Secular Brightness Research Articles

The JCMT Transient Survey: Stochastic and Secular Variability of Protostars and Disks In the Submillimeter Region Observed over 18 Months

Abstract We analyze results from the first 18 months of monthly submillimeter monitoring of eight star-forming regions in the JCMT Transient Survey. In our search for stochastic variability in 1643 bright peaks, only the previously identified source, EC 53, shows behavior well above the expected measurement uncertainty. Another four sources—two disks and two protostars—show moderately enhanced standard deviations in brightness, as expected for stochastic variables. For the two protostars, this apparent variability is the result of single epochs that are much brighter than the mean. In our search for secular brightness variations that are linear in time, we measure the fractional brightness change per year for 150 bright peaks, 50 of which are protostellar. The ensemble distribution of slopes is well fit by a normal distribution with σ ∼ 0.023. Most sources are not rapidly brightening or fading at submillimeter wavelengths. Comparison against time-randomized realizations shows that the width of the distribution is dominated by the uncertainty in the individual brightness measurements of the sources. A toy model for secular variability reveals that an underlying Gaussian distribution of linear fractional brightness change σ = 0.005 would be unobservable in the present sample, whereas an underlying distribution with σ = 0.02 is ruled out. Five protostellar sources, 10% of the protostellar sample, are found to have robust secular measures deviating from a constant flux. The sensitivity to secular brightness variations will improve significantly with a sample over a longer time duration, with an improvement by factor of two expected by the conclusion of our 36 month survey.

Secular brightness variations of periodic comets as a result of desintegration of their icy-dust nuclei and the solar activity

Results are presented from studies of the features of secular brightness fading for 18 short-period comets. The results by Sekanina are supplemented by new data on periodic-comet apparitions after 1964. Annual and orbital-period variations in the absolute magnitude for each comet, as well as the corresponding average values are determined. A statistically significant effect of the 90-year solar activity cycle on secular variations in the integrated magnitudes of short-period comets is found. The corresponding correlation coefficient is 0.81.

Dynamo model with a small number of modes and magnetic activity of T Tauri stars

We suggest a model based on the representation of the stellar magnetic field as a superposition of a finite number of poloidal and toroidal free decay modes to describe the dynamo action in fully convective stars. For the adopted law of stellar differential rotation, we determined the dynamo number in exceeding which the generation of a cyclically varying magnetic field is possible in stars without a radiative core and derived an expression for the period of the cycle. The dynamo cycles in fully convective stars and in stars with thin convective envelopes are shown to differ qualitatively: first, the distributions of spots in latitude during the cycle are different for these two types of stars and, second, the model predicts a great weakening of the spot formation in fully convective stars at certain phases of the cycle. To compare the theory with observations, we have analyzed the historical light curve for the weak-line T Tauri star V410 Tau and found that its long-term activity is not a well-defined cycle with a definite period—its activity is more likely quasi-cyclic with a characteristic time of ∼4 yr and with a chaotic component superimposed. we have also concluded that a redistribution of spots in longitude is responsible for the secular brightness variations in the star. This does not allow the results of photometric observations to be directly compared with predictions of ourmodel, in which, for simplicity, we assumed a symmetry in longitude and investigated the temporal evolution of the spot distribution in latitude. Therefore, we discuss the questions of what and how observations can be compared with predictions of the dynamo theory.

Solar backscattered ultraviolet (SBUV) observations of polar mesospheric clouds (PMCs) over two solar cycles

Previous satellite measurements have provided nearly complete seasonal and geographic coverage of polar mesospheric clouds (PMCs), but previous data sets have not been able to evaluate changes in PMC behavior on decadal timescales. The Solar Backscattered Ultraviolet (SBUV) series of ozone measuring instruments have been flying continuously since 1978. While the instrument design is not optimized for PMC detection, the radiance data can be analyzed to examine the occurrence frequency and intensity of relatively bright PMCs. In this paper, we present PMC results from five SBUV and SBUV/2 instruments covering more than 23 years (1978–2002), starting just before the maximum of solar cycle 21 and extending through the maximum of solar cycle 23. The overlapping data sets from nearly identical instruments give an accurate picture of long‐term variations. Multiple linear regression fits are used to examine solar and secular correlations. PMC occurrence frequency is anticorrelated with solar Lyman alpha irradiance, with an approximate 0.5‐year phase lag in the Northern Hemisphere (Rsolar = −0.87) and no phase lag in the Southern Hemisphere (Rsolar = −0.65). The distribution of cloud brightness by season appears to be changing over time. When the PMC brightness for each season is characterized using an exponential cumulative distribution function, the exponent decreases in magnitude by a factor of 2 from 1978 to 2002 in the Southern Hemisphere (Rtime = +0.85). This implies an increase in the relative proportion of the brightest PMCs. The secular brightness trend is less significant in the Northern Hemisphere (Rtime = +0.58). We discuss possible origins for these changes.

The evolutionary status of eta Car based on historical and modern optical and infrared photometry

We present a photometric study of the luminous early-type star η Car. The star’s secular brightness increase is due largely to the expansion of the Homunculus. η Car is a LBV with a possible B0.5V companion. The combined effect of Balmer-line emission, and of warm and cold dust radiation explains the star’s other light variations.

Secular brightness variation of the dwarf nova Z Cha during quiescence ⋆

Les resultats de la photometrie optique sont presentes pour la nova naine Z Cha observee durant 2 intervalles quiescents. La courbe de lumiere de Z Cha est analysee et des estimations des flux de la naine blanche, du disque d'accretion, et du point brillant sont derivees. Les variations inter-sursauts de ces flux sont discutees

Secular brightness decrease of periodic comets

Mean rates of the decrease in secular brightness of periodic comets are determined in two different ways: (1) from the statistics on their maximum absolute magnitudes reached at individual apparitions, and (2) from the statistics on their capture into and ejection from the inner planetary system, assuming a state of equilibrium on the time scale of a century. The results of both independent approaches are in very good agreement, indicating mean fading rates of about +0.01 magnitude per revolution at perihelion distances q < 1.5 AU and +0.04 magnitude per revolution at q > 1.5 AU. This progressive decrease in brightness is overwhelmed by much larger irregular fluctuations, which can even change its sign for a number of revolutions of each individual object. Implications for the total active lifetimes of periodic comets are also discussed.

Laboratory investigation of the sublimation of comet nucleus models

The results of the laboratory investigations of different versions of comet ice nucleus model sublimation under the conditions simulating 0.5–3 A.U. heliocentric distances are proposed. The sublimation constants of possible comet ices are experimentally studies, the laws for production and destruction of the dust matrix (mineral crust) on the nucleus model surface are investigated, and possible ejection of icy particles from the model surface is shown. The mineral crust properties of the nucleus model surfaces are studied and possible velocity of these crust fragments ejections are determined. The results of the experiments are used to interpret the observed of real comet desintegration, including the Halley comet. The possible mechanisms of dust and ice particles ejection out of the comet nuclei are given. On the basis of the observed dependence between the secular brightness decrease of short-period comets and perihelion distancrs of their orbits, and of nucleus model experiment data the conclusion were made of overgrowing the nucleus surfaces of some short-period comets with a mineral crust, and their further evolution into asteroid-like bodies. The perspective of laboratory modelling of the processes occuring on the comet nuclei and asteroid surfaces is considered.

Estimates of the bolometric albedos and radiation balance of Uranus and Neptune

We have attempted to bound the wavelength-averaged phase integrals and bolometric albedos of Uranus and Neptune by fitting a wide range of aerosol model atmospheres to their observed geometric albedo spectra. These models are characterized by an upper haze layer of finite optical depth and a lower cloud layer of infinite optical depth at discrete altitudes. Alternative models differ in the assumed value of the particles' single scattering phase function and the wavelength dependence of the haze optical depth. Phase functions ranging from isotropic to those characteristic of particles in the atmospheres of Titan, Jupiter, and Saturn are considered. We have partially tested the models of Uranus by comparing the dependence of their disk-integrated brightness on phase angle with that derived from a combination of ground-based and Voyager 1 data that span phase angles from 0 to 85° and by comparing their predicted shapes of several H 2 quadrupole lines with observed shapes. Predictions of the Neptune models were compared with determinations of the planet's disk-integrated brightness from 0 to 48° phase angle. The derived model parameters lie within useful bounds. In the case of Uranus, the cloud pressure for all seven models considered falls between 2.2 and 2.5 bars, implying methane mixing ratios in the deeper portion of the atmosphere that are at least 30 times higher than expected from solar elemental abundances if the cloud is interpreted as being a methane condensation cloud. The range of haze pressure (≤0.5 bars) and optical depth (0.06 to 0.6 at a wavelength of 0.6435 μm) imply that haze aerosols are a significant absorber of sunlight and hence constitute a significant heating source in Uranus's upper troposphere and stratosphere. The haze aerosols absorb strongly at both short and long visible wavelenths, unlike the aerosols in Titan's atmosphere. Qualitatively similar conclusions apply to our model atmospheres of Neptune, with the cloud pressure being somewhat higher than for Uranus (2.7–3.2 bars) and the methane abundance being at least 60 times higher than expected from solar elemental abundances. At the current epoch, the wavelength-averaged phase integrals of Uranus and Neptune equal 1.26 ± 0.11 and 1.25 ± 0.1, respectively. The corresponding bolometric albedos are 0.343 ± 0.055 and 0.282 ± 0.044, respectively. When averaged over an orbital period, these albedos may be 7% lower for Uranus and altered little for Neptune, based on measurements of their secular brightness variability. Comparison of these results with thermal observations implies that the internal heat source for Uranus is less than 0.27 times the solar input (specific luminosity ≤1.6 × 10 −7 erg/g/sec), while this value for Neptune is (1.85 ± 0.56) times its solar input (specific luminosity = 3.4 ± 1.1) × 10 −7 erg/ g/ sec). These results imply that the meteorological regimes in the observable atmospheres of Uranus and Neptune may be very different, with internal heat flux playing a much more important role for Neptune than for Uranus.

On the relationship between secular brightness changes of Titan and solar variability

Titan’ geometric albedo varied noticeably from 1972 to 1978, in phase with variations in solar activity [Lockwood and Thompson, 1979]. We carry out a series of radiative transfer and aerosol formation calculations in order to demonstrate the feasibility of the following scenario for these secular brightness changes: solar activity changes, especially in the UV output of the Sun, result in alterations to the mass production rate of aerosols in Titan’s atmosphere, which lead to modifications of their microphysical properties. The latter, in turn, cause the albedo to vary. Current estimates of the change in the solar UV radiation below the dissociation limit of methane imply alterations to the mean radius of the aerosols over an 11‐yr solar cycle that are consistent in sign and magnitude with those required to explain the observed secular brightness changes.

Secular brightness increases of Titan, Uranus, and Neptune, 1972–1976

Abstract The brightnesses of Titan, Uranus, and Neptune in b (4718 AA) and y (5508 AA) have increased linearly since 1972 at rates ranging from 0.005 to 0.025 mag yr −1 . The observations were made differentially on a number of nights each season with respect to a network of comparison stars whose relative magnitudes were determined by independent measurements. Solar phase coefficients were derived for each object, and all observations have been normalized to zero solar phase angle and mean heliocentric distances. No explanation for the changes has been found, but a possible influence of solar activity upon planetary albedo is suggested by the fact that all of the objects observed have brightened during the declining half of the solar cycle.

The secular and orbital brightness variations of Titan, 1972-74

The secular and orbital brightness variations of Titan, 1972-74

On the Sizes of Cometary Nuclei

Three methods have been developed for determining the sizes of cometary nuclei. They are all based on the spotted model for an icy nucleus. The first method follows from the relationship between the icy (unshielded) area and the total absolute brightness, although the quantity determined is actually a function of the radius and the shielding coefficient. The second method is based on analysis of the secular brightness decrease and the assumption that it is possible to establish when a new surface layer of dust starts to form; in practice, this method is found to be applicable only in the cases of P/Pons-Winnecke and P/Tuttle. The third method is also based on the secular brightness decrease, and it requires some assumption about the volume fraction of dust in the nucleus.

ON THE SECULAR BRIGHTNESS DECREASE OF COMETS

ON THE SECULAR BRIGHTNESS DECREASE OF COMETS