308 research outputs found
Galactic abundance gradients from Cepheids : On the iron abundance gradient around 10-12 kpc
Context: Classical Cepheids can be adopted to trace the chemical evolution of
the Galactic disk since their distances can be estimated with very high
accuracy. Aims: Homogeneous iron abundance measurements for 33 Galactic
Cepheids located in the outer disk together with accurate distance
determinations based on near-infrared photometry are adopted to constrain the
Galactic iron gradient beyond 10 kpc. Methods: Iron abundances were determined
using high resolution Cepheid spectra collected with three different
observational instruments: ESPaDOnS@CFHT, Narval@TBL and [email protected] ESO/MPG
telescope. Cepheid distances were estimated using near-infrared (J,H,K-band)
period-luminosity relations and data from SAAO and the 2MASS catalog. Results:
The least squares solution over the entire data set indicates that the iron
gradient in the Galactic disk presents a slope of -0.052+/-0.003 dex/kpc in the
5-17 kpc range. However, the change of the iron abundance across the disk seems
to be better described by a linear regime inside the solar circle and a
flattening of the gradient toward the outer disk (beyond 10 kpc). In the latter
region the iron gradient presents a shallower slope, i.e. -0.012+/-0.014
dex/kpc. In the outer disk (10-12 kpc) we also found that Cepheids present an
increase in the spread in iron abundance. Current evidence indicates that the
spread in metallicity depends on the Galactocentric longitude. Finally, current
data do not support the hypothesis of a discontinuity in the iron gradient at
Galactocentric distances of 10-12 kpc. Conclusions: The occurrence of a spread
in iron abundance as a function of the Galactocentric longitude indicates that
linear radial gradients should be cautiously treated to constrain the chemical
evolution across the disk.Comment: 5 tables, 8 figures, Accepted in A&
Post first dredge-up [C/N] ratio as age indicator. Theoretical calibration
We performed a detailed analysis of the use of [C/N] measured in red giant branch stars between the completion of the first dredge up and the red giant branch bump ([C/N]_{FDU}) as age indicator. [C/N]_{FDU} cannot give accurate ages for individual stars, but may provide a general chronology for the formation of composite populations and add constraints to analyses of red giants from surface gravity-effective temperature diagrams. We provide a theoretical calibration of [C/N]_{FDU} in terms of total metallicity [M/H] and age, for ages greater than 1 Gyr, which we tested against variations in the initial heavy element distribution (scaled-solar vs alpha-enhanced), efficiency of overshooting from MS convective cores and from the convective envelopes, variations in the initial He abundance and in the mixing length parameter. Our calibration is compared with a small sample of available measurements of [C/N]_{FDU} in star clusters and halo field stars, which at least qualitatively confirm the overall trend of the predicted [C/N]_{FDU} with age and [M/H]. The use of [C/N]_{FDU}-[M/H]-age relations obtained from independent sets of stellar evolution calculations cause age differences (for a given [C/N]_{FDU} and [M/H] pair) up to about 2~Gyr. More accurate spectroscopic measurements of [C/N]_{FDU} in star clusters with well-established ages and metallicities are required to better test theoretical calibrations of this age indicator
Multifrequency observations of XTE J0421+560/CI Cam in outburst
We report on two X-ray observations of the transient source XTE J0421+560
performed by BeppoSAX, and on a series of observations performed by the 0.7m
Teramo-Normale Telescope. Outburst peak occurrence time and duration depend on
photon energy: the outburst peak is achieved first in the X-ray band, then in
the optical and finally in the radio. An exponential decay law fits well the
X-ray data except in the TOO2 0.5-1.0 keV band, where erratic time variability
is detected. During TOO1 the e-folding time scale decreases with energy up to ~
20 keV, when it achieves a saturation; during TOO2 it decreases up to ~ 2 keV
and then increases. This change is correlated with a spectral change,
characterized by the onset of a soft (< 2 keV) component in TOO2 (Orr et al.
1998). This component might originate from the relativistic jets, while the
hard component is more likely associated to processes occurring in the
circumstellar matter and/or near the compact object. Optical observations show
that the object appears intrinsically red even during the outburst. The nature
of the compact object is discussed.Comment: Four pages. Accepted for publication in A&A Lette
On the distance of the Magellanic Clouds using Cepheid NIR and optical-NIR Period Wesenheit Relations
We present the largest near-infrared (NIR) data sets, , ever collected
for classical Cepheids in the Magellanic Clouds (MCs). We selected fundamental
(FU) and first overtone (FO) pulsators, and found 4150 (2571 FU, 1579 FO)
Cepheids for Small Magellanic Cloud (SMC) and 3042 (1840 FU, 1202 FO) for Large
Magellanic Cloud (LMC). Current sample is 2--3 times larger than any sample
used in previous investigations with NIR photometry. We also discuss optical
photometry from OGLE-III. NIR and optical--NIR Period-Wesenheit (PW)
relations are linear over the entire period range () and their slopes are, within the intrinsic dispersions, common between the
MCs. These are consistent with recent results from pulsation models and
observations suggesting that the PW relations are minimally affected by the
metal content. The new FU and FO PW relations were calibrated using a sample of
Galactic Cepheids with distances based on trigonometric parallaxes and Cepheid
pulsation models. By using FU Cepheids we found a true distance moduli of
mag (LMC) and
mag (SMC). These estimates
are the weighted mean over ten PW relations and the systematic errors account
for uncertainties in the zero-point and in the reddening law. We found similar
distances using FO Cepheids
( mag [LMC] and
mag [SMC]). These new MC
distances lead to the relative distance, mag (FU, ) and mag (FO, ),which agrees quite
well with previous estimates based on robust distance indicators.Comment: 17 pages, 7 figure
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