We report a Herschel detection of high-J rotational CO lines from a dense knot in the supernova remnant Cas A. Based on a combined analysis of these rotational lines and previously observed ro... Show moreWe report a Herschel detection of high-J rotational CO lines from a dense knot in the supernova remnant Cas A. Based on a combined analysis of these rotational lines and previously observed ro-vibrational CO lines, we find the gas to be warm (two components at ~{}400 and 2000 K) and dense (10$^{6-7}$ cm$^{-3}$), with a CO column density of ~{}5 { imes} 10$^{17}$ cm$^{-2}$. This, along with the broad line widths (~{}400 km s$^{-1}$), suggests that the CO emission originates in the post-shock region of the reverse shock. As the passage of the reverse shock dissociates any existing molecules, the CO has most likely reformed in the past several years in the post-shock gas. The CO cooling time is similar to the CO formation time, therefore we discuss possible heating sources (UV photons from the shock front, X-rays, electron conduction) that may maintain the high column density of warm CO. Show less
Kama, M.; López-Sepulcre, A.; Dominik, C.; Ceccarelli, C.; Fuente, A.; Caux, E.; ... ; Alonso-Albi, T. 2013
Context. Broadband spectral surveys of protostars offer a rich view of the physical, chemical and dynamical structure and evolution of star-forming regions. The Herschel Space Observatory opened... Show moreContext. Broadband spectral surveys of protostars offer a rich view of the physical, chemical and dynamical structure and evolution of star-forming regions. The Herschel Space Observatory opened up the terahertz regime to such surveys, giving access to the fundamental transitions of many hydrides and to the high-energy transitions of many other species. Aims: A comparative analysis of the chemical inventories and physical processes and properties of protostars of various masses and evolutionary states is the goal of the Herschel CHEmical Surveys of Star forming regions (CHESS) key program. This paper focusses on the intermediate-mass protostar, OMC-2 FIR 4. Methods: We obtained a spectrum of OMC-2 FIR 4 in the 480 to 1902 GHz range with the HIFI spectrometer onboard Herschel and carried out the reduction, line identification, and a broad analysis of the line profile components, excitation, and cooling. Results: We detect 719 spectral lines from 40 species and isotopologs. The line flux is dominated by CO, H$_{2}$O, and CH$_{3}$OH. The line profiles are complex and vary with species and upper level energy, but clearly contain signatures from quiescent gas, a broad component likely due to an outflow, and a foreground cloud. Conclusions: We find abundant evidence for warm, dense gas, as well as for an outflow in the field of view. Line flux represents 2% of the 7 L$_{⊙}$ luminosity detected with HIFI in the 480 to 1250 GHz range. Of the total line flux, 60% is from CO, 13% from H$_{2}$O and 9% from CH$_{3}$OH. A comparison with similar HIFI spectra of other sources is set to provide much new insight into star formation regions, a case in point being a difference of two orders of magnitude in the relative contribution of sulphur oxides to the line cooling of Orion KL and OMC-2 FIR 4. Appendix A is available in electronic form at http://www.aanda.orgShow less
Context. Over the last few years, the chemistry of molecules other than CO in the planet-forming zones of disks is starting to be explored with Spitzer and high-resolution ground-based data.... Show moreContext. Over the last few years, the chemistry of molecules other than CO in the planet-forming zones of disks is starting to be explored with Spitzer and high-resolution ground-based data. However, these studies have focused only on a few simple molecules. Aims: The aim of this study is to put observational constraints on the presence of more complex organic and sulfur-bearing molecules predicted to be abundant in chemical models of disks and to simulate high resolution spectra in view of future missions. Methods: High signal-to-noise ratio (S/N) Spitzer spectra of the near edge-on disks IRS 46 and GV Tau are used to search for mid-infrared absorption bands of various molecules. These disks are good laboratories because absorption studies do not suffer from low line/continuum ratios that plague emission data. Simple local thermodynamic equilibrium (LTE) slab models are used to infer column densities (or upper limits) and excitation temperatures. Results: Mid-infrared bands of HCN, C$_{2}$H$_{2}$ and CO$_{2}$ are clearly detected toward both sources. The HCN and C$_{2}$H$_{2}$ absorption arises in warm gas with excitation temperatures of 400-700 K, whereas the CO$_{2}$ absorption originates in cooler gas of ~{}250 K. Column densities and their ratios are comparable for the two sources. No other absorption features are detected at the 3{$σ$} level. Column density limits of the majority of molecules predicted to be abundant in the inner disk - C$_{2}$H$_{4}$, C$_{2}$H$_{6}$, C$_{6}$H$_{6}$, C$_{3}$H$_{4}$, C$_{4}$H$_{2}$, CH$_{3}$, HNC, HC$_{3}$N, CH$_{3}$CN, NH$_{3}$ and SO$_{2}$ - are determined and compared with disk models. Conclusions: The inferred abundance ratios and limits with respect to C$_{2}$H$_{2}$ and HCN are roughly consistent with models of the chemistry in high temperature gas. Models of UV irradiated disk surfaces generally agree better with the data than pure X-ray models. The limit on NH$_{3}$/HCN implies that evaporation of NH$_{3}$-containing ices is only a minor contributor. The inferred abundances and their limits also compare well with those found in comets, suggesting that part of the cometary material may derive from warm inner disk gas. The high resolution simulations show that future instruments on the James Webb Space Telescope (JWST), the Extremely Large Telescopes (ELTs), the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) can probe up to an order of magnitude lower abundance ratios and put important new constraints on the models, especially if pushed to high S/Ns. Appendices are available in electronic form at http://www.aanda.orgShow less
The spatial variations in polycyclic aromatic hydrocarbon (PAH) band intensities are normally attributed to the physical conditions of the emitting PAHs, however in recent years it has been... Show moreThe spatial variations in polycyclic aromatic hydrocarbon (PAH) band intensities are normally attributed to the physical conditions of the emitting PAHs, however in recent years it has been suggested that such variations are caused mainly by extinction. To resolve this question, we have obtained near-infrared (NIR), mid-infrared (MIR), and radio observations of the compact H II region IRAS 12063-6259. We use these data to construct multiple independent extinction maps and also to measure the main PAH features (6.2, 7.7, 8.6, and 11.2 {$μ$}m) in the MIR. Three extinction maps are derived: the first using the NIR hydrogen lines and case B recombination theory; the second combining the NIR data with radio data; and the third making use of the Spitzer/IRS MIR observations to measure the 9.8 {$μ$}m silicate absorption feature using the Spoon method and PAHFIT (as the depth of this feature can be related to overall extinction). The silicate absorption over the bright, southern component of IRAS 12063-6259 is almost absent while the other methods find significant extinction. While such breakdowns of the relationship between the NIR extinction and the 9.8 {$μ$}m absorption have been observed in molecular clouds, they have never been observed for H II regions. We then compare the PAH intensity variations in the Spitzer/IRS data after dereddening to those found in the original data. It was found that in most cases, the PAH band intensity variations persist even after dereddening, implying that extinction is not the main cause of the PAH band intensity variations. Show less
Chiar, J.; Tielens, A.G.G.M.; Adamson, A.; Ricca, A. 2013
Many materials have been considered for the carrier of the hydrocarbon absorption bands observed in the diffuse interstellar medium (ISM). In order to refine the model for ISM hydrocarbon grains,... Show moreMany materials have been considered for the carrier of the hydrocarbon absorption bands observed in the diffuse interstellar medium (ISM). In order to refine the model for ISM hydrocarbon grains, we analyze the observed aromatic (3.28, 6.2 {$μ$}m) and aliphatic (3.4 {$μ$}m) hydrocarbon absorption features in the diffuse ISM along the line of sight toward the Galactic center Quintuplet Cluster. Observationally, sp $^{2}$ bonds can be measured in astronomical spectra using the 6.2 {$μ$}m CC aromatic stretch feature, whereas the 3.4 {$μ$}m aliphatic feature can be used to quantify the fraction of sp $^{3}$ bonds. The fractional abundance of these components allows us to place the Galactic diffuse ISM hydrocarbons on a ternary phase diagram. We conclude that the Galactic hydrocarbon dust has, on average, a low H/C ratio and sp $^{3}$ content and is highly aromatic. We have placed the results of our analysis within the context of the evolution of carbon dust in the ISM. We argue that interstellar carbon dust consists of a large core of aromatic carbon surrounded by a thin mantle of hydrogenated amorphous carbon (a-C:H), a structure that is a natural consequence of the processing of stardust grains in the ISM. Show less
Optical and infrared emission lines from H II regions are an important diagnostic used to study galaxies, but interpretation of these lines requires significant modeling of both the internal... Show moreOptical and infrared emission lines from H II regions are an important diagnostic used to study galaxies, but interpretation of these lines requires significant modeling of both the internal structure and dynamical evolution of the emitting regions. Most of the models in common use today assume that H II region dynamics are dominated by the expansion of stellar wind bubbles, and have neglected the contribution of radiation pressure to the dynamics, and in some cases also to the internal structure. However, recent observations of nearby galaxies suggest that neither assumption is justified, motivating us to revisit the question of how H II region line emission depends on the physics of winds and radiation pressure. In a companion paper we construct models of single H II regions including and excluding radiation pressure and winds, and in this paper we describe a population synthesis code that uses these models to simulate galactic collections of H II regions with varying physical parameters. We show that the choice of physical parameters has significant effects on galactic emission line ratios, and that in some cases the line ratios can exceed previously claimed theoretical limits. Our results suggest that the recently reported offset in line ratio values between high-redshift star-forming galaxies and those in the local universe may be partially explained by the presence of large numbers of radiation-pressure-dominated H II regions within them. Show less
Infrared photometry and spectroscopy (1-25 {$μ$}m) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of grains and the composition of ices... Show moreInfrared photometry and spectroscopy (1-25 {$μ$}m) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of grains and the composition of ices before they are incorporated into circumstellar envelopes and disks. H$_{2}$O ices form at extinctions of A $_K$ = 0.25 {plusmn} 0.07 mag (A $_V$ = 2.1 {plusmn} 0.6). Such a low ice formation threshold is consistent with the absence of nearby hot stars. Overall, the Lupus clouds are in an early chemical phase. The abundance of H$_{2}$O ice (2.3 {plusmn} 0.1 { imes} 10$^{–5}$ relative to N $_H$) is typical for quiescent regions, but lower by a factor of three to four compared to dense envelopes of young stellar objects. The low solid CH$_{3}$OH abundance ({lt}3%-8% relative to H$_{2}$O) indicates a low gas phase H/CO ratio, which is consistent with the observed incomplete CO freeze out. Furthermore it is found that the grains in Lupus experienced growth by coagulation. The mid-infrared ({gt}5 {$μ$}m) continuum extinction relative to A $_K$ increases as a function of A $_K$. Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to R $_V$ ~{} 3.5 (A $_K$ = 0.71) and R $_V$ ~{} 5.0 (A $_K$ = 1.47). For lines of sight with A $_K$ {gt} 1.0 mag, the {$τ$}$_{9.7}$/A $_K$ ratio is a factor of two lower compared to the diffuse medium. Below 1.0 mag, values scatter between the dense and diffuse medium ratios. The absence of a gradual transition between diffuse and dense medium-type dust indicates that local conditions matter in the process that sets the {$τ$}$_{9.7}$/A $_K$ ratio. This process is likely related to grain growth by coagulation, as traced by the A $_{7.4}$/A $_K$ continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program IDs 083.C-0942 and 085.C-0620. Show less
Maaskant, K.M.; Honda, M.; Waters, L.; Tielens, A.G.G.M.; Dominik, C.; Min, M.; ... ; Ancker, M. van den 2013
Context. The evolution of young massive protoplanetary disks toward planetary systems is expected to correspond to structural changes in observational appearance, which includes the formation of... Show moreContext. The evolution of young massive protoplanetary disks toward planetary systems is expected to correspond to structural changes in observational appearance, which includes the formation of gaps and the depletion of dust and gas. Aims: A special group of disks around Herbig Ae/Be stars do not show prominent silicate emission features, although they still bear signs of flaring disks, the presence of gas, and small grains. We focus our attention on four key Herbig Ae/Be stars to understand the structural properties responsible for the absence of silicate feature emission. Methods: We investigate Q- and N-band images taken with Subaru/COMICS, Gemini South/T-ReCS, and VLT/VISIR. We perform radiative transfer modeling to examine the radial distribution of dust and polycyclic aromatic hydrocarbons (PAHs). Our solutions require a separation of inner- and outer- disks by a large gap. From this, we characterize the radial density structure of dust and PAHs in the disk. Results: The inner edge of the outer disk has a high surface brightness and a typical temperature between ~{}100-150 K and therefore, dominates the emission in the Q-band. All four disks are characterized by large gaps. We derive radii of the inner edge of the outer disk of 34$_{-4}$$^{+4}$, 23$_{-5}$$^{+3}$, 30$_{-3}$$^{+5}$ and 63$_{-4}$$^{+4}$ AU for HD 97048, HD 169142, HD 135344 B, and Oph IRS 48, respectively. For HD 97048 this is the first detection of a disk gap. The large gaps deplete the entire population of silicate particles with temperatures suitable for prominent mid-infrared feature emission, while small carbonaceous grains and PAHs can still show prominent emission at mid-infrared wavelengths. The continuum emission in the N-band is not due to emission in the wings of PAHs. This continuum emission can be due to very small grains or to thermal emission from the inner disk. We find that PAH emission is not always dominated by PAHs on the surface of the outer disk. Conclusions: The absence of silicate emission features is due to the presence of large gaps in the critical temperature regime. Many, if not all Herbig disks with spectral energy distribution classification ''group I'', are disks with large gaps and can be characterized as (pre-) transitional. An evolutionary path from the observed group I to the observed group II sources seems no longer likely. Instead, both might derive from a common ancestor. Tables A.1-A.4 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/555/A64Show less
Infrared photometry and spectroscopy (1-25 {$μ$}m) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of grains and the composition of ices... Show moreInfrared photometry and spectroscopy (1-25 {$μ$}m) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of grains and the composition of ices before they are incorporated into circumstellar envelopes and disks. H$_{2}$O ices form at extinctions of A $_K$ = 0.25 {plusmn} 0.07 mag (A $_V$ = 2.1 {plusmn} 0.6). Such a low ice formation threshold is consistent with the absence of nearby hot stars. Overall, the Lupus clouds are in an early chemical phase. The abundance of H$_{2}$O ice (2.3 {plusmn} 0.1 { imes} 10$^{–5}$ relative to N $_H$) is typical for quiescent regions, but lower by a factor of three to four compared to dense envelopes of young stellar objects. The low solid CH$_{3}$OH abundance ({lt}3%-8% relative to H$_{2}$O) indicates a low gas phase H/CO ratio, which is consistent with the observed incomplete CO freeze out. Furthermore it is found that the grains in Lupus experienced growth by coagulation. The mid-infrared ({gt}5 {$μ$}m) continuum extinction relative to A $_K$ increases as a function of A $_K$. Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to R $_V$ ~{} 3.5 (A $_K$ = 0.71) and R $_V$ ~{} 5.0 (A $_K$ = 1.47). For lines of sight with A $_K$ {gt} 1.0 mag, the {$τ$}$_{9.7}$/A $_K$ ratio is a factor of two lower compared to the diffuse medium. Below 1.0 mag, values scatter between the dense and diffuse medium ratios. The absence of a gradual transition between diffuse and dense medium-type dust indicates that local conditions matter in the process that sets the {$τ$}$_{9.7}$/A $_K$ ratio. This process is likely related to grain growth by coagulation, as traced by the A $_{7.4}$/A $_K$ continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program IDs 083.C-0942 and 085.C-0620. Show less
Organosilicon species such as silicon carbide and silicon dicarbide are considered as key molecular building blocks in the chemical evolution of the interstellar medium and are associated with the... Show moreOrganosilicon species such as silicon carbide and silicon dicarbide are considered as key molecular building blocks in the chemical evolution of the interstellar medium and are associated with the formation of silicon-carbide dust grains in the outflow of circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars. However, the formation mechanisms of even the simplest silicon-bearing organic molecules have remained elusive for decades. Here, we demonstrate in crossed molecular beam experiments combined with ab initio calculations that the silacyclopropenylidene molecule (c-SiC$_{2}$H$_{2}$) can be synthesized in the gas phase under single-collision conditions via the reaction of the silylidyne radical (SiH) with acetylene (C$_{2}$H$_{2}$). This system denotes the simplest representative of a previously overlooked reaction class, in which the formation of an organosilicon molecule can be initiated via barrierless and exoergic reactions of silylidyne radicals with hydrocarbon molecules in circumstellar envelopes of evolved carbon stars such as IRC+10216. Since organosilicon molecules like silacyclopropenylidene can be eventually photolyzed to carbon-silicon clusters such as silicon dicarbide (c-SiC$_{2}$), silacyclopropenylidene might even represent the missing link between simple molecular precursors and silicon-carbide-rich interstellar grains. Show less
The emission line ratios [O III] {$λ$}5007/H{$β$} and [N II] {$λ$}6584/H{$α$} have been adopted as an empirical way to distinguish between the fundamentally different mechanisms of ionization in... Show moreThe emission line ratios [O III] {$λ$}5007/H{$β$} and [N II] {$λ$}6584/H{$α$} have been adopted as an empirical way to distinguish between the fundamentally different mechanisms of ionization in emission-line galaxies. However, detailed interpretation of these diagnostics requires calculations of the internal structure of the emitting H II regions, and these calculations depend on the assumptions one makes about the relative importance of radiation pressure and stellar winds. In this paper, we construct a grid of quasi-static H II region models to explore how choices about these parameters alter H II regions' emission line ratios. We find that when radiation pressure is included in our models, H II regions reach a saturation point beyond which further increase in the luminosity of the driving stars does not produce any further increase in effective ionization parameter, and thus does not yield any further alteration in an H II region's line ratio. We also show that if stellar winds are assumed to be strong, the maximum possible ionization parameter is quite low. As a result of this effect, it is inconsistent to simultaneously assume that H II regions are wind-blown bubbles and that they have high ionization parameters; some popular H II region models suffer from this inconsistency. Our work in this paper provides a foundation for a companion paper in which we embed the model grids we compute here within a population synthesis code that enables us to compute the integrated line emission from galactic populations of H II regions. Show less
