469 research outputs found
Near-IR Spectroscopy of a Young Super-Star Cluster in NGC 6946: Chemical Abundances and Abundance Patterns
Using the NIRSPEC spectrograph at Keck II, we have obtained H and K-band
echelle spectra for a young (10-15 Myr), luminous (MV=-13.2) super-star cluster
in the nearby spiral galaxy NGC 6946. From spectral synthesis and equivalent
width measurements we obtain for the first time accurate abundances and
abundance patterns in an extragalactic super-star cluster. We find
[Fe/H]=-0.45+/-0.08 dex, an average alpha-enhancement of +0.22+/-0.1 dex, and a
relatively low 12C/13C~ 8+/-2 isotopic ratio. We also measure a velocity
dispersion of ~9.1 km/s, in agreement with previous estimates. We conclude that
integrated high-dispersion spectroscopy of massive star clusters is a promising
alternative to other methods for abundance analysis in extragalactic young
stellar populations.Comment: 5 pages, incl. 2 figures. Accepted for publication in MNRAS Letters.
The definitive version will be available at http://www.blackwell-synergy.co
Transformations between the theoretical and observational planes in the HST-NICMOS and WFPC2 photometric systems
Color-temperature relations and bolometric corrections in the HST-NICMOS
F1110W, F160W and F222M and in the WFPC2 F439W, F555W and F814W photometric
systems, using two different sets of model atmospheres, have been derived. This
database of homogeneous, self-consistent transformations between the
theoretical and observational planes also allows combinations of visual and
infrared quantities, without any further transformation between the two
different photometric systems. The behavior of the inferred quantities with
varying the stellar parameters, the adopted model atmospheres and the
instrumental configurations are investigated. Suitable relations to transform
colors and bolometric corrections from HST to ground-based photometric systems
are also provided.Comment: 22 pages, 14 figure
Probing the Galactic Bulge with deep Adaptive Optics imaging: the age of NGC 6440
We present first results of a pilot project aimed at exploiting the
potentiality of ground based adaptive optics imaging in the near infrared to
determine the age of stellar clusters in the Galactic Bulge. We have used a
combination of high resolution adaptive optics (ESO-VLT NAOS-CONICA) and
wide-field (ESO-NTT-SOFI) photometry of the metal rich globular cluster NGC
6440 located towards the inner Bulge, to compute a deep color magnitude diagram
from the tip of the Red Giant Branch down to J~22$, two magnitudes below the
Main Sequence Turn Off (TO). The magnitude difference between the TO level and
the red Horizontal Branch has been used as an age indicator. It is the first
time that such a measurement for a bulge globular cluster has been obtained
with a ground based telescope. From a direct comparison with 47 Tuc and with a
set of theoretical isochrones, we concluded that NGC 6440 is old and likely
coeval to 47 Tuc. This result adds a new evidence that the Galactic Bulge is ~2
Gyr younger at most than the pristine, metal poor population of the Galactic
Halo
High resolution infrared spectra of bulge globular clusters: Liller~1 and NGC 6553
Using the NIRSPEC spectrograph at Keck II, we have obtained echelle spectra
covering the range 1.5-1.8um for 2 of the brightest giants in Liller 1 and NGC
6553, old metal rich globular clusters in the Galactic bulge. We use spectrum
synthesis for the abundance analysis, and find [Fe/H]=-0.3 +/- 0.2 and
[O/H]=+0.3 +/- 0.2 dex. The composition of the clusters is similar to that of
field stars in the bulge and is consistent with a sceanrio in which the
clusters formed early, with rapid enrichment. We have dificulty achieveing a
good fit to the spectrum of NGC 6553 using either the low or the high values
recently reported in the literature, unless unusually large, or no
alpha-element enhancements are adopted, respectively.Comment: To appear in the Astronomical Journal, March 200
An empirical mass-loss law for Population II giants from the Spitzer-IRAC survey of Galactic globular clusters
The main aim of the present work is to derive an empirical mass-loss (ML) law
for Population II stars in first and second ascent red giant branches. We used
the Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8
micron range of a carefully chosen sample of 15 Galactic globular clusters
spanning the entire metallicity range and sampling the vast zoology of
horizontal branch (HB) morphologies. We complemented the IRAC photometry with
near-infrared data to build suitable color-magnitude and color-color diagrams
and identify mass-losing giant stars. We find that while the majority of stars
show colors typical of cool giants, some stars show an excess of mid-infrared
light that is larger than expected from their photospheric emission and that is
plausibly due to dust formation in mass flowing from them. For these stars, we
estimate dust and total (gas + dust) ML rates and timescales. We finally
calibrate an empirical ML law for Population II red and asymptotic giant branch
stars with varying metallicity. We find that at a given red giant branch
luminosity only a fraction of the stars are losing mass. From this, we conclude
that ML is episodic and is active only a fraction of the time, which we define
as the duty cycle. The fraction of mass-losing stars increases by increasing
the stellar luminosity and metallicity. The ML rate, as estimated from
reasonable assumptions for the gas-to-dust ratio and expansion velocity,
depends on metallicity and slowly increases with decreasing metallicity. In
contrast, the duty cycle increases with increasing metallicity, with the net
result that total ML increases moderately with increasing metallicity, about
0.1 Msun every dex in [Fe/H]. For Population II asymptotic giant branch stars,
we estimate a total ML of <0.1 Msun, nearly constant with varying metallicity.Comment: 17 pages, 9 figures, in press on A&
High precision radial velocities with GIANO spectra
Radial velocities (RV) measured from near-infrared (NIR) spectra are a
potentially excellent tool to search for extrasolar planets around cool or
active stars. High resolution infrared (IR) spectrographs now available are
reaching the high precision of visible instruments, with a constant improvement
over time. GIANO is an infrared echelle spectrograph at the Telescopio
Nazionale Galileo (TNG) and it is a powerful tool to provide high resolution
spectra for accurate RV measurements of exoplanets and for chemical and
dynamical studies of stellar or extragalactic objects. No other high spectral
resolution IR instrument has GIANO's capability to cover the entire NIR
wavelength range (0.95-2.45 micron) in a single exposure. In this paper we
describe the ensemble of procedures that we have developed to measure high
precision RVs on GIANO spectra acquired during the Science Verification (SV)
run, using the telluric lines as wavelength reference. We used the Cross
Correlation Function (CCF) method to determine the velocity for both the star
and the telluric lines. For this purpose, we constructed two suitable digital
masks that include about 2000 stellar lines, and a similar number of telluric
lines. The method is applied to various targets with different spectral type,
from K2V to M8 stars. We reached different precisions mainly depending on the H
-magnitudes: for H ~ 5 we obtain an rms scatter of ~ 10 m s-1, while for H ~ 9
the standard deviation increases to ~ 50 - 80 m s-1. The corresponding
theoretical error expectations are ~4 m s-1 and 30 m s-1, respectively. Finally
we provide the RVs measured with our procedure for the targets observed during
GIANO Science Verification.Comment: 26 pages, 15 figures, 6 table
An Accurate, Easy to Use Abundace Scale for Globular Clusters Based on 2.2um Spectra of Giant Stars
We present a new method for the determination of [Fe/H] for globular
clusters. This new method is based on moderate resolution (R~1500) near-IR
spectroscopy in the K-band of 6 to 10 of the brightest giants in a cluster. Our
calibration is derived from spectra of 105 stars in 15 globular clusters. From
measurements of the equivalent widths of three features in these spectra, Na,
Ca, and CO, we are able to reproduce the Zinn & West (1984) abundance scale as
updated by Harris (1996) to better than 0.10 dex for clusters with near solar
[Fe/H] down to an [Fe/H] of -1.8. Three advantages of this method are that it
can be used for metal rich, heavily reddened globulars in crowded fields, it
does not require any knowledge of any other cluster or stellar parameters such
as reddening, distance, or luminosity, and it requires only minimal telescope
time. If stellar (J-K)0 and MK values are available as well, the accuracy of
the [Fe/H] estimate is further improved. Observations of as few as three stars
per cluster still gives an [Fe/H] estimate wich is nearly as reliable as that
based on two to three times as many stars.
The accuracy of an [Fe/H] value based on observations of CO absorption alone
is significantly less than that which results from the three spectroscopic
indices. However, we predict that space-based observations of this feature in
the integrated light of stellar systems will prove to be of great value for
abundance determinations at distances as far as the Coma cluster of galaxies.Comment: 53 pages, 16 Postscript figures. Submitted to the A
Formation & evolution of the Galactic bulge: constraints from stellar abundances
We compute the chemical evolution of the Galactic bulge in the context of an
inside-out model for the formation of the Milky Way. The model contains updated
stellar yields from massive stars. The main purpose of the paper is to compare
the predictions of this model with new observations of chemical abundance
ratios and metallicity distributions in order to put constraints on the
formation and evolution of the bulge. We computed the evolution of several
alpha-elements and Fe and performed several tests by varying different
parameters such as star formation efficiency, slope of the initial mass
function and infall timescale. We also tested the effect of adopting a primary
nitrogen contribution from massive stars. The [alpha/Fe] abundance ratios in
the Bulge are predicted to be supersolar for a very large range in [Fe/H], each
element having a different slope. These predictions are in very good agreement
with most recent accurate abundance determinations. We also find a good fit of
the most recent Bulge stellar metallicity distributions. We conclude that the
Bulge formed on a very short timescale (even though timescales much shorter
than about 0.1 Gyr are excluded) with a quite high star formation efficiency of
about 20 Gyr and with an initial mass function more skewed toward high
masses (i.e. x <= 0.95) than the solar neighbourhood and rest of the disk. The
results obtained here are more robust than previous ones since they are based
on very accurate abundance measurements.Comment: 26 pages, 9 figures, accepted for publication in A&
TNG Near-IR Photometry of five Galactic Globular Clusters
We present near--infrared J and K observations of giant stars in five
metal-poor Galactic Globular Clusters (namely M3, M5, M10, M13 and M92)
obtained at the Telescopio Nazionale Galileo (TNG). This database has been used
to determine the main photometric properties of the red giant branch (RGB) from
the (K,J-K) and, once combined with the optical data, in the (K,V-K) Color
Magnitude Diagrams. A set of photometric indices (the RGB colors at fixed
magnitudes) and the major RGB evolutionary features (slope, bump, tip) have
been measured. The results have been compared with the relations obtained by
Ferraro et al. 2000 and with the theoretical expectations, showing a very good
agreement.Comment: 10 pages, 12 figures, accepted by Astronomy and Astrophysic
The Nuclear Stellar Cluster in the Seyfert~1 Galaxy NGC 3227: High Angular Resolution NIR Imaging and Spectroscopy
NIR high angular resolution speckle imaging and imaging spectroscopy of the
nuclear region (10'' ~ 840pc) of the Seyfert1 galaxy NGC3227 are presented. A
nuclear stellar cluster is slightly resolved in the J and H band with
increasing contribution to the NIR continuum from the K to the J band. The
stellar absorption lines are extended compared to the neighboring continuum
suggesting a cluster size of ~ 70pc FWHM. Analysis of those lines suggests that
the stars are contributing about 65% (40%) of the total continuum emission in
the H (K) band in a 3.6'' aperture. Population synthesis in conjunction with
NIR spectral synthesis indicates an age of 25 to 50 Myr when red supergiants
contribute most to the NIR light. This is supported by published optical data
on the MgIb line and the CaII triplet. Although a higher age of ~ 0.5 Gyr where
AGB stars dominate the NIR light can not be excluded, the observed parameters
are at the limit of those expected for a cluster dominated by AGB stars.
However, in either case the resolved stellar cluster contributes only about ~
15 % of the total dynamical mass in the inner 300pc implying another much older
stellar population. Pure constant star formation over the last 10 Gyr can be
excluded. Therefore, at least two star formation/starburst events took place in
the nucleus of NGC3227. Since such sequences in the nuclear star formation
history are also observed in the nuclei of other galaxies a link between the
activity of the star formation and the AGN itself seems likely.Comment: accepted for publication in the Astrophysical Journal, 46 pages, 15
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