17,903 research outputs found
The circumstellar environment of HD50138 revealed by VLTI/AMBER at high angular resolution
HD50138 is a Herbig B[e] star with a circumstellar disc detected at IR and mm
wavelength. Its brightness makes it a good candidate for NIR interferometry
observations. We aim to resolve, spatially and spectrally, the continuum and
hydrogen emission lines in the 2.12-2.47 micron region, to shed light on the
immediate circumstellar environment of the star. VLTI/AMBER K-band observations
provide spectra, visibilities, differential phases, and closure phases along
three long baselines for the continuum, and HI emission in Br and five
high-n Pfund lines. By computing the pure-line visibilities, we derive the
angular size of the different line-emitting regions. A simple LTE model was
created to constrain the physical conditions of HI emitting region. The
continuum region cannot be reproduced by a geometrical 2D elongated Gaussian
fitting model. We estimate the size of the region to be 1 au. We find the
Br and Pfund lines come from a more compact region of size 0.4 au. The
Br line exhibits an S-shaped differential phase, indicative of
rotation. The continuum and Br line closure phase show offsets of
-255 and 2010, respectively. This is evidence of an
asymmetry in their origin, but with opposing directions. We find that we cannot
converge on constraints for the HI physical parameters without a more detailed
model. Our analysis reveals that HD50138 hosts a complex circumstellar
environment. Its continuum emission cannot be reproduced by a simple disc
brightness distribution. Similarly, several components must be evoked to
reproduce the interferometric observables within the Br, line.
Combining the spectroscopic and interferometric data of the Br and
Pfund lines favours an origin in a wind region with a large opening angle.
Finally, our results point to an evolved source.Comment: accepted for publication in A&
Evolution and excitation conditions of outflows in high-mass star-forming regions
Theoretical models suggest that massive stars form via disk-mediated
accretion, with bipolar outflows playing a fundamental role. A recent study
toward massive molecular outflows has revealed a decrease of the SiO line
intensity as the object evolves. The present study aims at characterizing the
variation of the molecular outflow properties with time, and at studying the
SiO excitation conditions in outflows associated with massive YSOs. We used the
IRAM30m telescope to map 14 massive star-forming regions in the SiO(2-1),
SiO(5-4) and HCO+(1-0) outflow lines, and in several dense gas and hot core
tracers. Hi-GAL data was used to improve the spectral energy distributions and
the L/M ratio, which is believed to be a good indicator of the evolutionary
stage of the YSO. We detect SiO and HCO+ outflow emission in all the sources,
and bipolar structures in six of them. The outflow parameters are similar to
those found toward other massive YSOs. We find an increase of the HCO+ outflow
energetics as the object evolve, and a decrease of the SiO abundance with time,
from 10^(-8) to 10^(-9). The SiO(5-4) to (2-1) line ratio is found to be low at
the ambient gas velocity, and increases as we move to high velocities,
indicating that the excitation conditions of the SiO change with the velocity
of the gas (with larger densities and/or temperatures for the high-velocity gas
component). The properties of the SiO and HCO+ outflow emission suggest a
scenario in which SiO is largely enhanced in the first evolutionary stages,
probably due to strong shocks produced by the protostellar jet. As the object
evolves, the power of the jet would decrease and so does the SiO abundance.
During this process, however, the material surrounding the protostar would have
been been swept up by the jet, and the outflow activity, traced by entrained
molecular material (HCO+), would increase with time.Comment: 31 pages, 10 figures and 5 tables (plus 2 figures and 3 tables in the
appendix). Accepted for publication in A&A. [Abstract modified to fit the
arXiv requirements.
Chemical abundances of damped Lyman alpha systems in the XQ-100 survey
The XQ-100 survey has provided high signal-noise spectra of 100 redshift
3-4.5 quasars with the X-Shooter spectrograph. The metal abundances for 13
elements in the 41 damped Lyman alpha systems (DLAs) identified in the XQ-100
sample are presented, and an investigation into abundances of a variety of DLA
classes is conducted. The XQ-100 DLA sample contains five DLAs within 5000 km/s
of their host quasar (proximate DLAs; PDLAs) as well as three sightlines which
contain two DLAs within 10,000 km/s of each other along the same line-of-sight
(multiple DLAs; MDLAs). Combined with previous observations in the literature,
we demonstrate that PDLAs with logN(HI)<21.0 show lower [S/H] and [Fe/H]
(relative to intervening systems with similar redshift and N(HI)), whilst
higher [S/H] and [Si/H] are seen in PDLAs with logN(HI)>21.0. These abundance
discrepancies are independent of their line-of-sight velocity separation from
the host quasar, and the velocity width of the metal lines (v90). Contrary to
previous studies, MDLAs show no difference in [alpha/Fe] relative to single
DLAs matched in metallicity and redshift. In addition, we present follow-up
UVES data of J0034+1639, a sightline containing three DLAs, including a
metal-poor DLA with [Fe/H]=-2.82 (the third lowest [Fe/H] in DLAs identified to
date) at z=4.25. Lastly we study the dust-corrected [Zn/Fe], emphasizing that
near-IR coverage of X-Shooter provides unprecedented access to MgII, CaII and
TiII lines (at redshifts 3-4) to provide additional evidence for subsolar
[Zn/Fe] ratio in DLAs.Comment: Accepted to MNRAS. 19 pages plus Appendix material (102 pages total
Monitoring the influence of marine aquaculture on wild fish communities: benefits and limitations of fatty acid profiles
Fatty acids (FA) have been applied as indicators of the influence of coastal sea-cage fish farming on wild fish communities in several recent scientific publications. Due to the relatively high conservation of FA composition throughout the food web, they are useful for characterizing trophic relationships. The increasing utilization of vegetable or alternative animal oils in the production of aquafeeds results in cultivated fish exhibiting higher levels of terrestrial FAs in their tissues. As previously reported, wild fish ubiquitously aggregate around fish farms as a consequence of the introduction of new habitat and the easy availability of food—fish farms act as enhanced fish aggregation devices (FADs). The influence of food pellets on the composition of wild fish has been detected in recent studies on salmon, sea bass and sea bream aquaculture, with increased levels of linoleic acid (18:2n-6) and a low n-3/n-6 ratio as clear indicators of the consumption of food pellets from the farms. The potential ecological and physiological effects on wild fish are presently unknown. In the present article, guidelines are proposed for the investigation and use of terrestrial FAs to track the effects of coastal aquaculture on wild fish communities and local fisheries, as well as the benefits or limitations of this technique
Accuracy of a method based on atomic absorption spectrometry to determine inorganic arsenic in food : Outcome of the collaborative trial IMEP-41
Peer reviewedPublisher PD
Galaxy And Mass Assembly (GAMA): a deeper view of the mass, metallicity and SFR relationships
A full appreciation of the role played by gas metallicity (Z), star formation rate (SFR) and stellar mass (M*) is fundamental to understanding how galaxies form and evolve. The connections between these three parameters at different redshifts significantly affect galaxy evolution, and thus provide important constraints for galaxy evolution models. Using data from the Sloan Digital Sky Survey–Data Release 7 (SDSS–DR7) and the Galaxy and Mass Assembly (GAMA) surveys, we study the relationships and dependences between SFR, Z and M*, as well as the Fundamental Plane for star-forming galaxies. We combine both surveys using volume-limited samples up to a redshift of z ≈ 0.36. The GAMA and SDSS surveys complement each other when analysing the relationships between SFR, M* and Z. We present evidence for SFR and metallicity evolution to z ∼ 0.2. We study the dependences between SFR, M*, Z and specific SFR (SSFR) on the M*–Z, M*–SFR, M*–SSFR, Z–SFR and Z–SSFR relations, finding strong correlations between all. Based on those dependences, we propose a simple model that allows us to explain the different behaviour observed between low- and high-mass galaxies. Finally, our analysis allows us to confirm the existence of a Fundamental Plane, for which M* = f(Z, SFR) in star-forming galaxies
- …