Roche tomography of cataclysmic variables - V. A high-latitude star-spot on RU Pegasi

We present Roche tomograms of the secondary star in the dwarf nova system RU Pegasi derived from blue and red arm ISIS data taken on the 4.2-m William Herschel Telescope. We have applied the entropy landscape technique to determine the system parameters and obtained component masses of M1 = 1.06 Msun, M2 = 0.96 Msun, an orbital inclination angle of i = 43 degrees, and an optimal systemic velocity of gamma = 7 km/s. These are in good agreement with previously published values. Our Roche tomograms of the secondary star show prominent irradiation of the inner Lagrangian point due to illumination by the disc and/or bright spot, which may have been enhanced as RU Peg was in outburst at the time of our observations.We find that this irradiation pattern is axi-symmetric and confined to regions of the star which have a direct view of the accretion regions. This is in contrast to previous attempts to map RU Peg which suggested that the irradiation pattern was non-symmetric and extended beyond the terminator. We also detect additional inhomogeneities in the surface distribution of stellar atomic absorption that we ascribe to the presence of a large star-spot. This spot is centred at a latitude of about 82 degrees and covers approximately 4 per cent of the total surface area of the secondary. In keeping with the high latitude spots mapped on the cataclysmic variables AE Aqr and BV Cen, the spot on RU Peg also appears slightly shifted towards the trailing hemisphere of the star. Finally, we speculate that early mapping attempts which indicated non-symmetric irradiation patterns which extended beyond the terminator of CV donors could possibly be explained by a superposition of symmetric heating and a large spot.

THE VLT-FLAMES TARANTULA SURVEY: THE FASTEST ROTATING O-TYPE STAR AND SHORTEST PERIOD LMC PULSAR-REMNANTS OF A SUPERNOVA DISRUPTED BINARY?

We present a spectroscopic analysis of an extremely rapidly rotating late O-type star, VFTS102, observed during a spectroscopic survey of 30 Doradus. VFTS102 has a projected rotational velocity larger than 500 km s(-1) and probably as large as 600 km s-1; as such it would appear to be the most rapidly rotating massive star currently identified. Its radial velocity differs by 40 kms(-1) from the mean for 30 Doradus, suggesting that it is a runaway. VFTS102 lies 12 pc from the X-ray pulsar PSR J0537-6910 in the tail of its X-ray diffuse emission. We suggest that these objects originated from a binary system with the rotational and radial velocities of VFTS102 resulting from mass transfer from the progenitor of PSR J0537-691 and the supernova explosion, respectively.

The metamorphosis of supernova SN 2008D/XRF 080109: A link between supernovae and GRBs/hypernovae

The only supernovae (SNe) to show gamma-ray bursts ( GRBs) or early x-ray emission thus far are overenergetic, broad- lined type Ic SNe ( hypernovae, HNe). Recently, SN 2008D has shown several unusual features: (i) weak x-ray flash (XRF), (ii) an early, narrow optical peak, (iii) disappearance of the broad lines typical of SN Ic HNe, and (iv) development of helium lines as in SNe Ib. Detailed analysis shows that SN 2008D was not a normal supernova: Its explosion energy (E approximate to 6 x 10(51) erg) and ejected mass [similar to 7 times the mass of the Sun ( M.)] are intermediate between normal SNe Ibc and HNe. We conclude that SN 2008D was originally a similar to 30 M. star. When it collapsed, a black hole formed and a weak, mildly relativistic jet was produced, which caused the XRF. SN 2008D is probably among the weakest explosions that produce relativistic jets. Inner engine activity appears to be present whenever massive stars collapse to black holes.

Ruling out a massive asymptotic giant-branch star as the progenitor of supernova 2005cs

We calculate the predicted UBVRIJHK absolute magnitudes for models of supernova progenitors and apply the result to the case of supernova 2005cs. We agree with previous results that the initial mass of the star was low, around 6 to 8 M-circle dot. However, such stars are thought to go through a second dredge-up to become asymptotic giant branch (AGB) stars. We show that had this occurred to the progenitor of 2005cs it would have been observed in JHK pre-explosion images. The progenitor was not detected in these bands and therefore we conclude that it was not an AGB star. Furthermore, if some AGB stars do produce supernovae they will have a clear signature in pre-explosion near-infrared images. Electron-capture supernovae are thought to occur in AGB stars, hence the implication is that 2005cs was not an electron-capture supernova but was the collapse of an iron core.

Mass limits for the progenitor star of supernova 2001du and other Type II-P supernovae

The supernova SN 2001du was discovered in the galaxy NGC 1365 at a distance of 19 +/- 2 Mpc, and is a core-collapse event of Type II-P. Images of this galaxy, of moderate depth, have been taken with the Hubble Space Telescope approximately 6.6 yr before discovery and include the supernova position on the WFPC2 field of view. We have observed the supernova with the WFPC2 to allow accurate differential astrometry of SN 2001du on the pre-explosion frames. As a core-collapse event it is expected that the progenitor was a massive, luminous star. There is a marginal detection (3sigma) of a source close to the supernova position on the pre-discovery V -band frame, but it is not precisely coincident and we do not believe it to be a robust detection of a point source. We conclude that there is no stellar progenitor at the supernova position and derive sensitivity limits of the pre-discovery images that provide an upper mass limit for the progenitor star. We estimate that the progenitor had a mass of less than 15 M-circle dot . We revisit two other nearby Type II-P supernovae that have high-quality pre-explosion images, and refine the upper mass limits for the progenitor stars. Using a new distance determination for SN 1999gi from the expanding photosphere method, we revise the upper mass limit to 12 M-circle dot . We present new HST images of the site of SN 1999em, which validate the use of lower spatial resolution ground-based images in the progenitor studies and use a new Cepheid distance to the galaxy to measure an upper mass limit of 15 M-circle dot for that progenitor. Finally we compile all the direct information available for the progenitors of eight nearby core-collapse supernovae and compare their mass estimates. These are compared with the latest stellar evolutionary models of pre-supernova evolution which have attempted to relate metallicity and mass to the supernovae type. Although this is statistically limited at present, reasonable agreement is already found for the lower-mass events (generally the II-P), but some discrepancies appear at higher masses.

Non-radial pulsation, rotation and outburst in the Be star omega Orionis from the MuSiCoS 1998 campaign

omega Ori (HD37490, HR1934) is a Be star known to have presented variations. In order to investigate the nature and origin of its short-term and mid-term variability, a study is performed of several spectral lines (Halpha, Hdelta, HeI 4471, 4713, 4921, 5876, 6678, CII 4267, 6578, 6583, Mg II 4481, Si III 4553 and Si II 6347), based on 249 high signal-to-noise high-resolution spectra taken with 8 telescopes over 22 consecutive nights during the MuSiCoS (Multi SIte COntinuous Spectroscopy) campaign in November-December 1998. The stellar parameters are revisited and the projected rotational velocity (v sin i = 179 km s(-1)) is redetermined using several methods. With the MuSiCoS 98 dataset, a time series analysis of line-profile variations (LPVs) is performed using the Restricted Local Cleanest (RLC) algorithm and a least squares method. The behaviour of the velocity of the centroid of the lines, the equivalent widths and the apparent vsini for several lines, as well as Violet and Red components of photospheric lines affected by emission (red He i lines, Si II 6347, CII 6578, 6583) are analyzed. The non-radial pulsation (NRP) model is examined using phase diagrams and the Fourier-Doppler Imaging (FDI) method. The LPVs are consistent with a NRP mode with l = 2 or 3, \m\ = 2 with frequency 1.03 cd(-1). It is shown that an emission line outburst occurred in the middle of the campaign. Two scenarios are proposed to explain the behaviour of a dense cloud, temporarily orbiting around the star with a frequency 0.46 c d(-1), in relation to the outburst.

Infrared spectrum of an extremely cool white-dwarf star

White dwarfs are the remnant cores of stars that initially had masses of less than 8 solar masses. They cool gradually over billions of years, and have been suggested(1,2) to make up much of the 'dark matter' in the halo of the Milky way. But extremely cool white dwarfs have proved difficult to detect, owing to both their faintness and their anticipated similarity in colour to other classes of dwarf stars. Recent improved models(3-5) indicate that white dwarfs are much more blue than previously supposed, suggesting that the earlier searches may have been looking for the wrong kinds of objects. Here we report an infrared spectrum of an extremely cool white dwarf that is consistent with the new models. We determine the star's temperature to be 3,500 +/- 200 K, making it the coolest known white dwarf. The kinematics of this star indicate that it is in the halo of the Milky Way, and the density of such objects implied by the serendipitous discovery of this star is consistent with white dwarfs dominating the dark matter in the halo.

A metal deficient early B-type star near the edge of the galactic disk

High resolution spectra of an early B-type star associated with the H II region detected by de Geus et al. (1993) are analysed using LTE model atmosphere techniques to derive stellar atmospheric parameters and a chemical composition. A distance to the star of 8.2 kpc is estimated, placing it near the edge of the galactic disk and closer than the kinematic distance of 20 kpc to the H II region, calculated by de Geus et al. A differential line by line abundance analysis with respect to the spectroscopic standard tau Sco indicates a significant metal depletion, with elements down on average by -0.5 dex.

EFFECT OF WEAK PREPULSES ON COLLISIONAL X-RAY LASER PLASMAS

Through the use of time-integrated space-resolved keV spectroscopy, we characterize line plasmas showing gain in Ne-like Zn with prepulsed irradiation to explain the enhanced performances of x-ray lasers using the prepulse technique. It is observed that the value of the electron temperature does not vary significantly with prepulse level, nor does its spatially resolved profile along the line. The lateral width and density of the Ne-like region in the plasma are seen to increase with the prepulse level. (C) 1995 Optical Society of America

A comparison between star formation rate diagnostics and rate of core collapse supernovae within 11 Mpc

Aims. The core collapse supernova rate provides a strong lower limit for the star formation rate (SFR). Progress in using it as a cosmic SFR tracer requires some confidence that it is consistent with more conventional SFR diagnostics in the nearby Universe. This paper compares standard SFR measurements based on H alpha, far ultraviolet (FUV) and total infrared (TIR) galaxy luminosities with the observed core collapse supernova rate in the same galaxy sample. The comparison can be viewed from two perspectives. Firstly, by adopting an estimate of the minimum stellar mass to produce a core collapse supernova one can determine a SFR from supernova numbers. Secondly, the radiative SFR can be assumed to be robust and then the supernova statistics provide a constrain on the minimum stellar mass for core collapse supernova progenitors.

Weak interactions, omnivory and emergent food-web properties

Empirical studies have shown that, in real ecosystems, species-interaction strengths are generally skewed in their distribution towards weak interactions. Some theoretical work also suggests that weak interactions, especially in omnivorous links, are important for the local stability of a community at equilibrium. However, the majority of theoretical studies use uniform distributions of interaction strengths to generate artificial communities for study. We investigate the effects of the underlying interaction-strength distribution upon the return time, permanence and feasibility of simple Lotka-Volterra equilibrium communities. We show that a skew towards weak interactions promotes local and global stability only when omnivory is present. It is found that skewed interaction strengths are an emergent property of stable omnivorous communities, and that this skew towards weak interactions creates a dynamic constraint maintaining omnivory. Omnivory is more likely to occur when omnivorous interactions are skewed towards weak interactions. However, a skew towards weak interactions increases the return time to equilibrium, delays the recovery of ecosystems and hence decreases the stability of a community. When no skew is imposed, the set of stable omnivorous communities shows an emergent distribution of skewed interaction strengths. Our results apply to both local and global concepts of stability and are robust to the definition of a feasible community. These results are discussed in the light of empirical data and other theoretical studies, in conjunction with their broader implications for community assembly.

Severity staging by early features of age-related maculopathy exhibits weak relationships with functional deficits on SWS grating acuity

To examine the relationship between short-wavelength-sensitive (SWS) resolution acuity and epidemiologically defined stages of early age-related maculopathy (ARM).