• Brijesh Kumar

      Articles written in Journal of Astrophysics and Astronomy

    • X-ray Observations of Eight Young Open Star Clusters: I. Membership and X-ray Luminosity

      Himali Bhatt J. C. Pandey K. P. Singh Ram Sagar Brijesh Kumar

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      We present a detailed investigation of X-ray source contents of eight young open clusters with ages between 4 to 46 Myr using archival X-ray data from XMM-NEWTON. The probable cluster memberships of the X-ray sources have been established on the basis of multi-wavelength archival data, and samples of 152 pre-main sequence (PMS) low mass (< 2𝑀), 36 intermediate mass (2-10𝑀) and 16 massive (> 10𝑀) stars have been generated. X-ray spectral analyses of high mass stars reveal the presence of high temperature plasma with temperature < 2 keV, and mean 𝐿𝑋/𝐿bol of 10-6.9. In the case of PMS low mass stars, the plasma temperatures have been found to be in the range of 0.2 keV to 3 keV with a median value of ∼ 1.3 keV, with no significant difference in plasma temperatures during their evolution from 4 to 46 Myr. The X-ray luminosity distributions of the PMS low mass stars have been found to be similar in the young star clusters under study. This may suggest a nearly uniform X-ray activity in the PMS low mass stars of ages ∼ 4–14 Myr. These observed values of 𝐿𝑋/𝐿bol are found to have a mean value of 10-3.6 ± 0.4, which is below the X-ray saturation level. The 𝐿𝑋/𝐿bol values for the PMS low mass stars are well correlated with their bolometric luminosities, that implies its dependence on the internal structure of the low mass stars. The difference between the X-ray luminosity distributions of the intermediate mass stars and the PMS low mass stars has not been found to be statistically significant. Their 𝐿𝑋/𝐿bol values, however have been found to be significantly different from each other with a confidence level greater than 99.999% and the strength of X-ray activity in the intermediate mass stars is found to be lower compared to the low mass stars. However, the possibility of X-ray emission from the intermediate mass stars due to a low mass star in close proximity of the intermediate mass star can not be ruled out.

    • X-ray Flares Observed from Six Young Stars Located in the Region of Star Clusters NGC 869 and IC 2602

      Himali Bhatt J. C. Pandey K. P. Singh Ram Sagar Brijesh Kumar

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      We present, for the first time, an analysis of seven intense X-ray flares observed from six stars (LAV 796, LAV 1174, SHM2002 3734, 2MASS 02191082+5707324, V553 Car, V557 Car). These stars are located in the region of young open star clusters NGC 869 and IC 2602. These flares detected in the XMM-Newton data show a rapid rise (10–40 min) and a slow decay (20–90 min). The X-ray luminosities during the flares in the energy band 0.3–7.5 keV are in the range of 1029.9 to 1031.7 erg s-1. The strongest flare was observed with the ratio ∼ 13 for count rates at peak of the flare to the quiescent intensity. The maximum temperature during the flares has been found to be ∼ 100 MK. The semi-loop lengths for the flaring loops are estimated to be of the order of 1010 cm. The physical parameters of the flaring structure, the peak density, pressure and minimum magnetic field required to confine the plasma have been derived and found to be consistent with flares from pre-main sequence stars in the Orion and the Taurus-Auriga-Perseus region.

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      Posted on January 27, 2016

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