A Photometric Study of the Outer Halo Globular Cluster NGC 5824

Multi-wavelength CCD photometry over 21 years has been used to produce deep color-magnitude diagrams together with light curves for the variables in the Galactic globular cluster NGC 5824. Twenty-one new cluster RR Lyrae stars are identified, bringing the total to 47, of which 42 have reliable periods determined for the first time. The color-magnitude diagram is matched using BaSTI isochrones with age of $13$~Gyr. and reddening is found to be $E(B-V) = 0.15 \pm0.02$; using the period-Wesenheit relation in two colors the distance modulus is $(m-M)_0=17.45 \pm 0.07$ corresponding to a distance of 30.9 Kpc. The observations show no signs of populations that are significantly younger than the $13$~Gyr stars. The width of the red giant branch does not allow for a spread in [Fe/H] greater than $\sigma = 0.05$ dex, and there is no photometric evidence for widened or parallel sequences. The $V, c_{UBI}$ pseudo-color magnitude diagram shows a bifurcation of the red giant branch that by analogy with other clusters is interpreted as being due to differing spectral signatures of the first (75\%) and second (25\%) generations of stars whose age difference is close enough that main sequence turnoffs in the color-magnitude diagram are unresolved. The cluster main sequence is visible against the background out to a radial distance of $\sim17$ arcmin. We conclude that NGC 5824 appears to be a classical Oosterhoff Type II globular cluster, without overt signs of being a remnant of a now-disrupted dwarf galaxy.Comment: 26 pages, 15 figures, 4 tables, accepted for publication in Astronomical Journa

The distance to the LMC cluster Reticulum from the K-band Period-Luminosity-Metallicity relation of RR Lyrae stars

We present new and accurate Near-Infrared J and Ks-band data of the Large Magellanic Cloud cluster Reticulum. Data were collected with SOFI available at NTT and covering an area of approximately (5 x 5) arcmin^2 around the center of the cluster. Current data allowed us to derive accurate mean K-band magnitudes for 21 fundamental and 9 first overtone RR Lyrae stars. On the basis of the semi-empirical K-band Period-Luminosity-Metallicity relation we have recently derived, we find that the absolute distance to this cluster is 18.52 +- 0.005 (random) +- 0.117 (systematic). Note that the current error budget is dominated by systematic uncertainty affecting the absolute zero-point calibration and the metallicity scale.Comment: 14 pages, 2 figures, ApJ accepted. Full resolution figure 1 on request ([email protected]

Variable Stars in Local Group Galaxies. IV. RR Lyrae stars in the central regions of the low-density galaxy Crater II

We present a search and analysis of variable stars in the recently discovered Crater~II dwarf galaxy. Based on $B$, $V$, $I$ data collected with the Isaac Newton Telescope (FoV$\sim$0.44 square degrees) we detected 37 variable stars, of which 34 are bone-fide RR Lyrae stars of Crater~II (28 RRab, 4 RRc, 2 RRd). We applied the metal-independent ($V$, $B-V$) Period--Wesenheit relation and derived a true distance modulus ($\mu$ = 20.30$\pm$0.08 mag ($\sigma$=0.16 mag). Individual metallicities for RR Lyrae stars were derived by inversion of the predicted $I$-band Period-Luminosity relation. We find a mean metallicity of [Fe/H]=-1.64 and a standard deviation of $\sigma_{[Fe/H]}$ =0.21 dex, compatible with either negligible or vanishing intrinsic metallicity dispersion. The analysis of the Colour-Magnitude Diagram reveals a stark paucity of blue horizontal branch stars, at odds with other Galactic dwarfs, and globular clusters with similar metal abundances.Comment: 14 pages, 8 figures, accepted for publications on MNRAS. Time series photometry is available in the manuscript source ta

Constraints on the Formation of the Globular Cluster IC 4499 from Multi-Wavelength Photometry

We present new multiband photometry for the Galactic globular cluster IC 4499 extending well past the main sequence turn-off in the U, B, V, R, I, and DDO51 bands. This photometry is used to determine that IC4499 has an age of 12 pm 1 Gyr and a cluster reddening of E(B-V) = 0.22 pm 0.02. Hence, IC 4499 is coeval with the majority of Galactic GCs, in contrast to suggestions of a younger age. The density profile of the cluster is observed to not flatten out to at least r~800 arcsec, implying that either the tidal radius of this cluster is larger than previously estimated, or that IC 4499 is surrounded by a halo. Unlike the situation in some other, more massive, globular clusters, no anomalous color spreads in the UV are detected among the red giant branch stars. The small uncertainties in our photometry should allow the detection of such signatures apparently associated with variations of light elements within the cluster, suggesting that IC 4499 consists of a single stellar population.Comment: accepted to MNRA

TIC 43152097. The first eclipsing binary in NGC 2232

We report the discovery of a low-mass totally eclipsing system in the young (age$\simeq$28 Myr) open cluster NGC2232, during a scrutiny of their TESS light curves. The follow-up study of this detached system, TIC 43152097, is based on photometry and high-resolution spectra from the literature and purposely collected. The radial velocity of the center of mass, as well as the photospheric lithium abundance of the binary components, confirm its membership to NGC2232. By analyzing the existing photometric and spectroscopic data, we obtain orbital elements and fundamental stellar parameters for the two stars. The primary component of TIC 43152097 is a late F-type dwarf (Teff = 6070 K), while the lower-mass secondary results to be a late K-type star (Teff = 4130 K) that is still in the pre-main-sequence phase. The precise measurements of radii, masses, and effective temperatures, enabled by the simultaneous solution of light and radial velocity curves, indicate radius inflation for the K-type component, which turns out to be 7-11 % larger than predicted by standard evolutionary models. More sophisticated models incorporating both inhibition of convective energy transport caused by sub-photospheric magnetic fields and the effects by cool starspots covering a substantial fraction of the stellar surface (30-60 %) allow reproducing the position of the secondary component in the Hertzsprung-Russell and Mass-Radius diagrams.Comment: 11 pages, 15 figures; accepted for publication in Astronomy & Astrophysic

The RR Lyrae distance scale from near-infrared photometry: current results

We present new observational results on the RR Lyrae K-band Period-Luminosity relation (PLK). Data on the Galactic globular clusters NGC 3201 and NGC 4590 (M68), and on the Large Magellanic Cloud cluster Reticulum are shown. We compare the observed slopes of the PLK relations for these three clusters with those predicted by pulsational and evolutionary models, finding a fair agreement. Trusting on this finding we decided to adopt these theoretical calibrations to estimate the distance to the target clusters,finding a good agreement with optical-based RR Lyrae distances, but with a smaller formal scatter.Comment: Proceedings of the Stellar Pulsation and Evolution meeting, Rome, June 200

VLT FORS-1 observations of NGC 6397: Evidence for mass segregation

We present (V,V-I) VLT-FORS1 observations of the Galactic Globular Cluster NGC 6397. We derive accurate color--magnitude diagrams and luminosity functions (LFs) of the cluster Main Sequence (MS) for two fields extending from a region near the centre of the cluster out to ~ 10 arcmin. The photometry of these fields produces a narrow MS extending down to V ~ 27, much deeper than any previous ground based study on this system and comparable to previous HST photometry. The V, V-I CMD also shows a deep white dwarf cooling sequence locus, contaminated by many field stars and spurious objects. We concentrate the present work on the analysis of the MSLFs derived for two annuli at different radial distance from the center of the cluster. Evidence of a clear-cut correlation between the slope of the observed LFs before reaching the turn-over, and the radial position of the observed fields inside the cluster area is found. We find that the LFs become flatter with decreasing radius (x ~ 0.15 for 1'< R1 < 5.5'; x ~ 0.24 for 5.5'< R2 <9.8'; core radius, rc = 0.05'), a trend that is consistent with the interpretation of NGC 6397 as a dynamically relaxed system. This trend is also evident in the mass function.Comment: 11 pages; 15 figures; accepted for publication on Astronomy and Astrophysic