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1.
We review different surveys, in the optical and infrared, conducted in the very young (age 1–8 Myr), nearby (d ～ 350 pc) σ Orionis cluster aimed to characterize the substellar population. We describe spectral characteristics of very low mass stars, brown dwarfs and planetary mass objects in the cluster with spectral types from K7 to T6. We study the spatial distribution of the substellar population detected in a IZJ survey covering an area of 1.12 deg.2 We find that the radial distribution of substellar objects can be well fitted by an exponential law (ρ = ρo e ?r/ro ), with a central density (ρ o ) of 0.26 ± 0.03 objects/arcmin2 and a characteristic radius (r o) of 8.8 arcmin ± 0.6 (equivalent to 0.90 ± 0.06 pc at the distance of the cluster). We discuss the presence of possible inhomogeneities in this distribution due to the existence of subclustering. We also compare the spatial distribution of the substellar population with previously known stars in the cluster. We report the initial mass spectrum in the substellar domain.  相似文献   

2.
Because of the intense brightness of the OB‐type multiple star system σ Ori, the low‐mass stellar and substellar populations close to the centre of the very young σ Orionis cluster is poorly know. I present an IJHKs survey in the cluster centre, able to detect from the massive early‐type stars down to cluster members below the deuterium burning mass limit. The near‐infrared and optical data have been complemented with X‐ray imaging. Ten objects have been found for the first time to display high‐energy emission. Previously known stars with clear spectroscopic youth indicators and/or X‐ray emission define a clear sequence in the I vs. I – Ks diagram. I have found six new candidate cluster members that follow this sequence. One of them, in the magnitude interval of the brown dwarfs in the cluster, displays X‐ray emission and a very red J – Ks colour, indicative of a disc. Other three low‐mass stars have excesses in the Ks band as well. The frequency of X‐ray emitters in the area is 80±20 %. The spatial density of stars is very high, of up to 1.6±0.1 arcmin–2. There is no indication of lower abundance of substellar objects in the cluster centre. Finally, I also report two cluster stars with X‐ray emission located at only 8000–11000 AU to σ Ori AB, two sources with peculiar colours and an object with X‐ray emission and near‐infrared magnitudes similar to those of previously‐known substellar objects in the cluster. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
The temperatures, radii, and masses of 81 He-rich white dwarfs are calculated from photometric data. It is shown that, on the average, they are less massive than DA white dwarfs: 70% of He-rich white dwarfs have masses<0.55M ⊙. Space density and birth-rate for different mass groups of H-rich and He-rich white dwarfs are obtained. Birth-rate is 1×10?12 pc?3 yr?1 and 1.5×10?12pc?3yr?1 for He-rich and H-rich white dwarfs, respectively. The mean mass of nascent white dwarfs is about 0.55M ⊙. It is shown thatV Tand its dispersion σ are correlated with the mass of white dwars, and from this progenitors' masses — of different mass groups of white dwarfs are estimated.  相似文献   

4.
Recent observations of nearby star forming regions have offered evidence that young brown dwarfs undergo a period of mass accretion analogous to the T Tauri phase observed in young stars. Brown dwarf analogs to stellar protostars, however, have yet to be definitively observed. These young, accreting objects would shed light on the nature of the dominant brown dwarf formation process, as well as provide ideal laboratories to investigate the dependence of the accretion mechanism on protostellar mass. Recent near infrared surveys have identified candidate proto‐brown dwarfs and characterized low mass protostars in nearby star forming regions. These techniques allow near infrared spectra to diagnose the effective temperature, accretion luminosity, magnetic field strength and rotation velocity of young low mass stars across the stellar/substellar boundary. The lowest mass proto‐brown dwarfs (M < 40 MJup), however, will prove challenging to observe given current near IR observational capabilities. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

5.
We present the first results on variability of very low mass stars and brown dwarfs belonging to the ∼5 Myr Lambda Orionis cluster (Collinder 69). We have monitored almost continuously in the J filter a small area of the cluster which includes 12 possible members of the cluster during one night. Some members have turned to be short‐term variables. One of them, LOri167, has a mass close to the planetary mass limit and its variability might be due to instabilities produced by the deuterium burning, although other mechanism cannot be ruled out. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

6.
I have re-visited the spatial distribution of stars and high-mass brown dwarfs in the σ Orionis (σ Ori) cluster (∼3 Ma, ∼360 pc). The input was a catalogue of 340 cluster members and candidates at separations less than 30 arcmin to σ Ori AB. Of them, 70 per cent have features of extreme youth. I fitted the normalized cumulative number of objects counting from the cluster centre to several power-law, exponential and King radial distributions. The cluster seems to have two components: a dense core that extends from the centre to   r ≈  20 arcmin and a rarified halo at larger separations. The radial distribution in the core follows a power law proportional to r 1, which corresponds to a volume density proportional to   r −2  . This is consistent with the collapse of an isothermal spherical molecular cloud. The stars more massive than  3.7 M⊙  concentrate, however, towards the cluster centre, where there is also an apparent deficit of very low mass objects  ( M < 0.16 M⊙)  . Last, I demonstrated through Monte Carlo simulations that the cluster is azimuthally asymmetric, with a filamentary overdensity of objects that runs from the cluster centre to the Horsehead Nebula.  相似文献   

7.
The nearby young σ Orionis cluster (∼360 pc, ∼3 Myr) is becoming one of the most important regions for the study of ultra low‐mass star formation and its extension down to the mass regimes of the brown dwarfs and planetarymass objects. Here, I introduce the σ Orionis cluster and present three studies that the JOVIAN group is developing: a pilot programme of near‐infrared adaptive‐optics imaging of stars of the cluster, intermediate‐resolution optical spectroscopy of a large sample of stars of the cluster and a study of the mass function down to the planetary‐mass domain. This paper is a summary of the content of four posters that I presented in the Ultra low‐mass star formation and evolution Workshop, as single author or on behalf of different collaborations. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
The interior structure of Jupiter serves as a benchmark for an entire astrophysical class of liquid–metallic hydrogen-rich objects with masses ranging from ~0.1M J to ~80M J (1M J = Jupiter mass = 1.9e30 g), comprising hydrogen-rich giant planets (mass < 13M J) and brown dwarfs (mass > 13M J but ~ < 80M J), the so-called substellar objects (SSOs). Formation of giant planets may involve nucleated collapse of nebular gas onto a solid, dense core of mass ~0.04M J rather than a stellar-like gravitational instability. Thus, detection of a primordial core in Jupiter is a prime objective for understanding the mode of origin of extrasolar giant planets and other SSOs. A basic method for core detection makes use of direct modeling of Jupiter’s external gravitational potential terms in response to rotational and tidal perturbations, and is highly sensitive to the thermodynamics of hydrogen at multi-megabar pressures. The present-day core masses of Jupiter and Saturn may be larger than their primordial core masses due to sedimentation of elements heavier than hydrogen. We show that there is a significant contribution of such sedimented mass to Saturn’s core mass. The sedimentation contribution to Jupiter’s core mass will be smaller and could be zero.  相似文献   

9.
We present results of an optical and near‐infrared (IR) 1.8 deg2 survey in the Pleiades open cluster to search for substellar objects. From optical I ‐band images from the CFHT and J ‐band images from the 3.5m CAHA Telescope, we identify 18 faint and very red L brown dwarf candidates, with I > 20.9 and I – J > 3.2. The follow‐up observations of nine objects in the H ‐ and K s‐bands confirm that eight belong to the IR sequence of the cluster and the proper motion measurements of seven candidates confirm that they are Pleiades members. A preliminary estimation of the substellar mass spectrum dN/ dM in the form of a power law M c–α provides α = +0.57 ± 0.14. We extrapolate this function to estimate the number of planetary mass objects that could be present in the cluster down to 1 MJup. Sensitive searches combining far red and near‐IR observations may unveal these objects in a near future. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

10.
The massive OB-type binary σ Ori AB is in the centre of the very young σ Orionis cluster. I have computed the most probable distances and masses of the binary for several ages using a dynamical parallax-like method. It incorporates the BVRIH -band apparent magnitudes of both components, precise orbital parameters, interstellar extinction and a widely used grid of stellar models from the literature, Kepler's third law and a  χ2  minimization. The derived distance is  334+25−22 pc  for an age of 3 ± 2 Ma; larger ages and distances are unlikely. The masses of the primary and the secondary lie on the approximate intervals  16–20 and 10–12 M⊙  , respectively. I also discuss the possibility of σ Ori AB being a triple system at ∼ 385 pc. These results will help to constrain the properties of young stars and substellar objects in the σ Orionis cluster.  相似文献   

11.
We present the results of a survey of the open star cluster Melotte 111 in Coma Berenices, undertaken using the USNO‐B1.0 and 2MASS Point Source catalogues. On the basis of their astrometric and photometric properties, we have identified 60 new candidate members with masses in the range 1.007 < M < 0.269 M⊙. We estimate a membership probability for each by extracting control clusters from the proper motion vector diagram. All 60 are found to have greater than 60 per cent probability of being cluster members, which if they are confirmed as members, more than doubles the number of known cluster members. We also have I and Z photometry for 100 low mass candidate members of the cluster, 13 of which we suggest may be brown dwarfs. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

12.
Recent observations point to the presence of structured dust grains in the discs surrounding young brown dwarfs, thus implying that the first stages of planet formation take place also in the substellar regime. Here, we investigate the potential for planet formation around brown dwarfs and very low-mass stars according to the sequential core accretion model of planet formation. We find that, for a brown dwarf mass 0.05 M⊙, our models predict a maximum planetary mass of  ∼5   M⊕  , orbiting with semimajor axis ∼ 1 au. However, we note that the predictions for the mass–semimajor axis distribution are strongly dependent upon the models chosen for the disc surface density profiles and the assumed distribution of disc masses. In particular, if brown dwarf disc masses are of the order of a few Jupiter masses, Earth-mass planets might be relatively frequent, while if typical disc masses are only a fraction of Jupiter mass, we predict that planet formation would be extremely rare in the substellar regime. As the observational constraints on disc profiles, mass dependencies and their distributions are poor in the brown dwarf regime, we advise caution in validating theoretical models only on stars similar to the Sun and emphasize the need for observational data on planetary systems around a wide range of stellar masses. We also find that, unlike the situation around solar-like stars, Type II migration is totally absent from the planet formation process around brown dwarfs, suggesting that any future observations of planets around brown dwarfs would provide a direct measure of the role of other types of migration.  相似文献   

13.
We investigate the dependence of stellar properties on the mean thermal Jeans mass in molecular clouds. We compare the results from the two largest hydrodynamical simulations of star formation to resolve the fragmentation process down to the opacity limit, the first of which was reported by Bate, Bonnell & Bromm. The initial conditions of the two calculations are identical except for the radii of the clouds, which are chosen so that the mean densities and mean thermal Jeans masses of the clouds differ by factors of 9 and 3, respectively.
We find that the denser cloud, with the lower mean thermal Jeans mass, produces a higher proportion of brown dwarfs and has a lower characteristic (median) mass of the stars and brown dwarfs. This dependence of the initial mass function (IMF) on the density of the cloud may explain the observation that the Taurus star-forming region appears to be deficient in brown dwarfs when compared with the Orion Trapezium cluster. The new calculation also produces wide binaries (separations >20 au), one of which is a wide binary brown dwarf system.
Based on the hydrodynamical calculations, we develop a simple accretion/ejection model for the origin of the IMF. In the model, all stars and brown dwarfs begin with the same mass (set by the opacity limit for fragmentation) and grow in mass until their accretion is terminated stochastically by their ejection from the cloud through dynamically interactions. The model predicts that the main variation of the IMF in different star-forming environments should be in the location of the peak (due to variations in the mean thermal Jeans mass of the cloud) and in the substellar regime. However, the slope of the IMF at high masses may depend on the dispersion in the accretion rates of protostars.  相似文献   

14.
We suggest that planets, brown dwarfs, and even low mass stars can be formed by fragmentation of protoplanetary disks around very massive stars (M ? 100 M⊙). We discuss how fragmentation conditions make the formation of very massive planetary systems around very massive stars favorable. Such planetary systems are likely to be composed of brown dwarfs and low mass stars of ～0.1–0.3 M⊙, at orbital separations of ～ few × 100–104 AU. In particular, scaling from solar-like stars suggests that hundreds of Mercury-like planets might orbit very massive stars at ～103 AU where conditions might favor liquid water. Such fragmentation objects can be excellent targets for the James Webb Space Telescope and other large telescopes working in the IR bands. We predict that deep observations of very massive stars would reveal these fragmentation objects, orbiting in the same orbital plane in cases where there are more than one object.  相似文献   

15.
The knowledge of mass loss rates due to thermal winds in cool dwarfs is of crucial importance for modeling the evolution of physical parameters of main sequence single and binary stars. Very few, sometimes contradictory, measurements of such mass loss rates exist up to now. We present a new, independent method of measuring an amount of mass lost by a star during its past life. It is based on the comparison of the present mass distribution of solar type stars in an open cluster with the calculated distribution under an assumption that stars with masses lower than Mlim have lost an amount of mass equal to ΔM. The actual value of ΔM or its upper limit is found from the best fit. Analysis of four clusters: Pleiades, NGC 6996, Hyades and Praesepe gave upper limits for ΔM in three of them and the inconclusive result for Pleiades. The most restrictive limit was obtained for Praesepe indicating that the average mass loss rate of cool dwarfs in this cluster was lower than 6 × 10–11 M⊙/yr. With more accurate mass determinations of the solar type members of selected open clusters, including those of spectral type K, the method will provide more stringent limits for mass loss of cool dwarfs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
The masses and temperatures of young low-mass stars and brown dwarfs in star-forming regions are not yet well established because of uncertainties in the age of individual objects and the spectral type–temperature scale appropriate for objects with ages of only a few Myr. Using multi-object optical spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in Orion have been classified and 44 of these confirmed as bona fide cluster members. The spectral types obtained have been converted to effective temperatures using a temperature scale intermediate between those of dwarfs and giants, which is suitable for young pre-main-sequence objects. The objects have been placed on a Hertzsprung–Russell (HR) diagram overlaid with theoretical isochrones. The low-mass stars and the higher mass substellar objects are found to be clustered around the 1 Myr isochrone, while many of the lower mass substellar objects are located well above this isochrone. An average age of 1 Myr is found for the majority of the objects. Assuming coevality of the sources and an average age of 1 Myr, the masses of the objects have been estimated and range from  0.018 to 0.44 M⊙  . The spectra also allow an investigation of the surface gravity of the objects by measurement of the sodium doublet equivalent width. With one possible exception, all objects have low gravities, in line with young ages, and the Na indices for the Trapezium objects lie systematically below those of young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index may provide a sensitive means of estimating ages in young clusters.  相似文献   

17.
We study the spatial structure and sub‐structure of regions rich in Hipparcos stars with blue BT – VT colours. These regions, which comprise large stellar complexes, OB associations, and young open clusters, are tracers of on‐going star formation in the Galaxy. The DBSCAN (Density‐Based Spatial Clustering of Applications with Noise) data clustering algorithm is used to look for spatial overdensities of early‐type stars. Once an overdensity, “agglomerate”, is identified, we carry out a data and bibliographic compilation of their star member candidates. The actual membership in agglomerate of each early‐type star is studied based on its heliocentric distance, proper motion, and previous spectro‐photometric information. We identify 35 agglomerates of early‐type Hipparcos stars. Most of them are associated to previously known clusters and OB associations. The previously unknown P Puppis agglomerate is subject of a dedicated study with Virtual Observatory tools. It is actually a new, nearby, young open cluster (d ∼ 470 pc, age ∼ 20 Ma) with a clear radial density gradient.We list P Puppis and other six agglomerates (including NGC 2451 A, vdBH 23, and Trumpler 10) as new sites for substellar searches because of their youth, closeness, and spatial density. We investigate in detail the sub‐structure in the Orion, CMa‐Pup and Pup‐Vel OB complexes (“super‐agglomerates”). We confirm or discover some stellar overdensities in the Orion complex, like the 25 Ori group, the Horsehead region (including the σ Orionis cluster), and the η Orionis agglomerate. Finally, we derive accurate parallactic distances to the Pleiades, NGC 2451 A, and IC 2391, describe several field early‐type stars at d < 200 pc, and discuss the incompleteness of our search. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

18.
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19.
The method of effective temperatureT eff and logg determination for DA-white dwarfs by KSW model atmospheres is analyzed. The existence of systematic errors in logg determination, leading to lower mass values forT eff>15000 K, is demonstrated. With due account to logg corrections, masses for 355 DA-dwarfs were evaluated. The influence of the effects of observational selection on mass distribution has been considered. These effects are connected with the fact that such selection on effects favour discovery of white dwarfs of low masses. The distribution obtained is characterized by the average mass of ～0.75M ⊙ and the distribution width of ～0.20M ⊙.  相似文献   

20.
It is by now well established that open clusters contain a considerable fraction of brown dwarfs (BDs). This paper investigates the dynamical evolution of this substellar population by using simulations with Aarseth's (1994) NBODY5 code. A noticeable preferential escape of BDs is found, which may influence the determination of the IMF of substellar objects in dynamically evolved open clusters. This small dynamical-in-origin depletion may not explain, however, the scarcity of BDs observed in some evolved clusters, as the Hyades. On the other hand, BD cooling processes are able to reduce our ability to detect BDs in old clusters in a very significant way. Our results confirm that the probability of observing BDs in open clusters is almost the same over the whole cluster area because they are distributed quite uniformly even at late stages of the evolution of the cluster. This is expected to be a general feature as observed for low-mass stars in well studied open clusters (Pleiades, Praesepe). Our present calculations show that clusters as old as the Pleiades may have lost about 10% of their initial BD population but the number ratio of BDs to normal (not substellar) stars must remain almost unchanged. However, the long-term behavior of the relative percentage of BDs depends strongly on the initial mass function (IMF) assumed in the calculations. Clusters with a Salpeterian IMF evolve to reach relative percentages of BDs as low as 40% for a starting value around 70%. Our results suggest that BDs in clusters escape preferentially by evaporation. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

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