In-Flight Main Beam Reconstruction for Planck-LFI

In-flight measurements of the shape of the antenna main beam is a crucial input to the data analysis pipeline of each high resolution Cosmic Microwave Background (CMB) anisotropy experiment. We study the main beam reconstruction achievable by the PLANCK Low Frequency Instrument (LFI) through the observation of external planets. Although were strict our analysis to the 30 GHz LFI channel, the method can be easily extended to all the PLANCK frequency channels and to other CMB anisotropy experiments. We show that it is possible to fit the time ordered data from the external planets (mainly Jupiter and Saturn) to obtain an accurate, robust, simple and fast reconstruction of the main beam properties under very general conditions, almost independently of the calibration accuracy. In addition, we find that a bivariate Gaussian approximation of main beam shapes represents a significant improvement with respect to asymmetric representation. The impact of the most relevant systematic effects is also addressed. We demonstrate that by combining the recovery of the maximum signal at the planet transit with accurate in-flight calibration, it is possible to measure the intrinsic planet temperatures at millimetric wavelengths with < 1% accuracy. This work is based on PLANCK-LFI activities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physical conditions in the convective envelopes of stars

The physical conditions in the convective envelopes of Pop. II main-sequence stars are described with some detail. A particular care has been devoted to the effects due to the formation of hydrogen molecules and to the pressure ionization.

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Riassunto Vengono descritte in dettaglio le condizioni fisiche nelle zone convettive esterne delle stelle di sequenza principale di Popolazione II. In particolare vengono studiati gli effetti dovuti alla formazione della molecola dell'idrogeno ed alla ionizzazione per pressione.

     

Thermal history of ALH 84001 meteorite by Fe2+‐Mg ordering in orthopyroxene

Abstract— A single orthopyroxene crystal from the Martian meteorite Allan Hills (ALH) 84001 was studied by X‐ray diffraction (XRD) and electron microprobe analysis (EMPA) to retrieve information about its thermal history. Both sets of data were used to measure the Fe²⁺‐Mg order degree between the M1 and M2 sites expressed by the distribution coefficient k_D. The 529 ± 30°C closure temperature (T_c) of the Fe²⁺‐Mg ordering process of ALH 84001 orthopyroxene (Fs₂₈) was calculated using Stimpfl (2005a, 2005b) ln k_D versus 1/T equation obtained for intermediate iron sample. At this T_c, the orthopyroxene cooling rate, calculated by Ganguly's (1982) numerical method, was 0.1 °C/day. This study puts new constraints on the last high‐temperature thermal episode recorded by orthopyroxene. With reference to the geological history (Treiman 1998), we ascribe this episode to the I3 event, and we interpret the T_c of 529 °C as a lower limit for this impact heating. Our data confirm that experimentally defined physical conditions for the formation of magnetite from decomposition of carbonates took place on the Martian surface during event I3.     

High-pressure displacive phase transition of a natural Mg-rich pigeonite

A high-pressure single-crystal X-ray diffraction study has been carried out on a P2₁/c natural Mg-rich pigeonite sample with composition ca. Wo₆En₇₆Fs₁₈ using a diamond anvil-cell. The unit-cell parameters were determined at 14 different pressures to 7.14 GPa. The sudden disappearance of the b-type reflections (h + k = odd) and a strong discontinuity (about 2.8%) in the unit-cell volume indicated a first-order P2₁/c–C2/c phase transition between 4.66 and 4.88 GPa. The P(V) data of the P2₁/c phase were fitted to 4.66 GPa by a third-order Birch–Murnaghan equation of state (BM3 EoS), whereas the limited number of experimental data collected within the C2/c phase between 4.88 and 7.14 GPa were fitted using the same equation of state but with K′ constrained to the value obtained for the P2₁/c fitting. The equation of state coefficients are V ₀ = 424.66(6) ų, K _T0 = 104(2) GPa and K′ = 8(1) for the P2₁/c phase, and V ₀ = 423.6(1) ų, K _T0 = 112.4(8) GPa, and K′ fixed to 8(1) for the C2/c phase. The axial moduli for a, b, and c for the P2₁/c phase were obtained using also a BM3-EoS, while for the C2/c phase only a linear calculation could be performed, and therefore the same approach was applied for comparison also to the P2₁/c phase. In general the C2/c phase exhibits axial compressibilities (β _c > β _a >> β _b) lower than those of the P2₁/c phase (β _b > β _c ≈ β _a; similar to those found in previous studies in clinopyroxenes and orthopyroxenes). The lower compressibility of the C2/c phase compared with that of the P2₁/c could be ascribed to the greater stiffness along the b direction. A previously published relationship between P _c and M2 average cation radius (i.r.) has been updated using all the literature data on P2₁/c clinopyroxene containing large cations at M2 site and our new data. The following weighted regression was obtained: P _c (GPa) = 26(4) ? 28(5) ×  i.r (Å), R ² = 0.97. This improved equation can be used to predict the critical pressure of natural P2₁/c clinopyroxene samples just knowing the composition at M2 site.     

New evidence of mantle heterogeneity beneath the Hyblean Plateau (southeast Sicily,Italy) as inferred from noble gases and geochemistry of ultramafic xenoliths

We analyzed major and trace elements, Sr and Nd isotopes in ultramafic xenoliths in Miocenic age Hyblean diatremes, along with noble gases of CO₂-rich fluid inclusions hosted in the same products. The xenoliths consist of peridotites and pyroxenites, which are considered to be derived from the upper mantle. Although the mineral assemblage of peridotites and their whole-rock abundance of major elements (e.g., Al₂O₃ = 0.8–1.5 wt.%, TiO₂ = 0.03–0.08 wt.%) suggest a residual character of the mantle, a moderate enrichment in some incompatible elements (e.g., La_N/Yb_N = 9–14) highlights the presence of cryptic metasomatic events. In this context a deep silicate liquid is considered the metasomatizing agent, which is consistent with the occurrence of pyroxenites as veins in peridotites. Both the Zr/Nb and ¹⁴³Nd/¹⁴⁴Nd ratios of the investigated samples reveal two distinct compositional groups: (1) peridotites with Zr/Nb ≈ 4 and ¹⁴³Nd/¹⁴⁴Nd ≈ 0.5129, and (2) pyroxenites with Zr/Nb ≈ 20 and ¹⁴³Nd/¹⁴⁴Nd ≈ 0.5130. The results of noble-gas analyses also highlight the difference between the peridotite and pyroxenite domains. Indeed, the ³He/⁴He and ⁴He/⁴⁰Ar* ratios measured in the fluid inclusions of peridotites (respectively 7.0–7.4 ± 0.1 Ra and 0.5–8.2, where Ra is the atmospheric ³He/⁴He ratio of 1.38 × 10^? 6) were on average lower than those for the pyroxenites (respectively 7.2–7.6 Ra and 0.62–15). This mantle heterogeneity is interpreted as resulting from a mixing between two end-members: (1) a peridotitic layer with ³He/⁴He ≈ 7 Ra and ⁴He/⁴⁰Ar* ≈ 0.4, which is lower than the typical mantle ratio (~ 1–4) probably due to melt extraction events, and (2) metasomatizing mafic silicate melts that gave rise to pyroxenites characterized by ³He/⁴He ≈ 7.6 Ra, with a variable ⁴He/⁴⁰Ar* due to degassing processes connected with the ascent of magma at different levels in the peridotite wall rock. The complete geochemical data set also suggests two distinct mantle sources for the xenolithic groups highlighted above: (1) a HIMU (high-μ)-type source for the peridotites and (2) a DM (depleted mantle)-type source for the pyroxenites.