Detection of Keplerian dynamics in a disk around the post-AGB star AC Her

So far, only one rotating disk has been clearly identified and studied in AGB or post-AGB objects (in the Red Rectangle), by means of observations with high spectral and spatial resolution. However, disks are thought to play a key role in the late stellar evolution and are suspected to surround many evolved stars. We aim to extend our knowledge on these structures. We present interferometric observations of CO J=2-1 emission from the nebula surrounding the post-AGB star AC Her, a source belonging to a class of objects that share properties with the Red Rectangle and show hints of Keplerian disks. We clearly detect the Keplerian dynamics of a second disk orbiting an evolved star. Its main properties (size, temperature, central mass) are derived from direct interpretation of the data and model fitting. With this we confirm that there are disks orbiting the stars of this relatively wide class of post-AGB objectsComment: 4 pages, 3 figure

Extended rotating disks around post-AGB stars

There is a group of binary post-AGB stars that show a conspicuous NIR excess, usually assumed to arise from hot dust in very compact possibly rotating disks. These stars are surrounded by significantly fainter nebulae than the "standard", well studied protoplanetary and planetary nebulae (PPNe, PNe). We present high-sensitivity mm-wave observations of CO lines in 24 objects of this type. CO emission is detected in most observed sources and the line profiles show that the emissions very probably come from disks in rotation. We derive typical values of the disk mass between 1e-3 and 1e-2 Mo, about two orders of magnitude smaller than the (total) masses of "standard" PPNe. The high-detection rate (upper limits being in fact not very significant) clearly confirm that the NIR excess of these stars arises from compact disks in rotation, very probably the inner parts of those found here. Low-velocity outflows are also found in about eight objects, with moderate expansion velocities of ~ 10 km/s, to be compared with the velocities of about 100 km/s often found in "standard" PPNe. Except for two sources with complex profiles, the outflowing gas in our objects represents a minor nebular component. Our simple estimates of the disk typical sizes yields values ~ 0.5 - 1 arcsec, i.e. between 5e15 and 3e16 cm. Estimates of the linear momenta carried by the outflows, which can only be performed in a few well studied objects, also yield moderate values, compared with the linear momenta that can be released by the stellar radiation pressure (contrary, again, to the case of the very massive and fast bipolar outflows in "standard" PPNe, that are strongly overluminous). The mass and dynamics of nebulae around various classes of post-AGB stars differ very significantly, and we can expect the formation of PNe with very different properties.Comment: 19 pages, 26 figure

Interferometric observations of SiO thermal emission in the inner wind of M-type AGB stars IK Tauri and IRC+10011

Context. AGB stars go through a process of strong mass-loss that involves pulsations of the atmosphere, which extends to a region where the conditions are adequate for dust grains to form. Radiation pressure acts on these grains which, coupled to the gas, drive a massive outflow. The details of this process are not clear, including which molecules are involved in the condensation of dust grains. Aims. To study the role of the SiO molecule in the process of dust formation and mass-loss in M-type AGB stars. Methods. Using the IRAM NOEMA interferometer we observed the $^{28}$SiO and $^{29}$SiO $J=3-2$, $v=0$ emission from the inner circumstellar envelope of the evolved stars IK Tau and IRC+10011. We computed azimuthally averaged emission profiles to compare the observations to models using a molecular excitation and ray-tracing code for SiO thermal emission. Results. We observed circular symmetry in the emission distribution. We also found that the source diameter varies only marginally with radial velocity, which is not the expected behavior for envelopes expanding at an almost constant velocity. The adopted density, velocity, and abundance laws, together with the mass-loss rate, which best fit the observations, give us information on the chemical behavior of the SiO molecule and its role in the dust formation process. Conclusions. The results indicate that there is a strong coupling between the depletion of gas phase SiO and gas acceleration in the inner envelope. This could be explained by the condensation of SiO into dust grains

Two short mass-loss events that unveil the binary heart of Minkowski's Butterfly Nebula

Studying the appearance and properties of bipolar winds is critical to understand the stellar evolution from the AGB to the planetary nebula (PN) phase. Many uncertainties exist regarding the presence and role of binary stellar systems, mainly due to the deficit of conclusive observational evidences. We investigate the extended equatorial distribution around the early bipolar planetary nebula M 2-9 ("Minkowski's Butterfly Nebula") to gather new information on the mechanism of the axial ejections. Interferometric millimeter observations of molecular emission provide the most comprehensive view of the equatorial mass distribution and kinematics in early PNe. Here we present subarcsecond angular-resolution observations of the 12CO J=2-1 line and continuum emission with the Plateau de Bure interferometer. The data reveal two ring-shaped and eccentric structures at the equatorial basis of the two coaxial optical lobes. The two rings were formed during short mass-loss episodes (~ 40 yr), separated by ~ 500 yr. Their positional and dynamical imprints provide evidence of the presence of a binary stellar system at the center, which yields critical information on its orbital characteristics, including a mass estimate for the secondary of ~< 0.2 \ms. The presence of a stellar system with a modest-mass companion at the center of such an elongated bipolar PN strongly supports the binary-based models, because these are more easily able to explain the frequent axisymmetric ejections in PNe.Comment: 8 page

The chemical composition of the circumstellar envelopes around yellow hypergiant stars

The yellow hypergiant stars (YHGs) are extremely luminous and massive objects whose general properties are poorly known. Only two of this kind of star show massive circumstellar envelopes, IRC+10420 and AFGL2343. We aim to study the chemistry of the circumstellar envelopes around these two sources, by comparison with well known AGB stars and protoplanetary nebulae. We also estimate the abundances of the observed molecular species. We have performed single-dish observations of different transitions for twelve molecular species. We have compared the ratio of the intensities of the molecular transitions and of the estimated abundances in AFGL2343 and IRC+10420 with those in O-rich and C-rich AGB stars and protoplanetary nebulae. Both YHGs, AFGL2343, and IRC+10420, have been found to have an O-rich chemistry similar to that in O-rich AGB stars, though for AFGL2343 the emission of most molecules compared with 13CO lines is relatively weak. Clear differences with the other evolved sources appear when we compare the line intensity corrected for distance and the profile widths which are, respectively, very intense and very wide in YHGs. The abundances obtained for IRC+10420 agree with those found in AGB stars, but in general those found in AFGL2343, except for 13CO, are too low. This apparently low molecular abundance in AFGL2343 could be due to the fact that these molecules are present only in an inner region of the shell where the mass is relatively low.Comment: 14 pages, 12 figure

A disk inside the bipolar planetary nebula M2-9

Bipolarity in proto-planetary and planetary nebulae is associated with events occurring in or around their cores. Past infrared observations have revealed the presence of dusty structures around the cores, many in the form of disks. Characterising those dusty disks provides invaluable constraints on the physical processes that govern the final mass expulsion of intermediate-mass stars. We focus this study on the famous M2-9 bipolar nebula, where the moving lighthouse beam pattern indicates the presence of a wide binary. The compact and dense dusty core in the center of the nebula can be studied by means of optical interferometry. M2-9 was observed with VLTI/MIDI at 39-47 m baselines with the UT2-UT3 and UT3-UT4 baseline configurations. These observations are interpreted using a dust radiative transfer Monte Carlo code. A disk-like structure is detected perpendicular to the lobes and a good fit is found with a stratified disk model composed of amorphous silicates. The disk is compact, 25$\times$35 mas at 8$\rm \mu m$, and 37$\times$46 mas at 13$\rm \mu m$. For the adopted distance of 1.2 kpc, the inner rim of the disk is $\sim$15 AU. The mass represents a few percent of the mass found in the lobes. The compactness of the disk puts strong constraints on the binary content of the system, given an estimated orbital period 90-120yr. We derive masses of the binary components between 0.6--1.0M_{\sun} for a white dwarf and 0.6--1.4M_{\sun} for an evolved star. We present different scenarios on the geometric structure of the disk accounting for the interactions of the binary system, which includes an accretion disk as well.Comment: 9 figures, A&A accepte

Further ALMA observations and detailed modeling of the Red Rectangle

We present new high-quality ALMA observations of the Red Rectangle (a well known post-AGB object) in C17O J=6-5 and H13CN J=4-3 line emission and results from a new reduction of already published 13CO J=3-2 data. A detailed model fitting of all the molecular line data, including previous maps and single-dish spectra, was performed using a sophisticated code. These observations and the corresponding modeling allowed us to deepen the analysis of the nebular properties. We also stress the uncertainties in the model fitting. We confirm the presence of a rotating equatorial disk and an outflow, which is mainly formed of gas leaving the disk. The mass of the disk is ~ 0.01 Mo, and that of the CO-rich outflow is ~ 10 times smaller. High temperatures of ~ 100 K are derived for most components. From comparison of the mass values, we roughly estimate the lifetime of the rotating disk, which is found to be of about 10000 yr. Taking data of a few other post-AGB composite nebulae into account, we find that the lifetimes of disks around post-AGB stars typically range between 5000 and more than 20000 yr. The angular momentum of the disk is found to be high, ~ 9 Mo AU km/s, which is comparable to that of the stellar system at present. Our observations of H13CN show a particularly wide velocity dispersion and indicate that this molecule is only abundant in the inner Keplerian disk, at ~ 60 AU from the stellar system. We suggest that HCN is formed in a dense photodissociation region (PDR) due to the UV excess known to be produced by the stellar system, following chemical mechanisms that are well established for interstellar medium PDRs and disks orbiting young stars. We further suggest that this UV excess could lead to the efficient formation and excitation of PAHs and other C-bearing macromolecules, whose emission is very intense in the optical counterpart.Comment: Astronomy & Astrohysics, in press; 17 pages, 18 figures, 1 tabl

Optimal Control of Molecular Spin Qudits

We demonstrate, numerically, the possibility of manipulating the spin states of molecular nanomagnets with shaped microwave pulses designed with quantum optimal control theory techniques. The state-to-state or full gate transformations can be performed in this way in shorter times than using simple monochromatic resonant pulses. This enhancement in the operation rates can therefore mitigate the effect of decoherence. The optimization protocols and their potential for practical implementations are illustrated by simulations performed for a simple molecular cluster hosting a single Gd3+ ion. Its eight accessible levels (corresponding to a total spin S=7/2) allow encoding an eight-level qudit or a system of three coupled qubits. All necessary gates required for universal operation can be obtained with optimal pulses using the intrinsic couplings present in this system. The application of optimal control techniques can facilitate the implementation of quantum technologies based on molecular spin qudits