources resulted in twelve stars. We performed our own and standard periodogram time series analyses of all available data. The final results are, amongst others, the frequency of the maximum peak and the bootstrap probability of its reality. We detected that only two mCP candidates, 190.1 1581 and 190.1 15527, may show some weak rotationally modulated light variations with periods of 1.23 and 0.49 days; however, the 49% and 32% probabilities of their reality are not very satisfying. The variability of the other 10 mCP candidates is too low to be detectable by their V and I OGLE photometry. The relatively low amplitude variability of the studied LMC mCP candidates sample can be explained by the absence of photometric spots of overabundant optically active chemical elements. The unexpected LMC mCPs behaviour is probably caused by different conditions during the star formation in the LMC and the Galaxy.

The Lorentz force in atmospheres of chemically peculiar stars: 56 Arietis

The presence of electric currents in the atmospheres of magnetic chemically peculiar (mCP) stars could bring important theoretical constrains about the nature and evolution of magnetic field in these stars. The Lorentz force, which results from the interaction between the magnetic field and the induced currents, modifies the atmospheric structure and induces characteristic rotational variability of pressure-sensitive spectroscopic features, that can be analysed using phase-resolved spectroscopic observations. In this work we continue the presentation of results of the magnetic pressure studies in mCP stars focusing on the high-resolution spectroscopic observations of Bp star 56Ari. We have detected a significant variability of the Halpha, Hbeta, and Hgamma spectral lines during full rotation cycle of the star. Then these observations are interpreted in the framework of the model atmosphere analysis, which accounts for the Lorentz force effects. We used the LLmodels stellar model atmosphere code for the calculation of the magnetic pressure effects in the atmosphere of 56Ari taking into account realistic chemistry of the star and accurate computations of the microscopic plasma properties. The Synth3 code was employed to simulate phase-resolved variability of Balmer lines. We demonstrate that the model with the outward-directed Lorentz force in the dipole+quadrupole configuration is likely to reproduce the observed hydrogen lines variation. These results present strong evidences for the presence of non-zero global electric currents in the atmosphere of this early-type magnetic star.

Analysis of magnetic pressure effects in atmospheres of CP stars

AbstractSeveral dynamical processes may induce considerable Lorentz forces in atmospheres of mCP stars, thus modifying the hydrostatic structure of their atmospheres. This modification can be seen as characteristic rotational variability of certain spectral features such as hydrogen Balmer lines. In this work we present the first results of modeling the magnetic pressure effects in atmospheres of mCP stars in the framework of model atmosphere analysis with accurate treatment of microscopic properties of atmospheric plasma. We show that at least part of the rotational variability of hydrogen lines seen in high-resolution spectra of mCP stars could be attributed to the non-zero electrical currents flowing along stellar surfaces.

FCAPT uvby Photometry of the mCP Stars HR 1297, 25 Sex, BX Boo, and 49 Her

ABSTRACTI obtained additional differential Strömgren uvby observations of four magnetic CP stars HR 1297, 25 Sex, BX Boo, and 49 Her with the Four College Automated Photoelectric Telescope (FCAPT). The seasonal light curves of 25 Sex do not show definite differences. The period of Manfroid & Renson of 4.37900 days is confirmed. The other three stars usually have light curves in phase or nearly so, but there are differences between observing seasons. As the local properties of the photosphere change very slowly with time for these stars that have radiative atmospheres and strong magnetic fields, we are viewing slightly different hemispheres at the same phase. To produce this result, their rotational axes precess about their magnetic axes. For HR 1297, which has moderate amplitudes of variation, the period of Adelman is refined to 15.7505 days. The additional observations of BX Boo do not change the best estimate of its period, 2.88756 days. Although its amplitudes of variation are less than those of HR 1297, there are similar differences between observing seasons. For 49 Her the period of 0.93663 days of Adelman et al. is verified. There are probably small season-to-season light curve differences. Of the mCP stars for which I have FCAPT Strömgren observations, at least 13% exhibit variability among their light curves for different years.

Effective temperatures of magnetic chemically peculiar stars from full spectral energy distributions

New determinations of effective temperatures of 23 magnetic, chemically peculiar (mCP) stars were obtained from a fit of metal-enhanced model atmospheres to the observed spectral energy distributions (SED) from ultraviolet (UV) to red. Temperatures of four more CP stars without magnetic measurements were also obtained, of which three show light variations characteristic of magnetic variables, and the fourth may possibly be normal. The root-mean-square (rms) method was used to fit the theoretical SED to the observations corrected for reddening if necessary, with metallicity and effective temperature as the fitting parameters. Gravity was assumed to be equal to log g= 4 for main-sequence stars and to log g= 3 for two giants in the considered sample. Equal weights were given to the UV part and visual part of SED. An attempt was made to obtain a three-parameter fit (with reddening included) to the observed SED of HD 215441. It turned out that the three-parameter fit gave spurious results due to correlations among the parameters. We could determine the reddening of that star only by assuming that the metallicity of the model is known, hence reducing the number of fitting parameters to two. Independently of the formal quality of fit resulting from the rms method applied to the whole SED, the quality of fit was additionally checked for each star by determination of the temperature from the best-fitting model atmosphere to the UV part and the visual part of SED separately. Both temperatures should be close to one another if the global best-fitting model satisfactorily describes the full observed SED. This is the case for about a half of the investigated stars but the difference exceeds 750 K for the remaining stars with the extreme values above 2000 K. Possible reasons for such discrepancies are discussed. New revised calibrations of effective temperature and bolometric corrections of mCP stars in terms of reddening free Stromgren indices are given.

Uvby FCAPT Photometry of Six Small‐Amplitude mCP Stars

ABSTRACTIf magnetic fields cause elemental abundances to change in the stellar photosphere, then all magnetic chemically peculiar (mCP) stars should exhibit signatures of this effect in both their spectra and flux distributions. Given that all stars rotate, these stars should be magnetic, spectrum, and photometric variables, albeit sometimes of low amplitude. We study differential Strömgren observations from the Four College Automated Photoelectric Telescope (FCAPT) of α Psc, HR 5857, and HR 6709, which are small‐amplitude mCP stars, and AB Cet, HD 15890, and HR 8240, which were thought to be near‐constant mCP stars, to determine the periods and amplitudes of their photometric variability. We construct the photometric amplitude distribution functions of the mCP stars studied by the first author to obtain statistics on their photometric variability in Strömgren photometry. The star α Psc has a period of 0.74552 days, with its largest amplitude in u of 0.030 mag. The largest amplitude (0.035 mag) for HR 5857 is also for u. We confirmed the period of 1.29957 days found by Hatzes using Doppler imaging. For HR 6709 the largest amplitude observed (0.010 mag) is for u photometry. Its period is 1.20352 days. AB Cet might be minimally variable. HD 15980 appears to be minimally variable, with a period of at least 5 yr. HR 8240 is variable with a b amplitude about 0.05 mag and a period that is several years long. Additional observations of these six stars would be useful to confirm their characteristics. Two apparently constant stars studied with FCAPT Strömgren data, HD 11187 and HD 50169, should be checked to see whether they are long‐period variables. If additional observations show their variability, then all mCP stars observed photometrically by the first author with the FCAPT will be found to be variable.

On‐line database of photometric observations of magnetic chemically peculiar stars

AbstractWe present our extensive project of the On‐line database of photometric observations of magnetic chemically peculiar stars to collect published data of photometric observations of magnetic chemically peculiar (mCP) stars in the optical and near IR regions. Now the nascent database contains more than 107 000 photometric measurements of 102 mCP stars and will be continually supplemented with published or new photometric data on these and about 150 additional mCP stars. This report describes the structure and organization of the database. Moreover, for the all included data we estimated the error of measurements and the effective amplitudes of the light curves. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The variable light curves of some mCP stars

The use of the Four College Automatic Photometric Telescope at the Fairborn Observatory in Southern Arizona with its ability to observe stars every clear night and for continuous runs has played a major role in our very successful cooperative program of Stromgren differential uvby photometry of magnetic CP (mCP) stars. Out of about 100 stars observed only a few have variable light curves. But more may be found when additional observations are made. We review the published theoretical basis and observations as well as present new results for CU Vir, V1093 Ori, MW Vul, and HR 7224. We interpret some observations as possibly requiring a more robust theory of the variability of mCP star light curves. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

$\vec{uvby}$ photometry of the CP stars HR 149, HD 32966, HD 171782, and HR 7911

Dierential Stromgren uvby observations from the Four College Automated Photoelectric Telescope (FCAPT) are presented for the CP stars HR 149 and HR 7911 and the mCP stars HD 32966 and HD 171782. The Mercury-Manganese star HR 149 is found to be constant while HR 7911 with a period of 5.9617 days probably has a component which is a magnetic CP star. Improved periods were derived for HD 32966 and HD 171782 of 3.0927 and 4.4674 days, respectively. The former is a rather large amplitude mCP star. Both have at least one light curve dierent in shape from the others observed.

$\vec{uvby}$ photometry of the CP stars HR 5341, HD 142070, HR 6967, and HR 8434

(FCAPT) are presented for the mCP stars HD 142070, HR 6967, and HR 8434 and the CP star HR 5341. The latter star is found to be constant. Improved periods were derived for HD 142070, 3.37189 d, HR 6967, 3.91227 d, and HR 8434, 1.43237 d. Further observations of HD 142070 are needed to phase the magnetic data with the photometry, of HR 6967 to settle minor discrepancies between y and scaled GenevaV photometry, and of HR 8434 to resolve small discrepancies between two uvby photometric data sets.

Uvby photometry of the magnetic CP stars 20 Eridani, HR 1297, 12 Canis Majoris, and HD 134214

Dierential Stromgren uvby observations from the Four College Automated Photoelectric Telescope (FCAPT) are presented for the mCP stars 20 Eri, HR 1297, 12 CMa, and HD 134214. Adelman & Boyce found suggestions that the light curves of 20 Eri change between observing seasons. Observations for two additional seasons conrm this behavior and lead to an improved period of 1.928890 days. For HR 1297 a new period of 15.7490 days was derived. The uvby light curves now appear to be the same for dierent observing seasons. 12 CMa is found to be a photometric variable with the 2.18010 day period determined from magnetic measurements by Bohlender et al. Observations of the rapidly oscillating Ap star HD 134214 conrm the lack of photometric variability which is not associated with the rapid oscillations.

Uvby photometry of the mCP stars HD 35298, 19 Lyrae, HD 192678, and HR 8216

Dierential Stromgren uvby observations from the Four College Automated Photoelectric Telescope are presented for the mCP stars HD 35298, 19 Lyr, HD 192678, and HR 8216. The period for HD 35298 of 1.85457 days is a revision of North's value while that for 19 Lyr of 7.0980 days is alias of that found by Winzer. HD 192678 is found to be a small amplitude photomet- ric variable with the 6.4186 day period proposed by Leroy from polarization measurements. For HR 8216, observa- tions taken between 1995 and 1998 conrm that the star has remained constant at least since 1990.

Photometry from the Hipparcos catalogue: constant MCP stars, comparison and check stars

Photometry from the Hipparcos catalogue: constant MCP stars, comparison and check stars

A search for the age‐dependency of Ap star parameters

AbstractSome observational data of the sample of the magnetic chemically peculiar stars (MCP stars) are investigated statistically. For the MCP stars of spectral types later than A2 both the frequency distribution and the R ⋅ sin i‐values suggest the existence of a linear relation between stellar diameter and rotation period. The MCP stars of spectral types earlier than B9 show an overpopulation of small R ⋅ sin i which may indicate the existence of a second group with smaller radius in this sample. The equatorially symmetric rotator is used as the magnetic model. With respect to its temporal behaviour the effective magnetic field is separated into dipolar and quadrupolar contribution. Both signs of the axisymmetric quadrupole moment appear with equal frequency. The dipole moment which produces the amplitude of the Beff(t) curve forms for longer periods two groups which are separated by a distinct gap. Both of the groups exhibit magnetic fields which are the stronger the greater the stellar radius is, contrary to what is expected for frozen‐in fields. The dominance of magnetic curves without polarity reversal for longer‐period stars is in accordance with predictions of the dynamo theory.