Monthly Notices of the Royal Astronomical Society Letters

Villalobos, Á.; De Lucia, G.; Weinmann, S. M.; Borgani, S.; Murante, G.

doi: 10.1093/mnrasl/slt056pmid: N/A

We compare three analytical prescriptions for merger times available from the literature to simulations of isolated mergers. We probe three different redshifts, and several halo concentrations, mass ratios, orbital circularities and orbital energies of the satellite. We find that prescriptions available in the literature significantly underpredict long time-scales for mergers at high redshift. We argue that these results have not been highlighted previously either because the evolution of halo concentration of satellite galaxies has been neglected (in previous isolated merger simulations) or because long merger times and mergers with high initial orbital circularities are under-represented (for prescriptions based on cosmological simulations). Motivated by the evolution of halo concentration at fixed mass, an explicit dependence on redshift added as tmergermod(z) = (1 + z)0.44tmerger to the prescription based on isolated mergers gives a significant improvement in the predicted merger times up to ∼20 tdyn in the redshift range 0 ≤ z ≤ 2. When this modified prescription is used to compute galaxy stellar mass functions, we find that it leads up to a 25 per cent increase in the number of low-mass galaxies surviving at z = 0, and a 10 per cent increase for more massive galaxies. This worsens the known overprediction in the number of low-mass galaxies by hierarchical models of galaxy formation.

Artigue, Romain; Barret, Didier; Lamb, Frederick K.; Lo, Ka Ho; Miller, M. Coleman

doi: 10.1093/mnrasl/slt059pmid: N/A

Precise and accurate measurements of neutron star masses and radii would provide valuable information about the still uncertain properties of cold matter at supranuclear densities. One promising approach to making such measurements involves an analysis of the X-ray flux oscillations often seen during thermonuclear (type 1) X-ray bursts. These oscillations are almost certainly produced by emission from hotter regions on the stellar surface modulated by the rotation of the star. One consequence of the rotation is that the oscillation should appear earlier at higher photon energies than at lower energies. Ford found compelling evidence for such a hard lead in the tail oscillations of one type 1 burst from Aql X-1. Subsequently, Muno, Özel & Chakrabarty analysed oscillations in the tails of type 1 bursts observed using the Rossi X-ray Timing Explorer. They found significant evidence for variation of the oscillation phase with energy in 13 of the 51 oscillation trains they analysed and an apparent linear trend of the phase with energy in six of nine average oscillation profiles produced by folding the energy-resolved oscillation waveforms from five stars and then averaging them in groups. In four of these nine averaged energy-resolved profiles, the oscillation appeared to arrive earlier at lower energies than at higher energies. Such a trend is inconsistent with a simple rotating hotspot model of the burst oscillations and, if confirmed, would mean that this model cannot be used to constrain the masses and radii of these stars and would raise questions about its applicability to other stars. We have therefore re-analysed individually the oscillations observed in the tails of the four type 1 bursts from 4U 1636−536 that, when averaged, provided the strongest evidence for a soft lead in the analysis by Muno et al. We have also analysed the oscillation observed during the superburst from this star. We find that the data from these five bursts, treated both individually and jointly, are fully consistent with a rotating hotspot model. Unfortunately, the uncertainties in these data are too large to provide interesting constraints on the mass and radius of this star.

Porter, R L; Ferland, G J; Storey, P J; Detisch, M J

doi: 10.1093/mnrasl/slt049pmid: N/A

Morgan, H.

doi: 10.1093/mnrasl/slt066pmid: N/A

According to current models, solar closed-field active regions are isolated from contributing directly to the solar wind except through small- or large-scale magnetic reconnection, including coronal mass ejections. Here we show the first direct evidence of active regions contributing directly to the solar wind by a steady-state, quiescent expansion. Advanced image processing of coronagraph data reveals a large system of nested closed-field magnetic structures expanding and accelerating to heights of at least 14 R⊙ within a helmet streamer above a large active region. The system persists for several days, and must therefore be an important contribution to the slow solar wind.

Maund, J. R.; Spyromilio, J.; Höflich, P. A.; Wheeler, J. C.; Baade, D.; Clocchiatti, A.; Patat, F.; Reilly, E.; Wang, L.; Zelaya, P.

doi: 10.1093/mnrasl/slt050pmid: N/A

Spectropolarimetry provides the means to probe the 3D geometries of supernovae (SNe) at early times. We report spectropolarimetric observations of the Type Ia SN 2012fr at four epochs: −11, −5, +2 and +24 d, with respect to B-light-curve maximum. SN 2012fr is a normal Type Ia SN, similar to SNe 1990N, 2000cx and 2005hj (that all exhibit low velocity decline rates for the principal Si ii line). The SN displays high-velocity components at −11 d that are highly polarized. The polarization of these features decreases as they become weaker from −5 d. At +2 d, the polarization angles of the low-velocity components of silicon and calcium are identical and oriented at 90° relative to the high-velocity Ca component. In addition to having very different velocities, the high- and low-velocity Ca components have orthogonal distributions in the plane of the sky. The continuum polarization for the SN at all four epochs is low, <0.1 per cent. We conclude that the low level of continuum polarization is inconsistent with the merger-induced explosion scenario. The simple axial symmetry evident from the polarization angles of the high-velocity and low-velocity Ca components, along with the presence of high-velocity components of Si and Ca, is perhaps more consistent with the pulsating delayed detonation model. We predict that, during the nebular phase, SN 2012fr will display blueshifted emission lines of Fe-group elements.

Sesana, A.

doi: 10.1093/mnrasl/slt034pmid: N/A

In this Letter, we carry out the first systematic investigation of the expected gravitational wave (GW) background generated by supermassive black hole (SMBH) binaries in the nHz frequency band accessible to pulsar timing arrays (PTAs). We take from the literature several estimates of the redshift-dependent galaxy mass function and of the fraction of close galaxy pairs to derive a wide range of galaxy merger rates. We then exploit empirical black hole–host relations to populate merging galaxies with SMBHs. The result of our procedure is a collection of a large number of phenomenological SMBH binary merger rates consistent with current observational constraints on the galaxy assembly at z < 1.5. For each merger rate we compute the associated GW signal, eventually producing a large set of estimates of the nHz GW background that we use to infer confidence intervals of its expected amplitude. When considering the most recent SMBH–host relations, accounting for overmassive black holes in brightest cluster galaxies, we find that the nominal 1σ interval of the expected GW signal is only a factor of 3–10 below current PTA limits, implying a non-negligible chance of detection in the next few years.

Lelli, Federico; Fraternali, Filippo; Verheijen, Marc

doi: 10.1093/mnrasl/slt053pmid: N/A

For disc galaxies, a close relation exists between the distribution of light and the shape of the rotation curve. We quantify this relation by measuring the inner circular-velocity gradient dRV(0) for spiral and irregular galaxies with high-quality rotation curves. We find that dRV(0) correlates with the central surface brightness μ0 over more than two orders of magnitude in dRV(0) and four orders of magnitudes in μ0. This is a scaling relation for disc galaxies. It shows that the central stellar density of a galaxy closely relates to the inner shape of the potential well, also for low-luminosity and low-surface-brightness galaxies that are expected to be dominated by dark matter.

Sekhar, A.; Asher, D. J.

doi: 10.1093/mnrasl/slt065pmid: N/A

Many previous works have shown the relevance and dynamics of Jovian mean motion resonances (MMR) in various meteoroid streams. These resonant swarms are known to have produced spectacular meteor displays in the past. In this work, we investigate whether any MMR due to Saturn are feasible, and subsequently check whether such effects are strong enough to trap meteoroids so as to cause enhanced meteor phenomena on Earth. Extensive numerical simulations are done on two major meteoroid streams, which are known to exhibit exterior Jovian resonances. The roles of the 1:6 and 5:14 Jovian MMR have already been studied in the Orionids and Leonids, respectively. Now we find strong evidence of 1:3 and 8:9 Saturnian MMR in Orionids and Leonids, respectively. The presence of compact dust trails in real space due to these two Saturnian resonances is confirmed from our calculations.

Longmore, S. N.; Kruijssen, J. M. D.; Bally, J.; Ott, J.; Testi, L.; Rathborne, J.; Bastian, N.; Bressert, E.; Molinari, S.; Battersby, C.; Walsh, A. J.

doi: 10.1093/mnrasl/slt048pmid: N/A

Super star clusters are the end product of star formation under the most extreme conditions. As such, studying how their final stellar populations are assembled from their natal progenitor gas clouds can provide strong constraints on star formation theories. An obvious place to look for the initial conditions of such extreme stellar clusters is gas clouds of comparable mass and density, with no star formation activity. We present a method to identify such progenitor gas clouds and demonstrate the technique for the gas in the inner few hundred pc of our Galaxy. The method highlights three clouds in the region with similar global physical properties to the previously identified extreme cloud, G0.253 + 0.016, as potential young massive cluster (YMC) precursors. The fact that four potential YMC progenitor clouds have been identified in the inner 100 pc of the Galaxy, but no clouds with similar properties have been found in the whole first quadrant despite extensive observational efforts, has implications for cluster formation/destruction rates across the Galaxy. We put forward a scenario to explain how such dense gas clouds can arise in the Galactic Centre environment, in which YMC formation is triggered by gas streams passing close to the minimum of the global Galactic gravitational potential at the location of the central supermassive black hole, Sgr A*. If this triggering mechanism can be verified, we can use the known time interval since closest approach to Sgr A* to study the physics of stellar mass assembly in an extreme environment as a function of absolute time.

Clarke, C. J.; Owen, J. E.

doi: 10.1093/mnrasl/slt060pmid: N/A

We assume a scenario in which transition discs (i.e. discs around young stars that have signatures of cool dust but lack significant near-infrared emission from warm dust) are associated with the presence of planets (or brown dwarfs). These are assumed to filter the dust content of any gas flow within the planetary orbit and produce an inner ‘opacity hole’. In order to match the properties of transition discs with the largest (∼50 au scale) holes, we place such ‘planets’ at large radii in massive discs and then follow the evolution of the tidally coupled disc–planet system, comparing the system's evolution in the plane of mm flux against hole radius with the properties of observed transition discs. We find that, on account of the high disc masses in these systems, all but the most massive ‘planets’ (100 Jupiter masses) are conveyed to small radii by Type II migration without significant fading at millimetre wavelengths. Such behaviour would contradict the observed lack of mm bright transition discs with small (<10 au) holes. On the other hand, imaging surveys clearly rule out the presence of such massive companions in transition discs. We conclude that this is a serious problem for models that seek to explain transition discs in terms of planetary companions unless some mechanism can be found to halt inward migration and/or suppress mm flux production. We suggest that the dynamical effects of substantial accretion on to the planet/through the gap may offer the best prospect for halting such migration and that further long-term simulations are required to clarify this issue.