Magnetic braking in young late-type stars: the effect of polar spots

The concentration of magnetic flux near the poles of rapidly rotating cool stars has been recently proposed as an alternative mechanism to dynamo saturation in order to explain the saturation of angular momentum loss. In this work we study the effect of magnetic surface flux distribution on the coronal field topology and angular momentum loss rate. We investigate if magnetic flux concentration towards the pole is a reasonable alternative to dynamo saturation. We construct a 1D wind model and also apply a 2-D self-similar analytical model, to evaluate how the surface field distribution affects the angular momentum loss of the rotating star. From the 1D model we find that, in a magnetically dominated low corona, the concentrated polar surface field rapidly expands to regions of low magnetic pressure resulting in a coronal field with small latitudinal variation. We also find that the angular momentum loss rate due to a uniform field or a concentrated field with equal total magnetic flux is very similar. From the 2D wind model we show that there are several relevant factors to take into account when studying the angular momentum loss from a star. In particular, we show that the inclusion of force balance across the field in a wind model is fundamental if realistic conclusions are to be drawn from the effect of non-uniform surface field distribution on magnetic braking. This model predicts that a magnetic field concentrated at high latitudes leads to larger Alfven radii and larger braking rates than a smoother field distribution. From the results obtained, we argue that the magnetic surface field distribution towards the pole does not directly limit the braking efficiency of the wind.Comment: 11 pages, 10 figures, accepted in A&

Application of a MHD hybrid solar wind model with latitudinal dependences to Ulysses data at minimum

In a previous work, Ulysses data was analyzed to build a complete axisymmetric MHD solution for the solar wind at minimum including rotation and the initial flaring of the solar wind in the low corona. This model has some problems in reproducing the values of magnetic field at 1 AU despite the correct values of the velocity. Here, we intend to extend the previous analysis to another type of solutions and to improve our modelling of the wind from the solar surface to 1 AU. We compare the previous results to those obtained with a fully helicoidal model and construct a hybrid model combining both previous solutions, keeping the flexibility of the parent models in the appropriate domain. From the solar surface to the Alfven, point, a three component solution for velocity and magnetic field is used, reproducing the complex wind geometry and the well-known flaring of the field lines observed in coronal holes. From the Alfven radius to 1 AU and further, the hybrid model keeps the latitudinal dependences as flexible as possible, in order to deal with the sharp variations near the equator and we use the helicoidal solution, turning the poloidal streamlines into radial ones. Despite the absence of the initial flaring, the helicoidal model and the first hybrid solution suffer from the same low values of the magnetic field at 1 AU. However, by adjusting the parameters with a second hybrid solution, we are able to reproduce both the velocity and magnetic profiles observed by Ulysses and a reasonable description of the low corona, provided that a certain amount of energy deposit exists along the flow. The present paper shows that analytical axisymmetric solutions can be constructed to reproduce the solar structure and dynamics from 1 solar radius up to 1 AU.Comment: 12 pages, 16 figure

Disclosing Jackson Pollock’s palette in Alchemy (1947) by non-invasive spectroscopies

Alchemy (1947, Peggy Guggenheim Collection, Venice) is one of the most materic works by J. Pollock, whose palette is extensive, ranging from white to yellow, red, green, violet, blue, black, and silver. Each layer of color was laid on top of a previously dried one and effectively separated from the lower one forming a quite complex stratigraphy with colors intersecting each other. In this study, a non-invasive multi-technique method combining point analysis with Vis–NIR multispectral imaging has been exploited to give insights on the painting technique of the American abstract expressionist. The molecular identification of pigments, colorants and extenders contained in fifteen different paints has been achieved combining key spectral markers from elemental, electronic and vibrational spectroscopies. For those colors exhibiting similar hues but different chemical compositions, a mapping procedure based on false color rendering, obtained by properly mixing three spectral planes from the Vis–NIR multispectral imaging set, has been successfully applied. Relevant for the understanding of the evolution of Pollock's drip technique is the identification of both traditional oil-based paints and oil-modified alkyd media. Point analysis by reflection FTIR scattered throughout the painting enabled mapping the distinct use of traditional and new binding media among painted, squeezed and dripped paints

Stellar magnetism:empirical trends with age and rotation

We investigate how the observed large-scale surface magnetic fields of low-mass stars (~0.1 -- 2 Msun), reconstructed through Zeeman-Doppler imaging (ZDI), vary with age t, rotation and X-ray emission. Our sample consists of 104 magnetic maps of 73 stars, from accreting pre-main sequence to main-sequence objects (1 Myr < t < 10 Gyr). For non-accreting dwarfs we empirically find that the unsigned average large-scale surface field is related to age as $t^{-0.655 \pm 0.045}$. This relation has a similar dependence to that identified by Skumanich (1972), used as the basis for gyrochronology. Likewise, our relation could be used as an age-dating method ("magnetochronology"). The trends with rotation we find for the large-scale stellar magnetism are consistent with the trends found from Zeeman broadening measurements (sensitive to large- and small-scale fields). These similarities indicate that the fields recovered from both techniques are coupled to each other, suggesting that small- and large-scale fields could share the same dynamo field generation processes. For the accreting objects, fewer statistically significant relations are found, with one being a correlation between the unsigned magnetic flux and rotation period. We attribute this to a signature of star-disc interaction, rather than being driven by the dynamo.Comment: 15 pages, 7 figures. Accepted to MNRA