The Monitor project: searching for occultations in young open clusters

The Monitor project is a photometric monitoring survey of nine young (1–200 Myr) clusters in the solar neighbourhood to search for eclipses by very low mass stars and brown dwarfs and for planetary transits in the light curves of cluster members. It began in the autumn of 2004 and uses several 2- to 4-m telescopes worldwide. We aim to calibrate the relation between age, mass, radius and where possible luminosity, from the K dwarf to the planet regime, in an age range where constraints on evolutionary models are currently very scarce. Any detection of an exoplanet in one of our youngest targets (≲10 Myr) would also provide important constraints on planet formation and migration time-scales and their relation to protoplanetary disc lifetimes. Finally, we will use the light curves of cluster members to study rotation and flaring in low-mass pre-main-sequence stars.
The present paper details the motivation, science goals and observing strategy of the survey. We present a method to estimate the sensitivity and number of detections expected in each cluster, using a simple semi-analytic approach which takes into account the characteristics of the cluster and photometric observations, using (tunable) best-guess assumptions for the incidence and parameter distribution of putative companions, and we incorporate the limits imposed by radial velocity follow-up from medium and large telescopes. We use these calculations to show that the survey as a whole can be expected to detect over 100 young low and very low mass eclipsing binaries, and ∼3 transiting planets with radial velocity signatures detectable with currently available facilities.     

Reconciling optical and X-ray mass estimates: the case of the elliptical galaxy NGC 3379

NGC 3379 is a well-studied nearby elliptical for which optical investigations have claimed a little dark matter content, or even no dark matter. Recently, its total mass profile M ( r ) has been derived by exploiting Chandra observations of its extended and X-ray emitting interstellar medium, based on the hypothesis of hydrostatic equilibrium for the hot gas. The resulting total mass within the effective radius R _e has been claimed to be a few times larger than that found by optical studies. Here, we show that part of the discrepancy can be due to an underestimate of the optically derived mass, and the remaining discrepancy of a factor of ∼2 can be explained by deviations from hydrostatic equilibrium of the hot gas. By using hydrodynamical simulations tailored to reproduce the observed hot gas properties of NGC 3379, and by assuming as input for the simulations the total mass profile derived optically, we show that (i) the hot gas at the present time has X-ray properties consistent with those observed only if it is outflowing over most of the galactic body, and (ii) an overestimate of M of the same size found in the recent X-ray analysis is recovered when assuming hydrostatic equilibrium. We also show that the hot gas is outflowing even for a dark matter fraction within R _e as large as derived with the standard X-ray procedure based on the hydrostatic equilibrium assumption, which shows the unapplicability of the method for this galaxy. Finally, we find that the whole range of dark mass amount and distribution allowed for by optical studies is compatible with a hot gas flow with the observed X-ray properties.     

An analytic column density profile to fit prestellar cores

We present a new analytical three-parameter formula to fit observed column density profiles of prestellar cores. It represents a line-of-sight integral through a spherically symmetric or disc-like isothermal cloud. The underlying model resembles the Bonnor–Ebert model in that it features a flat central region leading into a power-law decline  ∝ r ⁻²  in density, and a well-defined outer radius. However, we do not assume that the cloud is in equilibrium, and can instead make qualitative statements about its dynamical state (expansion, equilibrium, collapse) using the size of the flat region as a proxy. Instead of having temperature as a fitting parameter, our model includes it as input, and thus avoids possible inconsistencies. It is significantly easier to fit to observational data than the Bonnor–Ebert sphere. We apply this model to L1689B and B68. We show that L1689B cannot be in equilibrium but instead appears to be collapsing, while our model verifies that B68 is not far from being a hydrostatic object.     

The hydrodynamics of dead radio galaxies

We present a numerical investigation of dead, or relic, radio galaxies and the environmental impact that radio galaxy activity has on the host galaxy or galaxy cluster. We perform axisymmetric hydrodynamical calculations of light, supersonic, back-to-back jets propagating in a β -model galaxy/cluster atmosphere. We then shut down the jet activity and let the resulting structure evolve passively. The dead source undergoes an initial phase of pressure driven expansion until it achieves pressure equilibrium with its surroundings. Thereafter, buoyancy forces drive the evolution and lead to the formation of two oppositely directed plumes that float high into the galaxy/cluster atmosphere. These plumes entrain a significant amount of low entropy material from the galaxy/cluster core and lift it high into the atmosphere. An important result is that a large fraction (at least half) of the energy injected by the jet activity is thermalized in the interstellar medium (ISM)/intracluster medium (ICM) core. The whole ISM/ICM atmosphere inflates in order to regain hydrostatic equilibrium. This inflation is mediated by an approximately spherical disturbance which propagates into the atmosphere at the sound speed. The fact that such a large fraction of the injected energy is thermalized suggests that radio galaxies may have an important role in the overall energy budget of rich ISM/ICM atmospheres. In particular, they may act as a strong and highly time-dependent source of negative feedback for galaxy/cluster cooling flows.     

Galactic Chemical Evolution of the Lighter Neutron Capture Elements Sr, Y and Zr

Based on a three-component Galaxy chemical evolutionary model satisfying a large set of Galactic and extragalactic constraints, we compute the chemical evolution of the lighter neutron capture elements (e.g., Sr, Y and Zr) taking into account contributions from three processes. We compare our model results with available observational results and find that the observed trends can be understood in the light of present knowledge of neutron capture nucleosynthesis.     

Testing Lorentz violation using propagating UHECRs

Lorentz invariant violation (LIV) test is important for studying modem physics.All the known astrophysical constraints either have a very small examinable parameter space or are only suitable for some special theoretical models. Here, we suggest that it is possible to directly detect the time-delay of ultra-high-energy cosmic-rays (UHECRs). We discuss some difficulties in our method, including the intergalactic magnetic fields. It seems that none of them are crucial, hence this method could give a larger examinable parameter space and a stronger constraint on LIV.     

Tailoring triaxial N-body models via a novel made-to-measure method

The made-to-measure N -body method slowly adapts the particle weights of an N -body model, whilst integrating the trajectories in an assumed static potential, until some constraints are satisfied, such as optimal fits to observational data. I propose a novel technique for this adaption procedure, which overcomes several limitations and shortcomings of the original method. The capability of the new technique is demonstrated by generating realistic N -body equilibrium models for dark matter haloes with prescribed density profile, triaxial shape and slowly outwardly growing radial velocity anisotropy.     

The inner magnetized regions of circumstellar accretion discs

In this lecture, I will briefly address several phenomena expected when magnetic fields are present in the innermost regions of circumstellar accretion discs: (i) the magneto-rotational instability and related “dead zones”; (ii) the formation of magnetically-driven jets and the observational constraints derived from Classical T Tauri stars; (iii) the magnetic star–disc interactions and their expected role in the stellar spin down.It should be noted that the magnetic fields invoked here are organized large scale magnetic fields, not turbulent small scale ones. I will therefore first argue why one can safely expect these fields to be present in circumstellar accretion discs. Objects devoid of such large scale fields would not be able to drive jets. A global picture is thus gradually emerging where the magnetic flux is an important control parameter of the star formation process as a whole. High angular resolution technics, by probing the innermost circumstellar disc regions should provide valuable constraints.     

Magnetic deformation of the white dwarf surface structure

The influence of strong, large‐scale magnetic fields on the structure and temperature distribution in white dwarf atmospheres is investigated. Magnetic fields may provide an additional component of pressure support, thus possibly inflating the atmosphere compared to the non‐magnetic case. Since the magnetic forces are not isotropic, atmospheric properties may significantly deviate from spherical symmetry. In this paper the magnetohydrostatic equilibrium is calculated numerically in the radial direction for either for small deviations from different assumptions for the poloidal current distribution. We generally find indication that the scale height of the magnetic white dwarf atmosphere enlarges with magnetic field strength and/or poloidal current strength. This is in qualitative agreement with recent spectropolarimetric observations of Grw+10°8247. Quantitatively, we .nd for e.g. a mean surface poloidal field strength of 100 MG and a toroidal field strength of 2‐10 MG an increase of scale height by a factor of 10. This is indicating that already a small deviation from the initial force‐free dipolar magnetic field may lead to observable effects. We further propose the method of finite elements for the solution of the two‐dimensional magnetohydrostatic equilibrium including radiation transport in the diffusive approximation. We present and discuss preliminary solutions, again indicating on an expansion of the magnetized atmosphere.     

Searching for tidal tails – investigating galaxy harassment

Galaxy harassment has been proposed as a physical process that morphologically transforms low surface density disc galaxies into dwarf elliptical galaxies in clusters. It has been used to link the observed very different morphology of distant cluster galaxies (relatively more blue galaxies with 'disturbed' morphologies) with the relatively large numbers of dwarf elliptical galaxies found in nearby clusters. One prediction of the harassment model is that the remnant galaxies should lie on low surface brightness tidal streams or arcs. We demonstrate in this paper that we have an analysis method that is sensitive to the detection of arcs down to a surface brightness of 29 B μ and we then use this method to search for arcs around 46 Virgo cluster dwarf elliptical galaxies. We find no evidence for tidal streams or arcs and consequently no evidence for galaxy harassment as a viable explanation for the relatively large numbers of dwarf galaxies found in the Virgo cluster.     

Equilibrium resurfacing of Venus: Results from new Monte Carlo modeling and implications for Venus surface histories

Venus’ impact crater population imposes two observational constraints that must be met by possible model surface histories: (1) near random spatial distribution of ～975 craters, and (2) few obviously modified impact craters. Catastrophic resurfacing obviously meets these constraints, but equilibrium resurfacing histories require a balance between crater distribution and modification to be viable. Equilibrium resurfacing scenarios with small incremental resurfacing areas meet constraint 1 but not 2, whereas those with large incremental resurfacing areas meet constraint 2 but not 1. Results of Monte Carlo modeling of equilibrium resurfacing (Strom et al., 1994) is widely cited as support for catastrophic resurfacing hypotheses and as evidence against hypotheses of equilibrium resurfacing. However, the Monte Carlo models did not consider intermediate-size incremental resurfacing areas, nor did they consider histories in which the era of impact crater formation outlasts an era of equilibrium resurfacing. We construct three suites of Monte Carlo experiments that examine incremental resurfacing areas not previously considered (5%, 1%, 0.7%, and 0.1%), and that vary the duration of resurfacing relative to impact crater formation time (1:1 [suite A], 5:6 [suite B], and 2:3 [suite C]). We test the model results against the two impact crater constraints.Several experiments met both constraints. The shorter the time period of equilibrium resurfacing, or the longer the time of crater formation following the cessation of equilibrium resurfacing, the larger the possible areas of incremental resurfacing that satisfy both constraints. Equilibrium resurfacing is statistically viable for suite A at 0.1%, suite B at 0.1%, and suite C for 1%, 0.7%, and 0.1% areas of incremental resurfacing.     

High-energy γ-rays from globular clusters

It is expected that specific globular clusters (GCs) can contain up to a hundred of millisecond pulsars. These pulsars can accelerate leptons at the shock waves originated in collisions of the pulsar winds and/or inside the pulsar magnetospheres. Energetic leptons diffuse gradually through the GC Comptonizing stellar and microwave background radiation. We calculate the GeV–TeV γ-ray spectra for different models of injection of leptons and parameters of the GCs assuming reasonable, of the order of 1 per cent, efficiency of energy conversion from the pulsar winds into the relativistic leptons. It is concluded that leptons accelerated in the GC cores should produce well localized γ-ray sources which are concentric with these GCs. The results are shown for four specific GCs (47 Tuc, Ter 5, M13 and M15), in which significant population of millisecond pulsars have been already discovered. We argue that the best candidates, which might be potentially detected by the present Cherenkov telescopes and the planned satellite telescopes (AGILE, GLAST), are 47 Tuc on the Southern hemisphere, and M13 on the Northern hemisphere. We conclude that detection (or non-detection) of GeV–TeV γ-ray emission from GCs by these instruments put important constraints on the models of acceleration of leptons by millisecond pulsars.     

A method to constrain the total mass of galaxy groups

We present a simple method to constrain the total mass of groups of galaxies. Tidal theory predicts that a limit to the mass of bound groups of galaxies can be obtained by using the fact that the tidal forces due to the external mass distributions are insufficient to disrupt the groups. To illustrate how the method works, we find tidal limits on the mass of 11 nearby galaxy groups. In most cases, tidal limits placed on these groups show that the mass estimations obtained from methods based either on the application of the virial theorem or moments of the projected mass are underestimated by a factor of ∼2 even if, in many cases, errors are large. Three groups show virial parameters fully concordant with the tidal constraints while two outlier groups show anomalous results. The “irregular” state of the latter suggests that the reliability of the method depends on the physical properties of the test groups, which should match the fundamental assumptions of spherical symmetry and dynamical equilibrium state.     

Models of flux tubes from constrained relaxation

We study the relaxation of a compressible plasma to an equilibrium with flow. The constraints of conservation of mass, energy, angular momentum, cross-helicity and relative magnetic helicity are imposed. Equilibria corresponding to the energy extrema while conserving these invariants for parallel flows yield three classes of solutions and one of them with an increasing radial density profile, relevant to solar flux tubes is presented.     

Tentative detection of warm intervening gas towards PKS 0548-322 with XMM–Newton

We present the results of a long (∼93 ks) XMM–Newton observation of the bright BL-Lac object  PKS 0548-322 ( z = 0.069)  . Our Reflection Grating Spectrometer (RGS) spectrum shows a single absorption feature at an observed wavelength  λ= 23.33 ± 0.01 Å  , which we interpret as O  vi Kα absorption at   z = 0.058  , i.e. ∼3000 km s⁻¹ from the background object. The observed equivalent width of the absorption line, ∼30 mÅ, coupled with the lack of the corresponding absorption edge in the EPIC pn data, implies a column density of   N _O VI∼ 2 × 10¹⁶ cm⁻²  and turbulence with a Doppler velocity parameter   b > 100 km s⁻¹  . Within the limitations of our RGS spectrum, no O  vii or O  v Kα absorption are detected. Under the assumption of ionization equilibrium by both collisions and the extragalactic background, this is only marginally consistent if the gas temperature is  ∼2.5 × 10⁵ K  , with significantly lower or higher values being excluded by our limits on O  v or O  vii . If confirmed, this would be the first X-ray detection of a large amount of intervening warm absorbing gas through O  vi absorption. The existence of such a high column density absorber, much stronger than any previously detected one in O  vi , would place stringent constraints on the large-scale distribution of baryonic gas in the Universe.     

Properties of the double β model for intracluster gas

We present an extensive study of the double β model for the X-ray surface brightness profiles of clusters, and derive analytically the gas density and total masses of clusters under the hydrostatic equilibrium hypothesis. It is shown that the employment of the double β model instead of the conventional single β model can significantly improve the goodness-of-fit to the observed X-ray surface brightness profiles of clusters, which will in turn lead to a better determination of the gas and total mass distributions in clusters. In particular, the observationally fitted β parameter for the extended component in a double β model may become larger. This opens a new possibility of resolving the long-standing β discrepancy for clusters. Using an ensemble of 33 ROSAT PSPC observed clusters drawn from the Mohr, Mathiesen & Evrard sample, we find that the asymptotic value of β _fit is 0.83±0.33 at large radii, consistent with both the average spectroscopic parameter β _spec=0.78±0.37 and the result given by numerical simulations.     

Pseudo-Newtonian models for the equilibrium structures of rotating relativistic stars

We obtain equilibrium solutions for rotating compact stars, including special relativistic effects. The gravity is assumed to be Newtonian, but we use the active mass density, which takes into account all energies such as the motion of the fluid, internal energy and pressure energy in addition to the rest-mass energy, in computing the gravitational potential using Poisson's equation. Such a treatment could be applicable to neutron stars with relativistic motions or a relativistic equation of state. We applied Hachisu's self-consistent field (SCF) method to find spheroidal as well as toroidal sequences of equilibrium solutions. Our solutions show better agreement with general relativistic solutions than the Newtonian relativistic hydrodynamic approach, which does not take into account the active mass. Physical quantities such as the peak density and equatorial radii in our solutions agree with the general relativistic ones to within 5 per cent. Therefore our approach can be used as a simple alternative to the fully relativistic one when a large number of model calculations is necessary, as it requires much fewer computational resources.