40 research outputs found
Fast Integrated Spectra Analyzer: A New Computational Tool For Age and Reddening Determination of Small Angular Diameter Open Clusters
We present a new algorithm called 'Fast Integrated Spectra Analyzer" (FISA)
that permits fast and reasonably accurate age and reddening determinations for
small angular diameter open clusters by using their integrated spectra in the
(3600-7400) \AA \ range and currently available template spectrum libraries.
This algorithm and its implementation help to achieve astrophysical results in
shorter times than from other methods. A brief review is given of the
integrated spectroscopic technique applied to the study of open clusters as
well as the basic assumptions that justify its use. We describe the numerical
algorithm employed in detail, show examples of its application, and provide a
link to the code. Our method has successfully been applied to integrated
spectroscopy of open clusters, both in the Galaxy and in the Magellanic Clouds,
to determine ages and reddenings.Comment: 27 Pages, 7 Figures, 1 table. Accepted to PAS
Temporal coherence of ultrashort high-order harmonic pulses
We have studied the temporal coherence of high-order harmonics (up to the 15th order) produced by focusing 100 fs laser pulses into an argon gas jet. We measure the visibility of the interference fringes, produced when two spatially separated harmonic sources interfere in the far field, as a function of the time delay between the two sources. In general, we find long coherence times, comparable to the expected pulse durations of the harmonics. For some of the harmonics, the interference pattern exhibits two regions, with significantly different coherence times. These results are interpreted in terms of different electronic trajectories contributing to harmonic generation. © 1998 American Physical Society
Temporal coherence of high-order harmonics
Systematic studies of the temporal coherence properties of high-order harmonic radiation are presented. These complement our previous investigations [Bellini et al., Phys. Rev. Lett. 81, 297 (1998)], where we showed the separation of the far-field pattern of high-order harmonics into two distinct spatial regions with different coherence times. Here we show how the coherence time of the inner and outer regions changes as a function of the harmonic order, the laser intensity, and the focusing conditions. Good agreement with the predictions of the semiclassical model of harmonic generation is obtained. © 1999 The American Physical Society
NGC 2401: A template of the Norma-Cygnus Arm's young population in the Third Galactic Quadrant
Based on a deep optical CCD (UBV(RI)_C) photometric survey and on the
Two-Micron All-Sky-Survey (2MASS) data we derived the main parameters of the
open cluster NGC 2401. We found this cluster is placed at 6.3 0.5 kpc
(V_O - M_V = 14.0 \pm 0.2) from the Sun and is 25 Myr old, what allows us to
identify NGC 2401 as a member of the young population belonging to the
innermost side of the extension of the Norma-Cygnus spiral--arm in the Third
Galactic Quadrant. A spectroscopic study of the emission star LSS 440 that lies
in the cluster area revealed it is a B0Ve star; however, we could not confirm
it is a cluster member. We also constructed the cluster luminosity function
(LF) down to and the cluster initial mass function (IMF) for all
stars with masses above M \sim 1-2 M_{\sun}. It was found that the slope of
the cluster IMF is . The presence of a probable PMS star
population associated to the cluster is weakly revealed.Comment: 10 paginas, 11 eps figures, accepted for publication in MNRA
Star clusters in the Carina complex: UBVRI photometry of NGC3114, Collinder228 and vdB-Hagen99
In this paper we present and analyze CCD photometry in the region of
the three young open clusters NGC 3114, Collinder~228, and vdB-Hagen~99,Comment: 11 pages, 15 figures, accepted for publication in A&
VUV frequency combs from below-threshold harmonics
Recent demonstrations of high-harmonic generation (HHG) at very high
repetition frequencies (~100 MHz) may allow for the revolutionary transfer of
frequency combs to the vacuum ultraviolet (VUV). This advance necessitates
unifying optical frequency comb technology with strong-field atomic physics.
While strong-field studies of HHG have often focused on above-threshold
harmonic generation (photon energy above the ionization potential), for VUV
frequency combs an understanding of below-threshold harmonic orders and their
generation process is crucial. Here we present a new and quantitative study of
the harmonics 7-13 generated below and near the ionization threshold in xenon
gas. We show multiple generation pathways for these harmonics that are
manifested as on-axis interference in the harmonic yield. This discovery
provides a new understanding of the strong-field, below-threshold dynamics
under the influence of an atomic potential and allows us to quantitatively
assess the achievable coherence of a VUV frequency comb generated through below
threshold harmonics. We find that under reasonable experimental conditions
temporal coherence is maintained. As evidence we present the first explicit VUV
frequency comb structure beyond the 3rd harmonic.Comment: 16 pages, 4 figures, 1 tabl
A study of the Galactic plane towards l = 305
We present optical () observations of a rich and complex field in the
Galactic plane towards and . Our
analysis reveals a significantly high interstellar absorbtion ()
and an abnormal extinction law in this line of sight. Availing a considerable
number of color combinations, the photometric diagrams allow us to derive new
estimates of the fundamental parameters of the two open clusters Danks~1 and
Danks~2. Due to the derived abnormal reddening law in this line of sight, both
clusters appear much closer (to the Sun) than previously thought. %
Additionally, we present the optical colors and magnitudes of the WR~48a star
and its main parameters were estimated. The properties of the two embedded
clusters DBS2003~130 and 131, are also addressed. We identify a number of Young
Stellar Objects which are probable members of these clusters. This new material
is then used to revisit the spiral structure in this sector of the Galaxy
showing evidence of populations associated with the inner Galaxy Scutum-Crux
arm
New isolated planetary mass objects and the stellar and substellar mass function of the sigma Orionis cluster
We report on our analysis of the VISTA Orion ZYJHKs photometric data
(completeness magnitudes Z=22.6 and J=21.0mag) focusing on a circular area of
2798.4 arcmin^2 around the young sigma Orionis star cluster (~3Myr, ~352pc,
solar metallicity). The combination of the VISTA photometry with optical, WISE
and Spitzer data allows us to identify a total of 210 cluster member candidates
with masses in the interval 0.25-0.004Msun, 23 of which are new planetary-mass
object findings. These discoveries double the number of cluster planetary-mass
candidates known so far. One object has colors compatible with a T spectral
type. The cluster harbors about as many brown dwarfs (69, 0.072-0.012Msun) and
planetary-mass objects (37, 0.012-0.004Msun) as very low-mass stars (104,
0.25-0.072Msun). Based on Spitzer data, we derive a disk frequency of ~40% for
very low-mass stars, brown dwarfs, and planetary mass objects in sigma Orionis.
The radial density distributions of these three mass intervals are alike: all
are spatially concentrated within an effective radius of 12arcmin (1.2pc)
around the multiple star sigma Ori, and no obvious segregation between
disk-bearing and diskless objects is observed. Using the VISTA data and the
Mayrit catalog, we derive the cluster mass spectrum (DeltaN/DeltaM ~
M^{-alpha}) from ~19 to 0.006Msun (VISTA ZJ completeness), which is reasonably
described by two power-law expressions with indices of alpha=1.7+/-0.2 for
M>0.35Msun, and alpha=0.6+/-0.2 for M<0.35Msun. The sigma Orionis mass spectrum
smoothly extends into the planetary-mass regime down to 0.004Msun. Our findings
of T-type sources (<0.004Msun) in the VISTA sigma Orionis exploration appear to
be smaller than what is predicted by the extrapolation of the cluster mass
spectrum down to the survey J-band completeness.Comment: Accepted for publication in ApJ. 69 pages, 15 figure
Building the cosmic distance scale: from Hipparcos to Gaia
Hipparcos, the first ever experiment of global astrometry, was launched by
ESA in 1989 and its results published in 1997 (Perryman et al., Astron.
Astrophys. 323, L49, 1997; Perryman & ESA (eds), The Hipparcos and Tycho
catalogues, ESA SP-1200, 1997). A new reduction was later performed using an
improved satellite attitude reconstruction leading to an improved accuracy for
stars brighter than 9th magnitude (van Leeuwen & Fantino, Astron. Astrophys.
439, 791, 2005; van Leeuwen, Astron. Astrophys. 474, 653, 2007).
The Hipparcos Catalogue provided an extended dataset of very accurate
astrometric data (positions, trigonometric parallaxes and proper motions),
enlarging by two orders of magnitude the quantity and quality of distance
determinations and luminosity calibrations. The availability of more than 20000
stars with a trigonometric parallax known to better than 10% opened the way to
a drastic revision of our 3-D knowledge of the solar neighbourhood and to a
renewal of the calibration of many distance indicators and age estimations. The
prospects opened by Gaia, the next ESA cornerstone, planned for launch in June
2013 (Perryman et al., Astron. Astrophys. 369, 339, 2001), are still much more
dramatic: a billion objects with systematic and quasi simultaneous astrometric,
spectrophotometric and spectroscopic observations, about 150 million stars with
expected distances to better than 10%, all over the Galaxy. All stellar
distance indicators, in very large numbers, will be directly measured,
providing a direct calibration of their luminosity and making possible detailed
studies of the impacts of various effects linked to chemical element
abundances, age or cluster membership. With the help of simulations of the data
expected from Gaia, obtained from the mission simulator developed by DPAC, we
will illustrate what Gaia can provide with some selected examples.Comment: 16 pages, 16 figures, Conference "The Fundamental Cosmic Distance
scale: State of the Art and the Gaia perspective, 3-6 May 2011, INAF,
Osservatorio Astronomico di Capodimonte, Naples. Accepted for publication in
Astrophysics & Space Scienc
X-ray harmonic comb from relativistic electron spikes
X-ray devices are far superior to optical ones for providing nanometre
spatial and attosecond temporal resolutions. Such resolution is indispensable
in biology, medicine, physics, material sciences, and their applications. A
bright ultrafast coherent X-ray source is highly desirable, for example, for
the diffractive imaging of individual large molecules, viruses, or cells. Here
we demonstrate experimentally a new compact X-ray source involving high-order
harmonics produced by a relativistic-irradiance femtosecond laser in a gas
target. In our first implementation using a 9 Terawatt laser, coherent soft
X-rays are emitted with a comb-like spectrum reaching the 'water window' range.
The generation mechanism is robust being based on phenomena inherent in
relativistic laser plasmas: self-focusing, nonlinear wave generation
accompanied by electron density singularities, and collective radiation by a
compact electric charge. The formation of singularities (electron density
spikes) is described by the elegant mathematical catastrophe theory, which
explains sudden changes in various complex systems, from physics to social
sciences. The new X-ray source has advantageous scalings, as the maximum
harmonic order is proportional to the cube of the laser amplitude enhanced by
relativistic self-focusing in plasma. This allows straightforward extension of
the coherent X-ray generation to the keV and tens of keV spectral regions. The
implemented X-ray source is remarkably easily accessible: the requirements for
the laser can be met in a university-scale laboratory, the gas jet is a
replenishable debris-free target, and the harmonics emanate directly from the
gas jet without additional devices. Our results open the way to a compact
coherent ultrashort brilliant X-ray source with single shot and high-repetition
rate capabilities, suitable for numerous applications and diagnostics in many
research fields