J. Alcolea

1'' Resolution Mapping of the Molecular Envelope of the Protoplanetary Nebula CRL 618

We present 1 00 resolution interferometric maps of the CO J ¼ 2 1a nd HC3N J ¼ 25 24 emission in the protoplanetary nebula CRL 618 obtained with the millimeter array of the Owens Valley Radio Observatory (OVRO). Our CO data trace with high accuracy the spatio-kinematic structure of different molecular components: (1) the fast, bipolar outflow, which is observed up to axial distances of � 2B 5( � 2:7 ; 10 16 cm) from the nebula center with a maximal deprojected expansion velocity of Vexp � 340 km s � 1 ; (2) the roughly round outer halo, which is slowly expanding at Vexp � 17 km s � 1 ; (3) an extended structure that is elongated in the polar direction � 6 00 (� 8:1 ; 10 16 cm),surroundingtheopticallobes,andthatexpandsatVexp � 22kms � 1 (slowaxialcomponent); and(4)a dense, inner torus-likecore (Rout � 1:6 ; 10 16 cm) expanding atVexpP12kms � 1 . Components (3) and (4) have been revealed for the first time by our high angular resolution data. The geometry, kinematics, density, and temperaturedistributionoftheindividualcomponentsofthemolecularenvelopeofCRL618havebeenconstrained by fitting a spatio-kinematic model to the data. We interpret our results in terms of nebular evolution as follows. In the last k2500 yr, there have been at least two distinct episodes of mass loss at a large scale in the form of a slow wind. The firsttookplace at a rate of ˙

The highly collimated bipolar outflow of OH 231.8+4.2 ?

We present high spatial resolution observations of the CO molecular emission (J =1 0a nd J =2 1 lines) in the post-AGB bipolar nebula OH 231.8+4.2. High-quality NIR images (J, H, K 0 bands) of light scattered by grains were also obtained. Our observations probe the bulk of the nebular material, providing maps with a resolution 1 00 of the mass distribution, both CO and NIR images being very closely coincident. The combination of the two 12 CO lines has been used to measure the distribution of the kinetic temperature in the nebula, which is found to be very low, ranging between 8 K, in the outer southern clumps, and 35 K, in the central region. A relative temperature increase is found in the northernmost condensation, probably associated to a strong bow-like shock. Since velocities are also measured in CO, the dynamic parameters (kinetic momentum and energy) are also measured with high resolution. Most of the nebular mass (0.64 M) is located in the central condensation and flows at expansion velocities40 km s 1 . The rest of the gas,0.3 M almost equally distributed in the two lobes, flows along the nebular axis at high velocities, that increase proportionally to the distance to the central star reaching values as large as 430 km s 1 , as a result of a sudden acceleration happened about 770 yr ago. The general mass distribution in OH 231.8+4.2 is found to be clumpy and very elongated, with a length/width ratio reaching a factor 20 in the southern tail. In the center, however, we nd a double hollow-lobe structure, similar to those found in other well studied protoplanetary nebulae. We stress the enormous kinetic linear momentum carried by the molecular nebula, about 27 M km s 1 (5:5 10 39 gc m s 1 ). The kinetic energy is also very high, 1700 M km 2 s 4 3:4 10 46 erg. Given the short time during which the acceleration of the molecular outflow took place, we conclude that the linear momentum carried by the stellar photons is about a factor 100 smaller than that carried by the outflow, even if the eects of multiple scattering are taken into account. We independently argue that radiation pressure directly acting onto grains (the mechanism thought to be responsible for the mass ejection in AGB envelopes) cannot explain the observed bipolar flow, since this would produce a signicant shift between the dust and gas features that is not observed. Finally, we review the uncertain nature and evolutionary status of this unique object.

High resolution observations of SiO masers: comparing the spatial distribution at 43 and 86 GHz

We present sub-milliarcsecond observations of SiO masers in the late-type stars IRC + 10011 and cCyg. We have used the NRAO Very Long Baseline Array (VLBA) to map the 43 GHz ( v = 1,2 J = 1-0) and the 86 GHz ( v = 1,2 J = 2-1) SiO masers. All the transitions have been imaged except the v = 2 J = 2-1 in IRC + 10011. We report the first VLBI map of the v = 1 J = 2-1 28SiO maser in IRC + 10011 as well as the first VLBA images of SiO masers in an S-type Mira variable, cCyg. In this paper we have focused on the study of the relative spatial distribution of the different obsd. lines. We have found that in some cases the observational results are not reproduced by the current theor. pumping models, either radiative or collisional. In particular, for IRC + 10011, the v = 1 J = 1-0 and J = 2-1 28SiO lines have different spatial distributions and emitting region sizes, the J = 2-1 emission being located in an outer region of the envelope. For cCyg, the distributions also differ, but the sizes of the masing regions are comparable. We suggest that the line overlaps between ro-vibrational transitions of two abundant mol. species, H2O and 28SiO, is a possible explanation for the discrepancies found between the observations and the theor. predictions. We have introduced this overlapping process in the calcns. of the excitation of the SiO mol. We conclude that the line overlaps can strongly affect the excitation of SiO and may reproduce the unexpected observational results for the two sources studied. [on SciFinder (R)]

Extended rotating disks around post-AGB stars

Context. There is a group of binary post-AGB stars that show a conspicu ous NIR excess, usually assumed to arise from hot dust in very compact possibly rotating disks. These stars are sur rounded by significantly fainter nebulae than the standard, well studied protoplanetary and planetary nebulae (PPNe, PNe), and, at l east some of them, have significantly lower luminosities. Aims. To identify and study extended rotating disks around these s tars, shedding light on the role of disks in the formation and shaping of planetary nebulae. Methods. We present high-sensitivity mm-wave observations of CO lin es in 24 objects of this type. The resulting CO lines are compared with profiles expected to arise from rotating disks , both from theoretical and observational grounds. We deriv e s mple formulae that allow determining the mass of the CO-emitting gas and estimating its extent; the reliability and uncertai nty of the methods are widely discussed. Results. 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 b etween 10−3 and 10−2 M⊙, about two orders of magnitude smaller than the (total) masses of standard PPNe. The high-detection rate (upper limits being in fact no t very significant) clearly confirm that the NIR excess of these stars arises from compact disks in rot ation, 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−1, to be compared with the velocities of about 100 km s −1 often found instandard PPNe. Except for two sources with complex profiles, the outflo wing 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 5 10 and 3 10 cm. Estimates of the linear momenta carried by the outflows, w hich can only be performed in a few well studied objects, also yield moderate values, compared with the line ar momenta that can be released by the stellar radiation pres sure (contrary, again, to the case of the very massive and fast bipolar outflow s in standard PPNe, that are strongly overluminous). The mass and dynamics of nebulae around various classes of post-AGB star s differ very significantly, and we can expect the formation of PNe w ith very different properties.

Arcsecond-resolution

IRC +10420 and AFGL 2343 are the unique, known yellow hypergiants (YHGs) presenting a heavy circumstellar envelope (CSE). We aim to study the morphology, exceptional kinematics, and excitation conditions of their CSEs, and the implications for mass-loss processes. We have mapped the 12CO J=2-1 and 1-0 emission in these YHGs with the IRAM Plateau de Bure interferometer and the 30m telescope. We developed LVG models in order to analyze their circumstellar characteristics. The maps show that the overall shape of both CSEs is approximately spherical, although they also reveal several aspherical features. The CSE around IRC +10420 shows a rounded extended halo surrounding a bright inner region, with both components presenting aspherical characteristics. It presents a brightness minimum at the center. The envelope around AFGL 2343 is a detached shell, showing spherical symmetry and clumpiness at a level of about 15% of the maximum brightness. The envelopes expand isotropically at about 35 km/s, about two or three times faster than typical CSEs around AGB stars. High temperatures (~ 200 K) are derived for the innermost regions in IRC +10420, while denser and cooler (~ 30 K) gas is found in AFGL 2343. The mass-loss processes in these YHGs have been found to be similar. The deduced mass-loss rates (~ 10E-4 - 10E-3 Msun/yr) are much higher than those obtained in AGB stars, and they present significant variations on time scales of ~ 1000 yr.

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We present high-resolution images obtained with the WFPC2, on board the HST, of the protoplanetary nebula (PPN) OH 231.8+4.2. H and NII line emission and scattered light in the continuum at 6750 and 7910 A were observed. We also discuss NIR NICMOS images from the HST archive. The images show with high accuracy the shape and excitation state of the shocks developed in the nebula. Our high-resolution images (and data from other works) allow a very detailed and quantitative description of the dierent nebular components and of the physical conditions in them. We interpret specic structures identied in our images using existing models of shock interaction. In the center of the nebula, there is a dense torus- or disk-like condensation continued by an hourglass-like structure, with relatively high densities (10 5 {10 6 cm 3 ) and temperatures (30 K). Inside this torus we have identied the location of the central star, from SiO maser observations. Two shock regions are detected from the optical line emission images, respectively in the north and south lobes. In both regions, a forward and a backward shock are identied. The densities of this hot gas vary between 40 and 250 cm 3 ,w ith the densest clumps being placed in the reverse shocks. The total mass of the shocked hot gas is210 3 M, both lobes showing similar masses in spite of their dierent extents. The relatively collimated jet that impinges on an originally slow shell, so producing the shocks, is identied from the scattered light images and in CO maps. This flow is signicantly denser and cooler than the shocked H regions. Its density decreases with the distance to the star, with typical values10 5 {10 4 cm 3 , and its temperature ranges between about 25 and 8 K. We explain the high H emission of the backward shock assuming that it propagates in a diuse gas component, entrained by the observed collimated flow and sharing its axial movement. The existence of shocks also in the collimated densest flow is suggested by the high abundance of some molecules like HCO + and its structure and kinematics in certain regions, but they are not seen in H emission, probably because of the absence of (well developed) hot components in this dense flow. We think that the exceptionally detailed and quantitative image derived for the wind interaction regions in OH 231.8+4.2 is a challenge to check and improve hydrodynamical models of wind interaction in PPNe.

CO mapping of the yellow hypergiants IRC +10420 and AFGL 2343

We aim to study equatorial disks in rotation and axial outflows in post-AGB objects, as to disclose the formation and shaping mechanisms in planetary nebulae. So far, both disks and outflows had not been observed simultaneously. We have obtained high-quality ALMA observations of 12CO and 13CO J=3-2 and 12CO J=6-5 line emission in the Red Rectangle, the only post-AGB/protoplanetary object in which a disk in rotation has been mapped up to date. These observations provide an unprecedented description of the complex structure of this source. Together with an equatorial disk in rotation, we find a low-velocity outflow that occupies more or less the region placed between the disk and the optical X-shaped nebula. From our observations and preliminary modeling of the data, we confirm the previously known properties of the disk and obtain a first description of the structure, dynamics, and physical conditions of the outflow.

HST observations of the protoplanetary nebula OH 231.8+4.2: The structure of the jets and shocks

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.

ALMA observations of the Red Rectangle, a preliminary analysis

Aims. 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. Methods. We present interferometric observations of 12 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. Results. 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 objects.

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

We present interferometric maps of CO emission in the Red Rectangle, a well known protoplanetary nebula. The CO emission is found to arise from a relatively thin equatorial disk, extending about 5 �� in the direction perpendicular to the symmetry axis of the optical nebula. The velocity dispersion of the emission clearly increases towards the center, in a pattern significantly coincident with that expected for a Keplerian velocity field. Modeling of the CO maps confirms that the emitting gas is probably rotating around the central star(s), with a Kepler-like velocity distribution (at least in the central re- gions) that would correspond to a central mass ∼0.9 M� . Other possible explanations to the observations are discussed, but are found to be unlikely. Our models also suggest that the density and temperature increase towards the center roughly proportion- ally to the inverse radius. The asymmetry observed in the line profile and intensity distribution (the red part being stronger) can be explained by self-absorption if, superimposed to the rotation velocity, there is a low radial expansion at a velocity of about 0.4 km s −1 , at least in the outer disk regions. This is the first probable detection of a gas disk in Kepler-like rotation around a post-AGB star.