Central Star Research Articles

Detection of extended gamma-ray emission in the vicinity of Cl Danks 1 and 2

Abstract We report the detection of high-energy gamma-ray emission towards the G305 star-forming region. Using almost 15 years of observation data from Fermi Large Area Telescope, we detected an extended gamma-ray source in this region with a significance of ∼13σ. The gamma-ray radiation reveals a clear pion-bump feature and can be fitted with the power law parent proton spectrum with an index of −2.5. The total cosmic ray (CR) proton energy in the gamma-ray production region is estimated to be the order of $10^{49}\ \rm erg$. We further derived the CR radial distribution from both the gamma-ray emission and gas distribution and found it roughly obeys the 1/r type profile, though a constant profile is not ruled out. This is consistent with other similar systems and expected from the continuous injection of CRs by the central powerful young massive star cluster Danks 1 or Danks 2 in this region. Together with former detections of similar gamma-ray structures, such as Cygnus cocoon, Westerlund 1, Westerlund 2, NGC 3603, and W40, the detection supports the hypothesis that young massive star clusters are CR accelerators.

Imaging the innermost circumstellar environment of the red supergiant WOH G64 in the Large Magellanic Cloud

Significant mass loss in the red supergiant (RSG) phase has great influence on the evolution of massive stars and their final fate as supernovae. We present near-infrared interferometric imaging of the circumstellar environment of the dust-enshrouded RSG WOH G64 in the Large Magellanic Cloud. WOH G64 was observed with the GRAVITY instrument at ESO's Very Large Telescope Interferometer (VLTI) at 2.0--2.45 mu m . We succeeded in imaging the innermost circumstellar environment of WOH G64 -- the first interferometric imaging of an RSG outside the Milky Way. The reconstructed image reveals elongated compact emission with a semimajor and semiminor axis of sim 2 and sim 1.5 mas (sim 13 and 9 $R_ star ), respectively. The GRAVITY data show that the stellar flux contribution at 2.2 mu m \ at the time of our observations in 2020 is much lower than predicted by the optically and geometrically thick dust torus model based on the VLTI/MIDI data taken in 2005 and 2007. We found a significant change in the near-infrared spectrum of WOH G64: while the (spectro)photometric data taken at 1--2.5 mu m \ before 2003 show the spectrum of the central RSG with H$_2$O absorption, the spectra and prime photometric data taken after 2016 are characterized by a monotonically rising continuum with very weak signatures of H$_2$O . This spectral change likely took place between December 2009 and 2016. On the other hand, the mid-infrared spectrum obtained in 2022 with VLT/VISIR agrees well with the spectra obtained before 2007. The compact emission imaged with GRAVITY and the near-infrared spectral change suggest the formation of hot new dust close to the star, which gives rise to the monotonically rising near-infrared continuum and the high obscuration of the central star. The elongation of the emission may be due to the presence of a bipolar outflow or effects of an unseen companion.

Low-mass Stellar and Substellar Candidate Companions around Massive Stars in Sco OB1 and M17

Massive stars are recognized for their high degree of multiplicity, yet the mass ratio regime below 0.1 remains insufficiently explored. It is therefore unknown whether extremely low-mass (possibly substellar) companions can form and survive in the direct UV-irradiated environment of massive stars. In this paper, we discuss Very Large Telescope/Spectro-Polarimetric High-contrast Exoplanet REsearch integral field spectrograph (0.″15–0.″85) observations of six massive O- and early B-type stars in Sco OB1 and M17 that each have a low-mass candidate companion (CC). Two targets have companions that are brown dwarf candidates. The other four have CCs in the low end of the stellar mass regime (≤0.30 M ⊙). For three of these, we have obtained a second epoch observation. At least two sources exhibit similar proper motion to that of their central star. However, given the expected proper motion of background objects, this does not imply certain companionship. We show how future follow-up observations of the brown dwarf CCs in the J, H, and L bands should allow for an unambiguous confirmation of their nature.

Modified method of round Gaussian rings. Application to the two-planetary problem

A scheme of the modified method of round Gaussian rings, designed to study the secular evolution of orbits in systems consisting of a central star and two planets, is presented. The reason for the secular evolution of the nodes and inclinations of the orbits of the planets is their mutual gravitational attraction. The orbits of the planets are modeled by homogeneous round Gaussian rings, to which the masses, sizes and angles of inclination of the orbits, as well as orbital angular momenta of the planets, are transferred. The method takes into account the fact that, in general, the ascending nodes of the orbits may not coincide. The mutual gravitational energy of the rings 𝑊𝑚𝑢𝑡 is represented as a series in the quadratic approximation in powers of small inclination angles. Using this function 𝑊𝑚𝑢𝑡, a closed system of four differential equations describing the secular evolution of the planets’ orbits is composed. The solution to the equations is obtained in finite analytic form, which simplifies the interpretation of the investigated planetary motions. The method was tested on the example of the Sun-Jupiter-Saturn system; for it, in particular, the difference in the longitudes of the nodes of the orbits of Jupiter and Saturn was calculated as a function of time. New approach is also used to study the precession of nodes in the exoplanetary system K2-36; graphs of all unknown quantities are obtained. It has been established that in the course of evolution the mutual inclination angle of the orbits remains constant, and the librations of the orbits in the inclination angle and in the motion of the nodes occur synchronously.

Novel constraints on companions to the Helix nebula central star

ABSTRACT The Helix is a visually striking and the nearest planetary nebula, yet any companions responsible for its asymmetric morphology have yet to be identified. In 2020, low-amplitude photometric variations with a periodicity of 2.8 d were reported based on Cycle 1 TESS observations. In this work, with the inclusion of two additional sectors, these periodic light curves are compared with lcurve simulations of irradiated companions in such an orbit. Based on the light-curve modelling, there are two representative solutions: (i) a Jupiter-sized body with 0.102 R$_\odot$ and an arbitrarily small orbital inclination $i=1^{\circ }$, and (ii) a 0.021 R$_\odot$ exoplanet with $i\approx 25^{\circ }$, essentially aligned with the Helix nebular inclination. Irradiated substellar companion models with equilibrium temperature 4970 K are constructed and compared with existing optical spectra and infrared photometry, where Jupiter-sized bodies can be ruled out, but companions modestly larger than Neptune are still allowed. Additionally, any spatially unresolved companions are constrained based on the multiwavelength, photometric spectral energy distribution of the central star. No ultracool dwarf companion earlier than around L5 is permitted within roughly 1200 au, leaving only faint white dwarfs and cold brown dwarfs as possible surviving architects of the nebular asymmetries. While a planetary survivor is a tantalizing possibility, it cannot be ruled out that the light-curve modulation is stellar in nature, where any substellar companion requires confirmation and may be possible with JWST observations.

Planetary nebulae seen with TESS: New and revisited short-period binary central star candidates from Cycles 1 to 4

Context. High-precision and high-cadence photometric surveys such as Kepler or TESS are making huge progress not only in the detection of new extrasolar planets but also in the study of a great number of variable stars. This is the case for central stars of planetary nebulae (PNe), which have similarly benefited from the capabilities of these missions, increasing the number of known binary central stars and helping us to constrain the relationship between binarity and the complex morphologies of their host PNe. Aims. In this paper, we analyse the TESS light curves of a large sample of central stars of PNe with the aim of detecting signs of variability that may hint at the presence of short-period binary nuclei. This will have important implications in understanding PN formation evolution as well as the common envelope phase. Methods. We analysed 62 central stars of true, likely, or possible PNe and modelled the detected variability through an MCMC approach accounting for three effects: reflection, ellipsoidal modulations due to tidal forces, and the so-called Doppler beaming. Among the 62 central stars, only 38 are amenable for this study. The remaining 24 show large contamination from nearby sources preventing an optimal analysis. Also, eight targets are already known binary central stars, which we revisit here with the new high precision of the TESS data. Results. In addition to recovering the eight already known binaries in our sample, we find that 18 further central stars show clear signs of periodic variability in the TESS data, probably resulting from different physical effects compatible with the binary scenario. We propose them as new candidate binary central stars. We also discuss the origin of the detected variability in each particular case by using the TESS_localize algorithm. Finally, 12 targets show no or only weak evidence of variability at the sensitivity of TESS. Conclusions. Our study demonstrates the power of space-based photometric surveys in searching for close binary companions of central stars of PNe. Although our detections can only be catalogued as candidate binaries, we find a large percentage of possible stellar pairs associated with PNe, supporting the hypothesis that binarity plays a key role in shaping these celestial structures.

Spectroscopic survey of faint planetary-nebula nuclei

We present an analysis of 17 H-rich central stars of planetary nebulae (PNe) observed in our spectroscopic survey of nuclei of faint Galactic PNe carried out at the 10-m Hobby-Eberly Telescope. Our sample includes ten O(H) stars, four DAO white dwarfs (WDs), two DA WDs, and one sdOB star. The spectra were analyzed by means of NLTE model atmospheres, allowing us to derive the effective temperatures, surface gravities, and He abundances of the central stars. Sixteen of them were analyzed for the first time, increasing the number of hot H-rich central stars with parameters obtained through NLTE atmospheric modeling by approximately 20%. We highlight a rare hot DA WD central star, Abell 24, which has a Teff likely in excess of 100 kK, as well as the unusually high gravity mass of 0.70 ± 0.05 M⊙ for the sdOB star Pa 3, which is significantly higher than the canonical extreme horizontal-branch star mass of ≈0.48 M⊙. By investigating Zwicky Transient Facility light curves, which were available for our 15 northern objects, we found none of them show a periodic photometric variability larger than a few hundredths of a magnitude. This could indicate that our sample mainly represents the hottest phase during the canonical evolution of a single star when transitioning from an asymptotic giant branch star into a WD. We also examined the spectral energy distributions, detecting an infrared excess in six of the objects, which could be due to a late-type companion or to hot (≈103 K) and/or cool (≈100 K) dust. We confirm previous findings that spectroscopic distances are generally higher than found through Gaia astrometry, a discrepancy that deserves to be investigated systematically.

Broadening the Canonical Picture of EUV-driven Photoevaporation of Accretion Disks

Photoevaporation driven by hydrogen-ionizing extreme-ultraviolet (EUV) radiation profoundly shapes the lives of diverse astrophysical objects. We construct an analytical model accounting for the finite timescales of photoheating and photoionization and apply it to the dispersal of protoplanetary disks. The model yields improved estimates for the ionization, temperature, and velocity versus distance from the central source when compared to the classical picture of fully ionized and isothermal winds with temperatures ≈104 K and speeds ≈10 km s−1. In contrast to the classical picture, the photoevaporative winds take on several distinct hydrodynamical and thermochemical states depending on the central star’s EUV emission rate and spectral hardness: T Tauri stars with EUV luminosities around 1030 erg s−1 drive nonisothermal ionized disk winds at lower temperatures than the classical value if the spectrum is soft, with an average deposited energy per photoionization less than about 3.7 eV. If, however, the spectrum is hard, the winds tend to be atomic and isothermal at most disk radii. For lower EUV intensities, even with soft spectra, atomic winds can emerge beyond ∼10 au through advection. We show that these predictions are in general agreement with detailed radiation hydrodynamics calculations. The model furthermore illustrates how the energy efficiency of photoevaporation varies with the intensity and spectral hardness of the EUV illumination, as well as addressing discrepancies in the literature around the effectiveness of X-ray photoevaporation. The findings highlight the importance of the photoheating and photoionization timescales both for modeling and for understanding winds’ observed behavior.

Are all starbursts equal? Star-formation-rate profiles in merger versus secular starbursts

Aims. Galaxy interactions can trigger drastic changes in the resolved star-forming properties of their constituents, but it remains unclear as to whether those changes are discernible from secular starburst triggers. In this Letter we investigate whether or not post-merger galaxies create unique star-forming trends on a kiloparsec scale. Methods. We present radial trends in star-formation-rate (SFR) surface density (ΣSFR) for 150 post-merger galaxies with moderate to extremely heightened global SFRs using observations from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. We juxtapose these profiles with those of noninteracting galaxies (excluding both galaxy pairs and post-merger galaxies) with similarly enhanced global SFRs. Results. Post-merger galaxies have a much stronger central starburst than isolated galaxies with similar global star-formation enhancements. Indeed, isolated starburst galaxies (SBs) lack a marked central enhancement and instead show a fairly uniform enhancement in ΣSFR with radius. Moreover, the difference in central star formation between post-merger galaxies and noninteracting galaxies is more radially extended and pronounced when the global enhancement in star formation is larger. We conclude that post-merger galaxies create a unique signature in their resolved star-forming properties that is distinct from secular processes that can trigger similar global SFR enhancements.

Asymmetric Dust Accumulation of the PDS 70 Disk Revealed by ALMA Band 3 Observations

The PDS 70 system, hosting two planets within its disk, is an ideal target for examining the effect of planets on dust accumulation, growth, and ongoing planet formation. Here, we present high-resolution (0.″07 = 8 au) dust continuum observations of the PDS 70 disk in ALMA Band 3 (3.0 mm). While previous Band 7 observations showed a dust ring with slight asymmetry, our Band 3 observations reveal a more prominent asymmetric peak in the northwest direction, where the intensity is 2.5 times higher than in other directions and the spectral index is at the local minimum with α SED ∼ 2.2. This indicates that a substantial amount of dust is accumulated both radially and azimuthally in the peak. We also detect point-source emission around the stellar position in the Band 3 image, which is likely to be free–free emission. We constrain the eccentricity of the outer ring to be e < 0.04 from the position of the central star and the outer ring. From the comparison with numerical simulations, we constrain the mass of PDS 70c to be less than 4.9 MJupiter if the gas turbulence strength α turb = 10−3. Then, we discuss the formation mechanism of the disk structures and further planet formation scenarios in the disk.

Short-lived gravitational instability in isolated irradiated discs

ABSTRACT Irradiation from the central star controls the temperature structure in protoplanetary discs. Yet simulations of gravitational instability typically use models of stellar irradiation with varying complexity, or ignore it altogether, assuming heat generated by spiral shocks is balanced by cooling, leading to a self-regulated state. In this paper, we perform simulations of irradiated, gravitationally unstable protoplanetary discs using 3D hydrodynamics coupled with live Monte-Carlo radiative transfer. We find that the resulting temperature profile is approximately constant in time, since the thermal effects of the star dominate. Hence, the disc cannot regulate gravitational instabilities by adjusting the temperatures in the disc. In a $0.1M_\odot$ disc, the disc instead adjusts by angular momentum transport induced by the spiral arms, leading to steadily decreasing surface density, and hence quenching of the instability. Thus, strong spiral arms caused by self-gravity would not persist for longer than ten thousand years in the absence of fresh infall, although weak spiral structures remain present over longer time-scales. Using synthetic images at 1.3 mm, we find that spirals formed in irradiated discs are challenging to detect. In higher mass discs, we find that fragmentation is likely because the dominant stellar irradiation overwhelms the stabilizing influence of $P\mathrm{d}V$ work and shock heating in the spiral arms.

Testing Cluster Membership of Planetary Nebulae with High-precision Proper Motions. I. HST Observations of JaFu 1 Near the Globular Cluster Palomar 6

If a planetary nebula (PN) is shown to be a member of a star cluster, we obtain important new constraints on the mass and chemical composition of the PN’s progenitor star, which cannot be determined for PNe in the field. Cluster membership can be tested by requiring the projected separation between the PN and cluster to be within the tidal radius of the cluster, and the objects to have nearly identical radial velocities (RVs) and interstellar extinctions, and nearly identical proper motions (PMs). In an earlier study, we used PMs to confirm that three PNe, which had already passed the other tests, are highly likely to be members of Galactic globular clusters (GCs). For a fourth object, the PN JaFu 1, which lies in the Galactic bulge near the GC Palomar 6 on the sky and has a similar RV, the available PM measurement gave equivocal results. We have now obtained new high-resolution images of the central star of JaFu 1 with the Hubble Space Telescope (HST), which, combined with archival HST frames taken 14 and 16 yr earlier, provide a high-precision PM. Unfortunately, we find that the PM of the central star differs from that of the cluster with high statistical significance. Nevertheless, JaFu 1 is of astrophysical interest because its nucleus appears to be a member of the rare class of “EGB 6-type” central stars, which are associated with compact emission-line knots.

Analyzing JWST/NIRSpec Hydrogen Line Detections at TWA 27B: Constraining Accretion Properties and Geometry

Hydrogen lines from forming planets are crucial for understanding planet formation. However, the number of planetary hydrogen line detections is still limited. Recent JWST/NIRSpec observations have detected Paschen and Brackett hydrogen lines at TWA 27 B (2M1207b). Although classified as a planetary- mass companison (PMC) rather than a planet due to its large mass ratio to the central star, TWA 27 B’s hydrogen line emissions are expected to be same as the planetary one, given its small mass (≈5M J). We aim to constrain the accretion properties and accretion geometry of TWA 27 B, contributing to our understanding of hydrogen-line emission mechanisms common to both PMCs and planets. We conduct spectral fitting of four bright hydrogen lines (Pa-α, Pa-β, Pa-γ, Pa-δ) with an accretion-shock emission model tailored for forming planets. We estimate the mass accretion rate at Ṁ≈3×10−9MJyr−1 with our fiducial parameters, though this is subject to an uncertainty of up to factor of ten. Our analysis also indicates a dense accretion flow, n ≳ 1013 cm−3 just before the shock, implying a small accretion-shock filling factor f f on the planetary surface (f f ≲ 5 × 10−4). This finding suggests that magnetospheric accretion is occurring at TWA 27 B. Additionally, we carry out a comparative analysis of hydrogen-line emission color to identify the emission mechanism, but the associated uncertainties proved too large for definitive conclusions. This underscores the need for further high-precision observational studies to elucidate these emission mechanisms fully.

Central molecular zones in galaxies: thirco (6-5) and molecular gas conditions in bright nearby galaxies

This paper summarizes all presently available $J_ upp thirco and accompanying co measurements of galaxy centers including new $J$=6-5 thirco and co observations of eleven galaxies with the Atacama Pathfinder EXperiment (APEX) telescope and also $Herschel$ high-$J$ measurements of both species in five galaxies. The observed $J$=6-5/$J$=1-0 co integrated temperature ratios range from 0.10 to 0.45 in matching beams. Multi-aperture data indicate that the emission of thirco (6-5) is more centrally concentrated than that of co (6-5). The intensities of co (6-5) suggest a correlation with those of $ but not with those of HCN. The new data are essential in refining and constraining the parameters of the observed galaxy center molecular gas in a simple two-phase model to approximate its complex multi-phase structure. In all galaxies except the Seyfert galaxy NGC 1068, high-$J$ emission from the center is dominated by a dense ($n and relatively cool (20-60 K) high-pressure gas. In contrast the low-$J$ lines are dominated by low-pressure gas of a moderate density ($n and more elevated temperature (60-150 K) in most galaxies. The three exceptions with significant high-pressure gas contributions to the low-$J$ emission are all associated with active central star formation.

Fe II] 1.644 µm imaging survey of planetary nebulae with low-ionisation structures

Low-ionisation structures (LISs) are commonly found in planetary nebulae (PNe), but they are still poorly understood. The recent discovery of unforeseen molecular hydrogen gas (H2) has impacted what we think we know about these microstructures and PNe. To obtain an overall understanding of LISs, we carried out an [Fe II] 1.644 µm imagery survey in PNe with LISs, with the aim to detect the [Fe II] 1.644µm emission line, a common tracer of shocks. We present the first detection of [Fe II] 1.644 µmline directly associated with the LISs in four out of five PNe. The theoretical H I 12-4 recombination line was also computed either from the Brγ or the Hβ line and subtracted from the observed narrow-band line fluxes. The [Fe II] 1.644 µm flux ranges from 1 to 40 ×10−15 ergs cm−2 s−1 and the intensity from 2 to 90 ×10−5 erg s−1 cm−2 sr−1. The R(Fe)=[Fe II] 1.644 µm/Brγ line ratio was also computed and found to range between 0.5 and 7. In particular, the [Fe II] 1.644 µm line was detected in NGC 6543 (R(Fe)&lt;0.15), along with the outer pairs of LISs in NGC 7009 (R(Fe)&lt;0.25) and the jet-like LISs in IC 4634 (R(Fe)~1), and in several LISs in NGC 6571 (2&lt;R(Fe)&lt;7). The low R(Fe) result for NGC 6543 is attributed to the UV radiation from the central star. In contrast, the higher values in NGC 6571 and IC 4634 are indicative of shocks. The moderate R(Fe) in NGC 7009 likely indicates the contribution of both mechanisms.

Sulfur monoxide (SO) as a shock tracer in protoplanetary disks: Case of AB Aurigae

Context. Sulfur monoxide (SO) is known to be a good shock tracer in molecular clouds and protostar environments, but its abundance is difficult to reproduce, even with state-of-the-art astrochemical models. Aims. We investigate the properties of the observed SO emission in the protoplanetary disk of AB Auriga, a Herbig Ae star of 2.4 M⊙ in mass, located at 156 pc. The AB Aur system is unique because it exhibits a dust trap and at least one young putative planet orbiting at about 30 au from the central star. Methods. We reduced ALMA archival data (projects 2019.1.00579.S, 2021.1.00690.S, and 2021.1.01216.S) and analyzed the three detected SO lines (SO 65 − 54, 67 − 56 and 56 − 45). We also used C17O and C18O 2–1 data to complement the interpretation of the SO data. Results. For the three SO lines, the maximum SO emission in the ring is not located in the dust trap. Moreover, the inner radius of the SO ring is significantly larger than the CO emission inner radius, ∼160 au versus ∼90 au. The SO emission traces gas located in part beyond the dust ring. This emission likely originates from shocks at the interface of the outer spirals, observed in CO and scattered light emission, as well as those in the molecular and dust ring. Also, SO is detected within the cavity, at a radius of ∼20 − 30 au and with a rotation velocity compatible with the protoplanet P1. We speculate that this SO emission originates from accretion shocks onto the circumplanetary disk of the putative protoplanet P1. Conclusions. These observations confirm that SO is a good tracer of shocks in protoplanetary disks and could serve as a powerful new tool for detecting embedded (proto)planets.

Magnetically Driven Turbulence in the Inner Regions of Protoplanetary Disks

Given the important role turbulence plays in the settling and growth of dust grains in protoplanetary disks, it is crucial that we determine whether these disks are turbulent and to what extent. Protoplanetary disks are weakly ionized near the midplane, which has led to a paradigm in which largely laminar magnetic field structures prevail deeper in the disk, with angular momentum being transported via magnetically launched winds. Yet, there has been little exploration of the precise behavior of the gas within the bulk of the disk. We carry out 3D, local shearing box simulations that include all three low-ionization effects (ohmic diffusion, ambipolar diffusion, and the Hall effect) to probe the nature of magnetically driven gas dynamics 1–30 au from the central star. We find that gas turbulence can persist with a generous yet physically motivated ionization prescription (order unity Elsässer numbers). The gas velocity fluctuations range from 0.03 to 0.09 of the sound speed c s at the disk midplane to ∼c s near the disk surface, and are dependent on the initial magnetic field strength. However, the turbulent velocities do not appear to be strongly dependent on the field polarity, and thus appear to be insensitive to the Hall effect. The midplane turbulence has the potential to drive dust grains to collision velocities exceeding their fragmentation limit, and likely reduces the efficacy of particle clumping in the midplane, though it remains to be seen if this level of turbulence persists in disks with lower ionization levels.

Dissecting the planetary nebula NGC 4361 with MUSE

Optical integral field spectroscopy of planetary nebulae (PNe) offers a unique tool to explore the spatial relationships between the complex mixture of the many components (neutral, low- and high-ionisation gas, dust, and the central star) and their underlying physical conditions. The optical line and continuum emission in the very-high-ionisation Galactic PN, NGC 4361, were mapped to study the distribution of ionisation, extinction, electron temperature, and density. Based on commissioning data, MUSE Wide Field (60times 60$''$) normal-mode (4750-9300\ observations of NGC 4361 were reduced. The PN is larger than a single MUSE field and only the central 1 arcmin2 of the PN was observed in good conditions. Emission images in recombination and collisionally excited lines were extracted and the line ratios provided the dust extinction, electron density and temperature, and ionic abundances using standard techniques. A family of compact low-ionisation knots (dubbed 'freckles') was discovered and techniques developed to measure their spectra, independently of the extended high-ionisation medium. The nebula is confirmed as optically thin in the H-ionising continuum, based on its very low He i emission, even to the edges of the field. The electron temperature, $T_ e $, is shown to have a large-scale spatially coherent structure, as indicated by a previous long-slit spectrum. Prior to this study, no low-ionisation emission had been positively detected, although MUSE revealed both weak extended N ii and O ii and $&gt;$100 spatially unresolved knots. There are several linear associations of these knots, but none of them point convincingly back to the central star. They have low-to-moderate ionisation with $T_ e $ sim 11000\,K e $ sim 1500 cm$^ $ and generally exhibit a higher extinction than the extended high-ionisation nebula. Within the MUSE field, a low-redshift emission-line galaxy was serendipitously found to be hiding behind NGC 4361. The spectrum of this dwarf galaxy was carefully extracted from the bright foreground nebular emission and the galaxy's line and continuum properties were then determined. NGC 4361 is not completely optically thin, as indicated by several extended regions and many compact features of lower ionisation emission. The low-ionisation 'freckles' identified here do not clearly appear to differ in (He, N, O, S) abundance with respect to the extended high-ionisation gas. The spatial distribution and radial velocities of these features suggest that they belong to a thick disk oriented perpendicular to the large-scale nebular gas, which may perhaps be remnants of an earlier structure. The low-luminosity disk galaxy at sim 87\,Mpc has bright H ii regions with metallicity 12+log(O/H) cong 8.4 and is suggested to be a Magellanic irregular or low-mass spiral.

FAST Detection of OH Emission in the Carbon-rich Planetary Nebula NGC 7027

We present the first detection of the ground-state OH emission line at 1612 MHz toward the prototypical carbon-rich planetary nebula (PN) NGC 7027, utilizing the newly installed ultrawideband (UWB) receiver of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). This emission is likely to originate from the interface of the neutral shell and the ionized region. The other three ground-state OH lines at 1665, 1667, and 1721 MHz are observed in absorption and have velocities well matched with that of HCO+ absorption. We infer that the OH absorption is from the outer shell of NGC 7027, although the possibility that they are associated with a foreground cloud cannot be completely ruled out. All the OH lines exhibit a single blueshifted component with respect to the central star. The formation of OH in carbon-rich environments might be via photodissociation-induced chemical processes. Our observations offer significant constraints for chemical simulations, and they underscore the potent capability of the UWB receiver of FAST to search for nascent PNe.

TRACE: a code for time-reversible astrophysical close encounters

Abstract We present TRACE, an almost time-reversible hybrid integrator for the planetary N-body problem. Like hybrid symplectic integrators, TRACE can resolve close encounters between particles while retaining many of the accuracy and speed advantages of a fixed timestep symplectic method such the Wisdom–Holman map. TRACE switches methods time-reversibly during close encounters following the prescription of Hernandez &amp; Dehnen. In this paper we describe the derivation and implementation of TRACE and study its performance for a variety of astrophysical systems. In all our test cases TRACE is at least as accurate and fast as the hybrid symplectic integrator MERCURIUS. In many cases TRACE’s performance is vastly superior to that of MERCURIUS. In test cases with planet-planet close encounters, TRACE is as accurate as MECURIUS with a 12x speedup. If close encounters with the central star are considered, TRACE achieves good error performance while MERCURIUS fails to give qualitatively correct results. In ensemble tests of violent scattering systems, TRACE matches the high-accuracy IAS15 while providing a 15x speed-up. In large N systems simulating lunar accretion, TRACE qualitatively gives the same results as IAS15 but at a 41x speedup. We also discuss some cases such as von Zeipel-Lidov-Kozai cycles where hybrid integrators perform poorly and provide some guidance on which integrator to use for which system. TRACE is freely available within the REBOUND package.