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J. Silvester

Publications and source records attributed to J. Silvester.

At least 19 recordsLinked to original sources
arXiv2022

Gaia Data Release 3: Astrophysical parameters inference system (Apsis) I -- methods and content overview

Gaia Data Release 3 contains a wealth of new data products for the community. Astrophysical parameters are a major component of this release. They were produced by the Astrophysical parameters inference system (Apsis) within the Gaia Data Processing and Analysis Consortium. The aim of this paper is to describe the overall content of the astrophysical parameters in Gaia Data Release 3 and how they were produced. In Apsis we use the mean BP/RP and mean RVS spectra along with astrometry and photometry, and we derive the following parameters: source classification and probabilities for 1.6 billion objects, interstellar medium characterisation and distances for up to 470 million sources, including a 2D total Galactic extinction map, 6 million redshifts of quasar candidates and 1.4 million redshifts of galaxy candidates, along with an analysis of 50 million outlier sources through an unsupervised classification. The astrophysical parameters also include many stellar spectroscopic and evolutionary parameters for up to 470 million sources. These comprise Teff, logg, and m_h (470 million using BP/RP, 6 million using RVS), radius (470 million), mass (140 million), age (120 million), chemical abundances (up to 5 million), diffuse interstellar band analysis (0.5 million), activity indices (2 million), H-alpha equivalent widths (200 million), and further classification of spectral types (220 million) and emission-line stars (50 thousand). This catalogue is the most extensive homogeneous database of astrophysical parameters to date, and it is based uniquely on Gaia data.

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arXiv2017

Mixed poloidal-toroidal magnetic configuration and surface abundance distributions of the Bp star 36 Lyn

Previous studies of the chemically peculiar Bp star 36 Lyn revealed a moderately strong magnetic field, circumstellar material, and inhomogeneous surface abundance distributions of certain elements. We present in this paper an analysis of 33 high-signal-to-noise ratio, high-resolution Stokes IV observations of 36 Lyn obtained with the Narval spectropolarimeter at the Bernard Lyot telescope at Pic du Midi Observatory. From these data, we compute new measurements of the mean longitudinal magnetic field, Bl, using the multi-line least-squares deconvolution (LSD) technique. A rotationally phased Bl curve reveals a strong magnetic field, with indications for deviation from a pure dipole field. We derive magnetic maps and chemical abundance distributions from the LSD profiles, produced using the Zeeman Doppler Imaging code InversLSD. Using a spherical harmonic expansion to characterize the magnetic field, we find that the harmonic energy is concentrated predominantly in the dipole mode (l = 1), with significant contribution from both the poloidal and toroidal components. This toroidal field component is predicted theoretically, but not typically observed for Ap/Bp stars. Chemical abundance maps reveal a helium enhancement in a distinct region where the radial magnetic field is strong. Silicon enhancements are located in two regions, also where the radial field is stronger. Titanium and iron enhancements are slightly offset from the helium enhancements, and are located in areas where the radial field is weak, close to the magnetic equator.

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arXiv2017

The complex magnetic field topology of the cool Ap star 49 Cam

49 Cam is a cool magnetic chemically peculiar star which has been noted for showing strong, complex Zeeman linear polarisation signatures. This paper describes magnetic and chemical surface maps obtained for 49 Cam using the INVERS10 magnetic Doppler imaging code and high-resolution spectropolarimetric data in all four Stokes parameters collected with the ESPaDOnS and Narval spectropolarimeters at the Canada-France-Hawaii Telescope and Pic du Midi Observatory. The reconstructed magnetic field maps of 49 Cam show a relatively complex structure. Describing the magnetic field topology in terms of spherical harmonics, we find significant contributions of modes up to l=3, including toroidal components. Observations cannot be reproduced using a simple low-order multipolar magnetic field structure. 49 Cam exhibits a level of field complexity that has not been seen in magnetic maps of other cool Ap stars. Hence we concluded that relatively complex magnetic fields are observed in Ap stars at both low and high effective temperatures. In addition to mapping the magnetic field, we also derive surface abundance distributions of nine chemical elements, including Ca, Sc, Ti, Cr, Fe, Ce, Pr, Nd, Eu. Comparing these abundance maps with the reconstructed magnetic field geometry, we find no clear relationship of the abundance distributions with the magnetic field for some elements. However, for other elements some distinct patterns are found. We discuss these results in the context of other recent magnetic mapping studies and theoretical predictions of radiative diffusion.

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arXiv2017

Magnetic field topology and chemical abundance distributions of the young, rapidly rotating chemically peculiar star HR5624

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.

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arXiv2015

The magnetic field topology and chemical abundance distributions of the Ap star HD 32633

Previous observations of the Ap star HD 32633 indicated that its magnetic field was unusually complex in nature and could not be characterised by a simple dipolar structure. Here we derive magnetic field maps and chemical abundance distributions for this star using full Stokes vector (Stokes $IQUV$) high-resolution observations obtained with the ESPaDOnS and Narval spectropolarimeters. Our maps, produced using the Invers10 magnetic Doppler imaging (MDI) code, show that HD 32633 has a strong magnetic field which features two large regions of opposite polarity but deviates significantly from a pure dipole field. We use a spherical harmonic expansion to characterise the magnetic field and find that the harmonic energy is predominately in the $\ell=1$ and $\ell=2$ poloidal modes with a small toroidal component. At the same time, we demonstrate that the observed Stokes parameter profiles of HD 32633 cannot be fully described by either a dipolar or dipolar plus quadrupolar field geometry. We compare the magnetic field topology of HD 32633 with other early-type stars for which MDI analyses have been performed, supporting a trend of increasing field complexity with stellar mass. We then compare the magnetic field topology of HD 32633 with derived chemical abundance maps for the elements Mg, Si, Ti, Cr, Fe, Ni and Nd. We find that the iron-peak elements show similar distributions, but we are unable to find a clear correlation between the location of local chemical enhancements or depletions and the magnetic field structure.

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arXiv2015

Revisiting the Rigidly Rotating Magnetosphere model for $σ$ Ori E - II. Magnetic Doppler imaging, arbitrary field RRM, and light variability

The initial success of the Rigidly Rotating Magnetosphere (RRM) model application to the B2Vp star sigma OriE by Townsend, Owocki & Groote (2005) triggered a renewed era of observational monitoring of this archetypal object. We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging (MDI) technique to simultaneously determine the magnetic configuration, which is predominately dipolar, with a polar strength Bd = 7.3-7.8 kG and a smaller non-axisymmetric quadrupolar contribution, as well as the surface distribution of abundance of He, Fe, C, and Si. We describe a revised RRM model that now accepts an arbitrary surface magnetic field configuration, with the field topology from the MDI models used as input. The resulting synthetic Ha emission and broadband photometric observations generally agree with observations, however, several features are poorly fit. To explore the possibility of a photospheric contribution to the observed photometric variability, the MDI abundance maps were used to compute a synthetic photospheric light curve to determine the effect of the surface inhomogeneities. Including the computed photospheric brightness modulation fails to improve the agreement between the observed and computed photometry. We conclude that the discrepancies cannot be explained as an effect of inhomogeneous surface abundance. Analysis of the UV light variability shows good agreement between observed variability and computed light curves, supporting the accuracy of the photospheric light variation calculation. We thus conclude that significant additional physics is necessary for the RRM model to acceptably reproduce observations of not only sigma Ori E, but also other similar stars with significant stellar wind-magnetic field interactions.

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arXiv2014

Stokes $IQUV$ Magnetic Doppler Imaging of Ap stars II: Next Generation Magnetic Doppler Imaging of $α^2$ CVn

We present updated magnetic field maps of the chemically peculiar B9p star $α^2$ CVn created using a series of time resolved observations obtained using the high resolution spectropolarimeters ESPaDOnS and Narval. We compare these new magnetic field maps with the original magnetic Doppler imaging maps based on spectra recorded with the MuSiCoS spectropolarimeter and taken a decade earlier. These new maps are inferred from line profiles in all four Stokes parameters using the magnetic Doppler imaging code INVERS10. With the addition of new lines exhibiting Stokes $IQUV$ signatures we have a unique insight into how the derived magnetic surface structure may be affected by the atomic lines chosen for inversion. We report new magnetic maps of $α^2$ CVn created using strong iron lines (directly comparable to the published MuSiCoS maps), weak iron lines and chromium lines, all of which yield a magnetic field structure roughly consistent with that obtained previously. We then derive an updated magnetic structure map for $α^2$ CVn based on the complete sample of Fe and Cr lines, which we believe to produce a more representative model of the magnetic topology of $α^2$ CVn. In agreement with the previous mapping, this new updated magnetic map shows a dipolar-like field which has complex sub-structure which cannot be explained by a simple low order multipolar geometry. Our new maps show that regardless of the atomic line or species choice, the reconstructed magnetic field is consistent with that published previously, suggesting that the reconstructed field is a realistic representation of the magnetic field of $α^2$ CVn. $α^2$ CVn is the first Ap star for which multiple, high resolution magnetic maps have been derived, providing important observational evidence for the stability of both the large and small-scale magnetic field.

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arXiv2013

Stokes IQUV mapping of $α^2$ CVn & other Ap stars using ESPaDOnS and NARVAL

New spectral line polarisation observations of 7 bright Ap stars have been obtained with the ESPaDOnS and Narval high resolution spectropolarimeters (Silvester et al. 2012). The aim of this data set is produce a series of surface magnetic field and surface chemistry maps for these Ap stars. We present new magnetic maps for the Ap star $α^2$ CVn using these new data and the MDI inversion code INVERS10. $α^2$ CVn is the first Ap star to be observed during two separate epochs using high resolution phase resolved spectropolarimetric $IQUV$ observations and as such allows us an insight into how stable the surface magnetic structure is over a decade timescale. We show that the new maps give a magnetic field structure consistent with the previous maps obtained by Kochukhov and Wade (2010) from lower quality MuSiCoS spectra taken a decade ago and that the field topology cannot be described by a dipolar or quadrupolar field.

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arXiv2012

Stokes IQUV Magnetic Doppler Imaging of Ap stars I. ESPaDOnS and NARVAL Observations

In this paper we describe and evaluate new spectral line polarisation observations obtained with the goal of mapping the surfaces of magnetic Ap stars in great detail. One hundred complete or partial Stokes IQUV sequences, corresponding to 297 individual polarised spectra, have been obtained for 7 bright Ap stars using the ESPaDOnS and NARVAL spectropolarimeters. The targets span a range of mass from approximately 1.8 to 3.4 solar mass, a range of rotation period from 2.56 to 6.80 days, and a range of maximum longitudinal magnetic field strength from 0.3 to over 4 kG. For 3 of the 7 stars, we have obtained dense phase coverage sampling the entire rotational cycle. These datasets are suitable for immediate magnetic and chemical abundance surface mapping using Magnetic Doppler Imaging (MDI). For the 4 remaining stars, partial phase coverage has been obtained, and additional observations will be required in order to map the surfaces of these stars. The median signal-to-noise ratio of the reduced observations is over 700 per 1.8 km\s pixel. Spectra of all stars show Stokes V Zeeman signatures in essentially all individual lines, and most stars show clear Stokes QU signatures in many individual spectral lines. The observations provide a vastly improved data set compared to previous generations of observations in terms of signal-to-noise ratio, resolving power and measurement uncertainties. Measurement of the longitudinal magnetic field demonstrates that the data are internally consistent within computed uncertainties typically at the 50 to 100 sigma level. Data are also shown to be in excellent agreement with published observations and in qualitative agreement with the predictions of published surface structure models. This study establishes the performance and stability of the ESPaDOnS and NARVAL high-resolution spectropolarimeters during the period 2006-2010.

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arXiv2011

Magnetic Field and Atmospheric Chemical Abundances of the Magnetic Ap Star HD 318107

New spectra have been obtained with the ESPaDOnS spectropolarimeter supplemented with unpolarised spectra from the ESO UVES, UVES-FLAMES, and HARPS spectrographs of the very peculiar large-field magnetic Ap star HD 318107, a member of the open cluster NGC 6405. The available data provide sufficient material with which to re-analyse the first-order model of the magnetic field geometry and to derive abundances of Si, Ti, Fe, Nd, Pr, Mg, Cr, Mn, O, and Ca. The magnetic field structure was modelled with a low-order colinear multipole expansion, using coefficients derived from the observed variations of the field strength with rotation phase. The abundances of several elements were determined using spectral synthesis. After experiments with a very simple model of uniform abundance on each of three rings of equal width in co-latitude and symmetric about the assumed magnetic axis, we decided to model the spectra assuming uniform abundances of each element over the stellar surface. The new magnetic field measurements allow us to refine the rotation period of HD 318107 to P = 9.7088 +/- 0.0007 days. Appropriate magnetic field model parameters were found that very coarsely describe the (apparently rather complex) field moment variations. Spectrum synthesis leads to the derivation of mean abundances for the elements Mg, Si, Ca, Ti, Cr, Fe, Nd, and Pr. All of these elements except for Mg and Ca are strongly overabundant compared to the solar abundance ratios. There is considerable evidence of non-uniformity, for example in the different values of B_z found using lines of different elements. The present data set, while limited, is nevertheless sufficient to provide a useful first-order assessment of both the magnetic and surface abundance properties of HD 318107, making it one of the very few magnetic Ap stars of well-known age for which both of these properties have been studied.

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arXiv2010

Magnetic field, chemical composition and line profile variability of the peculiar eclipsing binary star AR Aur

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. Modeling of the phase-resolve longitudinal field measurements leads to a 3 sigma upper limit on any dipole surface magnetic field of about 400 G.

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arXiv2009

Spots structure and stratification of helium and silicon in the atmosphere of He-weak star HD 21699

The magnetic star HD 21699 possesses a unique magnetic field structure where the magnetic dipole is displaced from the centre by 0.4 +/- 0.1 of the stellar radius (perpendicularly to the magnetic axis), as a result, the magnetic poles are situated close to one another on the stellar surface with an angular separation of 55$^o$ and not 180$^o$ as seen in the case of a centred dipole. Respectively, the two magnetic poles form a large "magnetic spot". High-resolution spectra were obtained allowing He I and Si II abundance variations to be studied as a function of rotational phase. The results show that the helium abundance is concentrated in one hemisphere of the star, near the magnetic poles and it is comparatively weaker in another hemisphere, where magnetic field lines are horizontal with respect to the stellar surface. At the same time, the silicon abundance is greatest between longitudes of 180 - 320$^o$, the same place where the helium abundance is the weakest. These abundance variations (with rotational phase) support predictions made by the theory of atomic diffusion in the presence of a magnetic field. Simultaneously, these result support the possibility of the formation of unusual structures in stellar magnetic fields. Analysis of vertical stratification of the silicon and helium abundances shows that the boundaries of an abundance jump (in the two step model) are similar for each element; $τ_{5000}$ = 0.8-1.2 for helium and 0.5-1.3 for silicon. The elemental abundances in the layers of effective formation of selected absorption lines for various phases are also correlated with the excitation energies of low transition levels: abundances are enhanced for higher excitation energy and higher optical depth within the applied model atmosphere.

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arXiv2009

On the incidence of magnetic fields in slowly-pulsating B, Beta Cephei and B-type emission line stars

We have obtained 40 high-resolution circular spectropolarimetric measurements of 12 slowly-pulsating B (SPB) stars, 8 Beta Cephei stars and two Be stars with the ESPaDOnS and NARVAL spectropolarimeters. The aim of these observations is to evaluate recent claims of a high incidence of magnetic field detections in stars of these types obtained using low-resolution spectropolarimetry by Hubrig (2006), Hubrig (2007) and Hubrig (2009). The precision achieved is generally comparable to or superior to that obtained by Hubrig et al., although our new observations are distinguished by their resolution of metallic and He line profiles, and their consequent sensitivity to magnetic fields of zero net longitudinal component. In the SPB stars we confirm the detection of magnetic field in one star (16 Peg), but find no evidence of the presence of fields in the remaining 11. In the Beta Cep stars, we detect a field in xi^1 CMa, but not in any of the remaining 7 stars. Finally, neither of the two B-type emission line stars shows any evidence of magnetic field. Based on our results, we conclude that fields are not common in SPB, Beta Cep and B-type emission line stars, consistent with the general rarity of fields in the broader population of main sequence B-type stars.

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arXiv2009

The MiMeS Project: Magnetism in Massive Stars

The Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration among the foremost international researchers of the physics of hot, massive stars, with the basic aim of understanding the origin, evolution and impact of magnetic fields in these objects. The cornerstone of the project is the MiMeS Large Program at the Canada-France-Hawaii Telescope, which represents a dedication of 640 hours of telescope time from 2008-2012. The MiMeS Large Program will exploit the unique capabilities of the ESPaDOnS spectropolarimeter to obtain critical missing information about the poorly-studied magnetic properties of these important stars, to confront current models and to guide theory.

astro-ph↗
arXiv2008

The magnetic field of the B3V star 16 Pegasi

The Slowly Pulsating B3V star 16 Pegasi was discovered by Hubrig (2006) to be magnetic, based on low-resolution spectropolarimetric observations with FORS1 at the VLT. We have confirmed the presence of a magnetic field with new measurements with the spectropolarimeters Narval at TBL, France and Espadons at CFHT, Hawaii during 2007. The most likely period is about 1.44 d for the modulation of the field, but this could not be firmly established with the available data set. No variability has been found in the UV stellar wind lines. Although the star was reported once to show H alpha in emission, there exists at present no confirmation that the star is a Be star.

astro-ph↗
arXiv2008

The MiMeS Project: First Results

Massive stars are those stars with initial masses above about 8 times that of the sun, eventually leading to catastrophic explosions in the form of supernovae. These represent the most massive and luminous stellar component of the Universe, and are the crucibles in which the lion's share of the chemical elements are forged. These rapidly-evolving stars drive the chemistry, structure and evolution of galaxies, dominating the ecology of the Universe - not only as supernovae, but also during their entire lifetimes - with far-reaching consequences. Although the existence of magnetic fields in massive stars is no longer in question, our knowledge of the basic statistical properties of massive star magnetic fields is seriously incomplete. The Magnetism in Massive Stars (MiMeS) Project represents a comprehensive, multidisciplinary strategy by an international team of recognized researchers to address the "big questions" related to the complex and puzzling magnetism of massive stars. This paper present the first results of the MiMeS Large Program at the Canada-France-Hawaii Telescope.

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arXiv2008

Magnetic topologies of the Herbig Ae/Be stars

Our recent discoveries of magnetic fields in a small number of Herbig Ae/Be (HAeBe) stars, the evolutionary progenitors of main sequence A/B stars, raise new questions about the origin of magnetic fields in the intermediate mass stars. The favoured fossil field hypothesis suggests that a few percent of magnetic pre-main sequence A/B stars should exhibit similar magnetic strengths and topologies to the magnetic Ap/Bp stars. In this talk I will present the methods that we have used to characterise the magnetic fields of the Herbig Ae/Be stars, as well as our first conclusions on the origin of magnetism in intermediate-mass stars.

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