Compositional characterisation of the Themis family

Context. It has recently been proposed that the surface composition of icy main-belt asteroids (B-,C-,Cb-,Cg-,P-,and D-types) may be consistent with that of chondritic porous interplanetary dust particles (CPIDPs). Aims. In the light of this new association, we re-examine the surface composition of a sample of asteroids belonging to the Themis family in order to place new constraints on the formation and evolution of its parent body. Methods. We acquired NIR spectral data for 15 members of the Themis family and complemented this dataset with existing spectra in the visible and mid-infrared ranges to perform a thorough analysis of the composition of the family. Assuming end-member minerals and particle sizes (<2\mum) similar to those found in CPIDPs, we used a radiative transfer code adapted for light scattering by small particles to model the spectral properties of these asteroids. Results. Our best-matching models indicate that most objects in our sample possess a surface composition that is consistent with the composition of CP IDPs.We find ultra-fine grained Fe-bearing olivine glasses to be among the dominant constituents. We further detect the presence of minor fractions of Mg-rich crystalline silicates. The few unsuccessfully matched asteroids may indicate the presence of interlopers in the family or objects sampling a distinct compositional layer of the parent body. Conclusions. The composition inferred for the Themis family members suggests that the parent body accreted from a mixture of ice and anhydrous silicates (mainly amorphous) and subsequently underwent limited heating. By comparison with existing thermal models that assume a 400km diameter progenitor, the accretion process of the Themis parent body must have occurred relatively late (>4Myr after CAIs) so that only moderate internal heating occurred in its interior, preventing aqueous alteration of the outer shell.Comment: 9 pages, 5 figures, accepted for publication in A&

Observation of a short-lived pattern in the solar chromosphere

In this work we investigate the dynamic behavior of inter-network regions of the solar chromosphere. We observed the chromosphere of the quiet Sun using a narrow-band Lyot filter centered at the Ca II K 2v emission peak with a bandpass of 0.3A. We achieved a spatial resolution of on average 0.7" at a cadence of 10s. In the inter-network we find a mesh-like pattern that features bright grains at the vertices. The pattern has a typical spatial scale of 1.95" and a mean evolution time scale of 53s with a standard deviation of 10s. A comparison of our results with a recent three-dimensional radiation hydrodynamical model implies that the observed pattern is of chromospheric origin. The measured time scales are not compatible with those of reversed granulation in the photosphere although the appearance is similar. A direct comparison between network and inter-network structure shows that their typical time scales differ by at least a factor of two. The existence of a rapidly evolving small-scale pattern in the inter-network regions supports the picture of the lower chromosphere as a highly dynamical and intermittent phenomenon.Comment: Letter A&A 4 pages 5 figure

Physical and dynamical properties of the main belt triple asteroid (87) Sylvia

We present the analysis of high angular resolution observations of the triple Asteroid (87) Sylvia collected with three 8-10 m class telescopes (Keck, VLT, Gemini North) and the Hubble Space Telescope. The moons' mutual orbits were derived individually using a purely Keplerian model. We computed the position of Romulus, the outer moon of the system, at the epoch of a recent stellar occultation which was successfully observed at less than 15 km from our predicted position, within the uncertainty of our model. The occultation data revealed that the Moon, with a surface-area equivalent diameter Ds=23.1$\pm$0.7km, is strongly elongated (axes ratio of 2.7$\pm$0.32.7$\pm$0.3), significantly more than single asteroids of similar size in the main-belt. We concluded that its shape is probably affected by the tides from the primary. A new shape model of the primary was calculated combining adaptive-optics observations with this occultation and 40 archived light-curves recorded since 1978. The difference between the J2=0.024-0.009+0.016 derived from the 3-D shape model assuming an homogeneous distribution of mass for the volume equivalent diameter Dv=273$\pm$10km primary and the null J2 implied by the Keplerian orbits suggests a non-homogeneous mass distribution in the asteroid's interior

Origin of molecular oxygen in Comet 67P/Churyumov-Gerasimenko

Molecular oxygen has been detected in the coma of comet 67P/Churyumov-Gerasimenko with abundances in the 1-10% range by the ROSINA-DFMS instrument on board the Rosetta spacecraft. Here we find that the radiolysis of icy grains in low-density environments such as the presolar cloud may induce the production of large amounts of molecular oxygen. We also show that molecular oxygen can be efficiently trapped in clathrates formed in the protosolar nebula, and that its incorporation as crystalline ice is highly implausible because this would imply much larger abundances of Ar and N2 than those observed in the coma. Assuming that radiolysis has been the only O2 production mechanism at work, we conclude that the formation of comet 67P/Churyumov-Gerasimenko is possible in a dense and early protosolar nebula in the framework of two extreme scenarios: (1) agglomeration from pristine amorphous icy grains/particles formed in ISM and (2) agglomeration from clathrates that formed during the disk's cooling. The former scenario is found consistent with the strong correlation between O2 and H2O observed in 67P/C-G's coma while the latter scenario requires that clathrates formed from ISM icy grains that crystallized when entering the protosolar nebula.Comment: The Astrophysical Journal Letters, in pres

Ultraviolet and visible photometry of asteroid (21) Lutetia using the Hubble Space Telescope

The asteroid (21) Lutetia is the target of a planned close encounter by the Rosetta spacecraft in July 2010. To prepare for that flyby, Lutetia has been extensively observed by a variety of astronomical facilities. We used the Hubble Space Telescope (HST) to determine the albedo of Lutetia over a wide wavelength range, extending from ~150 nm to ~700 nm. Using data from a variety of HST filters and a ground-based visible light spectrum, we employed synthetic photometry techniques to derive absolute fluxes for Lutetia. New results from ground-based measurements of Lutetia's size and shape were used to convert the absolute fluxes into albedos. We present our best model for the spectral energy distribution of Lutetia over the wavelength range 120-800 nm. There appears to be a steep drop in the albedo (by a factor of ~2) for wavelengths shorter than ~300 nm. Nevertheless, the far ultraviolet albedo of Lutetia (~10%) is considerably larger than that of typical C-chondrite material (~4%). The geometric albedo at 550 nm is 16.5 +/- 1%. Lutetia's reflectivity is not consistent with a metal-dominated surface at infrared or radar wavelengths, and its albedo at all wavelengths (UV-visibile-IR-radar) is larger than observed for typical primitive, chondritic material. We derive a relatively high FUV albedo of ~10%, a result that will be tested by observations with the Alice spectrograph during the Rosetta flyby of Lutetia in July 2010.Comment: 14 pages, 2 tables, 8 figure

Observations of a solar flare and filament eruption in Lyman <span class='mathrm'>α</span> and X-rays

&lt;p&gt;&lt;b&gt;Context&lt;/b&gt;: L&#945; is a strong chromospheric emission line, which has been relatively rarely observed in flares. The Transition Region and Coronal Explorer (TRACE) has a broad “Lyman &#945;” channel centered at 1216 Å used primarily at the beginning of the mission. A small number of flares were observed in this channel.&lt;/p&gt; &lt;p&gt;&lt;b&gt;Aims&lt;/b&gt;: We aim to characterise the appearance and behaviour of a flare and filament ejection which occurred on 8th September 1999 and was observed by TRACE in L&#945;, as well as by the Yohkoh Soft and Hard X-ray telescopes. We explore the flare energetics and its spatial and temporal evolution. We have in mind the fact that the L&#945; line is a target for the Extreme Ultraviolet Imaging telescope (EUI) which has been selected for the Solar Orbiter mission, as well as the LYOT telescope on the proposed SMESE mission.&lt;/p&gt; &lt;p&gt;&lt;b&gt;Methods&lt;/b&gt;: We use imaging data from the TRACE 1216 Å, 1600 Å and 171 Å channels, and the Yohkoh hard and soft X-ray telescopes. A correction is applied to the TRACE data to obtain a better estimate of the pure L&#945; signature. The L&#945;  power is obtained from a knowledge of the TRACE response function, and the flare electron energy budget is estimated by interpreting Yohkoh/HXT emission in the context of the collisional thick target model.&lt;/p&gt; &lt;p&gt;&lt;b&gt;Results&lt;/b&gt;: We find that the L&#945;  flare is characterised by strong, compact footpoints (smaller than the UV ribbons) which correlate well with HXR footpoints. The L&#945; power radiated by the flare footpoints can be estimated, and is found to be on the order of 1026 erg s-1 at the peak. This is less than 10% of the power inferred for the electrons which generate the co-spatial HXR emission, and can thus readily be provided by them. The early stages of the filament eruption that accompany the flare are also visible, and show a diffuse, roughly circular spreading sheet-like morphology, with embedded denser blobs.&lt;/p&gt; &lt;p&gt;&lt;b&gt;Conclusions&lt;/b&gt;: On the basis of this observation, we conclude that flare and filament observations in the L&#945; line with the planned EUI and LYOT telescopes will provide valuable insight into solar flare evolution and energetics, especially when accompanied by HXR imaging and spectroscopy.&lt;/p&gt

X-ray emission from dense plasma in CTTSs: Hydrodynamic modeling of the accretion shock

High spectral resolution X-ray observations of CTTSs demonstrate the presence of plasma at T~2-3X10^6 K and n_e~10^11-10^13 cm^-3, unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but they do not allow to analyze the stability of such material and its position in the stellar atmosphere. We investigate the dynamics and the stability of shock-heated accreting material in CTTSs and the role of the stellar chromosphere in determining the position and the thickness of the shocked region. We perform 1-D HD simulations of the impact of the accretion flow onto chromosphere of a CTTS, including the effects of gravity, radiative losses from optically thin plasma, thermal conduction and a well tested detailed model of the stellar chromosphere. Here we present the results of a simulation based on the parameters of the CTTS MP Mus. We find that the accretion shock generates an hot slab of material above the chromosphere with a maximum thickness of 1.8X10^9 cm, density n_e~10^11-10^2 cm^-3, temperature T~3X10^6 K and uniform pressure equal to the ram pressure of the accretion flow (~450 dyn cm^-2). The base of the shocked region penetrates the chromosphere and stays where the ram pressure is equal to the thermal pressure. The system evolves with quasi-periodic instabilities of the material in the slab leading to cyclic disappearance and re-formation of the slab. For an accretion rate of ~10^-10 M_sun yr^-1, the shocked region emits a time-averaged X-ray luminosity L_X~7X10^29 erg s^-1, which is comparable to the X-ray luminosity observed in CTTSs of the same mass. Furthermore, the X-ray spectrum synthesized from the simulation matches in detail all the main features of the O VIII and O VII lines of the star MP Mus.Comment: Accepted for publication as a Letter in Astronomy & Astrophysic

Gilbert Syndrome and the Development of Antiretroviral Therapy-Associated Hyperbilirubinemia

BackgroundUnconjugated hyperbilirubinemia results from Gilbert syndrome and from antiretroviral therapy (ART) containing protease inhibitors. An understanding of the interaction between genetic predisposition and ART may help to identify individuals at highest risk for developing jaundice MethodsWe quantified the contribution of UGT1A1*28 and ART to hyperbilirubinemia by longitudinally modeling 1386 total bilirubin levels in 96 human immunodeficiency virus (HIV)-infected individuals during a median of 6 years ResultsThe estimated average bilirubin level was 8.8 μmol/L (0.51 mg/dL). Atazanavir increased bilirubin levels by 15 μmol/L (0.87 mg/dL), and indinavir increased bilirubin levels by 8 μmol/L (0.46 mg/dL). Ritonavir, lopinavir, saquinavir, and nelfinavir had no or minimal effect on bilirubin levels. Homozygous UGT1A1*28 increased bilirubin levels by 5.2 μmol/L (0.3 mg/dL). As a consequence, 67% of individuals homozygous for UGT1A1*28 and receiving atazanavir or indinavir had ⩾2 episodes of hyperbilirubinemia in the jaundice range (>43 μmol/L [>2.5 mg/dL]), versus 7% of those with the common allele and not receiving either of those protease inhibitors (P<.001). Efavirenz resulted in decreased bilirubin levels, which is consistent with the induction of UDP-glucuronosyltransferase 1A1 ConclusionsGenotyping for UGT1A1*28 before initiation of ART would identify HIV-infected individuals at risk for hyperbilirubinemia and decrease episodes of jaundic