arXiv subjects

P. A. M. van Hoof

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

At least 19 recordsLinked to original sources

arXiv2026

The emergence of a [WC] star in Sakurai's object

Sakurai's object provides a rare opportunity to observe stellar evolution on human timescales. Since its born-again event and detection in 1996, its evolution has been extensively monitored, and recent optical spectroscopy has suggested the emergence of [WR]-type emission features. In this Letter, we present a secure spectroscopic identification of its central star based on new VLT/FORS2 observations and NLTE expanding atmosphere models. Several observed emission lines arise in a [WR]-type stellar wind, establishing the central star as a [WR] object. The strongest stellar features are due to C II-III and He I. We derive the stellar and wind parameters of Sakurai's object and discuss its evolutionary status. The [WR] star is much cooler (Teff ~ 30.5 kK) than the born-again star V605 Aql (Teff} ~ 95 kK), which erupted about 80 years earlier. The inferred temperature is lower than predicted by models suppressing convective mixing efficiency during the VLTP, but is consistent with higher time-resolution calculations involving slightly lower-mass (<~ 0.6Msun) remnants. Continued spectroscopic monitoring and atmospheric modelling will be crucial for tracing its reheating curve and constraining the physics of this rapid evolutionary phase.

astro-ph.SR↗

arXiv2023

MELCHIORS: The Mercator Library of High Resolution Stellar Spectroscopy

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

astro-ph.SR↗

arXiv2023

The 2017 Release of Cloudy

We describe the 2017 release of the spectral synthesis code Cloudy. A major development since the previous release has been exporting the atomic data into external data files. This greatly simplifies updates and maintenance of the data. Many large datasets have been incorporated with the result that we can now predict well over an order of magnitude more emission lines when all databases are fully used. The use of such large datasets is not realistic for most calculations due to the time and memory needs, and we describe the limited subset of data we use by default. Despite the fact that we now predict significantly more lines than the previous Cloudy release, this version is faster because of optimization of memory access patterns and other tuning. The size and use of the databases can easily be adjusted in the command-line interface. We give examples of the accuracy limits using small models, and the performance requirements of large complete models. We summarize several advances in the H- and He-like iso-electronic sequences. We use our complete collisional-radiative models of the ionization of these one and two-electron ions to establish the highest density for which the coronal or interstellar medium (ISM) approximation works, and the lowest density where Saha or local thermodynamic equilibrium can be assumed. The coronal approximation fails at surprisingly low densities for collisional ionization equilibrium but is valid to higher densities for photoionized gas clouds. Many other improvements to the physics have been made and are described. These include the treatment of isotropic continuum sources such as the cosmic microwave background (CMB) in the reported output, and the ability to follow the evolution of cooling non-equilibrium clouds.

astro-ph.GA↗

arXiv2021

H, He-like recombination spectra IV; H, He-like recombination spectra IV: clarification and refinement of methodology for $l$-changing collisions

Precise spectral diagnostic modelling of H~{\sc i} and He~{\sc ii} recombination spectra can constrain theoretical models which describe many astrophysical environments. Simple analytic expressions are of interest for collisional $l$-changing rate coefficients that are used by large-scale population modelling codes. We review, clarify and improve-upon the modified Pengelly \& Seaton formulae of Guzmán \etal We show that the recent poor results for it shown by Vrinceanu \etal are due to their misinterpretation of its usage. We also detail efficient numerical algorithms which should enable the full quantum mechanical expression for such rate coefficients to be used much more routinely by modelling codes. We illustrate with some collisional-radiative population modelling for hydrogen.

astro-ph.SR↗

arXiv2020

Improved Fe II emission line models for AGN using new atomic datasets

Understanding the Fe II emission from Active Galactic Nuclei (AGN) has been a grand challenge for many decades. The rewards from understanding the AGN spectra would be immense, involving both quasar classification schemes such as "Eigenvector 1" and tracing the chemical evolution of the cosmos. Recently, three large Fe II atomic datasets with radiative and electron collisional rates have become available. We have incorporated these into the spectral synthesis code Cloudy and examine predictions using a new generation of AGN Spectral Energy Distribution (SED), which indicates that the UV emission can be quite different depending on the dataset utilized. The Smyth et al dataset better reproduces the observed Fe II template of the I ZW 1 Seyfert galaxy in the UV and optical regions, and we adopt these data. We consider both thermal and microturbulent clouds and show that a microturbulence of $\approx$ 100 km/s reproduces the observed shape and strength of the so-called Fe II "UV bump". Comparing our predictions with the observed Fe II template, we derive a typical cloud density of $10^{11}$ cm$^{-3}$ and photon flux of $10^{20}$ cm$^{-2}$ s$^{-1}$, and show that these largely reproduce the observed Fe II emission in the UV and optical. We calculate the $I$(Fe II)/$I$(Mg II) emission-line intensity ratio using our best-fitting model and obtain log($I$(Fe II)/$I$(Mg II)) $\sim$ 0.7, suggesting many AGNs have a roughly solar Fe/Mg abundance ratio. Finally, we vary the Eddington ratio and SED shape as a step in understanding the Eigenvector 1 correlation.

astro-ph.GA↗

arXiv2020

Current and future development of the photoionization code Cloudy

The gas that is present in the interstellar medium is usually very far removed from (local) thermodynamic equilibrium, and in some cases may also not be in a steady-state equilibrium with its surroundings. The physics of this material is complex and one needs a sophisticated numerical code to study it. For this purpose the open-source photoionization code Cloudy was created. It models the physical state of the gas and predicts the spectrum that it emits. Cloudy is continually being developed to improve the treatment of the microphysical processes and the database of fundamental data that it uses. In this paper we will discuss how we are developing the code to improve our high-density predictions by implementing better collisional-radiative models for all ions. We will also briefly discuss the experimental mode in Cloudy to model gas that is not in steady-state equilibrium and present a preliminary model of recombining gas in a planetary nebula that is on the cooling track. We finish with a short discussion of how we are speeding up the code by using parallelization.

astro-ph.SR↗

arXiv2019

The Belgian repository of fundamental atomic data and stellar spectra (BRASS) II. Quality assessment of atomic data for unblended lines in FGK stars

Fundamental atomic transition parameters, such as oscillator strengths and wavelengths, play a key role in modelling and understanding the chemical composition of stars in the universe. Despite the significant work under way to produce these parameters for many ions, uncertainties in these parameters remain large and can limit the accuracy of chemical abundance determinations. The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide a large systematic and homogeneous quality assessment of the atomic data available for quantitative spectroscopy. BRASS shall compare synthetic spectra against extremely high quality observed spectra, at a resolution of ~85000 and signal-noise ratios of ~1000, for around 20 bright BAFGK spectral type stars, in order to evaluate the atomic data available for over a thousand potentially useful spectral lines. A large-scale homogeneous selection of atomic lines is performed by synthesising theoretical spectra of literature atomic lines, for FGK-type stars including the Sun, resulting in a selection of 1091 theoretically deep and unblended lines, in the wavelength range 4200-6800~Å, which may be suitable for quality assessment. Astrophysical log(gf) values are determined for the 1091 transitions using two commonly employed methods. The agreement of these log(gf) values are used to select well-behaving lines for quality assessment. 845 atomic lines were found to be suitable for quality assessment, of which 408 were found to be robust against systematic differences between analysis methods. Around 54% of the quality-assessed lines were found to have at least one literature log(gf) value in agreement with our derived values, though the remaining values can disagree by as much as 0.5 dex. Only 38% of FeI lines were found to have sufficiently accurate log(gf) values, increasing to ~70-75% for the remaining Fe-group lines.

astro-ph.SR↗

arXiv2019

H-, He-like recombination spectra III: $n$-changing collisions in highly-excited Rydberg states and their impact on the radio, IR and optical recombination lines

At intermediate to high densities, electron (de-)excitation collisions are the dominant process for populating or depopulating high Rydberg states. In particular, the accurate knowledge of the energy changing ($n$-changing) collisional rates is determinant for predicting the radio recombination spectra of gaseous nebula. The different datasets present in the literature come either from impact parameter calculations or semi-empirical fits and the rate coefficients agree within a factor of two. We show in this paper that these uncertainties cause errors lower than 5% in the emission of radio recombination lines (RRL) of most ionized plasmas of typical nebulae. However, in special circumstances where the transitions between Rydberg levels are amplified by maser effects, the errors can increase up to 20%. We present simulations of the optical depth and H$nα$ line emission of Active Galactic Nuclei (AGN) Broad Line Regions (BLRs) and the Orion Nebula Blister to showcase our findings.

astro-ph.GA↗

arXiv2018

Suppression of Dielectronic Recombination Due to Finite Density Effects II: Analytical Refinement and Application to Density-dependent Ionization Balances and AGN Broad-line Emission

We present improved fits to our treatment of suppression of dielectronic recombination at intermediate densities. At low densities, most recombined excited states eventually decay to the ground state, and therefore the total dielectronic recombination rate to all levels is preserved. At intermediate densities, on the other hand, collisions can lead to ionization of higher-lying excited states, thereby suppressing the dielectronic recombination rate. The improved suppression factors presented here, although highly approximate, allow summed recombination rate coefficients to be used to intermediate densities. There have been several technical improvements to our previously presented fits. For H- through B-like ions the activation log densities have been adjusted to better reproduce existing data. For B-, C-, Al-, and Si-like ions secondary autoionization is now included. The treatment of density discontinuity in electron excitations out of ground state H-, He-, and Ne-like ions has been improved. These refined dielectronic recombination suppression factors are used in the most recent version of the plasma simulation code Cloudy. We show how the ionization and emission spectrum change when this physics is included. Although these suppression factors improve the treatment of intermediate densities, they are highly approximate and are not a substitution for a complete collisional-radiative model of the ionization balance.

physics.atom-ph↗

arXiv2017

Thermodynamically-consistent semi-classical $\ell$-changing rates

We compare the results of the semi-classical (SC) and quantum-mechanical (QM) formalisms for angular-momentum changing transitions in Rydberg atom collisions given by Vrinceanu & Flannery, J. Phys. B 34, L1 (2001), and Vrinceanu, Onofrio & Sadeghpour, ApJ 747, 56 (2012), with those of the SC formalism using a modified Monte Carlo realization. We find that this revised SC formalism agrees well with the QM results. This provides further evidence that the rates derived from the QM treatment are appropriate to be used when modelling recombination through Rydberg cascades, an important process in understanding the state of material in the early universe. The rates for $Δ\ell=\pm1$ derived from the QM formalism diverge when integrated to sufficiently large impact parameter, $b$. Further to the empirical limits to the $b$ integration suggested by Pengelly & Seaton, MNRAS 127, 165 (1964), we suggest that the fundamental issue causing this divergence in the theory is that it does not fully cater for the finite time taken for such distant collisions to complete.

physics.atom-ph↗

arXiv2017

Testing atomic collision theory with the two-photon continuum of astrophysical nebulae

Accurate rates for energy-degenerate l-changing collisions are needed to determine cosmological abundances and recombination. There are now several competing theories for the treatment of this process, and it is not possible to test these experimentally. We show that the H I two-photon continuum produced by astrophysical nebulae is strongly affected by l-changing collisions. We perform an analysis of the different underlying atomic processes and simulate the recombination and two-photon spectrum of a nebula containing H and He. We provide an extended set of effective recombination coefficients and updated l-changing 2s-2p transition rates using several competing theories. In principle, accurate astronomical observations could determine which theory is correct.

astro-ph.CO↗

arXiv2017

The very fast evolution of Sakurai's object

V4334 Sgr (a.k.a. Sakurai's object) is the central star of an old planetary nebula that underwent a very late thermal pulse a few years before its discovery in 1996. We have been monitoring the evolution of the optical emission line spectrum since 2001. The goal is to improve the evolutionary models by constraining them with the temporal evolution of the central star temperature. In addition the high resolution spectral observations obtained by X-shooter and ALMA show the temporal evolution of the different morphological components.

astro-ph.SR↗

arXiv2016

H, He-like recombination spectra II: $l$-changing collisions for He Rydberg states

Cosmological models can be constrained by determining primordial abundances. Accurate predictions of the He I spectrum are needed to determine the primordial helium abundance to a precision of $< 1$% in order to constrain Big Bang Nucleosynthesis models. Theoretical line emissivities at least this accurate are needed if this precision is to be achieved. In the first paper of this series, which focused on H I, we showed that differences in $l$-changing collisional rate coefficients predicted by three different theories can translate into 10% changes in predictions for H I spectra. Here we consider the more complicated case of He atoms, where low-$l$ subshells are not energy degenerate. A criterion for deciding when the energy separation between $l$ subshells is small enough to apply energy-degenerate collisional theories is given. Moreover, for certain conditions, the Bethe approximation originally proposed by Pengelly & Seaton (1964) is not sufficiently accurate. We introduce a simple modification of this theory which leads to rate coefficients which agree well with those obtained from pure quantal calculations using the approach of Vrinceanu et al. (2012). We show that the $l$-changing rate coefficients from the different theoretical approaches lead to differences of $\sim 10$% in He I emissivities in simulations of H II regions using spectral code Cloudy.

astro-ph.CO↗

arXiv2016

H, He-like recombination spectra I: $l$-changing collisions for hydrogen

Hydrogen and helium emission lines in nebulae form by radiative recombination. This is a simple process which, in principle, can be described to very high precision. Ratios of He I and H I emission lines can be used to measure the He$^+$/H$^+$ abundance ratio to the same precision as the recombination rate coefficients. This paper investigates the controversy over the correct theory to describe dipole $l$-changing collisions ($nl\rightarrow nl'=l\pm 1$) between energy-degenerate states within an $n$-shell. The work of Pengelly & Seaton (1964) has, for half-a-century, been considered the definitive study which "solved" the problem. Recent work by Vrinceanu et al.(2012) recommended the use of rate coefficients from a semi-classical approximation which are nearly an order of magnitude smaller than those of Pengelly & Seaton (1964), with the result that significantly higher densities are needed for the $nl$ populations to come into local thermodynamic equilibrium. Here, we compare predicted H~I emissivities from the two works and find widespread differences, of up to $\approx 10$%. This far exceeds the 1% precision required to obtain the primordial He/H abundance ratio from observations so as to constrain Big Bang cosmologies. We recommend using the rate coefficients of Pengelly & Seaton (1964) for $l$-changing collisions, to describe the H recombination spectrum, based-on their quantum mechanical representation of the long-range dipole interaction.

astro-ph.GA↗

arXiv2015

Stout: Cloudy's Atomic and Molecular Database

We describe a new atomic and molecular database we developed for use in the spectral synthesis code Cloudy. The design of Stout is driven by the data needs of Cloudy, which simulates molecular, atomic, and ionized gas with kinetic temperatures 2.8 K < T < 1e10 K and densities spanning the low to high-density limits. The radiation field between photon energies $10^{-8}$ Ry and 100 MeV is considered, along with all atoms and ions of the lightest 30 elements, and ~100 molecules. For ease of maintenance, the data are stored in a format as close as possible to the original data sources. Few data sources include the full range of data we need. We describe how we fill in the gaps in the data or extrapolate rates beyond their tabulated range. We tabulate data sources both for the atomic spectroscopic parameters and for collision data for the next release of Cloudy. This is not intended as a review of the current status of atomic data, but rather a description of the features of the database which we will build upon.

astro-ph.IM↗

arXiv2015

Accurate determination of the free-free Gaunt factor. II - relativistic Gaunt factors

When modelling an ionised plasma, all spectral synthesis codes need the thermally averaged free-free Gaunt factor defined over a very wide range of parameter space in order to produce an accurate prediction for the spectrum. Until now no data set exists that would meet these needs completely. We have therefore produced a table of relativistic Gaunt factors over a much wider range of parameter space than has ever been produced before. We present tables of the thermally averaged Gaunt factor covering the range log10(gamma^2) = -6 to 10 and log10(u) = -16 to 13 for all atomic numbers Z = 1 through 36. The data were calculated using the relativistic Bethe-Heitler-Elwert (BHE) approximation and were subsequently merged with accurate non-relativistic results in those parts of the parameter space where the BHE approximation is not valid. These data will be incorporated in the next major release of the spectral synthesis code Cloudy. We also produced tables of the frequency integrated Gaunt factor covering the parameter space log10(gamma^2) = -6 to 10 for all values of Z between 1 and 36. All the data presented in this paper are available online.

astro-ph.HE↗

arXiv2015

Collisional excitation of [C II], [O I] and CO in Massive Galaxies

Many massive galaxies at the centres of relaxed galaxy clusters and groups have vast reservoirs of cool (~10,000 K) and cold (~100 K) gas. In many low redshift brightest group and cluster galaxies this gas is lifted into the hot ISM in filamentary structures, which are long lived and are typically not forming stars. Two important questions are how far do these reservoirs cool and if cold gas is abundant what is the cause of the low star formation efficiency? Heating and excitation of the filaments from collisions and mixing of hot particles in the surrounding X-ray gas describes well the optical and near infra-red line ratios observed in the filaments. In this paper we examine the theoretical properties of dense, cold clouds emitting in the far infra-red and submillimeter through the bright lines of [C II]157 μm , [O I]63 μm and CO, exposed to these energetic ionising particles. While some emission lines may be optically thick we find this is not sufficient to model the emission line ratios. Models where the filaments are supported by thermal pressure support alone also cannot account for the cold gas line ratios but a very modest additional pressure support, either from turbulence or magnetic fields can fit the observed [O I]/[C II] line ratios by decreasing the density of the gas. This may also help stabilise the filaments against collapse leading to the low rates of star formation. Finally we make predictions for the line ratios expected from cold gas under these conditions and present diagnostic diagrams for comparison with further observations. We provide our code as an Appendix.

astro-ph.GA↗

arXiv2014

Herschel imaging of the dust in the Helix Nebula (NGC 7293)

In our series of papers presenting the Herschel imaging of evolved planetary nebulae, we present images of the dust distribution in the Helix nebula (NGC 7293). Images at 70, 160, 250, 350, and 500 micron were obtained with the PACS and SPIRE instruments on board the Herschel satellite. The broadband maps show the dust distribution over the main Helix nebula to be clumpy and predominantly present in the barrel wall. We determined the spectral energy distribution of the main nebula in a consistent way using Herschel, IRAS, and Planck flux values. The emissivity index of 0.99 +/- 0.09, in combination with the carbon rich molecular chemistry of the nebula, indicates that the dust consists mainly of amorphous carbon. The dust excess emission from the central star disk is detected at 70 micron and the flux measurement agree with previous measurement. We present the temperature and dust column density maps. The total dust mass across the Helix nebula (without its halo) is determined to be 0.0035 solar mass at a distance of 216 pc. The temperature map shows dust temperatures between 22 and 42 K, which is similar to the kinetic temperature of the molecular gas, strengthening the fact that the dust and gas co-exist in high density clumps. Archived images are used to compare the location of the dust emission in the far infrared (Herschel) with the ionized (GALEX, Hbeta) and molecular hydrogen component. The different emission components are consistent with the Helix consisting of a thick walled barrel-like structure inclined to the line of sight. The radiation field decreases rapidly through the barrel wall.

astro-ph.SR↗