Significant Brightness Variations Research Articles

Mars Global Surveyor's Mars Orbiter Camera (MOC) Wide‐Angle Images (1999–2006): 2. Data Investigation Into North Polar Hood Formation, Broad Brightness Changes in Acidalia, and Seasonal Frost in Hellas

AbstractMars Orbiter Camera Wide Angle images were taken from late 1999 through most of 2006, in both red and blue, providing nearly daily, nearly global coverage of Mars for approximately four Mars Years. Significant work has been published from those data, but there did not exist in a public archive a uniform, full time series of the processed data for easier analysis. The companion work to this (Robbins, 2023a, https://doi.org/10.1029/2022EA002673) presents that processing and quality control of the data, producing the full time series in both colors, in equirectangular and polar stereographic projections, in mosaics that have been binned into four different time intervals (daily, ΔLs = 2°, 5°, and 10° [≈4 through 19 days]) and as approximately true‐color composites. The multidimensional data set can be mined for surface and atmospheric variations on Mars. To demonstrate agreement with past work, several well‐known trends are demonstrated in a review‐style narrative format in Supporting Information S1, including changes after a global dust storm, seasonal frost variations at the poles, and surface variations. The data are analyzed further in the main text to demonstrate new qualitative and quantitative analyses of atmospheric and surface features: the North Polar Hood can form earlier than previously demonstrated, Acidalia has significant seasonal brightness variations that can be quantified, and the seasonal frost deposition and sublimation of Hellas can be quantified and shown to vary from year‐to‐year. These different investigations represent a small slice of what can be done now that these data are made available and easily accessible.

Cyclic variations with twice the accretion disk precession period in the old nova V603 Aquilae

A dense series of long and high time resolution light curves of the old nova V603 Aql, covering 22 nights (19 of which are consecutive), are analyzed in order to identify and characterize variations on the time scale of hours and days. The well known 3.5 h modulation, observed many times in the past and considered to be due to a long lasting, albeit not entirely stable superhump, is recovered at a period of 0.1453 days and an amplitude of ∼ 0.m062. Most interesting, however, is the detection of highly significant brightness variations with an amplitude of ∼ 0.m050 and a period of 5.85 days which is to a very high precision equal to twice the beat period between the orbital and the superhump period. The latter is generally interpreted as the precession period of an eccentric accretion disk. The origin of these long term variations remains unknown.

Spontaneous flux concentrations from the negative effective magnetic pressure instability beneath a radiative stellar surface

Context. The formation of sunspots requires the concentration of magnetic flux near the surface. The negative effective magnetic pressure instability (NEMPI) might be a possible mechanism for accomplishing this, but it has mainly been studied in simple systems using an isothermal equation of state without a natural free surface. Aims. We study NEMPI in a stratified Cartesian mean-field model where turbulence effects are parameterized. We use an ideal equation of state and include radiation transport, which establishes selfconsistently a free surface. Methods. We use a Kramers-type opacity with adjustable exponents chosen such that the deeper layers are approximately isentropic. No convection is therefore possible in this model, allowing us to study NEMPI with radiation in isolation. We restrict ourselves to two-dimensional models. We use artificially enhanced mean-field coefficients to allow NEMPI to develop, thereby making it possible to study the reason why it is much harder to excite in the presence of radiation. Results. NEMPI yields moderately strong magnetic flux concentrations a certain distance beneath the surface where the optical depth is unity. The instability is oscillatory and in the form of upward traveling waves. This seems to be a new effect that has not been found in earlier models without radiative transport. The horizontal wavelength is about ten times smaller than what has previously been found in more idealized isothermal models. Conclusions. In our models, NEMPI saturates at field strengths too low to explain sunspots. Furthermore, the structures appear too narrow and too far beneath the surface to cause significant brightness variations at the radiative surface. We speculate that the failure to reproduce effects resembling sunspots may be related to the neglect of convection.

The Hubble Catalog of Variables (HCV)

AbstractThe Hubble Source Catalog (HSC) combines lists of sources detected on images obtained with the WFPC2, ACS and WFC3 instruments aboard the Hubble Space Telescope (HST) and now available in the Hubble Legacy Archive. The catalogue contains time-domain information for about two million of its sources detected using the same instrument and filter on at least five HST visits. The Hubble Catalog of Variables (HCV) aims to identify HSC sources showing significant brightness variations. A magnitude-dependent threshold in the median absolute deviation of photometric measurements (an outlier-resistant measure of light-curve scatter) is adopted as the variability detection statistic. It is supplemented with a cut in χred2 that removes sources with large photometric errors. A pre-processing procedure involving bad image identification, outlier rejection and computation of local magnitude zero-point corrections is applied to the HSC light-curves before computing the variability detection statistics. About 52 000 HSC sources have been identified as candidate variables, among which 7,800 show variability in more than one filter. Visual inspection suggests that ∼70% of the candidates detected in multiple filters are true variables, while the remaining ∼30% are sources with aperture photometry corrupted by blending, imaging artefacts or image processing anomalies. The candidate variables have AB magnitudes in the range 15–27m, with a median of 22m. Among them are the stars in our own and nearby galaxies, and active galactic nuclei.

HOT THERMAL X-RAY EMISSION FROM THE Be STAR HD 119682

We present an analysis of a series of four consecutive Chandra high resolution transmission gratings observations, amounting to a total of 150 ks, of the Be X-ray source HD119682 (= 1WGA J1346.5-6255), a member of the new class of gamma Cas analogs. The Chandra lightcurve shows significant brightness variations on timescales of hours. However, the spectral distribution appears rather stable within each observation and during the whole campaign. A detailed analysis is not able to detect any coherent pulsation up to a frequency of 0.05 Hz. The Chandra HETG spectrum seems to be devoid of any strong emission line, including Fe alpha fluorescence. The continuum is well described with the addition of two collisionally ionized plasmas of temperatures kT = 15 keV and 0.2 keV respectively, by the apec model. Models using photoionized plasma components (mekal) or non thermal components (powerlaw) give poorer fits, giving support to the pure thermal scenario. These two components are absorbed by a single column with N_H=(0.20^+0.15-0.03) x 10^22 cm^-2 compatible with the interstellar value. We conclude that HD119682 can be regarded as a pole-on gamma Cas analog.

Optical photometric and spectral study of the new FU Orionis object V2493 Cygni (HBC 722)

We present new results from optical photometric and spectroscopic observations of the eruptive pre-main sequence star V2493 Cyg (HBC 722). The object has continued to undergo significant brightness variations over the past few months and is an ideal target for follow-up observations. We carried out CCD BVRI photometric observations in the field of V2493 Cyg ("Gulf of Mexico") from August 1994 to April 2012, i.e. at the pre-outburst states and during the phases of the outburst. We acquired high, medium, and low resolution spectroscopy of V2493 Cyg during the outburst. To study the pre-outburst variability of the target and construct its historical light curve, we searched for archival observations in photographic plate collections. Both CCD and photographic observations were analyzed using 15 comparison stars in the field of V2493 Cyg. The pre-outburst photographic and CCD photometric observations of V2493 Cyg show low-amplitude light variations typical of T Tauri stars. The recent photometric data show a slow light decrease from October 2010 to June 2011 followed by an increase in brightness that continued until early 2012. The spectral observations of V2493 Cyg are typical of FU Orionis stars absorption spectra with strong P Cyg profiles of H alpha and Na I D lines. On the basis of photometric monitoring performed over the past two years, the spectral properties at the maximal light, as well as the shape of long-term light curves, we confirm that the observed outburst of V2493 Cyg is of FU Orionis type.

Relating white light and in situ observations of coronal mass ejections: A review

This paper provides a short review of some of the basic concepts related to the observations of coronal mass ejections (CMEs) in white light images and at large distances from the Sun. We review the various ideas which have been put forward to explain the dramatic changes in CME appearance in white light images from the Sun to 1 AU, focusing on results obtained by comparing white light observations of CMEs to the in situ measurements of Interplanetary CMEs (or ICMEs). We start with a list of definitions for the various in situ structures that form an ICME. A few representative examples of the formation of sheath regions and other interaction regions as well as the expansion of magnetic flux ropes are used to illustrate the basic phenomena which induce significant brightness variations during a CME's propagation to 1 AU and beyond. The white light signatures of a number of CMEs observed by the coronagraphs have been successfully simulated numerically by assuming that most of the coronal plasma observed in white light images is located on the surface of a croissant-shaped structure reminiscent of a magnetic flux rope. At large distances from the Sun, white light imagers show that the appearance of CMEs changes dramatically due to the changing position of the CME relative to the Thomson sphere, the expansion of the ejecta and the interaction of the ejecta with the ambient solar wind.

TOWARD UNDERSTANDING THE B[e] PHENOMENON. III. PROPERTIES OF THE OPTICAL COUNTERPART OF IRAS 00470+6429

FS CMa type stars are a group of Galactic objects with the B[e] phenomenon. They exhibit strong emission-line spectra and infrared excesses, which are most likely due to recently formed circumstellar dust. The group content and identification criteria were described in the first two papers of the series. In this paper we report our spectroscopic and photometric observations of the optical counterpart of IRAS 00470+6429 obtained in 2003--2008. The optical spectrum is dominated by emission lines, most of which have P Cyg type profiles. We detected significant brightness variations, which may include a regular component, and variable spectral line profiles in both shape and position. The presence of a weak Li {\sc I} 6708 \AA line in the spectrum suggests that the object is most likely a binary system with a B2--B3 spectral type primary companion of a luminosity $\log$ L/L$\odot$ = 3.9$\pm$0.3 and a late-type secondary companion. We estimate a distance toward the object to be 2.0$\pm$0.3 kpc from the Sun.

Five New Multicomponent Planetary Systems

We report Doppler measurements for six nearby G- and K-type main-sequence stars that show multiple low-mass companions, at least one of which has planetary mass. One system has three planets, the fourth triple-planet system known around a normal star, and another has an extremely low minimum mass of 18 M_⊕. HD 128311 (K0 V) has two planets (one previously known) with minimum masses (M sin i) of 2.18M_J and 3.21M_J and orbital periods of 1.26 and 2.54 yr, suggesting a possible 2 : 1 resonance. For HD 108874 (G5 V), the velocities reveal two planets (one previously known) having minimum masses and periods of (M sin i_b = 1.36M_J, P_b = 1.08 yr) and (M sin i_c = 1.02M_J, P_c = 4.4 yr). HD 50499 (G1 V) has a planet with P = 6.8 yr and M sin i = 1.7M_J, and the velocity residuals exhibit a trend of -4.8 m s^(-1) yr^(-1), indicating a more distant companion with P > 10 yr and minimum mass of 2M_J. HD 37124 (G4 IV-V) has three planets, one having M sin i = 0.61M_J and P = 154.5 days, as previously known. We find two plausible triple-planet models that fit the data, both having a second planet near P = 840 days, with the more likely model having its third planet in a 6 yr orbit and the other one in a 29 day orbit. For HD 190360, we confirm the planet having P = 7.9 yr and M sin i = 1.5M_J as found by the Geneva team, but we find a distinctly noncircular orbit with e = 0.36 ± 0.03, rendering this not an analog of as had been reported. Our velocities also reveal a second planet with P = 17.1 days and M sin i = 18.1 M_⊕. HD 217107 (G8 IV) has a previously known hot Jupiter with M sin i = 1.4M_J and P = 7.13 days, and we confirm its high eccentricity, e = 0.13. The velocity residuals reveal an outer companion in an eccentric orbit, having minimum mass of M sin i > 2M_J, eccentricity e ~ 0.5, and a period P > 8 yr, implying a semimajor axis α > 4 AU and providing an opportunity for direct detection. We have obtained high-precision photometry of five of the six planetary host stars with three of the automated telescopes at Fairborn Observatory. We can rule out significant brightness variations in phase with the radial velocities in most cases, thus supporting planetary reflex motion as the cause of the velocity variations. Transits are ruled out to very shallow limits for HD 217107 and are also shown to be unlikely for the prospective inner planets of the HD 37124 and HD 108874 systems. HD 128311 is photometrically variable with an amplitude of 0.03 mag and a period of 11.53 days, which is much shorter than the orbital periods of its two planetary companions. This rotation period explains the origin of periodic velocity residuals to the two-planet model of this star. All of the planetary systems here would be further constrained with astrometry by the Space Interferometry Mission.

Variability of stars in the Pulkovo spectrophotometric catalog

We present the results of a statistical study of brightness variability for 693 stars of the Pulkovo spectrophotometric database in five spectral bands in the range λλ 320–1080 nm. Significant brightness variations were detected in at least one spectral band against the background of the random noise for one-third of the stars not earlier believed to be variable. A comparison of the distributions of these variations in amplitude and spectral band for the normal and variable stars shows that variability is inherent to most stars to some extent and is often wavelength dependent.

Rotation and cometary activity of KBO (29981) 1999 TD 10

We describe observations of the scattered Kuiper Belt object (29981) 1999 TD 10 performed during five observing runs at two observatories, over 370 days from 2000 September to 2001 September. They show significant brightness variations that fit a double-peaked lightcurve with period 15.448±0.012 h in V and R bands. The phase effect in V band, 0.09±0.01 mag deg −1, is smaller than that of Pluto but larger than that of several KBOs, while in R band it is 0.030±0.005 mag deg −1. We find color variation between the two bands, which implies a non-homogeneous albedo distribution on the surface. Evidence of surface activity near perihelion in the form of a coma/tail is presented using radial image profiles and a 2D contour map.

On the detectability of lightcurves of Kuiper belt objects

We present a statistical study of the detectability of lightcurves of Kuiper belt objects (KBOs). Some Kuiper belt objects display lightcurves that appear “flat”; i.e., there are no significant brightness variations within the photometric uncertainties. Under the assumption that KBO lightcurves are mainly due to shape, the lack of brightness variations may be due to (1) the objects having very nearly spherical shapes or (2) their rotation axes coinciding with the line of sight. We investigate the relative importance of these two effects and relate it to the observed fraction of “flat” lightcurves. This study suggests that the fraction of KBOs with detectable brightness variations may provide clues about the shape distribution of these objects. Although the current database of rotational properties of KBOs is still insufficient to draw any statistically meaningful conclusions, we expect that, with a larger dataset, this method will provide a useful test for candidate KBO shape distributions.

High Angular Resolution Observations of Late‐Type Stars

This paper presents speckle observations of Mira (o Cet) and late-type stars with the PISCO speckle camera of Pic du Midi during the period 1995-1998. A survey for binarity among a sample of late-type stars was performed, which led to seven positive detections out of 36 objects. Photometric and color variations of the companion of Mira were searched for, but no significant brightness variations could be found over a timescale of ∼5-10 minutes. The position and photometry measurements, the restored images with high angular resolution of the binary system Mira A-B (ADS 1778) are in full agreement with Hubble Space Telescope data obtained at the same epoch. A new orbit has been derived for Mira A-B.

EVIDENCE ON HOT SPOT IN CONTACT BINARY VW CEPHEI

We study the nature of asymmetry and the intrinsic variability in the light curves of VW Cep. We analyze our own B,V light curves as well as other data from published sources. In view of the presence of significant intrinsic brightness variations at a level of 0 m :01-0 m :03 on time scales comparable to the or- bital period we deal only with individuallight curves sampled in one-two consecutive orbital cycles. The manifold evidence for the presence of a small hot spot region close to the neck con- necting both components is summarized: a) displacements of the brightness maxima from the predicted epochs of elongations suggestive of an additional energy input presumably of hot chro- mospheric origin, b) the overall pattern of asymmetry in bright- ness maxima and minima, c) systematic colour changes with the orbital phase, d) the presence of significant cosine odd harmon- ics in truncated series of the observed light curves. We find that a hot spot with a characteristic size of R 0:7-1:2 10 10 cm and the temperature contrast T=T =1 :3-1:4 located on the sur- face of the more massive star can explain the afore-mentioned peculiarities and model light curves based on our model give a rather good fit to the observed data studied so far. The possible physical nature of the hot spot in the light of our results com- pared with the spectroscopic data (specifically Mg II resonance doublet) and flare activity signatures are briefly discussed.

Imaging Observations of Asteroids with Hubble Space Telescope

We present the results of two Hubble Space Telescope (HST) observing programs, consisting of 11 imaging observations of 10 asteroids. The primary focus of the projects was to search for faint companions (satellites) of these asteroids. No binary systems were detected. More specifically, no companions were found at more than 0.1 arcsec separation of brightness down to 4 magnitudes fainter than the primary asteroid. No companions down to 6 magnitudes fainter than the primary asteroid were found at more than 0.5 arcsec separation. It is unlikely that companions more than 6 magnitudes fainter than a well-exposed primary would be detected. These nondetections place very stringent limits on the existence of companion bodies for these asteroids. A secondary goal was to resolve the illuminated portion of the asteroids. Nine of the 10 asteroids were marginally resolved, with three of these—9 Metis, 18 Melpomene, and 19 Fortuna—showing significant extension and brightness variations up to a factor of two across the illuminated portion of the restored image. The diameters of the resolved asteroids are generally in good agreement with those in the TRIAD II file (E. F. Tedesco, 1989, InAsteroids II(Binzel, Gehrels, and Matthews, Eds.)). Diameters for 19 Fortuna and 624 Hektor (which are not in the TRIAD file) have been measured: 225 and 370×195 km, respectively.

Coordinated observations of the red dwarf flare star EV LAC in 1994 and 1995

Abstract The results of photometric, spectral and radio studies of the flare star EV Lac we obtained during the course of cooperative observations in 1994 and 1995 are presented. A quantitative analysis of the radiation emitted by two powerful flares using the colour-colour diagram confirms the previous conclusion on the essential heterogeneity of matter radiating in optical flares. From simultaneous observations of the star in UBVRI, K and H bands no significant brightness variations in IR were found in coincidence with observed small-amplitude optical flares, except a gradual decrease in the K-band following a 1.0 mag flare in the U-band. A differential spottedness of bright and dark hemispheres of EV Lac is estimated using observations in separate seasons and the total stellar spottedness is determined within the framework of the zonal spottedness model. Spectra of quiet and active states of the star in the blue-green region in 1994 and in the red region in 1995 are described: the characteristics of the quiet chromosphere in the Hα, Hβ, HΔ lines, their widening during flares, the strengthening of neutral helium lines and the appearance of emission lines of He II and metals are presented. The monitoring of EV Lac in decametric wavelengths with the largest radio telescope UTR-2 led to the detection of 18 radio bursts, one of which satisfies the majority of criteria of signals of non-terrestrial origin and coincides in time with an optical flare.

Hubble Space TelescopeObservations of the Gravitationally Lensed Cloverleaf Broad Absorption Line QSO H1413+1143: Imaging

An analysis of Hubble Space Telescope (HST) Wide Field Planetary Camera (WFPC) and Wide Field Planetary Camera 2 (WFPC2) images of the gravitationally lensed Cloverleaf broad absorption line quasi-stellar object (QSO) H1413+1143 is presented. Astrometric and photometric measurements are derived for the four components of the lensed QSO for five different epochs over a baseline of 2.76 yr. Because of the replacement of WFPC with WFPC2 and the change in the purpose of the observation at the various epochs, the data were not always taken with the same filter. With the exception of the declination of component D, the relative positions of the four components are measured to within ≈ 3 mas; these results are consistent with but considerably more accurate than earlier measurements. The relative photometric measurements at any one epoch are typically accurate to ≈ 0.02-0.03 mag (1 σ). The initial HST WFPC images cover a baseline of 1.26 yr (1992.21-1993.47 [1992 March 16-1993 June 22]), and over this time interval there is little evidence for brightness variations of any of the components relative to one another at levels >0.06 mag (>2 σ). Photometric measurements of the more accurate WFPC2 data obtained with different filters extends this baseline an additional 1.50 yr (to 1994.97 [1994 December 22]). The WFPC2 data also fail to reveal significant brightness variations among the components. In addition, the WFPC2 data include both UV (F336W) and near-infrared (F814W) images. These color data indicate the presence of sight-line-dependent extinction, causing the F336W-F814W color index of component B (the most reddened component) to be 0.56 ± 0.04 mag redder than that of component C (the least reddened component). The lack of evidence for significant component brightness variations at all HST observation epochs suggests that the data could be reliably extinction-corrected to derive the relative amplifications of the four image components. This is done for several reasonable dust-extinction models. Thus, the derived astrometry along with the photometric analysis set clear constraints on models for the Cloverleaf. Since component D shows some evidence for microlensing, the results on its relative amplification should be used with caution. While existing models can successfully reproduce the relative positions, the relative amplifications have not yet been successfully modeled. The WFPC2 imaging data has also permitted a sensitive search for component structure and the gravitational lens itself. There is marginal evidence for elongated structure between components A and C that may be part of an Einstein ring. However, no significant evidence for the lensing object is found. The various measurements are quantified in ways useful for setting model constraints. Limits on the mass-to-light ratio and detectability of the lensing galaxy are also discussed.

Temperature and abundances in the Jovian auroral stratosphere: 1. Ethane as a probe of the millibar region

We report infrared heterodyne spectroscopy (λ/Δλ ∼ 106) of C2H6 emission at 11.9 μm from the northern Jovian auroral region, in observations conducted over December 2–7, 1989. Accurately measured line shapes provide information on C2H6 abundance as well as temperature and permit retrieval of the source pressure region. Enhanced emission was observed in the longitude range ∼150°–180° at ∼60° north latitude, approximately corresponding to the CH4 7.8‐μm hot spot and the region of brightest UV aurora. Significant brightness variations were observed in the hot spot emissions on a time scale of ∼20 hours. Analysis of the brightest hot spot spectra indicates C2H6 mole fractions of ∼(6.3–6.8) × 10−6 at temperatures of ∼182–184 K at 1 mbar, compared to mole fractions of (3.8 ± 1.4) × 10−6 averaged over spectra outside the hot spot at a temperature of ∼172 K at the same pressure. Fixing the mole fraction to the lower limit retrieved in the quiescent (non‐hot spot) region allows the temperature at 1 mbar to be as high as ∼200 K within the hot spot. These results provide upper limits to the temperature increase near the source of the C2H6 thermal infrared emission. Combined with results from similar measurements of ethylene emission probing the ∼10‐μbar region (Kostiuk et al., this issue), altitude information on the thermal structure of the Jovian auroral stratosphere can be obtained for the first time.

E1013-477: AN INTERMEDIATE POLAR

ABSTRACT Four nights of CCD white-light and one night of B and V observations of E1013-477 reveal significant brightness variations with three periods: 4.9 hrs, 68.1 min, and 88.8 min. Photometric behavior of the system and its known spectral characteristics are typical for an intermediate polar.