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1.
We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around  4.7 μm  , we determine the excitation conditions in the line-forming region. We find  12C/13C = 3.5+2.0−1.5  , consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of  2.2 × 10−6≤ M CO≤ 2.7 × 10−6 M⊙  of CO ejecta outside the dust, forming a high-velocity wind of  500 ± 80 km s−1  . We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor.  相似文献   

2.
The variation of the specific intensity across the stellar disc is an essential input parameter in surface brightness reconstruction techniques such as Doppler imaging, where the relative intensity contributions of different surface elements are important in detecting star-spots. We use phoenix and atlas model atmospheres to model light curves derived from high precision (signal-to-noise ratio ≃ 5000) Hubble Space Telescope ( HST ) data of the eclipsing binary SV Cam (F9V+K4V), where the variation of specific intensity across the stellar disc will determine the contact points of the binary system light curve. For the first time, we use χ2 comparison fits to the first derivative profiles to determine the best-fitting model atmosphere. We show the wavelength dependence of the limb darkening and that the first derivative profile is sensitive to the limb-darkening profile very close to the limb of the primary star. It is concluded that there is only a marginal difference (<1σ) between the χ2 comparison fits of the two model atmospheres to the HST light curve at all wavelengths. The usefulness of the second derivative of the light curve for measuring the sharpness of the primary's limb is investigated, but we find that the data are too noisy to permit a quantitative analysis.  相似文献   

3.
We present a state-of-the-art scenario for newly born magnetars as strong sources of gravitational waves (GWs) in the early days after formation. We address several aspects of the astrophysics of rapidly rotating, ultra-magnetized neutron stars (NSs), including early cooling before transition to superfluidity, the effects of the magnetic field on the equilibrium shape of NSs, the internal dynamical state of a fully degenerate, oblique rotator and the strength of the electromagnetic torque on the newly born NS. We show that our scenario is consistent with recent studies of supernova remnant surrounding Anomalous X-ray Pulsars (AXPs) and Soft Gamma-Ray Repeaters (SGRs) in the Galaxy that constrains the electromagnetic energy input from the central NS to be  ≤ 1051  erg. We further show that if this condition is met, then the GW signal from such sources is potentially detectable with the forthcoming generation of GW detectors up to Virgo cluster distances where an event rate ∼1 yr−1 can be estimated. Finally, we point out that the decay of an internal magnetic field in the 1016 G range couples strongly with the NS cooling at very early stages, thus significantly slowing down both processes: the field can remain this strong for at least 103 yr, during which the core temperature stays higher than several times 108 K.  相似文献   

4.
We present BeppoSAX observations of Nova Velorum 1999 (V382 Vel), carried out in a broad X-ray band covering 0.1–300 keV only 15 d after the discovery and again after 6 months. The nova was detected at day 15 with the BeppoSAX instruments which measured a flux F x≃1.8×10−11 erg cm−2 s−1 in the 0.1–10 keV range and a 2 σ upper limit F x<6.7×10−12 erg cm−2 s−1 in the 15–60 keV range. We attribute the emission to shocked nebular ejecta at a plasma temperature kT ≃6 keV . At six months no bright component emerged in the 15–60 keV range, but a bright central supersoft X-ray source appeared. The hot nebular component previously detected had cooled to a plasma temperature kT <1 keV . There was strong intrinsic absorption of the ejecta in the first observation and not in the second, because the column density of neutral hydrogen decreased from N (H)≃1.7×1023 to N (H)≃1021 cm−2 (close to the interstellar value). The unabsorbed X-ray flux also decreased from F x=4.3×10−11 to F x≃10−12 erg cm−2 s−1 .  相似文献   

5.
We present the Chandra ACIS-S3 data of the old classical nova RR Pic (1925). The source has a count rate of 0.067 ± 0.002 count s−1 in the 0.3–5.0 keV energy range. We detect the orbital period of the underlying binary system in the X-ray wavelengths. We also find that the neutral hydrogen column density differs for orbital minimum and orbital maximum spectra with values  0.25+0.23−0.18× 1022  and  0.64+0.13−0.14× 1022 cm−2  at 3σ confidence level. The X-ray spectrum of RR Pic can be represented by a composite model of bremsstrahlung with a photoelectric absorption, two absorption lines centered around 1.1–1.4 keV and five Gaussian lines centered at emission lines around 0.3–1.1 keV corresponding to various transitions of S, N, O, C, Ne and Fe. The bremsstrahlung temperature derived from the fits ranges from 0.99 to 1.60 keV and the unabsorbed X-ray flux is found to be  2.5+0.4−1.2× 10−13 erg  cm−2 s−1  in the 0.3–5.0 keV range with a luminosity of 1.1 ± 0.2  1031 erg s−1  at 600 pc. We also detect excess emission in the spectrum possibly originating from the reverse shock in the ejecta. A fit with a cooling flow plasma emission model shows enhanced abundances of He, C, N, O and Ne in the X-ray emitting region indicating existence of diffusive mixing.  相似文献   

6.
We present subarcsecond MERLIN 0.4-GHz (73 cm) and 1.6-GHz (18 cm) radio measurements of the nuclear region of the Seyfert galaxy NGC 4151. By comparison with higher frequency observations, we deduce that one component (C4) shows a low-frequency turnover which we interpret as evidence for free–free absorption by ionized gas with an emission measure between 3 × 105 and 106 pc cm−6. The free–free absorption appears to be localized to a region ∼50 pc in extent, and we consider models in which the ionized gas may be closely associated with a neutral molecular torus.  相似文献   

7.
CK Vul is classified as, amongst others, the slowest known nova, a hibernating nova or a very late thermal pulse object. Following its eruption in ad 1670, the star remained visible for 2 yr. A 15-arcsec nebula was discovered in the 1980s, but the star itself has not been detected since the eruption. We here present radio images which reveal a 0.1-arcsec radio source with a flux of 1.5 mJy at 5 GHz. Deep Hα images show a bipolar nebula with a longest extension of 70 arcsec, with the previously known compact nebula at its waist. The emission-line ratios show that the gas is shock-ionized, at velocities  >100 km s−1  . Dust emission yields an envelope mass of  ∼5 × 10−2 M⊙  . Echelle spectra indicate outflow velocities up to 360 km s−1. From a comparison of images obtained in 1991 and 2004 we find evidence for expansion of the nebula, consistent with an origin in the 1670 explosion; the measured expansion is centred on the radio source. No optical or infrared counterpart is found at the position of the radio source. The radio emission is interpreted as thermal free–free emission from gas with   T e∼ 104 K  . The radio source may be due to a remnant circumbinary disc, similar to those seen in some binary post-AGB stars. We discuss possible classifications of this unique outburst, including that of a sub-Chandrasekhar mass supernova, a nova eruption on a cool, low-mass white dwarf or a thermal pulse induced by accretion from a circumbinary disc.  相似文献   

8.
We employ spectra of resolution 20–35000 of seven SC stars, four S stars, two Ba stars and two K–M stars to derive abundances of a variety of elements from Sr to Eu relative to iron. Special attention is paid to Rb and Tc, and to the ratio of the heavy s-process species to the light s-process elements. Abundances are derived in LTE, both by using model atmospheres in which the carbon and oxygen abundances are nearly equal and by using curves of growth. Spectrum synthesis is used for critical lines such as the 5924-Å line of Tc and the 7800-Å line of Rb. For most of the heavy-element stars the enhancement of the s-process elements is about a factor of 10. The ratio of the heavy to light s-process species is not far from solar, except for RR Her for which the same ratio is +0.45 dex. For Tc the blending by other lines is severe. While we have probably detected the 5924-Å line, we can only present abundances in the less-than-or-equal-to category. For Rb, whose abundance is sensitive to the 85Rb/87Rb ratio and hence to the neutron density during s-process production, we find a considerable range of abundances, indicating a neutron density from 106 to ≳108 cm−3 for the SC stars. For the four S stars the range is from 107 to ≳108 cm−3. Recent calculations by Gallino et al. show that neutron densities near 107 cm−3 favour the 13C source for neutrons, while densities greater than 108 cm−3 may be associated with neutrons from the 22Ne source.  相似文献   

9.
We present radio interferometric observations of the Algol-type binary system RZ Cassiopeiae made with the VLA and MERLIN arrays at 6 cm over an incomplete orbital cycle of the system (1.195 d). We detected RZ Cas with both instruments. The images were unresolved in both cases, with angular extents comparable to the synthesized beams. The peak flux density in the VLA image was 1.14 mJy beam−1 and in the MERLIN image it was 0.93 mJy beam−1. The derived brightness temperatures are  4.02 × 108  and  4.35 × 108 K  and the effective electron energies are 0.347 and 0.346 MeV for the MERLIN and VLA data respectively. The radio light curve shows an interesting modulation centred close to the primary eclipse which seems to correlate with ASCA SIS observations of the system. The results can be interpreted as an emitting region on the outer hemisphere of the cool component aligned along the centroid axis of the binary system.  相似文献   

10.
We present new continuum VLA observations of the nearby Sy 1.5 galaxy NGC 5033, made at 4.9 and 8.4 GHz on 2003 April 8. Combined with VLA archival observations at 1.4- and 4.9-GHz made on 1993 August 7, 1999 August 29 and 1999 October 31, we sample the galaxy radio emission at scales ranging from the nuclear regions (≲100 pc) to the outer regions of the disc (∼40 kpc). The high-resolution VLA images show a core–jet structure for the Sy 1.5 nucleus. While the core has a moderately steep non-thermal radio spectrum ( S ν∝να; α4.91.5≈−0.4), the inner kpc region shows a steeper spectrum (α8.41.5≈−0.9). This latter spectrum is typical of galaxies where energy losses are high, indicating that the escape rate of cosmic ray electrons in NGC 5033 is low. The nucleus contributes little to the total 1.4-GHz radio power of NGC 5033 and, based on the radio to far-infrared (FIR) relation, it appears that the radio and FIR emission from NGC 5033 are dominated by a starburst that during the last 10 Myr produced stars at a rate of 2.8 M⊙ yr−1 yielding a supernova (type Ib/c and II) rate of 0.045 yr−1. This supernova rate corresponds to about 1 SN event every 22 yr. Finally, from our deep 8.4-GHz VLA-D image, we suggest the existence of a radio spur in NGC 5033, which could have been due to a hot superbubble formed as a consequence of sequential supernova explosions occurring during the lifetime of a giant molecular cloud.  相似文献   

11.
We study the evolution of supernova remnants in a low-metallicity medium   Z /Z⊙= 10−4 to 10−2  in the early universe, using one-dimensional hydrodynamics with non-equilibrium chemistry. Once a post-shock layer is able to cool radiatively, a dense shell forms behind the shock. If this shell becomes gravitationally unstable and fragments into pieces, next-generation stars are expected to form from these fragments. To explore the possibility of this triggered star formation, we apply a linear perturbation analysis of an expanding shell to our results and constrain the parameter range of ambient density, explosion energy and metallicity where fragmentation of the shell occurs. For the explosion energy of  1051 erg (1052 erg)  , the shell fragmentation occurs for ambient densities higher than  ≳102 cm−3 (10 cm−3  ), respectively. This condition depends little on the metallicity in the ranges we examined. We find that the mode of star formation triggered occurs only in massive  (≳108 M⊙)  haloes.  相似文献   

12.
Sakurai's object (V4334 Sgr) is a planetary nebula nucleus which is undergoing its final helium shell flash. This is the first of these rare and important events to be observable with non-optical instruments. We report the first radio detection, using a short (2-h) observation with the Very Large Array (VLA) at 4.86 GHz. The radio emission structure is coincident with the 34-arcsec diameter planetary nebula seen in optical emission lines. We find a statistical distance ∼ 3.8 ± 0.6 kpc, with a range of 1.9 <  D  < 5.3 kpc, depending on the planetary nebula (PN) mass. While we have no direct evidence for a new (post-flash) stellar wind, we estimate an upper limit to the mass-loss rate due to any such wind of 1.7 × 10−7 M⊙ yr−1. The number of emitting knots in the radio-visible nebula indicates an electron density of ∼ 2 × 108 m−3 in those knots, and a total emitting ionized mass of ∼ 0.15 M⊙, at an assumed distance of 3.8 kpc. The radio flux density indicates an Hβ flux of ∼ 6 × 10−16 W m−2, suggesting an extinction E ( B  −  V ) ∼ 1.15, comparable with reddening estimates in the direction of V4334 Sgr.  相似文献   

13.
RX J0720.4–3125 has recently been identified as a pulsating soft X-ray source in the ROSAT all-sky survey with a period of 8.391 s. Its spectrum is well characterized by a blackbody with a temperature of 8 × 105 K. We propose that the radiation from this object is thermal emission from a cooling neutron star. For this blackbody temperature we can obtain a robust estimate of the object's age of ∼ 3 × 105 yr, yielding a polar field ∼ 1014 G for magnetic dipole spin-down and a value of P compatible with current observations.  相似文献   

14.
We present the most complete multiwavelength coverage of any dwarf nova outburst: simultaneous optical, Extreme Ultraviolet Explorer and Rossi X-ray Timing Explorer observations of SS Cygni throughout a narrow asymmetric outburst. Our data show that the high-energy outburst begins in the X-ray waveband 0.9–1.4 d after the beginning of the optical rise and 0.6 d before the extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the extreme-ultraviolet flux rise, supporting the view that both components arise in the boundary layer between the accretion disc and white dwarf surface. The early rise of the X-ray flux shows that the propagation time of the outburst heating wave may have been previously overestimated.
The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve.
X-ray and extreme-ultraviolet luminosities imply accretion rates of  3 × 1015 g s−1  in quiescence,  1 × 1016 g s−1  when the boundary layer becomes optically thick, and  ∼1018 g s−1  at the peak of the outburst. The quiescent accretion rate is two and a half orders of magnitude higher than predicted by the standard disc instability model, and we suggest this may be because the inner accretion disc in SS Cyg is in a permanent outburst state.  相似文献   

15.
Although radio-quiet quasars (RQQs) constitute ≳ 90 per cent of optically identified quasar samples, their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (≤ 0.24 arcsec). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (α ∼ 0.7, where S  ∝ ν−α), and high brightness temperatures ( T B ≥ 105 K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity–absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample.  相似文献   

16.
We present the results of an analysis of ultraviolet observations of T Tauri stars (TTs). By analysing emission measures taken from the literature, we derive rates of ionizing photons from the chromospheres of five classical TTs in the range  ∼1041–1044  photon s−1, although these values are subject to large uncertainties. We propose that the He  ii /C  iv line ratio can be used as a reddening-independent indicator of the hardness of the ultraviolet spectrum emitted by TTs. By studying this line ratio in a much larger sample of objects, we find evidence for an ionizing flux which does not decrease, and may even increase, as TTs evolve. This implies that a significant fraction of the ionizing flux from TTs is not powered by the accretion of disc material on to the central object, and we discuss the significance of this result and its implications for models of disc evolution. The presence of a significant ionizing flux in the later stages of circumstellar disc evolution provides an important new constraint on disc photoevaporation models.  相似文献   

17.
We present infrared spectroscopy of the Be/X-ray binary HDE 245770/A0535+26 obtained over the period 1992–1995. The spectra show significant variability, reflecting changes in the circumstellar environment during this time. A reduction in the flux observed in the Paschen series lines between 1993 December and 1994 September correlates with a similar reduction in both the strength of Hα and the optical continuum emission, which can be attributed to a reduction in the emission measure of the disc. A turnover between optically thin and thick emission is seen for both Paschen and Brackett series lines, and allows an estimate of the disc density as ∼1012 cm−3. Echelle spectroscopy reveals strong similarities between the He I 1.008, 2.058 μm, Hα and Paschen series line profiles, suggesting their formation in a similar (and asymmetric) region of the disc. In contrast, the line profile of He I 6678 Å indicates that it is formed at smaller radii than the other transitions.  相似文献   

18.
We have used the Very Large Array (VLA) to search for the H92α radio recombination line (RRL) in four starburst galaxies. In NGC 660, the line was detected over a 17Å‐8 arcsec2 region near its starburst nucleus. The line and continuum emission indicate that the RRL-emitting gas is most likely in the form of a cluster of H ii regions with a small filling factor. Using a simple model we find that the total ionized mass in the nuclear region is in the range 2–8Å‐104 M⊙ and the rate of production of UV photons N Lyc∼1–3Å‐1053 s−1. The ratio of H92α and Brγ line intensities in NGC 660 indicates that extinction is significant even at λ=2 μm. The velocity field of the ionized gas is consistent with a rotating disc with an average velocity gradient of ∼15 km s−1 arcsec−1. The dynamical mass within the central 500 pc is ∼4Å‐108 M⊙ and may be about ∼6Å‐107 M⊙ within the central 120 pc. No line was detected in the other galaxies (NGC 520, NGC 1614 and NGC 6946) to a 3σ limit of 300 μJy. In the starburst galaxies in which RRLs have been detected, we find that there is a rough correlation between the integrated H92α line flux density and both the total far-infrared flux density and the radio continuum emission from the central region.  相似文献   

19.
We discuss the evolution of the magnetic flux density and angular velocity in a molecular cloud core, on the basis of three-dimensional numerical simulations, in which a rotating magnetized cloud fragments and collapses to form a very dense optically thick core of  >5 × 1010 cm−3  . As the density increases towards the formation of the optically thick core, the magnetic flux density and angular velocity converge towards a single relationship between the two quantities. If the core is magnetically dominated its magnetic flux density approaches  1.5( n /5 × 1010 cm−3)1/2 mG  , while if the core is rotationally dominated the angular velocity approaches  2.57 × 10−3 ( n /5 × 1010 cm−3)1/2 yr−1  , where n is the density of the gas. We also find that the ratio of the angular velocity to the magnetic flux density remains nearly constant until the density exceeds  5 × 1010 cm−3  . Fragmentation of the very dense core and emergence of outflows from fragments will be shown in the subsequent paper.  相似文献   

20.
I solve analytically the viscous evolution of an irradiated accretion disc, as seen during outbursts of soft X-ray transients. The solutions predict steep power-law X-ray decays L X ∼ (1 + t/tvisc)−4, changing to L X ∼ (1 − t/t'visc)4 at late times, where t visc, t 'visc are viscous time-scales. These forms closely resemble the approximate exponential and linear decays inferred by King and Ritter in these two regimes. The decays are much steeper than for unirradiated discs because the viscosity is a function of the central accretion rate rather than of local conditions in the disc.  相似文献   

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