Home About us Contact
|
Home About us Contact | ||
|
|
||
Dwarf Companions (dwarf + companion)
Kinds of Dwarf Companions Selected AbstractsThe dust-free symbiotic Mira K4,46 = LL CasMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2010U. 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] Near-infrared spectroscopy of the very low mass companion to the hot DA white dwarf PG 1234+482MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007P. 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] On the properties of young multiple starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004E. J. Delgado-Donate ABSTRACT We present numerical results on the properties of young binary and multiple stellar systems. Our analysis is based on a series of smoothed particle hydrodynamics (SPH) +N -body simulations of the fragmentation of small molecular clouds, which fully resolve the opacity limit for fragmentation. These simulations demonstrate that multiple star formation is a major channel for star formation in turbulent flows. We have produced a statistically significant number of stable multiple systems, with component separations in the range ,1,103 au. At the end of the hydrodynamic stage (0.5 Myr), we find that ,60 per cent of stars and brown dwarfs are members of multiples systems, with about a third of these being low-mass, weakly bound outliers in wide eccentric orbits. Our results imply that in the stellar regime most stars are in multiples (,80 per cent) and that this fraction is an increasing function of primary mass. After N -body integration to 10.5 Myr, the percentage of bound objects has dropped to about 40 per cent, this decrease arising mostly from very low-mass stars and brown dwarfs that have been released into the field. Brown dwarfs are never found to be very close companions to stars (the brown dwarf desert at very small separations), but one case exists of a brown dwarf companion at intermediate separations (10 au). Our simulations can accommodate the existence of brown dwarf companions at large separations, but only if the primaries of these systems are themselves multiples. We have compared the outcome of our simulations with the properties of real stellar systems as deduced from the infrared colour,magnitude diagram of the Praesepe cluster and from spectroscopic and high-resolution imaging surveys of young clusters and the field. We find that the spread of the observed main sequence of Praesepe in the 0.4,1 M, range appears to require that stars are indeed commonly assembled into high-order multiple systems. Similarly, observational results from Taurus and , Ophiuchus, or moving groups such as TW Hydrae and MBM 12, suggest that companion frequencies in young systems can indeed be as high as we predict. The comparison with observational data also illustrates two problems with the simulation results. First, low mass ratio (q < 0.2) binaries are not produced by our models, in conflict with both the Praesepe colour,magnitude diagram and independent evidence from field binary surveys. Secondly, very low-mass stars and brown dwarf binaries appear to be considerably underproduced by our simulations. [source] Editorial: Unexpected pleasure , Earthquake hits Hawaii , 25 years of the Herschel Museum , Giant radio rings round Abell 3376 , First brown dwarf companion , How low can you go?ASTRONOMY & GEOPHYSICS, Issue 6 2006Kavli Institute for Cambridge, Letters: Big breakthroughs of the 20th century, Meteor, astronomer from Keele' writes., not shower, over Bala Click HERE to view article. [source] Full evolution of low-mass white dwarfs with helium and oxygen coresMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007J. A. Panei ABSTRACT We study the full evolution of low-mass white dwarfs with helium and oxygen cores. We revisit the age dichotomy observed in many white dwarf companions to millisecond pulsar on the basis of white dwarf configurations derived from binary evolution computations. We evolve 11 dwarf sequences for helium cores with final masses of 0.1604, 0.1869, 0.2026, 0.2495, 0.3056, 0.3333, 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M,. In addition, we compute the evolution of five sequences for oxygen cores with final masses of 0.3515, 0.3844, 0.3986, 0.4160 and 0.4481 M,. A metallicity of Z= 0.02 is assumed. Gravitational settling, chemical and thermal diffusion are accounted for during the white dwarf regime. Our study reinforces the result that diffusion processes are a key ingredient in explaining the observed age and envelope dichotomy in low-mass helium-core white dwarfs, a conclusion we arrived at earlier on the basis of a simplified treatment for the binary evolution of progenitor stars. We determine the mass threshold where the age dichotomy occurs. For the oxygen white dwarf sequences, we report the occurrence of diffusion-induced, hydrogen-shell flashes, which, as in the case of their helium counterparts, strongly influence the late stages of white dwarf cooling. Finally, we present our results as a set of white dwarf mass,radius relations for helium and oxygen cores. [source] The dynamical formation of LMXBs in dense stellar environments: globular clusters and the inner bulge of M31MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007R. Voss ABSTRACT The radial distribution of luminous (LX > 1036 erg s,1) X-ray point sources in the bulge of M31 is investigated using archival Chandra observations. We find a significant increase in the specific frequency of X-ray sources, per unit stellar mass, within 1 arcmin from the centre of the galaxy. The radial distribution of surplus sources in this region follows the ,2* law, suggesting that they are low-mass X-ray binaries (LMXBs) formed dynamically in the dense inner bulge. We investigate dynamical formation of LMXBs, paying particular attention to the high-velocity regime characteristic for galactic bulges, which has not been explored previously. Our calculations suggest that the majority of the surplus sources are formed in tidal captures of black holes by main-sequence stars of low mass, M*, 0.3,0.4 M,, with some contribution of neutron star (NS) systems of same type. Due to the small size of the accretion discs, a fraction of such systems may be persistent X-ray sources. Some of the sources may be ultracompact X-ray binaries with helium star/white dwarf companions. We also predict a large number of faint transients, both NS and BH systems, within ,1 arcmin from the M31 galactic centre. Finally, we consider the population of dynamically formed binaries in Galactic globular clusters, emphasizing the differences between these two types of stellar environments. [source] New photometric and spectroscopic observations of the Seyfert galaxy Mrk 315MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005S. Ciroi ABSTRACT We present new important results about the intermediate-type Seyfert galaxy Mrk 315, recently observed through optical imaging and integral-field spectroscopy. Broad-band images were used to study the morphology of the host galaxy, narrow-band H, images to trace the star-forming regions, and middle-band [O iii] images to evidence the distribution of the highly ionized gas. Some extended emission regions were isolated and their physical properties studied by means of flux-calibrated spectra. High-resolution spectroscopy was used to separate different kinematic components in the velocity fields of gas and stars. Some peculiar features characterize this apparently undisturbed and moderately isolated active galaxy. Such features, already investigated by other authors, are re-analysed and discussed in the light of these new observations. The most relevant results we obtained are: the multitiers structure of the disc; the presence of a quasi-ring of regions with star formation much higher than previous claims; a secondary nucleus confirmed by a stellar component kinematically decoupled by the main galaxy; a new hypothesis about the controversial nature of the long filament, initially described as hook shaped, and more likely made of two independent filaments caused by interaction events between the main galaxy and two dwarf companions. [source] On the properties of young multiple starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004E. J. Delgado-Donate ABSTRACT We present numerical results on the properties of young binary and multiple stellar systems. Our analysis is based on a series of smoothed particle hydrodynamics (SPH) +N -body simulations of the fragmentation of small molecular clouds, which fully resolve the opacity limit for fragmentation. These simulations demonstrate that multiple star formation is a major channel for star formation in turbulent flows. We have produced a statistically significant number of stable multiple systems, with component separations in the range ,1,103 au. At the end of the hydrodynamic stage (0.5 Myr), we find that ,60 per cent of stars and brown dwarfs are members of multiples systems, with about a third of these being low-mass, weakly bound outliers in wide eccentric orbits. Our results imply that in the stellar regime most stars are in multiples (,80 per cent) and that this fraction is an increasing function of primary mass. After N -body integration to 10.5 Myr, the percentage of bound objects has dropped to about 40 per cent, this decrease arising mostly from very low-mass stars and brown dwarfs that have been released into the field. Brown dwarfs are never found to be very close companions to stars (the brown dwarf desert at very small separations), but one case exists of a brown dwarf companion at intermediate separations (10 au). Our simulations can accommodate the existence of brown dwarf companions at large separations, but only if the primaries of these systems are themselves multiples. We have compared the outcome of our simulations with the properties of real stellar systems as deduced from the infrared colour,magnitude diagram of the Praesepe cluster and from spectroscopic and high-resolution imaging surveys of young clusters and the field. We find that the spread of the observed main sequence of Praesepe in the 0.4,1 M, range appears to require that stars are indeed commonly assembled into high-order multiple systems. Similarly, observational results from Taurus and , Ophiuchus, or moving groups such as TW Hydrae and MBM 12, suggest that companion frequencies in young systems can indeed be as high as we predict. The comparison with observational data also illustrates two problems with the simulation results. First, low mass ratio (q < 0.2) binaries are not produced by our models, in conflict with both the Praesepe colour,magnitude diagram and independent evidence from field binary surveys. Secondly, very low-mass stars and brown dwarf binaries appear to be considerably underproduced by our simulations. [source] Evolution of dwarf galaxies in the Centaurus A groupASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009D. Crnojevi Abstract The study of (dwarf) galaxies in nearby groups is one among the most powerful tools that can be used to investigate galaxy evolution, chemical enrichment and environmental effects on these objects. The Centaurus A group (at a distance of ,4 Mpc) is dynamically evolved and contains about 30 dwarf companions of different morphologies and stellar contents. Here we present the first results for the dwarf spheroidal galaxy population in this group. We use archival HST/ACS data to study their resolved stellar content. The resulting metallicity distribution functions reveal metal-poor populations and wide metallicity spreads for each galaxy. We find no clear trend of the derived physical properties as a function of galaxy position in the group. Finally, we compare our results to the dwarf population of the Local Group, and find no outstanding differences (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||