TESS空间卫星单扇区变星的周期性研究

TESS (Transiting Exoplanet Survey Satellite)空间卫星提供的短曝光、高精度光度测量为寻找并区分变星与搜寻行星提供了良好的数据.利用变星源的光变曲线,使用周期频谱分析与光变折叠等一系列方法分析了TESS空间卫星21扇区19995颗拥有高质量光变数据的目标源,并对这些源进行了分类,共获得4624颗变星,其中食双星322颗、脉动变星470颗、行星凌星37颗.所得变星结果与VSX (The International Variable Star Index)变星表进行了交叉比较,共交叉匹配了625颗变星源,这些交叉源中共有131颗为食双星系统、31颗为脉动变星,并通过周期频谱分析获取了双星绕转以及脉动周期.另外在59颗变星中发现明显耀发现象,交叉源中有8颗变星为行星凌星并同样通过周期频谱分析获取了行星绕转周期,从而验证了TESS空间卫星数据对变星分析的可行性.通过利用TESS空间卫星21扇区获得的变星周期结果与VSX变星表中提供的变星周期对比,发现与VSX变星表中绝大部分变星的周期一致,有一部分结果与VSX变星表中的结果差别较大,对这些变星周期结果做了进一步修正,并给出了变星表未列出的变星周期结果.     

The use of frequency-separation ratios for asteroseismology

The systematic patterns of separations between frequencies of modes of different degree and order are a characteristic of p-mode oscillations of stars. The frequency separations depend on the internal structure of the star and so measuring them in the observed oscillation spectra of variable stars gives valuable diagnostics of the interior of a star. Roxburgh & Vorontsov proposed using the ratio of the so-called small frequency separation to the large frequency separation as a diagnostic of the stellar interior, and demonstrated that this ratio was less sensitive than the individual frequency separations themselves to uncertain details of the near-surface structure. Here we derive kernels relating the frequency separation ratio to structure, and show why the ratio is relatively insensitive to the near-surface structure in terms of the very small amplitude of the kernels in the near-surface layers. We also investigate the behaviour of the separation ratio for stars of different masses and ages, and demonstrate the usefulness of the ratio in the so-called asteroseismic Hertzsprung–Russell diagram.     

The eclipsing intermediate polar V597 Pup (Nova Puppis 2007)

JL 82 is a known binary, consisting of an sdB star and a companion which is not directly observable in the optical. Photometric measurements reported in this paper show it to variable with both the binary period (∼0.75 d), as well as on much shorter time-scales. The shorter periods are ascribed to pulsation of the sdB star, making it a member of the PG 1716 class of pulsating stars.     

A comprehensive variability study of the enigmatic WN8 stars: final results

As a conclusion of our all-sky variability survey of the 'enigmatic' variable WN8 stars, we have carried out coordinated multisite photometric and spectroscopic observations of WN8 stars in 1989 and 1994–1995. We confirm the leading role of the stellar core in restructuring the whole wind. This emerges as a statistical trend: the higher the level of the ∼continuum (i.e. ∼core) light variations, the higher the variability of the P Cygni edges of the optical emission lines. However, the form of the correlation between the light and profile variations is generally different for each individual star. The high level of activity of WN8 stars may be supported/induced by pulsational instability.     

Pre-main-sequence variability across the radiative–convective gap

We use I -band imaging to perform a variability survey of the 13-Myr-old cluster h Per. We find a significant fraction of the cluster members to be variable. Most importantly, we find that variable members lie almost entirely on the convective side of the gap in the cluster sequence between fully convective stars and those which have a radiative core. This result is consistent with a scenario in which the magnetic field changes topology when the star changes from being fully convective to one containing a radiative core. When the star is convective, the magnetic field appears dominated by large-scale structures, resulting in global-size spots that drive the observed variability. For those stars with radiative cores, we observe a marked absence of variability due to spots, which suggests a switch to a magnetic field dominated by smaller-scale structures, resulting in many smaller spots and thus less apparent variability. This implies that wide field variability surveys may only be sensitive to fully convective stars. On the one hand, this reduces the chances of picking out young groups (since the convective stars are the lower mass and therefore fainter objects), but conversely the absolute magnitude of the head of the convective sequence provides a straightforward measure of age for those groups which are discovered.     

The light curve of the semiregular variable L2 Puppis – I. A recent dimming event from dust

The nearby Mira-like variable L₂ Pup is shown to be undergoing an unprecedented dimming episode. The stability of the period rules out intrinsic changes to the star, leaving dust formation along the line of sight as the most likely explanation. Episodic dust obscuration events are fairly common in carbon stars but have not been seen in oxygen-rich stars. We also present a 10-μm spectrum, taken with the Japanese Infrared Telescope in Space satellite, showing strong silicate emission that can be fitted with a detached, thin dust shell, containing silicates and corundum.     

Multiperiodic semiregular variable stars in the ASAS data base: A pilot study

Based on a search for multi‐periodic variability among the semi‐regular red variable stars in the database of the All Sky Automated Survey (ASAS), a sample of 72 typical examples is presented. Their period analysis was performed using the Discrete Fourier Transform. In 41 stars we identified two significant periods each, simultaneously present, while the remaining 31 cases revealed even three such periods per star. They occur in a range roughly between 50 and 3000 days. Inter‐relationships between these periods were analyzed using the “double period diagram” which compares adjacent periods, and the so‐called “Petersen diagram”, the period ratio vs. the shorter period. In both diagrams we could identify six sequences of accumulation of the period values. For five of these sequences (containing 97 % of all data points) we found an almost perfect coincidence with those of previous studies which were based on very different samples of semiregular red variables. Therefore, existence and locations of these sequences in the diagrams seem to be universal features, which appear in any data set of semi‐regularly variable red giants of the AGB; we conclude that they are caused by different pulsation modes as the typical and consistent properties of similar stellar AGB configurations. Stellar pulsations can be considered as the principal cause of the observed periodic variability of these stars, and not binary, rotation of a spotted surface or other possible reasons suggested in the literature. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The exceptional Herbig Ae star HD 101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star

In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow‐up studies of magnetism among young pre‐main‐sequence stars. We obtained high‐resolution, high signal‐to‐noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb‐Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period P_rot = 13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non‐statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current‐driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate‐mass stars could be an alternative to a frozen‐in fossil field (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Pulsation–decline relationships in R Coronae Borealis stars

Decline onset times were measured in long-term visual light curves for five R Coronae Borealis (RCB) variable stars. These included RY Sgr and V854 Cen, the two RCB stars previously reported to have a relationship between dust formation events and pulsational variations. Analysis of the decline epochs showed that all decline onsets for a given star obey a linear ephemeris tied to the object's dominant radial pulsation period. Thus, in addition to confirming the pulsation–decline correlation for RY Sgr and V854 Cen, this same behaviour was demonstrated in UW Cen, R CrB and S Aps for the first time. This general result firmly establishes the connection between radial pulsation and mass loss in the RCB stars. The dominant pulsation period of ≈40 d for all of these objects therefore represents the characteristic time-scale on which these stars produce dust.     

贫金属富碳天琴RR变星的形成

贫金属富碳恒星(Carbon-Enhanced Metal-Poor, CEMP)是研究宇宙早期恒星性质和化学演化的极佳样本,通常认为来自双星.目前发现的贫金属富碳星中有9颗天琴RR变星(RR Lyrae star, RRL),其中至少7颗未表现出任何双星特征.传统双星物质转移模型不足以充分解释贫金属富碳天琴RR变星(CEMP-RR Lyrae)单星的形成.之前研究表明氦白矮星和赫氏空隙星(HG)的并合模型可以解释部分富碳红巨星单星的碳增丰现象,因此贫金属富碳星单星也可能来自氦白矮星和赫氏空隙星的并合模型渠道.通过详细计算的氦白矮星和赫氏空隙星并合模型来检验这一演化渠道,结果表明:该并合模型在后续的演化过程中,其重力加速度、温度、表面碳丰度均能与观测符合较好.由此,氦白矮星和赫氏空隙星并合模型极有可能是贫金属富碳天琴RR变星的形成渠道之一.     

Photometric variability of WC9 stars

Do some Wolf–Rayet stars owe their strong winds to something else besides radiation pressure? The answer to this question is still not entirely obvious, especially in certain Wolf–Rayet subclasses, mainly WN8 and WC9. Both of these types of Wolf–Rayet stars are thought to be highly variable, as suggested by observations, possibly due to pulsations. However, only the WN8 stars have so far been vigorously and systematically investigated for variability. We present here the results of a systematic survey during three consecutive weeks of 19 Galactic WC9 stars and one WC8 star for photometric variability in two optical bands, V and I . Of particular interest are the correlated variations in brightness and colour index in the context of carbon dust formation, which occurs frequently in WC9 and some WC8 stars. In the most variable case, WR76, we used this information to derive a typical dust grain size of  ∼ 0.1 μm  . However, most photometric variations occur at surprisingly low levels, and in fact almost half of our sample shows no significant variability at all above the instrumental level (  σ∼ 0.005– 0.01  mag).     

Mapping the β Cephei instability strip: photometric variability of stars in the central part of the Cygnus OB2 association

The central part of the Cygnus OB2 association was searched for the presence of short-period hot pulsators. Out of 16 O-type stars in the field, five turned out to be variable. None of the O-type variables showed short-period variations characteristic for the β Cephei-type stars. However, two β Cephei stars were found among the early B-type members of the association. Their pulsation periods are too long to be in agreement with the average association age. We note that the discrepancy can be explained by non-coeval star formation, which was already suggested by other authors from the investigation of the colour–magnitude diagrams of the association.   We found 29 new variables in all fields searched by us. About half of these are Cygnus OB2 members. By means of Hα photometry, we confirmed the presence of Hα emission in two stars and found one new Be star. A weak Hα emission is probably present in all observed O-type supergiants and giants. We also give the new BVRI Hα photometry for almost 300 stars. This multicolour photometry is used to determine a dereddened colour–magnitude diagram for the association and make a reddening map of the observed field. The extinction in the field, expressed as the total absorption in the Johnson V filter, A _v , ranges from 4.1 to 7.0 mag; the average A _V amounts to 5.8 mag. One O-type star is probably a background object.     

Automated classification of variable stars for All-Sky Automated Survey 1–2 data

With the advent of surveys generating multi-epoch photometry and the discovery of large numbers of variable stars, the classification of these stars has to be automatic. We have developed such a classification procedure for about 1700 stars from the variable star catalogue of the All-Sky Automated Survey 1–2 (ASAS 1–2) by selecting the periodic stars and by applying an unsupervised Bayesian classifier using parameters obtained through a Fourier decomposition of the light curve. For irregular light curves we used the period and moments of the magnitude distribution for the classification. In the case of ASAS 1–2, 83 per cent of variable objects are red giants. A general relation between the period and amplitude is found for a large fraction of those stars. The selection led to 302 periodic and 1429 semiperiodic stars, which are classified in six major groups: eclipsing binaries, 'sinusoidal curves', Cepheids, small amplitude red variables, SR and Mira stars. The type classification error level is estimated to be about 7 per cent.     

A catalogue of RR Lyrae stars from the Northern Sky Variability Survey

A search for RR Lyrae stars has been conducted in the publicly available data of the Northern Sky Variability Survey. Candidates have been selected by the statistical properties of their variation; the standard deviation, skewness and kurtosis with appropriate limits determined from a sample 314 known RRab and RRc stars listed in the General Catalogue of Variable Stars. From the period analysis and light-curve shape of over 3000 candidates 785 RR Lyrae have been identified of which 188 are previously unknown. The light curves were examined for the Blazhko effect and several new stars showing this were found. Six double-mode RR Lyrae stars were also found of which two are new discoveries. Some previously known variables have been reclassified as RR Lyrae stars and similarly some RR Lyrae stars have been found to be other types of variable, or not variable at all.     

The period–luminosity relation for type II Cepheids in globular clusters

We report the result of our near-infrared observations ( JHK _s) for type II Cepheids (including possible RV Tau stars) in galactic globular clusters. We detected variations of 46 variables in 26 clusters (10 new discoveries in seven clusters) and present their light curves. Their periods range from 1.2 d to over 80 d. They show a well-defined period–luminosity relation at each wavelength. Two type II Cepheids in NGC 6441 also obey the relation if we assume the horizontal branch stars in NGC 6441 are as bright as those in metal-poor globular clusters in spite of the high metallicity of the cluster. This result supports the high luminosity which has been suggested for the RR Lyr variables in this cluster. The period–luminosity relation can be reproduced using the pulsation equation     assuming that all the stars have the same mass. Cluster RR Lyr variables were found to lie on an extrapolation of the period–luminosity relation. These results provide important constraints on the parameters of the variable stars.
Using Two Micron All-Sky Survey (2MASS) data, we show that the type II Cepheids in the Large Magellanic Cloud (LMC) fit our period–luminosity relation within the expected scatter at the shorter periods. However, at long periods (   P > 40  d, i.e. in the RV Tau star range) the LMC field variables are brighter by about one magnitude than those of similar periods in galactic globular clusters. The long-period cluster stars also differ from both these LMC stars and galactic field RV Tau stars in a colour–colour diagram. The reasons for these differences are discussed.     

Rapidly oscillating M giant stars?

The Hipparcos mission discovered a few dozen M giant stars with periods P shorter than 10 d. Similar stars may be found in other large data bases of new variables (e.g., OGLE). The three possible sources of the magnitude variations – pulsation, starspots and ellipsoidal deformation – are discussed in general terms. The parallaxes and V − I colour indices are used to calculate radii and temperatures for all M giant variables with P <100 d. Masses are estimated from the positions of the stars in a Hertzsprung–Russell (HR) diagram, using evolutionary tracks. Using these data, it is shown that starspots can be ruled out as a variability mechanism in almost all cases, and ellipsoidal variations in about half of the stars. Pulsation in very high-overtone modes appears to be the only viable explanation for the stars with P <10 d. Many of the stars may be multiperiodic. IRAS data are used to deduce information about reddening and circumstellar dust. The apparently low level of mass-loss, as well as the kinematics and the spatial distribution of the stars, indicates that they are from a relatively young (i.e., thin disc) giant star population.     

A search for variable stars in the globular cluster M3

We describe here the results of a photometric time-sequence survey of the globular cluster M3 (NGC 5272), in a search for contact and detached eclipsing binary stars. We have discovered only one likely eclipsing binary and one SX Phe-type star in spite of monitoring 4077 stars with V  < 20.0 and observing 25 blue straggler stars (BSS). The newly identified SX Phe star, V237, shows a light curve with a variable amplitude. Variable V238 shows variability either with a period of 0.49 d or with a period of 0.25 d. On the cluster colour–magnitude diagram, the variable occupies a position a few hundredths of a magnitude to the blue of the base of the red giant branch. V238 is a likely descendant of a binary blue straggler.   As a side result we obtained high-quality data for 42 of the previously known RR Lyrae variables, including 33 of Bailey type ab, seven type c and two double-mode pulsators. We used equations that relate the physical properties of RRc stars to their pulsation periods and Fourier parameters in order to derive masses, luminosities, temperatures and helium parameters for five of the RRc stars. We also tested equations that relate the [Fe/H], M _V and temperature of RRab stars to pulsation period and Fourier parameters. We derived [Fe/H]= −1.42 in good agreement with spectroscopic determinations.     

Variable stars in the field of open cluster NGC 6819

We report on the discovery of 25 variable stars plus 13 suspected variables found in the field of the open cluster NGC 6819. The stars were identified from time-series photometric data obtained on the Isaac Newton Telescope, La Palma, during two observing runs covering the 19 nights between 1999 June  22–30  and 1999 July  22–31  . The variables found include 12 eclipsing binaries with an additional three suspected, nine BY Draconis systems, plus four variables of other types, including one star believed to be a Cepheid. Three of the 15 eclipsing binaries are believed to be cluster members. Details of a further 10 suspected variable stars are also included.     

Do red giant stars pulsate in high overtones?

About 30 photometrically variable red giant stars have periods less than 10 d, as determined by the compilers of the Hipparcos Catalogue from Hipparcos photometric measurements. These periods, when combined with estimates of the radii and masses of these stars, and with pulsation theory, imply that these stars are pulsating in very high overtones. We present several pieces of evidence which suggest that the periods may be spurious, as a result of the particular aliasing properties of the Hipparcos photometry. We conclude that the evidence for high-overtone pulsation in red giant stars is equivocal.     

Three new pulsating sdB stars from the Edinburgh–Cape survey

We report the discovery of very rapid pulsations in three sdB stars from the Edinburgh–Cape blue object survey. The short periods, small amplitudes and multi-periodicity clearly establish these stars as members of the EC 14026 class. EC 11583−2708 has pulsation periods near 149, 144 and 114 s, though evidence is presented that the 149-s period is resolved into two periods at 148.87 and 148.55 s by the full photoelectric data set. The amplitudes of the detected variations are in the range 0.002–0.006 mag. The light variation of EC 20338−1925 is dominated by a period near 147 s with a very large amplitude for a variable sdB star (0.025 mag), though four other frequencies are detected with periods near 168, 151, 141 and 135 s and amplitudes in the range 0.002–0.005 mag. The third star, EC 09582−1137, displays a light curve which is virtually a textbook example of frequency beating, being produced by two pulsations of almost equal amplitude (∼0.008 mag) and periods near 136.0 and 151.2 s.