Search

We have rediscussed the synchronism between rotation and revolution in close binaries by an inspection of the published projected rotational velocities V sin i of about 250 early-type (from 0 to F5) eclipsing and double-lined spectroscopic binaries. Corrections of the V sin i — values (which are mainly taken from the catalog of Uesugi and Fukuda, 1982) for the aspect effect is straightforward for the eclipsing binaries with analyzed light-curves; in the other cases we have estimated the value of the orbital inclination angle i from the primary’s minimum mass M1 sin3 i on the assumption that its mass follows Straižys and Kuriliene’s (1981) radius-spectrum relations for different luminosity classes. For the components of non-eclipsing binaries, for which the absolute radii are not directly known, we have adopted values of the absolute radii in accordance with Straižys and Kuriliene’s (1981) radius-spectrum relations for different luminosity classes. By using our estimates of the radii, for each component we have evaluated the synchronized velocity Vk(corresponding to the mean orbital angular velocity) and the pseudosynchronized velocity Ve, which corresponds to a synchronization with the instantaneous orbital angular velocity at periastron of an eccentric orbit; in close binaries with appreciably eccentric orbits synchronization is attained with V/Vk>1 and it is probably quickly reached at periastron (Hut 1981).

An updated version of the Active Control of Space Structure (ACOSS) Evaluation Model, ACOSS Model 2, has been designed and analyzed. This updated model reflects the need for closer correlation between model performance and actual system performance at the cost of increased model complexity. Using this updated model, two additional models have been created to fulfill the needs of the VCOSS program. The first VCOSS model is a minimum mass, flexible design. A vibration control system including hardware specifications will be designed for this model. This design will be compared with that based on the second VCOSS model. This model is a maximum mass, stiffness controlled design. The purpose of this design is to reduce control system requirements by stiffening the structure in order to raise critical modal frequencies. The MSC/NASTRAM finite element models for all three designs are listed in the appendices. (Author)

Ultra-low mass deployable and erectable truss structure designs for spacecraft are identified using computerized structural sizing techniques. Extremely slender strut proportions are shown to characterize minimum mass spacecraft which are designed for Shuttle transport to orbit. Analytical results are presented which demonstrate discrete element effects using a recently developed buckling theory for periodic lattice type structures. An analysis of fabrication imperfection effects on the surface accuracy of four different antenna reflector structures is summarized. This study shows the tetrahedral truss to have the greatest potential of the structures examined for application to accurate or large reflectors. A deployable module which can be efficiently transported is identified and shown to have significant potential for application to future antenna requirements. Recent investigations of erectable structure assembly are reviewed. Initial experiments simulating astronaut assembly by extra-vehicular activity (EVA) show that a pair of astronauts can achieve assembly times of 2-5 min/strut. Studies indicate that an automated assembler can achieve times of less than 1 min/strut on an around-the-clock basis.

We describe a fast trigger processor, developed for and used in Fermilab experiment E-537, for selecting high-mass dimuon events produced by negative pions and anti-protons. The processor finds candidate tracks by matching hit information received from drift chambers and scintillation counters, and determines their momenta. Invariant masses are calculated for all possible pairs of tracks and an event is accepted if any invariant mass is greater than some preselectable minimum mass. The whole process, accomplished within 5 to 10 microseconds, achieves up to a ten-fold reduction in trigger rate.

A method is presented for the minimum mass design of three dimensional space frames constructed of thin walled rectangular cross-section members. Constraints on nodal displacements and rotations, material stress, local buckling, and cross sectional dimensions are included. A high quality separable approximate problem is formed in terms of the reciprocals of the four section properties of the frame element cross section, replacing all implicit functions with simplified explicit relations. The cross sectional dimensions are efficiently calculated without using multilevel techniques. Several test problems are solved, demonstrating that a series of approximate problem solutions converge rapidly to an optimal design.

A procedure for sizing an airframe for flutter-free performance is demonstrated on a large, flexible supersonic transport aircraft. The procedure is based on using a two-level reduced basis or modal technique for reducing the computational cost of performing the repetitive flutter analyses. The supersonic transport aircraft exhibits complex dynamic behavior, has a well-known flutter problem and requires a large finite-element model to predict the vibratory and flutter response. Flutter-free designs are produced with small mass increases relative to the wing structural weight and aircraft payload. In view of the ability of the resizing procedure to handle this supersonic transport configuration, it seems likely that the method could be used for many other aircraft.

The problem of the axial vibration of a cantilever beam is investigated numerically. The mass distribution that minimizes the total mass for a given fundamental frequency constraint is determined using both the sequential ordinary gradient-restoration algorithm (SOGRA) and an ad hoc modification of the modified quasilinearization algorithm (MQA). (Author)

This study investigates the mass reduction potential of various materials which could be used in the primary structure of a front-wheel drive compact car. An in-house structural optimization computer program produced alternate designs with mild steel, HSLA steel and aluminum, as well as continuous fiber graphite-epoxy and glass-epoxy composites. For each alternate material, the design was optimized to find a minimum mass structure which would satisfy the comprehensive set of design criteria. An algorithm for selective replacement of strength constrained members with alternate high strength materials was developed and evaluated.

THE recent observation of a “quasi-sinusoidal” period variation in the pulsar NP 0532 has been attributed to perturbations produced by a planetary companion with an orbital period of about three months and a minimum mass of 1.0 M⊕ assuming the mass of the pulsar to be 1 M⊙ (ref. 1). It is therefore desirable to investigate the orbit of a planet after a supernova explosion of the central star, and the effect of the new orbit on the observed period of the pulsar.

PACZYNSKI has shown in some unpublished work that all stars in the interval 3.5 M⊙≲M≲8 M⊙ form degenerate carbon–oxygen cores (M ∼ 1.4 M⊙) which converge to the same central conditions before undergoing carbon ignition. The luminosities of these stars also converge, indicating that their cores are all essentially identical. Gunn and Ostriker (unpublished) have shown that the minimum mass for pulsar parent stars is about 4 M⊙ with a maximum parent mass of about 10 M⊙. Their study also indicates that the parameter B = Bia3 sin αI−1/2 (Bi is the initial magnetic field, a is the neutron star radius, α is the angle of inclination between the rotation and dipole axes, and I is the moment of inertia) varies only slightly among the fourteen pulsars the period of which has been observed to change, and for which B can thus be determined.

The method of Lagrange multipliers is used to design an optimum shadow shield for an isotropic point source. An exponential absorption without multiple scattering is assumed valid within the shield. If the shape of the outer surface is chosen, the inner surface is determined for a given attenuation. Once all the constraints are established (position, attenuation, solid-angle) the derived shape has a minimum mass.

The effects of shear deformation plate theory on the minimum mass design of composite prismatic plate assemblies of practical aircraft wing panel proportions are investigated using VICONOPT

The epochs of maximum light (EMLs) in the light curve of TU UMa, a halo RR Lyrae variable star, depart from a linear ephemeris over the 85 yr interval that the star has been under scrutiny. We have supplemented the list of EMLs assembled by Saha & White with new photoelectric (photomultiplier tube or CCD) measurements made during the period 1990–1999; in addition, we have collected other recent EMLs from the literature. There are now 83 EMLs in the list, of which 35 are reported for the first time here. We analyze these data, combining a quadratic pulsation ephemeris with an orbital timing model, as previously suggested by Szeidl et al. and by Saha & White. The favored model relies exclusively on photoelectric measurements of EMLs, and it yields residuals (2.4 minutes rms) that are comparable to the precision of measurement. We conclude that TU UMa is a member of a binary star system with a period of approximately 23 yr. This conclusion is supported by available high-quality center-of-mass radial velocity data. The inferred orbit is highly eccentric, e ≈ 0.79, but not extremely so. The minimum mass of the companion object in the favored model is ≈0.4 M⊙. The minimum projected size of TU UMa's orbital major axis is 10 mas. The binary system is presently near its widest orbital and angular separation. TU UMa is the only RR Lyrae star that can be considered with high probability to be a member of a relatively short-period binary and as such is the only RR Lyrae star for which there is a prospect for direct measurement of the mass.

In this paper we present giant planetary companions to the stars HD 28185 and HD 213240. Both candidates were discovered in the context of the CORALIE extra-solar planet search programme. The planet around HD 28185, with a minimum mass of 5.7MJup is one of the rst long period planets (P = 383 days) discovered so far orbiting its host star in a nearly circular trajectory. On the other hand, the planet around HD 213240, with a 951-day period orbit, an eccentricity of 0.45, and a minimum mass of 4.5MJup, is more typical of the up to now discovered long period systems.

A model is presented for the chemical evolution of the solar neighbourhood which takes into account three families of galactic objects, according to their condensation states: stars, refuses and gas. Stars are defined as every condensed objects with masses greater than or equal to the minimum mass which ignites hydrogen and which will give rise to an evolutionary track on the HR diagram to the left of Hayashi's limit; refuses include the remnants, which are compact objects resulting from stellar deaths, and the residues, which have masses not large enough to ignite hydrogen; gas is defined as the mass which can be condensed to form stars and/or residues. We have developed equations for the mass evolution of each family, and have studied the gas metallicity distribution within the framework of the instantaneous recycling approximation, adopting different initial conditions. In order to constrain the model parameters we have also used preliminary evaluations of comet cloud masses to investigate the role of the residues as sinks of heavy elements in the Galaxy., Comment: 11 pages (figures and table upon request), Tex, Ap.Spac.Sci. (in press)

We present IR K-band photometry of complete samples of VLM candidates constructed from IIIaF and IVN plates in 10 fields taken as part of the POSSII and UKSRC surveys. Using the I-K colors constructed for these stars we estimate a bolometric luminosity function which extends to M(Bol) = 13.75. We find significant evidence for a luminosity function decreasing toward these luminosities. We also find that our results are consistent with those of studies based on the Nearby Star sample, when those data are presented as a bolometric luminosity function. We convert our observed luminosity function into a mass function, which extends with reasonable statistics to 0.08 solar masses - the H-burning minimum mass. We find significant evidence for features in the mass function at these masses. Specifically, the mass function 'turns over' at 0.25 solar mass, goes through a local minimum at about 0.15 solar mass, and may increase again below 0.1 solar mass - none of these features are predicted by any of the current theories of star formation. Lastly, the mass density we observe just above the H-burning minimum mass makes it difficult to envisage brown dwarfs contributing significant quantities of missing mass without invoking either a mass function in this region significantly steeper than that seen for main-sequence stars, or an extremely low cutoff mass to the mass function.

We consider the thermal properties of cold, dense clouds of molecular hydrogen and atomic helium. For cloud masses below 10^-1.7 Msun, the internal pressure is sufficient to permit the existence of particles of solid or liquid hydrogen at temperatures above the microwave background temperature. Optically thin thermal continuum emission by these particles can balance cosmic-ray heating of the cloud, leading to equilibria which are thermally stable even though the heating rate is independent of cloud temperature. For the Galaxy, the known heating rate in the disk sets a minimum mass of order 10^-6 Msun necessary for survival. Clouds of this type may in principle comprise most of the dark matter in the Galactic halo. However, we caution that the equilibria do not exist at redshifts z > 1 when the temperature of the microwave background was substantially larger than its current value; the formation and survival of such clouds to the present epoch therefore remain open questions., 5 pp incl 2 figs, LaTeX, emulateapj.sty; ApJ Letters in press. Significant revisions, results qualitatively unchanged

Bridge structures have attracted the interest of Engineers throughout the history because they represent the attempt of men to overcome obstacles. Bridges allow easier connections between two different points and faster displacements of goods and people. The present work makes use of truss structures to search for the most efficient bridges. We first present numerical results for minimal mass truss bridge designs. Our approach creates a network of tensile and compressive members distributed throughout the system at many different scales. We leave the prestress calibration to a second step of our design strategy, to be carried out after the minimal mass topology has been determined, with the aim of optimizing the dynamic performance of the structure. A numerical example shows that the vibration modes of the examined bridge model can be tuned by playing with the prestress of the deck members.

We present J-band photometry and low-resolution optical spectroscopy (600-1000 nm) for one of the faintest substellar member candidates in the young sigma Ori cluster, SOri 47 (I=20.53, Bejar et al. 1999). Its very red (I-J)=3.3+/-0.1 color and its optical spectrum allow us to classify SOri 47 as an L1.5-type object which fits the low-luminosity end of the cluster photometric and spectroscopic sequences. It also displays atmospheric features indicative of low gravity such as weak alkaline lines and hydride and oxide bands, consistent with the expectation for a very young object still undergoing gravitational collapse. Our data lead us to conclude that SOri 47 is a true substellar member of the sigma Ori cluster. Additionally, we present the detection of LiI in its atmosphere which provides an independent confirmation of youth and substellarity. Using current theoretical evolutionary tracks and adopting an age interval of 1-5 Myr for the sigma Ori cluster, we estimate the mass of SOri 47 at 0.015+/-0.005 Msun, i.e. at the minimum mass for deuterium burning, which has been proposed as a definition for the boundary between brown dwarfs and giant planets. SOri 47 could well be the result of a natural extension of the process of cloud fragmentation down to the deuterium burning mass limit; a less likely alternative is that it has originated from a protoplanetary disc around a more massive cluster member and later ejected from its orbit due to interacting effects within this rather sparse (~12 objects/pc^3) young cluster., 9 pages, 3 figures, accepted for publication in ApJ Letters

We explore the variation in single star 15-30 $\rm{M}_{\odot}$, non-rotating, solar metallicity, pre-supernova MESA models due to changes in the number of isotopes in a fully-coupled nuclear reaction network and adjustments in the mass resolution. Within this two-dimensional plane we quantitatively detail the range of core masses at various stages of evolution, mass locations of the main nuclear burning shells, electron fraction profiles, mass fraction profiles, burning lifetimes, stellar lifetimes, and compactness parameter at core-collapse for models with and without mass loss. Up to carbon burning we generally find mass resolution has a larger impact on the variations than the number of isotopes, while the number of isotopes plays a more significant role in determining the span of the variations for neon, oxygen and silicon burning. Choice of mass resolution dominates the variations in the structure of the intermediate convection zone and secondary convection zone during core and shell hydrogen burning respectively, where we find a minimum mass resolution of $\approx$ 0.01 $\rm{M}_{\odot}$ is necessary to achieve convergence in the helium core mass at the $\approx$5% level. On the other hand, at the onset of core-collapse we find $\approx$30% variations in the central electron fraction and mass locations of the main nuclear burning shells, a minimum of $\approx$127 isotopes is needed to attain convergence of these values at the $\approx$10% level., Comment: 32 pages, 1 table, 25 figures, Accepted ApJS

This paper presents an application of the TwO-Dimensional CORrelation (TODCOR) algorithm to multi-order spec- tra. The combination of many orders enables the detection and measurement of the radial velocities of very faint companions. The technique is first applied here to the case of HD 41004, where the secondary is 3:68 mag fainter than the primary in the V band. When applied to CORALIE spectra of this system, the technique measures the secondary velocities with a precision of 0: 6k m s 1 and facilitates an orbital solution of the HD 41004 B subsystem. The orbit of HD 41004 B is nearly circular, with a companion of a 19 MJ minimum mass. The precision achieved for the primary is 10 m s 1 , allowing the measurement of a long-term trend in the velocities of HD 41004 A.

The problem of steady state heat conduction in a functionally graded open-cell metal foam thermal insulation is studied. The mass is minimized by varying the solidity profile for a given thickness. An optimality condition is derived and the optimization problem is reduced to that of an ordinary, nonlinear differential equation, which is solved numerically. The results include optimum cell size variation through the thickness of the insulation for given aerodynamic heating and the corresponding temperature distribution. It is shown that for a given thickness using a functionally graded insulation is predictably lighter than uniform one INTRODUCTION Metal foams [1] and advanced metallic thermal protection systems [2] are being investigated for use in multifunctional structures for reusable launch vehicles. Such multifunctional structures would insulate the vehicle interior from aerodynamic heating as well as carry primary vehicle loads. Varying the density, geometry, and/or material composition from point to point within the foam can produce functionally graded materials (FGM) that may be superior to uniform materials. To develop and test FGM for thermal protection systems, it is important to develop an understanding of what material property distributions offer significant efficiency gains. Satchi Venkataraman et al. [3] developed criterion for minimizing heat conduction through an open-cell titanium foam with variable cell size through its thickness. For a fixed inner wall temperature and foam thickness the outside wall temperature is maximized. Maximizing the outside wall temperature maximizes the heat radiated at the surface and therefore corresponds to minimizing the transmitted heat. The current study seeks to identify density profiles that may yield large improvements in weight efficiency compared to materials with uniform density. These results will be then used to direct research into improved modeling of FGM that will be used to refine the initial optimization. Finally, it is hoped that the results could be used to direct testing of promising configurations. The thermal protection systems (TPS) problem is inherently a transient one. However, we are first solving the simpler steady state problem to gain understanding of the effects of using functionally graded insulations. In this paper an optimality criterion is derived for minimizing the mass of an open-cell titanium foam with variable cell size through its thickness. The effective thermal conductivity of the foam is a function of temperature, pressure, properties of the foam material, and the foam geometry. The objective of optimization problem is to determine the density distribution that minimizes the mass of a titanium foam of given thickness for a fixed inner and outer wall temperatures. The optimality condition is developed and used to obtain the optimum density profile. The minimum mass obtained using a functionally graded foam and uniform density foam are compared to illustrate performance payoffs provided by optimization of graded foam properties. 43rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Con 22-25 April 2002, Denver, Colorado AIAA 2002-1425 Copyright © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

I integrated structural and mechanical systems, the design criteria may be governed by the fundamental frequency of the structure either to provide a certain precision in control or to prevent damage to the system due to resonance between the mechanical and structural systems. Tracking antennas, radio telescopes, and orbiting reflectors are a few examples of the types of structures for which the frequency consideration becomes exceptionally important. . Turner attempted to minimize the mass of a structure with a specific natural frequency. This problem is governed by a system of nonlinear equations and a solution was obtained iteratively by an adaptation of the Newton-Ralphson method. The procedure consists essentially of starting with a given mass distribution, and hence with a given natural frequency, and then obtaining a series of corrections to the mass distribution until minimum mass is achieved. In many physical problems, the original frequency may not be satisfactory and, hence, the problem of maximization of frequency for a given total mass of the structure becomes important/The objective of this paper is to present a method for the maximization of the lowest natural frequency of a structural system of a given weight by varying the stiffness of its component elements. This problem has been investigated by Taylor for the special case of a bar.

The results of numerical evolutionary calculations for stars with masses in the range of 0.01-0.10 solar mass are presented. The stellar models by which these stars are followed from the early stages of contraction through deuterium burning to the very late stages of degenerate cooling at ages comparable to that of the Galaxy are described, and the methodology used to investigate the major uncertainties in the input physics are discussed. It is found that, for brown dwarfs with masses substantially below the end of the hydrogen-burning main sequence, the evolution of the effective temperature and bolometric luminosity are fairly well determined, despite the residual uncertainties in the input physics. In particular, the evolution is remarkably insensitive to the choice of the atmospheric opacity law at low temperatures and to the amount of mismatch in specific entropy across the stellar envelope. The minimum mass for a star to attain main-sequence status is moderately sensitive to the assumed input physics, particularly the surface boundary conditions.

The evolutionary paths for Population I stars having initial masses 1, 2.5, 3, 4, and 5 solar masses were computed from the homogeneous main sequence to the onset of the first major thermal pulse to evaluate the minimum mass and the chemical stratification of the remnant white dwarf (WD) associated with each parent mass. The helium flash phase was followed in detail for a 2.5 solar masses star, whereas for the 1 solar mass star the flash was bypassed, and the models at the beginning of the steady central helium burning phase were obtained by means of a scaling procedure upon the properly computed total and core masses. The results show that for a parent ranging between 1-3 solar masses the core mass at the first thermal pulse ranges only from 0.64-0.69 solar mass. If some very fast mass-loss mechanism is triggered in connection with the early stages of the thermal pulse phase, as suggested by the observed deficiency of asymptotic giant branch stars, the relation between final and initial mass is almost flat at least up to an initial mass of 3 solar masses, and the mass spectrum of the WDs is narrow and heavily peaked around 0.65 solarmore » mass. 53 references.« less

Two nearby M dwarfs, CC 1228 and Wolf 922, give indication of duplicity from plates taken with the Sproul 61-cm refractor. The unseen companions cause a perturbation for CC 1228 with P = 6.3y and for Wolf 922 with P = 1.93y. The interpretation leads to a minimum mass of 0.02 solar mass for CC 1228B and a likely range in mass of 0.13-0.20 solar mass for Wolf 922B. Accurate parallaxes yield distances of 12.2 parsecs for CC 1228 and 8.2 parsec for Wolf 922.

Models of AGB evolution of intermediate mass stars are constructed that take into account overshooting by extrapolating models by Bertelli et al. (1985). Models with overshooting show an increased importance of second dredge-up compared with standard models. He production is increased, allowing dY/dZ to increase by roughly 0.7. Secondary N production is also increased in a way that can explain the observed N/O versus O diagram. Overshooting decreases the theoretical minimum mass for the formation of type I planetary nebulae (PN) down to roughly 2.6 solar masses, in good agreement with the observed value of 2.4 solar masses. Other effects of overshooting in type I PN are He/H abundances as high as 0.22 and higher depletion of O.

The asymptotic giant branch evolution of stars prior to the onset of thermal pulses is found to be highly sensitive to both initial composition and total stellar mass. Models investigated have masses in the range 3--11 M/sub sun/ and initial compositions of (Y,Z) = (0.28, 0.001), (0.28, 0.01), (0.28, 0.02), (0.28, 0.03), (0.20, 0.02), (0.36, 0.02), and (0.20, 0.001). The effects produced by varying the overall rate of neutrino emission and by varying the size of the mixing length in convective regions are studied; the role played by convective overshoot, semiconvection, and the energy cost of mixing across the hydrogen-helium discontinuity are discussed. a general description of the evolutionary behavior of our models from the start of the asymptotic giant branch to the first major thermal pulse is also presented.The inward advance of the base of the convective envelope results in a reduction in the size of the hydrogen-exhausted core for stellar masses in a specific range: M/sub UP/> or =M/sub asterisk/> or =M/sub CRIT/. The magnitudes of both M/sub CRIT/ and M/sub UP/ (defined as the minimum mass for nondegenerate carbon ignition) depend on the initial composition in ways that can be approximated analytically. The final size of themore » hydrogen-exhausted core that results after the dredge-up phase is, for a given initial mass, larger in models with a lower A or higher Y. The abundance changes that occur as a result of dredge-up are, for a given mass, greater in models with a lower Z or higher Y. Typically, /sup 4/He and /sup 14/N are enhanced while /sup 1/H, /sup 12/C, and /sup 16/O are depleted. For the most massive models undergoing dredge-up, significant enhancements in /sup 18/O also occur.« less

Near-infrared speckle interferometric observations of five pre-main-sequence stars reveal a core-halo structure around two of these stars: HL Tau and R Mon. The halo light distribution is shown to arise from scattered light from small circumstellar particles. Halo sizes of 320 x 200 AU (alpha x delta FWHM) and 1300 x 1300 AU are deduced for HL Tau and R Mon, respectively, and the halo light is substantially bluer than the stellar light. The minimum mass of small particles in the scattering regions is comparable to the earth's mass in HL Tau and ten times greater in R Mon. Mass loss from the stars is almost certainly insufficient to produce the halo matter. The halos probably consist of relatively slowly moving matter bound gravitationally to the stars. From the size and mass of the circumstellar matter, it appears likely that these halos are in the early stage in the formation of planet-forming disks around the young stars.

The nearby M3 V star CC 20,986, shows a perturbation from astrometric plates taken with the 61-cm Sproul refractor. The interpretation indicates an unseen companion revolving about the visible component in a period of 3.r72 with a likely range in minimum mass of 0.06 to 0.08 'At . Key words: astrometric binary-parallax

If half or more of the mass of a virialized system is lost in less than one dynamical crossing time, the system dissocites. If the mass loss occurs in a collapsing protosystem with uniform density, no angular momentum, and relatively little radiation of energy, the minimum fractional mass loss required for dissociation is reduced to ..delta..M/M/sub 0/=R/sub c//(2R/sub 0/). Here R/sub c/ is the radius of the system when the mass loss occurs and R/sub 0/ is the radius it would have attained after virialization if no mass loss had occurred. A situation of this type is expected in a protocluster that forms from a collapsing interstellar cloud. The stars form when the protocluster is near its point of maximum compression. Any newly formed OB stars produce an H II region whose expansion dissipates the residual gas in the protosystem before the system reaches dynamical equilibrium. The angular momentum of the protosystem and any radiative losses from it prior to star formation will tend to stabilize it against mass loss by limiting the compression factor 2R/sub 0//R/sub c/. The angular momentum places a lower limit on the radius R/sub c/, and the radiative losses reduce the final equilibrium radius R/submore » 0/. However, observed infantile clusters such as the Trapezium are sufficiently compressed at the present time that a loss of as little as 10% of their mass is sufficient to dissociate them. This may explain why most young stars are not located in gravitationally bound clusters. If the protocluster gas contains an appreciable magnetic field, the compression of this field in the collapsing cloud drains off some of the gravitational energy that otherwise would go into the kinetic energy of collapse. This increases the minimum mass loss required to dissociate the system, but it is still very small for most systems.« less

Spectroscopic and photometric observations were carried out for HD 77581, a B0.5 Ib star located in the error box of the X-ray source 2U 0900-40. HD 77581 is identified as spectroscopic binary with a provisional period of 7.0 days and a semiamplitude of 26 km/sec. The minimum mass of the secondary star is near 1.4 solar mass.

The purpose of this paper is to present a new approach to the problem of the minimum-mass design of two-dimension al continuous structures which are required to satisfy a constraint of a dynamic or aeroelastic nature, expressed in the form of one or more partial differential equations. The minimization of a functional subject to constraints of this form belongs to a wider class of problems, encountered in the theory of optimal control of systems with distributed parameters. Classical methods of optimal control theory are here extended to two dimensions in order to derive the set of necessary conditions for an extremum. They are then applied to the theoretical case of the minimum-mass design of a simply-supported shear plate for a given fundamental frequency of vibration under the structural mass assumption. An equation for the optimal displacement, which is the expression of a general optimality criterion in structural design due to Prager and rendering the necessary conditions also sufficient, is derived and solved uniquely in closed form for any shape of the plate. Results are presented for a square and rectangular shape, and for a circular plate. The case of an inequality constraint applied to the thickness (minimum thickness) is also examined.

Stellar models with carbon-rich interiors and helium envelopes of varying fractional mass have been constructed. The models have masses of 0.75, 1.02, and 1.37 M/sub sub solar /. A few 1.02 M/sub sub solar / models with hydrogen-r ich outer envelopes of small fractional mass have also been constructed. The evolution of these models is compared with the observed evolution of the nuclei of planetary nebulae (NPNs). Some models give good agreement with the luminosities and evolutionary time scales of NPNs, but the effective temperatures of these models are too high. We investigate the minimum mass required for carbon ignition in the stellar interior as a function of the fractional mass of the helium envelope. (auth)

The theory of intermediate screening is developed from the cluster expansion for the screening function. This systematic perturabation technique is used to extend weak-screening theory analytically to higher order in the scrcening parameters, and is solved numerically to yield screening-function data for intermediate and strong screening regions. for both the one-component system, equal charge reactions, and the two-component system in the form of 10 mixtures of low-Z elements, the resultant intermediate-screening function is shown to obey a simple power-law dependence on the screening parameter, H12(0) xf(z/sub i/) LAMBDA /sub 12/ sup b/, where b=0.860. This result is found to be equivalent to the general screening function presented in Paper I. The new screening thceory is studied in relation to various astrophysical situations. Three areas are found where screen~ng effects may produce quantitative changes in evolutionary star models: minimum mass limits for H, D, He, C, and 0 main sequences; the structure and extent of flash events in red-giant interiors; and the strength of detonations in degenerate carbon and oxygen cores. This latter case may require screening-factor increases of two orders of magnitude. (auth)

Dusty debris disks around pre- and main-sequence stars are potential signposts for the existence of planetesimals and exoplanets. Giant planet formation is therefore expected to play a key role in the evolution of the disk. This is indirectly confirmed by extant sub-millimeter near-infrared images of young protoplanetary and cool dusty debris disks around main sequence stars usually showing substantial spatial structures. A majority of recent discoveries of imaged giant planets have been obtained around young, early-type stars hosting a circumstellar disk. In this context, we have carried out a direct imaging program designed to maximize our chances of giant planet discovery and targeting twenty-two young, early-type stars. About half of them show indication of multi-belt architectures. Using the IRDIS dual-band imager and the IFS integral field spectrograph of SPHERE to acquire high-constrast coronagraphic differential near-infrared images, we have conducted a systematic search in the close environment of these young, dusty and early-type stars. We confirmed that companions detected around HIP 34276, HIP 101800 and HIP 117452 are stationary background sources and binary companions. The companion candidates around HIP 8832, HIP 16095 and HIP 95619 are determined as background contamination. For stars for which we infer the presence of debris belts, a theoretical minimum mass for planets required to clear the debris gaps can be calculated . The dynamical mass limit is at least $0.1 M_J$ and can exceed $1 M_J$. Direct imaging data is typically sensitive to planets down to $\sim 3.6 M_J$ at 1 $''$, and $1.7 M_J$ in the best case. These two limits tightly constrain the possible planetary systems present around each target. These systems will be probably detectable with the next generation of planet imagers., Accepted for publication in Astronomy & Astrophysics. 12 pages, 9 figures

The ESPRESSO spectrograph is a new powerful tool to detect and characterize extrasolar planets. Its design allows unprecedented radial velocity precision (down to a few tens of cm/s) and long-term thermo-mechanical stability. We present the first standalone detection of an extrasolar planet by blind radial velocity search using ESPRESSO and aim at showing the power of the instrument in characterizing planetary signals at different periodicities in long time spans. We use 41 ESPRESSO measurements of HD\,22496 within a time span of 895 days with a median photon noise of 18 cm/s. A radial velocity analysis is performed to test the presence of planets in the system and to account for the stellar activity of this K5-K7 main sequence star. For benchmarking and comparison, we attempt the detection with 43 archive HARPS measurements and compare the results yielded by the two datasets. We also use four TESS sectors to search for transits. We find radial velocity variations compatible with a close-in planet with an orbital period of $P=5.09071\pm0.00026$ days when simultaneously accounting for the effects of stellar activity at longer time scales ($P_{\rm rot}=34.99^{+0.58}_{-0.53}$ days). We characterize the physical and orbital properties of the planet and find a minimum mass of $5.57^{+0.73}_{-0.68}$ $\mathrm{M}_{\oplus}$, right in the dichotomic regime between rocky and gaseous planets. Although not transiting according to TESS data, if aligned with the stellar spin axis, the absolute mass of the planet must be below 16 $\mathrm{M}_{\oplus}$. We find no significant evidence for additional signals with semi-amplitudes above 56 cm/s at 95% confidence. With a modest set of radial velocity measurements, ESPRESSO is capable of detecting and characterizing low-mass planets and constrain the presence of planets in the habitable zone of K-dwarfs down to the rocky-mass regime., Comment: Accepted for publication in A&A. 12 pages, 8 figures, 3 tables