Amplitude Of Light Variation Research Articles

Stellar activity with LAMOST – I. Spot configuration in Pleiades

We use the spectra of Pleiades and field stars from LAMOST DR2 archive to study how spottedness and activity vary as a function of mass at young ages. We obtained standard TiO band strength by measuring TiO bands near 7050 $\AA$ from LAMOST spectra (R$\approx$1800) for large sample of field GKM dwarfs with solar metallicity. Analysis show that active dwarfs, including late G- and early K-type, have extra TiO absorption compare to inactive counterparts, indicating the presence of cool spots on their surface. Active late K- and M-dwarfs show deeper TiO2 and shallower TiO4 compare to inactive stars at a given TiO5, which could be partly explained through cool spots. We estimated cool spot fractional coverage for 304 Pleiades candidates by modelling their TiO2 (&TiO5) band strength with respect to standard value. Results show that surface of large fraction of K- and M-type members have very large spot coverage ($\sim$50%). We analysed a correlation between spot coverage, rotation and the amplitude of light variation, and found spot coverage on slow rotators ($R_{o}$ > 0.1) increases with decreasing Rossby Number $R_{o}$. Interestingly, we detected a saturation-like feature for spot coverage in fast rotators with a saturation level of 40%-50%. In addition, spot distribution in hotter fast rotators show more symmetrical compare to slow rotators. More interestingly, we detected large spot coverage in many M type members with no or little light variation. In bigger picture, these results provide important constraints for stellar dynamo on these cool active stars.

HD 240121 - An ACV variable showing anti-phase variations of the B and V light curves

The variability of HD 240121 = BD+59 2602 was first suspected by Särg and Wramdemark (1970) and later confirmed by Gröbel R. (1992a,b). Because of the observed anti-phase variations of the B and V light curves, the latter author tentatively suggested an ACV type. Apart from its inclusion in the catalog of New Suspected Variables (NSV 25977), no further investigations of the star have been published. HD 240121 was included into our target list of ACV candidates and investigated in order to determine the reason for the observed brightness variations. All available information on HD 240121 were collected via an exhaustive data mining procedure. Data from Gröbel (1992a,b) were re-analysed and photometric observations from the NSVS and Hipparcos archives were procured and investigated. Line-of-sight reddening and stellar parameters were calculated from archival photometric data. HD 240121 is a young, late B-type CP2 star of the silicon subgroup. The observed period, amplitude of light variations and variability pattern (anti-phase variations) are typical of ACV variables. The occurrence of anti-phase variations of the B and V light curves is rarely observed and points to the existence of a null wavelength in the visual spectrum. We therefore strongly encourage further multi-colour photometric observations of this star.

PHOTOMETRY OF VARIABLE STARS FROM THE THU-NAOC TRANSIENT SURVEY. I. THE FIRST TWO YEARS

In this paper, we report the detections of stellar variabilities from the first 2-year observations of sky area of about 1300 square degrees from the Tsinghua University-NAOC Transient Survey (TNTS). A total of 1237 variable stars (including 299 new ones) were detected with brightness < 18.0 mag and magnitude variation >= 0.1 mag on a timescale from a few hours to few hundred days. Among such detections, we tentatively identified 661 RR Lyrae stars, 431 binaries, 72 Semiregular pulsators, 29 Mira stars, 11 slow irregular variables, 11 RS Canum Venaticorum stars, 7 Gamma Doradus stars, 5 long period variables, 3 W Virginis stars, 3 Delta Scuti stars, 2 Anomalous Cepheids, 1 Cepheid, and 1 nove-like star based on their time-series variability index Js and their phased diagrams. Moreover, we found that 14 RR Lyrae stars show the Blazhko effect and 67 contact eclipsing binaries exhibit the O'Connell effect. Since the period and amplitude of light variations of RR Lyrae variables depend on their chemical compositions, their photometric observations can be used to investigate distribution of metallicity along the direction perpendicular to the Galactic disk. We find that the metallicity of RR Lyrae stars shows large scatter at regions closer to the Galactic plane (e.g., -3.0 < [Fe/H] < 0) but tends to converge at [Fe/H]~ -1.7 at larger Galactic latitudes. This variation may be related to that the RRAB Lyrae stars in the Galactic halo come from globular clusters with different metallicity and vertical distances, i.e. OoI and OoII populations, favoring for the dual-halo model.

An Analysis of the Radio Light Variations of Blazars With Low Peak Frequencies

The data of light variations of six blazars with low peak frequencies (i.e., BL Lacerta, 0235+164, OQ530, 0716+714, 3C 345, and 3C 273) at 4.8, 8, 14.5, 22 and 37GHz in the recent thirty years have been collected from immense amount of literature. By using the method of discrete correlation function the correlations and delays of the light curves of these six sources at the 5 wavebands are analyzed. With the method of structural function, the periods and timescales of the light variations of these six sources are analyzed, and their amplitudes of light variations are compared and analyzed. As revealed by the result of analysis, among the six blazars the amplitudes of light variations of 0716+714 and 0235+164 are relatively large, and those of 3C 345 and OQ 530 are less. The amplitudes of light variations of 3C 273 and BL Lacertae are relatively smaller. As shown by the multi-waveband analysis of delays between two neighboring wavebands, 0235+164 exhibits the tendency of variation that the high-frequency wavebands precede the low-frequency wavebands. On the whole, 3C 345 exhibits the tendency that the high-frequency wavebands lag be-hind the low-frequency wavebands. The analyses of the other blazars show that in a part of radio wavebands there appears the tendency of variation that the high-frequency waveband leads the low-frequency waveband. Yet in the other radio wavebands there appears the tendency that the high-frequency waveband lags behind the low-frequency waveband. As revealed by the results of analyses with the method of structural function, the timescale, fitted slope, and period of light variations of 3C 345 are larger than those of the other five blazars with low peak frequencies. This demonstrates that the activity of 3C 345 is comparatively weaker than the other five sources, and implies that in the interior of 3C 345 there may exist some physical processes which are different from those of the other five blazars with low peak frequencies.

High-dispersion Spectroscopic Observations of the T Tauri-type Stars with Weak Emission Lines

By using the 2.16 m telescope of Xinglong Observing Station of National Astronomical Observatories and its high-dispersion spectrograph, the high-dispersion spectroscopic observations of six T Tauri-type stars with weak emission lines (i.e., weak-line T Tauri-type stars, abbreviated as WTTS) were carried out. The lithium abundances of these weak-line T Tauri-type stars are calculated and the relationships of the lithium abundances with the rotation periods as well as the amplitudes of light variations of these stars are discussed. It is found by this study that the lithium abundance for the weak-line T Tauri-type stars with fast rotations tends to be less than that of those with slow rotations. However, for all these weak-line T Tauri-type stars, the lithium abundances have no conspicuous correlation with the amplitudes of light variations of these stars in the V waveband.

A multicolor photometric study of the neglected eclipsing binary FT Ursae Majoris

The multicolor photometric observations of the neglected eclipsing binary FT Ursae Majoris (FT UMa) were obtained in 2010. The 2003 version of the Wilson-Devinney code was used to analyze the light curves in the B, V and R bands simultaneously. Based on the spectroscopic mass ratio q = 0.984 published by Pribulla et al., it is found that FT UMa is an evolved contact binary with a contact degree of 15.3%. The low amplitude of light variations, ∼ 0.15 mag, arises mainly from a moderately low inclination angle of i = 62.80° and almost identical components in size rather than the light dilution of a third component, which contributes light of only ∼ 10%.

Sphericity Preference in Shapes of Sub-km-Sized Fast-Rotating Main-Belt Asteroids

Here, we report on results of statistical analyses on spin periods and shapes of main-belt asteroids (MBAs) smaller than $\ \sim\ $1 km in diameter, which were derived from lightcurve observations of a single night with the 8.2 m Subaru telescope. It is shown that, among our detected 68 MBAs giving reliable lightcurve periods, the fraction of fast-rotating asteroids (with spin periods of $\lt\ \sim\ $2.3 hr, candidates of monolithic bodies) is about 49%. In particular, statistical tests reveal with a high confidence level that, in terms of the shape (deduced from the light variation amplitude), the fast rotators belong to a population quite different from that for non fast-rotators; in other words, most of the fast-rotators show a strong trend that they are more spherical in shape than the non fast-rotator group. Consideration of several orbital and rotational evolution timescales for small asteroids in the main asteroid belt seems to indicate that the above-mentioned trend is not by coincidence, but primordial. Therefore, referring to the shape distributions of impact fragments produced in laboratory experiments, our discovered sphericity preference of small fast-rotating asteroids probably requires some spin deceleration mechanisms, which selectively worked on all elongated objects during their impact formation and/or subsequent evolution. We also note that our fast rotators are much more spherical compared to known small MBAs and near-Earth objects.

RED SUPERGIANT STARS IN THE LARGE MAGELLANIC CLOUD. I. THE PERIOD-LUMINOSITY RELATION

From previous samples of Red Supergiants (RSGs) by various groups, 191 objects are assembled to compose a large sample of RSG candidates in LMC. For 189 of them, the identity as a RSG is verified by their brightness and color indexes in several near- and mid-infrared bands related to the 2MASS JHKs bands and the Spitzer/IRAC and Spitzer/MIPS bands. From the visual time-series photometric observations by the ASAS and MACHO projects which cover nearly 8-10 years, the period and amplitude of light variation are analyzed carefully using both the PDM and Period04 methods. According to the properties of light variation, these objects are classified into five categories: (1) 20 objects are saturated in photometry or located in crowded stellar field with poor photometric results, (2) 35 objects with too complex variation to have any certain period, (3) 23 objects with irregular variation, (4) 16 objects with semi-regular variation, and (5) 95 objects with Long Secondary Period (LSP) among which 31 have distinguishable short period, and 51 have a long period shorter than 3000 days that can be determined with reasonable accuracy. For the semi-regular variables and the LSP variables with distinguishable short period, the period-luminosity relation is analyzed in the visual, near-infrared and mid-infrared bands. It is found that the P-L relation is tight in the infrared bands such as the 2MASS JHKs bands and the Spitzer/IRAC bands, in particular in the Spitzer/IRAC [3.6] and [4.5] bands; meanwhile, the P-L relation is relatively sparse in the V band which may be caused by the inhomogeneous interstellar extinction. The results are compared with others' P-L relationships for RSGs and the P-L sequences of red giants in LMC.

VARIABILITY IN PROTO-PLANETARY NEBULAE. I. LIGHT CURVE STUDIES OF 12 CARBON-RICH OBJECTS

We have carried out long-term (14 years) V and R photometric monitoring of 12 carbon-rich proto-planetary nebulae. The light and color curves display variability in all of them. The light curves are complex and suggest multiple periods, changing periods, and/or changing amplitudes, which are attributed to pulsation. A dominant period has been determined for each and found to be in the range of ~150 d for the coolest (G8) to 35-40 d for the warmest (F3). A clear, linear inverse relationship has been found in the sample between the pulsation period and the effective temperature and also an inverse linear relationship between the amplitude of light variation and the effective temperature. These are consistent with the expectation for a pulsating post-AGB star evolving toward higher temperature at constant luminosity. The published spectral energy distributions and mid-infrared images show these objects to have cool (200 K), detached dust shells and published models imply that intensive mass loss ended a few thousand years ago. The detection of periods as long as 150 d in these requires a revision in the published post-AGB evolution models that couple the pulsation period to the mass loss rate and that assume that intensive mass loss ended when the pulsation period had decreased to 100 d. This revision will have the effect of extending the time scale for the early phases of post-AGB evolution. It appears that real time evolution in the pulsation periods of individual objects may be detectable on the time scale of two decades.

Photometric survey of the very small near-Earth asteroids with the SALT telescope

Aims. We report first results from our extensive survey of the very small (H > 21.5 mag) near-Earth asteroids. Our aim was to obtain photometric lightcurves for these faint, fast moving objects and to measure their rotation periods and amplitudes of light variations. These parameters can be used to make statistical analysis of the still little known population of the smallest asteroids, test present theories of the YORP effect as well as to study their spin limits, which are connected with their internal structure. Methods. Due to the faintness of the targets and the expected short periods of rotation, observations were performed with the large, 10-m SALT telescope in SAAO (South Africa). For most asteroids, V filter images with exposure times of 5–60 s were obtained with the instrument’s SALTICAM’s CCD camera. Even though the non-sidereal tracking was not available, the SALTICAM’s relatively large field-of-view of 8 � × 8 � helped to perform the relative photometry of the fast-moving targets. Results. The presented asteroids have synodic periods ranging from 77 s to 44 min, effective diameters from 21 to 94 m, and significantly elongated shapes.

CCD Photometry of the MUSES-C Mission Target: Asteroid (25143) 1998 SF36

Abstract Near-Earth asteroid (25143) 1998 SF36 is now planned as the target for the Japanese MUSES-C sample return mission. We conducted extensive photometric observations at NAO-Mitaka from 2001 February 15 until March 19 to obtain its physical properties, such as asteroid’s colors, rotation period, phase curve, and light curves. The colors of this asteroid were found to be S-type and in agreement with the spectroscopic result by Binzel et al. (2001, Meteor. Planet. Sci., 36, 1167). From the phase curve of this object, the visual absolute magnitude is obtained to be 18.61 $\pm$ 0.18; we suggest that the albedo may be higher than that for typical S-type asteroids, after applying a known empirical relation between phase curve slope and geometric albedo. We also reveal that the synodic rotation period of this asteroid is $12.13 \pm 0.02 \,\mathrm{hr}$ and the peak-to-peak light variation amplitude at zero phase angle has been shown to be $\sim 0.49 \,\mathrm{mag}$, which implies the ratio of the longest and intermediate ellipsoidal axes for the asteroid $\gtrsim 1.6$.

Period analysis of secular optical light variations of the quasar PKS 2251+158

We have collected optical observational data of the blazar PKS 2251+158 for nearly 100 years and analyzed its long-tem periodicity in the light variation. The maximum amplitude of light variation in B was found to be 2.86 mag. Using the Jurkevich method, a period of 12.39 yr was found. It is probable that at the center of this object there is a black hole of mass 8.0×10 5 and that the γ amplification factor is not less than 7.2. The Jurkevich method was also discussed.

Lightcurve of asteroid Nereus

An Apollo asteroid (4660) Nereus is one of the most accessible asteroids by spacecraft. Photometric observations of light variation of Nereus were performed in order to support a mission scenario planning and to elucidate the origin of the asteroid. The results revealed that Nereus has a long rotational period, 15.1 ± 1.2 hours, and that the amplitude of light variation is fairly large. Its slow rotation is preferable for a landing or touch down scenario of spacecraft. On the other hand, the comparison of lightcurve data between small main-belt asteroids and comet nuclei suggests that Nereus may be an extinct cometary nucleus, rather than a migrated asteroid from the main-belt. The comparison of the observed data with laboratory impact experiments also implies that the asteroid may not be a fragment from a catastrophic impact event in the main-belt. All the other data already obtained, such as by colorimetry, are not inconsistent with the cometary origin, either.

Size and Shape of Trojan Asteroid Diomedes from Its Occultation and Photometry

The first Trojan asteroid occultation, the occultation of HIP 014402A by (1437) Diomedes, was successfully observed from Japan on November 7, 1997. From its occultation timings at six sites including two video observations, an elongated occultation silhouette of (180±28 km)×(96±5 km), at position angle PA=150±4° was revealed. Follow-up photometry of Diomedes obtained at Ondřejov on November 10 and 11 and at Mitaka on November 18, 19, and 24 revealed that its rotation period is 1.019±0.004 days, its amplitude of light variation is 0.70±0.15 mag, and its rotational phase at the occultation was almost at a minimum of the lightcurve. From these observations, a probability distribution ellipsoidal model for Diomedes is derived. It shows that two families of ellipsoidal models are possible. One is a triaxial ellipsoid of b/ a≈0.55; the other is a rather prolate ellipsoid of b/ a≈0.4, c/ b≈1. Possible orientation of the rotation axis is very restricted to two great circles on the celestial sphere. Mean lengths of the three principal axes of the model ellipsoid are (284±61 km)×(126±35 km)×(65±24 km), i.e., approximately a: b: c≈4:2:1.

The optical gravitational lensing experiment. Variable stars in globular clusters

We present V-band photometry for 141 variable stars from the field of ω Cen. All but one variables are either RR Lyr stars or Pop. II Cepheids. One object is a distant background RR Lyr star from the galactic halo. The presented sample includes 33 newly identified variables, mostly low amplitude RR Lyr stars. We identified also two Pop. II Cepheids with exceptionally low amplitudes of light variations . The presented data may be used to study period changes of ω Cen variables and/or for Fourier decomposition of light curves.

Population and Evolution of Pulsating A-F Stars

Pulsating A-F variables include all the stellar types listed in teh Table, as well as the pulsating white dwarfs. Stars near the zero-age-main-sequence have faster rotation velocity, which slows as expected with age (Villata 1992) and a smaller amplitude of light variation, so we suggest that rotation velocity be considered in Population classifications. Also, in the Galaxy, the galactic rotation constant A is related to stellar age T by: A(kms−1kpc−1) = (−2.4±0.8)T(109yr) + (32±2) (Kharchenko 1992). The linear rotation velocity is also a function of the Z coordinate of the object inside the Galaxy: the mean Z-gradient is −10kms−1kpc−1 (Malakhova &amp; Petrovskaya, 1992). Thus the population is strongly correlated with the rotation velocity and the evolutionary age.

New Ways of Revealing Cepheid Binaries

AbstractTwo methods involving the observed amplitudes of radial velocity and UBVR light variations for classical Cepheids have been analysed, both being implicitly known: their principle is trivial but these methods had not yet been used systematically as indicators of duplicity.The slope method is based on the alteration of the wavelength dependence of the light variation amplitude if either a blue or a red companion is added to the light of the Cepheid. The amplitude ratio (AR) method makes use of the fact that the companion reduces the amplitude of the light variation without observable effect on the pulsational radial velocity amplitude. This means that the ratio of these two amplitudes (Arad.vel./AB) has a larger value for binary Cepheids as compared with the single pulsators.Each method has been applied to more than 100 Cepheids, thus allowing to study how the uncontaminated parameters (amplitude ratio and slope) depend on the pulsation period. Binary Cepheids deviate from the regular pattern in these diagrams, and a number of new binaries can be discovered in this way. The effect of duplicity is revealed by both methods independently for VZ CMa, FM Cas, CR Cep, V402 Cyg, VI154 Cyg, V440 Per and DR Vel.

The Role of Technetium in the Evolution of Red Giants

AbstractTc is detected in many AGB stars providing unambiguous proof that recent nuclear s-processing and mixing (the third dredge-up) has taken place. During this evolutionary episode the atmospheres of AGB stars are progressively enhanced with helium burning products (primari ly l2C) and s-process elements as they evolve from M-&gt;MS-&gt;S-&gt;SC-&gt;C stars. The increase in s-process elements can be traced most easily by the presence and increasing strength of the Tc I lines accompanying this progression. We also find that the third dredge-up phase is accompanied by an increase in the amplitude of light variation since no non-variable or low amplitude variable M, MS, SC, or S (with one exception) have Tc lines. M star Mira variables show Tc if P &gt; 300d (low mass Pop I stars). No Pop II star is known to have Tc. Nor do supergiants show Tc I lines. The significant fraction of MS, S and C stars that do not show Tc, are surmised to be cooler analogues to the Ba II stars, i.e. binaries. The source to provide the neutrons for the s-process is most likely the 13C(a, n)16O reaction since most of the stars in which we observe Tc are thought to have masses less than 3 solar masses.

Absolute and relative amplitudes of variations in radius of classical cepheids

On the basis of observational data for the absolute ΔR and relative ΔR/R amplitudes of variations in radius of galactic classical cepheids (55 stars from Balona and Stobie (1979) and 30 stars from Sollazzoet al. (1981)), four kinds of empirical linear relations are obtained: log(P ΔV)−logΔR, logP−logΔR, log(P ΔV)−log(ΔR/R), and logP−log(ΔR/R);P, R, and ΔV are the pulsation periods, the mean stellar radii, and the amplitudes of light variations, respectively. Three groups of stars are considered: short-period cepheids (SPC)-with logP≤1.1; long-period cepheids (LPC)-with logP>1.1; and s-cepheids (sC). Both the ΔR values and the ΔR/R values increase withP andP ΔV, for a given group of variables. A comparison is performed with our results obtained from data in other sources (Kurochkin, 1966; Gieren, 1982; etc.). The investigated relations can be applied for determining ΔR and ΔR/R of galactic classical cepheids, by using their observedP and ΔV. All studied galactic classical cepheids have ΔR/R<0.35, ΔR<10R ⊙ for SPC and 10R ⊙<ΔR≲60R ⊙ for LPC. The sC have smaller ΔR and ΔR/R values than other classical cepheids, at the same periods (the difference is about 2 times for ΔR and 1.4–2.8 times for ΔR/R); the studied sC have ΔR/R in the range 0.025–0.075 and ΔR in the range 1–3R ⊙ (only Y Oph has ΔR≃8R ⊙).

The circumstellar gas of sigma orionis E

In order to explain the variable Hα emission and the eclipse-like light variation of σ Ori E, we investigated the circumstellar gas trapped by the stellar magnetic field and corotating with the star. By considering the potential along the magnetic field line, we found that the gas concentrates to a potential minimum. The circumstellar gas forms either two condensations or a disk, depending on the inclination of the magnetic dipole to the stellar rotation axis. The geometrical thickness of the circumstellar disk, of about 0.2 stellar radii, and the distance from the center of the star to the inner edge of the disk, of about 3 stellar radii, were obtained. The Hα emission line profile at its maximum phase and the amplitude of light variation were calculated by assuming the isothermal gas in LTE with the maximum gas density which the magnetic field can hold. The model gives good agreement with observation in the low obliquity case, and also explains the phase correlation among the Hα emission maximum, the light minimum, and the magnetic extreme. The model, however, failed to explain the large IR excess in theM band.