Cometary Science Newsletter

December 2017
Michael S. P. Kelley (

Refereed Articles

Abstracts of articles in press or recently published. Limited to 3000 characters.

Measuring molecular abundances in comet C/2014 Q2 (Lovejoy) using the APEX telescope

  • M. de Val-Borro 1,2
  • S. N. Milam 1
  • M. A. Cordiner 1,2
  • S. B. Charnley 1
  • I. M. Coulson 3
  • A. J. Remijan 4
  • G. L. Villanueva 1
  1. NASA Goddard Space Flight Center, Astrochemistry Laboratory, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
  2. Department of Physics, Catholic University of America, Washington, DC 20064, USA
  3. East Asian Observatory, Hilo, HI 96720, USA
  4. National Radio Astronomy Observatory, Charlottesville, VA 22903, USA

Comet composition provides critical information on the chemical and physical processes that took place during the formation of the Solar System. We report here on sub-millimeter spectroscopic observations of the long-period bright comet C/2014 Q2 (Lovejoy) using the Atacama Pathfinder Experiment (APEX) band 1 receiver between UT 16.948 to 18.120 January 2015, when the comet was at heliocentric distance of 1.30 au and geocentric distance of 0.53 au. Bright comets allow for sensitive observations of gaseous volatiles that sublimate in their coma. These observations allowed us to detect HCN, CH3OH (multiple transitions), H2CO and CO, and to measure precise production rates Additionally, sensitive upper limits were derived on the complex molecules acetaldehyde (CH3CHO) and formamide (NH2CHO) based on the average of the strongest lines in the targeted spectral range to improve the signal-to-noise ratio. Gas production rates are derived using a non-LTE molecular excitation calculation involving collisions with H2O and radiative pumping that becomes important in the outer coma due to solar radiation. We find a depletion of CO in C/2014 Q2 (Lovejoy) with a production rate relative to water of 1.9%, and relatively low abundances of Q(HCN)/Q(H2O), 0.09%, and Q(H2CO)/Q(H2O), 0.3%. In contrast the CH3OH relative abundance Q(CH3OH)/Q(H2O), 2.4%, is close to the mean value observed in other comets. The measured production rates are consistent with values derived for this object from other facilities at similar wavelengths taking into account the difference in the fields of view. Based on the observed mixing ratios of organic molecules in four bright comets including C/2014 Q2 (Lovejoy), we find some support for atom addition reactions on cold dust being the origin of some of the molecules.

Monthly Notices of the Royal Astronomical Society (In press)

DOI: 10.1093/mnras/stx2802 NASA ADS: arXiv:1710.11117 arXiv: 1710.11117

Prediscovery Observations and Orbit of Comet C/2017 K2 (PANSTARRS)

  • Man-To Hui 1
  • David Jewitt 1,2
  • David Clark 3,4
  1. Department of Earth, Planetary and Space Sciences, UCLA
  2. Department of Physics and Astronomy, UCLA
  3. Department of Physics and Astronomy, UWO
  4. Department of Earth Sciences, UWO

We present a study of comet C/2017 K2 (PANSTARRS) using prediscovery archival data taken from 2013 to 2017. Our measurements show that the comet has been marginally increasing in activity since at least 2013 May (heliocentric distance of rH = 23.7 AU pre-perihelion). We estimate the mass-loss rate during the period 2013–2017 as dM/dt ≈ (2.4 ± 1.1) × 102 kg s-1, which requires a minimum active surface area of ~10–102 km2 for sublimation of supervolatiles such as CO and CO2, by assuming a nominal cometary albedo pV = 0.04 ± 0.02. The corresponding lower limit to the nucleus radius is a few kilometers. Our Monte Carlo dust simulations show that dust grains in the coma are ≳ 0.5 mm in radius, with ejection speeds from ~1–3 m s-1, and have been emitted in a protracted manner since 2013, confirming estimates by Jewitt et al. (2017). The current heliocentric orbit is hyperbolic. Our N-body backward dynamical integration of the orbit suggests that the comet is most likely (with a probability of ~98%) from the Oort spike. The calculated median reciprocal of the semimajor axis 1 Myr ago was amed-1 = (3.61 ± 1.71) × 10-5 AU-1 (in a reference system of the solar-system barycentre).

Astronomical Journal (In press)