ALBERT

Description: The M 4 Core Project with HST is designed to exploit the Hubble Space Telescope ( HST ) to investigate the central regions of M 4, the Globular Cluster closest to the Sun. In this paper we combine optical and near-infrared photometry to study multiple stellar populations in M 4. We detected two sequences of M-dwarfs containing ~38 per cent ( MS I ) and ~62 per cent ( MS II ) of MS stars below the main-sequence (MS) knee. We compare our observations with those of NGC 2808, which is the only other GCs where multiple MSs of very low-mass stars have been studied to date. We calculate synthetic spectra for M-dwarfs, assuming the chemical composition mixture inferred from spectroscopic studies of stellar populations along the red giant branch, and different helium abundances, and we compare predicted and observed colours. Observations are consistent with two populations, one with primordial abundance and another with enhanced nitrogen and depleted oxygen.

Description: Blue stragglers in globular clusters are abnormally massive stars that should have evolved off the stellar main sequence long ago. There are two known processes that can create these objects: direct stellar collisions and binary evolution. However, the relative importance of these processes has remained unclear. In particular, the total number of blue stragglers found in a given cluster does not seem to correlate with the predicted collision rate, providing indirect support for the binary-evolution model. Yet the radial distributions of blue stragglers in many clusters are bimodal, with a dominant central peak: this has been interpreted as an indication that collisions do dominate blue straggler production, at least in the high-density cluster cores. Here we report that there is a clear, but sublinear, correlation between the number of blue stragglers found in a cluster core and the total stellar mass contained within it. From this we conclude that most blue stragglers, even those found in cluster cores, come from binary systems. The parent binaries, however, may themselves have been affected by dynamical encounters. This may be the key to reconciling all of the seemingly conflicting results found to date.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Knigge, Christian -- Leigh, Nathan -- Sills, Alison -- England -- Nature. 2009 Jan 15;457(7227):288-90. doi: 10.1038/nature07635.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉University of Southampton, School of Physics and Astronomy, Southampton SO17 1BJ, UK. christian@astro.soton.ac.uk〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/19148094" target="_blank"〉PubMed〈/a〉

Description: Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 +/- 12 parsecs measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source substantially closer at 114 +/- 2 parsecs. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Miller-Jones, J C A -- Sivakoff, G R -- Knigge, C -- Kording, E G -- Templeton, M -- Waagen, E O -- New York, N.Y. -- Science. 2013 May 24;340(6135):950-2. doi: 10.1126/science.1237145.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉International Centre for Radio Astronomy Research, Curtin University, Perth, WA 6845, Australia. james.miller-jones@curtin.edu.au〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23704566" target="_blank"〉PubMed〈/a〉

Description: Two types of supernova are thought to produce the overwhelming majority of neutron stars in the Universe. The first type, iron-core-collapse supernovae, occurs when a high-mass star develops a degenerate iron core that exceeds the Chandrasekhar limit. The second type, electron-capture supernovae, is associated with the collapse of a lower-mass oxygen-neon-magnesium core as it loses pressure support owing to the sudden capture of electrons by neon and/or magnesium nuclei. It has hitherto been impossible to identify the two distinct families of neutron stars produced in these formation channels. Here we report that a large, well-known class of neutron-star-hosting X-ray pulsars is actually composed of two distinct subpopulations with different characteristic spin periods, orbital periods and orbital eccentricities. This class, the Be/X-ray binaries, contains neutron stars that accrete material from a more massive companion star. The two subpopulations are most probably associated with the two distinct types of neutron-star-forming supernova, with electron-capture supernovae preferentially producing systems with short spin periods, short orbital periods and low eccentricities. Intriguingly, the split between the two subpopulations is clearest in the distribution of the logarithm of spin period, a result that had not been predicted and which still remains to be explained.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Knigge, Christian -- Coe, Malcolm J -- Podsiadlowski, Philipp -- England -- Nature. 2011 Nov 9;479(7373):372-5. doi: 10.1038/nature10529.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉University of Southampton, School of Physics and Astronomy, Southampton SO17 1BJ, UK. c.knigge@soton.ac.uk〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22080948" target="_blank"〉PubMed〈/a〉

Description: Astrophysical jets seem to occur in nearly all types of accreting objects, from supermassive black holes to young stellar objects. On the basis of x-ray binaries, a unified scenario describing the disc/jet coupling has evolved and been extended to many accreting objects. The only major exceptions are thought to be cataclysmic variables: Dwarf novae, weakly accreting white dwarfs, show similar outburst behavior to x-ray binaries, but no jet has yet been detected. Here we present radio observations of a dwarf nova in outburst showing variable flat-spectrum radio emission that is best explained as synchrotron emission originating in a transient jet. Both the inferred jet power and the relation to the outburst cycle are analogous to those seen in x-ray binaries, suggesting that the disc/jet coupling mechanism is ubiquitous.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kording, Elmar -- Rupen, Michael -- Knigge, Christian -- Fender, Rob -- Dhawan, Vivek -- Templeton, Matthew -- Muxlow, Tom -- New York, N.Y. -- Science. 2008 Jun 6;320(5881):1318-20. doi: 10.1126/science.1155492.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK. elmar@phys.soton.ac.uk〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/18535237" target="_blank"〉PubMed〈/a〉

Description: Interacting compact binary systems provide a natural laboratory in which to study irradiated substellar objects. As the mass-losing secondary (donor) in these systems makes a transition from the stellar to the substellar regime, it is also irradiated by the primary (compact accretor). The internal and external energy fluxes are both expected to be comparable in these objects, providing access to an unexplored irradiation regime. The atmospheric properties of donors are largely unknown, but could be modified by the irradiation. To constrain models of donor atmospheres, it is necessary to obtain accurate observational estimates of their physical properties (masses, radii, temperatures and albedos). Here we report the spectroscopic detection and characterization of an irradiated substellar donor in an accreting white-dwarf binary system. Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of 0.055 +/- 0.008 solar masses and an average spectral type of L1 +/- 1, supporting both theoretical predictions and model-dependent observational constraints that suggest that the donor is a brown dwarf. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the dayside and nightside of the substellar donor (57 kelvin) and the maximum difference between the hottest and coolest parts of its surface (200 kelvin). The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of less than 0.54 at the 2sigma confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the atmosphere of the brown-dwarf donor. These results add to our knowledge of binary evolution, in that the donor has survived the transition from the stellar to the substellar regime, and of substellar atmospheres, in that we have been able to test a regime in which the irradiation and the internal energy of a brown dwarf are comparable.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Santisteban, Juan V Hernandez -- Knigge, Christian -- Littlefair, Stuart P -- Breton, Rene P -- Dhillon, Vikram S -- Gansicke, Boris T -- Marsh, Thomas R -- Pretorius, Magaretha L -- Southworth, John -- Hauschildt, Peter H -- England -- Nature. 2016 May 18;533(7603):366-8. doi: 10.1038/nature17952.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK. ; Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK. ; Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK. ; Instituto de Astrofisica de Canarias, E-38205 La Laguna, Santa Cruz de Tenerife, Spain. ; Department of Physics, University of Warwick, Coventry CV4 7AL, UK. ; Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK. ; Astrophysics Group, Keele University, Newcastle ST5 5BG, UK. ; Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/27193683" target="_blank"〉PubMed〈/a〉

Description: In a virialized stellar system, the mean-square velocity is a direct tracer of the energy per unit mass of the system. Here, we exploit this to estimate and compare root-mean-square velocities for a large sample of nuclear star clusters and their host (late- or early-type) galaxies. Traditional observables, such as the radial surface brightness and second-order velocity moment profiles, are subject to short-term variations due to individual episodes of matter infall and/or star formation. The total mass, energy and angular momentum, on the other hand, are approximately conserved. Thus, the total energy and angular momentum more directly probe the formation of galaxies and their nuclear star clusters, by offering access to more fundamental properties of the nuclear cluster–galaxy system than traditional observables. We find that there is a strong correlation, in fact a near equality, between the root-mean-square velocity of a nuclear star cluster and that of its host. Thus, the energy per unit mass of a nuclear star cluster is always comparable to that of its host galaxy. We interpret this as evidence that nuclear star clusters do not form independently of their host galaxies, but rather that their formation and subsequent evolution are coupled. We discuss how our results can potentially be used to offer a clear and observationally testable prediction to distinguish between the different nuclear star cluster formation scenarios, and/or quantify their relative contributions.