Decrease In Luminosity Research Articles

The faint young sun climatic paradox: A simulation with an interactive seasonal climate-sea ice model

A seasonal one-and-a-half-dimensional (1 1 2 -D) energy-balance climate model including a detailed sea ice calculation and an interactive albedo formulation has been developed and is used to investigate the faint young sun climatic paradox. This model is shown to reproduce the present climate and sea ice observations. In spite of its greater complexity, its behavior is globally similar to simple energy-balance models with highly parameterized ice-albedo feedback used in previous studies of this question. It is found that when the solar luminosity drops by more than about 5% below its present value, the ice albedo feedback causes a global irreversible glaciation. Several sensitivity experiments show that the value of the critical solar constant and associated global surface temperature are only little sensitive to the set of model parameters describing the ice and snow albedo and meridional heat transport. In contrast, the absence or polar location of the continental mass introduce a nearly 10% decrease of the critical luminosity. The minimum level of atmospheric CO 2 needed to prevent a global glaciation through enhanced greenhouse warming is calculated as a function of the solar luminosity. A 30% drop in solar output requires a 2 × 10 4-fold increase in atmospheric CO 2, an unacceptably large value. However, in the absence of continents, a carbon dioxide partial pressure of 2000 times the present level is found to be sufficient to stabilize the climate. The effects of a reduced continental area, paleogeographic changes and higher CO 2 greenhouse effect combine to ensure a larger stability of the non-frozen configuration. Their cumulated and interactive effects may be able to solve the young sun paradox.

Force-free magnetic filaments and the cosmic background radiation

Dense, force-free magnetic filaments generated by quasars, active galactic nuclei, and Herbig-Haro objects can remain stable in the intergalactic medium for many Gy. They will be opaque to radiation with wavelengths longer than 100-400 mu m, and essentially transparent to shorter wavelengths. They are thus capable of thermalizing and isotropizing the cosmic background radiation, and of accounting for the observed decrease of radio luminosity of galaxies within increasing distance from the Earth.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Intensification and fading of auroral arcs in the dusk‐midnight sector of the polar cap

Observations of the aurora from South Pole station (magnetic latitude = −74.2°) have been used to study the intensification and fading of polar arcs observed near the dusk meridian. Most of the cases examined have the following features in common: (1) a preexisting auroral form intensifies for about 10 min; (2) this activation is followed by a pronounced decrease of luminosity; (3) the auroral fade terminates after 30–60 min with the onset of intense aurora which sweeps rapidly overhead. The brief intensifications involve O (630.0 nm) enhancements of 0.1–0.3 kR and N2+ (427.8 nm) enhancements of 0.3–1.5 kR giving an emission ratio which in each case is consistent with the precipitation of electrons with an average energy of 3–12 keV, assuming a Maxwellian distribution. The auroral brightening is not accompanied by riometer absorption. Ground magnetic perturbations of the order 20–40 nT occur, and in about half the instances, weak broadband impulsive hiss is present. The availability of all‐sky camera, auroral electrojet (AE) index and interplanetary magnetic field (IMF) data for some of the cases enables the following additional characterizations of these events. The preexisting form is a Sun‐ or oval‐aligned arc (or part of a multiple arc system) which disappears following the activation; equatorward drift of the arc (or system) accompanies the luminosity change. There is some evidence to suggest that the arc is poleward of the auroral oval. The brief intensification and/or the onset of fading occurs during the growth phase or near the start of the expansive phase of a substorm; termination of the fade is near the maximum in AE and is probably indicative of the beginning of the recovery phase of the substorm. For all three cases for which IMF data were available the onset of fading occurred 20–30 min after Bz turned southward. Two of these cases were preceded by auroral arc enhancement when Bx was sunward; in the third case there was no prior enhancement when Bx was zero or slightly antisunward. By was negative (dawnward) in each case. Sun‐aligned arcs are a common feature of the polar cap during northward Bz but disappear during the increasingly disturbed conditions that accompany southward Bz. The present results suggest that brief intensifications of southern hemisphere polar cap arcs near dusk may be linked in part to the sunward orientation of the IMF which favors enhanced electron fluxes in the southern lobe of the magnetotail.

The optical counterpart of the radio source close to the Seyfert 2 nucleus of NGC 5953 = ARP 91 B

view Abstract Citations (13) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Optical Counterpart of the Radio Source Close to the Seyfert 2 Nucleus of NGC 5953 = ARP 91 B Rafanelli, P. ; Osterbrock, D. E. ; Pogge, R. W. Abstract Spectroscopic and morphological observations of the nuclear region of the Seyfert 2 galaxy NGC 5953=Arp 91 B are presented. It is shown that an extended emitting region, with features typical of a supergiant H II region, is located to the west of the nucleus within a radio structure which is emitting as a nonthermal source with spectral index α=0.6 in the 1.4-5.0 GHz range. The faintness of the [0 III] λλ4959, 5007 emission lines in the spectrum of this region is discussed in terms of a state of low ionization, and high abundances of heavy elements in the emitting gas. In addition, the possibility that the ionization is produced by the Seyfert 2 nucleus and that its luminosity has undergone an abrupt decrease in luminosity has been taken into account. The nonthermal radio emission is tentatively interpreted as the result of frequent supernova blasts and the consistency of this interpretation with the optical data is discussed. Another extended emitting region, located close to the nucleus to its east, in a position symmetrically opposite to the western region, is revealed by our spectra. Its line ratios and fluxes are typical of a very luminous giant H II region. A bright stellar "knot," located 3" west of the nucleus, just within the emitting region associated with the extended radio source, is a foreground star of spectral type F8-G2 and magnitude V~16. Publication: The Astronomical Journal Pub Date: January 1990 DOI: 10.1086/115312 Bibcode: 1990AJ.....99...53R Keywords: Extragalactic Radio Sources; Galactic Nuclei; H Ii Regions; Radio Spectra; Seyfert Galaxies; Stellar Spectra; Emission Spectra; Infrared Astronomy Satellite; Line Spectra; Morphology; Astrophysics; GALAXIES: SEYFERT full text sources ADS | data products NED (3) SIMBAD (2)

Electromagnetic Processes in Relativistic Heavy Ion Collisions

Electromagnetic processes in peripheral nucleus-nucleus collisions are reviewed. First I discuss applications to radiative capture processes of astrophysical interest, the use of Coulomb-dissociation for the study of the structure of exotic secondary beams, and multiphoton excitations of multiphonon giant resonances and giant resonances coupled to low lying surface modes. In the second part, electromagnetic effects at relativistic heavy ions colliders are discussed, they are a serious source of loss for beam particles, which leads to a decrease of luminosity with time. Finally, the possibility to use ultra-relativistic heavy ion collision for fundamental studies in γ-γ physics is explored. The essential advantage over γ-γ-physics at e+ e-colliders is, due to the coherence effect, the Z4-factor (⩽ 108) in the cross section.

A comparison of ionospheric conductances and auroral luminosities observed simultaneously with the Chatanika Radar and the DE 1 auroral imagers

Auroral luminosities at vacuum ultraviolet (VUV) wavelengths are combined with simultaneous and coincident ionospheric electron density measurements made by the Chatanika radar to relate ionospheric conductances to optical emissions. The auroral luminosities are obtained along the magnetic meridian through Chatanika with the auroral imaging photometers on the Dynamics Explorer 1 (DE 1) satellite as the radar scans in the magnetic meridian to measure electron density and conductivity as a function of altitude and latitude. The observations are used to determine an empirical relationship between the luminosities measured at VUV wavelengths and the Hall and Pedersen conductances. Of particular interest is the response of the photometer when using the VUV filter designated 123W. This filter admits the 130.4‐ and 135.6‐nm emissions of atomic oxygen and the Lyman‐Birge‐Hopfield (LBH) bands of N2. Model calculations of the LBH and O I (135.6 nm) contributions to the total measured luminosity indicate that the relation between 123W luminosity and Pedersen conductance is less sensitive to the average energy of the precipitating electrons than the corresponding relation between the Hall conductance and 123W luminosity. This is because both the luminosity and Pedersen conductance decrease with increasing electron energy. The luminosity decreases with increasing energy because the emissions are more strongly absorbed by O2 above the region of production. The Pedersen conductance decreases with increasing energy because the Pedersen mobility maximizes at an altitude of about 140 km. In contrast, the Hall conductance increases with increasing electron energy, so that the relation between Hall conductance and luminosity depends on the hardness of the precipitation. These qualitative results are confirmed using the simultaneously and coincidentally obtained measurements of conductance and 123W luminosity. The absolute difference between the model calculations and the experimental results indicates that the 130.4‐nm emissions account for 50–65% of the total photometer response with the 123W filter.

Pangaean climates: Megamonsoons of the megacontinent

The climate for a highly idealized geographic representation of the megacontinent of Pangaea, corresponding to the period 250–200 m.y. ago, is explored using a low‐resolution atmospheric general circulation model that is coupled to a static mixed layer ocean of constant depth, with a provision for sea‐ice formation and decay. Soil moisture is included as a predicted variable on the continent. Because there are large uncertainties about key factors that might influence the Pangaean climate, sensitivity experiments are used to examine the climate for two different mean elevations (0 and 1000 m), two different snowcover prescriptions, and two different levels of implied greenhouse heating and solar luminosity (modern CO2 concentration and modern solar luminosity; fivefold increase in modern CO2 concentration and a 1% decrease in modern solar luminosity). In all of the experiments, the large bipolar land masses of Laurasia and Gondwanaland, joined near the equator in the west and separated by the Tethys Sea in the east, exhibit extreme continentality, with hot summers, cold winters, and large‐scale summer and winter monsoon circulations. Year‐round or seasonal aridity is simulated for all but the eastern coastal regions, the tropical western coast, and the regions poleward of 40° latitude. The simulated climate of the idealized Pangaean continent has many features that correspond qualitatively with geologic evidence for tropical warmth and aridity in the late Permian and Triassic. However, the model results serve mainly to indicate the importance of various climate‐forming factors (geography, topography, heating). Detailed comparisons will require new experiments, with better models and more accurate geologic boundary conditions.

Models of steady state cooling flows in elliptical galaxies

A comprehensive set of steady state models for spherically symmetric cooling flows in early-type galaxies is presented. It is found that a reduction of the supernova (SN) rate in ellipticals produces a decrease in the X-ray luminosity of galactic cooling flows and a steepening of the surface brightness profile. The mean X-ray temperature of the cooling flow is not affected noticeably by a change in the SN rate. The external pressure around a galaxy does not markedly change the luminosity of the gas within the galaxy but does change the mean temperature of the gas. The presence of a dark matter halo in a galaxy only changes the mean X-ray temperature slightly. The addition of a distribution of mass sinks which remove material from the general accretion flow reduces L(X) very slightly, flattens the surface brightness profile, and reduces the central surface brightness level to values close to those actually observed. A reduction in the stellar mass-loss rate only slightly reduces the X-ray luminosity of the cooling flow and flattens the surface brightness by a small amount.

The 1 Mpc giant head-tail radio galaxy IC 711: 3-dimensional simulation and additional constraints on the ram pressure theory

Recent observations of the longest known head-tail galaxy IC 711, at long wavelengths have shown it to extend farther than previously known. At δ 74 cm, Vallee and Roger (1987) found it to stretch 17′ (930 kpc) behind the optical nucleus of this elliptical galaxy. At δ92 cm, Vallee and Strom (1988) found it to stretch 18.3′ (1 Mpc) behind the optical nucleus. What new physics can be learned from it? To that end, a theoretical simulation in 3 dimensions of the longest head-tail galaxy has been performed, building the radio trail cell by cell inside a volume of space composed of 8 million cells, with each cell being assigned a proper radio emissivity. Matching ram pressure theory and recent observations yields a primary fast decrease of the radio luminosity with time or distance from the optical nucleus of the galaxy, and a secondary periodic variation of the radio luminosity every 500 kpc. Near the end of the tail, a sudden and unpredicted flare-up of the observed width of the tail requires a sudden change in the jet's physical conditions about 2 Gyr earlier, or that of the surroundings.

Optical color, polarimetric, and timing measurements of the 50 MS Large Magellanic Cloud pulsar, PSR 0540-69

view Abstract Citations (67) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optical Color, Polarimetric, and Timing Measurements of the 50 Millisecond Large Magellanic Cloud Pulsar, PSR 0540-69 Middleditch, J. ; Pennypacker, C. R. ; Burns, M. S. Abstract The 50-ms LMC pulsar, PSR 0540-69, is similar to the Crab pulsar in its negative (U-B)0 color of -0.83(0.37) mag (error), but is marginally redder than the Crab from B through R, (B-R)0 = 0.57(0.30). The optical pulse of PSR 0540-69 averages less than 15 percent linear polarization when resolved at 0.05 cycles (= 2.5 ms). An epoch and frequency for PSR 0540-69 are derived over a four-night span. When combined with frequencies derived from past optical and X-ray observations, these frequencies imply a braking index of 3.6(0.8), where 3.0 corresponds to braking by pure magnetic dipole radiation, provided frequency glitches of order one part in 10 million or larger are unlikely to occur during a 6-month interval. The secular decrease of optical luminosity of the Crab pulsar as measured by Kristian (1978) is confirmed. Publication: The Astrophysical Journal Pub Date: April 1987 DOI: 10.1086/165119 Bibcode: 1987ApJ...315..142M Keywords: Color; Magellanic Clouds; Polarimetry; Pulsars; Time Measurement; Linear Polarization; Magnetic Dipoles; Magnitude; Secular Variations; Ultraviolet Spectra; Astrophysics; GALAXIES: MAGELLANIC CLOUDS; POLARIZATION; PULSARS full text sources ADS | data products SIMBAD (4) NED (2)

X-ray observations of supernova remnants resembling the Crab Nebula

Out of the three SNRs G291.0 - 0.1, G308.7 + 0.0, and G328.4 + 0.2, which resemble the Crab Nebula in many of their radio properties, the present Einstein Observatory search was able to detect only the first, which is noted to have very extended X-ray emission with strong central brightening. Given the large distances and hydrogen column densities of the other two SNRs, their X-ray limits are consistent with membership in the Crab Nebula-like group. The evolutions of radio luminosity, X-ray luminosity, and the ratio of the two luminosities with remnant diameter are reassessed for this SNR class. No evidence is found for a radio luminosity decrease with diameter (age).

Radio emission from RS CVn binaries. I - VLA survey and period-radio luminosity relationship

A VLA survey of radio emission from 36 close binary stellar systems with RS CVn properties is reported. Eight new sources were detected. A summary of all published reports of radio emission from RS CVn systems is presented. There appears to be a correlation between maximum radio luminosity and rotational period, with a tentative functional form L(R) varies as P to the (-0.7) power. Rapid rotators (periods approximately 2 days) may be underluminous compared with the extrapolated trend from longer-period systems. The luminosity-period correlation probably results from a dynamo mechanism which produces strong magnetic fields and, in turn, enhances the nonthermal radio emission. The decrease in radio luminosity at short periods may be caused by a saturation of energy deposition in the chromosphere, possibly because the surface of the active star has become covered with spotted regions.

Effects of high CO2 levels on surface temperature and atmospheric oxidation state of the early Earth

One-dimensional radiative-convective and photochemical models are used to examine the effects of enhanced CO2 concentrations on the surface temperature of the early Earth and the composition of the prebiotic atmosphere. Carbon dioxide concentrations of the order of 100-1000 times the present level are required to compensate for an expected solar luminosity decrease of 25-30%, if CO2 and H2O were the only greenhouse gases present. The primitive stratosphere was cold and dry, with a maximum H2O volume mixing ratio of 10(-6). The atmospheric oxidation state was controlled by the balance between volcanic emission of reduced gases, photo-stimulated oxidation of dissolved Fe+2 in the oceans, escape of hydrogen to space, and rainout of H2O2 and H2CO. At high CO2 levels, production of hydrogen owing to rainout of H2O2 would have kept the H2 mixing ratio above 2x10(-4) and the ground-level O2 mixing ratio below 10(-11), even if no other sources of hydrogen were present. Increased solar UV fluxes could have led to small changes in the ground-level mixing ratios of both O2 and H2.

Variable extragalactic objects: identification and analysis of a complete sample to B = 21

A sample of faint variable objects detected on a series of UK 1.2-m Schmidt plates using the COSMOS measuring machine appears to comprise a population of quasars and active galaxies. The number–magnitude relation is found to show a constant increase by a factor of 5.8 per mag for 19 < B < 21. The colour–magnitude relation for the sample shows a marked correlation between apparent magnitude and B–R colour, which is tentatively interpreted as a reddening with decrease in luminosity.

Colliding ge and gg beams based on the single-pass e ±e − colliders (VLEPP type)

We discuss in detail our proposed [1,2] method of obtaining colliding γe and γγ beams with high energies and luminosities using the designed linear accelerators VLEPP and SLC with colliding e +e − beams at the energies 2 E ≳ 100 GeV. The intense γ beams are obtained by backward Compton scattering of laser light which is focused on the electron beams of these accelerators. This paper contains the scheme for conversion of an electron beam into a γ beam, a calculation of the conversion coefficient and of the total and spectral γe and γγ luminosities. To get the luminosity L γe , L γγ ∼ L ee, one needs a laser flash with energy ∼ 15J, and a pulse duration ∼ 30 ps at a repetition rate of 10 or 180 Hz. Such parameters seem to be achievable on the basis of current technology. The luminosity distribution over the γe or γγ invariant mass is broad. A method of monochromatization is described. It demands an increase of the laser flash energy (with a possible increase of pulse duration) and leads to a decrease of luminosity. We also describe a method for calibrating the total and spectral luminosities. Problems concerning the background are shown to be easier than in e +e − collisions. Some examples of interesting physical problems for γe and γγ collisions are enumerated.

Surface magnetic fields and the solar luminosity

Changes in the solar surface magnetic field alter the structure of the superadiabatic zone (outer 10/sup -7/ M/sub sun/) and produce luminosity fluctuations. Thr processes modeled in this paper include both the effect of magnetic pressure and the redistribution of the radiative flux. It is found that increases in the solar magnetic field (or sunspot area) cause a global decrease in the solar luminosity. The ''lost'' energy is stored as gravitational potential energy in the convection zone.

Observational Structure of Disk-like Envelope in the System DQ Her

The analysis of the eclipses in the system DQ Her based on the photometric observations of different authors during 1954, 1956 and 1975 allows us to obtain the photometric elements of the system and the structure of disk-like envelope surrounding the white dwarf. The orbit inclination of the system is i = 80° ± 3°, the major axis of the secondary component is 0.40 ± 0.03, the orbit radius is (10 ± 2)1010cm, the masses of the components are: mwd = (0.65 ± 0.35)M⊙, m2 = (0.50 ± 0.15)M⊙. A decrease in the optical luminosity of ≃20% and a total decrease in size of the disk-like envelope of ≃10% is observed from 1954 to 1975. The brightness of the disk-like envelope increases towards its center. The radiation temperature increases toward the center according to the laws T ≃ r−0.20 and T ≃ r−0.35 for 1954 and 1975, respectively. The average value of the radiation temperature of the disk-like envelope is about 104K. The bolometric luminosity of the envelope is (1 to 2)L⊙. The bolometric luminosity of the central white dwarf is anomalously high: ≃(20 to 200 L⊙) and the effective temperature of its surface is ≃(1.0 to 1.7)105K. All these results indicate that the disk-like envelope in the system DQ Her is an accreting disk.

The Secular Decrease of Optical and X-Ray Luminosity of Pulsars

The Secular Decrease of Optical and X-Ray Luminosity of Pulsars

Pulsar Distributions without Magnetic Decay

SEVERAL authors have recently argued for the necessity of decay of the surface magnetic field of neutron stars with a characteristic time of a few million years in order to explain the details of the observed pulsar sample. Gunn and Ostriker1 have summarized these arguments, pointing to the three empirical pieces of evidence which imply such decay: (1) the scarcity of long period pulsars, (2) the apparent decrease in surface magnetic field with age, and (3) the decrease in pulsed radio luminosity with age.

The Linear Polarization of Radio Sources Between 11 and 20 cm Wavelength. II. Polarization and Related Properties of Extragalactic Sources

The polarization properties of quasars, radio galaxies, and unidentified objects at 11 and 20 cm wavelength are discussed and compared with other source characteristics. For some quasars, scintillation and spectral data are in accord with current ideas that the sources comprise a small component radiating only at high frequencies and a larger component with a radiation spectrum similar to radio galaxies. The polarization is associated with this second component. For radio galaxies, the lower polarization of scintillators and a decrease of luminosity with increasing polarization suggest that sources of high brightness have low polarization. In spectral and polarization characteristics, the unidentified sources are similar to radio galaxies. Source evolution is discussed; the results suggest that quasars and radio galaxies evolve separately rather than represent different stages of a single evolutionary sequence.