Andromeda's New Clothes: Revealing the Causality Hidden in the Paradox

The Andromeda paradox is a popular version of the Rietdijk-Putnam argument based on unobservable spacetime events along planes of simultaneity. We show and prove that an extrapolation of the motions of frames of reference in a manner consistent with special relativity leads to a physically meaningful interpretation of the paradox in terms of future light-cone observables. The unobservable time shifts in the Andromeda paradox are thus found to be predictors of light travel time differences between moving observers upon the arrival of light from an event in that galaxy. This discovery of eventual causality not only undermines metaphysical arguments that rely on relative simultaneity to promote four-dimensionalism, but is also a timely contribution to counter recent claims in physics literature that imply the existence of rather exotic astronomical phenomena not yet observed by scientific means

High-Resolution Measurements of Intersystem Bands of Carbon Monoxide toward X Persei

In an echelle spectrum of X Per acquired with the Space Telescope Imaging Spectrograph we have identified individual rotational lines of 11 triplet-singlet (intersystem) absorption bands of ^12CO. Four bands provide first detections for interstellar clouds. From a comparison with the zeta Oph sight line we find that X Per is obscured by a higher 12CO column density of 1.4 x 10^16 cm-2. Together with the high spectral resolution of 1.3 km s-1, this allows (i) an improved measurement of previously published f-values for seven bands, and (ii) an extraction of the first astrophysical oscillator strengths for d-X (8-0), (9-0), and (10-0), as well as for e-X (12-0). The ^13CO d-X (12-0) band, previously suspected to exist toward zeta Oph, is now readily resolved and modeled. Our derived intersystem f-values for ^12CO include a few mild (leq 34%) disagreements with recent predictions from a perturbation analysis calculated for the interstellar excitation temperature. Overall, the comparison confirms the superiority of employing multiple singlet levels in the calculations of mixing coefficients over previous single-level predictions.Comment: 11 pages (incl. 1 figure). Accepted by ApJ Letter

FUSE Measurements of Interstellar Fluorine

The source of fluorine is not well understood, although core-collapse supernovae, Wolf-Rayet stars, and asymptotic giant branch stars have been suggested. A search for evidence of the nu process during Type II supernovae is presented. Absorption from interstellar F I is seen in spectra of HD 208440 and HD 209339A acquired with the Far Ultraviolet Spectroscopic Explorer. In order to extract the column density for F I from the line at 954 A, absorption from H2 has to be modeled and then removed. Our analysis indicates that for H2 column densities less than about 3 x 10^20 cm^-2, the amount of F I can be determined from lambda 954. For these two sight lines, there is no clear indication for enhanced F abundances resulting from the nu process in a region shaped by past supernovae.Comment: 17 pages, 4 figures, accepted for publication in Ap

HIGH-RESOLUTION MEASUREMENTS OF INTERSYSTEM BANDS OF CARBON MONOXIDE TOWARD X PERSEI 1

CO include a few mild (≤34%) disagreements with recent predictions from a perturbation analysis calculated for the interstellar excitation temperature. Overall, the comparison confirms the superiority of employing multiple singlet levels in the calculations of mixing coefficients over previous single-level predictions

C60_{60} in Photodissociation Regions

Recent studies have confirmed the presence of buckminsterfullerene (C$_{60}$) in different interstellar and circumstellar environments. However, several aspects regarding C$_{60}$ in space are not well understood yet, such as the formation and excitation processes, and the connection between C$_{60}$ and other carbonaceous compounds in the interstellar medium, in particular polycyclic aromatic hydrocarbons (PAHs). In this paper we study several photodissociation regions (PDRs) where C$_{60}$ and PAHs are detected and the local physical conditions are reasonably well constrained, to provide observational insights into these questions. C$_{60}$ is found to emit in PDRs where the dust is cool ($T_d = 20-40$ K) and even in PDRs with cool stars. These results exclude the possibility for C$_{60}$ to be locked in grains at thermal equilibrium in these environments. We observe that PAH and C$_{60}$ emission are spatially uncorrelated and that C$_{60}$ is present in PDRs where the physical conditions (in terms of radiation field and hydrogen density) allow for full dehydrogenation of PAHs, with the exception of Ced 201. We also find trends indicative of an increase in C$_{60}$ abundance within individual PDRs, but these trends are not universal. These results support models where the dehydrogenation of carbonaceous species is the first step towards C$_{60}$ formation. However, this is not the only parameter involved and C$_{60}$ formation is likely affected by shocks and PDR age

Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide

Ultraviolet absorption from interstellar 12CO and 13CO was detected toward rho Oph A and chi Oph. The measurements were obtained at medium resolution with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column density ratios, N(12CO)/N(13CO), of 125 \pm 23 and 117 \pm 35 were derived for the sight lines toward rho Oph A and chi Oph, respectively. A value of 1100 \pm 600 for the ratio N(12C16O)/N(12C18O) toward rho Oph A was also obtained. Absorption from vibrationally excited H_2 (v" = 3) was clearly seen toward this star as well. The ratios are larger than the isotopic ratios for carbon and oxygen appropriate for ambient interstellar material. Since for both carbon and oxygen the more abundant isotopomer is enhanced, selective isotopic photodissociation plays the key role in the fractionation process for these directions. The enhancement arises because the more abundant isotopomer has lines that are more optically thick, resulting in more self shielding from dissociating radiation. A simple argument involving the amount of self shielding [from N(12CO)] and the strength of the ultraviolet radiation field premeating the gas (from the amount of vibrationally excited H_2) shows that selective isotopic photodissociation controls the fractionation seen in these two sight lines, as well as the sight line to zeta Oph.Comment: 40 pages, 8 figures, to appear in 10 July 2003 issue of Ap

Dryland mechanisms could widely control ecosystem functioning in a drier and warmer world

Responses of terrestrial ecosystems to climate change have been explored in many regions worldwide. While continued drying and warming may alter process rates and deteriorate the state and performance of ecosystems, it could also lead to more fundamental changes in the mechanisms governing ecosystem functioning. Here we argue that climate change will induce unprecedented shifts in these mechanisms in historically wetter climatic zones, towards mechanisms currently prevalent in dry regions, which we refer to as ‘dryland mechanisms’. We discuss 12 dryland mechanisms affecting multiple processes of ecosystem functioning, including vegetation development, water flow, energy budget, carbon and nutrient cycling, plant production and organic matter decomposition. We then examine mostly rare examples of the operation of these mechanisms in non-dryland regions where they have been considered irrelevant at present. Current and future climate trends could force microclimatic conditions across thresholds and lead to the emergence of dryland mechanisms and their increasing control over ecosystem functioning in many biomes on Earth.The support of the Israel Science Foundation is acknowledged by J.M.G. (grant number 1796/19), O.A. (1185/17) and E.M. (1053/17). M.B. acknowledges funding through the ÖAW-ESS project ClimGrassHydro (Austrian Academy of Sciences).Peer reviewe