Lightcurve Amplitude Research Articles

Photometry of pluto in the last decade and before: evidence for volatile transport?

Photometric observations of Pluto in the BVR filter system were obtained in 1999 and in 1990–1993, and observations in the 0.89-μm methane absorption band were obtained in 2000. Our 1999 observations yield lightcurve amplitudes of 0.30 ± 0.01, 0.26 ± 0.01, and 0.21 ± 0.02 and geometric albedos of 0.44 ± 0.04, 0.52 ± 0.03, and 0.58 ± 0.02 in the B, V, and R filters, respectively. The low-albedo hemisphere of Pluto is slightly redder than the higher albedo hemisphere. A comparison of our results and those from previous epochs shows that the lightcurve of Pluto changes substantially through time. We developed a model that fully accounts for changes in the lightcurve caused by changes in the viewing geometry between the Earth, Pluto, and the Sun. We find that the observed changes in the amplitude of Pluto’s lightcurve can be explained by viewing geometry rather than by volatile transport. We also discovered a measurable decrease since 1992 of ∼0.03 magnitudes in the amplitude of Pluto’s lightcurve, as the model predicts. Pluto’s geometric albedo does not appear to be currently increasing, as our model predicts, although given the uncertainties in both the model and the measurements of geometric albedo, this result is not firm evidence for volatile transport. The maximum of methane-absorption lightcurve occurs near the minimum of the BVR lightcurves. This result suggests that methane is more abundant in the brightest regions of Pluto. Pluto’s phase coefficient exhibits a color dependence, ranging from 0.037 ± 0.01 in the B filter to 0.032 ± 0.01 in the R filter. Pluto’s phase curve is most like those of the bright, recently resurfaced satellites Triton and Europa. Although Pluto shows no strong evidence for volatile transport now (unlike Triton), it is important to continue to observe Pluto as it moves away from perihelion.

Adaptive optics observations of asteroid (216) Kleopatra

The large main-belt asteroid (216) Kleopatra has been for long suspected to be a binary object, mainly due to its large lightcurve amplitude. However, recent observations suggest that it is a single bone-shaped or bi-lobated body (Ostro et al. 2000; Tanga et al. 2001). We present results obtained from ground-based adaptive optics observations, and in agreement with the radar raw-observations, the images show two prominent lobes. Making use of the MISTRAL deconvolution technique, the restored images yield a well-separated binary object. Nevertheless, the spatial resolution of the 3.6m ESO telescope is limited and a dumbbell-shaped body could yield similar features. Further simulations show that adaptive optics observations with an 8-meter class telescope analyzed with the powerful MISTRAL deconvolution technique could overcome this limitation.

Photometry and Spectroscopy of the Optical Companion to the Pulsar PSR J1740−5340 in the Globular Cluster NGC 6397

We present photometric and spectroscopic observations of the optical companion to the millisecond radio pulsar PSR J1740-5340 in the globular cluster NGC 6397. An analysis of the photometric variability in the B, V, and I-bands indicates an inclination of the system of 43.9+-2.1 degrees if the optical companion fills its Roche lobe (a semi-detached configuration). The spectroscopic data show a radial velocity variation with a semi-amplitude of K=137.2 +- 2.4 km/sec, and a system velocity gamma=17.6 +- 1.5 km/sec, consistent with cluster membership. We use these results to derive a mass of the optical companion of M1=0.296 +- 0.034 Msol and M2=1.53 +- 0.19 Msol for the pulsar. There is evidence for secular change of the amplitude of the optical light curve of the variable measured over seven years. The change does not have interpretation and its presence complicates reliable determination of the absolute parameters of the binary.

Long- and Short-Term Photometric Behavior of Comet Hyakutake (1996 B2)

Narrowband filter photometry observations of Comet Hyakutake (1996 B2) were used to investigate this comet's short-term variability as well as its behavior for the apparition as a whole. Utilizing measurements obtained on a total of 13 nights between February 9, 1996, and April 14, 1996, we find that the heliocentric distance ( r H) dependence of the production rates of OH and NH were much shallower than those for either the carbon-bearing species or the visible dust. Based on the OH measurements, the derived water r H-dependence was also significantly less steep than expected from a basic water vaporization model and required an effective active surface area of about 29 km 2 at r H=1.8 AU, 16 km 2 at r H=1 AU, and only 13 km 2 at r H=0.6 AU. This decrease in active area may be due to seasonally induced variations of a heterogeneous surface, or due to a decreasing contribution of gas from icy grains in the innermost coma. The relative abundances of the minor gas species place Hyakutake into the “typical” category of comets in the A'Hearn et al. (1995, Icarus 118, 223–270) taxonomic classification system. The spectrum is generally redder at shorter wavelengths throughout the apparition; however, the dust color progressively changes from being significantly reddened (37%/1000 Å) at large r H to near-solar at small r H. This change of color with distance implies a significant change in grain sizes or a changing proportion between two or more grain populations. A major outburst was initiated near March 19.9, just prior to the comet's close approach to Earth. The characteristic recovery from the outburst differed among the observed species, with OH recovering most rapidly, essentially returning to its baseline values by March 25. The spatial radial fall-off of OH throughout this interval was consistent with the expected nominal spatial distribution, while CN and C 2 displayed fall-offs consistent with a distributed source, and the dust fall-off was significantly less steep than 1/ρ, possibly due to fragmenting grains. Rotational lightcurve amplitudes were largest for the OH, NH, and dust, again consistent with the carbon-bearing species primarily originating from a distributed source. Significant variations were observed in the lightcurve amplitude and phase shifts as functions of aperture size. Finally, a refined value for the rotation period of 0.2614±0.0003 day was determined.

The X-ray transient XTE J1859 + 226 in outburst and quiescence

We present optical photometry and spectroscopy of the X-ray transient XTE J1859+226, obtained during outburst and its subsequent decay to quiescence. Both the X-ray and optical properties are very similar to those of well-studied black hole soft X-ray transients. We have detected three mini-outbursts, when XTE J1859+226 was approaching quiescence, as has been previously detected in the soft X-ray transients GRO J0422+32 and GRS 1009-45. By 2000 August 24 the system had reached quiescence with R = 22.48 ′ 0.07. The estimated distance to the source is ∼11 kpc. Photometry taken during quiescence shows a sinusoidal modulation with a peak to peak amplitude of about 0.4 mag. A period analysis suggests that periods from 0.28 to 0.47 d are equally possible at the 68 per cent confidence level. The amplitude of the quiescent light curve and the relatively low ratio of X-ray to optical flux indicate that the binary inclination should be high. The measured colours during the outburst allow us to obtain the basic properties of the disc, which agrees well with irradiated disc model predictions.

Blazhko stars: Always distorted?

A reexamination of the amplitude ratios of the photometric and radial velocity changes of Blazhko RRab stars in their dierent Blazhko phases shows that a normal amplitude ratio occurs near the phase of maximum amplitude. Checking the Fourier parameters of maximum amplitude light curves with dierent methods proved, however, that none of these light curves can be regarded as a normal RR Lyrae type light curve.

The latitude distribution of star-spots on He 699

In this paper, the latitude distribution of star-spots is analysed for the rapidly rotating G dwarf He 699. An image has been reconstructed from data taken with the William Herschel Telescope on La Palma on 2000 October 8. The predominant magnetic field structure is a decentred polar spot at high latitude, with smaller low-latitude features also present. This result is verified by independent reconstructions using even- and odd-numbered spectra. This work confirms and extends that of Barnes et al., and provides further evidence that there is a correlation between the presence of low-latitude features and the amplitude of the photometric light curve. It is also a further step in the search for activity cycles on young G dwarfs.

How Mira Variables Change Visual Light by a Thousandfold

Mira variables change visual light by up to 8 mag over their roughly yearly cycle. Here we present a simple explanation for the extremely large amplitudes of light curves of oxygen-rich Mira variables. Metallic oxides, such as TiO, form throughout the stellar atmosphere as the star cools when approaching minimum light. When this happens, the visual light can be almost completely absorbed at large radii, extending the visual photosphere to nearly twice its nominal size. At these large radii, temperatures can fall to ≈1400 K and essentially all of the star's radiation emerges in the infrared. Since almost no optical light is emitted at these low temperatures, Mira variables can decrease their visual light by more than a thousandfold and almost "disappear" to the human eye.

Fluorescence Model for AG Draconis

AbstractA fluorescent light curve model (ionization-bounded wind and chromosphere with scale height) that previously found most fluorescent variation in symbiotic nova V1329 Cygni to be chromospheric, is applied to AG Draconis, where the strong wind of a well-detached red giant is illuminated by ultraviolet radiation from an accreting white dwarf. Bandpass dependence of light curve amplitude is even more striking than in V1329 Cyg and eliminates thermal re-radiation from the red star photosphere as a principal variation mechanism, since thermal re-radiation cannot change by large factors from U to B to V. Spectra show strong continuum and line emission, so both light curves and spectra show the importance of fluorescence. A small secular decay was included by fitting in time rather than phase, with dTwd/dt as a parameter. There is a 3σ indication of a large negative dP/dt. Chromospheric fluorescence accounts for the entire U variation, while B and V variations are too small for wind and chromosphere to be distinguished. We confirm a previously identified 355-day radial velocity period that is consistent with red star pulsation.

Ultraviolet Observations of Triton in 1999 with the Space Telescope Imaging Spectrograph: 2150–3180 Å Spectroscopy and Disk-integrated Photometry

We observed Triton using the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) on 1999 August 28, September 11, and September 13 UT. On all three occasions, we obtained images at both 271 and 374 nm, and spectra from 215 to 318 nm. We find that Triton's geometric albedo is high through the near- and mid-UV, decreasing from 0.64 ± 0.02 at 374 nm to 0.53 ± 0.05 at 223 nm. We compare these data with a previous UV data set obtained by the HST Faint Object Spectrograph (FOS), taken primarily during 1993 September 1–4. Between 250 and 320 nm, Triton was roughly 20% brighter in the STIS (1999) data set than in the FOS (1993) data set, but between 220 and 250 nm the albedos are essentially unchanged. As expected from the HST FOS and other data sets, we find that Triton had higher spatial contrast at shorter wavelengths in 1999, but it exhibited a light-curve amplitude that is as much as 33% lower than derived from Voyager 2 photometry taken in 1989. We discuss some possible causes for these observed changes.

The Lightcurve and Colors of Unusual Minor Planet 1998 WU 24

Minor planet 1998 WU 24 is unusual in having the orbital characteristics of a Halley family comet but showing no sign of cometary activity. We present optical data that reveal a double-peaked light-curve with a period of 7.283±0.003 h and a peak-to-peak amplitude of ∼0.54 mag. Infrared spectroscopy and quasi-simultaneous BVRIJHK photometry reveal a featureless K-band spectrum and colors typical of D-type asteroids, and suspected “bare” comet nuclei (B-V=0.78±0.034, V-R=0.53±0.037, V-I=0.99=0.035, V-J=1.67±0.043, V-H=2.10±0.076 and V-K=2.34±0.102). Image profiles from co-added frames in the R band indicate no apparent cometary activity, with an implied upper limit to the dust production rate of 150 g s −1. Assuming a D-type albedo of 0.04 we derive a spherical equivalent diameter of 5.28±0.07 km although the lightcurve amplitude implies an irregular body with an axial ratio of 1.64 : 1. We conclude that 1998 WU 24 is probably an inactive comet nucleus.

Coordinated 1996 HST and IRTF Imaging of Neptune and Triton: II. Implications of Disk-Integrated Photometry

Near-IR groundbased observations coordinated with Wide Field Planetary Camera 2 (WFPC2) HST observations (Sromovsky et al. Icarus 149, 416–434, 459–488) provide new insights into the variations of Neptune and Triton over a variety of time scales. From 1996 WFPC2 imaging we find that a broad circumpolar nonaxisymmetric dark band dominates Neptune's lightcurve at 0.467 μm, while three discrete bright features dominate the lightcurve at longer wavelengths, with amplitudes of 0.5% at 0.467 μm and 22% at 0.89 μm, but of opposite phases. The 0.89-μm modulation in 1994, estimated at 39%, is close to the 50% modulation observed during the 1986 “outburst” documented by Hammel et al. (1992, Icarus 99, 363–367), suggesting that the unusual 1994 cloud morphology might also have been present in 1986. Lightcurve amplitudes in J–K bands, from August 1996 IRTF observations, are comparable to those observed in 1977 (D. P. Cruikshank 1978, Astrophys. J. Lett. 220, 57–59) but significantly larger than the 1981 amplitudes of M. J. S. Belton et al. (1981, Icarus 45, 263–273). The 1996 disk-integrated albedos of Neptune at H–K wavelengths are 2–7 times smaller than the 1977 values of U. Fink and S. Larson (1979, Astrophys. J. 233, 1021–1040), which can be explained with about 1/2–1/4 of the upper level cloud opacity being present in 1996. A simplified three-layer model of cloud structure applied to CCD wavelengths implies ∼7% reflectivity at 1.3 bars (at λ=0.55 μm, decreasing as λ −0.94) and ∼1% at 100–150 mbars. To fit the WFPC2 observations and those of E. Karkoschka (1994, Icarus 111, 174–192), the putative H 2S cloud between 3.8 and 7–9 bars must have a strong decrease in reflectivity between 0.5 and 0.7 μm, as previously determined by K. H. Baines and W. H. Smith (1990, Icarus 85, 65–108). To match our 1996 IRTF results, this cloud must have another substantial drop in reflectivity at near-IR wavelengths, to a level of 0–5%, corresponding to single-scattering albedos of ∼0–0.3. The model that fits our near-IR observations on 13 August 1996 can reproduce the magnitudes of the dramatic 1976 “outburst” (R. R. Joyce et al. 1977, Astrophys. J. 214, 657–662) by increasing the upper cloud fraction to 6% (from ∼1%) and lowering its effective pressure to ∼90 mbars (from 151 mbars). Triton's disk-integrated albedo from HST imagery at 11 wavelengths from 0.25 to 0.9 μm are consistent with previous groundbased and Voyager measurements, thus providing no evidence for the albedo decrease suggested by Triton's recent warming (J. L. Elliot et al. 1998 Nature 393, 765–767). Triton's lightcurve inferred from 1994–1996 WFPC2 observations has about twice the amplitude inferred from 1989 Voyager models for the UV to long visible range (J. Hillier et al. 1991, J. Geophys. Res. 96, 19,211–19,215).

The "rotation-activity connection": Its extension to photospheric activity diagnostics

In this paper we present the results of a different approach in the study of the so-called rotation-activity connection, which is a well established correlation between rotation and magnetic activity at chromospheric and outer atmospheric levels. The present study concerns the photospheric level and was carried out by using V-band photometric light curve amplitudes as indicators of starspot coverage and of magnetic activity. A high degree of correlation between the envelope of maximum V-band light curve amplitudes and the rotation period is found for the active star members of young open clusters (IC 2602, IC 2391, Alpha Persei, Pleiades and Hyades), as well as for active field stars. This correlation shows a diffe rent behaviour in two different rotation period ranges. Moreover, some evidence of a possible activity "saturation"is found among the most rapidly rotating stars of the sample. Additional correlations between photospheric and other magnetic activity indicators in the chromosphere, transition region and corona are also investigated. The results presented here can be considered as an extension of the well established rotation-activity connection valid from the corona, transition region and chromosphere, down to the photosphere.

Analyses of the Currently Noneclipsing Binary SS Lacertae or SS Lacertae’s Eclipses

Confirmatory evidence for changing light-curve amplitude of the former eclipsing and current SB2 system SS Lac in the Open Cluster NGC 7209 has been uncovered. Remeasured Harvard plate data and published and compiled data sets reveal that the depth of the primary minimum increased between the 1890s and 1902 and decreased in the 1920s and 1930s. A parabolic fitting of the amplitude with phase predicts a maximum at 1911.5, with an eclipse onset at 1885.3 and eclipse cessation at 1937.8. We confirm the finding of Lehmann, that the system's inclination varies with time and that a central eclipse occurred ~1912, and we concur with Mossakovskaya that eclipses effectively ceased ~1940. Estimates of SS Lac on plates taken at Tashkent between 1937 and 1940 further serve to confirm the result. Thus, SS Lac belongs to a small but elite class of triple systems in which changes due to dynamical effects can be seen over a single human lifetime. In order to explore the properties of the SS Lac system, recent radial velocity curves and archival photographic and visual light curves have been analyzed with versions of the Wilson-Devinney code, augmented with a simplex routine to test solution uniqueness. The modeling solutions for the Dugan-Wright light curves ostensibly indicate that the former eclipsing system is composed of two early A stars of only slightly differing masses (2.57 ± 0.16 and 2.59 ± 0.19 M⊙) and effective surface temperatures (8750 ± 300 [assumed for component 1] and 8542 ± 309 K), but significantly different radii (2.38 ± 0.02 and 3.63 ± 0.07 R⊙) and luminosities (30 ± 4 and 63 ± 9 L⊙) for the hotter and cooler components, respectively. The light-curve solutions are compromised somewhat by variable eclipse depths over the ranges of dates of the data sets. This is especially true of the most complete light curve, that of Dugan & Wright; the others also suffer from incompleteness (that of Wachmann) and high scatter (that of Kordylewski, Pagaczewski, & Szafraniec). As a consequence, small, temporal variations in such system properties as the eccentricity, argument of periastron, modified Roche potentials, luminosities, and third light level, cannot be ruled out from currently available data. However, solutions with WD95, a self-iterating, damped–least squares version of the Wilson-Devinney program, reveal optimized inclinations for the data sets that project an inclination variation of 016 yr-1, but no evidence of apsidal motion. We find a distance for the system of 898 ± 95 pc, consistent with the value of Vansevicius et al. of 1040 ± 10 pc, and finally, on the bases of location on the sky, proper motion, radial velocity, photometry, and properties deduced in the present study, we confirm its membership in the cluster NGC 7209.

New BVR photometry of six prominent RS Canum Venaticorum binaries

An ewBVR photometry of six prominent RS CVn stars: V711 Tau, UX Ari, IM Peg, II Peg, Gem and And, carried out during 1995-97, is presented. The new results reveal signicant evolution in the shape as well as in the amplitude of light curves of these binaries. The traditional two-starspot model has been used to obtain the spot parameters from the observed light curves of the stars. Changes in spot area and their location on stellar surface can be noticed from extracted spot parameters. Some of these stars are found to have signicant variation in (B V )a nd ( V R) colour indices. The (V R) colour index variation found in IM Peg, II Peg and And have been used for determining starspot eective temperatures.

Doppler Imagery of the Spotted RS Canum Venaticorum Star HR 1099 (V711 Tauri) from 1981 to 1992

Doppler Imagery of the Spotted RS Canum Venaticorum Star HR 1099 (V711 Tauri) from 1981 to 1992

A Study of Hilda Asteroids: VI. Analysis of the Lightcurve Properties

The lightcurves of 47 Hilda asteroids presented in Dahlgrenet al. (1998,Icarus133, 247–285) have been analyzed. The Hilda population has mean lightcurve amplitudes significantly larger than those of main-belt asteroids of similar sizes. This is due to the very large mean amplitudes (0.31 mag) of the small (D<60 km) D-type Hilda asteroids. The spin rate distribution of the Hilda asteroids is markedly non-Maxwellian, mainly due to a large fraction of slowly rotating objects. More than half of the observed population have rotation periods longer than 12 h. The spin rate versus size diagram has a U-shape with slow spin rates at intermediate sizes. At smaller sizes (D<60 km) where the Hilda population is totally dominated by D-type objects (Dahlgren and Lagerkvist 1995,Astron. Astrophys.302, 907–914; Dahlgrenet al. 1997,Astron. Astrophys.323, 606–619) there is a large increase in the scatter of the spin rates, and an increase of the mean amplitudes of these (D-type) Hilda asteroids. This strongly suggests a connection between taxonomic type and collisional evolution among objects in the Hilda population. Possible interpretations of these results are discussed in terms of an early heating event in the Solar System and subsequent collisional evolution of the Hilda population.

The Mode Change of the RR Lyrae Variable V79 in M3

The RR Lyrae variable V79 in M3 has been studied on approximately 1000 photographs in the Sternberg Observatory plate collection and on 126 CCD frames. The photographs were obtained over the interval 1949-1992, and the CCD observations were made in 1998. A comparison of our data with observations made in 1992, 1993, and 1996 indicates that V79 changed its pulsation mode in 1992. Before 1992, it was a fundamental mode pulsator (RRab star) with an irregular light curve, but since then it has been a double-mode pulsator (RRd star) with the first overtone the dominant mode. Prior to the mode change, there were sporadic episodes when pulsation in the first overtone mode occurred, for example, for a few days in 1976 and again in 1989. In each case, however, the fundamental mode pulsation resumed afterward. During the first two weeks in April of 1989, the star appeared to stop varying, and when it began again, the period of the fundamental mode pulsation had decreased substantially, from 0.483 to 0.480 days. Although the first overtone has been the dominant mode of pulsation for V79 since 1992 when it became an RRd star, our 1998 observations show that the amplitude of the fundamental mode is greater than it was in 1996. Previously published investigations of the Oosterhoff type II cluster M15 indicate that some of its RRd stars have exhibited irregular behavior, but no mode switches have been observed. In addition, there are some short period RRab stars in other Oosterhoff type I clusters that have changed their light curve amplitude and shape on a timescale of years, but up to now, none of these have been shown to be RRd stars.

Near-Infrared Photometry of the Saturnian Satellites during Ring Plane Crossing

The first near-infrared (0.9 μm) photometric observations of Saturn's five medium-sized satellites were obtained during the August 1995 ring plane crossing with Palomar Observatory's 60-in. telescope. The albedos of the satellites are very high, indicating that a very low fraction of dark opaque contaminants is present on their icy surfaces. The geometric albedos for the leading sides of the satellites are Enceladus: 1.02; Tethys: 0.90; Dione: 0.72; and Rhea: 0.76. The geometric albedo of the trailing side of Mimas is 0.72. The amplitudes of the satellites' rotational lightcurves are somewhat less than those measured at visible wavelengths. They range from 0.40 (Dione) to 0.10 (Rhea) to 0.05 (Tethys). Both Enceladus and Mimas are brighter on the trailing side, unlike the other three satellites. These results suggest that the optical properties of all five satellites are determined by interactions with Saturn's E-ring.

Bipolar Jets and Orbital Dynamics of the Supersoft X‐Ray Source RX J0019.8+2156

Between 1994 July and 1997 February we monitored the optical spectrum of RX J0019.8+2156. This supersoft X-ray source is one of only two accreting white dwarfs in the Galaxy that are thought to be burning hydrogen on their surface as a consequence of a high rate of mass transfer from a binary companion. Accurate orbital ephemerides are derived from radial velocity measurements for the white dwarf, which are obtained from strong He II emission lines with a stable velocity semiamplitude of K = 71.2 ± 3.6 km s-1. We report the discovery of transient, low-velocity, bipolar jets. These jets are represented by redshifted-blueshifted pairs of emission lines from H and He II with an outflow velocity of v cos (i) ~ 815 km s-1, where i is the binary inclination angle. When present, the jet lines seen in Hα also exhibit an orbital modulation of 71 km s-1, which strengthens the interpretation that this is the orbital velocity of the white dwarf and also indicates that the jets are oriented nearly perpendicular to the orbital plane. On most occasions, the H emission line profiles are further altered by P Cygni absorption effects, and the strength of this absorption is also dependent on binary phase. We show that the jets and the P Cygni features have very different temporal characteristics and binary phase dependence; thus, we conclude that the outflowing material and the absorbing wind must have essentially different geometries. Finally, the measured mass function is combined with binary evolution models to suggest a limit on the inclination angle, i < 40°. A particular model invoked to explain a high rate of mass transfer requires 16° < i < 25°. However, at such small inclination it is difficult to explain the large amplitude of the orbital light curve (~0.5 mag). Alternatively, the results could signify substantial vertical structure in the accretions disks of supersoft X-ray sources. By contrast, a simple model fit to the jet outflow lines indicates an orbital inclination angle of 35° < i < 60°.