51,029 research outputs found
Using deuterated H3+ and other molecular species to understand the formation of stars and planets
The H3+ ion plays a key role in the chemistry of dense interstellar gas
clouds where stars and planets are forming. The low temperatures and high
extinctions of such clouds make direct observations of H3+ impossible, but lead
to large abundances of H2D+ and D2H+ which are very useful probes of the early
stages of star and planet formation. Maps of H2D+ and D2H+ pure rotational line
emission toward star-forming regions show that the strong deuteration of H3+ is
the result of near-complete molecular depletion of CNO-bearing molecules onto
grain surfaces, which quickly disappears as cores warm up after stars have
formed.
In the warmer parts of interstellar gas clouds, H3+ transfers its proton to
other neutrals such as CO and N2, leading to a rich ionic chemistry. The
abundances of such species are useful tracers of physical conditions such as
the radiation field and the electron fraction. Recent observations of HF line
emission toward the Orion Bar imply a high electron fraction, and we suggest
that observations of OH+ and H2O+ emission may be used to probe the electron
density in the nuclei of external galaxies.Comment: Proceedings of the H3+ centennial symposium, to be published in RSPTA
(editor: T. Oka
Universal Local symmetries and non-superposition in classical mechanics
In the Hilbert space formulation of classical mechanics (CM), pioneered by
Koopman and von Neumann (KvN), there are potentially more observables that in
the standard approach to CM. In this paper we show that actually many of those
extra observables are not invariant under a set of universal local symmetries
which appear once the KvN is extended to include the evolution of differential
forms. Because of their non-invariance, those extra observables have to be
removed. This removal makes the superposition of states in KvN, and as a
consequence also in CM, impossible
Can we trace very cold dust from its emission alone ?
Context. Dust is a good tracer of cold dark clouds but its column density is
difficult to quantify. Aims. We want to check whether the far-infrared and
submillimeter high-resolution data from Herschel SPIRE and PACS cameras
combined with ground-based telescope bolometers allow us to retrieve the whole
dust content of cold dark clouds. Methods. We compare far-infrared and
submillimeter emission across L183 to the 8 m absorption map from Spitzer
data and fit modified blackbody functions towards three different positions.
Results. We find that none of the Herschel SPIRE channels follow the cold dust
profile seen in absorption. Even the ground-based submillimeter telescope
observations, although more closely following the absorption profile, cannot
help to characterize the cold dust without external information such as the
dust column density itself. The difference in dust opacity can reach up to a
factor of 3 in prestellar cores of high extinction. Conclusions. In dark
clouds, the amount of very cold dust cannot be measured from its emission
alone. In particular, studies of dark clouds based only on Herschel data can
miss a large fraction of the dust content. This has an impact on core and
filament density profiles, masse and stability estimates.Comment: Letter to A&A (accepted for publication). must be viewed with ACROBAT
READER for full enhancement. Otherwise, check images in Appendix
Chemistry and kinematics of the pre-stellar core L1544: Constraints from H2D+
This paper explores the sensitivity of line profiles of H2D+, HCO+ and N2H+,
observed towards the center of L1544, to various kinematic and chemical
parameters. The total width of the H2D+ line can be matched by a static model
and by models invoking ambipolar diffusion and gravitational collapse. The
derived turbulent line width is b=0.15 km/s for the static case and <~ 0.05
km/s for the collapse case. However, line profiles of HC18O+ and N2H+ rule out
the static solution. The double-peaked H2D+ line shape requires either infall
speeds in the center that are much higher than predicted by ambipolar diffusion
models, or a shell-type distribution of H2D+, as is the case for HCO+ and N2H+.
At an offset of ~20 arcsec from the dust peak, the H2D+ abundance drops by a
factor of ~5.Comment: four pages, two colour figures; to appear in The Dense Interstellar
Medium in Galaxies, proceedings of the fourth Cologne-Bonn-Zermatt Symposium,
Sept 22-26, 200
Alkenone producers during late Oligocene-early Miocene revisited
This study investigates ancient alkenone producers among the late Oligocene–early Miocene coccolithophores recorded at Deep Sea Drilling Project (DSDP) Site 516. Contrary to common assumptions, Reticulofenestra was not the most important alkenone producer throughout the studied time interval. The comparison between coccolith species-specific absolute abundances and alkenone contents in the same sedimentary samples shows that Cyclicargolithus abundances explain 40% of the total variance of alkenone concentration and that the species Cyclicargolithus floridanus was a major alkenone producer, although other related taxa may have also contributed to the alkenone production at DSDP Site 516. The distribution of the different alkenone isomers (MeC37:2, EtC38:2, and MeC38:2) remained unchanged across distinct changes in species composition, suggesting similar diunsaturated alkenone compositions within the Noelaerhabdaceae family during the late Oligocene–early Miocene. However, the overall larger cell size of Cyclicargolithus may have implications for the alkenone-based reconstruction of past partial pressure of CO2. Our results underscore the importance of a careful evaluation of the most likely alkenone producers for periods (>1.85 Ma) predating the first occurrence of contemporary alkenone producers (i.e., Emiliania huxleyi and Gephyrocapsa oceanica)
Comments on the paper "The initial conditions of isolated star formation - VI. SCUBA mapping of prestellar cores" (Kirk et al. 2005)
In their survey paper of prestellar cores with SCUBA, Kirk et al. (2005) have
discarded two of our papers on L183 (Pagani et al. 2003, 2004). However these
papers bring two important pieces of information that they cannot ignore.
Namely, the real structure of L183 and the very poor correlation between
submillimeter and far infrared (FIR) dust emission beyond \Avb 15
mag. Making the erroneous assumption that it is the same dust that we are
seeing in emission at both 200 and 850 m, they derive constant
temperatures which are only approximate, and column densities which are too
low. In fact dust temperatures do decrease inside dark clouds and the FIR
emission is only tracing the outer parts of the dark clouds (Pagani et al.
2004
Insensitivity of alkenone carbon isotopes to atmospheric CO<sub>2</sub> at low to moderate CO<sub>2</sub> levels
Atmospheric pCO2 is a critical component of the global carbon system and is considered to be the major control of Earth’s past, present and future climate. Accurate and precise reconstructions of its concentration through geological time are, therefore, crucial to our understanding of the Earth system. Ice core records document pCO2 for the past 800 kyrs, but at no point during this interval were CO2 levels higher than today. Interpretation of older pCO2 has been hampered by discrepancies during some time intervals between two of the main ocean-based proxy methods used to reconstruct pCO2: the carbon isotope fractionation that occurs during photosynthesis as recorded by haptophyte biomarkers (alkenones) and the boron isotope composition (δ11B) of foraminifer shells. Here we present alkenone and δ11B-based pCO2 reconstructions generated from the same samples from the Plio-Pleistocene at ODP Site 999 across a glacial-interglacial cycle. We find a muted response to pCO2 in the alkenone record compared to contemporaneous ice core and δ11B records, suggesting caution in the interpretation of alkenone-based records at low pCO2 levels. This is possibly caused by the physiology of CO2 uptake in the haptophytes. Our new understanding resolves some of the inconsistencies between the proxies and highlights that caution may be required when interpreting alkenone-based reconstructions of pCO2
On the frequency of N2H+ and N2D+
Context : Dynamical studies of prestellar cores search for small velocity
differences between different tracers. The highest radiation frequency
precision is therefore required for each of these species. Aims : We want to
adjust the frequency of the first three rotational transitions of N2H+ and N2D+
and extrapolate to the next three transitions. Methods : N2H+ and N2D+ are
compared to NH3 the frequency of which is more accurately known and which has
the advantage to be spatially coexistent with N2H+ and N2D+ in dark cloud
cores. With lines among the narrowests, and N2H+ and NH3 emitting region among
the largests, L183 is a good candidate to compare these species. Results : A
correction of ~10 kHz for the N2H+ (J:1-0) transition has been found (~0.03
km/s) and similar corrections, from a few m/s up to ~0.05 km/s are reported for
the other transitions (N2H+ J:3-2 and N2D+ J:1-0, J:2-1, and J:3-2) compared to
previous astronomical determinations. Einstein spontaneous decay coefficients
(Aul) are included
BIMA N2H+ 1-0 mapping observations of L183 -- fragmentation and spin-up in a collapsing, magnetized, rotating, pre-stellar core
We have used the Berkeley-Illinois-Maryland Array (BIMA) to make deep N2H+
1-0 maps of the pre-stellar core L183, in order to study the spatial and
kinematic substructure within the densest part of the core. Three spatially and
kinematically distinct clumps are detected, which we label L183-N1, L183-N2 and
L183-N3. L183-N2 is approximately coincident with the submillimetre dust peak
and lies at the systemic velocity of L183. Thus we conclude that L183-N2 is the
central dense core of L183. L183-N1 and 3 are newly-discovered fragments of
L183, which are marked by velocity gradients that are parallel to, but far
stronger than, the velocity gradient of L183 as a whole, as detected in
previous single-dish data. Furthermore, the ratio of the large-scale and
small-scale velocity gradients, and the ratio of their respective size-scales,
are consistent with the conservation of angular momentum for a rotating,
collapsing core undergoing spin-up. The inferred axis of rotation is parallel
to the magnetic field direction, which is offset from its long axis, as we have
seen in other pre-stellar cores. Therefore, we propose that we have detected a
fragmenting, collapsing, filamentary, pre-stellar core, rotating about its
B-field, which is spinning up as it collapses. It will presumably go on to form
a multiple protostellar system.Comment: 7 figures, 1 table, 21 pages, accepted for publication in Ap
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