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1.
In this paper we present numerical results on the decay of small stellar systems under different initial conditions (multiplicity 3 ≤  N  ≤ 10, and various mass spectra, initial velocities and initial configurations). The numerical treatment uses the CHAIN1 code (Mikkola &38; Aarseth). Particular attention is paid to the distribution of high-velocity escapers: we define these as stars with velocity above 30 km s−1. These numerical experiments confirm that small N -body systems are dynamically unstable and produce cascades of escapers in the process of their decay. It is shown that the fraction of stars that escape from small dense stellar systems with an escape velocity greater than 30 km s−1 is ∼1 per cent for all systems treated here. This relatively small fraction must be considered in relation to the rate of star formation in the Galaxy in small groups: this could explain some moderately high-velocity stars observed in the Galactic disc and possibly some young stars with relatively high metallicity in the thick disc.  相似文献   

2.
We present 13 CO J  = 1 − 0 line observations of the H  ii region complex W51B located in the high-velocity (HV) stream. These observations reveal a filamentary and clumpy structure in the molecular gas. The mean local standard of rest (LSR) velocity ∼ + 65 km s−1 of the molecular gas in this region is greater than the maximum velocities allowed by kinematic Galactic rotation curves. The size and mass of the molecular clouds are ∼ 48 × 17 pc2 and ∼ 2.4 × 105 M⊙ respectively. In a position–velocity diagram, molecular gas in the southern part comprises a redshifted ring structure with v LSR=+ 60 to +73 km s−1. The velocity gradient of this ring is ∼ 0.5 km s−1 pc−1, and the mass is ∼ 6.2 × 104 M⊙. If we assume that the ring is expanding with a uniform velocity, the expansion velocity, radius and kinetic energy are ∼ 7 km s−1, ∼ 13 pc and ∼ 3.0 × 10 49 erg respectively. The kinetic energy and mass spectrum of the ring could be explained by an expanding cylindrical cloud with a centrally condensed mass distribution. The locations of two compact H  ii regions, G49.0−0.3 and G48.9−0.3, coincide with the two molecular clumps in this ring. We discuss star formation, and the mechanism that produced the ring structure.  相似文献   

3.
Using new and archival radio data, we have measured the proper motion of the black hole X-ray binary V404 Cyg to be  9.2 ± 0.3 mas yr−1  . Combined with the systemic radial velocity from the literature, we derive the full three-dimensional heliocentric space velocity of the system, which we use to calculate a peculiar velocity in the range 47–102 km s−1, with a best-fitting value of 64 km s−1. We consider possible explanations for the observed peculiar velocity and find that the black hole cannot have formed via direct collapse. A natal supernova is required, in which either significant mass  (∼11 M⊙)  was lost, giving rise to a symmetric Blaauw kick of up to ∼65 km s−1, or, more probably, asymmetries in the supernova led to an additional kick out of the orbital plane of the binary system. In the case of a purely symmetric kick, the black hole must have been formed with a mass  ∼9 M⊙  , since when it has accreted  0.5–1.5 M⊙  from its companion.  相似文献   

4.
A statistical study of 233 pulsar proper motions   总被引：2，自引：0，他引：2  
We present and analyse a catalogue of 233 pulsars with proper motion measurements. The sample contains a wide variety of pulsars including recycled objects and those associated with globular clusters or supernova remnants. After taking the most precise proper motions for those pulsars for which multiple measurements are available, the majority of the proper motions (58 per cent) are derived from pulsar timing methods, 41 per cent using interferometers and the remaining 1 per cent using optical telescopes. Many of the one-dimensional (1D) and two-dimensional (2D) speeds (referring to speeds measured in one coordinate only and the magnitudes of the transverse velocities, respectively) derived from these measurements are somewhat lower than earlier estimates because of the use of the most recent electron density model in determining pulsar distances. The mean 1D speeds for the normal and recycled pulsars are 152(10) and 54(6) km s−1, respectively. The corresponding mean 2D speeds are 246(22) and 87(13) km s−1. PSRs B2011+38 and B2224+64 have the highest inferred 2D speeds of  ∼1600 km s−1  . We study the mean speeds for different subsamples and find that, in general, they agree with previous results. Applying a novel deconvolution technique to the sample of 73 pulsars with characteristic ages less than 3 Myr, we find the mean three-dimensional (3D) pulsar birth velocity to be 400(40) km s−1. The distribution of velocities is well described by a Maxwellian distribution with  1D rms σ= 265 km s−1  . There is no evidence for a bimodal velocity distribution. The proper motions for PSRs B1830−08 and B2334+61 are consistent with their proposed associations with the supernova remnants W41 and G114.3+0.3, respectively.  相似文献   

5.
We present time-series echelle spectra of the Paβ line of the T Tauri star SU Aur, observed over three consecutive nights. The line shows strong variability (∼10 per cent) over the velocity range 100–420 km s−1 in the red broad absorption component, and weaker variability (∼2 per cent) over the velocity range  −200–0 km s−1  in the blue wing. The variability in the velocity range  −200–0 km s−1  is correlated with that in  200–400 km s−1  , and the variability in these velocity ranges anticorrelates with that in  0–100 km s−1  . The mean spectrum from the second night shows the suggestion of a blueshifted absorption component at about  −150 km s−1  , similar to that found in the Hα and Hβ lines. We find the position of the subpeak in the red absorption component changes steadily with time, and its motion modulates at half the rotational period. We also find that the modulation of the line equivalent width is possibly associated with a half and a third of the rotational period, which is consistent with the surface Doppler images of SU Aur. Radiative transfer models of a rotationally modulated Paβ line, produced in the shock-heated magnetospheric accretion flow, are also presented. Models with a magnetic dipole offset reproduce the overall characteristics of the observed line variability, including the line equivalent width and the motion of the subpeak in the red absorption trough.  相似文献   

6.
We have detected the   v = 1 → 0 S(1) (λ= 2.1218 μm)  and   v = 2 → 1 S(1) (λ= 2.2477 μm)  lines of H2 in the Galactic Centre, in a  90 × 27 arcsec2  region between the north-eastern boundary of the non-thermal source Sgr A East, and the giant molecular cloud (GMC)  M−0.02 − 0.07  . The detected  H2 v = 1 → 0  S(1) emission has an intensity of  1.6–21 × 10−18 W m−2 arcsec−2  and is present over most of the region. Along with the high intensity, the large linewidths  (FWHM = 40–70 km s−1)  and the  H2 v = 2 → 1 S(1)  to   v = 1 → 0 S(1)  line ratios (0.3–0.5) can be best explained by a combination of C-type shocks and fluorescence. The detection of shocked H2 is clear evidence that Sgr A East is driving material into the surrounding adjacent cool molecular gas. The H2 emission lines have two velocity components at ∼+50 and  ∼0 km s−1  , which are also present in the NH3(3, 3) emission mapped by McGary, Coil & Ho. This two-velocity structure can be explained if Sgr A East is driving C-type shocks into both the  GMC M−0.02 − 0.07  and the northern ridge of McGary et al.  相似文献   

7.
We use ideal axisymmetric relativistic magnetohydrodynamic simulations to calculate the spin-down of a newly formed millisecond,   B ∼ 1015 G  , magnetar and its interaction with the surrounding stellar envelope during a core-collapse supernova (SN) explosion. The mass, angular momentum and rotational energy lost by the neutron star are determined self-consistently given the thermal properties of the cooling neutron star's atmosphere and the wind's interaction with the surrounding star. The magnetar drives a relativistic magnetized wind into a cavity created by the outgoing SN shock. For high spin-down powers  (∼1051–1052 erg s−1)  , the magnetar wind is superfast at almost all latitudes, while for lower spin-down powers  (∼1050 erg s−1)  , the wind is subfast but still super-Alfvénic. In all cases, the rates at which the neutron star loses mass, angular momentum and energy are very similar to the corresponding free wind values (≲30 per cent differences), in spite of the causal contact between the neutron star and the stellar envelope. In addition, in all cases that we consider, the magnetar drives a collimated  (∼5–10°)  relativistic jet out along the rotation axis of the star. Nearly all of the spin-down power of the neutron star escapes via this polar jet, rather than being transferred to the more spherical SN explosion. The properties of this relativistic jet and its expected late-time evolution in the magnetar model are broadly consistent with observations of long duration gamma-ray bursts (GRBs) and their associated broad-lined Type Ic SN.  相似文献   

8.
We have analysed the kinematical parameters of Cir X-1 to constrain the nature of its companion star, the eccentricity of the binary and the pre-supernova parameter space. We argue that the companion is most likely to be a low-mass (≲2.0 M⊙) unevolved star and that the eccentricity of the orbit is 0.94±0.04. We have evaluated the dynamical effects of the supernova explosion and we find it must have been asymmetric. On average , we find that a kick of ∼740 km s−1 is needed to account for the recently measured radial velocity of +430 km s−1 (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is ∼500 km s−1. This is the largest kick needed to explain the motion of any observed binary system. If Cir X-1 is associated with the supernova remnant G321.9-0.3 then we find a limiting minimum age of this remnant of ∼60 000 yr. Furthermore, we predict that the companion star has lost ∼10 per cent of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion.  相似文献   

9.
We present radial velocities for 2045 stars in the Small Magellanic Cloud (SMC), obtained from the 2dF survey by Evans et al. The great majority of these stars are of OBA type, tracing the dynamics of the young stellar population. Dividing the sample into ad hoc 'bar' and 'wing' samples (north and south, respectively, of the line:  δ=−77°50'+[4α]'  , where α is the right ascension in minutes of time) we find that the velocities in the SMC bar show a gradient of 26.3 ± 1.6 km s−1 deg−1 at a position angle of 126°± 4°. The derived gradient in the bar is robust to the adopted line of demarcation between the two samples. The largest redshifts are found in the SMC wing, in which the velocity distribution appears distinct from that in the bar, most probably a consequence of the interaction between the Magellanic Clouds that is predicted to have occurred 0.2 Gyr ago. The mean velocity for all stars in the sample is +172.0 ± 0.2 km s−1 (redshifted by ∼20 km s−1 when compared to published results for older populations), with a velocity dispersion of 30 km s−1.  相似文献   

10.
Continuum-subtracted dereddened images in the light of several atomic lines show the presence of an extended bipolar nebula surrounding η Carinae with size ∼100×45 arcsec2 (1.3×0.5 pc2). This feature is best delineated in [O  iii ] 5007. The geometrical disposition and mass of the shell suggest that it was formed by mass ejections from η Carinae. The dynamic age of the nebula is ∼13 000/ V 7 yr, where V 7 is the mean expansion velocity in 100 km s−1, and its mass is between 5 and 10 M⊙. The nebula is photoionized and composed of unprocessed material. The major axes of the nebula and of the Homunculus are nearly perpendicular. We also report the discovery of elongated emission knots prominent in [N  ii ] located 64 to 100 arcsec away from η Carinae, which implies that they were ejected either centuries ago or at a more recent date but with extremely large velocities.  相似文献   

11.
GGD30 has been suggested to be either a small reflection nebulosity or a Herbig–Haro (HH) object formed in the outflow from a nearby obscured star. Observations to date have not been able to distinguish between these two scenarios. In addition, there are conflicting proposals for the location of the exciting source for GGD30. To resolve these questions, we have carried out optical spectroscopy and near-infrared ( J , K and 3.6-μm) imaging of GGD30. Taken together, these observations reveal that the bright optical knot in GGD30 must be a HH object, excited by the outflow from an optically obscured pre-main-sequence (PMS) star located ∼3 arcsec to the southwest. Based on mid-infrared fluxes from the Mid-course Space Experiment ( MSX ) satellite, we estimate the luminosity of this PMS star to be  ∼12.5 L⊙  which suggests it is an intermediate-mass object rather than low-mass as previously proposed. The optical spectroscopy indicates projected velocities of  ∼−270 km s−1  associated with the HH object. The fact that these velocities are blueshifted and relatively high compared to the velocities typical of HH flows suggests that the outflow from the PMS star must be almost aligned with the line of sight. There is an additional low-velocity  (∼−70 km s−1) Hα  component but its origin is not clear.  相似文献   

12.
To study the kinematics of O-B5 giant stars (luminosity class III), 290 non-Gould belt stars with proper motions taken from the Hipparcos catalogue are used, of which 107 have radial velocities taken from other sources. Semidefinite programming solves for the kinematical parameters and the coefficients of the velocity ellipsoid. The condition that both solutions must yield the same solar velocity is enforced. The results obtained are reasonable: solar velocity of 13.83 ± 0.17 km s−1; Oort's constants, in units of km s−1 kpc−1, A = 16.08 ± 0.72 and   B =−10.74 ± 0.65,  implying a rotational velocity of 228.0 ± 21.4 km s−1 if we take the distance to the Galactic Centre as 8.5 ± 1.1 kpc; velocity dispersions, in units of km s−1, of  σ x = 32.44 ± 5.04, σ y = 26.16 ± 2.75, σ z = 18.71 ± 2.39  with a vertex deviation of      相似文献   

13.
High-resolution long-slit Hα spectra of the shell of the old nova DQ Herculis have been obtained with the William Herschel Telescope using the ISIS spectrograph. An equatorial expansion velocity of  370 ± 14   km s−1  is derived from the spectra which, in conjunction with a narrow-band Hα image of the remnant, allows a distance estimate of 525 ± 28 pc. An equatorial ring which exhibits enhanced [N  ii ] emission has also been detected and the inclination angle of the shell is found to be     with respect to the line of sight. The spectra also reveal tails extending from the clumps in the shell, which have a radial velocity increasing along their length. This suggests the presence of a stellar wind, collimated in the polar direction, which ablates fragments of material from the clumps and accelerates them into its stream up to a terminal velocity of the order of 800–900 km s−1.  相似文献   

14.
We present high- and medium-resolution spectroscopic observations of the cataclysmic variable BF Eridani (BF Eri) during its low and bright states. The orbital period of this system was found to be 0.270881(3) d. The secondary star is clearly visible in the spectra through the absorption lines of the neutral metals Mg  i , Fe  i and Ca  i . Its spectral type was found to be K3±0.5. A radial velocity study of the secondary yielded a semi-amplitude of   K 2= 182.5 ± 0.9 km s−1  . The radial velocity semi-amplitude of the white dwarf was found to be   K 1= 74 ± 3 km s−1  from the motion of the wings of the Hα and Hβ emission lines. From these parameters, we have obtained that the secondary in BF Eri is an evolved star with a mass of  0.50–0.59 M⊙  , whose size is about 30 per cent larger than a zero-age main-sequence single star of the same mass. We also show that BF Eri contains a massive white dwarf  ( M 1≥ 1.2 M⊙)  , which allows us to consider the system as a Type Ia supernova progenitor. BF Eri also shows a high γ-velocity  (γ=−94 km s−1)  and substantial proper motion. With our estimation of the distance to the system  ( d ≈ 700 ± 200 pc)  , this corresponds to a space velocity of ∼350 km s−1 with respect to the dynamical local standard of rest. The cumulative effect of repeated nova eruptions with asymmetric envelope ejection might explain the high space velocity of the system. We analyse the outburst behaviour of BF Eri and question the current classification of the system as a dwarf nova. We propose that BF Eri might be an old nova exhibiting 'stunted' outbursts.  相似文献   

15.
Using data from the Sloan Digital Sky Survey data release 3 (SDSS DR3), we investigate how narrow (<700 km s−1) C  iv and Mg  ii quasar absorption-line systems are distributed around quasars. The C  iv absorbers lie in the redshift range 1.6 < z < 4 and the Mg  ii absorbers in the range 0.4 < z < 2.2. By correlating absorbers with quasars on different but neighbouring lines of sight, we measure the clustering of absorbers around quasars on comoving scales between 4 and 30 Mpc. The observed comoving correlation lengths are   r o∼ 5 h −1Mpc  , similar to those observed for bright galaxies at these redshifts. Comparing correlations between absorbers and the quasars, in whose spectra they are identified, then implies: (i) that quasars destroy absorbers to comoving distances of ∼300 kpc (C  iv ) and ∼800 kpc (Mg  ii ) along their lines of sight; (ii) that ≳40 per cent of C  iv absorbers within 3000 km s−1 of the quasi-stellar object are not a result of large-scale clustering but rather are directly associated with the quasar itself; (iii) that this intrinsic absorber population extends to outflow velocities of the order of 12 000 km s−1; (iv) that this outflow component is present in both radio-loud and radio-quiet quasars and (v) that a small high-velocity outflow component is also observed in the Mg  ii population. We also find an indication that absorption systems within 3000 km s−1 are more abundant for radio-loud quasars than for radio-quiet quasars. This suggests either that radio-loud objects live in more massive haloes, or that their radio activity generates an additional low-velocity outflow, or both.  相似文献   

16.
We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s−1. The highest energy lines require a column density of   N H∼ 5 × 1023 cm−2  , at an ionization parameter of  log ξ∼ 3.4  . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143.  相似文献   

17.
We present MERLIN neutral hydrogen absorption measurements against supernova remnants in the central starburst region of M82 with an angular resolution of ∼ 0.4 arcsec. We detect H  I absorption or set significant upper limits against 33 supernova remnants from which we have been able to deduce column densities. Hence, using these measurements, we are able to probe the neutral hydrogen distribution and dynamics of the interstellar medium in M82 along 33 lines of sight on linear scales of order 1 pc.   Our results show column densities ranging from <1.6 to >30 × 1021 atom cm−2 with the highest values seen towards the edge of the 250-pc 'ring'. The absorption velocities show a gradient of 7.3 ± 4 km s−1 arcsec−1, consistent with rotation parameters of this 'ring' inferred from other measurements. The absorption velocities against individual remnants show deviations of typically 30 km s−1 from simple solid body rotation, and a number show multiple velocity absorption features. Although some of these deviations may be the result of the remnants being embedded at different depths within the neutral gas, the velocities cannot be explained by a simple rotating ring.  相似文献   

18.
In this paper, an attempt is made to integrate the propeller ejection of material by the fast rotating white dwarf in AE Aquarii with the highly transient thermal and non-thermal emission in a single unifying model. It has been shown that the violent interaction between the fast rotating magnetosphere and a clumpy fragmented stream, in AE Aquarii specifically, may result in the growth of unstable modes of the Kelvin–Helmholtz instability and associated turbulence over length scales comparable to the stream radius on time-scales  τK-H ∼  t dyn (∼ 600 s  ). For all conversion efficiencies of magnetohydrodynamic (MHD) power to mechanical energy ε≥ 0.1, these instabilities result in the effective azimuthal acceleration of the gas parcels to the escape velocity over time-scales   t acc≤ 1000 s (∼ t dyn)  . Further, it has been shown that the turbulence in the flow will cascade down to the dissipative level over time-scales  τcas∼ 3 h  . If released through dissipative shocks, this reservoir can drive a luminosity of   L ∼ 1033 erg s−1  , which can significantly contribute to the total emission when blobs collide in the exit stream, resulting in shock heating and associated flares. During the propeller process, particles can also be accelerated to high energies, which may be the driving mechanism behind the non-thermal radio to mid-infrared emission. The confluence of these ejected magnetized clouds may result in radio remnant surrounding AE Aquarii, which is optically thin between frequencies ν≥ 100 MHz–1 GHz.  相似文献   

19.
We present a new analysis of an archived Chandra HETGS X-ray spectrum of the WR+O colliding wind binary γ2 Velorum. The spectrum is dominated by emission lines from astrophysically abundant elements: Ne, Mg, Si, S and Fe. From a combination of broad-band spectral analysis and an analysis of line flux ratios we infer a wide range of temperatures in the X-ray-emitting plasma (∼4–40 MK). As in the previously published analysis, we find the X-ray emission lines are essentially unshifted, with a mean FWHM of  1240 ± 30 km s−1  . Calculations of line profiles based on hydrodynamical simulations of the wind–wind collision predict lines that are blueshifted by a few hundred  km s−1  . The lack of any observed shift in the lines may be evidence of a large shock-cone opening half-angle (>85°), and we suggest this may be evidence of sudden radiative braking. From the R and G ratios measured from He-like forbidden-intercombination-resonance triplets we find evidence that the Mg  xi emission originates from hotter gas closer to the O star than the Si  xiii emission, which suggests that non-equilibrium ionization may be present.  相似文献   

20.
We present high-resolution spectra (1.0 km s−1 FWHM) of the circumstellar Ca K line towards β Pictoris obtained on 1997 June 19 and 20. On the former date a strong absorption component was found at a heliocentric velocity of v helio = +8 km s−1, that is blueshifted by 14 km s−1 with respect to the main, 'stable', circumstellar component at v helio = +22 km s−1. To our knowledge, this is the first detection of a blueshifted Ca  ii component with a strength comparable to the more frequently observed redshifted events. On the following night a blueshifted component was still present, but its strength had decreased significantly; in addition, a strong redshifted component had appeared at v helio = +54 km s−1 which was absent on the previous night. The implications of these observations for the evaporating 'comet' model of spectral variations in the β Pictoris disc are discussed.  相似文献   

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