601 research outputs found
AL 3 (BH 261): a new globular cluster in the Galaxy
AL~3 (BH 261), previously classified as a faint open cluster candidate, is
shown to be a new globular cluster in the Milky Way, by means of B, V and I
Color-Magnitude Diagrams. The main feature of AL~3 is a prominent blue extended
Horizontal Branch. Its Color-Magnitude Diagrams match those of the intermediate
metallicity cluster M~5. The cluster is projected in a rich bulge field, also
contaminated by the disk main sequence. The globular cluster is located in the
Galactic bulge at a distance from the Sun d = 6.00.5 kpc. The
reddening is E(B-V)=0.360.03 and the metallicity is estimated to be [Fe/H]
-1.30.25. AL~3 is probably one of the least massive globular
clusters of the Galaxy.Comment: 6 figures. Astrophysical Journal Letters, in pres
The old metal-poor open cluster ESO 92-SC05: accreted from a dwarf galaxy?
The study of old open clusters outside the solar circle can bring constraints
on formation scenarios of the outer disk. In particular, accretion of dwarf
galaxies has been proposed as a likely mechanism in the area. We use BVI
photometry for determining fundamental parameters of the faint open cluster ESO
92-SC05. Colour-Magnitude Diagrams are compared with Padova isochrones, in
order to derive age, reddening and distance. We derive a reddening E(B-V)=
0.17, and an old age of 6.0 Gyr.
It is one of the rare open clusters known to be older than 5 Gyr. A
metallicity of Z0.004 or [M/H]-0.7 is found. The rather low
metallicity suggests that this cluster might be the result of an accretion
episode of a dwarf galaxy.Comment: 11 figures: 1, 2a,b,c, 3a,b, 4a,b, 5, 6, 7 6 pages to compile with
mn2e.cls. Monthly Notices of the Royal Astronomical Society, in pres
Chemodynamical history of the Galactic Bulge
The Galactic Bulge can uniquely be studied from large samples of individual
stars, and is therefore of prime importance for understanding the stellar
population structure of bulges in general. Here the observational evidence on
the kinematics, chemical composition, and ages of Bulge stellar populations
based on photometric and spectroscopic data is reviewed. The bulk of Bulge
stars are old and span a metallicity range -1.5<~[Fe/H]<~+0.5. Stellar
populations and chemical properties suggest a star formation timescale below ~2
Gyr. The overall Bulge is barred and follows cylindrical rotation, and the more
metal-rich stars trace a Box/Peanut (B/P) structure. Dynamical models
demonstrate the different spatial and orbital distributions of metal-rich and
metal-poor stars. We discuss current Bulge formation scenarios based on
dynamical, chemical, chemodynamical and cosmological models. Despite impressive
progress we do not yet have a successful fully self-consistent chemodynamical
Bulge model in the cosmological framework, and we will also need more extensive
chrono-chemical-kinematic 3D map of stars to better constrain such models.Comment: 9 figures, 55 pages final version to appear in the Annual Reviews of
Astronomy & Astrophysics, volume 5
Detection of Ks-excess stars in the 14Myr open cluster NGC4755
We derive the structure, distribution of MS and PMS stars and dynamical state
of the young open cluster NGC 4755. We explore the possibility that, at the
cluster age, some MS and PMS stars still present infrared excesses related to
dust envelopes and proto-planetary discs. The radial density profile follows
King's law with a core radius \rm\rc=0.7\pm0.1 pc and a limiting radius
\rm\rl=6.9\pm0.1 pc; the cluster age is . Field-star
decontamination reveals a low-MS limit at \rm\approx1.4 \ms. The core MF
() is flatter than the halo's (). NGC 4755
contains candidate PMS stars of age , and a few
evolved stars. The mass locked up in PMS, MS and evolved stars amounts to
\rm\sim1150 \ms. Proper motions show that \ks-excess MS and PMS stars are
cluster members. \ks-excess fractions in PMS and MS stars are
and respectively, consistent with the cluster age. The core is
deficient in PMS stars, as compared with MS ones. NGC 4755 hosts binaries in
the halo but they are scarce in the core. Compared to open clusters in
different dynamical states studied with similar methods, NGC 4755 fits
relations involving structural and dynamical parameters in the expected locus
for its age and mass. On the other hand, the flatter core MF probably
originates from primordial processes related to parent molecular cloud
fragmentation and mass segregation over . Star formation in NGC
4755 began ago and proceeded for about the same length of
time. Detection of \ks-excess emission in member MS stars suggests that some
circumstellar dust discs survived for , occurring both in some
MS and PMS stars for the age and spread observed in NGC 4755.Comment: 10 figs. Astronomy & Astrophysics, in pres
Abundances in the Galactic bulge: results from planetary nebulae and giant stars
Our understanding of the chemical evolution of the Galactic bulge requires
the determination of abundances in large samples of giant stars and planetary
nebulae (PNe). We discuss PNe abundances in the Galactic bulge and compare
these results with those presented in the literature for giant stars. We
present the largest, high-quality data-set available for PNe in the direction
of the Galactic bulge (inner-disk/bulge). For comparison purposes, we also
consider a sample of PNe in the Large Magellanic Cloud (LMC). We derive the
element abundances in a consistent way for all the PNe studied. By comparing
the abundances for the bulge, inner-disk, and LMC, we identify elements that
have not been modified during the evolution of the PN progenitor and can be
used to trace the bulge chemical enrichment history. We then compare the PN
abundances with abundances of bulge field giant. At the metallicity of the
bulge, we find that the abundances of O and Ne are close to the values for the
interstellar medium at the time of the PN progenitor formation, and hence these
elements can be used as tracers of the bulge chemical evolution, in the same
way as S and Ar, which are not expected to be affected by nucleosynthetic
processes during the evolution of the PN progenitors. The PN oxygen abundance
distribution is shifted to lower values by 0.3 dex with respect to the
distribution given by giants. A similar shift appears to occur for Ne and S. We
discuss possible reasons for this PNe-giant discrepancy and conclude that this
is probably due to systematic errors in the abundance derivations in either
giants or PNe (or both). We issue an important warning concerning the use of
absolute abundances in chemical evolution studies.Comment: 23 pages, 15 figures, 16 pages of online material, A&A in pres
Origin of the heavy elements in HD 140283. Measurement of europium abundance
HD 140283 is a nearby (V=7.7) subgiant metal-poor star, extensively analysed
in the literature. Although many spectra have been obtained for this star, none
showed a signal-to-noise (S/N) ratio high enough to enable a very accurate
derivation of abundances from weak lines. The detection of europium proves that
the neutron-capture elements in this star originate in the r-process, and not
in the s-process, as recently claimed in the literature. Based on the OSMARCS
1D LTE atmospheric model and with a consistent approach based on the spectrum
synthesis code Turbospectrum, we measured the europium lines at 4129 {\AA} and
4205 {\AA}, taking into account the hyperfine structure of the transitions. The
spectrum, obtained with a long exposure time of seven hours at the
Canada-France-Hawaii Telescope (CFHT), has a resolving power of 81000 and a S/N
ratio of 800 at 4100 {\AA}. We were able to determine the abundance A(Eu)=-2.35
dex, compatible with the value predicted for the europium from the r-process.
The abundance ratio [Eu/Ba]=+0.58 dex agrees with the trend observed in
metal-poor stars and is also compatible with a strong r-process contribution to
the origin of the neutron-capture elements in HD 140283.Comment: 10 pages, 7 figures. To be published in A\&
Measuring stellar oscillations using equivalent widths of absorption lines
Kjeldsen et al. (1995, AJ 109, 1313; astro-ph/9411016) have developed a new
technique for measuring stellar oscillations and claimed a detection in the G
subgiant eta Boo. The technique involves monitoring temperature fluctuations in
a star via their effect on the equivalent width of Balmer lines. In this paper
we use synthetic stellar spectra to investigate the temperature dependence of
the Balmer lines, Ca II, Fe I, the Mg b feature and the G~band. We present a
list of target stars likely to show solar-like oscillations and estimate their
expected amplitudes. We also show that centre-to-limb variations in Balmer-line
profiles allow one to detect oscillation modes with l<=4, which accounts for
the detection by Kjeldsen et al. of modes with degree l=3 in integrated
sunlight.Comment: MNRAS (accepted); 7 pages, LaTeX with necessary style file and
PostScript figures in a single uuencoded Z-compressed .tar fil
Magnesium Isotopes in Metal-Poor Dwarfs, the Rise of AGB Stars and the Formation Timescale of the Galactic Halo
We have determined magnesium isotopic ratios (^{25,26}Mg/Mg) in metal-poor
(-2.6 [Fe/H] -1.3) halo dwarfs employing high S/N (90-280) high spectral
resolution (R = 10^5) Keck HIRES spectra. Unlike previous claims of an
important contribution from intermediate-mass AGB stars at low metallicities,
we find that the rise of the AGB contribution in the Galactic halo did not
occur until intermediate metallicities ([Fe/H] ~> -1.5).Comment: ApJ Letters, in pres
Evolution of O Abundance Relative to Fe
We present a three-component mixing model for the evolution of O abundance
relative to Fe, taking into account the contributions of the first very massive
(> 100 solar masses) stars formed from Big Bang debris. We show that the
observations of O and Fe abundances in metal-poor stars in the Galaxy by
Israelian et al. and Boesgaard et al. can be well represented both
qualitatively and quantitatively by this model. Under the assumption of an
initial Fe ([Fe/H] = -3) and O inventory due to the prompt production by the
first very massive stars, the data at -3 < [Fe/H] < -1 are interpreted to
result from the addition of O and Fe only from type II supernovae (SNII) to the
prompt inventory. At [Fe/H] = -1, SNII still contribute O while both SNII and
type Ia supernovae contribute Fe. During this later stage, (O/Fe) sharply drops
off to an asymptotic value of 0.8(O/Fe)_sun. The value of (O/Fe) for the prompt
inventory at [Fe/H] = -3 is found to be (O/Fe) = 20(O/Fe)_sun. This result
suggests that protogalaxies with low ``metallicities'' should exhibit high
values of (O/Fe). The C/O ratio produced by the first very massive stars is
expected to be much less than 1 so that all the C should be tied up as CO and
that C dust and hydrocarbon compounds should be quite rare at epochs
corresponding to [Fe/H] < -3.Comment: 25 pages, 8 postscript figures, to appear in Ap
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