Three dimensional maps of the Magellanic Clouds using RR Lyrae Stars and Cepheids II. The Small Magellanic Cloud

We use data on variable stars from the Optical Gravitational Lensing Experiment (OGLE III) survey to determine the three-dimensional structure of the Small Magellanic Cloud (SMC). Deriving individual distances to RR Lyrae stars and Cepheids we investigate the distribution of these tracers of the old and young population in the SMC. Photometrically estimated metallicities are used to determine the distances to 1494 RR Lyrae stars, which have typical ages greater than 9 Gyr. For 2522 Cepheids, with ages of a few tens to a few hundred Myr, distances are calculated using their period-luminosity relation. Individual reddening estimates from the intrinsic color of each star are used to obtain high precision three-dimensional maps. The distances of RR Lyrae stars and Cepheids are in very good agreement with each other. The median distance of the RR Lyrae stars is found to be 61.5 +/- 3.4 kpc. For the Cepheids a median distance of 63.1 +/- 3.0 kpc is obtained. Both populations show an extended scale height, with 2.0 +/- 0.4 kpc for the RR Lyrae stars and 2.7 +/- 0.3 kpc for the Cepheids. This confirms the large depth of the SMC suggested by a number of earlier studies. The young population is very differently oriented than the old stars. While we find an inclination angle of 7{\deg} +/- 15{\deg} and a position angle of 83{\deg} +/- 21{\deg} for the RR Lyrae stars, for the Cepheids an inclination of 74{\deg} +/- 9{\deg} and a position angle of 66{\deg} +/- 15{\deg} is obtained. The RR Lyrae stars show a fairly homogeneous distribution, while the Cepheids follow roughly the distribution of the bar with their northeastern part being closer to us than the southwestern part of the bar. Interactions between the SMC, LMC, and Milky Way are presumably responsible for the tilted, elongated structure of the young population of the SMC.Comment: 13 pages, 6 figures, 4 tables, published in A

Structure, Abundances, and Reddening of the Magellanic Clouds from its Variable Stars

Mit Hilfe von Daten des Optical Gravitational Lensing Experiment werden die Eigenschaften der Magellanischen Wolken untersucht. Für die alte Population werden RR Lyrae Sternen genutzt und für die junge Population Cepheiden. Die Messung der Entfernungen von RR Lyrae Sternen und Cepheiden werden mit unseren eigenen Verfärbungskarten korrigiert. Mit diesen Entfernungen errechnen wir die Skalenhöhe, der Positionswinkel und die Inklination der Großen und Kleinen Magellanischen Wolke (G/KMW). Die Skalenhöhe der KMW ist signifikant höher als die der GMW. Die jungen Cepheiden der KMW haben eine andere Orientierung als die alten RR Lyrae Sterne. Eines der interessantesten Resultate ist der versetzte Balken der GMW, welcher, in beiden Populationen, bis zu ∼ 5 kpc aus der Scheibe herausragt. Die Metallizitätsverteilungfunktion wird aus Metallizitäten einzelner RR Lyrae Sterne bestimmt und zeigt signifikante Unterschiede zwischen GMW and KMW. Die mittlere Metallizität der KMW ist um ∼ 0.2 dex niedriger. Dies ist ein Hinweis auf eine unterschiedliche Anreicherungsgeschichte der frühen Wolken. Die Untersuchung der RR Lyrae Sterne in den OGLE Feldern des Galaktischen Bulge erlaubt es Verfärbungen, Metallizitäten und Entfernungen zu RRLyrae Sternen im Bulge und in einem kleinen Teil des Sagittarius Stroms zu bestimmen. Der Strom hat eine signifikant niedrigere Metallizität und scheint geneigt zu sein. Mit Hilfe einer spectroskopischen Analyse von individuellen Elementhäufigkeiten von neun möglichen metallarmen RRLyrae Sternen, finden wir den bisher metallärmsten Stern ([Fe/H] = −2.7 dex) in den Magellanischen Wolken überhaupt. Die Elementhäufigkeiten der anderen Sterne ermöglichen die Schlußfolgerung, dass der Galaktische Halo durch die Akkretion einer Galaxie von Magellanischem Typus entstanden sein könnte

New Optical Reddening Maps of the Large and Small Magellanic Clouds

We present new reddening maps of the SMC and LMC based on the data of the third phase of the Optical Gravitational Lensing Experiment (OGLE III). We have used two different methods to derive optical reddening maps. We adopt a theoretical mean unreddened colour for the red clump in the SMC and LMC, respectively. We subdivide the photometric data for both Clouds into subfields and calculate the difference between the observed red clump position and the theoretical value for each field, which provides us with the reddening value in (V-I). Furthermore reddening values are obtained for 13490 LMC RR Lyrae ab and 1529 SMC RR Lyrae ab stars covering the whole OGLE III region of the MCs. The observed colours (V-I) of the RR Lyrae stars are compared with the colour from the absolute magnitudes. The absolute magnitude of each RR Lyrae star is computed using its period and metallicity derived from Fourier decomposition of its lightcurve. In general we find a low and uniform reddening distribution in both Magellanic Clouds. The red clump method indicates a mean reddening of the LMC of E(V-I) = 0.09 +/- 0.07 mag, while for the SMC E(V-I) = 0.04 +/- 0.06 mag is obtained. With RR Lyrae stars a median value of E(V-I) = 0.11 +/- 0.06 mag for the LMC and E(V-I) = 0.07 +/- 0.06 mag for the SMC is found. The LMC shows very low reddening in the bar region, whereas the reddening in the star-forming leading edge and 30 Doradus is considerably higher. In the SMC three pronounced regions with higher reddening are visible. Two are located along the bar, while the highest reddening is found in the star-forming wing of the SMC. In general the regions with higher reddening are in good spatial agreement with infrared reddening maps as well as with reddening estimations of other studies. The position-dependent reddening values from the red clump method are available via the Virtual Observatory interface.Comment: 15 pages, 22 Figures, AJ publishe

Metallicity distribution functions of the old populations of the Magellanic Clouds from RR Lyrae stars

We present the first metallicity distribution functions of the old field populations in the Magellanic Clouds. Our metallicities are based on the Fourier decomposition of Type ab RR Lyrae light curves from the Optical Gravitational Lensing Experiment (OGLE-III). On the metallicity scale of Zinn & West; we find a mean metallicity of [Fe/H] = -1.50 +/- 0.24 dex based on 16776 RR Lyrae stars in the Large Magellanic Cloud (LMC). For the Small Magellanic Cloud (SMC) we obtain -1.70 +/- 0.27 dex based on 1831 RR Lyrae stars. These uncertainties represent the intrinsic spread in the population rather than the standard deviation of the mean. Our results are in good agreement with the few existing spectroscopic metallicity determinations for LMC RR Lyrae stars from the literature. For both the LMC and the SMC the metallicity spread exceeds 1 dex in [Fe/H]. The distribution of metallicities in both Clouds is very uniform, and no significant metallicity gradient is detectable. We also do not find any pronounced populations of extremely metal-poor RR Lyrae candidates with metallicities well below -2 dex, although we need to caution that the photometric method used may overestimate the metallicities of metal-deficient stars. Moreover, because of stellar evolutionary effects one does not expect to observe many RR Lyrae stars among very metal-poor horizontal branch stars. We suggest that the Magellanic Clouds experienced fairly rapid and efficient early enrichment involving pre-enriched gas as well as possibly gas infall, while metal loss through outflows does not seem to have played a significant role. Moreover we suggest that the differences in the metallicities of the old population of LMC and SMC make an origin from a single, common progenitor unlikely, unless the separation happened very early on.Comment: Accepted for publication in AJ, 12 pages, 13 figure

Chemical abundances of metal-poor RR Lyrae stars in the Magellanic Clouds

We present for the first time a detailed spectroscopic study of chemical element abundances of metal-poor RR Lyrae stars in the Large and Small Magellanic Cloud (LMC and SMC). Using the MagE echelle spectrograph at the 6.5m Magellan telescopes, we obtain medium resolution (R ~ 2000 - 6000) spectra of six RR Lyrae stars in the LMC and three RR Lyrae stars in the SMC. These stars were chosen because their previously determined photometric metallicities were among the lowest metallicities found for stars belonging to the old populations in the Magellanic Clouds. We find the spectroscopic metallicities of these stars to be as low as [Fe/H]_{spec} = -2.7dex, the lowest metallicity yet measured for any star in the Magellanic Clouds. We confirm that for metal-poor stars, the photometric metallicities from the Fourier decomposition of the lightcurves are systematically too high compared to their spectroscopic counterparts. However, for even more metal-poor stars below [Fe/H]_{phot} < -2.8dex this trend is reversed and the spectroscopic metallicities are systematically higher than the photometric estimates. We are able to determine abundance ratios for ten chemical elements, which extend the abundance measurements of chemical elements for RR Lyrae stars in the Clouds beyond [Fe/H] for the first time. For the overall [alpha/Fe] ratio, we obtain an overabundance of 0.36dex, which is in very good agreement with results from metal-poor stars in the Milky Way halo as well as from the metal-poor tail in dwarf spheroidal galaxies. Comparing the abundances with those of the stars in the Milky Way halo we find that the abundance ratios of stars of both populations are consistent with another. Therefore we conclude that from a chemical point of view early contributions from Magellanic-type galaxies to the formation of the Galactic halo as claimed in cosmological models are plausible.Comment: accepted for publication in AJ, 19 pages, 12 figure

An OGLE view of the bulge and Sagittarius

We use observations of OGLE-III ab-type RRLyrae stars towards the Galactic centre (Soszyński et al. 2011, AcA 6,1) to study the metallicity and 3D spatial distribution of the old and metal-poor component of the Galactic bulge. Metallicities and distances to the RRLyrae stars are derived photometrically from Fourier-component analyses of their light curves, allowing the determination of distances accurate to ~7% from 0 to 40 kpc. The distance distribution of the RRLyrae stars peaks at 8.8 kpc, with the data indicating the presence of a bar-like structure inclined at ~30° to the line of sight. The dataset also exhibits a secondary concentration of stars beyond the Galactic centre at ~27 kpc. This is consistent with the distance to the Sagittarius (Sgr) dwarf galaxy, and can be attributed to the serendipitous alignment of the Sgr stream with the bulge. This dataset allows the Sgr stream to be traced in this part of the sky for the first time. The underlying metallicity distributions are determined to have mean +/- intrinsic width of [Fe/H]bulge = -1.24+/-0.23 dex and [Fe/H]Sgr = -1.53+/-0.14 dex

An OGLE view of the bulge and Sagittarius

We use observations of OGLE-III ab-type RRLyrae stars towards the Galactic centre (Soszyński et al. 2011, AcA 6,1) to study the metallicity and 3D spatial distribution of the old and metal-poor component of the Galactic bulge. Metallicities and distances to the RRLyrae stars are derived photometrically from Fourier-component analyses of their light curves, allowing the determination of distances accurate to ~7% from 0 to 40 kpc. The distance distribution of the RRLyrae stars peaks at 8.8 kpc, with the data indicating the presence of a bar-like structure inclined at ~30° to the line of sight. The dataset also exhibits a secondary concentration of stars beyond the Galactic centre at ~27 kpc. This is consistent with the distance to the Sagittarius (Sgr) dwarf galaxy, and can be attributed to the serendipitous alignment of the Sgr stream with the bulge. This dataset allows the Sgr stream to be traced in this part of the sky for the first time. The underlying metallicity distributions are determined to have mean +/- intrinsic width of [Fe/H]bulge = -1.24+/-0.23 dex and [Fe/H]Sgr = -1.53+/-0.14 dex