Kn 26, a New Quadrupolar Planetary Nebula

Once classified as an emission line source, the planetary nebula (PN) nature of the source Kn 26 has been only recently recognized in digital sky surveys. To investigate the spectral properties and spatio-kinematical structure of Kn 26, we have obtained high spatial-resolution optical and near-IR narrow-band images, high-dispersion long-slit echelle spectra, and intermediate-resolution spectroscopic observations. The new data reveal an hourglass morphology typical of bipolar PNe. A detailed analysis of its morphology and kinematics discloses the presence of a second pair of bipolar lobes, making Kn 26 a new member of the subclass of quadrupolar PNe. The time-lap between the ejection of the two pairs of bipolar lobes is much smaller than their dynamical ages, implying a rapid change of the preferential direction of the central engine. The chemical composition of Kn 26 is particularly unusual among PNe, with a low N/O ratio (as of type II PNe) and a high helium abundance (as of type I PNe), although not atypical among symbiotic stars. Such an anomalous chemical composition may have resulted from the curtail of the time in the Asymptotic Giant Branch by the evolution of the progenitor star through a common envelope phase.Comment: 9 pages, 5 figures, accepted for publication in A&

The Unusual Distributions of Ionized Material and Molecular Hydrogen in NGC 6881: Signposts of Multiple Events of Bipolar Ejection in a Planetary Nebula

The planetary nebula NGC 6881 displays in the optical a quadrupolar morphology consisting of two pairs of highly collimated bipolar lobes aligned along different directions. An additional bipolar ejection is revealed by the hydrogen molecular emission, but its wide hourglass morphology is very different from that of the ionized material. To investigate in detail the spatial distribution of molecular hydrogen and ionized material within NGC 6881, and to determine the prevalent excitation mechanism of the H2 emission, we have obtained new near-IR Br-gamma and H2 and optical H-alpha and [N II] images, as well as intermediate resolution JHK spectra. These observations confirm the association of the H2 bipolar lobes to NGC 6881 and find that the prevalent excitation mechanism is collisional. The detailed morphology and very different collimation degree of the H2 and ionized bipolar lobes of NGC 6881 not only imply that multiple bipolar ejections have occurred in this nebula, but also that the dominant shaping agent is different for each bipolar ejection: a bipolar stellar wind most likely produced the H2 lobes, while highly collimated outflows are carving out the ionized lobes into the thick circumstellar envelope. The asymmetry between the southeast and northwest H2 bipolar lobes suggests the interaction of the nebula with an inhomogeneous interstellar medium. We find evidence that places NGC 6881 in the H II region Sh 2-109 along the Orion local spiral arm.Comment: 9 pages, 7 figures, 4 table

VISIR-VLT high resolution study of the extended emission of four obscured post-AGB candidates

The onset of the asymmetry of planetary nebulae (PNe) is expected to occur during the late Asymptotic Giant Branch (AGB) and early post-AGB phases of low- and intermediate-mass stars. Among all post-AGB objects, the most heavily obscured ones might have escaped the selection criteria of previous studies detecting extreme axysimmetric structures in young PNe. Since the most heavily obscured post-AGB sources can be expected to descend from the most massive PN progenitors, these should exhibit clear asymmetric morphologies. We have obtained VISIR-VLT mid-IR images of four heavily obscured post-AGB objects barely resolved in previous Spitzer IRAC observations to analyze their morphology and physical conditions across the mid-IR. The VISIR-VLT images have been deconvolved, flux calibrated, and used to construct RGB composite pictures as well as color and optical depth maps that allow us to study the morphology and physical properties of the extended emission of these sources. We have detected extended emission from the four objects in our sample and resolved it into several structural components that are greatly enhanced in the temperature and optical depth maps. They reveal the presence of asymmetry in three young PNe (IRAS 15534-5422, IRAS 17009-4154, and IRAS 18454+0001), where the asymmetries can be associated with dusty torii and slightly bipolar outflows. The fourth source (IRAS 18229-1127), a possible post-AGB star, is better described as a rhomboidal detached shell. The heavily obscured sources in our sample do not show extreme axisymmetric morphologies. This is at odds with the expectation of highly asymmetrical morphologies in post-AGB sources descending from massive PN progenitors. The sources presented in this paper may be sampling critical early phases in the evolution of massive PN progenitors, before extreme asymmetries develop.Comment: 9 pages, 4 figure

NGC 6309, a Planetary Nebula that Shifted from Round to Multipolar

We present new narrow-band Ha, [N II], and [O III] high-resolution images of the quadrupolar planetary nebula (PN) NGC 6309 that show in great detail its bipolar lobes and reveal new morphological features. New high- and low-dispersion long-slit spectra have been obtained to help in the investigation of the new nebular components. The images and spectra unveil two diffuse blobs, one of them located at 55 arcsec from the central star along the NE direction (PA= +71) and the other at 78 arcsec in the SW direction (PA= -151). Therefore, these structures do not share the symmetry axes of the inner bipolar outflows. Their radial velocities relative to the system are quite low: +3 and -4 km/s, respectively. Spectroscopic data confirm a high [O III] to Ha ratio, indicating that the blobs are being excited by the UV flux from the central star. Our images convincingly show a spherical halo 60 arcsec in diameter encircling the quadrupolar nebula. The expansion velocity of this shell is low, 66 km/s. The software SHAPE has been used to construct a morpho-kinematic model for the ring and the bipolar flows that implies an age of 4,000 yrs, the expansion of the halo sets a lower limit for its age 46,000 yrs, and the very low expansion of the blobs suggests they are part of a large structure corresponding to a mass ejection that took place 150,000 yrs ago. In NGC 6309 we have direct evidence of a change in the geometry of mass-loss, from spherical in the halo to axially-symmetric in the two pairs of bipolar lobes.Comment: 8 pages, 7 figures, Accepted for publication in MNRA

A search for water maser emission toward obscured post-AGB star and planetary nebula candidates

Water maser emission at 22 GHz is a useful probe to study the transition between the nearly spherical mass-loss in the AGB to a collimated one in the post-AGB phase. In their turn, collimated jets in the post-AGB phase could determine the shape of planetary nebulae (PNe) once photoionization starts. We intend to find new cases of post-AGB stars and PNe with water maser emission, including water fountains or water-maser-emitting PNe. We observed water maser emission in a sample of 133 objects, with a significant fraction being post-AGB and young PN candidate sources with strong obscuration. We detected this emission in 15 of them, of which seven are reported here for the first time. We identified three water fountain candidates: IRAS 17291-2147, with a total velocity spread of ~96 km/s in its water maser components and two sources (IRAS 17021-3109 and IRAS 17348-2906) that show water maser emission outside the velocity range covered by OH masers. We have also identified IRAS 17393-2727 as a possible new water-maser-emitting PN. The detection rate is higher in obscured objects (14%) than in those with optical counterparts (7%), consistent with previous results. Water maser emission seems to be common in objects that are bipolar in the near-IR (43% detection rate). The water maser spectra of water fountain candidates like IRAS 17291-2147 show significantly less maser components than others (e.g., IRAS 18113-2503). We speculate that most post-AGBs may show water maser emission with wide enough velocity spread (> 100 km/s) when observed with enough sensitivity and/or for long enough periods of time. Therefore, it may be necessary to single out a special group of "water fountains", probably defined by their high maser luminosities. We also suggest that the presence of both water and OH masers in a PN is a better tracer of its youth, rather than the presence of just one of these species.Comment: To be published in Astronomy & Astrophysics. 16 pages, 1 figure (spanning 5 pages). This version includes some minor language corrections and fixes some errors in Table

Sub-arcsecond Morphology of Planetary Nebulae

Planetary nebulae (PNe) can be roughly categorized into several broad morphological classes. The high quality images of PNe acquired in recent years, however, have revealed a wealth of fine structures that preclude simplistic models for their formation. Here we present narrow-band, sub-arcsecond images of a sample of relatively large PNe that illustrate the complexity and variety of small-scale structures. This is especially true for bipolar PNe, for which the images reveal multi-polar ejections and, in some cases, suggest turbulent gas motions. Our images also reveal the presence or signs of jet-like outflows in several objects in which this kind of component has not been previously reported.Comment: 7 pages, 7 figures, Accepted for publication in PAS

Icnología de vertebrados de la facies Buntsandstein de Mallorca

Se describen, por primera vez en Mallorca, hasta cinco tipos diferentes de icnitas y pistas de vertebrados continentales dentro de las facies  cuarzoareniticas y limosas rojas de la facies Buntsandstein del Triásico inferio