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We present UBVRI CCD photometry in the region of the open clusters NGC 637 and NGC 957. The radii are found to be 4.2 arcmin and 4.3 arcmin. Their reddenings E(B-V) are 0.64+/-0.05 mag and 0.71+/-0.05 mag and their distances, from main sequence fitting are 2.5+/-0.2 kpc and 2.2+/-0.2 kpc. Comparison with Z=0.02 isochrones leads to an age of 10+/-5 Myr for both clusters. Combining our photometry with 2MASS JHK shows the reddening law in these directions to be normal. Mass function slopes of x=1.65+/-0.20 and 1.31+/-0.50 are derived for the clusters, both of which are found to be dynamically relaxed. Spectral and photometric characteristics of three Be stars, two in NGC 957 and one (newly discovered) in NGC 637 indicate them to be of Classical Be type., Comment: Accepted for publication in MNRAS

The solar-age open cluster M67 (C0847+120, NGC2682) is a touchstone in studies of the old Galactic disk. Despite its outstanding role, the census of cluster membership for M67 at fainter magnitudes and their properties are not well-established. Using the proprietary and archival ESO data, we have obtained astrometric, photometric, and radial velocities of stars in a 34'x 33' field centered on the old open cluster M67. The two-epoch archival observations separated by 4 years and acquired with the Wide Field Imager at the 2.2m MPG/ESO telescope have been reduced with our new astrometric techniques, as described in the first paper of this series. The same observations served to derive calibrated BVI photometry in M67. Radial velocities were measured using the archival and new spectroscopic data obtained at VLT. We have determined relative proper motions and membership probabilities for ~2,400 stars. The precision of proper motions for optimally exposed stars is ~2 mas/yr, gradually degrading down to ~5 mas/yr at V= 20. Our relatively precise proper motions at V>16 are first obtained in this magnitude range for M67. Radial velocities are measured for 211 stars in the same field. We also present a detailed comparison with recent theoretical isochrones from several independent groups. For M67 area we provide positions, calibrated BVI photometry, relative proper motions, membership probabilities, and radial velocities. We demonstrate that the ground-based CCD mosaic observations just a few years apart are producing proper motions, allowing a reliable membership determination. We produced a catalogue that is made electronically available to the astronomical community., Comment: 15 pages, 11 figures, 5 table. Accepted (2008 February 25) on A&A

We present the $UBVRI$ CCD photometry in the region of the open cluster NGC 2421. Radius of the cluster is determined as $\sim$ 3$^\prime$.0 using stellar density profile. Our Study indicates that metallicity of the cluster is $Z \sim$ 0.004. The reddening $E(B-V) = 0.42\pm$0.05 mag is determined using two colour $(U-B)$ versus $(B-V)$ diagram. By combining the 2MASS $JHK$ data with the optical data we determined $E(J-K) = 0.20\pm$0.20 mag and $E(V-K) = 1.15\pm$0.20 mag for this cluster. Colour-excess diagrams show normal interstellar extinction law in the direction of the cluster. We determined the distance of the cluster as 2.2$\pm$0.2 Kpc by comparing the ZAMS with the intrinsic CM diagram of the cluster. The age of the cluster has been estimated as 80$\pm$20 Myr using the stellar isochrones of metallicity $Z = 0.004$. The mass function slope $x = 1.2\pm0.3$ has been derived by applying the corrections of field stars contamination and data incompleteness. Our analysis indicate that the cluster NGC 2421 is dynamically relaxed., Comment: 10 pages, 13 figures, Accepted for publication in MNRAS

In this paper we present UBVRI CCD photometry in the region of two young open star clusters Basel 4 and NGC 7067 for the first time. Our sample consists of ~ 4000 stars down to V ~ 21 mag. Stellar surface density profile indicates that radius of Basel 4 and NGC 7067 are about 1.8 and 3.0 arcmin respectively.The (U-B) versus (B-V) diagrams indicate that metallicity of NGC 7067 is solar while that of Basel 4 is Z ~ 0.008. We estimate the mean value of E(B-V) = 0.45$\pm$0.05 and 0.75$\pm$0.05 mag for Basel 4 and NGC 7067 respectively. The analysis of 2MASS JHK data in combination with the optical data in both the clusters yields E(J-K) = 0.30$\pm$0.20 mag and E(V-K) = 1.60$\pm$0.20 mag for Basel 4 while E(J-K) = 0.40$\pm$0.20 mag and E(V-K) = 2.10$\pm$0.20 mag for NGC 7067. Furthermore, colour excess diagrams show a normal interstellar extinction law towards both the clusters. Using the intrinsic colour-magnitude diagrams of the cluster members, we estimated the distances of the clusters as 3.0$\pm$0.2 and 3.6$\pm$0.2 Kpc for Basel 4 and NGC 7067 respectively. By fitting the proper metallicity isochrones to the bright cluster members we estimated the age of the clusters as 200$\pm$50 and 100$\pm$25 Myr for Basel 4 and NGC 7067 respectively. The mass function slope which is derived by applying the corrections of field star contamination and data incompleteness are $1.55\pm0.25$ and $1.68\pm0.47$ for Basel 4 and NGC 7067 respectively. The values of mass function slopes are thus not too different from the Salpeter's (1955) value. Mass segregation is observed in both the clusters which may be due to the dynamical evolutions or imprint of star formation processes or both., Comment: 14 pages, 12 figures, Accepted for publication in MNRAS

We report the first deep $UBVRI$ CCD photometry of 2460 stars in the field of two poorly studied OB associations Bochum 1 and Bochum 6. We selected 15 and 14 probable members in Bochum 1 and Bochum 6 respectively using photometric criteria and proper motion data of Tycho 2. Our analysis indicates variable reddening having mean value of $E(B-V)=$ 0.47$\pm$0.10 and 0.71$\pm$0.13 mag for Bochum 1 and Bochum 6 respectively. Using the zero-age main-sequence fitting method, we derive a distance of 2.8$\pm$0.4 and 2.5$\pm$0.4 Kpc for Bochum 1 and Bochum 6 respectively. We obtain an age of 10$\pm$5 Myrs for both the associations from isochrone fitting. In both associations high and low mass stars have probably formed together. Within the observational uncertainties, mass spectrum of the both associations appears to be similar to the Salpeter's one., Comment: 14 pages, 7 figures, 6 tables. Accepted for Bull. Astr. Soc. India

We have obtained Johnson U, B, V and Cousins R, I photometry and low resolution spectra of a small sample of hot post-AGB candidates. Using the present data in combination with JHK data from 2MASS, infrared data from the MSX catalog and the IRAS fluxes, we have studied the spectral energy distribution (SED) of these stars. Using the DUSTY code we have estimated the dust temperatures, the distances to the stars, the mass-loss rates, angular radii of the inner boundary of the dust envelopes and dynamical ages from the tip of the AGB. These candidates have also been imaged through a narrow band H-alpha filter, to search for nebulosity around the central stars. Our H-alpha images revealed the bipolar morphology of the low excitation PN IRAS 17395-0841 with an angular extent of 2.8arcsec. The bipolar lobes of IRAS 17423-1755 in H-alpha were found to have an angular extent of 3.5arcsec (south-east lobe) and 2.2arcsec (north-west lobe). The dust envelope characteristics, low resolution spectrum and IRAS colors suggest that IRAS 18313-1738 is similar to the proto-planetary nebula (PPN) HD 51585. The SED of IRAS 17423-1755, IRAS 18313-1738 and IRAS 19127+1717 show a warm dust component (in addition to the cold dust) which may be due to recent and ongoing mass-loss., Comment: 20 pages, 6 figures, h-alpha figure compressed with XV, paper accepted for publication in Astronomy & Astrophysics

We present deep $UBVRI$ CCD photometry for the young open star clusters Tr 1 and Be 11. The CCD data for Be 11 is obtained for the first time. The sample consists of $\sim$ 1500 stars reaching down to $V$ $\sim$ 21 mag. Analysis of the radial distribution of stellar surface density indicates that radius values for Tr 1 and Be 11 are 2.3 and 1.5 pc respectively. The interstellar extinction across the face of the imaged clusters region seems to be non-uniform with a mean value of $E(B-V)$ = 0.60$\pm$0.05 and 0.95$\pm$0.05 mag for Tr 1 and Be 11 respectively. A random positional variation of $E(B-V)$ is present in both the clusters. In the cluster Be 11, the reason of random positional variation may be apparent association of the HII region (S 213). The 2MASS $JHK$ data in combination with the optical data in the cluster Be 11 yields $E(J-K)$ = 0.40$\pm$0.20 mag and $E(V-K)$ = 2.20$\pm$0.20 mag. Colour excess diagrams indicate a normal interstellar extinction law in the direction of cluster Be 11. The distances of Tr 1 and Be 11 are estimated as 2.6$\pm$0.10 and 2.2$\pm$0.10 Kpc respectively, while the theoretical stellar evolutionary isochrones fitted to the bright cluster members indicate that the cluster Tr 1 and Be 11 are 40$\pm$10 and 110$\pm$10 Myr old. The mass functions corrected for both field star contamination and data incompleteness are derived for both the clusters. The slopes $1.50\pm0.40$ and $1.22\pm0.24$ for Tr 1 and Be 11 respectively are in agreement with the Salpeter's value. Observed mass segregations in both clusters may be due to the result of dynamical evolutions or imprint of star formation processes or both., Comment: 15 pages, 13 figures. Accepted for publication in MNRAS

The extinction law and the variation of colour excess with position, luminosity as well as spectral class in young open star clusters NGC 663, NGC869, NGC 884, NGC 1502, NGC 1893, NGC 2244, NGC 2264, NGC 6611, Tr 14, Tr 15,Tr 16, Coll 228, Tr 37 and Be 86 have been studied. The difference in the minimum and maximum values of E(B-V) of cluster members has been considered as a measure of the presence of non-uniform gas and dust inside the clusters. Its value ranges from 0.22 to 1.03 mag in clusters under study, which indicates that non-uniform extinction is present in all the clusters. It has been noticed for the first time in NGC 1502 and Tr 37. It is also found that the differential colour excess in open clusters, which may be due to the presence of gas and dust, decreases systematically with the age of clusters indicating that matter is used either in star formation or blown away by hot stars or both. There is no uniformity in the variation of E(B-V) with either position or spectral class or luminosity., Comment: 11 pages, 8 figures, 4 tables; accepted for publication in MNRAS, typos added

Gamma-ray bursts (GRBs) are thought to result from the interaction of an extremely relativistic outflow interacting with a small amount of material surrounding the site of the explosion. Multi-wavelength observations covering the gamma-ray to radio wavebands allow investigations of this `fireball' model. On 23 January 1999 optical emission was detected while the gamma-ray burst was still underway. Here we report the results of gamma-ray, optical/infra-red, sub-mm, mm and radio observations of this burst and its afterglow, which indicate that the prompt and afterglow emissions from GRB 990123 are associated with three distinct regions in the fireball. The afterglow one day after the burst has a much lower peak frequency than those of previous bursts; this explains the short-lived nature of the radio emission, which is not expected to reappear. We suggest that such differences reflect variations in the magnetic-field strengths in the afterglow emitting regions., Comment: Accepted for publication in Nature. Under press embargo

The CCD magnitudes in Johnson $BV$ and Cousins $RI$ photometric passbands are determined for 18 stars in the field of GRB 990123. These measurements can be used in carrying out precise CCD photometry of the optical transient of GRB 990123 using differential photometric techniques during non--photometric sky conditions. A comparison with previous photometry indicates that the present photmetry is more precise., Comment: Tex file, 5 pages with 1 figure. Bull. Astron. Society India, Vol. 27 (accepted)

The CCD magnitudes in Johnson $BV$ and Cousins $R$ photometric passbands are determined for the optical transient of GRB 990123. These observations have been taken approximately 12 hours after trigger of the gamma-ray burst. BVR photometric light curves are obtained by combining the published data with the present measurements. They indicate that the flux decay constants in $BV$ and $R$ passbands are almost the same. Its value is $1.1\pm 0.06$. Such decay is quite consistent with the fireball models for the optical transient., Comment: LaTex file, 5 pages with 1 figure. Bull. Astron. Society India, Vol. 27 (accepted)