Radiative Transfer Effects in He I Emission Lines

We consider the effect of optical depth of the 2 ^{3}S level on the nebular recombination spectrum of He I for a spherically symmetric nebula with no systematic velocity gradients. These calculations, using many improvements in atomic data, can be used in place of the earlier calculations of Robbins. We give representative Case B line fluxes for UV, optical, and IR emission lines over a range of physical conditions: T=5000-20000 K, n_{e}=1-10^{8} cm^{-3}, and tau_{3889}=0-100. A FORTRAN program for calculating emissivities for all lines arising from quantum levels with n < 11 is also available from the authors. We present a special set of fitting formulae for the physical conditions relevant to low metallicity extragalactic H II regions: T=12,000-20,000 K, n_{e}=1-300 cm^{-3}, and tau_{3889} < 2.0. For this range of physical conditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A, and 6678 A, are changed less than 1%, in agreement with previous studies. However, the 7065 A corrections are much smaller than those calculated by Izotov & Thuan based on the earlier calculations by Robbins. This means that the 7065 A line is a better density diagnostic than previously thought. Two corrections to the fitting functions calculated in our previous work are also given.Comment: To be published in 10 April 2002 ApJ; relevant code available at ftp://wisp.physics.wisc.edu/pub/benjamin/Heliu

Improving Predictions for Helium Emission Lines

We have combined the detailed He I recombination model of Smits with the collisional transitions of Sawey & Berrington in order to produce new accurate helium emissivities that include the effects of collisional excitation from both the 2 (3)S and 2 (1) S levels. We present a grid of emissivities for a range of temperature and densities along with analytical fits and error estimates. Fits accurate to within 1% are given for the emissivities of the brightest lines over a restricted range for estimates of primordial helium abundance. We characterize the analysis uncertainties associated with uncertainties in temperature, density, fitting functions, and input atomic data. We estimate that atomic data uncertainties alone may limit abundance estimates to an accuracy of 1.5%; systematic errors may be greater than this. This analysis uncertainty must be incorporated when attempting to make high accuracy estimates of the helium abundance. For example, in recent determinations of the primordial helium abundance, uncertainties in the input atomic data have been neglected.Comment: ApJ, accepte

Point Source C-Band Mueller Matrices for the Green Bank Telescope

C-Band Mueller matrices for the Green Bank Telescope are presented here which enable on-sky Stokes parameters for point sources at the beam center to be determined. Standard calibrators, 3C138 and 3C286, were observed using the Spider program to steer the telescope across a broad range of Right Ascensions on both sides of the zenith transit. For this analysis, only the observations at the peak of the Spider pattern were used rather than the full sweep of the runs. Therefore, the results presented here only apply to point sources at the beam center. The Mueller matrices are shown to vary with frequency and with use of the Hi-Cal or Lo-Cal noise diodes, due to the relative calibration gain between the X and Y components of the feed. However, the relative calibration gain can be determined from observations of a source with known polarization. Correcting the data for the relative calibration gain prior to data analysis allows for use of a frequency independent Mueller matrix. This generic Mueller matrix is shown to provide reliable C-Band polarization measurements.Comment: Acceptance for publication in The Astronomical Journa

Multiwavelength OH observations of Mon R2 during 4765 MHz OH maser flaring

During the spectacular 4765 MHz excited OH maser flaring episodes observed in Mon R2 from 1994 to 1998 (Smits et al. 1998), observations were made at the line frequencies of other OH transitions. No excited OH lines were detected, nor were any 1612 or 1720 MHz OH masers found in Mon R2. 1665 and 1667 MHz OH masers were present and underwent small changes. With a peak flux of ∼ 75 Jy, the null detections and small variations at other wavelengths put strong constraints on models of OH masers.</jats:p

An nl-model with radiative transfer for hydrogen recombination line masers

Abstract Atomic hydrogen masers occur in recombination plasmas in sufficiently dense H ii regions. These hydrogen recombination line (HRL) masers have been observed in a handful of objects to date and the analysis of the atomic physics involved has been rudimentary. In this work a new model of HRL masers is presented which uses an nl-model to describe the atomic populations interacting with free-free radiation from the plasma, and an escape probability framework to deal with radiative transfer effects. The importance of including the collisions between angular momentum quantum states and the free-free emission in models of HRL masers is demonstrated. The model is used to describe the general behaviour of radiative transfer of HRLs and to investigate the conditions under which HRL masers form. The model results show good agreement with observations collected over a broad range of frequencies. Theoretical predictions are made regarding the ratio of recombination lines from the same upper quantum level for these objects.</jats:p

Photoionization Modelling of H II Region for Oxygen Ions

We present phoionization model calculations of ionization fraction of hydrogen, helium and oxy-gen at low density of photoionized nebulae using the photoionization code CLOUDY. The ionization parameter is between −2.0 ≤ log(U) ≤ 1.0, and we study them by changing the abundances of oxy-gen from −2.5 ≤ log(O/H) ≤ −5.0. This was tested first by Campbell (1988) [1]. But, this was differ-ent in highest abundances of oxygen, filling factor and carbon abundances. We change the abun-dances of carbon by 0.5 dex of solar value, to avoid formation CO in the process. We include the charge transfer interaction between oxygen ions and hydrogen atom. Based on this process, we have calculated the ionization profiles of O0, O+, O2+ O3+ and O4+ in the presence and absence of charge exchange between hydrogen and oxygen. In this work, we assumed the effective rate of co-efficient for the charge transfer is the order of ≈10−9 cm−3∙s−1