600 research outputs found
Properties and origin of the old, metal rich, star cluster, NGC 6791
In this contribution I summarize the unique properties of the old, metal
rich, star cluster NGC 6791, with particular emphasis on its population of
extreme blue horizontal branch stars. I then conclude providing my personal
view on the origin of this fascinating star cluster.Comment: 7 pages, 4 eps figures, paper presented at the 10th Pacific Rim
Conference on Stellar Astrophysics, Seoul, May 27-31, 2013, ASP Conference
Series, in pres
Morphological transformation of dwarf galaxies in the Local Group
In the Local Group there are three main types of dwarf galaxies: Dwarf Irregulars, Dwarf Spheroidals, and Dwarf Ellipticals. Intermediate/transitional types are present as well. This contribution reviews the idea that the present day variety of dwarf galaxy morphologies in the Local Group might reveal the existence of a transformation chain of events, of which any particular dwarf galaxy represents a manifestation of a particular stage. In other words, all dwarf galaxies that now are part of the Local Group would have formed identically in the early universe, but then evolved differently because of morphological transformations induced by dynamical processes like galaxy harassment, ram pressure stripping, photo-evaporation, and so forth. We start describing the population of dwarf galaxies and their spatial distribution in the LG. Then, we describe those phenomena that can alter the morphology of a dwarf galaxies, essentially by removing, partially or completely, their gas content. Lastly, we discuss morphological signatures in the Local Group Dwarf Galaxies that can be attributed to different dynamical phenomena. While it is difficult to identify a unique and continuous transformation sequence, we have now a reasonable understanding of the basic evolutionary paths that lead to the various dwarf galaxy type
Gaia 1 cannot be a Thick Disk Galactic cluster
In this note I show how the recently suggested membership of the open cluster
Gaia 1 to the Galactic thick disk is based on incorrect assumptions about the
structure of the disk itself, and neglect well-known observational evidences on
the disk warp and flare.Comment: 2 pages, 1 figure, in press as a Research Note of the American
Astronomical Societ
The Milky Way disk
This review summarises the invited presentation I gave on the Milky Way disc.
The idea underneath was to touch those topics that can be considered hot
nowadays in the Galactic disk research: the reality of the thick disk, the
spiral structure of the Milky Way, and the properties of the outer Galactic
disk. A lot of work has been done in recent years on these topics, but a
coherent and clear picture is still missing. Detailed studies with high quality
spectroscopic data seem to support a dual Galactic disk, with a clear
separation into a thin and a thick component. Much confusion and very
discrepant ideas still exist concerning the spiral structure of the Milky Way.
Our location in the disk makes it impossible to observe it, and we can only
infer it. This process of inference is still far from being mature, and depends
a lot on the selected tracers, the adopted models and their limitations, which
in many cases are neither properly accounted for, nor pondered enough. Finally,
there are very different opinions on the size (scale length, truncation radius)
of the Galactic disk, and on the interpretation of the observed outer disk
stellar populations in terms either of external entities (Monoceros,
Triangulus-Andromeda, Canis Major), or as manifestations of genuine disk
properties (e.g., warp and flare).Comment: 7 pages, 9 figures. Full text in English. To be published in the 57
Bulletin of the Argentinian Association of Astronomy (BAAA 57
The Milky Way thin disk structure as revealed by stars and young open clusters
In this contribution I shall focus on the structure of the Galactic thin
disk. The evolution of the thin disk and its chemical properties have been
discussed in detail by T. Bensby's contribution in conjunction with the
properties of the Galactic thick disk, and by L.Olivia in conjunction with the
properties of the Galactic bulge. I will review and discuss the status of our
understanding of three major topics, which have been the subject of intense
research nowadays, after long years of silence: (1) the spiral structure of the
Milky Way, (2) the size of the Galactic disk, and (3) the nature of the Local
arm (Orion spur), where the Sun is immersed. The provisional conclusions of
this discussion are that : (1) we still have quite a poor knowledge of the
Milky Way spiral structure, and the main dis-agreements among various tracers
are still to be settled; (2) the Galactic disk does clearly \textit{not} have
an obvious luminous cut-off at about 14 kpc from the Galactic center, and next
generation Galactic models need to be updated in this respect, and (3) the
Local arm is most probably an inter-arm structure, similar to what we see in
several external spirals, like M~74. Finally, the impact of GAIA and LAMOST in
this field will be briefly discussed as well.Comment: 12 pages, 8 eps figure, invited review, to appear in Proceedings of
the IAU Symposium No. 298, "Setting the scene for Gaia and LAMOST", eds. S.
Feltzing, G. Zhao, N. A. Walton, and P. A. Whiteloc
A photometric study of the intermediate age open cluster King 5
We report on near IR (J and K bands) observations of an region centered on the poorly studied open cluster King 5, for
which only optical photometry existed. We found that the cluster is of moderate
age (1.0 Gry old), intermediate in age between the Hyades and NGC 752.
Combining optical and IR photometry we obtain estimates for cluster parameters.
The color excess E(J-K), E(V-I) and E(V-K) are 0.50, 1.10 and 2.45,
respectively. The true distance modulus turns out to be . As a consequence, King 5 is 1.9 kpc far from the Sun.Comment: 7 pages, 6 eps figures, accepted for pubblication in Astronomy and
Astrophysic
The old open clusters Saurer A, B and C revisited
We report on deep (V 24.0) CCD photometry of 3 fields centered
in the regions of the old open clusters Saurer A, B and C. In the case of
Saurer A, which is considered one of the oldest known open cluster, we also
provide a comparison field. From the analysis of the photometry we claim that
Saurer A is as old as M 67 ( 5 Gyrs), but more metal poor (Z=0.008).
Moreover it turns out to be the open cluster with the largest galactocentric
distance so far detected. As for Saurer B, it closely resembles NGC 2158, and
indeed is of intermediate-age (1.8-2.2 Gyrs) and significantly reddened. In
this case we revise both the age and the distance with respect to previous
studies, but we are not able to clearly establish the cluster metal abundance.
Finally, Saurer C has an age of about 2 Gyrs, but we emphasize that the precise
determination of its properties is hampered by the heavy fieldstars
contamination.Comment: 10 pages, 16 eps figures, in press in MNRA
Photometry of the five marginally studied open clusters Collinder 74, Berkeley 27, Haffner 8, NGC 2509 and VdB-Hagen 4
The stellar populations in the outer Galactic disk are nowadays a subject of
wide interest. To contribute to a better picture of this part of the Galaxy, we
have studied the nature of five marginally investigated star clusters
(Collinder 74, Berkeley 27, Haffner 8, NGC 2509, and VdB-Hagen 4) by means of
accurate CCD photometry in the V and I pass-bands. These clusters are in fact
located in the Third Galactic Quadrant. We aim to obtain the basic parameters
of these objects, which in some cases are still disputed in the literature. In
the case of VdB-Hagen 4 we provide the first estimate of its fundamental
parameters, while for Haffner 8 we present the first CCD photometry. The
analysis is based on the comparison between field stars decontaminated Color
Magnitude Diagrams and stellar models. Particular care is devoted to the
assessment of the data quality, and the statistical field stars
decontamination. The library of stellar isochrones from Girardi et al. (2000)
is adopted in this study. The analysis we carried out allowed us to solve a few
inconsistencies in the literature regarding Haffner 8 and NGC 2509. Collinder
74 is found to be significantly older than reported before. VdB-Hagen 4 is a
young open cluster located more than 20 kpc from the Galactic center. Such an
extreme distance is compatible with the cluster belonging to the
Norma-Cygnusarm.Comment: 10 pages, 17 eps figures (some of them degraded in resolution), in
press in Astronomy and Astrophysic
On the formation and evolution of the globular cluster Omega Centauri
By means of N-body/hydrodynamical simulations we model the evolution of a
primordial 10^{8} solar masses density peak which ends up in an object closely
resembling the present day globular cluster Omega Centauri. We succeed to
reproduce the main features of the cluster, namely the structure, kinematics
and metallicity distribution. We suggest that Omega Centauri might be a
cosmological dwarf elliptical, formed at high redshift, evolved in isolation
and self-enriched, and eventually fallen inside the potential well of the Milky
Way, in agreement with the Searle-Zinn (1978) paradigm for galactic globular
clusters formation. We finally suggest that Omega Centauri is probably
surrounded by an extended Dark Matter (DM) halo, for which no observational
evidence is at present available. We expect that signatures, if any, of the DM
halo can be found in the kinematics of stars outside about 20 arcmin.Comment: 5 pages, 7 figures, in press in Astronomy and Astrophysics Main
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