Nebular Mass Research Articles

The nebula around the binary post-AGB star 89 Herculis

Context. There is a class of binary post-asymptotic giant branch (post-AGB) stars that exhibit remarkable near-infrared (NIR) excess. These stars are surrounded by disks with Keplerian or quasi-Keplerian dynamics and outflows composed of gas escaping from the rotating disk. Depending on the dominance of these components, there are two subclasses of binary post-AGB stars: disk-dominated and outflow-dominated. Aims. We aim to properly study the hourglass-like structure that surrounds the Keplerian disk around 89 Her. Methods. We present total-power on-the-fly maps of 12CO and 13CO J = 2 − 1 emission lines in 89 Her. Previous studies are known to suffer from flux losses in the most extended components. We merge these total-power maps with previous NOEMA maps. The resulting combined maps are expected to detect the whole nebula extent of the source. Results. Our new combined maps contain the entirety of the detectable flux of the source and at the same time are of high spatial resolution thanks to the interferometric observations. We find that the hourglass-like extended outflow around the rotating disk is larger and more massive than suggested by previous works. The total nebular mass of this very extended nebula is 1.8 × 10−2 M⊙, of which ∼65% comes from the outflow. The observational data and model results lead us to classify the envelope around 89 Her as an outflow-dominated nebula, together with R Sct and IRAS 19125+0343 (and very probably AI CMi, IRAS 20056+1834, and IRAS 18123+0511). The updated statistics on the masses of the two post-AGB main components reveal that there are two distinct subclasses of nebulae around binary post-AGB stars depending on which component is the dominant one. We speculate that the absence of an intermediate subclass of sources is due to the different initial conditions of the stellar system and not because both subclasses are in different stages of the post-AGB evolution.

Rotating and Expanding Gas in Binary Post-AGB Stars

There is a class of binary post-AGB stars (binary system including a post-AGB star) that are surrounded by Keplerian disks and outflows resulting from gas escaping from the disk. To date, there are seven sources that have been studied in detail through interferometric millimeter-wave maps of CO lines (ALMA/NOEMA). For the cases of the Red Rectangle, IW Carinae, IRAS 08544-4431, and AC Herculis, it is found that around ≥85% of the total nebular mass is located in the disk with Keplerian dynamics. The remainder of the nebular mass is located in an expanding component. This outflow is probably a disk wind consisting of material escaping from the rotating disk. These sources are the disk-dominated nebulae. On the contrary, our maps and modeling of 89 Herculis, IRAS 19125+0343, and R Scuti, which allowed us to study their morphology, kinematics, and mass distribution, suggest that, in these sources, the outflow clearly is the dominant component of the nebula (∼75% of the total nebular mass), resulting in a new subclass of nebulae around binary post-AGB stars: the outflow-dominated sources.Besides CO, the chemistry of this type of source has been practically unknown thus far. We also present a very deep single-dish radio molecular survey in the 1.3, 2, 3, 7, and 13 mm bands (∼600 h of telescope time). Our results and detections allow us to classify our sources as O- or /C-rich. We also conclude that the calculated abundances of the detected molecular species other than CO are particularly low, compared with AGB stars. This fact is very significant in those sources where the rotating disk is the dominant component of the nebula.

FAST Search for Circumstellar Atomic Hydrogen. I. The Young Planetary Nebula IC 4997

Using the Five-hundred-meter Aperture Spherical radio Telescope in Guizhou, China, we detect the 21 cm neutral atomic hydrogen absorption in the young planetary nebula IC 4997. The absorption arises from a shell that is also associated with Na i D lines. The H i shell has a mass of 1.46 × 10−2 M ⊙ and a dynamic age of 990 yr. The column density of H i is estimated to be 7.1 × 1020 cm−2, which can be well explained in terms of a photodissociation region around the ionized nebula, limited by the self-shielding of H2. We find that the atomic-to-ionized hydrogen ratio is 0.6, suggesting that H i substantially contributes to the overall nebular mass.

M 1–92 Revisited: New Findings and Open Questions: New NOEMA Observations of Minkowski’s Footprint

PN M 1–92, also known as Minkowski’s Footprint, is a textbook example of a massive pre-planetary nebula. It presents all the characteristics of this type of source: non-spherical symmetry (bipolar cylindrical symmetry in this case), high-velocity gas emission, large amounts of linear momentum and kinetic energy (momentum excess), and a self-similar growing structure. We have revisited this object by performing new NOEMA observations (with half arc-second resolution) of a wealth of molecules, including the rare isotopologues of CO, as well as other less abundant species. These maps provide new insights into the origin of this source. Our findings include the discovery of molecular species in the ionised regions of the nebula, confirming its shocked origin; the structure of the massive equatorial component, including the presence of active wind collisions; and the strong evidence that the AGB evolution of the source was terminated prematurely, probably due to the huge mass loss event that resulted in the formation/acceleration of the present nebula.

Chemistry of nebulae around binary post-AGB stars: A molecular survey of mm-wave lines

Context. There is a class of binary post-asymptotic giant branch (post-AGB) stars that exhibit remarkable near-infrared excess. Such stars are surrounded by Keplerian or quasi-Keplerian disks, as well as extended outflows composed of gas escaping from the disk. This class can be subdivided into disk- and outflow-dominated sources, depending on whether it is the disk or the outflow that represents most of the nebular mass, respectively. The chemistry of this type of source has been practically unknown thus far. Aims. Our objective is to study the molecular content of nebulae around binary post-AGB stars that show disks with Keplerian dynamics, including molecular line intensities, chemistry, and abundances. Methods. We focused our observations on the 1.3, 2, 3 mm bands of the 30mIRAM telescope and on the 7 and 13 mm bands of the 40 m Yebes telescope. Our observations add up ~600 h of telescope time. We investigated the integrated intensities of pairs of molecular transitions for CO, other molecular species, and IRAS fluxes at 12, 25, and 60 μm. Additionally, we studied isotopic ratios, in particular 17O/18O, to analyze the initial stellar mass, as well as 12CO/13CO, to study the line and abundance ratios. Results. We present the first single-dish molecular survey of mm-wave lines in nebulae around binary post-AGB stars. We conclude that the molecular content is relatively low in nebulae around binary post-AGB stars, as their molecular lines and abundances are especially weaker compared with AGB stars. This fact is very significant in those sources where the Keplerian disk is the dominant component of the nebula. The study of their chemistry allows us to classify nebulae around AC Her, the Red Rectangle, AI CMi, R Sct, and IRAS 20056+1834 as O-rich, while that of 89 Her is probably C-rich. The calculated abundances of the detected species other than CO are particularly low compared with AGB stars. The initial stellar mass derived from the 17O/18O ratio for the Red Rectangle and 89 Her is compatible with the central total stellar mass derived from previous mm-wave interferometric maps. The very low 12CO/13CO ratios found in binary post-AGB stars reveal a high 13CO abundance compared to AGB and other post-AGB stars.

LBV phenomenon and binarity: The environment of HR Car

Luminous blue variable stars (LBVs) are of great interest in massive-star evolution as they experience very high mass-loss episodes within short periods of time. HR Car is a famous member of this class in the Galaxy. It has a large circumstellar nebula and has also been confirmed as being in a binary system. One means of gaining information about the evolutionary status and physical nature of LBVs is studying their environments. We investigated the stellar content within ∼100 pc of HR Car and also its circumstellar nebula. Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations of a 2′×2′ region around the star highlight the incompleteness of stellar classification for stars with magnitudes of V > 13 mag. Eight B0 to B9 stars have been identified which may lie in close spatial vicinity to HR Car. For a region with a radius of r = 1.2° (∼100 pc at a distance of 4.8 kpc) around HR Car, existing catalogs list several late O-type and early B-type stars, but only one early O-type star. Given the relatively low stellar and nebular masses in the HR Car system, no early O-type stars and only a few late O-type stars would be expected in association with HR Car. Instead, HR Car’s location in a point vector diagram suggests that HR Car is not isolated, but is part of a moving group with a population of B-type stars in a spiral arm, and it has not received a strong kick from a supernova explosion of a companion star or a merger event. Potential binary evolution pathways for the HR Car system cannot be fully explored because of the unknown nature of the companion star. Furthermore, the MUSE observations reveal the presence of a fast outflow and “bullets” that have been ejected at intervals of about 400 years. These features may have been caused by recurrent mass transfer in the system.

Dust in RCW 58: Clues to common envelope channel formation?

ABSTRACT We present a characterization of the dust in the Wolf–Rayet (WR) nebula RCW 58 around the WN8h star WR 40 using archival infrared (IR) observations from WISE and Herschel and radio observations from ATCA. We selected two clumps, free from contamination from material along the line of sight and located towards southern regions in RCW 58, as representative of the general properties of this WR nebula. Their optical, IR, and radio properties are then modelled using the photoionization code cloudy, which calculates a self-consistent spatial distribution of dust and gas properties. Two populations of dust grains are required to model the IR SED: a population of small grains with sizes 0.002–0.01 $\mu$m, which is found throughout the clumps, and a population of large grains, with sizes up to 0.9 $\mu$m, located further from the star. Moreover, the clumps have very high dust-to-gas ratios, which present a challenge for their origin. Our model supports the hypothesis that RCW 58 is distributed in a ring-like structure rather than a shell, and we estimate a mass of ∼2.5 M⊙. This suggests that the mass of the progenitor of WR 40 was about $\approx 40^{+2}_{-3}$ M⊙. The ring morphology, low nebular mass, large dust grain size, and high dust-to-gas ratio lead us to propose that RCW 58 has formed through a common envelope channel, similar to what has been proposed for M 1-67.

Keplerian disks and outflows in post-AGB stars: AC Herculis, 89 Herculis, IRAS 19125+0343, and R Scuti

Context. There is a class of binary post-AGB stars with a remarkable near-infrared excess that are surrounded by Keplerian or quasi-Keplerian disks and extended outflows composed of gas escaping from the disk. The Keplerian dynamics had been well identified in four cases, namely the Red Rectangle, AC Her, IW Car, and IRAS 08544−4431. In these objects, the mass of the outflow represents ~10% of the nebular mass, the disk being the dominant component of the nebula. Aims. We aim to study the presence of rotating disks in sources of the same class in which the outflow seems to be the dominant component. Methods. We present interferometric NOEMA maps of 12CO and 13CO J = 2–1 in 89 Her and 12CO J = 2–1 in AC Her, IRAS 19125+0343, and R Sct. Several properties of the nebula are obtained from the data and model fitting, including the structure, density, and temperature distributions, as well as the dynamics. We also discuss the uncertainties on the derived values. Results. The presence of an expanding component in AC Her is doubtful, but thanks to new maps and models, we estimate an upper limit to the mass of this outflow of ≲3 × 10−5 M⊙, that is, the mass of the outflow is ≲5% of the total nebular mass. For 89 Her, we find a total nebular mass of 1.4 × 10−2 M⊙, of which ~50% comes from an hourglass-shaped extended outflow. In the case of IRAS 19125+0343, the nebular mass is 1.1 × 10−2 M⊙, where the outflow contributes ~70% of the total mass. The nebular mass of R Sct is 3.2 × 10−2 M⊙, of which ~75% corresponds to a very extended outflow that surrounds the disk. Conclusions. Our results for IRAS 19125+0343 and R Sct lead us to introduce a new subclass of binary post-AGB stars, for which the outflow is the dominant component of the nebula. Moreover, the outflow mass fraction found in AC Her is smaller than those found in other disk-dominated binary post-AGB stars. 89 Her would represent an intermediate case between both subclasses.

The Nature of the Young Supernova Remnant S8 in the Dwarf Galaxy IC 1613

Abstract We present subarcsecond optical images and low- to moderate-resolution spectra of the compact, X-ray, and optically bright supernova remnant known as S8 in the nearby dwarf galaxy IC 1613. Deep Hα images of the remnant show a sharply defined crescent-shaped nebula, while narrow passband images reveal a coincident and unexpectedly bright continuum nebulosity exhibiting a size and morphology like that seen for the remnant’s line emissions. Low-dispersion spectra covering 3600–9000 Å show numerous low-ionization line emissions such as [O i] and [Fe ii], along with higher-ionization emission lines including He ii and optical coronal lines [Fe vii], [Fe x], [Fe xi], and [Fe xiv]. This suggests the presence of a wide range of shock velocities from ∼50 to over 350 km s−1, corresponding to preshock densities of ∼1–30 cm−3. Higher-resolution spectra indicate an expansion velocity around 180 km s−1 with an ∼45 km s−1 wide central cavity. The Hα emission spans rest-frame velocities of +120 to −240 km s−1, and we estimate a total nebula mass of 119 ± 34 M ⊙. We conclude that S8 is a relatively young supernova remnant (≃2700–4400 yr) exhibiting properties remarkably like those seen in the young LMC remnant N49, including age, physical size, shock velocities, filament densities, optical line strengths, X-ray and optical luminosities, and coronal line and continuum emissions.

Can Chondrules Be Produced by the Interaction of Jupiter with the Protosolar Disk?

Abstract Chondrules are crystallized droplets of silicate melt formed by rapid heating to high temperatures (>1800 K) of solid precursors followed by hours or days of cooling. The time interval of chondrule formation is consistent with the formation timescale of Jupiter in the core-accretion model (1–4 Myr). Here we investigate if the shocks generated by a massive planet could generate flash heating episodes necessary to form chondrules using high-resolution 2D simulations with the multifluid code RoSSBi. We use different radiative cooling prescriptions, planet masses, orbits, and disk models. Temperatures reached during flash heating can be deduced from chondrule observations and are achieved in a Minimum Mass Solar Nebula (MMSN) for a massive protoplanet (>0.75 M ♃) but only in cases in which radiative cooling is low enough to lead to nearly adiabatic conditions. More realistic thermodynamics undershoot the temperatures required in shocks for chondrule formation. However, these temperatures are reached when considering more massive disks (e.g., five MMSN), but these conditions lead to fast planet migration and too low cooling rates compared to those deduced from chondrule textures. Thus, it seems unlikely that shocks from Jupiter can form chondrules in most cases. Independent of the nebular mass, the simulations demonstrate that a massive planet that forms a gap triggers vortices, which act as dust traps for chondrule precursors. These vortices also provide a high-pressure environment consistent with cosmochemical evidence from chondrules. They only lack the flash heating source for a potential chondrule formation environment.

An explanation of the titius-bode law resulted by a Kant-Laplace type model of the solar system’s forming

In the paper is shows that the Titius-Bode relation of the distances Sun-planet may be explained by a Kant-Laplace type model of planetary system forming, considering a linear decreasing of the lenticular protoplanetary nebula’s density and the constancy of its rotation speed. The resulted conclusions is that the protoplanetary material rings were formed by the successive halving of the nebular mass MN and thereafter- of the remained part and may be generalized for the star clusters, for the galaxies forming but also for the expansion of the galaxies super-clusters and of the Universe.

A massive nebula around the luminous blue variable star RMC 143 revealed by ALMA

The luminous blue variable (LBV) RMC 143 is located in the outskirts of the 30 Doradus complex, a region rich with interstellar material and hot luminous stars. We report the 3σ sub-millimetre detection of its circumstellar nebula with ALMA. The observed morphology in the sub-millimetre is different than previously observed with HST and ATCA in the optical and centimetre wavelength regimes. The spectral energy distribution (SED) of RMC 143 suggests that two emission mechanisms contribute to the sub-mm emission: optically thin bremsstrahlung and dust. Both the extinction map and the SED are consistent with a dusty massive nebula with a dust mass of 0.055 ± 0.018 M⊙ (assuming κ850 = 1.7 cm2 g−1). To date, RMC 143 has the most dusty LBV nebula observed in the Magellanic Clouds. We have also re-examined the LBV classification of RMC 143 based on VLT/X-shooter spectra obtained in 2015/16 and a review of the publication record. The radiative transfer code CMFGEN is used to derive its fundamental stellar parameters. We find an effective temperature of ∼8500 K, luminosity of log(L/L⊙) = 5.32, and a relatively high mass-loss rate of 1.0 × 10−5 M⊙ yr−1. The luminosity is much lower than previously thought, which implies that the current stellar mass of ∼8 M⊙ is comparable to its nebular mass of ∼5.5 M⊙ (from an assumed gas-to-dust ratio of 100), suggesting that the star has lost a large fraction of its initial mass in past LBV eruptions or binary interactions. While the star may have been hotter in the past, it is currently not hot enough to ionize its circumstellar nebula. We propose that the nebula is ionized externally by the hot stars in the 30 Doradus star-forming region.

Comprehensive Panchromatic Data Analyses and Photoionization Modeling of NGC 6781

AbstractWe characterized the dusty circumstellar nebula and central star of the C-rich bipolar planetary nebula (PN) NGC 6781 using our own Herschel data augmented with the archival data from UV to radio and constructed one of the most comprehensive photoionization PN models ever produced consisting of the ionized, atomic and molecular gas components as well as the dust component. We reproduced the observed spectral energy distribution (SED), constrained by 136 observational data points. The total nebula mass was estimated to be 0.41 M⊙, with a significant fraction (about 70 %) of it existing in the photo-dissociation region (PDR) surrounding the ionized nebula. This finding demonstrates the critical importance of the PDR in PNe, which are typically recognized as the hallmark of ionized/H+ region. It is therefore essential to characterize the PDR of the circumstellar nebula to understand material recycling in the Milky Way and other galaxies.

Radio observations of planetary nebulae: no evidence for strong radial density gradients

Radio continuum observations trace thermal emission of ionized plasma in planetary nebulae and bring useful information on nebular geometries. A model of homogeneous sphere or shell cannot fit the nebular spectra and brightness temperatures. Two alternative models have been proposed in the literature: the first one consists of two homogeneous components, while the other one is a model of a shell with a significant radial density gradient. On the other side, prolate ellipsoidal shell model can successfully fit the surface brightness distribution of selected objects. We verify the existing models using data collected in radio surveys covering wide range of frequencies. In about 50% cases, density gradient can be excluded, and none of the remaining objects could be confirmed. None of the observed planetary nebulae show the spectral index of 0.6 in the optically thick part of the spectrum, which is a value predicted for a shell containing strong radial density gradient. Radio spectra can be fitted with a model of prolate ellipsoidal shell, but also by a shell containing temperature variations in planetary nebulae. At least eight planetary nebulae show two component spectra, with one compact component showing much higher optical thickness than the other one. Unexpectedly, a group of planetary nebulae with lowest surface brightness show non-negligible optical thickness. Their emission comes from compact and dense structures, comprising only small part of total nebular mass.

High-resolution observations of IRAS 08544-4431: Detection of a disk orbiting a post-AGB star and of a slow disk wind.

In order to study the effects of rotating disks in the post-asymptotic giant branch (post-AGB) evolution, we observe a class of binary post-AGB stars that seem to be systematically surrounded by equatorial disks and slow outflows. Although the rotating dynamics had only been well identified in three cases, the study of such structures is thought to be fundamental to the understanding of the formation of disks in various phases of the late evolution of binary stars and the ejection of planetary nebulae from evolved stars. We present ALMA maps of 12CO and 13CO J=3-2 lines in the source IRAS 08544-4431, which belongs to the above mentioned class of objects. We analyzed the data by means of nebula models, which account for the expectedly composite source and can reproduce the data. From our modeling, we estimated the main nebula parameters, including the structure and dynamics and the density and temperature distributions. We discuss the uncertainties of the derived values and, in particular, their dependence on the distance. Our observations reveal the presence of an equatorial disk in rotation; a low-velocity outflow is also found, probably formed of gas expelled from the disk. The main characteristics of our observations and modeling of IRAS 08544-4431 are similar to those of better studied objects, confirming our interpretation. The disk rotation indicates a total central mass of about 1.8 M⊙, for a distance of 1100 pc. The disk is found to be relatively extended and has a typical diameter of ~ 4 1016 cm. The total nebular mass is ~ 2 10-2M⊙, of which ~ 90% corresponds to the disk. Assuming that the outflow is due to mass loss from the disk, we derive a disk lifetime of ~ 10000 yr. The disk angular momentum is found to be comparable to that of the binary system at present. Assuming that the disk angular momentum was transferred from the binary system, as expected, the high values of the disk angular momentum in this and other similar disks suggest that the size of the stellar orbits has significantly decreased as a consequence of disk formation.

A minimum mass nebula for M dwarfs

Abstract Recently revealed differences in planets around M dwarf versus solar-type stars could arise from differences in their primordial discs and surveys of T Tauri stars find a correlation between stellar mass and disc mass. ‘Minimum’ discs have been reconstructed for the Solar system and solar-type stars and here this exercise is performed for M dwarfs using Kepler-detected planets. Distribution of planet mass between current orbits produces a disc with total mass of ≈0.009 M⊙ and a power-law profile with index α = 2.2. Disc reconstruction from the output of a forward model of planet formation indicates that the effect of detection bias on disc profile is slight and that the observed scatter in planet masses and semimajor axes are consistent with a universal disc profile. This nominal M dwarf disc is more centrally concentrated than those inferred around the solar-type stars observed by Kepler, and the mass surface density beyond 0.02 au is sufficient for in situ accretion of planets as single embryos. The mass of refractory solids within 0.5 au is 5.6 M⊕ compared to 4 M⊕ for solar-type stars in contrast with the trend with total disc mass. The total solid beyond 0.5 au is sufficient for the core of at least one giant planet.

Masses of the Planetary Nebula Central Stars in the Galactic Globular Cluster System from HST Imaging and Spectroscopy∗

Abstract The globular cluster (GC) system of our Galaxy contains four planetary nebulae (PNe): K 648 (or Ps 1) in M15, IRAS 18333-2357 in M22, JaFu 1 in Pal 6, and JaFu 2 in NGC 6441. Because single-star evolution at the low stellar mass of present-epoch GCs was considered incapable of producing visible PNe, their origin presented a puzzle. We imaged the PN JaFu 1 with the Hubble Space Telescope (HST) to obtain photometry of its central star (CS) and high-resolution morphological information. We imaged IRAS 18333-2357 with better depth and resolution, and we analyzed its archival HST spectra to constrain its CS temperature and luminosity. All PNe in Galactic GCs now have quality HST data, allowing us to improve CS mass estimates. We find reasonably consistent masses between 0.53 and 0.58 M ⊙ for all four objects, though estimates vary when adopting different stellar evolutionary calculations. The CS mass of IRAS 18333-2357, though, depends strongly on its temperature, which remains elusive due to reddening uncertainties. For all four objects, we consider their CS and nebula masses, their morphologies, and other incongruities to assess the likelihood that these objects formed from binary stars. Although generally limited by uncertainties (∼0.02 M ⊙) in post-AGB tracks and core mass versus luminosity relations, the high-mass CS in K 648 indicates a binary origin. The CS of JaFu 1 exhibits compact, bright [O iii] and Hα emission, like EGB 6, suggesting a binary companion or disk. Evidence is weaker for a binary origin of JaFu 2.

A SEARCH FOR MASS LOSS ON THE CEPHEID INSTABILITY STRIP USING H i 21 cm LINE OBSERVATIONS

ABSTRACT We present the results of a search for H i 21 cm line emission from the circumstellar environments of four Galactic Cepheids (RS Pup, X Cyg, ζ Gem, and T Mon) based on observations with the Karl G. Jansky Very Large Array. The observations were aimed at detecting gas associated with previous or ongoing mass loss. Near the long-period Cepheid T Mon, we report the detection of a partial shell-like structure whose properties appear consistent with originating from an earlier epoch of Cepheid mass loss. At the distance of T Mon, the nebula would have a mass (H i+He) of , or ∼6% of the stellar mass. Assuming that one-third of the nebular mass comprises swept-up interstellar gas, we estimate an implied mass-loss rate of yr−1. No clear signatures of circumstellar emission were found toward ζ Gem, RS Pup, or X Cyg, although in each case, line-of-sight confusion compromised portions of the spectral band. For the undetected stars, we derive model-dependent upper limits on the mass-loss rates, averaged over their lifetimes on the instability strip, of yr−1 and estimate the total amount of mass lost to be less than a few percent of the stellar mass.

New ATCA, ALMA and VISIR observations of the candidate LBV SK -67 266 (S61): the nebular mass from modelling 3D density distributions

We present new observations of the nebula around the Magellanic candidate Luminous Blue Variable S61. These comprise high-resolution data acquired with the Australia Telescope Compact Array (ATCA), the Atacama Large Millimetre/Submillimetre Array (ALMA), and VISIR at the Very Large Telescope (VLT). The nebula was detected only in the radio, up to 17 GHz. The 17 GHz ATCA map, with 0.8 arcsec resolution, allowed a morphological comparison with the H$\alpha$ Hubble Space Telescope image. The radio nebula resembles a spherical shell, as in the optical. The spectral index map indicates that the radio emission is due to free-free transitions in the ionised, optically thin gas, but there are hints of inhomogeneities. We present our new public code RHOCUBE to model 3D density distributions, and determine via Bayesian inference the nebula's geometric parameters. We applied the code to model the electron density distribution in the S61 nebula. We found that different distributions fit the data, but all of them converge to the same ionised mass, ~0.1 $\rm M\odot$, which is an order of magnitude smaller than previous estimates. We show how the nebula models can be used to derive the mass-loss history with high-temporal resolution. The nebula was probably formed through stellar winds, rather than eruptions. From the ALMA and VISIR non-detections, plus the derived extinction map, we deduce that the infrared emission observed by space telescopes must arise from extended, diffuse dust within the ionised region.

ELEMENT MASSES IN THE CRAB NEBULA

ABSTRACT Using our previously published element abundance or mass-fraction distributions in the Crab Nebula, we derived actual mass distributions and estimates for overall nebular masses of hydrogen, helium, carbon, nitrogen, oxygen and sulfur. As with the previous work, computations were carried out for photoionization models involving constant hydrogen density and also constant nuclear density. In addition, employing new flux measurements for [Ni ii] λ7378, along with combined photoionization models and analytic computations, a nickel abundance distribution was mapped and a nebular stable nickel mass estimate was derived.