Narrow Results

Under Newtonian dynamics, the relative motion of the components of a binary star should follow a Keplerian scaling with separation. Once orientation effects and a distribution of ellipticities are accounted for, dynamical evolution can be modeled to include the effects of Galactic tides and stellar mass perturbers, over the lifetime of the solar neighborhood. This furnishes a prediction for the relative velocity between the components of a binary and their projected separation. Taking a carefully selected small sample of 81 solar neighborhood wide binaries from the Hipparcos catalog, we identify these same stars in the recent Gaia DR2, to test the prediction mentioned using the latest and most accurate astrometry available. The results are consistent with the Newtonian prediction for projected separations below 7000 AU, but inconsistent with it at larger separations, where accelerations are expected to be lower than the critical $a_0=1.2\times 10^{-10}$m.s${}^{-2}$ value of MONDian gravity. This result challenges Newtonian gravity at low accelerations and shows clearly the appearance of gravitational anomalies of the type usually attributed to dark matter at galactic scales, now at much smaller stellar scales.

I outline methods for calculating the solution of Monte Carlo Radiative Transfer (MCRT) in scattering, absorption and emission processes of dust and gas, including polarization. I provide a bibliography of relevant papers on methods with astrophysical applications.

Gravity is so different from other fundamental forces that it is now essentially treated as a nonfundamental force of entropic origin. Several good studies have been carried out in this direction. Quantum gravity is a theory which combines gravity well with quantum physics. However, there are still many impediments to our understanding, especially in the limits of extreme. The effects of quantum gravity start appearing on the scene at Planck length which is the smallest length in nature idealized so far. Although this study incorporates a model which is valid for potential energy corrections at small distances, we have also given a bold try to use it consistently for the corrections at very large distances as well. The model uses two techniques, namely, Boltzmann and Tsallis statistical approaches to explore the thermodynamics within the ambit of the braneworld model giving its further modified version. We have computed the partition function by using both Boltzmann and Tsallis statistical approaches and then used it to study the thermodynamics of the braneworld model. We have analyzed both analytically and graphically, the thermodynamic quantities like Helmholtz free energy and specific heat.

The baryon acoustic oscillation (BAO) peak, seen in the cosmic matter distribution at redshifts up to $\sim$3.5, reflects the continued expansion of the sonic horizon first identified in temperature anisotropies of the cosmic microwave background. The BAO peak position can now be measured to better than $\sim$1% accuracy using galaxies and $\sim$1.4–1.6% precision with Ly-$\alpha$ forests and the clustering of quasars. In conjunction with the Alcock–Paczyński (AP) effect, which arises from the changing ratio of angular to spatial/redshift size of (presumed) spherically-symmetric source distributions with distance, the BAO measurement is viewed as one of the most powerful tools to use in assessing the geometry of the Universe. In this paper, we employ five BAO peak measurements from the final release of the Sloan Digital Sky Survey IV, at average redshifts $〈z〉=0.38$, $0.51$, $0.70$, $1.48$ and $2.33$, to carry out a direct head-to-head comparison of the standard model, $\mathrm{\Lambda }$CDM, and one of its principal competitors, known as the $R_{\mathrm{\text{h}}}=ct$ universe. For completeness, we complement the AP diagnostic with a volume-averaged distance probe that assumes a constant comoving distance scale $r_{\mathrm{\text{d}}}$. Both probes are free of uncertain parameters, such as the Hubble constant, and are therefore ideally suited for this kind of model selection. We find that $R_{\mathrm{\text{h}}}=ct$ is favored by these measurements over the standard model based solely on the AP effect, with a likelihood $\sim$75% versus $\sim$25%, while Planck-$\mathrm{\Lambda }$CDM is favored over $R_{\mathrm{\text{h}}}=ct$ based solely on the volume-averaged distance probe, with a likelihood $\sim 80\%$ versus $\sim$20%. A joint analysis using both probes produces an inconclusive outcome, yielding comparable likelihoods to both models. We are therefore not able to confirm with this work that the BAO data, on their own, support an accelerating Universe.

The Tolman test for surface brightness (SB) dimming was originally proposed as a test for the expansion of the universe. The test, which is independent of the details of the assumed cosmology, is based on comparisons of the SB of identical objects at different cosmological distances. Claims have been made that the Tolman test provides compelling evidence against a static model for the universe. In this paper we reconsider this subject by adopting a static Euclidean universe (SEU) with a linear Hubble relation at all z (which is not the standard Einstein–de Sitter model), resulting in a relation between flux and luminosity that is virtually indistinguishable from the one used for ΛCDM models. Based on the analysis of the UV SB of luminous disk galaxies from HUDF and GALEX datasets, reaching from the local universe to z ~ 5, we show that the SB remains constant as expected in a static universe.

A re-analysis of previously published data used for the Tolman test at lower redshift, when treated within the same framework, confirms the results of the present analysis by extending our claim to elliptical galaxies. We conclude that available observations of galactic SB are consistent with a SEU model.

We do not claim that the consistency of the adopted model with SB data is sufficient by itself to confirm what would be a radical transformation in our understanding of the cosmos. However, we believe this result is more than sufficient reason to examine this combination of hypotheses further.

Extremely wide binary stars represent ideal systems to probe Newtonian dynamics in the low acceleration regimes ($<10^{-10}$ms${}^{-2}$) typical of the external regions of galaxies. Here, we present a study of 60 alleged wide binary stars with projected separation ranging from 0.004pc to 1pc, probing gravitational accelerations well below the limit where dark matter or modified dynamics theories set in. Radial velocities with accuracy $\sim 100$m/s were obtained for each star, in order to constrain their orbital velocity, that, together with proper motion data, can distinguish bound from unbound systems. It was found that about half of the observed pairs do have velocity in the expected range for bound systems, out of the largest separations probed here. In particular, we identified five pairs with projected separation $>0.15$pc that are useful for the proposed test. While it would be premature to draw any conclusion about the validity of Newtonian dynamics at these low accelerations, our main result is that very wide binary stars seem to exist in the harsh environment of the solar neighborhood. This could provide a tool to test Newtonian dynamics versus modified dynamics theories in the low acceleration conditions typical of galaxies. In the near future the GAIA satellite will provide data to increase significantly the number of wide pairs that, with the appropriate follow-up spectroscopic observations, will allow the implementation of this experiment with unprecedented accuracy.

Pulsars are very stable clocks in space which have many applications to problems in physics and astrophysics. Observations of double-neutron-star binary systems have given the first observational evidence for the existence of gravitational waves (GWs) and shown that Einstein’s general theory of relativity is an accurate description of gravitational interactions in the regime of strong gravity. Observations of a large sample of pulsars spread across the celestial sphere forming a “Pulsar Timing Array” (PTA), can in principle enable a positive detection of the GW background in the Galaxy. The Parkes Pulsar Timing Array (PPTA) is making precise timing measurements of 20 millisecond pulsars at three radio frequencies and is approaching the level of timing precision and data spans which are needed for GW detection. These observations will also allow us to establish a “Pulsar Timescale” and to detect or limit errors in the Solar System ephemerides used in pulsar timing analyses. Combination of PPTA data with that of other groups to form an International Pulsar Timing Array (IPTA) will enhance the sensitivity to GWs and facilitate reaching other PTA goals. The principal source of GWs at the nanoHertz frequencies to which PTAs are sensitive is believed to be super-massive binary black holes in the cores of distant galaxies. Current results do not signficantly limit models for formation of such black-hole binary systems, but in a few years we expect that PTAs will either detect GWs or seriously constrain current ideas about black-hole formation and galaxy mergers. Future instruments such as the Square Kilometre Array (SKA) should not only detect GWs from astrophysical sources but also enable detailed studies of the sources and the gravitational theories used to account for the GW emission.

Strongly Lensed systems, and in particular gravitational arcs, are useful tools for a variety of astrophysical applications. Finding arcs in wide-field surveys such as the Dark Energy Survey (DES) requires automated algorithms to select arc candidates due to the large amount of data. In this contribution we present a new arcfinding method that uses the Mediatrix filamentation method coupled to a neural network to select arc candidates. We carry out a systematic comparison between this method and three other arcfinders available in the literature — Lenzen et al. (2004), Horesh et al. (2005), and More et al. (2012) — on a sample of arc simulated with the PaintArcs method.

New $0^{+}$ resonance parameters have been extracted and interpreted in terms of the experimental neutron–proton branching ratio of $\mathrm{\text{d+d}}$ reactions in strontium and tantalum metallic lattices. The material-dependent cross-section formula, which includes the new $0^{+}$ resonance parameters, the screening effect, and the effective mass, have been applied to $d{(d,p)}^{3}H$ and ${}^{2}H{(d,n)}^{3}He$ reactions in metallic environments. The contributions of the $0^{+}$ resonance and effective electron mass are crucial to the understanding of the discrepancy between the theoretical and experimental screening values in metallic environments. As a different and connected part of this research, we have proposed a new target material: deuterated metal wire mesh structures. Experimentally confirmed numerical results for Pendry’s effective electron mass for aluminum metal wire have been used to calculate the cross-section and screening energy.

The binary system AR Scorpii hosts an M-type main sequence cool star orbiting around a magnetic white dwarf in the Milky Way Galaxy. The broadband non-thermal emission over radio, optical and X-ray wavebands observed from AR Scorpii indicates strong modulations on the spin frequency of the white dwarf as well as the spin-orbit beat frequency of the system. Therefore, AR Scorpii is also referred to as a white dwarf pulsar wherein a fast spinning white dwarf star plays very crucial role in the broadband non-thermal emission. Several interpretations for the observed features of AR Scorpii appear in the literature without firm conclusions. In this paper, we investigate connection between some of the important physical properties like spin-down power, surface magnetic field, equation of state, temperature and gravity associated with the white dwarf in the binary system AR Scorpii and its observational characteristics. We explore the plausible effects of white dwarf surface magnetic field on the absence of substantial accretion in this binary system and also discuss the gravitational wave emission due to magnetic deformation mechanism.

High energy transients make up a diverse and exotic class of objects, from terrestrial lightning to $\gamma$-ray bursts at cosmological distances. In this review, we provide a detailed look at some of the more exciting transients observed over the last few years by Swift and other high energy missions.

The Chandrasekhar limit is of key importance for the evolution of white dwarfs in binary systems and for the formation of neutron stars and black holes in binaries. Mass transfer can drive a white dwarf in a binary over the Chandrasekhar limit, which may lead to a Type Ia supernova (in case of a CO white dwarf) or an Accretion-Induced Collapse (AIC, in the case of an O-Ne-Mg white dwarf; and possibly also in some CO white dwarfs) which produces a neutron star. The direct formation of neutron stars or black holes out of degenerate stellar cores that exceed the Chandrasekhar limit, occurs in binaries with components that started out with masses ≥ 8 M_⊙.

This paper first discusses possible models for Type Ia supernovae, and then focusses on the formation of neutron stars in binary systems, by direct core collapse and by the AIC of O-Ne-Mg white dwarfs in binaries. Observational evidence is reviewed for the existence of two different direct neutron-star formation mechanisms in binaries: (i) by electron-capture collapse of the degenerate O-Ne-Mg core in stars with initial masses in the range of 8 to about 12 M_⊙, and (ii) by iron-core collapse in stars with inital masses above this range. Observations of neutron stars in binaries are consistent with a picture in which neutron stars produced by e-capture collapse have relatively low masses, ˜1.25M_⊙, and received hardly any velocity kick at birth, whereas neutron stars produced by iron-core collapses are more massive and received large velocity kicks at birth. Many of the globular cluster neutron stars and also some of the neutron stars in low-mass binaries in the Galactic disk are likely to have been produced by AIC of O-Ne-Mg white dwarfs in binaries. AIC is expected to produce normal strongly magnetized neutron stars, which in binaries can evolve into millisecond pulsars through the usual recycling scenario.

The discovery of dynamical friction was Chandrasekhar’s best known contribution to the theory of stellar dynamics, but his work ranged from the fewbody problem to the limit of large N (in effect, galaxies). Much of this work was summarised in the text “Principles of Stellar Dynamics“ (Chandrasekhar 1942, 1960), which ranges from a precise calculation of the time of relaxation, through a long analysis of galaxy models, to the behaviour of star clusters in tidal fields. The later edition also includes the work on dynamical friction and related issues. In this review we focus on progress in the collisional aspects of these problems, i.e. those where few-body interactions play a dominant role, and so we omit further discussion of galaxy dynamics. But we try to link Chandrasekhar’s fundamental discoveries in collisional problems with the progress that has been made in the 50 years since the publication of the enlarged edition.