Statistical-Equilibrium Model Atmospheres for Early-Type Stars. I. Hydrogen Continua

Mihalas, Dimitri

doi:10.1086/149239

Cited by 45 publications

(23 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…We solve the partial redistribution (PRD) only for Ly (in the future we plan to include PRD in other lines as well). 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).…”

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

View full text Add to dashboard Cite

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.

show abstract

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

View full text Add to dashboard Cite

show abstract

“…The short-wavelength continuum benefits from the use of ff-cross sections from Sutherland (1998) 3 which replace the older fit formula from Mihalas (1967). More importantly, the continuum flux depends on the treatment of the b f -transitions.…”

Section: Nlte Models For Pure Hydrogen Atmospheresmentioning

confidence: 99%

Establishing HZ43 A, Sirius B, and RX J185635-3754 as soft X-ray standards: a cross-calibration between the Chandra LETG+HRC-S, the EUVE spectrometer, and the ROSAT PSPC

Beuermann¹,

Burwitz

Rauch³

2006

A&A

View full text Add to dashboard Cite

Context. The absolute calibration of space-borne instruments in the soft X-ray regime rests strongly on model spectra of hot white dwarfs. Aims. We analyze the Chandra LETG+HRC observations of the white dwarfs HZ43 A and Sirius B and of the neutron star RX J185635−3754 with the aim of resolving current uncertainties in the soft X-ray spectral fluxes and photospheric parameters of the three stars. We apply the derived photon spectra to a cross-calibration of the LETG+HRC-S with the short-wavelength EUVE spectrometer and the ROSAT PSPC. Methods. We tie HZ43 A to the flux of RX J1856 in the 44−48 Å range and perform a simultaneous least squares fit to the LETG+HRC spectra of the three stars. This allows us to determine an internally consistent set of spectral energy distributions and an empirically derived wavelength-dependent correction to the LETG+HRC-S effective area. We employ NLTE model atmospheres calculated with TMAP for the white dwarfs and a two-blackbody model for RX J1856, tied to the respective optical fluxes. Results. The two-blackbody model for RX J1856 features a hot spot on a cooler star and yields kT spot = 62.8 ± 0.4 eV and kT star = 32.3 ± 0.7 eV with a stellar angular radius as seen from infinity of 0.1371 ± 0.0010 km pc −1 . For HZ43 A, our fit yields T eff = 51 126 ± 660 K and log g = 7.90 ± 0.080 (cgs) with anti-correlated errors (1-σ) which include not only the statistical but also the systematic uncertainties of the fit. HZ43AB displays a previously detected bremsstrahlung component with a temperature kT 0.6 keV. For Sirius B, we find T eff = 24 923 ± 115 K for fixed log g = 8.6. The calibration of the short-wavelength EUVE spectrometer differs from that of the LETG+HRC-S by 15 ± 7%. The ROSAT PSPC is found to be correctly calibrated within a few percent and reports of a major miscalibration are unfounded. Conclusions. We have obtained improved parameters for RX J185635−3754, HZ43 A, and Sirius B which fit the observations from the optical to the soft X-ray regime. Our approach allows us to quote their absolute spectral fluxes at selected wavelengths which may aid the calibration of other space-borne instruments.

show abstract

“…There polynomial fits in wavelength λ and temperature T , sometimes also in Θ = 5040/T , to the opacities are taken from Carbon & Gingerich (1969). The Gaunt factors are calculated, also via analytical expressions in λ and Θ, following Mihalas (1967). With known continuum opacities it is now easy to locate the position in the atmosphere where τ 5000 = 1.…”

Section: Opacitiesmentioning

confidence: 99%

Formation of granular intensity fluctuations on the Sun

Rodríguez

Kneer

2002

A&A

View full text Add to dashboard Cite

Abstract. We deal with granular intensity fluctuations in the Sun and how they are caused by temperature fluctuations. We calculate temperature response functions RF T (λ, z) in the solar photosphere. We apply the Diagonal Element Lambda Operator (DELO) method to calculate the response functions for the continuum (λλ = 380 nm-2.5 µ). LTE is assumed throughout. It is demonstrated that, within the framework of the linear approximation, the H − continuum absorption plays an important rôle for the formation of the fluctuations in continuum images. It counteracts substantially the intensity fluctuations caused by variations of the Planck function. The temperature response functions are used to calculate, for three models of temperature fluctuations, the continuum intensity contrasts and their formation height for the above wavelength range.

show abstract

Statistical-Equilibrium Model Atmospheres for Early-Type Stars. I. Hydrogen Continua

Cited by 45 publications

References 0 publications

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

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

Establishing HZ43 A, Sirius B, and RX J185635-3754 as soft X-ray standards: a cross-calibration between the Chandra LETG+HRC-S, the EUVE spectrometer, and the ROSAT PSPC

Formation of granular intensity fluctuations on the Sun

Contact Info

Product

Resources

About