Mean Surface Magnetic Field Research Articles

The Mean Magnetic Field Strength of CI Tau

Abstract We present a blind comparison of two methods to measure the mean surface magnetic field strength of the classical T Tauri star CI Tau based on Zeeman broadening of sensitive spectral lines. Our approach takes advantage of the greater Zeeman broadening at near-infrared compared to optical wavelengths. We analyze a high signal-to-noise, high spectral resolution spectrum from 1.5 to 2.5 μm observed with IGRINS (Immersion GRating INfrared Spectrometer) on the Discovery Channel Telescope. Both stellar parameterization with MoogStokes (which assumes a uniform magnetic field) and modeling with SYNTHMAG (which includes a distribution of magnetic field strengths) yield consistent measurements for the mean magnetic field strength of CI Tau is B of ∼2.2 kG. This value is typical compared with measurements for other young T Tauri stars and provides an important contribution to the existing sample given that it is the only known developed planetary system hosted by a young classical T Tauri star. Moreover, we potentially identify an interesting and suggestive trend when plotting the effective temperature and the mean magnetic field strength of T Tauri stars. While a larger sample is needed for confirmation, this trend only appears for a subset of the sample, which may have implications regarding the magnetic field generation.

Features of magnetic field structures in chemically peculiar stars. IV

We present results of modeling of the sample of magnetic stars. We have obtained such important for magnetic star physics parameters as the mean surface magnetic field Bs, the magnetic field at magnetic poles—Bp, the dipole inclination to the rotation equatorial plane α, and the distance to monopoles from the center of the star Δa. We present some information onmagnetic star physics that helps to understand the derived results better.

Measuring the surface magnetic fields of magnetic stars with unresolved Zeeman splitting

High-dispersion, archival spectra of magnetic Ap stars with resolved Zeeman components in Stokes I are used to derive a simple relation that can be utilised to estimate the mean surface field strengths of stars with v sin i > 10 km/s. For each star, the mean surface field, as measured from the observed splitting in Fe II at 6149 A, is compared to the differential broadening of spectral lines with large and small Lande factors in order to produce a relation to estimate the field strengths of magnetic stars with unresolved Zeeman patterns. The method is shown to be reliable for rotational velocities up to about 50 km/s for field strengths down to about 5 kG. These results should allow for better constraints to be placed on the mean surface magnetic fields of Ap stars where Zeeman patterns are unresolved.

Exploring the magnetic field complexity in M dwarfs at the boundary to full convection

Based on detailed spectral synthesis we carry out quantitative measurements of the strength and complexity of surface magnetic fields in the four well-known M-dwarfs GJ 388, GJ 729, GJ 285, and GJ 406 populating the mass regime around the boundary between partially and fully convective stars. Very high resolution R=100000, high signal-to-noise (up to 400) near-infrared Stokes I spectra were obtained with CRIRES at ESO's Very Large Telescope covering regions of the FeH Wing-Ford transitions at 1mum. The field distributions in all four stars are characterized by three distinct groups of field components, the data are neither consistent with a smooth distribution of different field strengths, nor with one average field strength covering the full star. We find evidence of a subtle difference in the field distribution of GJ 285 compared to the other three targets. GJ 285 also has the highest average field of 3.5kG and the strongest maximum field component of 7-7.5kG. The maximum local field strengths in our sample seem to be correlated with rotation rate. While the average field strength is saturated, the maximum local field strengths in our sample show no evidence for saturation. We find no difference between the field distributions of partially and fully convective stars. The one star with evidence for a field distribution different to the other three is the most active star (i.e. with largest x-ray luminosity and mean surface magnetic field) rotating relatively fast. A possible explanation is that rotation determines the distribution of surface magnetic fields, and that local field strengths grow with rotation even in stars in which the average field is already saturated.

EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars

Aims. We observe the slowly-rotating, active, single giant, EK Eri, to study and infer the nature of its magnetic field directly. Methods. We used the spectropolarimeter NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least Square Deconvolution method to create high signal-to-noise ratio Stokes V profiles. We fitted the Stokes V profiles with a model of the large-scale magnetic field. We studied the classical activity indicators, the Ca ii H and K lines, the Ca ii infrared triplet, and Hα line. Results. We detected the Stokes V signal of EK Eri securely and measured the longitudinal magnetic field Bl for seven individual dates spanning 60% of the rotational period. The measured longitudinal magnetic field of EK Eri reached about 100 G and was as strong as fields observed in RSCVn or FK Com type stars: this was found to be extraordinary when compared with the weak fields observed at the surfaces of slowly-rotating MS stars or any single red giant previously observed with NARVAL. From our modeling, we infer that the mean surface magnetic field is about 270 G, and that the large scale magnetic field is dominated by a poloidal component. This is compatible with expectations for the descendant of a strongly magnetic Ap star.

Model atmospheres of magnetic chemically peculiar stars

In the last few years we have developed stellar model atmospheres which included effects of anomalous abundances and strong magnetic field. The full treatment of anomalous Zeeman splitting and polarized radiative transfer were introduced in the model atmosphere calculations for the first time. In this investigation we present results of modelling the atmosphere of one of the most extreme magnetic chemically peculiar stars, HD137509. This Bp SiCrFe star has the mean surface magnetic field modulus of about 29kG. We use the recent version of the line-by-line opacity sampling stellar model atmosphere code LLmodels, which incorporates the full treatment of Zeeman splitting of spectral lines, detailed polarized radiative transfer and arbitrary abundances. We compare model predictions with photometric and spectroscopic observations of the star, aiming to reach a self-consistency between the abundance pattern derived from high-resolution spectra and abundances used for model atmosphere calculation. Based on magnetic model atmospheres, we redetermined abundances and fundamental parameters of HD137509 using spectroscopic and photometric observations. This allowed us to obtain a better agreement between observed and theoretical parameters compared to non-magnetic models with individual or scaled-solar abundances. We confirm that the magnetic field effects should be taken into account in the stellar parameter determination and abundance analysis.

Comments on the magnetic field structure of the star CU Virginis

The model of the magnetic field structure of the CP2 star CU Vir (HD 124224) constructed by the method of “magnetic charge (MCD-method) has shown that it is consistent with the model of a displaced dipole. The displacement from the center of the star to the negative monopole is $d = 0.3$ of the radius, the inclination angle of the dipole axis to the rotational axis is $\beta = 87^{\rm o}$, and the field strength at the poles amounts to $B_{\rm p}(-) = 7.9$ kG and $B_{\rm p}(+) = 1.2$ kG. The mean surface magnetic field varies within 1.2–3.2 kG. The dipole axis points away from the zero meridian by an angle of $+30^{\rm o}$. Using the MCD-method we derived the distribution of the field intensity over the surface, which has been compared to the distribution of the chemical elements He and Si, taken from literature. Silicon has turned out to concentrate around the strong negative magnetic pole, whereas helium concentrates in the region of the weak positive pole, where the orientation of magnetic lines of force is mostly vertical. The presence of a double silicon spot suggests a more complex magnetic field structure than the dipolar one, however, the small number of data makes it impossible so far to confirm such an assumption.

Dependence of the magnetic field of chemically peculiar stars on the rotation period

In this paper, the dependence of the mean surface magnetic field B /sub s/ of chemically peculiar stars on the rotation period P is found. In the region P is less than 8 days, this dependence has a direct correlation and in the region P is greater than 8 days, an inverse correlation. Data given indicate that rapidly rotating chemically peculiar stars have on the average weaker fields than slowly rotating stars. An anticorrelation was found between the strength and the rotation period, in disagreement with the conclusions of the dynamo hypothesis. In the work of the author and Didelon data showing that the distribution of the angles beta is arbitrary were given. It is assumed that the characteristic form of the dependence of B /sub s/ (P) carries traces of magnetic field generation by means of a dynamo.

Photometrically determined mean surface magnetic fields of Ap stars

AbstractThe photometrically determined mean surface magnetic fields BS need a revision. None of the stars for which BS can be measured directly by Zeeman line splitting fulfils the relation between the photometric parameter Δ(V1 – G) and the mean surface field BS, which is used by NORTH, CRAMER and MAEDER to determine BS for other B2 – A3 stars. The ratio BeffMax/BS for stars, which define North's relation, shows unreasonable large values.

Lunar Magnetic Anomalies and the Cayley Formation

THE observation of the existence of local magnetic fields on the Moon is striking since it is known from the Explorer 35 satellite that the global magnetic dipole moment of the Moon is less than 1020 gauss cm3 implying a mean surface magnetic field of less than 2 gamma (ref. 1). Apollo subsatellite data suggest that the mean dipole moment may be even less than this2. Local magnetic anomalies have been measured, however, by three independent techniques: (1) surface magnetometers showed values of 36 gamma at the Apollo 12 landing site; 43 gamma and 104 gamma at two positions near the Apollo 14 site; less than 5 gamma at the Apollo 15 site; 121, 125, 180, 231 and 313 gamma at various positions near the Apollo 16 site3–8; (2) the magnetometers aboard the Apollo 15 and 16 subsatellites recorded anomalies as large as 1 gamma at a flight height of about 100 km apparently associated with large basins on the back side of the Moon9–12; (3) limb compression effects detected by the Explorer 35 satellite reveal many magnetic sources most of which are located in the lunar highlands; more detailed results were obtained by the Apollo 15 subsatellite due to its lower flight height1,9,13,14. We shall consider here the material which causes these anomalies.