共查询到20条相似文献,搜索用时 15 毫秒
1.
In this paper, we present simulation results of a ground-layer correction adaptive optics system (GLAO), based on four laser guide stars and a single deformable mirror. The goal is to achieve a seeing improvement over an 8-arcmin field of view, in the near-infrared (from 1.06 to 2.2 μm). We show results on the scaling of this system (number of subapertures, frame rates), and the required number of tip-tilt stars. We investigate the use for GLAO of both sodium and Rayleigh guide stars. We also show that if the lasers can be repositioned, the performance of the adaptive optics can be tailored to the astronomical observations. 相似文献
2.
Ren Rutten Maarten Blanken Richard McDermid Thomas Gregory Paul Jolley Tim Morris Richard Myers Johan Pragt Ton Schoenmaker Remko Stuik Gordon Talbot 《New Astronomy Reviews》2006,49(10-12):632
The scientific exploitation of adaptive optics (AO) with natural guide stars is severely constrained by the limited presence of bright guide stars for wavefront sensing. Use of a laser beam as an alternative means to provide a source for wavefront sensing has the potential of drastically improving the sky coverage for AO. For this reason at the 4.2-m William Herschel Telescope a project was started to develop a Rayleigh laser beacon to work together with the existing NAOMI adaptive optics instrumentation and the OASIS integral field spectrograph. This paper presents the rationale for this development, highlights some of the technical aspects, and gives some expected performance measures. 相似文献
3.
To accomplish its mission, the spaceborne observatory SNAP (SuperNova Acceleration Probe) requires a pointing stability of
<0.03 arcseconds during exposures lasting up to 500 sec. A Monte Carlo simulation of the photoelectron statistics from the
guiding star investigates geometrical (such as the pixel size of the detector or the plate scale) and physical parameters
(such as the magnitude of the star). It is shown that simple centroiding calculations can lead to the desired accuracy with
guide stars as faint as magnitude 16. Availability of these stars is verified thanks to the HST Guide Star Catalog complemented
with a statistical model of the distribution of stars. Thus a through-the-lens sensor that uses stars as faint as magnitude
16 to provide the necessary guiding signals is feasible.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
用于自适应光学系统的激光引导星 总被引:2,自引:0,他引:2
自适应光学系统应用于天文观测时应满足一些技术要求,其中的关键技术之一是自适应光学系统为了对畸变的光波进行采样,需要在其很小的等晕角内有足够的信标强度。激光引导星,即人造信标,就是解决这种问题的方法之一。特别是在观测天文暗目标时,其自身的光强度不能为有探测提供信息,激光引导星就蛤得非常重要。根据国际上的最新进展对激光引导星技术给予了介绍。重点介绍了激光引导星的局限性,对近几年提出的对激光引导星可能的 相似文献
5.
We present a multicolour catalogue of faint galaxies situated close to bright stars, V ≲ 15 , with the aim of identifying high-redshift galaxies suitable for study with adaptive optics-equipped near-infrared imagers and spectrographs. The catalogue is constructed from archival calibration observations of the United Kingdom Infrared Telescope (UKIRT) Faint Standard stars with the UKIRT Fast Track Imager (UFTI) camera on UKIRT. We have analysed the deepest 16 fields from the archive to provide a catalogue of galaxies brighter than K ∼ 20.3 lying between 3 and 25 arcsec of the guide stars. We identify 111 objects in a total survey area of 8.7 arcmin2 . Of these, 87 are classified as galaxies based on their light profiles in our ∼0.5 arcsec median seeing K -band images. 12 of the galaxies have ( J − K ) ≥ 2.0 consistent with them lying at high redshifts, z ≳ 2 . These 12 very red galaxies have K -band magnitudes of K = 18.1–20.1 and separations from the guide stars of 4–20 arcsec and hence are very well suited to adaptive optics studies to investigate their morphologies and spectral properties on sub-kpc scales. We provide coordinates and JHK photometry for all catalogued objects. 相似文献
6.
G. Handler 《Monthly notices of the Royal Astronomical Society》1999,309(2):L19-L23
70 new γ Doradus candidates were identified from Hipparcos photometry, which represents more than a doubling of the presently known number. Selecting the objects with good evidence for multiperiodicity, it is found that these stars, together with bona fide members of the class, occupy a well-defined region in a colour–magnitude diagram. This domain corresponds to a range of 7200–7700 K on the zero-age main sequence (ZAMS) and 6900–7500 K one magnitude above it, which partly overlaps with the instability strip of δ Scuti stars.
For the first time, γ Doradus stars can be discussed as a group. They can be found over a significant fraction of the main sequence lifetimes for objects in the relevant temperature range. An upper limit on the surface metallicity of γ Doradus stars is apparent, which may guide the search for their pulsation driving mechanism. The importance of possible objects exhibiting both γ Doradus and δ Scuti-type pulsations is discussed. 相似文献
For the first time, γ Doradus stars can be discussed as a group. They can be found over a significant fraction of the main sequence lifetimes for objects in the relevant temperature range. An upper limit on the surface metallicity of γ Doradus stars is apparent, which may guide the search for their pulsation driving mechanism. The importance of possible objects exhibiting both γ Doradus and δ Scuti-type pulsations is discussed. 相似文献
7.
The daytime atmospheric turbulence profile is crucial for the design of both optical systems and the control algorithm of a solar Multi-Conjugate Adaptive Optics(MCAO) system. The Multi-Aperture Seeing Profiler(MASP) is a portable instrument which can measure the daytime turbulence profile up to^30 km. It consists of two portable small telescopes that can deliver performance similar to a SolarDifferential Image Motion Monitor +(S-DIMM+) on a 1.0 m solar telescope. In the original design of MASP, only two guide stars are used to retrieve the turbulence profile. In this paper, we studied the usage of multiple guide stars in MASP using numerical simulation, and found that there are three main advantages.Firstly, the precision of the turbulence profile can be increased, especially at a height of about 15 km, which is important for characterizing turbulence at the tropopause. Secondly, the equivalent diameter of MASP can be increased up to 30%, which will reduce the cost and weight of the instruments. Thirdly, the vertical resolution of the turbulence profile near the ground increases with the help of multiple guide stars. 相似文献
8.
F. Larsen H. Olofsson K. Eriksson B. Gustafsson 《Astrophysics and Space Science》1997,251(1-2):235-238
We have searched for line emission from various transitions of a number of molecules, including CO, HCN, CN, CS and SiO, as
well as some of their isotopic variants, towards a sample of 68 bright carbon stars. Part of the data has already been published
in Olofsson et al. (1993a,b). The aim of the project is to obtain a better understanding of the carbon star phenomenon and
the processes involved. In particular, we would like to obtain reliable mass loss rates and molecular abundances for these
objects. This requires careful and detailed modelling, which is currently underway. Our hope is that the study of these bright
carbon stars will serve as a guide to the study of higher mass loss rate objects. Some preliminary observational results are
presented here.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
9.
New challenges for adaptive optics: extremely large telescopes 总被引:1,自引:0,他引:1
M. Le Louarn N. Hubin M. Sarazin A. Tokovinin 《Monthly notices of the Royal Astronomical Society》2000,317(3):535-544
The performance of an adaptive optics (AO) system on a 100-m diameter ground-based telescope working in the visible range of the spectrum is computed using an analytical approach. The target Strehl ratio of 60 per cent is achieved at 0.5 μm with a limiting magnitude of the AO guide source near R magnitude~10, at the cost of an extremely low sky coverage. To alleviate this problem, the concept of tomographic wavefront sensing in a wider field of view using either natural guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use three or four reference sources and up to three deformable mirrors, which increase up to 8-fold the corrected field size (up to 60 arcsec at 0.5 μm). Operation with multiple NGS is limited to the infrared (in the J band this approach yields a sky coverage of 50 per cent with a Strehl ratio of 0.2). The option of open-loop wavefront correction in the visible using several bright NGS is discussed. The LGS approach involves the use of a faint ( R ~22) NGS for low-order correction, which results in a sky coverage of 40 per cent at the Galactic poles in the visible. 相似文献
10.
D. H. Morgan R. D. Cannon D. Hatzidimitriou B. F. W. Croke 《Monthly notices of the Royal Astronomical Society》2003,341(2):534-550
A sample of 1497 carbon stars in the Large Magellanic Cloud (LMC) has been observed in the red part of the spectrum with the 2dF facility on the Anglo-Australian Telescope. Of these, 156 have been identified as J-type (i.e. 13 C-rich) carbon stars using a technique which provides a clear distinction between J stars and the normal N-type carbon stars that comprise the bulk of the sample, and yields few borderline cases. A simple two-dimensional classification of the spectra, based on their spectral slopes in different wavelength regions, has been constructed and found to be related to the more conventional c and j indices, modified to suit the spectral regions observed. Most of the J stars form a photometric sequence in the K − ( J − K ) colour–magnitude diagram, parallel to and 0.6 mag fainter than the N-star sequence. A subset of the J stars (about 13 per cent) are brighter than this J-star sequence; most of these are spectroscopically different from the other J stars. The bright J stars have stronger CN bands than the other J stars and are found strongly concentrated in the central regions of the LMC. Most of the rather few stars in common with Hartwick and Cowley's sample of suspected CH stars are J stars. Overall, the proportion of carbon stars identified as J stars is somewhat lower than has been found in the Galaxy. The Na D lines are weaker in the LMC J stars than in either the Galactic J stars or the LMC N stars, and do not seem to depend on temperature. 相似文献
11.
Brian L. Rachford † 《Monthly notices of the Royal Astronomical Society》2000,315(1):24-30
We report observations of the He i λ 5876 (D3) line in the late A- and early F-type stars in the Pleiades and Alpha Persei star clusters used to determine chromospheric activity levels. This represents the first sample of young stars in this temperature range with chromospheric activity measurements. We find the same average activity level in the young early F stars as in Hyades-age stars and field stars. In addition, the young star sample shows the same large star-to-star variation in activity as seen in the older stars. Thus, as a whole, chromospheric activity in this photospheric temperature range remains the same over nearly a factor of 100 in stellar age (50 Myr to 3 Gyr), in striking contrast to the behaviour of later-type stars. In the five late A stars we find three certain detections of D3 and one likely detection. This includes the bluest star yet observed with a chromospheric D3 line, Pleiades star HII 1362 at ( B − V )0 =0.22, making it one of the earliest stars with an observed chromosphere. The late A stars have D3 equivalent widths comparable to the weakest early F stars. However, when comparing D3 measurements in the young late A stars with older late A stars, we find evidence for a slight decrease in activity with age based on the large number of non-detections in the older stars. We find an apparently linear relationship between the activity upper limit and B − V over our entire range of B − V . Extrapolated blueward, this relationship predicts that the chromospheric D3 line would disappear for all stars at B − V ≈0.13. 相似文献
12.
W.-R. Hamann 《Astrophysics and Space Science》2010,329(1-2):151-158
Stellar winds appear as a persistent feature of hot stars, irrespective of their wide range of different luminosities, masses, and chemical composition. Among the massive stars, the Wolf–Rayet types show considerably stronger mass loss than the O stars. Among hot low-mass stars, stellar winds are seen at central stars of planetary nebulae, where again the hydrogen-deficient stars show much stronger winds than those central stars with “normal” composition. We also studied mass-loss from a few extreme helium stars and sdOs. Their mass-loss rate roughly follows the same proportionality with luminosity to the power 1.5 as the massive O stars. This relation roughly marks a lower limit for the mass loss from hot stars of all kinds, and provides evidence that radiation pressure on spectral lines is the basic mechanism at work. For certain classes of stars the mass-loss rates lie significantly above this relation, for reasons that are not yet fully understood. Mass loss from low-mass stars may affect their evolution, by reducing the envelope mass, and can easily prevent diffusion from establishing atmospheric abundance patterns. In close binary systems, their winds can feed the accretion onto a companion. 相似文献
13.
《New Astronomy》2016
The evolution of massive stars is only partly understood. Observational constraints can be obtained from the study of massive stars located in young massive clusters. The ESO Public Survey “VISTA Variables in the Vía Lácteá (VVV)” discovered several new clusters hosting massive stars. We present an analysis of massive stars in four of these new clusters. Our aim is to provide constraints on stellar evolution and to better understand the relation between different types of massive stars. We use the radiative transfer code CMFGEN to analyse K-band spectra of twelve stars with spectral types ranging from O and B to WN and WC. We derive the stellar parameters of all targets as well as surface abundances for a subset of them. In the Hertzsprung–Russell diagram, the Wolf–Rayet stars are more luminous or hotter than the O stars. From the log(C/N)–log(C/He) diagram, we show quantitatively that WN stars are more chemically evolved than O stars, WC stars being more evolved than WN stars. Mass loss rates among Wolf–Rayet stars are a factor of 10 larger than for O stars, in agreement with previous findings. 相似文献
14.
This study is part of an investigation of the possibility of using chemically peculiar (CP) stars to map local galactic structure.
Correct luminosities of these stars are therefore crucial. CP stars are generally regarded as main-sequence or near-main-sequence
objects. However, some CP stars have been classified as giants.
A selection of stars, classified in literature as CP giants, are compared to normal stars in the same effective temperature
interval and to ordinary ‘non giant’ CP stars. We find no clear confirmation of a higher luminosity for ‘CP giants’, than
for CP stars in general. In addition, CP characteristics seem to be individual properties not repeated in a component star
or other cluster members. 相似文献
15.
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17.
本文给出十颗Me星的低色散光谱资料,其中有六颗星以前没有人给出发射线资料,四颗前人没有给出光谱资料;还有一颗是新发现的Me星。这里我们均给出发射线资料及光谱型。另外还给出了每颗星的光谱描迹图及其中三颗星的证认图。 相似文献
18.
Blesson Mathew Annapurni Subramaniam Bhuwan Chandra Bhatt 《Monthly notices of the Royal Astronomical Society》2008,388(4):1879-1888
Emission-line stars in young open clusters are identified to study their properties, as a function of age, spectral type and evolutionary state. 207 open star clusters were observed using the slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission-line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20 per cent clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0–10 and 20–30 Myr age bin (∼40 per cent each). Most of the emission stars in our survey belong to Classical Be class (∼92 per cent) while a few are Herbig Be stars (∼6 per cent) and Herbig Ae stars (∼2 per cent). The youngest clusters to have Classical Be stars are IC 1590, NGC 637 and 1624 (all 4 Myr old) while NGC 6756 (125–150 Myr) is the oldest cluster to have Classical Be stars. The Classical Be stars are located all along the main sequence (MS) in the optical colour–magnitude diagrams (CMDs) of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn-off. The distribution of Classical Be stars as a function of spectral type shows peaks at B1–B2 and B6–B7 spectral types. The Be star fraction [N(Be)/N(B+Be)] is found to be less than 10 per cent for most of the clusters and NGC 2345 is found to have the largest fraction (∼26 per cent). Our results indicate there could be two mechanisms responsible for the Classical Be phenomenon. Some are born Classical Be stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to Classical Be stars, within the MS lifetime, as indicated by the enhancement in the fraction of clusters with Classical Be stars in the 20–30 Myr age bin. 相似文献
19.
Takashi Tsuji 《Journal of Astrophysics and Astronomy》1981,2(3):253-276
On the basis of the effective temperature scale proposed previously for cool carbon stars (Paper I), other intrinsic properties
of them are examined in detail. It is shown that the major spectroscopic properties of cool carbon stars, including those
of molecular bands due to polyatomic species (SiC2, HCN, C2H2
etc.), can most consistently be understood on the basis of our new effective temperature scale and the theoretical prediction of
chemical equilibrium. Various photometric indices of cool carbon stars also appear to be well correlated with the new effective
temperatures. Furthermore, as effective temperatures of some 30 carbon stars are now obtained, the calibration of any photometric
index is straightforward, and some examples of such a calibration are given. In general, colour index-effective temperature
calibrations for carbon stars are quite different from those for K-M giant stars. It is found that the intrinsic (R —I)0 colour is nearly the same for N-irregular variables in spite of a considerable spread in effective temperatures, and this
fact is used to estimate the interstellar reddening of carbon stars.
An observational HR diagram of red giant stars, including carbon stars as well as K-M giant stars, is obtained on the basis
of our colour index-effective temperature calibrations and the best estimations of luminosities. It is shown that carbon stars
and M giant stars are sharply divided in the HR diagram by a nearly vertical line at aboutT
eff = 3200 K (logT
eff = 3.50) and the carbon stars occupy the upper right region of M giant stars (except for some high luminosity, high temperature
J-type stars in the Magellanic Clouds; also Mira variables are not considered). Such an observational HR diagram of red giant
stars shows rather a poor agreement with the current stellar evolution models. Especially, a more efficient mixing process
in red giant stars, as compared with those ever proposed, is required to explain the formation of carbon stars. 相似文献
20.
The energy distribution curves of eleven Ap stars, three Am stars, four normal A stars and one F0 V magnetic star have been obtained between 478 nm and 680 nm. For four of the Ap stars, two Am stars and all the four normal A stars, the effective temperatures are believed to have been estimated for the first time. For the rest, these estimates are expected to be an improvement over previously available values.It is concluded that the Ap and Am stars are not much different from the normal A stars in so far as their temperatures are concerned. 相似文献