arXiv subjects

R. Meijerink

Publications and source records attributed to R. Meijerink.

58 records · Page 4Linked to original sources

arXiv2007

A PDR-Code Comparison Study

We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution. A number of benchmark models have been created, covering low and high gas densities and far ultraviolet intensities. The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-processes, and a second set determining the temperature selfconsistently. We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly.

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arXiv2006

Diagnostics of irradiated dense gas in galaxy nuclei. II. A grid of XDR and PDR models

\abridged The nuclei of active galaxies harbor massive young stars, an accreting central black hole, or both. In order to determine the physical conditions that pertain to molecular gas close to the sources of radiation, numerical models are constructed. These models determine the thermal and chemical balance of molecular gas that is exposed to X-rays and far-ultraviolet radiation, as a function of depth. We present a grid of XDR and PDR models that span ranges in density, irradiation and column density. We find that the fine-structure line ratios of e.g. [SiII] 35mum/[CII] 158 mum are larger in XDRs than in PDRs, for a given density, column and irradiation strength. We find that the line ratios HCN/HCO+ and HNC/HCN, as well as the column density ratio CN/HCN, discriminate between PDRs and XDRs. The HCN/HCO+ 1-0 ratio is <1 (>1) for XDRs (PDRs) if the density exceeds 10^5 cm^-3 and if the column density is >10^23 cm^-2. For columns <10^ 22.5 cm^-2 the XDR HCN/HCO+ 1-0 ratio becomes larger than one, although the individual HCN 1-0 and HCO+ 1-0 line intensities are weaker. For modest densities, n=10^4-10^5 cm^-3, and strong radiation fields (>100 erg s^-1 cm^-2), HCN/HCO+ ratios can become larger in XDRs than PDRs as well. The HCN/CO 1-0 ratio is typically smaller in XDRs, and the HCN emission in XDRs is boosted with respect to CO only for columns >10^{23} cm^{-2} and densities <10^4 cm^-3. CO is typically warmer in XDRs than in PDRs, for the same total energy input. This leads to higher CO J=N+1-N/CO 1-0, N>=1, line ratios in XDRs. Lines with N>=10, like CO(16-15) and CO(10-9) observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. Column density ratios indicate that CH, CH+, NO, HOC+ and HCO are good PDR/XDR discriminators.

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arXiv2006

Irradiated ISM: Discriminating between Cosmic Rays and X-rays

The ISM of active galaxy centers is exposed to a combination of cosmic ray, FUV and X-ray radiation. We apply PDR models to this ISM with both `normal' and highly elevated (5\times 10^{-15}s^-1) cosmic-ray rates and compare the results to those obtained for XDRs. Our existing PDR-XDR code is used to construct models over a 10^3-10^5 cm^-3 density range and for 0.16-160 erg s^-1 cm^-2 impingent fluxes. We obtain larger high J (J>10) CO ratios in PDRs when we use the highly elevated cosmic ray rate, but these are always exceeded by the corresponding XDR ratios. The [CI] 609 mum/13CO(2-1) line ratio is boosted by a factor of a few in PDRs with n~10^3 cm^-3 exposed to a high cosmic ray rate. At higher densities ratios become identical irrespective of cosmic ray flux, while XDRs always show elevated [CI] emission per CO column. The HCN/CO and HCN/HCO+ line ratios, combined with high J CO emission lines, are good diagnostics to distinguish between PDRs under either low or high cosmic ray irradiation conditions, and XDRs. Hence, the HIFI instrument on Herschel, which can detect these CO lines, will be crucial in the study of active galaxies.

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arXiv2004

A submillimeter exponential disk in M~51: evidence for an extended cold dust disk

A 850 micron map of the interacting spiral galaxy M51 shows well-defined spiral arms, closely resembling the structures seen in CO and HI emission. However, most of the 850 micron emission originates in an underlying exponential disk, a component that has not been observed before in a face-on galaxy at these wavelengths. The scale-length of this disk is 5.45 kpc, which is somewhat larger than the scale-length of the stellar disk, but somewhat smaller than that of atomic hydrogen. Its profile can not be explained solely by a radial disk temperature gradient but requires the underlying dust to have an exponential distribution as well. This reinforces the view that the submm emission from spiral galaxy disks traces total hydrogen column density, i.e.the sum of H2 and HI. A canonical gas-to-dust ratio of 100+/-26 is obtained for kappa(850)=1.2 g**-1 cm**2, where kappa(850) is the dust opacity at 850 micron.

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