109 research outputs found
The Flux Calibration of Gaia
The Gaia mission is described, along with its scientific potential and its updated science perfomances. Although it is often described as a self-calibrated mission, Gaia still needs to tie part of its measurements to external scales (or to convert them in physical units). A detailed decription of the Gaia spectro-photometric standard stars survey is provided, along with a short description of the Gaia calibration model. The model requires a grid of approximately 200 stars, calibrated to a few percent with respect to Vega, and covering different spectral types
Blue Horizontal Branch Stars in the Sagittarius dwarf spheroidal galaxy
We report on the recovery of a Blue Horizontal Branch (BHB) population
belonging to the Sagittarius Dwarf Spheroidal Galaxy (Sgr). The sequence is
clearly identified in the (V, V-I) Color Magnitude Diagram (CMD) obtained for
about 500,000 stars in the region of the globular cluster M~54. The BHB
morphology is similar to the analogous sequence in M~54, but it is
unambiguously associated with Sgr since {\it(i)} it is detected well outside
the main body of the cluster, up to more than 5 tidal radii from the cluster
center and {\it(ii)} the BHB stars follow the radial distribution of the other
stellar populations of Sgr. This finding finally demonstrates that the Sgr
galaxy hosts a significant (of the order of 10%) old and metal-poor
stellar population ([Fe/H]\ltsima -1.3; age \gtsima 10 Gyr), similar to that of
its oldest clusters (M~54, Ter~8). We also show that the Sgr BHB sequence found
here is the counterpart of the analogous feature observed by Newberg et al.
(2002) in the Sgr Stream, in a field more than 80\degr away from the center
of the galaxy.Comment: 12 pages, 4 figures. Accepted for publication in ApJ Letter
Lithium abundances in globular clusters
Lithium is created during the Big Bang nucleosynthesis and it is destroyed in stellar interiors at relatively low temperatures. However, it should be preserved in the stellar envelopes of unevolved stars and progressively diluted during mixing processes. In particular, after the first dredge-up along the RGB, lithium should be completely destroyed, but this is not what we observe today in globular clusters. This element allows to test stellar evolutionary models, as well as different types of polluters for second population stars in the multiple population scenarios. Due to the difficulty in the measurement of the small available lithium line, few GCs have been studied in details so far. Literature results are not homogeneous for what concerns type of stars, sample sizes, and chemical analysis methods. The Gaia-ESO survey allows us to study the largest sample of GCs stars (about 2000, both dwarfs and giants) for which the lithium has been analysed homogeneously
DOOp, an automated wrapper for DAOSPEC
Large spectroscopic surveys such as the Gaia-ESO Survey produce huge
quantities of data. Automatic tools are necessary to efficiently handle this
material. The measurement of equivalent widths in stellar spectra is
traditionally done by hand or with semi-automatic procedures that are
time-consuming and not very robust with respect to the repeatability of the
results. The program DAOSPEC is a tool that provides consistent measurements of
equivalent widths in stellar spectra while requiring a minimum of user
intervention. However, it is not optimised to deal with large batches of
spectra, as some parameters still need to be modified and checked by the user.
Exploiting the versatility and portability of BASH, we have built a pipeline
called DAOSPEC Option Optimiser (DOOp) automating the procedure of equivalent
widths measurement with DAOSPEC. DOOp is organised in different modules that
run one after the other to perform specific tasks, taking care of the
optimisation of the parameters needed to provide the final equivalent widths,
and providing log files to ensure better control over the procedure. In this
paper, making use of synthetic and observed spectra, we compare the performance
of DOOp with other methods, including DAOSPEC used manually. The measurements
made by DOOp are identical to the ones produced by DAOSPEC when used manually,
while requiring less user intervention, which is convenient when dealing with a
large quantity of spectra. DOOp shows its best performance on high-resolution
spectra (R>20 000) and high signal-to-noise ratio (S/N>30), with uncertainties
ranging from 6 m{\AA} to 2 m{\AA}. The only subjective parameter that remains
is the normalisation, as the user still has to make a choice on the order of
the polynomial used for the continuum fitting. As a test, we use the equivalent
widths measured by DOOp to re-derive the stellar parameters of four
well-studied stars
The Gaia-ESO Survey Astrophysical Calibration
The Gaia-ESO Survey is a wide field spectroscopic survey recently started with the FLAMES@VLT in Cerro Paranal, Chile. It will produce radial velocities more accurate than Gaia's for faint stars (down to V ≃ 18), and astrophysical parameters and abundances for approximately 100 000 stars, belonging to all Galactic populations. 300 nights were assigned in 5 years (with the last year subject to approval after a detailed report). In particular, to connect with other ongoing and planned spectroscopic surveys, a detailed calibration program — for the astrophysical parameters derivation — is planned, including well known clusters, Gaia benchmark stars, and special equatorial calibration fields designed for wide field/multifiber spectrographs
Globular clusters with Gaia
The treatment of crowded fields in Gaia data will only be a reality in a few years from now. In particular, for globular clusters, only the end-of-mission data (public in 2022-2023) will have the necessary full crowding treatment and will reach sufficient quality for the faintest stars. As a consequence, the work on the deblending and decontamination pipelines is still ongoing. We describe the present status of the pipelines for different Gaia instruments, and we model the end-of-mission crowding errors on the basis of available information. We then apply the nominal post-launch Gaia performances, appropriately worsened by the estimated crowding errors, to a set of 18 simulated globular clusters with different concentration, distance and field contamination. We conclude that there will be 103-104 stars with astrometric performances virtually untouched by crowding (contaminated by -1; (II) internal kinematics will be of unprecedented quality, cluster masses will be determined to ≃10 per cent up to 15 kpc and beyond, and it will be possible to identify differences of a few km s-1 or less in the kinematics (if any) of cluster sub-populations up to 10 kpc and beyond; (III) the brightest stars (V ≃ 17 mag) will have space-quality, wide-field photometry (mmag errors), and all Gaia photometry will have 1-3 per cent errors on the absolute photometric calibration
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