5 research outputs found
High-Resolution Infrared Spectroscopic Measurements of Comet 2PlEncke: Unusual Organic Composition and Low Rotational Temperatures
We present high-resolution infrared spectroscopic measurements of the ecliptic comet 2P/Encke, observed on 4-6 Nov. 2003 during its close approach to the Earth, using the Near Infrared Echelle Spectrograph on the Keck II telescope. We present flux-calibrated spectra, production rates, and mixing ratios for H2O, CH3OH, HCN, H2CO, C2H2, C2H6, CH4 and CO. Comet 2P/Encke is a dynamical end-member among comets because of its short period of 3.3 years. Relative to "organics-normal" comets, we determined that 2PlEncke is depleted in HCN, H2CO, C2H2, C2H6, CH4 and CO, but it is enriched in CH3OH. We compared mixing ratios of these organic species measured on separate dates, and we see no evidence of macroscopic chemical heterogeneity in the nucleus of 2P/Encke, however, this conclusion is limited by sparse temporal sampling. The depleted abundances of most measured species suggest that 2P/Encke may have formed closer to the young Sun, before its insertion to the Kuiper belt, compared with "organics-normal" comets - as was previously suggested for other depleted comets (e.g. C/1999 S4 (LINEAR)). We measured very low rotational temperatures of 20 - 30 K for H2O, CH3OH and HCN in the near nucleus region of 2P/Encke, which correlate with one of the lowest cometary gas production rates (approx. 2.6 x 10(exp 27) molecules/s) measured thus far in the infrared. This suggests that we are seeing the effects of more efficient radiative cooling, insufficient collisional excitation, and/or inefficient heating by fast H-atoms (and icy grains) in the observed region of the coma. Its extremely short orbital period, very low gas production rate, and classification as an ecliptic comet, make 2PlEncke an important addition to our growing database, and contribute significantly to the establishment of a chemical taxonomy of comets
The Peculiar Volatile Composition of Comet 8P/Tuttle: A Contact Binary of Chemically Distinct Cometesimals?
We report measurements of eight native (i.e., released directly from the
comet nucleus) volatiles (H2O, HCN, CH4, C2H2, C2H6, CO, H2CO, and CH3OH) in
comet 8P/Tuttle using NIRSPEC at Keck 2. Comet Tuttle reveals a truly unusual
composition, distinct from that of any comet observed to date at infrared
wavelengths. The prominent enrichment of methanol relative to water contrasts
the depletions of other molecules, especially C2H2 and HCN. We suggest that the
nucleus of 8P/Tuttle may contain two cometesimals characterized by distinct
volatile composition. The relative abundances C2/CN, C2/OH, and CN/OH in
8P/Tuttle (measured at optical/near-UV wavelengths) differ substantially from
the mixing ratios of their potential parents (C2H2/HCN, C2H2/H2O, and HCN/H2O)
found in this work. Based on this comparison, our results do not support C2H2
and HCN being the principal precursors for respectively C2 and CN in Tuttle.
The peculiar native composition observed in 8P/Tuttle (compared to other
comets) provides new strong evidence for chemical diversity in the volatile
materials stored in comet nuclei. We discuss the implications of this diversity
for expected variations in the deuterium enrichment of water among comets.Comment: Accepted for Astrophysical Journal Letter
Cometary diversity and cometary families
Comets are classified from their orbital characteristics into two separate
classes: nearly-isotropic, mainly long-period comets and ecliptic, short-period
comets. Members from the former class are coming from the Oort cloud. Those of
the latter class were first believed to have migrated from the Kuiper belt
where they could have been accreted in situ, but recent orbital evolution
simulations showed that they rather come from the trans-Neptunian scattered
disc. These two reservoirs are not where the comets formed: they were expelled
from the inner Solar System following interaction with the giant planets. If
comets formed at different places in the Solar System, one would expect they
show different chemical and physical properties. In the present paper, I review
which differences are effectively observed: chemical and isotopic compositions,
spin temperatures, dust particle properties, nucleus properties... and
investigate whether these differences are correlated with the different
dynamical classes. The difficulty of such a study is that long-period,
nearly-isotropic comets from the Oort cloud are better known, from Earth-based
observations, than the weak nearly-isotropic, short-period comets. On the other
hand, only the latter are easily accessed by space missions.Comment: Proceedings of the XVIIIemes Rencontres de Blois: Planetary Science:
Challenges and Discoveries, 28th May - 2nd June 2006, Blois, Franc
A Newly Developed Fluorescence Model for C2H6 v5 and Application to Cometary Spectra Acquired with NIRSPEC at Keck II
Accurate rotational temperatures are essential for extracting production rates for parent volatiles in comets. Two strong bands of ethane (v7 at 2985.39/cm and v5 at 2895.67/cm) are seen in infrared cometary spectra, but the Q-branches of v7 are not resolved by current instruments and cannot provide an accurate rotational temperature with current models.We developed a fluorescence model for the C2H6 v5 band that can be used to derive a rotational temperature.We applied our C2H6 5 model to high-resolution infrared spectra of the comets C/2004 Q2 Machholz and C/2000 WM1 (LINEAR), acquired with the Near-infrared Echelle Spectrograph on the Keck II telescope. We demonstrate agreement among the rotational temperatures derived from C2H6 v5 and other species, and between mixing ratios derived from C2H6 v5 and C2H6 v7. As a symmetric hydrocarbon, C2H6 is observed only in the infrared, and it is now the fifth molecule (along with H2O, HCN, CO, and H2CO) for which we can derive a reliable rotational temperature from cometary infrared spectra