High-resolution (R~60,000) spectra of the CN BSUP2/SUPSigmaSUP+/SUP-XSUP2/SUPSigmaSUP+/SUP (0, 0) band (near 3880 Å) in the Halley-type comet 122P/1995 S1 de Vico (with a period of 74 yr) and the ...``intermediate-period'' comet 153P/2002 C1 Ikeya-Zhang (P~370 yr) were obtained with the 2dcoudé spectrograph at the 2.7 m Harlan J. Smith telescope of the McDonald Observatory. The comets were within 1 AU from the Sun (0.66 and 0.92 AU, respectively) at the time of the observations. While the measured SUP12/SUPC/SUP13/SUPC isotope ratios of both comets (90+/-10 and 90+/-25, respectively) are in very good agreement with the solar system value, the SUP14/SUPN/SUP15/SUPN ratios (140+/-20 and 170+/-50, respectively) are approximately half the value in Earth's atmosphere. The similarity is striking between these ratios and those obtained recently for two other long-period Oort Cloud comets, C/1995 O1 (Hale-Bopp) and C/2000 WM1 (LINEAR). While these optical determinations of SUP14/SUPN/SUP15/SUPN are consistent with each other, they disagree with those obtained in comet Hale-Bopp from submillimeter measurements of HCN, generally believed to be the main parent of CN. This puzzling difference points toward the existence of (an)other unknown parent(s) of CN, with an even higher SUP15/SUPN excess. Organic compounds like those found in interplanetary dust particles are good candidates.
Theoretical resonance fluorescence calculations are presented of the triatomic C3 radical and are compared with observations of the C3 emission in comets Hale-Bopp and de Vico. A theoretical model of ...the C3 vibration-rotational structure in the $A^1\Pi_u - X^1\Sigma_g^+$ electronic system is introduced. The model takes into account the detailed structure of the bending mode $\nu_2$ which is responsible for the emission of the 4050 Ågroup. A total of 1959 levels are considered, with 515 levels in the ground state. The main effort is to model high-resolution spectra of the 4050 Å emission in comets C/1995 O1 Hale-Bopp and 122P/1995 S1 de Vico. The agreement between observed and theoretical spectra is good for a value of the dipole moment derivative of ${{\rm d}\mu\over {\rm d}r} \approx 2.5$ Debye Å-1. The modeled C3 emission exhibits a pronounced Swings effect.
We observed comet C/1995 O1 (Hale-Bopp) at 4.6–2.9 AU pre-perihelion and 2.8–12.8 AU post-perihelion with optical long-slit spectroscopy. Emission bands of CN, C3, C2 and NH2 have been covered. ...Emission of C3 was detected up to 7.0 AU, and CN could be followed up to 9.8 AU post-perihelion. Spatial column density profiles of the radicals have been used to derive effective parent Haser scale lengths for heliocentric distances beyond 3 AU. Production rates were derived based on these Haser scale lengths. The observations of CN are in agreement with HCN as the major parent molecule of this radical at large distances from the Sun (i.e. beyond ~3 AU). We compare the measured CN production rate to sublimation rates of HCN from a simple nucleus sublimation model. The variation of CN production rates with changing heliocentric distance gives no indication for sublimation from the interior and is consistent with very little thermal lag of the nucleus.
The Al-to-Mg abundance ratio is redetermined in two extreme metal-poor stars, HD 19445 and HD 140283, on the basis of accurate spectral material. It is found that: (1) one of the two lines used for ...the Al abundance determination is blended by CH lines; (2) no anomalous difference appears between the hotter and the cooler star, both showing an Al overdeficiency; (3) the observations are in fair agreement with predictions from the theory of explosive burning.
Laboratory data taken from the laser-induced fluorescence (LIF) spectrum of C
2(X
1
Σ
g
+) radicals produced during the 193 nm laser photolysis of C
2H
2 has been used to explain the band profiles of ...the Hubble Space Telescope (HST) on nucleus spectra of the C
2 Mulliken system (X
1
Σ
g
+ ← D
1
Σ
u
+) in comet Hyakutake (C/1996 B2). The authors propose, using laboratory data and
ab initio calculations, that photolysis of C
2H
2 in the laboratory and in comets occurs via a sequential mechanism, first producing C
2H and then C
2. The
ab initio calculations were used to identify the two excited electronic states (2
2
Σ
+ and 2
2Π) of C
2H through which photodissociation in the second step proceeds. In both the laboratory and cometary studies, the photodissociation of C
2H appears to proceed via the same electronic surfaces. The C
2 Mulliken bands obtained in the laboratory and the HST observations of these bands are used to explain the internal energy distribution and origin of C
2(X
1
Σ
g
+) in comet Hyakutake. The off nucleus HST spectrum at 16 arcsec shows that the C
2 radicals are much colder vibrationally than they are on the nucleus. This vibrational cooling is inconsistent with literature reports and is discussed in the text.
New independent constraints on the amount of water delivered to Earth by comets are derived using the 15N/14N isotopic ratio, measured to be roughly twice as high in cometary CN and HCN as in the ...present Earth. Under reasonable assumptions, we find that no more than a few percent of Earth's water can be attributed to comets, in agreement with the constraints derived from D/H. Our results also suggest that a significant part of Earth's atmospheric nitrogen might come from comets. Since the 15N/14N isotopic ratio is not different in Oort-cloud and Kuiper-belt comets, our estimates apply to the contribution of both types of objects.
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