The 2200 Å extinction hump and porous graphite grains

We discuss in this paper the possibility of interpreting the 2200 Å band occurring in the interstellar extinction curves as being attributed to porous graphite. The results show that grains with radii smaller than 0.015 m and a porosity degree within the values 0.02f0.25 are able to fit satisfactorily the peak at 4.6 m^–1 and the band shape between 4 and 5.2 m^–1. Consideration of the expected density for such particles seems to confirm that interstellar grains may be porous but, at the same time, suggests that care must be taken when data concerning dust in the solar system are extrapolated to the interstellar space.     

Interpretation of the 6818.9-Å methane feature observed on Jupiter,Saturn, and Uranus

High-resolution (0.1-Å) spectra of the 6818.9-Å methane feature obtained for Jupiter, Saturn, and Uranus by K. H. Baines, W. V. Schempp, and W. H. Smith ((1983). Icarus56, 534–542) are modeled using a doubling and adding code after J. H. Hansen ((1969). Astrophys. J.155, 565–573). The feature's rotational quantum number is estimated using the relatively homogeneous atmosphere of Saturn, with only J = 0 and J = 1 fitting the observational constraints. The aerosol content within Saturn's northern temperate region is shown to be substantially less than at the equator, indicating a haze only half as optically thick. Models of Jupiter's atmosphere are consistent with the rotational quantum-number assignment. Synthetic line profiles of the 6818.9-Å feature observed on Uranus reveal that a substantial haze exists at or above the methane condensation region with an optical depth eight times greater than previously reported. Seasonal effects are indicated. The methane column abundance is 5 ± 1 km-am. The mixing ratio of methane to hydrogen within the deep unsaturated region of the planet is 0.045 ± 0.025, based on an H₂ column abundance of 240 ± 60 km-am (W. H. Smith, W. Macy, and C. B. Pilcher (1980). Icarus43, 153–160), thus indicating that the methane comprises between one-sixth and one-half of the planet's mass. However, proper reevaluation of H₂ quadrupole features accounting for the haze reported here may significantly reduce the relative methane abundance.     

On the λ 2800 Å interstellar extinction feature

We answer our critics and confirm the reality of a feature in the interstellar extinction curve near 2800 Å for stars showing abnormally low graphite absorption.     

Extinction coefficients of amorphous carbon grains from 2100 Å to 340 μm

Extinction measurements were made for three kinds of amorphous carbon grains in the range 2100 -340 m. Absolute values of extinction coefficients from different sources vary by almost constant factors. Wavelength dependences of extinction curves show a ^–1 or less steep fall off, against a ^–2 or steeper fall off of graphite grains. Small humps are found around 6.3, 8, 13.3 and 90 m, although their origins are not yet clear. The infrared properties of amorphous carbon grains suggest that they could supply a significant amount of far-infrared emission noted in recent astronomical observations.     

Identification of the λ2200 Å Interstellar Absorption Feature

We show that heterocyclic aromatic compounds could explain the interstellar absorption feature at 2200 Å, requiring 10% of the available interstellar C and N to be tied up in this form.     

A unified model for the 3.28 μ emission and the λ2200 å interstellar extinction feature

The 3.28 m feature observed in the diffuse galactic emission and the 2200 Å interstellar extinction feature must have a common source. This requirement rules out graphitic-type PAH molecules such as coronone and tends to favour naturally occurring bi-cyclic ring structures typified by quinoline and its derivatives.     

The 4.6 micron feature of –SiH groups in silicate dust grains and infrared cometary spectra

Amorphous silicate dust grains have been produced in the laboratory by means of laser ablation of solid targets in different ambient atmospheres. In this work we show that, if the condensation occurs in the presence of hydrogen, the spectra of silicate grains, together with the characteristic 10 and 20 μm features, exhibit an absorption band around 4.6 μm. Such features, absent in the spectra of the same silicate grains produced in an oxygen atmosphere, may be attributed to a fundamental stretching vibration of –SiH functional groups bound into the grains or on their surface.Based on the cosmic abundance of the elements, silicates are expected to condense in the atmospheres of oxygen-rich stars where hydrogen is also abundant. This means that –SiH functional groups may be present also in the circumstellar and interstellar silicate dust grains. An absorption feature at 4.6 μm has already been observed in the absorbing dust of several protostellar embedded sources. The observation of a similar feature in comets can give important information on the origin and evolution of cometary material. © 1999 Elsevier Science Ltd. All rights reserved.     

A Unified Model for The 3.28 μ Emission and The 2200 Å Interstellar Extinction Feature

The 3.28 μm feature observed in the diffuse galactic emission and the 2200 Å interstellar extinction feature must have a common source. This requirement rules out graphitic-type PAH molecules such as coronone and tends to favour naturally occurring bi-cyclic ring structures typified by quinoline and its derivatives.     

Line width observation of Heii 4686 Å and Hei 4713 Å in the chromosphere

Line profiles of He ii 4686 Å and He i 4713 Å from active regions in the chromosphere were observed during the total solar eclipse of February 16, 1980, with a grazing incidence objective grating spectrograph. The Doppler width of the He i triplet line of 4713 Å increases with height and the average width is compatible with width of metallic and hydrogen lines, suggesting that the kinetic temperature of He i triplet emitting region is T 8000 K. This can only be explained by recombination after photo-ionization due to coronal UV radiation. The Doppler width of the Paschen line of He ii 4686 is, without any correction for the separation of subcomponents of the line nor non-thermal velocity, 18.4 km s^-1. This line width also shows a tendency to increase with height. After comparison with Doppler widths of He i 4713 and the EUV lines, and a necessary subtraction of non-thermal velocity, it is shown that this line is emitted in a 2 × 10⁴ K temperature region, which again supports the view that this line is emitted through the recombination process after photoionization due to coronal XUV radiation below 228 Å.     

Correlations between OH,NaD and OI 5577 Å emissions in the airglow

Simultaneous measurements of the night airglow OI 5577 Å and OH (8,3) band have been carried out at Cachoeira Paulista (23°S, 45°W), Brazil since 1976. Cross correlation analyses between the nocturnal variations of these emissions and also with the OH rotational temperature (ROT) for various time shifts are presented. It is found that OH (8,3) is well correlated with ROT but with a time lag of about 1 h. The variations of OI 5577 Å lead OH (8,3) by about 2–3 h and ROT covaries with 5577 Å with a time lag less than 1 h. For the sake of comparison, OI 5577 Å, OH and NaD data from a number of IQSY stations have been analysed. It is noted that (1) OI 5577 Å leads OH by about 2 h at mid-latitude stations and (2) OH is well correlated with Na 5893 Å with a time lag of less than 1 h. The presence of the phase lagged correlation patterns between OH/5577 Å, OH/ROT and OH/NaD indicates vertical propagation of a wavelike perturbation of the upper atmosphere.     

Silicon monoxide and the 10 μm interstellar feature

Laboratory spectra of SiO particles of 1 m radius show a broad structureless extinction peak at 9.6m. The wavelength dependence of extinction from SiO, an amorphous silicon oxide, provides a good match to that of interstellar dust.     

Mineral grains and the 10 and 20 μm spectral features in the Trapezium nebula

The infrared emission observed for dust in the Trapezium nebula over the waveband 8–40 m cannot be explained on the basis of a simple silicate model involving either terrestrial or lunar silicates. Laboratory measurements were carried out to determine the opacity coefficient of finely crushed Murchison meteorite material. Murchison material (including organics within it) provides a better fit to the Trapezium data than any known silicate, but the agreement is inferior to that obtained for a diatom model.     

On the behavior of the Cii 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars

With the aim of investigating the possible particular behavior of carbon in a sample of chemically peculiar stars of the main sequence without turning to modeling, we performed spectroscopic observations of three important and usually prominent single ionized carbon lines: 4267.261, 6578.052 and 6582.882 Å. In addition, we observed a large number of standard stars in order to define a kind of normality strip, useful for comparing the observed trend for the peculiar stars. We paid particular attention to the problem of the determination of fundamental atmospheric parameters, especially for the chemically peculiar stars for which the abundance anomalies change the flux distribution in such a way that the classical photometric methods to infer effective temperatures and gravities parameter cannot be applied. Regarding CP stars, we found a normal carbon abundance in Hg–Mn, Si (with some exceptions) and He strong stars. He weak stars are normal too, but with a large spread out of the data around the mean value. A more complicated behavior has been noted in the group of SrCrEu stars: four out of seven show a strong overabundance, being the others normal.     

The interstellar band at λ 4430 Å and the abundance of interstellar iron,titanium, and molecular hydrogen

For the direction to a number of stars the depletion of interstellar gaseous iron and titanium as well as the relative abundance of molecular hydrogen and the strength of the interstellar band at 4430 Å were determined by different authors and can be found in the literature. In this paper it is shown that the difference (A _c-A_co) is a more reliable measurement of the column density of 4430 Å absorbers than the usually used central depth,A _c, because the positive valueA _co ofA _c forE(B-V)=0, i.e. the intercept with theA _c axis of a least-squares fit to the observedA _c vsE(B-V) data, is with a high probability not caused by an interstellar effect.There was no correlation found between the interstellar depletion of iron and titanium, respectively, and (A _c-A_co), whereas a tendency exists that with increasing relative molecular hydrogen abundance the number of 4430 Å absorbers per hydrogen atom decreases. If the carriers of the 4430 Å absorbers are interstellar grains, then these grains must be altered during the same process in which molecular hydrogen is built. The found correlation is also compatible with the assumption that the 4430 Å absorber is related to an interstellar gaseous species.     

High resolution scans of Comet Kohoutek in the vicinity of 5015, 5890, and 6563 Å

High resolution scans were made of Comet Kohoutek (1973f) using the McMath solar telescope at Kitt Peak National Observatory. The data were taken on January 1 and 4, 1974 UT, just after the comet perihelion. Hα emission (～4.1 × 10²⁷ photon sec^?1) was observed from the head of the Comet. An upper limit on the He I(5015) radiation was determined to be less than 2% of the observed Hα emission. The Na D1/D2 line intensities on both nights were approximately 0.5, indicating an optically thin emission region.     

IUE detector saturation and the new 2800 Å absorption featre ‘discovered’ by Karim,Hoyle, and Wickramasinghe

The broad 2800 Å absorption feature discovered by Karim, Hoyle, and Wickramasingle is instrumental in origin. The feature is produced by IUE detector saturation effects in overexposed spectra.     

Periodic variations of the Ba II 4554 Å and Ca II 8542 Å line profiles in coronal holes

The oscillations of the half-width of the Ba II 4554 ? and Ca II 8542 ? spectral lines have been analyzed using observations at the base of solar coronal holes (CHs). The observed variations (~50 m ? for Ca II and ~4 m ? for Ba II) exceed considerably the thermal broadenings of these lines calculated from the measured intensity oscillations, suggesting their nonthermal nature. We point out a number of observational facts that hamper an unambiguous interpretation of the periodic Ba II and Ca II profile variations solely by the manifestation of torsional Alfve´ n waves in the lower solar atmosphere.     

Spectropolarimetry of the methane and ammonia bands of Jupiter between 6800 and 8200 Å

Spectropolarimetry of Jupiter at resolutions between 22 and 35 Å reveals a strong increase of linear polarization in the 7250-$\text{A}\text{?}$ CH₄ band. This is very probably due to the decreasing contribution toward the band center of the higher orders of scattering, which have a smaller net polarization than the first few orders. The linear polarization is also enhanced in the band at 7900 $\text{A}\text{?}$ comprising the 7920-$\text{A}\text{?}$ NH₃ and 7600- to 8200-$\text{A}\text{?}$ CH₄ bands. The normalized circular polarization shows a feature at 7250 $\text{A}\text{?}$ with a dispersion shape. This is most probably produced in a double-scattering process involving either a solid or liquid aerosol with an absorption at 7250 $\text{A}\text{?}$. Methane aerosols, the obvious candidates from a spectroscopic point of view, are, however, forbidden if current estimates of the Jovian atmospheric temperature are correct.     

Infrared spectral variations of three Mira variable carbon stars with the 11.3 μm SiC feature

The spectral variations of three Mira variable carbon stars, V CrB, T Dra and V Cyg in the infrared are investigated based on ISO SWS data. It is found that either continua or molecular/dust features were variable with time in the infrared for these carbon stars during one and a half year observations. When stars were brighter the infrared continuum spectra became blue while stars were fainter the infrared continuum spectra became red. In addition, during spectral variations there were the correlation between the 3.05 μm HCN+C₂H₂ and the 5.2 μm C₃ molecular band strengths and the anti-correlation between the 3.05 μm HCN+C₂H₂ molecular band strengths and 13.7 μm C₂H₂ band strengths while during variations the 11.3 μm SiC dust emission strengths were not clearly changed.     

Changes of the CaI λ6102.7 Å line profile and the magnetic field structure during the August 12, 1981 solar flare

We study the changes of the CaI λ6102.7 Å line profile and the magnetic field structure during the 1B/M2.2 while-light flare of August 12, 1981. The two brightest flare knots located in the penumbra of a sunspot with a δ configuration are investigated. The 1 ± V line profiles are analyzed. The reduction and analysis of our observations have yielded the following results. (1) The line profiles changed significantly during the flare, especially at the time of optical continuum emission observed near the flare maximum. In addition to the significant decrease in the depth, a narrow polarized emission whose Zeeman splitting corresponded to a longitudinal magnetic field strength of 3600 Gs was observed. This is much larger than the magnetic field strength in the underlying sunspot determined from the Zeeman splitting of absorption lines. (2) The largest changes of the CaI λ6102.7 Å line profile observed during the flare can lead to an underestimation of the longitudinal magnetic field strength measured with a video magnetograph by a factor of 4.5, but they cannot be responsible for the polarity reversal. (3) A sharp short-term displacement of the neutral line occurred at a time close to the flare maximum, which gave rise to a reversed-polarity magnetic field on a small area of the active region, i.e., a magnetic transient. This can be interpreted as a change in the inclination of the magnetic field lines to the line of sight during the flare. The short-term depolarization of the CaI λ6102.7 Å line emission observed at the other flare knot can also be the result of a change in the magnetic field structure. (4) These fast dynamic changes of the magnetic field lines occurred after the maximum of the impulsive flare phase and were close in time to the appearance of type II radio emission.