We present homogeneous and accurate iron abundances for 42 Galactic Cepheids based on high resolution (R ~ 38 000) high signal-to-noise ratio (S/N ≥ 100) optical spectra collected with UVES at VLT ...(128 spectra). The above abundances were complemented with high-quality iron abundances provided either by our group (86) or available in the literature. We were careful to derive a common metallicity scale and ended up with a sample of 450 Cepheids. We also estimated accurate individual distances for the entire sample by using homogeneous near-infrared photometry and the reddening free period-Wesenheit relations. The new metallicity gradient is linear over a broad range of Galactocentric distances (RG ~ 5–19 kpc) and agrees quite well with similar estimates available in the literature (–0.060 ± 0.002 dex/kpc). We also uncover evidence that suggests that the residuals of the metallicity gradient are tightly correlated with candidate Cepheid groups (CGs). The candidate CGs have been identified as spatial overdensities of Cepheids located across the thin disk. They account for a significant fraction of the residual fluctuations, and also for the large intrinsic dispersion of the metallicity gradient. We performed a detailed comparison with metallicity gradients based on different tracers: OB stars and open clusters. We found very similar metallicity gradients for ages younger than 3 Gyr, while for older ages we found a shallower slope and an increase in the intrinsic spread. The above findings rely on homogeneous age, metallicity, and distance scales. Finally, by using a large sample of Galactic and Magellanic Cepheids for which accurate iron abundances are available, we found that the dependence of the luminosity amplitude on metallicity is vanishing.
ABSTRACT We present a detailed investigation of the Large Magellanic Cloud (LMC) disk using classical Cepheids. Our analysis is based on optical (I, V; OGLE-IV), near-infrared (NIR: J, H, ) and ...mid-infrared (MIR: w1; WISE) mean magnitudes. By adopting new templates to estimate the NIR mean magnitudes from single-epoch measurements, we build the currently most accurate, largest, and homogeneous multi-band data set of LMC Cepheids. We determine Cepheid individual distances using optical and NIR Period-Wesenheit relations (PWRs), to measure the geometry of the LMC disk and its viewing angles. Cepheid distances based on optical PWRs are precise at 3%, but accurate to 7%, while the ones based on NIR PWRs are more accurate (to 3%), but less precise (2%-15%), given the higher photometric error on the observed magnitudes. We found an inclination of i = 25.05 0.02 (stat.) 0.55 (syst.) deg, and a position angle of the lines of nodes P.A. = 150.76 0.02 (stat.) 0.07 (syst.) deg. These values agree well with estimates based either on young (Red Supergiants) or on intermediate-age (Asymptotic Giant Branch, Red Clump) stellar tracers, but they significantly differ from evaluations based on old (RR Lyrae) stellar tracers. This indicates that young/intermediate and old stellar populations have different spatial distributions. Finally, by using the reddening-law fitting approach, we provide a reddening map of the LMC disk, which is 10 times more accurate and 2 times larger than similar maps in the literature. We also found an LMC true distance modulus of (stat. and syst.) mag, in excellent agreement with the currently most accurate measurement.
We have combined deep photometry in the B, V and I bands from CTIO/MOSAIC of the Sculptor dwarf spheroidal galaxy, going down to the oldest main sequence turn-offs, with spectroscopic metallicity ...distributions of red giant branch stars. This allows us to obtain the most detailed and complete star formation history to date, as well as an accurate timescale for chemical enrichment. The star formation history shows that Sculptor is dominated by old (>10 Gyr), metal-poor stars, but that younger, more metal-rich populations are also present. Using star formation histories determined at different radii from the centre we show that Sculptor formed stars with an increasing central concentration with time. The old, metal-poor populations are present at all radii, while more metal-rich, younger stars are more centrally concentrated. We find that within an elliptical radius of 1 degree, or 1.5 kpc from the centre, a total mass in stars of 7.8 × 106 M⊙ was formed, between 14 and 7 Gyr ago, with a peak at 13−14 Gyr ago. We use the detailed star formation history to determine age estimates for individual red giant branch stars with high resolution spectroscopic abundances. Thus, for the first time, we can directly determine detailed timescales for the evolution of individual chemical elements. We find that the trends in alpha-elements match what is expected from an extended, relatively uninterrupted period of star formation continuing for 6−7 Gyr. The knee in the alpha-element distribution occurs at an age of 10.9 ± 1Gyr, suggesting that SNe Ia enrichment began ≈2 ± 1 Gyr after the start of star formation in Sculptor.
We present deep photometry of the Carina dwarf spheroidal galaxy in the B and V filters from CTIO/MOSAIC out to and beyond the tidal radius of rell ≈ 0.48 degrees. The accurately calibrated ...photometry is combined with spectroscopic metallicity distributions of red giant branch (RGB) stars to determine the detailed star formation and chemical evolution history of Carina. The star formation history (SFH) confirms the episodic formation history of Carina and quantifies the duration and strength of each episode in great detail as a function of radius from the centre. Two main episodes of star formation occurred at old (>8 Gyr) and intermediate (2−8 Gyr) ages, both enriching stars starting from low metallicities (Fe/H < − 2 dex). By dividing the SFH into two components, we determine that 60 ± 9 percent of the total number of stars formed within the intermediate-age episode. Furthermore, within the tidal radius (0.48 degrees or 888 pc) a total mass in stars of 1.07 ± 0.08 × 106 M⊙ was formed, giving Carina a stellar mass-to-light ratio of 1.8 ± 0.8. By combining the detailed SFH with spectroscopic observations of RGB stars, we determined the detailed age-metallicity relation of each episode and the timescale of α-element evolution of Carina from individual stars. The oldest episode displays a tight age-metallicity relation during ≈6 Gyr with steadily declining α-element abundances and a possible α-element “knee” visible at Fe/H ≈ − 2.5 dex. The intermediate-age sequence displays a more complex age-metallicity relation starting from low metallicity and a sequence in α-element abundances with a slope much steeper than observed in the old episode, starting from Fe/H = −1.8 dex and Mg/Fe ≈ 0.4 dex and declining to Mg-poor values (Mg/Fe ≤ − 0.5 dex). This clearly indicates that the two episodes of star formation formed from gas with different abundance patterns, which is inconsistent with simple evolution in an isolated system.
Context. Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group. The Fornax field star population is dominated by intermediate age stars but star formation was going on over ...almost its entire history. It has been proposed that Fornax experienced a minor merger event. Aims. Despite recent progress, only the high metallicity end of Fornax field stars (Fe/H > –1.2 dex) has been sampled in larger number via high resolution spectroscopy. We want to better understand the full chemical evolution of this galaxy by better sampling the whole metallicity range, including more metal poor stars. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode to determine the abundances of several α, iron-peak and neutron-capture elements in a sample of 47 individual red giant branch stars in the Fornax dwarf spheroidal galaxy. We combine these abundances with accurate age estimates derived from the age probability distribution from the colour-magnitude diagram of Fornax. Results. Similar to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are typically α-rich while the young metal-rich stars are α-poor. In the classical scenario of the time delay between Type II (SNe II) and Type Ia Supernovae (SNe Ia), we confirm that SNe Ia started to contribute to the chemical enrichment at Fe/H between –2.0 and –1.8 dex. We find that the onset of SNe Ia took place between 12–10 Gyr ago. The high values of Ba/Fe, La/Fe reflect the influence of SNe Ia and AGB stars in the abundance pattern of the younger stellar population of Fornax. Conclusions. Our findings of low α/Fe and enhanced Eu/Mg are compatible with an initial mass function that lacks the most massive stars and with star formation that kept going on throughout the whole history of Fornax. We find that massive stars kept enriching the interstellar medium in α-elements, although they were not the main contributor to the iron enrichment.
Context. Standard spectroscopic analyses of Cepheid variables are based on hydrostatic one-dimensional model atmospheres, with convection treated using various formulations of mixing-length theory. ...Aims. This paper aims to carry out an investigation of the validity of the quasi-static approximation in the context of pulsating stars. We check the adequacy of a two-dimensional time-dependent model of a Cepheid-like variable with focus on its spectroscopic properties. Methods. With the radiation-hydrodynamics code CO5BOLD, we construct a two-dimensional time-dependent envelope model of a Cepheid with Teff = 5600 K, log g = 2.0, solar metallicity, and a 2.8-day pulsation period. Subsequently, we perform extensive spectral syntheses of a set of artificial iron lines in local thermodynamic equilibrium. The set of lines allows us to systematically study effects of line strength, ionization stage, and excitation potential. Results. We evaluate the microturbulent velocity, line asymmetry, projection factor, and Doppler shifts. The microturbulent velocity, averaged over all lines, depends on the pulsational phase and varies between 1.5 and 2.7 km s-1. The derived projection factor lies between 1.23 and 1.27, which agrees with observational results. The mean Doppler shift is non-zero and negative, −1 km s-1, after averaging over several full periods and lines. This residual line-of-sight velocity (related to the “K-term”) is primarily caused by horizontal inhomogeneities, and consequently we interpret it as the familiar convective blueshift ubiquitously present in non-pulsating late-type stars. Limited statistics prevent firm conclusions on the line asymmetries. Conclusions. Our two-dimensional model provides a reasonably accurate representation of the spectroscopic properties of a short-period Cepheid-like variable star. Some properties are primarily controlled by convective inhomogeneities rather than by the Cepheid-defining pulsations. Extended multi-dimensional modelling offers new insight into the nature of pulsating stars.
Context. Cepheids are excellent tracers of young stellar populations. They play a crucial role in astrophysics as standard candles. The chemistry of classical Cepheids in the Milky Way is now quite ...well-known, however despite a much larger sample, the chemical composition of Magellanic Cepheids has been only scarcely investigated. Aims. For the first time, we study the chemical composition of several Cepheids located in the same populous cluster: NGC 1866, in the Large Magellanic Cloud (LMC). To also investigate the chemical composition of Cepheids at lower metallicity, we look at four targets located in the Small Magellanic Cloud (SMC). Our sample allows us to increase the number of Cepheids with known metallicities in the LMC/SMC by 20%/25% and the number of Cepheids with detailed chemical composition in the LMC/SMC by 46%/50%. Methods. We use canonical spectroscopic analysis to determine the chemical composition of Cepheids and provide abundances for a good number of α, iron-peak, and neutron-capture elements. Results. We find that six Cepheids in the LMC cluster NGC 1866 have a very homogeneous chemical composition, also consistent with red giant branch (RGB) stars in the cluster. Period–age relations that include no or average rotation indicate that all the Cepheids in NGC 1866 have a similar age and therefore belong to the same stellar population. Our results are in good agreement with theoretical models accounting for luminosity and radial velocity variations. Using distances based on period-luminosity relations in the near- or mid-infrared, we investigate for the first time the metallicity distribution of the young population in the SMC in the depth direction. Preliminary results show no metallicity gradient along the SMC main body, but our sample is small and does not contain Cepheids in the inner few degrees of the SMC.
Abstract
We collected the largest spectroscopic catalog of RR Lyrae (RRLs) including ≈20,000 high-, medium-, and low-resolution spectra for ≈10,000 RRLs. We provide the analytical forms of radial ...velocity curve (RVC) templates. These were built using 36 RRLs (31 fundamental—split into three period bins—and five first-overtone pulsators) with well-sampled RVCs based on three groups of metallic lines (Fe, Mg, Na) and four Balmer lines (H
α
, H
β
, H
γ
, H
δ
). We tackled the long-standing problem of the reference epoch to anchor light-curve and RVC templates. For the
V
-band, we found that the residuals of the templates anchored to the phase of the mean magnitude along the rising branch are ∼35% to ∼45% smaller than those anchored to the phase of maximum light. For the RVC, we used two independent reference epochs for metallic and Balmer lines and we verified that the residuals of the RVC templates anchored to the phase of mean RV are from 30% (metallic lines) up to 45% (Balmer lines) smaller than those anchored to the phase of minimum RV. We validated our RVC templates by using both the single-point and the three phase point approaches. We found that barycentric velocities based on our RVC templates are two to three times more accurate than those available in the literature. We applied the current RVC templates to Balmer lines RVs of RRLs in the globular NGC 3201 collected with MUSE at VLT. We found the cluster barycentric RV of
V
γ
= 496.89 ± 8.37(error) ± 3.43 (standard deviation) km s
−1
, which agrees well with literature estimates.
We present abundances for seven stars in the (extremely) low-metallicity tail of the Sculptor dwarf spheroidal galaxy, from spectra taken with X-shooter on the ESO VLT. Targets were selected from the ...Ca II triplet (CaT) survey of the dwarf abundances and radial velocities team (DART) using the latest calibration. Of the seven extremely metal-poor candidates, five stars are confirmed to be extremely metal-poor (i.e., Fe/H < −3 dex), with Fe/H = –3.47 ± 0.07 for our most metal-poor star. All have Fe/H ≤ −2.5 dex from the measurement of individual Fe lines. These values are in agreement with the CaT predictions to within error bars. None of the seven stars is found to be carbon-rich. We estimate a 2–13% possibility of this being a pure chance effect, which could indicate a lower fraction of carbon-rich extremely metal-poor stars in Sculptor compared to the Milky Way halo. The α/Fe ratios show a range from +0.5 to –0.5, a larger variation than seen in Galactic samples although typically consistent within 1–2σ. One star seems mildly iron-enhanced. Our program stars show no deviations from the Galactic abundance trends in chromium and the heavy elements barium and strontium. Sodium abundances are, however, below the Galactic values for several stars. Overall, we conclude that the CaT lines are a successful metallicity indicator down to the extremely metal-poor regime and that the extremely metal-poor stars in the Sculptor dwarf galaxy are chemically more similar to their Milky Way halo equivalents than the more metal-rich population of stars.
ABSTRACT
Radial abundance gradients provide sound constraints for chemo-dynamical models of galaxies. Azimuthal variations of abundance ratios are solid diagnostics to understand their chemical ...enrichment. In this paper, we investigate azimuthal variations of abundances in the Milky Way using Cepheids. We provide the detailed chemical composition (25 elements) of 105 Classical Cepheids from high-resolution SALT spectra observed by the MAGIC project. Negative abundance gradients, with abundances decreasing from the inner to the outer disc, have been reported both in the Milky Way and in external galaxies, and our results are in full agreement with literature results. We find azimuthal variations of the oxygen abundance O/H. While a large number of external spirals show negligible azimuthal variations, the Milky Way seems to be one of the few galaxies with noticeable O/H azimuthal asymmetries. They reach ≈0.2 dex in the inner Galaxy and in the outer disc, where they are the largest, thus supporting similar findings for nearby spiral galaxies, as well as recent 2D chemo-dynamical models.
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