Abstract:Context. Once ALMA full polarization capabilities are offered, it will become possible to perform detailed studies of polarized maser emission towards star-forming regions and late-type stars, such as (post-) asymptotic giant branch stars and young planetary nebulae. To derive the magnetic field orientation from maser linear polarization, a number of conditions involving the rate of stimulated emission R, the decay rate of the molecular state Γ, and the Zeeman frequency gΩ need to be satisfied. Aims. The goal … Show more
“…Vlemmings et al 2011) and requires strong anisotropic pumping of the maser transition (Nedoluha & Watson 1994). The low polarization fraction of the H 2 O maser however, fits with the expectation for a non-saturated maser (Pérez-Sánchez & Vlemmings 2013). Dedicated observations over a large range of parallactic angles will allow us to determine both velocity resolved polarization fraction and linear polarization direction, providing constraints on both the circumstellar magnetic field as well as the anisotropic radiation field.…”
Aims. The aim is to investigate the use of 183 GHz H 2 O masers for characterization of the physical conditions and mass loss process in the circumstellar envelopes of evolved stars. Methods. We used APEX SEPIA Band 5 (an ALMA Band 5 receiver on the APEX telescope) to observe the 183 GHz H 2 O line towards two Red Supergiant (RSG) and three Asymptotic Giant Branch (AGB) stars. Simultaneously, we observed the J=4−3 line for 28 SiO v=0, 1, 2 and 3, and for 29 SiO v=0 and 1. We compared the results with simulations and radiative transfer models for H 2 O and SiO, and examined data for the individual linear orthogonal polarizations. Results. We detected the 183 GHz H 2 O line towards all the stars with peak flux densities >100 Jy, including a new detection from VY CMa. Towards all five targets, the water line had indications of being due to maser emission and had higher peak flux densities than for the SiO lines. The SiO lines appear to originate from both thermal and maser processes. Comparison with simulations and models indicate that 183 GHz maser emission is likely to extend to greater radii in the circumstellar envelopes than SiO maser emission and to similar or greater radii than water masers at 22, 321 and 325 GHz. We speculate that a prominent blue-shifted feature in the W Hya 183 GHz spectrum is amplifying the stellar continuum, and is located at a similar distance from the star as mainline OH maser emission. We note that the coupling of an SiO maser model to a hydrodynamical pulsating model of an AGB star yields qualitatively similar simulated results to the observations. From a comparison of the individual polarizations, we find that the SiO maser linear polarization fraction of several features exceeds the maximum fraction allowed under standard maser assumptions and requires strong anisotropic pumping of the maser transition and strongly saturated maser emission. The low polarization fraction of the H 2 O maser however, fits with the expectation for a non-saturated maser. Conclusions. 183 GHz H 2 O masers can provide strong probes of the mass loss process of evolved stars. Higher angular resolution observations of this line using ALMA Band 5 will enable detailed investigation of the emission location in circumstellar envelopes and can also provide information on magnetic field strength and structure.
“…Vlemmings et al 2011) and requires strong anisotropic pumping of the maser transition (Nedoluha & Watson 1994). The low polarization fraction of the H 2 O maser however, fits with the expectation for a non-saturated maser (Pérez-Sánchez & Vlemmings 2013). Dedicated observations over a large range of parallactic angles will allow us to determine both velocity resolved polarization fraction and linear polarization direction, providing constraints on both the circumstellar magnetic field as well as the anisotropic radiation field.…”
Aims. The aim is to investigate the use of 183 GHz H 2 O masers for characterization of the physical conditions and mass loss process in the circumstellar envelopes of evolved stars. Methods. We used APEX SEPIA Band 5 (an ALMA Band 5 receiver on the APEX telescope) to observe the 183 GHz H 2 O line towards two Red Supergiant (RSG) and three Asymptotic Giant Branch (AGB) stars. Simultaneously, we observed the J=4−3 line for 28 SiO v=0, 1, 2 and 3, and for 29 SiO v=0 and 1. We compared the results with simulations and radiative transfer models for H 2 O and SiO, and examined data for the individual linear orthogonal polarizations. Results. We detected the 183 GHz H 2 O line towards all the stars with peak flux densities >100 Jy, including a new detection from VY CMa. Towards all five targets, the water line had indications of being due to maser emission and had higher peak flux densities than for the SiO lines. The SiO lines appear to originate from both thermal and maser processes. Comparison with simulations and models indicate that 183 GHz maser emission is likely to extend to greater radii in the circumstellar envelopes than SiO maser emission and to similar or greater radii than water masers at 22, 321 and 325 GHz. We speculate that a prominent blue-shifted feature in the W Hya 183 GHz spectrum is amplifying the stellar continuum, and is located at a similar distance from the star as mainline OH maser emission. We note that the coupling of an SiO maser model to a hydrodynamical pulsating model of an AGB star yields qualitatively similar simulated results to the observations. From a comparison of the individual polarizations, we find that the SiO maser linear polarization fraction of several features exceeds the maximum fraction allowed under standard maser assumptions and requires strong anisotropic pumping of the maser transition and strongly saturated maser emission. The low polarization fraction of the H 2 O maser however, fits with the expectation for a non-saturated maser. Conclusions. 183 GHz H 2 O masers can provide strong probes of the mass loss process of evolved stars. Higher angular resolution observations of this line using ALMA Band 5 will enable detailed investigation of the emission location in circumstellar envelopes and can also provide information on magnetic field strength and structure.
“…Large Keplerian disks around stars can be used to infer a binary past (e.g., Bujarrabal et al, 2013) even when a binary companion is not seen. Finally, the detection of magnetic fields strengths and geometries in giant stars (e.g., Pérez-Sánchez & Vlemmings, 2013) is a fundamental step to understand how they are generated and the interplay between duplicity and magnetic fields (Nordhaus & Blackman, 2006; Nordhaus, Blackman, & Frank, 2007). Figure 3. Left panel: ALMA (Section 4.1) observation of the AGB giant R Sculptoris (Maercker et al, 2012).…”
Astrophysicists are increasingly taking into account the effects of orbiting companions on stellar evolution. New discoveries have underlined the role of binary star interactions in a range of astrophysical events, including some that were previously interpreted as being due uniquely to single stellar evolution. We review classical binary phenomena, such as type Ia supernovae, and discuss new phenomena, such as intermediate luminosity transients, gravitational wave-producing double black holes, and the interaction between stars and their planets. Finally, we reassess well-known phenomena, such as luminous blue variables, in light of interpretations that include both single and binary stars. At the same time we contextualise the new discoveries within the framework of binary stellar evolution. The last decade has seen a revival in stellar astrophysics as the complexity of stellar observations is increasingly interpreted with an interplay of single and binary scenarios. The next decade, with the advent of massive projects such as the Square Kilometre Array, the James Webb Space Telescope, and increasingly sophisticated computational methods, will see the birth of an expanded framework of stellar evolution that will have repercussions in many other areas of astrophysics such as galactic evolution and nucleosynthesis.
“…For all other sources, polarisation has not been measured. The cause of this polarisation is currently unknown and further measurements are required in order to ascertain the cause (Goldsmith et al 1988;Pérez-Sánchez & Vlemmings 2013). Due to this, it is difficult to draw firm conclusions about variability with pulsation phase.…”
We have identified a new vibrational HCN maser at 89.087 GHz in the asymptotic giant branch (AGB) star IRAS 15082−4808, a maser which is thought to trace the innermost region of an AGB envelope. The observations of this maser at three epochs are presented: two positive detections and one null detection. The line profile has varied between the positive detections, as has the intensity of the maser. The major component of the maser is found to be offset by −2.0 ± 0.9 km/s with respect to the systemic velocity of the envelope, as derived from the 88.631 GHz transition of HCN. Similar blueshifts are measured in the other 9 sources where this maser has been detected. Maser variability with pulsation phase has been investigated for the first time using the 10 stars now available. Comparisons with AGB model atmospheres constrain the position of the formation region of the maser to the region between the pulsation shocks and the onset of dust acceleration, between 2 and 4 stellar radii.
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