Central Star Research Articles

About the loss of a primordial atmosphere of super-Earths by planetesimal impacts

ABSTRACT We consider planets composed of water ice and rock, located far from a central star. In an earlier study, computing the growth of planets by continuous accretion, we found that a large fraction of the ice evaporates upon accretion, creating a water vapor atmosphere. Here, we consider accretion as a discrete series of planetesimal impacts (of the order of 108), at the same time-dependent accretion rate, and investigate the fate of the vapor, as a result of its interaction with the accreting planetesimals. We find that a large fraction of the vapor escapes. The remaining fraction may form an outer layer of ice after the termination of accretion and cooling of the surface. The escaped water mass may significantly alter the ice-to-rock ratio of the planet. We investigate the effect of different choices of parameters such as the ice-to-rock ratio, the planetesimal size distribution, and the impact velocities. We find that the planetesimal size distribution has a negligible effect and explain why. By contrast, the ice-to-rock ratio and impact velocities affect the fraction of retained water masses considerably.

A survey for variable young stars with small telescopes: VI – Analysis of the outbursting Be stars NSW 284, gaia 19eyy, and VES 263

ABSTRACT This paper is one in a series reporting results from small telescope observations of variable young stars. Here, we study the repeating outbursts of three likely Be stars based on long-term optical, near-infrared, and mid-infrared photometry for all three objects, along with follow-up spectra for two of the three. The sources are characterized as rare, truly regularly outbursting Be stars. We interpret the photometric data within a framework for modelling light-curve morphology, and find that the models correctly predict the burst shapes, including their larger amplitudes and later peaks towards longer wavelengths. We are thus able to infer the start and end times of mass loading into the circumstellar discs of these stars. The disc sizes are typically 3 – 6 times the areas of the central star. The disc temperatures are ∼40 per cent, and the disc luminosities are ∼10 per cent of those of the central Be star, respectively. The available spectroscopy is consistent with inside-out evolution of the disc. Higher excitation lines have larger velocity widths in their double-horned shaped emission profiles. Our observations and analysis support the decretion disc model for outbursting Be stars.

On the Evolution of Angles between the Magnetic Moment and the Axis of Rotation in Radio Pulsars

Three methods were used to calculate the angles β between the magnetic moment and the axis of rotation of central neutron stars for 307 known radio pulsars. There is no explicit statistical dependence of the calculated angles β on the kinematic age of pulsars. It follows from this that either the indicated angle does not change noticeably with the age of the pulsar, or pulsars are born with an arbitrary inclination angle.

A New Study on a Type Iax Stellar Remnant and its Probable Association with SN 1181

We report observations and modeling of the stellar remnant and presumed double-degenerate merger of Type Iax supernova Pa 30, which is the probable remnant of SN 1181 AD. It is the only known bound stellar SN remnant and the only star with Wolf–Rayet features that is neither a planetary nebula central star nor a massive Population I progenitor. We model the unique emission-line spectrum with broad, strong O vi and O viii lines as a fast stellar wind and shocked, hot gas. Non-LTE wind modeling indicates a mass-loss rate of ∼10−6 M ⊙ yr−1 and a terminal velocity of ∼15,000 km s−1, consistent with earlier results. O viii lines indicate shocked gas temperatures of T ≃ 4 MK. We derive a magnetic field upper limit of B < 2.5 MG, below earlier suggestions. The luminosity indicates a remnant mass of 1.0–1.65 M ⊙ with ejecta mass 0.15 ± 0.05 M ⊙. Archival photometry suggests the stellar remnant has dimmed by ∼0.5 mag over 100 yr. A low Ne/O < 0.15 argues against an O-Ne white dwarf in the merger. A cold dust shell is only the second detection of dust in an SN Iax and the first of cold dust. Our ejecta mass and kinetic energy estimates of the remnant are consistent with Type Iax extragalactic sources.

The Binary and the Disk: The Beauty is Found within NGC3132 with JWST

The planetary nebula (PN) NGC 3132 is a striking example of the dramatic but poorly understood mass-loss phenomena that (1–8) M ⊙ stars undergo during their death throes as they evolve into white dwarfs (WDs). From an analysis of JWST multiwavelength (0.9–18 μm) imaging of NGC 3132, we report the discovery of an extended dust cloud around the WD central star (CS) of NGC 3132, seen most prominently in the 18 μm image, with a surface-brightness-limited radial extent of ≳2″. We show that the A2V star located 1.″7 to CS’s northeast (and 0.75 kpc from Earth) is gravitationally bound to the latter, as evidenced by the detection of relative orbital angular motion of 0.°24 ± 0.°045 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 μm) range. We find that fitting the SED of the CS and the radial intensity distributions at 7.7, 12.8, and 18 μm with thermal emission from dust requires a cloud that extends to a radius of ≳1785 au, with a dust mass of ∼1.3 × 10−2 M ⊕ 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.

Centrifugal barriers in magnetospheric accretion

ABSTRACT We reconsider the dynamics of accretion flows onto magnetized central star. For dipolar magnetically aligned case, the centrifugal barrier is at Rcb = (2/3)1/3Rc = 0.87Rc, where Rc = (GM/Ω2)1/3 is the corotation radius. For oblique dipole direct accretion from the corotation radius Rc is possible only for magnetic obliquity satisfying $\tan \theta _\mu \ge 1/(2 \sqrt{3})$ (θμ ≥ 16.1°). The accretion proceeds in a form of funnel flows – along two streams centred on the μ–Ω plane, with azimuthal opening angle $\cos (\Delta \phi) = { \cot ^ 2 {\theta _\mu } }/{12}$. For the magnetosphere distorted by the diamagnetic disc, the centrifugal barrier can be at as small radius as Rcb = 0.719Rc for the fully confined dipole, extending out to Rcb ∼ Rc for the magnetically balanced case. Type-II X-ray bursts in accreting neutron stars may be mediated by the centrifugal barrier; this requires nearly aligned configuration. Centrifugally barriered material trapped in the magnetosphere may lead to periodic obscuration (‘dips’) in the light curve of the host star, e.g. as observed in accreting young stellar objects and X-ray binaries.

Editorial: Planetary nebulae as tools for astrophysics

Planetary nebulae (PNe) represent the ejected outer layers of an evolved low-to-intermediate mass 11star, and consist of ionized, neutral, and molecular components, along with dust. In total, the nebular 12 material contains the stellar envelope that was ejected during the AGB phase or through binary 13 interactions with a companion. The PN central star is formed by the former core of the AGB or RGB 14 progenitor of the PNe, which is now contracting towards the white dwarf phase. The PN central star 15 is a post-AGB object which has evolved crossing the HR diagram, and has an effective temperature 16 of 28,000 K up to more than 250,000 K. It emits a large fraction of its light as UV photons that ionize 17 the envelope. PNe are mainly studied through imaging, photometry and low-and high-resolution 18 spectroscopy techniques which allow determination of the physical conditions and chemical 19 abundances in the nebula. Through this kind of analysis it is possible to study the physical processes 20 in ionized plasmas, the evolution and characteristics of the central star, along with the mechanisms of 21 ejection of the nebula and the evolution of the interstellar medium (ISM) -in external galaxies as 22 well as in the Milky Way -which will be enriched with elements produced in the stellar 23 nucleosynthesis and brought to the stellar surface via several dredge-up episodes. Further, an accurate 24 census of PNe provides an important constraint on a galaxy's star formation history.

Vortex weighing and dating of planets in protoplanetary discs

ABSTRACT High-resolution sub-mm observations of some protoplanetary discs reveal non-axisymmetric features, which can often be interpreted as dust concentrations in vortices that form at the edges of gaps carved out by the embedded planets. We use recent results on the time-scale for the planet-driven vortex development in low-viscosity discs to set constraints on the mass and age of a planet producing the vortex. Knowledge of the age of the central star in a vortex-bearing protoplanetary disc system allows one to set a lower limit on the planetary mass at the level of several tens of M⊕. Also, an independent upper limit on the planetary mass would constrain the planetary age, although given the current direct imaging detection limits this constraint is not yet very stringent (it is also sensitively dependent on the disc scale height). These results can be extended to account for the history of planetary mass accretion if it is known. We apply our calculations to several protoplanetary discs harbouring vortex-like features as revealed by ALMA and set limits of (30–50) M⊕ (for disc aspect ratio of 0.1) on the minimum masses of putative planets that could be responsible for these vortices. Our vortex-based method provides an independent way of constraining the properties of embedded planets, complementary to other approaches.

The GRAVITY young stellar object survey

Context. Hot atomic hydrogen emission lines in pre-main sequence stars serve as tracers for physical processes in the innermost regions of circumstellar accretion disks, where the interaction between a star and disk is the dominant influence on the formation of infalls and outflows. In the highly magnetically active T Tauri stars, this interaction region is particularly shaped by the stellar magnetic field and the associated magnetosphere, covering the inner five stellar radii around the central star. Even for the closest T Tauri stars, a region as compact as this is only observed on the sky plane at sub-mas scales. To resolve it spatially, the capabilities of optical long baseline interferometry are required. Aims. We aim to spatially and spectrally resolve the Brγ hydrogen emission line with the methods of interferometry in order to examine the kinematics of the hydrogen gas emission region in the inner accretion disk of a sample of solar-like young stellar objects. The goal is to identify trends and categories among the sources of our sample and to discuss whether or not they can be tied to different origin mechanisms associated with Brγ emission in T Tauri stars, chiefly and most prominently magnetospheric accretion. Methods. We observed a sample of seven T Tauri stars for the first time with VLTI GRAVITY, recording spectra and spectrally dispersed interferometric quantities across the Brγ line at 2.16 µm in the near-infrared K-band. We used the visibilities and differential phases to extract the size of the Brγ emission region and the photocentre shifts on a channel-by-channel basis, probing the variation of spatial extent at different radial velocities. To assist in the interpretation, we also made use of radiative transfer models of magnetospheric accretion to establish a baseline of expected interferometric signatures if accretion is the primary driver of Brγ emission. Results. From among our sample, we find that five of the seven T Tauri stars show an emission region with a half-flux radius in the four to seven stellar radii range that is broadly expected for magnetospheric truncation. Two of the five objects also show Brγ emission primarily originating from within the co-rotation radius, which is an important criterion for magnetospheric accretion. Two objects exhibit extended emission on a scale beyond 10 R*, one of them is even beyond the K-band continuum half-flux radius of 11.3 R*. The observed photocentre shifts across the line can be either similar to what is expected for disks in rotation or show patterns of higher complexity. Conclusions. Based on the observational findings and the comparison with the radiative transfer models, we find strong evidence to suggest that for the two weakest accretors in the sample, magnetospheric accretion is the primary driver of Brγ radiation. The results for the remaining sources imply either partial or strong contributions coming from additional, spatially extended emission components in the form of outflows, such as stellar or disk winds. We expect that in actively accreting T Tauri stars, these phenomena typically occur simultaneously on different spatial scales. Through more advanced modelling, interferometry will be a key factor in disentangling their distinct contributions to the total Brγ flux arising from the innermost disk regions.

A Survey for C ii Emission-line Stars in the Large Magellanic Cloud. II. Final Results and the Origin of C ii Emission in [WC] Spectra

We present the final results of an imaging and spectroscopic search for stars in the Large Magellanic Cloud (LMC) with C ii λλ7231, 7236 emission lines. The goal is to identify and study [WC11] stars, the coolest of the low-mass Wolf–Rayet sequence, and a subset of central stars of planetary nebulae where the C ii lines are known to be especially prominent. A recent serendipitous discovery of an LMC [WC11] raised the possibility that these objects, although difficult to identify, might in fact be more common than previously believed. Several new members of this rare class have been found in this survey. It now seems clear, however, that a significant number of these stars are not hiding among the general [WC] population. We point out that the C ii doublet intensity ratio observed in our spectra proves to neatly divide the objects into two distinct groups, with the C ii emission likely originating from either the stellar wind or a surrounding nebula. The physics of the C ii emission mechanism correctly explains this bifurcation. Spectral subtypes are suggested for most of the objects. The numerous spectroscopic clues now available for these objects should facilitate future detailed modeling.

Hot white dwarfs and pre-white dwarfs discovered with SALT

ABSTRACT The Southern African Large Telescope survey of helium-rich hot subdwarfs aims to explore evolutionary pathways among groups of highly evolved stars. The selection criteria mean that several hot white dwarfs and related objects have also been included. This paper reports the discovery and analysis of eight new very hot white dwarf and pre-white dwarf stars with effective temperatures exceeding 100 000 K. They include two PG1159 stars, one DO white dwarf, three O(He), and two O(H) stars. One of the O(H) stars is the central star of a newly discovered planetary nebula, and the other is the hottest ‘naked’ O(H) star. Both of the PG1159 stars are GW Vir variables, one being the hottest GW Vir star measured and a crucial test for pulsation stability models. The DO white dwarf is also the hottest in its class.

Panchromatic HST/WFC3 Imaging Studies of Young, Rapidly Evolving Planetary Nebulae. II. NGC 7027

The iconic planetary nebula (PN) NGC 7027 is bright, nearby (D ∼ 1 kpc), highly ionized, intricately structured, and well observed. This nebula is hence an ideal case study for understanding PN shaping and evolution processes. Accordingly, we have conducted a comprehensive imaging survey of NGC 7027 comprised of 12 HST Wide Field Camera 3 images in narrow-band and continuum filters spanning the wavelength range 0.243–1.67 μm. The resulting panchromatic image suite reveals the spatial distributions of emission lines covering low-ionization species such as singly ionized Fe, N, and Si, through H recombination lines, to more highly ionized O and Ne. These images, combined with available X-ray and radio data, provide the most extensive view of the structure of NGC 7027 obtained to date. Among other findings, we have traced the ionization structure and dust extinction within the nebula in subarcsecond detail; uncovered multipolar structures actively driven by collimated winds that protrude through and beyond the PN’s bright inner core; compared the ionization patterns in the WFC3 images to X-ray and radio images of its interior hot gas and to its molecular outflows; pinpointed the loci of thin, shocked interfaces deep inside the nebula; and more precisely characterized the central star. We use these results to describe the recent history of this young and rapidly evolving PN in terms of a series of shaping events. This evolutionary sequence involves both thermal and ram pressures, and is far more complex than predicted by extant models of UV photoionization or winds from a single central progenitor star, thereby highlighting the likely influence of an unseen binary companion.

Mapping NGC 7027 in New Light: CO+ and HCO+ Emission Reveal Its Photon- and X-Ray-dominated Regions

The young and well-studied planetary nebula (PN) NGC 7027 harbors significant molecular gas that is irradiated by luminous, pointlike UV (central star) and diffuse (shocked nebular) X-ray emission. This nebula represents an excellent subject to investigate the molecular chemistry and physical conditions within photon- and X-ray-dominated regions (PDRs and XDRs). As yet, the exact formation routes of CO+ and HCO+ in PN environments remain uncertain. Here we present ∼2″ resolution maps of NGC 7027 in the irradiation tracers CO+ and HCO+ obtained with the IRAM NOEMA interferometer, along with SMA CO and HST 2.12 μm H2 data for context. The CO+ map constitutes the first interferometric map of this molecular ion in any PN. Comparison of CO+ and HCO+ maps reveals strikingly different emission morphologies, as well as a systematic spatial displacement between the two molecules; the regions of brightest HCO+, found along the central waist of the nebula, are radially offset by ∼1″ (∼900 au) outside the corresponding CO+ emission peaks. The CO+ emission furthermore precisely traces the inner boundaries of the nebula’s PDR (as delineated by near-IR H2 emission), suggesting that central star UV emission drives CO+ formation. The displacement of HCO+ radially outward with respect to CO+ is indicative that dust-penetrating soft X-rays are responsible for enhancing the HCO+ abundance in the surrounding molecular envelope, forming an XDR. These interferometric CO+ and HCO+ observations of NGC 7027 thus clearly establish the spatial distinction between the PDR and XDR formed (respectively) by intense UV and X-ray irradiation of molecular gas.

Orbital structure of planetary systems formed by giant impacts: stellar mass dependence

ABSTRACT Recent exoplanet surveys revealed that for solar-type stars, close-in Super-Earths are ubiquitous and many of them are in multiplanet systems. These systems are more compact than the Solar system’s terrestrial planets. However, there have been few theoretical studies on the formation of such planets around low-mass stars. In the standard model, the final stage of terrestrial planet formation is the giant impact stage, where protoplanets gravitationally scatter and collide with each other and then evolve into a stable planetary system. We investigate the effect of the stellar mass on the architecture of planetary systems formed by giant impacts. We perform N-body simulations around stars with masses of 0.1–2 times the solar mass. Using the isolation mass of protoplanets, we distribute the initial protoplanets in 0.05–0.15 au from the central star and follow the evolution for 200 million orbital periods of the innermost protoplanet. We find that for a given protoplanet system, the mass of planets increases as the stellar mass decreases, while the number of planets decreases. The eccentricity and inclination of orbits and the orbital separation of adjacent planets increase with decreasing the stellar mass. This is because as the stellar mass decreases, the relative strength of planetary scattering becomes more effective. We also discuss the properties of planets formed in the habitable zone using the minimum-mass extrasolar nebula model.

Using debris disk observations to infer substellar companions orbiting within or outside a parent planetesimal belt

Context. Alongside a debris disk, substellar companions often exist in the same system. The companions influence the dust dynamics via their gravitational potential. Aims. We analyze whether the effects of secular perturbations, originating from a substellar companion, on the dust dynamics can be investigated with spatially resolved observations. Methods. We numerically simulated the collisional evolution of narrow and eccentric cold planetesimal belts around a star of spectral type A3 V that are secularly perturbed by a substellar companion that orbits either closer to or farther from the star than the belt. Our model requires a perturber on an eccentric orbit (e ≳ 0.3) that is both far from and more massive than the collisionally dominated belt around a luminous central star. Based on the resulting spatial dust distributions, we simulated spatially resolved maps of their surface brightness in the K, N, and Q bands and at wavelengths of 70 µm and 1300 µm. Results. Assuming a nearby debris disk seen face-on, we find that the surface brightness distribution varies significantly with observing wavelength, for example between the N and Q band. This can be explained by the varying relative contribution of the emission of the smallest grains near the blowout limit. The orbits of both the small grains that form the halo and the large grains close to the parent belt precess due to the secular perturbations induced by a substellar companion orbiting inward of the belt. The halo, being composed of older grains, trails the belt. The magnitude of the trailing decreases with increasing perturber mass and hence with increasing strength of the perturbations. We recovered this trend in synthetic maps of surface brightness by fitting ellipses to lines of constant brightness. Systems with an outer perturber do not show a uniform halo precession since the orbits of small grains are strongly altered. We identified features of the brightness distributions suitable for distinguishing between systems with a potentially detectable inner or outer perturber, especially with a combined observation with JWST/MIRI in the Q band tracing small grain emission and with ALMA at millimeter wavelengths tracing the position of the parent planetesimal belt.

The origin of stars in the inner 500 parsecs in TNG50 galaxies

ABSTRACT We investigate the origin of stars in the innermost 500 pc of galaxies spanning stellar masses of $5\times 10^{8-12}\, \mathrm{M}_{\odot }$ at z = 0 using the cosmological magnetohydrodynamical TNG50 simulation. Three different origins of stars comprise galactic centres: (1) in situ (born in the centre), (2) migrated (born elsewhere in the galaxy and ultimately moved to the centre), (3) ex situ (accreted from other galaxies). In situ and migrated stars dominate the central stellar mass budget on average with 73 and 23 per cent, respectively. The ex situ fraction rises above 1 per cent for galaxies $\gtrsim 10^{11}\, \mathrm{M}_{\odot }$. Yet, only 9 per cent of all galaxies exhibit no ex situ stars in their centres and the scatter of ex situ mass is significant ($4\!-\!6\, \mathrm{dex}$). Migrated stars predominantly originate closely from the centre ($1\!-\!2\, \mathrm{kpc}$), but if they travelled together in clumps distances reach $\sim \! 10\, \mathrm{kpc}$. Central and satellite galaxies possess similar amounts and origins of central stars. Star-forming galaxies ($\gtrsim 10^{10}\, \mathrm{M}_{\odot }$) have on average more ex situ mass in their centres than quenched ones. We predict readily observable stellar population and dynamical properties: (1) migrated stars are distinctly young ($\sim \! 2\, \mathrm{Gyr}$) and rotationally supported, especially for Milky Way-mass galaxies, (2) in situ stars are most metal-rich and older than migrated stars, (3) ex situ stars are on random motion dominated orbits and typically the oldest, most metal-poor and α-enhanced population. We demonstrate that the interaction history with other galaxies leads to diverse pathways of building up galaxy centres in a Lambda cold dak matter universe. Our work highlights the necessity for cosmological context in formation scenarios of central galactic components and the potential to use galaxy centres as tracers of overall galaxy assembly.

DISCOVERY OF A NEW [WR] STAR

Wolf–Rayet (WR) stars are evolved massive objects, famous for their spectra rich with emission lines. For the first time WR stars were described more than 150 years ago, and now in our Galaxy we know around 667 objects of this type. However, the discovery of a new WR star, especially based on spectral observations, is still a rare event. The paper is devoted to J040901.83+323955.6 star, discovered a few years ago as WR star in LAMOST spectral survey by machine learning methods. We were interested that different researchers based on different methods classify this object in different ways – ranging from RR Lyr variable to nitrogen rich WR – and so we began our study. We combined spectral and archival photometric data, analysed the spatial location of the star in the Galaxy and concluded what J040901 is [WR] – central star of a planetary nebula.

The messy death of a multiple star system and the resulting planetary nebula as observed by JWST

Planetary nebulae—the ejected envelopes of red giant stars—provide us with a history of the last, mass-losing phases of 90% of stars initially more massive than the Sun. Here we analyse images of the planetary nebula NGC 3132 from the James Webb Space Telescope (JWST) Early Release Observations. A structured, extended hydrogen halo surrounding an ionized central bubble is imprinted with spiral structures, probably shaped by a low-mass companion orbiting the central star at about 40–60 au. The images also reveal a mid-infrared excess at the central star, interpreted as a dusty disk, which is indicative of an interaction with another closer companion. Including the previously known A-type visual companion, the progenitor of the NGC 3132 planetary nebula must have been at least a stellar quartet. The JWST images allow us to generate a model of the illumination, ionization and hydrodynamics of the molecular halo, demonstrating the power of JWST to investigate complex stellar outflows. Furthermore, new measurements of the A-type visual companion allow us to derive the value for the mass of the progenitor of a central star with excellent precision: 2.86 ± 0.06 M⊙. These results serve as pathfinders for future JWST observations of planetary nebulae, providing unique insight into fundamental astrophysical processes including colliding winds and binary star interactions, with implications for supernovae and gravitational-wave systems. JWST observations of the planetary nebula NGC 3132 reveal a rich molecular environment containing evidence of multiple stellar interactions on a variety of scales.

Spatially resolved mid-infrared observations of the circumstellar environment of the born-again object FG Sge

Context. FG Sge has evolved from the hot central star of the young planetary nebula Hen 1–5 to a G–K supergiant in the last 100 yr. It is one of the three born-again objects that have been identified as of yet, and they are considered to have undergone a thermal pulse in the post-asymptotic giant branch evolution. Aims. We present mid-infrared spectro-interferometric observations of FG Sge and probe its dusty environment. Methods. FG Sge was observed with MIDI at the Very Large Telescope Interferometer at baselines of 43 and 46 m between 8 and 13 µm. Results. The circumstellar dust environment of FG Sge was spatially resolved, and the Gaussian fit to the observed visibilities results in a full width at half maximum of ~10.5 mas. The observed mid-infrared visibilities and the spectral energy distribution can be fairly reproduced by optically thick (τV ≈ 8) spherical dust shell models consisting of amorphous carbon with an inner radius rin of ~30 R★ (corresponding to a dust temperature of 1100 ± 100 K). The dust shell is characterized with a steep density profile proportional to r−3.5±0.5 from the inner radius rin to (5–10) × rin, beyond which it changes to r−2. The dust mass is estimated to be ~ 7 × 10−7 M⊙, which translates into an average total mass-loss rate of ~ 9 × 10−6 M⊙ yr−1 as of 2008 with a gas-to-dust ratio of 200 being adopted. In addition, the 8–13 µm spectrum obtained with MIDI with a field of view of 200 mas does not show a signature of the polycyclic aromatic hydrocarbon (PAH) emission, which is in marked contrast to the spectra taken with the Spitzer Space Telescope six and 20 months before the MIDI observations with wide slit widths of 3″.6–10″. This implies that the PAH emission originates from an extended region of the optically thick dust envelope. Conclusions. The dust envelope of FG Sge is much more compact than that of the other born-again stars’ Sakurai’s object and V605 Aql, which might reflect the difference in the evolutionary status. The PAH emission from the extended region of the optically thick dust envelope likely originates from the material ejected before the central star became H-deficient, and it may be excited by the UV radiation from the central star escaping through gaps among dust clumps and/or the bipolar cavity of a disk-like structure.

Nascent planetary nebulae: new identifications and extraordinary evolution

AbstractPlanetary nebulae (PNe) harbouring masers of H2O (H2OPNe) and/or OH (OHPNe) are thought to be nascent PNe. They are extremely scarce, and so far only eight members are know to date. Here we explain our current effort to identify new H2OPNe and/or OHPNe. We report IRAS 07027–7934 as a new bona fine OHPN. Its 1612 MHz OH spectrum seems to be changing from double- to single-peaked since the redshifted emission has vanished almost completely, and the 1667 MHz OH maser emission has disappeared. For the OHPN Vy 2-2, we found that its central star is unexpectedly carbon (C)-rich, has a low-mass progenitor, and could be a post-common envelope binary system. Moreover, we confirm Vy 2-2 as a nascent PN. We speculate that low-mass C-rich central stars in post-common envelope systems could be a common end of H2OPNe and OHPNe.