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1.
G. Thuillier M. DeLand A. Shapiro W. Schmutz D. Bolsée S. M. L. Melo 《Solar physics》2012,277(2):245-266
We present a new method to reconstruct the solar spectrum irradiance in the Ly α – 400 nm region, and its variability, based on the Mg ii index and neutron-monitor measurements. Measurements of the solar spectral irradiance available in the literature have been
made with different instruments at different times and different spectral ranges. However, climate studies require harmonised
data sets. This new approach has the advantage of being independent of the absolute calibration and aging of the instruments.
First, the Mg ii index is derived using solar spectra from Ly α (121 nm) to 410 nm measured from 1978 to 2010 by several space missions. The variability of the spectra with respect to a
chosen reference spectrum as a function of time and wavelength is scaled to the derived Mg ii index. The set of coefficients expressing the spectral variability can be applied to the chosen reference spectrum to reconstruct
the solar spectra within a given time frame or Mg ii index values. The accuracy of this method is estimated using two approaches: direct comparison with particular cases where
solar spectra are available from independent measurements, and calculating the standard deviation between the measured spectra
and their reconstruction. From direct comparisons with measurements we obtain an accuracy of about 1 to 2%, which degrades
towards Ly α. In a further step, we extend our solar spectral-irradiance reconstruction back to the Maunder Minimum introducing the relationship
between the Mg ii index and the neutron-monitor data. Consistent measurements of the Mg ii index are not available prior to 1978. However, we remark that over the last three solar cycles, the Mg ii index shows strong correlation with the modulation potential determined from the neutron-monitor data. Assuming that this
correlation can be applied to the past, we reconstruct the Mg ii index from the modulation potential back to the Maunder Minimum, and obtain the corresponding solar spectral-irradiance reconstruction
back to that period. As there is no direct measurement of the spectral irradiance for this period we discuss this methodology
in light of the other proposed approaches available in the literature. The use of the cosmogenic-isotope data provides a major
advantage: it provides information about solar activity over several thousands years. Using technology of today, we can calibrate
the solar irradiance against activity and thus reconstruct it for the times when cosmogenic-isotope data are available. This
calibration can be re-assessed at any time, if necessary. 相似文献
2.
High dispersion CCD spectra are used to study the profiles of the Hα and Hβ lines of 48 stars in early spectral classes in
the young diffuse binary cluster h/χ Per. In addition, moderate resolution spectra of 15 B and Be stars were taken over the
interval 4400–4960 Å. One or, possibly, two new Be stars are discovered. The major parameters of the Hα line are measured
for the observed B and Be stars. The spectra over 4400–4960 Å are used to estimate Teff, logg, and υsin i. No traces of emission are detected in the Hα line profiles for 28 of the stars and emission is observed in the Hα line
for 20. During our observations an absorption profile of the Hα line was observed for some of the stars, such as Oo146, Oo566,
Oo922, and Oo1268, although they have previously been identified as Be stars. A significant long-term variability of the Hα
line is discovered for the Be stars Oo1161 and Oo2242. Oo2371 manifests a variability in its faint emission spectrum which
is typical of close binary systems that include a Be star.
__________
Translated from Astrofizika, Vol. 51, No. 2, pp. 305–319 (May 2008). 相似文献
3.
The populations of B and Be-stars in the young open clusters NGC 7419 and NGC 659 are studied. Lowresolution spectral observations
in the region of the Hα line are used to study 34 members of NGC 7419 and additional spectra in the range λ3700-6200 ? are
obtained for 12 stars. During the period of our observations, four previously identified Be-stars manifested an absorption
profile for the Hα line. Emission in the Hα line is observed in the spectra of 21 objects. The members of the cluster NGC
7419 are classified as to spectral type. The position of the Be-stars on a two-color IR diagram and a comparison with the
position of the objects in other clusters with similar ages cast doubt on the existence of two starformation waves. Moderate
resolution spectra were obtained for 22 stars in the cluster NGC 659 over wavelengths of λ4050-5200 ? and for 7 objects in
the neighborhood of the Hα line. Emission is observed in both the Hα and Hβ lines for two of the five Be-stars that were studied,
while faint emission appears only in the Hα line in one of the stars. The other members of the cluster show no signs of emission
in these spectra. Teff, log g, and Vsini are estimated for the observed stars. It is found that 5 of the objects are not members of the cluster. 相似文献
4.
We present an analysis of the proximity effect in a sample of 10 2-Å-resolution QSO spectra of the Ly α forest at . Rather than investigating variations in the number density of individual absorption lines, we employ a novel technique that is based on the statistics of the transmitted flux itself. We confirm the existence of the proximity effect at the > 99 per cent confidence level. We derive a value for the mean intensity of the extragalactic background radiation at the Lyman limit of . This value assumes that QSO redshifts measured from high-ionization lines differ from the true systemic redshifts by . We find evidence at a level of 2.6 σ that the significance of the proximity effect is correlated with QSO Lyman limit luminosity. Allowing for known QSO variability, the significance of the correlation reduces to 2.1 σ .
The QSOs form a close group on the sky and the sample is thus well suited for an investigation of the foreground proximity effect, where the Ly α forest of a background QSO is influenced by the UV radiation from a nearby foreground QSO. From the complete sample we find no evidence for the existence of this effect, implying either that or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example. Our sample includes the fortunate constellation of a foreground QSO surrounded by four nearby background QSOs. These four spectra all show underdense absorption within ±3000 km s−1 of the redshift of the foreground QSO. 相似文献
The QSOs form a close group on the sky and the sample is thus well suited for an investigation of the foreground proximity effect, where the Ly α forest of a background QSO is influenced by the UV radiation from a nearby foreground QSO. From the complete sample we find no evidence for the existence of this effect, implying either that or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example. Our sample includes the fortunate constellation of a foreground QSO surrounded by four nearby background QSOs. These four spectra all show underdense absorption within ±3000 km s
5.
John K. Webb Michael T. Murphy Victor V. Flambaum Stephen J. Curran 《Astrophysics and Space Science》2003,283(4):565-575
We report results from a third sample of quasar absorption line spectra from the Keck telescope which has been studied to
search for any possible variation of the fine structure constant, α. This third sample, which is larger than the sum of the
two previously published samples, shows the same effect, and also gives, as do the previous two samples, a significant result.
The combined sample yields a highly significant effect, Δα = (αz - α0 )/α0 = -0.57 ± 0.10 × 10-5, averaged over the redshift range 0.2 < z < 3.7. We include a brief discussion of small-scale kinematic structure in quasar
absorbing clouds. However, kinematics are unlikely to impact significantly on the averagednon-zeroΔα /α above, and we have
so far been unable to identify any systematic effect which can explain it. New measurements of quasar spectra obtained using
independent instrumentation and telescopes are required to properly check the Keck results.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
We have previously presented 5.7 σ evidence for a smaller αat redshifts 0.2 < zabs < 3.7 from a sample of 128 Keck/HIRES quasar absorption systems: Δα/α = (-0.57 ± 0.10) x10-5. A non-zero <Δα/α manifests itself as a distinct pattern of shifts in the measured absorption line wavelengths. The statistical
error is now small: we do detect small line shifts in the HIRES data. Whether these shifts are due to systematic errors or
due to real variation in α is now the central question. Here we summarize the two potentially most important systematic effects:
atmospheric dispersion and isotopic abundance evolution. Previously, these have been difficult to quantify/model but here
we find that neither of them can explain our results. Furthermore, the HIRES spectra themselves contain no evidence for these
effects. Independent measurements of Δα/α with a different telescope and spectrograph are now crucial if we are to rule out
or confirm the present evidence for a variable α.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
8.
Joop Schaye † Tom Theuns Michael Rauch George Efstathiou Wallace L.W. Sargent 《Monthly notices of the Royal Astronomical Society》2000,318(3):817-826
At redshifts z ≳2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Ly α forest. This photoheated gas follows a tight temperature–density relation which introduces a cut-off in the distribution of widths of the Ly α absorption lines ( b -parameters) as a function of column density. We have measured this cut-off in a sample of nine high-resolution, high signal-to-noise ratio quasar spectra and determined the thermal evolution of the intergalactic medium in the redshift range 2.0–4.5. At a redshift z ∼3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of He ii . 相似文献
9.
J. A. Yates E. M. L. Humphreys A. M. S. Richards 《Astrophysics and Space Science》1997,251(1-2):285-288
The combination of a time-dependent spherically symmetric hydrodynamic model of stellar atmosphere pulsation and a radiation
transport code, which incorporates maser saturation theory, enabled us to synthesise maps and spectra of H2O maser emission from the circumstellar envelopes of long period variable stars.
The synthetic maps and spectra compare favourably with observed 22, 321 and 325 GHz H2O maser emission. As is observed in H2O maser regions the peak emission occurs between 3–8 stellar radii from the star. The calculated H2O maser regions are in conditions of nH2 = 106 − 108 cm−3, assuming a fractional abundance of 10−4; kinetic temperatures of 550–3000 K; dust ensemble temperatures of 500–1200 K and an accelerating velocity field. The IR
radiation field is explicitly included in the radiation transport model, incorporating the latest absorption efficiency data
for silicates from Draine. We reproduce the features seen in high angular resolution MERLIN spectral line datacubes. This
shows that a mass outflow model which extends the photosphere using pulsations and incorporates radiation pressure on silicate
based dust particles can produce the observed data on small (10-mas) angular scales.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
10.
K. R. Anantharamaiah 《Journal of Astrophysics and Astronomy》1985,6(4):177-201
A survey of the H 272 α recombination line at 325 MHz has been made towards 53 directions in the galactic plane using the
Ooty Radio Telescope (ORT). 34 of these directions correspond to well-known Hn regions, 12 to SNRs and 6 to ‘blank’ areas
selected so that the 5 GHz continuum is a minimum over the telescope beam of 2ℴ x 6 arcmin. Observing procedure and spectra
of 47 sources towards which lines are detected are presented. Hydrogen recombination lines have been detected towards all
the observed directions havingl <40ℴ. Carbon recombination lines are identified in 12 of the directions. The hydrogen line intensities are found to correlate
well with the total continuum intensity (which includes the nonthermal galactic background) indicating that most of the lines
arise due to stimulated emission by the background radiation. A preliminary discussion on the nature of the line-emitting
regions is also presented. 相似文献
11.
Based on optical–NIR spectra, we discuss the nebular properties and stellar populations of starburst nuclei. Starbursts are
found to have higher electron densities and higher excitations than HII nuclei. The emission lines have been used to estimate
the nebular oxygen abundances, ionization parameter and radiation softness parameter. From a study of the Hα emission-line equivalent widths and the CaII triplet absorption line equivalent widths, we infer the ages and stellar content
of the starburst regions. Most of the nuclei show evidence for a composite population – a young, ionizing population co-existing
with evolved, non-ionizing stars of about 5–7 Myr, which are evolving towards the peak of RSG distribution.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
R. Carballo J.I. Gonzalez-Serrano S. Sanchez C.R. Benn M. Vigotti 《Astrophysics and Space Science》1998,263(1-4):63-66
We present some results based on optical photometry of an unbiased subsample of 64 sources, from a sample of radio quasars
80 per cent complete. The quasars (z=0.4-2.8) show approximately a similar spectral energy distribution (SED). Only two quasars
appear clearly differenciated, exhibiting redder colours than the rest, and they have low or moderate redshifts (z=0.50 and
1.12). Composite broad-band spectra for the remaining 62 quasars reveal the presence of a break in the SED at ∼ 3000 Å, where
the average slope α (defined by Sν ∝ να) changes from αblue=0.11±0.16 at λ>3000 Å to αUV=-0.66±0.15 at λ<3000 Å. Composite spectra from a low and a high redshift subsample, with the redshift separation at the median
value z=1.2, yields αUV=-0.87±0.20, for z<1.2 and αUV=-0.48±0.12 for z>1.2 and the difference is 98 per cent significant. This trend could be intrinsic or could arise from a dependence
between αUV and the blue/UV luminosity, since the three quantities αUV - Lblue/UV - z are correlated. Finally, an intrinsic correlation is found between blue/UV and radio luminosity.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
There is observational evidence showing that stellar and solar flares occur with a similar circumstance, although the former are usually much more energetic. It is expected that the bombardment by high-energy electrons is one of the chief heating processes of the flaring atmosphere. In this paper we study how a precipitating electron beam can influence the line profiles of Ly α , H α , Ca ii K and λ 8542. We use a model atmosphere of a dMe star and make non-LTE computations taking into account the non-thermal collisional rates owing to the electron beam. The results show that the four lines can be enhanced to different extents. The relative enhancement increases with increasing formation height of the lines. Varying the energy flux of the electron beam has different effects on the four lines. The wings of Ly α and H α become increasingly broad with the beam flux; change of the Ca ii K and λ 8542 lines, however, is most significant in the line centre. Varying the electron energy (i.e. the low-energy cut-off for a power-law beam) has a great influence on the Ly α line, but little on the H α and Ca ii lines. An electron beam of higher energy precipitates deeper, thus producing less enhancement of the Ly α line. The Ly α /H α flux ratio is thus sensitive to the electron energy. 相似文献
14.
Archna Sagdeo S. K. Rai Gyan S. Lodha K. P. Singh Nisha Yadav R. Dhawan Umesh Tonpe M. N. Vahia 《Experimental Astronomy》2010,28(1):11-23
X-ray reflectivity measurements were performed on several thin foil gold mirrors fabricated in TIFR for a Soft X-ray Imaging
Telescope. The mirrors were made from thin aluminum foils with a reflecting layer of sputtered gold transferred from a smooth
glass mandrel using an epoxy. X-ray reflectivity measurements were performed on a sample of randomly selected mirrors using
CuK
α
(8.05 keV), CrK
α
(5.41 keV) X-rays and also at several energies in the energy range of 155–300 eV using the synchrotron source Indus-1. It
was found that the roughness of the low-density top gold layer as obtained from the fitting of X-ray reflectivity data for
CuK
α
radiation is relatively more as compared to that obtained from the CrK
α
radiation. This indicates that in the mirrors made by this process, the upper surfaces are smoother as compared to the deeper
layers. It was also observed that the critical angle almost vanishes in the very low energy range of 290–300 eV due to strong
absorption effects of the low density material sitting on top of these mirrors. Due to this absorption effect, efficiency
of these mirrors reduces in this energy range. This is first time that reflectivity measurements are being reported for very
soft X-rays (≤ 300 eV) for mirrors made for any X-ray astronomy mission. 相似文献
15.
Results from a spectrophotometric study of the galaxy Kaz 701 are presented. Spectra of this galaxy were obtained on the 2.6-m
telescope at the Byurakan Observatory with the SCORPIO spectral camera. In the spectra of Kaz 701, strong Hα, [OIII] λλ5007,
4959, and Hβ emission were observed, and the [SII]λλ6731, 6717 lines were observed with moderate intensities, while the [NII]λ6584
line was scarcely noticeable in the spectra obtained in 2004, but was much more intense in the spectra of 2009. The lines
observed in the spectrum of this galaxy are typical of type Sy2 galaxies. A component was observed in the red wings of both
the Hα and Hβ lines in the data of 2004, but, although these lines were stronger, these components were absent in the spectra
taken in 2009. The relative intensities, equivalent widths, and half widths of these lines are given. These data, obtained
in different observation periods (2004 and 2009), differ greatly; that is, the emission is variable. 相似文献
16.
The sensitivity of stellar spectra to α/Fe abundance changes is investigated with the aim to be detected photometrically and
employed for the scientific goals of the GAIA mission. A grid of plane parallel, line blanketed, flux constant, LTE model
atmospheres with different [α/Fe] ratios was calculated. As a first step, the modeled stellar energy fluxes for solar-type
stars and giants were computed and intercompared. The spectral sensitivity to α/Fe abundance changes is noticeable and has
to be taken into account when selecting photometric filters for GAIA. The Ca II H and K lines and Mg I b triplet are the most
sensitive direct indicators ofα/Fe abundance changes.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
In an earlier research the employment of a radiation transport model with angle-dependent partial frequency redistribution,
self-absorption by interplanetary hydrogen, realistic solar HLyαemission profile, and a time dependent `hot' hydrogen model to analyze 5 interplanetary HLyα glow spectra obtained with theHubble–Space–Telescope–GHRS spectrometer, has not resulted in unequivocal determination of
a set of thermodynamical parameters of the interstellar hydrogen The residual discrepancies between the model and the data
concern the observations performed within an interval of 1 year close to the solar minimum from very similar lines of sight.
In this paper we investigate by calculating interplanetary HLyα lines with the use of a one hydrogen distribution and several solar HLyα line profiles whether this residual may be caused by possible variations in time of the shape of the solar HLyα emission line profile which cause variable illuminations of the interplanetary gas. These variations of illuminations cause
variations in Doppler shift of the resonant interplanetary HLyα line that can amount to ≃ 4 km s-1in the line peak. Consequently we conclude that without adequate knowledge of the solar HLyα emission line profile during spectral observations of the interplanetary hydrogen gas it is impossible to obtain an agreement
between models and observations better than by this value.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
By introducing an asymmetry between the two hemispheres, we study whether the solar dynamo solutions in the two hemispheres
remain coupled with each other. Our calculations are based on the solar dynamo code SURYA, which incorporates the helioseismically-determined
solar-rotation profile, a Babcock–Leighton α effect concentrated near the surface, and a meridional circulation. When the magnetic coupling between the hemispheres is
enhanced by either increasing the diffusion or introducing an α effect distributed throughout the convection zone, we find that the solutions in the two hemispheres evolve together with
a single period even when we make the meridional circulation or the α effect different in the two hemispheres. On the other hand, when the hemispheric coupling is weaker for other values of parameters,
an asymmetry between the hemispheres can make solutions in the two hemispheres evolve independently with different periods. 相似文献
19.
An LRS Bianchi type I string dust cosmological model with and without bulk viscosity following a method used by Letelier and
Stachel, is investigated. To get a determine solution, it is assumed that σ∝θ whereσ is shear and θ is scalar of expansion
and which leads to A =αBn were n is a constant. The physical and geometrical aspects of the model are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
We have developed a model which aims to reproduce observational data of many kinds related to cosmic-ray (CR) origin and propagation:
direct measurements of nuclei, antiprotons, electrons and positrons,γ-rays, and synchrotron radiation. Our main results include
evaluation of diffusion/convection and reacceleration models, estimates of the halo size, calculations of the interstellar
positron and antiproton spectra, evaluation of alternative hypotheses of nucleon and electron interstellar spectra, and computation
of the Galactic diffuseγ-ray emission. Recently our CR propagation code has been generalized to include fragmentation networks
of arbitrary complexity. The code can now provide an alternative to leaky-box calculations for full isotopic abundance calculations
and has the advantage of including the spatial dimension which is essential for radioactive nuclei. Preliminary predictions
for sub-Fe/Fe, 10Be/9Be and 26Al/27Al are presented in anticipation of new experimental isotopic data. We show that combining information from classical CR studies
with γ-ray and other data leads to tighter constraints on CR origin and propagation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献