Upper limits to the magnetic field in central stars of planetary nebulae

More than about twenty central stars of planetary nebulae (CSPN) have been observed spectropolarimetrically, yet no clear, unambiguous signal of the presence of a magnetic field in these objects has been found. We perform a statistical (Bayesian) analysis of all the available spectropolarimetric observations of CSPN to constrain the magnetic fields on these objects. Assuming that the stellar field is dipolar and that the dipole axis of the objects are oriented randomly (isotropically), we find that the dipole magnetic field strength is smaller than 400 G with 95% probability using all available observations. The analysis introduced allows integration of future observations to further constrain the parameters of the distribution, and it is general, so that it can be easily applied to other classes of magnetic objects. We propose several ways to improve the upper limits found here.Comment: 7 pages, 3 figures, accepted for publication in Ap

A near-IR line of Mn I as a diagnostic tool of the average magnetic energy in the solar photosphere

We report on spectropolarimetric observations of a near-IR line of Mn I located at 15262.702 A whose intensity and polarization profiles are very sensitive to the presence of hyperfine structure. A theoretical investigation of the magnetic sensitivity of this line to the magnetic field uncovers several interesting properties. The most important one is that the presence of strong Paschen-Back perturbations due to the hyperfine structure produces an intensity line profile whose shape changes according to the absolute value of the magnetic field strength. A line ratio technique is developed from the intrinsic variations of the line profile. This line ratio technique is applied to spectropolarimetric observations of the quiet solar photosphere in order to explore the probability distribution function of the magnetic field strength. Particular attention is given to the quietest area of the observed field of view, which was encircled by an enhanced network region. A detailed theoretical investigation shows that the inferred distribution yields information on the average magnetic field strength and the spatial scale at which the magnetic field is organized. A first estimation gives ~250 G for the mean field strength and a tentative value of ~0.45" for the spatial scale at which the observed magnetic field is horizontally organized.Comment: 42 pages, 17 figures, accepted for publication in the Astrophysical Journal. Figures 1 and 9 are in JPG forma

Estimating the magnetic field strength from magnetograms

A properly calibrated longitudinal magnetograph is an instrument that measures circular polarization and gives an estimation of the magnetic flux density in each observed resolution element. This usually constitutes a lower bound of the field strength in the resolution element, given that it can be made arbitrarily large as long as it occupies a proportionally smaller area of the resolution element and/or becomes more transversal to the observer and still produce the same magnetic signal. Yet, we know that arbitrarily stronger fields are less likely --hG fields are more probable than kG fields, with fields above several kG virtually absent-- and we may even have partial information about its angular distribution. Based on a set of sensible considerations, we derive simple formulae based on a Bayesian analysis to give an improved estimation of the magnetic field strength for magnetographs.Comment: 8 pages, 7 figures, accepted for publication in A&

On the magnetism and dynamics of prominence legs hosting tornadoes

Solar tornadoes are dark vertical filamentary structures observed in the extreme ultraviolet associated with prominence legs and filament barbs. Their true nature and relationship to prominences requires understanding their magnetic structure and dynamic properties. Recently, a controversy has arisen: is the magnetic field organized forming vertical, helical structures or is it dominantly horizontal? And concerning their dynamics, are tornadoes really rotating or is it just a visual illusion? Here, we analyze four consecutive spectropolarimetric scans of a prominence hosting tornadoes on its legs which help us shed some light on their magnetic and dynamical properties. We show that the magnetic field is very smooth in all the prominence, probably an intrinsic property of the coronal field. The prominence legs have vertical helical fields that show slow temporal variation probably related to the motion of the fibrils. Concerning the dynamics, we argue that 1) if rotation exists, it is intermittent, lasting no more than one hour, and 2) the observed velocity pattern is also consistent with an oscillatory velocity pattern (waves).Comment: accepted for publication in Ap

Near-IR internetwork spectro-polarimetry at different heliocentric angles

The analysis of near infrared spectropolarimetric data at the internetwork at different regions on the solar surface could offer constraints to reject current modeling of these quiet areas. We present spectro-polarimetric observations of very quiet regions for different values of the heliocentric angle for the Fe I lines at 1.56 micron, from disc centre to positions close to the limb. The spatial resolution of the data is 0.7-1". We analyze direct observable properties of the Stokes profiles as the amplitude of circular and linear polarization as well as the total degree of polarization. Also the area and amplitude asymmetries are studied. We do not find any significant variation of the properties of the polarimetric signals with the heliocentric angle. This means that the magnetism of the solar internetwork remains the same regardless of the position on the solar disc. This observational fact discards the possibility of modeling the internetwork as a Network-like scenario. The magnetic elements of internetwork areas seem to be isotropically distributed when observed at our spatial resolution.Comment: Sorry, this is the version with the correct bibliography. Some figures had to be compressed. Accepted for publication in A&

Temperature dependent charge transport mechanisms in carbon sphere/polymer composites

Carbon spheres (CS) with diameters in the range $2 - 10 \mu m$ were prepared via hydrolysis of a sucrose solution at $200^o C,$ and later annealed in $N_2$ at $800^o C.$ The spheres were highly conducting but difficult to process into thin films or pressed pellets. In our previous work, composite samples of CS and the insulating polymer polyethylene oxide (PEO) were prepared and their charge transport was analyzed in the temperature range $80 K < T < 300 K.$ Here, we analyze charge transport in CS coated with a thin polyaniline (PANi) film doped with hydrochloric acid (HCl), in the same temperature range. The goal is to study charge transport in the CS using a conducting polymer (PANi) as a binder and compare with that occurring at CS/PEO. A conductivity maxima was observed in the CS/PEO composite but was absent in CS/PANi. Our data analysis shows that variable range hopping of electrons between polymeric chains in PANi-filled gaps between CS takes on a predominant part in transport through CS/PANi composites, whereas in CS/PEO composites, electrons travel through gaps between CS solely by means of direct tunneling. This difference in transport mechanisms results in different temperature dependences of the conductivity.Comment: 7 pages, 6 figure

Movimentação de inseticidas de tratamento de sementes no solo sob efeito da irrigação.

Este trabalho foi desenvolvido no Centro Nacional de Pesquisa de Milho e Sorgo (CNPMS), EMBRAPA, Sete Lagoas, MG, e teve como objetivo avaliar o movimento de inseticidas de tratamento de sementes de milho no perfil do solo em areas irrigadas. Foram utilizadas tres laminas de irrigacao: deficiente (50% da adequada), adequada e excessiva (100% superior a adequada), e dois inseticidas de tratamento de semente de milho: Semevin 350 RA (Thiodicarb) e Furadan 350 TS (Carbofuran), nas doses recomendadas pelos fabricantes. O monitoramento da lixiviacao desses inseticidas no solo foi feito em camadas de 5 cm e em quatro intervalos de 10 dias, a partir semeadura. A analise de residuo das amostras de solo foi feita utilizando-se cromatografia liquida de alta performance (HPLC). Nas condicoes em que foi realizado o experimento obtiveram-se as seguintes conclusoes: o Thiodicarb apresentou-se o mais instavel (com relacao a sua atividade) no solo com baixo risco de contaminacao do lencol freatico; o Carbofuran apresentou-se com um potencial maior de lixiviacao no perfil do solo devido, possivelmente, a sua resistencia a degradacao e, com baixo risco de contaminacao do lencol freatico para as condicoes apresentadas. Para todos os tratamentos, houve maior retencao do principio ativo dos inseticidas nas camadas mais superficiais (0 a 15 cm) do solo