共查询到20条相似文献，搜索用时 15 毫秒
1.
2.
3.
Stuart Clark Alan McCall Antonio Chrysostomou Tim Gledhill Jeremy Yates James Hough 《Monthly notices of the Royal Astronomical Society》2000,319(2):337-349
Near-infrared linear imaging polarimetry of the young stellar objects R CrA and T CrA in the J , H and K n bands, and circular imaging polarimetry in the H band, is presented. The data are modelled with the Clark and McCall scattering model. The R CrA and T CrA system is shown to be a particularly complex scattering environment. In the case of R CrA there is evidence that the wavelength dependence of polarization changes across the nebula. MRN dust grain models do not explain this behaviour. Depolarization by line emission is considered as an alternative explanation. The dust grain properties could also be changing across the nebula.
Although surrounded by reflection nebulosity, there is a region of particularly low polarization surrounding R CrA that is best modelled by the canonical bipolar outflow being truncated by an evacuated spherical cavity surrounding the star. The symmetry axis of the nebula appears inclined by 50° to the plane of the sky.
The H -band circular polarimetry of R CrA clearly shows a quadrupolar structure of positive and negative degrees of circular polarization that reach peak magnitudes of ∼5 per cent within our limited map. It is shown that spherical MRN grains are incapable of producing this circular polarization given the observed linear polarization of the R CrA system. Instead, scattering from aligned non-spherical grains is proposed as the operating mechanism.
T CrA is a more archetypical bipolar reflection nebula, and this object is modelled as a canonical parabolic reflection nebula that lies in the plane of the sky. The wavelength independence of linear polarization in the T CrA reflection nebula suggests that the scattering particles are Rayleigh sized. This is modelled with the MRN interstellar grain size distribution. 相似文献
Although surrounded by reflection nebulosity, there is a region of particularly low polarization surrounding R CrA that is best modelled by the canonical bipolar outflow being truncated by an evacuated spherical cavity surrounding the star. The symmetry axis of the nebula appears inclined by 50° to the plane of the sky.
The H -band circular polarimetry of R CrA clearly shows a quadrupolar structure of positive and negative degrees of circular polarization that reach peak magnitudes of ∼5 per cent within our limited map. It is shown that spherical MRN grains are incapable of producing this circular polarization given the observed linear polarization of the R CrA system. Instead, scattering from aligned non-spherical grains is proposed as the operating mechanism.
T CrA is a more archetypical bipolar reflection nebula, and this object is modelled as a canonical parabolic reflection nebula that lies in the plane of the sky. The wavelength independence of linear polarization in the T CrA reflection nebula suggests that the scattering particles are Rayleigh sized. This is modelled with the MRN interstellar grain size distribution. 相似文献
4.
T. R. Bedding Albert A. Zijlstra A. Jones & G. Foster 《Monthly notices of the Royal Astronomical Society》1998,301(4):1073-1082
We discuss visual observations spanning nearly 70 years of the nearby semiregular variable R Doradus. Using wavelet analysis, we show that the star switches back and forth between two pulsation modes having periods of 332 d and about 175 d, the latter with much smaller amplitude. Comparison with model calculations suggests that the two modes are the first and third radial overtone, with the physical diameter of the star making fundamental-mode pulsation unlikely. The mode changes occur on a time-scale of about 1000 d, which is too rapid to be related to a change in the overall thermal structure of the star and may instead be related to weak chaos. The Hipparcos distance to R Dor is 62.4 ± 2.8 pc which, taken with its dominant 332-d period, places it exactly on the period–luminosity (P–L) relation of Miras in the Large Magellanic Cloud. Our results imply first-overtone pulsation for all Miras which fall on the P–L relation. We argue that semiregular variables with long periods may largely be a subset of Miras and should be included in studies of Mira behaviour. The semiregulars may contain the immediate evolutionary Mira progenitors, or stars may alternate between periods of semiregular and Mira behaviour. 相似文献
5.
Geoffrey C. Clayton 《Astrophysics and Space Science》2002,279(1-2):167-170
The R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich supergiants which undergo spectaculardeclines in brightness of up to 8 magnitudes at irregular intervals as dust forms along the line of sight.Understanding the RCB stars is a key test for any theory whichaims to explain hydrogen deficiency in post-Asymptotic Giant Branch (AGB) stars. There are two major evolutionary models for the origin of RCB stars: the Double Degenerate and the Final Helium ShellFlash. In the final flashmodel, there is a close relationship between RCB stars and Planetary Nebulae (PNe). The connection between RCB stars and PNe has recentlybecome stronger, since the central stars of three old PNe (Sakurai's Object, V605 Aql and FG Sge) have had observedoutbursts that transformed them from hot evolved central stars into cool giants with the spectral properties of an RCB star. 相似文献
6.
A. P. Jacob T. R. Bedding J. G. Robertson M. Scholz 《Monthly notices of the Royal Astronomical Society》2000,312(4):733-746
We discuss the wavelength dependence of angular diameters of M giants from an observational perspective. Observers cannot directly measure an optical-depth radius for a star, despite this being a common theoretical definition. Instead, they can use an interferometer to measure the square of the fringe visibility. We present new plots of the wavelength-dependent centre-to-limb variation (CLV) of intensity of the stellar disc as well as visibility for Mira and non-Mira M giant models. We use the terms 'CLV spectra' and 'visibility spectra' for these plots. We discuss a model-predicted extreme limb-darkening effect (also called the narrow-bright-core effect) in very strong TiO bands which can lead to a misinterpretation of the size of a star in these bands. We find no evidence as yet that this effect occurs in real stars. Our CLV spectra can explain the similarity in visibilities of R Dor (M8IIIe) that have been observed recently, despite the use of two different passbands. We compare several observations with models, and find that the models generally underestimate the observed variation in visibility with wavelength. We present CLV and visibility spectra for a model that is applicable to the M supergiant α Ori. 相似文献
7.
8.
O. Hashimoto H. Izumiura D. J. M. Kester Tj. R. Bontekoe 《Astrophysics and Space Science》1995,224(1-2):477-478
Extended emission components are clearly found in the IRAS scan data of optically visible oxygen-rich AGB stars which show no 10µm silicate band feature in the IRAS LRS spectra but a strong infrared excess in the IRAS photometric data. It is most likely that these stars really have their circumstellar dust envelopes, which are detached from the central stars, indicating a halting of mass loss for a significant period. 相似文献
9.
10.
R. J. Ivison J. A. Yates & P. J. Hall 《Monthly notices of the Royal Astronomical Society》1998,295(4):813-818
We report high spatial and spectral resolution measurements of masers towards R Aqr and H1−36, both of which are examples of the sub-class of symbiotic stars that contain a long-period Mira-type variable. Our observations have resulted in the first detection of 321-GHz H2 O maser action towards a symbiotic Mira — R Aqr. Comparison with simultaneous 22-GHz H2 O maser data suggests that the masers do not have the same properties as those in the circumstellar envelopes of field Miras. R Aqr's 22-/321-GHz peak flux density and luminosity ratios are low, as is the line width ratio. Continuum and spectral-line maps indicate that the 22-GHz maser and free–free emission are aligned. Three mechanisms can reproduce the data with varying degrees of success. All three lead naturally to normal levels of maser emission in SiO and 321-GHz H2 O and anomalously weak OH and 22-GHz H2 O masers. In the most convincing model, UV radiation and a fast wind from the companion remove the Mira's envelope of dusty, molecular gas, leaving a relatively small cavity of dense, neutral material within a large, ionized nebula. Excitation temperatures suggest that 321-GHz masers are normally excited close to the Mira whilst 22-GHz masers are more remote; in R Aqr, therefore, the 22-GHz masers do not form under optimum conditions. Instead, we see weak and narrow lines that form closer to the Mira, consistent with our high-resolution maps. 相似文献
11.
Yakiv Pavlenko 《Astrophysics and Space Science》2002,279(1-2):91-102
Procedures and results of computations of model atmospheres andspectral energy distributions of R CrB, V4334 Sgr and relatedobjects are discussed. The sequence of SEDs computed for a gridof model atmospheres with 4000 < Teff < 7000, 0 < logg < 1 provides a unique possibility to analyze the temporalchanges of physical parameters of V 4334 Sgr and related objectsin the framework of a self-consistent approach. 相似文献
12.
13.
P. Petit B. Dintrans S. K. Solanki J.-F. Donati M. Aurière F. Lignières J. Morin F. Paletou J. Ramirez C. Catala R. Fares 《Monthly notices of the Royal Astronomical Society》2008,388(1):80-88
From a set of stellar spectropolarimetric observations, we report the detection of surface magnetic fields in a sample of four solar-type stars, namely HD 73350, HD 76151, HD 146233 (18 Sco) and HD 190771. Assuming that the observed variability of polarimetric signal is controlled by stellar rotation, we establish the rotation periods of our targets, with values ranging from 8.8 d (for HD 190771) to 22.7 d (for HD 146233). Apart from rotation, fundamental parameters of the selected objects are very close to the Sun's, making this sample a practical basis to investigate the specific impact of rotation on magnetic properties of Sun-like stars.
We reconstruct the large-scale magnetic geometry of the targets as a low-order (ℓ < 10) spherical harmonic expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (i) The magnetic energy of the large-scale field increases with rotation rate. The increase in chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (ii) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ≈12 d is necessary for the toroidal magnetic energy to dominate over the poloidal component. 相似文献
We reconstruct the large-scale magnetic geometry of the targets as a low-order (ℓ < 10) spherical harmonic expansion of the surface magnetic field. From the set of magnetic maps, we draw two main conclusions. (i) The magnetic energy of the large-scale field increases with rotation rate. The increase in chromospheric emission with the mean magnetic field is flatter than observed in the Sun. Since the chromospheric flux is also sensitive to magnetic elements smaller than those contributing to the polarimetric signal, this observation suggests that a larger fraction of the surface magnetic energy is stored in large scales as rotation increases. (ii) Whereas the magnetic field is mostly poloidal for low rotation rates, more rapid rotators host a large-scale toroidal component in their surface field. From our observations, we infer that a rotation period lower than ≈12 d is necessary for the toroidal magnetic energy to dominate over the poloidal component. 相似文献
14.
15.
Saul J. Adelman 《Monthly notices of the Royal Astronomical Society》1999,310(1):146-156
Elemental abundances of the moderately rotating B9–A3 stars λ UMa, 59 Her, 14 Cyg and 29 Cyg have been derived in a consistent manner with previous studies of this series from spectrograms obtained with Reticon and CCD detectors. The derived elemental abundances show that λ UMa is a mild Am star, while 59 Her is slightly metal-rich. Although 14 Cyg has values closer to solar than these stars, its subsolar Ca and Sc abundances indicate that it might be the hottest known hot-Am star. 29 Cyg is a metal-poor λ Boo star. 相似文献
16.
17.
D. Burns J. E. Baldwin R. C. Boysen C. A. Haniff P. R. Lawson C. D. Mackay J. Rogers T. R. Scott D. St.-Jacques P. J. Warner D. M. A. Wilson & J. S. Young 《Monthly notices of the Royal Astronomical Society》1998,297(2):462-466
We report the first direct detection of long-term periodic diameter variations in a Mira variable. Angular diameter measurements of the 313-d period variable R Leonis at 833 nm and 940 nm obtained between 1996 February and 1997 June using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT) show a cyclic modulation of the apparent stellar diameter by approximately 35 per cent. The agreement between these new data and archival measurements from 1992 January suggests coherence in the modulation over a 5-yr period. Our data are consistent with recent models which suggest that, in photometric bands with only weak to moderate molecular contamination, periodic variations in stellar diameter of order 50 per cent can be maintained. The measurements indicate that the apparent stellar diameter was largest at visual phase 0.5 and that any phase shifts between the visual light curve and those at 833 and 940 nm were at most 0.05. The large offset (∼ 0.25) between the phase of the observed diameter maximum and that predicted for the photospheric continuum diameter variations suggests that our observations are more sensitive to the changing temperature structure of the outer atmosphere than to the deeper continuum-forming layers. 相似文献
18.
19.
S. P. Littlefair V. S. Dhillon Steve B. Howell David R. Ciardi 《Monthly notices of the Royal Astronomical Society》2000,313(1):117-128
We present time-resolved, J ‐band (1.025–1.340 μm) infrared spectra of the short-period dwarf novae (DNe) WZ Sge and VY Aqr, and single spectra of the short-period DN EF Peg and the nova-like variable PX And. There is some evidence in the spectra of VY Aqr and EF Peg that we have detected the secondary star, both in the continuum slope and also through the possible presence of spectral features. The spectra of WZ Sge and PX And, on the other hand, show no evidence for the secondary star, with upper limits for its contribution to the J ‐band light of 10 and 20 per cent respectively. The spectral type of the secondary in WZ Sge is constrained to be later than M7.5V. Using skew mapping, we have been able to derive a value for the radial velocity semi-amplitude of the secondary star in VY Aqr of K R =320±70 km s−1 , which in conjunction with K W from Thorstensen & Taylor gives a mass ratio of q =0.15±0.04. 相似文献