CO+ in M 82: A Consequence of Irradiation by X-rays

Based on its strong CO+ emission it is argued that the M 82 star-burst galaxy is exposed to a combination of FUV and X-ray radiation. The latter is likely to be the result of the star-burst superwind, which leads to diffuse thermal emission at ~0.7 keV, and a compact hard, 2-10 keV, source (but not an AGN). Although a photon-dominated region (FUV) component is clearly present in the nucleus of M 82, and capable of forming CO+, only X-ray irradiated gas of density 10^3-10^5 cm^-3 can reproduce the large, ~(1-4)x10^13 cm^-2, columns of CO+ that are observed toward the proto-typical star-burst M 82. The total X-ray luminosity produced by M 82 is weak, ~10^41 erg s^-1, but this is sufficient to drive the formation of CO+.Comment: added discussion on more recent X-ray observation

The irradiated ISM of ULIRGs

The nuclei of ULIRGs harbor massive young stars, an accreting central black hole, or both. Results are presented for molecular gas that is exposed to X-rays (1-100 keV, XDRs) and far-ultraviolet radiation (6-13.6 eV, PDRs). Attention is paid to species like HCO+, HCN, HNC, OH, H2O and CO. Line ratios of HCN/HCO+ and HNC/HCN discriminate between PDRs and XDRs. Very high J (>10) CO lines, observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. In XDRs, it is easy to produce large abundances of warm (T>100 K) H2O and OH. In PDRs, only OH is produced similarly well.Comment: 5 pages, 6 figures, to appear in: IAU Symposium 242 Astrophysical Masers and their Environment

Diagnostics of the molecular component of PDRs with mechanical heating. II: line intensities and ratios

CO observations in active galactic nuclei and star-bursts reveal high kinetic temperatures. Those environments are thought to be very turbulent due to dynamic phenomena such as outflows and high supernova rates. We investigate the effect of mechanical heating (MH) on atomic fine-structure and molecular lines, and their ratios. We use those ratios as a diagnostic to constrain the amount of MH in an object and also study its significance on estimating the H2 mass. Equilibrium PDRs models were used to compute the thermal and chemical balance for the clouds. The equilibria were solved for numerically using the optimized version of the Leiden PDR-XDR code. Large velocity gradient calculations were done as post-processing on the output of the PDR models using RADEX. High-J CO line ratios are very sensitive to MH. Emission becomes at least one order of magnitude brighter in clouds with n~10^5~cm^-3 and a star formation rate of 1 Solar Mass per year (corresponding to a MH rate of 2 * 10^-19 erg cm^-3 s^-1). Emission of low-J CO lines is not as sensitive to MH, but they do become brighter in response to MH. Generally, for all of the lines we considered, MH increases excitation temperatures and decreases the optical depth at the line centre. Hence line ratios are also affected, strongly in some cases. Ratios involving HCN are a good diagnostic for MH, such as HCN(1-0)/CO(1-0) and HCN(1-0)/HCO^+(1-0). Both ratios increase by a factor 3 or more for a MH equivalent to > 5 percent of the surface heating, as opposed to pure PDRs. The first major conclusion is that low-J to high-J intensity ratios will yield a good estimate of the MH rate (as opposed to only low-J ratios). The second one is that the MH rate should be taken into account when determining A_V or equivalently N_H, and consequently the cloud mass. Ignoring MH will also lead to large errors in density and radiation field estimates.Comment: 38 pages, to appear in A&

FUV and X-ray irradiated protoplanetary disks: a grid of models I. The disk structure

Context. Planets are thought to eventually form from the mostly gaseous (~99% of the mass) disks around young stars. The density structure and chemical composition of protoplanetary disks are affected by the incident radiation field at optical, FUV, and X-ray wavelengths, as well as by the dust properties. Aims. The effect of FUV and X-rays on the disk structure and the gas chemical composition are investigated. This work forms the basis of a second paper, which discusses the impact on diagnostic lines of, e.g., C+, O, H2O, and Ne+ observed with facilities such as Spitzer and Herschel. Methods. A grid of 240 models is computed in which the X-ray and FUV luminosity, minimum grain size, dust size distribution, and surface density distribution are varied in a systematic way. The hydrostatic structure and the thermo-chemical structure are calculated using ProDiMo. Results. The abundance structure of neutral oxygen is stable to changes in the X-ray and FUV luminosity, and the emission lines will thus be useful tracers of the disk mass and temperature. The C+ abundance distribution is sensitive to both X-rays and FUV. The radial column density profile shows two peaks, one at the inner rim and a second one at a radius r=5-10 AU. Ne+ and other heavy elements have a very strong response to X-rays, and the column density in the inner disk increases by two orders of magnitude from the lowest (LX = 1e29 erg/s) to the highest considered X-ray flux (LX = 1e32 erg/s). FUV confines the Ne+ ionized region to areas closer to the star at low X-ray luminosities (LX = 1e29 erg/s). H2O abundances are enhanced by X-rays due to higher temperatures in the inner disk and higher ionization fractions in the outer disk. The line fluxes and profiles are affected by the effects on these species, thus providing diagnostic value in the study of FUV and X-ray irradiated disks around T Tauri stars. (abridged)Comment: 47 pages, accepted by Astronomy and Astrophysics, a high resolution version of the paper is located at http://www.astro.rug.nl/~meijerink/disk_paperI_xrays.pd

Diagnostics of the Molecular Component of PDRs with Mechanical Heating

Context. Multitransition CO observations of galaxy centers have revealed that significant fractions of the dense circumnuclear gas have high kinetic temperatures, which are hard to explain by pure photon excitation, but may be caused by dissipation of turbulent energy. Aims. We aim to determine to what extent mechanical heating should be taken into account while modelling PDRs. To this end, the effect of dissipated turbulence on the thermal and chemical properties of PDRs is explored. Methods. Clouds are modelled as 1D semi-infinite slabs whose thermal and chemical equilibrium is solved for using the Leiden PDR-XDR code. Results. In a steady-state treatment, mechanical heating seems to play an important role in determining the kinetic temperature of the gas in molecular clouds. Particularly in high-energy environments such as starburst galaxies and galaxy centers, model gas temperatures are underestimated by at least a factor of two if mechanical heating is ignored. The models also show that CO, HCN and H2 O column densities increase as a function of mechanical heating. The HNC/HCN integrated column density ratio shows a decrease by a factor of at least two in high density regions with n \sim 105 cm-3, whereas that of HCN/HCO+ shows a strong dependence on mechanical heating for this same density range, with boosts of up to three orders of magnitude. Conclusions. The effects of mechanical heating cannot be ignored in studies of the molecular gas excitation whenever the ratio of the star formation rate to the gas density is close to, or exceeds, 7 \times 10-6 M yr-1 cm4.5 . If mechanical heating is not included, predicted column densities are underestimated, sometimes even by a few orders of magnitude. As a lower bound to its importance, we determined that it has non-negligible effects already when mechanical heating is as little as 1% of the UV heating in a PDR.Comment: 26 pages, 14 figures in the text and 13 figures as supplementary material. Accepted for publication in A&

Star Formation in Extreme Environments: The Effects of Cosmic Rays and Mechanical Heating

Context: Molecular data of extreme environments, such as Arp 220, but also NGC 253, show evidence for extremely high cosmic ray (CR) rates (10^3-10^4 * Milky Way) and mechanical heating from supernova driven turbulence. Aims: The consequences of high CR rates and mechanical heating on the chemistry in clouds are explored. Methods: PDR model predictions are made for low, n=10^3, and high, n=10^5.5 cm^-3, density clouds using well-tested chemistry and radiation transfer codes. Column densities of relevant species are discussed, and special attention is given to water related species. Fluxes are shown for fine-structure lines of O, C+, C, and N+, and molecular lines of CO, HCN, HNC, and HCO+. A comparison is made to an X-ray dominated region model. Results: Fine-structure lines of [CII], [CI], and [OI] are remarkably similar for different mechanical heating and CR rates, when already exposed to large amounts of UV. HCN and H2O abundances are boosted for very high mechanical heating rates, while ionized species are relatively unaffected. OH+ and H2O+ are enhanced for very high CR rates zeta > 5 * 10^-14 s^-1. A combination of OH+, OH, H2O+, H2O, and H3O+ trace the CR rates, and are able to distinguish between enhanced cosmic rays and X-rays.Comment: 13 pages, 8 figures, A&A accepte

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.Comment: accepted by Astrophysical Journal Letter

Molecular gas heating in Arp 299

Understanding the heating and cooling mechanisms in nearby (Ultra) luminous infrared galaxies can give us insight into the driving mechanisms in their more distant counterparts. Molecular emission lines play a crucial role in cooling excited gas, and recently, with Herschel Space Observatory we have been able to observe the rich molecular spectrum. CO is the most abundant and one of the brightest molecules in the Herschel wavelength range. CO transitions are observed with Herschel, and together, these lines trace the excitation of CO. We study Arp 299, a colliding galaxy group, with one component harboring an AGN and two more undergoing intense star formation. For Arp 299 A, we present PACS spectrometer observations of high-J CO lines up to J=20-19 and JCMT observations of $^{13}$CO and HCN to discern between UV heating and alternative heating mechanisms. There is an immediately noticeable difference in the spectra of Arp 299 A and Arp 299 B+C, with source A having brighter high-J CO transitions. This is reflected in their respective spectral energy line distributions. We find that photon-dominated regions (PDRs) are unlikely to heat all the gas since a very extreme PDR is necessary to fit the high-J CO lines. In addition, this extreme PDR does not fit the HCN observations, and the dust spectral energy distribution shows that there is not enough hot dust to match the amount expected from such an extreme PDR. Therefore, we determine that the high-J CO and HCN transitions are heated by an additional mechanism, namely cosmic ray heating, mechanical heating, or X-ray heating. We find that mechanical heating, in combination with UV heating, is the only mechanism that fits all molecular transitions. We also constrain the molecular gas mass of Arp 299 A to 3e9 Msun and find that we need 4% of the total heating to be mechanical heating, with the rest UV heating

Radiative and mechanical feedback into the molecular gas of NGC 253

Starburst galaxies are undergoing intense periods of star formation. Understanding the heating and cooling mechanisms in these galaxies can give us insight to the driving mechanisms that fuel the starburst. Molecular emission lines play a crucial role in the cooling of the excited gas. With SPIRE on the Herschel Space Observatory we have observed the rich molecular spectrum towards the central region of NGC 253. CO transitions from J=4-3 to 13-12 are observed and together with low-J line fluxes from ground based observations, these lines trace the excitation of CO. By studying the CO excitation ladder and comparing the intensities to models, we investigate whether the gas is excited by UV radiation, X-rays, cosmic rays, or turbulent heating. Comparing the $^{12}$CO and $^{13}$CO observations to large velocity gradient models and PDR models we find three main ISM phases. We estimate the density, temperature,and masses of these ISM phases. By adding $^{13}$CO, HCN, and HNC line intensities, we are able to constrain these degeneracies and determine the heating sources. The first ISM phase responsible for the low-J CO lines is excited by PDRs, but the second and third phases, responsible for the mid to high-J CO transitions, require an additional heating source. We find three possible combinations of models that can reproduce our observed molecular emission. Although we cannot determine which of these are preferable, we can conclude that mechanical heating is necessary to reproduce the observed molecular emission and cosmic ray heating is a negligible heating source. We then estimate the mass of each ISM phase; $6\times 10^7$ M$_\odot$ for phase 1 (low-J CO lines), $3\times 10^7$ M$_\odot$ for phase 2 (mid-J CO lines), and $9\times 10^6$ M$_\odot$ for phase 3 (high-J CO lines) for a total system mass of $1\times10^{8}$ M$_\odot$

Mechanical feedback in the molecular ISM of luminous IR galaxies

Aims: Molecular emission lines originating in the nuclei of luminous infra-red galaxies are used to determine the physical properties of the nuclear ISM in these systems. Methods: A large observational database of molecular emission lines is compared with model predictions that include heating by UV and X-ray radiation, mechanical heating, and the effects of cosmic rays. Results: The observed line ratios and model predictions imply a separation of the observedsystems into three groups: XDRs, UV-dominated high-density (n>=10^5 cm-3) PDRs, and lower-density (n=10^4.5 cm-3) PDRs that are dominated by mechanical feedback. Conclusions: The division of the two types of PDRs follows naturally from the evolution of the star formation cycle of these sources, which evolves from deeply embedded young stars, resulting in high-density (n>=10^5 cm-3) PDRs, to a stage where the gas density has decreased (n=10^4.5 cm-3) and mechanical feedback from supernova shocks dominates the heating budget.Comment: 4 pages, 3 figures, published as Letter to the Editor in A&A (see http://www.aanda.org/articles/aa/abs/2008/34/aa10327-08/aa10327-08.html