arXiv subjects

A. G. G. M Tielens

Publications and source records attributed to A. G. G. M Tielens.

7 recordsLinked to original sources

arXiv2026

PDRs4All XXIII. JWST-MIRI template spectra of PAHs in the Orion Bar obtained with blind signal separation

The aromatic infrared bands (AIBs) are observed in a wide variety of astrophysical sources, and now in unprecedented detail by the James Webb space telescope (JWST). These bands are generally attributed to the emission from polycyclic aromatic hydrocarbon (PAHs). Our aim is to extract mid-infrared AIB template spectra from JWST observations of the Orion Bar photo-dissociation region that can be used to derive PAH properties from observations of other objects. We used the JWST MIRI-MRS observations from the PDRs4All program. We first isolated the AIBs from the data using a multi-component fitting approach. We then applied a non-negative matrix factorization to the isolated AIB spectra and extracted two templates, named AIBirrad and AIBshield following previous work. The spatial distribution and spectral characteristics of these two spectra suggest that they originate from two chemical distinct populations. We interpret AIBirrad as emission originating from a PAH population strongly exposed to ultra-violet (UV) radiation and mainly composed of large, compact ionized PAHs. Conversely, AIBshield is interpreted as originating from a more shielded population of PAHs, which is less exposed to the UV by being more embedded in the cloud. This population consists of small, asymmetric and mainly neutral PAHs with bonded aliphatic side groups. These templates can serve to analyze PAH emission in a wide variety of objects observed with MIRI-MRS.

astro-ph.GA↗

arXiv2023

The SOFIA FEEDBACK Legacy Survey: Rapid molecular cloud dispersal in RCW 79

It has long been discussed whether stellar feedback in the form of winds and/or radiation can shred the nascent molecular cloud, thereby controlling the star formation rate. However, directly probing and quantifying the impact of stellar feedback on the neutral gas of the nascent clouds is challenging. We present an investigation doing exactly that toward the RCW 79 HII region using the ionized carbon line at 158 $μ$m ([CII]) from the FEEDBACK Legacy Survey. We combine this data with information on the dozen ionizing O stars responsible for the evolution of the region, and observe in [CII] for the first time both blue- and red-shifted mostly neutral high-velocity gas which reaches velocities up to 25 km s$^{-1}$ relative to the bulk emission of the molecular cloud. This high-velocity gas mostly contains neutral gas and partly forms a fragmented shell, similar to recently found shells in a few Galactic HII regions. However, this shell does not account for all of the observed neutral high-velocity gas. We also find high-velocity gas streaming out of the nascent cloud through holes and obtain a range of dynamical timescales below 1.0 Myr for the high-velocity gas which is well below the 2.3$\pm$0.5 Myr age of the OB cluster. This suggests a different scenario for the evolution of RCW 79, where the high-velocity gas is not solely stemming from a spherical expanding bubble, but also from gas recently ablated at the edge of the turbulent molecular cloud into the surrounding interstellar medium through low-pressure holes or chimneys. The resulting mass ejection rate estimate for the cloud is 0.9-3.5$\times$10$^{-2}$ M$_{\odot}$~yr$^{-1}$, which leads to short erosion timescales, i.e. $<$5 Myr, for the nascent molecular cloud. This finding provides direct observational evidence of rapid molecular cloud dispersal.

astro-ph.GA↗

arXiv2015

Large dust gaps in the transitional disks of HD 100453 and HD 34282

The formation of dust gaps in protoplanetary disks is one of the most important signposts of disk evolution and possibly the formation of planets. We aim to characterize the 'flaring' disk structure around the Herbig Ae/Be stars HD 100453 and HD 34282. Their spectral energy distributions (SEDs) show an emission excess between 15-40μm, but very weak (HD 100453) and no (HD 34282) signs of the 10 and 20 μm amorphous silicate features. We investigate whether this implies the presence of large dust gaps. In this work, spatially resolved mid-infrared Q-band images taken with Gemini North/MICHELLE are investigated. We perform radiative transfer modeling and examine the radial distribution of dust. We simultaneously fit the Q-band images and SEDs of HD 100453 and HD 34282. Our solutions require that the inner-halos and outer-disks are likely separated by large dust gaps that are depleted wih respect to the outer disk by a factor of 1000 or more. The inner edges of the outer disks of HD 100453 and HD 34282 have temperatures of about $160 \pm 10$ K and $60 \pm 5$ K respectively. Because of the high surface brightnesses of these walls, they dominate the emission in the Q-band. Their radii are constrained at 20+2 AU and 92+31 AU, respectively. We conclude that, HD 100453 and HD 34282 likely have disk dust gaps and the upper limit on the dust mass in each gap is estimated to be about $10^{-7}$M$_{\odot}$. We find that the locations and sizes of disk dust gaps are connected to the SED, as traced by the mid-infrared flux ratio F30/F13.5. We propose a new classification scheme for the Meeus groups (Meeus et al. 2001) based on the F30/F13.5 ratio. The absence of amorphous silicate features in the observed SEDs is caused by the depletion of small (smaller than 1 μm) silicate dust at temperatures above 160 K, which could be related to the presence of a dust gap in that region of the disk.

astro-ph.SR↗

arXiv2011

Coupled Blind Signal Separation and Spectroscopic Database Fitting of the Mid Infrared PAH Features

The aromatic infrared bands (AIBs) observed in the mid infrared spectrum are attributed to Polycyclic Aromatic Hydrocarbons (PAHs). We observe the NGC 7023-North West (NW) PDR in the mid-infrared (10 - 19.5 micron) using the Infrared Spectrometer (IRS), on board Spitzer. Clear variations are observed in the spectra, most notably the ratio of the 11.0 to 11.2 micron bands, the peak position of the 11.2 and 12.0 micron bands, and the degree of asymmetry of the 11.2 micron band. The observed variations appear to change as a function of position within the PDR. We aim to explain these variations by a change in the abundances of the emitting components of the PDR. A Blind Signal Separation (BSS) method, i.e. a Non-Negative Matrix Factorization algorithm is applied to separate the observed spectrum into components. Using the NASA Ames PAH IR Spectroscopic Database, these extracted signals are fit. The observed signals alone were also fit using the database and these components are compared to the BSS components. Three component signals were extracted from the observation using BSS. We attribute the three signals to ionized PAHs, neutral PAHs, and Very Small Grains (VSGs). The fit of the BSS extracted spectra with the PAH database further confirms the attribution to ionized and neutral PAHs and provides confidence in both methods for producing reliable results. The 11.0 micron feature is attributed to PAH cations while the 11.2 micron band is attributed to neutral PAHs. The VSG signal shows a characteristically asymmetric broad feature at 11.3 micron with an extended red wing. By combining the NASA Ames PAH IR Spectroscopic Database fit with the BSS method, the independent results of each method can be confirmed and some limitations of each method are overcome.

astro-ph.GA↗

arXiv2010

A coincidence between a hydrocarbon plasma absorption spectrum and the lambda 5450 DIB

The aim of this work is to link the broad lambda 5450 diffuse interstellar band (DIB) to a laboratory spectrum recorded through an expanding acetylene plasma. Cavity ring-down direct absorption spectra and astronomical observations of HD 183143 with the HERMES spectrograph on the Mercator Telescope in La Palma and the McKellar spectrograph on the DAO 1.2 m Telescope are compared. In the 543-547 nm region a broad band is measured with a band maximum at 545 nm and FWHM of 1.03(0.1) nm coinciding with a well-known diffuse interstellar band at lambda 5450 with FWHM of 0.953 nm. A coincidence is found between the laboratory and the two independent observational studies obtained at higher spectral resolution. This result is important, as a match between a laboratory spectrum and a - potentially lifetime broadened - DIB is found. A series of additional experiments has been performed in order to unambiguously identify the laboratory carrier of this band. This has not been possible. The laboratory results, however, restrict the carrier to a molecular transient, consisting of carbon and hydrogen.

astro-ph.IM↗

arXiv2008

A spatially resolved study of photoelectric heating and [CII] cooling in the LMC

(abridged) We study photoelectric heating throughout the Large Magellanic Cloud. We quantify the importance of the [CII] cooling line and the photoelectric heating process of various environments in the LMC and investigate which parameters control the extent of photoelectric heating. We use the BICE [CII] map and the Spitzer/SAGE infrared maps. We examine the spatial variations in the efficiency of photoelectric heating: photoelectric heating rate over power absorbed by grains. We correlate the photoelectric heating efficiency and the emission from various dust constituents and study the variations as a function of Hαemission, dust temperatures, and the total infrared luminosity. From this we estimate radiation field, gas temperature, and electron density. We find systematic variations in photoelectric efficiency. The highest efficiencies are found in the diffuse medium, while the lowest coincide with bright star-forming regions (~1.4 times lower). The [CII] line emission constitutes 1.32% of the far infrared luminosity across the whole of the LMC. We find correlations between the [CII] emission and ratios of the mid infrared and far infrared bands, which comprise various dust constituents. The correlations are interpreted in light of the spatial variations of the dust abundance and by the local environmental conditions that affect the dust emission properties. As a function of the total infrared surface brightness, S_{TIR}, the [CII] surface brightness can be described as: S_{[CII]}=1.25 S_{TIR}^{0.69} [10^{-3} erg s^{-1} cm^{-2} sr^{-1}]. The [CII] emission is well-correlation with the 8 micrometer emission, suggesting that the polycyclic aromatic hydrocarbons play a dominant role in the photoelectric heating process.

astro-ph↗

arXiv2004

Metallicity and the spectral energy distribution and spectral types of dwarf O-stars

We present a systematic study of the effect of metallicity on the stellar spectral energy distribution (SED) of O main sequence (dwarf) stars, focussing on the hydrogen and helium ionizing continua, and on the optical and near-IR lines used for spectral classification. The spectra are based on non-LTE line blanketed atmosphere models with stellar winds calculated using the CMFGEN code of Hillier & Miller (1998). We draw the following conclusions. First, we find that the total number of Lyman photons emitted is almost independent of line blanketing effects and metallicity for a given effective temperature. This is because the flux that is blocked by the forest of metal lines at wavelengths shorter than 600 Angstrom is redistributed mainly within the Lyman continuum. Second, the spectral type, as defined by the ratio of the equivalent widths of HeI 4471 Angstrom and HeII 4542 Angstrom, is shown to depend noticeably on the microturbulent velocity in the atmosphere, on metallicity and, within the luminosity class of dwarfs, on gravity. Third, we confirm the decrease in the effective temperature for a given spectral type due to the inclusion of line blanketing recently found by e.g. Martins et al. (2002). Finally, we find that the SED below ~450 Angstrom is highly dependent on metallicity. This is reflected in the behaviour of nebular fine-structure line ratios such as [NeIII]/[NeII] 15.5/12.8 and [ArIII]/[ArII] 9.0/7.0 micron. This dependence complicates the use of these nebular ratios as diagnostic tools for the effective temperature determination of the ionizing stars in HII regions and for age dating of starburst regions in galaxies.

astro-ph↗