Chemical Spots Research Articles

The enigmatic magnetic field of the planet hosting Herbig Ae/Be star HD 169142

Recent observations of the accretion disk around the Herbig Ae/Be star HD,169142 have revealed its complex and asymmetric morphology that indicates the presence of planets. The knowledge of the magnetic field structure in host stars is indispensable for our understanding of the magnetospheric interaction between the central stars, the circumstellar (CS) environment, and the planetary companions. We want to study the geometry of the magnetic field of HD,169142. We measured the mean longitudinal magnetic field from high-resolution ESPaDOnS and HARPS-pol spectra of HD,169142 using the least-squares deconvolution technique. Additionally, the spectral variability of hydrogen lines is studied using dynamical spectra. Our analysis of the Stokes V spectra reveals the presence of definitely detected narrow Zeeman features observed using line masks with neutral iron lines. On two observing epochs, we also obtain marginally detected broad Zeeman features. To explain the simultaneous appearance of narrow and broad Zeeman features, we discuss different scenarios, including one scenario related to a non-photospheric origin of the narrow Zeeman features due to magnetospheric interaction with warm CS matter. In an environment such as a wind or an accretion disk, spectral lines may form over a relatively large volume, and the field topology may therefore be complex not only in latitude and azimuth, but in radius as well. Dynamical plots of the Hβ line show an intriguing very complex structure with appearing and disappearing absorption features, which can be related to the complex morphology of the CS matter with asymmetric dust clump structures. The profiles of spectral lines belonging to different elements are variable, indicating the presence of chemical spots.

Magnetic field of the roAp star KIC 10685175: Observations versus theory

Context. KIC 10685175 is a roAp star whose polar magnetic field is predicted to be 6 kG through a nonadiabatic axisymmetric pulsation theoretical model. Aims. In this work, we aim to measure the magnetic field strength of KIC 10685175 using high-resolution spectropolarimetric observations, and compare it with the one predicted by the theoretical model. Methods. Two high-resolution unpolarized spectra have been analyzed to ascertain the presence of magnetically split lines and derive the iron abundance of this star through equivalent width measurements of 10 Fe lines. One polarized spectrum has been used to measure the mean longitudinal magnetic field with the least-squares deconvolution technique. Further, to examine the presence of chemical spots on the stellar surface, we have measured the mean longitudinal magnetic fields using different lines belonging to different elements. Results. From the study of two high-resolution unpolarized spectra, we obtained the spectroscopic atmospheric parameters including the effective temperature (Teff), surface gravity (log 𝑔), iron abundance ([Fe/H]), abundance ratio of alpha elements to iron ([α/Fe]), and micro-turbulent velocity (Vmic). The final result is [Teff, log g, [Fe/H], [α/Fe], Vmic)]=[8250 ± 200 K, 4.4 ± 0.1, −0.4 ± 0.2, 0.16 ± 0.1, 1.73 ± 0.2 km s−1]. Although the Fe absorption lines appear relatively weak in comparison to typical Ap stars with similar Teff, the lines belonging to rare earth elements (Eu and Nd) are stronger than those in chemically normal stars, indicating the peculiar nature of KIC 10685175. The mean longitudinal magnetic field, 〈Bℓ〉 = −226 ± 39 G, was measured in the polarized spectrum, but magnetically split lines were not detected. No significant line profile variability is evident in our spectra. Also, the longitudinal magnetic field strengths measured using line masks constructed for different elements are rather similar. Due to the poor rotation phase coverage of our data, additional spectroscopic and polarimetric observations are needed to allow us to come to any conclusions about the inhomogeneous element distribution over the stellar surface. Conclusions. The estimated polar magnetic field is 4.8 ± 0.8 kG, which is consistent with the predicted polar magnetic field strength of about 6kG within 3σ. This work therefore provides support for the pulsation theoretical model.

Surface structure of 45 Hercules: an otherwise unremarkable Ap star with a surprisingly weak magnetic field

ABSTRACTThe origin of magnetic fields and their role in chemical spot formation on magnetic Ap stars is currently not understood. Here, we contribute to solving this problem with a detailed observational characterization of the surface structure of 45 Her, a weak-field Ap star. We find this object to be a long-period, single-lined spectroscopic binary and determine the binary orbit as well as fundamental and atmospheric parameters of the primary. We study magnetic field topology and chemical spot distribution of 45 Her with the help of the Zeeman Doppler imaging technique. Magnetic mapping reveals the stellar surface field to have a distorted dipolar topology with a surface-averaged field strength of 77 G and a dipolar component strength of 119 G – confirming it as one of the weakest well-characterized Ap-star fields known. Despite its feeble magnetic field, 45 Her shows surface chemical inhomogeneities with abundance contrasts of up to 6 dex. Of the four chemical elements studied, O concentrates at the magnetic equator, whereas Ti, Cr, and Fe avoid this region. Apart from this trend, the positions of Fe-peak element spots show no apparent correlation with the magnetic field geometry. No signs of surface differential rotation or temporal evolution of chemical spots on the time-scale of several years were detected. Our findings demonstrate that chemical spot formation does not require strong magnetic fields to proceed and that both the stellar structure and the global field itself remain stable for sub-100 G field strengths contrary to theoretical predictions.

Modeling the Photometric Variability of Alpha2 CVn with a Dynamical Magnetosphere

Alpha2 Canum Venaticorum (α 2 CVn) is a strongly magnetic star with peculiar chemical signatures and periodic variability that have been long attributed to the diffusion of magnetic elements through the photosphere, leading to chemical spots across the stellar surface. However, recent studies of other magnetic hot stars are consistent with magnetospheric clouds above the surface. Here we take a renewed approach to modeling α 2 CVn with a simplified dynamical magnetosphere and a tilted, offset magnetic dipole to reproduce its Transiting Exoplanet Survey Satellite variability. Our dipole model also reproduces well the magnetic surface map of α 2 CVn from Silvester et al. Its ultraviolet variability, from IUE archival spectra, is also consistent with traditional reddening models. However, other observable quantities from the system contradict the expectations of a magnetosphere, and we conclude that it is unlikely to be present in α 2 CVn.

The magnetic field of the chemically peculiar star V352 Peg

ABSTRACTWe present a spectropolarimetric analysis of the hot star V352 Peg. We have acquired 18 spectropolarimetric observations of the star with ESPaDOnS at the CFHT between 2018 and 2019 and completed our data set with one archival ESPaDOnS measurement obtained in 2011. Our analysis of the spectra shows that the star is on the main sequence and chemically peculiar, i.e. it is a Bp star, with overabundances of iron peak elements (Ti, Cr, and Fe) and underabundance of He and O. Through a Least-Square Deconvolution of each spectrum, we extracted the mean Zeeman signature and mean line profile of thousands of spectral lines and detected a magnetic field in V352 Peg. By modelling the Stokes I and V profiles and using the Oblique Rotator Model, we determined the geometrical configuration of V352 Peg. We also performed Zeeman–Doppler Imaging (ZDI) to provide a more detailed characterization of the magnetic field of V352 Peg and its surface chemical distributions. We find a magnetic field that is mainly dipolar, dominantly poloidal, and largely non-axisymmetric with a dipole field strength of ∼9 kG and a magnetic axis almost perpendicular to the rotation axis. The strong variability of Stokes I profiles also suggests the presence of chemical spots at the stellar surface.

A spectroscopic test of the rotational modulation origin of periodic Kepler photometric variability of A-type stars

ABSTRACT High-precision space-based photometry obtained by the Kepler and TESS missions has revealed evidence of rotational modulation associated with main-sequence (MS) A- and late B-type stars. Generally, such variability in these objects is attributed to inhomogeneous surface structures (e.g. chemical spots), which are typically linked to strong magnetic fields ($B\gtrsim 100\, {\rm G}$) visible at the surface. It has been reported that ≈44 per cent of all A-type stars observed during the Kepler mission exhibit rotationally modulated light curves. This is surprising considering that ≲10 per cent of all MS A-type stars are known to be strongly magnetic (i.e. they are Ap/Bp stars). We present a spectroscopic monitoring survey of 44 A- and late B-type stars reported to exhibit rotational modulation in their Kepler light curves. The primary goal of this survey is to test the hypothesis that the variability is rotational modulation by comparing each star’s rotational broadening (vsin i) with the equatorial velocities (veq) inferred from the photometric periods. We searched for chemical peculiarities and binary companions in order to provide insight into the origin of the apparent rotational modulation. We find that 14 stars in our sample have vsin i > veq and/or have low-mass companions that may contribute to or be responsible for the observed variability. Our results suggest that more than 10 per cent of all MS A- and late B-type stars may exhibit inhomogeneous surface structures; however, the incidence rate is likely ≲30 per cent.

The enigmatic highly peculiar binary system HD 66051

HD 66051 (V414 Pup) is an eclipsing and spectroscopic double-lined binary, hosting two chemically peculiar stars: a highly peculiar B star as primary and an Am star as secondary. It also shows out-of-eclipse variability that is due to chemical spots. Using a set of high-resolution spectropolarimetric observations, a weak magnetic field on the primary was found. The investigation of the new high-resolution UVES spectrum of HD 66051 allowed us to decide on the chemical peculiarity type of both components with more reliability.

MOBSTER – III. HD 62658: a magnetic Bp star in an eclipsing binary with a non-magnetic ‘identical twin’

ABSTRACT HD 62658 (B9p V) is a little-studied chemically peculiar star. Light curves obtained by the Kilodegree Extremely Little Telescope (KELT) and Transiting Exoplanet Survey Satellite (TESS) show clear eclipses with a period of about 4.75 d, as well as out-of-eclipse brightness modulation with the same 4.75 d period, consistent with synchronized rotational modulation of surface chemical spots. High-resolution ESPaDOnS circular spectropolarimetry shows a clear Zeeman signature in the line profile of the primary; there is no indication of a magnetic field in the secondary. PHOEBE modelling of the light curve and radial velocities indicates that the two components have almost identical masses of about 3 M⊙. The primary’s longitudinal magnetic field 〈Bz〉 varies between about +100 and −250 G, suggesting a surface magnetic dipole strength Bd = 850 G. Bayesian analysis of the Stokes V profiles indicates Bd = 650 G for the primary and Bd < 110 G for the secondary. The primary’s line profiles are highly variable, consistent with the hypothesis that the out-of-eclipse brightness modulation is a consequence of rotational modulation of that star’s chemical spots. We also detect a residual signal in the light curve after removal of the orbital and rotational modulations, which might be pulsational in origin; this could be consistent with the weak line profile variability of the secondary. This system represents an excellent opportunity to examine the consequences of magnetic fields for stellar structure via comparison of two stars that are essentially identical with the exception that one is magnetic. The existence of such a system furthermore suggests that purely environmental explanations for the origin of fossil magnetic fields are incomplete.

12,660 Spotted Stars toward the OGLE Galactic Bulge Fields

Abstract We present the discovery and statistical analysis of 12,660 spotted variable stars toward and inside the Galactic bulge from the Optical Gravitational Lensing Experiment (OGLE) data that are over two decades long. We devise a new method of dereddening of individual stars toward the Galactic bulge where strong and highly nonuniform extinction is present. In effect, 11,812 stars were classified as giants and 848 as dwarfs. Well-defined correlations among the luminosity, variability amplitude, and rotation period were found for the giants. Rapidly rotating dwarfs with periods P ≤ 2 days show I-band amplitudes <0.2 mag, which is substantially less than the amplitudes of up to 0.8 mag observed in giants and slowly rotating dwarfs. We also notice that amplitudes of stars brighter than I 0 ≈ 16 mag do not exceed 0.3–0.4 mag. We divide the stars into three groups characterized by correlation between light and color variations. The positive correlation is characteristic for stars that are cooler when fainter, which results from the variable coverage of the stellar surface with spots similar to the sunspots. The variability of stars that are cooler when brighter (negative correlation) can be characterized by chemical spots with an overabundance of heavy elements inside and a variable line-blanketing effect, which is observed in chemically peculiar stars. The null correlation may result from a very high level of the magnetic activity with rapidly variable magnetic fields. This division is readily visible on the color–magnitude diagram (CMD), which suggests that it may depend on the radius of the stars. We detect 79 flaring objects and discuss briefly their properties. Among others, we find that relative brightening during flares is correlated with brightness amplitude.

Global properties of the light curves of magnetic, chemically peculiar stars as a testbed for the existence of dipole-like symmetry in surface structures

Context. Magnetic, chemically peculiar stars are known for exhibiting surface abundance inhomogeneities (chemical spots) that lead to photometric and spectroscopic variability with the rotation period. It is commonly assumed that the surface structures are causally connected with the global magnetic field that dominates the photospheric and subphotospheric layers of these stars. As a rule, the observed magnetic fields show a simple dipole-like geometry, with the magnetic axis being noncollinear to the rotational one. Aims. The present study aims at detecting underlying patterns in the distribution of photometric spots in a sample of 650 magnetic, chemically peculiar stars and examines their link to the magnetic field topology. Methods. Photometric time-series observations from the ASAS-3 archive were employed to inspect the light-curve morphology of our sample stars and divide them into representative classes described using a principal component analysis. Theoretical light curves were derived from numerous simulations assuming different spot parameters and following the symmetry of a simple dipole magnetic field. These were subsequently compared with the observed light curves. Results. The results from our simulations are in contradiction with the observations and predict a much higher percentage of double-wave light curves than is actually observed. We thereby conclude that the distribution of the chemical spots does not follow the magnetic field topology, which indicates that the role of the magnetic field in the creation and maintenance of the surface structures may be more subsidiary than what is predicted by theoretical studies.

Trace picture of grievous bodily harm causing victim’s death

The article investigates the trace of intentional grave bodily injuries that caused the victim's death, analyzes the views of scientists in relation to various types of mappings that remain in the course of intentional attacks on human health. Material tracks are always objective media about event information and therefore the best sources for data accumulation within the forensic characteristics of a particular category of crimes. Together with information on the methods and other pivotal evidence for crime, they form a system of interconnected model summaries used to promote versions. Once discovered, recorded and seized during surveys and other investigative (search) actions, these sources of information are analyzed and summarized by scientists to obtain information about the types of traces. Their systematization allows more efficient use of the given complex of data for the purpose of investigation of a crime. Traces of deliberate grave bodily harm that caused the victim's death are divided into two groups, taking into account the acting entity ("traces showing the actions of the offender" and "traces showing the actions of the victim"). Accordingly, within each of them, subgroups of tracks are identified, taking into account their localization, in particular, those that remain on the body and clothing of the offender, on the body and clothing of the victim, on the tools of the crime and surrounding objects, as well as changes in the environment. The study of archival cases made it clear that most law enforcement officers at the crime scene showed traces of biological or other origin (45% of cases), including traces of blood, saliva, sperm, urine, skin particles, chemical spots. In addition, cases of removal of "remaining" objects (25.5%), including objects found by the offender on the scene or near him and used for damages, were not uncommon; instruments; bottles; personal things; cold and firearms; chemicals. It should be noted that at certain places of events no significant changes were observed in the situation at all (16.5%). The reason for this is attempts by criminals to conceal the facts of committing unlawful acts and their consequences. Traces of reflection, in particular, traces of the hands, footprints or footwear, traces of tools, microobjects, were removed quite rarely (only 13%).

Orbital parameters and evolutionary status of the highly peculiar binary system HD 66051

Context. The spectroscopic binary system HD 66051 (V414 Pup) consists of a highly peculiar CP3 (HgMn) star and an A-type component. It also shows out-of-eclipse variability that is due to chemical spots. This combination allows the derivation of tight constraints for the testing of time-dependent diffusion models. Aims. We aim at deriving astrophysical parameters, information on age, and an orbital solution of the system. Methods. We analysed radial velocity and photometric data using two different methods to determine astrophysical parameters and the orbit of the system. Appropriate isochrones were used to derive the age of the system. Results. The orbital solution and the estimates from the isochrones are in excellent agreement with the estimates from a prior spectroscopic study. The system is very close to the zero-age main sequence and younger than 120 Myr. Conclusions. HD 66051 is a most important spectroscopic binary system that can be used to test the predictions of the diffusion theory explaining the peculiar surface abundances of CP3 stars.

The magnetic early B-type stars I: magnetometry and rotation

Abstract The rotational and magnetic properties of many magnetic hot stars are poorly characterized, therefore the Magnetism in Massive Stars and Binarity and Magnetic Interactions in various classes of Stars collaborations have collected extensive high-dispersion spectropolarimetric data sets of these targets. We present longitudinal magnetic field measurements 〈Bz〉 for 52 early B-type stars (B5–B0), with which we attempt to determine their rotational periods Prot. Supplemented with high-resolution spectroscopy, low-resolution Dominion Astrophysical Observatory circular spectropolarimetry, and archival Hipparcos photometry, we determined Prot for 10 stars, leaving only five stars for which Prot could not be determined. Rotational ephemerides for 14 stars were refined via comparison of new to historical magnetic measurements. The distribution of Prot is very similar to that observed for the cooler Ap/Bp stars. We also measured v sin i and vmac for all stars. Comparison to non-magnetic stars shows that v sin i is much lower for magnetic stars, an expected consequence of magnetic braking. We also find evidence that vmac is lower for magnetic stars. Least-squares deconvolution profiles extracted using single-element masks revealed widespread, systematic discrepancies in 〈Bz〉 between different elements: this effect is apparent only for chemically peculiar stars, suggesting it is a consequence of chemical spots. Sinusoidal fits to H line 〈Bz〉 measurements (which should be minimally affected by chemical spots), yielded evidence of surface magnetic fields more complex than simple dipoles in six stars for which this has not previously been reported; however, in all six cases, the second- and third-order amplitudes are small relative to the first-order (dipolar) amplitudes.

Magnetic stars with wide depressions in the continuum. 2. The silicon star with a complex field structure HD 27404

Observations of the chemically peculiar star HD 27404 with the 6-m SAO RAS telescope showed a strong magnetic field with the longitudinal field component varying in a complicated way in the range of −2.5 to 1 kG. Fundamental parameters of the star (Teff = 11 300 K, log g = 3.9) were estimated analyzing photometric indices in the Geneva and in the Stro¨ mgren–Crawford photometric systems. We detected weak radial velocity variations which can be due to the presence of a close star companion or chemical spots in the photosphere. Rapid estimation of the key chemical element abundance allows us to refer HD 27404 to a SiCr or Si+ chemically peculiar A0–B9 star.

Magnetic field topology and chemical abundance distributions of the young, rapidly rotating, chemically peculiar star HR 5624

The young, rapidly rotating Bp star HR5624 (HD133880) shows an unusually strong non-sinusoidal variability of its longitudinal magnetic field. This behaviour was previously interpreted as the signature of an exceptionally strong, quadrupole-dominated surface magnetic field geometry. We studied the magnetic field structure and chemical abundance distributions of HR5624 with the aim to verify the unusual quadrupolar nature of its magnetic field and to investigate correlations between the field topology and chemical spots. We analysed high resolution, time series Stokes parameter spectra of HR5624 with the help of a magnetic Doppler imaging inversion code based on detailed polarised radiative transfer modelling of the line profiles. Our magnetic Doppler inversions reveal that the field structure of HR5624 is considerably simpler and the field strength is much lower than proposed by previous studies. We find a maximum local field strength of 12 kG and a mean field strength of 4 kG. Our model implies that overall large-scale field topology of HR5624 is better described as a distorted, asymmetric dipole. The chemical abundance maps of Mg, Si, Ti, Cr, Fe, and Nd obtained in our study are characterised by large-scale, high-contrast abundance patterns. These structures correlate weakly with the magnetic field geometry and show no distinct element concentrations in the horizontal field regions predicted by theoretical atomic diffusion calculations. We conclude that the surface magnetic field topology of HR5624 is not as unusual as previously proposed. Considering these results together with other recent magnetic mapping analyses of early-type stars suggests that predominantly quadrupolar magnetic field topologies, invoked to be present in a significant number of stars, probably do not exist in real stars.

Diffusion in plasma: The Hall effect, compositional waves, and chemical spots

We consider diffusion caused by a combined influence of the electric current and the Hall effect, and argue that such diffusion can form inhomogeneities of the chemical composition in plasma. The considered mechanism can be responsible for a formation of element spots in laboratory and astrophysical plasmas. This current-driven diffusion can be accompanied by propagation of a particular type waves in which the impurity number density oscillate alone. These compositional waves exist if the magnetic pressure in plasma is much greater than the gas pressure,

Doppler imaging of chemical spots on magnetic Ap/Bp stars

Doppler imaging (DI) is a powerful spectroscopic inversion technique that enables conversion of a line profile time series into a two-dimensional map of the stellar surface inhomogeneities. In this paper we investigate the accuracy of chemical abundance DI of Ap/Bp stars and assess the impact of several different systematic errors on the reconstructed spot maps. We simulate spectroscopic observational data for different spot distributions in the presence of a moderately strong dipolar magnetic field. We then reconstruct chemical maps using different sets of spectral lines and making different assumptions about line formation in the inversion calculations. Our numerical experiments demonstrate that a modern DI code successfully recovers the input chemical spot distributions comprised of multiple circular spots at different latitudes or an element overabundance belt at the magnetic equator. For the optimal reconstruction the average reconstruction errors do not exceed ~0.10 dex. The errors increase to about 0.15 dex when abundance distributions are recovered from a few and/or blended spectral lines. Ignoring a 2.5 kG dipolar magnetic field in chemical abundance DI leads to an average relative error of 0.2 dex and maximum errors of 0.3 dex. Similar errors are encountered if a DI inversion is carried out neglecting a non-uniform continuum brightness distribution and variation of the local atmospheric structure. This series of numerical DI simulations proves that inversions with simplifying assumptions of the non-magnetic radiative transfer and a single model atmosphere are generally reliable. We assess magnetic field strengths of several dozen Ap/Bp stars previously studied with DI methods, concluding that the vast majority of the published chemical spot maps are unaffected by the systematic errors addressed in this paper. (Abridged)

Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

Abstract Due to the knowledge of the rotation period and the presence of a rather strong surface magnetic field, the sharp‐lined young Herbig Ae star HD 101412 with a rotation period of 42 d has become one of the most well‐studied targets among the Herbig Ae stars. High‐resolution HARPS polarimetric spectra of HD 101412 were recently obtained on seven different epochs. Our study of the spectral variability over the part of the rotation cycle covered by HARPS observations reveals that the line profiles of the elements Mg, Si, Ca, Ti, Cr, Mn, Fe, and Sr are clearly variable while He exhibits variability that is opposite to the behaviour of the other elements studied. Since classical Ap stars usually show a relationship between the magnetic field geometry and the distribution of element spots, we used in our magnetic field measurements different line samples belonging to the three elements with the most numerous spectral lines, Ti, Cr, and Fe. Over the time interval covered by the available spectra, the longitudinal magnetic field changes sign from negative to positive polarity. The distribution of field values obtained using Ti, Cr, and Fe lines is, however, completely different compared to the magnetic field values determined in previous low‐resolution FORS 2 measurements, where hydrogen Balmer lines are the main contributors to the magnetic field measurements, indicating the presence of concentration of the studied iron‐peak elements in the region of the magnetic equator. Further, we discuss the potential role of contamination by the surrounding warm circumstellar matter in the appearance of Zeeman features obtained using Ti lines. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Element spots in Ap and Hg‐Mn stars from current‐driven diffusion

Abstract The stars of the middle main sequence often have spot‐like chemical structures at their surfaces. We consider the diffusion process caused by electric currents that can lead to the formation of such chemical spots. Diffusion is considered using the partial momentum equations derived by the Chapman‐Enskog method. We argue that diffusion caused by electric currents can substantially change the surface chemistry of stars and form spotted chemical structures even in a relatively weak magnetic field. The considered mechanism can be responsible for a formation of element spots in Hg‐Mn and Ap‐stars. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Visual and ultraviolet flux variability of the bright CP starθAurigae

Chemically peculiar stars of the upper part of the main sequence show periodical variability in line intensities and continua, modulated by the stellar rotation, which is attributed to the existence of chemical spots on the surface of these stars. The flux variability is caused by the changing redistribution rate of the radiative flux predominantly from the short-wavelength part of the spectra to the long-wavelength part, which is a result of abundance anomalies. We study the nature of the multi-spectral variability of one of the brightest chemically peculiar stars, $\theta$ Aur. We predict the flux variability of $\theta$ Aur from the emerging intensities calculated for individual surface elements of the star taking into account horizontal variation of chemical composition derived from Doppler abundance maps. The simulated optical variability in the Str\"omgren photometric system and the ultraviolet flux variability agree well with observations. The IUE flux distribution is reproduced in great detail by our models. The resonance lines of magnesium and possibly also some lines of silicon are relatively weak in the ultraviolet domain, which indicates non-negligible vertical abundance gradients in the atmosphere. We also derive a new period of the star, $P=3.618\,664(10)$ d, from all available photometric and magnetic measurements and show that the observed rotational period is constant over decades. The ultraviolet and visual variability of $\theta$ Aur is mostly caused by silicon bound-free absorption and chromium and iron line absorption. These elements redistribute the flux mainly from the far-ultraviolet region to the near-ultraviolet and optical regions in the surface abundance spots. The light variability is modulated by the stellar rotation. The ultraviolet domain is key for understanding the properties of chemically peculiar stars. (abridged)