Radio OH Observations of Recent Bright Comets from Arecibo

We obtained OH spectra of recent comets with the Arecibo 305m radiotelescope. C/2012 X1 LINEAR was observed between 03 November 2013 and 13 January 2014, C/2014 E2 Jacques on 14 dates between 10 May and 31 July 2014, and C/2012 K1 PANSTARRS on 12 dates between 16 June and 23 August 2014. Spectra at 1667 and 1665 MHz (18cm wavelength) were obtained with an on-sky beam size of 2.9 arcminutes, mapping 7 positions of the OH within 4 arcminutes of the nucleus when the coma is sufficiently large. Radio OH spectra are seen via a Λ-doublet, with the excitation of the lines depending on the heliocentric velocity of the comet. We interpret the spectra via a Monte Carlo model, taking into account the OH inversion predictions of Despois et al. (1981) or Schleicher & A'Hearn (1988). In highly productive comets, high densities thermalize the lines, reducing the line strength near the nucleus. Models of mapping observations can directly constrain the radius within which quenching is active, and thus yield a more accurate estimate of the gas production rate, while radio observations at high spectral resolution place excellent constraints on the gas outflow velocity whether or not the coma is resolved. We present gas production rates, quenching radius estimates and outflow velocities for comets C/2012 X1 LINEAR, C/2014 E2 Jacques and C/2012 K1 PANSTARRS. Near its noteworthy sungrazing perihelion, comet C/2012 S1 ISON presented too small a gas coma for mapping observations, so we present only estimated gas production rates and outflow velocities for these unresolved observations.