On the highly reddened members in 6 young galactic star clusters - a multiwavelength study

The spectral and reddening properties of 211 highly reddened proper motion members with $V < 15$ mag in 6 young galactic star clusters are investigated using low resolution spectroscopic, broad-band $UBVRIJHK$ and mid-IR data. We report emission features in CaII HK and HI lines for a sample of 29 stars including 11 stars reported for the first time and also provide either a new or more reliable spectral class for a sample of 24 stars. CaII triplet width measurements are used to indicate the presence of an accretion disk for a dozen stars and to hint luminosity for a couple of stars. On the basis of spectral features, near-IR excesses, dereddened color-color diagrams and mid-IR spectral indices we identify a group of 28 pre-main sequence cluster members including 5 highly probable Herbig Ae/Be and 6 classical T Tauri star. A total of 25 non-emission MS stars, amounting to $\sim$ 10 % early type MS members, appears to show Vega-like characteristics or are precursors to such a phenomenon. The various membership indicators suggest that $\sim$ 16% of the PM members are non-members. A significant fraction ($>$70%) of program stars in NGC 1976, NGC 2244, NGC 6530 and NGC 6611 show anomalous reddening with $R_{V}$ = $5.11\pm0.11$, $3.60\pm0.05$, $3.87\pm0.05$ and $3.56\pm 0.02$, respectively, indicating the presence of grain size dust larger than that typical to the diffuse medium. A small number of stars in NGC 1976, NGC 2244 and NGC 6611 also show normal behavior while the cluster NGC 6823 appears to have a normal reddening. Three highly luminous late type giants, one in NGC 2244 and two in NGC 6530, appears to be member and are in post-hydrogen-core-burning stages suggesting a prolonged duration ($\sim$ 25 Myrs) of star formation.Comment: 12 pages, 13 figures, submitted to MNRA

A spectroscopic study of the hybrid pulsator Gamma Pegasi

The recent detection of both pressure and high-order gravity modes in the classical B-type pulsator Gamma Pegasi offers promising prospects for probing its internal structure through seismic studies. To aid further modelling of this star, we present the results of a detailed NLTE abundance analysis based on a large number of time-resolved, high-quality spectra. A chemical composition typical of nearby B-type stars is found. The hybrid nature of this star is consistent with its location in the overlapping region of the instability strips for beta Cephei and slowly pulsating B stars computed using OP opacity tables, although OPAL calculations may also be compatible with the observations once the uncertainties in the stellar parameters and the current limitations of the stability calculations are taken into account. The two known frequencies f1 = 6.58974 and f2 = 0.68241 c/d are detected in the spectroscopic time series. A mode identification is attempted for the low-frequency signal, which can be associated to a high-order g -mode. Finally, we re-assess the binary status of Gamma Peg and find no evidence for variations that can be ascribed to orbital motion, contrary to previous claims in the literature.Comment: 28 pages, 16 figure

Variability of the Vela Pulsar-wind Nebula Observed with Chandra

The observations of the pulsar-wind nebula (PWN) around the Vela pulsar with the Advanced CCD Imaging Spectrometer aboard the Chandra X-ray Observatory, taken on 2000 April 30 and November 30, reveal its complex morphology reminiscent of that of the Crab PWN. Comparison of the two observations shows changes up to 30% in the surface brightness of the PWN features. Some of the PWN elements show appreciable shifts, up to a few arcseconds (about 10^{16} cm), and/or spectral changes. To elucidate the nature of the observed variations, further monitoring of the Vela PWN is needed.Comment: 7 pages (incl. 3 embedded PS figures), AASTEX, uses emulateapj5.sty. Submitted to ApJ Lett. For a high-resolution color PS image of Figure 3 (6.3 Mby), see http://www.astro.psu.edu/users/divas/velaneb_fig3.p

The Compact Central Object in Cas A: A Neutron Star with Hot Polar Caps or a Black Hole?

The central pointlike X-ray source of the Cas A supernova remnant was discovered in the Chandra First Light Observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. The power-law fit yields the photon index 2.6-4.1, and luminosity (2-60)e34 erg/s, for d=3.4 kpc. The power-law index is higher, and the luminosity lower, than those observed fromvery young pulsars. One can fit the spectrum equally well with a blackbody model with T=6-8 MK, R=0.2-0.5 km, L=(1.4-1.9)e33 erg/s. The inferred radii are too small, and the temperatures too high, for the radiationcould be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. One cannot exclude, however, that the observed emission originates from hot spots on a cooler neutron star surface. Because of strong interstellar absorption, the possible low-temperature component gives a small contribution to the observed spectrum; an upper limit on the (gravitationally redshifted) surface temperature is < 1.9-2.3 MK. Amongst several possible interpretations, we favor a model of a strongly magnetized neutron star with magnetically confined hydrogen or helium polar caps on a cooler iron surface. Alternatively, the observed radiation may be interpreted as emitted by a compact object (more likely, a black hole) accreting from a fossil disk or from a late-type dwarf in a close binary.Comment: 12 pages, 2 figures, submitted to ApJ

Mechanisms of physical and biochemical basis of resistance against leaf-hopper (Amrasca biguttula biguttula) in different okra (Abelmoschus esculentus) genotypes

Different biophysical parameters, viz. trichomes, leaf length, angle between the mid-ribs, angle between the midribs and vein, mid-rib thickness, leaf angle, number of leaves/plant, plant height and biochemical parameter like total phenol content of leaves were studied in relation to the expression of reaction towards leaf hopper (Amrasca biguttula biguttula) in 10 okra [Abelmoschus esculentus (L.) Moench] genotypes. It was observed that genotype SB-6 had relatively lower number of trichomes on leaf lamina (10.11), mid-rib (7.17) and vein (8.05) as showed highly susceptible to jassids (17.57 jassids/leaf) as compared to tolerant genotype VROB-181 (5.43 jassids/leaf) which had 11.85, 9.17 and 9.95 trichomes per cm2, respectively. Susceptible cultivar SB-10 possessed higher mid-rib thickness (1.75 mm) and leaf length (25.65 cm) as compared to tolerant genotype VROB-181 (1.58 mm and 22.43 cm, respectively). Similarly, higher number of total leaves (31.9) per plant was also recorded from SB-6 as compared to other tolerant lines (VROB-181, VROB-178, VROR-160). Leaf length, plant height, angle between mid-ribs and total leaves showed a strong positive correlation (r value = 0.493, 0.499, 0.723 and 0.474, respectively) with jassid incidence. Amongst the biochemical parameter, total phenol content was highest in VROB-181 (75.04 mg/100 g) and the incidence of jassids was lowest than those of susceptible genotypes, viz. SB-6 (42.61 mg/100 g) and SB-10 (48.35 mg/100 g) and thereby establishing the significant negative correlation (r = - 0.577) with the jassid incidence and total phenol content