Chemical abundances in the young galaxy at z=2.309 towards PHL 957

We present high-resolution Utrecht Echelle Spectrograph spectra of the quasar PHL 957, obtained in order to study the foreground damped Lyα (DLA) galaxy at z =2.309. Measurements of absorption lines lead to accurate abundance determinations of Fe, S and N which complement measurements of Zn, Cr and Ni already available for this system. We find [Fe/H]=−2.0±0.1, [S/H]=−1.54±0.06 and [N/H]=−2.76±0.07. The ratio [Fe/Zn]=−0.44 provides evidence that ≈74 per cent of iron and ≈28 per cent of zinc are locked into dust grains with a dust-to-gas ratio of ≈3 per cent of the Galactic one. The total iron content in both gas and dust in the DLA system is [Fe/H]=−1.4. This confirms a rather low metallicity in the galaxy, which is in the early stages of its chemical evolution. The detection of S ii allows us to measure the S ii /Zn ii ratio, which is a unique diagnostic tool for tracing back its chemical history, since it is not affected by the presence of dust. Surprisingly, the resulting relative abundance is [S/Zn]=0.0±0.1, at variance with the overabundance found in the Galactic halo stars with similar metallicity. We emphasize that the [S/Zn] ratio is solar in all the three DLA absorbers with extant data. Upper limits are also found for Mn, Mg, O and P and, once the dust depletion is accounted for, we obtain [Mg/Fe]_c<+0.2, [O/Fe]_c<+0.4, [Mn/Fe]_c<+0.0 and [P/Fe]_c<−0.7. The [α/Fe] values do not support Galactic halo-like abundances, implying that the chemical evolution of this young galaxy is not reproducing the evolution of our own Galaxy.     

Cluster AgeS Experiment (CASE): RR Lyrae stars from the globular cluster ω Centauri as standard candles

New photometry of RRab and RRc stars in ω Centauri is used to calibrate their absolute magnitudes M_V as a function of (a) metallicity and (b) the Fourier parameters of light curves in the V band. The zero point of both calibrations relies on the distance modulus to the cluster derived earlier by the Cluster AgeS Experiment (CASE) project based on observations of the detached eclipsing binary OGLE GC17. For RRab variables, we obtained a relation of   M_V = (0.26 ± 0.08)[ Fe/H ] + (0.91 ± 0.13)  . A dereddened distance modulus to the Large Magellanic Cloud (LMC) based on that formula is  μ₀= 18.56 ± 0.14 mag  . The second calibration of M_V , which is based on Fourier coefficients of decomposed light curves, results in the LMC distance of  μ₀= 18.51 ± 0.07 mag  .     

WIYN Open Cluster Study – XVI. Optical/infrared photometry and comparisons with theoretical isochrones

We present combined optical/near-infrared photometry ( BVIK ) for six open clusters – M35, M37, NGC 1817, NGC 2477, NGC 2420 and M67. The open clusters span an age range from 150 Myr to 4 Gyr and have metal abundances from  [Fe/H]=−0.27  to +0.09 dex. We have utilized these data to test the robustness of theoretical main sequences constructed by several groups as denoted by the following designations – Padova, Baraffe, Y², Geneva and Siess. The comparisons of the models with the observations have been performed in the  [ M_V , ( B − V )₀], [ M_V , ( V − I )₀]  and  [ M_V , ( V − K )₀]  colour–magnitude diagrams as well as the distance-independent  [( V − K )₀, ( B − V )₀]  and  [( V − K )₀, ( V − I )₀]  two-colour diagrams. We conclude that none of the theoretical models reproduces the observational data in a consistent manner over the magnitude and colour range of the unevolved main sequence. In particular, there are significant zero-point and shape differences between the models and the observations. We speculate that the crux of the problem lies in the precise mismatch between theoretical and observational colour–temperature relations. These results underscore the importance of pursuing the study of stellar structure and stellar modelling with even greater intensity.     

Harmonizing the RR Lyrae and clump distance scales – stretching the short distance scale to intermediate ranges?

I explore the consequences of making the RR Lyrae and clump giant distance scales consistent in the solar neighbourhood, Galactic bulge and Large Magellanic Cloud (LMC). I employ two major assumptions: (i) that the absolute magnitude–metallicity, M _V (RR)–[Fe/H], relation for RR Lyrae stars is universal, and (ii) that absolute I magnitudes of clump giants, M _I (RC), in Baade's Window are known (e.g. can be inferred from the local Hipparcos -based calibration or theoretical modelling). A comparison between the solar neighbourhood and Baade's Window sets M _V (RR) at [Fe/H]=−1.6 in the range (0.59±0.05, 0.70±0.05), somewhat brighter than the statistical parallax solution. More luminous RR Lyrae stars imply younger globular clusters, which would be in better agreement with the conclusions from the currently favoured stellar evolution and cosmological models. A comparison between Baade's Window and the LMC sets M ^LMC(RC) _I in the range (−0.33±0.09,−0.53±0.09). The distance modulus to the LMC is μ ^LMC∈(18.24±0.08,18.44±0.07). Unlike M ^LMC(RC) _I , this range in μ ^LMC does not depend on the adopted value of the dereddened LMC clump magnitude, I ^LMC(RC)₀. I argue that the currently available information is insufficient to select the correct distance scale with high confidence.     

Intermediate-age Galactic open clusters: fundamental parameters of NGC 2627

Charge-coupled device (CCD) photometry in the Johnson V , Kron–Cousins I and Washington CMT ₁ systems is presented in the field of the poorly known open cluster NGC 2627. Four independent Washington abundance indices yield a mean cluster metallicity of  [Fe/H]=−0.12 ± 0.08  , which is compatible with the existence of a radial gradient in the Galactic disc. The resultant colour–magnitude diagrams indicate that the cluster is an intermediate-age object of 1.4 Gyr. Based on the best fits of the Geneva group's isochrones to the ( V , V − I ) and  ( T ₁, C − T ₁)  diagrams, we estimate   E ( V − I ) = 0.25 ± 0.05  and   V − M_V = 11.80 ± 0.25  for  log  t = 9.15  , and   E ( C − T ₁) = 0.23 ± 0.07  and   T ₁− M _{T 1}= 11.85 ± 0.25  for  log  t = 9.10  , respectively, assuming solar metal content. The derived reddening value   E ( C − T ₁)  implies   E ( B − V ) = 0.12 ± 0.07  and a distance from the Sun of  2.0 ± 0.4 kpc  . Using the WEBDA data base and the available literature, we re-examined the overall properties of all the open clusters with ages between 0.6 and 2.5 Gyr. We identified peaks of cluster formation at 0.7–0.8, 1.0–1.1, 1.6–1.7 and 2.0–2.1 Gyr, separated by relative quiescent epochs of ∼0.2–0.3 Gyr. We also estimated a radial abundance gradient of  −0.08 ± 0.02  , which is consistent with the most recent determinations for the Galactic disc, but no clear evidence for a gradient perpendicular to the Galactic plane is found.     

The star formation histories of two northern LMC fields

Ground-based UBV photometry of two fields in the northern disc of the Large Magellanic Cloud (LMC) is presented. A distance modulus of ( m − M )₀=18.41±0.04 and an extinction of A _V =0.30±0.05 have been calculated for these fields. The measurable star formation history of the LMC began no more than 12 Gyr ago with a strong star‐forming episode with [Fe/H]=−1.63±0.10 that accounted for approximately half (by mass) of the total star formation of the LMC in the first 3 Gyr. The data do not give accurate star formation rates during intermediate ages, but there appears to have been a recent increase in the star formation rate in these fields, beginning approximately 2.5 Gyr ago, with the current metallicity in the region being [Fe/H]=−0.38±0.10. The two fields have had very similar star formation rates until 200 Myr ago, at which point one shows a large increase.     

A near-infrared and optical photometric study of the Sculptor dwarf spheroidal galaxy: implications for the metallicity spread

The Sculptor dwarf spheroidal galaxy has a giant branch with a significant spread in colour, symptomatic of an intrinsic age–metallicity spread. We present here a detailed study of the Sculptor giant branch and horizontal branch (HB) morphology, combining new near-infrared photometry from the Cambridge Infrared Survey Instrument (CIRSI), with optical data from the European Southern Observatory Wide Field Imager. For a Sculptor-like old and generally metal-poor system, the position of red giant branch (RGB) and asymptotic giant branch (AGB) stars on the colour–magnitude diagram (CMD) is mainly metallicity dependent. The advantage of using optical–near-infrared colours is that the position of the RGB locus is much more sensitive to metallicity than with optical colours alone. In contrast the HB morphology is strongly dependent on both metallicity and age. Therefore a detailed study of both the RGB in optical–near-infrared colours and the HB can help break the age–metallicity degeneracy. Our measured photometric width of the Sculptor giant branch corresponds to a range in metallicity of 0.75 dex. We detect the RGB and AGB bumps in both the near-infrared and the optical luminosity functions, and derive from them a mean metallicity of  [M/H]=−1.3 ± 0.1  . From isochrone fitting we derive a mean metallicity of  [Fe/H]=−1.42  with a dispersion of 0.2 dex. These photometric estimators are for the first time consistent with individual metallicity measurements derived from spectroscopic observations. No spatial gradient is detected in the RGB morphology within a radius of 13 arcmin, twice the core radius. On the other hand, a significant gradient is observed in the HB morphology index, confirming the 'second parameter problem' present in this galaxy. These observations are consistent with an early extended period of star formation continuing in time for a few Gyr.     

A catalogue of helium abundance indicators from globular cluster photometry

We present a survey of helium abundance indicators derived from a comprehensive study of globular cluster photometry in the literature. For each of the three indicators used, we conduct a thorough error analysis, and identify systematic errors in the computational procedures. For the population ratio R N _HB N _RGB, we find that there is no evidence of a trend with metallicity, although there appears to be real scatter in the values derived. Although this indicator is the one best able to provide useful absolute helium abundances, the mean value is Y ≈0.20, indicating the probable presence of additional systematic error.
For the magnitude difference from the horizontal branch to the main sequence Δ and the RR Lyrae mass–luminosity exponent A , it is only possible to determine relative helium abundances reliably. This is due to continuing uncertainties in the absolute metallicity scale for Δ, and uncertainty in the RR Lyrae temperature scale for A . Both indicators imply that the helium abundance is approximately constant as a function of [Fe/H]. According to the A indicator, both Oosterhoff I and II group clusters have constant values independent of [Fe/H] and horizontal branch type. In addition, the two groups have slopes d log〈 P _ab 〉/d[Fe/H] that are consistent with each other, but significantly smaller than the slope for the combined sample.     

Analysis and calibration of Ca ii triplet spectroscopy of red giant branch stars from VLT/FLAMES observations

We demonstrate that low-resolution Ca  ii triplet (CaT) spectroscopic estimates of the overall metallicity ([Fe/H]) of individual red giant branch (RGB) stars in two nearby dwarf spheroidal galaxies (dSphs) agree to ±0.1–0.2 dex with detailed high-resolution spectroscopic determinations for the same stars over the range  −2.5 < [Fe/H] < −0.5  . For this study, we used a sample of 129 stars observed in low- and high-resolution modes with VLT/FLAMES in the Sculptor and Fornax dSphs. We also present the data-reduction steps we used in our low-resolution analysis and show that the typical accuracy of our velocity and CaT [Fe/H] measurement is ∼2 km s⁻¹ and 0.1 dex, respectively. We conclude that CaT–[Fe/H] relations calibrated on globular clusters can be applied with confidence to RGB stars in composite stellar populations over the range  −2.5 < [Fe/H] < −0.5  .     

Self-enrichment by asymptotic giant branch stars in globular clusters: comparison between intermediate and high metallicities

We present theoretical evolutionary sequences of intermediate-mass stars  ( M = 3 − 6.5 M_⊙)  with metallicity   Z = 0.004  . Our goal is to test whether the self-enrichment scenario by massive asymptotic giant branch stars may work for the high-metallicity globular clusters, after previous works by the same group showed that the theoretical yields by this class of objects can reproduce the observed trends among the abundances of some elements, namely the O–Al and O–Na anticorrelations, at intermediate metallicities, i.e.  [Fe/H]=−1.3  . We find that the increase in the metallicity favours only a modest decrease of the luminosity and the temperature at the bottom of the envelope for the same core mass, and also the efficiency of the third dredge-up is scarcely altered. On the contrary, differences are found in the yields, due to the different impact that processes with the same efficiency have on the overall abundance of envelopes with different metallicities. We expect the same qualitative patterns as in the intermediate-metallicity case, but the slopes of some of the relationships among the abundances of some elements are different. We compare the sodium–oxygen anticorrelation for clusters of intermediate metallicity ( Z ≈ 10⁻³) and clusters of metallicity large as in these new models. Although the observational data are still too scarce, the models are consistent with the observed trends, provided that only stars of   M ≳ 5 M_⊙  contribute to self-enrichment.     

The red giant branch tip and bump of the Leo II dwarf spheroidal galaxy

We present V and I photometry of a  9.4 × 9.4 arcmin²  field centred on the dwarf spheroidal galaxy Leo II. The tip of the red giant branch (TRGB) is identified at   I ^TRGB= 17.83 ± 0.03  and adopting  〈[M/H]〉=−1.53 ± 0.2  from the comparison of RGB stars with Galactic templates, we obtain a distance modulus  ( m − M )₀= 21.84 ± 0.13  , corresponding to a distance   D = 233 ± 15 kpc  . Two significant bumps have been detected in the luminosity function of the RGB. The fainter bump (B1, at   V = 21.76 ± 0.05  ) is the RGB bump of the dominant stellar population while the actual nature of the brightest one (B2, at   V = 21.35 ± 0.05  ) cannot be firmly assessed on the basis of the available data; it may be due to the asymptotic giant branch clump of the main population or it may be a secondary RGB bump. The luminosity of the main RGB bump (B1) suggests that the majority of RGB stars in Leo II belong to a population that is ≳4 Gyr younger than the classical Galactic globular clusters. The stars belonging to the He-burning red clump are shown to be significantly more centrally concentrated than RR Lyrae and blue horizontal branch stars, probing the existence of an age/metallicity radial gradient in this remote dwarf spheroidal.     

Magnetically collimated jets with high mass flux

We present HST /WFPC2 observations of UGC 4483, an irregular galaxy in the M81/NGC 2403 complex. Stellar photometry was carried out with HSTphot, and is complete to V ≃26.0 and I ≃24.7. We measure the red giant branch tip at I =23.56±0.10, and calculate a distance modulus of μ ₀=27.53±0.12 (corresponding to a distance of 3.2±0.2 Mpc), placing UGC 4483 within the NGC 2403 subgroup. We were able to measure properties of a previously known young star cluster in UGC 4483, finding integrated magnitudes of V =18.66±0.21 and I =18.54±0.10 for the stellar contribution (integrated light minus H α and [O  iii ] contribution), corresponding to an age of ∼10–15 Myr and an initial mass of ∼10⁴ M_⊙. This is consistent with the properties of the cluster's brightest stars, which were resolved in the data for the first time. Finally, a numerical analysis of the galaxy's stellar content yields a roughly constant star formation rate of 1.3×10⁻³ M_⊙ yr⁻¹ and mean metallicity of [Fe/H]=−1.3 dex from 15 Gyr ago to the present.     

Deep Gemini/GMOS imaging of an extremely isolated globular cluster in the Local Group

We report on deep imaging of a remote M31 globular cluster, MGC1, obtained with Gemini/GMOS. Our colour–magnitude diagram for this object extends ∼5 mag below the tip of the red-giant branch and exhibits features consistent with an ancient metal-poor stellar population, including a long, well-populated horizontal branch. The red-giant branch locus suggests MGC1 has a metal abundance  [M/H]≈−2.3  . We measure the distance to MGC1 and find that it lies ∼160 kpc in front of M31 with a distance modulus  μ= 23.95 ± 0.06  . Combined with its large projected separation of   R _p= 117 kpc  from M31, this implies a deprojected radius of   R _gc= 200 ± 20 kpc  , rendering it the most isolated known globular cluster in the Local Group by some considerable margin. We construct a radial brightness profile for MGC1 and show that it is both centrally compact and rather luminous, with   M_V =−9.2  . Remarkably, the cluster profile shows no evidence for a tidal limit and we are able to trace it to a radius of at least 450 pc, and possibly as far as ∼900 pc. The profile exhibits a power-law fall-off with exponent  γ=−2.5  , breaking to  γ=−3.5  in its outermost parts. This core-halo structure is broadly consistent with expectations derived from numerical models, and suggests that MGC1 has spent many gigayears in isolation.     

The metal homogeneity of the globular star cluster M5

This work is concerned with the precision measurement of the magnitudes of the stars across a large field of the galactic globular cluster M5. The colours of the red giants are subjected to a thorough statistical analysis in order to quantify the extent of any star-to-star variations in metal abundance within the cluster. Use is made of an extensive optical/near-infrared observational data set acquired with a large-array CCD detector and the Johnson V and I filters. The results of this photometry, which is accurate to within a few millimagnitudes for the brighter stars, are used to compile a colour–magnitude diagram.
The analysis finds that in a metal-rich M5 ([Fe/H]=−1.2) the extent of any metal inhomogeneity, Δ[Fe/H], is less than ±0.1 dex, and in a metal-poor M5 ([Fe/H]=−1.5) then     . (The literature contains a range of derived values of metallicity for M5.)     

Secular variability of the longitudinal magnetic field of the Ap star γ Equ

We present an analysis of the secular variability of the longitudinal magnetic field B _e in the roAp star γ Equ (HD 201601). Measurements of the stellar magnetic field B _e were mostly compiled from the literature, and we appended also our 33 new B _e measurements which were obtained with the 1-m optical telescope of the Special Astrophysical Observatory (Russia). All the available data cover the time period of 58 yr, and include both phases of the maximum and minimum B _e. We determined that the period of the long-term magnetic B _e variations equals  91.1 ± 3.6 yr  , with   B _e(max) =+577 ± 31 G  and   B _e(min) =−1101 ± 31 G  .     

The distance to Galactic globular clusters through RR Lyrae pulsational properties

By adopting the same approach outlined by De Santis & Cassisi, we evaluate the absolute bolometric magnitude of the zero-age horizontal branch (ZAHB) at the level of the RR Lyrae variable instability strip in selected Galactic globular clusters. This allows us to estimate the ZAHB absolute visual magnitude for these clusters and to investigate its dependence on the cluster metallicity. The derived M _V (ZAHB)–[Fe/H] relation, corrected in order to account for the luminosity difference between the ZAHB and the mean RR Lyrae magnitude, has been compared with some of the most recent empirical determinations in this field, such as the one provided by Baade–Wesselink analyses, RR Lyrae periods, Hipparcos data for field variables and main-sequence fitting based on Hipparcos parallaxes for field subdwarfs. As a result, our relation provides a clear support to the 'long' distance scale. We discuss also another method for measuring the distance to Galactic globular clusters. This method is quite similar to the one adopted for estimating the absolute bolometric magnitude of the ZAHB but it relies only on the pulsational properties of the Lyrae variables in each cluster. The reliability and accuracy of this method have been tested by applying it to a sample of globular clusters for which, owing to the morphology of their horizontal branch (HB), the use of the commonly adopted ZAHB fitting is a risky procedure. We notice that the two approaches for deriving the cluster distance modulus provide consistent results when applied to globular clusters, the RR Lyrae instability strip is well populated. As the adopted method relies on theoretical predictions on both the fundamental pulsational equation and the allowed mass range for fundamental pulsators, we give an estimate of the error affecting present results, owing to systematic uncertainties in the adopted theoretical framework.     

Keck spectroscopy of the faint dwarf elliptical galaxy population in the Perseus Cluster core: mixed stellar populations and a flat luminosity function

We present the result of a photometric and Keck low-resolution imaging spectrometer (LRIS) spectroscopic study of dwarf galaxies in the core of the Perseus Cluster, down to a magnitude of   M _B =−12.5  . Spectra were obtained for 23 dwarf-galaxy candidates, from which we measure radial velocities and stellar population characteristics from absorption line indices. From radial velocities obtained using these spectra, we confirm 12 systems as cluster members, with the remaining 11 as non-members. Using these newly confirmed cluster members, we are able to extend the confirmed colour–magnitude relation for the Perseus Cluster down to   M _B =−12.5  . We confirm an increase in the scatter about the colour–magnitude relationship below   M _B =−15.5  , but reject the hypothesis that very red dwarfs are cluster members. We measure the faint-end slope of the luminosity function between   M _B =−18  and −12.5, finding  α=−1.26 ± 0.06  , which is similar to that of the field. This implies that an overabundance of dwarf galaxies does not exist in the core of the Perseus Cluster. By comparing metal and Balmer absorption line indices with α-enhanced single stellar population models, we derive ages and metallicities for these newly confirmed cluster members. We find two distinct dwarf elliptical populations: an old, metal-poor population with ages ∼8 Gyr and metallicities  [Fe/H] < −0.33  , and a young, metal-rich population with ages <5 Gyr and metallicities  [Fe/H] > −0.33  . Dwarf galaxies in the Perseus Cluster are therefore not a simple homogeneous population, but rather exhibit a range in age and metallicity.     

Pulsational MV versus [Fe/H] relation(s) for globular cluster RR Lyrae variables

We use the results from recent computations of updated non-linear convective pulsating models to constrain the distance modulus of Galactic globular clusters through the observed periods of first-overtone (RR _c ) pulsators. The resulting relation between the mean absolute magnitude of RR Lyrae stars 〈 M _V (RR)〉 and the heavy element content [Fe/H] appears well in the range of several previous empirical calibrations, but with a non-linear dependence on [Fe/H] so that the slope of the relation increases when moving towards larger metallicities. On this ground, our results suggest that metal-poor ([Fe/H]<−1.5) and metal-rich ([Fe/H]>−1.5) variables follow two different linear 〈 M _V (RR)〉−[Fe/H] relations. Application to RR Lyrae stars in the metal-poor globular clusters of the Large Magellanic Cloud (LMC) provides an LMC distance modulus of the order of 18.6 mag, thus supporting the 'long' distance scale. The comparison with recent predictions based on updated stellar evolution theory is briefly presented and discussed.     

A search for previously unrecognized metal-poor subdwarfs in the Hipparcos astrometric catalogue

We have identified 317 stars included in the Hipparcos astrometric catalogue that have parallaxes measured to a precision of better than 15 per cent, and the location of which in the ( M _V ,( B − V ) _T ) diagram implies a metallicity comparable to or less than that of the intermediate-abundance globular cluster M5. We have undertaken an extensive literature search to locate Strömgren, Johnson/Cousins and Walraven photometry for over 120 stars. In addition, we present new UBV ( RI )_C photometry of 201 of these candidate halo stars, together with similar data for a further 14 known metal-poor subdwarfs. These observations provide the first extensive data set of R _C I _C photometry of metal-poor, main-sequence stars with well-determined trigonometric parallaxes. Finally, we have obtained intermediate-resolution optical spectroscopy of 175 stars.
47 stars still lack sufficient supplementary observations for population classification; however, we are able to estimate abundances for 270 stars, or over 80 per cent of the sample. The overwhelming majority have near-solar abundance, with their inclusion in the present sample stemming from errors in the colours listed in the Hipparcos catalogue. Only 44 stars show consistent evidence of abundances below [Fe/H]=−1.0 . Nine are additions to the small sample of metal-poor subdwarfs with accurate photometry. We consider briefly the implication of these results for cluster main-sequence fitting.     

[Fe/H] relations for c-type RR Lyrae variables based upon Fourier coefficients

[Fe/H]–φ₃₁– P relations are found for c-type RR Lyrae stars in globular clusters. The relations are analogous to that found by Jurcsik & Kovács for field ab-type RR Lyrae stars, where a longer period correlates with lower metallicity values for similar values of the Fourier coefficient φ₃₁. The relations obtained here are used to determine the metallicity of field c-type RR Lyrae stars, those within ω Cen, the Large Magellanic Cloud and toward the galactic bulge. The results are found to compare favourably to metallicity values obtained elsewhere.