Distribution by Scientific Domains

Kinds of Dwarfs

  • brown dwarf
  • white dwarf

  • Terms modified by Dwarfs

  • dwarf candidate
  • dwarf companion
  • dwarf elliptical galaxy
  • dwarf galaxy
  • dwarf hamster
  • dwarf irregular galaxy
  • dwarf nova
  • dwarf phenotype
  • dwarf pine
  • dwarf rat
  • dwarf shrub
  • dwarf shrub heath
  • dwarf spheroidal galaxy
  • dwarf star
  • dwarf virus
  • dwarf viruse

  • Selected Abstracts


    Summary. Ancient medical discourse conveys a mainly negative view of children's bodies. From Hippocrates to Galen, newborn children are defined as imperfect and ugly beings, associating an excessive softness and weakness with various anomalies. Aristotle links their physical disproportions with those of dwarfs and animals. These disproportions induce physiological troubles and mental incapacities. Hot-tempered and moist, children are dominated by their emotions and sensations. Often authors group them with other beings regarded as inferior, such as women, the old, the sick, the insane, the drunk. How are mythical and human children rendered in iconography? Do their characteristics correspond to the medical discourse? The image of children's bodies changes with the passing of time; the miniature adult of archaic Greece gradually turns into the plump toddler of the Hellenistic period. How can we interpret these transformations? Does the evolution of iconography reflect the transformation of society or does it mirror the progress of medical knowledge? [source]


    ABSTRACT Studies on seasonal variation in oil and fatty acid profile of developing solid endosperm of two cultivars, West Coast Tall (WCT) and Chowghat Orange Dwarf (COD), and their hybrids indicated that oil percentage increased from 30% in 6-month-old nuts to 63% in matured nuts (12 months old). Nuts sampled during July from different levels of maturity had high oil percentage and followed by those sampled during April, October and January. During nut development to maturity, the percentages and contents of medium and long chain saturated fatty acids increased except that of palmitic and myristic acids. Concentration of long chain unsaturated fatty acids (LCUFAs) in developing coconut kernel were high at 5 and 6 months after fertilization and then decreased toward maturity. The LCUFAs were high in nuts developing during October; consequently, saturated to unsaturated fatty acid ratios were low during October. Results indicated that nuts matured during October had better nutritional quality for human consumption and those matured during January are more suitable for industrial purpose due to higher medium chain fatty acid concentrations. PRACTICAL APPLICATIONS Coconut is consumed either as the tender nut (5,6 months after fertilization) or as the kernel from mature nut (12 months after fertilization). Recent technologies of making snowball tender nut use the nuts aged 7,8 months old. Kernel also is consumed in this product. Apart from this, the coconut is being increasingly used for making different kernel-based value-added products. This information is useful, as the value-added products are being developed using different maturities of coconut. Hence, it is of paramount importance that the fatty acid profile of coconut kernel is known in detail for assessing the safety of food consumption from the human health point of view. Apart from this, information on the seasonal variation in fatty acid profile of developing endosperm gives an integrated knowledge so as to optimize the usage of coconut kernel for both human consumption and industrial exploitation. [source]

    Analysis of mitochondrial DNA diversity in Burkina Faso populations confirms the maternal genetic homogeneity of the West African goat

    ANIMAL GENETICS, Issue 3 2009
    L. J. Royo
    Summary To date, no comprehensive study has been performed on mitochondrial genetic diversity of the West African goat. Here, we analysed a 481-bp fragment of the HVI region of 111 goats representing four native West African populations, namely the three main Burkina Faso breeds, zoo-farm kept Dwarf goats and endangered Spanish goat breeds used as the outgroup. Analyses gave 83 different haplotypes with 102 variable sites. Most haplotypes (65) were unique. Only three haplotypes were shared between populations. Haplotypes were assigned to cluster A except for H45 (belonging to the Spanish Bermeya goat) which was assigned to cluster C. amova analysis showed that divergence between groups (,CT) was not statistically significant regardless of whether the partition in two hierarchical levels that was fitted included Spanish samples or not. The West African goat scenario shown here is consistent with that previously reported for the species: haplogroup A is predominant and has a very high haplotype diversity regardless of the geographic area or sampled breed. The large phenotypic differences observable between the West African Dwarf and Sahelian long-legged goat populations are not detectable with mitochondrial markers. Moreover, a previously suggested introgression of Sahelian goat southwards because of desertification could not be assessed using mtDNA information. [source]

    Pollen Morphology of Tundra Shrubs and Submarginal Plants from Barrow, Alaska

    Ling-Yu Tang
    Abstract Investigation of plant morphological features, pollen, and habitat have been made for two shrub species from Barrow, Alaska, namely Dryas integrifolia M. Vahl and Salix rotundifolia Trautv., both of which are endemic to the Arctic floristic area. The former species has small lanceolate or plate leaves, whereas the latter has rounded leaves with distinct veins, rich in vitamin C. Both have dwarf and sprawling habits. Pollen studies showed that the pollen grains of the two species are spheroidal to sub-spheroidal or prolate. The type of aperture was tricolporate; pollen size 26.3,31.3 ,m; ornamentation finely reticulate under a light microscope (LM) and striate-reticulate under a scanning electron microscope (SEM) for D. integrafolia and finely reticulate under the LM and SEM for S. rotundifolia. Comparisons were made between the pollen from the same species from Arctic collections with those from China and Japan. Investigation of pollen morphology of tundra plants can provide significant data for comparative studies of fossil pollen and for the reconstruction of paleovegetation and paleoclimate in the Barrow area. (Managing editor: Ya-Qin Han) [source]

    Ongoing ecological divergence in an emerging genomic model

    MOLECULAR ECOLOGY, Issue 14 2009
    Much of Earth's biodiversity has arisen through adaptive radiation. Important avenues of phenotypic divergence during this process include the evolution of body size and life history (Schluter 2000). Extensive adaptive radiations of cichlid fishes have occurred in the Great Lakes of Africa, giving rise to behaviours that are remarkably sophisticated and diverse across species. In Tanganyikan shell-brooding cichlids of the tribe Lamprologini, tremendous intraspecific variation in body size accompanies complex breeding systems and use of empty snail shells to hide from predators and rear offspring. A study by Takahashi et al. (2009) in this issue of Molecular Ecology reveals the first case of genetic divergence between dwarf and normal-sized morphs of the same nominal lamprologine species, Telmatochromis temporalis. Patterns of population structure suggest that the dwarf, shell-dwelling morph of T. temporalis might have arisen from the normal, rock-dwelling morph independently in more than one region of the lake, and that pairs of morphs at different sites may represent different stages early in the process of ecological speciation. The findings of Takahashi et al. are important first steps towards understanding the evolution of these intriguing morphs, yet many questions remain unanswered about the mating system, gene flow, plasticity and selection. Despite these limitations, descriptive work like theirs takes on much significance in African cichlids due to forthcoming resources for comparative genomics. [source]

    The transcriptomics of life-history trade-offs in whitefish species pairs (Coregonus sp.)

    MOLECULAR ECOLOGY, Issue 7 2008
    J. ST-CYR
    Abstract Despite the progress achieved in elucidating the ecological mechanisms of adaptive radiation, there has been little focus on documenting the extent of adaptive differentiation in physiological functions during this process. Moreover, a thorough understanding of the genomic basis underlying phenotypic adaptive divergence is still in its infancy. One important evolutionary process for which causal genetic mechanisms are largely unknown pertains to life-history trade-offs. We analysed patterns of gene transcription in liver tissue of sympatric dwarf and normal whitefish from two natural lakes, as well as from populations reared in controlled environments, using a 16 006-gene cDNA microarray in order to: (i) document the extent of physiological adaptive divergence between sympatric dwarf and normal species pairs, and (ii) explore the molecular mechanisms of differential life history trade-offs between growth and survival potentially involved in their adaptive divergence. In the two natural lakes, 6.45% of significantly transcribed genes showed regulation either in parallel fashion (2.39%) or in different directions (4.06%). Among genes showing parallelism in regulation patterns, we observed a higher proportion of over-expressed genes in dwarf relative to normal whitefish (70.6%). Patterns observed in controlled conditions were also generally congruent with those observed in natural populations. Dwarf whitefish consistently showed significant over-expression of genes potentially associated with survival through enhanced activity (energy metabolism, iron homeostasis, lipid metabolism, detoxification), whereas more genes associated with growth (protein synthesis, cell cycle, cell growth) were generally down-regulated in dwarf relative to normal whitefish. Overall, parallelism in patterns of gene transcription, as well as patterns of interindividual variation across controlled and natural environments, provide strong indirect evidence for the role of selection in the evolution of differential regulation of genes involving a vast array of potentially adaptive physiological processes between dwarf and normal whitefish. Our results also provide a first mechanistic, genomic basis for the observed trade-off in life-history traits distinguishing dwarf and normal whitefish species pairs, wherein enhanced survival via more active swimming, necessary for increased foraging and predator avoidance, engages energetic costs that translate into slower growth rate and reduced fecundity in dwarf relative to normal whitefish. [source]

    The dust-free symbiotic Mira K4,46 = LL Cas

    U. Munari
    ABSTRACT Accurate BVRCIC light and colour curves of the Mira variable in the dust-free symbiotic system K4,46 are presented and discussed. They cover several consecutive pulsations cycles. The Mira mean period is 284.2 d, the reddening sums to E(B,V) = 0.35 and the distance is 10 kpc. Absolute spectrophotometry at both maximum and minimum brightness of the Mira is presented. The rich emission line and continuum spectrum of the H ii region ionized by the hard radiation of the white dwarf dominates at Mira minimum. From its photoionization analysis, it is found that the H ii region extends to a radius of 210 au, has a mass 1.8 × 10,4 M, and it is ionized by a white dwarf of 158 000 K temperature and 0.06 R, radius, stably burning hydrogen at its surface. The derived [Fe/H]=,0.45 well match the ambient metallicity expected at K4,46 at its 15 kpc galactocentric distance, and the overabundances in N, Ne and He are those expected if the H ii region is fed by the wind of the Mira and polluted by nuclearly processed material lost by the white dwarf companion. [source]

    PG 1258+593 and its common proper motion magnetic white dwarf counterpart

    J. Girven
    ABSTRACT We confirm SDSS J130033.48+590407.0 as a common proper motion companion to the well-studied hydrogen-atmosphere (DA) white dwarf PG 1258+593 (GD322). The system lies at a distance of 68 ± 3 pc, where the angular separation of 16.1 ± 0.1 arcsec corresponds to a minimum binary separation of 1091 ± 7 au. SDSS J1300+5904 is a cool (Teff= 6300 ± 300 K) magnetic white dwarf (B, 6 mG). PG 1258+593 is a DA white dwarf with Teff= 14790 ± 77 K and log g= 7.87 ± 0.02. Using the white dwarf mass,radius relation implies the masses of SDSS J1300+5904 and PG 1258+593 are 0.54 ± 0.06 and 0.54 ± 0.01 M,, respectively, and therefore a cooling age difference of 1.67 ± 0.05 Gyr. Adopting main-sequence lifetimes from stellar models, we derive an upper limit of 2.2 M, for the mass of the progenitor of PG 1258+593. A plausible range of initial masses is 1.4,1.8 M, for PG 1258+593 and 2,3 M, for SDSS J1300+5904. Our analysis shows that white dwarf common proper motion binaries can potentially constrain the white dwarf initial mass,final mass relation and the formation mechanism for magnetic white dwarfs. The magnetic field of SDSS J1300+5904 is consistent with an Ap progenitor star. A common envelope origin of the system cannot be excluded, but requires a triple system as progenitor. [source]

    Life in the last lane: star formation and chemical evolution in an extremely gas rich dwarf

    Ayesha Begum
    ABSTRACT We present an analysis of H i, H, and oxygen abundance data for NGC 3741. This galaxy has a very extended gas disc (,8.8 times the Holmberg radius), and a dark-to-luminous (i.e. stellar) mass ratio of ,149, which makes it one of the ,darkest' dwarf irregular galaxies known. However, its ratio of baryon (i.e. gas + stellar) mass to dark mass is typical of that in galaxies. Our new high-resolution H i images of the galaxy show evidence for a large-scale (purely gaseous) spiral arm and central bar. From our H i data, a rotation curve can be derived out to ,37,44 disc scalelengths in the J and B bands, respectively. This is just slightly short of the radius at which one would expect a Navarro,Frenk,White type rotation curve to start falling. The galaxy has an integrated star formation rate (SFR) of ,0.0034 M, yr,1, while the average SFR within the optical disc is ,0.0049 M, yr,1 kpc,2. Despite the gaseous spiral feature and the ongoing star formation, we find that the global gas density in NGC 3741 is significantly lower than the Toomre instability criterion. This is consistent with the behaviour seen in other dwarf galaxies. We also find that the SFR is consistent with that expected from the observed correlations between H i mass and SFR and the global Kennicutt,Schmidt law, respectively. We measure the oxygen abundance to be 12 + log(O/H) = 7.66 ± 0.10, which is consistent with that expected from the metallicity,luminosity relation, despite its extreme gas mass ratio. We also examine the issue of chemical evolution of NGC 3741 in the context of the closed-box model of chemical evolution. The effective oxygen yield of NGC 3741 is consistent with recent model estimates of closed-box yields, provided one assumes that the gas has been efficiently mixed all the way to the edge of the H i disc (i.e. greater than eight times the optical radius). This seems a priori unlikely. On the other hand, using a sample of galaxies with both interferometric H i maps and chemical abundance measurements, we find that the effective yield is anticorrelated with the total dynamical mass, as expected in leaky box models. [source]

    GHASP: an H, kinematic survey of spiral and irregular galaxies , V. Dark matter distribution in 36 nearby spiral galaxies

    M. Spano
    ABSTRACT The results obtained from a study of the mass distribution of 36 spiral galaxies are presented. The galaxies were observed using Fabry,Perot interferometry as part of the GHASP survey. The main aim of obtaining high-resolution H, 2D velocity fields is to define more accurately the rising part of the rotation curves which should allow to better constrain the parameters of the mass distribution. The H, velocities were combined with low resolution H i data from the literature, when available. Combining the kinematical data with photometric data, mass models were derived from these rotation curves using two different functional forms for the halo: an isothermal sphere (ISO) and a Navarro,Frenk,White (NFW) profile. For the galaxies already modelled by other authors, the results tend to agree. Our results point at the existence of a constant density core in the centre of the dark matter haloes rather than a cuspy core, whatever the type of the galaxy from Sab to Im. This extends to all types the result already obtained by other authors studying dwarf and low surface brightness galaxies but would necessitate a larger sample of galaxies to conclude more strongly. Whatever model is used (ISO or NFW), small core radius haloes have higher central densities, again for all morphological types. We confirm different halo scaling laws, such as the correlations between the core radius and the central density of the halo with the absolute magnitude of a galaxy: low-luminosity galaxies have small core radius and high central density. We find that the product of the central density with the core radius of the dark matter halo is nearly constant, whatever the model and whatever the absolute magnitude of the galaxy. This suggests that the halo surface density is independent from the galaxy type. [source]

    Near-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG 1234+482

    P. R. Steele
    ABSTRACT We present a near-infrared spectrum of the hot (Teff, 55 000 K) hydrogen atmosphere (DA) white dwarf PG 1234+482. We confirm that a very low mass companion is responsible for the previously recognized infrared photometric excess. We compare spectra of M and L dwarfs, combined with an appropriate white dwarf model, to the data to constrain the spectral type of the secondary. We find that uncertainties in the Two-Micron All-Sky Survey HK photometry of the white dwarf prevent us from distinguishing whether the secondary is stellar or substellar, and assign a spectral type of L0±1 (M9,L1).Therefore, this is the hottest and youngest (,106 yr) DA white dwarf with a possible brown dwarf companion. [source]

    Mass modelling of dwarf spheroidal galaxies: the effect of unbound stars from tidal tails and the Milky Way

    Jaros, aw Klimentowski
    ABSTRACT We study the origin and properties of the population of unbound stars in the kinematic samples of dwarf spheroidal (dSph) galaxies. For this purpose we have run a high-resolution N -body simulation of a two-component dwarf galaxy orbiting in a Milky Way potential. In agreement with the tidal stirring scenario of Mayer et al., the dwarf is placed on a highly eccentric orbit, its initial stellar component is in the form of an exponential disc and it has a NFW-like dark matter (DM) halo. After 10 Gyr of evolution the dwarf produces a spheroidal stellar component and is strongly tidally stripped so that mass follows light and the stars are on almost isotropic orbits. From this final state, we create mock kinematic data sets for 200 stars by observing the dwarf in different directions. We find that when the dwarf is observed along the tidal tails the kinematic samples are strongly contaminated by unbound stars from the tails. We also study another source of possible contamination by adding stars from the Milky Way. We demonstrate that most of the unbound stars can be removed by the method of interloper rejection proposed by den Hartog & Katgert and recently tested on simulated DM haloes. We model the cleaned-up kinematic samples using solutions of the Jeans equation with constant mass-to-light ratio (M/L) and velocity anisotropy parameter. We show that even for such a strongly stripped dwarf the Jeans analysis, when applied to cleaned samples, allows us to reproduce the mass and M/L of the dwarf with accuracy typically better than 25 per cent and almost exactly in the case when the line of sight is perpendicular to the tidal tails. The analysis was applied to the new data for the Fornax dSph galaxy. We show that after careful removal of interlopers the velocity dispersion profile of Fornax can be reproduced by a model in which mass traces light with a M/L of 11 solar units and isotropic orbits. We demonstrate that most of the contamination in the kinematic sample of Fornax probably originates from the Milky Way. [source]

    A thousand and one nova outbursts

    Noya Epelstain
    ABSTRACT A full nova cycle includes mass accretion, thermonuclear runaway resulting in outburst and mass-loss, and finally, decline. Resumed accretion starts a new cycle, leading to another outburst. Multicycle nova evolution models have been calculated over the past twenty years, the number being limited by numerical constraints. Here we present a long-term evolution code that enables a continuous calculation through an unlimited number of nova cycles for an unlimited evolution time, even up to 1.5 × 1010 yr. Starting with two sets of the three independent nova parameters , the white dwarf (WD) mass, the temperature of its isothermal core, and the rate of mass transfer on to it , we have followed the evolution of two models, with initial masses of 1 M, and 0.65 M, through over 1000 and over 3000 cycles, respectively. The accretion rate was assumed constant throughout each calculation: 10,11 M, yr,1 for the 1 M, WD, and 10,9 M, yr,1 for the 0.65 M, one. The initial temperatures were taken to be relatively high: 30 × 106 and 50 × 106 K, respectively, as they are likely to be at the onset of the outburst phase. The results show that although on the short-term consecutive outbursts are almost identical, on the long-term scale the characteristics change. This is mainly due to the changing core temperature, which decreases very similarly to that of a cooling WD for a time, but at a slower rate thereafter. As the WD's mass continually decreases, since both models lose more mass than they accrete, the central pressure decreases accordingly. The outbursts on the massive WD change gradually from fast to moderately fast, and the other characteristics (velocity, abundance ratios, isotopic ratios) change, too. Very slowly, a steady state is reached, where all characteristics, both in quiescence and in outburst, remain almost constant. For the less massive WD accreting at a high rate, outbursts are similar throughout the evolution. [source]

    A new interpretation of the remarkable X-ray spectrum of the symbiotic star CH Cyg

    Peter J. Wheatley
    ABSTRACT We have re-analysed the ASCA X-ray spectrum of the bright symbiotic star CH Cyg, which exhibits apparently distinct hard and soft X-ray components. Our analysis demonstrates that the soft X-ray emission can be interpreted as scattering of the hard X-ray component in a photoionized medium surrounding the white dwarf. This is in contrast to previous analyses in which the soft X-ray emission was fitted separately and assumed to arise independently of the hard X-ray component. We note the striking similarity between the X-ray spectra of CH Cyg and Seyfert 2 galaxies, which are also believed to exhibit scattering in a photoionized medium. [source]

    VW Hyi: optical spectroscopy and Doppler tomography

    Amanda J. Smith
    ABSTRACT We present high-quality optical spectroscopy of the SU UMa-subtype dwarf nova, VW Hyi taken while the system was in quiescence. An S-wave is executed by the emission cores of the hydrogen Balmer lines and by the emission lines of He i, Ca ii, Fe ii and He ii. Using Doppler tomography, we show it originates in the accretion stream,disc impact region. The He ii emission is strongly phase-dependent, suggesting it originates exclusively within a hot cavity at the initial impact. We map the ionization structure of the stream,disc interaction region. One possible interpretation of this is that the Balmer hotspot lies downstream of the He ii hotspot in the outer accretion disc, with the He i and Ca ii hotspots at intermediate locations between the two. This suggests that Balmer emission is suppressed until material has cooled somewhat downstream of the impact site and is able to recombine. We favour a phase offset of 0.15 ± 0.04 between the photometric ephemeris and inferior conjunction of the mass donor. The white dwarf contributes significantly to the optical continuum, with broad Balmer absorption and narrow Mg ii ,4481 absorption clearly apparent. This latter feature yields the gravitational redshift: vgrav= 38 ± 21 km s ,1, so M1= 0.71+0.18,0.26 M,. This implies M2= 0.11 ± 0.03 M, and hence the donor is not a brown dwarf. A prominent Balmer jump is also observed. We note that the previously accepted system parameters for both VW Hyi and WX Hyi incorporate an algebraic error, and we provide a recalculated M1(q) plane for WX Hyi. [source]

    The white dwarf in AE Aqr brakes harder

    Christopher W. Mauche
    ABSTRACT Taking advantage of the very precise de Jager et al. optical white dwarf orbit and spin ephemerides; ASCA, XMM,Newton and Chandra X-ray observations spread over 10 yr; and a cumulative 27-yr baseline, we have found that in recent years the white dwarf in AE Aqr is spinning down at a rate that is slightly faster than predicted by the de Jager et al. spin ephemeris. At the present time, the observed period evolution is consistent with either a cubic term in the spin ephemeris with , which is inconsistent in sign and magnitude with magnetic dipole radiation losses, or an additional quadratic term with , which is consistent with a modest increase in the accretion torques spinning down the white dwarf. Regular monitoring, in the optical, ultraviolet and/or X-rays, is required to track the evolution of the spin period of the white dwarf in AE Aqr. [source]

    Simultaneous ram pressure and tidal stripping; how dwarf spheroidals lost their gas

    Lucio Mayer
    ABSTRACT We perform high-resolution N -body+SPH (smoothed particle hydrodynamics) simulations of gas-rich dwarf galaxy satellites orbiting within a Milky Way-sized halo and study for the first time the combined effects of tides and ram pressure. The structure of the galaxy models and the orbital configurations are chosen in accordance with those expected in a Lambda cold dark matter (,CDM) universe. While tidal stirring of disky dwarfs produces objects whose stellar structure and kinematics resembles that of dwarf spheroidals after a few orbits, ram pressure stripping is needed to entirely remove their gas component. Gravitational tides can aid ram pressure stripping by diminishing the overall potential of the dwarf, but tides also induce bar formation which funnels gas inwards making subsequent stripping more difficult. This inflow is particularly effective when the gas can cool radiatively. Assuming a low density of the hot Galactic corona consistent with observational constraints, dwarfs with Vpeak < 30 km s,1 can be completely stripped of their gas content on orbits with pericenters of 50 kpc or less. Instead, dwarfs with more massive dark haloes and Vpeak > 30 km s,1 lose most or all of their gas content only if a heating source keeps the gas extended, partially counteracting the bar-driven inflow. We show that the ionizing radiation from the cosmic ultraviolet (UV) background at z > 2 can provide the required heating. In these objects, most of the gas is removed or becomes ionized at the first pericenter passage, explaining the early truncation of the star formation observed in Draco and Ursa Minor. Galaxies on orbits with larger pericenters and/or falling into the Milky Way halo at lower redshift can retain significant amounts of the centrally concentrated gas. These dwarfs would continue to form stars over a longer period of time, especially close to pericenter passages, as observed in Fornax and other dwarf spheroidal galaxies (dSphs) of the Local Group. The stripped gas breaks up into individual clouds pressure confined by the outer gaseous medium that have masses, sizes and densities comparable to the H i clouds recently discovered around M31. [source]

    A ZZ Ceti white dwarf in SDSS J133941.11+484727.5

    B. T. Gänsicke
    ABSTRACT We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSS J133941.11+484727.5 (SDSS 1339) which has been discovered in the Sloan Digital Sky Survey (SDSS) Data Release 4. The orbital period determined from radial velocity studies is 82.524(24) min, close to the observed period minimum. The optical spectrum of SDSS 1339 is dominated to 90 per cent by emission from the white dwarf (WD). The spectrum can be successfully reproduced by a three-component model (white dwarf, disc, secondary) with TWD=12 500 K for a fixed log g= 8.0, d= 170 pc, and a spectral type of the secondary later than M8. The mass-transfer rate corresponding to the optical luminosity of the accretion disc is very low, , 1.7 × 10,13 M, yr,1. Optical photometry reveals a coherent variability at 641 s with an amplitude of 0.025 mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photometric variation with a period of either 320 or 344 min and an amplitude of 0.025 mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photometric signals have been found in the CVs SDSS J123813.73,033933.0, SDSS J204817.85,061044.8, GW Lib and FS Aur, but so far no working model for this behaviour is available. [source]

    A non-main-sequence secondary in SY Cancri

    Robert Connon Smith
    ABSTRACT Simultaneous spectroscopic and photometric observations of the Z Cam type dwarf nova SY Cancri were used to obtain absolute flux calibrations. A comparison of the photometric calibration with a wide-slit spectrophotometric calibration showed that either method is equally satisfactory. A radial velocity study of the secondary star, made using the far-red Na i doublet, yielded a semi-amplitude of K2= 127 ± 23 km s,1. Taking the published value of 86 ± 9 km s,1 for K1 gives a mass ratio of q=M2/M1= 0.68 ± 0.14; this is very different from the value of 1.13 ± 0.35 quoted in the literature. Using the new lower mass ratio, and constraining the mass of the white dwarf to be within reasonable limits, then leads to a mass for the secondary star that is substantially less than would be expected for its orbital period if it satisfied a main-sequence mass,radius relationship. We find a spectral type of M0 that is consistent with that expected for a main-sequence star of the low mass we have found. However, in order to fill its Roche lobe, the secondary must be significantly larger than a main-sequence star of that mass and spectral type. The secondary is definitely not a normal main-sequence star. [source]

    Star cluster ecology , V. Dissection of an open star cluster: spectroscopy

    Simon F. Portegies Zwart
    ABSTRACT We have modelled in detail the evolution of rich open star clusters such as NGC 2516, NGC 2287, Pleiades, Praesepe, Hyades, NGC 2660 and 3680, using simulations that include stellar dynamics as well as the effects of stellar evolution. The dynamics is modelled via direct N -body integration, while the evolution of single stars and binaries is followed through the use of fitting formulae and recipes. The feedback of stellar and binary evolution on the dynamical evolution of the stellar system is taken into account self-consistently. Our model clusters dissolve in the tidal field of the Galaxy in a time-span of the order of a billion years. The rate of mass loss is rather constant, ,1 M, per million years. The binary fraction at first is nearly constant in time, then increases slowly near the end of a cluster's lifetime. For clusters which are more than about 108 yr old the fractions of stars in the form of binaries, giants and merger products in the inner few core radii are considerably higher than in the outer regions, beyond the cluster's half-mass radius. When stars with masses ,2 M, escape from the cluster, they tend to do so with velocities higher than average. The stellar merger rate in our models is roughly one per 30 million years. Most mergers are the result of unstable mass transfer in close binaries (,70 per cent), but a significant minority are caused by direct encounters between single and binary stars. While most mergers occur within the cluster core, even beyond the half-mass radius stellar mergers occasionally take place. We notice a significant birth rate of X-ray binaries, most containing a white dwarf as the mass acceptor. We also find one high-mass X-ray binary with a neutron-star accretor. If formed and retained, black holes participate in many (higher-order) encounters in the cluster centre, resulting in a large variety of exotic binaries. The persistent triple and higher-order systems formed in our models by dynamical encounters between binaries and single stars are not representative for the multiple systems observed in the Galactic disc. We conclude that the majority of multiples in the disc probably formed when the stars were born, rather than through later dynamical interactions. [source]

    Is the dark halo of our Galaxy spherical?

    Amina Helmi
    ABSTRACT It has been recently claimed that the confined structure of the debris from the Sagittarius dwarf implies that the dark matter halo of our Galaxy should be nearly spherical, in strong contrast with predictions from cold dark matter simulations, where dark haloes are found to have typical density axis ratios of 0.6,0.8. In this paper, numerical simulations are used to show that the Sagittarius streams discovered thus far are too young dynamically to be sensitive to the shape of the dark halo of the Milky Way. The data presently available are entirely consistent with a Galactic dark matter halo that could be either oblate or prolate, with minor-to-major density axis ratios as low as 0.6 within the region probed by the orbit of the Sagittarius dwarf. [source]

    Formation of , Centauri from an ancient nucleated dwarf galaxy in the young Galactic disc

    K. Bekki
    ABSTRACT We first present a self-consistent dynamical model in which , Cen is formed from an ancient nucleated dwarf galaxy merging with the first generation of the Galactic thin disc in a retrograde manner with respect to the Galactic rotation. Our numerical simulations demonstrate that during merging between the Galaxy and the , Cen host dwarf with MB,,14 mag and its nucleus mass of 107 M,, the outer stellar envelope of the dwarf is nearly completely stripped, whereas the central nucleus can survive from the tidal stripping because of its compactness. The developed naked nucleus has a very bound retrograde orbit around the young Galactic disc, as observed for , Cen, with apocentre and pericentre distances of ,8 and ,1 kpc, respectively. The Galactic tidal force can induce radial inflow of gas to the centre of the dwarf and consequently triggers moderately strong nuclear starbursts in a repetitive manner. This result implies that efficient nuclear chemical enrichment resulting from the later starbursts can be closely associated with the origin of the observed relatively young and metal-rich stars in , Cen. Dynamical heating by the , Cen host can transform the young thin disc into the thick disc during merging. [source]

    X-ray spectroscopy of the intermediate polar PQ Gem

    Cynthia H. James
    Abstract Using RXTE and ASCA data, we investigate the roles played by occultation and absorption in the X-ray spin pulse profile of the intermediate polar PQ Gem. From the X-ray light curves and phase-resolved spectroscopy, we find that the intensity variations are the result of a combination of varying degrees of absorption and the accretion regions rotating behind the visible face of the white dwarf. These occultation and absorption effects are consistent with those expected from the accretion structures calculated from optical polarization data. We can reproduce the changes in absorber covering fraction either from geometrical effects, or by considering that the material in the leading edge of the accretion curtain is more finely fragmented than in other parts of the curtain. We determine a white dwarf mass of , 1.2 using the RXTE data. [source]

    On the evolution of the nova-like variable AE Aquarii

    P. J. Meintjes
    Abstract A possible evolution for the enigmatic cataclysmic variable AE Aquarii is considered that may put into context the long orbital period and short white dwarf rotation period compared with other DQ Her systems. It has been shown that mass transfer could have been initiated when the secondary KIV,V star was already somewhat evolved when it established Roche lobe contact. In this initial phase the orbital period of the system was probably Porb,i, 8.5 h, and the white dwarf rotation period P*,i > 1 h. Mass transfer in the form of diamagnetic gas blobs will result in an initial discless accretion process, resulting in an efficient drain of the binary orbital angular momentum. Since the initial mass ratio of the binary was probably qi, 0.8, a high mass transfer rate and a slow expansion of the Roche lobe of the secondary star followed, accompanied by a fast expanding secondary following the mass loss. This could have resulted in the KIV,V secondary flooding its Roche surface, causing a run-away mass transfer of that lasted for approximately , during which time the binary expanded to an orbital period of approximately Porb, 11 h. During this phase the mass accretion rate on to the surface of the white dwarf most probably exceeded the critical value for stable nuclear burning , which could have resulted in AE Aqr turning into an ultrasoft X-ray source. The high mass transfer terminated when a critical mass ratio of qcrit= 0.73 was reached. Disc torques spun-up the white dwarf to a period close to 33 s within the time-scale before the high mass transfer shut down when qcrit was reached. The decrease in the mass loss of the secondary allowed it to re-establish hydrostatic equilibrium on the dynamical time-scale (fraction of a day). From this point when qcrit is reached the mass transfer and binary evolution proceed at a slower rate since mass transfer from the secondary star is driven by magnetic braking of the secondary on a time-scale , which is the same as the thermal time-scale tth, 6.3 × 107 yr, i.e. the time-scale on which the secondary shrinks to restore its perturbed thermal equilibrium after the high mass loss. The significantly lower mass transfer in this phase will result in mass ejection from the system. This propeller,ejector action erodes the rotational kinetic energy of the white dwarf, channelling it into mass ejection and non-thermal activity, which explains the non-thermal outbursts that are observed at radio wavelengths and occasionally also at TeV energies. [source]

    The mass of the white dwarf in the recurrent nova U Scorpii

    T.D. Thoroughgood
    We present spectroscopy of the eclipsing recurrent nova U Sco. The radial velocity semi-amplitude of the primary star was found to be from the motion of the wings of the He ii,4686-Å emission line. By detecting weak absorption features from the secondary star, we find its radial velocity semi-amplitude to be . From these parameters, we obtain a mass of for the white dwarf primary star and a mass of for the secondary star. The radius of the secondary is calculated to be , confirming that it is evolved. The inclination of the system is calculated to be , consistent with the deep eclipse seen in the light-curves. The helium emission lines are double-peaked, with the blueshifted regions of the disc being eclipsed prior to the redshifted regions, clearly indicating the presence of an accretion disc. The high mass of the white dwarf is consistent with the thermonuclear runaway model of recurrent nova outbursts, and confirms that U Sco is the best Type Ia supernova progenitor currently known. We predict that U Sco is likely to explode within ,700 000 yr. [source]

    Velocity dispersions of dwarf spheroidal galaxies: dark matter versus MOND

    Ewa L.
    We present predictions for the line-of-sight velocity dispersion profiles of dwarf spheroidal galaxies and compare them to observations in the case of the Fornax dwarf. The predictions are made in the framework of standard dynamical theory of spherical systems with different velocity distributions. The stars are assumed to be distributed according to Sérsic laws with parameters fitted to observations. We compare predictions obtained assuming the presence of dark matter haloes (with density profiles adopted from N -body simulations) with those resulting from Modified Newtonian Dynamics (MOND). If the anisotropy of velocity distribution is treated as a free parameter, observational data for Fornax are reproduced equally well by models with dark matter and with MOND. If stellar mass-to-light ratio of 1 M,/L, is assumed, the required mass of the dark halo is , two orders of magnitude larger than the mass in stars. The derived MOND acceleration scale is . In both cases a certain amount of tangential anisotropy in the velocity distribution is needed to reproduce the shape of the velocity dispersion profile in Fornax. [source]

    Formation of blue compact dwarf galaxies from merging and interacting gas-rich dwarfs

    Kenji Bekki
    ABSTRACT We present the results of numerical simulations which show the formation of blue compact dwarf (BCD) galaxies from merging between very gas-rich dwarfs with extended H i gas discs. We show that dwarf,dwarf merging can trigger central starbursts and form massive compact cores dominated by young stellar populations. We also show that the pre-existing old stellar components in merger precursor dwarfs can become diffuse low surface brightness components after merging. The compact cores dominated by younger stellar populations and embedded in more diffusely distributed older ones can be morphologically classified as BCDs. Since new stars can be formed from gas transferred from the outer part of the extended gas discs of merger precursors, new stars can be very metal-poor ([Fe/H] < ,1). Owing to very high gaseous pressure exceeding 105 kB (where kB is the Boltzmann constant) during merging, compact star clusters can be formed in forming BCDs. The BCDs formed from merging can still have extended H i gas discs surrounding their blue compact cores. We discuss whether tidal interaction of gas-rich dwarfs without merging can also form BCDs. [source]

    Mitigation of establishment of Brassica napus transgenes in volunteers using a tandem construct containing a selectively unfit gene

    Hani Al-Ahmad
    Summary Transgenic oilseed rape (Brassica napus) plants may remain as ,volunteer' weeds in following crops, complicating cultivation and contaminating crop yield. Volunteers can become feral as well as act as a genetic bridge for the transfer of transgenes to weedy relatives. Transgenic mitigation using genes that are positive or neutral to the crop, but deleterious to weeds, should prevent volunteer establishment, as previously intimated using a tobacco (Nicotiana tabacum) model. A transgenically mitigated (TM), dwarf, herbicide-resistant construct using a gibberellic acid-insensitive (,gai) gene in the B. napus crop was effective in offsetting the risks of transgene establishment in volunteer populations of B. napus. This may be useful in the absence of herbicide, e.g. when wheat is rotated with oilseed rape. The TM dwarf B. napus plants grown alone had a much higher yield than the non-transgenics, but were exceedingly unfit in competition with non-transgenic tall cohorts. The reproductive fitness of TM B. napus was 0% at 2.5-cm and 4% at 5-cm spacing between glasshouse-grown plants relative to non-transgenic B. napus. Under screen-house conditions, the reproductive fitness of TM B. napus relative to non-transgenic B. napus was less than 12%, and the harvest index of the TM plants was less than 40% of that of the non-transgenic competitors. The data clearly indicate that the ,gai gene greatly enhances the yield in a weed-free transgenic crop, but the dwarf plants can be eliminated when competing with non-transgenic cohorts (and presumably other species) when the selective herbicide is not used. [source]

    Flores hominid: New species or microcephalic dwarf?

    Robert D. Martin
    Abstract The proposed new hominid "Homo floresiensis" is based on specimens from cave deposits on the Indonesian island Flores. The primary evidence, dated at , 18,000 y, is a skull and partial skeleton of a very small but dentally adult individual (LB1). Incomplete specimens are attributed to eight additional individuals. Stone tools at the site are also attributed to H. floresiensis. The discoverers interpreted H. floresiensis as an insular dwarf derived from Homo erectus, but others see LB1 as a small-bodied microcephalic Homo sapiens. Study of virtual endocasts, including LB1 and a European microcephalic, purportedly excluded microcephaly, but reconsideration reveals several problems. The cranial capacity of LB1 (, 400 cc) is smaller than in any other known hominid < 3.5 Ma and is far too small to derive from Homo erectus by normal dwarfing. By contrast, some associated tools were generated with a prepared-core technique previously unknown for H. erectus, including bladelets otherwise associated exclusively with H. sapiens. The single European microcephalic skull used in comparing virtual endocasts was particularly unsuitable. The specimen was a cast, not the original skull (traced to Stuttgart), from a 10-year-old child with massive pathology. Moreover, the calotte does not fit well with the rest of the cast, probably being a later addition of unknown history. Consideration of various forms of human microcephaly and of two adult specimens indicates that LB1 could well be a microcephalic Homo sapiens. This is the most likely explanation for the incongruous association of a small-brained recent hominid with advanced stone tools. Anat Rec Part A, 288A:1123,1145, 2006. © 2006 Wiley-Liss, Inc. [source]

    Expanding atmosphere models for SSS spectra of novae

    D.R. van Rossum
    Abstract Super Soft Source (SSS) spectra are powered by nuclear burning on the surface of a white dwarf. The released energy causes a radiatively-driven wind that leads to a radially extended atmosphere around the white dwarf. Significant blue shifts in photospheric absorption lines are found in the spectra of novae during their SSS phase, being an evidence of continued mass loss in this phase. We present spherically symmetric PHOENIX models that account for the expansion of the ejecta. A comparison to a plane parallel, hydrostatic atmosphere model demonstrates that the mass loss can have a significant impact on the model spectra. The dynamic model yields less pronounced absorption edges, and harder X-ray spectra are the result. Therefore, lower effective temperatures are needed to explain the observed spectra. Although both types of models are yet to be fine-tuned in order to accurately determine best fit parameters, the implications on the chemical abundances are going in opposite directions. With the expanding models the requirement for strong depletion of the crucial elements that cause these edges is now avoidable (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]