Cluster formation versus star formation rates around six regions in the LMC

The stellar population and star clusters around six regions in the Large Magellanic Cloud (LMC) are studied to understand the correlation between star formation and cluster formation rates. We used the stellar database of the OGLE II LMC survey and the star cluster catalogues. The observed distribution of stellar density on the colour-magnitude diagrams (CMDs) were compared with the synthetic ones generated from the stellar evolutionary models. By minimising the reduced chi-square values, the star formation history of the regions were obtained in terms of the star formation rates (SFRs). All the regions were found to show large SFRs between the ages 500 Myr - 2 Gyr with lower values for younger and older ages. The correlated peak in the cluster and SFRs is found for ages $sim$ 1 Gyr, and for ages less than 100 Myr. Five out of six regions show significant cluster formation in the 100 - 300 Myr, while the SFRs were found to be very low. This indicates anti-correlation between star and cluster formation rates for the 100 - 300 Myr age range. A possible reason may be that the stars are predominantly formed in clusters, whether bound or unbound, as a result of star formation during the above age range. The enhanced cluster formation rate at 100 - 300 Myr age range could be correlated with the encounter of LMC with the Small Magellanic Cloud, while the enhanced star and cluster formation at $\sim$ 1 Gyr does not correspond to any interaction. This could indicate that the star formation induced by interaction is biased towards group or cluster formation of stars.Comment: to appear in A&A, 12 pages including 7 figure

Structure of the Large Magellanic Cloud from the Near Infrared magnitudes of Red Clump stars

The structural parameters, like the inclination, i and the position angle of the line of nodes (PA_lon) of the disk of the Large Magellanic Cloud (LMC) are estimated using the JH photometric data of red clump stars from the Infrared Survey Facility - Magellanic Cloud Point Source Catalog (IRSF-MCPSC). The observed LMC region is divided into several sub-regions and stars in each region are cross identified with the optically identified red clump stars to obtain the near infrared magnitudes. The peak values of H magnitude and (J-H) colour of the observed red clump distribution are obtained by fitting a profile to the distributions and also by taking the average value of magnitude and colour of the red clump stars in the bin with largest number. Then the dereddened peak H0 magnitude of the red clump stars in each sub-region is obtained. The RA, Dec and relative distance from the center of each sub-region are converted into x, y & z Cartesian coordinates. A weighted least square plane fitting method is applied to this x,y,z data to estimate the structural parameters of the LMC disk. A reddening map based on (J-H) colour of the RC stars is presented. When the peaks of the red clump distribution were identified by averaging, an inclination of 25.7 +/- 1.6 and PA_lon = 141.5 +/- 4.5 were obtained. We estimate a distance modulus of 18.47 +/- 0.1 mag to the LMC. Extra-planar features which are in front as well as behind the fitted plane are identified which match with the optically identified extra-planar features. The bar of the LMC is found to be part of the disk within 500 pc. The estimates of the structural parameters are found to be independent of the photometric bands used for the analysis. We find that the inner disk, within 3.0, is less inclined and has larger value of PA_lon when compared to the outer disk.Comment: 13 pages, 16 figures, Accepted for publication in the journal Astronomy & Astrophysic

RR Lyrae stars in the inner LMC: Where did they form?

RR Lyrae stars (RRLS) belong to population II and are generally used as a tracer of the host galaxy halo. The surface as well as vertical distribution of RRLS in the inner Large Magellanic Cloud (LMC) are studied to understand whether these stars are actually formed in the halo. RRLS identified by the OGLE III survey are used to estimate their number density distribution. The scale-height of their distribution is estimated using extinction corrected average magnitudes of ab type stars. The density distribution mimics the bar, confirming results in the literature. The distribution of their scale height indicates that there may be two populations, one with smaller scale-height, very similar to the red clump stars and the other, much larger. The distribution of the reddening-corrected magnitude along the minor axis shows variation, suggesting an inclination. The inclination is estimated to be i = 31.3 (3.5) degrees, very similar to the inclination of the disk. Thus, the RRLS in the inner LMC mimic the bar and inclination of the disk, suggesting that a major fraction of RRLS is formed in the disk of the LMC. The results indicate that the RRLS in the inner LMC trace the disk and probably the inner halo. They do not trace the extended metal-poor halo of the LMC. We suggest that a major star formation event happened in the LMC at 10-12 Gyrs ago, resulting in the formation of most of the inner RRLS, as well as probably the globular clusters, inner halo and the disk of the LMC.Comment: A&A Letters (in press

Recent star formation history of the Large and Small Magellanic Clouds

We traced the age of the last star formation event (LSFE) in the inner Large & Small Magellanic Cloud (L&SMC) using the photometric data from the Optical Gravitational Lensing Experiment (OGLE-III) and the Magellanic Cloud Photometric Survey (MCPS). The LSFE is estimated from the main-sequence turn off point in the color-magnitude diagram (CMD) of a region. Extinction corrected turn off magnitude is converted to age, which represents the LSFE in a region. The spatial map of the LSFE age shows that the star formation has shrunk to the central regions in the last 100Myr in both the galaxies. The location and age of LSFE is found to correlate well with those of the star cluster in both the Clouds. The SMC map shows two separate concentrations of young star formation. We detect peaks of star formation at 0-10, 90-100Myr in the LMC, and 0-10, 50- 60Myr in the SMC. The quenching of star formation in the LMC is found to be asymmetric with respect to the optical center such that most of the young star forming regions are located to the north and east. On deprojecting the data on the LMC plane, the recent star formation appears to be stretched in the north-east direction and the HI gas is found to be distributed preferentially in the North. The centroid is found to shift to north in 200-40Myr, and to north-east in the last 40Myr. In the SMC, we detect a shift in centroid of population of 500-40Myr in the direction of the LMC. We propose that the HI gas in the LMC is pulled to the north of the LMC in the last 200Myr due to the gravitational attraction of our Galaxy at the time of perigalactic passage. The shifted HI gas is preferentially compressed in the north during 200-40Myr and in the north-east in the last 40Myr, due to the motion of the LMC in the Galactic halo. The recent star formation in the SMC is due to the combined gravitational effect of the LMC and the perigalactic passage.Comment: Accepted to A&A on August 31, 201

Disk of the Small Magellanic Cloud as traced by Cepheids

The structure and evolution of the disk of the Small Magellanic Cloud (SMC) are traced by studying the Cepheids. We aim to estimate the orientation measurements of the disk, such as the inclination and the position angle of the line of nodes, and the depth of the disk. We used the V and I band photometric data of the fundamental and first-overtone Cepheids from the Optical Gravitational Lensing Experiment survey. The period-luminosity relations were used to estimate the relative distance and reddening of each Cepheid. A weighted least-square plane fitting method was then applied to estimate the structural parameters. The line-of-sight depth and then the orientation corrected depth or thickness of the disk were estimated from the relative distance measurements. The period-age-colour relation of Cepheids were used to derive the age of the Cepheids. A break in the PL relations of the fundamental-mode and first-overtone Cepheids at P ~ 2.95 days and P ~ 1 day are observed. An inclination of 64$^o$.4$\pm$0$^o$.7 and a PA$_{lon}$=155$^o$.3$\pm$6$^o$.3 are obtained from the full sample. A reddening map of the SMC disk is also presented. The orientation-corrected depth or thickness of the SMC disk is found to be 1.76 $\pm$ 0.6 kpc. The scale height is estimated to be 0.82 $\pm$ 0.3 kpc. The age distribution of Cepheids matches the SMC cluster age distribution. The radial variation of the disk parameters mildly indicate structures/disturbances in the inner SMC (0.5 $<$ r $<$ 2.5 degree). Some of the Cepheids found in front of the fitted plane in the eastern regions are possibly the youngest tidally stripped counterpart of the H {{\sc i}} gas of the Magellanic Bridge. The Cepheids behind the fitted plane are most likely the population in the Counter Bridge predicted in recent numerical simulations. Different scenarios for the origin of the extra-planar Cepheids are also discussed.Comment: Accepted for publication in the journal Astronomy & Astrophysics, 18 pages, 18 figure

UV observations of globular clusters and nearby galaxies

We propose to identify the hot stellar population of the galactic globular clusters (GCs) using the TAUVEX. These stars can be easily identified in the core of the GCs where the crowding due to the main-sequence stars get dramatically reduced in the UV broad bands. We propose to observe and create a first complete and homogeneous UV data on Magellanic Clouds. Studies of extra-galactic globular clusters in the UV is the only way to estimate their age and metallicity without ambiguity. Such studies have not been done even for M 31. Therefore, we propose that extra-galactic globular clusters should be observed in the UV broad band filters

Large Magellanic Cloud bar: evidence of a warped bar

The geometry of the LMC bar is studied using the dereddened mean magnitudes of the red clump stars (I0) from the Optical Gravitational Lensing Experiment II catalog. The value of I0 is found to vary in the east-west direction such that both the east and the west end of the bar are closer to us with respect to the center of the bar. The maximum observed variation has a statistical significance of more than 7.6 σ with respect to the maximum value of random error. The variation in I0 indicates the presence of warp in the bar of the LMC. The warp and the structures seen in the bar indicate that the bar could be a dynamically disturbed structure

Simulation of old open clusters for UVIT on ASTROSAT

Ultra Violet Imaging Telescope (UVIT) is one of the payloads on the first Indian multi wavelength satellite ASTROSAT expected to be launched by Indian Space Research Organisation (ISRO) in the year 2015. We have performed simulations of UV studies of old open clusters for the UVIT. The colour magnitude diagrams (CMDs) and spatial appearances have been created using 10 filters of FUV channel (130 - 180 nm) and NUV channel (200 - 300nm) available for observations on the UVIT, for three old open clusters M67, NGC 188 and NGC 6791. The CMDs are simulated for different filter combinations, and they are used to identify the loci of various evolutionary sequences, white dwarfs, blue stragglers, red giants, sub giants, turn off stars and the main sequence of the clusters. The present work helps in identifying the potential area of study in the case of three old open clusters, by considering the availability of filters and the detection limits of the instrument. We also recommend filter combinations, which can be used to detect and study the above mentioned evolutionary stages. The simulations and the results presented here are essential for the optimal use of the UVIT for studies of old open clusters.Comment: Accepted for publication in RAA; 25 pages, 19 figures, 1 tabl