Small Star Research Articles

The Sirius Supercluster and Missing Mass near the Sun

The Hipparcos results confirm some 50 members of the Sirius supercluster in the Bright Star Catalogue. The resulting, well-defined color-luminosity array indicates an age of 4 × 108 yr from conventional (no overshoot) models. A comparison of the luminosity function obtained from members in the Bright Star Catalogue, unbiased as to proper motion, and in the Catalogue of Nearby Stars (∼27 pc), which are strongly biased toward large proper motions, reveals that either most of the small proper-motion stars near the Sun remain to be identified or the luminosity function of the supercluster is drastically different from that of the field stars. A search for low-mass members in a 6° × 6° field in the center of the Ursa Major cluster yielded 10 possible members that lie on the white dwarf sequence, 5 mag below the main sequence. Luyten (field LP 131) found 368 stars in this field with proper motion exceeding 008, and only 19 of these have a position angle of their proper motion between 0° and 98° (i.e., the quadrant containing UMa cluster members). The assumption of cluster membership yields a parallax very close to that obtained for bright members. Aside from white dwarfs, the possibilities are discussed that these stars are (1) brown dwarfs, (2) members of a cluster in the line of sight to UMa but 100 pc more distant, or (3) reflecting a chain of very unlikely coincidences.

CLC]Ca[/CLC] [CSC]ii[/CSC] Triplet Spectroscopy of Giants in Small Magellanic Cloud Star Clusters: Abundances, Velocities, and the Age-Metallicity Relation

We have obtained spectra at the Ca II triplet of individual red giants in seven Small Magellanic Cloud (SMC) star clusters whose ages range from ∼4 to 12 Gyr. The spectra have been used to determine mean abundances for six of the star clusters to a typical precision of 0.12 dex. When combined with existing data for other objects, the resulting SMC age-metallicity relation is generally consistent with that for a simple model of chemical evolution, scaled to the present-day SMC mean abundance and gas mass fraction. Two of the clusters (Lindsay 113 and NGC 339), however, have abundances that are ∼0.5 dex lower than that expected from the mean age-metallicity relation. It is suggested that the formation of these clusters, which have ages of ∼5 Gyr, may have involved the infall of unenriched gas, perhaps from the Magellanic Stream. The spectra also yield radial velocities for the seven clusters. The resulting velocity dispersion is 16 ± 4 km s-1, consistent with those of the SMC planetary nebula and carbon star populations.

Absolute Dimensions and Masses of V541 Cygni and the General Theory of Relativity

Absolute dimensions and masses of V541 Cyg (B9.5 V + B9.5 V) have been computed from its first spectroscopic orbit and the excellent light curve published by Khaliullin in 1985. This binary has a long period (15.34 days) and a highly eccentric orbit (e = 0.479), with very small stars relative to their separation. It shows apsidal motion of 060 ± 010 per 100 years, significantly slower than the rate predicted by theory [089 ± 003 (100 yr)-1 ]. In this system the rate of apsidal motion due to general relativistic effects is 5 times that from the classical Newtonian causes (tidal and rotational distortions of the stars). The discrepancy in the apsidal motion rate is difficult to explain, and is in the same sense as the discrepancies found for two other eclipsing binaries, DI Her and AS Cam, in which the relativistic contributions to apsidal motion are also significant.

The influence of turbulence pressure on the late stages of evolution of medium and small mass stars (I) Calculation of the turbulence pressure, equation of state and thermodynamic quantities

Using the mixing length theory we give expressions for turbulence pressure and the equation of state and various thermodynamic quantities when turbulence is included. On this basis we examined the size of turbulence in the evolution of two stars of masses 2.8 and 7.0M ⊙ from the main sequence to the red giant and the AGB stages, Our results show that in the late stages the turbulence pressure near the surface of the star can be as much as 30% of the total pressure.

Melkomasshtabnaya struktura temnoi materii i mikrolinzirovanie

It has been revealed using microlensing that a considerable part, possibly more than half, of the dark matter in the halo of our Galaxy consists of objects with a mass spectrum ranging from 0.05 to 0.8 of the solar mass. What is the nature of these objects? There exist two hypotheses. According to one, these are Jupiter type planets or small stars (brown and white dwarfs) consisting of normal baryonic matter. According to the other, these are non-compact objects, i.e., small-scale formations in non-baryonic dark matter. Here, a theory is proposed describing the possibility of the existence of non-compact objects in the halo of our Galaxy, their structure and formation from non-baryonic matter. The theory of microlensing on compact and non-compact objects is considered in detail. The results of microlensing observations are described and compared with theory. Possible astrophysical manifestations of the presence of small-scale structure are pointed out. The field is being extensively studied and is of fundamental interest for cosmology and astrophysics.

Stars and Stripes in Manganese Oxide

Charge ordering in a La0.33Ca0.67MnO3 lattice can be seen as regularly spaced stripes in this low‐temperature high‐resolution (0.5‐nm) electron micrograph made by Cheng Hsuan Chen and Sang‐Wook Cheong of Bell Laboratories, Lucent Technologies. In the strategically placed inset at the upper left, charge ordering is evidenced by the smallest stars in the electron diffraction pattern.

Effects of Shocks on Emission from the Central Engines of Active Galactic Nuclei. I.

In this paper we show that perturbations of the accretion flow within the central engines of some active galactic nuclei (AGNS) are likely to form shock waves in the accreting plasma. Such shocks, which may be either collisional or collisionless, can contribute to the observed high-energy temporal and spectral variability. Our rationale is the following: Observations show that the continuum emission probably originates in an optically thin, hot plasma in the AGN central engine. The flux and spectrum from this hot plasma varies significantly over light crossing timescales. Several authors have suggested that macroscopic perturbations contained within this plasma are the sources of this variability. In order to produce the observed emission the perturbations must be radiatively coupled with the optically thin hot matter and must also move with high velocities. We suggest that shocks, which can be very effective in randomizing the bulk motion of the perturbations, are responsible for this coupling. Shocks should form in the central engine, because the temperatures and magnetic fields are probably reduced below their virial values by radiative dissipation. Perturbations moving at Keplerian speeds, or strong non-linear excitations, result in supersonic and super-Alfvenic velocities leading to shock waves within the hot plasma. We show that even a perturbation smaller than the emitting region can form a shock that significantly modifies the continuum emission in an AGN, and that the spectral and temporal variability from such a shock generally resembles those of radio-quiet AGNS. As an example, the shock inducing perturbation in our model is a small main-sequence star, the capturing and eventual accretion of which are known to be a plausible process. We argue that shocks in the central engine may also provide a natural triggering mechanism for the cold component of Guilbert & Rees two-phase medium and an efficient mecha- nism for angular momentum transfer. Current and future missions, such as ASCA, XTE, XMM, AXAF, and ASTRO-E may determine the importance of shock-related emission from the central engines of AGNS.

Dark matter detectors

A fundamental question of astrophysics and cosmology is the nature of dark matter. Astrophysical observations show clearly the existence of some kind of dark matter, though they cannot yet reveal its nature. Dark matter can consist of baryonic particles, or of other (known or unknown) elementary particles. Baryonic dark matter probably exists in the form of dust, gas, or small stars. Other elementary particles constituting the dark matter can possibly be measured in terrestrial experiments. Possibilities for dark matter particles are neutrinos, axions, and weakly interacting massive particles (WIMPs). While a direct detection of relic neutrinos seems at the moment impossible, there are experiments looking for baryonic dark matter in the form of Massive Compact Halo Objects, and for particle dark matter in the form of axions and WIMPS.

Dark Matter Detectors

A fundamental question of astrophysics and cosmology is the nature of dark matter. Astrophysical observations show clearly the existence of some kind of dark matter, though they cannot yet reveal its nature. Dark matter can consist of baryonic particles, or of other (known or unknown) elementary particles. Baryonic dark matter probably exists in the form of dust, gas, or small stars. Other elementary particles constituting the dark matter can possibly be measured in terrestrial experiments. Possibilities for dark matter particles are neutrinos, axions, and weakly interacting massive particles (WIMPs). While a direct detection of relic neutrinos seems at the moment impossible, there are experiments looking for baryonic dark matter in the form of Massive Compact Halo Objects, and for particle dark matter in the form of axions and WIMPS.

The Nuclear Cluster of the Milky Way: Star Formation and Velocity Dispersion in the Central 0.5 Parsec

We report the first results of an extensive new study of the Galactic center stellar cluster. The central parsec is powered by a cluster of about two dozen luminous and helium-rich blue supergiants/Wolf-Rayet stars (Teff ~ 20,000-30,000 K) with ZAMS masses up to ~100 M☉. The most likely scenario for the formation of the massive stars is a small star formation burst between 3 × 106 and 7 × 106 years ago. In this scenario the Galactic center is presently in a short-lived, post-main-sequence wind phase. In addition, there is evidence for another star formation event about 108 years ago, as well as for recently formed massive stars that may have been transported into the central core along with orbiting gas streamers. The radial velocity dispersion of 35 early- and late-type stars with distances of 1''-12'' from Sgr A* is 154 ± 19 km s-1. Our new results strongly favor the existence of a central dark mass of ~3 × 106 M☉ (density ≥ 108.5 M☉ pc-3, M/L ≥ 10 M☉/L☉) within 0.14 pc of the dynamic center.

A SPECTROSCOPIC ORBIT FOR THE O8 IF STAR 9 SGE

ABSTRACT We performed a period search of the 109 published radial velocities of 9 Sge (HD 188001) obtained since 1921 and found a dominant period of 78.74 day plus its first harmonic. We confirm earlier suggestions that the O8 If star 9 Sge is an SB1 binary of small amplitude (K ~ 11 km s -1). Our improved set of orbital elements indicate the companion may be a small star whose mass is probably in the range 1.2 - 1.9 solar mass. This permits a non-degenerate dwarf companion as early as A-type, which would not be detectable in contrast with the bright primary star.

Changes in Larval Morphology in the Evolution of Benthic Development by Patiriella exigua (Asteroidea: Asterinidae), a Comparison with the Larvae of Patiriella Species with Planktonic Development.

Patiriella exigua (Lamarck) is a small asterinid sea star that deposits large eggs onto the substratum. Development is lecithotrophic and entirely benthic, proceeding without parental care. The embryos develop to the brachiolaria stage before hatching and there is no trace of a bipinnaria larva. In its early stage the larva of P. exigua resembles a planktonic brachiolaria in having one long central brachium and two short lateral brachia. By hatching, the brachia are equal in length, giving the larva a tripod-like appearance. Comparison of the larva of P. exigua with the brachiolaria of Patiriella species with planktonic development supports the hypothesis that the tripod larval form results from differential growth of the lateral brachia. At hatching, the P. exigua larva has a well-developed attachment complex composed of a large adhesive disk and three muscular brachia; the latter bear a striking resemblance to adult tube feet. This hypertrophic elaboration of the brachiolar complex is an adaptation for permanent benthic attachment. Internally, one large enterocoel forms at the anterior end of the archenteron. The archenteron then closes to form the rudiment for the adult gut. Metamorphosis involves gradual resorption of the brachiolar complex concomitant with formation of the first tube feet. The adhesive disk plays a major role in attachment during late metamorphosis but is gradually reduced to a plug of tissue as the tube feet become functional. Juvenile P. exigua are negatively geotactic and float on the water surface, behavior that may act as a mechanism for dispersal. The similarity of the early larva of P. exigua to planktonic brachiolariae suggests that the evolution of benthic lecithotrophy by this species involved modification of a planktonic larval form. These modifications include elimination and reduction of larval feeding structures, formation of one rather than three enterocoels, and hypertrophy of the brachiolar complex to form a tripod larva. Heterochronies in the ontogeny of P. exigua include the delay in hatching to the brachiolaria stage and the accelerated development of the juvenile form and adult skeleton.

Lithium Abundances and Other Clues to Envelope Burning in Small Magellanic Cloud Asymptotic Giant Branch Stars

We present a chemical analysis of seven luminous asymptotic giant branch stars and one M supergiant of the Small Magellanic Cloud. The abundances are derived from high-resolution spectra by spectrum synthesis using new, spherically symmetric, opacity sampling model atmospheres. The average metallicity is [Fe/H]=-0.5, in accordance with other determinations of the SMC's metallicity. The AGB stars show signs of envelope burning, being Li rich, C poor, and with a low 12 C/ 13 C ratio. The s-process elements (Rb, Zr, Nd) abundance pattern is different from that in the solar system, and is characteristic of a high exposure at low neutron density

Small and Intermediate Mass Stellar Evolution - Main Sequence and Close to It

AbstractWe present the standard models for small and intermediate main sequence stars and we discuss some of the problems arising with semiconvection and overshooting. The surface abundance of Li serves as a test for other physical mechanisms, including microscopic and turbulent diffusion, rotation and mass loss.

X-rays from the eclipsing pulsar 1957 +20

Two examples of eclipsing pulsars, PSRs 1957 + 20 and 1744 − 25, have been discovered and studied1–7. In these systems, radiation from a pulsar drives a particle wind from a low-mass degenerate companion, creating an outflow which eclipses the radio pulsar once each orbital period. Ultimately the secondaries in both of these systems may be eroded away entirely, but the process by which the radiative wind ablates the smaller star remains controver-sial. Here we report the detection of soft X-rays, of ∼1 keV energy, from PSR1957 +20. This high-energy radiation, also observed by Kulkarni et al.8, should be a valuable diagnostic of the wind in this recycled pulsar system. Possible sources of the X-ray emission are the interstellar nebula driven by the pulsar wind, the interaction between the pulsar and its evaporating companion, and the pulsar itself. The small apparent size of the X-ray object argues against the first of these possibilities, and suggests that the X-rays are produced within the binary.

FORMATION OF A GIANT GALACTIC GASEOUS HALO: METAL-ABSORPTION LINES AND HIGH-VELOCITY CLOUDS

FORMATION OF A GIANT GALACTIC GASEOUS HALO: METAL-ABSORPTION LINES AND HIGH-VELOCITY CLOUDS

Can bees generalise?

During the last 3 years we tested the perception of different sized lines in Apis melli/era L. (CELLI & ANGELINI 1988). Starting from these studies we decided to test the bees' capacity for abstract reasoning. To reach this goal we trained bees for a total of 12 hr to choose the shorter of two segments measuring 2 and 4 em by placing a sugar solution under the shorter segment. Afterwords we introduced them to different shapes: small squares and stars, large squares and stars: now the feeder was placed under all shapes. The correct choice was, naturally, the smaller one and they selected it in 86% of their visits. On the other hand bees with only 6 hr training were not able to make the right choice; in fact they selected the smaller one in 50% of their visits.

The myoepithelial and basal cells of ducts of human major salivary glands: a SEM study.

The cytoarchitecture and distribution of myoepithelial (mecs) and basal (bc) cells of intralobular (intercalated, striated) and interlobular (excretory) ducts of human major salivary glands were studied by SEM through a variety of maceration and microdissection techniques. Intercalated ducts are covered by mecs which, unlike the large stellate cells of acini, are spindle shaped. Small star shaped mecs are rarely observed even in the most distal striated ducts, while no such cells are seen in excretory ducts. Basal cells form a more or less continuous row of small, basally placed cells in excretory ducts. Sparse bc are occasionally present in proximal striated ducts as well. Following microdissection, bc exhibit a cup-shaped apex which embraces the convex base of principal cells. This configuration may explain why, in TEM, bc seem to possess lateral processes which may be mistaken for mecs processes. Moreover, the lateral surfaces of bc do not exhibit the complex system of plasmalemma folds typical of principal cells. The present study demonstrates that fusate mecs are present in intercalated ducts and that basal cells are distinct from myoepithelial cells. These results may have some relevance in histogenetic studies of salivary gland neoplasms.

Cold nuclear fusion in dense metallic hydrogen

view Abstract Citations (4) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cold Nuclear Fusion in Dense Metallic Hydrogen Horowitz, Charles J. Abstract Fusion rates for hydrogen isotopes in both liquid and solid metallic hydrogen are calculated as a function of temperature and density. For densities of 5-25 g/cu cm and temperatures below 20 eV the rate is greatly enhanced by both electron and ion screening. Deuterium burning could contribute noticeable heating to large planets/very small stars (brown dwarfs) at temperatures of only 10-15 eV. A more complicated mechanism, which somewhat enhances the deuterium fusion rate, could provide heating in Jupiter. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169627 Bibcode: 1991ApJ...367..288H Keywords: Brown Dwarf Stars; Hydrogen Isotopes; Metallic Hydrogen; Nuclear Astrophysics; Nuclear Fusion; Stellar Interiors; Electron Scattering; Ion Scattering; Liquid Hydrogen; Nuclear Reactions; Astrophysics; NUCLEAR REACTIONS; PLANETS: INTERIORS; STARS: BROWN DWARFS; STARS: INTERIORS full text sources ADS |

Astron observations of the Rapid Burster MXB 1730-335 and constraints on burster parameters from spectra of trailing bursts

We report on eight X-ray bursts detected by ASTRON from the Rapid Burster (RB) on 13 and 28 April and 16 August, 1983. Six of them (trailing bursts), with durations of 1.5–2 min, rise times of 5–10 s and intervals of 1–1.5 hours, exhibit spectral softening during the burst decay and may be related to the type I bursts. Two of the bursts (triangle bursts) observed on 28 April at interval of ∼28 min with much longer rise times (30–50 s) and longer durations (≃3 min), do not show distinct spectral softening. Persistent flux from RB on 16 August was estimated asFp≃(2.0–2.4)×10−9 erg cm−2 s−1. Spectral evolution of two trailing bursts was investigated by fitting their spectra in consecutive time intervals with the blackbody (BB), isothermal scattering photosphere (SP) and thermal bremsstrahlung (TB) models. Around the burst maxima the SP model fits the data best whereas in the burst tails the TB model is generally better. The BB model is worse than at least one of the two others. Interpretation of the burst spectra in terms of the BB radiation leads to improbably small neutron star mass and radius (M<0.86M⊙,RNS<5 km) if the peak luminosity does not exceed the Eddington limit. Interpretation of the spectra around the burst maxima (3–15 s from the burst onset) in terms of an isothermal SP yields reasonable constraints onM,RNS, and distanceD. For instance, for the hydrogen photosphere we obtainedM=(1.0–2.1)M⊙RNS=(7.1–16.4) km ifD=11 kpc. If one postulatesM=1.4M⊙, thenD=(8.5–13) kpc for hydrogen photosphere; if, besides,D=11 kpc, thenRNS=(8.1–13.3) km. It follows also from the SP-interpretation that the photosphere radius may increase up to 20–30 km in maxima of the trailing bursts when the luminosity becomes close to the Eddington luminosity.