2,399 research outputs found
The origin of the spurious iron spread in the globular cluster NGC 3201
NGC 3201 is a globular cluster suspected to have an intrinsic spread in the
iron content. We re-analysed a sample of 21 cluster stars observed with
UVES-FLAMES at the Very Large Telescope and for which Simmerer et al. found a
0.4 dex wide [Fe/H] distribution with a metal-poor tail. We confirmed that when
spectroscopic gravities are adopted, the derived [Fe/H] distribution spans ~0.4
dex. On the other hand, when photometric gravities are used, the metallicity
distribution from Fe I lines remains large, while that derived from Fe II lines
is narrow and compatible with no iron spread. We demonstrate that the
metal-poor component claimed by Simmerer et al. is composed by asymptotic giant
branch stars that could be affected by non local thermodynamical equilibrium
effects driven by iron overionization. This leads to a decrease of the Fe I
abundance, while leaving the Fe II abundance unaltered. A similar finding has
been already found in asymptotic giant branch stars of the globular clusters M5
and 47 Tucanae. We conclude that NGC 3201 is a normal cluster, with no evidence
of intrinsic iron spread.Comment: Accepted for publication by ApJ, 7 pages, 4 figure
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
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
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&
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
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
A chemical trompe-l'\oe{}il: no iron spread in the globular cluster M22
We present the analysis of high-resolution spectra obtained with UVES and
UVES-FLAMES at the Very Large Telescope of 17 giants in the globular cluster
M22, a stellar system suspected to have an intrinsic spread in the iron
abundance. We find that when surface gravities are derived spectroscopically
(by imposing to obtain the same iron abundance from FeI and FeII lines) the
[Fe/H] distribution spans ~0.5 dex, according to previous analyses. However,
the gravities obtained in this way correspond to unrealistic low stellar masses
(0.1-0.5 Msun) for most of the surveyed giants. Instead, when photometric
gravities are adopted, the [FeII/H] distribution shows no evidence of spread at
variance with the [FeI/H] distribution. This difference has been recently
observed in other clusters and could be due to non-local thermodynamical
equilibrium effects driven by over-ionization mechanisms, that mainly affect
the neutral species (thus providing lower [FeI/H]) but leave [FeII/H]
unaltered. We confirm that the s-process elements show significant star-to-star
variations and their abundances appear to be correlated with the difference
between [FeI/H] and [FeII/H]. This puzzling finding suggests that the peculiar
chemical composition of some cluster stars may be related to effects able to
spuriously decrease [FeI/H]. We conclude that M22 is a globular cluster with no
evidence of intrinsic iron spread, ruling out that it has retained the
supernovae ejecta in its gravitational potential well.Comment: Accepted for publication to ApJ; 33 pages, 10 figures, 6 table
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