A bimodal dust grain distribution in the IC 434 HII region

Recent studies of dust in the interstellar medium have challenged the capabilities and validity of current dust models, indicating that the properties of dust evolve as it transits between different phases of the interstellar medium. We conduct a multi-wavelength study of the dust emission from the ionized gas of the IC 434 emission nebula, and combine this with modeling, from large scales that provide insight into the history of the IC 434/L1630 region, to small scales that allow us to infer quantitative properties of the dust content inside the H II region. The dust enters the H II region through momentum transfer with a champagne flow of ionized gas, set up by a chance encounter between the L1630 molecular cloud and the star cluster of $\sigma$ Ori. We observe two clearly separated dust populations inside the ionized gas, that show different observational properties, as well as contrasting optical properties. Population A is colder ($\sim$ 25 K) than predicted by widely-used dust models, its temperature is insensitive to an increase of the impinging radiation field, is momentum-coupled to the gas, and efficiently absorbs radiation pressure to form a dust wave at 1.0 pc ahead of $\sigma$ Ori AB. Population B is characterized by a constant [20/30] flux ratio throughout the HII region, heats up to $\sim$ 75 K close to the star, and is less efficient in absorbing radiation pressure, forming a dust wave at 0.1 pc from the star. We conclude that the dust inside IC 434 is bimodal. The characteristics of population A are remarkable and can not be explained by current dust models. Population B are grains that match the classical description of spherical, compact dust. Our results confirm recent work that stress the importance of variations in the dust properties between different regions of the interstellar medium.Comment: 18 pages, 10 figures, proposed for acceptance in A&

High-Resolution 4.7 Micron Keck/NIRSPEC Spectra of Protostars. II. Detection of the ^(13)CO Isotope in Icy Grain Mantles

The high-resolution (R = 25,000) infrared M-band spectrum of the massive protostar NGC 7538 IRS 9 shows a narrow absorption feature at 4.779 μm (2092.3 cm^(-1)) that we attribute to the vibrational stretching mode of the ^(13)CO isotope in pure CO icy grain mantles. This is the first detection of ^(13)CO in icy grain mantles in the interstellar medium. The ^(13)CO band is a factor of 2.3 narrower than the apolar component of the ^(12)CO band. With this in mind, we discuss the mechanisms that broaden solid-state absorption bands. It is shown that ellipsoidally shaped pure CO grains fit the bands of both isotopes at the same time. Slightly worse but still reasonable fits are also obtained by CO embedded in N_2-rich ices and thermally processed O_2-rich ices. In addition, we report new insights into the nature and evolution of interstellar CO ices by comparing the very high resolution multicomponent solid ^(12)CO spectrum of NGC 7538 IRS 9 with that of the previously studied low-mass source L1489 IRS. The narrow absorption of apolar CO ices is present in both spectra but much stronger in NGC 7538 IRS 9. It is superposed on a smooth broad absorption feature well fitted by a combination of CO_2 and H_2O-rich laboratory CO ices. The abundances of the latter two ices, scaled to the total H_2O ice column, are the same in both sources. We thus suggest that thermal processing manifests itself as evaporation of apolar ices only and not the formation of CO_2 or polar ices. Finally, the decomposition of the ^(12)CO band is used to derive the ^(12)CO/^(13)CO abundance ratio in apolar ices. A ratio of ^(12)CO/^(13)CO = 71 ± 15 (3 σ) is deduced, in good agreement with gas-phase CO studies (~77) and the solid ^(12)CO_2/^(13)CO_2 ratio of 80 ± 11 found in the same line of sight. The implications for the chemical path along which CO_2 is formed are discussed

Polycyclic Aromatic Hydrocarbons with armchair edges and the 12.7 {\mu}m band

In this Letter we report the results of density functional theory calculations on medium-sized neutral Polycyclic Aromatic Hydrocarbon (PAH) molecules with armchair edges. These PAH molecules possess strong C-H stretching and bending modes around 3 {\mu}m and in the fingerprint region (10-15 {\mu}m), and also strong ring deformation modes around 12.7 {\mu}m. Perusal of the entries in the NASA Ames PAHs Database shows that ring deformation modes of PAHs are common - although generally weak. We then propose that armchair PAHs with NC >65 are responsible for the 12.7 {\mu}m Aromatic Infrared Band in HII regions and discuss astrophysical implications in the context of the PAH life-cycle.Comment: Minor editin

Nested shells reveal the rejuvenation of the Orion-Eridanus superbubble

The Orion-Eridanus superbubble is the prototypical superbubble due to its proximity and evolutionary state. Here, we provide a synthesis of recent observational data from WISE and Planck with archival data, allowing to draw a new and more complete picture on the history and evolution of the Orion-Eridanus region. We discuss the general morphological structures and observational characteristics of the superbubble, and derive quantitative properties of the gas- and dust inside Barnard's Loop. We reveal that Barnard's Loop is a complete bubble structure which, together with the lambda Ori region and other smaller-scale bubbles, expands within the Orion-Eridanus superbubble. We argue that the Orion-Eridanus superbubble is larger and more complex than previously thought, and that it can be viewed as a series of nested shells, superimposed along the line of sight. During the lifetime of the superbubble, HII region champagne flows and thermal evaporation of embedded clouds continuously mass-load the superbubble interior, while winds or supernovae from the Orion OB association rejuvenate the superbubble by sweeping up the material from the interior cavities in an episodic fashion, possibly triggering the formation of new stars that form shells of their own. The steady supply of material into the superbubble cavity implies that dust processing from interior supernova remnants is more efficient than previously thought. The cycle of mass-loading, interior cleansing, and star formation repeats until the molecular reservoir is depleted or the clouds have been disrupted. While the nested shells come and go, the superbubble remains for tens of millions of years.Comment: 20 pages, 6 figures, accepted for publication in Ap

Evaluation of the Multiplane Method for Efficient Simulations of Reaction Networks

Reaction networks in the bulk and on surfaces are widespread in physical, chemical and biological systems. In macroscopic systems, which include large populations of reactive species, stochastic fluctuations are negligible and the reaction rates can be evaluated using rate equations. However, many physical systems are partitioned into microscopic domains, where the number of molecules in each domain is small and fluctuations are strong. Under these conditions, the simulation of reaction networks requires stochastic methods such as direct integration of the master equation. However, direct integration of the master equation is infeasible for complex networks, because the number of equations proliferates as the number of reactive species increases. Recently, the multiplane method, which provides a dramatic reduction in the number of equations, was introduced [A. Lipshtat and O. Biham, Phys. Rev. Lett. 93, 170601 (2004)]. The reduction is achieved by breaking the network into a set of maximal fully connected sub-networks (maximal cliques). Lower-dimensional master equations are constructed for the marginal probability distributions associated with the cliques, with suitable couplings between them. In this paper we test the multiplane method and examine its applicability. We show that the method is accurate in the limit of small domains, where fluctuations are strong. It thus provides an efficient framework for the stochastic simulation of complex reaction networks with strong fluctuations, for which rate equations fail and direct integration of the master equation is infeasible. The method also applies in the case of large domains, where it converges to the rate equation results

Physical conditions in photodissociation regions: Application to galactic nuclei

Infrared and sub-millimeter observations are used in a simple procedure to determine average physical properties of the neutral interstellar medium in Galactic photodissociation regions as well as in ensembles of clouds which exist in the nuclei of luminous infrared galaxies. The relevant observations include the Infrared Astronomy Satellite (IRAS) infrared continuum measurements, infrared spectroscopy of the fine-structure lines of SiII 35 microns, OI 63 microns, and CII 158 microns, and the 2.6 mm CO (J=1-0) rotational transition. The diagnostic capabilities of the OI 145 microns line is also addressed. Researchers attribute these emission lines as well as the continuum to the atomic/molecular photodissociation region on the surfaces of molecular clouds which are illuminated by strong ultraviolet fields. They use the theoretical photodissociation region models of Tielens and Hollenbach (1985, Ap. J., 291, 722) to construct simple diagrams which utilize line ratios and line to continuum ratios to determine the average gas density n, the average incident far-ultraviolet flux G sub o, and the temperature of the atomic gas T

Mapping PAH sizes in NGC 7023 with SOFIA

NGC 7023 is a well-studied reflection nebula, which shows strong emission from polycyclic aromatic hydrocarbon (PAH) molecules in the form of aromatic infrared bands (AIBs). The spectral variations of the AIBs in this region are connected to the chemical evolution of the PAH molecules which, in turn, depends on the local physical conditions. We use the capabilities of SOFIA to observe a 3.2' x 3.4' region of NGC 7023 at wavelengths that we observe with high spatial resolution (2.7") at 3.3 and 11.2 um. We compare the SOFIA images with existing images of the PAH emission at 8.0 um (Spitzer), emission from evaporating very small grains (eVSG) extracted from Spitzer-IRS spectral cubes, the ERE (HST and CFHT), and H_2 (2.12 um). We create maps of the 11.2/3.3 um ratio to probe the morphology of the PAH size distribution and the 8.0/11.2 um ratio to probe the PAH ionization. We make use of an emission model and of vibrational spectra from the NASA Ames PAHdb to translate the 11.2/3.3 um ratio to PAH sizes. The 11.2/3.3 um map shows the smallest PAH concentrate on the PDR surface (H_2 and extended red emission) in the NW and South PDR. We estimated that PAHs in the NW PDR bear, on average, a number of carbon atoms (N_c) of ~70 in the PDR cavity and ~50 at the PDR surface. In the entire nebula, the results reveal a factor of 2 variation in the size of the PAH. We relate these size variations to several models for the evolution of the PAH families when they traverse from the molecular cloud to the PDR. The PAH size map enables us to follow the photochemical evolution of PAHs in NGC 7023. Small PAHs result from the photo-evaporation of VSGs as they reach the PDR surface. Inside the PDR cavity, the PAH abundance drops as the smallest PAH are broken down. The average PAH size increases in the cavity where only the largest species survive or are converted into C_60 by photochemical processing.Comment: accepted for publication in A&

ISO Spectroscopy of the Young Bipolar Nebulae S106 IR and Cep A East

We present the results of ISO SWS and LWS grating scans towards the embedded Young Stellar Objects (YSOs) S106 IR and Cep A East. Emission from the pure rotational lines of H2 and the infrared fine structure lines of [C II], [O I], [S I], [Si II] and [Fe II], as well as absorption bands due to H2O, CO and CO2 ice were detected toward Cep A. In S106 we detected emission lines of H2, CO, H I, and a large number of ionized species including Fe, O, N, C, Si, S, Ne and Ar. S106 also shows many of the infrared PAH bands in emission. Excitation temperatures and molecular hydrogen masses were derived from the low-lying pure rotational levels of H2 and are 500 and 730 K and 8 and 3 x 10^{-3} solar masses for S106 and Cep A, respectively. Since both objects are expected to have several solar masses of H2 in their environment, we conclude that in both cases the bulk of the H2 is cooler than a few hundred Kelvins. Excitation temperatures and line ratios were compared with those predicted by theoretical models for PDRs and dissociative and non-dissociative shocks. The [S I] 25.2 micron/[Si II] 34.8 micron ratio is a particularly useful shock versus PDR discriminant and we conclude that S106 IR is dominated by PDR emission while Cep A East has a large shock component. From an analysis of the ionic lines in S106 we conclude that the central star must have a temperature around 37,000 K, corresponding to a spectral type of O8. From its luminosity it is concluded that the driving source of Cep A must also be a massive early-type star. The absence of strong high-ionization ionic lines in its ISO spectrum shows that Cep A has not yet created a significant H II region and must be younger than S106, illustrating the process of the clearing of the surroundings of a massive young star.Comment: 15 pages (including 10 figures), to appear in Astronomy & Astrophysic