303 research outputs found
Photometric catalog of nearby globular clusters (I)
We present the first part of the first large and homogeneous CCD
color-magnitude diagram (CMD) data base, comprising 52 nearby Galactic globular
clusters (GGC) imaged in the V and I bands using only two telescopes (one for
each hemisphere). The observed clusters represent 75% of the known Galactic
globulars with (m-M)v<=16.15 mag, cover most of the globular cluster
metallicity range (-2.2 <= [Fe/H] <= -0.4), and span Galactocentric distances
from ~1.2 to ~18.5 kpc.
In this paper, the CMDs for the 39 GGCs observed in the southern hemisphere
are presented. The remaining 13 northern hemisphere clusters of the catalog are
presented in a companion paper. For four clusters (NGC 4833, NGC 5986, NGC
6543, and NGC 6638) we present for the first time a CMD from CCD data. The
typical CMD spans from the 22nd V magnitude to the tip of the red giant branch.
Based on a large number of standard stars, the absolute photometric calibration
is reliable to the ~0.02 mag level in both filters.
This catalog, because of its homogeneity, is expected to represent a useful
data base for the measurement of the main absolute and relative parameters
characterizing the CMD of GGCs.Comment: 34 pages, 108 figures, Astronomy & Astrophysics Supplement Series, in
press. Full resolution figures can be obtained from the authors upon reques
Chemical composition of the stellar cluster Gaia1: No surprise behind Sirius
IndexaciĂłn: Web of Science; Scopus.We observed six He-clump stars of the intermediate-Age stellar cluster Gaia1 with the MIKE/Magellan spectrograph. A possible extra-galactic origin of this cluster, recently discovered thanks to the first data release of the ESA Gaia mission, has been suggested, based on its orbital parameters. Abundances for Fe, α, proton-And neutron-capture elements have been obtained. We find no evidence of intrinsic abundance spreads. The iron abundance is solar ([FeI/H] = + 0.00 ± 0.01; Ï = 0.03 dex). All the other abundance ratios are generally solar-scaled, similar to the Galactic thin disk and open cluster stars of similar metallicity. The chemical composition of Gaia1 does not support an extra-galactic origin for this stellar cluster, which can be considered as a standard Galactic open cluster.https://www.aanda.org/articles/aa/abs/2017/07/aa31009-17/aa31009-17.htm
The emission distribution of RR Pictoris
We present time-resolved optical spectroscopy of the old nova RR Pic. Two
emission lines (Halpha and He I) are present in the observed part of the
spectrum and both show strong variability. Halpha has been used for Doppler
tomography in order to map the emission distribution in this system for the
first time. The resulting map shows the emission from the disc as well as two
additional emission sources on the leading and trailing side of the disc.
Furthermore we find evidence for the presence of either a disc-overflow or an
asymmetric outflow from the binary with velocities up to +- 1200km/s. The
origin of the outflow would be the emission source on the leading side of the
accretion disc.Comment: Latex, 7 pages, 10 figure
The peculiar Na-O anticorrelation of the bulge globular cluster NGC 6440
Context. Galactic Globular Clusters (GCs) are essential tools to understand
the earliest epoch of the Milky Way, since they are among the oldest objects in
the Universe and can be used to trace its formation and evolution. Current
studies using high resolution spectroscopy for many stars in each of a large
sample of GCs allow us to develop a detailed observational picture about their
formation and their relation with the Galaxy. However, it is necessary to
complete this picture by including GCs that belong to all major Galactic
components, including the Bulge. Aims. Our aim is to perform a detailed
chemical analyses of the bulge GC NGC 6440 in order to determine if this object
has Multiple Populations (MPs) and investigate its relation with the Bulge of
the Milky Way and with the other Galactic GCs, especially those associated with
the Bulge, which are largely poorly studied. Methods. We determined the stellar
parameters and the chemical abundances of light elements (Na, Al), iron-peak
elements (Fe, Sc, Mn, Co, Ni), -elements (O, Mg, Si, Ca, Ti) and heavy
elements (Ba, Eu) in seven red giant members of NGC 6440 using high resolution
spectroscopy from FLAMES@UVES. Results. We found a mean iron content of
[Fe/H]=-0.500.03 dex in agreement with other studies. We found no internal
iron spread. On the other hand, Na and Al show a significant intrinsic spread,
but the cluster has no significant O-Na anticorrelation nor exhibits a Mg-Al
anticorrelation. The -elements show good agreement with the Bulge field
star trend, although they are at the high alpha end and are also higher than
those of other GCs of comparable metallicity. The heavy elements are dominated
by the r-process, indicating a strong contribution by SNeII. The chemical
analysis suggests an origin similar to that of the Bulge field stars.Comment: 12 pages, 13 figures, Accepted for publication in A&
The Calcium Triplet metallicity calibration for galactic bulge stars
We present a new calibration of the Calcium II Triplet equivalent widths
versus [Fe/H], constructed upon K giant stars in the Galactic bulge. This
calibration will be used to derive iron abundances for the targets of the GIBS
survey, and in general it is especially suited for solar and supersolar
metallicity giants, typical of external massive galaxies. About 150 bulge K
giants were observed with the GIRAFFE spectrograph at VLT, both at resolution
R~20,000 and at R~6,000. In the first case, the spectra allowed us to perform
direct determination of Fe abundances from several unblended Fe lines, deriving
what we call here high resolution [Fe/H] measurements. The low resolution
spectra allowed us to measure equivalent widths of the two strongest lines of
the near infrared Calcium II triplet at 8542 and 8662 A. By comparing the two
measurements we derived a relation between Calcium equivalent widths and [Fe/H]
that is linear over the metallicity range probed here, -1<[Fe/H]<+0.7. By
adding a small second order correction, based on literature globular cluster
data, we derived the unique calibration equation [Fe/H], with a rms dispersion of 0.197 dex, valid across the
whole metallicity range -2.3<[Fe/H]<+0.7.Comment: Accepted for publication in A&
The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem
We present Keck/DEIMOS spectroscopy of stars in 8 of the newly discovered
ultra-faint dwarf galaxies around the Milky Way. We measure the velocity
dispersions of Canes Venatici I and II, Ursa Major I and II, Coma Berenices,
Hercules, Leo IV and Leo T from the velocities of 18 - 214 stars in each galaxy
and find dispersions ranging from 3.3 to 7.6 km/s. The 6 galaxies with absolute
magnitudes M_V < -4 are highly dark matter-dominated, with mass-to-light ratios
approaching 1000. The measured velocity dispersions are inversely correlated
with their luminosities, indicating that a minimum mass for luminous galactic
systems may not yet have been reached. We also measure the metallicities of the
observed stars and find that the 6 brightest of the ultra-faint dwarfs extend
the luminosity-metallicity relationship followed by brighter dwarfs by 2 orders
of magnitude in luminosity; several of these objects have mean metallicities as
low as [Fe/H] = -2.3 and therefore represent some of the most metal-poor known
stellar systems. We detect metallicity spreads of up to 0.5 dex in several
objects, suggesting multiple star formation epochs. Having established the
masses of the ultra-faint dwarfs, we re-examine the missing satellite problem.
After correcting for the sky coverage of the SDSS, we find that the ultra-faint
dwarfs substantially alleviate the discrepancy between the predicted and
observed numbers of satellites around the Milky Way, but there are still a
factor of ~4 too few dwarf galaxies over a significant range of masses. We show
that if galaxy formation in low-mass dark matter halos is strongly suppressed
after reionization, the simulated circular velocity function of CDM subhalos
can be brought into approximate agreement with the observed circular velocity
function of Milky Way satellite galaxies. [slightly abridged]Comment: 22 pages, 15 figures (12 in color), 6 tables, minor revisions in
response to referee report. Accepted for publication in Ap
The Draco and Ursa Minor dwarf spheroidals. A comparative study
We present (V,I) photometry of two wide (~25 X 25 arcmin^2) fields centered
on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor.
New estimates of the distance to these galaxies are provided
((m-M)_0(UMi)=19.41 \pm 0.12 and (m-M)_0(Dra)=19.84 \pm 0.14) and a comparative
study of their evolved stellar population is presented. We detect for the first
time the RGB-bump in the Luminosity Function of UMi (V_{RGB}^{Bump}=19.40\pm
0.06) while the feature is not detected in Draco. Photometric metallicity
distributions are obtained for the two galaxies and an accurate analysis to
determine the intrinsic metallicity spread is performed by means of artificial
stars experiments. (Abridged) We demonstrate that the inner region of UMi is
significantly structured, at odds with what expected for a system in dynamical
equilibrium. In particular we show that the main density peak of UMi is
off-centered with respect to the center of symmetry of the whole galaxy and it
shows a much lower ellipticity with respect to the rest of the galaxy.
Moreover, UMi stars are shown to be clustered according to two different
characteristic clustering scales, as opposite to Draco, which instead has a
very symmetric and smooth density profile. The possible consequences of this
striking structural difference on our ideas about galaxy formation are briefly
discussed. Combining our distance modulus with the more recent estimates of the
total luminosity of UMi, we find that the mass to light (M/L) ratio of this
galaxy may be as low as M/L ~ 7, a factor 5-10 lower than current estimates.Comment: 31 pages, 18 .ps figures. AAS Latex. emulateapj5.sty macro included.
Fig. 3 is provided in a low res. version. The full resolution version is
available upon request to [email protected]. Accepted by The
Astronomical Journa
The Sagittarius dwarf irregular galaxy: metallicity and stellar populations
We present deep observations of the dwarf irregular galaxy UKS1927-177
in Sagittarius. Statistically cleaned , CMDs clearly display the key
evolutionary features in this galaxy. Previously detected C stars are located
in the CMDs and shown to be variable, thus confirming the presence of a
significant upper-AGB intermediate age population. A group of likely red
supergiants is also identified, whose magnitude and color is consistent with a
30 Myr old burst of star formation. The observed colors of both blue and red
stars in SagDIG are best explained by introducing a differential reddening
scenario in which internal dust extinction affects the star forming regions.
Adopting a low reddening for the red giants, , gives
[Fe/H]= for the mean stellar metallicity, a value consistent with
the [O/H] abundance measured in the HII regions. This revised metallicity,
which is in accord with the trend of metallicity against luminosity for dwarf
irregular galaxies, is indicative of a ``normal'', although metal-poor, dIrr
galaxy. A quantitative description is given of the spatial distribution of
stars in different age intervals, in comparison with the distribution of the
neutral hydrogen. We find that the youngest stars are located near the major
peaks of emission on the HI shell, whereas the red giants and intermediate-age
C stars define an extended halo or disk with scale length comparable to the
size of the hydrogen cloud. The relationship between the distribution of ISM
and star formation is briefly discussed.Comment: 10 pages, 7 figures, accepted A&
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