arXiv subjects

Peter A. M. van Hoof

Publications and source records attributed to Peter A. M. van Hoof.

At least 19 recordsLinked to original sources

arXiv2025

Fe XVIII-XXIV K beta Inner-shell Absorption Lines in the X-ray Spectra of Neutron Star and Black Hole Binaries with XRISM

The advent of the X-ray microcalorimeter spectrometer Resolve onboard the XRISM space telescope opened a new era for high-resolution X-ray spectroscopy of astrophysical plasmas. Many spectral features were newly detected, including the K alpha and K beta inner-shell transition lines of mildly ionized (F- to Li-like) Fe at 6-8 keV in the spectra of X-ray binaries and active galactic nuclei. The widely used atomic databases contain information on the K alpha but not K beta lines of these ions. We conducted the atomic structure calculation using FAC to derive the Fe K alpha and K beta lines and verified the result against ground experiments and other calculations of the Fe K alpha lines. We then implemented the Fe K beta lines in a radiative transfer code (cloudy) and compared the synthesized and observed spectra with XRISM. A reasonably good agreement was obtained between the observation and the ab initio calculations. This exemplifies the need to expand the atomic databases to interpret astrophysical spectra.

astro-ph.HE↗

arXiv2025

Detection of CH3+ in the O-rich planetary nebula NGC 6302

Planetary nebulae are sites where ejected stellar material evolves into complex molecules, but the precise physical conditions and chemical routes that govern these processes are unclear. The presence of abundant carbon-rich molecules in O-rich environments poses particular challenges. Here we report the first detection of methyl cation (CH3+) in any planetary nebula, observed in the O-rich nebula NGC 6302 using JWST MIRI/MRS observations. CH3+ is a key driver of organic chemistry in UV-irradiated environments. Spatially resolved observations reveal that CH3+ is co-located with 12CO, H2, H II, HCO+, and Polycyclic aromatic hydrocarbons (PAHs). LTE modelling of the CH3+ emission yields excitation temperatures of 500-800K in the inner bubble and torus, rising to 1000-2000K in the outer bubble of NGC 6302, with column densities ranging from ~10^11 to 10^13 cm^-2. This detection demonstrates that hydrocarbon radical chemistry must be incorporated into planetary nebulae chemical models. Further near-IR observations are crucial to map different chemical networks operating in these environments.

astro-ph.GA↗

arXiv2025

The JWST/MIRI view of the planetary nebula NGC 6302 I.: a UV irradiated torus and a hot bubble triggering PAH formation

NGC 6302 is a spectacular bipolar planetary nebula (PN) whose spectrum exhibits fast outflows and highly ionized emission lines, indicating the presence of a very hot central star (~220,000 K). Its infrared spectrum reveals a mixed oxygen and carbon dust chemistry, displaying both silicate and polycyclic aromatic hydrocarbon (PAH) features. Using the JWST Mid-Infrared Instrument (MIRI) and Medium Resolution Spectrometer, a mosaic map was obtained over the core of NGC 6302, covering the wavelength range of 5--28 micron and spanning an area of ~18.5 arcsec x15 arcsec. The spatially resolved spectrum reveals ~200 molecular and ionized lines from species requiring ionisation potentials of up to 205 eV. The spatial distributions highlight a complex structure at the nebula's centre. Highly ionized species such as Mg VII and Si VII show compact structures, while lower-ionization species such as H^+ extend much farther outwards, forming filament-defined rims that delineate a bubble. Within the bubble, the H^+ and H_2 emission coincide, while the PAH emission appears farther out, indicating an ionization structure distinct from typical photodissociation regions, such as the Orion Bar. This may be the first identification of a PAH formation site in a PN. This PN appears to be shaped not by a steady, continuous outflow, but by a series of dynamic, impulsive bubble ejections, creating local conditions conducive to PAH formation. A dusty torus surrounds the core, primarily composed of large (micron-sized) silicate grains with crystalline components. The long-lived torus contains a substantial mass of material, which could support an equilibrium chemistry and a slow dust-formation process.

astro-ph.SR↗

arXiv2025

The 2025 Release of Cloudy

We present the 2025 release of the spectral synthesis code Cloudy, highlighting significant enhancements to the scope and accuracy of the physics which have been made since the previous release. A major part of this development involves resolving the Lyman $α$ line into $j$-resolved fine-structure doublets, making Cloudy of use to the X-ray community. On this front, we have also updated inner-shell ionization line energies and incorporated the 1 keV feature commonly observed in X-ray binaries. Additionally, we update our in-house database, Stout, for the carbon isoelectronic sequence, improving Cloudy microphysical calculations for all wavelengths. We have also extended the molecular network by adding new silicon-bearing species, titanium-related reactions, and phosphorus-containing molecules, enhancing Cloudy's ability to model the complex chemistry relevant to rapidly growing field of exoplanet atmospheres. Finally, we outline future developments aimed at maximizing the scientific return from the current and upcoming generation of observatories, including XRISM, JWST, Roman, the Habitable Worlds Observatory (HWO) and NewAthena.

astro-ph.GA↗

arXiv2025

Cloudy and the High-Resolution Microcalorimeter Revolution: Optical, UV, and X-ray Spectra of One-electron Systems

With the launch of the XRISM microcalorimeter mission, space-based X-ray observations will achieve a record spectral resolving power of $R\equiv E/ΔE\sim$1200. With this resolving power, emission features associated with fine-structure energy levels of some species will be resolved, sometimes for the first time. The plasma code, Cloudy, was not originally designed for high-resolution X-ray spectroscopy and throughout its history did not resolve fine-structure components of Lyman lines. Here we expand Cloudy to resolve these fine-structure energy levels and obtain predicted X-ray spectra that match the resolution of new microcalorimeter observations. We show how the Lyman lines can be used as column density indicators in the hot X-ray emitting gas in a cluster of galaxies such as Perseus, and examine their sensitivity to external radiation fields and turbulence.

astro-ph.HE↗

arXiv2025

New Insights with XRISM & Cloudy: A novel Column Density Diagnostic

We present a simple, yet powerful column density diagnostic for plasmas enabled by X-ray microcalorimeter observations. With the recent developments of the spectral simulation code Cloudy, inspired by the high spectral resolution of the X-Ray Imaging and Spectroscopy Mission (XRISM) and the Advanced Telescope for High Energy Astrophysics (Athena), we make predictions for the intensity ratio of the resolved fine-structure lines Ly$α_1$ and Ly$α_2$ of H-like ions. We show that this ratio can be observationally constrained and used as a plasma column density indicator. We demonstrate this with a XRISM observation of the high-mass X-ray binary Centaurus X-3. This diagnostic is useful for a wide range of X-ray emitting plasmas, either collisionally or radiatively ionized.

astro-ph.HE↗

arXiv2024

Non-thermal radio emission in Sakurai's Object

The very late thermal pulse (VLTP) affects the evolution of $\sim$20\% of 1--8\,$\mathrm M_\odot$ stars, repeating the last phases of the red giant within a few years and leading to the formation of a new, but hydrogen-poor nebula within the old planetary nebula (PN). The strong dust formation in the latter obscures the optical and near-infrared radiation of the star. We aimed to determine the reheating timescale of the central star in Sakurai's object, which is an important constraint for the poorly understood VLTP evolution. We observed the radio continuum emission of Sakurai's object for almost 20 years from 2004 to 2023. Continuous, multi-frequency observations proved to be essential to distinguish between phases dominated by photoionization and shock ionization. The flux density fluctuates by more than a factor 40 within months to years. The spectral index remained negative between 2006 and 2017 and is close to zero since 2019. The emission region is barely resolved since 2021. Non-thermal radio emission observed from 2004 to 2017 traces shocks induced by wind interactions due to discrete mass-loss events. Thermal emission dominates during the period 2019--2023 and may indicate photoionization of the nebula by the central star.

astro-ph.SR↗

arXiv2023

The 23.01 release of Cloudy

We announce the C23.01 update of Cloudy. This corrects a simple coding error, present since $\sim$ 1990, in one routine that required a conversion from the line-center to the mean normalization of the Ly$α$ optical depth. This affects the destruction of H I Ly$α$ by background opacities. Its largest effect is upon the Ly$α$ intensity in high-ionization dusty clouds, where the predicted intensity is now up to three times stronger. Other properties that depend on Ly$α$ destruction, such as grain infrared emission, change in response.

astro-ph.GA↗

arXiv2023

JWST observations of the Ring Nebula (NGC 6720): I. Imaging of the rings, globules, and arcs

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

astro-ph.SR↗

arXiv2023

The heart of Sakurai's Object revealed by ALMA

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

astro-ph.SR↗

arXiv2023

The 2023 release of Cloudy

We describe the 2023 release of the spectral synthesis code Cloudy. Since the previous major release, migrations of our online services motivated us to adopt git as our version control system. This change alone led us to adopt an annual release scheme, accompanied by a short release paper, the present being the inaugural. Significant changes to our atomic and molecular data have improved the accuracy of Cloudy predictions: we have upgraded our instance of the Chianti database from version 7 to 10; our H- and He-like collisional rates to improved theoretical values; our molecular data to the most recent LAMDA database, and several chemical reaction rates to their most recent UDfA and KiDA values. Finally, we describe our progress on upgrading Cloudy's capabilities to meet the requirements of the X-ray microcalorimeters aboard the upcoming XRISM and Athena missions, and outline future development that will make Cloudy of use to the X-ray community.

astro-ph.GA↗

arXiv2022

Recombination of hot ionized nebulae: The old planetary nebula around V4334 Sgr (Sakurai's star)

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

astro-ph.SR↗

arXiv2022

First images of the molecular gas around a born-again star revealed by ALMA

Born-again stars allow probing stellar evolution in human timescales and provide the most promising path for the formation of hydrogen-deficient post-asymptotic giant branch objects, but their cold and molecular components remain poorly explored. Here we present ALMA observations of V605 Aql that unveil for the first time the spatio-kinematic distribution of the molecular material associated to a born-again star. Both the continuum and molecular line emission exhibit a clumpy ring-like structure with a total extent of $\approx$1$^{\prime\prime}$ in diameter. The bulk of the molecular emission is interpreted as being produced in a radially-expanding disk-like structure with an expansion velocity v$_{\rm exp}$$\sim$90 km s$^{-1}$ and an inclination $i$$\approx$60$^{\circ}$ with respect to the line-of-sight. The observations also reveal a compact high-velocity component, v$_{\rm exp}$$\sim$280 km s$^{-1}$, that is aligned perpendicularly to the expanding disk. This component is interpreted as a bipolar outflow with a kinematical age $τ$$\lesssim$20 yr, which could either be material that is currently being ejected from V605 Aql, or it is being dragged from the inner parts of the disk by a stellar wind. The dust mass of the disk is in the range $M_{\rm dust}$$\sim$0.2-8$\times$10$^{-3}$ M$_{\odot}$, depending on the dust absorption coefficient. The mass of the CO is $M_{\rm CO}$$\approx$1.1$\times10^{-5}$ $M_{\odot}$, which is more than three orders of magnitude larger than the mass of the other detected molecules. We estimate a $^{12}$C/$^{13}$C ratio of 5.6$\pm$0.6, which is consistent with the single stellar evolution scenario in which the star experienced a very late thermal pulse instead of a nova-like event as previously suggested.

astro-ph.GA↗

arXiv2020

The cooling-down central star of the planetary nebula SwSt\,1: a late thermal pulse in a massive post-AGB star?

SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing a late [WC]-type central star. Previous studies suggested that the nebular and stellar lines are slowly changing with time. We studied new and archival optical and ultraviolet spectra of the object. The [OIII] 4959 and 5007 A to $\mathrm{H}β$ line flux ratios decreased between about 1976 and 1997/2015. The stellar spectrum also shows changes between these epochs. We modeled the stellar and nebular spectra observed at different epochs. The analyses indicate a drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and 1993. We do not detect significant changes between 1993 and 2015. The observations show that the star performed a loop in the H-R diagram. This is possible when a shell source is activated during its post-AGB evolution. We infer that a late thermal pulse (LTP) experienced by a massive post-AGB star can explain the evolution of the central star. Such a star does not expand significantly as the result of the LTP and does not became a born-again red giant. However, the released energy can remove the tiny H envelope of the star.

astro-ph.SR↗

arXiv2018

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.

astro-ph.GA↗

arXiv2018

Herschel Planetary Nebula Survey (HerPlaNS): Hydrogen Recombination Laser Lines in Mz 3

The bipolar nebula Menzel 3 (Mz 3) was observed as part of the \textit{Herschel} Planetary Nebula Survey (\textit{HerPlaNS}), which used the PACS and SPIRE instruments aboard the \textit{Herschel Space Observatory} to study a sample of planetary nebulae (PNe). In this paper, one of the series describing \textit{HerPlaNS} results, we report the detection of H I recombination lines (HRLs) in the spectrum of Mz 3. Inspection of the spectrum reveals the presence of 12 HRLs in the 55 to 680 $μ$m range covered by the PACS and SPIRE instruments (H11$α$ to H21$α$ and H14$β$). The presence of HRLs in this range is unusual for PNe and has not been reported in Mz 3 before. Our analysis indicates that the HRLs we observed are enhanced by laser effect occurring in the core of Mz 3. Our arguments for this are: (i) the available Mz 3 optical to submillimetre HRL $α$ line intensity ratios are not well reproduced by the spontaneous emission of optically thin ionized gas, as would be typical for nebular gas in PNe; (ii) the compact core of Mz 3 is responsible for a large fraction of the Herschel HRLs emission; (iii) the line intensity ratios for Mz 3 are very similar to those in the core emission of the well known star MWC 349A, where laser effect is responsible for the enhancement of HRLs in the Herschel wavelength range; (iv) the physical characteristics relevant to cause laser effect in the core of MWC 349A are very similar to those in the core of Mz 3.

astro-ph.GA↗

arXiv2017

The Herschel Planetary Nebula Survey (HerPlaNS) - a comprehensive dusty photoionization model of NGC6781

We perform a comprehensive analysis of the planetary nebula (PN) NGC6781 to investigate the physical conditions of each of its ionized, atomic, and molecular gas and dust components and the object's evolution, based on panchromatic observational data ranging from UV to radio. Empirical nebular elemental abundances, compared with theoretical predictions via nucleosynthesis models of asymptotic giant branch (AGB) stars, indicate that the progenitor is a solar-metallicity, 2.25-3.0 Msun initial-mass star. We derive the best-fit distance of 0.46 kpc by fitting the stellar luminosity (as a function of the distance and effective temperature of the central star) with the adopted post-AGB evolutionary tracks. Our excitation energy diagram analysis indicate high excitation temperatures in the photodissociation region (PDR) beyond the ionized part of the nebula, suggesting extra heating by shock interactions between the slow AGB wind and the fast PN wind. Through iterative fitting using the Cloudy code with empirically-derived constraints, we find the best-fit dusty photoionization model of the object that would inclusively reproduce all of the adopted panchromatic observational data. The estimated total gas mass (0.41 Msun) corresponds to the mass ejected during the last AGB thermal pulse event predicted for a 2.5 Msun initial-mass star. A significant fraction of the total mass (about 70 percent) is found to exist in the PDR, demonstrating the critical importance of the PDR in PNe that are generally recognized as the hallmark of ionized/H+ regions.

astro-ph.SR↗

arXiv2015

Ultraviolet emission lines of Si II in cool star and solar spectra

Recent atomic physics calculations for Si II are employed within the Cloudy modelling code to analyse Hubble Space Telescope (HST) STIS ultraviolet spectra of three cool stars, Beta-Geminorum, Alpha-Centauri A and B, as well as previously published HST/GHRS observations of Alpha-Tau, plus solar quiet Sun data from the High Resolution Telescope and Spectrograph. Discrepancies found previously between theory and observation for line intensity ratios involving the 3s$^{2}$3p $^{2}$P$_{J}$--3s3p$^{2}$ $^{4}$P$_{J^{\prime}}$ intercombination multiplet of Si II at 2335 Angs are significantly reduced, as are those for ratios containing the 3s$^{2}$3p $^{2}$P$_{J}$--3s3p$^{2}$ $^{2}$D$_{J^{\prime}}$ transitions at 1816 Angs. This is primarily due to the effect of the new Si II transition probabilities. However, these atomic data are not only very different from previous calculations, but also show large disagreements with measurements, specifically those of Calamai et. al. (1993) for the intercombination lines. New measurements of transition probabilities for Si II are hence urgently required to confirm (or otherwise) the accuracy of the recently calculated values. If the new calculations are confirmed, then a long-standing discrepancy between theory and observation will have finally been resolved. However, if the older measurements are found to be correct, then the agreement between theory and observation is simply a coincidence and the existing discrepancies remain.

astro-ph.SR↗