X-ray emission from O-type stars : DH Cep and HD 97434

We present X-ray emission characteristics of the massive O-type stars DH Cep and HD 97434 using archival XMM-Newton observations. There is no convincing evidence for short term variability in the X-ray intensity during the observations. However, the analysis of their spectra reveals X-ray structure being consistent with two-temperature plasma model. The hydrogen column densities derived from X-ray spectra of DH Cep and HD 97434 are in agreement with the reddening measurements for their corresponding host clusters NGC 7380 and Trumpler 18, indicating that the absorption by stellar wind is negligible. The X-ray emission from these hot stars is interpreted in terms of the standard instability-driven wind shock model.Comment: 13 pages ; 2 figures; 2 tables (Accepted for publication in New Astronomy

Stellar contents and Star formation in the young cluster Stock 18

We have carried out deep (V$\sim$21 mag) \ubvri photometric study of the star cluster Stock 18. These along with archival Infrared data have been used to derive the basic cluster parameters and also to study the star formation processes in and around the cluster region. The distance to the cluster is derived as 2.8$\pm$0.2 kpc while its age is estimated as $6.0\pm2.0$ Myr. Present study indicates that interstellar reddening is normal in the direction of the cluster. The mass function slope is found to be -1.37$\pm$0.27 for the mass range 1$<M/M_\odot<$11.9. There is no evidence found for the effect of mass segregation in main-sequence stars of the cluster. A young stellar population with age between 1-2 Myr have been found in and around the cluster region. The presence of IRAS and AKARI sources with MSX intensity map also show the youth of the Sh2-170 region.Comment: 25 pages, 11 figures, 3 tables (Accepted for publication in New Astronomy

Multi-wavelength study of a young open cluster NGC 7419

Using UBVRI Halpha CCD photometric observations and the archival NIR and X-ray data, we have carried out a multi-wavelength study of a young star cluster NGC 7419. An age of 22.5+/-3.0 Myr and a distance of 3230^{+330}_{-430} pc are derived for the cluster with a higher value of color excess ratio E(U-B)/E(B-V) than the normal one. There is an evidence for mass segregation in this dynamically relaxed cluster with mass function slope is in agreement with the Salpeter value. NIR and Halpha excess support the existence of a young (< 2 Myr) stellar population of Herbig Ae/Be stars (> 3.0 M_sun) indicating a second episode of star formation in the cluster region. Using XMM-Newton observations, we found several X-ray sources in the cluster region but none of the Herbig Ae/Be stars is detected in X-rays. We compare the distribution of upper limits for Herbig Ae/Be stars with the X-ray distribution functions of the T-Tauri and the Herbig Ae/Be stars from previous studies, and found that the X-ray emission level of these Herbig Ae/Be stars is not more than L_X ~5.2 x 10^{30} erg/s, which is not significantly higher than for the T-Tauri stars. Therefore, X-ray emission from Herbig Ae/Be stars could be the result of either unresolved companion stars or a process similar to T-Tauri stars. We report an extended X-ray emission from the cluster region NGC 7419, with a total L_X estimate of ~ 1.8 x 10^31 erg/s/arcmin^2. Investigation of dust and CO map of 1 degree region around the cluster indicates the presence of a foreground dust cloud which is most likely associated with star forming region Sh2-154. This cloud harbors uniformly distributed pre main sequence stars (0.1-2.0M_sun) and the star formation in this cloud depend mostly upon the primordial fragmentation.Comment: 23 pages, 23 figures, 13 tables; Accepted for publication in MNRA

X-ray emission characteristics of two Wolf-Rayet binaries: V444 Cyg and CD Cru

Using data from observations made with XMM-Newton, we present an X-ray analysis of two Wolf-Rayet (WR) binaries: V444 Cyg and CD Cru. The X-ray light curves show the phase-locked variability in both binaries, where the flux increased by a factor of ~2 for V444 Cyg and ~1.5 for CD Cru, from minimum to maximum. The maximum luminosities in the 0.3-7.5 keV energy band were found to be 5.8×10<SUP>32</SUP> and 2.8×10<SUP>32</SUP> erg s<SUP>−1</SUP> for V444 Cyg and CD Cru, respectively. The X-ray spectra of these stars confirmed large extinction and revealed hot plasma with prominent emission-line features of highly ionized Ne, Mg, Si, S, Ar, Ca and Fe; these are found to be consistent with a two-temperature plasma model. At a temperature of ~0.6 keV, the cooler plasma was found to be constant at all phases for both binaries, which could be the result of a distribution of small-scale shocks in radiation-driven outflows. The hot components in these binaries were found to be phase-dependent. They varied from 1.85 to 9.61 keV for V444 Cyg and from 1.63 to 4.27 keV for CD Cru. The absorption of the hard component varied with the orbital phase and was found to be maximum during the primary eclipse of V444 Cyg. The high plasma temperature and variability with orbital phase suggest that the hard-component emission is caused by a colliding wind shock between the binary components