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1.
Direct N -body calculations are presented of the formation of Galactic clusters using GasEx , which is a variant of the code Nbody6 . The calculations focus on the possible evolution of the Orion nebula cluster (ONC) by assuming that the embedded OB stars explosively drove out 2/3 of its mass in the form of gas about 0.4 Myr ago. A bound cluster forms readily and survives for 150 Myr despite additional mass loss from the large number of massive stars, and the Galactic tidal field. This is the very first time that cluster formation is obtained under such realistic conditions. The cluster contains about 1/3 of the initial 104 stars, and resembles the Pleiades cluster to a remarkable degree, implying that an ONC-like cluster may have been a precursor of the Pleiades. This scenario predicts the present expansion velocity of the ONC, which will be measurable by upcoming astrometric space missions. These missions should also detect the original Pleiades members as an associated expanding young Galactic-field subpopulation. The results arrived at here suggest that Galactic clusters form as the nuclei of expanding OB associations.
The results have wide implications, also for the formation of globular clusters and the Galactic-field and halo stellar populations. In view of this, the distribution of binary orbital periods and the mass function within and outside the model ONC and Pleiades is quantified, finding consistency with observational constraints. Advanced mass segregation is evident in one of the ONC models. The calculations show that the primordial binary population of both clusters could have been much the same as is observed in the Taurus–Auriga star-forming region. The computations also demonstrate that the binary proportion of brown dwarfs is depleted significantly for all periods, whereas massive stars attain a high binary fraction.  相似文献   

2.
We present the results of a proper motion survey of the Hyades to search for brown dwarfs, based on UKIRT Deep Sky Survey (UKIDSS) and Two-Micron All Sky Survey (2MASS) data. This survey covers  ∼275 deg2  to a depth of   K ∼ 15  mag, equivalent to a mass of  ∼0.05 M⊙  assuming a cluster age of 625 Myr. The discovery of 12 L dwarf Hyades members is reported. These members are also brown dwarfs, with masses between  0.05 < M < 0.075 M⊙  . A high proportion of these L dwarfs appear to be photometric binaries.  相似文献   

3.
A membership catalogue for the Pleiades is divided into four mass bins, and a tidally truncated King profile is fitted to each bin with good agreement with the data. The tidal radius of the cluster is found to be 13.1 pc, and the total mass of the cluster down to the stellar limit is calculated to be 735 M. The spread of stars in each bin, as well as the relaxation and crossing times, shows the Pleiades to be an approximately relaxed cluster with equilibrium between the density and velocity distributions. The cluster kinetic energy and binding energy are consistent with the virial theorem, indicating no large unseen population of brown dwarfs. However, the 1 σ errors in the cluster parameters provide an upper limit to the mass of any brown dwarf population of 131 M, which would show up in deep CCD surveys as ≤ 5.5 brown dwarfs per 10 × 10 arcminute field in the cluster centre.  相似文献   

4.
We present the results of a deep optical–near-infrared (optical–NIR) multi-epoch survey covering 2.5 deg2 of the Pleiades open star cluster to search for new very low-mass brown dwarf members. A significant (∼5 yr) epoch difference exists between the optical (CFH12k I -, Z -band) and NIR (UKIRT WFCAM J -band) observations. We construct I , I − Z and Z , Z − J colour–magnitude diagrams to select candidate cluster members. Proper motions are computed for all candidate members and compared to the background field objects to further refine the sample. We recover all known cluster members within the area of our survey. In addition, we have discovered nine new candidate brown dwarf cluster members. The seven faintest candidates have red Z − J colours and show blue NIR colours. These are consistent with being L- and T-type Pleiads. Theoretical models predict their masses to be around 11 M Jup.  相似文献   

5.
We report on the results of the spectroscopy of 10 objects previously classified as brown dwarf candidates via RIJHK colors by Eisenbeiss et al. (2009), who performed deep imaging observations on a ∼0.4 sq.deg. field at the edge of the Pleiades. We describe and judge on classification techniques in the region of M‐type stars. To classify and characterise the objects, visual and near infrared spectra have been obtained with VLT FORS and ISAAC. The spectral classification was performed using the shape of the spectra as well as spectral indices that are sensitive to the spectral type and luminosity class of M‐type stars and late M‐type brown dwarfs. Furthermore a spectrophotometric distance was calculated and compared the distance of the Pleiades to investigate the membership probability. As a second argument we analyzed the proper motion. The brown dwarf candidates were found not to be brown dwarfs, but late‐K to mid‐M‐type dwarf stars. Based on the obtained distance and tabulated proper motions we conclude that all objects are background dwarf stars (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

6.
We report the spectroscopic confirmation of four further white dwarf members of Praesepe. This brings the total number of confirmed white dwarf members to 11, making this the second largest collection of these objects in an open cluster identified to date. This number is consistent with the high-mass end of the initial mass function of Praesepe being Salpeter in form. Furthermore, it suggests that the bulk of Praesepe white dwarfs did not gain a substantial recoil kick velocity from possible asymmetries in their loss of mass during the asymptotic giant branch phase of evolution. By comparing our estimates of the effective temperatures and the surface gravities of WD0833+194, WD0840+190, WD0840+205 and WD0843+184 to modern theoretical evolutionary tracks, we have derived their masses to be in the range  0.72–0.76 M⊙  and their cooling ages ∼300 Myr. For an assumed cluster age of 625 ± 50 Myr, the inferred progenitor masses are between 3.3 and  3.5 M⊙  . Examining these new data in the context of the initial mass–final mass relation, we find that it can be adequately represented by a linear function  ( a 0= 0.289 ± 0.051,  a 1= 0.133 ± 0.015)  over the initial mass range 2.7–6  M⊙  . Assuming an extrapolation of this relation to larger initial masses is valid and adopting a maximum white dwarf mass of  1.3 M⊙  , our results support a minimum mass for core-collapse supernovae progenitors in the range  ∼6.8–8.6 M⊙  .  相似文献   

7.
We present results from high-resolution hydrodynamical simulations that explore the effects of small-scale clustering in star-forming regions. A large ensemble of small- N clusters with five stellar seeds have been modelled and the resulting properties of stars and brown dwarfs statistically derived and compared with observational data.
Close dynamical interactions between the protostars and competitive accretion driven by the cloud collapse are shown to produce a distribution of final masses that is bimodal, with most of the mass residing in the binary components. When convolved with a suitable core mass function, the final distribution of masses resembles the observed initial mass function, in both the stellar and substellar regimes. Binaries and single stars are found to constitute two kinematically distinct populations, with about half of the singles attaining velocities ≥2 km s−1, which might deprive low-mass star-forming regions of their lightest members in a few crossing times. The eccentricity distribution of binaries and multiples is found to follow a distribution similar to that of observed long-period (uncircularized) binaries.
The results obtained support a mechanism in which a significant fraction of brown dwarfs form under similar circumstances as those of normal stars but are ejected from the common envelope of unstable multiple systems before their masses exceed the hydrogen burning limit. We predict that many close binary stars should have wide brown dwarf companions. Brown dwarfs, and, in general, very low-mass stars, would be rare as pure binary companions. The binary fraction should be a decreasing function of primary mass, with low-mass or substellar primaries being scarce. Where such binaries exist, they are expected either to be close enough (semimajor axis ∼10 au) to survive strong interactions with more massive binaries or to be born in very small molecular cloud cores.  相似文献   

8.
We present the results of a long-term high-resolution spectroscopy campaign on the O-type stars in NGC 6231. We revise the spectral classification and multiplicity of these objects and we constrain the fundamental properties of the O-star population. Almost three quarters of the O-type stars in the cluster are members of a binary system. The minimum binary fraction is 0.63, with half the O-type binaries having an orbital period of the order of a few days. The eccentricities of all the short-period binaries are revised downward, and henceforth match a normal period–eccentricity distribution. The mass ratio distribution shows a large preference for O + OB binaries, ruling out the possibility that, in NGC 6231, the companion of an O-type star is randomly drawn from a standard initial mass function. Obtained from a complete and homogeneous population of O-type stars, our conclusions provide interesting observational constraints to be confronted with the formation and early evolution theories of O-stars.  相似文献   

9.
The stars that populate the solar neighbourhood were formed in stellar clusters. Through N -body simulations of these clusters, we measure the rate of close encounters between stars. By monitoring the interaction histories of each star, we investigate the singleton fraction in the solar neighbourhood. A singleton is a star which formed as a single star, has never experienced any close encounters with other stars or binaries, or undergone an exchange encounter with a binary. We find that, of the stars which formed as single stars, a significant fraction is not singletons once the clusters have dispersed. If some of these stars had planetary systems, with properties similar to those of the Solar System, the planets' orbits may have been perturbed by the effects of close encounters with other stars or the effects of a companion star within a binary. Such perturbations can lead to strong planet–planet interactions which eject several planets, leaving the remaining planets on eccentric orbits. Some of the single stars exchange into binaries. Most of these binaries are broken up via subsequent interactions within the cluster, but some remain intact beyond the lifetime of the cluster. The properties of these binaries are similar to those of the observed binary systems containing extrasolar planets. Thus, dynamical processes in young stellar clusters will alter significantly any population of Solar System-like planetary systems. In addition, beginning with a population of planetary systems exactly resembling the Solar System around single stars, dynamical encounters in young stellar clusters may produce at least some of the extrasolar planetary systems observed in the solar neighbourhood.  相似文献   

10.
We examine the effects of dynamical evolution in clusters on planetary systems or protoplanetary discs orbiting the components of binary stars. In particular, we look for evidence that the companions of host stars of planetary systems or discs could have their inclination angles raised from zero to between the threshold angles (39.23° and 140.77°) that can induce the Kozai mechanism. We find that up to 20 per cent of binary systems have their inclination angles increased to within the threshold range. Given that half of all extrasolar planets could be in binary systems, we suggest that up to 10 per cent of extrasolar planets could be affected by this mechanism.  相似文献   

11.
N -body simulations are widely used to simulate the dynamical evolution of a variety of systems, among them star clusters. Much of our understanding of their evolution rests on the results of such direct N -body simulations. They provide insight in the structural evolution of star clusters, as well as into the occurrence of stellar exotica. Although the major pure N -body codes starlab/kira and nbody4 are widely used for a range of applications, there is no thorough comparison study yet.
Here, we thoroughly compare basic quantities as derived from simulations performed either with starlab/kira or nbody4 .
We construct a large number of star cluster models for various stellar mass function settings (but without stellar/binary evolution, primordial binaries, external tidal fields, etc.), evolve them in parallel with starlab/kira and nbody4 , analyse them in a consistent way and compare the averaged results quantitatively. For this quantitative comparison, we develop a bootstrap algorithm for functional dependencies.
We find an overall excellent agreement between the codes, both for the clusters' structural and energy parameters as well as for the properties of the dynamically created binaries. However, we identify small differences, like in the energy conservation before core collapse and the energies of escaping stars, which deserve further studies.
Our results reassure the comparability and the possibility to combine results from these two major N -body codes, at least for the purely dynamical models (i.e. without stellar/binary evolution) we performed. Further detailed comparison studies for more complex systems, e.g. including stellar/binary evolution, are required.  相似文献   

12.
We have applied the “moving cluster” method to an archive of L and T brown dwarf stars to identify those stars which are members of the Ursa Major moving group.We show that five stars have proper motion directions which agree with the direction of motion expected for a cluster member, and which have proper motion distances in agreement with distances determined by trigonometrical parallax observations. We then use 2MASS data to produce an M K versus J ‐ K S colour magnitude diagram. The group members define an empirical 400 Myr isochrone, which is compared to theoretical models. This is the first cluster/group to have a known T dwarf member. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

13.
Follow-up spectroscopy of the four brown dwarf candidates discovered in an 850 arcmin2 optical/near-infrared survey of the Pleiades cluster is presented. All four candidates show spectra consistent with Pleiades membership and should be considered to be very probable members, as the possibility of their being field stars has been shown to be small. At l  = 20.55 and I  −  J  = 3.40 our faintest candidate, NPL40, is the lowest mass brown dwarf spectroscopically identified in the Pleiades at present. Its mass was estimated to be 0.040 M⊙.  相似文献   

14.
We examine the spatial distribution of brown dwarfs produced by the decay of small‐N stellar systems as expected from the embryo ejection scenario. We model a cluster of several hundred stars grouped into ‘cores’ of a few stars/brown dwarfs. These cores decay, preferentially ejecting their lowest‐mass members. Brown dwarfs are found to have a wider spatial distribution than stars, however once the effects of limited survey areas and unresolved binaries are taken into account it can be difficult to distinguish between clusters with many or no ejections. A large difference between the distributions probably indicates that ejections have occurred, however similar distributions sometimes arise even with ejections. Thus the spatial distribution of brown dwarfs is not necessarily a good discriminator between ejection and non‐ejection scenarios. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
We have performed deep, wide‐field imaging on a ∼0.4 deg2 field in the Pleiades (Melotte 22). The selected field was not yet target of a deep search for low mass stars and brown dwarfs. Our limiting magnitudes are R ∼ 22 mag and I ∼ 20 mag, sufficient to detect brown dwarf candidates down to 40 MJ. We found 197 objects, whose location in the (I, R – I) color magnitude diagram is consistent with the age and the distance of the Pleiades. Using CTK R and I as well as JHK photometry from our data and the 2MASS survey we were able to identify 7 new brown dwarf candidates. We present our data reduction technique, which enables us to resample, calibrate, and co‐add many images by just two steps. We estimate the interstellar extinction and the spectral type from our optical and the NIR data using a two‐dimensional χ2 fitting (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
We present the result of a photometric and Keck low-resolution imaging spectrometer (LRIS) spectroscopic study of dwarf galaxies in the core of the Perseus Cluster, down to a magnitude of   M B =−12.5  . Spectra were obtained for 23 dwarf-galaxy candidates, from which we measure radial velocities and stellar population characteristics from absorption line indices. From radial velocities obtained using these spectra, we confirm 12 systems as cluster members, with the remaining 11 as non-members. Using these newly confirmed cluster members, we are able to extend the confirmed colour–magnitude relation for the Perseus Cluster down to   M B =−12.5  . We confirm an increase in the scatter about the colour–magnitude relationship below   M B =−15.5  , but reject the hypothesis that very red dwarfs are cluster members. We measure the faint-end slope of the luminosity function between   M B =−18  and −12.5, finding  α=−1.26 ± 0.06  , which is similar to that of the field. This implies that an overabundance of dwarf galaxies does not exist in the core of the Perseus Cluster. By comparing metal and Balmer absorption line indices with α-enhanced single stellar population models, we derive ages and metallicities for these newly confirmed cluster members. We find two distinct dwarf elliptical populations: an old, metal-poor population with ages ∼8 Gyr and metallicities  [Fe/H] < −0.33  , and a young, metal-rich population with ages <5 Gyr and metallicities  [Fe/H] > −0.33  . Dwarf galaxies in the Perseus Cluster are therefore not a simple homogeneous population, but rather exhibit a range in age and metallicity.  相似文献   

17.
We investigate whether the recently observed population of high-velocity white dwarfs can be derived from a population of binaries residing initially within the thin disc of the Galaxy. In particular, we consider binaries where the primary is sufficiently massive to explode as a Type II supernova. A large fraction of such binaries are broken up when the primary then explodes as a supernova, owing to the combined effects of the mass loss from the primary and the kick received by the neutron star on its formation. For binaries where the primary evolves to fill its Roche lobe, mass transfer from the primary leads to the onset of a common envelope phase during which the secondary and the core of the primary spiral together as the envelope is ejected. Such binaries are the progenitors of X-ray binaries if they are not broken up when the primary explodes. For those systems that are broken up, a large number of the secondaries receive kick velocities ∼100–200 km s−1 and subsequently evolve into white dwarfs. We compute trajectories within the Galactic potential for this population of stars and relate the birth rate of these stars over the entire Galaxy to those seen locally with high velocities relative to the local standard of rest (LSR) . We show that for a reasonable set of assumptions concerning the Galactic supernova rate and the binary population, our model produces a local number density of high-velocity white dwarfs compatible with that inferred from observations. We therefore propose that a population of white dwarfs originating in the thin disc may make a significant contribution to the observed population of high-velocity white dwarfs.  相似文献   

18.
We present the discovery of the widest known ultracool dwarf–white dwarf binary. This binary is the first spectroscopically confirmed widely separated system from our target sample. We have used the Two-Micron All-Sky Survey (2MASS) and SuperCOSMOS archives in the southern hemisphere, searching for very widely separated ultracool dwarf–white dwarf binaries, and find one common proper motion system, with a separation of 3650–5250 au at an estimated distance of 41–59 pc, making it the widest known system of this type. Spectroscopy reveals 2MASS J0030−3740 is a DA white dwarf with   T eff= 7600 ± 100 K, log( g ) = 7.79–8.09  and   M WD= 0.48–0.65 M⊙  . We spectroscopically type the ultracool dwarf companion (2MASS J0030−3739) as M9 ± 1 and estimate a mass of  0.07–0.08 M⊙,  T eff= 2000–2400 K  and  log( g ) = 5.30–5.35  , placing it near the mass limit for brown dwarfs. We estimate the age of the system to be >1.94 Gyr (from the white dwarf cooling age and the likely length of the main-sequence lifetime of the progenitor) and suggest that this system and other such wide binaries can be used as benchmark ultracool dwarfs.  相似文献   

19.
It is textbook knowledge that open clusters are conspicuous members of the thin disk of our Galaxy, but their role as contributors to the stellar population of the disk was regarded as minor. Starting from a homogenous stellar sky survey, the ASCC‐2.5, we revisited the population of open clusters in the solar neighbourhood from scratch. In the course of this enterprise we detected 130 formerly unknown open clusters, constructed volume‐ and magnitude‐limited samples of clusters, re‐determined distances, motions, sizes, ages, luminosities and masses of 650 open clusters. We derived the present‐day luminosity and mass functions of open clusters (not the stellar mass function in open clusters), the cluster initial mass function CIMF and the formation rate of open clusters. We find that open clusters contributed around 40 percent to the stellar content of the disk during the history of our Galaxy. Hence, open clusters are important building blocks of the Galactic disk (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

20.
The results of CCD photometric survey performed with the 90/180 cm Schmidt‐Cassegrain Telescope of the Nicolaus Copernicus University Astronomical Observatory in Piwnice (Poland) and the 70/172 cm Schmidt Telescope of the National Astronomical Observatory (NAO) at Rozhen (Bulgaria) of the field of the 1 Gyr old open cluster NGC 6939 are presented. Twenty two variable stars were detected, four of them previously known. Four eclipsing systems (3 detached and 1 contact binary) were found to be members of the cluster. Analysis of the brightness of the contact binary V20 strongly supports the distance to the cluster of 1.74 ± 0.20 kpc. The small population of contact binaries in NGC 6939 confirms also the relatively young age of the cluster. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

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