Electron collision cross sections at low energies for all transitions between the n=1, 2, 3, 4 and 5 levels of atomic hydrogen

Aggarwal, K. M.; Berrington, K A; Burke, P G; Kingston, A E; Pathak, A

doi:10.1088/0953-4075/24/6/024

Cited by 59 publications

(53 citation statements)

References 17 publications

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“…Cooling and heating mechanisms include -radiative cooling by H 2 lines excited by collisions with H, H 2 , He, and electrons (Le Bourlot et al 1999); -radiative cooling by CO, H 2 O, and 13 CO in the Large Velocity Gradient approximation (Neufeld & Kaufman 1993), and by OH and NH 3 in the low-density limit (Flower et al 1985); -atomic cooling by fine-structure and metastable lines of C, N, O, S, Si, C + , N + , O + , S + , Si + and Fe + (Giannini et al 2004); -inelastic scattering of electrons on H and H 2 (Aggarwal et al 1991;Hummer 1963;Rapp & Englander-Golden 1965); -energy released by collisional ionization and dissociation and exo/endo-thermicity of chemical reactions (Flower et al 1985); -energy heat/loss through thermalization with grains (Tielens & Hollenbach 1985); -ambipolar-diffusion heating by elastic scattering between the neutral fluid and charged ions and grains (Garcia et al 2001a, see Sect. 2.3); -ohmic heating arising from the drift between electrons and other fluids (Garcia et al 2001a, see Sect.…”

Section: Thermo-chemical Evolutionmentioning

confidence: 99%

Molecule survival in magnetized protostellar disk winds

Panoglou¹,

Cabrit²,

Forêts³

et al. 2012

A&A

108

144

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Context. Molecular counterparts to atomic jets have recently been detected within 1000 AU of young stars at early evolutionary stages. Reproducing these counterparts is an important new challenge for proposed ejection models. Aims. We explore whether molecules may survive in the magneto-hydrodynamic (MHD) disk wind solution currently invoked to reproduce the kinematics and tentative rotation signatures of atomic jets in T Tauri stars. Methods. The coupled ionization, chemical, and thermal evolution along dusty flow streamlines is computed for the prescribed MHD disk wind solution, using a method developed for magnetized shocks in the interstellar medium. Irradiation by (wind-attenuated) coronal X-rays and far-ultraviolet photons from accretion hot spots is included, with an approximate self-shielding of H 2 and CO. Disk accretion rates of 5 × 10 −6 , 10 −6 and 10 −7 M yr −1 are considered, representative of low-mass young protostars (so-called "Class 0"), evolved protostars ("Class I") and very active T Tauri stars ("Class II") respectively. Results. The disk wind has an "onion-like" thermo-chemical structure, with streamlines launched from larger radii having lower temperature and ionization, and higher H 2 abundance. The coupling between charged and neutral fluids is sufficient to eject molecules from the disk out to at least 9 AU. The launch radius beyond which most H 2 survives moves outward with evolutionary stage, from 0.2 AU (sublimation radius) in the Class 0 disk wind, to 1 AU in the Class I, and >1 AU in the Class II. In this molecular wind region, CO survives in the Class 0 but is significantly photodissociated in the Class I/II. Balance between ambipolar heating and molecular cooling establishes a moderate asymptotic temperature 700−3000 K, with cooler jets at earlier protostellar stages. As a result, endothermic formation of H 2 O is efficient, with abundances up to 10 −4 , while CH + and SH + can reach ≥10 −6 in the hotter and more ionised Class I/II winds. Conclusions. A centrifugal MHD disk wind launched from beyond 0.2−1 AU can produce molecular jets/winds up to speeds 100 km s −1 in young low-mass stars ranging from Class 0 to active Class II. The model predicts a high abundance ratio of H 2 to CO and an increase of molecular launch radius, temperature, and flow width as the source evolves, in promising agreement with current observed trends. Calculations of synthetic maps and line profiles in H 2 , CO and H 2 O will allow detailed tests of the model against observations.

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Section: Thermo-chemical Evolutionmentioning

confidence: 99%

Molecule survival in magnetized protostellar disk winds

Panoglou¹,

Cabrit²,

Forêts³

et al. 2012

A&A

108

144

View full text Add to dashboard Cite

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“…2 rate, we use the results from Scholz et al (1990) for n ! n 0 with n 0 5 the values from Aggarwal et al (1991) and the remaining ones from Johnson (1972). (See Chang et al 1991 for a comparison of various rates, including those of Giovanardi et al 1987 andPalla (1989.…”

Section: Hydrogenmentioning

confidence: 99%

Models of the Solar Chromosphere and Transition Region from SUMER and HRTS Observations: Formation of the Extreme‐Ultraviolet Spectrum of Hydrogen, Carbon, and Oxygen

Avrett

Loeser

2008

ASTROPHYS J SUPPL S

367

234

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We present the results of optically thick non-LTE radiative transfer calculations of lines and continua of H, C iYiv, and O iYvi and other elements using a new one-dimensional, time-independent model corresponding to the average quiet-Sun chromosphere and transition region. The model is based principally on the Curdt et al. SUMER atlas of the extreme ultraviolet spectrum. Our model of the chromosphere is a semiempirical one, with the temperature distribution adjusted to obtain optimum agreement between calculated and observed continuum intensities, line intensities, and line profiles. Our model of the transition region is determined theoretically from a balance between (a) radiative losses and (b) the downward energy flow from the corona due to thermal conduction and particle diffusion, and using boundary conditions at the base of the transition region established at the top of the chromosphere from the semiempirical model. The quiet-Sun model presented here should be considered as a replacement of the earlier model C of Vernazza et al., since our new model is based on an energy-balance transition region, a better underlying photospheric model, a more extensive set of chromospheric observations, and improved calculations. The photospheric structure of the model given here is the same as in Table 3 of Fontenla, Avrett, Thuiller, & Harder. We show comparisons between calculated and observed continua, and between the calculated and observed profiles of all significant lines of H, C iYiv, and O iYvi in the wavelength range 67Y173 nm. While some of the calculated lines are not in emission as observed, we find reasonable general agreement, given the uncertainties in atomic rates and cross sections, and we document the sources of the rates and cross sections used in the calculation. We anticipate that future improvements in the atomic data will give improved agreement with the observations.

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“…For the results shown here we used the ionization collisional rates of Mihalas (1967; see references therein for the original sources) and the collisional excitation rates of Burke et al (1967) and Sampson & Golden (1970), which are relatively large compared to other published ones. For estimating effects of collision rates uncertainties, we have also performed NLTE calculations using collisional excitation and ionization rates from Johnson (1972), with the following exception: for transitions between the lowest five levels we used the values of Aggarwal et al (1991). The choice of collisional rates has some effect on the H results in the low chromosphere by decreasing the LTE departure coefficient of level 1 by a factor of 2 at the temperature minimum, but does not change the conclusions in this paper.…”

Section: The H and H-nlte Calculationsmentioning

confidence: 99%

Semiempirical Models of the Solar Atmosphere. II. The Quiet‐Sun Low Chromosphere at Moderate Resolution

Fontenla

Balasubramaniam

Harder

2007

ApJ

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We present a new, one-dimensional model of the solar atmosphere (called SRPM 305) at moderate angular resolution ($1 00 Y2 00 ). Key characteristics of the SRPM 305 model include (1) a minimum temperature of $3800 K at a gas pressure of $80 dyne cm À2 and (2) a rapid temperature rise above the temperature-minimum layer that results in substantial overionization of most elements when compared with LTE calculations. The model calculations reproduce the $4300 K minimum brightness temperature of the UV continuum ( between 1400Y1500 8) observed by SUMER and the $4400 K observed minimum radio-continuum brightness temperature (between wavelengths 0.01 and 100 mm). Neither the UV nor the radio continuum bear on the low-temperature minimum value because their broad intensity contribution functions cause the higher temperatures of the upper chromospheric layers to effectively hide the low minimum temperature region. The SRPM 305 model reproduces the observed intensities of CO lines at 4.466 m, at both the disk center and near the limb, by using C and O abundances consistent with recent literature low values. The model also reproduces observed intensities of C i spectral lines at 5381 and 8337 8, CH lines at about 4306 8, the CN band head at 3883 8, and the O i lines at 7772, 7774, and 7776 8, respectively. Using the SRPM 305 model, we find no significant abundance variations between the photosphere and the low chromosphere. Consequently, the single-component model presented here matches several apparently contradictory observations and thereby resolves the controversy about the temperature minimum value.

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Electron collision cross sections at low energies for all transitions between the n=1, 2, 3, 4 and 5 levels of atomic hydrogen

Cited by 59 publications

References 17 publications

Molecule survival in magnetized protostellar disk winds

Molecule survival in magnetized protostellar disk winds

Models of the Solar Chromosphere and Transition Region from SUMER and HRTS Observations: Formation of the Extreme‐Ultraviolet Spectrum of Hydrogen, Carbon, and Oxygen

Semiempirical Models of the Solar Atmosphere. II. The Quiet‐Sun Low Chromosphere at Moderate Resolution

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