Nuevos métodos observacionales de estrellas post-agb y nebulosas planetarias

Resumen

Introduction. Planetary Nebulae (PNe) are the descendants of low- and intermediate-mass evolved stars (0.8 - 10 solar masses). Among the astrophysical phenomena, PNe display an impressive variety of complex morphologies, as has been demonstrated by observations obtained in the last years by the new generation of telescopes, e.g. Hubble Space Telescope (HST), Very Large Telescope (VLT), Very Large Array (VLA). PNe show morphologies from spherical to bipolar or multipolar, as well as knots and/or jets collimated at high velocities, reaching 600 km/s in the most extreme cases. Furthermore, it has been estimated that nearly 80% of the PNe are not spherical, a controversial result since PNe are formed from the spherical envelope ejected of an asymptotic giant branch (AGB) star and ionized afterwards by the UV photons produced by its central star. The shaping of asymmetrical planetary nebulae is yet under debate since these objects defy the standard models for the formation of spherical or elliptical PNe. The onset of the asymmetry of PNe is expected to occur during the short transition from the late AGB and early post-AGB phase. Consequently, a proto-PN frequently displays a bipolar or multipolar morphology, like the proto-PN AFGL915 (The Red Rectangle) or the two young PNe M2-9 (The Butterfly Nebula) and Mz 3 (The Ant Nebula). The existence of disks and/or torii in the equatorial regions and high velocity jets has been proposed to sculpt such morphologies in PNe, since the stellar radiation pressure is not able to collimate these extremely axisymmetric morphologies. The study of disks and jets as shaping agents of PNe requires the use of high-resolution observational techniques able to observe and resolve the innermost regions of post-AGB sources, proto-PNe, and young PNe. Furthermore, these innermost regions are often embedded in dust and are thus optically thick, nonetheless they emit copiously at infrared (IR) and radio wavelengths. Motivation and aims. This PhD thesis aims to prove high-resolution observational techniques using new generation telescopes, as is the case of the 8.2m VLT, to observe in the IR post-AGB, proto-PNe, and PNe at high-resolution. Based in previous studies, we have selected different samples of compact sources in transition to the PNe phase, as well as young PNe in which their innermost regions, where the shaping mechanism lays, were not resolved due to the lack of resolution. In addition, we have selected axisymmetric sources whose innermost zones have been observed and modeled. We aim to prove and compare previous results with the ones obtained using a spectroscopic high-resolution technique applied to the search of the shaping mechanism of extremely axisymmetric PNe. Considering the characteristics of the evolved stars studied in this thesis, beside the use of powerful instruments capable to achieve high-resolutions, they must also be able to observe structures embedded in dusty environments, since post-AGB stars are important dust producers. These conditions also favour the apparition of molecules whose emission are mainly detected towards IR. Inspired by all these facts, we aim to obtain observing time using two instruments of the VLT: VISIR (VLT Imager and Spectrometer for the mid-IR) and CRIRES (Cryogenic high-resolution Infra-Red Echelle Spectrograph). Results and conclusions. Among all post-AGB objects, the most heavily obscured ones might have escaped the selection criteria of previous studies that detected extreme axisymmetric 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. High-resolution observations of these sources should reveal marked bipolar morphologies, confirming the link between progenitor mass and nebular morphology. We have obtained VISIR-VLT mid-IR images of a sample of four heavily obscured post-AGB objects that are barely resolved in previous Spitzer IRAC observations in order to analyze their morphology and physical conditions across the mid-IR. The images obtained in four different mid-IR filters have been deconvolved, flux-calibrated, and used to construct RGB composite pictures as well as color (temperature) and optical depth maps that allow us to study the morphology and physical properties of the extended emission of the sources in our sample. We have detected the 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. The morphologies of the sample, as well as their physical conditions, 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, which is otherwise supported by observations of bright mid-IR unobscured sources. The sources studied may be sampling critical early phases in the evolution of massive PN progenitors, before extreme asymmetries develop. On the other hand, the young PN K3-35 represents a unique case where a small-sized water maser ring has been linked to the launch of collimated outflows that would be shaping the nebula. The contrasting optical and radio continuum morphologies of K3-35 indicate that they disclose different structural components which are apparently unconnected. To bridge the gap between optical and radio continuum observations, we present here new broad- and narrow-band near- and VISIR-VLT mid-IR images of K3-35. These images, and their comparison with optical and radio continuum images, are revealing. The radio continuum and mid-IR images are dominated by a compact source at the core of K3-35 whose emission gives evidence of very dense ionized material embedded within a dust cocoon. The emission from the core, obscured at optical wavelengths, is faintly detected in the Ks band. We suggest that the dust may shield the water molecules at the inner ring from the central star ionizing radiation. The precessing collimated outflows, very prominent in radio continuum, are also detected in mid-IR, very particularly in the [S IV] image. The mid-IR emission from these outflows consists mostly of ionized material, although the broad-band filter at 11.85 m seems to imply that a small amount of dust may be carried out by the outflow. The interactions of these outflows with the nebular shell result in shocks which excite the emission of H2 and low-excitation lines from ionized species such as [N II] at the tips of the outflows. Regarding the spectroscopic observations here presented, we have used and analysed high-resolution long-slit spectra from CRIRES-VLT commissioning time. These data have been used to investigate the implementation of the observational technique known as high-resolution spectro-astrometry, not used before in this research field. We have selected the spectro-astrometry technique motivated by the limitations of the IR interferometric techniques in the study of the asymmetries of PNe. High-resolution spectro-astrometry technique using CRIRES-VLT has proven its efficiency in the search for proto-planetary disks. CRIRES-VLT commissioning data of the proto-PN IRAS 17516-2525 and the young PN SwSt 1 have been used to develop the methodology and tools needed to apply the spectro-astrometric analysis to sources transiting towards or at early stages of the PN phase. Previous studies lacked the spatial resolution required to disclose the morphology of these two sources. Our exploratory study using CRIRES-VLT commissioning data has revealed small sized structures after the spectro-astrometric analysis of these two sources. The structures found in SwSt 1 have sizes of 230 mas in the [Fe III] line and 130 mas in the Brackett gamma line. As for the proto-PN IRAS 17516-2525, the spectro-astrometric signal can be interpreted as compact bipolar lobes of 12 mas in size arising from the innermost regions. In the second phase of the implementation of the spectro-astrometry technique we have obtained observations carefully planed aiming to prove the feasibility of this technique in the search of disks and other compact innermost structures in extremely axisymmetric PNe. We have selected the well studied proto-PN AFGL915, from which past studies have provided strong evidences of a Keplerian gaseous CO disk as well as a disk of crystalline silicates aligned with its minor axis of symmetry. Based in the previous studies and in the C/O chemistry of this source, we have acquired CRIRES-VLT high-resolution spectra of the near-IR CO fundamental band at 4.99 microns. The spectro-astrometric analysis has revealed the following preliminary results. The spectro-astrometric signatures and the line profiles of the 12CO isotope (4.99 microns) resemble Keplerian sources. The presence of a thick torus-like orientated along the minor axis of symmetry, that is, at 101º, has been also disclosed by the spectro-astrometric analysis. These results are in concordance with previous observations representative for the outermost regions of the Keplerian disk and with models predicting the existence of this thick toroidal structure at the innermost regions of AFGL915, whose origins are related with the common envelope binary system progenitor. It has been proposed that this thick innermost torus is massive and is accreting material. Finally, we emphasize that these results must be properly modeled in order to corroborate the properties of the disk found by our spectro-astrometric analysis at the central region of AFGL915.