Search

Aims .Over the past decades, libraries of stellar spectra have been used in a large variety of science cases, including as sources of reference spectra for a given object or a given spectral type. Despite the existence of large libraries and the increasing number of projects of large-scale spectral surveys, there is to date only one very high-resolution spectral library offering spectra from a few hundred objects from the southern hemisphere (UVES-POP). We aim to extend the sample, offering a finer coverage of effective temperatures and surface gravity with a uniform collection of spectra obtained in the northern hemisphere. Methods .Between 2010 and 2020, we acquired several thousand echelle spectra of bright stars with the Mercator-HERMES spectrograph located in the Roque de Los Muchachos Observatory in La Palma, whose pipeline offers high-quality data reduction products. We have also developed methods to correct for the instrumental response in order to approach the true shape of the spectral continuum. Additionally, we have devised a normalisation process to provide a homogeneous normalisation of the full spectral range for most of the objects. Results .We present a new spectral library consisting of 3256 spectra covering 2043 stars. It combines high signal-to-noise and high spectral resolution over the entire range of effective temperatures and luminosity classes. The spectra are presented in four versions: raw, corrected from the instrumental response, with and without correction from the atmospheric molecular absorption, and normalised (including the telluric correction)., SCOPUS: ar.j, info:eu-repo/semantics/published

We present JWST images of the well-known planetary nebula NGC 6720 (the Ring Nebula), covering wavelengths from 1.6$\mu$m to 25 $\mu$m. The bright shell is strongly fragmented with some 20 000 dense globules, bright in H$_2$, with a characteristic diameter of 0.2 arcsec and density $n_{\rm H} \sim 10^5$-$10^6$ cm$^{-3}$. The shell contains a thin ring of polycyclic aromatic hydrocarbon (PAH) emission. H$_2$ is found throughout the shell and in the halo. H$_2$ in the halo may be located on the swept-up walls of a biconal polar flow. The central cavity is shown to be filled with high ionization gas and shows two linear structures. The central star is located 2 arcsec from the emission centroid of the cavity and shell. Linear features (`spikes') extend outward from the ring, pointing away from the central star. Hydrodynamical simulations are shown which reproduce the clumping and possibly the spikes. Around ten low-contrast, regularly spaced concentric arc-like features are present; they suggest orbital modulation by a low-mass companion with a period of about 280 yr. A previously known much wider companion is located at a projected separation of about 15 000 au; we show that it is an M2-M4 dwarf. The system is therefore a triple star. These features, including the multiplicity, are similar to those seen in the Southern Ring Nebula (NGC 3132) and may be a common aspect of such nebulae., Comment: 25 pages, 23 figures. Submitted to Monthly Notices of the Royal Astronomical Society. Corrected typos in metadata

We present high angular-resolution observations of Sakurai's object using the Atacama Large Millimeter Array, shedding new light on its morpho-kinematical structure. The millimetre continuum emission, observed at an angular resolution of 20 milliarcsec (corresponding to 70 AU), reveals a bright compact central component whose spectral index indicates that it composed of amorphous carbon dust. Based on these findings, we conclude that this emission traces the previously suggested dust disc observed in mid-infrared observations. Therefore, our observations provide the first direct imaging of such a disc. The H$^{12}$CN($J$=4$\rightarrow$3) line emission, observed at an angular resolution of 300 milliarcsec (corresponding to 1000 AU), displays bipolar structure with a north-south velocity gradient. From the position-velocity diagram of this emission we identify the presence of an expanding disc and a bipolar molecular outflow. The inclination of the disc is determined to be $i$=72$^\circ$. The derived values for the de-projected expansion velocity and the radius of the disc are $v_{\rm exp}$=53 km s$^{-1}$ and $R$=277 AU, respectively. On the other hand, the de-projected expansion velocity of the bipolar outflow detected in the H$^{12}$CN($J$=4$\rightarrow$3) emission of approximately 1000 km s$^{-1}$. We propose that the molecular outflow has an hourglass morphology with an opening angle of around 60$^{\circ}$. Our observations unambiguously show that an equatorial disc and bipolar outflows formed in Sakurai's object in less than 30 years after the born-again event occurred, providing important constraints for future modelling efforts of this phenomenon., Comment: 9 pages, 5 figures. Accepted for publication as a Letter in Astronomy and Astrophysics

Planetary nebulae (PNe), the ejected envelopes of red giant stars, provide us with a history of the last, mass-losing phases of 90 percent of stars initially more massive than the Sun. Here, we analyse James Webb Space Telescope (JWST) Early Release Observation (ERO) images of the PN NGC3132. A structured, extended H2 halo surrounding an ionised central bubble is imprinted with spiral structures, likely shaped by a low-mass companion orbiting the central star at 40-60 AU. The images also reveal a mid-IR excess at the central star interpreted as a dusty disk, indicative of an interaction with another, closer companion. Including the previously known, A-type visual companion, the progenitor of the NGC3132 PN must have been at least a stellar quartet. The JWST images allow us to generate a model of the illumination, ionisation and hydrodynamics of the molecular halo, demonstrating the power of JWST to investigate complex stellar outflows. Further, new measurements of the A-type visual companion allow us to derive the value for the mass of the progenitor of a central star to date with excellent precision: 2.86+/-0.06 Mo. These results serve as path finders for future JWST observations of PNe providing unique insight into fundamental astrophysical processes including colliding winds, and binary star interactions, with implications for supernovae and gravitational wave systems., Comment: 32 pages, 5 figures for the main article. 12 pages 8 figures for the supplementary material

The planetary nebula (PN) NGC3132 is a striking example of the dramatic but poorly understood, mass-loss phenomena that (1-8) Msun stars undergo during their death throes as they evolve into white dwarfs (WDs). From an analysis of JWST multiwavelength (0.9-18 micron) imaging of NGC3132, we report the discovery of an asymmetrical dust cloud around the WD central star (CS) of NGC3132, seen most prominently in the 18 micron~image, with a surface-brightness limited radial extent of >~2 arcsec. We show that the A2V star located 1.7 arcsec to CS's North-East (and 0.75 kpc from Earth) is gravitationally-bound to the latter, by the detection of relative orbital angular motion of (0.24+/-0.045) deg between these stars over ~20 yr. Using aperture photometry of the CS extracted from the JWST images, together with published optical photometry and an archival UV spectrum, we have constructed the spectral-energy distribution (SED) of the CS and its extended emission over the UV to mid-IR (0.091-18 micron) range. We find that fitting the SED of the CS and the radial intensity distributions at 7.7, 12.8 and 18 micron with thermal emission from dust requires a cloud that extends to a radius of >~1785 au, with a dust mass of ~1.3 x 10^(-2) M(Earth) and grains that are 70% silicate and 30% amorphous carbon. We propose plausible origins of the dust cloud and an evolutionary scenario in which a system of three stars -- the CS, a close low-mass companion, and a more distant A2V star -- forms a stable hierarchical triple system on the main-sequence but becomes dynamically unstable later, resulting in the spectacular mass-ejections that form the current, multipolar PN., Comment: 23 pages, 5 tables, 8 figures (ApJ, in press) references added, minor revision of Fig 5 caption, minor typos corrected

After becoming ionized, low-density astrophysical plasmas will begin a process of slow recombination. Models for this still have significant uncertainties. The recombination cannot normally be observed in isolation, because the ionization follows the evolutionary time scale of the ionizing source. Laboratory experiments are unable to reach the appropriate conditions because of the required very long time scales. The extended nebula around the very late helium flash (VLTP) star V4334 Sgr provides a unique laboratory for this kind of study. The sudden loss of the ionizing UV radiation after the VLTP event has allowed the nebula to recombine free from other influences. More than 290 long slit spectra taken with FORS1/2 at the ESO VLT between 2007 and 2022 are used to follow the time evolution of lines of H, He, N, S, O, Ar. Hydrogen and helium lines, representing most of the ionized mass, do not show significant changes. A small increase is seen in [N II] (+2.8 %/yr; significance 2.7 sigma), while we see a decrease in [O III] (-1.96 %/yr; 2.0 sigma). The [S II] lines show a change of +3.0 %/yr; 1.6 sigma). The lines of [S III] and of Ar III] show no significant change. For [S III], the measurement differs from the predicted decrease by 4.5 sigma. A possible explanation is that the fraction of [S IV] and higher is larger than expected. Such an effect could provide a potential solution for the sulfur anomaly in planetary nebulae., Comment: 13 pages, 8 figures, 3 tables; Accepted for Publication in ApJ

Context. Hundreds of candidate hybrid pulsators of intermediate type A-F were revealed by recent space missions. Hybrid pulsators allow us to study the full stellar interiors, where both low-order p- and high-order g-modes are simultaneously excited. The true hybrid stars must be identified since other processes, related to stellar multiplicity or rotation, might explain the presence of (some) low frequencies observed in their periodograms. Aims. We measured the radial velocities of 50 candidate δ Scuti -γ Doradus hybrid stars from the Kepler mission with the Hermes and ace spectrographs over a time span of months to years. We aim to derive the fraction of binary and multiple systems and to provide an independent and homogeneous determination of the atmospheric properties and v sin i for all targets. The long(er)-term objective is to identify the (probable) physical cause of the low frequencies. Methods. We computed one-dimensional cross-correlation functions (CCFs) in order to find the best set of parameters in terms of the number of components, spectral type(s), and v sin i for each target. Radial velocities were measured using spectrum synthesis and a two-dimensional cross-correlation technique in the case of double- and triple-lined systems. Fundamental parameters were determined by fitting (composite) synthetic spectra to the normalised median spectra corrected for the appropriate Doppler shifts. Results. We report on the analysis of 478 high-resolution Hermes and 41 ace spectra of A/F-type candidate hybrid pulsators from the Kepler field. We determined their radial velocities, projected rotational velocities, and atmospheric properties and classified our targets based on the shape of the CCFs and the temporal behaviour of the radial velocities. We derived orbital solutions for seven new systems. Three preliminary long-period orbital solutions are confirmed by a photometric time-delay analysis. Finally, we determined a global multiplicity fraction of 27% in our, SCOPUS: ar.j, info:eu-repo/semantics/published

We have obtained contemporaneous light, color, and radial velocity data for three proto-planetary nebulae (PPNe) over the years 2007 to 2015. The light and velocity curves of each show similar periods of pulsation, with photometric periods of 42 and 50 days for IRAS 17436+5003, 102 days for IRAS 18095+2704, and 35 days for IRAS 19475+3119. The light and velocity curves are complex with multiple periods and small, variable amplitudes. Nevertheless, at least over limited time intervals, we were able to identify dominant periods in the light, color, and velocity curves and compare the phasing of each. The color curves appear to peak with or slightly after the light curves while the radial velocity curves peak about a quarter of a cycle before the light curves. Similar results were found previously for two other PPNe, although for them the light and color appeared to be in phase. Thus it appears that PPNe are brightest when smallest and hottest. These phase results differ from those found for classical Cepheid variables, where the light and velocity differ by half a cycle, and are hottest at about average size and expanding. However, they do appear to have similar phasing to the larger amplitude pulsations seen in RV Tauri variables. Presently, few pulsation models exist for PPNe, and these do not fit the observations well, especially the longer periods observed. Model fits to these new light and velocity curves would allow masses to be determined for these post-AGB objects, and thereby provide important constraints to post-AGB stellar evolution models of low and intermediate-mass stars., Comment: Accepted for publication in Astron. J.: 33 pages, 12 figures

We have obtained contemporaneous light, color, and radial velocity data for three proto-planetary nebulae (PPNe) over the years 2007 to 2015. The light and velocity curves of each show similar periods of pulsation, with photometric periods of 42 and 50 days for IRAS 17436+5003, 102 days for IRAS 18095+2704, and 35 days for IRAS 19475+3119. The light and velocity curves are complex with multiple periods and small, variable amplitudes. Nevertheless, at least over limited time intervals, we were able to identify dominant periods in the light, color, and velocity curves and compare the phasing of each. The color curves appear to peak with or slightly after the light curves while the radial velocity curves peak about a quarter of a cycle before the light curves. Similar results were found previously for two other PPNe, although for them the light and color appeared to be in phase. Thus it appears that PPNe are brightest when smallest and hottest. These phase results differ from those found for classical Cepheid variables, where the light and velocity differ by half a cycle, and are hottest at about average size and expanding. However, they do appear to have similar phasing to the larger amplitude pulsations seen in RV Tauri variables. Presently, few pulsation models exist for PPNe, and these do not fit the observations well, especially the longer periods observed. Model fits to these new light and velocity curves would allow masses to be determined for these post-AGB objects, and thereby provide important constraints to post-AGB stellar evolution models of low and intermediate-mass stars., Comment: Accepted for publication in Astron. J.: 33 pages, 12 figures

We present the results of an expanded, long-term radial velocity search (25 yrs) for evidence of binarity in a sample of seven bright proto-planetary nebulae (PPNe). The goal is to investigate the widely-held view that the bipolar or point-symmetric shapes of planetary nebulae (PNe) and PPNe are due to binary interactions. Observations from three observatories were combined from 2007-2015 to search for variations on the order of a few years and then combined with earlier observations from 1991-1995 to search for variations on the order of decades. All seven show velocity variations due to periodic pulsation in the range of 35-135 days. However, in only one PPN, IRAS 22272+5435, did we find even marginal evidence found for multi-year variations that might be due to a binary companion. This object shows marginally-significant evidence of a two-year period of low semi-amplitude which could be due to a low-mass companion, and it also displays some evidence of a much longer period of >30 years. The absence of evidence in the other six objects for long-period radial velocity variations due to a binary companion sets significant constraints on the properties of any undetected binary companions: they must be of low mass, <0.2 M(sun), or long period, >30 years. Thus the present observations do not provide direct support for the binary hypothesis to explain the shapes of PNe and PPNe and severely constrains the properties of any such undetected companions., Comment: 28 pages, 5 figures

This article presents an overview of the published results for planetary nebulae based on images and spectroscopy from the PACS, SPIRE, and HIFI instruments on board the Herschel satellite., Comment: 8 pages, 2 figures Invited review to appear in the Proceedings of the IAU Symp. 323: "Planetary nebulae: Multi-wavelength probes of stellar and galactic evolution". Eds. X.-W. Liu, L. Stanghellini and A. Karakas

We present the results of an expanded, long-term radial velocity search (25 yrs) for evidence of binarity in a sample of seven bright proto-planetary nebulae (PPNe). The goal is to investigate the widely-held view that the bipolar or point-symmetric shapes of planetary nebulae (PNe) and PPNe are due to binary interactions. Observations from three observatories were combined from 2007-2015 to search for variations on the order of a few years and then combined with earlier observations from 1991-1995 to search for variations on the order of decades. All seven show velocity variations due to periodic pulsation in the range of 35-135 days. However, in only one PPN, IRAS 22272+5435, did we find even marginal evidence found for multi-year variations that might be due to a binary companion. This object shows marginally-significant evidence of a two-year period of low semi-amplitude which could be due to a low-mass companion, and it also displays some evidence of a much longer period of >30 years. The absence of evidence in the other six objects for long-period radial velocity variations due to a binary companion sets significant constraints on the properties of any undetected binary companions: they must be of low mass, <0.2 M(sun), or long period, >30 years. Thus the present observations do not provide direct support for the binary hypothesis to explain the shapes of PNe and PPNe and severely constrains the properties of any such undetected companions., Comment: 28 pages, 5 figures

This article presents an overview of the published results for planetary nebulae based on images and spectroscopy from the PACS, SPIRE, and HIFI instruments on board the Herschel satellite., Comment: 8 pages, 2 figures Invited review to appear in the Proceedings of the IAU Symp. 323: "Planetary nebulae: Multi-wavelength probes of stellar and galactic evolution". Eds. X.-W. Liu, L. Stanghellini and A. Karakas

We present new Herschel photometric and spectroscopic observations of Supernova 1987A, carried out in 2012. Our dedicated photometric measurements provide new 70 mu m data and improved imaging quality at 100 and 160 mu m compared to previous observations in 2010. Our Herschel spectra show only weak CO line emission, and provide an upper limit for the 63 mu m [O-I] line flux, eliminating the possibility that line contaminations distort the previously estimated dustmass. The far-infrared spectral energy distribution (SED) is well fitted by thermal emission from cold dust. The newly measured 70 mu m flux constrains the dust temperature, limiting it to nearly a single temperature. The far-infrared emission can be fitted by 0.5 +/- 0.1M(circle dot) of amorphous carbon, about a factor of two larger than the current nucleosynthetic mass prediction for carbon. The observation of SiO molecules at early and late phases suggests that silicates may also have formed and we could fit the SED with a combination of 0.3M(circle dot) of amorphous carbon and 0.5M(circle dot) of silicates, totalling 0.8M(circle dot) of dust. Our analysis thus supports the presence of a large dust reservoir in the ejecta of SN 1987A. The inferred dust mass suggests that supernovae can be an important source of dust in the interstellar medium, from local to high-redshift galaxies., AuthorCount:36

We present new Herschel photometric and spectroscopic observations of Supernova 1987A, carried out in 2012. Our dedicated photometric measurements provide new 70 micron data and improved imaging quality at 100 and 160 micron compared to previous observations in 2010. Our Herschel spectra show only weak CO line emission, and provide an upper limit for the 63 micron [O I] line flux, eliminating the possibility that line contaminations distort the previously estimated dust mass. The far-infrared spectral energy distribution (SED) is well fitted by thermal emission from cold dust. The newly measured 70 micron flux constrains the dust temperature, limiting it to nearly a single temperature. The far-infrared emission can be fitted by 0.5+-0.1 Msun of amorphous carbon, about a factor of two larger than the current nucleosynthetic mass prediction for carbon. The observation of SiO molecules at early and late phases suggests that silicates may also have formed and we could fit the SED with a combination of 0.3 Msun of amorphous carbon and 0.5 Msun of silicates, totalling 0.8 Msun of dust. Our analysis thus supports the presence of a large dust reservoir in the ejecta of SN 1987A. The inferred dust mass suggests that supernovae can be an important source of dust in the interstellar medium, from local to high-redshift galaxies., Comment: ApJ accepted, 8 pages

We present new Herschel photometric and spectroscopic observations of Supernova 1987A, carried out in 2012. Our dedicated photometric measurements provide new 70 micron data and improved imaging quality at 100 and 160 micron compared to previous observations in 2010. Our Herschel spectra show only weak CO line emission, and provide an upper limit for the 63 micron [O I] line flux, eliminating the possibility that line contaminations distort the previously estimated dust mass. The far-infrared spectral energy distribution (SED) is well fitted by thermal emission from cold dust. The newly measured 70 micron flux constrains the dust temperature, limiting it to nearly a single temperature. The far-infrared emission can be fitted by 0.5+-0.1 Msun of amorphous carbon, about a factor of two larger than the current nucleosynthetic mass prediction for carbon. The observation of SiO molecules at early and late phases suggests that silicates may also have formed and we could fit the SED with a combination of 0.3 Msun of amorphous carbon and 0.5 Msun of silicates, totalling 0.8 Msun of dust. Our analysis thus supports the presence of a large dust reservoir in the ejecta of SN 1987A. The inferred dust mass suggests that supernovae can be an important source of dust in the interstellar medium, from local to high-redshift galaxies., Comment: ApJ accepted, 8 pages

We surveyed the central 4 x 4 degrees of the Galactic center for planetary nebulae in the light of [S III] 9532 A and found 94 PNe that were not previously known, plus 3 that were previously identified as possible candidates. For 63 of these 97 objects, we obtained spectra that are consistent with highly reddened PN while the other 34 could not be recovered spectroscopically and remain unverified. Of the 94 candidates, 54 and 57 were detected via radio at 3 and 6 cm, respectively. An additional 20 PNe candidates were found during follow-up Halpha imaging but have not yet been verified spectroscopically. Based on the properties of IRAS sources in this region of the Galaxy, and on the total luminosity of the Galactic bulge, the expected number of PNe is ~250, only 50 % more than the 160 PNe candidates now known. Thus, surveys for PNe in the bulge are approximately two-thirds complete with the remainder likely hidden behind dust., Comment: A&A in press; the full version of the paper, including the appendix, can be found at: http://homepage.oma.be/gsteene/publications.html

We surveyed the central 4 x 4 degrees of the Galactic center for planetary nebulae in the light of [S III] 9532 A and found 94 PNe that were not previously known, plus 3 that were previously identified as possible candidates. For 63 of these 97 objects, we obtained spectra that are consistent with highly reddened PN while the other 34 could not be recovered spectroscopically and remain unverified. Of the 94 candidates, 54 and 57 were detected via radio at 3 and 6 cm, respectively. An additional 20 PNe candidates were found during follow-up Halpha imaging but have not yet been verified spectroscopically. Based on the properties of IRAS sources in this region of the Galaxy, and on the total luminosity of the Galactic bulge, the expected number of PNe is ~250, only 50 % more than the 160 PNe candidates now known. Thus, surveys for PNe in the bulge are approximately two-thirds complete with the remainder likely hidden behind dust., Comment: A&A in press; the full version of the paper, including the appendix, can be found at: http://homepage.oma.be/gsteene/publications.html

We present the basic properties of the multipolar post-AGB star IRAS 16594-4656, and discuss in particular its near infrared spectrum which shows shock excited H2 and [Fe II] emission lines., Comment: 4 pages, 5 figures; to appear in "Asymmetrical Planetary Nebulae III" editors M. Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series)

We present the basic properties of the multipolar post-AGB star IRAS 16594-4656, and discuss in particular its near infrared spectrum which shows shock excited H2 and [Fe II] emission lines., Comment: 4 pages, 5 figures; to appear in "Asymmetrical Planetary Nebulae III" editors M. Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series)

We observed 64 newly identified galactic bulge planetary nebulae in the radio continuum at 3 and 6 cm with the Australia Telescope Compact Array. We present their radio images, positions, flux densities, and angular sizes. The survey appears to have detected a larger ratio of more extended planetary nebulae with low surface brightness than in previous surveys. We calculated their distances according to Van de Steene & Zijlstra (1995). We find that most of the new sample is located on the near side around the galactic center and closer in than the previously known bulge PNe. Based on H-alpha images and spectroscopic data, we calculated the total H-alpha flux. We compare this flux value with the radio flux density and derive the extinction. We confirm that the distribution of the extinction values around the galactic center rises toward the center, as expected., Comment: accepted for publication in A&A

We observed 64 newly identified galactic bulge planetary nebulae in the radio continuum at 3 and 6 cm with the Australia Telescope Compact Array. We present their radio images, positions, flux densities, and angular sizes. The survey appears to have detected a larger ratio of more extended planetary nebulae with low surface brightness than in previous surveys. We calculated their distances according to Van de Steene & Zijlstra (1995). We find that most of the new sample is located on the near side around the galactic center and closer in than the previously known bulge PNe. Based on H-alpha images and spectroscopic data, we calculated the total H-alpha flux. We compare this flux value with the radio flux density and derive the extinction. We confirm that the distribution of the extinction values around the galactic center rises toward the center, as expected., Comment: accepted for publication in A&A

We have investigated the infrared properties of a sample of objects with IRAS colors typical of planetary nebulae. The selected objects have not yet evolved to the planetary nebula stage since they have no detectable radio continuum emission and they are therefore likely post-AGB stars., Comment: 4 pages, 2 figures,7 pages, 5 figures, to appear in the proceedings of: "Post-AGB Objects (Proto-Planetary Nebulae) as a Phase of Stellar Evolution", held in Torun, Poland, July 5-7, 2000; eds. R. Szczerba, R. Tylenda, and S.K. Gorny. See also the accepted A&A paper at http://xxx.lanl.gov/abs/astro-ph/0008034

We have investigated the infrared properties of a sample of objects with IRAS colors typical of planetary nebulae. The selected objects have not yet evolved to the planetary nebula stage since they have no detectable radio continuum emission and they are therefore likely post-AGB stars., Comment: 4 pages, 2 figures,7 pages, 5 figures, to appear in the proceedings of: "Post-AGB Objects (Proto-Planetary Nebulae) as a Phase of Stellar Evolution", held in Torun, Poland, July 5-7, 2000; eds. R. Szczerba, R. Tylenda, and S.K. Gorny. See also the accepted A&A paper at http://xxx.lanl.gov/abs/astro-ph/0008034

We present H alpha emission line spectra of 7 post-AGB stars. Typically, they have a P-Cygni type profile with a very strong emission component and a relatively narrow absorption component. These lines are formed in the fast post-AGB wind which in most cases has a velocity of around 100 km/s. Hence the emission originates close to the central star and the absorption occurs in the fast wind itself or in the region where the fast wind sweeps up the slow wind. The emission profile has very broad wings. It is still unclear what causes these wings., Comment: 4 pages, 1 figure. To appear in ``Asymmetrical Planetary Nebulae II: from Origins to Microstructures,'' ASP Conference Series, Vol. 199, 2000; J.H. Kastner, N. Soker, & S.A. Rappaport, eds

We present H alpha emission line spectra of 7 post-AGB stars. Typically, they have a P-Cygni type profile with a very strong emission component and a relatively narrow absorption component. These lines are formed in the fast post-AGB wind which in most cases has a velocity of around 100 km/s. Hence the emission originates close to the central star and the absorption occurs in the fast wind itself or in the region where the fast wind sweeps up the slow wind. The emission profile has very broad wings. It is still unclear what causes these wings., Comment: 4 pages, 1 figure. To appear in ``Asymmetrical Planetary Nebulae II: from Origins to Microstructures,'' ASP Conference Series, Vol. 199, 2000; J.H. Kastner, N. Soker, & S.A. Rappaport, eds

We present $\textit{JWST}$ images of the well-known planetary nebula NGC 6720 (the Ring Nebula), covering wavelengths from 1.6$\mu$m to 25$\mu$m. The bright shell is strongly fragmented with some 20 000 dense globules, bright in $H_2$, with a characteristic diameter of 0.2 arcsec and density $n_H$ ∼ $10^5$–$10^6$ $\textit{cm}$$^{-3}$. The shell contains a narrow ring of polycyclic aromatic hydrocarbon (PAH) emission. $H_2$ is found throughout the shell and also in the halo. $H_2$ in the halo may be located on the swept-up walls of a biconal polar flow. The central cavity is filled with high ionization gas and shows two linear structures which we suggest are the edges of a biconal flow, seen in projection against the cavity. The central star is located 2 arcsec from the emission centroid of the cavity and shell. Linear features (‘spikes’) extend outward from the ring, pointing away from the central star. Hydrodynamical simulations reproduce the clumping and possibly the spikes. Around ten low-contrast, regularly spaced concentric arc-like features are present; they suggest orbital modulation by a low-mass companion with a period of about 280 yr. A previously known much wider companion is located at a projected separation of about 15 000 au; we show that it is an M2–M4 dwarf. NGC 6720 is therefore a triple star system. These features, including the multiplicity, are similar to those seen in the Southern Ring Nebula (NGC 3132) and may be a common aspect of such nebulae.