Surface-averaged Magnetic Field Research Articles

Star-crossed Lovers DI Tau A and B: Orbit Characterization and Physical Properties Determination

The stellar companion to the weak-line T Tauri star DI Tau A was first discovered by the lunar occultation technique in 1989 and was subsequently confirmed by a speckle imaging observation in 1991. It has not been detected since, despite being targeted by five different studies that used a variety of methods and spanned more than 20 yr. Here, we report the serendipitous rediscovery of DI Tau B during our Young Exoplanets Spectroscopic Survey (YESS). Using radial velocity data from YESS spanning 17 yr, new adaptive optics observations from Keck II, and a variety of other data from the literature, we derive a preliminary orbital solution for the system that effectively explains the detection and (almost all of the) non-detection history of DI Tau B. We estimate the dynamical masses of both components, finding that the large mass difference (q ∼ 0.17) and long orbital period (≳35 yr) make the DI Tau system a noteworthy and valuable addition to studies of stellar evolution and pre-main-sequence models. With a long orbital period and a small flux ratio (f2/f1) between DI Tau A and B, additional measurements are needed for a better comparison between these observational results and pre-main-sequence models. Finally, we report an average surface magnetic field strength () for DI Tau A, of ∼0.55 kG, which is unusually low in the context of young active stars.

Long-term Activity in Photospheres of Low-mass Stars with Strong Magnetic Fields

The behavior of the average annual luminosity of K-M dwarfs OU Gem, EQ Vir, V1005 Ori and AU Mic was studied at time intervals of several decades. The main sources of photometric data for 1989-2019 were the Hipparcos, ASAS, KWS databases. An analysis of long-term series showed that the average annual brightness of all stars varies cyclical. The durations of possible cycles for OU Gem, V1005 Ori and AU Mic are 40-42 years, for EQ Vir - 16.6 years, and amplitudes of cycles are of 0.09$^m$ - 0.2$^m$. The selected stars belong to the group of red dwarfs for which the average surface magnetic field <B> exceeds of several kilogauss. We examined the type of the relationship between the parameters of the cycle, its duration and amplitude, for 9 stars with <B> < 4 kG. A tendency to increasing of the amplitude and duration of the cycle when the value of <B> decrease has been noted. It may be suggested that with an increasing of the surface magnetic field, it becomes more uniform and the level of activity changes in this case in less degrees than on stars with strong local fields concentrated in large spots.

Absolute Dimensions of the F-type Eclipsing Binary V2154 Cygni

Abstract We report spectroscopic observations of the 2.63 day, detached, F-type main-sequence eclipsing binary V2154 Cyg. We use our observations together with existing uvby photometric measurements to derive accurate absolute masses and radii for the stars that are good to better than 1.5%. We obtain masses of and , radii of and , and effective temperatures of 6770 ± 150 K and 5020 ± 150 K for the primary and secondary stars, respectively. Both components appear to have their rotations synchronized with the motion in the circular orbit. A comparison of the properties of the primary with current stellar evolution models gives good agreement for a metallicity of , which is consistent with photometric estimates, and an age of about 2.2 Gyr. On the other hand, the K2 secondary is larger than predicted for its mass by about 4%. Similar discrepancies are known to exist for other cool stars, and are generally ascribed to stellar activity. The system is in fact an X-ray source, and we argue that the main site of the activity is the secondary star. Indirect estimates give a strength of about 1 kG for the average surface magnetic field on that star. A previously known close visual companion to V2154 Cyg is shown to be physically bound, making the system a hierarchical triple.

The magnetic cataclysmic variable LSQ1725-64

We present new photometry and spectroscopy of the 94m eclipsing binary LSQ1725-64 that provide insight into the fundamental parameters and evolutionary state of this system. We confirm that LSQ1725-64 is a magnetic cataclysmic variable whose white dwarf has a surface-averaged magnetic field strength of $12.5 \pm 0.5$ MG measured from Zeeman splitting. The spectral type and colour of the secondary, as well as the eclipse length, are consistent with other secondaries that have not yet evolved through the period minimum expected for cataclysmic variables. We observe two different states of mass transfer and measure the transition between the two to occur over about 45 orbital cycles. In the low state, we observe photometric variations that we hypothesize to arise predominantly from two previously heated magnetic poles of the white dwarf. Our precise eclipse measurements allow us to determine binary parameters of LSQ1725-64 and we find it contains a high mass ($0.97 \pm 0.03\ M_{\odot}$) white dwarf if we assume a typical mass-radius relationship for a CO core white dwarf. We also measure an eclipse of the accretion stream after the white dwarf eclipse, and use it to estimate an upper limit of the mass transfer rate. This derived limit is consistent with that expected from angular momentum loss via gravitational radiation alone.

MEASUREMENTS OF STELLAR MAGNETIC FIELDS USING AUTOCORRELATION OF SPECTRA

We present a novel technique that uses the autocorrelation of the spectrum of a star to measure the line broadening caused by the modulus of its average surface magnetic field. The advantage of the autocorrelation comes from the fact that it can detect very small spectral line broadening effects because it averages over many spectral lines and therefore gives an average with a very high signal to noise ratio. We validate the technique with the spectra of known magnetic stars and obtain autocorrelation curves that are in full agreement with published magnetic curves obtained with Zeeman splitting. The autocorrelation also gives less noisy curves so that it can be used to obtain very accurate curves. We degrade the resolution of spectra of these magnetic stars to lower spectral resolutions where the Zeeman splitting is undetectable. At these resolutions, the autocorrelation still gives good quality curves, thereby showing that it can be used to measure magnetic fields in spectra where the Zeeman splitting is significantly smaller than the width of the spectral line. This would therefore allow observing magnetic fields in very faint Ap stars with low-resolution spectrographs, thereby greatly increasing the number of known magnetic stars. It also demonstrates that the autocorrelation can measure magnetic fields in rapidly rotating stars as well as weak magnetic fields that give a Zeeman splitting smaller than the intrinsic width of the spectral lines. Finally, it shows that the autocorrelation can be used to find unknown magnetic stars in low resolution spectroscopic surveys.

Magnetic field measurements of ϵ Eridani from Zeeman broadening

AbstractWe present new magnetic field measurements of the K2 main‐sequence star ϵ Eri based on principal components analysis (PCA) line‐profile reconstructions. The aim of this paper is to quantify the surface‐averaged magnetic field and search for possible variations. A total of 338 optical échelle spectra from our robotic telescope facility STELLA with a spectral resolution of 55 000 were available for analysis. This time‐series was used to search for the small line‐profile variations due to a surface magnetic field with the help of a PCA. Evidence for a spatial and temporal inhomogeneous magnetic field distribution is presented. The mean, surface averaged, magnetic field strength was found tobe 〈B〉 = 186 ± 47 G in good agreement with previous Zeeman‐broadening measurements. Clear short‐term variations of the surface averaged magnetic field of up to few tens Gauss were detected together with evidence for a three‐year cycle in the surface‐averaged magnetic field of ϵ Eri. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

CONSTRAINTS ON THE SURFACE MAGNETIC FIELDS AND AGE OF A COOL HYPERGIANT:XMM-NEWTONX-RAY OBSERVATIONS OF VY CMa

The complex circumstellar ejecta of highly evolved, cool hypergiants are indicative of multiple, asymmetric mass loss events. To explore whether such episodic, non-isotropic mass loss may be driven by surface magnetic activity, we have observed the archetypical cool hypergiant VY CMa with the XMM-Newton X-ray satellite observatory. The hypergiant itself is not detected in these observations. From the upper limit on the X-ray flux from VY CMa at the time of our observations ($F_{X,UL}~\approx~8\times10^{-14} {\rm~erg~cm}^{-2} {\rm~s}^{-1}$, corresponding to $\log~L_X/L_{bol}\leq-8$), we estimate an average surface magnetic field strength $fB \leq 2\times10^{-3}$~G (where $f$ is the filling factor of magnetically active surface regions). These X-ray results for VY CMa represent the most stringent constraints to date on the magnetic field strength near the surface of a hypergiant. VY CMa's mass loss is episodic and may have been in a state of low surface magnetic activity during the XMM observations. The XMM observations also yield detections of more than 100 X-ray sources within $\sim$15$^{\prime}$ of VY CMa, roughly 50 of which have near-infrared counterparts. Analysis of X-ray hardness ratios and IR colors indicates that some of these field sources may be young, late-type stars associated with VY CMa, its adjacent molecular cloud complex, and the young cluster NGC 2362. Further study of the VY CMa field is warranted, given the potential to ascertain the evolutionary timescale of this enigmatic, massive star.

MODELING THE NON-RECYCLEDFERMIGAMMA-RAY PULSAR POPULATION

We use Fermi Gamma-ray Space Telescope detections and upper limits on non-recycled pulsars obtained from the Large Area Telescope (LAT) to constrain how the gamma-ray luminosity L depends on the period P and the period derivative \dot{P}. We use a Bayesian analysis to calculate a best-fit luminosity law, or dependence of L on P and \dot{P}, including different methods for modeling the beaming factor. An outer gap (OG) magnetosphere geometry provides the best-fit model, which is L \propto P^{-a} \dot{P}^{b} where a=1.36\pm0.03 and b=0.44\pm0.02, similar to but not identical to the commonly assumed L \propto \sqrt{\dot{E}} \propto P^{-1.5} \dot{P}^{0.5}. Given upper limits on gamma-ray fluxes of currently known radio pulsars and using the OG model, we find that about 92% of the radio-detected pulsars have gamma-ray beams that intersect our line of sight. By modeling the misalignment of radio and gamma-ray beams of these pulsars, we find an average gamma-ray beaming solid angle of about 3.7{\pi} for the OG model, assuming a uniform beam. Using LAT-measured diffuse fluxes, we place a 2{\sigma} upper limit on the average braking index and a 2{\sigma} lower limit on the average surface magnetic field strength of the pulsar population of 3.8 and 3.2 X 10^{10} G, respectively. We then predict the number of non-recycled pulsars detectable by the LAT based on our population model. Using the two-year sensitivity, we find that the LAT is capable of detecting emission from about 380 non-recycled pulsars, including 150 currently identified radio pulsars. Using the expected five-year sensitivity, about 620 non-recycled pulsars are detectable, including about 220 currently identified radio pulsars. We note that these predictions significantly depend on our model assumptions.

Evolution of the magnetic fields of main-sequence CP-stars. I.

A study of the average surface magnetic fields Bs shows that the damping time for the magnetic field of chemically peculiar stars is τ ~ 108 −109 years, which is consistent with the assumption of ohmic dissipation. Besides ohmic losses, there are evidently no other causes of the magnetic field decay, such as differential rotation, meridional circulation, turbulence, etc. An earlier idea that the average surface magnetic field of CP-stars varies as the reciprocal of the cube of the increasing radius of the evolving star, as it should for a dipole magnetic field configuration, is confirmed. There are signs, however, of a simultaneous gradual increase in the magnetic field beginning with the ZAMS and extending up to the time a star leaves the main sequence. Most likely there is a gradual simplification and ordering of the structure of the surface magnetic field because of the instability of fine structures. CP-stars appear to be divided into two groups, with normal and extreme values of Bs. Most are in the normal group, with the extreme group forming a small part of the whole. They may have developed from the most highly magnetized protostellar clouds.

Magnetic field models of nine CP stars from “accurate” measurements

The dipole models of magnetic fields in nine CP stars are constructed based on the measurements of metal lines taken from the literature, and performed by the LSD method with an accuracy of 10–80 G. The model parameters are compared with the parameters obtained for the same stars from the hydrogen line measurements. For six out of nine stars the same type of structure was obtained. Some parameters, such as the field strength at the poles Bp and the average surface magnetic field Bs differ considerably in some stars due to differences in the amplitudes of phase dependences Be(Φ) and Bs(Φ), obtained by different authors. It is noted that a significant increase in the measurement accuracy has little effect on the modelling of the large-scale structures of the field. By contrast, it is more important to construct the shape of the phase dependence based on a fairly large number of field measurements, evenly distributed by the rotation period phases. It is concluded that the Zeeman component measurement methods have a strong effect on the shape of the phase dependence, and that the measurements of the magnetic field based on the lines of hydrogen are more preferable for modelling the large-scale structures of the field.

Impact of magnetic field inhomogeneity on electron cyclotron radiative loss in tokamak reactors

The potential importance of electron cyclotron (EC) emission in the local electron power balance in the steady-state regimes of ITER operation with high temperatures, as well as in the DEMO reactor, requires accurate calculation of the one-dimensional (over magnetic surfaces) distribution of the net radiated power density, P EC(ρ). When the central electron temperature increases to ∼30 keV, the local EC radiative loss comprises a substantial fraction of the heating power from fusion alphas and is close to the total auxiliary NBI heating power, P EC(0) ≃ 0.3P α(0) ≃ P aux(0). In the present paper, the model of EC radiative transport in an axisymmetric toroidal plasma is extended to the case of an inhomogeneous magnetic field B(R, Z). The impact of such inhomogeneity on local and total power losses is analyzed in the framework of this model by using the CYNEQ code. It is shown that, for the magnetic field B, temperature T e , density n e , and wall reflection coefficient R w expected in ITER and DEMO, accurate simulations of the EC radiative loss require self-consistent 1.5D transport analysis (i.e., one-dimensional simulations of plasma transport and two-dimensional simulations of plasma equilibrium). It is shown that EC radiative transport can be described with good accuracy in the 1D approximation with the surface-averaged magnetic field, B(ρ) = 〈B(R, Z)〉 ms . This makes it possible to substantially reduce the computational time required for time-dependent self-consistent 1.5D transport analysis. Benchmarking of the CYNEQ results with available results of the RAYTEC, EXACTEC, and CYTRAN codes is performed for various approximations of the magnetic field.

The cool magnetic DAZ white dwarf NLTT 10480

We have identified a new, cool magnetic white dwarf in the New Luyten Two-Tenths (NLTT) catalogue. The high proper-motion star NLTT 10480 (mu=0.5"/year) shows weak Zeeman-split lines of calcium, as well as characteristic Halpha and beta Zeeman triplets. Using VLT X-shooter spectra we measured a surface-averaged magnetic field B_S ~ 0.5 MG. The relative intensity of the pi and sigma components of the calcium and hydrogen lines imply a high inclination (i > 60 deg). The optical-to-infrared V-J colour index and the CaI/CaII ionization balance indicate a temperature between 4900 and 5200 K, while the Balmer line profiles favour a higher temperature of 5400 K. The discrepancy is potentially resolved by increasing the metallicity to 0.03x solar, hence increasing the electron pressure. However, the measured calcium abundance and abundance upper limits for other elements (Na, Al, Si. and Fe) imply a low photospheric metallicity < 10^{-4} solar. Assuming diffusion steady-state, a calcium accretion rate of log(dM/dt)(g/s)=5.6+/-0.3 is required to sustain a calcium abundance of log(n(Ca)/n(H))=-10.30+/-0.30 in the white dwarf atmosphere. We examine the implications of this discovery for the incidence of planetary debris and weak magnetic fields in cool white dwarf stars.

Rotation periods of CP-stars with different magnetic fields

Data on the average surface magnetic field Bs for chemically peculiar stars are used to study the magnetic field dependence of their rotation periods. In order to exclude the effect of the age dependence of Bs (owing to the change in radius), the data were reduced to equal relative radii in the V-band luminosity. The resulting dependence is described, with sufficient confidence, by a second degree polynomial with a maximum at periods near P = 10 days. It is suggested that this dependence is a result of several processes that take place the early stages of evolution.

The magnetic fields of millisecond pulsars in globular clusters

Many of the characteristic properties of the millisecond pulsars found in globular clusters are markedly different from those in the Galactic disc. We find that one such physical parameter is the surface magnetic field strength. Even though the average spin-periods do not differ much the average surface magnetic field is 2-5 times larger in the globular cluster pulsars. This effect could be apparent, arising due to one or more of several biases. Alternatively, if future observations confirm this effect to be real, then this could be interpreted as a preferential recycling of pulsars in tight binaries where the mass transfer takes place at high accretion rates.

Some properties of the He-w star HD 35502

This is a preliminary study of the star HD 35502. Its magnetic field has been measured in different phases of its period. Preliminary values of the magnetic field parameters have been obtained based on a central quadrupole model. The effective magnetic field Be varies over 0-5000 G, the average surface magnetic field ranges over 6300-6700 G, the field at the poles is Bp=7000 G, and the angle between the quadrupole axis and the axis of rotation is β = 80o. As a first approximation, the surface helium is concentrated around the (negative) pole and for τ > 1 its abundance is reduced by approximately 2-4 dex, which confirms the hypothesis of helium diffusion under the action of gravitation and wind in a stable atmosphere. The chemical elements Si and Cr are concentrated in four spots on the magnetic equator between the magnetic poles, or in a ring coincident with the magnetic equator; precisely which is not clear at present.

Magnetic-field dependence of chemical anomalies in CP stars

The dependence of the degree of anomaly of parameter Z of Geneva photometry (Z0 = ZCPℒZnorm.) on the average surface magnetic field Bs is analyzed. The Z0 value is proportional to the degree of anomaly of chemical composition. It was found that Bs → 0 corresponds Z0 → −0.010÷ −0.015, i.e., part of CP stars are virtually devoid of magnetic field, but exhibit chemical anomalies. This effect may be due to selection whereby only objects with strong chemical anomalies are classified as CP stars, thereby producing a deficit of stars with relatively weak anomalies. Moreover, CP stars have other sources of stabilization of their atmospheres besides the magnetic field, e.g., slow rotation. Formulas relating Z0 to Bs are derived.

Composite model for the magnetic field of HD 21699

A model is constructed for the magnetic field of the He-weak CP star HD 21699. This star has the field structure of a dipole displaced by 0.4 radii from the center perpendicular to its axis. The magnetic poles appear to be close to one another on the surface; they are separated by 55°, not by 180° as in the case of a central dipole. The phase dependences of the equivalent widths of He and Si lines have extrema at the phases corresponding to passage through the visible meridian of zero magnetic field between the magnetic poles. At the magnetic poles, the intensity of the helium lines is maximal and of the silicon lines, minimal. The silicon abundance is maximal in the regions where the magnetic field is predominantly tangential to the star’s surface. Because of averaging over the visible hemisphere and owing to the closeness of the magnetic poles, only one wave of variation in the intensity of the spectral lines of these chemical elements, one wave of photometric variability, and an average surface magnetic field Bs are observed.

Magnetic field model for slowly rotating CP-stars. γEqu= HD201601

A magnetic field model is constructed for the extremely slow rotator γEqu based on measurements of its magnetic field over many years and using the “magnetic charge” method. An analysis of γEqu and of all the data accumulated up to the present on the magnetic field parameters of chemically peculiar stars leads to some interesting conclusions, of which the main ones are: the fact that the axis of rotation and the dipole axis are not parallel in γEqu and the other slowly rotating magnetic stars which we have studied previously is one of the signs that the braking of CP stars does not involve the participation of the magnetic field as they evolve “to the main sequence.” The axes of the magnetic field dipole in slow rotators are oriented arbitrarily with respect to their axes of rotation. The substantial photometric activity of these CP stars also argues against these axes being close. The well-known absence of sufficiently strong magnetic fields in the Ae/Be Herbig stars also presents difficulties for the hypothesis of “magnetic braking” in the “pre-main sequence” stages of evolution. The inverse relation between the average surface magnetic field Bs and the rotation period P is yet another fact in conflict with the idea that the magnetic field is involved in the braking of CP stars. We believe that angular momentum loss involving the magnetic field can hardly have taken place during evolution immediately prior “to the main sequence,” rather the slow rotation of CP stars most likely originates from protostellar clouds with low angular momentum. Some of the slowly rotating stars have a central dipole magnetic field configuration, while others have a displaced dipole configuration, where the displacement can be toward the positive or the negative magnetic pole.

Basic Parameters Determining X-Ray Emission Level in Stars of Spectral Type Later than F5

This is a merely exploratory search for estimating the importance of those stellar parameters which we believe to be relevant to the X-ray emission from stars possessing outer convective envelopes.Belvedere et al.(1981,1982) have shown that a mechanism which converts magnetic into thermal energy is plausible for explaining the X-ray emission level in late type main sequence and giant stars. One of the main conclusions of their work is that the average surface X-ray flux Fx depends on the square of the average surface magnetic field strength B. The surface magnetic activity,in stars possessing outer convective envelopes, is likely due to the interaction of rotation with convection which produces both differential rotation (Belvedere et al. 1980a) and dynamo action by the α-effect (Belvedere et al. 1980b).Therefore we would expect that the level of magnetic field intensity depends on star's rotation Ω and the depth of the convection zone(c.z.) D,because both parameters are very important in determining the strength of the interaction.