Extreme-mass-ratio solar-type binaries and their implications for cluster environs

Results are reported from a search for short-period variables among blue stragglers in the central region of NGC 5466, based on analysis of 248 B and V CCD images obtained with the U.S. Naval Observatory 1-m, Palomar Observatory 1.5-m, and Steward Observatory 2.3-m telescopes during 1987-1989. The data are presented in extensive tables and graphs and characterized in detail. Nine variable blue stragglers are identified, of which three are eclipsing binaries with periods 0.298-0.511 d (two contact binaries of W UMa type and one detached or semidetached binary) and six are pulsating SX Phe stars. Theoretical models indicate that all of the noneclipsing blue stragglers could be merged close binaries, although other formation mechanisms cannot be completely ruled out. 111 refs.

view Abstract Citations (146) References (144) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Starbursts, Blue Stragglers, and Binary Stars in Local Superclusters and Groups. II. The Old Disk and Halo Populations Eggen, Olin J. ; Iben, Icko, Jr. Abstract A study of the distribution in the HR diagram of stars in several old-disk population aggregates and in several halo-population aggregates indicates that the blue straggler phenomenon is ubiquitous. In all the cases considered, several stars are found to lie far from the locus defined by a semiempirical isochrone that best fits the distribution of the bulk of the stars in the aggregate. The results support the starburst interpretation of blue stragglers in young-disk aggregates. Publication: The Astronomical Journal Pub Date: February 1989 DOI: 10.1086/114993 Bibcode: 1989AJ.....97..431E Keywords: Blue Stars; Cepheid Variables; Eclipsing Binary Stars; Star Formation; Starburst Galaxies; Stellar Spectrophotometry; Main Sequence Stars; Open Clusters; Star Clusters; Star Distribution; Stellar Luminosity; Stellar Winds; Astrophysics; STARS: BINARIES; STARS: PROPER-MOTION full text sources ADS | data products SIMBAD (182) Related Materials (1) Part 1: 1988AJ.....96..635E

In this work we present a newly constructed equation of state (EoS) --applicable to stellar core collapse and neutron star mergers--, including the entire baryon octet. Our EoS is compatible with the main constraints from nuclear physics and, in particular, with a maximum mass for cold beta-equilibrated neutron stars of 2 solar masses in agreement with recent observations. As an application of our new EoS, we compute numerical stationary models for rapidly (rigidly) rotating hot neutron stars. We consider maximum masses of hot stars, such as proto-neutron stars or hypermassive neutron stars in the post-merger phase of binary neutron star coalescence. The universality of I-Q-relations at nonzero temperature for fast rotating models, comparing a purely nuclear EoS with its counterparts containing Lambda-hyperons or the entire baryon octet, respectively, is discussed, too. We find that the I-Q universality is broken when thermal effects become important, whatever the value of entropy gradients in our models. Thus, the use of I-Q relations for the analysis of proto-neutron stars or merger remnant data, including gravitational wave signals from the last stages of binary neutron star mergers, should be regarded with care.

view Abstract Citations (2) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Period Determinations for Southern Hemisphere Red Variables Bord, D. J. ; Shawl, S. J. Abstract As part of a search for binaries containing long-period, red variables in the Southern Hemisphere, the periods for ten such objects based on photographic and new photoelectric observations made within the last decade have been determined. Results for the three stars having no prior periods are as follows: FU Hya, 190 days; EI TrA, 274 days; and V1161 Sgr, 175 days. Finding charts and light curves for some of these variables are presented, and the uncertainties in the results discussed. The unreliability of predicted times of maximum brightness, based on older ephemerides, is demonstrated by showing such predictions to be in error by more than a quarter of a period in individual instances, when compared with results from recent observations. Publication: The Astronomical Journal Pub Date: April 1987 DOI: 10.1086/114375 Bibcode: 1987AJ.....93..898B Keywords: Binary Stars; Southern Sky; Stellar Color; Stellar Spectrophotometry; Variable Stars; Ephemerides; Light Curve; Spectrum Analysis; Stellar Luminosity; Stellar Magnitude; Astronomy; STARS: PULSATION full text sources ADS | data products SIMBAD (10)

A prevailing open problem in planetary nebulae (PNe) research, and photoionized gaseous nebulae research at large, is the systematic discrepancies in electron temperatures and ionic abundances as derived from recombination and collisionally excited lines. Peimbert proposed the presence of “temperature fluctuations” in these nebulae, but the apparent amplitude of such fluctuations, as deduced from spectral diagnostics and/or abundance discrepancy factors (ADFs), remain unexplained by standard photoionization modeling. While this and other alternative models to explain the temperature and abundance discrepancies remain inconclusive, recent observations seem to point at a connection between nebular ADFs and a binary nature of photoionizing stars. In this paper, we show that large amplitude temperature fluctuations are expected to form in PNe photoionized by short-period binary stars. Resonant temperature fluctuations (RTFs) are first formed along the orbital disk around the binary stars, as the periodically varying ionizing radiation field induces periodic oscillations in the heating-minus-cooling function. Then, the temperature fluctuations propagate vertically to the disk as thermal waves that later steepen into radiative shocks. The binary period of the ionizing stars is determinant in the formation and propagation of temperature fluctuations, as well as in associated density fluctuations. Fluctuations propagate efficiently only in systems with binary periods significantly shorter than the gas thermalization time, of the order of 10 days. Furthermore, we propose temperature diagnostic line ratios that combine [O iii] collisionally excited lines and O ii recombination lines to determine the equilibrium temperature and the magnitude of RTFs in nebulae.

In this talk, we will simply show the consequences of binary population synthesis for blue stragglers, such as the integrated spectral energy distribution (ISED), the color-magnitude diagram, the specific frequency, and the influences on colors. Blue stragglers have been found in all stellar populations and they are an important population component in both stellar evolution and star clusters. Much evidence shows that blue stragglers are relevant to primordial binaries. The binary population synthesis study shows that primordial binary evolution may produce blue stragglers at any given times. The specific frequency in this way decreases with time first, then increases when the age is larger than 10 Gyr, while that from angular momentum loss induced by magnetic braking in low-mass binaries increases with time and exceeds that of primordial binary evolution in a population older than 3 Gyr. Meanwhile, blue stragglers resulting from primordial binary evolution are dominant contributors to the ISEDs in ultraviolet and blue bands in a population between 0.3 and 2.0 Gyr. The mass fraction of the lost matter from the primary accreted by the secondary, beta, significantly affects on the final results, e.g. the specific frequency of blue stragglers decreasing with beta, blue stragglers produced from a high value of beta being more massive, then contributing more to the ISEDs of the host clusters. For old open clusters, it is appropriate to adopt a higher value of beta when the primary is in HG at the onset of mass transfer. Our study also shows that, for most Galactic open clusters, the specific frequency of blue stragglers obtained from our simulations is much lower than that of observations, which has been discussed in this talk.

We have made an asteroseismic analysis of the variable blue stragglers in the open cluster M67. The data set consists of photometric time-series from eight sites using nine 0.6-2.1 m telescopes with a time-baseline of 43 d. In two stars, EW Cnc and EX Cnc, we detect the highest number of frequencies (41 and 26) detected in δ Scuti stars belonging to a stellar cluster, and EW Cnc has the second highest number of frequencies detected in any δ Scuti star. We have computed a grid of pulsation models that take the effects of rotation into account. The distribution of observed and theoretical frequencies shows that in a wide frequency range a significant fraction of the radial and non-radial low-degree modes are excited to detectable amplitudes. Despite the large number of observed frequencies we cannot constrain the fundamental parameters of the stars. To make progress we need to identify the degrees of some of the modes from either multicolour photometry or spectroscopy.

view Abstract Citations (20) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Significance of the Interstellar 7Li/ 6Li Ratio Steigman, Gary Abstract The present abundances of Li-6 and Li-7, when compared with the solar system values, can provide valuable information on stellar and cosmic-ray nucleosynthesis, stellar structure and evolution, and Galactic chemical evolution in the solar vicinity in the last 4.6 Gyr. Unfortunately, the interpretation of stellar data from observations of main-sequence stars in young clusters and of pre-main-sequence stars are clouded by large and uncertain corrections for burning of lithium and for non-LTE effects, and the interpretation of observations of interstellar Li are rendered uncertain by large and unknowable corrections for ionization and depletion onto grains and/or molecules. In this Letter it is shown that the singular origin for Population I Li-6 offers the potential for exploiting bounds to interstellar beryllium and boron to infer an upper bound to the present interstellar Li-6 abundance. Then the recent observations (Lemoine et al., 1992; Meyer et al., 1993) of the interstellar Li-7/Li-6 ratio may be utilized to bound the present abundance of Li-7. This approach is independent of any model for the evolution of Li-7. The current data suggest that the Li abundance has barely increased, and may well have decreased, in the 4.6 Gyr since the formation of the solar system. This result is inconsistent with the predictions of chemical evolution models and raises the question of the reliability of the sun as a probe of the abundances in the local ISM 4.6 Gyr ago. Publication: The Astrophysical Journal Pub Date: August 1993 DOI: 10.1086/186962 Bibcode: 1993ApJ...413L..73S Keywords: Interstellar Matter; Lithium Isotopes; Metallicity; Milky Way Galaxy; Abundance; Solar System; Astrophysics; GALAXY: ABUNDANCES; ISM: ABUNDANCES; SUN: ABUNDANCES full text sources ADS |

Recent high-resolution observations with the Hubble Space Telescope (HST) reveal that star clusters of extraordinary luminosity and compactness are commonly found in a variety of starburst systems. There has been much speculation that these clusters represent present-day analogs of young globular clusters. Using the HIRES echelle spectrograph on the Keck 10 m telescope, we obtained high-dispersion optical spectra of one of the ``super star clusters'' (cluster ``A'') in the nearby dwarf galaxy NGC 1569. The size of the cluster is known from published HST images. The line-of-sight velocity dispersion (15.7+-1.5 km/s) has been measured from a cross-correlation analysis of its integrated spectrum at visual wavelengths. If the cluster is gravitationally bound and the velocities are isotropic, application of the virial theorem implies that the cluster has a total stellar mass of (3.3+-0.5) X 10$^5$ solar masses. This object's mass, mass density, and probable mass-to-light ratio after aging 10--15 Gyr are fully consistent with the typical values of Galactic globular clusters. Our result strongly suggests that at least some of the luminous, compact, young star clusters being discovered with {\it HST} will indeed evolve into normal globular clusters of the type seen in the Milky Way.

view Abstract Citations (62) References (66) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The blue stragglers of M 67. Peterson, R. C. ; Carney, B. W. ; Latham, D. W. Abstract Photometry and echelle spectroscopy are used to show that at least two, and probably all, of the five brightest blue stragglers in the open cluster M67 are binaries. One of the stars, F190, has radial velocity variations with an amplitude of 30 km/s or more. The average velocities for F156 and F280 depart by 5-7 km/s from the cluster mean, which is more than twice the 1 sigma uncertainty in each determination. The rotational velocity values range from less than 10 to 120 km/s, somewhat lower than average for stars of this temperature in other, necessarily younger, open clusters. These results constitute strong support for the hypothesis that stragglers owe their current existence beyond the main sequence turnoff to mass transfer from an evolving companion. Previous observations are discussed which suggest that the scarcity of stragglers in globular clusters is due to the ease of disruption of the widely separated components of the progenitor systems. Publication: The Astrophysical Journal Pub Date: April 1984 DOI: 10.1086/161887 Bibcode: 1984ApJ...279..237P Keywords: Binary Stars; Blue Stars; Star Clusters; Stellar Evolution; Stellar Spectra; Stellar Spectrophotometry; Astronomical Photometry; Eccentric Orbits; Main Sequence Stars; Mass Transfer; Open Clusters; Radial Velocity; Stellar Rotation; Astrophysics full text sources ADS | data products SIMBAD (30)

We identified blue straggler stars (BSSs) in 53 open clusters utilizing data from Gaia DR3. Most of these clusters are situated in the outer regions of the Galactic disk, encompassing structures such as the warp and the Outer arm. We analyzed their astrometric parameters and determined that 48 of them demonstrate high reliability in the radial density profile. Furthermore, through manual isochrone fitting and visual inspection, we confirmed 119 BSS candidates and identified 328 additional possible candidates within these clusters. Our results contribute to a 46% increase in the sample size of BSSs in open clusters for regions of the Galactic disk where R gc > 12 kpc. We observed that the new samples are fainter compared to those identified in the past. Additionally, we investigated the maximum fractional mass excess (M e ) of the BSSs in open clusters, including previously published BSS samples. Our findings indicate a strong correlation between the capability to produce highest-M e BSSs and the mass of their host clusters. This observation appears to reinforce a fundamental principle whereby an increase in the mass of a star cluster correlates with a higher likelihood of stellar mergers. In contrast, we observe minimal correlation between maximum-M e and the cluster age. Among clusters containing BSSs, younger clusters (0.5–1 Gyr) display a scarcity of high-M e BSSs. This scarcity may be attributed to the absence of more massive clusters within this age range.

We perform a numerical-relativity simulation for the merger of binary neutron stars with 6 nuclear-theory-based equations of state (EOSs) described by piecewise polytropes. Our purpose is to explore the dependence of the dynamical behavior of the binary neutron star merger and resulting gravitational waveforms on the EOS of the supernuclear-density matter. The numerical results show that the merger process and the first outcome are classified into three types; (i) a black hole is promptly formed, (ii) a short-lived hypermassive neutron star (HMNS) is formed, (iii) a long-lived HMNS is formed. The type of the merger depends strongly on the EOS and on the total mass of the binaries. For the EOS with which the maximum mass is larger than 2Msun, the lifetime of the HMNS is longer than 10 ms for a total mass m_0=2.7Msun. A recent radio observation suggests that the maximum mass of spherical neutron stars is M_max \geq 1.97\pm 0.04Msun in one \sigma level. This fact and our results support the possible existence of a HMNS soon after the onset of the merger for a typical binary neutron star with m_0=2.7Msun. We also show that the torus mass surrounding the remnant black hole is correlated with the type of the merger process; the torus mass could be large, \geq 0.1Msun, in the case that a long-lived HMNS is formed. We also show that gravitational waves carry information of the merger process, the remnant, and the torus mass surrounding a black hole.

The one‐dimensional magnetohydrodynamics of shocked flows subjected to significant mass loading are considered. Recent observations at comets Giacobini‐Zinner and Halley suggest that simple nonreacting MHD is an inappropriate description for active cometary bow shocks. The thickness of the observed cometary shock implies that mass loading represents an important dynamical process within the shock itself, thereby requiring that the Rankine‐Hugoniot condition for the mass flux possess a source term. In a formal sense, this renders mass‐loading shocks qualitatively similar to combustion shocks, except that mass loading induces the shocked flow to shear. Nevertheless, a large class of stable shocks exist, identified by means of the Lax conditions appropriate to MHD. Thus mass‐loading shocks represent a new and interesting class of shocks, which, although found frequently in the solar system, both at the head of comets and, under suitable conditions, upsteam of weakly magnetized and nonmagnetized planets, has not been discussed in any detail. Owing to the shearing of the flow, mass‐loading shocks can behave like switch‐on shocks regardless of the magnitude of the plasma beta. Thus the behavior of the magnetic field in mass‐loading shocks is significantly different from that occurring in nonreacting classical MHD shocks. It is demonstrated that there exist two types of mass‐loading fronts for which no classical MHD analogue exists, these being the fast and slow compound mass‐loading shocks. These shocks are characterized by an initial deceleration of the fluid flow to either the fast or the slow magnetosonic speed followed by an isentropic expansion to the final decelerated downstream state. Thus these transitions take the flow from a supersonic to a supersonic, although decelerated, downstream state, unlike shocks which occur in classical MHD or gasdynamics. It is possible that such structures have been observed during the Giotto‐Halley encounter, and a brief discussion of the appropriate Halley parameters is therefore given, together with a short discussion of the determination of the shock normal from observations. A further interesting new form of mass‐loading shock is the “slow‐intermediate” shock, a stable shock which possesses many of the properties of intermediate MHD shocks yet which propagates like a slow mode MHD shock. An important property of mass‐loading shocks is the large parameter regime (compared with classical MHD) which does not admit simple or stable transitions from a given upstream to a downstream state. This suggests that it is often necessary to construct compound structures consisting of shocks, slip waves, rarefactions, and fast and slow compound waves in order to connect given upstream and downstream states. Thus the Riemann problem is significantly different from that of classical MHD.

What is believed to be the first direct evidence of a violent ejection, or jet, from a star relatively close to the sun has been detected by astronomers at the Goddard Space Flight Center, Greenbelt, Md.; the University of Maryland; and the Lick Observatory, University of California.Observable violent ejections are thought to be matter from the center of active galaxies or quasars. This most recent observation, however, reveals an extraordinary jet structure, known as the star R Aquarii, in a binary star system near our sun. In contrast to the only other star known to have jet structure in our galaxy, SS 433, R. Aquarii is much closer to our sun, a distance of only 750 light years.

The widest binary star systems pose a challenge to theory: true stellar twins could not form so far apart. Simulations suggest these twins are in fact triplets, two of which masquerade as one star and cast out the third. See Letter p.221 Very wide binary star systems—widely separated pairs of gravitationally bound stars—are relatively common in the Milky Way, but they challenge current theories of star formation. The problem is that their separation can exceed the typical size of the collapsing cloud of dust and gas from which the stars form. Recent observations have shown that very wide binaries are frequently members of triple systems and that close binaries often have a distant third companion. Bo Reipurth and Seppo Mikkola report N-body simulations of the dynamical evolution of newborn triple systems that demonstrate that although the triple systems are compact when they are born, they can develop extreme hierarchical architectures on timescales of millions of years as one component is dynamically scattered into a very distant orbit. The energy of ejection comes from shrinking the orbits of the other two stars, often making them look like a single star. Such loosely bound triple systems will therefore appear as very wide binaries.