J. C. Pandey

A study of X-ray flares – II. RS CVn-type binaries

We present an analysis of seven flares detected from five RS CVn-type binaries (UZ Lib, σ Gem, λ And, V711 Tau and EI Eri) observed with XMM–Newton observatory. The quiescent state X-ray luminosities in the energy band of 0.3–10.0 keV of these stars were found to be 10 30.7-30.9 erg s −1 . The exponential decay time in all the sample of flares range from ∼1 to 8 h. The luminosity at peak of the flares in the energy band of 0.3–10.0 keV was found to be in the range of 10 30.8 –10 31.8 erg s −1 . The great sensitivity of the XMM-European Photon Imaging Camera instrument allowed us to perform time resolved spectral analysis during the flares and also in the subsequent quiescent phases. The derived metal abundances of coronal plasma were found to vary during the flares observed from σ Gem, V771 Tau and EI Eri. In these flares elemental abundances found to be enhanced by factors of ∼1.3–1.5 to the quiescent states. In most of the flares, the peak temperature was found to be more than 100 MK, whereas emission measure increased by factors of 1.5–5.5. Significant sustained heating was present in the majority of flares. The loop lengths (L) derived for flaring structure were found to be in the order of 1010-11 cm and are smaller than the stellar radii (R★), i.e. L/R★≤ 1. The flare from σ Gem showed a high and variable absorption column density during the flare.

A multiwavelength polarimetric study towards the open cluster NGC 1893

We present multiwavelength linear polarimetric observations for 44 stars of the NGC 1893 young open cluster region along with V-band polarimetric observations of stars of four other open clusters located between l ∼ 160° and 175°. We found evidence for the presence of two dust layers located at a distance of ∼170 and ∼360 pc. The dust layers produce a polarization P v ∼ 2.2 per cent. It is evident from the clusters studied in this work that, in the Galactic longitude range from l ∼ 160° to 175° and within the Galactic plane (|b| < 2°), the polarization angles remain almost constant, with a mean of ∼163° and a dispersion of 6°. The small dispersion in polarization angle could be due to the presence of a uniform dust layer beyond 1 kpc. Present observations reveal that in the case of NGC 1893, the foreground two dust layers, in addition to the intracluster medium, seem to be responsible for the polarization effects. It is also found that towards the direction of NGC 1893, the dust layer that exists between 2 and 3 kpc has a negligible contribution towards the total observed polarization. The weighted mean for percentage of polarization (P max ) and the wavelength at maximum polarization (λ max ) are found to be 2.59 ± 0.02 per cent and 0.55 ± 0.01 μm, respectively. The estimated mean value of λmax indicates that the average size of the dust grains within the cluster is similar to that in the general interstellar medium. The spatial variation of the polarization is found to decrease towards the outer region of the cluster. In this work, we support the notion, as has already been shown in previous studies, that polarimetry, in combination with the (U - B)-(B - V) colour-colour diagram, is a useful tool for identifying non-members in a cluster.

Optical polarimetric study of open clusters: Distribution of Interstellar matter towards NGC 654

ABSTRACT We present new B,V and R linear polarimetric observations for 61 starstowards the region of the young open cluster NGC 654. In this study we foundevidence for the presence of at least two layers of dust along the line of sight tothe cluster. The distances to the two dust layers are estimated to be ∼ 200 pcand ∼ 1 kpc which are located much closer to the Sun than the cluster (∼ 2.4kpc). Both the dust layers have their local magnetic field orientation nearlyparallel to the direction of the Galactic plane. The foreground dust layer isfound to have a ring morphology with the central hole coinciding with thecenter of the cluster. The foreground dust grains are suggested to be mainlyresponsible for both the observed differential reddening and the polarizationtowards the cluster.Keywords: polarization-dust, extinction-openclustersandassociations:individual:NGC 654 1 INTRODUCTIONThe wavelength dependence of interstellar extinction and polarization provides constraintson the characteristics of interstellar grains. Interstellar polarization strongly varies withwavelength (Serkowski, Mathewson & Ford 1975; Wilking et al. 1980). In particular, thewavelength of maximum interstellar polarization (λ

LO Pegasi: an investigation of multiband optical polarization

We presentBVR polarimetric study of the cool active star LO Pegasi (LO Peg) for the first time. LO Peg was found to be highly polarized among the cool active stars. Our observations yield average values of polarization in LO Peg: PB = 0.387 ± 0.004 per cent, θ B = 88 ◦ ± 1 ◦ ; PV = 0.351 ± 0.004 per cent, θ V = 91 ◦ ± 1 ◦ and PR = 0.335 ± 0.003 per cent, θ R = 91 ◦ ± 1 ◦ .B oth the degree of polarization and the position angle are found to be variable. The semi-amplitude of the polarization variability in B, V and R bands is found to be 0.18 ± 0.02, 0.13 ± 0.01 and 0.10 ± 0.02 per cent, respectively. We suggest that the levels of polarization observed in LO Peg could be the result of scattering of an anisotropic stellar radiation field by an optically thin circumstellar envelope or scattering of the stellar radiation by prominence-like structures.

Optical and X-Ray Studies of Chromospherically Active Stars: FR Cancri, HD 95559, and LO Pegasi

We present a multiwavelength study of three chromospherically active stars, namely FR Cnc (= BD +16 ◦ 1753), HD 95559 and LO Peg (=BD +22 ◦ 4409), including newly obtained optical photometry, low-resolution optical spectroscopy for FR Cnc, as well as archival IR and X-ray observations. The BVR photometry carried out during the years 2001 - 2004 has found significant photometric variability to be present in all three stars. For FR Cnc, a photometric period 0.8267±0.0004 d has been established. The strong variation in the phase and amplitude of the FR Cnc light curves when folded on this period implies the presence of evolving and migrating spots or spot groups on its surface. Two independent spots with migration periods of 0.97 and 0.93 years respectively are inferred. The photometry of HD 95559 suggests the formation of a spot (group) during the interval of our observations. We infer the existence of two independent spots or groups in the photosphere of LO Peg, one of which has a migration period of 1.12 years. The optical spectroscopy of FR Cnc carried out during 2002-2003, reveals the

Polarization towards the young open cluster NGC 6823

We present multiwavelength linear polarimetric observations of 104 stars towards the region of young open cluster NGC 6823. The polarization towards NGC 6823 is dominated by foreground dust grains and we found evidence for the presence of several layers of dust towards the line of sight. The first layer of dust is approximately located within 200 pc towards the cluster, which is much closer to the Sun than the cluster itself (~2.1 kpc). The radial distribution of the position angles for the member stars is found to show a systematic change, while the polarization is found to reduce towards the outer parts of the cluster and the average position angle of the coronal region of the cluster is very close to the inclination of the Galactic parallel (~32°). The size distribution of the grains within NGC 6823 is similar to those in the general interstellar medium. The patchy distribution of foreground dust grains is suggested to be mainly responsible for both differential reddening and polarization towards NGC 6823. The majority of the observed stars do not show evidence of intrinsic polarization in their light.

Multiwavelength study of a young open cluster NGC 7419

Using new UBVRI Hα CCD photometric observations and the archival infrared and X-ray data, we have carried out a multiwavelength study of a Perseus arm young open 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. Our photometric data indicate a higher value of colour excess ratio E(U - B)/E(B - V) than the normal one. There is an evidence for mass segregation in this dynamically relaxed cluster and in the range 1.4-8.6 M ⊙ , the mass function slope is in agreement with the Salpeter value. Excess emissions in near-infrared and Ha support the existence of a young (≤2 Myr) stellar population of Herbig Ae/Be stars (≥3.0 M ⊙ ) 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 -1 , 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 X-ray luminosity estimate of ∼1.8 x 10 31 erg s -1 arcmin -2 . If the extended emission is due to unresolved emission from the point sources then we estimate ∼288 T Tauri stars in the cluster region each having X-ray luminosity ∼1.0 x 10 30 erg s -1 . Investigation of dust attenuation and 12 CO emission map of a square degree region around the cluster indicates the presence of a foreground dust cloud which is most likely associated with the local arm star-forming region (Sh2-154). This cloud harbours uniformly distributed pre-main-sequence stars (0.1-2.0 M ⊙ ), with no obvious trend of their distribution with either (H - K) excess or A V . This suggests that the star formation in this cloud depend mostly upon the primordial fragmentation.

Phase-resolved XMM-Newton and Swift Observations of WR 25

We present an analysis of long-term X-ray and optical observations of the Wolf-Rayet binary WR 25. Using archival data from observations with the XMM-Newton and the Swift observatories spanning over ~10 yr, we show that WR 25 is a periodic variable in X-rays with a period of

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

208 \pm 3

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

days. X-ray light curves in the 0.5-10.0 keV energy band show phase-locked variability, where the flux increased by a factor of ~2 from minimum to maximum, being maximum near periastron passage. The light curve in the soft energy band (0.5-2.0 keV) shows two minima indicating the presence of two eclipses. However, the light curve in the hard energy band (2.0-10.0 keV) shows only one minimum during the apastron passage. The X-ray spectra of WR 25 were explained by a two-temperature plasma model. Both the cool and the hot plasmas were constant at 0.628+/-0.008 and 2.75+/-0.06 keV throughout an orbital cycle, where the cooler plasma could be due to the small scale shocks in a radiation-driven outflow and the high temperature plasma could be due to the collision of winds. The column density varied with the orbital phase and was found to be maximum after the periastron passage, when the WN star is in front of the O star. The abundances of WR 25 were found to be non-solar. Optical V-band data of WR 25 also show the phase-locked variability, being at maximum near periastron passage. The results based on the present analysis indicate that WR 25 is a colliding wind binary where the presence of soft X-rays is attributed to individual components; however, hard X-rays are due to the collision of winds.