Hydrogen Isocyanide in Comet 73P/Schwassmann-Wachmann (Fragment B)

We present a sensitive 3-sigma upper limit of 1.1% for the HNC/HCN abundance ratio in comet 73P/Schwassmann-Wachmann (Fragment B), obtained on May 10-11, 2006 using Caltech Submillimeter Observatory (CSO). This limit is a factor of ~7 lower than the values measured previously in moderately active comets at 1 AU from the Sun. Comet 73P/Schwassmann-Wachmann was depleted in most volatile species, except of HCN. The low HNC/HCN ratio thus argues against HNC production from polymers produced from HCN. However, thermal degradation of macromolecules, or polymers, produced from ammonia and carbon compounds, such as acetylene, methane, or ethane appears a plausible explanation for the observed variations of the HNC/HCN ratio in moderately active comets, including the very low ratio in comet 73P/Schwassmann-Wachmann reported here. Similar polymers have been invoked previously to explain anomalous 14N/15N ratios measured in cometary CN.Comment: 6 pages, 5 figures, 2 table

Search for CO gas in Pluto, Centaurs and Kuiper Belt objects at radio wavelengths

We have searched for several rotational lines of CO in the Pluto-Charon system, Centaurs (Chiron, Pholus, Nessus, Asbolus, Chariklo and 1998 SG35) and Kuiper Belt objects (1994 TB, 1996 TL66, 1996 TO66, 1996 TP66 and 1998 WH4). The observations were performed with the 30 m telescope of the Institut de Radioastronomie Millimétrique for Pluto/Charon, and with the James Clerk Maxwell Telescope and Caltech Submillimeter Observatory for Centaurs and Kuiper Belt objects. A tentative 4.5-σ J(2-1) CO line is present in the Pluto/Charon spectrum, which requires further confirmation. Assuming that Charon does not contribute to the CO emission, an upper limit of 1.2% and 7% is obtained for the CO/N_2 mixing ratio in Pluto's atmosphere, using the atmospheric thermal structure derived from the Stansberry et al. (1994, Icarus 111, 503) and Strobel et al. (1996, Icarus 120, 266) models, respectively. These upper limits are more constraining (by more than a factor of 6) than the upper limits reported by Young et al. (2001, Icarus, in press) from near-IR spectroscopy. None of the Centaurs or Kuiper Belt objects (KBO) were detected in CO. The CO production rate upper limit obtained for Chiron (3-5 x 10^(27) mol s^(-1)) over 1998-2000 years is a factor of 10 lower than the CO production rate derived from the marginal CO detection obtained in June 1995 by Womack & Stern ([CITE], Astron. Vestnik 33, 216), using same modelling of CO emission. Upper limits obtained for other Centaurs are typically ~10^(28) mol s^(-1), and between 1 and 5 x 10^(28) mol s^(-1) for the best observed KBOs. The comparison between these upper limits and the CO outgassing rates of comet C/1995 O1 (Hale-Bopp) measured at large distances from the Sun shows that Centaurs and KBOs underwent significant CO-devolatilization since their formation

An upper limit for the water outgassing rate of the main-belt comet 176P/LINEAR observed with Herschel/HIFI

176P/LINEAR is a member of the new cometary class known as main-belt comets (MBCs). It displayed cometary activity shortly during its 2005 perihelion passage that may be driven by the sublimation of sub-surface ices. We have therefore searched for emission of the H2O 110-101 ground state rotational line at 557 GHz toward 176P/LINEAR with the Heterodyne Instrument for the Far Infrared (HIFI) on board the Herschel Space Observatory on UT 8.78 August 2011, about 40 days after its most recent perihelion passage, when the object was at a heliocentric distance of 2.58 AU. No H2O line emission was detected in our observations, from which we derive sensitive 3-sigma upper limits for the water production rate and column density of < 4e25 molec/s and of < 3e10 cm^{-2}, respectively. From the peak brightness measured during the object's active period in 2005, this upper limit is lower than predicted by the relation between production rates and visual magnitudes observed for a sample of comets by Jorda et al. (2008) at this heliocentric distance. Thus, 176P/LINEAR was likely less active at the time of our observation than during its previous perihelion passage. The retrieved upper limit is lower than most values derived for the H2O production rate from the spectroscopic search for CN emission in MBCs.Comment: 5 pages, 2 figures. Minor changes to match published versio

Detection of CO and HCN in Pluto's atmosphere with ALMA

Observations of the Pluto-Charon system, acquired with the ALMA interferometer on June 12-13, 2015, have yielded a detection of the CO(3-2) and HCN(4-3) rotational transitions from Pluto, providing a strong confirmation of the presence of CO, and the first observation of HCN, in Pluto's atmosphere. The CO and HCN lines probe Pluto's atmosphere up to ~450 km and ~900 km altitude, respectively. The CO detection yields (i) a much improved determination of the CO mole fraction, as 515+/-40 ppm for a 12 ubar surface pressure (ii) clear evidence for a well-marked temperature decrease (i.e., mesosphere) above the 30-50 km stratopause and a best-determined temperature of 70+/-2 K at 300 km, in agreement with recent inferences from New Horizons / Alice solar occultation data. The HCN line shape implies a high abundance of this species in the upper atmosphere, with a mole fraction >1.5x10-5 above 450 km and a value of 4x10-5 near 800 km. The large HCN abundance and the cold upper atmosphere imply supersaturation of HCN to a degree (7-8 orders of magnitude) hitherto unseen in planetary atmospheres, probably due to the slow kinetics of condensation at the low pressure and temperature conditions of Pluto's upper atmosphere. HCN is also present in the bottom ~100 km of the atmosphere, with a 10-8 - 10-7 mole fraction; this implies either HCN saturation or undersaturation there, depending on the precise stratopause temperature. The HCN column is (1.6+/-0.4)x10^14 cm-2, suggesting a surface-referred net production rate of ~2x10^7 cm-2s-1. Although HCN rotational line cooling affects Pluto's atmosphere heat budget, the amounts determined in this study are insufficient to explain the well-marked mesosphere and upper atmosphere's ~70 K temperature. We finally report an upper limit on the HC3N column density (< 2x10^13 cm-2) and on the HC15N / HC14N ratio (< 1/125).Comment: Revised version. Icarus, in press, Oct. 11, 2016. 57 pages, including 13 figures and 4 table

Large excess of heavy nitrogen in both hydrogen cyanide and cyanogen from comet 17P/Holmes

From millimeter and optical observations of the Jupiter-family comet 17P/Holmes performed soon after its huge outburst of October 24, 2007, we derive 14 N/15N = 139 +/- 26 in HCN, and 14N/15N = 165 +/- 40 in CN, establishing that HCN has the same non-terrestrial isotopic composition as CN. The same conclusion is obtained for the long-period comet C/1995 O1 (Hale-Bopp) after a reanalysis of previously published measurements. These results are compatible with HCN being the prime parent of CN in cometary atmospheres. The 15N excess relative to the Earth atmospheric value indicates that N-bearing volatiles in the solar nebula underwent important N isotopic fractionation at some stage of Solar System formation. HCN molecules never isotopically equilibrated with the main nitrogen reservoir in the solar nebula before being incorporated in Oort-cloud and Kuiper-belt comets. The 12C/13C ratios in HCN and CN are measured to be consistent with the terrestrial value.Comment: Accepted for publication in the Astrophysical Journal (Letters) 4 page

A Herschel Study of D/H in Water in the Jupiter-Family Comet 45P/Honda-Mrkos-Pajdušáková and Prospects for D/H Measurements with CCAT

We present Herschel observations of water isotopologues in the atmosphere of the Jupiter-family comet 45P/Honda–Mrkos–Pajdušáková. No HDO emission is detected, with a 3σ upper limit of 2.0 × 10^(−4) for the D/H ratio. This value is consistent with the earlier Herschel measurement in the Jupiter-family comet 103P/Hartley 2. The canonical value of 3 × 10^(−4) measured pre-Herschel in a sample of Oort-cloud comets can be excluded at a 4.5σ level. The observations presented here further confirm that a diversity of D/H ratios exists in the comet population and emphasize the need for additional measurements with future ground-based facilities, such as CCAT, in the post-Herschel era

Search for CO gas in Pluto, Centaurs and Kuiper Belt objects at radio wavelengths

We have searched for several rotational lines of CO in the Pluto-Charon system, Centaurs (Chiron, Pholus, Nessus, Asbolus, Chariklo and 1998 SG35) and Kuiper Belt objects (1994 TB, 1996 TL66, 1996 TO66, 1996 TP66 and 1998 WH4). The observations were performed with the 30 m telescope of the Institut de Radioastronomie Millimétrique for Pluto/Charon, and with the James Clerk Maxwell Telescope and Caltech Submillimeter Observatory for Centaurs and Kuiper Belt objects. A tentative 4.5-σ J(2-1) CO line is present in the Pluto/Charon spectrum, which requires further confirmation. Assuming that Charon does not contribute to the CO emission, an upper limit of 1.2% and 7% is obtained for the CO/N_2 mixing ratio in Pluto's atmosphere, using the atmospheric thermal structure derived from the Stansberry et al. (1994, Icarus 111, 503) and Strobel et al. (1996, Icarus 120, 266) models, respectively. These upper limits are more constraining (by more than a factor of 6) than the upper limits reported by Young et al. (2001, Icarus, in press) from near-IR spectroscopy. None of the Centaurs or Kuiper Belt objects (KBO) were detected in CO. The CO production rate upper limit obtained for Chiron (3-5 x 10^(27) mol s^(-1)) over 1998-2000 years is a factor of 10 lower than the CO production rate derived from the marginal CO detection obtained in June 1995 by Womack & Stern ([CITE], Astron. Vestnik 33, 216), using same modelling of CO emission. Upper limits obtained for other Centaurs are typically ~10^(28) mol s^(-1), and between 1 and 5 x 10^(28) mol s^(-1) for the best observed KBOs. The comparison between these upper limits and the CO outgassing rates of comet C/1995 O1 (Hale-Bopp) measured at large distances from the Sun shows that Centaurs and KBOs underwent significant CO-devolatilization since their formation

Earth-based detection of the millimetric thermal emission of the nucleus of comet 8P/Tuttle

Little is known about the physical properties of cometary nuclei. Apart from space mission targets, measuring the thermal emission of a nucleus is one of the few means to derive its size, independently of its albedo, and to constrain some of its thermal properties. This emission is difficult to detect from Earth but space telescopes (Infrared Space Observatory, Spitzer Space Telescope, Herschel Space Observatory) allow reliable measurements in the infrared and the sub-millimetre domains. We aim at better characterizing the thermal properties of the nucleus of comet 8P/Tuttle using multi-wavelentgh space- and ground-based observations, in the visible, infrared, and millimetre range. We used the Plateau de Bure Interferometer to measure the millimetre thermal emission of comet 8P/Tuttle at 240 GHz (1.25 mm) and analysed the observations with the shape model derived from Hubble Space Telescope observations and the nucleus size derived from Spitzer Space Telescope observations. We report on the first detection of the millimetre thermal emission of a cometary nucleus since comet C/1995 O1 Hale-Bopp in 1997. Using the two contact spheres shape model derived from Hubble Space Telescope observations, we constrained the thermal properties of the nucleus. Our millimetre observations are best match with: i) a thermal inertia lower than ~10 J K-1 m-2 s-1/2, ii) an emissivity lower than 0.8, indicating a non-negligible contribution of the colder sub-surface layers to the outcoming millimetre flux.Comment: 7 pages. Accepted for publication in Astronomy & Astrophysic

Water, hydrogen cyanide, carbon monoxide, and dust production from distant comet 29P/Schwassmann-Wachmann 1

peer reviewedContext. 29P/Schwassmann-Wachmann 1 is a distant Centaur/comet, showing persistent CO-driven activity and frequent outbursts. Aims: We aim to better characterize its gas and dust activity from multiwavelength observations performed during outbursting and quiescent states. Methods: We used the HIFI, PACS and SPIRE instruments of the Herschel space observatory on several dates in 2010, 2011, and 2013 to observe the H2O 557 GHz and NH3 573 GHz lines and to image the dust coma in the far-infrared. Observations with the IRAM 30 m telescope were undertaken in 2007, 2010, 2011, and 2021 to monitor the CO production rate through the 230 GHz line, and to search for HCN at 89 GHz. The 70 and 160 µm PACS images were used to measure the thermal flux from the nucleus and the dust coma. Modeling was performed to constrain the size of the sublimating icy grains and to derive the dust production rate. Results: HCN is detected for the first time in comet 29P (at 5σ in the line area). H2O is detected as well, but not NH3. H2O and HCN line shapes differ strongly from the CO line shape, indicating that these two species are released from icy grains. CO production rates are in the range (2.9-5.6) × 10E28 s−1 (1400-2600 kg s−1). A correlation between the CO production rate and coma brightness is observed, as is a correlation between CO and H2O production. The correlation obtained between the excess of CO production and excess of dust brightness with respect to the quiescent state is similar to that established for the continuous activity of comet Hale-Bopp. The measured Q(H2O)/Q(CO) and Q(HCN)/Q(CO) production rate ratios are 10.0 ± 1.5 % and 0.12 ± 0.03 %, respectively, averaging the April-May 2010 measurements (Q(H2O) = (4.1 ± 0.6) × 10E27 s−1, Q(HCN) = (4.8 ± 1.1) × 10E25 s−1). We derive three independent and similar values of the effective radius of the nucleus, ~31 ± 3 km, suggesting an approximately spherical shape. The inferred dust mass-loss rates during quiescent phases are in the range 30-120 kg s−1, indicating a dust-to-gas mass ratio <0.1 during quiescent activity. We conclude that strong local heterogeneities exist on the surface of 29P, with quenched dust activity from most of the surface, but not in outbursting regions. Conclusions: The volatile composition of the atmosphere of 29P strongly differs from that of comets observed within 3 au from the Sun. The observed correlation between CO, H2O and dust activity may provide important constraints for the outburst-triggering mechanism