Infrared photometry of low-mass stars in Praesepe     

S. T. Hodgkin  D. J. Pinfield  R. F. Jameson  I. A. Steele  M. R. Cossburn  N. C. Hambly 《Monthly notices of the Royal Astronomical Society》1999,310(1):87-96

We present follow-up infrared photometry for a sample of low-mass and very low-mass stars in the Praesepe open cluster. Our sample is selected from two sources: (i) 90 stars selected from the Hambly et al. photometric and proper-motion survey of Praesepe; (ii) 17 stars selected from the CCD imaging survey presented by Pinfield et al. We investigate cluster membership using infrared colour–magnitude and colour–colour diagrams. We find 81 likely and two possible members in the Hambly et al. sample, in line with predictions. Contamination in the Pinfield et al. sample is higher, and we find nine probable cluster members. We investigate the non-grey models of Baraffe et al., which are found to be in good agreement with the data. Multiplicity in Praesepe is also examined, and we find the multiple star fraction to be 0.51 from analysis of the I , I − K diagram. We investigate individual object masses, and find that the faintest candidate cluster members have masses close to the substellar limit.  相似文献   

On the class of Oe stars'     

I. Negueruela  I. A. Steele  G. Bernabeu 《Astronomische Nachrichten》2004,325(9):749-760

We present high‐quality spectra of the majority of stars that have been classified as Oe and find that their published spectral types are generally too early, most likely due to infilling of He I lines. As a matter of fact, all stars classified as Oe actually fall inside the range O9–B0 with the important exception of HD 155806 (O7.5 III) and perhaps HD 39680 (difficult to classify, but likely O8.5V). Observations of a sample of objects with published spectral types in the O9–B0 range previously classified as peculiar or emission‐line stars fail to reveal any new Oe star with spectral type earlier than O9.5. Most objects classified as peculiar in “classical” literature show signs of binarity in our spectra, but no spectral anomalies. We conclude that there is likely a real decline in the fraction of Be stars for spectral types earlier than B0, not due to observational bias. The few Oe stars with spectral types earlier than O9.5 deserve detailed investigation in order to provide constraints on the physical reasons of the Be phenomenon. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

The role of Pacific water in the dramatic retreat of arctic sea ice during summer 2007     

Mike Steele  Rebecca Woodgate 《极地研究(英文版)》2008,19(2):93-107

A model study is conducted to examine the role of Pacific water in the dramatic retreat of arctic sea ice during summer 2007. The model generally agrees with the observations in showing considerable seasonal and interannual variability of the Pacific water inflow at Bering Strait in response to changes in atmospheric circulation. During summer 2007 anomalously strong southerly winds over the PaCific sector of the Arctic Ocean strengthen the ocean circulation and bring more Pacific water into the Arctic than the recent （2000-2006） average. The simulated summer （3 months ） 2007 mean Pacific water inflow at Bering Strait is 1.2 Sv, which is the highest in the past three decades of the simulation and is 20% higher than the recent average. Particularly, the Pacific water inflow in September 2007 is about 0.5 Sv or 50% above the 2000-2006 average. The strengthened warm Pacific water inflow carries an additional 1.0 x 1020 Joules of heat into the Arctic, enough to melt an additional 0.5 m of ice over the whole Chukchi Sea. In the model the extra summer oceanic heat brought in by the Pacific water mainly stays in the Chukchi and Beaufort region, contributing to the warming of surface waters in that region. The heat is in constant contact with the ice cover in the region in July through September. Thus the Pacific water plays a role in ice melting in the Chukchi and Beaufort region all summer long in 2007, likely contributing to up to O. 5 m per month additional ice melting in some area of that region.  相似文献   

Regime shifts: Can ecological theory illuminate the mechanisms?   总被引：2，自引：0，他引：2  

Jeremy S. Collie  Katherine Richardson  John H. Steele 《Progress in Oceanography》2004,60(2-4):281

“Regime shifts” are considered here to be low-frequency, high-amplitude changes in oceanic conditions that may be especially pronounced in biological variables and propagate through several trophic levels. Three different types of regime shift (smooth, abrupt and discontinuous) are identified on the basis of different patterns in the relationship between the response of an ecosystem variable (usually biotic) and some external forcing or condition (control variable). The smooth regime shift is represented by a quasi-linear relationship between the response and control variables. The abrupt regime shift exhibits a nonlinear relationship between the response and control variables, and the discontinuous regime shift is characterized by the trajectory of the response variable differing when the forcing variable increases compared to when it decreases (i.e., the occurrence of alternative “stable” states). Most often, oceanic regime shifts are identified from time series of biotic variables (often commercial fish), but this approach does not allow the identification of discontinuous regime shifts. Recognizing discontinuous regime shifts is, however, particularly important as evidence from terrestrial and freshwater ecosystems suggests that such regime shifts may not be immediately reversible. Based on a review of various generic classes of mathematical models, we conclude that regime shifts arise from the interaction between population processes and external forcing variables. The shift between ecosystem states can be caused by gradual, cumulative changes in the forcing variable(s) or it can be triggered by acute disturbances, either anthropogenic or natural. A protocol for diagnosing the type of regime shift encountered is described and applied to a data set on Georges Bank haddock, from which it is concluded that a discontinuous regime shift in the abundance of haddock may have occurred. It is acknowledged that few, if any, marine data are available to confirm the occurrence of discontinuous regime shifts in the ocean. Nevertheless, we argue that there is good theoretical evidence for their occurrence as well as some anecdotal evidence from data collection campaigns and that the possibility of their occurrence should be recognized in the development of natural resource management strategies.  相似文献   

Thermal equilibria of isobaric coronal magnetic arcades     

C. D. C. Steele  E. R. Priest 《Solar physics》1991,134(1):73-97

A coronal magnetic arcade can be thought of as consisting of an assembly of coronal loops. By solving equations of isobaric thermal equilibrium along each loop and assuming a base temperature of 2 × 10⁴ K, the thermal structure of the arcade can be found. The possible thermal equilibria can be shown to depend on two parameters L ^* p ^* and h ^*/p ^* representing the ratios of cooling (radiation) to condu and heating to cooling, respectively. Arcades can contain four types of loops: hot loops with summits hotter than 400000 K; cool loops at temperatures less than 80000 K along their lengths; hot-cool loops with cool summits and cool footpoints but hotter intermediate portions; and warm loops, cooler than 80000 K along most of their lengths but with summits as hot as 400000 K. Two possibilities for coronal heating are considered, namely a heating that is independent of magnetic field and a heating that is proportional to the square of the local magnetic field. When the arcade is sheared the thermal structure of the arcade may change, leading in some cases to non-equilibrium or in other cases to the formation of a cool core.  相似文献   

High-resolution array processing using implicit eigenvectorweighting techniques     

Steele  A.K. Byrne  C.L. 《Oceanic Engineering, IEEE Journal of》1990,15(1):8-13

Many high-resolution bearing estimators require the explicit calculation of the eigenvectors and eigenvalues of the cross-spectral matrix of the sensor outputs. Once the eigenvectors have been calculated, various estimators can be derived by altering the eigenvalues to give a reweighting of the eigenvectors. These weighting functions are reminiscent of ideal filter responses in analog filter theory, where practical filters are designed by using polynomial approximations to the ideal desired response. The approximation theory developed for filter design is used to derive high-resolution bearing estimators that do not require explicit calculation of the eigenvectors  相似文献   

Comments on `Theory and application of calibration techniques foran NDBC directional wave measurements buoy' by K.E. Steele, et al.:nonlinear effects     

Barrick  D.E. Lipa  B.J. Steele  K.E. 《Oceanic Engineering, IEEE Journal of》1989,14(3):268-272

For original paper see ibid., vol. OE-10, no.4, p.382-96 (1985). The authors of the above mentioned paper present an extensive set of linear calibration techniques that are applied to National Data Buoy Center wave-buoy sensor spectral output before calculating and disseminating directional wave spectra. The commentators identify and estimate the nonlinear effects that produce biases still present in the output, due both to wave nonlinearities themselves and to constraints on the buoy and mooring system to the driving forces. Simple models show that these nonlinearities can produce spectral energy biases of 5-15% at and above the spectral peak frequency, and even greater errors below it. NDBC presently records wave data from vertically stabilized and fixed accelerometers and slope sensors. Calculations show that these sensors all incur bias due to wave nonlinearities: this is greater for vertically stabilized accelerometers and least for slope sensors. Effects of the resulting inconsistencies between the different sensors are most pronounced below the spectral peak where the nonlinear terms dominate; these effects are illustrated with measured data  相似文献   

Directional ocean wave spectra using buoy azimuth, pitch, and rollderived from magnetic field components     

Steele  K.E. Earle  M.D. 《Oceanic Engineering, IEEE Journal of》1991,16(4):427-433

Buoy azimuth, pitch, and roll, when used with measurements of buoy vertical acceleration, can provide directional wave spectra. Earlier work, which considered effects of buoy hull magnetism, showed that azimuth can be determined from magnetic field measurements (K.E. Steele and J.C. Lau, 1986). This work is extended to show that buoy pitch and roll, and thus buoy slopes, can also be determined from the same measurements. These slopes can be determined from measurements of the magnetic field components inside the hull along two orthogonal axes parallel to the deck of a buoy. Algorithms are developed for estimation of azimuth, pitch, and roll angles using these measurements. The algorithms account for residual and induced hull magnetism. Azimuth, pitch, roll, and estimates of directional wave spectra are determined both from the magnetic field measurements and from a conventional wave measurement system on the same buoy. Comparisons show that estimates of directional spectra based on magnetometer-derived pitch and roll agree well  相似文献   

Infrared spectroscopy of the Be/X-ray transient A0535+26     

J. S. Clark  I. A. Steele  M. J. Coe  & P. Roche 《Monthly notices of the Royal Astronomical Society》1998,297(3):657-666

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   

Radio observations of IRAS-selected Southern hemisphere classical Be stars     

Clark  Steele  & Fender 《Monthly notices of the Royal Astronomical Society》1998,299(4):1119-1122

We present the first radio observations of a sample of 13 optically and IR-bright Southern hemisphere classical Be stars made from the Australian Telescope Compact Array at 3.5 and 6.3 cm simultaneously. One star, δ Cen, was detected at 3.5 cm, and a second, μ Cen, was also thought to have been detected; further observations of this source are required to confirm this detection. No sources were detected at 6.3 cm, although δ Cen was previously detected at this wavelength by other observers at a higher flux than our detection limit. The radio observations show that the spectral energy distribution undergoes a turnover between the far-IR and radio wavelengths, as was seen in previous studies. Likewise we find no simple correlation between far-IR and radio flux. Lower limits to the outer disc radius were found to be of the order of a few hundred solar radii i.e. of the order of those found previously by Taylor et al.  相似文献