Large-amplitude variables near the Galactic Centre

We report here the results of a 4-yr K -band (2.2 μm) survey for large-amplitude variable stars in a     area centred on the Galactic Centre. A total of 409 likely long-period variables (LPVs) were detected, for which positions, amplitudes, average magnitudes and periods were obtained whenever possible. The surface density of LPVs is more than ten times greater than in the Sgr I Baade window at        
The limits of completeness arising from interstellar and circumstellar absorption are discussed. Most of the area suffers interstellar extinction of     The shorter-period LPVs are less luminous than the longer-period ones and may be slightly under-represented in the data. Extremely heavy extinction     which affects the probability of detecting variables, occurs in less than 25 per cent of the area.
Almost all of the LPVs are Miras or OH/IR stars, with periods ranging from 150 d to about 800 d. K -band counterparts have been found for 59 per cent of the 109 known OH sources in the field. The average period of the variables found is 427 d, while that of the OH/IR stars is 524 d. For comparison, the average period in the Sgr I window, which contains no known OH/IR stars, is 333 d and only two stars are detected with     The survey field also contains a number of long-period, large-amplitude variables that are not OH emitters.     

Period–luminosity relations for red supergiant variables – I.The calibration

We present CCD photometry of red supergiant long-period variables (LPVs) in the Per OB1 association, the Large Magellanic Cloud (LMC) and M33. The photometry was obtained in the Kron–Cousins R and I bandpasses and in a narrow bandpass ( λ ₀=8250 Å, FWHM=300 Å) chosen to avoid TiO bands in the spectral energy distribution of the LPVs. Because the strength of the TiO bands varies greatly with temperature, which varies with the phase of an LPV, avoiding TiO reduces the amplitude of the photometric variations seen in LPVs. The result is a lower dispersion and a well defined period–luminosity (PL) relation.
For the LMC sample we find an rms dispersion of 0.27 mag in the narrow-band PL relation and slightly larger dispersions for the LPVs in Per OB1 and M33. This dispersion is comparable to that of the Cepheid PL relation at similar wavelengths. Adopting a distance modulus of 18.5±0.1 mag for the LMC, we obtain distance moduli of 11.68±0.15 mag for Per OB1 and 24.85±0.13 mag for M33. These distances agree well with those based on main sequence fitting for Per OB1 and the Cepheid distance for M33. Since LPVs are ∼ 5 times more common than Cepheids and have a well defined PL relation, LPVs provide a promising method for estimating Galactic and extra galactic distances.     

中等质量富金属AGB长周期变星的振动研究

本文采用恒星演化计算与恒星振动计算相结合的方法，对中等质量富金属恒星演化到渐近巨星分支时的振动性质进行了分析研究，从理论上得出这类恒星的振动方式是处于一阶谐频振动，而振动的激发则是在氢电离区和氦的二次电离区由多种机制共同作用造成的，同时提出很长周期的ＡＧＢ长周期变星只能是由中等质量恒星演化到ＡＧＢ阶段形成的。我们的理论计算结果还比较支持在ＡＧＢ顶端存在巨大星风物质损失的观点，且这种星风物质损失很可能与恒星振动有关。     

Search for variables in six Galactic open clusters

Variables in open cluster (known distance, age, and metallicity) fields play an important role in stellar astrophysics because they allow to investigate the interior of stars. Therefore, six Galactic open clusters were selected to search for new variables and to complement data for already known variables. As five of these clusters are younger than 40 Myr, we aim at finding variable high-mass stars such as β Cephei and Slowly Pulsating B-type stars as well as classical pulsating stars within the instability strip. About 26 000 images (312 h) photometric images were taken at the 0.8 m (Vienna, Austria) and 1.0 m (Hvar, Croatia) telescope using V and I filters. The differential light curves were analyzed with standard time series analysis methods. In total, 11 variables were found in all investigated clusters. For nine of them, we were able to determine their nature and period. In addition, the membership probabilities from the literature were analyzed.     

Observations of Stellar Wind Instabilities and Variability on Small and Large Scales

I review various observations which suggest that the winds of hot stars are inhomogeneous because of instabilities in the wind flow. On large scales, local wind overdensities are indirectly detected in the form of excess in the infra-red (IR) and radio free-free continuum. The X-ray detection of a hot (T ∼ 10⁶) wind component suggests that the wind is pervaded with strong shocks. The small-scale density structure of the wind can be studied from observations of Line-Profile Variations (LPVs) in optical and UV spectral lines, which are formed close to the stellar surface. LPVs in lines of the P Cygni type consist of blue-edge variations in saturated profiles, and Discrete Absorption Components (DACs) and Periodic Absorption Modulations (PAMs) in unsaturated profiles. These LPVs are shown to be recurrent, and thought to result from instabilities propagating through the wind and generated at the stellar surface. LPVs in recombination lines appear as stochastic subpeaks, which suggest that wind instabilities have a clump-like, rather than shell-like, structure. The kinematics of LPVs in both line types is consistent with wind propagating shocks generated from radiative instabilities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Database of molecular masers and variable stars

We present the database of maser sources in H2 O, OH and Si O lines that can be used to identify and study variable stars at evolved stages. Detecting the maser emission in H2 O, OH and Si O molecules toward infrared-excess objects is one of the methods for identifing long-period variables(LPVs, including miras and semiregulars), because these stars exhibit maser activity in their circumstellar shells. Our sample contains 1803 known LPV objects. Forty-six percent of these stars(832 objects) manifest maser emission in the line of at least one molecule: H2 O, OH or Si O. We use the database of circumstellar masers in order to search for LPVs which are not included in the General Catalogue of Variable Stars(GCVS). Our database contains 4806 objects(3866 objects without associations in GCVS) with maser detection in at least one molecule. Therefore it is possible to use the database in order to locate and study the large sample of LPV stars. The database can be accessed at http://maserdb.net.     

Searching for Variable Stars in the Field of NGC 7789

We present the results of a time-series CCD photometric survey of variable stars in the field of the open cluster NGC 7789. In a field of about one degree centering on the cluster, a total of 28 new variable stars are discovered (14 W UMa systems, nine EA-type eclipsing binaries, one RR Lyr star, and four unclassified). In addition, we recovered 11 old variables previously discovered by other authors. Preliminary parameters are given for some of these variables.     

Velocity Space of Galactic O-B Stars

Based on the Hipparcos proper motions and available radial velocity data of O-B stars, we have re-examined the local kinematical structure of the young disk population of-1500 O-B stars not including the Gould-belt stars. A systematic warping motion of the stars about the direction to the Galactic center has been reconfirmed. A negative K-term implying a systematic contraction of stars in the solar vicinity has been detected. Two different distance scales are used in order to find out their impact on the kinematical parameters, and we conclude that the adopted distance scale plays an important role in characterizing the kinematical parameters at the present level of the measurement uncertainty.     

Variable stars in the globular cluster NGC 4590 (M68)

We present results of time series photometry to search for variable stars in the field of metal-poor globular cluster NGC 4590 (M68). Periods have been revised for 40 known variables and no significant changes were found. A considerable change in Blazhko effect for V25 has been detected. Among nine newly discovered variable candidates, 5 stars are of RRc Bailey type variables while 4 stars are unclassified. The variable stars V10, V21, V50 and V51 are found to be cluster members based on the radial velocity data taken from literature.     

RAT J1953+1859: a dwarf nova discovered through high amplitude QPOs in quiescence

We report the discovery of an accreting binary, RAT J1953+1859, made during the RApid Temporal Survey (RATS) on the Isaac Newton Telescope. It showed high amplitude (0.3 mag) quasi-periodic oscillations on a time-scale of ∼20 min. Further observations made using the Nordic Optical Telescope showed it to be ∼4 mag brighter than in the discovery images. These photometric observations, together with radial velocity data taken using the William Herschel Telescope, point to an orbital period of ∼90 min. These data suggest that RAT J1953+1859 is a dwarf novae of the SU UMa type. What makes RAT J1953+1859 unusual is that it is the first such system to be discovered as a result of high amplitude QPOs during quiescence. This suggests that high-cadence wide-field surveys could be another means to discover cataclysmic variables as a result of their short period variability.     

Sungrazing Comets Discovered with the SOHO/LASCO Coronagraphs 1996-1998

The Kreutz sungrazing family of comets is unique because of its small perihelion distance and because of the large number of known members of this family. SOHO/LASCO coronagraph observations beginning in 1996 have revealed an unprecedented number of Kreutz comets. These new coronagraph observations improve upon earlier observations because of a larger field-of-view, increased image cadence, and better photometric measurements. This paper presents the lightcurves of the 141 Kreutz family comets observed from 1996 through 1998. Throughout this period, the number of family members discovered each year is shown to be constant. None of the comets were detected postperihelion. The lightcurves show distinctive characteristics which reveal much about the properties of the nuclei. It is shown that the individual fragments can be related to one of two “standard candles,” which we call Universal Curves. The comets all reach a peak brightness at one of two characteristic distances (both near 12 R_⊙) and that the comets fragment at another characteristic distance (about 7 R_⊙). Also, evidence is seen for line emission, which varies with heliocentric distance.     

Determination of the Proper Motions and Membership of the Globular Cluster M3 and of its Orbit in the Galaxy

We examine 14 plates of the globular cluster M3 (NGC 5272) taken with the 40 cm refractor at the Sheshan station of Shanghai Astronomical Observatory. The plates span over a period of about 77 years. The positions and absolute proper motions of eight stars in the Hipparcos Catalogue and of 49 stars in the Tycho-2 Catalogue are used as the reference frame. The astrometric reduction is made with the central overlapping principle. The absolute proper motions of 534 stars in a region of about 100' × 100' around the cluster are measured. With the new proper motion data the membership probabilities of the stars are determined. The average absolute proper motion obtained for the cluster is -0.06@0.30 mas yr-1 in R.A. and -2.6@0.30 mas yr-1 in Decl. By combining this result with the known distance and radial velocity of the cluster, we also obtained the Galactic orbit of M3 for a chosen three-component Galactic potential.     

小行星(469219) Kamo`oalewa轨道的确定与误差分析

作为一颗与地球共轨道的小行星,(469219)Kamo'oalewa是一个具有很高研究价值的近地小天体,也是中国首次小行星探测计划的目标天体之一.针对其轨道特性,建立了兼顾太阳、地球和月球非球形引力作用的小行星动力学模型.并在该模型的基础上,利用国际小行星中心(Minor Planet Center,MPC)提供的2004|2018年间的光学观测数据对该小行星的轨道进行确定.拟合后观测残差的均方根误差约为0:2″(与美国喷气推进实验室的Horizons在线历表系统相当),其中2004年期间数据的观测残差有所改进.最后,对小行星(469219)Kamo'oalewa的轨道误差进行了详细分析,并预报了2020-2025年期间该小行星的轨道误差.     

Analysis of the Difference Light Curve of the Gravitational Mirage QSO 0957+561

From optical data of Q0957+561 for 1995/1996 seasons (in the g band) and 1996/1997 seasons (in the R band) they are derived upper limits on the amplitude of the rapid microlensing fluctuations of 0.07 mag and 0.05 mag, respectively. The photometry in the R band (1996/1997 seasons) was taken at Teide Observatory (IAC-80 Telescope), Instituto de Astrofisica de Canarias, Spain. In this contribution, we also study the microlensing history (from 1981 to 1996) of the Twin QSO, which is a curious system with two optical images of similar brightness. This revised version was published online in July 2006 with corrections to the Cover Date.     

A study of active galactic nuclei in low surface brightness galaxies with Sloan Digital Sky Survey spectroscopy

Active galactic nuclei (AGNs) in low surface brightness galaxies (LSBGs) have received little attention in previous studies. We present a detailed spectral analysis of 194 LSBGs from the Impey et al. (1996) APM LSBG sample which has been observed spec-troscopically by the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). Our elaborate spectral analysis enables us to carry out, for the first time, reliable spectral classification of nuclear processes in LSBGs based on the standard emission line diagnostic diagrams in a rigorous way. Star-forming galaxies are common, as found in about 52% of LSBGs. We find that, contrary to some previous claims, the fraction of galaxies that contain AGNs is significantly lower than that found in nearby normal galaxies of high surface brightness. This is qualitatively in line with the finding of Impey et al. This result holds true even within each morphological type from Sa to Sc. LSBGs that have larger central stellar ve-locity dispersions or larger physical sizes tend to have a higher chance of harboring an AGN. For three AGNs with broad emission lines, the black hole masses estimated from the emission lines are broadly consistent with the well known M-σ* relation established for normal galaxies and AGNs.     

Automated detection of EUV Polar Coronal Holes during Solar Cycle 23

A new method for automated detection of polar coronal holes is presented. This method, called perimeter tracing, uses a series of 171, 195, and 304 Å full disk images from the Extreme ultraviolet Imaging Telescope (EIT) on SOHO over solar cycle 23 to measure the perimeter of polar coronal holes as they appear on the limbs. Perimeter tracing minimizes line-of-sight obscurations caused by the emitting plasma of the various wavelengths by taking measurements at the solar limb. Perimeter tracing also allows for the polar rotation period to emerge organically from the data as 33 days. We have called this the Harvey rotation rate and count Harvey rotations starting 4 January 1900. From the measured perimeter, we are then able to fit a curve to the data and derive an area within the line of best fit. We observe the area of the northern polar hole area in 1996, at the beginning of solar cycle 23, to be about 4.2% of the total solar surface area and about 3.6% in 2007. The area of the southern polar hole is observed to be about 4.0% in 1996 and about 3.4% in 2007. Thus, both the north and south polar hole areas are no more than 15% smaller now than they were at the beginning of cycle 23. This compares to the polar magnetic field measured to be about 40% less now than it was a cycle ago.     

Longitude-resolved imaging of Jupiter at λ=2 cm

We present a technique for creating a longitude-resolved image of Jupiter's thermal radio emission. The technique has been applied to VLA data taken on 25 January 1996 at a wavelength of 2 cm. A comparison with infrared data shows a good correlation between radio hot spots and the 5 μm hot spots seen on IRTF images. The brightest spot on the radio image is most likely the hot spot through which the Galileo probe entered Jupiter's atmosphere. We derived the ammonia abundance (= volume mixing ratio) in the hot spot, which is ∼3×10⁻⁵, about half that seen in longitude-averaged images of the NEB, or less than 1/3 of the longitude-averaged ammonia abundance in the EZ. This low ammonia abundance probably extends down to at least the 4 bar level.     

Photometric Calibration of the Lasco-C3 Coronagraph Using Stars

The LASCO-C3 coronagraph on SOHO, launched in December 1995, has been collecting images of the corona and background star fields in a regular manner since 1996. This instrument contains a number of broadband filters with various passbands in the range between 400 and 1100 nm. The filter used most often has been the Clear filter (400–900 nm) but there are four other filters with about 100 nm passbands that are also used periodically. Preliminary calibration of the C3 optical system was done before flight and a number of techniques that use star intensities or magnitudes and position have been applied during flight. In order to understand the long-term behavior of the C3 instrument, we have recently performed an analysis of LASCO data that examines the observed intensities of a set of moderately bright stars whose spectra is known from 13 color photometry. Using these star spectra and the observed count rates we have derived the photometric calibration factors of the C3 coronagraph for all five color filters with an absolute precision of about ± 7%. Observations with the Clear filter have been used to look for long-term trends in the instrument sensitivity. The observations indicate a very slight decrease in the instrument sensitivity of about 3.5% over the 8 years studied here.     

A 408-MHz variability study of two fields in the region 6h30m, +45°

Variability in 408-MHz flux density, over a 1-yr interval between 1995 and 1996, has been investigated using the DRAO Synthesis Telescope for a complete sample of 322 sources with S _408 MHz > 80 mJy at galactic latitude b  = 15°; four sources are found to show significant flux density variations. These results, taken with those from a similar study at b  = 58°, indicate that 408-MHz variability over a time-scale of 1 yr is more uncommon than has previously been thought; variations at a level >10 per cent could have been detected in a statistically complete sample of about 150 of these sources and none was found. Variability over a period of 18 yr has also been investigated by comparing the DRAO data with those from the B3 catalogue for 182 sources with S _408 MHz > 200 mJy; five variables are found. Large flux density variations over a period of 18 yr are much more common than over 1 yr, with roughly one source in 15 showing variations at the 15 per cent level or greater. The statistics are not good enough to show any conclusive evidence for a dependence of variability on galactic latitude. Roughly half of the variable sources have spectra that are steep at high frequencies and flatten at low frequencies; the other half have flat radio spectra over the entire frequency range. Simultaneous observations at 1420 MHz, also with the DRAO Synthesis Telescope, have been used to investigate variability at this frequency as well, and indicate that over a period of 1 yr roughly half of the flat-spectrum sources vary at the 5 per cent level or greater.     

Time-resolved optical observations of five cataclysmic variables detected by INTEGRAL

The European Space Agency γ-ray telescope, INTEGRAL , is detecting relatively more intrinsically rare cataclysmic variables (CVs) than were found by surveys at lower energies. Specifically, a large fraction of the CVs that are INTEGRAL sources consists of asynchronous polars and intermediate polars (IPs). IP classifications have been proposed for the majority of CVs discovered by INTEGRAL , but, in many cases, there is very little known about these systems. In order to address this, I present time-resolved optical data of five CVs discovered through INTEGRAL observations. The white dwarf spin modulation is detected in high-speed photometry of three of the new CVs (IGR J15094−6649, IGR J16500−3307 and IGR J17195−4100), but two others (XSS J12270−4859 and IGR J16167−4957) show no evidence of magnetism, and should be considered unclassified systems. Spectroscopic orbital period ( P _orb) measurements are also given for IGR J15094−6649, IGR J16167−4957, IGR J16500−3307 and IGR J17195−4100.