Dynamic features of Io's extended sodium distributions

We report on two-dimensional imaging observations of D-line emissions from the extended distribution of iogenic sodium atoms with two fields of view (±20 RJ (narrow FOV) and ±400 RJ (wide FOV)) simultaneously by using a portable small telescope or camera lens. We derived dynamic feature of the band-shaped and spray-shaped distributions near Io's orbit by means of continuous observation. The observations confirm the phenomenological behavior of the sodium cloud on two spatial scales, as previously observed by Pilcher et al. [Pilcher, C.B., Smyth, W.H., Combi, M.R., Fertel, J.H., 1984. Astrophys. J. 287, 427-444], Schneider et al. [Schneider, N.M., Trauger, J.T., Wilson, J.K., Brown, D.I., Evans, R.W., Shemansky, D.E., 1991. Science 253, 1394-1397], and Mendillo et al. [Mendillo, M., Baumgartner, J., Flynn, B., Hughes, W.S., 1990. Nature 348, 312-314]. We also confirm an elongated oval emission distribution of the sodium nebula and derivation of its detailed east-west asymmetry depending on Io's phase angle, which was first noted by Flynn et al. [Flynn, B., Mendillo, M., Baumgartner, J., 1994. J. Geophys. Res. 99, 8403-8409]. We then did model analyses to investigate the source process for sodium atoms and the dynamics behind their distribution. We conclude that the essential of molecular ion mechanisms to the band-shaped distribution is in agreement with Wilson and Schneider [Wilson, J.K., Schneider, N.M., 1999. J. Geophys. Res. 104, 16567-16583]. We differ from Wilson et al. [Wilson, J.K., Mendillo, M., Baumgartner, J., Schneider, N.M., Trauger, J.T., Flynn, B., 2002. Icarus 157, 476-489] in finding that charge exchange process contributes more to the spray-shaped distribution and sodium nebula than sputtering does. These results derived the double-peaked velocity distribution of released sodium atoms, and re-confirmed the source rates in agreement with past studies.     

Magnetic hydrogen atmosphere models and the neutron star RX J1856.5–3754

RX J1856.5−3754 is one of the brightest nearby isolated neutron stars (INSs), and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5−3754, within the uncertainties. In our simplest model, the best-fitting parameters are an interstellar column density   N _H≈ 1 × 10²⁰ cm⁻²  and an emitting area with   R ^∞≈ 17 km  (assuming a distance to RX J1856.5−3754 of 140 pc), temperature   T ^∞≈ 4.3 × 10⁵ K  , gravitational redshift   z _g ∼ 0.22  , atmospheric hydrogen column   y _H≈ 1 g cm⁻²  , and magnetic field   B ≈ (3–4) × 10¹² G  ; the values for the temperature and magnetic field indicate an effective average over the surface. We also calculate a more realistic model, which accounts for magnetic field and temperature variations over the NS surface as well as general relativistic effects, to determine pulsations; we find that there exist viewing geometries that produce pulsations near the currently observed limits. The origin of the thin atmospheres required to fit the data is an important question, and we briefly discuss mechanisms for producing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5−3754.     

The rotational temperature and column density of H3 in Uranus

H₃⁺ emission from Uranus has been observed repeatedly for over a decade. However, the details of the emission mechanisms are still poorly understood. In this paper, we discuss our findings from the observations we made in September 2000 and September 2001. The spectrum of Uranus was recorded at the NASA Infrared Telescope Facility using the SpeX instrument between 3 and 5 μm, with a resolving power of 1000. The 3.4–4.1 μm range permits a determination of both the H₃⁺ column density and its rotational temperature. The H₃⁺ emission, measured at 3.986 μm in the 0.8×3.7 arcsec aperture, was 0.031 Jy in September 2000 and 0.053 Jy in September 2001. The rotational temperature was found to be 560±40 K and 640±40 K in 2000 and 2001 respectively, with corresponding column densities of 5.1 (+3.2,−1.4) 10¹¹ and 4.0 (+1.8,−1.0) 10¹¹ cm⁻². These results extend the baseline for the variability study of the H₃⁺ emission (Astrophys. J. 524 (1999) 1059). Previous observations between 1992 and 1998 seemed to indicate a correlation between the H₃⁺ intensity and the solar cycle. The current data for 2000 and 2001 appear to be consistent with this general tendency.     

Updated Opacity Project radiative accelerations

Updated data for the calculation of radiative accelerations, g _rad, referred to as OP 2005, have been made generally available on the Web. They differ from the earlier 1997 OP data mainly in the inclusion of contributions from inner-shell processes. The frequency resolution used for OP 2005 is shown to be satisfactory except for extreme cases of low concentrations and low densities. The 2005 data are in reasonably good agreement with results from OPAL, given by Richer et al. in graphical form, for a model with   T _eff= 10⁴  and  log ( R ) =−3  . They are in less good agreement with OPAL results of Turcotte et al. for the solar radiative interior.     

The influence of atmospheric dust on limb darkening of M-type Mira variables

We explore the occurrence of dust in M-type Mira atmospheres and its effect on limb darkening under schematic assumptions about dust temperatures and dust particle properties. Dust particles that are thermodynamically coupled to the surrounding gas may form and may affect limb darkening, though only by very little in infrared continuum bandpasses. Dust particles that assume the equilibrium temperature given by the mean intensity of the radiation field only form under rare circumstances. Unexpectedly large or wavelength-dependent infrared continuum radii observed by interferometry are unlikely to be caused by atmospheric dust, except possibly near 1 μm; however, radius measurements may be significantly affected by molecular band contamination.     

Ionization freeze-out and hydrogen excitation in the SN IIP atmosphere

We study the nonstationary recombination of hydrogen in the atmosphere of SN 1987A by taking into account ion-molecular processes. The hydrogen excitation due to nonstationary recombination is shown to be enough to explain the observed hydrogen lines in a time interval until day 30 in the absence of additional excitation mechanisms. Thus, the problem of a deficit in the hydrogen excitation that has recently been found in modeling the hydrogen spectrum of SN 1987A at an early photospheric stage by assuming statistical ionization equilibrium is resolved. The mass of the radioactive ⁵⁶Ni with a spherically symmetric distribution in the outer layers is shown to be close to 10^?6 M _⊙. Our model predicts the appearance of a blue peak in the Hα profile between days 20 and 30. This peak bears a close similarity to the observed peak known as the Bochum event. The presence of this peak in the model is attributable to nonstationary recombination and to a substantial contribution of hydrogen neutralization involving H^? and H₂.     

Stability of jets on Jupiter and Saturn

The linear stability of a zonal jet that decays with depth is investigated under the assumption that the thermal stratification is very small. A westerly cosine jet is found to be more stable than it is in a thin fluid shell with two-dimensional flow. This is in agreement with observations of Jupiter and Saturn, where jet curvature exceeds the barotropic stability criterion. This result constitutes an alternative hypothesis to that of Ingersoll and Pollard [Icarus 52 (1982) 62], who showed that deep jets extending through the interior are also more stable than thin shell jets. The flow regime assumed in the present work requires that a small stratification can exist and persist even in the presence of horizontal temperature gradients. Further work will be needed to test whether this is realistic.