arXiv subjects

P. A. M. van Hoof

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

At least 37 records · Page 2Linked to original sources

arXiv2014

A stubbornly large mass of cold dust in the ejecta of Supernova 1987A

We present new Herschel photometric and spectroscopic observations of Supernova 1987A, carried out in 2012. Our dedicated photometric measurements provide new 70 micron data and improved imaging quality at 100 and 160 micron compared to previous observations in 2010. Our Herschel spectra show only weak CO line emission, and provide an upper limit for the 63 micron [O I] line flux, eliminating the possibility that line contaminations distort the previously estimated dust mass. The far-infrared spectral energy distribution (SED) is well fitted by thermal emission from cold dust. The newly measured 70 micron flux constrains the dust temperature, limiting it to nearly a single temperature. The far-infrared emission can be fitted by 0.5+-0.1 Msun of amorphous carbon, about a factor of two larger than the current nucleosynthetic mass prediction for carbon. The observation of SiO molecules at early and late phases suggests that silicates may also have formed and we could fit the SED with a combination of 0.3 Msun of amorphous carbon and 0.5 Msun of silicates, totalling 0.8 Msun of dust. Our analysis thus supports the presence of a large dust reservoir in the ejecta of SN 1987A. The inferred dust mass suggests that supernovae can be an important source of dust in the interstellar medium, from local to high-redshift galaxies.

astro-ph.SR↗

arXiv2014

Implications of Coronal Line Emission in NGC 4696

We announce a new facility in the spectral code CLOUDY that enables tracking the evolution of a cooling parcel of gas with time. For gas cooling from temperatures relevant to galaxy clusters, earlier calculations estimated the [Fe XIV] λ5303 / [Fe X] λ6375 luminosity ratio, a critical diagnostic of a cooling plasma, to slightly less than unity. By contrast, our calculations predict a ratio ~3. We revisit recent optical coronal line observations along the X-ray cool arc around NGC 4696 by Canning et al. (2011), which detected [Fe X] λ6375, but not [Fe XIV] λ5303. We show that these observations are not consistent with predictions of cooling flow models. Differential extinction could in principle account for the observations, but it requires extinction levels (A_V > 3.625) incompatible with previous observations. The non-detection of [Fe XIV] implies a temperature ceiling of 2.1 million K. Assuming cylindrical geometry and transonic turbulent pressure support, we estimate the gas mass at ~1 million solar masses. The coronal gas is cooling isochorically. We propose that the coronal gas has not condensed out of the intracluster medium, but instead is the conductive or mixing interface between the X-ray plume and the optical filaments. We present a number of emission lines that may be pursued to test this hypothesis and constrain the amount of intermediate temperature gas in the system.

astro-ph.GA↗

arXiv2014

The Very Fast Evolution of the VLTP Object V4334 Sgr

V4334 Sgr (Sakurai's object) is an enigmatic evolved star that underwent a very late thermal pulse a few years before its discovery in 1996. It ejected a new, hydrogen-deficient nebula in the process. Emission lines from the newly ejected gas were first discovered in 1998 (He I 1083 nm) and 2001 (optical). We have monitored the optical emission spectrum since. From 2001 through 2007 the optical spectrum showed an exponential decline in flux, consistent with a shock that occurred around 1998 and started cooling soon after that. In this paper we show that since 2008 the line fluxes have been continuously rising again. Our preliminary interpretation is that this emission comes from a region close to the central star, and is excited by a second shock. This shock may have been induced by an increase in the stellar mass loss and wind velocity associated with a rise in the stellar temperature.

astro-ph.SR↗

arXiv2014

A Far-Infrared Spectral Sequence of Galaxies: Trends and Models

We present a framework for the interpretation of the far-infrared spectra of galaxies in which we have expanded the model parameters compared with previous work by varying the ionization parameter $U$, column density $N$(H), and gas density at the cloud face $n$(H$^{+}$) for a central starburst or AGN. We compare these models carried out with the $Cloudy$ spectral synthesis code to trends in line-to-total far-infrared luminosity ratios, far-infrared fine-structure line ratios, IRAS colors, and OH and H$_{2}$O column densities with trends found in the well-studied sample of ten nearby galaxies from the IRAS Bright Galaxy Sample with infrared luminosities greater than 10$^{10}$ L$_{\odot}$ and IRAS 60 micron fluxes equal to or greater than that of the nearby ULIRG Arp 220. We find that the spectral sequence extending from normal starburst-type emission line spectra to ULIRG-type absorption-dominated spectra with significant absorption from excited levels, can be best explained by simultaneously increasing the hydrogen column density, from as low as 10$^{21}$ cm$^{-2}$ to as high as 10$^{24.8}$ cm$^{-2}$ or greater, and the ionization parameter, from as low as 10$^{-4}$ to as high as 1. The starburst models best reproduce most of the sequence, while AGN models are somewhat better able to produce the high OH and H$_{2}$O column densities in Arp 220. Our results suggest that the molecular ISM in ULIRG-like, absorption-dominated systems is located close to and at least partially obscures the source of power throughout much of the far-infrared, which must be taken into account in order to properly interpret diagnostics of both their sources of power and of feedback.

astro-ph.GA↗

arXiv2014

Accurate determination of the free-free Gaunt factor; I - non-relativistic Gaunt factors

Modern 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 emitted by an ionized plasma. Until now no set of data exists that would meet this need in a fully satisfactory way. We have therefore undertaken to produce a table of very accurate non-relativistic Gaunt factors over a much wider range of parameters than has ever been produced before. We first produced a table of non-averaged Gaunt factors, covering the parameter space log10(epsilon_i) = -20 to +10 and log10(w) = -30 to +25. We then continued to produce a table of thermally averaged Gaunt factors covering the parameter space log10(gamma^2) = -6 to +10 and log10(u) = -16 to +13. Finally we produced a table of the frequency integrated Gaunt factor covering the parameter space log10(gamma^2) = -6 to +10. All the data presented in this paper are available online.

astro-ph.HE↗

arXiv2014

Herschel Planetary Nebula Survey (HerPlaNS) - First Detection of OH+ in Planetary Nebulae

We report the first detections of OH$^+$ emission in planetary nebulae (PNe). As part of an imaging and spectroscopy survey of 11 PNe in the far-IR using the PACS and SPIRE instruments aboard the Herschel Space Observatory, we performed a line survey in these PNe over the entire spectral range between 51 and 672$μ$m to look for new detections. OH$^+$ rotational emission lines at 152.99, 290.20, 308.48, and 329.77$μ$m were detected in the spectra of three planetary nebulae: NGC 6445, NGC 6720, and NGC 6781. Excitation temperatures and column densities derived from these lines are in the range of 27 to 47 K and 2$\times$10$^{10}$ to 4 $\times$10$^{11}$ cm$^{-2}$, respectively. In PNe, the OH+ rotational line emission appears to be produced in the photodissociation region (PDR) in these objects. The emission of OH+ is observed only in PNe with hot central stars (T$_{eff}$ > 100000 K), suggesting that high-energy photons may play a role in the OH+ formation and its line excitation in these objects, as it seems to be the case for ultraluminous galaxies.

astro-ph.GA↗

arXiv2014

Radiative Cooling II: Effects of Density and Metallicity

This work follows Lykins et al. discussion of classic plasma cooling function at low density and solar metallicity. Here we focus on how the cooling function changes over a wide range of density (n_H<10^12 cm^(-3)) and metallicity (Z<30Z _sun ). We find that high densities enhance the ionization of elements such as hydrogen and helium until they reach local thermodynamic equilibrium. By charge transfer, the metallicity changes the ionization of hydrogen when it is partially ionized. We describe the total cooling function as a sum of four parts: those due to H&He, the heavy elements, electron-electron bremsstrahlung and grains. For the first 3 parts, we provide a low-density limit cooling function, a density dependence function, and a metallicity dependence function. These functions are given with numerical tables and analytical fit functions. For grain cooling, we only discuss in ISM case. We then obtain a total cooling function that depends on density, metallicity and temperature. As expected, collisional de-excitation suppresses the heavy elements cooling. Finally, we provide a function giving the electron fraction, which can be used to convert the cooling function into a cooling rate.

astro-ph.IM↗

arXiv2014

The evolving spectrum of the planetary nebula Hen 2-260

We analysed the planetary nebula Hen 2-260 using optical spectroscopy and photometry. We compared our observations with the data from literature to search for evolutionary changes. The nebular line fluxes were modelled with the Cloudy photoionization code to derive the stellar and nebular parameters. The planetary nebula shows a complex structure and possibly a bipolar outflow. The nebula is relatively dense and young. The central star is just starting $\rm O^+$ ionization ($\rm T_{eff} \approx 30,000 \, K$). Comparison of our observations with literature data indicates a 50% increase of the [OIII] 5007 Å line flux between 2001 and 2012. We interpret it as the result of the progression of the ionization of $\rm O^{+}$. The central star evolves to higher temperatures at a rate of $\rm 45 \pm 7\,K\, yr^{-1}$. The heating rate is consistent with a final mass of $\rm 0.626 ^{+0.003}_{-0.005} \, M_{\odot}$ or $\rm 0.645 ^{+0.008}_{-0.008} \, M_{\odot}$ for two different sets of post-AGB evolutionary tracks from literature. The photometric monitoring of Hen 2-260 revealed variations on a timescale of hours or days. The variability may be caused by pulsations of the star. The temperature evolution of the central star can be traced using spectroscopic observations of the surrounding planetary nebula spanning a timescale of roughly a decade. This allows us to precisely determine the stellar mass, since the pace of the temperature evolution depends critically on the core mass. The kinematical age of the nebula is consistent with the age obtained from the evolutionary track. The final mass of the central star is close to the mass distribution peak for central stars of planetary nebulae found in other studies. The object belongs to a group of young central stars of planetary nebulae showing photometric variability.

astro-ph.SR↗

arXiv2014

Herschel spectral-mapping of the Helix Nebula (NGC 7293): Extended CO photodissociation and OH+ emission

The Helix Nebula (NGC 7293) is the closest planetary nebulae. Therefore, it is an ideal template for photochemical studies at small spatial scales in planetary nebulae. We aim to study the spatial distribution of the atomic and the molecular gas, and the structure of the photodissociation region along the western rims of the Helix Nebula as seen in the submillimeter range with Herschel. We use 5 SPIRE FTS pointing observations to make atomic and molecular spectral maps. We analyze the molecular gas by modeling the CO rotational lines using a non-local thermodynamic equilibrium (non-LTE) radiative transfer model. For the first time, we have detected extended OH+ emission in a planetary nebula. The spectra towards the Helix Nebula also show CO emission lines (from J= 4 to 8), [NII] at 1461 GHz from ionized gas, and [CI] (2-1), which together with the OH+ lines, trace extended CO photodissociation regions along the rims. The estimated OH+ column density is (1-10)x1e12 cm-2. The CH+ (1-0) line was not detected at the sensitivity of our observations. Non-LTE models of the CO excitation were used to constrain the average gas density (n(H2)=(1-5)x1e5 cm-3) and the gas temperature (Tk= 20-40 K). The SPIRE spectral-maps suggest that CO arises from dense and shielded clumps in the western rims of the Helix Nebula whereas OH+ and [CI] lines trace the diffuse gas and the UV and X-ray illuminated clumps surface where molecules reform after CO photodissociation. [NII] traces a more diffuse ionized gas component in the interclump medium.

astro-ph.SR↗

arXiv2014

The Herschel Planetary Nebula Survey (HerPlaNS) I. Data Overview and Analysis Demonstration with NGC 6781

This is the first of a series of investigations into far-IR characteristics of 11 planetary nebulae (PNs) under the Herschel Space Observatory Open Time 1 program, Herschel Planetary Nebula Survey (HerPlaNS). Using the HerPlaNS data set, we look into the PN energetics and variations of the physical conditions within the target nebulae. In the present work, we provide an overview of the survey, data acquisition and processing, and resulting data products. We perform (1) PACS/SPIRE broadband imaging to determine the spatial distribution of the cold dust component in the target PNs and (2) PACS/SPIRE spectral-energy-distribution (SED) and line spectroscopy to determine the spatial distribution of the gas component in the target PNs. For the case of NGC 6781, the broadband maps confirm the nearly pole-on barrel structure of the amorphous carbon-richdust shell and the surrounding halo having temperatures of 26-40 K. The PACS/SPIRE multi-position spectra show spatial variations of far-IR lines that reflect the physical stratification of the nebula. We demonstrate that spatially-resolved far-IR line diagnostics yield the (T_e, n_e) profiles, from which distributions of ionized, atomic, and molecular gases can be determined. Direct comparison of the dust and gas column mass maps constrained by the HerPlaNS data allows to construct an empirical gas-to-dust mass ratio map, which shows a range of ratios with the median of 195+-110. The present analysis yields estimates of the total mass of the shell to be 0.86 M_sun, consisting of 0.54 M_sun of ionized gas, 0.12 M_sun of atomic gas, 0.2 M_sun of molecular gas, and 4 x 10^-3 M_sun of dust grains. These estimates also suggest that the central star of about 1.5 M_sun initial mass is terminating its PN evolution onto the white dwarf cooling track.

astro-ph.GA↗

arXiv2013

CKVul: evolving nebula and three curious background stars

We analyse the remnants of CK Vul (Nova Vul 1670) using optical imaging and spectroscopy. The imaging, obtained between 1991 and 2010, spans 5.6% of the life-time of the nebula. The flux of the nebula decreased during the last 2 decades. The central source still maintains the ionization of the innermost part of the nebula, but recombination proceeds in more distant parts of the nebula. Surprisingly, we discovered two stars located within 10 arcsec of the expansion centre of the radio emission that are characterized by pronounced long term variations and one star with high proper motion. The high proper motion star is a foreground object, and the two variable stars are background objects. The photometric variations of two variables are induced by a dusty cloud ejected by CK Vul and passing through the line of sight to those stars. The cloud leaves strong lithium absorption in the spectra of the stars. We discuss the nature of the object in terms of recent observations.

astro-ph.SR↗

arXiv2013

Herschel SPIRE and PACS observations of the red supergiant VY CMa: analysis of the molecular line spectra

We present an analysis of the far-infrared and submillimetre molecular emission line spectrum of the luminous M-supergiant VY CMa, observed with the SPIRE and PACS spectrometers aboard the Herschel Space Observatory. Over 260 emission lines were detected in the 190-650-micron SPIRE FTS spectra, with one-third of the observed lines being attributable to H2O. Other detected species include CO, 13CO, H2^18O, SiO, HCN, SO, SO2, CS, H2S, and NH3. Our model fits to the observed 12CO and 13CO line intensities yield a 12C/13C ratio of 5.6+-1.8, consistent with measurements of this ratio for other M supergiants, but significantly lower than previously estimated for VY CMa from observations of lower-J lines. The spectral line energy distribution for twenty SiO rotational lines shows two temperature components: a hot component at 1000 K, which we attribute to the stellar atmosphere and inner wind, plus a cooler ~200 K component, which we attribute to an origin in the outer circumstellar envelope. We fit the line fluxes of 12CO, 13CO, H2O and SiO, using the SMMOL non-LTE line transfer code, with a mass-loss rate of 1.85x10^-4 Msun yr^-1 between 9 R* and 350 R*. To fit the observed line fluxes of 12CO, 13CO, H2O and SiO with SMMOL non-LTE line radiative transfer code, along with a mass-loss rate of 1.85x10^-4 Msun yr^-1. To fit the high rotational lines of CO and H2O, the model required a rather flat temperature distribution inside the dust condensation radius, attributed to the high H2O opacity. Beyond the dust condensation radius the gas temperature is fitted best by an r^-0.5 radial dependence, consistent with the coolant lines becoming optically thin. Our H2O emission line fits are consistent with an ortho:para ratio of 3 in the outflow.

astro-ph.GA↗

arXiv2013

A Herschel study of NGC 650

As part of the Herschel Guaranteed Time Key Project MESS (Mass loss of Evolved StarS) we have imaged a sample of planetary nebulae. In this paper we present the PACS and SPIRE images of the classical bipolar planetary nebula NGC 650. We used these images to derive a temperature map of the dust. We also constructed a photoionization and dust radiative transfer model using the spectral synthesis code Cloudy. To constrain this model, we used the PACS and SPIRE fluxes and combined these with hitherto unpublished IUE and Spitzer IRS spectra as well as various other data from the literature. The temperature map combined with the photoionization model were used to study various aspects of the central star, the nebula, and in particular the dust grains in the nebula. The central star parameters are determined to be T_eff = 208 kK and L = 261 L_sol assuming a distance of 1200 pc. The stellar temperature is much higher than previously published values. We confirm that the nebula is carbon-rich with a C/O ratio of 2.1. The nebular abundances are typical for a type IIa planetary nebula. With the photoionization model we determined that the grains in the ionized nebula are large (assuming single-sized grains, they would have a radius of 0.15 micron. Most likely these large grains were inherited from the asymptotic giant branch phase. The PACS 70/160 micron temperature map shows evidence for two radiation components heating the grains. The first component is direct emission from the central star, while the second component is diffuse emission from the ionized gas (mainly Ly alpha). We show that previous suggestions that there is a photo-dissociation region surrounding the ionized region are incorrect. The neutral material resides in dense clumps inside the ionized region. These may also harbor stochastically heated very small grains in addition to the large grains.

astro-ph.SR↗

arXiv2013

Carbon Monoxide in the Cold Debris of Supernova 1987A

We report spectroscopic and imaging observations of rotational transitions of cold CO and SiO in the ejecta of SN1987A, the first such emission detected in a supernova remnant. In addition to line luminosities for the CO J=1-0, 2-1, 6-5, and 7-6 transitions, we present upper limits for all other transitions up to J=13-12, collectively measured from the Atacama Large Millimeter Array (ALMA), the Atacama Pathfinder EXperiment (APEX), and the Herschel Spectral and Photometric Imaging REceiver (SPIRE). Simple models show the lines are emitted from at least 0.01 solar masses of CO at a temperature > 14 K, confined within at most 35% of a spherical volume expanding at ~ 2000 km/s. Moreover, we locate the emission within 1'' of the central debris. These observations, along with a partial observation of SiO, confirm the presence of cold molecular gas within supernova remnants and provide insight into the physical conditions and chemical processes in the ejecta. Furthermore, we demonstrate the powerful new window into supernova ejecta offered by submillimeter observations.

astro-ph.SR↗

arXiv2013

Near-infrared photometry of Galactic planetary nebulae with the VVV Survey

Planetary nebulae (PNe) are powerful tracers of evolved stellar populations. Among the 3000 known PNe in the Galaxy, about 600 are located within the 520 square-degree area covered by the VVV survey. The VVV photometric catalogue provides an important new dataset for the study of PNe, with high-resolution imaging in five near-infrared bands. Aperture photometry of known PNe in the VVV area was retrieved from source catalogues. Care was taken to minimise any confusion with field stars. The colours of the PNe we are determined for H-Ks, J-H, Z-Y, and Y-J, and compared to stars and to other types of emission line objects. Cloudy photo-ionisation models were used to predict colours for typical PNe. We present near-infrared photometry for 353 known PNe. The best separation from other objects is obtained in the H-Ks vs. J-H diagram. We calculated the emission-line contribution to the in-band flux based on a model for NGC 6720: we find that this is highest in the Z and Y bands at over 50%, lower in the J band at 40%, and lowest in the H and Ks bands at 20%. A new view of PNe in the wavelength domain of the Z and Y bands is shown. Photo-ionisation models are used to explore the observed colours in these bands. The Y band is shown to be dominated by HeI 1.083 mu and HeII 1.012 mu, and colours involving this band are very sensitive to the temperature of the ionizing star. The VVV survey represents a unique dataset for studing crowded and obscured regions in the Galactic plane. The diagnostic diagrams presented here allow one to study the properties of known PNe and to uncover objects not previously classified.

astro-ph.GA↗

arXiv2013

Expanded Iron UTA spectra -- probing the thermal stability limits in AGN clouds

The Fe unresolved transition array (UTAs) produce prominent features in the 15-17?A wavelength range in the spectra of Active Galactic Nuclei (AGN). Here we present new calculations of the energies and oscillator strengths of inner- shell lines from Fe XIV, Fe XV, and Fe XVI. These are crucial ions since they are dominant at inflection points in the gas thermal stability curve, and UTA excitation followed by autoionization is an important ionization mechanism for these species. We incorporate these, and data reported in previous papers, into the plasma simulation code Cloudy. This updated physics is subsequently employed to reconsider the thermally stable phases in absorbing media in Active Galactic Nuclei. We show how the absorption profile of the Fe XIV UTA depends on density, due to the changing populations of levels within the ground configuration.

astro-ph.CO↗

arXiv2013

The 2013 Release of Cloudy

This is a summary of the 2013 release of the plasma simulation code Cloudy. Cloudy models the ionization, chemical, and thermal state of material that may be exposed to an external radiation field or other source of heating, and predicts observables such as emission and absorption spectra. It works in terms of elementary processes, so is not limited to any particular temperature or density regime. This paper summarizes advances made since the last major review in 1998. Much of the recent development has emphasized dusty molecular environments, improvements to the ionization / chemistry solvers, and how atomic and molecular data are used. We present two types of simulations to demonstrate the capability of the code. We consider a molecular cloud irradiated by an X-ray source such as an Active Nucleus and show how treating EUV recombination lines and the full SED affects the observed spectrum. A second example illustrates the very wide range of particle and radiation density that can be considered.

astro-ph.GA↗

arXiv2012

Pumping up the [N I] nebular lines

The optical [N I] doublet near 5200 Å is anomalously strong in a variety of emission-line objects. We compute a detailed photoionization model and use it to show that pumping by far-ultraviolet (FUV) stellar radiation previously posited as a general explanation applies to the Orion Nebula (M42) and its companion M43; but, it is unlikely to explain planetary nebulae and supernova remnants. Our models establish that the observed nearly constant equivalent width of [N I] with respect to the dust-scattered stellar continuum depends primarily on three factors: the FUV to visual-band flux ratio of the stellar population; the optical properties of the dust; and the line broadening where the pumping occurs. In contrast, the intensity ratio [N I]/Hβ depends primarily on the FUV to extreme-ultraviolet ratio, which varies strongly with the spectral type of the exciting star. This is consistent with the observed difference of a factor of five between M42 and M43, which are excited by an O7 and B0.5 star respectively. We derive a non-thermal broadening of order 5 km/s for the [N I] pumping zone and show that the broadening mechanism must be different from the large-scale turbulent motions that have been suggested to explain the line-widths in this H II region. A mechanism is required that operates at scales of a few astronomical units, which may be driven by thermal instabilities of neutral gas in the range 1000 to 3000 K. In an appendix, we describe how collisional and radiative processes are treated in the detailed model N I atom now included in the Cloudy plasma code.

astro-ph.GA↗