322 research outputs found
A single intrinsic Josephson junction with double-sided fabrication technique
We make stacks of intrinsic Josephson junctions (IJJs) imbedded in the bulk
of very thin (~nm) single crystals.
By precisely controlling the etching depth during the double-sided fabrication
process, the stacks can be reproducibly tailor-made to be of any microscopic
height (), i.e. enclosing a specified number of IJJ (0-6),
including the important case of a single junction. We discuss reproducible
gap-like features in the current-voltage characteristics of the samples at high
bias.Comment: 3 pages, 4 figures, to be published in APL May. 2
Planar infall of CH3OH gas around Cepheus A HW2
Aims: In order to test the nature of an (accretion) disk in the vicinity of
Cepheus A HW2, we measured the three-dimensional velocity field of the CH3OH
maser spots, which are projected within 1000au of the HW2 object, with an
accuracy of the order of 0.1km/s. Methods: We made use of the European VLBI
Network (EVN) to image the 6.7GHz CH3OH maser emission towards Cepheus A HW2
with 4.5 milli-arcsecond resolution (3au). We observed at three epochs spaced
by one year between 2013 and 2015. During the last epoch, on mid-march 2015, we
benefited from the new deployed Sardinia Radio Telescope. Results: We show that
the CH3OH velocity vectors lie on a preferential plane for the gas motion with
only small deviations of 12+/-9 degrees away from the plane. This plane is
oriented at a position angle of 134 degrees east of north, and inclined by 26
degrees with the line-of-sight, closely matching the orientation of the
disk-like structure previously reported by Patel et al.(2005). Knowing the
orientation of the equatorial plane, we can reconstruct a face-on view of the
CH3OH gas kinematics onto the plane. CH3OH maser emission is detected within a
radius of 900au from HW2, and down to a radius of about 300au, the latter
coincident with the extent of the dust emission at 0.9mm. The velocity field is
dominated by an infall component of about 2km/s down to a radius of 300au,
where a rotational component of 4km/s becomes dominant. We discuss the nature
of this velocity field and the implications for the enclosed mass. Conclusions:
These findings bring direct support to the interpretation that the high-density
gas and dust emission, surrounding Cepheus A HW2, trace an accretion disk.Comment: 9 pages, 4 figures, 2 tables, accepted by Astronomy & Astrophysic
Modelling of railway curve squeal including effects of wheel rotation
Railway vehicles negotiating tight curves may emit an intense high-pitch noise. The underlying mechanisms of this squeal noise are still a subject of research. Simulation models are complex since they have to consider the non-linear, transient and high-frequency interaction between wheel and rail. Often simplified models are used for wheel and rail to reduce computational effort, which involves the risk of oversimplifications. This paper focuses on the importance to include a rotating wheel instead of a stationary wheel in the simulation models. Two formulations for a rotating wheel are implemented in a previously published wheel/rail interaction model: a realistic model based on an Eulerian modal coordinate approach and a simplified model based on a rotating load and moving Green's functions. The simulation results for different friction coefficients and values of lateral creepage are compared with results obtained for the stationary wheel. Both approaches for the rotating wheel give almost identical results for the rolling speed considered. Furthermore, it can be concluded that a model of a stationary flexible wheel is sufficient to simulate curve squeal
VALES: IV. Exploring the transition of star formation efficiencies between normal and starburst galaxies using APEX/SEPIA Band-5 and ALMA at low redshift
In this work we present new APEX/SEPIA Band-5 observations targeting the CO
() emission line of 24 Herschel-detected galaxies at .
Combining this sample {with} our recent new Valpara\'iso ALMA Line Emission
Survey (VALES), we investigate the star formation efficiencies (SFEs =
SFR/) of galaxies at low redshift. We find the SFE of our sample
bridges the gap between normal star-forming galaxies and Ultra-Luminous
Infrared Galaxies (ULIRGs), which are thought to be triggered by different star
formation modes. Considering the as the SFR and the
ratio, our data show a continuous and smooth increment as a function of
infrared luminosity (or star formation rate) with a scatter about 0.5 dex,
instead of a steep jump with a bimodal behaviour. This result is due to the use
of a sample with a much larger range of sSFR/sSFR using LIRGs, with
luminosities covering the range between normal and ULIRGs. We conclude that the
main parameters controlling the scatter of the SFE in star-forming galaxies are
the systematic uncertainty of the conversion factor, the gas
fraction and physical size.Comment: 9pages, 7 figures, 1 table, accepted for publication in MNRA
Sea-ice microbial communities in the Central Arctic Ocean: Limited responses to short-term pCO(2) perturbations
The Arctic Ocean is more susceptible to ocean acidification than other marine environments due to its weaker buffering capacity, while its cold surface water with relatively low salinity promotes atmospheric CO 2 uptake. We studied how sea-ice microbial communities in the central Arctic Ocean may be affected by changes in the carbonate system expected as a consequence of ocean acidification. In a series of four experiments during late summer 2018 aboard the icebreaker Oden, we addressed microbial growth, production of dissolved organic carbon (DOC) and extra- cellular polymeric substances (EPS), photosynthetic activity, and bacterial assemblage structure as sea-ice microbial communities were exposed to elevated partial pressures of CO 2 (pCO 2 ). We incubated intact, bottom ice-core sections and dislodged, under-ice algal aggregates (dominated by Melosira arctica) in separate experiments under approximately 400, 650, 1000, and 2000 micro atm pCO 2 for 10 d under different nutrient regimes. The results indicate that the growth of sea-ice algae and bacteria was unaffected by these higher pCO 2 levels, and concentrations of DOC and EPS were unaffected by a shifted inorganic C/N balance, resulting from the CO 2 enrichment. These central Arctic sea-ice microbial communities thus appear to be largely insensitive to short-term pCO 2 perturbations. Given the natural, seasonally driven fluctuations in the carbonate system of sea ice, its resident microorganisms may be sufficiently tolerant of large variations in pCO 2 and thus less vulnerable than pelagic communities to the impacts of ocean acidification, increasing the ecological importance of sea-ice microorganisms even as the loss of Arctic sea ice continue
GASP. X: APEX detection of molecular gas in the tails and in the disks of ram-pressure stripped galaxies
Jellyfish galaxies in clusters are key tools to understand environmental
processes at work in dense environments. The advent of Integral Field
Spectroscopy has recently allowed to study a significant sample of stripped
galaxies in the cluster environment at z, through the GAs Stripping
Phenomena in galaxies with MUSE (GASP) survey. However, optical spectroscopy
can only trace the ionized gas component through the H emission that
can be spatially resolved on kpc scale at this redshift. The complex interplay
between the various gas phases (ionized, neutral, molecular) is however yet to
be understood. We report here the detection of large amounts of molecular gas
both in the tails and in the disks of 4 jellyfish galaxies from the GASP sample
with stellar masses , showing
strong stripping. The mass of molecular gas that we measure in the tails
amounts to several and the total mass of molecular gas ranges
between 15 and 100 \% of the galaxy stellar mass. The molecular gas content
within the galaxies is compatible with the one of normal spiral galaxies,
suggesting that the molecular gas in the tails has been formed in-situ. We find
a clear correlation between the ionized gas emission and the
amount of molecular gas. The CO velocities measured from APEX data are not
always coincident with the underlying emitting knots, and the
derived Star Formation Efficiencies appear to be very low.Comment: 14 pages, 7 figures, Submitted to MNRA
Milliarcsecond structure of water maser emission in two young high-mass stellar objects associated with methanol masers
The 22.2 GHz water masers are often associated with the 6.7 GHz methanol
masers but owing to the different excitation conditions they likely probe
independent spatial and kinematic regions around the powering young massive
star. We compared the emission of these two maser species on milliarcsecond
scales to determine in which structures the masers arise and to test a
disc-outflow scenario where the methanol emission arises in a circumstellar
disc while the water emission comes from an outflow. We obtained high-angular
and spectral resolution 22.2 GHz water maser observations of the two sources
G31.581+00.077 and G33.641-00.228 using the EVN. In both objects the water
maser spots form complex and filamentary structures of sizes 18-160 AU. The
emission towards the source G31.581+00.077 comes from two distinct regions of
which one is related to the methanol maser source of ring-like shape. In both
targets the main axis of methanol distribution is orthogonal to the water maser
distribution. Most of water masers appear to trace shocks on a working surface
between an outflow/jet and a dense envelope. Some spots are possibly related to
the disc-wind interface which is as close as 100-150 AU to the regions of
methanol emission.Comment: 10 pages, accepted to Astronomy and Astrophysic
The properties and polarization of the H2O and CH3OH maser environment of NGC7538-IRS1
NGC7538 is a complex massive star-forming region. The region is composed of
several radio continuum sources, one of which is IRS1, a high-mass protostar,
from which a 0.3 pc molecular bipolar outflow was detected. Several maser
species have been detected around IRS1. The CH3OH masers have been suggested to
trace a Keplerian-disk, while the H2O masers are almost aligned to the outflow.
More recent results suggested that the region hosts a torus and potentially a
disk, but with a different inclination than the Keplerian-disk that is supposed
to be traced by the CH3OH masers. Tracing the magnetic field close to
protostars is fundamental for determining the orientation of the disk/torus.
Recent studies showed that during the protostellar phase of high-mass star
formation the magnetic field is oriented along the outflows and around or on
the surfaces of the disk/torus. The observations of polarized maser emissions
at milliarcsecond resolution can make a crucial contribution to understanding
the orientation of the magnetic field and, consequently, the orientation of the
disk/torus in NGC7538-IRS1. The NRAO Very Long Baseline Array was used to
measure the linear polarization and the Zeeman-splitting of the 22GHz H2O
masers toward NGC7538-IRS1. The European VLBI Network and the MERLIN telescopes
were used to measure the linear polarization and the Zeeman-splitting of the
6.7GHz CH3OH masers toward the same region. We detected 17 H2O masers and 49
CH3OH masers at high angular resolution. We detected linear polarization
emission toward two H2O masers and toward twenty CH3OH masers. The CH3OH
masers, most of which only show a core structure, seem to trace rotating and
potentially infalling gas in the inner part of a torus. Significant
Zeeman-splitting was measured in three CH3OH masers. [...] We also propose a
new description of the structure of the NGC7538-IRS1 maser region.Comment: 13 pages, 9 figures, 4 Tables, accepted by Astronomy & Astrophysic
Intermittent maser flare around the high mass young stellar object G353.273+0.641 I: data & overview
We have performed VLBI and single-dish monitoring of 22 GHz HO maser
emission from the high mass young stellar object G353.273+0.641 with VERA (VLBI
Exploration of Radio Astrometry) and Tomakamai 11-m radio telescope. Two maser
flares have been detected, separated almost two years. Frequent VLBI monitoring
has revealed that these flare activities have been accompanied by structural
change of the prominent shock front traced by H2O maser alignments. We have
detected only blue-shifted emissions and all maser features have been
distributed within very small area of 200 200 au in spite of
wide velocity range (> 100 km s). The light curve shows notably
intermittent variation and suggests that the HO masers in G353.273+0.641
are excited by episodic radio jet. The time-scale of \sim2 yr and
characteristic velocity of \sim500 km s also support this
interpretation. Two isolated velocity components of C50 (-53 \pm 7 km s)
and C70 (-73 \pm 7 km s) have shown synchronised linear acceleration of
the flux weighted V_{\rmn{LSR}} values (\sim-5 km s yr) during
the flare phase. This can be converted to the lower-limit momentum rate of 1.1
\times 10 M_{\sun} km s yr. Maser properties are quite
similar to that of IRAS 20126+4104 especially. This corroborates the previous
suggestion that G353.273+0.641 is a candidate of high mass protostellar object.
The possible pole-on geometry of disc-jet system can be suitable for direct
imaging of the accretion disc in this case.Comment: 13 pages, 5 figures accepted for publication in MNRA
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