The Black Hole Binary Nova Scorpii 1994 (GRO J1655-40): An improved chemical analysis

The chemical analysis of secondary stars of low mass X-ray binaries provides an opportunity to study the formation processes of compact objects, either black holes or neutron stars. Following the discovery of overabundances of $\alpha$-elements in the HIRES/Keck spectrum of the secondary star of Nova Scorpii 1994 (Israelian et al. 1999), we obtained UVES/VLT high-resolution spectroscopy with the aim of performing a detailed abundance analysis of this secondary star. Using a $\chi2$-minimization procedure and a grid of synthetic spectra, we derive the stellar parameters and atmospheric abundances of O, Mg, Al, Ca, Ti, Fe and Ni, using a new UVES spectrum and the HIRES spectrum.The abundances of Al, Ca, Ti, Fe and Ni seem to be consistent with solar values, whereas Na, and especially O, Mg, Si and S are significantly enhanced in comparison with Galactic trends of these elements. A comparison with spherically and non-spherically symmetric supernova explosion models may provide stringent constraints to the model parameters as mass-cut and the explosion energy, in particular from the relative abundances of Si, S, Ca, Ti, Fe and Ni. Most probably the black hole in this system formed in a hypernova explosion of a 30--35 \Msun progenitor star with a mass-cut in the range 2--3.5 \Msun. However, these models produce abundances of Al and Na almost ten times higher than the observed values.Comment: New Accepted version for publication in Astronomy and Astrophysics Table 2: Correcte

The Formation of a Star Cluster: Predicting the Properties of Stars and Brown Dwarfs

We present results from the largest numerical simulation of star formation to resolve the fragmentation process down to the opacity limit. The simulation follows the collapse and fragmentation of a large-scale turbulent molecular cloud to form a stellar cluster and, simultaneously, the formation of circumstellar discs and binary stars. This large range of scales enables us to predict a wide variety of stellar properties for comparison with observations. The calculation clearly demonstrates that star formation is a highly-dynamic and chaotic process. Star-disc encounters form binaries and truncate discs. Stellar encounters disrupt bound multiple systems. The cloud produces roughly equal numbers of stars and brown dwarfs, with masses down to the opacity limit for fragmentation (~5 Jupiter masses). The initial mass function is consistent with a Salpeter slope (Gamma=-1.35) above 0.5 Msun, a roughly flat distribution (Gamma=0) in the range 0.006-0.5 Msun, and a sharp cutoff below ~0.005 Msun. This is consistent with recent observational surveys. The brown dwarfs form by the dynamical ejection of low-mass fragments from dynamically unstable multiple systems before the fragments have been able to accrete to stellar masses. Close binary systems (with separations <10 AU) are not formed by fragmentation in situ. Rather, they are produced by hardening of initially wider multiple systems through a combination of dynamical encounters, gas accretion, and/or the interaction with circumbinary and circumtriple discs. Finally, we find that the majority of circumstellar discs have radii less than 20 AU due to truncation in dynamical encounters. This is consistent with observations of the Orion Trapezium Cluster and implies that most stars and brown dwarfs do not form large planetary systems.Comment: Accepted by MNRAS, 27 pages, 15 figures (7 degraded). Paper with high-resolution figures and animations available at http://www.astro.ex.ac.uk/people/mbate

Brown Dwarfs in the Pleiades Cluster. III. A deep IZ survey

We present the results of a deep CCD-based IZ photometric survey of a ~1 sq. deg area in the central region of the Pleiades Galactic open cluster. The magnitude coverage of our survey (from I~17.5 down to 22) allows us to detect substellar candidates with masses between 0.075 and 0.03 Msol. Details of the photometric reduction and selection criteria are given. Finder charts prepared from the I-band images are provided.Comment: 11 pages with 8 figures, 4 of them are finder charts given in gif format. Accepted for publication in A&AS. Also available at http://www.iac.es/publicaciones/preprints.htm

Lithium Production in Companions of Accreting X-Ray Binaries by Neutron Spallation of C,N,O Elements

We examine the processes which could lead to the observed enhancement of Li and possibly other light elements (Be, B) in the companions of a number of X-ray novae. We conclude that one of the most promising mechanisms is the spallation of CNO elements on the surface of the companion induced by the neutron flux produced in the hot accretion flow onto the compact object. Direct production of the observed Li and its deposition onto the dwarf companion seem less likely, mainly because of the possibility of its destruction in the production region itself and difficulties in its deposition associated with the configuration of the companion's magnetic field. We discuss other potential observables of the above scenario.Comment: 23 pages Latex, of which 5 pages of tables, to appear in the Astrophysical Journal, Vol 512, Feb 10 issu

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

Whether BDs form as stars through gravitational collapse ("star-like") or BDs and some very low-mass stars constitute a separate population which form alongside stars comparable to the population of planets, e.g. through circumstellar disk ("peripheral") fragmentation, is one of the key questions of the star-formation problem. For young stars in Taurus-Auriga the binary fraction is large with little dependence on primary mass above ~0.2Msun, while for BDs it is <10%. We investigate a case in which BDs in Taurus formed dominantly through peripheral fragmentation. The decline of the binary frequency in the transition region between star-like and peripheral formation is modelled. A dynamical population synthesis model is employed in which stellar binary formation is universal. Peripheral objects form separately in circumstellar disks with a distinctive initial mass function (IMF), own orbital parameter distributions for binaries and a low binary fraction. A small amount of dynamical processing of the stellar component is accounted for as appropriate for the low-density Taurus-Auriga embedded clusters. The binary fraction declines strongly between the mass-limits for star-like and peripheral formation. The location of characteristic features and the steepness depend on these mass-limits. Such a trend might be unique to low density regions hosting dynamically unprocessed binary populations. The existence of a strong decline in the binary fraction -- primary mass diagram will become verifiable in future surveys on BD and VLMS binarity in the Taurus-Auriga star forming region. It is a test of the (non-)continuity of star formation along the mass-scale, the separateness of the stellar and BD populations and the dominant formation channel for BDs and BD binaries in regions of low stellar density hosting dynamically unprocessed populations.Comment: accepted for publication in A&A, 11 pages, 4 figures, 1 tabl