Photometric Model Research Articles

One Moon, many measurements 2: Photometric corrections

Observations of the lunar surface within the past 10years have been made with various lunar remote sensing instruments, the Moon Mineralogy Mapper (M3) onboard the Chandrayaan-1 mission, the Spectral Profiler (SP), the Multiband Imager (MI), the Terrain Camera (TC) onboard the SELENE mission, and the ground based USGS Robotic Lunar Observatory (ROLO) for some of them. The lunar phase functions derived from these datasets, which are used in the photometric modeling to correct for the various illumination conditions of the data, are compared to assess their differences and similarity in order to improve interpretations of lunar surface spectra. The phase functions are found to be similar across various phase angles except in the 0–20° range. Differences across the 0–20° range likely result from two different inputs in the photometric modeling of the M3 and SP data: (1) M3 has larger emission angles due to the characteristics of the instrument and the attitude of the spacecraft, and (2) M3 viewing geometry was derived from the local topography whereas SP used a spherical Moon (no topography). The combination of these two different inputs affects the phase function at small phase angles where shadows play a more substantial role, with spatial resolution differences between M3 and SP being another possible source for the differences. SP data are found to be redder (i.e., steeper slope with increasing wavelengths) than MI, M3 and ROLO. Finally, the M3 overall reflectance is also found to be lower than that the other instruments (i.e., MI, SP, and ROLO), generally at least 10% darker than MI. These differences can be observed at local scales in specific examples at hundreds of meters resolutions. At regional and global scales, the same differences are found, which demonstrates the overall stability of the various datasets. The observations from M3, TC, SP and MI are very stable and agree well; however caution should be used when making interpretations based on the spectral slope of SP data or on the absolute reflectance of M3 data.

The ATLAS3D project – XXI. Correlations between gradients of local escape velocity and stellar populations in early-type galaxies

We explore the connection between the local escape velocity, Vesc, and the stellar population properties in the ATLAS3D survey, a complete, volume-limited sample of nearby early-type galaxies. We make use of ugriz photometry to construct Multi-Gaussian Expansion models of the surface brightnesses of our galaxies. We are able to fit the full range of surface brightness profiles found in our sample, and in addition we reproduce the results of state-of-the-art photometry in the literature with residuals of 0.04 mag. We utilize these photometric models and SAURON integral-field spectroscopy, combined with Jeans dynamical modelling, to determine the local Vesc derived from the surface brightness. We find that the local Vesc is tightly correlated with the Mg b and Fe5015 line strengths and optical colours, and anti-correlated with the Hβ line strength. In the case of the Mg b and colour–Vesc relations we find that the relation within individual galaxies follows the global relation between different galaxies. We intentionally ignored any uncertain contribution due to dark matter since we are seeking an empirical description of stellar population gradients in early-type galaxies that is ideal for quantitative comparison with model predictions. We also make use of single stellar population (SSP) modelling to transform our line strength index measurements into the SSP-equivalent parameters age (t), metallicity ([Z/H]) and α-enhancement [α/Fe]. The residuals from the relation are correlated with age, [α/Fe], molecular gas mass and local environmental density. We identify a population of galaxies that occur only at low Vesc that exhibit negative gradients in the Mg b– and Colour–Vesc relations. These galaxies typically have young central stellar populations and contain significant amounts of molecular gas and dust. Combining these results with N-body simulations of binary mergers we use the Mg b–Vesc relation to constrain the possible number of dry mergers experienced by the local early-type galaxy population – a typical massive early-type galaxy can have experienced only ∼1.5 major mergers before becoming a significant outlier in the Mg b–Vesc relation.

COMPARISON OFKEPLERPHOTOMETRIC VARIABILITY WITH THE SUN ON DIFFERENT TIMESCALES

We utilize Kepler data to study the precision differential photometric variability of solar-type and cooler stars at different timescales, ranging from half an hour to 3 months. We define a diagnostic that characterizes the median differential intensity change between data bins of a given timescale. We apply the same diagnostics to SOHO data that has been rendered comparable to Kepler. The Sun exhibits similar photometric variability on all timescales as comparable solar-type stars in the Kepler field (it is not unusually quiet). The previously-defined photometric "range" serves as our activity proxy (driven by starspot coverage). We revisit the fraction of comparable stars in the Kepler field that are more active than the Sun. The exact active fraction depends on what is meant by "more active than the Sun", and on the magnitude limit of the sample of stars considered. This active fraction is between a quarter and a third (depending on the timescale). We argue that a reliable result requires timescales of half a day or longer and stars brighter than Kepler magnitude of 14, otherwise non-stellar noise distorts it. We also analyze main sequence stars grouped by temperature from 6500-3500K. As one moves to cooler stars, the active fraction of stars becomes steadily larger (greater than 90% for early M dwarfs). The Sun is a good photometric model at all timescales for those cooler stars that have long-term variability within the span of solar variability.

New Photometric Models of Four Near-Contact Binaries: EP Cassiopeiae, AK Canis Minoris, FG Geminorum, and DF Puppis

Abstract We present new photometric observations of EP Cas, AK CMi, and FG Gem, which were obtained during the period from 2008 October to 2011 March using the 60 cm telescope at the Xinglong station. Several light-minimum times were determined, and orbital periods were revised. By using the 2003-version Wilson–Devinney code, photometric solutions of EP Cas, AK CMi, FG Gem, and DF Pup were deduced from multicolor light curves. The results imply that the four systems are Algol-type near-contact binaries (NCBs), whose less-massive components are filling their Roche lobes. With mass and angular-momentum loss, the separation between both components will decrease, and the primary will finally fill its Roche lobe. Algol-type NCBs of this kind, such as EP Cas, AK CMi, FG Gem, and DF Pup, may evolve into the contact configuration as a progenitor of a W UMa-type binary.

Micrometer-sized ice particles for planetary-science experiments – II. Bidirectional reflectance

We have measured the bidirectional reflectance of spherical micrometer-sized water–ice particles in the visible spectral range over a wide range of incidence and emission angles. The small ice spheres were produced by spraying fine water droplets directly into liquid nitrogen. The resulting mean particle radii are 1.47+0.96−0.58μm. Such a material shares many properties with ice in comets and at the surface of icy satellites. Measurements show that the fresh sample material is highly backscattering, contrasting with natural terrestrial snow and frost. The formation of agglomerates of particles during the sample production results in a noticeable variability of the photometric properties of the samples in their initial state. We have also observed significant temporal evolutions of the scattering behavior of the samples, shifting towards more forward scattering within some tens of hours, resulting most likely from sintering processes. All reflectance data are fitted by the Hapke photometric model (1993 and 2002 formulation) with a one/two/three-parameter Henyey–Greenstein phase function and the resulting Hapke parameters are provided. These parameters can be used to compare laboratory results with the observed photometric behaviors of astronomical objects. We show, in particular, that the optical properties of the fresh micrometer-sized ice samples can be used to reproduce the predominant backscattering in the phase curves of Enceladus and Europa.

CONFIRMATION OF HOT JUPITER KEPLER-41b VIA PHASE CURVE ANALYSIS

We present high precision photometry of Kepler-41, a giant planet in a 1.86 day orbit around a G6V star that was recently confirmed through radial velocity measurements. We have developed a new method to confirm giant planets solely from the photometric light curve, and we apply this method herein to Kepler-41 to establish the validity of this technique. We generate a full phase photometric model by including the primary and secondary transits, ellipsoidal variations, Doppler beaming and reflected/emitted light from the planet. Third light contamination scenarios that can mimic a planetary transit signal are simulated by injecting a full range of dilution values into the model, and we re-fit each diluted light curve model to the light curve. The resulting constraints on the maximum occultation depth and stellar density combined with stellar evolution models rules out stellar blends and provides a measurement of the planet's mass, size, and temperature. We expect about two dozen Kepler giant planets can be confirmed via this method.

PHOTOMETRIC PROPERTIES OF THE INTERACTING BINARY BO MONOCEROTIS: EVIDENCE FOR MAGNETIC ACTIVITY

BO Monocerotis (BO Mon) is a severely neglected short-period (2.23 days) Algol-type eclipsing binary star system undergoing angular momentum variations that are likely due to the evolved secondary star experiencing cycles of magnetic activity. We present the first CCD light curves of BO Mon, which were observed at the Kutztown University Observatory (Kutztown, PA) in 2012 using B, V, and I filters. The analysis presented here is the first of its kind for BO Mon and provides the first physical model of the system's parameters. We also incorporate over 40 yr of published times of minimum light to provide a new ephemeris curve and perform a period study that greatly improves, while differing significantly from, an earlier ephemeris analysis that was done more than 13 yr ago. The observed variations in BO Mon's orbital period supply evidence for mass transfer and magnetic activity and our photometric model affords the basic properties of the system for use in future photometric and spectroscopic studies.

Improving the uniformity of luminous system in radial imaging capsule endoscope system

This study concerns the illumination system in a radial imaging capsule endoscope (RICE). Uniformly illuminating the object is difficult because the intensity of the light from the light emitting diodes (LEDs) varies with angular displacement. When light is emitted from the surface of the LED, it first encounters the cone mirror, from which it is reflected, before directly passing through the lenses and complementary metal oxide semiconductor (CMOS) sensor. The light that is strongly reflected from the transparent view window (TVW) propagates again to the cone mirror, to be reflected and to pass through the lenses and CMOS sensor. The above two phenomena cause overblooming on the image plane. Overblooming causes nonuniform illumination on the image plane and consequently reduced image quality. In this work, optical design software was utilized to construct a photometric model for the optimal design of the LED illumination system. Based on the original RICE model, this paper proposes an optimal design to improve the uniformity of the illumination. The illumination uniformity in the RICE is increased from its original value of 0.128 to 0.69, greatly improving light uniformity.

The opposition effect in Saturn’s main rings as seen by Cassini ISS: 1. Morphology of phase functions and dependence on the local optical depth

During the Prime mission (2004–2008), the Cassini cameras captured the opposition effect in Saturn’s main rings with a fine radial resolution at extremely small phase angles. Phase functions were extracted for 211 different radial locations in the main ring system (135 in the broadband clear filter of the Wide Angle Camera at 635nm, 76 in four narrowband color filters (at the respective central wavelengths 440, 568, 650, and 752nm for the Narrow Angle Camera and 460, 567, 649, and 742nm for the Wide Angle Camera). The morphology of the observed opposition surges is fitted with a simple linear model in order to derive three useful parameters – the amplitude A, the width of the surge HWHM, and the slope S at larger phase angles – throughout the whole main ring system. In agreement with previous studies, the opposition surge is found to vary substantially across the ring system: whereas values of A and HWHM are nearly constant across the A and B Rings, they show strong local variations for the C Ring and the Cassini Division. Thanks to the high spatial resolution of Cassini images, the opposition surges are obtained for very narrow regions, allowing us to study the effect of the local normal optical depth (τ) on the surge. For τ>0.7, A(τ) and HWHM(τ) are nearly constant (1.3° and 0.25°, respectively) across the whole ring system. For τ<0.7, A(τ) and HWHM(τ) exhibit clear decreasing trends for the A, B and C Rings only. Conversely, points from the Cassini Division show a very large dispersion in both A and HWHM, with no visible trend. The slope S is found to be an increasing function of the optical depth across the full ring system. A trend S∝τ0.5 is observed, in qualitative agreement with a geometrical description of the shadow hiding effect. No strong trend of A, HWHM and S with the wavelength of observation was found. We also tabulate the fit parameters for the opposition surge of 139 regions in the clear filter for future comparison with numerical simulations and photometric models.

PHOTOMETRIC STUDIES OF THREE NEGLECTED SHORT-PERIOD CONTACT BINARIES GN BOOTIS, BL LEONIS, AND V1918 CYGNI

We present new photometry for three short-period contact binaries, GN Boo, BL Leo, and V1918 Cyg, observed from 2008 December to 2012 April using several small telescopes in China. Photometric models were deduced from new observations using the updated Wilson-Devinney Code. The results show that GN Boo and BL Leo are W-type contact binaries, while V1918 Cyg is an A-type one. The mass ratios and fill-out factors are q = 0.320(+/- 0.002) and f = 5.8(+/- 0.1)% for GN Boo, q = 0.476(+/- 0.005) and f = 21.3(+/- 1.1)% for BL Leo, q = 0.264(+/- 0.002), and f = 49.7(+/- 0.7)% for V1918 Cyg, respectively. From the (O - C) curves, it is discovered that the orbital periods of three binaries have varied in a complicated way, i.e., cyclic oscillation for GN Boo, long-term period decrease for BL Leo, and both for V1918 Cyg. The cyclic variations for GN Boo and V1918 Cyg may probably be attributed to the magnetic activity of the primary component or light-time effect due to the third body. Meanwhile, the secular period decreases for BL Leo and V1918 Cyg may result from mass transfer from the primary to the secondary, accompanying the mass and angular momentum loss from the central system. Finally, GN Boo, BL Leo, and V1918 Cyg will evolve into deep contact binaries. Additionally, a statistical study of 37 contact binaries with decreasing periods is given. We obtained the relations of q - f and q - d ln P/dt, and preliminarily determined the mass loss rate of d ln M/dt from the binary system.

Research on photometric correction of lunar image from CCD stereo camera onboard Chang'E-1 satellite

为更加精确拟合相函数系数, 得到较为准确的光度校正结果, 针对嫦娥一号CCD立体相机数据的特点, 对改进的Lommel-Seeliger模型中的Lommel-Seeliger因子进行修订, 使去除太阳入射角和传感器观测角影响的遥感图像数据能够更好地契合相位角的变化; 利用加入修订后Lommel-Seeliger因子的模型对嫦娥一号CCD立体相机数据进行逐像素光度校正; 选取同一轨道不同纬度和同一区域不同视角两组数据来验证算法的有效性和适用性. 实验结果表明, 该方法能够有效校正由于几何观测条件变化而引起的目标光谱特性的不一致性, 对于较亮和较暗区域的光度校正效果更为有效.

YONSEI EVOLUTIONARY POPULATION SYNTHESIS (YEPS) MODEL. I. SPECTROSCOPIC EVOLUTION OF SIMPLE STELLAR POPULATIONS

We present a series of papers on the year-2012 version of Yonsei Evolutionary Population Synthesis (YEPS) model which is constructed on over 20 years of heritage. This first paper delineates the spectroscopic aspect of integrated light from stellar populations older than 1 Gyr. The standard YEPS is based on the most up-to-date Yonsei-Yale stellar evolutionary tracks and BaSel 3.1 flux libraries, and provides absorption line indices of the Lick/IDS system and high-order Balmer lines for simple stellar populations as functions of stellar parameters, such as metallicity, age and {\alpha}-element mixture. Special care has been taken to incorporate systematic contribution from horizontal-branch stars which alters the temperature-sensitive Balmer lines significantly, resulting in up to 5 Gyr difference in age estimation of old, metal-poor stellar populations. We also find that the horizontal branches exert an appreciable effect not only on the Balmer lines but also on the metallicity-sensitive lines including the magnesium index. This is critical to explain the intriguing bimodality found in index distributions of globular clusters in massive galaxies and to derive spectroscopic metallicities accurately from various indices. A full set of the spectroscopic and photometric YEPS model data of the entire parameter space is currently downloadable at http://web.yonsei.ac.kr/cosmic/data/YEPS.htm

Searching for non-axisymmetries in NGC 6503: a weak end-on bar

The isolation, simple apparent structure, and low luminosity of the nearby spiral galaxy NGC 6503 make it an ideal candidate for an in-depth kinematic and photometric study. We introduce a new publicly available code, DiskFit, that implements procedures for fitting non-axisymmetries in either kinematic or photometric data. We use DiskFit to analyze new Halpha and CO velocity field data as well as HI kinematics from Greisen et al. to search for non-circular motions in the disc of NGC 6503. We find NGC 6503 to have remarkably regular gas kinematics that are well-described by rotation only. We also use DiskFit and a new Ks-band image of NGC 6503 to constrain photometric models of the disc, bar and bulge. We find the galaxy to be photometrically dominated by the disc. We find NGC 6503 to contain a faint bar and an exponential bulge which together contribute only ~5% of the total galaxy light. The combination of our kinematic and photometric DiskFit models suggest that NGC 6503 contains a weak, end-on bar that may have produced its Type II surface brightness profile but is unlikely to be responsible for its strong sigma-drop.

A visible and near-infrared photometric correction for Moon Mineralogy Mapper (M3)

Observations of the Moon obtained by the Moon Mineralogy Mapper (M3) instrument were acquired at various local viewing geometries. To compensate for this, a visible near-infrared photometric correction for the M3 observations of the lunar surface has been derived. Images are corrected to the standard geometry of 30° phase angle with an incidence of 30° and an emission of 0°. The photometric correction is optimized for highland materials but is also a good approximation for mare deposits. The results are compared with ground-based observations of the lunar surface to validate the absolute reflectance of the M3 observations. This photometric model has been used to produce the v1.0 Level 2 delivery of the entire set of M3 data to the Planetary Data System (PDS). The photometric correction uses local topography, in this case derived from an early version of the Lunar Orbiter Laser Altimeter data, to more accurately determine viewing geometry. As desired, this photometric correction removes most of the topography of the M3 measurements. In this paper, two additional improvements of the photometric modeling are discussed: (1) an extrapolated phase function long ward of 2500nm to avoid possible misinterpretation of spectra in the wavelength region that includes possible OH/H2O absorptions and (2) an empirical correction to remove a residual cross-track gradient in the data that likely is an uncorrected instrumental effect. New files for these two effects have been delivered to PDS and can be applied to the M3 observations.

Response to the comment by B. Hapke on “A critical assessment of the Hapke photometric model”

In this note we address critical comments given by Hapke (in this issue) [1] on our paper “A critical assessment of the Hapke photometric model” (Shkuratov et al., 2012 [2]) elaborating further on weak points of this model. One of the main problems of the H-model is the too free juggling with formulas, leading, in particular, to violation of energy conservation. The model is filled with ungrounded approximations and unsupported conclusions. Some of the model assumptions are mutually contradictory, e.g., it considers anisotropic single-particle phase functions to describe incoherent multiple scattering and simultaneously uses the isotropic function in order to calculate coherent backscattering. Another well-known problem of the H-model is that some of the model parameters have similar effects on the reflectance, which makes it difficult to retrieve unique parameter values. The model uses empirical parameters and, hence, is empirical.

Comment on “A critical assessment of the Hapke photometric model” by Y. Shkuratov et al.

Criticism of theories and models is an important part of science and can lead to increased understanding and improvement of the models. Unfortunately, the paper by Shkuratov et al. [1], which severely criticizes a model for the reflectance of a particulate medium, does little to advance this goal. The paper contains a large number of misrepresentations and statements that are completely false. Important references that address the supposed errors are ignored. Many of the criticisms appear to arise from a profound misunderstanding of the model. The paper correctly criticizes the model for being approximate. However, the assumptions and approximations have been clearly identified and justified in the relevant papers, and it should be noted that all extant models for the reflectance of a close-packed particulate medium are approximate. One of the problems that the model shares with all physically-based models is that some of the parameters have similar effects on the reflectance, which sometimes makes it difficult to retrieve unique values. However, this arises from the properties of the physical processes involved in reflectance. Thus, it is a property of nature and does not mean that the model is wrong. This problem can often be overcome by judicious choice of parameter retrieval methods.

Evidence for spin alignment of spiral and elliptical/S0 galaxies in filaments

Galaxies are not distributed randomly in the cosmic web but are instead arranged in filaments and sheets surrounding cosmic voids. Observationally there is still no convincing evidence of a link between the properties of galaxies and their host structures. However, by the tidal torque theory (our understanding of the origin of galaxy angular momentum), such a link should exist. Using the presently largest spectroscopic galaxy redshift survey (SDSS) we study the connection between the spin axes of galaxies and the orientation of their host filaments. We use a three dimensional field of orientations to describe cosmic filaments. To restore the inclination angles of galaxies, we use a 3D photometric model of galaxies that gives these angles more accurately than traditional 2D models. We found evidence that the spin axes of bright spiral galaxies have a weak tendency to be aligned parallel to filaments. For elliptical/S0 galaxies, we have a statistically significant result that their spin axes are aligned preferentially perpendicular to the host filaments; we show that this signal practically does not depend on the accuracy of the estimated inclination angles for elliptical/S0 galaxies.

NEW PHOTOMETRIC STUDY OF THE INTERACTING BINARY STAR SYSTEM Y PISCIUM

We present a new photometric study of Y Piscium (Y Psc), which includes BVI light curves and a period analysis. With an orbital period of 3.77 days, this neglected system undergoes period changes described by an overall quadratic ephemeris with oscillating variability superimposed. Variations in the light curve and ephemeris curve, possibly resulting from changes in the accretion structure and mass transfer rate, suggest that Y Psc may be a direct-impact system like U Coronae Borealis and U Sagittae, which are known to exhibit variable accretion states. The 0.46 m modified Cassegrain telescope at the Kutztown University Observatory (Kutztown, PA) was used to obtain the new CCD photometry over 18 nights of observation between 2011 October 5 and 2012 January 15. We present a new photometric model that was determined using the computer program PHOEBE and suggest a reclassification of the spectral type of the primary star. An analysis of previously recorded times of minimum, in conjunction with our observations, was used to suggest possible physical mechanisms intrinsic to the system. These include magnetic activity (Applegate mechanism) and angular momentum transfer, while the possibility of a third body is ruled out. The results presented here demonstrate that Y Psc is an excellent candidate for a high-resolution spectroscopic study.

PHOTOMETRIC PROPERTIES FOR SELECTED ALGOL-TYPE BINARIES. V. V1241 TAURI

We present new photometry for the eclipsing binary V1241 Tau, which was obtained on six nights between 2011 December and 2012 January using the 85 cm telescope at the Xinglong station of the National Astronomical Observatories of China. By using the updated Wilson-Devinney code, photometric models with third lights were deduced from two sets of light curves. The result implies that V1241 Tau is an Algol-type near-contact binary (NCB), whose mass ratio and filling-out of the primary are q = 0.545 (± 0.003) and f 1 = 82.4% (± 0.2%), respectively. Based on all available times of minimum light spanning over 80 yr, the O – C curve of V1241 Tau appears to show a quasi-sinusoidal oscillation, i.e., a light-time orbit. The modulated period and amplitude are P mod = 47.4 (± 1.7) yr and A = 0.0087 (± 0.0005) days, respectively. This kind of period variation may be more likely attributed to the light-time effect via a presence of an unseen third body. From an analysis of 23 Algol-type NCBs with EB-type light curves, we determine that the fill-out for the primary f 1 will increase as the orbital period P decreases. With angular momentum loss, the orbit of the binary will shrink, which causes f 1 to increase. The primary component finally fills its Roche lobe, and the binary evolves into contact configuration. Therefore, this kind of Algol-type NCB with EB-type light curves, such as V1241 Tau, may be a progenitor of the A-type W UMa binary.

THE NEPTUNE-SIZED CIRCUMBINARY PLANET KEPLER-38b

We discuss the discovery and characterization of the circumbinary planet Kepler-38b. The stellar binary is single-lined, with a period of 18.8 days, and consists of a moderately evolved main-sequence star (M_A = 0.949 +/- 0.059 solar masses and R_A = 1.757 +/- 0.034 solar radii) paired with a low-mass star (M_B = 0.249 +/- 0.010 solar masses and R_B = 0.2724 +/- 0.0053 solar radii) in a mildly eccentric (e=0.103) orbit. A total of eight transits due to a circumbinary planet crossing the primary star were identified in the Kepler light curve (using Kepler Quarters 1 through 11), from which a planetary period of 105.595 +/- 0.053 days can be established. A photometric dynamical model fit to the radial velocity curve and Kepler light curve yields a planetary radius of 4.35 +/- 0.11 Earth radii, or 1.12 +/- 0.03 Neptune radii. Since the planet is not sufficiently massive to observably alter the orbit of the binary from Keplerian motion, we can only place an upper limit on the mass of the planet of 122 Earth masses (7.11 Neptune masses or 0.384 Jupiter masses) at 95% confidence. This upper limit should decrease as more Kepler data become available.