Aims. The iron abundance gradient in the Galactic stellar disk provides fundamental constraints on the chemical evolution of this important Galaxy component, however the spread around the mean slope ...is, at fixed Galactocentric distance, more than the estimated uncertainties. Methods. To provide quantitative constraints on these trends, we adopted iron abundances for 265 classical Cepheids (more than 50% of the currently known sample) based either on high-resolution spectra or on photometric metallicity indices. Homogeneous distances were estimated using near-infrared period-luminosity relations. The sample covers the four disk quadrants, and their Galactocentric distances range from ~5 to ~17 kpc. We provided a new theoretical calibration of the metallicity-index-color (MIC) relation based on Walraven and NIR photometric passbands. Results. We estimated the photometric metallicity of 124 Cepheids. Among them 66 Cepheids also have spectroscopic iron abundances and we found that the mean difference is $-0.03\pm0.15$ dex. We also provide new iron abundances, based on high-resolution spectra, for four metal-rich Cepheids located in the inner disk. The remaining iron abundances are based on high-resolution spectra collected by our group (73) or available in the literature (130). A linear regression over the entire sample provides an iron gradient of $-0.051 \pm 0.004$ dex kpc-1. The above slope agrees quite well, within the errors, with previous estimates based either on Cepheids or on open clusters covering similar Galactocentric distances. However, Cepheids located in the inner disk systematically appear more metal-rich than the mean metallicity gradient. Once we split the sample into inner ($R_{\rm G} <8$ kpc) and outer disk Cepheids, the slope ($-0.130\pm0.015$ dex kpc-1) in the former region is ≈3 times steeper than the slope in the latter one ($-0.042 \pm 0.004$ dex kpc-1). In the outer disk the radial distribution of metal-poor (MP, Fe/H $<-0.02$ dex) and metal-rich (MR) Cepheids across the four disk quadrants does not show a clear trend when moving from the innermost to the external disk regions. The relative fractions of MP and MR Cepheids in the 1st and in the 3rd quadrants differ at the 8σ (MP) and 15σ (MR) levels. Finally, we found that iron abundances in two local overdensities of the 2nd and of the 4th quadrant cover individually a range in iron abundance of ≈0.5 dex. Conclusions. Current findings indicate that the recent chemical enrichment across the Galactic disk shows a clumpy distribution.
We present new and accurate near-infrared (NIR) J, K-band time series data for the Galactic globular cluster (GC) M5 = NGC 5904. Data were collected with SOFI at the NTT (71 J+ 120 K images) and with ...NICS at the TNG (25 J+ 22 K images) and cover two orthogonal strips across the centre of the cluster of ≈5 × 10 arcmin2 each. These data allowed us to derive accurate mean K-band magnitudes for 52 fundamental (RR
ab
) and 24 first overtone (RR
c
) RR Lyrae stars. Using this sample of RR Lyrae stars, we find that the slope of the K-band period-luminosity (PLK) relation (−2.33 ± 0.08) agrees quite well with similar estimates available in the literature. We also find, using both theoretical and empirical calibrations of the PLK relation, a true distance to M5 of 14.44 ± 0.02 mag. This distance modulus agrees very well (1σ) with distances based on main-sequence fitting method and on kinematic method (14.44 ± 0.41 mag), while it is systematically smaller than the distance based on the white dwarf cooling sequence (14.67 ± 0.18 mag), even if with a difference slightly larger than 1σ. The true distance modulus to M5 based on the J-band period-luminosity relation (14.50 ± 0.08 mag) is in quite good agreement with the distance based on the PLK relation, further supporting the use of NIR PL relations for RR Lyrae stars to improve the precision of the GC distance scale.
We present the largest near-infrared (NIR) data sets, JHKs, 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. 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 EU and FO PW relations were calibrated using a sample of Galactic Cepheids with distances based on trigonometric parallaxes and Cepheid pulsation models. These new MC distances lead to the relative distance, Delta mu = 0.48 + or - 0.03 mag (FU, log P = 1) and Delta mu = 0.52 + or - 0.03 mag (FO, log P = 0.5), which agrees quite well with previous estimates based on robust distance indicators.
Abstract
We present a new complete near-infrared (NIR,
JHK
s
) census of RR Lyrae stars (RRLs) in the globular
ω
Cen (NGC 5139). We collected 15,472
JHK
s
images with 4–8 m class telescopes over 15 ...years (2000–2015) covering a sky area around the cluster center of 60 × 34 arcmin
2
. These images provided calibrated photometry for 182 out of the 198 cluster RRL candidates with 10 to 60 measurements per band. We also provide new homogeneous estimates of the photometric amplitude for 180 (
J
), 176 (
H
) and 174 (
K
s
) RRLs. These data were supplemented with single-epoch
JK
s
magnitudes from VHS and with single-epoch
H
magnitudes from 2MASS. Using proprietary optical and NIR data together with new optical light curves (ASAS-SN) we also updated pulsation periods for 59 candidate RRLs. As a whole, we provide
JHK
s
magnitudes for 90 RRab (fundamentals), 103 RRc (first overtones) and one RRd (mixed-mode pulsator). We found that NIR/optical photometric amplitude ratios increase when moving from first overtone to fundamental and to long-period (
P
> 0.7 days) fundamental RRLs. Using predicted period–luminosity–metallicity relations, we derive a true distance modulus of 13.674 ± 0.008 ± 0.038 mag (statistical error and standard deviation of the median) based on spectroscopic iron abundances, and of 13.698 ± 0.004 ± 0.048 mag based on photometric iron abundances. We also found evidence of possible systematics at the 5%–10% level in the zero-point of the period–luminosity relations based on the five calibrating RRLs whose parallaxes had been determined with the
HST
.
Context. Classical Cepheids are excellent tracers of intermediate- mass stars, since their distances can be estimated with very high accuracy. In particular, they can be adopted to trace the chemical ...evolution of the Galactic disk. 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 FEROS-2.2m 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\pm0.003 dex kpc super(-1) 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\pm0.014 dex kpc super(-1). 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.
We present new intermediate-band Stromgren photometry based on more than 300 u, v, b, y images of the Galactic globular cluster omega Cen. Optical data were supplemented with new multiband ...near-infrared (NIR) photometry (350 J, H, Ks images). The final optical-NIR catalog covers a region of more than 20 X 20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper-motion data available in the literature to identify candidate cluster red-giant (RG) stars. By adopting different Stromgren metallicity indices, we estimate the photometric metallicity for 4000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks (Fe/Hphot = -1.73 +/- 0.08, -1.29 +/- 0.03, -1.05 +/- 0.02, -0.80 +/- 0.04, -0.42 +/- 0.12, and -0.07 +/- 0.08 dex) and a sharp cutoff in the metal-poor (MP) tail (Fe/Hphot -2 dex) that agree quite well with spectroscopic measurements. We identify four distinct subpopulations, namely, MP (Fe/H < = -1.49), metal-intermediate (MI; -1.49 < Fe/H < = -0.93), metal-rich (MR; -0.95 < Fe/H < = -0.15), and solar metallicity (Fe/H 0). The last group includes only a small fraction of stars (~8% +/- 5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19% +/- 1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Stromgren metallicity indices display a robust correlation with alpha-elements (Ca+Si/H) when moving from the MI to the MR regime (Fe/H -1.7 dex).
We present near-infrared (NIR) J-, H-, and K-band time series observations of the Galactic globular cluster (GGC) M92. On the basis of these data, we derive well-sampled light curves for 11 out of ...the 17 cluster RR Lyrae variables and, in turn, accurate mean NIR magnitudes. The comparison between the predicted and empirical slopes of the NIR period-luminosity (PL) relations indicates a very good agreement. Cluster distance determinations based on independent theoretical NIR PL relations present uncertainties smaller than 5% and agree quite well with recent distance estimates based on different distance indicators. We also obtain accurate and deep NIR color-magnitude diagrams ranging from the tip of the red giant branch (RGB) down to the main-sequence turnoff. We detect the RGB bump, and the NIR luminosities of this evolutionary feature are, within theoretical and empirical uncertainties, in good agreement with each other.
We present a new method to estimate the absolute ages of stellar systems. This method is based on the difference in magnitude between the main-sequence turnoff (MSTO) and a well-defined knee located ...along the lower main sequence (MSK). This feature is caused by the collisionally induced absorption of molecular hydrogen, and it can easily be identified in near-infrared (NIR) and in optical-NIR color-magnitude diagrams of stellar systems. We took advantage of deep and accurate NIR images collected with the Multi-Conjugate Adaptive Optics Demonstrator temporarily available on the Very Large Telescope and of optical images collected with the Advanced Camera for Surveys Wide Field Camera on the Hubble Space Telescope and with ground-based telescopes to estimate the absolute age of the globular NGC 3201 using both the MSTO and the Delta *D(MSTO-MSK). We have adopted a new set of cluster isochrones, and we found that the absolute ages based on the two methods agree to within 1 Delta *s. However, the errors of the ages based on the Delta *D(MSTO-MSK) method are potentially more than a factor of 2 smaller, since they are not affected by uncertainties in cluster distance or reddening. Current isochrones appear to predict slightly bluer (0.05 mag) NIR and optical-NIR colors than observed for magnitudes fainter than the MSK.
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