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Renaud F.,University of Surrey | Agertz O.,University of Surrey | Agertz O.,Lund Observatory | Gieles M.,University of Surrey
Monthly Notices of the Royal Astronomical Society | Year: 2017

We present a cosmological zoom-in simulation of a Milky Way-like galaxy used to explore the formation and evolution of star clusters. We investigate in particular the origin of the bimodality observed in the colour and metallicity of globular clusters, and the environmental evolution through cosmic times in the form of tidal tensors. Our results self-consistently confirm previous findings that the blue, metal-poor clusters form in satellite galaxies that are accreted on to the Milky Way, while the red, metal-rich clusters form mostly in situ, or, to a lower extent, in massive, self-enriched galaxies merging with the Milky Way. By monitoring the tidal fields these populations experience, we find that clusters formed in situ (generally centrally concentrated) feel significantly stronger tides than the accreted ones, both in the present day, and when averaged over their entire life. Furthermore, we note that the tidal field experienced by Milky Way clusters is significantly weaker in the past than at present day, confirming that it is unlikely that a power-law cluster initial mass function like that of young massive clusters, is transformed into the observed peaked distribution in the Milky Way with relaxation-driven evaporation in a tidal field. © 2016 The Authors.

News Article | April 9, 2016

Planet Nine, as its name suggests, is the ninth planet of the solar system discovered earlier this year by the same scientist who demoted Pluto to a dwarf planet. A new study said, however, that Planet Nine is actually an imposter that lurked near the solar system and got stolen from its host star. In January, Konstantin Batygin and Michael Brown found a never before seen planet around 10 times the mass of Earth lurking at the edge of the solar system. Some scientists hypothesized that the planet could have originated within the solar system and migrated toward its edges. A team of astronomers now suggests the opposite - the planet may have been stolen by the sun from a nearby star. A new study, published in the open journal Arvix, and led by Alexander Mustill from the Lund Observatory, proposes that the Planet Nine might actually be an exoplanet outside the solar system and orbiting its own host star. The chances of this happens in 0.1 to 2 percent, which is very low, but the team believes this is possible. The probability "Although these probabilities seem low, you have to compare them to each other, and not absolutely," said Mustill. "Because ultimately any very specific outcome is very unlikely," he added. One of the biggest mysteries in the solar system is probably the presence of a distant planet travelling around the sun in a 20,000-year orbit. This planet is found far beyond Pluto. It has been previously suggested that the sun could have captured objects from other stars while they are passing nearby, including planets and even comets. Though its presence is deemed mysterious, it could have been forcibly ejected from its own orbit or formed on its own in its location today, considering the complexities of cosmic bodies in the universe. "I think it's premature to say what's most likely," said Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. This discovery will spur new questions about Planet Nine, where it really came from and how it might affect Earth in the future. © 2016 Tech Times, All rights reserved. Do not reproduce without permission.

Bensby T.,Lund Observatory | Feltzing S.,Lund Observatory | Oey M.S.,University of Michigan
Astronomy and Astrophysics | Year: 2014

Aims. The aim of this paper is to explore and map the age and abundance structure of the stars in the nearby Galactic disk. Methods. We have conducted a high-resolution spectroscopic study of 714 F and G dwarf and subgiant stars in the Solar neighbourhood. The star sample has been kinematically selected to trace the Galactic thin and thick disks to their extremes, the metal-rich stellar halo, sub-structures in velocity space such as the Hercules stream and the Arcturus moving group, as well as stars that cannot (kinematically) be associated with either the thin disk or the thick disk. The determination of stellar parameters and elemental abundances is based on a standard analysis using equivalent widths and one-dimensional, plane-parallel model atmospheres calculated under the assumption of local thermodynamical equilibrium (LTE). The spectra have high resolution (R = 40 000-110 000) and high signal-to-noise (S/N = 150-300) and were obtained with the FEROS spectrograph on the ESO 1.5 m and 2.2 m telescopes, the SOFIN and FIES spectrographs on the Nordic Optical Telescope, the UVES spectrograph on the ESO Very Large Telescope, the HARPS spectrograph on the ESO 3.6 m telescope, and the MIKE spectrograph on the Magellan Clay telescope. The abundances from individual Fe i lines were were corrected for non-LTE effects in every step of the analysis. Results. We present stellar parameters, stellar ages, kinematical parameters, orbital parameters, and detailed elemental abundances for O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, and Ba for 714 nearby F and G dwarf stars. Our data show that there is an old and α-enhanced disk population, and a younger and less α-enhanced disk population. While they overlap greatly in metallicity between-0.7 < [Fe/H] â‰+0.1, they show a bimodal distribution in [α/Fe]. This bimodality becomes even clearer if stars where stellar parameters and abundances show larger uncertainties (Teff â‰5400 K) are discarded, showing that it is important to constrain the data set to a narrow range in the stellar parameters if small differences between stellar populations are to be revealed. In addition, we find that the α-enhanced population has orbital parameters placing the stellar birthplaces in the inner Galactic disk while the low-α stars mainly come from the outer Galactic disk, fully consistent with the recent claims of a short scale-length for the α-enhanced Galactic thick disk. We have also investigated the properties of the Hercules stream and the Arcturus moving group and find that neither of them presents chemical or age signatures that could suggest that they are disrupted clusters or extragalactic accretion remnants from ancient merger events. Instead, they are most likely dynamical features originating within the Galaxy. We have also discovered that a standard 1D, LTE analysis, utilising ionisation and excitation balance of Fe i and Fe ii lines produces a flat lower main sequence. As the exact cause for this effect is unclear we chose to apply an empirical correction. Turn-off stars and more evolved stars appear to be unaffected. © 2014 ESO.

Miller M.C.,University of Maryland University College | Davies M.B.,Lund Observatory
Astrophysical Journal | Year: 2012

Massive black holes have been discovered in all closely examined galaxies with high velocity dispersion. The case is not as clear for lower-dispersion systems such as low-mass galaxies and globular clusters. Here we suggest that above a critical velocity dispersion 40kms-1, massive central black holes will form in relaxed stellar systems at any cosmic epoch. This is because above this dispersion primordial binaries cannot support the system against deep core collapse. If, as previous simulations show, the black holes formed in the cluster settle to produce a dense subcluster, then given the extremely high densities reached during core collapse the holes will merge with each other. For low velocity dispersions and hence low cluster escape speeds, mergers will typically kick out all or all but one of the holes due to three-body kicks or the asymmetric emission of gravitational radiation. If one hole remains, it will tidally disrupt stars at a high rate. If none remain, one is formed after runaway collisions between stars, and then it tidally disrupts stars at a high rate. The accretion rate after disruption is many orders of magnitude above Eddington. If, as several studies suggest, the hole can accept matter at that rate because the generated radiation is trapped and advected, then it will grow quickly and form a massive central black hole. © 2012 The American Astronomical Society. All rights reserved.

Feltzing S.,Lund Observatory | Chiba M.,Tohoku University
New Astronomy Reviews | Year: 2013

We present a review of elemental abundances in the Milky Way stellar disk, bulge, and halo with a focus on data derived from high-resolution stellar spectra. These data are fundamental in disentangling the formation history and subsequent evolution of the Milky Way. Information from such data is still limited and confined to narrowly defined stellar samples. The astrometric Gaia satellite will soon be launched by the European Space Agency. Its final data set will revolutionize information on the motions of a billion stars in the Milky Way. This will be complemented by several ground-based observational campaigns, in particular spectroscopic follow-up to study elemental abundances in the stars in detail. Our review shows the very rich and intriguing picture built from rather small and local samples. The Gaia data deserve to be complemented by data of the same high quality that have been collected for the solar neighborhood. © 2013 Elsevier B.V.

O'Shaughnessy R.,Pennsylvania State University | Kim C.,Lund Observatory
Astrophysical Journal | Year: 2010

One ingredient in an empirical birthrate estimate for pulsar binaries is the fraction of sky subtended by the pulsar beam: the pulsar beaming fraction. This fraction depends on both the pulsar's opening angle and the misalignment angle between its spin and magnetic axes. The current estimates for pulsar binary birthrates are based on an average value of beaming fractions for only two pulsars, i.e., PSRs B1913+16 and B1534+12. In this paper, we revisit the observed pulsar binaries to examine the sensitivity of birthrate predictions to different assumptions regarding opening angle and alignment. Based on empirical estimates for the relative likelihood of different beam half-opening angles and misalignment angles between the pulsar rotation and magnetic axes, we calculate an effective beaming correction factor, f b,eff, whose reciprocal is equivalent to the average fraction of all randomly selected pulsars that point toward us. For those pulsars without any direct beam geometry constraints, we find that f b,eff is likely to be smaller than 6, a canonically adopted value when calculating birthrates of Galactic pulsar binaries. We calculate f b,eff for PSRs J0737-3039A and J1141-6545, applying the currently available constraints for their beam geometry. As in previous estimates of the posterior probability density function P() for pulsar binary birthrates , PSRs J0737-3039A and J1141-6545 still significantly contribute to, if not dominate, the Galactic birthrate of tight pulsar-neutron star (NS) and pulsar-white dwarf (WD) binaries, respectively. Our median posterior present-day birthrate predictions for tight PSR-NS binaries, wide PSR-NS binaries, and tight PSR-WD binaries given a preferred pulsar population model and beaming geometry are 89 Myr-1, 0.5 Myr-1, and 34 Myr-1, respectively. For long-lived PSR-NS binaries, these estimates include a weak (×1.6) correction for slowly decaying star formation in the galactic disk. For pulsars with spin period between 10 ms and 100 ms, where few measurements of misalignment and opening angle provide a sound basis for extrapolation, we marginalized our posterior birthrate distribution P() over a range of plausible beaming correction factors. We explore several alternative beaming geometry distributions, demonstrating that our predictions are robust except in (untestable) scenarios with many highly aligned recycled pulsars. Finally, in addition to exploring alternative beam geometries, we also briefly summarize how uncertainties in each pulsar binary's lifetime and in the pulsar luminosity distribution can be propagated into P(. © 2010. The American Astronomical Society. All rights reserved.

Ryde N.,Lund Observatory
Astronomische Nachrichten | Year: 2010

In 2006 ESO Council authorized a Phase B study of a European AO-telescope with a 42 m segmented primary with a 5-mirror design, the E-ELT. Several reports and working groups have already presented science cases for an E-ELT, specifically exploiting the new capabilities of such a large telescope. One of the aims of the design has been to find a balance in the performances between an E-ELT and the James Webb Space Telescope, JWST. Apart from the larger photon-collecting area, the strengths of the former is the higher attainable spatial and spectral resolutions. The E-ELT AO system will have an optimal performance in the near-IR, which makes it specially advantageous. High-resolution spectroscopy in the near-infrared has, however, not been discussed much. This paper aims at filling that gap, by specifically discussing spectroscopy of stellar (mainly red giant), photospheric abundances. Based on studies in the literature of stellar abundances, at the needed medium to high spectral resolutions in the near-infrared (0.8-2.4 μm), I will try to extrapolate published results to the performance of the E-ELT and explore what could be done at the E-ELT in this field. A discussion on what instrument characteristics that would be needed for stellar abundance analyses in the near-IR will be given. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Dittrich K.,Max Planck Institute for Astronomy | Klahr H.,Max Planck Institute for Astronomy | Johansen A.,Lund Observatory
Astrophysical Journal | Year: 2013

Recent numerical simulations have shown long-lived axisymmetric sub- and super-Keplerian flows in protoplanetary disks. These zonal flows are found in local as well as global simulations of disks unstable to the magnetorotational instability. This paper covers our study of the strength and lifetime of zonal flows and the resulting long-lived gas over- and underdensities as functions of the azimuthal and radial size of the local shearing box. We further investigate dust particle concentrations without feedback on the gas and without self-gravity. The strength and lifetime of zonal flows increase with the radial extent of the simulation box, but decrease with the azimuthal box size. Our simulations support earlier results that zonal flows have a natural radial length scale of 5-7 gas pressure scale heights. This is the first study that combines three-dimensional MHD simulations of zonal flows and dust particles feeling the gas pressure. The pressure bumps trap particles with St = 1 very efficiently. We show that St = 0.1 particles (of some centimeters in size if at 5 AU in a minimum mass solar nebula) reach a hundred-fold higher density than initially. This opens the path for particles of St = 0.1 and dust-to-gas ratio of 0.01 or for particles of St ≥ 0.5 and dust-to-gas ratio 10-4 to still reach densities that potentially trigger the streaming instability and thus gravoturbulent formation of planetesimals. © 2013. The American Astronomical Society. All rights reserved..

News Article | September 14, 2016

Astronomers the world over are about to get their first taste of a tool that will transform their working lives. Gaia, a space telescope launched by the European Space Agency (ESA) in late 2013, will release its first map of the Milky Way on 14 September. The catalogue will show the 3D positions of 2,057,050 stars and other objects, and how those positions have changed over the past two decades. Eventually, the map will contain one billion objects or more and will be 1,000 times more extensive and at least 10 times more precise than anything that came before. The release next week will also include 19 papers by Gaia astronomers who have already seen the data. But independent teams are getting ready for their first glimpse. Lennart Lindegren, an astronomer at the Lund Observatory in Sweden and a major driving force in the Gaia project since it was first proposed in 1993, expects astronomers to produce 100 or so papers just in the weeks following the draft catalogue release. Some groups have planned ‘Gaia hacking’ and ‘Gaia sprint’ events, at which researchers will collectively work out how best to exploit the sudden manna. “Gaia is going to revolutionize what we know about stars and the Galaxy,” says David Hogg, an astronomer at New York University who is leading some of these efforts. So what are some of the revelations that Gaia could make? Gaia’s 3D view of the Milky Way in motion will reveal how stars move under its combined gravitational pull. This will add to knowledge of the Galaxy’s structure, including parts that are not directly visible from Earth, such as the ‘bar’ — two arms that stick straight out of the Galactic Centre and join it to the spiral arms. Researchers will be able to identify ‘outlier’ groups of stars which stream together at high speeds, and which are thought to be remnants of mergers with smaller galaxies, says Michael Perryman, an astronomer at University College Dublin and a former senior scientist for Gaia at ESA. Combined with existing information about factors including stars’ colour, temperature and chemical composition, this detailed map will enable researchers to reconstruct the Galaxy’s archaeology: how it got to its present state over the past 13 billion years. “Over its lifetime, Gaia is going to radically impact our understanding of the structure of the Milky Way and its evolutionary history,” says Monica Valluri, an astronomer at the University of Michigan in Ann Arbor. The details of star trajectories inside the Galaxy will reveal the distribution not only of visible matter, but also of dark matter, which constitutes the bulk of most galaxies’ mass. And that in turn could help to reveal what dark matter is. Gaia might also put some exotic theories to the test. Standard dark-matter theory predicts that the gravitational field of the Galaxy is spherically symmetrical near the Galactic Centre but then becomes elongated “like an American football” farther out, Valluri explains. But an alternative theory called MOND (modified Newtonian dynamics) implies that the field is shaped more like a pancake. By looking at the velocities of stars, which depend on the gravitational field, Gaia will be able to test which theory is right. The probe’s data might even reveal evidence for the idea that dark matter killed the dinosaurs. If dark matter is concentrated in a relatively thin ‘dark disk’ near the Galactic plane, says the audacious theory, it could trigger asteroid impacts that cause mass extinctions when the Solar System periodically crosses the disk. Precise measurements of how far individual stars lie from the Sun will enable astrophysicists to fine-tune their models of how stars evolve. That is because current theories rely heavily on estimates of distance to understand how a star’s intrinsic brightness changes during its lifetime. One of the first groups of stars that researchers will want to check is the Pleiades, a cluster in the constellation Taurus. Most observations, including one1 made with the Hubble Space Telescope, put the cluster about 135 parsecs (440 light years) away. But results based on data from Hipparcos, an ESA space mission that preceded Gaia, suggest 2 that it is only 120 parsecs away. Some have said that the discrepancy casts doubt on the accuracy of Hipparcos. Gaia uses a similar, but much more evolved, method to Hipparcos, so astronomers will be watching its observations closely. “I believe that the Hipparcos result will very likely be proved wrong by Gaia,” says David Soderblom of the Space Telescope Science Institute in Baltimore, Maryland, who is an author on the Hubble study. Astronomers have discovered thousands of planets orbiting other stars, in most cases by detecting tiny dips in a star’s brightness when an orbiting planet passes in front of, or ‘transits’, it. Gaia will detect planets using another method: measuring slight wobbles in the star’s position caused by a planet’s gravitational pull. “It seems like a good bet that the mission will reveal thousands of new worlds,” says Gregory Laughlin, an astronomer at Yale University in New Haven, Connecticut. Gaia’s technique is best suited to detecting large planets in relatively wide orbits, says Alessandro Sozzetti, a Gaia researcher at the Astrophysical Observatory of Turin in Italy. And unlike the transit method, it directly measures a planet’s mass. If it works, it will be a striking comeback for a technique that has seen many false starts. But finding planets in this way will require several years of observation, with a sneak preview expected by 2018, Sozzetti says. Although Gaia is primarily an explorer of the Milky Way, its influence will reach across the entire observable Universe. Gaia’s direct distance measurements work only for objects in the Galaxy or its immediate vicinity; to estimate the distances to faraway galaxies, astronomers typically wait for stellar explosions called Type Ia supernovae. The apparent brightness of such a supernova reveals how far away the corresponding galaxy is. Such signposts, or ‘standard candles’, have been the main tool for estimating the rate of expansion of the Universe. The measurements have led astronomers to propose that a mysterious ‘dark energy’ has been accelerating that expansion. But to use supernovae as signposts, astronomers must compare them with other types of standard candle in our Galaxy. In its first release, Gaia will measure the distances of thousands of such stars to high accuracy. Eventually, the probe’s measurements will enable cosmologists to improve their maps of the entire Universe and perhaps to resolve some conflicting estimates of its rate of expansion. As it constantly scans the sky, Gaia will also track and discover things much closer to home. It is ultimately expected to observe some 350,000 asteroids inside the Solar System, and to discover hundreds of new ones, says Gaia astronomer Paolo Tanga of the Côte d’Azur Observatory in Nice, France. These will include near-Earth objects (NEOs), those whose orbits bring them within about 200 million kilometres of Earth. When it spots an NEO, Gaia can alert observatories, which can then use ground-based telescopes to establish whether the object is a threat. From its vantage point in space, Gaia will scan nearly the entire sky and so might reveal objects that, during certain times, are too close to the Sun to be observed from Earth, says Anthony Brown, an astronomer at the Leiden Observatory in the Netherlands who chairs Gaia’s data-processing collaboration. “We can observe in areas you cannot normally reach from the ground at the same time.” By tracking the way certain asteroids orbit the Sun over several years, Gaia will also be able to perform sensitive tests of Albert Einstein's description of gravity, his general theory of relativity.

News Article | October 26, 2016

Lonely planets can blame big, pushy bullies. Giant planets may bump off most of their smaller brethren, partly explaining why the Kepler space telescope has seen so many single-planet systems. Of the thousands of planetary systems Kepler has discovered, about 80 per cent appear as single planets passing in front of their stars. The rest feature as many as seven planets – a distinction dubbed the Kepler dichotomy. Recent studies suggest even starker differences. While multiple-planet systems tend to have circular orbits that all lie in the same plane – like our solar system – the orbits of singletons tend to be more elliptical and are often misaligned with the spins of their stars. Now, a pair of computer simulations suggest that hidden giants may lurk in these single systems. We wouldn’t be able to see them; big, Jupiter-like planets in wide orbits would take too long for Kepler to catch, and they may not have orbits that cause them to pass in front of their stars in our line of sight. But if these unseen bullies are there, they may have removed many of the smaller planets in closer orbits, leaving behind the solitary worlds that Kepler sees. The simulations show that gravitational interactions involving giants in outer orbits can eject smaller planets from the system, nudge them into their stars or send them crashing into each other. “There are bigger things out there trying to pull you around,” says Chelsea Huang at the University of Toronto, Canada. She and her team also showed the giants pull the few remaining inner planets into more elliptical and inclined orbits – the same kind seen in many of the single systems Kepler has spotted. Alex Mustill at Lund Observatory in Sweden and his colleagues mimicked more general scenarios, including planets orbiting a binary star system, and got similar results. The studies complement each other, say Huang and Mustill. “We know these configurations have to occur in some fraction of exoplanet systems,” Mustill says. But that doesn’t mean they’re universal. “They don’t occur all the time, and this is one reason why you can’t explain the large number of single planets purely through this mechanism,” Mustill says. According to his analysis, bullying giants can only account for about 18 per cent of Kepler’s singles. To confirm their proposed mechanism, the researchers must wait until next year for the launch of the Transiting Exoplanet Survey Satellite (TESS), which will target closer and brighter systems – and thus be easier for follow-up observations to uncover the bully planets.

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