10,778 research outputs found
A comparison of the local spiral structure from Gaia DR2 and VLBI maser parallaxes
Context. The Gaia mission has released the second data set (Gaia DR2), which
contains parallaxes and proper motions for a large number of massive, young
stars. Aims. We investigate the spiral structure in the solar neighborhood
revealed by Gaia DR2 and compare it with that depicted by VLBI maser
parallaxes. Methods. We examined three samples with different constraints on
parallax uncertainty and distance errors and stellar spectral types: (1) all OB
stars with parallax errors of less than 10%; (2) only O-type stars with 0.1 mas
errors imposed and with parallax distance errors of less than 0.2 kpc; and (3)
only O-type stars with 0.05 mas errors imposed and with parallax distance
errors of less than 0.3 kpc. Results. In spite of the significant distance
uncertainties for stars in DR2 beyond 1.4 kpc, the spiral structure in the
solar neighborhood demonstrated by Gaia agrees well with that illustrated by
VLBI maser results. The O-type stars available from DR2 extend the spiral arm
models determined from VLBI maser parallaxes into the fourth Galactic quadrant,
and suggest the existence of a new spur between the Local and Sagittarius arms.Comment: 4 pages, 3 figures, 1 table, accepted for publication in A&
On the dilaton and the axion potentials
We extend the Vecchia-Veneziano-Witten (VVW) model of QCD in the chiral limit
and for large colour number , by introducing an effective dilaton-gluon
coupling from which we derive both the axion and dilaton potentials.
Furthermore, using a string inspired model, we determine a new interquark
potential as a perturbative series in terms of the interquark distance . Our
potential goes beyond Dick one obtained in [8] and shares the same features as
the Bian-Huang-Shen potential which depends only on odd powers of
[22].Comment: 15 pages, Late
Understanding AGN-Host Connection in Partially Obscured Active Galactic Nuclei. Part I: The Nature of AGN+HII Composites
The goal of our serial papers is to examine the evolutionary connection
between AGN and star formation in its host galaxy in the partially obscured
AGNs (i.e., Seyfert 1.8 and 1.9 galaxies). Taking advantage of these galaxies,
the properties of both components can be studied together by direct
measurements. In this paper, we focus on the broad-line composite galaxies
(composite AGNs) which are located between the theoretical and empirical
separation lines in the [NII]/Ha vs. [OIII]/Hb diagram. These galaxies are
searched for from the composite galaxies provided by the SDSS DR4 MPA/JHU
catalogs. After re-analyze the spectra, we perform a fine classification for
the 85 composite AGNs in terms of the BPT diagrams. All the objects located
below the three theoretical separation lines are associated with a young
stellar population (<1Gyrs), while either a young or old stellar population is
identified in the individual multiply-classified object. The
multiply-classified objects with a very old stellar population are located in
the LINER region in the [OI]/Ha vs. [OIII]/Hb diagram. We then consider the
connection between AGN and star formation to derive the key results. The
Eddington ratio inferred from the broad Ha emission, the age of the stellar
population of AGN's host as assessed by D_n(4000), and the line ratio [OI]/Ha
are found to be related with each other. These relations strongly support the
evolutionary scenario in which AGNs evolve from high L/L_Edd state with soft
spectrum to low L/L_Edd state with hard spectrum as young stellar population
ages and fades. The significant correlation between the line ratio [OI]/Ha and
D_n(4000) leads us to suggest that the line ratio could be used to trace the
age of stellar population in type I AGNs.Comment: 39 pages, 11 figures, 1 table, accepted by Ap
Oscillatory surface dichroism of an insulating topological insulator Bi2Te2Se
Using circular dichroism-angle resolved photoemission spectroscopy
(CD-ARPES), we report a study of the effect of angular momentum transfer
between polarized photons and topological surface states on the surface of
highly bulk insulating topological insulator Bi2Te2Se. The photoelectron
dichroism is found to be strongly modulated by the frequency of the helical
photons including a dramatic sign-flip. Our results suggest that the observed
dichroism and its sign-flip are consequences of strong coupling between the
photon field and the spin-orbit nature of the Dirac modes on the surface. Our
studies reveal the intrinsic dichroic behavior of topological surface states
and point toward the potential utility of bulk insulating topological
insulators in device applications.Comment: 5 pages, 4 figure
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