Showing papers by "Ram Sagar published in 2007"

Star formation in young star cluster NGC 1893

Abstract: We present a comprehensive multiwavelength study of the star-forming region NGC 1893 to explore the effects of massive stars on low-mass star formation. Using near-infrared colours, slitless spectroscopy and narrow-band Hα photometry in the cluster region we have identified candidate young stellar objects (YSOs) distributed in a pattern from the cluster to one of the nearby nebulae Sim 129. The V, (V- I) colour-magnitude diagram of the YSOs indicates that majority of these objects have ages between 1 and 5 Myr. The spread in the ages of the YSOs may indicate a non-coeval star formation in the cluster. The slope of the K-band luminosity function for the cluster is estimated to be 0.34 ± 0.07, which agrees well with the average value (∼0.4) reported for young clusters. For the entire observed mass range 0.6 < M/M ⊙ ≤ 17.7 the value of the slope of the initial mass function, 'Γ', comes out to be -1.27 ± 0.08, which is in agreement with the Salpeter value of - 1.35 in the solar neighbourhood. However, the value of 'Γ' for pre-main-sequence phase stars (mass range 0.6 < M/M ⊙ ≤ 2.0) is found to be -0.88 ± 0.09 which is shallower than the value (-1.71 ± 0.20) obtained for main-sequence stars having mass range 2.5 < M/M ⊙ ≤ 17.7 indicating a break in the slope of the mass function at ∼2M ⊙ . Estimated 'Γ' values indicate an effect of mass segregation for main-sequence stars, in the sense that massive stars are preferentially located towards the cluster centre. The estimated dynamical evolution time is found to be greater than the age of the cluster, therefore, the observed mass segregation in the cluster may be the imprint of the star formation process. There is evidence for triggered star formation in the region, which seems to govern initial morphology of the cluster.

South Asian dust episode in June 2006: Aerosol observations in the central Himalayas

Abstract: [1] A dust storm blew through the Thar Desert on 12 June 2006, which has significantly influenced aerosol physical and optical properties over the central Himalayas (Nainital, 29.4°N; 79.5°E, 1958 m amsl) on 13 June 2006. Aerosol number concentrations in the coarse and giant modes on 13 June 2006 are found to be five (26 × 106 m−3) and ten (17.2 × 103 m−3) times higher compared to their respective monthly mean values. Aerosol optical depth values also showed two to four times increase, particularly at longer wavelengths suggesting increase in the concentrations of coarse and giant particles. This is supported by three to five times increase in Angstrom turbidity coefficient (β) and significant reduction in Angstrom wavelength exponent (α). Absence of enhancements in black carbon and accumulation mode particles suggests negligible changes in the influences of anthropogenic activities at the site during the study period.

Star formation in young star cluster NGC 1893

Abstract: We present a comprehensive multi-wavelength study of the star-forming region NGC 1893 to explore the effects of massive stars on low-mass star formation. Using near-infrared colours, slitless spectroscopy and narrow-band $H\alpha$ photometry in the cluster region we have identified candidate young stellar objects (YSOs) distributed in a pattern from the cluster to one of the nearby nebulae Sim 129. The $V, (V-I)$ colour-magnitude diagram of the YSOs indicates that majority of these objects have ages between 1 to 5 Myr. The spread in the ages of the YSOs may indicate a non-coeval star formation in the cluster. The slope of the KLF for the cluster is estimated to be $0.34\pm0.07$, which agrees well with the average value ($\sim 0.4$) reported for young clusters. For the entire observed mass range $0.6 < M/M_\odot \le 17.7$ the value of the slope of the initial mass function, $`\Gamma$', comes out to be $-1.27\pm0.08$, which is in agreement with the Salpeter value of -1.35 in the solar neighborhood. However, the value of $`\Gamma$' for PMS phase stars (mass range $0.6 < M/M_\odot \le 2.0$) is found to be $-0.88\pm0.09$ which is shallower than the value ($-1.71\pm0.20$) obtained for MS stars having mass range $2.5 < M/M_\odot \le 17.7$ indicating a break in the slope of the mass function at $\sim 2 M_\odot$. Estimated $`\Gamma$' values indicate an effect of mass segregation for main-sequence stars, in the sense that massive stars are preferentially located towards the cluster center. The estimated dynamical evolution time is found to be greater than the age of the cluster, therefore the observed mass segregation in the cluster may be the imprint of the star formation process. There is evidence for triggered star formation in the region, which seems to govern initial morphology of the cluster.

Broad‐band optical polarimetric study of IC 1805

Abstract: We present the BVR broad-band polarimetric observations of 51 stars belonging to the young open cluster IC 1805. Along with the photometric data from the literature, we have modelled and subtracted the foreground dust contribution from the maximum polarization (P max ) and colour excess [E (B - V)]. The mean value of the P max for intracluster medium and the foreground are found to be 5.008 ± 0.005 and 4.865 ± 0.022 per cent, respectively. Moreover, the mean value of the wavelength of maximum polarization (λ max ) for intracluster medium is 0.541 ± 0.003 m, which is quite similar as the general interstellar medium (ISM). The resulting intracluster dust component is found to have negligible polarization efficiency as compared to interstellar dust. Some of the observed stars in IC 1805 have shown the indication of intrinsic polarization in their measurements.

Quasi-periodic oscillations in XTE J0111.2-7317: highest frequency among the HMXB pulsars

Abstract: We report the discovery of quasi-periodic oscillations (QPOs) in the high-mass X-ray binary (HMXB) pulsar XTE J0111.2-7317 during a transient outburst of the source in 1998 December. Using observations made with the Proportional Counter Array of the Rossi X-Ray Timing Explorer during the second peak and the declining phase of the outburst, we have discovered a QPO feature at a frequency of 1.27 Hz. We have ruled out the possibility that the observed feature could instead be from the neighboring bright X-ray pulsar SMC X-1. This is the highest frequency QPO ever detected in an HMXB pulsar. In the absence of a cyclotron absorption feature in the X-ray spectrum, the QPO, along with the pulse period and X-ray flux measurement, helps us to constrain the magnetic field strength of the neutron star.

Broad Band Optical Polarimetric Study of IC 1805

Abstract: We present the BVR broad band polarimetric observations of 51 stars belonging to the young open cluster IC 1805. Along with the photometric data from the literature we have modeled and subtracted the foreground dust contribution from the maximum polarization (P_{max}) and colour excess (E_{B-V}). The mean value of the P_max for intracluster medium and the foreground are found to be 5.008 +/-0.005 % and 4.865 +/-0.022 % respectively. Moreover, the mean value of the wavelength of maximum polarization (lambda_{max}) for intracluster medium is 0.541 +/- 0.003 micro m, which is quite similar as the general interstellar medium (ISM). The resulting intracluster dust component is found to have negligible polarization efficiency as compared to interstellar dust. Some of the observed stars in IC 1805 have shown the indication of intrinsic polarization in their measurements.

Further evidence for intra-night optical variability of radio-quiet quasars

Abstract: Although well established for BL Lac objects and radio-loud quasars, the occurrence of intra-night optical variability (INOV) in radio-quiet quasars is still debated, primarily since only a handful of INOV events with good statistical significance, albeit small amplitude, have been reported so far. This has motivated us to continue intra-night optical monitoring of bona-fide radio-quiet quasars (RQQs). Here we present the results for a sample of 11 RQQs monitored by us on 19 nights. On 5 of these nights a given RQQ was monitored simultaneously from two well separated observatories. In all, two clear cases and two probable cases of INOV events were detected. From these data, we estimate an INOV duty cycle of sim8% for RQQs which would increase to 19% if the `probable variable' cases are also included. Such comparatively small INOV duty cycles for RQQs, together with the small INOV amplitudes (sim1%), are in accord with the previously deduced characteristics of this phenomenon.