Characterizing maser polarization: effects of saturation, anisotropic pumping and hyperfine structure

The polarization of masers contains information on the magnetic field strength and direction of the regions they occur in. Many maser polarization observations have been performed over the last 30 years. However, versatile maser polarization models that can aide in the interpretation of these observations are not available. We aim to develop a program suite that can compute the polarization by a magnetic field of any non-paramagnetic maser specie at arbitrarily high maser saturation. Furthermore, we aim to investigate the polarization of masers by non-Zeeman polarizing effects. We aim to present a general interpretive structure for maser polarization observations. We expand existing maser polarization theories of non-paramagnetic molecules and incorporate these in a numerical modeling program suite. We present a modeling program that CHAracterizes Maser Polarization (CHAMP) that can examine the polarization of masers of arbitrarily high maser saturation and high angular momentum. Also, hyperfine multiplicity of the maser-transition can be incorporated. The user is able to investigate non-Zeeman polarizing mechanisms such as anisotropic pumping and polarized incident seed radiation. We present an analysis of the polarization of v = 1 SiO masers and the 22 GHz water maser. We comment on the underlying polarization mechanisms, and also investigate non-Zeeman effects. We identify the regimes where different polarizing mechanisms will be dominant and present the polarization characteristics of the SiO and water masers. From the results of our calculations, we identify markers to recognize alternative polarization mechanisms.Comment: 67 pages, 27 figures. Accepted to be published in A&

Magnetic fields of AGB and post-AGB stars

There is ample evidence for strong magnetic fields in the envelopes of (Post-)Asymptotic Giant Branch (AGB) stars as well as supergiant stars. The origin and role of these fields are still unclear. This paper updates the current status of magnetic field observations around AGB, post-AGB stars and describes their possible role during these stages of evolution. The discovery of magnetically aligned dust around a supergiant star is also highlighted. In our search for the origin of the magnetic fields, recent observations show the signatures of possible magnetic activity and rotation, indicating that the magnetic fields might be intrinsic to the AGB stars.Comment: 8 pages, 6 figures; invited review to appear in IAU Symposium 343, Why Galaxies Care About AGB Stars, (eds.) F. Kerschbaum, M. Groenewegen and H. Olofsson. Updates and partly reproduces previous reviews (Vlemmings 2014, 2018

Detection of thermal radio emission from a single coronal giant

We report the detection of thermal continuum radio emission from the K0 III coronal giant Pollux ($\beta$ Gem) with the Karl G. Jansky Very Large Array (VLA). The star was detected at 21 and 9 GHz with flux density values of $150\pm21$ and $43\pm8\,\mu$Jy, respectively. We also place a $3\sigma_{\mathrm{rms}}$ upper limit of $23\,\mu$Jy for the flux density at 3 GHz. We find the stellar disk-averaged brightness temperatures to be approximately 9500, 15000, and $<71000\,$K, at 21, 9, and 3 GHz, respectively, which are consistent with the values of the quiet Sun. The emission is most likely dominated by optically thick thermal emission from an upper chromosphere at 21 and 9 GHz. We discuss other possible additional sources of emission at all frequencies and show that there may also be a small contribution from gyroresonance emission above active regions, coronal free-free emission and free-free emission from an optically thin stellar wind, particularly at the lower frequencies. We constrain the maximum mass-loss rate from Pollux to be less than $3.7\times 10^{-11}\,M_{\odot}$ yr$^{-1}$ (assuming a wind terminal velocity of 215 km s$^{-1}$), which is about an order of magnitude smaller than previous constraints for coronal giants and is in agreement with existing predictions for the mass-loss rate of Pollux. These are the first detections of thermal radio emission from a single (i.e., non-binary) coronal giant and demonstrate that low activity coronal giants like Pollux have atmospheres at radio frequencies akin to the quiet Sun

Collisional polarization of molecular ions: a signpost of ambipolar diffusion

Magnetic fields play a role in the dynamics of many astrophysical processes, but they are hard to detect. In a partially ionized plasma, a magnetic field works directly on the ionized medium but not on the neutral medium, which gives rise to a velocity drift between them: ambipolar diffusion. This process is suggested to be important in the process of star formation, but has never been directly observed. We introduce a method that could be used to detect ambipolar diffusion and the magnetic field that gives rise to it, where we exploit the velocity drift between the charged and neutral medium. By using a representative classical model of the collision dynamics, we show that molecular ions partially align themselves when a velocity drift is present between the molecular ion and its main collision partner H2. We demonstrate that ambipolar diffusion potently aligns molecular ions in regions denser than their critical density, which subsequently leads to partially polarized emission from these species. We include a model for HCO+ and show that collisional polarization could be detectable for the ambipolar drifts predicted by numerical simulations of the inner protostellar disk regions. The polarization vectors are aligned perpendicular to the magnetic field direction projected on the plane of the sky.Comment: 5 pages, 2 figures. Accepted and published in A&

Magnetic fields around evolved stars

There has long been evidence of magnetic fields in the extended envelopes of asymptotic giant branch (AGB) and red supergiant (RSG) stars. These stars are important contributors to the enrichment of the interstellar medium by dust and heavy elements. Magnetic fields might play a role in the mass loss process responsible. Additionally, magnetic fields, typically in combination with binary companions, have often be suggested to be involved in shaping strongly a-spherical planetary nebulae (PNe). New telescopes and instruments are increasing our knowledge about magnetic fields around these evolved, mass losing, stars and their descendants

A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars

We have performed a comprehensive study of the UV emission detected from AGB stars by the Galaxy Evolution Explorer (GALEX). Of the 468 AGB stars in our sample, 316 were observed by GALEX. In the NUV bandpass ($\lambda_{\rm eff} \sim 2310$z A), 179 AGB stars were detected and 137 were not detected. Only 38 AGB stars were detected in the FUV bandpass ($\lambda_{\rm eff} \sim1528$ A). We find that NUV emission is correlated with optical to near infrared emission leading to higher detection fractions among the brightest, hence closest, AGB stars. Comparing the AGB time-variable visible phased light curves to corresponding GALEX NUV phased light curves we find evidence that for some AGB stars the NUV emission varies in phase with the visible light curves. We also find evidence that the NUV emission, and possibly, the FUV emission are anti-correlated with the circumstellar envelope density. These results suggest that the origin of the GALEX-detected UV emission is an inherent characteristic of the AGB stars that can most likely be traced to a combination of photospheric and chromospheric emission. In most cases, UV detections of AGB stars are not likely to be indicative of the presence of binary companions.Comment: Accepted by ApJ; go spurs go

Photodissociation of CO in the outflow of evolved stars

Context. Ultraviolet (UV) photodissociation of carbon monoxide (CO) controls the abundances and distribution of CO and its photodissociation products. This significantly influences the gas-phase chemistry in the circumstellar material around evolved stars. A better understanding of CO photodissociation in outflows also provides a more precise estimate of mass-loss rates. Aims. We aim to update the CO photodissociation rate in an expanding spherical envelope assuming that the interstellar radiation field (ISRF) photons penetrate through the envelope. This will allow us to precisely estimate the CO abundance distributions in circumstellar envelope around evolved stars. Methods. We used the most recent CO spectroscopic data to precisely calculate the depth dependency of the photodissociation rate of each CO dissociating line. We calculated the CO self- and mutual-shielding functions in an expanding envelope. We investigated the dependence of the CO profile on the five fundamental parameters mass-loss rate, the expansion velocity, the CO initial abundance, the CO excitation temperature, and the strength of the ISRF. Results. Our derived CO envelope size is smaller than the commonly used radius derived by Mamon et al. 1988. The difference between results varies from 1% to 39% and depends on the H2 and CO densities of the envelope. We list two fitting parameters for a large grid of models to estimate the CO abundance distribution. We demonstrate that the CO envelope size can differ between outflows with the same effective content of CO, but different CO abundance, mass-loss rate, and the expansion velocity as a consequence of differing amounts of shielding by H2 and CO. Conclusions. Our study is based on a large grid of models employing an updated treatment of the CO photodissociation, and in it we find that the abundance of CO close to the star and the outflow density both can have a significant effect on the size of the molecular envelope. We also demonstrate that modest variations in the ISRF can cause measurable differences in the envelope extent

Polarization of thermal molecular lines in the envelope of IK Tauri

Molecular line polarization is a unique source of information about the magnetic fields and anisotropies in the circumstellar envelopes of evolved stars. Here we present the first detection of thermal CO(J = 2 -> 1) and SiO(J = 5 -> 4, nu = 0) polarization, in the envelope of the asymptotic giant branch star IK Tau. The observed polarization direction does not match predictions for circumstellar envelope polarization induced only by an anisotropic radiation field. Assuming that the polarization is purely due to the Goldreich-Kylafis effect, the linear polarization direction is defined by the magnetic field as even the small Zeeman splitting of CO and SiO dominates the molecular collisional and spontaneous emission rates. The polarization was mapped using the Submillimeter Array (SMA) and is predominantly north-south. There is close agreement between the CO and SiO observations, even though the CO polarization arises in the circumstellar envelope at similar to 800 AU and the SiO polarization at less than or similar to 250 AU. If the polarization indeed traces the magnetic field, we can thus conclude that it maintains a large-scale structure throughout the circumstellar envelope. We propose that the magnetic field, oriented either east-west or north-south is responsible for the east-west elongation of the CO distribution and asymmetries in the dust envelope. In the future, the Atacama Large Millimeter/submillimeter Array will be able to map the magnetic field using CO polarization for a large number of evolved stars