Far-ultraviolet Radiation Field Research Articles

2.12 Micron Molecular Hydrogen Emission from Circumstellar Disks Embedded in the Orion Nebula

We present narrowband images of two externally illuminated circumstellar disks embedded in the Orion Nebula. The images were taken with camera 2 of the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) as part of the Hubble Space Telescope Early Release Observations program. Molecular hydrogen emission is confined to the silhouettes embedded in the objects HST 10 (182-413) and HST 17 (183-419). This emission appears to trace the surfaces of nearly edge-on circumstellar disks and is likely to be excited by the intense far-ultraviolet (FUV) radiation field produced by nearby O stars. The presence of this emission confirms that FUV radiation penetrates to the disk surface. The absence of H2 emission in the region between the ionization front and the disk surface provides evidence that this medium is predominantly atomic. Our observations constitute the first demonstration that the dense circumstellar disks embedded in the proplyds are predominately molecular. While the central star is marginally detected at 2.15 μm in HST 10, the central star of HST 17 is clearly detected.

DENSE PHOTODISSOCIATION REGIONS (PDRs)

▪ Abstract All neutral atomic hydrogen gas and a large fraction of the molecular gas in the Milky Way Galaxy and external galaxies lie in PDRs, and PDRs are the origin of most of the nonstellar infrared (IR) and the millimeter CO emission from a galaxy. On the surfaces (Av< 1−3) of interstellar clouds, the absorption of far ultraviolet (FUV) photons (hν < 13.6 eV) by gas and dust grains leads to intense emission of [C II] 158 μm, [O I] 63, 146 μm, and H2rovibrational transitions, as well as IR dust continuum and polycyclic aromatic hydrocarbon (PAH) emission features. Deeper in PDRs, CO rotational and [C I] 370, 609 μm lines originate. The transition of H to H2and C+to CO occurs within PDRs. Theoretical models compared with observations diagnose such physical parameters as the density and temperature structure, the elemental abundances, and the FUV radiation field in PDRs. Applications include clouds next to H II regions, reflection nebulae, planetary nebulae, red giant outflows, circumstellar gas around young stars, diffuse clouds, the warm neutral medium (WNM), and molecular clouds in the interstellar radiation field: in summary, much of the interstellar medium in galaxies. This review focuses on dense PDRs in the Milky Way Galaxy. Theoretical PDR models help explain the observed correlation of the CO J = 1–0 luminosity with the molecular mass and also suggest FUV-induced feedback mechanisms that may regulate star formation rates and the column density through giant molecular clouds.

Variations of the 8.6 and 11.3 μm Emission Bands within NGC 1333: Evidence for Polycyclic Aromatic Hydrocarbon Cations

We have obtained 8-13 μm spectra of three positions in the IR reflection nebula associated with SVS 3 in NGC 1333. These observations reveal systematic variations in the relative intensities of the IR emission features at 8.6 μm and 11.3 μm. In particular, the [8.6]/[11.3] ratio is highest at the position of the exciting star and decreases with increasing angular distance from the star. We attribute these variations to changes in the relative populations of ionized and neutral polycyclic aromatic hydrocarbons (PAHs), driven by the strong far-ultraviolet radiation field near the star. From experiments and quantum chemical theoretical studies, the intrinsic strength of these two bands is known to vary with the charge state of the emitting PAH. We have developed a simple model for the relative intensity of the 8.6 and 11.3 μm bands taking the charge state of the carrier into account. This model is in good agreement with the data. We point out several observational and experimental tests of this model.

Chemistry in Dense Photon-dominated Regions

view Abstract Citations (433) References (102) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Chemistry in Dense Photon-dominated Regions Sternberg, A. ; Dalgarno, A. Abstract We present a detailed chemical model of a photon-dominated region produced in a dense molecular cloud exposed to an intense far-ultraviolet radiation field. We use it to illustrate the gas-phase processes that lead to the production of atomic and molecular species in the several distinct zones that occur in the cloud. We identify molecular diagnostics of photon-dominated chemistry in dense molecular clouds. Publication: The Astrophysical Journal Supplement Series Pub Date: August 1995 DOI: 10.1086/192198 Bibcode: 1995ApJS...99..565S Keywords: ISM: MOLECULES; MOLECULAR PROCESSES; GALAXIES: ISM full text sources ADS |

Time-dependent fluorescent H2 emission

We present computations of the time-dependent infrared fluorescent emission-line response of molecular hydrogen (H2) gas in interstellar clouds exposed to far-ultraviolet radiation fields. In clouds exposed to intense fields the H2 line intensities may be much larger at early times immediately following the onset of irradiation than under conditions of static equilibrium. Time-dependent effects in evolving photon-dominated regions may be important in the fluorescent H2 emitting regions in planetary and reflection nebulae and possibly in starburst galaxies and active galactic nuclei.

Voyager Observations of Diffuse Far-Ultraviolet Continuum and Line Emission in Eridanus

view Abstract Citations (16) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Voyager Observations of Diffuse Far-Ultraviolet Continuum and Line Emission in Eridanus Murthy, Jayant ; Im, M. ; Henry, R. C. ; Holberg, J. B. Abstract We present Voyager observations of the diffuse far-ultraviolet radiation field (912 Å-1150 Å) in two regions of the Eridanus superbubble A strong continuum is present in both locations which we have identified with starlight scattered by interstellar dust. From the level of this emission (1620‰150 photons cm-2 s-1 sr-1 Å-1) we have placed a lower limit of about 0.3 on the albedo if the scattering is isotropic and an upper limit of about 0.8 on the phase function asymmetry factor (g) if the grains are perfectly reflecting. In addition, O VI (λλ1032/1038) emission with an intensity of 28.7±2.8 × 104 photons cm-2 s-1 sr-1 is apparent in one of the two locations, and C III (977 Å) is visible in both locations with intensities of 2.14±0.75 × 104 and 18.9±1.7 × 104 photons cm-2 s-1 sr-1. The line ratios indicate shock velocities on the order of 90 km s-1 in one of the targets and 180 km s-1 in the other, which is located near a peak of the C-band X-ray emission. Publication: The Astrophysical Journal Pub Date: December 1993 DOI: 10.1086/173524 Bibcode: 1993ApJ...419..739M Keywords: COSMOLOGY: DIFFUSE RADIATION; ISM: BUBBLES; ISM: INDIVIDUAL NAME: ERIDANUS; ULTRAVIOLET: INTERSTELLAR full text sources ADS | data products SIMBAD (9)

Anatomy of the Photodissociation Region in the Orion Bar

Much of the interstellar gas resides in photodissociation regions whose chemistry and energy balance is controlled by the flux of far-ultraviolet radiation upon them. These photons can ionize and dissociate molecules and heat the gas through the photoelectric effect working on dust grains. These regions have been extensively modeled theoretically, but detailed observational studies are few. Mapping of the prominent Orion Bar photodissociation region at wavelengths corresponding to the carbon-hydrogen stretching mode of polycyclic aromatic hydrocarbons, the 1-0 S(1) line of molecular hydrogen, and the J = 1-0 rotational line of carbon monoxide allows the penetration of the far-ultraviolet radiation into the cloud to be traced. The results strongly support the theoretical models and show conclusively that the incident far-ultraviolet radiation field, not shocks as has sometimes been proposed, is responsible for the emission in the Orion Bar.

A possible connection between enhanced CH(+) and intense far-ultraviolet radiation fields

Evidence favoring a radiative formation mechanism for CH + is described, based largely on new observations of the star BD +31°643 in the young cluster IC 348; a region associated with reflection nebulosity and large grains. The CH + column density in this line of sight is 8.3×10 13 cm −2 , larger than can be easily explained by shock formation models, and the velocity displacement between the lines of CH + and cold-gas two obverved lines of CH + is inconsistent with the expectations of the shock formation hypothesis

The integrated ultraviolet radiation field from QSOs

The contribution of QSOs to the integrated diffuse far-ultraviolet radiation field as a function of redshift is calculated. As expected, the calculated value of the specific intensity, Jv(z), is strongly dependent on the QSO luminosity function, especially at high redshifts, whereas the spectral shape is mostly dependent on the adopted form for the intrinsic UV spectrum of the individual QSOs. It is estimated that uncertainties in the luminosity function of the low-luminosity AGN introduce uncertainties of up to a factor of nine in Jv. Continuous absorption by the neutral hydrogen associated with intervening galaxies attenuates Jv by a factor of 3-5 at 912 A. At shorter wavelengths, Jv is attenuated by a factor of 2-5 depending on the value of He I:He II adopted for the intervening systems. The implications of the uncertainties in Jv(z) for the analysis of observations of the Lyman-alpha forest clouds and the detectability of the associated He I and He II absorption with the Hubble Space Telescope are discussed. 79 references.