TX Cam Research Articles

Maser polarization simulation in an evolving star: effect of magnetic field on SiO maser in the circumstellar envelope

AbstractMaser polarization changes during a pulsation in the CSE of an AGB star are related in a complicated way to the magnetic field structure. 43 GHz SiO maser transitions are useful for polarization study because of their relatively simple Zeeman splitting structure and their location. This work uses 3D maser simulation to investigate the effect of the magnetic field on maser polarization with different directions. The results show that linear polarization depends on the magnetic direction while circular polarization is less significant. The EVPA changes through π/2 at an angle of around 50 degrees, approximately the Van Vleck angle. The EVPA rotation result from 3D maser simulation is consistent with results from 1D simulations, and may explain the 90 degree change of the EVPA within a single cloud in the observational cases of TX Cam and R Cas.

Constraining Theories of Polarized SiO Maser Transport: Multi-epoch Analysis of a π/2 Electric Vector Rotation Feature

Abstract The detailed polarization mechanisms of SiO masers originating from the near-circumstellar environment of asymptotic giant branch stars are not yet definitively known. Prevailing theories are broadly classified as either Zeeman or non-Zeeman in origin, the latter including effects such as anisotropic pumping or anisotropic resonant scattering. The predicted behavior of the linear and circular polarization fractions and the electric vector position angle (EVPA) vary by theory. In particular, individual maser features that exhibit a rotation in linear polarization of ∼π/2 as a function of frequency over their extent can be utilized as a test of several maser polarization transport theories. In this paper, we analyze one SiO (ν = 1, J = 1 − 0) maser feature toward the Mira variable, TX Cam, that exhibits this internal polarization rotation and persists across five epochs (spanning ∼3 months). We compare our results to the predictions by several maser polarization theories and find that the linear polarization across the feature is consistent with a geometric effect for a saturated maser originating when the angle between the projected magnetic field and the line of sight (θ) crosses the Van Vleck angle θ F ∼ 55°. However, the EVPA exhibits a smooth rotation across the spatial extent of the feature rather than the expected abrupt π/2 flip. We discuss possible explanations for this discrepancy and alternative theories. Circular polarization across the feature is also analyzed, and it is most accurately described by Zeeman effects giving rise to a circular polarization fraction of the form .

New Circumstellar Sources of PO and PN: The Increasing Role of Phosphorus Chemistry in Oxygen-rich Stars

Abstract PO and PN have been newly identified in several oxygen-rich circumstellar envelopes, using the Submillimeter Telescope of the Arizona Radio Observatory. The J = 5 → 4 and J = 6 → 5 transitions of PN near 235 and 282 GHz, and the lambda doublets originating in the J = 5.5 → 4.5 and J = 6.5 → 5.5 lines of PO at 240 and 284 GHz, have been detected toward the shells of asymptotic giant branch (AGB) stars TX Cam and R Cas. A similar set of lines has been observed toward the supergiant NML Cyg, and new transitions of these two molecules were also measured toward the AGB star IK Tau. Along with the previous data from VY Canis Majoris (VY CMa), these spectral lines were analyzed using the non-local thermodynamic equilibrium (non-LTE) circumstellar modeling code, ESCAPADE. For the AGB stars, peak abundances found for PN and PO were f ∼ (1–2) × 10−8 and (0.5–1) × 10−7, respectively, while those for the supergiants were f(PN) ∼ (0.3–0.7) × 10−8 and f(PO) ∼ (5–7) × 10−8. PN was well modeled with a spherical radial distribution, suggesting formation near the stellar photosphere, perhaps enhanced by shocks. PO was best reproduced by a shell model, indicating a photochemical origin, except for VY CMa. Overall, the abundance of PO is a factor of 5–20 greater than that of PN. This study suggests that phosphorus-bearing molecules are common in O-rich envelopes, and that a significant amount of phosphorus (>20%) remains in the gas phase.

Constraining Theories of SiO Maser Polarization: Analysis of a π/2 EVPA Change

AbstractThe full theory of polarized SiO maser emission from the near-circumstellar environment of Asymptotic Giant Branch stars has been the subject of debate, with theories ranging from classical Zeeman origins to predominantly non-Zeeman anisotropic excitation or propagation effects. Features with an internal electric vector position angle (EVPA) rotation of ∼π/2 offer unique constraints on theoretical models. In this work, results are presented for one such feature that persisted across five epochs of SiO ν = 1, J = 1 − 0 VLBA observations of TX Cam. We examine the fit to the predicted dependence of linear polarization and EVPA on angle (θ) between the line of sight and the magnetic field against theoretical models. We also present results on the dependence of mc on θ and their theoretical implications. Finally, we discuss potential causes of the observed differences, and continuing work.

Search for aluminium monoxide in the winds of oxygen-rich AGB stars

Aluminium monoxide, AlO, is likely efficiently depleted from the gas around oxygen-rich evolved stars to form alumina clusters and dust seeds. Its presence in the extended atmospheres of evolved stars has been derived from optical spectroscopy. More recently, AlO gas was also detected at long wavelengths around the supergiant VY CMa and the oxygen-rich asymptotic giant branch (AGB) star o Cet (Mira A). In search of AlO, we mined data obtained with APEX, the IRAM 30m telescope, Herschel/HIFI, SMA, and ALMA, which were primarily aimed at studying other molecular species. We report here on observations of AlO towards a sample of eight oxygen-rich AGB stars in different rotational transitions, up to seven for some stars. We present definite detections of one rotational transition of AlO for o Cet and R Aqr, and tentative detections of one transition for R Dor and o Cet, and two for IK Tau and W Hya. The presented spectra of WX Psc, R Cas, and TX Cam show no signature of AlO. For o Cet, R Aqr, and IK Tau, we find that the AlO(N=9-8) emission likely traces the inner parts of the wind, out to only a few tens of AU, where the gas has not yet reached its terminal velocity. The conclusive detections of AlO emission in the case of o Cet and R Aqr confirm the presence of AlO gas in outflows of AGB stars. The tentative detections further support this. Since most of the observations presented in this study were obtained with stronger emission from other species than AlO in mind, observations with higher sensitivity in combination with high angular resolution will improve our understanding of the presence and behaviour of AlO. From the current data sets we cannot firmly conclude whether there is a direct correlation between the wind properties and the detection rate of AlO emission. We hope that this study can serve as a stimulus to perform sample studies in search of AlO in oxygen-rich outflows.

NEAR-INFRARED OBSERVATIONS OF SiO MASER-EMITTING ASYMPTOTIC GIANT BRANCH (AGB) STARS

ABSTRACT Near-infrared (NIR) monitoring observations of asymptotic giant branch stars exciting bright SiO masers have been made with the 1 m telescope of Kagoshima University. In order to investigate the properties of these stars and their envelopes, we combined our NIR photometric data with mid- and far-infrared flux data obtained by the IRAS satellite, SiO maser flux data provided by the Nobeyama Radio Observatory, visual magnitude data provided by the AAVSO, and the reported data on the expansion velocities of the circumstellar envelopes. The absolute magnitudes at the K-band and the distances are estimated using the period–luminosity relation of Mira variables determined by Feast et al. Then, mass-loss rates and isotropic luminosities of an SiO maser are estimated. The mass-loss rates range from approximately 10−8 M ⊙ yr − 1 ?> to over 10−5 M ⊙ yr − 1 ?> . We found that the NIR pulsation amplitudes are correlated with the pulsation periods and the observed wavelengths. We also found correlations of the isotropic luminosities of SiO masers with the mass-loss rates and absolute magnitudes at the K-band. These results will help us to understand the pumping mechanism of SiO masers. We measured, for the first time, the periods and/or NIR magnitudes of TX Cam, BW Cam, IRAS 06297+4045, IRAS 18387–0423, and RT Cep.

FIRST DETECTION OF 22 GHZ H2O MASERS IN TX CAMELOPARDALIS

Simultaneous time monitoring observations of <TEX>$H_2O$</TEX> <TEX>$6_{16}-5_{23}$</TEX>, SiO J = 1-0, 2-1, 3-2, and <TEX>$^{29}SiO$</TEX> <TEX>${\upsilon}=0$</TEX>, J = 1-0 lines are carried out in the direction of the Mira variable star TX Cam with the Korean VLBI Network single dish radio telescopes. For the first time, the <TEX>$H_2O$</TEX> maser emission from TX Cam is detected near the stellar velocity at five epochs from April 10, 2013 (<TEX>${\phi}=3.13$</TEX>) to June 4, 2014 (<TEX>${\phi}=3.89$</TEX>) including minimum optical phases. The intensities of <TEX>$H_2O$</TEX> masers are very weak compared to SiO masers. The variation of peak antenna temperature ratios among SiO <TEX>${\upsilon}=1$</TEX>, J = 1-0, J = 2-1, and J = 3-2 masers is investigated according to their phases. The shift of peak velocities of <TEX>$H_2O$</TEX> and SiO masers with respect to the stellar velocity is also investigated according to observed optical phases. The <TEX>$H_2O$</TEX> maser emission occurs around the stellar velocity during our monitoring interval. On the other hand, the peak velocities of SiO masers show a spread compared to the stellar velocity. The peak velocities of SiO J = 2-1, and J = 3-2 masers show a smaller spread with respect to the stellar velocity than those of SiO J = 1-0 masers. These simultaneous observations of multi-frequencies will provide a good constraint for maser pumping models and a good probe for investigating the stellar atmosphere and envelope according to their different excitation conditions.

A long-term VLBA monitoring campaign of the v = 1, J = 1 →0 SiO masers towards TX Cam – I. Morphology and shock waves

We present the latest and final version of the movie of the SiO masers toward the Mira variable TX Cam. The new version consists of 112 frames (78 successfully reduced epochs) with data covering almost three complete stellar cycles between 24 May 1997 to 25 January 2002, observed with the VLBA. In this paper we examine the global morphology, kinematics and variability of the masering zone. The morphology of the emission is confined in a structure that usually resembles a ring or an ellipse, with occasional deviations due to localised phenomena. The ring appears to be contracting and expanding, although for the first cycle contraction is not observed. The width and outer boundary of the masering zone follow the stellar pulsation. Our data seem to be consistent with a shock being created once per stellar cycle at maximum that propagates with a velocity of ~7 km/s. The difference in velocities along different axes strongly suggests that the outflow in TX Cam is bipolar. The contribution of projection is examined and our results are compared with the latest theoretical model.

The final 112-frame movie of the 43 GHz SiO masers around the Mira Variable TX Cam

AbstractThe proximity of the SiO masers to the star, makes them a powerful tool for studying the properties of the extended stellar atmosphere. This project is a long monitoring campaign of the 43 GHz SiO masers (v=1 J=1→0) around the Mira Variable TX Cam. The target source was observed with the Very Long Baseline Array (VLBA) from the 24th of May 1997 to the 25th of January 2002 in bi-weekly or monthly intervals, covering 3.06 stellar cycles. The time-span and frequency of observations helped us examine the long and short-term properties of the emission and study the dynamics of the gas, the existence of shock waves and their contribution to the morphology and kinematics. Maps from each epoch were concatenated into a 112-frame movie showing the evolution of the emission around TX Cam.

CIRCUMSTELLAR ION-MOLECULE CHEMISTRY: OBSERVATIONS OF HCO+IN THE ENVELOPES OF O-RICH STARS AND IRC+10216

Millimeter-wave observations of HCO+ have been conducted toward oxygen-rich circumstellar envelopes, as well as IRC+10216, using the facilities of the Arizona Radio Observatory (ARO). The J = 1 → 0 and 2 → 1 transitions of this molecule were measured with the ARO 12 m antenna, while the J = 3 → 2 and 4 → 3 lines were observed using the ARO Sub-Millimeter Telescope. HCO+ was detected toward the supergiant NML Cyg and the asymptotic giant branch (AGB) stars IK Tau, TX Cam, and W Hya in at least two transitions. The J = 2 → 1 and 3 → 2 lines of this ion were also detected toward IRC+10216, confirming the identification of HCO+ in this object. The line profiles measured for HCO+ toward NML Cyg consist of red- and blueshifted components, suggesting a non-spherical shell. Based on a radiative transfer analysis, the abundances in the O-rich envelopes were f(HCO+/H2) ∼ 0.15–1.3 × 10−7, with the AGB stars typically exhibiting the higher values. In IRC+10216, f(HCO+/H2) ∼ 4.1 × 10−9, lower than the O-rich counterparts. The abundances of HCO+ were also found to peak at considerable distances from the star, indicative of an outer envelope molecule. Comparison with H2O and CO, the main precursor species, suggests that HCO+ is more prevalent in envelopes that have substantial water, but CO also plays a role in its formation. The abundance of HCO+ appears to increase inversely with mass-loss rate, provided the rate is >10−6 M☉ yr−1. The common appearance of HCO+ in circumstellar gas indicates that, at some level, ion–molecule reactions influence the chemistry of evolved stellar envelopes.

Mapping the12CO J = 1−0 andJ = 2−1 emission in AGB and early post-AGB circumstellar envelopes

We present COSAS (CO Survey of late AGB Stars), a project to map and analyze the 12CO J = 1−0 and J = 2−1 line emission in a representative sample of circumstellar envelopes around AGB and post-AGB stars. The survey was undertaken with the aim of investigating small- and large-scale morphological and kinematical properties of the molecular environment surrounding stars in the late AGB and early post-AGB phases. For this, COSAS combines the high sensitivity and spatial resolving power of the IRAM Plateau de Bure interferometer with the better capability of the IRAM 30 m telescope to map extended emission. The global sample encompasses 45 stars selected to span a range in chemical type, variability type, evolutionary state, and initial mass. COSAS provides means to quantify variations in the mass-loss rates, assess morphological and kinematical features, and to investigate the appearance of fast aspherical winds in the early post-AGB phase. This paper, which is the first of a series of COSAS papers, presents the results from the analyses of a first sample of 16 selected sources. The envelopes around late AGB stars are found to be mostly spherical, often mingled with features such as concentric arcs (R Cas and TX Cam), a broken spiral density pattern (TX Cam), molecular patches testifying to aspherical mass-loss (WX Psc, IK Tau, V Cyg, and S Cep), and also with well-defined axisymmetric morphologies and kinematical patterns (X Her and RX Boo). The sources span a wide range of angular sizes, from relatively compact (CRL 2362, OH 104.9+2.4 and CRL 2477) to very large (χ Cyg and TX Cam) envelopes, sometimes partially obscured by self-absorption features, which particularly for IK Tau and χ Cyg testifies to the emergence of aspherical winds in the innermost circumstellar regions. Strong axial structures with more or less complex morphologies are detected in four early post-AGB stars (IRAS 20028+3910, IRAS 23321+6545, IRAS 19475+3119 and IRAS 21282+5050) of the sub-sample.

Kinematics of the v=1, J=1→0 SiO masers at 43 GHz towards TX Cam - a new 73-frame movie

We present a new version of the movie of the SiO masers around the Mira variable TX Cam. It consists of 73 frames (60 observed epochs) with data covering almost two complete stellar cycles between 1997 May 24 and 2000 May 21. Observations were carried out using the Very Long Baseline Array in biweekly and later at monthly intervals. The purpose of this paper is to discuss the global properties of the emission in the extended atmosphere of the star as well as the kinematics of individual components. The existence of shock waves and their contribution to the morphology and the kinematics of the gas is also examined and compared with theoretical models. Contrary to the dominant expansion observed in the first cycle, both expansion and contraction are observed in the second cycle. The maser variability is correlated with the pulsation of the star but not with the optical intensity. Most of the individual components identified follow the behaviour of the ring, but there are exceptions with more complex three-dimensional trajectories.

MULTI-EPOCH IMAGING POLARIMETRY OF THE SiO MASERS IN THE EXTENDED ATMOSPHERE OF THE MIRA VARIABLE TX CAM

We present a time series of synoptic images of the linearly-polarized v=1, J=1-0 SiO maser emission toward the Mira variable, TX Cam. These data comprise 43 individual epochs at an approximate biweekly sampling over an optical pulsation phase range of 0.68 to 1.82. The images have an angular resolution of ~500 microarcsec and were obtained using the Very Long Baseline Array (VLBA), operating in the 43 GHz band in spectral-line, polarization mode. We have previously published the total intensity time series for this pulsation phase range; this paper serves to present the linearly-polarized image sequence and an associated animation representing the evolution of the linear polarization morphology over time. We find a predominantly tangential polarization morphology, a high degree of persistence in linear polarization properties over individual component lifetimes, and stronger linear polarization in the inner projected shell than at larger projected shell radii. We present an initial polarization proper motion analysis examining the possible dynamical influence of magnetic fields in component motions in the extended atmospheres of late-type, evolved stars.

CARBON CHEMISTRY IN THE ENVELOPE OF VY CANIS MAJORIS: IMPLICATIONS FOR OXYGEN-RICH EVOLVED STARS

Observations of the carbon-bearing molecules CO, HCN, CS, HNC, CN, and HCO+ have been conducted toward the circumstellar envelope of the oxygen-rich red supergiant star, VY Canis Majoris (VY CMa), using the Arizona Radio Observatory (ARO). CO and HCN were also observed toward the O-rich shells of NML Cyg, TX Cam, IK Tau, and W Hya. Rotational transitions of these species at 1 mm, 0.8 mm, and 0.4 mm were measured with the ARO Submillimeter Telescope, including the J = 6 → 5 line of CO at 691 GHz toward TX Cam and W Hya. The ARO 12 m was used for 2 mm and 3 mm observations. Four transitions were observed for HCO+ in VY CMa, the first definitive identification of this ion in a circumstellar envelope. Molecular line profiles from VY CMa are complex, indicating three separate outflows: a roughly spherical flow and separate red- and blueshifted winds, as suggested by earlier observations. Spectra from the other sources appear to trace a single outflow component. The line data were modeled with a radiative transfer code to establish molecular abundances relative to H2 and source distributions. Abundances for CO derived for these objects vary over an order of magnitude, f ∼ 0.4–5 × 10−4, with the lower values corresponding to the supergiants. For HCN, a similar range in abundance is found (f ∼ 0.9–9 × 10−6), with no obvious dependence on the mass-loss rate. In VY CMa, HCO+ is present in all three outflows with f ∼ 0.4–1.6 × 10−8 and a spatial extent similar to that of CO. HNC is found only in the red- and blueshifted components with [HCN]/[HNC] ∼ 150–190, while [CN]/[HCN] ∼ 0.01 in the spherical flow. All three velocity components are traced in CS, which has a confined spatial distribution and f ∼ 2–6 × 10−7. These observations suggest that carbon-bearing molecules in O-rich shells are produced by a combination of photospheric shocks and photochemistry. Shocks may play a more prominent role in the supergiants because of their macroturbulent velocities.

Variable Asymmetry of the Circumstellar Envelope in IK Tauri Traced by SiO Maser Emission

Abstract We observed the SiO ($v=1$ and $v=2$, $J=$ 1-0) maser emissions toward the Mira variable IK Tauri (IK Tau, NML Tau, IRC$+$10050) over nine epochs from 2003 November to 2005 April using the VLBI Exploration of Radio Astrometry (VERA) telescope array. We found that the SiO maser distributions around IK Tau are highly variable, depending on the stellar pulsation phase. The velocity structures were complex and also time-variable, and are inconsistent with a simply rotating shell that was suggested by previous observations. Based on fittings of the SiO maser distributions to elliptical ring models, we found that the ring size seems to vary in correlation with the pulsation phase, as previously found for TX Cam. Comparisons of the $v=1$ and $v=2$ maser distributions reveal that 42-50% of the maser pairs have a positional offset less than 0.5mas, suggesting that the maser pumping in these spots is likely to be dominated by line overlapping or collisional pumping, rather than pure radiative pumping. In order to model the velocity structure of the spoke-like features observed in IK Tau, we propose a ballistic-orbit model, in which a gas clump ejected from the stellar surface moves radially outward with a gravitational deceleration, and argue that the stellar mass can be deduced from that model.

First Detection of the SiO (v= 3,J= 2–1) Maser Emission from χ Cygni

From the simultaneous observations of SiO v = 3, J = 2-1 and J = 3-2 masers, SiO v = 3, J = 2-1 maser emission was detected for the first time toward the S-type Mira variable χ Cyg. The line was a single spike and was redshifted with respect to the stellar velocity. The SiO v = 3, J = 3-2 maser emission was not detected from this star. However, for the O-rich Mira variable TX Cam, SiO v = 3, J = 3-2 maser emission was detected, while v = 3, J = 2-1 maser was not detected. Two possible line overlaps were adopted as an explanation of these observational results.

Time Monitoring Observations of SiO J = 2–1 and J = 3–2 Maser Emission toward Late‐Type Stars

We present the results of simultaneous time monitoring observations of SiO J = 2-1 and J = 3-2 maser emission for 10 late-type stars (8 Mira variables, 1 OH/IR star, and 1 supergiant) with the 14 m radio telescope at Taeduk Radio Astronomy Observatory from 1999 January to 2001 February. The SiO v = 1, J = 2-1 and J = 3-2 maser emission was detected at almost all observational epochs. The SiO v = 2, J = 2-1 maser was detected from 4 late-type stars (VY CMa, R Cas, χ Cyg, R Leo) and the v = 2, J = 3-2 maser was detected from 7 stars (R Aqr, TX Cam, R Cas, χ Cyg, W Hya, R Leo, IK Tau). The v = 3, J = 2-1 and J = 3-2 masers were also detected from χ Cyg and TX Cam, respectively. Based on these observational data, line profile and peak velocity variations with respect to stellar velocity, antenna temperatures, and their ratio variations as a function of optical phase of central star were investigated. As main results, the line profile and the peak velocity variation of the v = 1, J = 3-2 maser with pulsation phase was found to differ from the v = 1, J = 2-1 transition. Similarly, the J = 2-1 and J = 3-2 transitions also differ between rovibrational transitions at a given pulsation phase. However, it is difficult to find significant correlations between the peak velocity variation relative to the stellar velocity of either the J = 3-2 or J = 2-1 transitions over pulsation phase, due to limited time sampling in these data. The peak and integrated antenna temperature (PT and IT) ratios among rotational ladders and vibrational states are investigated. These ratios between rotational ladders of the v = 1, J = 2-1, and J = 3-2 masers are averaged to be the peak antenna temperature ratio, PT(v = 1, J = 3-2)/PT(v = 1, J = 2-1) ≈ 0.29, and the integrated antenna temperature ratio, IT(v = 1, J = 3-2)/IT(v = 1, J = 2-1) ≈ 0.21, respectively. In the v = 2 state, these ratios are PT(v = 2, J = 3-2)/PT(v = 2, J = 2-1) ≈ 7.94 and IT(v = 2, J = 3-2)/IT(v = 2, J = 2-1) ≈ 8.50, respectively. The peak and integrated antenna temperature ratios between vibrational states are also averaged to be PT(v = 2, J = 3-2)/PT(v = 1, J = 3-2) ≈ 1.29, IT(v = 2, J = 3-2)/IT(v = 1, J = 3-2) ≈ 1.02, PT(v = 2, J = 2-1)/PT(v = 1, J = 2-1) ≈ 0.06, and IT(v = 2, J = 2-1)/IT(v = 1, J = 2-1) ≈ 0.05, respectively. These intensity ratios for the v = 2, J = 2-1 and v = 2, J = 3-2 masers suggest that line overlaps operating in the v = 2, J = 2-1 transition do not similarly affect the v = 2, J = 3-2 transition.

No Methane Here. The HCN Puzzle: Searching for CH3OH and C2H in Oxygen-rich Stars

I present results from a sensitive imaging search for CH3OH and C2H in two oxygen-rich stars (IK Taurus and TX Camelopardalis) carried out with the Owens Valley Millimeter Array. Neither CH3OH nor C2H are detected in these stars, in agreement with single-dish observations by Charnley and Latter. The observations place upper limits on the fractional abundance of C2H of 9.7 × 10-9 in IK Tau and 3.2 × 10-8 in TX Cam and also place upper limits on the fractional abundance of CH3OH of 4.5 × 10-9 in IK Tau and 1.1 × 10-8 in TX Cam. A chemical model developed by Charnley and coworkers explaining the presence of HCN in these objects is clearly ruled out by these observations. The HCN distribution in these oxygen-rich stars has been mapped with a resolution of ≈3''. The HCN emission is marginally resolved, centrally concentrated, and does not appear to be well fitted by a thin-shell model. Continuum fluxes for the sources have been measured. At 96.7 GHz, the observed continuum fluxes were 6.1 ± 0.4 mJy for IK Tau and 3.7 ± 0.4 mJy for TX Cam. The continuum emission is unresolved at the ≈3'' level, as expected from a model for the emission mechanism proposed by Reid and Menten.

SiO masers in TX Cam

SiO masers in TX Cam

Dichroic Masers Due to Radiation Anisotropy and the Influence of the Hanle Effect on the Circumstellar SiO Polarization

The theory of the generation and transfer of polarized radiation, mainly developed for interpreting solar spectropolarimetric observations, allows us to reconsider, in a more rigorous and elegant way, a physical mechanism that has been suggested some years ago to interpret the high degree of polarization often observed in astronomical masers. This mechanism, for which the name of "dichroic maser" is proposed, can operate when a low-density molecular cloud is illuminated by an anisotropic source of radiation (e.g., a nearby star). Here we investigate completely unsaturated masers and show that selective stimulated emission processes are capable of producing highly polarized maser radiation in a nonmagnetic environment. The polarization of the maser radiation is linear and is directed tangentially to a ring equidistant to the central star. We show that the Hanle effect due to the presence of a magnetic field can produce a rotation (from the tangential direction) of the polarization by more that 45° for some selected combinations of the strength, inclination, and azimuth of the magnetic field vector. However, these very same conditions produce a drastic inhibition of the maser effect. The rotations of about 90° observed in SiO masers in the evolved stars TX Cam by Kemball & Diamond and IRC +10011 by Desmurs and coworkers may then be explained by a local modification of the anisotropy of the radiation field, being transformed from mainly radial to mainly tangential.