8,444 research outputs found
A new method for estimating the pattern speed of spiral structure in the Milky Way
In the last few decades many efforts have been made to understand the effect
of spiral arms on the gas and stellar dynamics in the Milky Way disc. One of
the fundamental parameters of the spiral structure is its angular velocity, or
pattern speed , which determines the location of resonances in the
disc and the spirals' radial extent. The most direct method for estimating the
pattern speed relies on backward integration techniques, trying to locate the
stellar birthplace of open clusters. Here we propose a new method based on the
interaction between the spiral arms and the stars in the disc. Using a sample
of around 500 open clusters from the {\it New Catalogue of Optically Visible
Open Clusters and Candidates}, and a sample of 500 giant stars observed by
APOGEE, we find km s kpc, for a local
standard of rest rotation ~km s and solar radius ~kpc.
Exploring a range in and within the acceptable values, 200-240 km
s and 7.5-8.5 kpc, respectively, results only in a small change in our
estimate of , that is within the error. Our result is in close
agreement with a number of studies which suggest values in the range 20-25 km
s kpc. An advantage of our method is that we do not need
knowledge of the stellar age, unlike in the case of the birthplace method,
which allows us to use data from large Galactic surveys. The precision of our
method will be improved once larger samples of disk stars with spectroscopic
information will become available thanks to future surveys such as 4MOST.Comment: 10 pages, 6 figures, 4 tables, accepted for publication in MNRA
Bridge over troubled gas: clusters and associations under the SMC and LMC tidal stresses
We obtained SOAR telescope B and V photometry of 14 star clusters and 2
associations in the Bridge tidal structure connecting the LMC and SMC. These
objects are used to study the formation and evolution of star clusters and
associations under tidal stresses from the Clouds. Typical star clusters in the
Bridge are not richly populated and have in general relatively large diameters
(~30-35 pc), being larger than Galactic counterparts of similar age. Ages and
other fundamental parameters are determined with field-star decontaminated
photometry. A self-consistent approach is used to derive parameters for the
most-populated sample cluster NGC 796 and two young CMD templates built with
the remaining Bridge clusters. We find that the clusters are not coeval in the
Bridge. They range from approximately a few Myr (still related to optical HII
regions and WISE and Spitzer dust emission measurements) to about 100-200 Myr.
The derived distance moduli for the Bridge objects suggests that the Bridge is
a structure connecting the LMC far-side in the East to the foreground of the
SMC to the West. Most of the present clusters are part of the tidal dwarf
candidate D 1, which is associated with an H I overdensity. We find further
evidence that the studied part of the Bridge is evolving into a tidal dwarf
galaxy, decoupling from the Bridge.Comment: 15 pages, 15 figures, MNRAS, Accepted 2015 July 2
Wave Equation for Sound in Fluids with Vorticity
We use Clebsch potentials and an action principle to derive a closed system
of gauge invariant equations for sound superposed on a general background flow.
Our system reduces to the Unruh (1981) and Pierce (1990) wave equations when
the flow is irrotational, or slowly varying. We illustrate our formalism by
applying it to waves propagating in a uniformly rotating fluid where the sound
modes hybridize with inertial waves.Comment: RevTeX, 27page
The globular cluster NGC 6642: Evidence for a depleted mass function in a very old cluster
We present photometry for the globular cluster NGC 6642 using the F606W and
F814W filters with the ACS/WFC third generation camera on board of Hubble Space
Telescope. The Colour Magnitude Diagram shows sources reaching ~ 6 mags below
the turn-off in m_F606W. A theoretical isochrone fitting was performed and
evolutionary parameters were obtained, such as the metallicity [Fe/H] = -1.80
+/- 0.2 and age log(Age) = 10.14 +/- 0.05. We confirm that NGC 6642 is located
in the Galactic bulge, with a distance to the Sun d_{\odot} = 8.05 +/- 0.66 ~
kpc$ and the reddening E(B-V) = 0.46 +/- 0.02. These values are in general
agreement with those of previous authors. About 30 blue stragglers were found
within the central 1.6 pc of NGC 6642. They are strongly concentrated to the
very central regions. The cluster displays a well-developed horizontal branch,
with a much redder morphology than that of typical old halo globular clusters
of similar metallicity. Completeness corrected luminosity and mass functions
were obtained for different annuli centred on NGC 6642. Their spatial variation
indicates the existence of mass segregation and depletion of low mass stars.
Most striking is the inverted shape of the mass function itself, with an
increase in number as a function of increasing mass. This has been previously
observed in other globular clusters and is also the result of N-body
simulations of stellar systems which have undergone ~ 90% of their lifetime and
which are subjected to strong tidal effects. We also analysed the density
profile and concluded that NGC 6642 has a collapsed core, provided completeness
effects are correctly accounted for. We thus conclude from independent means
that NGC 6642 is a very old, highly-evolved, core-collapsed globular cluster
with an atypical HB morphology.Comment: Paper, contains 8 figures, 1 table and 8 page
Transmission of viruses via our microbiomes.
BackgroundBacteria inhabiting the human body have important roles in a number of physiological processes and are known to be shared amongst genetically-related individuals. Far less is known about viruses inhabiting the human body, but their ecology suggests they may be shared between close contacts.ResultsHere, we report the ecology of viruses in the guts and mouths of a cohort and demonstrate that substantial numbers of gut and oral viruses were shared amongst genetically unrelated, cohabitating individuals. Most of these viruses were bacteriophages, and each individual had distinct oral and gut viral ecology from their housemates despite the fact that some of their bacteriophages were shared. The distribution of bacteriophages over time within households indicated that they were frequently transmitted between the microbiomes of household contacts.ConclusionsBecause bacteriophages may shape human oral and gut bacterial ecology, their transmission to household contacts suggests they could have substantial roles in shaping the microbiota within a household
Rede Brasil Arroz: transferência de tecnologia para a orizicultura brasileira.
O foco principal do projeto é a equiparação da qualidade dos grãos e a competitividade da orizicultura das diversas regiões produtoras com a do Mercosul. Para alcançar esses objetivos, o projeto atua em todo território nacional, porém de maneira mais efetiva em Goiás, Mato Grosso, Mato Grosso do Sul, Alagoas, Tocantins, Maranhão, Rio Grande do Sul e Santa Catarina, onde estão sendo utilizados os princÃpios e métodos descritos
Network Management System for (FUTON-like) Radio-over- Fiber Infrastructure
EU-Project FUTON Radio-over-Fiber (RoF) infrastructure proposes high transmission rates at small antenna costs, implying competitive CAPEX for next generation networks. But to be cost-efficient, it needs to employ new network architectures and intelligent technology solutions for decreasing network operational costs. The RoF Network Manager manages the network equipment on the optical front haul between the Central Unit (CU) and all Remote Antenna Units (RAU)s connected by it, as well as the communication links, while enabling end-to-end service problem resolution and service quality management by the FUTON Middleware. Although a significant amount of prior research work can be found in the literature related to RoF, there is still significant lack of technologies concerning RoF networks management. RoF Manager and its sub-systems target to fill such gap, proposing a novel concept in the form of Channel Forwarding Table (CFT). RoF Manager follows an autonomous and generic network management framework, designed to be scalable in terms of adding new network elements (NEs). It targets multitechnology, multi-service and multi-vendor NEs in the network using Simple Network Management Protocol (SNMP). It can also provide alternative paths in case of failure. This work puts forward a new paradigm towards RoF management solution managing network performance, network faults, network security and configurations for convergent networks
The Angular Momentum Evolution of Very Low Mass Stars
We present theoretical models of the angular momentum evolution of very low
mass stars (0.1 - 0.5 M_sun) and solar analogues (0.6 - 1.1 M_sun). We
investigate the effect of rotation on the effective temperature and luminosity
of these stars. We find that the decrease in T_eff and L can be significant at
the higher end of our mass range, but becomes negligible below 0.4 M_sun.
Formulae for relating T_eff to mass and v_rot are presented.
We compare our models to rotational data from young open clusters of
different ages to infer the rotational history of low mass stars, and the
dependence of initial conditions and rotational evolution on mass. We find that
the qualitative conclusions for stars below 0.6 M_sun do not depend on the
assumptions about internal angular momentum transport, which makes these low
mass stars ideal candidates for the study of the angular momentum loss law and
distribution of initial conditions. We find that neither models with solid body
nor differential rotation can simultaneously reproduce the observed stellar
spin down in the 0.6 to 1.1 M_sun mass range and for stars between 0.1 and 0.6
M_sun. The most likely explanation is that the saturation threshold drops more
steeply at low masses than would be predicted with a simple Rossby scaling. In
young clusters there is a systematic increase in the mean rotation rate with
decreased temperature below 3500 K (0.4 M_sun). This suggests either
inefficient angular momentum loss or mass-dependent initial conditions for
stars near the fully convective boundary. (abridged)Comment: To appear in the May 10, 2000 Ap
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