P. D. Singh

The FeH Wing-Ford Band in Spectra of M Stars*

We study the FeH Wing-Ford band at 9850-10200 A by fitting synthetic spectra to the observations of M stars, employing recent model atmospheres. On the basis of the spectral synthesis, we analyze the dependence of the band upon atmospheric parameters. FeH lines are a very sensitive surface gravity indicator, being stronger in dwarfs. They are also sensitive to metallicity. The blending with CN lines, which are stronger in giants, does not affect the response of the Wing-Ford band to surface gravity at low resolution (or high velocity dispersions), because CN lines, which are spread all along the spectrum, are smeared out at convolutions of FWHM 3 A. We conclude that the Wing-Ford band is a suitable dwarf/giant indicator for the study of composite stellar populations.

Dust release rates and dust-to-gas mass ratios of eight comets

Mass release rates of dust and mass ratios of dust-to-gas release rates of Comets Thiele (1985m), Wilson (1986l), P/Borrelly (1987p), Liller (1988a), Bradfield (1987s), Hartley-Good (1985l), P/Giacobini-Zinner (1984e), and P/Halley (1982i) are estimated from the analysis of continuum flux measurements at optical wavelengths. An attempt is made to estimate the size of each comet nucleus on the basis of water-ice sublimation (vaporization), assuming that the nucleus is spherical and only a fraction of its surface area is active. Where possible, the dust mass release rates are compared with those obtained by other investigators in the optical and IR wavelength regions. Good agreement with results based on IR observations is found.

Photodissociation lifetime of 32S2 molecule in comets

Photodissociation lifetime of 32s2in comets is calculated by absorption of solar photons into the B3Σ− state and velocity distributions of sulphur atoms are determined. Absorption of solar photons of wavelength ~ 280 nm leads to a photodissociation lifetime of about 250 s for 32S2 molecule in comets when sun-comet distance is 1 AU. Forbidden lines corresponding to 1D-3P transitions of neutral sulphur atom may be detectable at about 11 306 and 10 821 Å in comets. The production rate of 32S2 dimer in comet IRAS-Araki-Alcock 1983d compares well with the production rate of CS, observed in comet Bradfield, when compared at the same heliocentric distance. The chemistry of 32S2 dimer formation in the inner coma of a comet is discussed in the framework of some gas phase reactions.

Water release rates, active areas, and minimum nuclear radius derived from visual magnitudes of comets—an application to comet 46P/Wirtanen

Abstract A semi-empirical photometric method is developed to estimate water release rates from visual magnitude measurements of “normal” comets of abundance ratio C 2 :CN ≈ 1.46 ± 0.68 for the heliocentric distance range 2.5-0.5 AU for which the aperture of the instrument, projected on the coma, is larger than the effective scale length of C 2 . The method is applied to comets Austin (1982 VI), Bradfield (1979 X), Bradfield (1980 XV), Bradfield (1987 XXIX), P/Brorsen-Metcalf (1989 X), Levy (1990 XX), Liller (1988 V), and 46P/Wirtanen. The water release rates are compared with those derived from observed OH or OI (6300 A) line fluxes. The H 2 O release rates estimated from visual magnitude measurements may have typical errors of about 2–4, while maximum errors may be as high as a factor of 10. The total active area ( AA ) on the sunlit side of the nucleus has been estimated from derived water release rates combined with theoretical estimates of H 2 O flux rates (molecules cm −2 s −1 ) by vaporization theory for an assumed nuclear visual albedo p v = 0.03. The constraint AA ≤ 2 π ( R N ) 2 has been used for estimation of the lower limit of the nuclear radius of a comet. For comet 46P/Wirtanen, the variation of water release rates with heliocentric distance ( r ), total active area, lower limit of nuclear radius, and fractional active area are: Q (H 2 O) = 1.8 × 10 28 r −7.0±0.4 mols −1 , 6 ± 3 km 2 , 1.0 km, and 0.15, respectively.

THE FORMATION OF CN AND CN' BY DIRECT RADIATIVE ASSOCIATION

Rate coefficients for direct radiative association of carbon and nitrogen atoms to form CN, and of carbon ions and nitrogen atoms to form CN + ions, are calculated for temperatures in the range of 300 to 14,700 K. For the CN molecule, the rate coefficients can be represented by the standard expression, kCN(T) = 7.87 × 10-19(T/300)0.056 exp(-96.0/T) cm3 s-1 for temperatures between 300 and 2700 K and kCN(T) = 1.37 × 10-18(T/300)-0.128 exp(-520.1/T) cm-3 s-1 at T > 2700 K. For the CN+ ion, the corresponding expression is k(T) = 1.08 × 10-18(T/300)0.071 exp(-57.5/T) cm-3 s-1 for the temperature range studied. Calculated rate coefficients kCN are about 2 orders of magnitude lower than the canonical value used in the modeling of the chemistry of various astrophysical environments.

The photodissociation lifetimes of the OH and OD radicals in comets

Abstract The rates of photodissociation of the OH and OD molecules from absorption of solar radiation in he X 2 Π-A 2 Σ + electronic transition are calculated to lie between 3.5 and 6.7 × 10 −6 sec −1 for OH for heliocentric velocities between -60 and +60 km sec −1 and to be about 4.7 × 10 −7 sec −1 for OD at 1 AU from the Sun. The corresponding lifetimes, which are upper bounds to the actual lifetimes, are generally consistent with the observational cometary data.

Predictions on finding the SiH+ and SiH in interstellar space

Frequencies are computed for transitions in the SiH+ and SiH molecules which are expected to occur in the millimeter region usually covered by radiowave astronomy. These molecules are expected to be in low-temperature sources whose state temperatures are not greater than about 20 K. On the basis of binary gas-phase reactions, the formation and destruction of the SiH+ and the SiH radicals are also discussed.

Haser model CN, C2 and C3 production rates in some comets

Emission fluxes of CN, C2 and C3 species observed in the coma of some comets are analysed in the framework of Haser model. CN, C2 and C3 production rates are determined using recently derived fluorescence efficiencies and dependence of CN, C2 and C3 production rates on the heliocentric distance is studied. Evidence for a burst type activity around January 15, 1974 in the post-perihelion period of comet Kohoutek (1973f) is observed.

Gas and dust release rates and color of dust in comets P/Halley (1986 III), P/Giacobini-Zinner (1985 XIII), and P/Hartley-Good (1985 XVII)

Abstract CN, C2, and C3 emission fluxes observed at wavelengths of 388, 516, and 405 nm in the spectra of the comae of comets P/Halley (1986 III), P/Giacobini-Zinner (1985 XIII), and P/Hartley-Good (1985 XVII) were analyzed within the framework of the Haser model (Haser, Bull. Acad. R. Belg., Classe de Sciences 43, 740, 1957). Column densities and production rates of these gaseous species are determined and compared with those from other observations. For pre-perihelion heliocentric distances it is found that production rates/expansion velocity ( Q ν ) for species vary proportional to r−n, where n is 3.7 ± 0.8, 1.7 ± 0.7, and 4.7 ± 1.4 for CN, C2, and C3, respectively. Continuum fluxes measured at wavelengths of 484.5 and 412.0 nm were analyzed, and dust release rates in the coma of these comets are determined. The dust release rates decrease with increase of heliocentric distance for comet Halley, and follow a r−3.0±0.7 power law. Color excesses of grains were determined in comets P/Halley (1986 III), P/Giacobini-Zinner (1985 XIII), and P/Hartley-Good (1985 XVII).

Atomic Emission Lines in the Spectrum Observed after the Impact between the L-Fragment of Comet D/Shoemaker-Levy 9 and Jupiter

Emission lines of neutral Na, Fe, Ca, and Li atoms were detected in the wavelength range 575-700 nm in the spectra of the plume produced by the L-fragment impact of comet D/Shoemaker-Levy 9 with Jupiter. If the fluorescence mechanism is responsible for excitation of the Na I and Li I resonance lines, then the ratio Li/Na = (1.2 ± 0.3) × 10-3 is quite compatible with meteoritic values. A lower limit of about 1013 g was estimated for the mass of the L-fragment.