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Register a new userThe VMC Survey - X. Cepheids, RR Lyrae stars and binaries as probes of the Magellanic Systems structure
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Maria Ida Moretti Dr
Publication date
2013
fields
Physics
and research's language is
English
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The VMC survey is obtaining multi-epoch photometry in the Ks band of the Magellanic System down to a limiting magnitude of Ks ~ 19.3 for individual epoch data. The observations are spaced in time such as to provide optimal sampling of the light curves for RR Lyrae stars and for Cepheids with periods up to 20-30 days. We present examples of the Ks-band light curves of Classical Cepheids and RR Lyrae stars we are obtaining from the VMC data and outline the strategy we put in place to measure distances and infer the System three-dimensional geometry from the variable stars. For this purpose the near-infrared Period-Luminosity, Period-Wesenheit, and Period-Luminosity-Colour relations of the system RR Lyrae stars and Cepheids are used. We extensively exploit the catalogues of the Magellanic Clouds variable stars provided by the EROS-2 and OGLE III/IV microlensing surveys. By combining these surveys we present the currently widest-area view of the Large Magellanic Cloud as captured by the galaxy Cepheids, RR Lyrae stars and binaries. This reveals the full extent of the main structures (bar/s - spiral arms) that have only been vaguely guessed before. Our work strengthens the case for a detailed study of the Large Magellanic Cloud three-dimensional geometry.
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We present results from the analysis of 2997 fundamental mode RR Lyrae variables located in the Small Magellanic Cloud (SMC). For these objects near-infrared time-series photometry from the VISTA survey of the Magellanic Clouds system (VMC) and visual light curves from the OGLE IV survey are available. In this study the multi-epoch $K_{rm s}$-band VMC photometry was used for the first time to derive intensity-averaged magnitudes of the SMC RR Lyrae stars. We determined individual distances to the RR Lyrae stars from the near-infrared period-absolute magnitude-metallicity ($PM_{K_{rm s}}Z$) relation, which has a number of advantages in comparison with the visual absolute magnitude-metallicity ($M_{V}-{rm [Fe/H]}$) relation, such as a smaller dependence of the luminosity on interstellar extinction, evolutionary effects and metallicity. The distances we have obtained were used to study the three-dimensional structure of the SMC. The distribution of the SMC RR Lyrae stars is found to be ellipsoidal. The actual line-of-sight depth of the SMC is in the range from 1 to 10 kpc, with an average depth of 4.3 $pm$ 1.0 kpc. We found that RR Lyrae stars in the eastern part of the SMC are affected by interactions of the Magellanic Clouds. However, we do not see a clear bimodality in the distribution of RR Lyrae stars as observed for red clump (RC) stars.
RR Lyrae stars being distance indicators and tracers of old population serve as excellent probes of the structure, formation, and evolution of our Galaxy. Thousands of them are being discovered in ongoing wide-field surveys. The OGLE project conducts the Galaxy Variability Survey with the aim to detect and analyze variable stars, in particular of RRab type, toward the Galactic bulge and disk, covering a total area of 3000 deg^2. Observations in these directions also allow detecting background halo variables and unique studies of their properties and distribution at distances from the Galactic Center to even 40 kpc. In this contribution, we present the first results on the spatial distribution of the observed RRab stars, their metallicity distribution, the presence of multiple populations, and relations with the old bulge. We also show the most recent results from the analysis of RR Lyrae stars of the Sgr dwarf spheroidal galaxy, including its center, the globular cluster M54.
We present results from an analysis of $sim$ 29,000 RR Lyrae stars located in the Large Magellanic Cloud (LMC). For these objects, near-infrared time-series photometry from the VISTA survey of the Magellanic Clouds system (VMC) and optical data from the OGLE (Optical Gravitational Lensing Experiment) IV survey and the Gaia Data Release 2 catalogue of confirmed RR Lyrae stars were exploited. Using VMC and OGLE IV magnitudes we derived period-luminosity (PL), period-luminosity-metallicity (PLZ), period-Wesenheit (PW) and period-Wesenheit-metallicity (PWZ) relations in all available bands. More that ~7,000 RR Lyrae were discarded from the analysis because they appear to be overluminous with respect to the PL relations. The $PL_{K_{mathrm{s}}}$ relation was used to derive individual distance to $sim 22,000$ RR Lyrae stars, and study the three-dimensional structure of the LMC. The distribution of the LMC RR Lyrae stars is ellipsoidal with the three axis $S_1$=6.5 kpc, $S_2$=4.6 kpc and $S_3$=3.7 kpc, inclination i=$22pm4^{circ }$ relative to the plane of the sky and position angle of the line of nodes $theta=167pm7^{circ }$ (measured from north to east). The north-eastern part of the ellipsoid is closer to us and no particular associated substructures are detected as well as any metallicity gradient.
The VISTA near-infrared YJKs survey of the Magellanic System (VMC) is collecting deep Ks-band time-series photometry of pulsating stars hosted by the two Magellanic Clouds and their connecting Bridge. Here we present YJKs light curves for a sample of 717 Small Magellanic Cloud (SMC) Classical Cepheids (CCs). These data, complemented with our previous results and V magnitude from literature, allowed us to construct a variety of period-luminosity and period-Wesenheit relationships, valid for Fundamental, First and Second Overtone pulsators. These relations provide accurate individual distances to CCs in the SMC over an area of more than 40 sq. deg. Adopting literature relations, we estimated ages and metallicities for the majority of the investigated pulsators, finding that: i) the age distribution is bimodal, with two peaks at 120+-10 and 220+-10 Myr; ii) the more metal-rich CCs appear to be located closer to the centre of the galaxy. Our results show that the three-dimensional distribution of the CCs in the SMC, is not planar but heavily elongated for more than 25-30 kpc approximately in the east/north-east towards south-west direction. The young and old CCs in the SMC show a different geometric distribution. Our data support the current theoretical scenario predicting a close encounter or a direct collision between the Clouds some 200 Myr ago and confirm the presence of a Counter-Bridge predicted by some models. The high precision three-dimensional distribution of young stars presented in this paper provides a new testbed for future models exploring the formation and evolution of the Magellanic System.
Galaxy interactions distort the distribution of baryonic matter and can affect star formation. The nearby Magellanic Clouds are a prime example of an ongoing galaxy interaction process. Here we use the intermediate-age ($sim1$-$10$ Gyr) red clump stars to map the three-dimensional structure of the Small Magellanic Cloud (SMC) and interpret it within the context of its history of interaction with the Large Magellanic Cloud (LMC) and the Milky Way. Red clump stars are selected from near-infrared colour-magnitude diagrams based on data from the VISTA survey of the Magellanic Clouds. Interstellar reddening is measured and removed, and the corrected brightness is converted to a distance, on a star-by-star basis. A flat plane fitted to the spatial distribution of red clump stars has an inclination $i=35deg$-$48deg$ and position angle PA$=170deg$-$186deg$. However, significant deviations from this plane are seen, especially in the periphery and on the eastern side of the SMC. In the latter part, two distinct populations are present, separated in distance by as much as 10 kpc. Distant red clump stars are seen in the North of the SMC, and possibly also in the far West; these might be associated with the predicted `Counter-Bridge. We also present a dust reddening map, which shows that dust generally traces stellar mass. The structure of the intermediate-age stellar component of the SMC bears the imprints of strong interaction with the LMC a few Gyr ago, which cannot be purely tidal but must have involved ram pressure stripping.
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