arXiv subjects

H. M. J. Boffin

Publications and source records attributed to H. M. J. Boffin.

87 records · Page 5Linked to original sources

arXiv2004

Wind accretion in binary stars - I. Mass accretion ratio

Three-dimensional hydrodynamic calculations are performed in order to investigate mass transfer in a close binary system, in which one component undergoes mass loss through a wind. The mass ratio is assumed to be unity. The radius of the mass-losing star is taken to be about a quarter of the separation between the two stars. Calculations are performed for gases with a ratio of specific heats gamma=1.01 and 5/3. Mass loss is assumed to be thermally driven so that the other parameter is the sound speed of the gas on the mass-losing star. Here, we focus our attention on two features: flow patterns and mass accretion ratio, which we define as the ratio of the mass accretion rate onto the companion to the mass loss rate from the mass-losing primary star. We characterize the flow by the mean normal velocity of wind on the critical Roche surface of the mass-losing star, Vr. When Vr<0.4 A Omega, where A and Omega are the separation between the two stars and the angular orbital frequency of the binary, respectively, we obtain Roche-lobe over-flow (RLOF), while for Vr>0.7 A Omega we observe wind accretion. We find very complex flow patterns in between these two extreme cases. We derive an empirical formula of the mass accretion ratio in the low and in the high velocity regime.

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arXiv2003

Time-resolved photometry of cataclysmic variables

We present time-resolved photometry of two cataclysmic variables whose CCD photometric observations were obtained with the 1m telescope at the South African Astronomical Observatory in October 2002 and August 2003 and with the 1m telescope at Hoher List in Germany. Concerning MCT 2347-3144 we detect for the first time a period of 6.65h. For V1193 Ori the 3.96 h periodicity has for the first time been confirmed through time-resolved photometry.

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arXiv2003

Spectroscopic Binary Orbits from Photoelectric Radial Velocities -- Paper 171: HD 152028 and Hde 284195 (GK Draconis and V1094 Tauri)

The two stars that form the subject of this paper are both short-period double- lined eclipsing binaries having non-circular orbits despite their short periods. Although the HD type of HD 152028 is G0, the integrated spectral type of the system must actually be much earlier: the B-V colour index is only about 0.37 mag and the parallax indicates an integrated absolute magnitude as bright as +1.4 mag. A published photometric investigation suggests that the primary star exhibits delta Scuti pulsations, with a period of 0.1138 days. That period is not present in the radial velocities, but we have identified a comparable periodicity in the initially excessive residuals (sigma of the order of 2 km/s) in the radial velocities of the primary star: there is an asymmetrical pulsational velocity curve with a semi-amplitude of about 3 km/s and a period of 0.1178 days. HDE 284195 was not observed by Hipparcos, but its HDE type of G0 is in reasonable agreement with its colour and the nature of its radial-velocity traces. The rotations of both stars appear to be pseudo-synchronized to the orbit. The orbital inclinations are not formally determined but they must be very high,however and there is specific evidence that the inclination of HD 152028 is very close to 90 degrees,so it is permissible to assume that the masses are scarcely above the minimum values, which in the case of HD 152028 are 1.78 and 1.42 solar mass and in that of HDE 284195 are 1.10 and 1.01 solar mass, with uncertainties below 1%.(ABRIDGED)

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arXiv2002

Reprocessing the Hipparcos Intermediate Astrometric Data of spectroscopic binaries: II. Systems with a giant component

By reanalyzing the Hipparcos Intermediate Astrometric Data of a large sample of spectroscopic binaries containing a giant, we obtain a sample of 29 systems fulfilling a carefully derived set of constraints and hence for which we can derive an accurate orbital solution. Of these, one is a double-lined spectroscopic binary and six were not listed in the DMSA/O section of the catalogue. Using our solutions, we derive the masses of the components in these systems and statistically analyze them. We also briefly discuss each system individually.

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arXiv2002

A warped accretion disc in PX And ?

We have undertaken a photometric study of the SW Sex star, PX And. We clearly identify a negative superhump signal which might be regarded as the signature of a nodal precessing disc, possibly warped. PX And is also observed to possess highly variable eclipse depth and we discuss two possible explanations.

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arXiv2002

PX Andromedae: Superhumps and variable eclipse depth

Results of a photometric study of the SW Sex novalike PX And are presented. The periodogram analysis of the observations obtained in October 2000 reveals the presence of three signals with periods of 0.142, 4.8 and 0.207 days. The first two periods are recognized as "negative superhumps" and the corresponding retrograde precession period of the accretion disk. The origin of the third periodic signal remains unknown. The observations in September-October 2001 point only to the presence of "negative superhumps" and possibly to the precession period. The origin of the "negative superhumps" is discussed and two possible mechanisms are suggested. All light curves show strong flickering activity and power spectra with a typical red noise shape. PX And shows eclipses with highly variable shape and depth. The analysis suggests that the eclipse depth is modulated with the precession period and two possible explanations of this phenomenon are discussed. An improved orbital ephemeris is also determined: T_min[HJD]=49238.83662(14)+0.146352739(11)E.

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arXiv2001

Spiral waves in accretion discs

In the first part of this article, we review the observational evidence for spirals in the accretion discs of cataclysmic variables. It is shown that with the increasing amount of data available, spirals appear to be an omnipresent feature of accretion discs in outburst. Spirals seem to live until decline that is, for several tens of orbital periods. We then study the formation of spiral shocks from a theoretical side, using the results of various numerical simulations. We make a comparison between observations and theory and briefly discuss the implications of the presence of spirals in the discs of cataclysmic variables.

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arXiv2000

IY UMa: accretion disc evolution after superoutburst

CCD photometry of the newly discovered eclipsing dwarf nova IY UMa is used to study the physical properties of the accretion disc in the late decline and quiescence stages. Eclipse mapping analysis shows that in these stages the accretion disc is cool with approximately flat radial brightness temperature distribution and T_BR about 5000-5500 K. The hot spot is found to lie close along the stream trajectory at distance 0.36a and 0.25a from the disc center in February and March. The orbital hump strength decreases by about 30 % in a month. Taking the smallest possible size of the hot spot, we derive an upper limit for its brightness temperature, 15900 K and 13800 K, respectively.

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arXiv2000

Numerical simulation of accretion discs in close binary systems and discovery of spiral shocks

The history of hydrodynamic numerical simulations for accretion disks in close binary systems is reviewed, in which emphasis is placed, in particular, on the facts that spiral shock waves were numerically found in 1986 by researchers including one of the present authors and that spiral structure was discovered in IP Pegasi in 1997 by Steeghs et al. The results of our two and three-dimensional numerical simulations in recent years are then summarized, with comparison being made with observations.

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arXiv2000

Delta Scuti Stars in Stellar Systems: a Review

We present a list of delta Scuti stars in double and multiple systems, ranging from the very wide binaries to the very close ones such as spectroscopic and eclipsing systems including the optical visual pairs which are of no further use here. Our aim is to group the information from the binarity on the one hand and the pulsational characteristics on the other hand for as complete a sample as possible of delta Scuti stars in stellar systems. A selection of 18 well-documented cases, taking care that every type of binary is being represented, is discussed more extensively.

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arXiv1998

The structure of young star clusters

We analyse and compare the clustering of young stars in Chamaeleon I and Taurus. We compute the mean surface-density of companion stars \bar{N} as a function of angular displacement θfrom each star. We then fit \bar{N}(θ) with two simultaneous power laws. For Chamaeleon I, the exponents of the power laws are 1.97 and 0.28, with the elbow at ~0.011 degrees. For Taurus, we obtain 2.02 and 0.87, with the elbow at ~0.013 deg. For both star clusters the observational data make quite large systematic excursions from the best fitting curve in the binary regime. These excursions may be attributable to evolutionary effects of the types discussed recently by Nakajima et al. and Bate et al. In the clustering regime the data conform to the best fitting curve very well. We also calculate the box-dimensions for the two star clusters. However, the limited dynamic range makes these estimates simply descriptors of the large-scale clustering, and not admissible evidence for fractality. We also propose two algorithms for objectively generating maps of constant stellar surface-density in young star clusters. Such maps are useful for comparison with molecular-line and dust-continuum maps of star-forming clouds, and with the results of numerical simulations. These maps retain information which is suppressed in the evaluation of \bar{N}(θ). Analyzing such maps can be used to discriminate between fractal structure and single-level clustering, and to determine the degree of central condensation in small-N clusters. Algorithm I uses a universal smoothing length, and therefore has a restricted dynamic range, but it is implicitly normalized. Algorithm II uses a local smoothing length, which gives it much greater dynamic range, but it has to be normalized explicitly.

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arXiv1998

Numerical simulations of protostellar encounters I. Star-disc encounters

It appears that most stars are born in clusters, and that at birth most stars have circumstellar discs which are comparable in size to the separations between the stars. Interactions between neighbouring stars and discs are therefore likely to play a key role in determining disc lifetimes, stellar masses, and the separations and eccentricities of binary orbits. Such interactions may also cause fragmentation of the discs, thereby triggering the formation of additional stars. We have carried out a series of simulations of disc-star interactions using an SPH code which treats self-gravity, hydrodynamic and viscous forces. We find that interactions between discs and stars provide a mechanism for removing energy from, or adding energy to, the orbits of the stars, and for truncating the discs. However, capture during such encounters is unlikely to be an important binary formation mechanism. A more significant consequence of such encounters is that they can trigger fragmentation of the disc, via tidally and compressionally induced gravitational instabilities, leading to the formation of additional stars. When the disc-spins and stellar orbits are randomly oriented, encounters lead to the formation of new companions to the original star in 20% of encounters. If most encounters are prograde and coplanar, as suggested by simulations of dynamically-triggered star formation, then new companions are formed in approximately 50% of encounters.

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arXiv1998

Numerical simulations of protostellar encounters II. Coplanar disc-disc encounters

It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. Such interactions must have a significant impact upon the evolution of the protostars and their discs. We have carried out a series of simulations of coplanar encounters between two stars, each possessing a massive circumstellar disc, using an SPH code that models gravitational, hydrodynamic and viscous forces. We find that during a coplanar encounter, disc material is swept up into a shock layer between the two interacting stars, and the layer then fragments to produce new protostellar condensations. The truncated remains of the discs may subsequently fragment; and the outer regions of the discs may be thrown off to form circumbinary disc-like structures around the stars. Thus coplanar disc-disc encounters lead efficiently to the formation of multiple star systems and small-N clusters.

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arXiv1998

Numerical simulations of protostellar encounters III. Non-coplanar disc-disc encounters

It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. If protostars are formed individually within a cluster before falling together and interacting, there should be no preferred orientation for such interactions. As star formation within clusters is believed to be coeval, it is probable that during interactions, both protostars possess massive, extended discs. We have used an SPH code to carry out a series of simulations of non-colpanar disc-disc interactions. We find that non-coplanar interactions trigger gravitational instabilities in the discs, which may then fragment to form new companions to the existing stars. (This is different from coplanar interactions, in which most of the new companion stars form after material in the discs has been swept up into a shock layer, and this then fragments.) The original stars may also capture each other, leading to the formation of a small-N cluster. If every star undergoes a randomly oriented disc-disc interaction, then the outcome will be the birth of many new stars. Approximately two-thirds of the stars will end up in multiple systems.

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arXiv1998

Gas-cooling by dust during dynamical fragmentation

We suggest that the abrupt switch, from hierarchical clustering on scales larger than 0.04 pc, to binary (and occasionally higher multiple) systems on smaller scales, which Larson has deduced from his analysis of the grouping of pre-Main-Sequence stars in Taurus, arises because pre-protostellar gas becomes thermally coupled to dust at sufficiently high densities. The resulting change from gas-cooling by molecular lines at low densities to gas-cooling by dust at high densities enables the matter to radiate much more efficiently, and hence to undergo dynamical fragmentation. We derive the domain where gas-cooling by dust facilitates dynamical fragmentation. Low-mass (i.e. solar mass) clumps - those supported mainly by thermal pressure - can probably access this domain spontaneously, albeit rather quasistatically, provided they exist in a region where external perturbations are few and far between. More massive clumps probably require an impulsive external perturbation, for instance a supersonic collision with another clump, in order for the gas to reach sufficiently high density to couple thermally to the dust. Impulsive external perturbations should promote fragmentation, by generating highly non-line ar substructures which can then be amplified by gravity during the subsequent collapse.

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