Carbon monoxide in the distantly active Centaur (60558) 174P/Echeclus at 6 AU

(60558) 174P/Echeclus is an unusual object that belongs to a class of minor planets called Centaurs, which may be intermediate between Kuiper Belt Objects and Jupiter Family comets. It is sporadically active throughout its orbit at distances too far for water ice to sublimate, the source of activity for most comets. Thus, its coma must be triggered by another mechanism. In 2005, Echeclus had a strong outburst with peculiar behavior that raised questions about the nucleus homogeneity. In order to test nucleus models, we performed the most sensitive search to date for the highly volatile CO molecule via its J=2-1 emission toward Echeclus during 2016 May-June (at 6.1 astronomical units from the Sun) using the Arizona Radio Observatory 10-m Submillimeter Telescope. We obtained a 3.6-sigma detection with a slightly blue-shifted (delta v = -0.55 +- 0.1 km/s) and narrow (FWHM = 0.53 +- 0.23 km/s) line. The data are consistent with emission from a cold gas from the sunward side of the nucleus, as seen in two other comets at 6 AU. We derive a production rate of Q(CO) = (7.7 +- 3.3)x10^26 mol/s, which is capable of driving the estimated dust production rates. Echeclus CO outgassing rate is ~40 times lower than what is typically seen for another Centaur at this distance, 29P/Schwassmann-Wachmann 1. We also used the IRAM 30-m telescope to search for the CO J=2-1 line, and derive an upper limit that is above the SMT detection. Compared to the relatively unprocessed comet C/1995 O1 (Hale-Bopp), Echeclus produces significantly less CO, as do Chiron and four other Centaurs.Comment: 27 pages, 3 figures, 1 table. Accepted for publication in The Astronomical Journa

A search for interstellar CH3D: Limits to the methane abundance in Orion-KL

A search has been performed for interstellar CH3D via its J(K) = 1(0) - 0(0) transition at 230 GHz and its J(K) = 2(0) - l(0) and J(K) = 2(1) - 1(1) lines at 465 GHz using the NRAO 12 m and CSO 10 m telescopes towards Orion-KL. This search was done in conjunction with laboratory measurements of all three transitions of CH3D using mm/sub-mm direct absorption spectroscopy. The molecule was not detected down to a 3 sigma level of T(A) less than 0.05 K towards Orion, which suggests an upper limit to the CH3D column density of N less than 6 x 10(exp 18)/sq cm in the hot core region and a fractional abundance (with respect to H2) of less than 6 x 10(exp -6). These measurements suggest that the methane abundance in the Orion hot core is f less than 6 x 10-4, assuming D/H approximately 0.01. Such findings are in agreement with recent hot core chemical models, which suggest CH4/H2 approximately 10(exp -4)

Impact Rates in the Outer Solar System

Previous studies of cometary impacts in the outer Solar System used the spatial distribution of ecliptic comets (ECs) from dynamical models that assumed ECs began on low-inclination orbits (<5 deg) in the Kuiper belt. In reality, the source population of ECs - the trans-Neptunian scattered disk - has orbital inclinations reaching up to ~30 deg. In Nesvorny et al. (2017), we developed a new dynamical model of ECs by following comets as they evolved from the scattered disk to the inner Solar System. The model was absolutely calibrated from the population of Centaurs and active ECs. Here we use our EC model to determine the steady-state impact flux of cometary/Centaur impactors on Jupiter, Saturn, Uranus, and their moons. Relative to previous work (Zahnle et al. 2003), we find slightly higher impact probabilities on the outer moons and lower impact probabilities on the inner moons. The impact probabilities are smaller when comet disruption is accounted for. The results provide a modern framework for the interpretation of the cratering record in the outer Solar System.Comment: PSJ, in pres

NH3 and NH2 in the coma of Comet Brorsen-Metcalf

Evidence consistent with NH3 ice in the nucleus of Comet Brorsen-Metcalf as the source of the NH2 observed in the comet coma is presented. The distribution of NH2 is symmetric and shows no evidence for jet structure at the 3-sigma significance level above background emission. An azimuthal average of the NH2 image produces an NH2 surface brightness profile for Comet Brorsen-Metcalf which yields a factor of about-10 improvement in the signal-to-noise ratio over previous 1D long-slit NH2 observations, and provides a significant constraint on the NH2 photodissociation time scale in comets. A Monte Carlo simulation of the comet coma, assuming that NH2 is the primary source of NH2, is described and compared with the observations. For an observed production rate, Q(H2O) is approximately equal to 7 x 10 exp 28 molecules/s, collisional effects on the NH3 and NH2 outflow had at most an approximately 10-percent effect on the NH2 surface brightness profile. Because Comet Brorsen-Metcalf showed no significant dust or gas production rate variability, it is argued that steady state conditions best match the comet at the time of the observations

Ion abundances and implications for photochemistry in comets Halley (1986 III) and Bradfield (1987 XXIX).

Spectra of the plasma tails of comets P/Halley (1986 III) and P/Bradfield (1987 XXIX, also 1987s) were recorded using the Ohio State University Image Dissector Scanner (IDS) on the Perkins 1.8 m telescope at the Lowell Observatory. The ionic species CO+, N2+, CH+, and H2O+ were identified in these spectra and column densities for them were calcd. from measured fluxes. The obsd. abundance ratios of CO+/H2O+ in comets Halley and Bradfield are consistent with predictions by photochem. models, provided that CO+ and H2O+ are produced primarily from photoionization of CO and H2O, resp. However, the obsd. N2+/H2O+ ratios are at least an order of magnitude lower and the obsd. CH+/H2O+ ratios are a factor of 100 higher, than theor. results. The abundance ratio N2/CO was derived in the plasma tail of Bradfield from N2+ and CO+ data, and found to be an order of magnitude higher than a value measured in Halley. The relative ion abundances of CH+, N2+, and H2O+ in Halley are consistent with in situ measurements obtained from the Giotto spacecraft. Fluorescence efficiencies have been recalcd. for optical transitions of N2+, CH+, NH, CH, OH+, and CO and are also presented. [on SciFinder(R)

Pre- and postperihelion abundances of gas and dust in comet Halley.

Photometrically calibrated spectra of comet P/Halley (1986 III) were recorded between 1985 Sept. 12-1986 June 10 using the Ohio State University Image Dissector Scanner on the Perkins 72 in. telescope at the Lowell Observatory. Column densities of CN, C3, CH, C2, and NH2 were calcd. from measured fluxes in these spectra, and mol. scale lengths were deduced from the radial distribution of CN, C3, C2, and NH2. Prodn. rates were computed using the new scale lengths and a Haser model anal. Continuum emission at 4260 Å was used to derive gas-to-dust ratios. The data indicate than comet Halley was ∼2-5 times more abundant in gas and dust at postperihelion than preperihelion. On 1986 June 8 the authors obsd. the onset of a cometary outburst which appeared very strong in dust prodn. The gas-to-dust ratios appeared to be subject to changes as a result of short-term outbursts but otherwise did not exhibit any systematic dependence on heliocentric distance. Reflectivity gradients of the continuum were also measured from the spectra. While most of the continua were red, blue continua were also obsd. which may be correlated with dust outbursts. [on SciFinder(R)

Unveiling the chemical fingerprint of phosphorus-rich stars I. In the infrared region of APOGEE-2

The origin of phosphorus, one of the essential elements for life on Earth, is currently unknown. Prevalent models of Galactic chemical evolution (GCE) underestimate the amount of P compared to observations. The recently discovered P-rich ([P/Fe] > 1 dex) and metal-poor giants further challenge current theories on stellar nucleosynthesis. Since the observed stars are low-mass giants, our primary goal is to find clues on their progenitor. By increasing the number of known P-rich stars, we aim to narrow down a reliable chemical abundance pattern and to place robust constraints on the responsible nucleosynthetic mechanism. In the long term, identifying the progenitor of the P-rich stars may contribute to the search for the source of P in our Galaxy. We performed a detailed chemical abundance analysis based on the H-band spectra from APOGEE-2 (DR17). Employing the BACCHUS code, we measured the abundances of 13 elements in the sample, which is mainly composed of a recent collection of Si-enhanced giants. We also analyzed the orbital motions and compared the abundance results to possible nucleosynthetic formation scenarios, and also to detailed GCE models. We enlarged the sample of confirmed P-rich stars from 16 to 78 giants, which represents the largest sample of P-rich stars to date. Significant enhancements in O, Al, Si and Ce, as well as systematic correlations among the elements, unveil the chemical fingerprint of the P-rich stars. The high Mg and C+N found in some of the P-rich stars with respect to P-normal stars is not confirmed over the full sample. Strikingly, the strong over-abundance in the $\alpha$-element Si is accompanied by normal Ca and S abundances. Our analysis of the orbital motion showed that the P-rich stars do not belong to a specific sub-population. In addition, we confirm that the majority of the sample stars are not part of binary systems.Comment: 29 pages, 18 figures, 8 tables, accepted for publication in Astronomy & Astrophysic

The Distribution, Excitation and Formation of Cometary Molecules: Methanol, Methyl Cyanide and Ethylene Glycol

We present an interferometric and single dish study of small organic species toward Comets C/1995 O1 (Hale-Bopp) and C/2002 T7 (LINEAR) using the BIMA interferometer at 3 mm and the ARO 12m telescope at 2 mm. For Comet Hale-Bopp, both the single-dish and interferometer observations of CH3OH indicate an excitation temperature of 105+/-5 K and an average production rate ratio Q(CH3OH)/Q(H2O)~1.3% at ~1 AU. Additionally, the aperture synthesis observations of CH3OH suggest a distribution well described by a spherical outflow and no evidence of significant extended emission. Single-dish observations of CH3CN in Comet Hale-Bopp indicate an excitation temperature of 200+/-10 K and a production rate ratio of Q(CH3CN)/Q(H2O)~0.017% at ~1 AU. The non-detection of a previously claimed transition of cometary (CH2OH)2 toward Comet Hale-Bopp with the 12m telescope indicates a compact distribution of emission, D<9'' (<8500 km). For the single-dish observations of Comet T7 LINEAR, we find an excitation temperature of CH3OH of 35+/-5 K and a CH3OH production rate ratio of Q(CH3OH)/Q(H2O)~1.5% at ~0.3 AU. Our data support current chemical models that CH3OH, CH3CN and (CH2OH)2 are parent nuclear species distributed into the coma via direct sublimation off cometary ices from the nucleus with no evidence of significant production in the outer coma.Comment: accepted for publication in Ap