In this work we investigate the inversion accuracy of stellar magnetic fields through the analysis of spectropolarimetric data. We performed several noise free tests to untangle the impact among the atmospheric and magnetic parameters in the data analysis. Using a single spectral line, we found that under ideal scenario, without noise, the magnetic parameters can be recovered with very high accuracy, while some of the atmospheric ones show a considerable incertitude. If multi-line profiles are considered instead, then the accuracy of the recovered atmospheric parameters increases significantly. Nonetheless, it is quite common that for the inversion of stellar spectropolarimetric data, the atmospheric parameters and the stellar inclination angle of the star are fixed; we show that this procedure could induce very high errors in the inference of the magnetic properties, specially in what concerns to fix the inclination angle. We show that even small deviations from the true inclination angles could have as consequence that the recovery of the magnetic properties of the star are no longer reliable. This is because we demonstrate that given a set of Stokes profiles observed along the rotational phase, they can be satisfactorily fitted regardless of the assumed stellar inclination angle, thereby revealing a degeneracy in the solution. Although this work validates the solution generation solely for a de-centered dipolar geometry, a comparable degeneracy is anticipated when modeling the magnetic field with higher order spherical harmonics, an effect that remains to be formally characterized.

We present the polarimetric measurements of the asteroid (16) Psyche for its whole rotational phase. Observations were conducted in the facilities of the OAN-SPM with a double beam polarizer (POLIMA 2) coupled to the 84 cm telescope in September 2019. Polarimetric lightcurves are valuable because they might provide useful information about the geometric albedo and surface properties of atmosphereless bodies. The upcoming NASA mission to visit Psyche will verify most of the ground-based observations, however, there is a lack of polarimetric lightcurves, which could be useful for a more precise determination of the surface properties. Polarimetry relies on relative flux variation of orthogonal polarization states to determine Stokes parameters. The aim, in this work, is to measure the amount of linearly polarized light reflected by the asteroid (16) Psyche. The polarimetric lightcurve was obtained for a rotation period, and an amplitude of 0.14 +/- 0.03% was measured for a phase angle of 17 degrees. From a comparison between the polarization curve with the corresponding photometric lightcurve, we found that the latter is possibly affected by both albedo and shape. Our results show evidence of albedo variegation, which is in agreement with the albedo map of (16) Psyche obtained with different observational techniques.

ABSTRACT The big majority of the reported measurements of the stellar magnetic fields that have analysed spectropolarimetric data have employed the least-squares deconvolution method (LSD) and the first-order moment approach. We present a series of numerical tests in which we review some important aspects of this technique. First, we show that the selection of the profile widths, i.e. integration range in the first-order moment equation, is independent of the accuracy of the magnetic measurements, meaning that for any arbitrary profile width it is always possible to properly determine the longitudinal magnetic field. We also study the interplay between the line depth limit adopted in the line mask and the normalization values of the LSD profiles. We finally show that the rotation of the stars has to be considered to correctly infer the intensity of the magnetic field, something that has been neglected up to now. We show that the latter consideration is crucial, and our test shows that the magnetic intensities differ by a factor close to 3 for a moderate fast rotator star with vsini of 50 ${\rm km\, s^{-1}}$. Therefore, it is expected that in general the stellar magnetic fields reported for fast rotators are stronger than what was believed. All the previous results shows that the first-order moment can be a very robust tool for measurements of magnetic fields, provided that the weak magnetic field approximation is secured. We also show that when the magnetic field regime breaks down, the use of the first-order moment method becomes uncertain.

In this work we present the preliminary results for the automatic determination of the mean longitudinal magnetic field of polarized spectra through the analysis of spectropolarimetric observations. In order to achieve this goal, we first developed an artificial database encompassing a set of different stellar spectra each one defined by a set of free parameters. Then we used supervised learning for artificial neural networks, a computational intelligence paradigm, in order to determine this important parameter.

In this work we present the results for the automatic determination of the mean longitudinal magnetic field in polarized stellar spectra through the analysis of spectropolarimetric observations. In order to determine this important parameter, we first developed a synthetic database encompassing a set of different stellar spectra, each one defined by a set of free parameters. Then, we used supervised learning for artificial neural networks, a machine learning approach, to achieve our goal.

We present the latest instrumental and theoretical developments performed at the Universidad Nacional AutOnoma de Mexico (UNAM) regarding the search and analysis of magnetic fields in various type of stars among the more evolved ones. This work is the prelude to a wider assessment of magnetism across the Hertzsprung-Russell diagram.

With its bright and wide equatorial waist seen almost edge-on ('the butterfly body') and the faint and broad bipolar extensions ('the butterfly wings'), NGC 650-1 is the archetypical example of bipolar planetary nebula (PN) with butterfly morphology. We present here deep high-resolution broad-and narrow-band optical images that expose the rich and intricate fine structure of this bipolar PN, with small-scale bubble-like features and collimated outflows. A SHAPE spatio-kinematic model indicates that NGC 650-1 has a broad central torus with an inclination angle of 75 degrees with respect to the line of sight, whereas that of the bipolar lobes, which are clearly seen in the position-velocity maps, is 85 degrees. Large field of view deep images show, for first time, an arc-like diffuse envelope in low-and high-excitation emission lines located up to 180 arcsec towards the east-south-east of the central star, well outside the main nebula. This morphological component is confirmed by Spitzer MIPS and WISE infrared imaging, as well as by long-slit low-and high-dispersion optical spectroscopic observations. Hubble Space Telescope images of NGC 650-1 obtained at two different epochs similar to 14 yr apart reveal the proper motion of the central star along this direction. We propose that this motion of the star through the interstellar medium compresses the remnant material of a slow asymptotic giant branch wind, producing this bow-shock-like feature.

Context.Regression methods based on machine learning algorithms (MLA) have become an important tool for data analysis in many different disciplines.Aims.In this work, we use MLA in an astrophysical context; our goal is to measure the mean longitudinal magnetic field in stars (Heff) from polarized spectra of high resolution, through the inversion of the so-called multi-line profiles.Methods.Using synthetic data, we tested the performance of our technique considering different noise levels: In an ideal scenario of noise-free multi-line profiles, the inversion results are excellent; however, the accuracy of the inversions diminish considerably when noise is taken into account. We therefore propose a data pre-process in order to reduce the noise impact, which consists of a denoising profile process combined with an iterative inversion methodology.Results.Applying this data pre-process, we find a considerable improvement of the inversions results, allowing to estimate the errors associated to the measurements of stellar magnetic fields at different noise levels.Conclusions.We have successfully applied our data analysis technique to two different stars, attaining for the first time the measurement ofHefffrom multi-line profiles beyond the condition of line autosimilarity assumed by other techniques.

We present new optical broad- band (UBVRI) aperture polarimetric observations of 53 postasymptotic giant branch (AGB) stars selected to exhibit a large near- infrared excess. 24 out of the 53 stars (45 per cent of our sample) are presented for the first time. A statistical analysis shows four distinctive groups of polarized post- AGB stars: unpolarized or very lowly polarized (degree of polarization or DoP < 1 per cent), lowly polarized (1 per cent < DoP < 4 per cent), moderately polarized (4 per cent< DoP< 8 per cent) and highly polarized (DoP> 8 per cent). 23 out of the 53 (66 per cent) belong to the first group, 10 (19 per cent) to the second, five (9 per cent) to the third and only three (6 per cent) to the last group. Approximately 34 per cent of our sample was found to be unpolarized objects, which is close to the percentage of round planetary nebulae. On average, the low and moderate groups show a wavelength- dependent polarization that increases towards shorter wavelengths, implying an intrinsic origin of the polarization, which signifies a Rayleigh- like scattering spectrum typical for non- symmetrical envelopes composed principally of small dust grains. The moderately polarized stars exhibit higher K -W3 andW1 - W3 colour indices compared with the group of lowly polarized stars, suggesting a possible relation between DoP and mass- loss rate. Moreover, they are found to be systematically colder (redder in B - V), which may be associated with the condensation process close to these stars that results in a higher degree of polarization. We also provide evidence that multiple scattering in optically thin polar outflows is the mechanism that gives high DoP in post- AGB stars with bipolar or multi- polar envelopes.

Context. A lot of effort has been put into the detection and determination of stellar magnetic fields using the spectral signal obtained from the combination of hundreds or thousands of individual lines, an approach known as a multi-line analysis. So far, however, most of the developed multi-line techniques that retrieve mean stellar longitudinal magnetic fields can sometimes entail substantial simplifications concerning line shapes and Zeeman splittings.Aims. In this paper we determine stellar longitudinal magnetic fields by means of the Principal Components Analysis and Zeeman Doppler Imaging (PCA-ZDI) multi-line technique, based on accurate polarised spectral line synthesis.Methods. In this paper we present the methodology for performing inversions of profiles obtained using PCA-ZDI.Results. Inversions with various magnetic geometries, field strengths and rotational velocities show that we can correctly determine the effective longitudinal magnetic field in stars using the PCA-ZDI method.

We present the results of a major high-resolution spectropolarimetric BCool project magnetic survey of 170 solar-type stars. Surface magnetic fields were detected on 67 stars, with 21 classified as mature solar-type stars, a result that increases by a factor of 4 the number of mature solar-type stars on which magnetic fields have been observed. In addition, a magnetic field was detected for 3 out of 18 of the subgiant stars surveyed. For the population of K-dwarfs, the mean value of vertical bar B-l vertical bar (vertical bar B-l vertical bar(mean)) was also found to be higher (5.7 G) than vertical bar B-l vertical bar(mean) measured for the G-dwarfs (3.2 G) and the F-dwarfs (3.3 G). For the sample as a whole, vertical bar B-l vertical bar(mean) increases with rotation rate and decreases with age, and the upper envelope for vertical bar B-l vertical bar correlates well with the observed chromospheric emission. Stars with a chromospheric S-index greater than about 0.2 show a high magnetic field detection rate and so offer optimal targets for future studies. This survey constitutes the most extensive spectropolarimetric survey of cool stars undertaken to date, and suggests that it is feasible to pursue magnetic mapping of a wide range of moderately active solar-type stars to improve our understanding of their surface fields and dynamos.

Abstract We present the design of a stellar spectropolarimeter to measure the magnetic field of point sources. The polarization module interfaces the Boller and Chivens (B&Ch) intermediate-low resolution (R ~ 500 − 4000) spectrograph to the 2.1-m telescope of the San Pedro Martir National Astronomical Observatory in Mexico. The module uses a Savart plate to split the beam into two orthogonal states of polarization and a quarter (half) waveplate to measure circular (linear) polarization. The module is mounted to the telescope and it feeds the spectrograph through a set of four fibers, two for the polarized star images and two for the spectrograph calibration lamp. The instrument will be capable of measuring polarization in spectral lines to determine the longitudinal and transversal fields in magnetic stars.

We present optical broad-band (UBVRI) aperture polarimetry of 52 post-AGB stars, selected from De Ruyter et al. (2006) and the Torun Catalog, based on the shape of their SED and near-infrared excess. We find 10 (19%) of the stars in our sample to have high polarization (P > 5%), 30 (56%) intermediate/low polarization (1% < P < 5%) and 13 (25%) very low (or non–polarized) polarization (P < 1%). Our observations show clear evidence of asymmetric circumstellar envelopes or equatorial density enhancement around post-AGB stars, probably formed at the beginning of the AGB phase. Some stars exhibit wavelength-independent polarization suggesting scattered light by large dust grains or free electrons (Thomson scattering), while others show wavelength-dependent polarization originated from scattering by small dust grains (Rayleigh scattering). Finally, we conclude that highly polarized sources (P > 3%), show systematically [12] − [25] > 1.5, J − H > 0.5 and J − K > 0.5, clearly separated from the group of RV Tauri stars, which are found to have very low polarization (P < 3%).

AbstractWe present the preliminary results of the measurements of longitudinal magnetic field of the massive white dwarf 1658+441. This star have an hydrogen pure atmosphere (e.g. Dupuis & Chayer, 2003). We have observed the target in a total of 18 hrs during 3 consecutive nights in June 2010 and one more in May 2011. The data was acquired with a prototypical spectropolarimeter at the San Pedro Martir Telescope in Mexico. We have tested the magnetic field measurements with our instrument using the famous Babcock's star obtaining consistent results with previous studies. For our object of study, the WD 1658+441, we have measured variable intensities of the longitudinal magnetic field of Blong = 720 kG that oscillates with an amplitude of 130 kG.

We analyze the properties of 98 weak interplanetary shocks measured by the dual STEREO spacecraft over approximately 3 years during the past solar minimum. We study the occurrence of whistler waves associated with these shocks, which on average are high beta shocks (0.2 < β< 10). We have compared the waves properties upstream and downstream of the shocks. In the upstream region the waves are mainly circularly polarized, and in most of the cases (∼75%) they propagate almost parallel to the ambient magnetic field (<30°). In contrast, the propagation angle with respect to the shock normal varies in a broad range of values (20° to 90°), suggesting that they are not phase standing. We find that the whistler waves can extend up to 100,000 km in the upstream region but in most cases (88%) are contained in a distance within 30,000 km from the shock. This corresponds to a larger region with upstream whistlers associated with IP shocks than previously reported in the literature. The maximum amplitudes of the waves are observed next to the shock interface, and they decrease as the distance to the shock increases. In most cases the wave propagation direction becomes more aligned with the magnetic field as the distance to the shock increases. These two facts suggest that most of the waves in the upstream region are Landau damping as they move away from the shock. From the analysis we also conclude that it is likely that the generation mechanism of the upstream whistler waves is taking place at the shock interface. In the downstream region, the waves are irregularly polarized, and the fluctuations are very compressive; that is, the compressive component of the wave clearly dominates over the transverse one. The majority of waves in the downstream region (95%) propagate at oblique angles with respect to the ambient magnetic field (>60°). The wave propagation with respect to the shock‐normal direction has no preferred direction and varies similarly to the upstream case. It is possible that downstream fluctuations are generated by ion relaxation as suggested in previous hybrid simulation shocks.

[1] We investigate the characteristics of 9 interplanetary shocks associated with stream interaction regions observed by both STEREO-A and STEREO-B spacecraft during the years 2007–2008. Interplanetary shocks modify the plasma both upstream and downstream of the front shock. As they propagate, interplanetary shocks encounter solar wind with different characteristics (density, velocity) and different orientations of ambient magnetic field relative to the shock normal. Thus, it is interesting to compare dual observations of stream interaction shocks at the locations of the two spacecraft to determine the role of these parameters in controlling both the structure at the shock but also in the regions upstream and downstream from the shock. The range of shock normal angle (ΘBn) values observed by spacecraft covered the range from ∼20° to ∼81°. The largest difference in ΘBn for the same shock observed at two different longitudinal locations was ∼39°. The shock magnetosonic Mach numbers covered the range of ∼1.1 to ∼2.2, having a largest change for the same shock of ∼0.9. The jump in the field magnitude, i.e., the ratio of downstream magnetic field intensity to upstream magnetic field intensity (Bd/Bu), ranged from ∼1.1 to ∼2.25. The largest difference in the jump in field magnitude for the same shock at two different locations was ∼0.72. These variations with longitude of shock properties observed with the STEREO dual mission show the non-homogeneous character of the plasma in the heliosphere, and they need to be taken into account to understand in detail how these shocks modify the solar wind, and affect the acceleration processes of energetic particles in the solar wind.

We present the photometry and spectropolarimetry of the pre-main-sequence star HD 106506. A photometric rotational period of similar to 1.416 +/- 0.133 d has been derived using observations at Mount Kent Observatory (MKO). Spectropolarimetric data obtained with the 3.9-m Anglo-Australian Telescope (AAT) were used to derive spot occupancy and magnetic maps of the star through the technique of Zeeman Doppler imaging (ZDI). The resulting brightness maps indicate that HD 106506 displays photospheric spots at all latitudes including a predominant polar spot. Azimuthal and radial magnetic images of this star have been derived, and a significant azimuthal magnetic field is indicated, in line with other active young stars. A solar-like differential rotation law was incorporated into the imaging process. Using Stokes I information the equatorial rotation rate, (eq), was found to be 4.54 +/- 0.01 rad d-1, with a photospheric shear delta of 0.21+0.02(-0.03) rad d-1. This equates to an equatorial rotation period of similar to 1.39 +/- 0.01 d, with the equatorial region lapping the poles every similar to 30+5(-3) d. Using the magnetic features, the equatorial rotation rate, (eq), was found to be 4.51 +/- 0.01 rad d-1, with a photospheric shear delta of 0.24 +/- 0.03 rad d-1. This differential rotation is approximately four times that observed on the Sun.

Spectropolarimetric observations of the pre-main sequence early-G star HD 141943 were made at three observing epochs (2007, 2009 and 2010). The observations were made using the 3.9-m Anglo-Australian Telescope with the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor instrument. The brightness and surface magnetic field topologies (given in Paper I) were used to determine the star’s surface differential rotation and reconstruct the coronal magnetic field of the star. The coronal magnetic field at the three epochs shows on the largest scales that the field structure is dominated by the dipole component with possible evidence for the tilt of the dipole axis shifting between observations. We find very high levels of differential rotation on HD 141943 (∼8 times the solar value for the magnetic features and ∼5 times solar for the brightness features), similar to that evidenced by another young early-G star, HD 171488. These results indicate that a significant increase in the level of differential rotation occurs for young stars around a spectral type of early-G. We also find for the 2010 observations that there is a large difference in the differential rotation measured from the brightness and magnetic features, similar to that seen on early-K stars, but with the difference being much larger. We find only tentative evidence for temporal evolution in the differential rotation of HD 141943.

Spectroscopic and spectropolarimetric observations of the pre-main sequence early-G star HD 141943 were obtained at four observing epochs (in 2006, 2007, 2009 and 2010). The observations were undertaken at the 3.9-m Anglo-Australian Telescope using the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor instrument. Brightness and surface magnetic field topologies were reconstructed for the star using the technique of least-squares deconvolution to increase the signal-to-noise ratio of the data.The reconstructed brightness maps show that HD 141943 had a weak polar spot and a significant amount of low-latitude features, with little change in the latitude distribution of the spots over the 4 yr of observations. The surface magnetic field was reconstructed at three of the epochs from a high-order (l < 30) spherical harmonic expansion of the spectropolarimetric observations. The reconstructed magnetic topologies show that in 2007 and 2010 the surface magnetic field was reasonably balanced between poloidal and toroidal components. However, we find tentative evidence of a change in the poloidal/toroidal ratio in 2009 with the poloidal component becoming more dominant. At all epochs the radial magnetic field is predominantly non-axisymmetric while the azimuthal field is predominantly axisymmetric with a ring of positive azimuthal field around the pole similar to that seen on other active stars.