Why chromatic imaging matters

During the last two decades, the first generation of beam combiners at the Very Large Telescope Interferometer has proved the importance of optical interferometry for high-angular resolution astrophysical studies in the near- and mid-infrared. With the advent of 4-beam combiners at the VLTI, the u ? v coverage per pointing increases significantly, providing an opportunity to use reconstructed images as powerful scientific tools. Therefore, interferometric imaging is already a key feature of the new generation of VLTI instruments, as well as for other interferometric facilities like CHARA and JWST. It is thus imperative to account for the current image reconstruction capabilities and their expected evolutions in the coming years. Here, we present a general overview of the current situation of optical interferometric image reconstruction with a focus on new wavelength-dependent information, highlighting its main advantages and limitations. As an Appendix we include several cookbooks describing the usage and installation of several state-of-the art image reconstruction packages. To illustrate the current capabilities of the software available to the community, we recovered chromatic images, from simulated MATISSE data, using the MCMC software SQUEEZE. With these images, we aim at showing the importance of selecting good regularization functions and their impact on the reconstruction.     

Le passage Ordovicien–Silurien et la partie inférieure du Silurien (Sud-Est du Massif armoricain, France)The Ordovician–Silurian transition and the lower part of the Silurian (southeastern Armorican Massif, France)

A new section in the Silurian graptolitic ‘phtanites’ (black cherts) of Les Fresnaies at Chalonnes-sur-Loire (SE Armorican Massif) shows for the first time that these rocks (1) succeed conformably to Uppermost Ordovician (Hirnantian) glaciomarine deposits and (2) contain successive graptolite assemblages that characterise the base of the Silurian, the whole Rhuddanian and Aeronian stages and the lower part of the Telychian. To cite this article: J.M. Piçarra et al., C. R. Geoscience 334 (2002) 1177–1183.     

Characterization of the negative carbon isotope shift in segment C2, its global implications as a harbinger of OAE1a

Lower Cretaceous C-isotope records show intermittent negative/positive spikes, and consistent patterns of coeval chemostratigraphic curves thus document shifts that signal simultaneous responses of temporal changes in the global carbon reservoir. The standard pattern registered by the δ ¹³C_org and δ ¹³C_carb in Lower Aptian sediments includes distinct isotope segments C1 to C8 (Menegatti et al., 1998). In the El Pui section, Organyà Basin, Spain, C-isotope segment C2 is the longest interval preceding segments C3–C6 associated with oceanic anoxic event 1a (OAE 1a), and reveals a distinct negative shift of ~1.8‰ to ~2.23‰ defining the C-isotope pattern within that interval. Total inorganic carbon (TIC), total organic carbon (TOC), δ ¹³C_org, microfacies, n-alkanes show no difference before, during, or after the negative inflection. The biomarkers indicate that organic matter (OM) mainly originates from algal/microbial sources because short-chain length homologues (≤nC₁₉) dominate. nC₂₀ through nC₂₅ indicate some contribution from aquatic vegetation, but little from higher plants (>nC₂₅), as also suggested by the terrestrial/aquatic ratio of n-alkanes or (TAR) = [(nC₂₇+nC₂₉+nC₃₁)/(nC₁₅+nC₁₇+nC₁₉)] (averages 0.085). We suggest that conjoint pulses of contemporaneous LIPs (Ontong Java) and massive explosive volcanism in northeast Asia, the Songliao Basin (SB-V), best conform to plausible causes of the negative intra-C2 carbon isotopic excursion (CIE) at that time. Because of its apparent common occurrence the intra-C2 inflection could be a useful marker harbinger to the more pronounced CIE C3, the hallmark of OAE1a.     

Role of cyanobacteria in Corg-rich deposits: an example from the Indidura Formation (Cenomanian-Turonian), northeastern Mexico

We present evidence of accumulation of calcareous cyanobacterial “microspheroids” as predominant components of the Cenomanian-Turonian Indidura Formation of northeastern Mexico. The unit at Parras de la Fuente includes a sequence of limestones and marls with well-defined light-dark rhythms at the decimetric to millimetric scale, in which CaCO₃ and total organic carbon vary between 43–78% and 0.3–3.6%, respectively.A distinctive feature of the section is the presence of abundant millimeter-scale microlaminae arranged in nearly even-parallel white and dark gray “varve like” dual lamination less than 3 mm thick, in which the darker units contain scattered planktonic foraminifera and radiolaria, whereas the lighter microlaminae are dominated by calcitic microspheroids (20–40 μm). The white laminae are evidently the result of recurring cycles of calcareous cyanobacterial blooms, possibly associated with fluvial dilution of surface waters.The organic carbon-rich laminated marlstones and laminated biocalcilutites of the Indidura Formation document paleoceanographic conditions favorable to unusual cyanobacterial productivity cycles that were also characterized by strong dysoxic/anoxic bottom conditions.     

Chitinozoans from the Fenxiang Formation (Early Ordovician) of Yichang, Hubei Province, China

Chitinozoans collected from upper Tremadocian to lower Floian strata of Chenjiahe section, Yichang, western Hubei, China comprise six species belonging respectively to the genera Euconochitina including a new species, Euconochitina fenxiangensis, Lagenochitina and Bursachitina, together with Desmochitina sp. and Eremochitina sp. The chitinozoan succession across the interval is correlated with relevant conodont and chitinozoan biozones and two new regional chitinozoan biozones, the Lagenochitina destombesi Biozone and the Euconochitina symmetrica Biozone are proposed based on their stratigraphic ranges in the Fenxiang to Honghuayuan formations in the Chenjiahe section.     

Perspectives of a visible instrument on the VLTI

In this paper we present the most promising science cases for a new generation visible instrument on the VLTI and the conceptual idea for the instrumental configuration. We also present a statistical study of the potential targets that may be accessible for the different classes of objects and for the required spectral resolutions.     

Geochemistry of basalts from the Dumisseau Formation, southern Haiti: implications for the origin of the Caribbean Sea crust

Basalt and diabase from the Cretaceous Dumisseau Formation, southern Haiti have Mg-numbers of 43–63, TiO₂ contents of 1.6–3.9% and La abundances of 3.6–15.3 ppm.La/Ta ratios average 10, and indicate that the basalts are oceanic in character, distinct from the arc associations forming the northern part of Haiti. Oldest lavas have low TiO₂ (1.6%) and are LREE-depleted, similar to N-MORBs, whereas overlying lavas have higher TiO₂ (2–3.9%) and are LREE-enriched, similar to E-MORBs or hotspot basalts.⁸⁷Sr⁸⁶Sr ratios vary from 0.70280 to 0.70316,¹⁴³Nd¹⁴⁴Nd from 0.512929 to 0.513121, and²⁰⁶Pb²⁰⁴Pb from 19.00 to 19.27. LREE-depleted lavas have high¹⁴³Nd¹⁴⁴Nd (0.51309–0.51310) typical of MORBs, whereas¹⁴³Nd¹⁴⁴Nd in the LREE-enriched lavas varies widely (0.512929–0.513121).Chemical features of the Dumisseau basalts are equivalent to those of Caribbean seafloor basalts recovered on DSDP Leg 15, and support the contention that the Dumisseau is an uplifted section of Caribbean Sea crust. Oldest lavas are analogous to MORB-like basalts cored at Leg 15 Sites 146, 150, 152 and 153, and the overlying lavas are analogous to incompatible-element-enriched basalts cored at Site 151 on the Beata Ridge. Isotopic compositions of the Dumisseau basalts overlap with those of the eastern Pacific Galapagos and Easter Island hotspots. However, the presence of N-MORB basalts in the lower part of the Dumisseau and at the majority of Leg 15 Sites indicates that the anomalously thick Caribbean crust probably did not originate as a hotspot-related basaltic plateau, but may have been generated by on-ridge or near-ridge hotspot magmatism.     

Interferometry basics in practice: Exercises

The following exercises aim to learn the link between the object intensity distribution and the corresponding visibility curves of a long-baseline optical interferometer. They are also intended to show the additional constraints on observability that an interferometer has.This practical session is meant to be carried out with the ASPRO software, from the Jean-Marie Mariotti Center, but can also be done using other observation preparation software, such as viscalc from ESO.There are two main parts with series of exercises and the exercises corrections. The first one aims at understanding the visibility and its properties by practicing with simple examples, and the second one is about UV coverage.     

All you ever wanted to know about optical long baseline stellar interferometry, but were too shy to ask your adviser

I try to present a small view of the properties and issues related to astronomical interferometry observations. I recall a bit of history of the technique, give some basic assessments to the principle of interferometry, and finally, describe physical processes and limitations that affect optical long baseline interferometry and which are, in general, very useful for everyday work. Therefore, this text is not intended to perform strong demonstrations and show accurate results, but rather to transmit the general “feeling” one needs to have to not be destabilised by the first contact to real world interferometry.