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Register a new userWater production in comet 81P/Wild 2 as determined by Herschel/HIFI
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Added by
Miguel de Val-Borro
Publication date
2010
fields
Physics
and research's language is
English
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The high spectral resolution and sensitivity of Herschel/HIFI allows for the detection of multiple rotational water lines and accurate determinations of water production rates in comets. In this letter we present HIFI observations of the fundamental 110-101 (557 GHz) ortho and 111-000 (1113 GHz) para rotational transitions of water in comet 81P/Wild 2 acquired in February 2010. We mapped the extent of the water line emission with five point scans. Line profiles are computed using excitation models which include excitation by collisions with electrons and neutrals and solar infrared radiation. We derive a mean water production rate of $1.0 times 10^{28}$ molecules s$^{-1}$ at a heliocentric distance of 1.61 AU about 20 days before perihelion, in agreement with production rates measured from the ground using observations of the 18-cm OH lines. Furthermore, we constrain the electron density profile and gas kinetic temperature, and estimate the coma expansion velocity by fitting the water line shapes.
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Individual particles from comet 81P/Wild 2 collected by NASAs Stardust mission vary in size from small sub-$mu$m fragments found in the walls of the aerogel tracks, to large fragments up to tens of $mu$m in size found towards the termini of tracks. The comet, in an orbit beyond Neptune since its formation, retains an intact a record of early-Solar-System processes that was compromised in asteroidal samples by heating and aqueous alteration. We measured the O isotopic composition of seven Stardust fragments larger than $sim$2 $mu$m extracted from five different Stardust aerogel tracks, and 63 particles smaller than $sim$2 $mu$m from the wall of a Stardust track. The larger particles show a relatively narrow range of O isotopic compositions that is consistent with $^{16}$O-poor phases commonly seen in meteorites. Many of the larger Stardust fragments studied so far have chondrule-like mineralogy which is consistent with formation in the inner Solar System. The fine-grained material shows a very broad range of O isotopic compositions ($-70<Delta^{17}$O$<+60$) suggesting that Wild 2 fines are either primitive outer-nebula dust or a very diverse sampling of inner Solar System compositional reservoirs that accreted along with a large number of inner-Solar-System rocks to form comet Wild 2.
We predict the amount of cometary, interplanetary, and interstellar cosmic dust that is to be measured by the Cometary and Interstellar Dust Analyzer (CIDA) and the aerogel collector on-board the Stardust spacecraft during its fly-by of comet P/Wild 2 and during the interplanetary cruise phase. We give the dust flux on the spacecraft during the encounter with the comet using both, a radially symmetric and an axially symmetric coma model. At closest approach, we predict a total dust flux of $10^{6.0} m^{-2} s^{-1}$ for the radially symmetric case and $10^{6.5} m^{-2} s^{-1}$ for the axially symmetric case. This prediction is based on an observation of the comet at a heliocentric distance of $1.7 {rm AU}$. We reproduce the measurements of the Giotto and VEGA missions to comet P/Halley using the same model as for the Stardust predictions. The planned measurements of {em interstellar} dust by Stardust have been triggered by the discovery of interstellar dust impacts in the data collected by the Ulysses and Galileo dust detector. Using the Ulysses and Galileo measurements we predict that 25 interstellar particles, mainly with masses of about $10^{-12} g$, will hit the target of the CIDA experiment. The interstellar side of the aerogel collector will contain 120 interstellar particles, 40 of which with sizes greater than $1 mu m$. We furthermore investigate the ``contamination of the CIDA and collector measurements by interplanetary particles during the cruise phase.
High-resolution far-infrared and sub-millimetre spectroscopy of water lines is an important tool to understand the physical and chemical properties of cometary atmospheres. We present observations of several rotational ortho- and para-water transitions in comet C/2008 Q3 (Garradd) performed with HIFI on Herschel. These observations have provided the first detection of the 2_{12}-1_{01} (1669 GHz) ortho and 1_{11}-0_{00} (1113 GHz) para transitions of water in a cometary spectrum. In addition, the ground-state transition 1_{10}-1_{01} at 557 GHz is detected and mapped. By detecting several water lines quasi-simultaneously and mapping their emission we can constrain the excitation parameters in the coma. Synthetic line profiles are computed using excitation models which include excitation by collisions, solar infrared radiation, and radiation trapping. We obtain the gas kinetic temperature, constrain the electron density profile, and estimate the coma expansion velocity by analyzing the map and line shapes. We derive water production rates of 1.7-2.8 x 10^{28} s^{-1} over the range r_h = 1.83-1.85 AU.
We report the petrology, O isotopic composition, and Al-Mg isotope systematics of a chondrule fragment from the Jupiter-family comet Wild 2, returned to Earth by NASAs Stardust mission. This object shows characteristics of a type II chondrule that formed from an evolved oxygen isotopic reservoir. No evidence for extinct 26Al was found, with (26Al/ 27Al)0 < 3.0 x 10^-6. Assuming homogenous distribution of 26Al in the solar nebula, this particle crystallized at least 3 Myr after the earliest solar system objects--relatively late compared to most chondrules in meteorites. We interpret the presence of this object in a Kuiper Belt body as evidence of late, large-scale transport of small objects between the inner and outer solar nebula. Our observations constrain the formation of Jupiter (a barrier to outward transport if it formed further from the Sun than this cometary chondrule) to be more than 3 Myr after calcium-aluminum-rich inclusionss.
We present Herschel/HIFI observations of fourteen water lines in W43-MM1, a massive protostellar object in the luminous star cluster-forming region W43. We analyze the gas dynamics from the line profiles using Herschel-HIFI observations (WISH-KP) of fourteen far-IR water lines (H2O, H217O, H218O), CS(11-10), and C18O(9-8) lines, and using our modeling of the continuum spectral energy distribution. As for lower mass protostellar objects, the molecular line profiles are a mix of emission and absorption, and can be decomposed into medium, and broad velocity components. The broad component is the outflow associated with protostars of all masses. Our modeling shows that the remainder of the water profiles can be well fitted by an infalling and passively heated envelope, with highly supersonic turbulence varying from 2.2 km/s in the inner region to 3.5 km/s in the outer envelope. Also, W43-MM1 has a high accretion rate, between 4.0 x 10^{-4} and 4.0 x 10^{-2} msun /yr, derived from the fast (0.4-2.9 km/s) infall observed. We estimate a lower mass limit of gaseous water of 0.11 msun and total water luminosity of 1.5 lsun (in the 14 lines presented here). The central hot core is detected with a water abundance of 1.4 x 10^{-4} while the water abundance for the outer envelope is 8 x10^{-8}. The latter value is higher than in other sources, most likely related to the high turbulence and the micro-shocks created by its dissipation. Examining water lines of various energies, we find that the turbulent velocity increases with the distance to the center. While not in clear disagreement with the competitive accretion scenario, this behavior is predicted by the turbulent core model. Moreover, the estimated accretion rate is high enough to overcome the expected radiation pressure.
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