The magnetic field and the evolution of element spots on the surface of the HgMn eclipsing binary AR Aur★ (original) (raw)
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Monthly Notices of the Royal Astronomical Society, 2010
AR Aur is the only eclipsing binary known to contain a HgMn star, making it an ideal case for a detailed study of the HgMn phenomenon. HgMn stars are a poorly understood class of chemically peculiar stars, which have traditionally been thought not to possess significant magnetic fields. However, the recent discovery of line profile variability in some HgMn stars, apparently attributable to surface abundance patches, has brought this belief into question. In this paper we investigate the chemical abundances, line profile variability, and magnetic field of the primary and secondary of the AR Aur system, using a series of high-resolution spectropolarimetric observations. We find the primary is indeed a HgMn star, and present the most precise abundances yet determined for this star. We find the secondary is a weak Am star, and is possibly still on the pre-main sequence. Line profile variability was observed in a range of lines in the primary, and is attributed to inhomogeneous surface distributions of some elements. No magnetic field was detected in any observation of either stars, with an upper limit on the longitudinal magnetic field in both stars of 100 G. Modelling of the phase-resolve longitudinal field measurements leads to a 3σ upper limit on any dipole surface magnetic field of about 400 G.
Monthly Notices of the Royal Astronomical Society, 2006
We present the results of a high spectral resolution study of the eclipsing binary AR Aur. AR Aur is the only known eclipsing binary with a HgMn primary star exactly on the ZAMS and a secondary star still contracting towards the ZAMS. We detect for the first time in the spectra of the primary star that for many elements the line profiles are variable over the rotation period. The strongest profile variations are found for the elements Pt, Hg, Sr, Y, Zr, He and Nd, while the line profiles of O, Na, Mg, Si, Ca, Ti, and Fe show only weak distortions over the rotation period. The slight variability of He and Y is also confirmed by the study of high resolution spectra of another HgMn star: α And. A preliminary modelling of the inhomogeneous distribution has been carried out for Sr and Y. Our analysis shows that these elements are very likely concentrated in a fractured ring along the rotational equator. It may be an essential clue for the explanation of the origin of the chemical anomalies in HgMn stars (which are very frequently found in binary and multiple systems) that one large fraction of the ring is missing exactly on the surface area which is permanently facing the secondary, and another small one on the almost opposite side. The results presented about the inhomogeneous distribution of various chemical elements over the stellar surface of the primary suggest new directions for investigations to solve the question of the origin of abundance anomalies in B-type stars with HgMn peculiarity.
Magnetic Fields and Dynamical Evolution of Chemical Spots on the Surface of HgMn Stars
The discovery of exotic abundances, chemical inhomogeneities, and weak magnetic fields on the surface of HgMn stars, which are frequently the primaries in spectroscopic binaries, has important implications not only for our understanding of the formation mechanisms of stars with Hg and Mn peculiarities themselves, but also to the general understanding of the B-type star formation. The recent results on the Doppler mapping of several elements on the surface of HgMn stars at different epochs, and magnetic field measurements suggest new directions in the studies of these stars to be followed in view of improving our understanding of the physical processes occurring in their atmospheres.
Spectroscopic variability and magnetic fields of HgMn stars
Astronomische Nachrichten, 2011
The discovery of exotic abundances, chemical inhomogeneities, and weak magnetic fields on the surface of late B-type primaries in spectroscopic binaries has important implications not only for our understanding of the formation mechanisms of stars with Hg and Mn peculiarities themselves, but also for the general understanding of B-type star formation in binary systems. The origin of the abundance anomalies observed in late B-type stars with HgMn peculiarity is still poorly understood. The connection between HgMn peculiarity and membership in binary and multiple systems is supported by our observations during the last decade. The important result achieved in our studies of a large sample of HgMn stars is the finding that most HgMn stars exhibit spectral variability of various chemical elements, proving that the presence of an inhomogeneous distribution on the surface of these stars is a rather common characteristic and not a rare phenomenon. Further, in the studied systems, we found that all components are chemically peculiar with different abundance patterns. Generally, He and Si variable Bp stars possess large-scale organised magnetic fields that in many cases appear to occur essentially in the form of a single large dipole located close to the centre of the star. The presence of magnetic fields in the atmospheres of HgMn stars has been demonstrated in several studies. In addition to the measurements of longitudinal and quadratic magnetic fields, this work also showed evidence for a relative magnetic intensification of Fe II lines produced by different magnetic desaturations induced by different Zeeman-split components.
Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412
Astronomische Nachrichten, 2016
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.
Astronomy & Astrophysics, 2012
Context. The frequent presence of weak magnetic fields on the surface of spotted late-B stars with HgMn peculiarity in binary systems has been controversial during the two last decades. Recent studies of magnetic fields in these stars using the least-squares deconvolution (LSD) technique have failed to detect magnetic fields, indicating an upper limit on the longitudinal field between 8 and 15 G. In these LSD studies, assumptions were made that all spectral lines are identical in shape and can be described by a scaled mean profile. Aims. We re-analyse the available spectropolarimetric material by applying the moment technique on spectral lines of inhomogeneously distributed elements separately. Furthermore, we present new determinations of the mean longitudinal magnetic field for the HgMn star HD 65949 and the hotter analog of HgMn stars, the PGa star HD 19400, using FORS 2 installed at the VLT. We also give new measurements of the eclipsing system AR Aur with a primary star of HgMn peculiarity, which were obtained with the SOFIN spectropolarimeter installed at the Nordic Optical Telescope. Methods. We downloaded from the European Southern Observatory (ESO) archive the publically available HARPS spectra for eight HgMn stars and one normal and one superficially normal B-type star obtained in 2010. Out of this sample, three HgMn stars belong to spectroscopic double-lined systems. The application of the moment technique to the HARPS and SOFIN spectra allowed us to study the presence of the longitudinal magnetic field, the crossover effect, and quadratic magnetic fields. Results for the HgMn star HD 65949 and the PGa star HD 19400 are based on a linear regression analysis of low-resolution spectra obtained with FORS 2 in spectropolarimetric mode. Results. Our measurements of the magnetic field with the moment technique using spectral lines of several elements separately reveal the presence of a weak longitudinal magnetic field, a quadratic magnetic field, and the crossover effect on the surface of several HgMn stars as well as normal and superficially normal B-type stars. Furthermore, our analysis suggests the existence of intriguing correlations between the strength of the magnetic field, abundance anomalies, and binary properties. The results are discussed in the context of possible mechanisms responsible for the development of the element patches and complex magnetic fields on the surface of late B-type stars.
The magnetic field of the pre-main sequence Herbig Ae star HD 190073
Astronomy and Astrophysics, 2007
Context. The general context of this paper is the study of magnetic fields in the pre-main sequence intermediate mass Herbig Ae/Be stars. Magnetic fields are likely to play an important role in pre-main sequence evolution at these masses, in particular in controlling the gains and losses of stellar angular momentum. Aims. The particular aim of this paper is to announce the detection of a structured magnetic field in the Herbig Ae star HD 190073, and to discuss various scenarii for the geometry of the star, its environment and its magnetic field. Methods. We have used the ESPaDOnS spectropolarimeter at CFHT in 2005 and 2006 to obtain high-resolution, high signal-to-noise circular polarization spectra which demonstrate unambiguously the presence of a magnetic field in the photosphere of this star. Results. Nine circular polarization spectra were obtained, each one showing a clear Zeeman signature. This signature is suggestive of a magnetic field structured on large scales. The signature, which corresponds to a longitudinal magnetic field of 74 ± 10 G, does not vary detectably on a one-year timeframe, indicating either an azimuthally symmetric field, a zero inclination angle between the rotation axis and the line of sight, or a very long rotation period. The optical spectrum of HD 190073 exhibits a large number of emission lines. We discuss the formation of these emission lines in the framework of a model involving a turbulent heated region at the base of the stellar wind, possibly powered by magnetic accretion.
Magnetic Doppler imaging of the roAp star HD 24712
Astronomy & Astrophysics, 2010
Aims. We present the first magnetic Doppler images of a rapidly oscillating Ap (roAp) star. Methods. We deduce information about magnetic field geometry and abundance distributions of a number of chemical elements on the surface of the hitherto best studied roAp star, HD 24712, using the magnetic Doppler imaging (MDI) code, INVERS10, which allows us to reconstruct simultaneously and consistently the magnetic field geometry and elemental abundance distributions on a stellar surface. For this purpose we analyse time series spectra obtained in Stokes I and V parameters with the SOFIN polarimeter at the Nordic Optical Telescope and recover surface abundance structures of sixteen different chemical elements, respectively ions, including Mg, and Dy. For the rare earth elements (REE) Pr and Nd separate maps were obtained using lines of the first and the second ionization stage. Results. We find and confirm a clear dipolar structure of the surface magnetic field and an unexpected correlation of elemental abundance with respect to this field: one group of elements accumulates solely where the positive magnetic pole is visible, whereas the other group avoids this region and is enhanced where the magnetic equatorial region dominates the visible stellar surface. We also observe relative shifts of abundance enhancement-or depletion regions between the various elements exhibiting otherwise similar behaviour.
Photometric/spectroscopic analyses and magnetic activity in young late-type stars
We present preliminary results of a study in progress based on photometric and spectroscopic observations of young weak-line T Tauri and post-T Tauri stars just arriving on the Zero Age Main Sequence. The study is part of a project based on high-resolution spectra obtained with FOCES@CAHA (Spain) and SARG@TNG (Spain) and contemporaneous photometry performed at Catania (Italy) and TUBITAK (Turkey) observatories. The main aim is to investigate the topology of magnetic active regions at photospheric and chromospheric levels in young single stars. Since our targets are slow rotators, with rotation periods longer than about 2 days, we are able to apply the spectroscopic technique based on line-depth ratio for the measure of the photospheric temperature modulation. These stars, possible members of Stellar Kinematic Groups, display emission cores in the CaII H&K and IRT lines, as well as a conspicuous filling-in of the Hα core. Moreover, we derive lithium abundance, and measure rotational and radial velocities. In several cases we detect a clear rotational modulation of brightness. In this work we briefly mention some preliminary results obtained for SAO 51891. We have also developed a spot/plage model to be applied to the data deriving the spot parameters (filling factor and temperature) and recovering information about the chromospheric inhomogeneities (flux contrast and filling factor). This study will contribute to explore the correlations between global stellar parameters and spot/plage characteristics in stars with different activity level and evolutionary stage.
Chemical abundances of magnetic and non-magnetic Herbig Ae/Be stars
Monthly Notices of the Royal Astronomical Society, 2012
The photospheres of about 10-20% of main sequence A-and B-type stars exhibit a wide range of chemical peculiarities, often associated with the presence of a magnetic field. It is not exactly known at which stage of stellar evolution these chemical peculiarities develop. To investigate this issue, in this paper we study the photospheric compositions of a sample of Herbig Ae and Be stars, which are considered to be the pre-main sequence progenitors of A and B stars. We have performed a detailed abundance analysis of 20 Herbig stars (three of which have confirmed magnetic fields), and one dusty young star. We have found that half the stars in our sample show λ Boo chemical peculiarities to varying degrees, only one star shows weak Ap/Bp peculiarities, and all the remaining stars are chemically normal. The incidence of λ Boo chemical peculiarities we find in Herbig stars is much higher than what is seen on the main sequence. We argue that a selective accretion model for λ Boo star formation is a natural explanation for the remarkably large number of λ Boo stars in our sample. We also find that the magnetic Herbig stars do not exhibit a range of chemical compositions remarkably different from the normal stars: one magnetic star displays λ Boo chemical peculiarities (HD 101412), one displays weak Ap/Bp peculiarities (V380 Ori A), and one (HD 190073) is chemically normal. This is completely different from what is seen on the main sequence, where all magnetic A and cool B stars show Ap/Bp chemical peculiarities, and this is consistent with the idea that the magnetic field precedes the formation of the chemical peculiarities typical of Ap and Bp stars.