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1.
碎片盘通常被描述为贫气盘, 其气尘比显著低于原行星盘, 因此很少在碎片盘中探测到气体. 得益于各种灵敏的望远镜, 探测到气体的碎片盘的数量正在不断增加, 寻找更多的含有气体的碎片盘成为碎片盘研究的重点课题. 然而大范围的搜寻非常耗时且低效, 因而为了更快更好地遴选探测的目标, 需要根据这类源的特性择选更小范围样本. 通过统计探测到气体的碎片盘及其宿主恒星的参数总结出这些源的总体特性. 为此搜集了已经发表的探测到气体的碎片盘的文献, 总计找到37个源, 包括探测到CO等冷气体的12个源、CaII等热气体的14个源以及冷热气体共存的11个源. 通过统计其宿主恒星的光谱类型、年龄、离地球距离和碎片盘的相对光度、气体质量、尘埃质量等信息, 得出了主要结论: 宿主恒星多为B型和A型恒星, 年龄大部分都小于50Myr, 相对光度分布相比于已知碎片盘更为集中, 在$10^{ - 5  相似文献   

2.
The formation of a planetary system from the protoplanetary disk leads to destruction of the latter; however, a debris disk can remain in the form of asteroids and cometary material. The motion of planets can cause the formation of coorbital structures from the debris disk matter. Previous calculations have shown that such a ring-like structure is more stable if there is a binary star in the center of the system, as opposed to a single star. To analyze the properties of the coorbital structure, we have calculated a grid of models of binary star systems with a circumbinary planet moving in a planetesimal disk. The calculations are performed considering circular orbits of the stars and the planet; the mass and position of the planet, as well as the mass ratio of the stars, are varied. The analysis of the models shows that the width of the coorbital ring and its stability significantly depend on the initial parameters of the problem. Additionally, the empirical dependences of the width of the coorbital structure on the parameters of the system have been obtained, and the parameters of the models with the most stable coorbital structures have been determined. The results of the present study can be used for the search of planets around binary stars with debris disks.  相似文献   

3.
A large number of terrestrial planets in the classical habitable zone of stars of different spectral types have already been discovered and many are expected to be discovered in the near future. However, owing to the lack of knowledge on the atmospheric properties, the ambient environment of such planets are unknown. It is known that sufficient amount of Extreme Ultraviolet (EUV) radiation from the star can drive hydrodynamic outflow of hydrogen that may drag heavier species from the atmosphere of the planet. If the rate of mass loss is sufficiently high, then substantial amount of volatiles would escape causing the planet to become uninhabitable. Considering energy-limited hydrodynamical mass loss with an escape rate that causes oxygen to escape alongwith hydrogen, an upper limit for the ratio between the EUV and the bolometric luminosities of stars which constrains the habitability of planets around them is presented here. Application of the limit to planet-hosting stars with known EUV luminosities implies that many M-type of stars should not have habitable planets around them.  相似文献   

4.
We investigate the populations of main-sequence stars within 25 pc that have debris discs and/or giant planets detected by Doppler shift. The metallicity distribution of the debris sample is a very close match to that of stars in general, but differs with >99 per cent confidence from the giant planet sample, which favours stars of above average metallicity. This result is not due to differences in age of the two samples. The formation of debris-generating planetesimals at tens of au thus appears independent of the metal fraction of the primordial disc, in contrast to the growth and migration history of giant planets within a few au. The data generally fit a core accumulation model, with outer planetesimals forming eventually even from a disc low in solids, while inner planets require fast core growth for gas to still be present to make an atmosphere.  相似文献   

5.
Planets result from a series of processes within a circumstellar disk. Evidence comes from the near planar orbits in the Solar System and other planetary systems, observations of newly formed disks around young stars, and debris disks around main-sequence stars. As planet-hunting techniques improve, we approach the ability to detect systems like the Solar System, and place ourselves in context with planetary systems in general. Along the way, new classes of planets with unexpected characteristics are discovered. One of the most recent classes contains super Earth-mass planets orbiting a few AU from low-mass stars. In this contribution, we outline a semi-analytic model for planet formation during the pre-main sequence contraction phase of a low-mass star. As the star contracts, the “snow line”, which separates regions of rocky planet formation from regions of icy planet formation, moves inward. This process enables rapid formation of icy protoplanets that collide and merge into super-Earths before the star reaches the main sequence. The masses and orbits of these super-Earths are consistent with super-Earths detected in recent microlensing experiments.  相似文献   

6.
Debris disks are optically thin, almost gas-free dusty disks observed arounda significant fraction of main-sequence stars older than about 10 Myr. Since the circumstellar dust is short-lived, the very existence of these disks is considered as evi-dence that dust-producing planetesimals are still present in mature systems, in whichplanets have formed – or failed to form – a long time ago. It is inferred that theseplanetesimals orbit their host stars at asteroid to Kuiper-belt distances and continuallysupply ...  相似文献   

7.
We present the results of numerical simulations of stationary, spherically outflowing, e ± pair winds, with total luminosities in the range 1034–1042 ergs s?1. In the concrete example described here, the wind injection source is a hot, bare, strange star, predicted to be a powerful source of e ± pairs created by the Coulomb barrier at the quark surface. We find that photons dominate in the emerging emission, and the emerging photon spectrum is rather hard and differs substantially from the thermal spectrum expected from a neutron star with the same luminosity. This might help distinguish the putative bare strange stars from neutron stars.  相似文献   

8.
Stellar abundance pattern of n-capture elements such as barium is used as a powerful tool to infer how the star formation proceeded in dwarf spheroidal (dSph) galaxies. It is found that the abundance correlation of barium with iron in stars belonging to dSph galaxies orbiting the Milky Way, i.e., Draco, Sextans, and Ursa Minor have a feature similar to that in Galactic metal-poor stars. The common feature of these two correlations can be realized by our in homogeneous chemical evolution model based on the supernova-driven star formation scenario if dSph stars formed from gas with a velocity dispersion of ∼ 26 km s-1. This velocity dispersion together with the stellar luminosities strongly suggest that dark matter dominated dSph galaxies. The tidal force of the Milky Way links this velocity dispersion with the currently observed value ≲ 10 km s-1 by stripping the dark matter in dSph galaxies. As a result, the total mass of each dSph galaxy is found to have been originally ∼ 25 times larger than at present. In this model, supernovae immediately after the end of the star formation can expel the remaining gas over the gravitational potential of the dSph galaxy. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

9.
Two A-stars that are moving in the opposite directions with respect to a system with radial velocities +100 and ?200 km/s were detected in the θ1 Ori C system (HD 37022). These stars are considered to be escaping stars. An analysis of the high-quality spectra using the cross-correlation method allowed us to determine the physical characteristics of these stars: effective temperature, gravitational acceleration, relative and absolute luminosities, mass, chemical composition, and the locations of the stars on the Hertzsprung-Russell diagram.  相似文献   

10.
Massive stars are of interest as progenitors of supernovae, i.e. neutron stars and black holes, which can be sources of gravitational waves. Recent population synthesis models can predict neutron star and gravitational wave observations but deal with a fixed supernova rate or an assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernova progenitors, i.e. with O‐ and early B‐type stars, and also all supergiants within 3 kpc. We restrict our sample to those massive stars detected both in 2MASS and observed by Hipparcos, i.e. only those stars with parallax and precise photometry. To determine the luminosities we calculated the extinctions from published multi‐colour photometry, spectral types, luminosity class, all corrected for multiplicity and recently revised Hipparcos distances. We use luminosities and temperatures to estimate the masses and ages of these stars using different models from different authors. Having estimated the luminosities of all our stars within 3 kpc, in particular for all O‐ and early B‐type stars, we have determined the median and mean luminosities for all spectral types for luminosity classes I, III, and V. Our luminosity values for supergiants deviate from earlier results: Previous work generally overestimates distances and luminosities compared to our data, this is likely due to Hipparcos parallaxes (generally more accurate and larger than previous ground‐based data) and the fact that many massive stars have recently been resolved into multiples of lower masses and luminosities. From luminosities and effective temperatures we derived masses and ages using mass tracks and isochrones from different authors. From masses and ages we estimated lifetimes and derived a lower limit for the supernova rate of ≈20 events/Myr averaged over the next 10 Myr within 600 pc from the sun. These data are then used to search for areas in the sky with higher likelihood for a supernova or gravitational wave event (like OB associations) (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

11.
In this study we investigated the effects of external trigger on the characteristics of young stellar objects (YSOs) associated with cometary globules (CGs). We made optical spectroscopy of stars associated with star-forming CGs. We find that the masses of the most massive stars associated with CGs are correlated with the masses of the parent cloud but they are systematically larger than expected for clouds of similar mass from the relation M max-star=0.33M cl 0.43 given by Larson (Mon. Not. R. Astron. Soc. 200:159, 1982). We have also estimated the luminosities of the IRAS sources found associated with CGs as a function of cloud mass and then compared them with those of the IRAS sources found associated with isolated opacity class 6 clouds (isolated and relatively away from large star forming regions). We find that the luminosities of IRAS sources associated with CGs are larger than those of the opacity class 6 clouds. These findings support results from recent simulations in which it was shown that the Radiation Driven Implosion (RDI) process, believed to be responsible for the cometary morphology and star formation, can increase the luminosity 1–2 orders of magnitudes higher than those of protostars formed without external triggering due to an increase in accretion rates. Thus implying that the massive stars can have profound influence on the star formation in clouds located in their vicinity.  相似文献   

12.
The clumpy structure in the Vega's debris disk was reported at millimeter wavelengths previously, and attributed to the concentration of dust grains trapped in resonances with a potential high-eccentricity planet. However, current imaging at multi-wavelengths with higher sensitivity indicates that the Vega's debris disk has a smooth structure. But a planet orbiting Vega could not be neglected, and the present-day observations may place a severe constraint on the orbital parameters for the potential planet. Herein, we utilize the modi- fied MERCURY codes to numerically simulate the Vega system, which consists of a debris disk and a planet. In our simulations, the initial inner and outer boundaries of the debris disk are assumed to be 80 AU and 120 AU, respectively. The dust grains in the disk have the sizes from 10 μm to 100 μm, and the nearly coplanar orbits. From the outcomes, we show that the evolution of debris disk is consistent with recent observations, if there is no planet orbiting Vega. However, if Vega owns a planet with a high eccentricity (e.g., e = 0.6), the planet's semi- major axis cannot be larger than 60 AU, otherwise, an aggregation phenomenon will occur in the debris disk due to the existence of the postulated planet. In addition, the 2:1 mean motion resonances may play a significant role in forming the structure of debris disk.  相似文献   

13.
We present, for the first time, an analysis of seven intense X-ray flares observed from six stars (LAV 796, LAV 1174, SHM2002 3734, 2MASS 02191082+5707324, V553 Car, V557 Car). These stars are located in the region of young open star clusters NGC 869 and IC 2602. These flares detected in the XMM-Newton data show a rapid rise (10–40 min) and a slow decay (20–90 min). The X-ray luminosities during the flares in the energy band 0.3–7.5 keV are in the range of 1029.9 to 1031.7 erg s?1. The strongest flare was observed with the ratio ～13 for count rates at peak of the flare to the quiescent intensity. The maximum temperature during the flares has been found to be ～100 MK. The semi-loop lengths for the flaring loops are estimated to be of the order of 1010 cm. The physical parameters of the flaring structure, the peak density, pressure and minimum magnetic field required to confine the plasma have been derived and found to be consistent with flares from pre-main sequence stars in the Orion and the Taurus-Auriga-Perseus region.  相似文献   

14.
Many nearby main sequence stars are surrounded by cool dust radiating strongly in the far-IR. The characteristics of some of the prototype systems will be discussed. The dust is understood to represent debris related to planet formation, and the known disk systems may be dense analogs of the Kuiper Belt structure in our outer solar system. ISO observing programs plan to address questions regarding how common planetary material is around normal stars and how its amount and location depend on system age. These questions are central to an understanding of the place of the "Vega/β Pic" disks in stellar evolution. Preliminary results from the first year of ISO operation will be reported. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

15.
Summary In the solar neighborhood, approximately half of all intermediate mass main sequence stars with initially between 1 and about 5 Mbecome carbon stars with luminosities near 104 L for typically less than 106 years. These high luminosity carbon stars lose mass at rates nearly always in excess of 10–7 M yr–1 and sometimes in excess of 10–5 M yr–1. Locally, close to half of the mass returned into the interstellar medium by intermediate mass stars before they become white dwarfs is during the carbon star phase. A much greater fraction of lower metallicity stars become carbon-rich before they evolve into planetary nebulae than do higher metallicity stars; therefore, carbon stars are much more importan t in the outer than in the inner Galaxy.  相似文献   

16.
A catalog of massive (⩾10 M ⊙) stars in binary and multiple systems with well-known masses and luminosities has been compiled. The catalog is analyzed using a theoretical mass-luminosity relation. This relation allows both normal main-sequence stars and stars with peculiarities: with clear manifestations of mass transfer, mass accretion, and axial rotation, to be identified. Least-squares fitting of the observational data in the range of stellar masses 10M ⊙ ⩽ M ≲ 50 M ⊙ yields the relation L ∼ M 2.76. An erratum to this article is available at .  相似文献   

17.
The evolution of the family of binaries with a low-mass star and a compact neutron star companion (low-mass X-ray binaries (LMXBs) with neutron stars) ismodeled by the method of population synthesis. Continuous Roche-lobe filling by the optical star in LMXBs is assumed to be maintained by the removal of orbital angular momentum from the binary by a magnetic stellar wind from the optical star and the radiation of gravitational waves by the binary. The developed model of LMXB evolution has the following significant distinctions: (1) allowance for the effect of the rotational evolution of a magnetized compact remnant on themass transfer scenario in the binary, (2) amore accurate allowance for the response of the donor star to mass loss at the Roche-lobe filling stage. The results of theoretical calculations are shown to be in good agreement with the observed orbital period-X-ray luminosity diagrams for persistent Galactic LMXBs and their X-ray luminosity function. This suggests that the main elements of binary evolution, on the whole, are correctly reflected in the developed code. It is shown that most of the Galactic bulge LMXBs at luminosities L x > 1037 erg s?1 should have a post-main-sequence Roche-lobe-filling secondary component (low-mass giants). Almost all of the models considered predict a deficit of LMXBs at X-ray luminosities near ～1036.5 erg s?1 due to the transition of the binary from the regime of angular momentum removal by a magnetic stellar wind to the regime of gravitational waves (analogous to the widely known period gap in cataclysmic variables, accreting white dwarfs). At low luminosities, the shape of the model luminosity function for LMXBs is affected significantly by their transient behavior-the accretion rate onto the compact companion is not always equal to the mass transfer rate due to instabilities in the accretion disk around the compact object. The best agreement with observed binaries is achieved in the models suggesting that heavy neutron stars with masses 1.4–1.9M ⊙ can be born.  相似文献   

18.
1 INTRODUCTION In the past years, we were thrilled to the reports of discoveries of many planets around stars.These planetary systems outside the solar system (if exist) provide not only an independenttest of the formation theory of the solar system but also a chance to search for extraterrestriallife in the universe. Many studies have been made to identify the particularities of these stars,among which spectroscopic studies (e.g. Gonzalez et al. 2001; Santos et al. 2001; Zhao etaL. 2001…  相似文献   

19.
We present deep Hα+[NII] narrowband imaging of FCC046 and FCC207, two dwarf ellipticals (dEs) in the Fornax Cluster. Although one does not expect much of an interstellar medium (ISM) to be present in dEs, FCC207 shows a central emission region, whereas FCC046 also contains fainter emission regions. The central emission can be explained as photo-ionization by post-AGB stars. Some of the emission clouds in FCC046 are resolved and have diameters of the order of 50-150 pc and Hα luminosities of 1030 W, comparable to supernova remnants (SNRs), pointing to recent star formation. Dwarfs like FCC046 could be the evolutionary link between normal, quiescent dEs and more vigorously starforming blue compact dwarfs (BCDs). This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

20.
The results of investigations of a number of eclipsing Wolf-Rayet binaries are presented. The ‘core’ radiuses, the ‘core’ temperatures and masses of WR stars in the eclipsing WR+OB binary systems V 444 Cyg, CX Cep, CQ Cep, and CV Ser are obtained (see Table I). The results obtained from the light curves analysis of the V 444 Cyg in the range λλ2460 Å-3.5μ give strong evidence for the Beals (1944) model of WR phenomenon. The chromospheric-coronal effects in the WN5 extended atmosphere are not observed up to a distance ofr?20R ⊙. In the Hertzsprung—Russell diagram all the WR stars lie on the left side from the main sequence between the main sequence and the sequence of uniform helium stars (see Figure 9). Their locations are close to those of the helium remnants formed as a result of mass exchange in massive close binary systems. The period variations in the systems V 444 Cyg and CQ Cep have been discovered and a reliable value of the mass loss rateM=10?5 M ⊙yr?1 is obtained, for the two WR stars. The results of the photometric and spectroscopic investigations of the WR stars with low mass companions (post X-ray binary stage?) are presented too (see Table II). The masses of the companions are (1–2)M ⊙, their optical luminosity is ～1036, erg s?1 which implies that these companions cannot be the normal stars. It is possible that these companions are neutron stars accreting from the stellar wind of the WR stars. Low values of the X-ray luminosities of such WR stars with low mass companions imply that the accretion of matter in such systems is distinct from the accretion process in classical X-ray binary systems. It is noted also that the parameters of low massive companions coupled with WR stars are close to those of helium stars.  相似文献   

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