Fallback in bipolar planetary nebulae?

The subclass of bipolar Planetary Nebulae (PNe) exhibits well-defined low-power outflows and some shows shock-related equatorial spiderweb structures and hourglass structures surrounding these outflows. These structures are distinctly different from the phenomena associated with spherical and elliptical PNe and suggest a non-standard way to simultaneously energise both kinds of structures. This paper presents evidence from the published literature on bipolar PN Hb 12 and other sources in support of an alternative scenario for energising these structures by means of accretion from material shells deposited during earlier post-AGB and pre-PNe evolutionary stages. In addition to energising the bipolar outflow, a sub-Eddington accretion scenario could hydrodynamically explain the spiderweb and outer hourglass structures as oblique shockwaves for guiding the accreting material into the equatorial region of the source. Estimates of the accretion rate resulting from fallback-related spherical accretion could indeed help to drive a low-power outflow and contribute to the total luminosity of these sources.

Discovery of a New Population of Galactic H ii Regions with Ionized Gas Velocity Gradients

We investigate the kinematic properties of Galactic H ii regions using radio recombination line (RRL) emission detected by the Australia Telescope Compact Array at 4–10 GHz and the Jansky Very Large Array at 8–10 GHz. Our H ii region sample consists of 425 independent observations of 374 nebulae that are relatively well isolated from other, potentially confusing sources and have a single RRL component with a high signal-to-noise ratio. We perform Gaussian fits to the RRL emission in position-position–velocity data cubes and discover velocity gradients in 178 (42%) of the nebulae with magnitudes between 5 and 200 m s − 1 arcsec − 1 . About 15% of the sources also have an RRL width spatial distribution that peaks toward the center of the nebula. The velocity gradient position angles appear to be random on the sky with no favored orientation with respect to the Galactic plane. We craft H ii region simulations that include bipolar outflows or solid body rotational motions to explain the observed velocity gradients. The simulations favor solid body rotation since, unlike the bipolar outflow kinematic models, they are able to produce both the large, >40 m s − 1 arcsec − 1 , velocity gradients and also the RRL width structure that we observe in some sources. The bipolar outflow model, however, cannot be ruled out as a possible explanation for the observed velocity gradients for many sources in our sample. We nevertheless suggest that most H ii region complexes are rotating and may have inherited angular momentum from their parent molecular clouds.

FLASHING: New high-velocity H2O masers in IRAS 18286−0959

We discovered new high-velocity components of H2O maser emission in one of the “water fountain” sources, IRAS 18286−0959, which has been monitored using the Nobeyama 45 m telescope in the FLASHING (Finest Legacy Acquisitions of SiO- and H2O-maser Ignitions by Nobeyama Generation) project since 2018 December. The maser spectra show new components with extremely high expansion velocities (>200 km s−1 projected in the line of sight), some of which are located symmetrically in the spectrum with respect to the systemic velocity. They were also mapped with KaVA (KVN and VERA Combined Array) in 2019 March. We located some of these maser components closer to the central stellar system than other high-velocity components (50–200 km s−1) that have been confirmed to be associated with the known bipolar outflow. The new components would have flashed in the fast collimated jet at a speed of over 300 km s−1 (soon) after 2011 when they had not been detected. The fastest of the new components seem to indicate rapid deceleration in these spectra; however, our present monitoring is still too sparse to unambiguously confirm it (up to 50 km s−1 yr−1) and too short to reveal their terminal expansion velocity, which will be equal to the expansion velocity that has been observed ($v$exp ∼ 120 km s−1). Future occurrences of such extreme-velocity components may provide a good opportunity to investigate possible recurrent outflow ignitions. Thus, the sculpture of the parental envelope will be traced by the dense gas that is entrained by the fast jet and exhibits spectacular distributions of the relatively stable maser features.

The near-infrared polarization of the pre-planetary nebula Frosty Leo

ABSTRACT We present a near-infrared imaging polarimetric study of the pre-planetary nebula: Frosty Leo. The observations were carried out in J, H, and K′ bands using the new polarimeter POLICAN mounted on the 2.1-m telescope of the Guillermo Haro Astrophysical Observatory, Sonora, Mexico. The most prominent result observed in the polarization maps is a large and well-defined dusty envelope (35 arcsec diameter in H band). The polarization position angles in the envelope are particularly well ordered and nearly parallel to the equator of the nebula (seen in J and H bands). The nebula presents a known bipolar outflow and the envelope completely wraps around it. Within the bipolar lobes, we find high polarization levels ranging from $60{{\ \rm per\ cent}}$ (J band) to $90{{\ \rm per\ cent}}$ (K′ band) and the polarization angles trace a centrosymmetric pattern. We found the remnants of superwind shells at the edges of the bipolar lobes and the duration of this phase is around 600 yr. The origin of polarization features in the nebula is most likely due to a combination of single and multiple scattering. Our results clearly demonstrate new structures that provide new hints on the evolution of Frosty Leo from its previous asymptotic giant branch phase.

Astrometry of H2O masers in the W 48 A (G35.20−01.74) H ii region with VERA: A compact disk outflow inside core H-2a

W 48 A core H-2a is one of the young massive protostellar objects in the W 48 region. We conducted multi-epoch astrometric observations of the water (H2O) masers associated with the W 48 A core H-2a with VLBI Exploration of Radio Astrometry (VERA). The trigonometric annual parallax of W 48 A core H-2a was measured to be 0.433 ± 0.026 mas, corresponding to a distance of $2.31^{+0.15}_{-0.13}$ kpc. This agrees with the revised parallax of 0.412 ± 0.014 mas by Wu et al. (2019, ApJ, 874, 94). We obtained the systemic proper motion and local standard of rest velocity to be (μαcos δ, μδ) = (0.26 ± 0.73, −3.87 ± 0.33) mas yr−1 and vLSR = 41.9 ± 0.9 km s−1, respectively. The distribution of the H2O masers covers an area of 70 mas × 80 mas, corresponding to 160 au × 180 au at the distance of 2.31 kpc. The internal proper motions of the H2O masers trace an east–west bipolar outflow. With the recent absolute position measurement of the 6.7 GHz methanol (CH3OH) masers and their elliptical distribution, whose major axis is perpendicular to the axis of the bipolar outflow, we suggest the presence of a disk outflow system in core H-2a. The spectral energy distribution (SED) of the driving source of core H-2a was previously reported to yield a luminosity and envelope mass of 8000 ± 1000 $W_{\odot}$ and 170 ± 30 $M_{\odot}$, respectively. Refitting the SED with the new distance, we obtained the luminosity to be 3100 ± 388 $L_{\odot}$ and derived the zero age main sequence (ZAMS) stellar mass to be 9 ± 1 $M_{\odot}$. Using our distance measurement, we derived the peculiar motion of W 48 A to be (Us, Vs, Ws) = (1 ± 4, 5 ± 6, −15 ± 5) km s−1.

The extended molecular envelope of the asymptotic giant branch star π1 Gruis as seen by ALMA

Context. This study is a follow up to the previous analysis of lower-angular resolution data in which the kinematics and structure of the circumstellar envelope (CSE) around the S-type asymptotic giant branch (AGB) star π1 Gruis were investigated. The AGB star has a known companion (at a separation of ~400 AU) that cannot explain the strong deviations from spherical symmetry of the CSE. Recently, hydrodynamic simulations of mass transfer in closer binary systems have successfully reproduced the spiral-shaped CSEs found around a handful of sources. There is growing evidence for an even closer, undetected companion complicating the case of π1 Gruis further. Aims. The improved spatial resolution allows for the investigation of the complex circumstellar morphology and the search for imprints on the CSE of the third component. Methods. We have observed the 12CO J = 3–2 line emission from π1 Gruis using both the compact and extended array of Atacama Large Millimeter/submillimeter Array (ALMA). The interferometric data have furthermore been combined with data from the ALMA total power array. The imaged brightness distribution has been used to constrain a non-local, non-local thermodynamic equilibrium 3D radiative transfer model of the CSE. Results. The high-angular resolution ALMA data have revealed the first example of a source on the AGB where both a faster bipolar outflow and a spiral pattern along the orbital plane can be seen in the gas envelope. The spiral can be traced in the low- to intermediate-velocity (13–25 km s−1) equatorial torus. The largest spiral-arm separation is ≈5.′′5 and consistent with a companion with an orbital period of ≈330 yr and a separation of less than 70 AU. The kinematics of the bipolar outflow is consistent with it being created during a mass-loss eruption where the mass-loss rate from the system increased by at least a factor of five for 10–15 yr. Conclusions. The spiral pattern is the result of an undetected companion. The bipolar outflow is the result of a rather recent mass-loss eruption event.

Time variations of H2O and SiO masers in the proto-Planetary Nebula OH 231.8+4.2

H2O (22 GHz) and SiO masers (43, 86, 129 GHz) in the bipolar proto-planetary nebula OH 231.8+4.2 were simultaneously monitored using the 21-m antennas of the Korean VLBI Network in 2009–2015. Both species exhibit periodic flux variations that correlate with the central star’s optical light curve, with a phase delay of up to 0.15 for the maser flux variations with respect to the optical light curve. The flux densities of SiO v = 2, J = 1→0 and H2O masers decrease with time, implying that they may disappear in 10–20 years. However, there seems to have been a transient episode of intense H2O maser emission around 2010. We also found a systematic behaviour in the velocity profiles of these masers. The velocities of the H2O maser components appear to be remarkably constant, suggesting ballistic motion for the bipolar outflow in this nebula. On the other hand, those of the SiO maser clumps show a systematic radial acceleration of the individual clumps, converging to the outflow velocity of the H2O maser clumps. Measuring the full widths at zero power of the detected lines, we estimated the expansion velocities of the compact bipolar outflow traced by H2O maser and SiO thermal line, and discussed the possibility of the expanding SiO maser region in the equatorial direction. All of our analyses support that the central host star of OH231.8 is close to the tip of the AGB phase, and that the mass-loss rate recently started to decrease because of incipient post-AGB evolution.