1,881 research outputs found
Optimization of Multiple-Rendezvous Low-Thrust Missions on General-Purpose Graphics Processing Units
A massively parallel method for the identification of optimal sequences of targets in multiple-rendezvous low-thrust missions is presented. Given a list of possible targets, a global search of sequences compatible with the mission requirements is performed. To estimate the feasibility of each transfer, a heuristic model based on Lambert's transfers is evaluated in parallel for each target, making use of commonly available general-purpose graphics processing units such as the Nvidia Tesla cards. The resulting sequences are ranked by user-specified criteria such as length or fuel consumption. The resulting preliminary sequences are then optimized to a full low-thrust trajectory using classical methods for each leg. The performance of the method is discussed as a function of various parameters of the algorithm. The efficiency of the general-purpose graphics processing unit implementation is demonstrated by comparing it with a traditional CPU-based branch-and-bound method. Finally, the algorithm is used to compute asteroid sequences used in a solution submitted to the seventh edition of the Global Trajectory Optimization Competition
Exploiting the Gaia EDR3 photometry to derive stellar temperatures
We present new colour -- effective temperature (Teff) transformations based
on the photometry of the early third data release (EDR3) of the Gaia/ESA
mission. These relations are calibrated on a sample of about 600 dwarf and
giant stars for which Teff have been previously determined with the InfraRed
Flux Method from dereddened colours. The 1 dispersion of the
transformations is of 60-80 K for the pure Gaia colours BP-RP, BP-G , G-RP,
improving to 40-60 K for colours including the 2MASS K-band, namely BP-K, RP-K
and G-K, We validate these relations in the most challenging case of dense
stellar fields, where the Gaia EDR3 photometry could be less reliable,
providing guidance for a safe use of Gaia colours in crowded environments . We
compare the Teff from the Gaia EDR3 colours with those obtained from standard
V-K colours for stars in three Galactic globular clusters of different
metallicity, namely NGC 104, NGC 6752 and NGC 7099. The agreement between the
two estimates of Teff is excellent, with mean differences between -50 and +50
K, depending on the colour, and with 1 dispersions around the mean Teff
differences of 25-50 K for most of the colours and below 10 K for BP-K and G-K
. This demonstrates that these colours are analogue to V-K , as Teff
indicators.Comment: 12 pages. Accepted for publication on A&
Probing the MSP prenatal stage: the optical identification of the X-ray burster EXO 1745-248 in Terzan 5
We report on the optical identification of the neutron star burster EXO
1745-248 in Terzan 5. The identification was performed by exploiting HST/ACS
images acquired in Director's Discretionary Time shortly after (approximately 1
month) the Swift detection of the X-ray burst. The comparison between these
images and previous archival data revealed the presence of a star that
currently brightened by ~3 magnitudes, consistent with expectations during an
X-ray outburst. The centroid of this object well agrees with the position, in
the archival images, of a star located in the Turn-Off/Sub Giant Branch region
of Terzan 5. This supports the scenario that the companion should has recently
filled its Roche Lobe. Such a system represents the pre-natal stage of a
millisecond pulsar, an evolutionary phase during which heavy mass accretion on
the compact object occurs, thus producing X-ray outbursts and re-accelerating
the neutron star.Comment: ApJ Letter, in pres
The Terzan 5 puzzle: discovery of a third, metal-poor component
We report on the discovery of 3 metal-poor giant stars in Terzan 5, a complex
stellar system in the the Galactic bulge, known to have two populations at
[Fe/H]=-0.25 and +0.3. For these 3 stars we present new echelle spectra
obtained with NIRSPEC at Keck II, which confirm their radial velocity
membership and provide average [Fe/H]=-0.79 dex iron abundance and
[alpha/Fe]=+0.36 dex enhancement. This new population extends the metallicity
range of Terzan~5 0.5 dex more metal poor, and it has properties consistent
with having formed from a gas polluted by core collapse supernovae.Comment: Accepted for publication on ApJ Lette
First evidence of fully spatially mixed first and second generations in globular clusters: the case of NGC 6362
We present the first evidence of multiple populations in the Galactic
globular cluster NGC 6362. We used optical and near-UV Hubble Space Telescope
and ground based photometry, finding that both the sub giant and red giant
branches are split in two parallel sequences in all color magnitude diagrams
where the F336W filter (or U band) is used. This cluster is one of the least
massive globulars (M_tot~5x10^4 M_sun) where multiple populations have been
detected so far. Even more interestingly and at odds with any previous finding,
we observe that the two identified populations share the same radial
distribution all over the cluster extension. NGC 6362 is the first system where
stars from different populations are found to be completely spatially mixed.
Based on N-body and hydrodynamical simulations of multiple stellar generations,
we argue that, to reproduce these findings, NGC 6362 should have lost up to the
80% of its original massComment: Accepted for publication by ApJ Letters; 6 pages, 5 figure
Gaia Data Release 2. The kinematics of globular clusters and dwarf galaxies around the Milky Way (Corrigendum)
Proper motions in Terzan 5: membership of the multi-iron sub-populations and first constrain to the orbit
By exploiting two sets of high-resolution images obtained with HST ACS/WFC
over a baseline of ~10 years we have measured relative proper motions of
~70,000 stars in the stellar system Terzan 5. The results confirm the
membership of the three sub-populations with different iron abudances
discovered in the system. The orbit of the system has been derived from a first
estimate of its absolute proper motion, obtained by using bulge stars as
reference. The results of the integration of this orbit within an axisymmetric
Galactic model exclude any external accretion origin for this cluster. Terzan 5
is known to have chemistry similar to the Galactic bulge; our findings support
a kinematic link between the cluster and the bulge, further strengthening the
possibility that Terzan 5 is the fossil remnant of one of the pristine clumps
that originated the bulge.Comment: 25 pages, 14 figures, accepted for publication by Ap
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