arXiv subjects

Ram Sagar

Publications and source records attributed to Ram Sagar.

78 records · Page 5Linked to original sources

arXiv2002

Aerosol contents at an altitude of ~2 km in central Himalayas

Aerosols, both natural and anthropogenic, play an important role in the atmospheric science, by imparting radiative forcing and perturbing the radiative balance of the Earth atmosphere system as well as by degrading the environment. To understand the effect of aerosols on our geo/biosphere system, it is essential to characterize their physical and chemical properties regionally because of the regional nature of their properties and the short lifetime. As most of the aerosol sources are of terrestrial origin the variability of their properties will be very large close to the surface. At higher altitudes, above the mixing region, and in the free troposphere, the aerosol characteristics have a more synoptic perspective; would be indicative of the background level and are useful to understand long-term impacts. Such systematic measurements of aerosols at high altitudes are practically non-existing in India. Realising the potential and need for such studies, an activity has been initiated at Manora Peak, Nainital in the Shivalik Hills of Central Himalayas at an altitude of $\sim$2 km. The present paper provides the preliminary results of these aerosol measurements. These daytime measurements are compared with the existing earlier nighttime measurements.

physics.ao-ph↗

arXiv2002

A deep UVBRI CCD photometric study of open clusters Tr 1 and Be 11

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.

astro-ph↗

arXiv2001

HD 12098 - A new norhtern hemisphere roAp star

We present the analysis of 65 hours of high speed photometric observations of HD 12098 taken from State Observatory, Naini Tal and Gurushikhar Observatory, Mt.Abu on sixteen nights spanning from November 1999 to November 2000. HD 12098 is the first rapidly oscillating Ap star discovered from the `Naini Tal-Cape survey for northern hemisphere roAp stars'. It is the 32nd in the complete list. HD 12098 exhibits one predominant mode of oscillation at nu_1 = 2.1738 mHz. The second-most significant frequency in our data is at nu_2 = 2.1641 mHz with a 1 cycle/day alias ambiguity. We argue that nu_2 is a rotational sidelobe of nu_1, rather than an independent pulsation mode. Evidence for the presence of two other frequencies at 2.1807 and 2.3056 mHz is also presented.

astro-ph↗

arXiv2001

Non-uniform extinction in young open star clusters

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.

astro-ph↗

arXiv2001

A study of the old galactic star cluster Berkeley 32

We present new CCD photometry of the distant old open star cluster Berkeley 32 in Johnson V and Cousins I passbands. A total of about 3200 stars have been observed in a field of 13X13 arcmin**2. The colour-magnitude diagram in V, (V-I) has been generated down to V = 22 mag. A broad but well defined main sequence is clearly visible. Some blue stragglers, a well developed subgiant branch and a Red Clump are also seen. By fitting isochrones to this CMD as well as to other CMDs available in the literature, and using the Red Clump location, the reddening, distance and age of the star cluster have been determined. The cluster has a distance of 3.3 kpc, its radius is about 2.4 pc; the reddening E(B-V) is 0.08 mag and the age is 6.3 Gyr. By comparison with theoretical isochrones, a metallicity of [Fe/H]= -0.2 dex has been estimated. We find a much flatter mass function than what has been found for young clusters. If the mass function is a power law dN/dm = const.*m**alpha, then we get alpha = -0.5+-0.3 in the mass range 0.6-1.1 solar masses. This may be seen as a signature of the highly evolved dynamical state of the cluster.

astro-ph↗

arXiv1994

Young LMC star clusters as a test for stellar evolutionary models:

We compare the observed colour-magnitude diagrams (CMD) and the main sequence luminosity functions (LFs) of four Large Magellanic Cloud (LMC) star clusters namely, NGC 1711, NGC 2004, NGC 2164 and NGC 2214 with the synthetic ones derived from the stellar evolutionary models. Four different types of stellar evolutionary models have been used for comparison. The comparison of the features of the observed CMDs with the synthetic ones favour the overshoot models from Bressan et al. (1993). The synthetic integrated luminosity functions from the models can be matched with the observed ones by varying the value of mass function slopes. In order to constrain the models from the comparison of the synthetic ILFs with the observed ones, reliable estimates of mass function slope and binary fraction are desired.

astro-ph↗