62 research outputs found
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
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
NGC 6994 - clearly not a physical stellar ensemble
The sparse stellar ensemble NGC 6994 = M73 has recently been discussed as a
possible remnant of an old open cluster. In order to solve the controversy on
the nature of this object we have taken high-resolution spectra of the six
brightest stars within 6' angular distance of its nominal position. These stars
are the only obvious member candidates for an eventual cluster or cluster
remnant since fainter stars do not show any significant concentration in the
plane of the sky. The radial velocities, atmospheric parameters, and absolute
magnitudes derived from the spectra reveal that the six candidates do not share
the same kinematics and lie at different distances from the Sun. The proper
motions provided by the Tycho-2 catalogue show that there is also a large
spread in the tangential motions of these stars, in agreement with the
spectroscopic results. This leads to the conclusion that the few bright stars
that constitute NGC 6994 are not a physical system (cluster, cluster remnant,
or group). They must instead be understood as a projective chance alignment of
physically unrelated field stars.Comment: 9 pages, 4 figures, accepted for publication in A&
Characterisation of 15 overlooked Ruprecht clusters with ages within 400Myr and 3Gyr
We derive fundamental, structural, and photometric parameters of 15
overlooked Ruprecht (hereafter Ru) star clusters by means of 2MASS photometry
and field-star decontamination. Ru\,1, 10, 23, 26, 27, 34, 35, 37, 41, 54, 60,
63, 66, and 152 are located in the third Galactic quadrant, while Ru\,174 is in
the first. With the constraints imposed by the field-decontaminated
colour-magnitude diagrams (CMDs) and stellar radial density profiles (RDPs), we
derive ages in the range 400\,Myr --- 1\,Gyr, except for the older Ru\,37, with
\,Gyr. Distances from the Sun are within \rm1.5\la\ds(kpc)\la8.0. The
RDPs are well-defined and can be described by a King-like profile for most of
the radial range, except for Ru\,23, 27, 41, 63, and 174, which present a
conspicuous stellar density excess in the central region. The clusters dwell
between (or close to) the Perseus and Sagittarius-Carina arms. We derive
evidence in favour of cluster size increasing with distance to the Galactic
plane (\zgc), which is consistent with a low frequency of tidal stress
associated with high-|\zgc| regions. The clusters are rather faint even in
the near-infrared, with apparent integrated \jj\ magnitudes within 6.4\la
m_J\la9.8, while their absolute magnitudes are -6.6\la M_J\la-2.6.
Extrapolation of the relation between and , derived for globular
clusters, suggests that they are low-luminosity optical clusters, with -5\la
M_V\la-1.Comment: The paper contains 11 figures and 3 tables. Accepted by MNRAS
Astrophysical parameters of 14 open clusters projected close to the Galactic plane
Astrophysical parameters (\textit{age, reddening, distance, core and cluster
radii}) of 14 open clusters (OCs) projected close to the Galactic plane are
derived with 2MASS photometry. The OCs are Be 63, Be 84, Cz 6, Cz 7, Cz 12, Ru
141, Ru 144, Ru 172, FSR 101, FSR 1430, FSR 1471, FSR 162, FSR 178 and FSR 198.
The OCs Be 63, Be 84, Ru 141, Ru 144, and Ru 172 are studied in more detail
than in previous works, while the others have astrophysical parameters derived
for the first time. The open clusters of the sample are located at
kpc from the Sun and at Galactocentric distances
kpc, with age in the range 10 Myr to 1.5 Gyr and reddening in the
range mag. The core and cluster radii are in the range
pc and pc, respectively. Cz 6 and FSR 198 are the youngest OCs of
this sample, with a population of pre-main sequence (PMS) stars, while FSR 178
is the oldest cluster.Comment: 11 pages, 14 figures - accepted by 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
Towards a census of the Galactic anticentre star clusters: colour-magnitude diagram and structural analyses of a sample of 50 objects
In this work we investigate the nature of 50 overdensities from the catalogue
of Froebrich, Scholz, and Raftery (FSR) projected towards the Galactic
anticentre, in the sector 160{\deg} \leq \ell \leq 200{\deg}. The sample
contains candidates with |b| \leq 20{\deg} classified by FSR as probable open
cluster (OC) and labelled with quality flags 2 and 3. Our main purpose is to
determine the nature of these OC candidates and the fraction of these objects
that are unknown OCs, as well as to derive astrophysical parameters (age,
reddening, distance, core and cluster radii) for the clusters and to
investigate the relationship among parameters. The analysis is based on 2MASS
J, (J-H), and (J-Ks) colour-magnitude diagrams (CMDs), and stellar radial
density profiles (RDPs) built with decontamination tools. The tools are a field
star decontamination algorithm, used to uncover the cluster's intrinsic CMD
morphology, and colour-magnitude filters to isolate stars with a high
probability of being cluster members. Out of the 50 objects, 16 (32%) are star
clusters. We show that 9 (18%) overdensities are new OCs (FSR 735, FSR 807, FSR
812, FSR 826, FSR 852, FSR 904, FSR 941, FSR 953, and FSR 955) and 7 (14%) are
previously studied or catalogued OCs (KKC1, FSR 795, Cz 22, FSR 828, FSR 856,
Cz 24, and NGC 2234). These are OCs with ages in the range 5 Myr to 1 Gyr, at
distances from the Sun 1.28 \precnapprox d_Sun(kpc) \precnapprox 5.78 and
Galactocentric distances 8.5 R_GC(kpc) \precnapprox 12.9. We also derive
parameters for the previously analysed OCs Cz 22 and NGC 2234. Five (10%)
candidates are classified as uncertain cases, and the remaining objects are
probable field fluctuations.Comment: 14 pages, 15 figure
Probing the age and structure of the nearby very young open clusters NGC 2244 and NGC 2239
The very young open cluster (OC) NGC 2244 in the Rosette Nebula was studied
with field-star-decontaminated 2MASS photometry, which shows the main-sequence
(MS) stars and an abundant pre-MS (PMS) population. Fundamental and structural
parameters were derived with colour-magnitude diagrams (CMDs), stellar radial
density profiles (RDPs) and mass functions (MFs). Most previous studies centred
NGC 2244 close to the bright K0V star 12 Monocerotis, which is not a cluster
member. Instead, the near-IR RDP indicates a pronounced core near the O5 star
HD 46150. We derive an age within 1--6 Myr, an absorption \aV=1.7\pm0.2, a
distance from the Sun \ds=1.6\pm0.2 kpc ( kpc outside the Solar
circle), an MF slope and a total (MS+PMS) stellar mass of
\sim625 \ms. Its RDP is characterised by the core and cluster radii
\rc\approx5.6\arcmin ( pc) and \rl\approx10\arcmin
( pc), respectively. Departure from dynamical equilibrium is
suggested by the abnormally large core radius and the marked central stellar
excess. We also investigate the elusive neighbouring OC NGC 2239, which is
low-mass (m_{MS+PMS}\approx301 \ms), young ( Myr) rather absorbed
(\aV=3.4\pm0.2), and located in the background of NGC 2244 at \ds=3.9\pm0.4
kpc. Its RDP follows a King-like function of \rc\approx0.5\arcmin\approx0.5
pc and \rl\approx5.0\arcmin\approx5.6 pc. The MF slope, ,
is essentially Salpeter's IMF. NGC 2244 is probably doomed to dissolution in a
few yr. Wide-field extractions and field-star decontamination increase
the stellar statistics and enhance both CMDs and RDPs, which is essential for
faint and bright star clusters.Comment: Accepted by MNRA
XUV interferometry using high-order harmonics: Application to plasma diagnostics
In this paper, we present and compare the two different XUV interferometric techniques using high-order harmonics that have been developed so far. The first scheme is based on the interference between two spatially separated phase-locked harmonic sources while the second uses two temporally separated harmonic sources. These techniques have been applied to plasma diagnostics in feasibility experiments where electron densities up to a few 1020 e[minus sign/cm3 have been measured with a temporal resolution of 200 fs. We present the main characteristics of each technique and discuss their respective potentials and limitations
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