40 research outputs found
Evidence for ubiquitous carbon grain destruction in hot protostellar envelopes
Earth is deficient in carbon and nitrogen by up to orders of
magnitude compared with the Sun. Destruction of (carbon- and nitrogen-rich)
refractory organics in the high-temperature planet forming regions could
explain this deficiency. Assuming a refractory cometary composition for these
grains, their destruction enhances nitrogen-containing oxygen-poor molecules in
the hot gas (K) after the initial formation and sublimation of
these molecules from oxygen-rich ices in the warm gas (K). Using
observations of high-mass protostars with ALMA, we find that
oxygen-containing molecules (CHOH and HNCO) systematically show no
enhancement in their hot component. In contrast, nitrogen-containing,
oxygen-poor molecules (CHCN and CHCN) systematically show an
enhancement of a factor in their hot component, pointing to
additional production of these molecules in the hot gas. Assuming only thermal
excitation conditions, we interpret these results as a signature of destruction
of refractory organics, consistent with the cometary composition. This
destruction implies a higher C/O and N/O in the hot gas than the warm gas,
while, the exact values of these ratios depend on the fraction of grains that
are effectively destroyed. This fraction can be found by future chemical models
that constrain C/O and N/O from the abundances of minor carbon, nitrogen and
oxygen carriers presented here.Comment: Accepted for publication in ApJ Letter
"Drop in" gastroscopy outpatient clinic - experience after 9 months
<p>Abstract</p> <p>Background</p> <p>Logistics handling referrals for gastroscopy may be more time consuming than the examination itself. For the patient, "drop in" gastroscopy may reduce uncertainty, inadequate therapy and time off work.</p> <p>Methods</p> <p>After an 8-9 month run-in period we asked patients, hospital staff and GPs to fill in a questionnaire to evaluate their experience with "drop in" gastroscopy and gastroscopy by appointment, respectively. The diagnostic gain was evaluated.</p> <p>Results</p> <p>112 patients had "drop in" gastroscopy and 101 gastroscopy by appointment. The number of "drop in" patients varied between 3 and 12 per day (mean 6.5). Mean time from first GP consultation to gastroscopy was 3.6 weeks in the "drop in" group and 14 weeks in the appointment group. The half-yearly number of outpatient gastroscopies increased from 696 before introducing "drop in" to 1022 after (47% increase) and the proportion of examinations with pathological findings increased from 42% to 58%. Patients and GPs expressed great satisfaction with "drop in". Hospital staff also acclaimed although it caused more unpredictable working days with no additional staff.</p> <p>Conclusions</p> <p>"Drop in" gastroscopy was introduced without increase in staff. The observed increase in gastroscopies was paralleled by a similar increase in pathological findings without any apparent disadvantages for other groups of patients. This should legitimise "drop in" outpatient gastroscopies, but it requires meticulous observation of possible unwanted effects when implemented.</p
Robustness of NâHâş as tracer of the CO snowline
Context. Snowlines in protoplanetary disks play an important role in planet formation and composition. Since the CO snowline is difficult to observe directly with CO emission, its location has been inferred in several disks from spatially resolved ALMA observations of DCOâş and NâHâş. Aims. NâHâş is considered to be a good tracer of the CO snowline based on astrochemical considerations predicting an anti-correlation between NâHâş and gas-phase CO. In this work, the robustness of NâHâş as a tracer of the CO snowline is investigated. Methods. A simple chemical network was used in combination with the radiative transfer code LIME to model the NâHâş distribution and corresponding emission in the disk around TW Hya. The assumed CO and Nâ abundances, corresponding binding energies, cosmic ray ionization rate, and degree of large-grain settling were varied to determine the effects on the NâHâş emission and its relation to the CO snowline. Results. For the adopted physical structure of the TW Hya disk and molecular binding energies for pure ices, the balance between freeze-out and thermal desorption predicts a CO snowline at 19 AU, corresponding to a CO midplane freeze-out temperature of 20 K. The NâHâş column density, however, peaks 5â30 AU outside the snowline for all conditions tested. In addition to the expected NâHâş layer just below the CO snow surface, models with an Nâ/CO ratio 0.2 predict an NâHâş layer higher up in the disk due to a slightly lower photodissociation rate for Nâ as compared to CO. The influence of this NâHâş surface layer on the position of the emission peak depends on the total CO and Nâ abundances and the disk physical structure, but the emission peak generally does not trace the column density peak. A model with a total (gas plus ice) CO abundance of 3 Ă 10â6 with respect to Hâ fits the position of the emission peak previously observed for the TW Hya disk. Conclusions. The relationship between NâHâş and the CO snowline is more complicated than generally assumed: for the investigated parameters, the NâHâş column density peaks at least 5 AU outside the CO snowline. Moreover, the NâHâş emission can peak much further out, as far as âź50 AU beyond the snowline. Hence, chemical modeling, as performed here, is necessary to derive a CO snowline location from NâHâş observations
Ecological commonalities among pelagic fishes: comparison of freshwater ciscoes and marine herring and sprat
Systematic comparisons of the ecology between functionally similar fish species from freshwater and marine aquatic systems are surprisingly rare. Here, we discuss commonalities and differences in evolutionary history, population genetics, reproduction and life history, ecological interactions, behavioural ecology and physiological ecology of temperate and Arctic freshwater coregonids (vendace and ciscoes, Coregonus spp.) and marine clupeids (herring, Clupea harengus, and sprat, Sprattus sprattus). We further elucidate potential effects of climate warming on these groups of fish based on the ecological features of coregonids and clupeids documented in the previous parts of the review. These freshwater and marine fishes share a surprisingly high number of similarities. Both groups are relatively short-lived, pelagic planktivorous fishes. The genetic differentiation of local populations is weak and seems to be in part correlated to an astonishing variability of spawning times. The discrete thermal window of each species influences habitat use, diel vertical migrations and supposedly also life history variations. Complex life cycles and preference for cool or cold water make all species vulnerable to the effects of global warming. It is suggested that future research on the functional interdependence between spawning time, life history characteristics, thermal windows and genetic differentiation may profit from a systematic comparison of the patterns found in either coregonids or clupeids