We present the analysis of ≈100 molecular maps of the coma of comet 67P/Churyumov-Gerasimenko that were obtained with the MIRO submillimeter radiotelescope on board the Rosetta spacecraft. From the ...spectral line mapping of H 216 $_2^{16}$ 216 O, H 218 $_2^{18}$218 O, H 217 $_2^{17}$217 O, CH3OH, NH3, and CO and some fixed nadir pointings, we retrieved the outgassing pattern and total production rates for these species. The analysis covers the period from July 2014, inbound to perihelion, to June 2016, outbound, and heliocentric distances rh = 1.24–3.65 AU. A steep evolution of the outgassing rates with heliocentric distance is observed, typically in rh−16 $r_{\textrm{h}}^{-6}$rh−6 , with significant differences between molecules (e.g. steeper variation for H2O post-perihelion than for methanol). As a consequence, the abundances relative to water in the coma vary. The CH3OH and CO abundances increase after perihelion, while the NH3 abundance peaks around perihelion and then decreases. Outgassing patterns have been modeled as 2D Gaussian jets. The width of these jets is maximum around the equinoxes when the bulk of the outgassing is located near the equator. From July 2014 to February 2015, the outgassing is mostly restricted to a narrower jet (full width at half-maximum ≈80°) originating from high northern latitudes, while around perihelion, most of the gaseous production comes from the southernmost regions ( − 80 ± 5° cometocentric latitude) and forms a 100°–130° (full width at half-maximum) wide fan. We find a peak production of water of 0.8 × 1028 molec. s−1, 2.5 times lower than measured by the ROSINA experiment, and place an upper limit to a 50% additional production that could come from the sublimation of icy grains. We estimate the total loss of ices during this perihelion passage to be 4.18 ± 0.18 × 109 kg. We derive a dust-to-gas ratio in the lost material of 0.7–2.3 (including all sources of errors) based on the nucleus mass loss of 10.5 ± 3.4 × 109 kg estimated by the RSI experiment. We also obtain an estimate of the H 218 $_2^{18}$218 O/H 217 $_2^{17}$217 O ratio of 5.6 ± 0.8.
Aims. Using spectroscopic and continuum data measured by the MIRO instrument on board Rosetta of comet 67P/Churyumov-Gerasimenko, it is possible to derive and track the change in the water production ...rate, to learn how the outgassing evolves with heliocentric distance. The MIRO data are well suited to investigate the evolution of 67P, in unprecedented spatial and temporal detail. Methods. To obtain estimates of the local effective Haser production rates we developed an efficient and reliable retrieval approach with precalculated lookup tables. We employed line area ratios (H216O/H218O) from pure nadir observations as the key variable, along with the Doppler shift velocity, and continuum temperature. This method was applied to the MIRO data from August 2014 until April 2016. Perihelion occurred on August 13, 2015 when the comet was 1.24 AU from the Sun. Results. During the perihelion approach, the water production rates increased by an order of magnitude, and from the observations, the derived maximum for a single observation on August 29, 2015 is (1.42 ± 0.51) ×1028. Modeling the data indicates that there is an offset in the peak outgassing, occurring 34 ± 10 days after perihelion. During the pre-perihelion phase, the production rate changes with heliocentric distance as rh−3.8±0.2; during post-perihelion, the dependence is rh−4.3±0.2. The comet is calculated to have lost 0.12 ± 0.06 % of its mass during the perihelion passage, considering only water ice sublimation. Additionally, this method provides well sampled data to determine the spatial distribution of outgassing versus heliocentric distance. The time evolution is definitely not uniform across the surface. Pre- and post-perihelion, the surface temperature on the southern hemisphere changes rapidly, as does the sublimation rate with an exponent of ~−6. There is a strong latitudinal dependence on the rh exponent with significant variation between northern and southern hemispheres, and so the average over the comet surface may only be of limited importance. We present more detailed regional variation in the outgassing, demonstrating that the highest derived production rates originate from the Wosret, Neith and Bes regions during perihelion.
CONFIRMATION OF CIRCUMSTELLAR PHOSPHINE AGUNDEZ, M; Cernicharo, J; Decin, L ...
Astrophysical journal. Letters,
08/2014, Letnik:
790, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Phosphine (PH sub(3)) was tentatively identified a few years ago in the carbon star envelopes IRC +10216 and CRL 2688 from observations of an emission line at 266.9 GHz attributable to the J = 1-0 ...rotational transition. We report the detection of the J = 2-1 rotational transition of PH sub(3) in IRC +10216 using the HIFI instrument on board Herschel, which definitively confirms the identification of PH sub(3). Radiative transfer calculations indicate that infrared pumping in excited vibrational states plays an important role in the excitation of PH sub(3) in the envelope of IRC +10216, and that the observed lines are consistent with phosphine being formed anywhere between the star and 100 Rlow * from the star, with an abundance of 10 super(-8) relative to H sub(2). The detection of PH sub(3) challenges chemical models, none of which offer a satisfactory formation scenario. Although PH sub(3) holds just 2% of the total available phosphorus in IRC +10216, it is, together with HCP, one of the major gas phase carriers of phosphorus in the inner circumstellar layers, suggesting that it could also be an important phosphorus species in other astronomical environments. This is the first unambiguous detection of PH sub(3) outside the solar system, and is a further step toward a better understanding of the chemistry of phosphorus in space.
Aims. We present the spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko using the H216O rotational transition line at 556.936 GHz observed from Rosetta/MIRO in ...August 2014. Methods. The water line was analyzed with a non-LTE radiative transfer model and an optimal estimation method to retrieve the H216O outgassing intensity, expansion velocity, and gas kinetic temperature. On August 7−9, 2014 and August 18−19, 2014, MIRO performed long steady nadir-pointing observations of the nucleus while it was rotating around its spin axis. The ground track of the MIRO beam during the observation was mostly on the northern hemisphere of comet 67P, covering its three distinct parts: the so-called head, body, and neck areas. Results. The MIRO spectral observation data show that the water-outgassing intensity varies by a factor of 30, from 0.1 × 1025 molecules s-1 sr-1 to 3.0 × 1025 molecules s-1 sr-1, the terminal gas expansion velocity varies by 0.17 km s-1 from 0.61 km s-1 to 0.78 km s-1, and the terminal gas temperature varies by 27 K from 47 K to 74 K. The retrieved coma parameters are co-registered with local environment variables such as the subsurface temperatures, measured in the MIRO continuum bands, the local solar time, illumination condition, and beam location on nucleus. The spatial variation of the outgassing activity is very noticeable, and the largest outgassing activity in August 2014 occurs near the neck region of the nucleus. The outgassing activity in the neck region is also found to be correlated with the local solar hour, which is related to the local illumination condition.
We present the first ~7.5'×11.5' velocity-resolved (~0.2 km s
) map of the C ii 158
m line toward the Orion molecular cloud 1 (OMC 1) taken with the
/HIFI instrument. In combination with far-infrared ...(FIR) photometric images and velocity-resolved maps of the H41
hydrogen recombination and CO
=2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/PDR/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the C ii luminosity (~85 %) is from the extended, FUV-illuminated face of the cloud (
>500,
>5×10
cm
) and from dense PDRs (
≳10
,
≳10
cm
) at the interface between OMC 1 and the H ii region surrounding the Trapezium cluster. Around ~15 % of the C ii emission arises from a different gas component without CO counterpart. The C ii excitation, PDR gas turbulence, line opacity (from
C ii) and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the
C ii/
and
/
ratios and show that
C ii/
decreases from the extended cloud component (~10
-10
) to the more opaque star-forming cores (~10
-10
). The lowest values are reminiscent of the "C ii deficit" seen in local ultra-luminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing
C ii/
ratio correlates better with the column density of dust through the molecular cloud than with
/
. We conclude that the C ii emitting column relative to the total dust column along each line of sight is responsible for the observed
C ii/
variations through the cloud.
The Microwave Instrument on the Rosetta Orbiter (MIRO) has been observing the coma of comet 67P/Churyumov-Gerasimenko almost continuously since June 2014 at wavelengths near 0.53 mm. We present here ...a map of the water column density in the inner coma (within 3 km from nucleus center) when the comet was at 3.4 AU from the Sun. Based on the analysis of the H216O and H218O (110-101) lines, we find that the column density can vary by two orders of magnitude in this region. The highest column density is observed in a narrow region on the dayside, close to the neck and north pole rotation axis of the nucleus, while the lowest column density is seen against the nightside of the nucleus where outgassing seems to be very low. We estimate that the outgassing pattern can be represented by a Gaussian distribution in a solid angle with FWHM ≈ 80°.
We present a ~52-671 mu m spectral scan toward Sgr A* taken with the PACS and SPIRE spectrometers on board Herschel. The achieved angular resolution allows us to separate, for the first time at ...far-IR wavelengths, the emission toward the central cavity (gas in the inner central parsec of the galaxy) from that of the surrounding circumnuclear disk. The spectrum toward Sgr A* is dominated by strong OIII, OI, CII, NIII, NII, and CI fine-structure lines (in decreasing order of luminosity) arising in gas irradiated by UV photons from the central stellar cluster. In addition, rotationally excited lines of super(12)CO (from J = 4-3 to 24-23), super(13)CO, H sub(2)O, OH, H sub(3)O super(+), HCO super(+), and HCN, as well as ground-state absorption lines of OH super(+), H sub(2)O super(+), H sub(3)O super(+), CH super(+), H sub(2)O, OH, HF, CH, and NH are detected. The excitation of the super(12)CO ladder is consistent with a hot isothermal component at T sub(k) Asymptotically = to 10 super(3.1) K and n(H sub(2)) <, ~ 10 super(4) cm super(-3). It is also consistent with a distribution of temperature components at higher density with most CO at T sub(k) <, ~ 300 K. The detected molecular features suggest that, at present, neither very enhanced X-ray nor cosmic-ray fluxes play a dominant role in the heating of the hot molecular gas. The hot CO component (either the bulk of the CO column or just a small fraction depending on the above scenario) results from a combination of UV- and shock-driven heating. If irradiated dense clumps/clouds do not exist, shocks likely dominate the heating of the hot molecular gas. This is consistent with the high-velocity gas detected toward Sgr A*.
Aims. Permittivity measurements on porous samples of volcanic origin have been performed in the 0.05–190 GHz range under laboratory conditions in support of the Rosetta mission to comet ...67P/Churyumov–Gerasimenko, specifically with the MIRO radiometric experiment and CONSERT radar experiment. Methods. The samples were split into several subsamples with different size ranges covering a few μm to 500 μm. Bulk densities of the subsamples were estimated to be in the 800 to 1500 kg/m3 range. The porosities were in the range of 48% to 65%. From 50 MHz to 6 GHz and at 190 GHz, permittivity has been determined with a coaxial cell and with a quasi-optical bench, respectively. Results. Without taking into account the volume-scattering effect at 190 GHz, the real part of the permittivity, normalized by the bulk density, is in the range of 2.1 to 2.6. The results suggest that the real part of the permittivity of an ice-free dust mantle covering the nucleus is in the 1.5−2.2 range at 190 GHz. From these values, a lower limit for the absorption length for the millimeter receiver of MIRO has been estimated to be between 0.6 and 2 cm, in agreement with results obtained from MIRO in September 2014. At frequencies of interest for CONSERT experiment, the real part of the permittivity of a suspected ice-free dust mantle should be below 2.2. It may be in the range of 1.2 to 1.7 for the nucleus, in agreement with first CONSERT results, taking into account a mean temperature of 110 K and different values for the dust-to-ice volumetric ratio. Estimations of contributions of the different parameters to the permittivity variation may indicate that the porosity is the main parameter.
The lakes of Titan Stofan, E. R; Elachi, C; Lunine, J. I ...
Nature,
01/2007, Letnik:
445, Številka:
7123
Journal Article
Recenzirano
The surface of Saturn's haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface. Initial visible and radar imaging ...failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface. Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T16). The radar imaging polewards of 70° north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface. The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan's surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface 'liquid methane' table.
The European Space Agency Rosetta Spacecraft, launched on March 2, 2004 toward Comet 67P/Churyumov-Gerasimenko, carries a relatively small and lightweight millimeter-submillimeter spectrometer ...instrument, the first of its kind launched into deep space. The instrument will be used to study the evolution of outgassing water and other molecules from the target comet as a function of heliocentric distance. During flybys of the asteroids (2867) Steins and (21) Lutetia in 2008 and 2010 respectively, the instrument will measure thermal emission and search for water vapor in the vicinity of these asteroids.The instrument, named MIRO (Microwave Instrument for the Rosetta Orbiter), consists of a 30-cm diameter, offset parabolic reflector telescope followed by two heterodyne receivers. Center-band operating frequencies of the receivers are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband continuum channels are implemented in both frequency bands for the measurement of near surface temperatures and temperature gradients in Comet 67P/Churyumov-Gerasimenko and the asteroids (2867) Steins and (21) Lutetia. A 4096 channel CTS (Chirp Transform Spectrometer) spectrometer having 180 MHz total bandwidth and 44 kHz resolution is, in addition to the continuum channel, connected to the submillimeter receiver. The submillimeter radiometer/spectrometer is fixed tuned to measure four volatile species – CO, CH3OH, NH3 and three, oxygen-related isotopologues of water, H216O, H217O and H218O. The basic quantities measured with the MIRO instrument are surface temperature, gas production rates and relative abundances, and velocity and excitation temperature of each species, along with their spatial and temporal variability. This paper provides a short discussion of the scientific objectives of the investigation, and a detailed discussion of the MIRO instrument system.
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