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Planetary nebulae (PNs) were first discovered over 200 years ago and our understanding of these objects has undergone significant evolution over the years. Developments in astronomical optical spectroscopy and atomic physics have shown that PNe are gaseous objects photoionized by UV radiation from a hot central star. Studies of the kinematics of the nebulae coupled with progress in theories of stellar evolution have led to the identification that PNe are evolved stars and progenitors of white dwarfs. Development of infrared and millimeter-wave technology in the 1970s made us realize that there is significant amount of neutral matter (molecules and dust) in PNe. The link of PNe to the stellar winds from their progenitor asymptotic giant branch (AGB) stars and subsequent dynamical interactions are now believed to be the underlying causes of the morphological structures of PNe. The role of PNe as prolific molecular factories producing complex molecules and organic solids has significant implications on the chemical enrichment of the Galaxy.
In this paper, we discuss the misconceptions and errors that we have encountered in our journey of understanding the nature of PN. The various detours and dead ends that had happened during our quest to pin down the evolutionary status and causes of nebulae ejection will be discussed. As there are still many unsolved problems in PN research, these lessons of history have much to offer for future progress in this field.
Over the last decade Galactic planetary nebula discoveries have entered a golden age due to the emergence of high sensitivity, high resolution narrow-band surveys of the Galactic plane. These have been coupled with access to complimentary, deep, multi-wavelength surveys across near-IR, mid-IR and radio regimes in particular from both ground-based and space-based telescopes. These have provided powerful diagnostic and discovery capabilities. In this review these advances are put in the context of what has gone before, what we are uncovering now and through the window of opportunity that awaits in the future. The astrophysical potential of this brief but key phase of late stage stellar evolution is finally being realised.
We report on the current status of the search for planetary nebulae (PNe) in the IPHAS survey, the 1800 square degree H-alpha survey of the Northern Galactic Plane carried out at La Palma Observatory. The first major installment of the “IPHAS Catalogue of Extended Nebulae” is presented. This includes a complete census of emission-line nebulae in 320 square degrees toward the inner regions of the Galaxy. In this area, more than 450 emission nebulae have been catalogued including 100 known PNe, and 155 new IPHAS candidate PNe.
A vigorous programme of spectroscopic confirmation of IPHAS candidate PNe over the entire survey area is underway using a number of telescopes worldwide. So far, around one hundred new PNe over the whole IPHAS area have been spectroscopically confirmed. Their main properties, some outstanding examples, as well as the relevance of the IPHAS survey to determine the total PN population in the Galactic disc and its chemical gradient is discussed.
The Spitzer Space Telescope has three science instruments (IRAC, MIPS, and IRS) that can take images at 3.6, 4.5, 5.8, 8.0, 24, 70, and 160 μm, spectra over 5–38 μm, and spectral energy distribution over 52–100 μm. The Spitzer archive contains targeted imaging observations for more than 100 PNe. Spitzer legacy surveys, particularly the GLIMPSE survey of the Galactic plane, contain additional serendipitous imaging observations of PNe. Spitzer imaging and spectroscopic observations of PNe allow us to investigate atomic/molecular line emission and dust continuum from the nebulae as well as circumstellar dust disks around the central stars. Highlights of Spitzer observations of PNe are reviewed in this paper.
A sample of ~150 compact Galactic PNe has been observed with the Spitzer/IRS spectrograph to characterize their dust properties. These PNe are likely to be at the onset of the PN evolutionary phase, and are therefore ideal for probing dust evolution. The molecular emission features in these Galactic PN spectra are similar to those found in our Magellanic Cloud sample, except that we found a sizable fraction of PNe with mixed-chemistry dust which are not observed in the Clouds. We also found that the distribution among dust types depends strongly on the metallicity of the parent population, implying that the metallicity of the progenitors affects the evolution of a PN from its early stages.
Herschel, an ESA space observatory equipped with science instruments provided by European-led Principal Investigator consortia with important participation from NASA, was launched on 14 May 2009. With its 3.5m diameter primary mirror, Herschel is the largest space telescope ever launched into space, and carries onboard three science instruments, whose focal plane units are cryogenically cooled inside a superfluid helium cryostat. The PACS and SPIRE instruments provide broadband imaging photometry in six bands centred at 75, 100, 160, 250, 350, and 500 microns and imaging spectroscopy over the range 55–672 microns. The HIFI instrument provides very high-resolution heterodyne spectroscopy over the ranges 157–212 and 240–625 microns. The results obtained already in the first year and a half of routine science operations demonstrate that Herschel will have strong impact on all research fields, from Solar System studies to the area of Cosmology, from the analysis of star formation to the mysteries of galaxy formation. In this talk I will review the Herschel highlights in the area of evolved stars in general and of planetary nebulae more in particular, resulting from observations performed with the three instruments onboard Herschel since launch. This will be exemplified by a few observational results, just the tip of the iceberg of what is yet to come in the remaining year and a half of science operations.
The ultraviolet (UV) domain, in particular shortwards of Lyα, provides unique information to unravel the physical parameters of Central Stars of Planetary Nebulae (CSPNe) and the paths for this elusive final stage of stellar evolution, thanks to a wealth of diagnostic transitions from ionic species not observable at other wavelengths. Intermediate mass stars are the major providers of important elements like C and N. Understanding how they shed most of their initial mass is critical for understanding the chemical enrichment of the ISM. Mass-loss diagnostic lines abound at UV wavelengths, and when the CSPN reaches the hottest Teff before turning on the WD-cooling sequence, and the wind fades, the last wind lines to disappear are found in the far-UV, as well as diagnostic lines for elements such as Ne. This domain also offers a host of H2 transitions, tracing the circum-stellar material expelled in previous phases. UV images and spectra of PNe add critical constraints to their ionization structure and to some abundances. Finally, the recent GALEX sky surveys in two UV bands afforded the first unbiased census of hot white dwarfs (WD) and post-AGB objects in the Milky Way, significantly expanding known catalogs and providing statistical constraints to the initial-final mass relation.
The Hubble Space Telescope has served the critical roles of microscope and movie camera in the past 20 years of research on planetary nebulae (“PNe”). We have glimpsed the details of the evolving structures of neutral and ionized post-AGB objects, built ingenious heuristic models that mimic these structures, and constrained most of the relevant physical processes with careful observations and interpretation. We have searched for close physical binary stars with spatial resolution ~50 AU at 1 AU, located jets emerging from the nucleus at speeds up to 2000 km s−1 and matched newly discovered molecular and X-ray emission regions to physical substructures in order to better understand how stellar winds and ionizing radiation interact to form the lovely symmetries that are observed. Ultraviolet spectra of CNO in PNe help to uncover how stars process deep inside AGB stars with unstable nuclear burning zones. HST broadband imaging has been at the forefront of uncovering surprisingly complex wind morphologies produced at the tip of the AGB, and has led to an increasing realization of the potentially vital roles of close binary stars and emerging magnetic fields in shaping stellar winds.
Post-AGB stars are key objects for the study of the dramatic morphological changes of low- to intermediate-mass stars on their evolution from the Asymptotic Giant Branch (AGB) towards the planetary nebula stage. There is growing evidences that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of post-AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid-infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of post-AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid-infrared using VISIR/VLT, T-Recs/Gemini South and Michelle/Gemini North. We found that all the the proto-planetary nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non resolved. The resolved targets can be divided in two categories. The nebulae with a dense central core, that are either bipolar and multipolar. The nebulae with no central core have an elliptical morphology. The dense central torus observed likely host binary systems which triggered fast outflows that shaped the nebulae.
The San Pedro Mártir kinematic catalogue of galactic planetary nebulae provides spatially resolved, long-slit, Echelle spectra for about 600 planetary nebulae, representing 55 observing runs and about 4000 individual integrations to date in this first release. The project is ongoing and will continue adding spectra to the database. The data are presented wavelength calibrated and corrected for heliocentric motion. This is the most extensive and homogeneous single source of data concerning the internal kinematics of the ionized nebular material in planetary nebulae. The catalogue is available through the world wide web at http://kincatpn.astrosen.unam.mx and an article will a full description of the catalogue will soon appear in the RevMexAA.
We report spectrophotometric observations made with SOFIA/FORCAST on 2011 June 2 UT. Optical measurements have previously shown that the abundance discrepancy factor (adf) varies with position in several high-adf PNe, and is highest close to the central star. The very low electron temperature inclusions postulated to explain the abundance discrepancy, must be cooled predominantly by fine structure IR lines. These SOFIA data will map mid-IR FS lines (and our related Herschel program will add several far-IR FS lines) in the bright, well-characterized, high-adf PN NGC 7009. We will compare these IR results with FS optical line measurements in order to correlate ratios of IR to optical fluxes with position, and thus correlate with where the adf peaks.
Mass loss on the AGB removes most of the envelope and leaves a compact remnant to become a white dwarf and perhaps first the central star of a planetary nebula. The envelope mass provides an upper limit on the material available to form the PN, and the terminal mass loss rate plus the small remnant mass left on the core determines how much of that would still be available to form the PN after the star has evolved far enough to the blue. Given a mass loss formula based on observations or models, we can find the deathline where −dMstar/dt = (M/L) dL/dt and can find the contours of constant mass loss rate on a plot of M vs. L. From such plots we can derive the mass available for a PN and the lowest mass single star that can produce a PN of a given mass. However, some details important for PN formation remain uncertain, including the maximum mass loss rate achieved and the envelope mass left when AGB mass loss ceases.
It is important to properly describe the mass-loss rate of AGB stars, in order to understand their evolution from the AGB to PN phase. The primary goal of this study is to investigate the influence of metallicity on the mass-loss rate, under well determined luminosities. The luminosity of the star is a crucial parameter for the radiative driven stellar wind. Many efforts have been invested to constrain the AGB mass-loss rate, but most of the previous studies use Galactic objects, which have poorly known distances, thus their luminosities. To overcome this problem, we have studied mass loss from AGB stars in the Galaxies of the Local Group. The distance to the stars have been independently measured, thus AGB stars in these galaxies are ideal for understanding the mass-loss rate. Moreover, these galaxies have a lower metallicity than the Milky Way, providing an ideal target to study the influence of metallicity on the mass-loss rate. We report our analysis of mass loss, using the Spitzer Space Telescope and the Herschel Space Observatory. We will discuss the influence of AGB mass-loss on stellar evolution, and explore AGB and PN contribution to the lifecycle of matter in galaxies.
We present a complete study of the morphology of post-Asymptotic Giant Branch (post-AGB) stars. The post-AGB stage is a very short evolutionary phase between the end of the AGB and the beginning of the Planetary Nebula (PN) stage (between 100 and 10,000 yrs). Post-AGB stars do not show variability and are not hot enough to fully ionize the hydrogen envelope. We have defined the end of the post-AGB phase and the beginning of the PN phase when the star has a temperature of 30000 K. Post-AGB stars have a circumstellar shell that is illuminated by the central stars or partially ionized. However, this circumstellar shell is too small to be resolved by ground-based observations. Thus, we have used the Hubble Space Telescope (HST) database to resolve these shells. 117 post-AGBs were found in this database. Here we present the preliminary results on their morphological classification and the correlation with the galactic latitude. Our preliminary results show that 38% of the sample are stellar-like (S), 31% bipolar (B), 12% multipolar (M) and 19% elliptical (E).
I provide a synthetic overview of the present status of stellar models for the asymptotic giant branch phase, one of the most complex and still uncertain stages of stellar evolution. In particular I will focus on two aspects that are most relevant in the context of the planetary nebulæ progeny, namely: the chemical composition of the AGB ejecta, and the mass of the bare CO core left after the ejection of the stellar mantle at the AGB tip. Recent progress, present uncertainties, and future perspectives to constrain AGB models are briefly discussed.
Many binary star systems are not wide enough to contain the progenitor stars from which they were made. One explanation for this is that when one star becomes a red giant a common envelope forms around both stars in the binary system. The core of the giant and its companion star continue to orbit one another inside the envelope. Frictional energy deposited into the common envelope may lead to its ejection and, if so, a close binary system is formed from the core of the former giant star and its relatively untouched companion. When the primary is an asymptotic giant branch star the core becomes a hot carbon-oxygen white dwarf which may ionise the ejected envelope and illuminate a planetary nebula. Many other types of binary systems form through common envelope evolution such as low-mass X-ray binaries and cataclysmic variables. In the case of a failed envelope ejection when the cores merge, rapidly-rotating solitary giants similar to FK Comae stars form. In this short review we focus on attempts to constrain parameters of common envelope evolution models and also describe the latest efforts to model this elusive phase of binary stellar evolution.
We present the results of 3D hydrodynamic simulations aimed to explore the binary scenario for shaping bipolar, point- and mirror-symmetric proto-Planetary Nebulae. We consider a jet launched by the secondary star of a binary system, located at the center of the PPN, which propagates within a circumstellar medium swept previously up by the wind of the giant companion. As a result of the presence of the companion star, the accretion disk around the jet source is likely to precess. We have carried out 3D hydrodynamical simulations with the YGUAZÚ-A code including the combination of an orbital motion plus a precession. Our results show that to produce a multipolar nebula, it is necessary to have a precessing jet in a binary system with a time-dependent ejection velocity.
Binary central stars have long been invoked to explain the vexing shapes of planetary nebulae (PNe) despite there being scant direct evidence to support this hypothesis. Modern large-scale surveys and improved observing strategies have allowed us to significantly boost the number of known close binary central stars and estimate at least 20% of PNe have close binary nuclei that passed through a common-envelope (CE) phase. The larger sample of post-CE nebulae appears to have a high proportion of bipolar nebulae, low-ionisation structures (especially in SN1987A-like rings) and polar outflows or jets. These trends are guiding our target selection in ongoing multi-epoch spectroscopic and photometric surveys for new binaries. Multiple new discoveries are being uncovered that further strengthen the connection between post-CE trends and close binaries. These ongoing surveys also have wider implications for understanding CE evolution, low-ionisation structure and jet formation, spectral classification of central stars, asymptotic giant branch (AGB) nucleosynthesis and dust obscuration events in PNe.
There is at present no viable theory whereby a single star with MMS < 2.3 ± 0.3 M⊙ can sustain a PN-forming superwind with Ṁ > 3×10−5 M⊙ yr−1 at the tip of the AGB that can eject the last ~0.3 M⊙ of envelope mass. We propose that a binary companion can not only result in non-spherical PN, but more importantly provide the AGB mass loss rate enhancement that is required to create a visible PN. We provide an overview of our binary population synthesis calculations of the PN formation rates from common envelope (CE) interactions, CE mergers with substellar companions, tidally synchronized systems that avoid CE, gravitationally focused winds, and double degenerate systems. The predicted number of Galactic PN with radii < 0.9 pc shaped and created by a binary companion is 8,100±2,300 which is (71±20)% of the observationally-estimated total. We demonstrate that the observed close central star of PN binary fraction of 15–20% is consistent with our overall binary fraction, considering we predict two binary populations with period distributions centred at log P(days) ~0 and 4. Finally, we discuss the impact of binarity on the PN luminosity function, central star mass distribution, chemical abundances, morphologies, etc., and why these distributions predicted in the binary scenario are close to observations while the single star paradigm produces distributions which are measurably discrepant.