Showing papers in "Monthly Notices of the Royal Astronomical Society: Letters in 2011"

Too big to fail? The puzzling darkness of massive Milky Way subhaloes

Abstract: We show that dissipationless CDM simulations predict that the majority of themost massive subhalos of the Milky Way are too dense to host any of its brightsatellites (L V > 10 5 L ). These dark subhalos have circular velocities at infall ofV infall = 30 1070kms 1 and infall masses of [0:2 4] 10 M . Unless the Milky Way isa statistical anomaly, this implies that galaxy formation becomes e ectively stochasticat these masses. This is in marked contrast to the well-established monotonic relationbetween galaxy luminosity and halo circular velocity (or halo mass) for more massivehalos. We show that at least two (and typically four) of these massive dark subhalosare expected to produce a larger dark matter annihilation ux than Draco. It maybe possible to circumvent these conclusions if baryonic feedback in dwarf satellites ordi erent dark matter physics can reduce the central densities of massive subhalos byorder unity on a scale of 0.3 { 1 kpc.Key words: Galaxy: halo { galaxies: abundances { dark matter { cosmology: theory

Efficient Generation of Jets from Magnetically Arrested Accretion on a Rapidly Spinning Black Hole

Abstract: We describe global, 3D, time-dependent, non-radiative, general-relativistic, magnetohydrodynamic simulations of accreting black holes (BHs). The simulations are designed to transport a large amount of magnetic flux to the centre, more than the accreting gas can force into the BH. The excess magnetic flux remains outside the BH, impedes accretion, and leads to a magnetically arrested disc. We find powerful outflows. For a BH with spin parameter a = 0.5, the efficiency with which the accretion system generates outflowing energy in jets and winds is η ≈ 30 per cent. For a = 0.99, we find η ≈ 140 per cent, which means that more energy flows out of the BH than flows in. The only way this can happen is by extracting spin energy from the BH. Thus the a = 0.99 simulation represents an unambiguous demonstration, within an astrophysically plausible scenario, of the extraction of net energy from a spinning BH via the Penrose–Blandford–Znajek mechanism. We suggest that magnetically arrested accretion might explain observations of active galactic nuclei with apparent η ≈ few × 100 per cent.

Cooling neutron star in the Cassiopeia A supernova remnant: evidence for superfluidity in the core

Abstract: According to recent results of Ho & Heinke, the Cassiopeia A supernova remnant contains a young (?330-yr-old) neutron star (NS) which has carbon atmosphere and shows notable decline of the effective surface temperature. We report a new (2010 November) Chandra observation which confirms the previously reported decline rate. The decline is naturally explained if neutrons have recently become superfluid (in triplet state) in the NS core, producing a splash of neutrino emission due to Cooper pair formation (CPF) process that currently accelerates the cooling. This scenario puts stringent constraints on poorly known properties of NS cores: on density dependence of the temperature Tcn(?) for the onset of neutron superfluidity [Tcn(?) should have a wide peak with maximum ? (7–9) × 108 K]; on the reduction factor q of CPF process by collective effects in superfluid matter (q > 0.4) and on the intensity of neutrino emission before the onset of neutron superfluidity (30–100 times weaker than the standard modified Urca process). This is serious evidence for nucleon superfluidity in NS cores that comes from observations of cooling NSs

Preparation of Kepler light curves for asteroseismic analyses

Abstract: The Kepler mission is providing photometric data of exquisite quality for the asteroseismic study of different classes of pulsating stars. These analyses place particular demands on the pre-processing of the data, over a range of time-scales from minutes to months. Here, we describe processing procedures developed by the Kepler Asteroseismic Science Consortium to prepare light curves that are optimized for the asteroseismic study of solar-like oscillating stars in which outliers, jumps and drifts are corrected.

How neutral is the intergalactic medium surrounding the redshift z = 7.085 quasar ULAS J1120+0641?

Abstract: The quasar ULAS J1120+0641 at redshift z = 7.085 has a highly ionized near zone which is smaller than those around quasars of similar luminosity at z ≃ 6. The spectrum also exhibits evidence for a damping wing extending redward of the systemic Lyα redshift. We use radiative transfer simulations in a cosmological context to investigate the implications for the ionization state of the inhomogeneous intergalactic medium (IGM) surrounding this quasar. Our simulations show that the transmission profile is consistent with an IGM in the vicinity of the quasar with a volume averaged H i fraction of and that ULAS J1120+0641 has been bright for 106–107 yr. The observed spectrum is also consistent with smaller IGM neutral fractions, , if a damped Lyα system in an otherwise highly ionized IGM lies within 5 proper Mpc of the quasar. This is, however, predicted to occur in only ∼5 per cent of our simulated sightlines for a bright phase of 106–107 yr. Unless ULAS J1120+0641 grows during a previous optically obscured phase, the low age inferred for the quasar adds to the theoretical challenge of forming a 2 × 109 M⊙ black hole at this high redshift.

The causal connection between disc and power-law variability in hard state black hole X-ray binaries

Abstract: We use the XMM–Newton EPIC-pn instrument in timing mode to extend spectral time-lag studies of hard state black hole X-ray binaries into the soft X-ray band. We show that variations of the disc blackbody emission substantially lead variations in the power-law emission, by tenths of a second on variability time-scales of seconds or longer. The large lags cannot be explained by Compton scattering but are consistent with time delays due to viscous propagation of mass accretion fluctuations in the disc. However, on time-scales less than a second the disc lags the power-law variations by a few milliseconds, consistent with the disc variations being dominated by X-ray heating by the power law, with the short lag corresponding to the light traveltime between the power-law emitting region and the disc. Our results indicate that instabilities in the accretion disc are responsible for continuum variability on time-scales of seconds or longer and probably also on shorter time-scales.

The small covering factor of cold accretion streams

Abstract: Theoretical models of galaxy formation predict that galaxies acquire most of their baryons via cold mode accretion. Observations of high-redshift galaxies, while showing ubiquitous outflows, have so far not revealed convincing traces of the predicted cold streams, which has been interpreted as a challenge for the current models. Using high-resolution, zoom-in smooth particle hydrodynamics simulations of Lyman break galaxy (LBG) haloes combined with ionizing radiative transfer, we quantify the covering factor of the cold streams at z= 2–4. We focus specifically on Lyman limit systems (LLSs) and damped Lyα absorbers (DLAs), which can be probed by absorption spectroscopy using a background galaxy or quasar sightline, and which are closely related to low-ionization metal absorbers. We show that the covering factor of these systems is relatively small and decreases with time. At z= 2, the covering factor of DLAs within the virial radius of the simulated galaxies is ∼3 per cent (∼1 per cent within twice this projected distance), and arises principally from the galaxy itself. The corresponding values for LLSs are ∼10 and 4 per cent. Because of their small covering factor compared to the order unity covering fraction expected for galactic winds, the cold streams are naturally dominated by outflows in stacked spectra. We conclude that the existing observations are consistent with the predictions of cold mode accretion, and outline promising kinematic and chemical diagnostics to separate out the signatures of galactic accretion and feedback.

The distinction between star clusters and associations

Abstract: In Galactic studies a distinction is made between (open) star clusters and associations. For barely resolved objects at a distance of several Mpc this distinction is not trivial to make. Here we provide an objective definition by comparing the age of the stars to the crossing time of nearby stellar agglomerates. We find that a satisfactory separation can be made where this ratio equals unity. Stellar agglomerates for which the age of the stars exceeds the crossing time are bound, and are referred to as star clusters. Alternatively, those for which the crossing time exceeds the stellar age are unbound and are referred to as associations. This definition is useful whenever reliable measurements for the mass, radius and age are available.

Imprints of the anisotropic inflation on the cosmic microwave background

Abstract: We study the imprints of the anisotropic inflation on the CMB temperature fluctuations and polarizations. The statistical anisotropy stems not only from the direction dependence of curvature and tensor perturbations, but also from the cross-correlation between curvature and tensor perturbations, and the linear polarization of tensor perturbations. We show that off-diagonal TB and EB spectra as well as on- and off-diagonal TT, EE, BB and TE spectra are induced from the anisotropic inflation. We emphasize that the off-diagonal spectra induced by the cross-correlation could be a characteristic signature of the anisotropic inflation.

Large-scale outflows in galaxies

Abstract: We discuss massive outflows in galaxy bulges, particularly the ones driven by accretion episodes where the central supermassive black hole reaches the Eddington limit. We show that the quasar radiation field Compton-cools the wind shock until this reaches distances ∼1 kpc from the black hole, but becomes too dilute to do this at larger radii. Radiative processes cannot cool the shocked gas within the flow time at any radius. Outflows are therefore momentum driven at small radii (as required to explain the M–σ relation). At large radii, they are energy driven, contrary to recent claims. We solve analytically the motion of an energy-driven shell after the central source has turned off. This shows that the thermal energy in the shocked wind can drive further expansion for a time ∼10 times longer than the active time of the central source. Outflows observed at large radii with no active central source probably result from an earlier short (few Myr) active phase of this source.

Runaway accretion of metals from compact discs of debris on to white dwarfs

Abstract: It was recently proposed that metal-rich white dwarfs (WDs) accrete their metals from compact discs of debris found to exist around more than a dozen of them. At the same time, elemental abundances measured in atmospheres of some WDs imply vigorous metal accretion at rates up to 1011 g s−1, far in excess of what can be supplied solely by Poynting–Robertson drag acting on such discs of debris. To explain this observation we propose a model, in which rapid transport of metals from the disc on to the WD naturally results from interaction between this particulate disc and a spatially coexisting disc of metallic gas. The latter is fed by evaporation of debris particles at the sublimation radius located at several tens of WD radii. Because of pressure support the gaseous disc orbits the WD slower than the particulate disc. Resultant azimuthal drift between them at speed ≲1 m s−1 causes aerodynamic drag on the disc of solids and drives inward migration of its constituent particles. Upon reaching the sublimation radius, particles evaporate, enhancing the density of the metallic gaseous disc and leading to positive feedback. Under favourable circumstances (low viscosity in the disc of metallic gas and efficient aerodynamic coupling between the discs) a system evolves in a runaway fashion, destroying the discs of debris on time-scale of ∼105 yr, and giving rise to high metal accretion rates up to g s−1, in agreement with observations.

The Milky Way's Fermi bubbles: Echoes of the last quasar outburst?

Abstract: Fermi-LAT has recently detected two gamma-ray bubbles disposed symmetrically with respect to the Galactic plane. The bubbles have been suggested to be in a quasi-steady state, inflated by ongoing star formation over the age of the Galaxy. Here we propose an alternative picture where the bubbles are the remnants of a large-scale wide-angle outflow from Sgr A ∗ , the supermassive black hole (SMBH) of our Galaxy. Such an outflow would be a natural consequence of a short but bright accretion event on to Sgr A ∗ if it happened concurrently with the well-known star formation event in the inner 0.5 pc of the Milky Way ∼6 Myr ago. We find that the hypothesized near-spherical outflow is focused into a pair of symmetrical lobes by the greater gas pressure along the Galactic plane. The outflow shocks against the interstellar gas in the Galaxy bulge. Gamma-ray emission could be powered by cosmic rays created by either Sgr A ∗ directly or accelerated in the shocks with the external medium. The Galaxy disc remains unaffected, agreeing with recent observational evidence that SMBHs do not correlate with galaxy disc

Low‐resolution sodium D absorption is a bad proxy for extinction

Abstract: Dust extinction is generally the least tractable systematic uncertainty in astronomy, and particularly in supernova science. Often in the past, studies have used the equivalent width of Na I D absorption measured from low-resolution spectra as proxies for extinction, based on tentative correlations that were drawn from limited data sets. We show here, based on 443 low-resolution spectra of 172 Type Ia supernovae for which we have measured the dust extinction as well as the equivalent width of Na I D, that the two barely correlate. We briefly examine the causes for this large scatter that effectively prevents one from inferring extinction using this method.

The subaru Lyα blob survey: A sample of 100-kpc Lyα blobs at z = 3

Abstract: We present results of a survey for giant Lyα blobs (LABs) at z = 3 with Subaru/Suprime-Cam. We obtained Lyα imaging at z = 3.09 ± 0.03 around the SSA22 protocluster and in several blank fields. The total survey area is 2.1 deg 2 , corresponding to a comoving volume of 1.6 × 10 6 Mpc 3 . Using a uniform detection threshold of 1.4 × 10 −18 erg s −1 cm −2 arcsec −2 for the Lyα images, we construct a sample of 14 LAB candidates with major-axis diameters larger than 100 kpc, including five previously known blobs and two known quasars. This survey triples the number of known LABs over 100 kpc. The giant LAB sample shows a possible ‘morphology–density relation’: filamentary LABs reside in average density environments as derived from compact Lyα emitters, while circular LABs reside in both average density and overdense environments. Although it is hard to examine the formation mechanisms of LABs only from the Lyα morphologies, more filamentary LABs may relate to cold gas accretion from the surrounding intergalactic medium (IGM) and more circular LABs may relate to large-scale gas outflows, which are driven by intense starbursts and/or by active galactic nucleus activities. Our survey highlights the potential usefulness of giant LABs to investigate the interactions between galaxies and the surrounding IGM from the field to overdense environments at high redshift.

The shocking transit of WASP-12b: modelling the observed early ingress in the near-ultraviolet

Abstract: Near-ultraviolet (near-UV) observations of WASP-12b have revealed an early ingress compared to the optical transit light curve. This has been interpreted as due to the presence of a magnetospheric bow shock which forms when the relative velocity of the planetary and stellar material is supersonic. We aim to reproduce this observed early ingress by modelling the stellar wind (or coronal plasma) in order to derive the speed and density of the material at the planetary orbital radius. From this, we determine the orientation of the shock and the density of compressed plasma behind it. With this model for the density structure surrounding the planet we perform Monte Carlo radiation transfer simulations of the near-UV transits of WASP-12b with or without bow shock. We find that we can reproduce the transit light curves with a wide range of plasma temperatures, shock geometries and optical depths. Our results support the hypothesis that a bow shock could explain the observed early ingress.

Detection of a planetary system orbiting the eclipsing polar HU Aqr

Abstract: Using the precise times of mid-egress of the eclipsing polar HU Aqr, we discovered that this polar is orbited by two or more giant planets. The two planets detected so far have masses of at least 5.9 and 4.5M(Jup). Their respective distances from the polar are 3.6 and 5.4 au with periods of 6.54 and 11.96 yr, respectively. The observed rate of decrease of period derived from the downward parabolic change in the observed - calculated (O - C) curve is a factor of 15 larger than the value expected for gravitational radiation. This indicates that it may be only a part of a long-period cyclic variation, revealing the presence of one more planet. It is interesting to note that the two detected circumbinary planets follow the Titus-Bode law of solar planets with n = 5 and 6. We estimate that another 10 yr of observations will reveal the presence of the predicted third planet.

Negative X-ray reverberation time delays from MCG–6-30-15 and Mrk 766★

Abstract: We present an X-ray time lag analysis, as a function of Fourier frequency, for MCG–6-30-15 and Mrk 766 using long-termXMM–Newton light curves in the 0.5–1.5 and the 2–4 keV energy bands, together with some physical modelling of the corresponding time lag spectra. Both the time lag spectra of MCG–6-30-15 and Mrk 766 show negative values (i.e. soft band variations lag behind the corresponding hard band variations) at high frequencies, around 10 −3 Hz, similar to those previously observed from 1H 0707−495. The remarkable morphological resemblance between the time lag spectra of MCG–6-30-15 and Mrk 766 indicate that the physical processes responsible for the observed soft time delays are very similar in the two sources, favouring a reflection scenario from material situated very nearby to the central black hole.

A dynamical analysis of the proposed HU Aquarii planetary system

Abstract: It has recently been suggested that the eclipsing polar HU Aquarii is a host to at least two giant planets. We have performed highly detailed dynamical analyses of the orbits of those planets and showed that the proposed system is highly unstable on time-scales of <5 × 10 3 yr. For the coplanar orbits suggested in the discovery Letter, we find stable orbital solutions for the planetary system only if the outer body moves on an orbit that brings it no closer to the host star than ~6 au. The required periastron distance for the outer planet lies approximately 5 Hill radii beyond the orbit of the inner planet, and well beyond the 1σ error bars placed on the orbit of the outer planet in the discovery Letter. If the orbits of the proposed planets are significantly inclined with respect to one another, the median stability increases slightly, but such systems still become destabilized on astronomically minute time-scales (typically within a few 10 4 yr). Only in the highly improbable scenario where the outer planet follows a retrograde but coplanar orbit (i.e. inclined by 180° to the orbit of the inner planet) is there any significant region of stability within the original 1σ orbital uncertainties. Our results suggest that, if there is a second (and potentially, a third planet) in the HU Aquarii system, its orbit is dramatically different from that suggested in the discovery Letter, and that more observations are critically required in order to constrain the nature of the suggested orbital bodies.

Evidence of major dry mergers at M* > 2 × 1011M⊙ from curvature in early-type galaxy scaling relations?

Abstract: For early-type galaxies, the correlations between stellar mass and size, velocity dispersion, surface brightness, colour, axial ratio and colour gradient all indicate that two mass scales, M*= 3 × 1010 and 2 × 1011 M⊙, are special. The smaller scale could mark the transition between wet and dry mergers, or it could be related to the interplay between supernovae (SNe) and active galactic nuclei (AGNs) feedback, although quantitative measures of this transition may be affected by morphological contamination. At the more massive scale, mean axial ratios and colour gradients are maximal, and above it, the colours are redder, the sizes larger and the velocity dispersions smaller than expected based on the scaling at lower M*. In contrast, the colour–σ relation, and indeed, most scaling relations with σ, are not curved: they are well described by a single power law, or in some cases, are almost completely flat. When major dry mergers change masses, sizes, axial ratios and colour gradients, they are expected to change the colours or velocity dispersions much less. Therefore, the fact that scaling relations at σ > 150 km s −1 show no features, whereas the size–M*, b/a–M*, colour–M* and colour gradient–M* relations do, suggests that M*= 2 × 1011 M⊙ is the scale above which major mergers dominate the assembly histories of early-type galaxies.

The impact of primordial supersonic flows on early structure formation, reionization and the lowest-mass dwarf galaxies

Abstract: Tseliakhovich and Hirata recently discovered that higher order corrections to the cosmological linear-perturbation theory lead to supersonic coherent baryonic flows just after recombination (i.e. z ≈ 1020), with rms velocities of ˜30 km s-1 relative to the underlying dark matter distribution, on comoving scales of ≲3 Mpc h-1. To study the impact of these coherent flows, we performed high-resolution N-body plus smoothed particle hydrodynamic simulations in boxes of 5.0 and 0.7 Mpc h-1, for bulk-flow velocities of 0 (as reference), 30 and 60 km s-1. The simulations follow the evolution of cosmic structures by taking into account detailed, primordial, non-equilibrium gas chemistry (i.e. H, He, H2, HD, HeH, etc.), cooling, star formation and feedback effects from stellar evolution. We find that these bulk flows suppress star formation in low-mass haloes (i.e. Mvir≲ 108 M⊙ until z ˜ 13), lower the abundance of the first objects by ˜1-20 per cent and as a consequence delay cosmic star formation history by ˜2 × 107 yr. The gas fractions in individual objects can change by up to a factor of 2 at very early times. Coherent bulk flow therefore has implications for (i) the star formation in the lowest-mass haloes (e.g. dSphs); (ii) the start of reionization by suppressing it in some patches of the Universe; and (iii) the heating (i.e. spin temperature) of neutral hydrogen. We speculate that the patchy nature of reionization and heating on several Mpc scales could lead to enhanced differences in the H I spin temperature, giving rise to stronger variations in the H I brightness temperatures during the late dark ages.

Can rotation explain the multiple main-sequence turn-offs of Magellanic Cloud star clusters?

Abstract: Many intermediate-age star clusters in the Magellanic Clouds present multiple main-sequence turn-offs (MMSTOs), which challenge the classical idea that star formation in such objects took place over short time-scales. It has been recently suggested that the presence of fast rotators among main-sequence stars could be the cause of such features, hence relaxing the need for extended periods of star formation. In this Letter, we compute evolutionary tracks and isochrones of models with and without rotation. We find that, for the same age and input physics, both kinds of models present turn-offs with an almost identical position in the colour–magnitude diagrams (CMDs). As a consequence, a dispersion of rotational velocities in coeval ensembles of stars could not explain the presence of MMSTOs. We construct several synthetic CMDs for the different kinds of tracks and combinations of them. The models that best reproduce the morphology of observed MMSTOs are clearly those assuming a significant spread in the stellar ages – as long as ∼400 Myr – added to a moderate amount of convective core overshooting. Only these models produce the detailed ‘golf club’ shape of observed MMSTOs. A spread in rotational velocities alone cannot do anything similar. We also discuss models involving a mixture of stars with and without overshooting, as an additional scenario to producing MMSTOs with coeval populations. We find that they produce turn-offs with a varying extension in the CMD direction perpendicular to the lower main sequence, which are clearly not present in observed MMSTOs.

Discovery of a possibly old galaxy at z= 6.027, multiply imaged by the massive cluster Abell 383

Abstract: We report the discovery of a unique z= 6.027 galaxy, multiply imaged by the cluster Abell 383 and detected in new Hubble Space Telescope ACS and WFC3 imaging, as well as in Warm Spitzer observations. This galaxy was selected as a pair of i-dropouts; its suspected high redshift was confirmed by the measurement of a strong Lyman α line in both images using Keck/DEIMOS. Combining Hubble and Spitzer photometry after correcting for contamination by line emission (estimated to be a small effect), we identify a strong Balmer break of 1.5 mag. Taking into account the magnification factor of 11.4 ± 1.9 (2.65 ± 0.17 mag) for the brightest image, the unlensed AB magnitude for the source is 27.2 ± 0.05 in the H band, corresponding to a 0.4 L* galaxy, and 25.7 ± 0.08 at 3.6 μm. The UV slope is consistent with β˜ 2.0, and from the rest-frame UV continuum we measure a current star formation rate of 2.4 ± 1.1 Ms yr-1. The unlensed half-light radius is measured to be 300 pc, from which we deduce a star-forming surface density of ˜10 Ms yr-1 kpc-2. The Lyman α emission is found to be extended over ˜3 arcsec along the slit, corresponding to ˜5 kpc in the source plane. This can be explained by the presence of a much larger envelope of neutral hydrogen around the star-forming region. Finally, fitting the spectral energy distribution (SED) using seven photometric data points with simple SED models, we derive the following properties: very little reddening, an inferred stellar mass of M*= 6 × 109 Ms, and an inferred age of ˜800 Myr (corresponding to a redshift of formation of ˜18). The star formation rate of this object was likely much stronger in the past than at the time of observation, suggesting that we may be missing a fraction of galaxies at z˜ 6 which have already faded in rest-frame UV wavelengths.

Prospects for detection of exoplanet magnetic fields through bow‐shock observations during transits

Abstract: An asymmetry between the ingress and egress times was observed in the near-UV light curve of the transit planet WASP-12b. Such asymmetry led us to suggest that the early ingress in the UV light curve of WASP-12b, compared to the optical observations, is caused by a shock around the planet, and that shocks should be a common feature in transiting systems. Here, we classify all the transiting systems known to date according to their potential for producing shocks that could cause observable light curve asymmetries. We found that 36/92 of known transiting systems would lie above a reasonable detection threshold and that the most promising candidates to present shocks are: WASP-19b, WASP-4b, WASP-18b, CoRoT-7b, HAT-P-7b, CoRoT-1b, TrES-3 and WASP-5b. For prograde planets orbiting outside the corotation radius of fast rotating stars, the shock position, instead of being ahead of the planetary motion as in WASP-12b, trails the planet. In this case, we predict that the light curve of the planet should present a late-egress asymmetry. We show that CoRoT-11b is a potential candidate to host such a behind shock and show a late egress. If observed, these asymmetries can provide constraints on planetary magnetic fields. For instance, for a planet that has a magnetic field intensity similar to Jupiter's field (∼14 G) orbiting a star whose magnetic field is between 1 and 100 G, the stand-off distance between the shock and the planet, which we take to be the size of the planet's magnetosphere, ranges from 1 to 40 planetary radii.

Dating the formation of the counter‐rotating stellar disc in the spiral galaxy NGC 5719 by disentangling its stellar populations★

Abstract: We present the results of the VLT/VIMOS integral-field spectroscopic observations of the inner 28 × 28 arcsec 2 (3.1 × 3.1 kpc 2 ) of the interacting spiral NGC 5719, which is known to host two cospatial counter-rotating stellar discs. At each position in the field of view, the observed galaxy spectrum is decomposed into the contributions of the spectra of two stellar and one ionized-gas components. We measure the kinematics and the line strengths of the Lick indices of the two stellar counter-rotating components. We model the data of each stellar component with single stellar population models that account for the α/Fe overabundance. We also derive the distribution and kinematics of the ionized-gas disc, that is associated with the younger, less rich in metals, more α-enhanced, and less luminous stellar component. They are both counter rotating with respect the main stellar body of the galaxy. These findings prove the scenario where gas was accreted first by NGC 5719 on to a retrograde orbit from the large reservoir available in its neighbourhoods as a result of the interaction with its companion NGC 5713, and subsequently fuelled the in situ formation of the counter-rotating stellar disc.

Magneto-elastic oscillations and the damping of crustal shear modes in magnetars

Abstract: In a realistic model of magneto-elastic oscillations in magnetars, we find that crustal shear oscillations, often invoked as an explanation of quasi-periodic oscillations (QPOs) seen after giant flares in soft gamma-ray repeaters (SGRs), are damped by resonant absorption on timescales of at most 0.2s, for a lower limit on the dipole magnetic field strength of 5 10 13 G. At higher magnetic field strengths (typical in magnetars) the damping timescale is even shorter, as anticipated by earlier toy-models. We have investigated a range of equations of state and masses and if magnetars are dominated by a dipole magnetic field, our findings exclude torsional shear oscillations of the crust from explaining the observed low-frequency QPOs. In contrast, we find that the Alfv´ en QPO model is a viable explanation of observed QPOs, if the dipole magnetic field strength exceeds a minimum strength of about several times 10 14 G to

Detection of visible light from the darkest world

Abstract: We present the detection of visible light from the planet TrES-2b, the darkest exoplanet currently known. By analysis of the orbital photometry from publicly available Kepler data (0.4–0.9 μm), we determine a day–night contrast amplitude of 6.5 ± 1.9 ppm (parts per million), constituting the lowest amplitude orbital phase variation discovered. The signal is detected to 3.7σ confidence and persists in six different methods of modelling the data and thus appears robust. In contrast, we are unable to detect ellipsoidal variations or beaming effects, but we do provide confidence intervals for these terms. If the day–night contrast is interpreted as being due to scattering, it corresponds to a geometric albedo of Ag= 0.0253 ± 0.0072. However, our models indicate that there is a significant emission component to dayside brightness, and the true albedo is even lower (<1 per cent). By combining our measurement with Spitzer and ground-based data, we show that a model with moderate redistribution (Pn≃ 0.3) and moderate extra optical opacity (κ′≃ 0.3–0.4) provide a compatible explanation to the data.

Radio jets from stellar tidal disruptions

Abstract: A star that passes too close to a massive black hole will be torn apart by tidal forces. The flare of photons emitted during the accretion of the stellar debris is predicted to be observable, and candidates of such events have been observed at optical to X-ray frequencies. If a fraction of the accreted material is fed into a jet, tidal flares should be detectable at radio frequencies too, thus comprising a new class of rare radio transients. Using the well-established scaling between accretion power and jet luminosity and basic synchrotron theory, we construct an empirically rooted model to predict the jet luminosity for a time-dependent accretion rate. We apply this model to stellar tidal disruptions and predict the snapshot rate of these events. For a small angle between the observer and the jet, our model reproduces the observed radio flux of the tidal flare candidate GRB 110328A. We find that future radio surveys will be able to test whether the majority of tidal disruptions are accompanied by a jet.

The link between the star formation history and [α/Fe ]

Abstract: The abundance ratios between key elements such as iron and α-process elements carry a wealth of information on the star formation history (SFH) of galaxies. So far, simple chemical evolution models have linked [α/Fe ] with the SFH time-scale, correlating large abundance ratios with short-lived SFH. The incorporation of full spectral fitting to the analysis of stellar populations allows for a more quantitative constraint between [α/Fe ] and the SFH. In this letter, we provide, for the first time, an empirical correlation between [α/Fe ] (measured from spectral indices) and the SFH (determined via a non-parametric spectral-fitting method). We offer an empirical version of the iconic outline of Thomas et al., relating star formation time-scale with galaxy mass, although our results suggest, in contrast, a significant population of old (≳10 Gyr) stars even for the lowest mass ellipticals (M/dyn ∼ 3 × 1010 M⊙). In addition, the abundance ratio is found to be strongly correlated with the time to build up the stellar component, showing that the highest [α/Fe ] (≳+0.2) are attained by galaxies with the shortest half-mass formation time (≲2 Gyr), or equivalently, with the smallest (≲40 per cent) fraction of populations younger than 10 Gyr. These observational results support the standard hypothesis that star formation incorporates the Fe-enriched interstellar medium into stars, lowering the high abundance ratio of the old populations.

Weak- and strong-field dynamos: from the Earth to the stars

Abstract: Observations of magnetism in very low mass stars recently made important progress, revealing characteristics that are now to be understood in the framework of dynamo theory. In parallel, there is growing evidence that dynamo processes in these stars share many similarities with planetary dynamos. We investigate the extent to which the weak-field versus strong-field bistability predicted for the geodynamo can apply to recent observations of two groups of very low mass fully-convective stars sharing similar stellar parameters but generating radically different types of magnetic fields. Our analysis is based on previously published spectropolarimetric and spectroscopic data. We argue that these can be interpreted in the framework of weak- and strong-field dynamos.

On the alignment of debris discs and their host stars' rotation axis - implications for spin-orbit misalignment in exoplanetary systems

Abstract: It has been widely thought that measuring the misalignment angle between the orbital plane of a transiting exoplanet and the spin of its host star was a good discriminator between different migration processes for hot-Jupiters. Specifically, well-aligned hot-Jupiter systems (as measured by the Rossiter–McLaughlin effect) were thought to have formed via migration through interaction with a viscous disc, while misaligned systems were thought to have undergone a more violent dynamical history. These conclusions were based on the assumption that the planet-forming disc was well-aligned with the host star. Recent work by a number of authors has challenged this assumption by proposing mechanisms that act to drive the star–disc interaction out of alignment during the pre-main-sequence phase. We have estimated the stellar rotation axis of a sample of stars which host spatially resolved debris discs. Comparison of our derived stellar rotation axis inclination angles with the geometrically measured debris–disc inclinations shows no evidence for a misalignment between the two.