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1.
The kHz quasi‐periodic oscillations (QPOs) have been detected by the RXTE satellite in about thirty neutron stars (NSs) in low mass X‐ray binaries (LMXBs), which are usually interpreted to be related to the Keplerian motions in the orbit close to NS surface where the accreted matter is sucked onto the star. Based on the MHD Alfvén wave oscillation model and the relativistic precession model for the neutron star (NS) kHz QPOs, estimations of mass M and radius R of some NSs are given, which can give clues to evaluate the models. Furthermore, comparisons with theoretical M ‐R relations by stellar equations of state (EOSs) are presented (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

2.
This article summarizes the processes of high‐energy emission in young stellar objects. Stars of spectral type A and B are called Herbig Ae/Be (HAeBe) stars in this stage, all later spectral types are termed classical T Tauri stars (CTTS). Both types are studied by high‐resolution X‐ray and UV spectroscopy and modeling. Three mechanisms contribute to the highenergy emission from CTTS: 1) CTTS have active coronae similar to main‐sequence stars, 2) the accreted material passes through an accretion shock at the stellar surface, which heats it to a few MK, and 3) some CTTS drive powerful outflows. Shocks within these jets can heat the plasma to X‐ray emitting temperatures. Coronae are already well characterized in the literature; for the latter two scenarios models are shown. The magnetic field suppresses motion perpendicular to the field lines in the accretion shock, thus justifying a 1D geometry. The radiative loss is calculated as optically thin emission. A mixture of shocked and coronal gas is fitted to X‐ray observations of accreting CTTS. Specifically, the model explains the peculiar line‐ratios in the He‐like triplets of Ne IX and O VII. All stars require only small mass accretion rates to power the X‐ray emission. In contrast, the HAeBe HD 163296 has line ratios similar to coronal sources, indicating that neither a high density nor a strong UV‐field is present in the region of the X‐ray emission. This could be caused by a shock in its jet. Similar emission is found in the deeply absorbed CTTS DG Tau. Shock velocities between 400 and 500 km s–1 are required to explain the observed spectrum (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

3.
We present X‐shooter observations of two brown dwarf candidates. We focus on the determination of stellar parameters and their errors. The targets, an accreting class II and a non‐accreting class III objects, are members of the σ Orionis star‐forming region. We derive the spectroscopic spectral types from the VIS spectrum and the stellar parameters. We find that the uncertainties on the stellar parameters have a minor effect on the determination of the mass accretion rate for the accreting star, thus confirming that the discrepancies between the mass accretion rate estimates found with different (simultaneous) tracers are probably due to different physical conditions where the accretion/wind indicators are produced (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
We have carried out global three‐dimensional magnetohydrodynamic simulations of the star‐disc interaction region around a young solar‐type star. The magnetic field is generated and maintained by dynamos in the star as well as in the disc. The developing mass flows possess non‐periodic time‐variable azimuthal structure and are controlled by the nonaxisymmetric magnetic fields. Since the stellar field drives a strong stellar wind, accretion is anti‐correlated with the stellar field strength and disc matter is spiraling onto the star at low latitudes, both contrary to the generally assumed accretion picture. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

5.
In this paper, we improve the previous work on the MHD Alfvén wave oscillation model for the neutron star (NS) kHz quasi‐periodic oscillations (QPOs), and compare the model with the updated twin kHz QPO data. For the 17 NS X‐ray sources with the simultaneously detected twin kHz QPO frequencies, the stellar mass M and radius R constraints are given by means of the derived parameter A in the model, which is associated with the averaged mass density of the star as 〈ρ 〉 = 3M /(4πR3) ≃ 2.4 × 1014 (A /0.7)2 g/cm3, and we also compare the M ‐R constraints with the stellar equations of state. Moreover, we also discuss the theoretical maximum kHz QPO frequency and maximum twin peak separation, and some expectations on SAX J1808.4–3658 are mentioned, such as its highest kHz QPO frequency ∼ 870 Hz, which is about 1.4–1.5 times less than those of the other known kHz QPO sources. The estimated magnetic fields for both Z sources (about Eddington accretion rate ) and Atoll sources (∼ 1% ) are approximately ∼109 G and ∼108 G, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

6.
The kilo‐Hertz Quasi‐Periodic Oscillations in X‐ray binaries could originate within the accretion flow, and be a signature of non–linear fluid oscillations and mode coupling in strong gravity. The possibility to decipher these systems will impact our knowledge of fundamental parameters such as the neutron star mass, radius, and spin. Thus they offer the possibility to constrain the nuclear equation of state and the rotation parameter of stellar–mass black holes. We review the general properties of these oscillations from a hydrodynamical point of view, when the accretion flow is subject to external perturbations and summarize recent results. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

7.
We present preliminary results of our X‐shooter survey in star forming regions. In this contribution we focus on subsamples of young stellar and sub‐stellar objects (YSOs) in the Lupus star forming region and in the TW Hya association. We show that the X‐shooter spectra are suitable for conducting several parallel studies such as YSO + disk fundamental parameters, accretion and outflow activity in the very low‐mass (VLM) and sub‐stellar regimes, as well as magnetic activity in young VLM YSOs, and Li abundance determinations. The capabilities of X‐shooter in terms of wide spectral coverage, resolution and limiting magnitudes, allow us to assess simultaneously the accretion/outflow, magnetic activity, and disk diagnostics, from the UV and optical to the near‐IR, avoiding ambiguities due to possible YSO variability (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

8.
In an attempt to model the accretion on to a neutron star in low-mass X-ray binaries, we present 2D hydrodynamical models of the gas flow in close vicinity of the stellar surface. First, we consider a gas pressure-dominated case, assuming that the star is non-rotating. For the stellar mass we take   M star= 1.4 × 10−2 M⊙  and for the gas temperature   T = 5 × 106 K  . Our results are qualitatively different in the case of a realistic neutron star mass and a realistic gas temperature of T ≃ 108 K, when the radiation pressure dominates. We show that to get the stationary solution in a latter case, the star most probably has to rotate with the considerable velocity.  相似文献   

9.
We present a method of determining lower limits on the masses of pre-main-sequence (PMS) stars and so constraining the PMS evolutionary tracks. This method uses the redshifted absorption feature observed in some emission-line profiles of T Tauri stars, indicative of infall. The maximum velocity of the accreting material measures the potential energy at the stellar surface, which, combined with an observational determination of the stellar radius, yields the stellar mass. This estimate is a lower limit owing to uncertainties in the geometry and projection effects. Using available data, we show that the computed lower limits can be larger than the masses derived from PMS evolutionary tracks for M   0.5 M. Our analysis also supports the notion that accretion streams do not impact near the stellar poles but probably hit the stellar surface at moderate latitudes.  相似文献   

10.
We have investigated the influence of the r-mode instability on hypercritically accreting neutron stars in close binary systems during their common envelope phases, based on the scenario proposed by Brown et al. On the one hand, neutron stars are heated by the accreted matter at the stellar surface, but on the other hand they are also cooled down by the neutrino radiation. At the same time, the accreted matter transports its angular momentum and mass to the star. We have studied the evolution of the stellar mass, temperature and rotational frequency.
The gravitational-wave-driven instability of the r-mode oscillation strongly suppresses spinning up of the star, the final rotational frequency of which is well below the mass-shedding limit, in fact typically as low as 10 per cent of that of the mass-shedding state. On a very short time-scale the rotational frequency tends to approach a certain constant value and saturates there, as long as the amount of accreted mass does not exceed a certain limit to collapse to a black hole. This implies that a similar mechanism of gravitational radiation to that in the so-called 'Wagoner star' may work in this process. The star is spun up by accretion until the angular momentum loss by gravitational radiation balances the accretion torque. The time-integrated dimensionless strain of the radiated gravitational wave may be large enough to be detectable by gravitational wave detectors such as LIGO II.  相似文献   

11.
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   

12.
The landscape of Galactic X‐ray sources made of accreting binaries, isolated objects and active stellar coronae has been significantly modified by the advent of the Chandra, XMM‐Newton and INTEGRAL satellites. New types of relatively low X‐ray luminosity X‐ray binaries have been unveiled in the Galactic disc, while deep observations of the central regions have revealed large numbers of X‐ray binaries of so far poorly constrained nature. Because of the high spatial resolution needed and faint X‐ray luminosities generally emitted, studying the dependency of the X‐ray source composition with parent stellar population, Galactic disc, bulge, nuclear bulge, etc., is only practicable in our Galaxy. The evolutionary links between low LX X‐ray binaries and classical X‐ray luminous accreting systems are still open in many cases. In addition, the important question of the nature of the compact sources contributing to the Galactic ridge hard X‐ray emission remains unresolved. We review the most important results gathered by XMM‐Newton over the last years in this domain and show how future observations could be instrumental in addressing several of these issues. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

13.
We have identified three possible ways in which future XMM‐Newton observations can provide significant constraints on the equation of state of neutron stars. First, using a long observation of the neutron star X‐ray transient Cen X‐4 in quiescence one can use the RGS spectrum to constrain the interstellar extinction to the source. This removes this parameter from the X‐ray spectral fitting of the pn and MOS spectra and allows us to investigate whether the variability observed in the quiescent X‐ray spectrum of this source is due to variations in the soft thermal spectral component or variations in the power law spectral component coupled with variations in NH. This will test whether the soft thermal spectral component can indeed be due to the hot thermal glow of the neutron star. Potentially such an observation could also reveal redshifted spectral lines from the neutron star surface. Second, XMM‐Newton observations of radius expansion type I Xray bursts might reveal redshifted absorption lines from the surface of the neutron star. Third, XMM‐Newton observations of eclipsing quiescent low‐mass X‐ray binaries provide the eclipse duration. With this the system inclination can be determined accurately. The inclination determined from the X‐ray eclipse duration in quiescence, the rotational velocity of the companion star and the semi‐amplitude of the radial velocity curve determined through optical spectroscopy, yield the neutron star mass. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
We collect the data of twin kilohertz quasi‐periodic oscillations (kHz QPOs) published before 2012 from 26 neutron star (NS) low‐mass X‐ray binary (LMXB) sources, then we analyze the centroid frequency (ν) distribution of twin kHz QPOs (lower frequency ν1 and upper frequency ν2) both for Atoll and Z sources. For the data without shift‐and‐add, we find that Atoll and Z sources show different distributions of ν1, ν2 and ν2/ν1, but the same distribution of Δν (difference of twin kHz QPOs), which indicates that twin kHz QPOs may share the common properties of LXMBs and have the same physical origins. The distribution of Δν is quite different from a constant value, so is ν 2/ν1 from a constant ratio. The weighted mean values and maxima of ν1 and ν2 in Atoll sources are slightly higher than those in Z sources. We also find that shift‐and‐add technique can reconstruct the distributions of ν1 and Δν. The K‐S test results of ν1 and Δν between Atoll and Z sources from data with shift‐and‐add are quite different from those without it, and we think that this may be caused by the selection biases of the sample. We also study the properties of the quality factor (Q) and the root‐meansquared (rms) amplitude of 4U 0614+09 with data from the two observational methods, but the errors are too big to make a robust conclusion. The NS spin frequency (νs) distribution of 28 NS‐LMXBs show a bigger mean value (∼408 Hz) than that (∼281 Hz) of the radio binary millisecond pulsars (MSPs), which may be due to the lack of the spin detections from Z sources (systematically lower than 281 Hz). Furthermore, on the relations between the kHz QPOs and NS spin frequency νs, we find the approximate correlations of the mean values of Δν with NS spin and its half, respectively. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
In this paper, we develop a spectral differential technique with which the dynamical mass of low‐mass companions can be found. This method aims at discovering close companions to late‐type stars by removing the stellar spectrum through a subtraction of spectra obtained at different orbital phases and discovering the companion spectrum in the difference spectrum in which the companion lines appear twice (positive and negative signal). The resulting radial velocity difference of these two signals provides the true mass of the companion, if the orbital solution for the radial velocities of the primary is known. We select the CO line region in the K band for our study, because it provides a favourable star‐to‐companion brightness ratio for our test case GJ 1046, an M2V dwarf with a low‐mass companion that most likely is a brown dwarf. Furthermore, these lines remain largely unblended in the difference spectrum so that the radial velocity amplitude of the companion can be measured directly. Only if the companion rotates rapidly and has a small radial velocity due to a high mass, does blending occur for all lines so that our approach fails. We also consider activity of the host star, and show that the companion difference flux can be expected to have larger amplitude than the residual signal from the active star so that stellar activity does not inhibit the determination of the companion mass. In addition to determining the companion mass, we restore the single companion spectrum from the difference spectrum using singular value decomposition. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
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   

17.
Star‐disc coupling is considered in numerical models where the stellar field is not an imposed perfect dipole, but instead a more irregular self‐adjusting dynamo‐generated field. Using axisymmetric simulations of the hydromagnetic mean‐field equations, it is shown that the resulting stellar field configuration is more complex, but significantly better suited for driving a stellar wind. In agreement with recent findings by a number of people, star‐disc coupling is less efficient in braking the star than previously thought. Moreover, stellar wind braking becomes equally important. In contrast to a perfect stellar dipole field, dynamo‐generated stellar fields favor field‐aligned accretion with considerably higher velocity at low latitudes, where the field is weaker and originating in the disc. Accretion is no longer nearly periodic (as it is in the case of a stellar dipole), but it is more irregular and episodic. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

18.
Stellar magnetic fields govern key aspects of the evolution of a young star, from controlling accretion to regulating the angular momentum evolution of the system. Spectro‐polarimetric studies of T Tauri stars have revealed a surprising range of magnetic field topologies. Meanwhile multi‐wavelength campaigns have probed T Tauri star systems from stellar photosphere to inner disk, allowing us to study magnetospheric accretion in unprecedented detail. We review recent results and discuss their implications for understanding the evolution of young stars (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

19.
Low frequency oscillation, typical for γ Doradus g‐mode type stellar core sensitive pulsation, as well as higher frequency δ Scuti type pulsation typical for p ‐modes, sensitive to the envelope, make HD 8801 a remarkable hybrid pulsator with the potential to probe a stellar structure over a wide range of radius. In addition HD 8801 is a rare pulsating metallic line (Am) star. We determined the astro‐physical fundamental parameters to locate HD 8801 in the H‐R diagram. We analyzed the element abundances, paying close attention to the errors involved, and confirm the nature of HD 8801 as a metallic line (Am) star. We also determined an upper limit on the magnetic field strength. Our abundance analysis is based on classical techniques, but uses for the final step a model atmosphere calculated with the abundances determined by us. We also discuss spectropolarimetric observations obtained for HD 8801. This object is remarkable in several respects. It is a nonmagnetic metallic line (Am) star, pulsating simultaneously in p‐ and g‐modes, but also shows oscillations with periods in between these two domains, whose excitation requires explanation. Overall, the pulsational incidence in unevolved classical Am stars is believed to be quite low; HD 8801 does not conform to this picture. Finally, about 75 % of Am stars are located in short‐period binaries, but there is no evidence that HD 8801 has a companion. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

20.
A total of 28 young nearby stars (ages ≤60 Myr) have been observed in the Ks‐band with the adaptive optics imager Naos‐Conica of the Very Large Telescope at the Paranal Observatory in Chile. Among the targets are ten visual binaries and one triple system at distances between 10 and 130 pc, all previously known. During a first observing epoch a total of 20 faint stellar or sub‐stellar companion‐candidates were detected around seven of the targets. These fields, as well as most of the stellar binaries, were re‐observed with the same instrument during a second epoch, about one year later. We present the astrometric observations of all binaries. Their analysis revealed that all stellar binaries are co‐moving. In two cases (HD 119022 AB and FG Aqr B/C) indications for significant orbital motions were found. However, all sub‐stellar companion candidates turned out to be non‐moving background objects except PZ Tel which is part of this project but whose results were published elsewhere. Detection limits were determined for all targets, and limiting masses were derived adopting three different age values; they turn out to be less than 10 Jupiter masses in most cases, well below the brown dwarf mass range. The fraction of stellar multiplicity and of the sub‐stellar companion occurrence in the star forming regions in Chamaeleon are compared to the statistics of our search, and possible reasons for the observed differences are discussed. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

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