2,028 research outputs found
OB Stars in the Solar Neighborhood I: Analysis of their Spatial Distribution
We present a newly-developed, three-dimensional spatial classification
method, designed to analyze the spatial distribution of early type stars within
the 1 kpc sphere around the Sun. We propose a distribution model formed by two
intersecting disks -the Gould Belt (GB) and the Local Galactic Disk (LGD)-
defined by their fundamental geometric parameters. Then, using a sample of
about 550 stars of spectral types earlier than B6 and luminosity classes
between III and V, with precise photometric distances of less than 1 kpc, we
estimate for some spectral groups the parameters of our model, as well as
single membership probabilities of GB and LGD stars, thus drawing a picture of
the spatial distribution of young stars in the vicinity of the Sun.Comment: 28 pages including 9 Postscript figures, one of them in color.
Accepted for publication in The Astronomical Journal, 30 January 200
The Nature of the Gould Belt from a Fractal Analysis of its Stellar Population
The Gould Belt (GB) is a system of gas and young, bright stars distributed
along a plane that is inclined with respect to the main plane of the Milky Way.
Observational evidence suggests that the GB is our closest star formation
complex, but its true nature and origin remain rather controversial. In this
work we analyze the fractal structure of the stellar component of the GB. In
order to do this, we tailor and apply an algorithm that estimates the fractal
dimension in a precise and accurate way, avoiding both boundary and small data
set problems. We find that early OB stars (of spectral types earlier than B4)
in the GB have a fractal dimension very similar to that of the gas clouds in
our Galaxy. On the contrary, stars in the GB of later spectral types show a
larger fractal dimension, similar to that found for OB stars of both age groups
in the local Galactic disk (LGD). This result seems to indicate that while the
younger OB stars in the GB preserve the memory of the spatial structure of the
cloud where they were born, older stars are distributed following a similar
morphology as that found for the LGD stars. The possible causes for these
differences are discussed.Comment: 20 pages including 7 figures and 1 table. ApJ (in press
Pure diffusional creep of NiO polycrystals
Compressive creep testing of polycrystalline NiO has been performed in air and argon, at temperatures between 1 100 and 1 500 °C (0.61 to 0.79 TM) under loads corresponding to initial applied stresses from 6 to 90 MPa. Nabarro creep was observed in the lower stress range; creep rate and oxygen diffusivity show a good correlation.Des essais de fluage en compression ont été faits sur des polycristaux de NiO, dans l'air et l'argon, à des températures entre 1 100 et 1 500 °C (0,61 à 0,79 TF) sous des charges correspondant à des contraintes initiales entre 6 et 90 MPa. Le fluage Nabarro a été observé à faible contrainte; la vitesse de déformation et la diffusivité de l'oxygène montrent une bonne corrélation
Fluage de monocristaux de NiO
NiO single crystals have been subjected to constant-load compressive creep along (100). Stresses between 50 MPa and 120 MPa and temperatures between 950 °C and 1 200 °C were studied. The activation energy observed suggests that oxygen diffusion is the rate-controlling process.Des expériences de compression à charge constante, selon , ont été réalisées sur des monocristaux de NiO. Les contraintes utilisées vont de 50 MPa à 120 MPa et les températures de 950 °C à 1200 °C. L'énergie d'activation du fluage suggère que la diffusion de l'oxygène joue un rôle prédominant sur la cinétique de déformation plastique
Membership Determination of Open Cluster M48 Based on BATC Thirteen-Band Photometry
Beijing-Arizona-Taiwan-Connecticut (BATC) multi-band photometric data in the
field of open cluster M48 are used to determine its membership. By comparing
observed spectral energy distributions (SEDs) of stars with theoretical ones,
membership probabilities of 750 stars with limiting magnitude of 15.0 in BATC
band ( \AA) are determined. 323 stars with membership
probabilities higher than 30% are considered as candidate members of M48.
Comparing membership probabilities of 229 common stars obtained by the present
method and the proper-motion based methods, a 80% agreement among these methods
is obtained.Comment: 27 pages,7figures, accepted for publication in PAS
Spectroscopy and Orbital Analysis of Bright Bolides Observed over the Iberian Peninsula from 2010 to 2012
We present the analysis of the atmospheric trajectory and orbital data of
four bright bolides observed over Spain, one of which is a potential meteorite
dropping event. Their absolute magnitude ranges from -10 to -11. Two of these
are of sporadic origin, although a Geminid and a kappa-Cygnid fireball are also
considered. These events were recorded in the framework of the continuous
fireball monitoring and spectroscopy campaigns developed by the SPanish Meteor
Network (SPMN) between 2010 and 2012. The tensile strength of the parent
meteoroids is estimated and the abundances of the main rock-forming elements in
these particles are calculated from the emission spectrum obtained for three of
these events. This analysis revealed a chondritic nature for these meteoroids.Comment: Manuscript in press in Monthly Notices of the Royal Astronomical
Societ
Discovery of the broad-lined Type Ic SN 2013cq associated with the very energetic GRB 130427A
Long-duration gamma-ray bursts (GRBs) at z < 1 are in most cases found to be
accompanied by bright, broad-lined Type Ic supernovae (SNe Ic-BL). The
highest-energy GRBs are mostly located at higher redshifts, where the
associated SNe are hard to detect observationally. Here we present early and
late observations of the optical counterpart of the very energetic GRB 130427A.
Despite its moderate redshift z = 0.3399+/-0.0002, GRB 130427A is at the high
end of the GRB energy distribution, with an isotropic-equivalent energy release
of Eiso ~ 9.6x10^53 erg, more than an order of magnitude more energetic than
other GRBs with spectroscopically confirmed SNe. In our dense photometric
monitoring, we detect excess flux in the host-subtracted r-band light curve,
consistent with what expected from an emerging SN, ~0.2 mag fainter than the
prototypical SN 1998bw. A spectrum obtained around the time of the SN peak
(16.7 days after the GRB) reveals broad undulations typical of SNe Ic-BL,
confirming the presence of a SN, designated SN 2013cq. The spectral shape and
early peak time are similar to those of the high expansion velocity SN 2010bh
associated with GRB 100316D. Our findings demonstrate that high-energy
long-duration GRBs, commonly detected at high redshift, can also be associated
with SNe Ic-BL, pointing to a common progenitor mechanism.Comment: 12 pages, 4 figures, 1 table, accepted for publication in Ap
Quasi-stellar objects in the ALHAMBRA survey. I. Photometric redshift accuracy based on a 23 optical-NIR filter photometry
We characterize the ability of the ALHAMBRA survey to assign accurate
photo-z's to BLAGN and QSOs based on their ALHAMBRA very-low-resolution
optical-NIR spectroscopy. A sample of 170 spectroscopically identified BLAGN
and QSOs have been used together with a library of templates (including SEDs
from AGN, normal, starburst galaxies and stars) in order to fit the 23
photometric data points provided by ALHAMBRA in the optical and NIR (20
medium-band optical filters plus the standard JHKs). We find that the ALHAMBRA
photometry is able to provide an accurate photo-z and spectral classification
for ~88% of the spectroscopic sources over 2.5 deg^2 in different areas of the
survey, all of them brighter than m678=23.5 (equivalent to r(SLOAN)~24.0). The
derived photo-z accuracy is better than 1% and comparable to the most recent
results in other cosmological fields. The fraction of outliers (~12%) is mainly
caused by the larger photometric errors for the faintest sources and the
intrinsic variability of the BLAGN/QSO population. A small fraction of outliers
may have an incorrectly assigned spectroscopic redshift. The definition of the
ALHAMBRA survey in terms of the number of filters, filter properties, area
coverage and depth is able to provide photometric redshifts for BLAGN/QSOs with
a precision similar to any previous survey that makes use of medium-band
optical photometry. In agreement with previous literature results, our analysis
also reveals that, in the 0<z<4 redshift interval, very accurate photo-z can be
obtained without the use of near-IR broadband photometry at the expense of a
slight increase of outliers. The NIR importance is expected to increase at
higher redshifts (z>4). These results are relevant for the design of future
optical follow-ups of surveys with a large fraction of BLAGN, as it is the case
for X-rays or radio surveys.Comment: 17 pages, 12 figures. Accepted for publication in A&
Combined Effects of Rotation and Age Spreads on Extended Main-Sequence Turn Offs
The extended main-sequence turn offs (eMSTOs) of several young to intermediate age clusters are examined in the Magellanic Clouds and the Milky Way. We explore the effects of extended star formation (eSF) and a range of stellar rotation rates on the behavior of the color–magnitude diagram, paying particular attention to the MSTO. We create synthetic stellar populations based on MESA stellar models to simulate observed Hubble Space Telescope and Gaia star cluster data. We model the effect of rotation as a nonparametric distribution, allowing for maximum flexibility. In our models the slow rotators comprise the blueward, and fast rotators the redward portion of the eMSTO. We simulate data under three scenarios: nonrotating eSF, a range of rotation rates with a single age, and a combination of age and rotation effects. We find that two of the five clusters (the youngest and oldest) favor an age spread, but these also achieve the overall worst fits of all clusters. The other three clusters show comparable statistical evidence between rotation and an age spread. In all five cases, a rotation-rate distribution alone is capable of qualitatively matching the observed eMSTO structure. In future work, we aim to compare our predicted with observations in order to better constrain the physics related to stellar rotation
The ALHAMBRA survey: Discovery of a faint QSO at z = 5.41
Aims. We aim to illustrate the potentiality of the Advanced Large, Homogeneous Area, Medium-Band Redshift Astronomical (ALHAMBRA) survey to investigate the high-redshift universe through the detection of quasi stellar objects (QSOs) at redshifts higher than 5.
Methods. We searched for QSOs candidates at high redshift by fitting an extensive library of spectral energy distributions – including active and non-active galaxy templates, as well as stars – to the photometric database of the ALHAMBRA survey (composed of 20 optical medium-band plus the 3 broad-band JHKs near-infrared filters).
Results. Our selection over ≈1 square degree of ALHAMBRA data (~1/4 of the total area covered by the survey), combined with GTC/OSIRIS spectroscopy, has yielded identification of an optically faint QSO at very high redshift (z = 5.41). The QSO has an absolute magnitude of ~−24 at the 1450 Å continuum, a bolometric luminosity of ≈2 × 1046 erg s-1, and an estimated black hole mass of ≈108 M⊙. This QSO adds itself to a reduced number of known UV faint sources at these redshifts. The preliminary derived space density is compatible with the most recent determinations of the high-z QSO luminosity functions. This new detection shows how ALHAMBRA, as well as forthcoming well-designed photometric surveys, can provide a wealth of information on the origin and early evolution of this kind of object
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