181 research outputs found
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
Detailed Analysis of Nearby Bulgelike Dwarf Stars III. Alpha and Heavy-element abundances
The present sample of nearby bulgelike dwarf stars has kinematics and
metallicities characteristic of a probable inner disk or bulge origin. Ages
derived by using isochrones give 10-11 Gyr for these stars and metallicities
are in the range -0.80< [Fe/H]< +0.40. We calculate stellar parameters from
spectroscopic data, and chemical abundances of Mg, Si, Ca, Ti, La, Ba, Y, Zr
and Eu are derived by using spectrum synthesis.
We found that [alpha-elements/Fe] show different patterns depending on the
element. Si, Ca and Ti-to-iron ratios decline smoothly for increasing
metallicities, and follow essentially the disk pattern. O and Mg, products of
massive supernovae, and also the r-process element Eu, are overabundant
relative to disk stars, showing a steeper decline for metallicities [Fe/H] >
-0.3 dex. [s-elements/Fe] roughly track the solar values with no apparent trend
with metallicity for [Fe/H] < 0, showing subsolar values for the metal rich
stars. Both kinematical and chemical properties of the bulgelike stars indicate
a distinct identity of this population when compared to disk stars.Comment: 21 pages, 9 figures, to appear in Ap
Chemical abundances for 11 bulge stars from high-resolution, near-IR spectra
It is debated whether the Milky Way bulge has the characteristics of a
classical bulge sooner than those of a pseudobulge. Detailed abundance studies
of bulge stars is a key to investigate the origin, history, and classification
of the bulge. The aim is to add to the discussion on the origin of the bulge
and to study detailed abundances determined from near-IR spectra for bulge
giants already investigated with optical spectra, the latter also providing the
stellar parameters which are very significant for the results of the present
study. Especially, the important CNO elements are better determined in the
near-IR. High-resolution, near-infrared spectra in the H band are recorded
using the CRIRES spectrometer on the Very Large Telescope. The CNO abundances
can all be determined from the numerous molecular lines in the wavelength range
observed. Abundances of the alpha elements are also determined from the near-IR
spectra. [O/Fe], [Si/Fe] and [S/Fe] are enhanced up to metallicities of at
least [Fe/H]=-0.3, after which they decline. This suggests that the Milky Way
bulge experienced a rapid and early star-formation history like that of a
classical bulge. However, a similarity between the bulge trend and the trend of
the local thick disk seems present. Such a similarity could suggest that the
bulge has a pseudobulge origin. Our [C/Fe] trend does not show any increase
with [Fe/H] which could have been expected if W-R stars have contributed
substantially to the C abundances. No "cosmic scatter" can be traced around our
observed abundance trends; the scatter found is expected, given the
observational uncertainties.Comment: Accepted for publication in A&
High-resolution abundance analysis of red giants in the metal-poor bulge globular cluster HP~1
The globular cluster HP~1 is projected at only 3.33 degrees from the Galactic
center. Together with its distance, this makes it one of the most central
globular clusters in the Milky Way. It has a blue horizontal branch (BHB) and a
metallicity of [Fe/H]~-1.0. This means that it probably is one of the oldest
objects in the Galaxy. Abundance ratios can reveal the nucleosynthesis pattern
of the first stars as well as the early chemical enrichment and early formation
of stellar populations. High-resolution spectra obtained for six stars were
analyzed to derive the abundances of the light elements C, N, O, Na, and Al,
the alpha-elements Mg, Si, Ca, and Ti, and the heavy elements Sr, Y , Zr, Ba,
La, and Eu.} High-resolution spectra of six red giants that are confirmed
members of the bulge globular cluster HP~1 were obtained with the 8m VLT
UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration.
The spectroscopic parameter derivation was based on the excitation and
ionization equilibrium of FeI and FeII. We confirm a mean metallicity of [Fe/H]
= -1.06~0.10, by adding the two stars that were previously analyzed in HP~1.
The alpha-elements O and Mg are enhanced by about +0.3<[O,Mg/Fe]<+0.5 dex, Si
is moderately enhanced with +0.15<[Si/Fe]<+0.35dex, while Ca and Ti show lower
values of -0.04<[Ca,Ti/Fe]<+0.28dex. The r-element Eu is also enhanced with
[Eu/Fe]~+0.4, which together with O and Mg is indicative of early enrichment by
type II supernovae. Na and Al are low, but it is unclear if Na-O are
anticorrelated. The heavy elements are moderately enhanced, with
-0.20<[La/Fe]<+0.43dex and 0.0<[Ba/Fe]<+0.75~dex, which is compatible with
r-process formation. The spread in Y, Zr, Ba, and La abundances, on the other
hand, appears to be compatible with the spinstar scenario or other additional
mechanisms such as the weak r-process.Comment: 15 pages, 8 figures In press in Astronomy & Astrophysics (2016
Iron-peak elements Sc, V, Mn, Cu and Zn in Galactic bulge globular clusters
Globular clusters are tracers of the history of star formation and chemical
enrichment in the early Galaxy. Their abundance pattern can help understanding
their chemical enrichment processes. In particular, the iron-peak elements have
been relatively little studied so far in the Galactic bulge. The abundances of
iron-peak elements are derived for the sample clusters, and compared with bulge
field, and thick disk stars. We derive abundances of the iron-peak elements Sc,
V, Mn, Cu, and Zn in individual stars of five bulge globular clusters (NGC
6528, NGC 6553, NGC 6522, NGC 6558, HP1), and of the reference thick disk/inner
halo cluster 47 Tucanae (NGC 104). High resolution spectra were obtained with
the UVES spectrograph at the Very Large Telescope over the years. The sample
globular clusters studied span metallicities in the range -1.2<Fe/H]<0.0. V and
Sc appear to vary in lockstep with Fe, indicating that they are produced in the
same supernovae as Fe. We find that Mn is deficient in metal-poor stars,
confirming that it is underproduced in massive stars; Mn-over-Fe steadily
increases at the higher metallicities due to a metallicity-dependent enrichment
by supernovae of type Ia. Cu behaves as a secondary element, indicating its
production in a weak-s process in massive stars. Zn has an alpha-like behaviour
at low metallicities, which can be explained in terms of nucleosynthesis in
hypernovae. At the metal-rich end, Zn decreases with increasing metallicity,
similarly to the alpha-elements.Comment: article resubmitted to Astronomy & Astrophysics, taking into account
referee's comment
The E-ELT Multi-Object Spectrograph: latest news from MOSAIC
There are 8000 galaxies, including 1600 at z larger than 1.6, which could be
simultaneously observed in an E-ELT field of view of 40 sq. arcmin. A
considerable fraction of astrophysical discoveries require large statistical
samples, which can only be obtained with multi-object spectrographs (MOS).
MOSAIC will provide a vast discovery space, enabled by a multiplex of 200 and
spectral resolving powers of R=5000 and 20000. MOSAIC will also offer the
unique capability of more than 10 "high-definition" (multi-object adaptive
optics, MOAO) integral-field units, optimised to investigate the physics of the
sources of reionization. The combination of these modes will make MOSAIC the
world-leading MOS facility, contributing to all fields of contemporary
astronomy, from extra-solar planets, to the study of the halo of the Milky Way
and its satellites, and from resolved stellar populations in nearby galaxies
out to observations of the earliest "first-light" structures in the Universe.
It will also study the distribution of the dark and ordinary matter at all
scales and epochs of the Universe. Recent studies of critical technical issues
such as sky-background subtraction and MOAO have demonstrated that such a MOS
is feasible with state-of-the-art technology and techniques. Current studies of
the MOSAIC team include further trade-offs on the wavelength coverage, a
solution for compensating for the non-telecentric new design of the telescope,
and tests of the saturation of skylines especially in the near-IR bands. In the
2020s the E-ELT will become the world's largest optical/IR telescope, and we
argue that it has to be equipped as soon as possible with a MOS to provide the
most efficient, and likely the best way to follow-up on James Webb Space
Telescope (JWST) observations.Comment: 10 pages, 3 Figures, in Ground-based and Airborne Instrumentation for
Astronomy VI, 2016, Proc. SPI
A MUSE study of the inner bulge globular cluster Terzan 9: a fossil record in the Galaxy
Context. Moderately metal-poor inner bulge globular clusters are relics of a
generation of long-lived stars that formed in the early Galaxy. Terzan 9,
projected at 4d 12 from the Galactic center, is among the most central globular
clusters in the Milky Way, showing an orbit which remains confined to the inner
1 kpc. Aims. Our aim is the derivation of the cluster's metallicity, together
with an accurate measurement of the mean radial velocity. In the literature,
metallicities in the range between have been estimated for Terzan 9 based on
color-magnitude diagrams and CaII triplet (CaT) lines. Aims. Our aim is the
derivation of the cluster's metallicity, together with an accurate measurement
of the mean radial velocity. In the literature, metallicities in the range
between -2.0 and -1.0 have been estimated for Terzan 9 based on color-magnitude
diagrams and CaII triplet (CaT) lines.
Methods. Given its compactness, Terzan 9 was observed using the Multi Unit
Spectroscopic Explorer (MUSE) at the Very Large Telescope. The extraction of
spectra from several hundreds of individual stars allowed us to derive their
radial velocities, metallicities, and [Mg/Fe]. The spectra obtained with MUSE
were analysed through full spectrum fitting using the ETOILE code.
Results. We obtained a mean metallicity of [Fe/H] -1.10 0.15, a heliocentric
radial velocity of vhr = 58.1 1.1 km/s , and a magnesium-to-iron [Mg/Fe] = 0.27
0.03. The metallicity-derived character of Terzan 9 sets it among the family of
the moderately metal-poor Blue Horizontal Branch clusters HP 1, NGC 6558, and
NGC 6522
An Abundance Analysis for Five Red Horizontal Branch Stars in the Extremely Metal Rich Globular Cluster NGC 6553
We provide a high dispersion line-by-line abundance analysis of five red HB
stars in the extremely metal rich galactic globular cluster NGC 6553. These red
HB stars are significantly hotter than the very cool stars near the tip of the
giant branch in such a metal rich globular cluster and hence their spectra are
much more amenable to an abundance analysis than would be the case for red
giants.
We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the
mean abundance in the galactic bulge found by McWilliam & Rich (1994) and
considerably higher than that obtained from an analysis of two red giants in
this cluster by Barbuy etal (1999). The relative abundance for the best
determined alpha process element (Ca) indicates an excess of alpha process
elements of about a factor of two. The metallicity of NGC 6553 reaches the
average of the Galactic bulge and of the solar neighborhood.Comment: 29 pages, 6 figures, accepted for publication in the Ap
HST-NICMOS Observations of Terzan 5: Stellar Content and Structure of the Core
We report results from HST-NICMOS imaging of the extremely dense core of the
globular cluster Terzan 5. This highly obscured bulge cluster has been
estimated to have one of the highest collision rates of any galactic globular
cluster, making its core a particularly conducive environment for the
production of interacting binary systems. We have reconstructed high-resolution
images of the central 19"x19" region of Terzan 5 by application of the drizzle
algorithm to dithered NIC2 images in the F110W, F187W, and F187N near-infrared
filters. We have used a DAOPHOT/ALLSTAR analysis of these images to produce the
deepest color-magnitude diagram (CMD) yet obtained for the core of Terzan 5. We
have also analyzed the parallel 11"X11" NIC1 field, centered 30" from the
cluster center and imaged in F110W and F160W, and an additional NIC2 field that
is immediately adjacent to the central field. This imaging results in a clean
detection of the red-giant branch and horizontal branch in the central NIC2
field, and the detection of these plus the main-sequence turnoff and the upper
main sequence in the NIC1 field. We have constructed an H versus J-H CMD for
the NIC1 field. We obtain a new distance estimate of 8.7 kpc, which places
Terzan 5 within less than 1 kpc of the galactic center. We have also determined
a central surface-density profile which results in a maximum likelihood
estimate of 7.9" +/- 0.6" for the cluster core radius. We discuss the
implications of these results for the dynamical state of Terzan 5.Comment: 17 pages, 9 figures, accepted for publication in ApJ, for May 20,
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