post-AGB Nebula Research Articles

A second post-AGB nebula that contains gas in rotation and in expansion: ALMA maps of IW Carinae

We aim to study the presence of both rotation and expansion in post-AGB nebulae, in particular around IW Car, a binary post-AGB star that was suspected to be surrounded by a Keplerian disk. We obtained high-quality ALMA observations of 12CO and 13CO J=3-2 lines in IW Car. The maps were analyzed by means of a simplified model of CO emission, based on those used for similar objects. Our observations clearly show the presence of gas components in rotation, in an equatorial disk, and expansion, which shows an hourglass-like structure with a symmetry axis perpendicular to the rotation plane and is probably formed of material extracted from the disk. Our modeling can reproduce the observations and shows moderate uncertainties. The rotation velocity corresponds to a central stellar mass of approximately 1 Mo. We also derive the total mass of the molecule-rich nebula, found to be of ~ 4 10e-3 Mo; the outflow is approximately eight times less massive than the disk. From the kinematical age of the outflow and the mass values derived for both components, we infer a (future) lifetime of the disk of approximately 5000--10000 yr.

28SiOv= 0J= 1–0 emission from evolved stars

Observations of 28SiO v=0 J=1-0 line emission (7-mm wavelength) from AGB stars show in some cases peculiar profiles, composed of a central intense component plus a wider plateau. Very similar profiles have been observed in CO lines from some AGB stars and most post-AGB nebulae and, in these cases, they are clearly associated with the presence of conspicuous axial symmetry and bipolar dynamics. We present systematic observations of 28SiO v=0 J=1-0 emission in 28 evolved stars, performed with the 40~m radio telescope of the IGN in Yebes, Spain. We find that the composite core plus plateau profiles are almost always present in O-rich Miras, OH/IR stars, and red supergiants. They are also found in one S-type Mira ($\chi$ Cyg), as well as in two semiregular variables (X Her and RS Cnc) that are known to show axial symmetry. In the other objects, the profiles are simpler and similar to those of other molecular lines. The composite structure appears in the objects in which SiO emission is thought to come from the very inner circumstellar layers, prior to dust formation. The central spectral feature is found to be systematically composed of a number of narrow spikes, except for X Her and RS Cnc, in which it shows a smooth shape that is very similar to that observed in CO emission. These spikes show a significant (and mostly chaotic) time variation, while in all cases the smooth components remain constant within the uncertainties. The profile shape could come from the superposition of standard wide profiles and a group of weak maser spikes. Alternatively, we speculate that the very similar profiles detected in objects that are axisymmetric may be indicative of the systematic presence of a significant axial symmetry in the very inner circumstellar shells around AGB stars; the presence of such symmetry would be independent of the probable weak maser effects in the central spikes.

FIR and sub-mm line observations of AGB and post-AGB nebulae

The existing information on asymptotic giant branch (AGB) and post-AGB objects from FIR and submm spectroscopy is still scarce. Observations from the ground are often very difficult to calibrate and show low S/N ratios. However, we expect to obtain in the near future an impressive amount of high-quality data, from the Herschel space telescope. ALMA is also expected to provide high-quality maps of submm lines.

Magnetic fields in planetary nebulae and post-AGB nebulae

Magnetic fields are an important but largely unknown ingredient of planetary nebulae. They have been detected in oxygen-rich asymptotic giant branch (AGB) and post-AGB stars, and may play a role in the shaping of their nebulae. Here we present SCUBA submillimetre polarimetric observations of four bipolar planetary nebulae and post-AGB stars, including two oxygen-rich and two carbon-rich nebulae, to determine the geometry of the magnetic field by dust alignment. Three of the four sources (NGC 7027, 6537 and 6302) present a well-defined toroidal magnetic field oriented along their equatorial torus or disc. NGC 6302 may also show field lines along the bipolar outflow. CRL 2688 shows a complex field structure, where part of the field aligns with the torus, whilst an other part approximately aligns with the polar outflow. It also presents marked asymmetries in its magnetic structure. NGC 7027 shows evidence for a disorganized field in the south-west corner, where the SCUBA shows an indication for an outflow. The findings show a clear correlation between field orientation and nebular structure.

Adaptive Optics Imaging of IRAS 18276−1431: A Bipolar Preplanetary Nebula with Circumstellar “Searchlight Beams” and “Arcs”

We present high angular resolution images of the post-AGB nebula IRAS 18276-1431 (also known as OH 17.7-2.0) obtained with the Keck II adaptive optics (AO) system in its natural guide star (NGS) mode in the K_P, L_P, and M_S near-infrared bands. We also present supporting optical F606W and F814W HST images as well as interferometric observations of the 12^CO J = 1-0, 13^CO J = 1-0, and 2.6 mm continuum emission with OVRO. The envelope of IRAS 18276-1431 displays a clear bipolar morphology in our optical and NIR images with two lobes separated by a dark waist and surrounded by a faint 4.5'' × 3.4'' halo. Our K_P-band image reveals two pairs of radial searchlight beams emerging from the nebula center and several intersecting, arclike features. From our CO data we derive a mass of M > 0.38 (D/3 kpc)^2 M_☉ and an expansion velocity V_exp = 17 km s^-1 for the molecular envelope. The density in the halo follows a radial power law ∝r^-3, which is consistent with a mass-loss rate increasing with time. Analysis of the NIR colors indicates the presence of a compact central source of ~300-500 K dust illuminating the nebula in addition to the central star. Modeling of the thermal IR suggests a two-shell structure in the dust envelope: (1) an outer shell with inner and outer radius R_in ~ 1.6 × 10^16 and R_out ≥1.25 × 10^17 cm, dust temperature T_dust ~ 105-50 K, and a mean mass-loss rate of M ≈ 10^3 M_☉ yr^-1; and (2) an inner shell with R_in ~ 6.3 × 10 cm, T_dust ~ 500-105 K, and M ~ 3 × 10^-5 M_☉ yr^-1. An additional population of big dust grains (radius a ≥0.4 mm) with T_dust = 150-20 K and mass M_dust = (0.16-1.6) × 10^-3 (D/3 kpc)^2 M_☉ can account for the observed submillimeter and millimeter flux excess. The mass of the envelope enclosed within R_out = 1.25 × 10^17 cm derived from SED modeling is ~1 (D/3 kpc)^2 M_☉.

The structure and dynamics of the molecular envelope of M 2-56

M 2-56 is a protoplanetary nebula (PPN) in which strong shocks are taking place, therefore, useful to study the post-AGB wind interaction. It is well known that molecular observations allow studying the mass distribution of PPNe, even in those regions that have been recently shocked. We present high-resolution maps of the emission of 12CO J = 2-1 and J = 1-0 in M 2-56. Such maps show a bipolar, molecular nebula that extends ~28 ´´ along the symmetry axis. The nebula is composed of two contiguous, incomplete shells located along the symmetry axis, which has an inclination of ~17o with respect to the plane of the sky. Those empty lobes intersect in the center of the nebula, where there is a small and dense ring perpendicular to the axis. This central ring expands radially at about 8 and seems to be the remnant of the circumstellar envelope of the AGB star, that has not been accelerated by the interaction with the fast post-AGB jets. The radius of the central ring is of ~ cm, for a distance of 2.1 kpc (deduced from an analysis of the main properties of the object). At ~ cm from the nebular center, the tips of the lobes reach axial expansion velocities of ~200 . We have developed a model for the spatio-kinematical distribution and the excitation conditions of the molecular gas in M 2-56. From the best fitting of the observations with the predictions of the model for both lines, we have estimated the physical conditions of the molecular nebula. It is found that the density varies from to cm-3 from the nebular center to the lobe tips, and that the part of the lobes that has not been detected is probably composed of photodissociated gas, due to the effect of interstellar photons on low-density regions. The rotational temperature is estimated to be approximately constant, ~13-16 K. For the assumed geometry, a velocity field composed by a dominant radial component plus an axial contribution has been deduced. The emission of both lines is found to be optically thin, and therefore probes the whole molecular gas, which has a mass of ~0.05 . The “scalar” momentum and the kinetic energy of the different regions of the molecular nebula have been calculated, finding that the high momentum won by the gas in the post-AGB phase cannot have been supplied by the radiation pressure mechanism. Although the central star of M 2-56 is not very hot yet (~20 000 K), this PPN has a large kinematical age, between 1000 yr and 1700 yr, in comparison with other PPNe that have hotter central stars. M 2-56 may not be a typical PPN, but an intermediate object between the known low-mass post-AGB nebulae and the standard PPNe.

Submilliarcsecond-resolution mapping of the 43 GHz SiO maser emission in the bipolar post-AGB nebula OH231.8+4.2

We present ∼0.3 milliarcsec-resolution maps of the SiO (v = 2, J = 1-0) maser emission in the bipolar post-AGB nebula OH 231.8+4.2 obtained with the Very Long Baseline Array. These observations have provided for the firsttime the structure and kinematics of the close stellar environment in a proto-Planetary Nebula. Our observations reveal the SiO maser emission arising in several bright spots of less than ∼10 1 3 cm in size forming a structure elongated in the direction perpendicular to the symmetry axis of the nebula. Such a distribution is consistent with an equatorial torus with a radius of ∼6 AU around the central star. A complex velocity gradient is found along the torus, which suggests rotation and infall of material towards the star. The rotation and infalling velocities deduced are of the same order and range between ∼7 and ∼10 km s - 1 . From our data, we estimate the mass of the SiO torus and the central star, as well as a stringent upper limit to the present stellar mass-loss rate.