Physical Processes in Gaseous Nebulae. III. The Balmer Decrement.

Baker, James G.; Menzel, Donald H.

doi:10.1086/143959

Cited by 225 publications

(175 citation statements)

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“…The fractional populations vary significantly below 10 4 cm −3 and converge to the thermalized values at higher densities. Baker & Menzel (1938) define the Cases A and B with reference to the recombination spectrum of hydrogen. In N ii, there are five low-lying levels belonging to the ground configuration 2s 2 2p 2 , 3 P e 0,1,2 , 1 D e 2 and 1 S e 0 .…”

Section: Figmentioning

confidence: 99%

New effective recombination coefficients for nebular N ii lines

Fang

Storey

Liu

2011

A&A

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Aims. In nebular astrophysics, there has been a long-standing dichotomy in plasma diagnostics between abundance determinations using the traditional method based on collisionally excited lines (CELs), on the one hand, and (optical) recombination lines/continuum, on the other. A number of mechanisms have been proposed to explain the dichotomy. Deep spectroscopy and recombination line analysis of emission line nebulae (planetary nebulae and H ii regions) in the past decade have pointed to the existence of another previously unknown component of cold, H-deficient material as the culprit. Better constraints are needed on the physical conditions (electron temperature and density), chemical composition, mass, and spatial distribution of the postulated H-deficient inclusions in order to unravel their astrophysical origins. This requires knowledge of the relevant atomic parameters, most importantly the effective recombination coefficients of abundant heavy element ions such as C ii, O ii, N ii, and Ne ii, appropriate for the physical conditions prevailing in those cold inclusions (e.g. T e ≤ 1000 K). Methods. Here we report new ab initio calculations of the effective recombination coefficients for the N ii recombination spectrum. We have taken into account the density dependence of the coefficients arising from the relative populations of the fine-structure levels of the ground term of the recombining ion ( 2 P • 1/2 and 2 P • 3/2 in the case of N iii), an elaboration that has not been attempted before for this ion, and it opens up the possibility of electron density determination via recombination line analysis. Photoionization cross-sections, bound state energies, and the oscillator strengths of N ii with n ≤ 11 and l ≤ 4 have been obtained using the closecoupling R-matrix method in the intermediate coupling scheme. Photoionization data were computed that accurately map out the near-threshold resonances and were used to derive recombination coefficients, including radiative and dielectronic recombination. Also new is including the effects of dielectronic recombination via high-n resonances lying between the 2 P • 1/2 and 2 P • 3/2 levels. The new calculations are valid for temperatures down to an unprecedentedly low level (approximately 100 K). The newly calculated effective recombination coefficients allow us to construct plasma diagnostics based on the measured strengths of the N ii optical recombination lines (ORLs). Results. The derived effective recombination coefficients are fitted with analytic formulae as a function of electron temperature for different electron densities. The dependence of the emissivities of the strongest transitions of N ii on electron density and temperature is illustrated. Potential applications of the current data to electron density and temperature diagnostics for photoionized gaseous nebulae are discussed. We also present a method of determining electron temperature and density simultaneously.

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Section: Figmentioning

confidence: 99%

New effective recombination coefficients for nebular N ii lines

Fang

Storey

Liu

2011

A&A

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“…Sources assigned the "YSO" identification are radio-quiet mid-IR point sources associated with 13 CO emission which do not look isolated in near-IR images. Where near-IR spectroscopy is available, these sources usually show H i emission, though not at Case-B ratios (Baker & Menzel 1938), and can also show H 2 , He i, Fe ii and [Fe ii] emission, as well as CO bandhead emission and/or absorption. "H ii region" sources are either extended in ∼1 resolution mid-IR imaging and/or radio loud, with Case-B H i line ratios in the near-IR.…”

Section: The Rms Sample Of Massive Young Starsmentioning

confidence: 99%

The RMS survey: far-infrared photometry of young massive stars

Mottram

Hoare²,

Lumsden³

et al. 2010

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Context. The Red MSX Source (RMS) survey is a multi-wavelength campaign of follow-up observations of a colour-selected sample of candidate massive young stellar objects (MYSOs) in the galactic plane. This survey is returning the largest well-selected sample of MYSOs to date, while identifying other dust contaminant sources with similar mid-infrared colours including a large number of new ultra-compact (UC) H ii regions.Aims. To measure the far-infrared (IR) flux, which lies near the peak of the spectral energy distribution (SED) of MYSOs and UCH ii regions, so that, together with distance information, the luminosity of these sources can be obtained.Methods. Less than 50% of RMS sources are associated with IRAS point sources with detections at 60 μm and 100 μm, though the vast majority are visible in Spitzer MIPSGAL or IRAS Galaxy Atlas (IGA) images. However, standard aperture photometry is not appropriate for these data due to crowding of sources and strong spatially variable far-IR background emission in the galactic plane. A new technique using a 2-dimensional fit to the background in an annulus around each source is therefore used to obtain far-IR photometry for young RMS sources. Results. Far-IR fluxes are obtained for a total of 1113 RMS candidates identified as young sources. Of these 734 have flux measurements using IGA 60 μm and 100 μm images and 724 using MIPSGAL 70 μm images, with 345 having measurements in both data sets.

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“…Most analytical work done on HII regions heavily depends on the on-the-spot approximation, part of which has its origin in Baker & Menzel (1938), and has been discussed and used in the standard texts Spitzer (1978) and Osterbrock (1989). In this argument, one considers the gas within the ionisation front, or within the Strömgren sphere when the static equilibrium solution has already been reached, and does some bookkeeping to equate the number of recombinations within that volume to the number of ionising photons.…”

Section: The On-the-spot Approximationmentioning

confidence: 99%

The diffuse nature of Strömgren spheres

Ritzerveld¹

2005

A&A

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Abstract. In this Letter, we argue that the standard analytical derivations of properties of HII regions, such as the speed, shape and asymptotic position of ionisation fronts require a more precise treatment. These derivations use the on the spot approximation, which in effect ignores the diffuse component of the radiation field. We show that, in fact, HII regions are diffusion dominated. This has as a result that the morphology of inhomogeneous HII regions will be drastically different, because shadowing effects have a less profound impact on the apparent shape. Moreover, it will have influence on the propagation speed of ionisation fronts. We quantify our claims by analytically deriving the internal radiation structure of HII regions, taking diffusion fully into account for several different cosmologically relevant density distributions.

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Physical Processes in Gaseous Nebulae. III. The Balmer Decrement.

Cited by 225 publications

References 0 publications

New effective recombination coefficients for nebular N ii lines

New effective recombination coefficients for nebular N ii lines

The RMS survey: far-infrared photometry of young massive stars

The diffuse nature of Strömgren spheres

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