共查询到20条相似文献，搜索用时 453 毫秒
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S. C. O. Glover P. W. J. L. Brand 《Monthly notices of the Royal Astronomical Society》2001,321(3):385-397
The first star formation in the Universe is expected to take place within small protogalaxies, in which the gas is cooled by molecular hydrogen. However, if massive stars form within these protogalaxies, they may suppress further star formation by photodissociating the H2 . We examine the importance of this effect by estimating the time-scale on which significant H2 is destroyed. We show that photodissociation is significant in the least massive protogalaxies, but becomes less so as the protogalactic mass increases. We also examine the effects of photodissociation on dense clumps of gas within the protogalaxy. We find that while collapse will be inhibited in low-density clumps, denser ones may survive to form stars. 相似文献
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W. K. M. Rice Philip J. Armitage D. F. Hogg 《Monthly notices of the Royal Astronomical Society》2008,384(3):1242-1248
We use numerical simulations to model the migration of massive planets at small radii and compare the results with the known properties of 'hot Jupiters' (extrasolar planets with semimajor axes a < 0.1 au). For planet masses M pl sin i > 0.5 M J , the evidence for any 'pile-up' at small radii is weak (statistically insignificant), and although the mass function of hot Jupiters is deficient in high-mass planets as compared to a reference sample located further out, the small sample size precludes definitive conclusions. We suggest that these properties are consistent with disc migration followed by entry into a magnetospheric cavity close to the star. Entry into the cavity results in a slowing of migration, accompanied by a growth in orbital eccentricity. For planet masses in excess of 1 Jupiter mass we find eccentricity growth time-scales of a few ×105 yr, suggesting that these planets may often be rapidly destroyed. Eccentricity growth appears to be faster for more massive planets which may explain changes in the planetary mass function at small radii and may also predict a pile-up of lower mass planets, the sample of which is still incomplete. 相似文献
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Volker Bromm 《Astrophysics and Space Science》2003,284(2):349-352
Paramount among the processes that ended the cosmic dark ages must have been the emergence of a first generation of stars.
I review recent results on their formation out of metal-free gas, and discuss related open questions that still defy our understanding.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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Using the “Updated Nearby Galaxy Catalog”, we consider different properties of companion galaxies around luminous hosts in the Local Volume. The data on stellar masses, linear diameters, surface brightnesses, HI‐richness, specific star formation rate (sSFR), and morphological types are discussed for members of the nearest groups, including the Milky Way and M 31 groups, as a function of their separation from the hosts. Companion galaxies in groups tend to have lower stellar masses, smaller linear diameters, and fainter mean surface brightnesses as the distance to their host decreases. The hydrogen‐to‐stellar mass ratio of the companions increases with their linear projected separation from the dominant luminous galaxy. This tendency is more expressed around the bulge‐dominated hosts. While linear separation of the companions decreases, their mean sSFR becomes lower, accompanied with the increasing sSFR scatter. the typical linear projected separation of dSphs around the bulge‐dominated hosts, 350 kpc, is substantially larger than that around the disk‐dominated ones, 130 kpc. This difference probably indicates the presence of larger hot/warm gas haloes around the early‐type host galaxies. The mean fraction of dSph (quenched) companions in the 11 nearest groups as a function of their projected separation Rp can be expressed as ƒ(E) = (0.55–0.69)×Rp. The fraction of dSphs around the Milky Way and M 31 looks much higher than in other nearby groups because the quenching efficiency dramatically increases towards the ultra‐low mass companions. We emphasize that the observed properties of the Local Group are not typical for other groups in the Local Volume due to the role of selection effects caused by our location inside the Local Group. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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On corotation torques, horseshoe drag and the possibility of sustained stalled or outward protoplanetary migration 总被引：1，自引：0，他引：1
S.-J. Paardekooper J. C. B. Papaloizou 《Monthly notices of the Royal Astronomical Society》2009,394(4):2283-2296
We study the torque on low-mass protoplanets on fixed circular orbits, embedded in a protoplanetary disc in the isothermal limit. We consider a wide range of surface density distributions including cases where the surface density increases smoothly outwards. We perform both linear disc response calculations and non-linear numerical simulations. We consider a large range of viscosities, including the inviscid limit, as well as a range of protoplanet mass ratios, with special emphasis on the co-orbital region and the corotation torque acting between disc and protoplanet.
For low-mass protoplanets and large viscosity, the corotation torque behaves as expected from linear theory. However, when the viscosity becomes small enough to enable horseshoe turns to occur, the linear corotation torque exists only temporarily after insertion of a planet into the disc, being replaced by the horseshoe drag first discussed by Ward. This happens after a time that is equal to the horseshoe libration period reduced by a factor amounting to about twice the disc aspect ratio. This torque scales with the radial gradient of specific vorticity, as does the linear torque, but we find it to be many times larger. If the viscosity is large enough for viscous diffusion across the co-orbital region to occur within a libration period, we find that the horseshoe drag may be sustained. If not, the corotation torque saturates leaving only the linear Lindblad torques. As the magnitude of the non-linear co-orbital torque (horseshoe drag) is always found to be larger than the linear torque, we find that the sign of the total torque may change even for mildly positive surface density gradients. In combination with a kinematic viscosity large enough to keep the torque from saturating, strong sustained deviations from linear theory and outward or stalled migration may occur in such cases. 相似文献
For low-mass protoplanets and large viscosity, the corotation torque behaves as expected from linear theory. However, when the viscosity becomes small enough to enable horseshoe turns to occur, the linear corotation torque exists only temporarily after insertion of a planet into the disc, being replaced by the horseshoe drag first discussed by Ward. This happens after a time that is equal to the horseshoe libration period reduced by a factor amounting to about twice the disc aspect ratio. This torque scales with the radial gradient of specific vorticity, as does the linear torque, but we find it to be many times larger. If the viscosity is large enough for viscous diffusion across the co-orbital region to occur within a libration period, we find that the horseshoe drag may be sustained. If not, the corotation torque saturates leaving only the linear Lindblad torques. As the magnitude of the non-linear co-orbital torque (horseshoe drag) is always found to be larger than the linear torque, we find that the sign of the total torque may change even for mildly positive surface density gradients. In combination with a kinematic viscosity large enough to keep the torque from saturating, strong sustained deviations from linear theory and outward or stalled migration may occur in such cases. 相似文献
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Evgeny Griv 《Monthly notices of the Royal Astronomical Society》2006,365(3):1007-1011
The possibility that protoplanetary gaseous discs are dynamically unstable to axisymmetric and non-axisymmetric gravity perturbations (e.g. those produced by spontaneous disturbances) with characteristic scales larger than the vertical scale height is discussed analytically, using a local Wentzel–Kramers–Brillouin (WKB) approach. It is shown that such discs might be clumpy, and these gravitationally bound clumps may later collapse to become giant planets ('hot Jupiters'). The chief aim in this paper is to underscore a fact of vital importance for application in the planetary formation process: gravitationally unstable non-axisymmetric (spiral) perturbations can effectively transport both the angular momentum and the mass in a spatially inhomogeneous disc. 相似文献
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S.-J. Paardekooper J. C. B. Papaloizou 《Monthly notices of the Royal Astronomical Society》2009,394(4):2297-2309
We study the coorbital flow for embedded, low-mass planets. We provide a simple semi-analytic model for the corotation region, which is subsequently compared to high-resolution numerical simulations. The model is used to derive an expression for the half-width of the horseshoe region, x s , which in the limit of zero softening is given by x s / r p = 1.68( q / h )1/2 , where q is the planet to central star mass ratio, h is the disc aspect ratio and r p is the orbital radius. This is in very good agreement with the same quantity measured from simulations. This result is used to show that horseshoe drag is about an order of magnitude larger than the linear corotation torque in the zero-softening limit. Thus, the horseshoe drag, the sign of which depends on the gradient of specific vorticity, is important for estimates of the total torque acting on the planet. We further show that phenomena, such as the Lindblad wakes, with a radial separation from corotation of approximately a pressure scaleheight H can affect x s , even though for low-mass planets x s ≪ H . The effect is to distort streamlines and reduce x s through the action of a back pressure. This effect is reduced for smaller gravitational softening parameters and planets of higher mass, for which x s becomes comparable to H . 相似文献
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Noah Brosch Carmiya Bar-Or Doron Malka 《Monthly notices of the Royal Astronomical Society》2006,368(2):864-876
We report the discovery of small groups of uncatalogued, compact, star forming (SF) dwarf galaxies (DGs) by Hα mapping of the neighbourhoods of apparently isolated, catalogued, SF DGs. Our sample consists of dwarf ( M ≥−18 mag) galaxies at least 2 Mpc away from any other catalogued galaxy. The galaxies were selected to exhibit Hα emission of any intensity, i.e. not selecting only strong starbursts, as an indicator of recent or on-going star formation with the goal of understanding why are they presently forming stars. We identified possible neighbours by imaging the galaxies and their surroundings through Hα filters centred at or near the redshift of the galaxy, and searching for localized Hα emission with the characteristics of the line emission from the sample galaxies.
We identified 20 possible SF neighbour galaxies, 17 of them not previously catalogued, in three of the five search fields where we had good quality data, and present here their positions and, images and morphology, as well as some indications of binarity. The relatively large number of possible neighbour candidates, combined with their relative faintness, argue that it would be virtually impossible to identify truly isolated galaxies. It seems that the objects we selected as extremely isolated are probably the brightest members of sparse groups of galaxies, where the other members are also DGs that are presently forming stars. In order to enhance the confidence of this statement regular redshifts are required for our candidate neighbours. 相似文献
We identified 20 possible SF neighbour galaxies, 17 of them not previously catalogued, in three of the five search fields where we had good quality data, and present here their positions and, images and morphology, as well as some indications of binarity. The relatively large number of possible neighbour candidates, combined with their relative faintness, argue that it would be virtually impossible to identify truly isolated galaxies. It seems that the objects we selected as extremely isolated are probably the brightest members of sparse groups of galaxies, where the other members are also DGs that are presently forming stars. In order to enhance the confidence of this statement regular redshifts are required for our candidate neighbours. 相似文献
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We develop a simple model for computing planetary formation based on the core instability model for the gas accretion and the oligarchic growth regime for the accretion of the solid core. In this model several planets can form simultaneously in the disc, a fact that has important implications especially for the changes in the dynamic of the planetesimals and the growth of the cores since we consider the collision between them as a source of potential growth. The type I and type II migration of the embryos and the migration of the planetesimals due to the interaction with the disc of gas are also taken into account. With this model we consider different initial conditions to generate a variety of planetary systems and analyse them statistically. We explore the effects of using different type I migration rates on the final number of planets formed per planetary system such as on the distribution of masses and semimajor axis of extrasolar planets, where we also analyse the implications of considering different gas accretion rates. A particularly interesting result is the generation of a larger population of habitable planets when the gas accretion rate and type I migration are slower. 相似文献
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Marios Kampakoglou Joseph Silk 《Monthly notices of the Royal Astronomical Society》2007,380(2):646-656
Galaxy discs are characterized by star formation histories that vary systematically along the Hubble sequence. We study global star formation, incorporating supernova feedback, gas accretion and enriched outflows in discs modelled by a multiphase interstellar medium in a fixed gravitational potential. The star formation histories, gas distributions and chemical evolution can be explained in a simple sequence of models which are primarily regulated by the cold gas accretion history. 相似文献
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Half of the energy ever emitted by stars and accreting objects comes to us in the far-infrared (FIR) waveband and has yet
to be properly explored. We propose a powerful Far-InfraRed Interferometer mission, FIRI, to carry out high-resolution imaging spectroscopy in the FIR. This key observational capability is essential to reveal how
gas and dust evolve into stars and planets, how the first luminous objects in the Universe ignited, how galaxies formed, and
when super-massive black holes grew. FIRI will disentangle the cosmic histories of star formation and accretion onto black holes and will trace the assembly and evolution
of quiescent galaxies like our Milky Way. Perhaps most importantly, FIRI will observe all stages of planetary system formation and recognise the birth of planets via its ability to image the dust
structures in planetary systems. FIRI is an observatory-class mission concept: three cold, 3.5-m apertures, orbiting a beam-combining module, with separations
of up to 1 km, free-flying or tethered, operating between 25 and 385 μm, using the interferometric direct-detection technique
to ensure μJy sensitivity and 0.02” resolution at 100 μm, across an arcmin2 instantaneous field of view, with a spectral resolution, R ~ 5,000 and a heterodyne system with R ~ 1 million. Although FIRI is an ambitious mission, we note that FIR interferometry is appreciably less demanding than at shorter wavelengths.
On behalf of the following scientists: Javier Alcolea, David Alexander, Philippe Andre, Rafael Bachiller, Mike Barlow, Andrey
Baryshev, Alain Baudry, Carlton Baugh, Dominic Benford, Ted Bergin, Frank Bertoldi, Geoffrey Blake, Andrew Blain, Sylvain
Bontemps, Franois Bouchet, Francois Boulanger, Jeroen Bouwman, Jonathan Braine, Bernhard Brandl, Valentin Bujarrabal, Sylvie
Cabrit, Martin Caldwell, Daniela Calzetti, John Carpenter, Emmanuel Caux, Cecilia Ceccarelli, Jose Cernicharo Quintanilla,
Pierre Chanial, Andrea Cimatti, John Conway, Pierre Cox, Dave Clements, Bill Dent, Gianfranco De Zotti, Carsten Dominik, Loretta
Dunne, Steve Eales, Nicholas Elias, Duncan Farrah, Mike Fich, Jacqueline Fischer, Alberto Franceschini, David Frayer, Carlos
Frenk, Gary Fuller, Maryvonne Gerin, Martin Giard, Thijs de Graauw, Gian Luigi Granato, Jane Greaves, Matt Griffin, Rolf Guesten,
Stephane Guilloteau, Martin Harwit, Thomas Henning, Eric Herbst, Michiel Hogerheijde, Kate Isaak, Christine Joblin, Eelco
van Kampen, Hubert Klahr, Kirsten Kraiberg Knudsen, Oliver Krause, Cedric Lacey, Robert Laing, Jean-Michel Lamarre, Huib Jan
van Langevelde, David Leisawitz, Simon Lilly, Darek Lis, Rene Liseau, Enrico Lorenzini, Dieter Lutz, Michael Macintyre, Suzanne
Madden, Roberto Maiolino, Jesus Martin-Pintado, Karine Mercier, Karl Menton, Vincent Minier, Raphael Moreno, Harvey Moseley,
Takao Nakagawa, David Neufeld, Michael Olberg, Seb Oliver, Luca Olmi, Goran Olofsson, Hans Olofsson, Mat Page, Pantelis Papadopoulos,
Juan Pardo, John Pearson, Ismael Perez-Fournon, Sabine Philipp, Rene Plume, Albrecht Poglitsch, Jean-Loup Puget, John Richer,
Dimitra Rigopoulou, Stephen Rinehart, Peter Roelfsema, Paolo Saraceno, Rudolf Schieder, Karl-Friedrich Schuster, Gene Serabyn,
Stephen Serjeant, Robert Simon, Ian Smail, Luigi Spinoglio, Jason Stevens, Eckhard Sturm, Juergen Stutzki, Bruce Swinyard,
Emma Taylor, Floris van der Tak, Leonardo Testi, Wing-Fai Thi, Mattia Vaccari, Lars Venema, Laurent Vigroux, Serena Viti,
Catherine Vlahakis, Christoffel Waelkens, Malcolm Walmsley, Fabian Walter, Rens Waters, Paul van der Werf, Paul Wesselius,
Glenn White, Wolfgang Wild, Tom Wilson, Adam Woodcraft, Gillian Wright, FriedrichWyrowski, Harold Yorke, Min Yun, Jonas Zmuidzinas 相似文献
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