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1.
S. Chandrasekhar 《Journal of Astrophysics and Astronomy》1994,15(2):105-109
The density of the white dwarf stars is reconsidered from the point of view of the theory of the poly tropic gas spheres,
and gives for themean density of a white dwarf (under ideal conditions) the formula ρ=2.162 × 106 × (M/⊙)2.
The above formula is derived on considerations which are a much nearer approximation to the conditionsactually existent in a white dwarf than the previous calculations of Stoner based on uniform density distribution in the star and which gave
for the limiting density the formula ρ=3.977 × 106 × (M/⊙)2. 相似文献
2.
V. P. Gaur B. M. Tripathi G. C. Joshi M. C. Pande 《Astrophysics and Space Science》1988,147(1):107-113
Molecular dissociation equilibrium calculations were done for the model atmospheres of DA and non-DA white dwarfs. Our calculations show that He
2
+
and HeH+ appear as most abundant molecules in the atmospheres of non-DA white dwarfs while H2 and H
2
+
are most abundant molecules in DA white dwarfs. It is suggested that these molecules should be searched for in the atmospheres of white dwarfs. 相似文献
3.
The propagation of an instantaneous burst of isotropic radiation from the time of its onset at some redshift z 0 to the time of its detection at the present epoch (at z = 0) is considered within the framework of a flat Universe. Thomson scattering by free electrons and scattering in primordial hydrogen lines (Hα, Hβ, Pα, and Pβ) are believed to be the sources of opacity, with the single-scattering albedo in the lines being calculated by taking into account the deexcitation of the upper levels of the transitions being considered under the action of background blackbody radiation. The profiles of these lines in the burst spectrum at the present epoch have been constructed for various z0 at various distances from the burst center. To a first approximation, these profiles do not depend on the burst radiation spectrum and intensity. It is shown that the lines are purely absorption ones at a sufficiently large distance, but an emission component can appear with decreasing distance, which strengthens as the distance decreases, while the absorption component weakens. The absorption depth in the combined profile can reach 2 ×10?4 for the Hα and Hβ lines and 7 × 10?6 for the Pα and Pβ lines. In this case, the relative amplitude of the temperature fluctuations lies within the range 10?7?10?9. The calculations have been performed for bursts with different characteristic initial sizes. At the same z 0, the hydrogen line profiles essentially coincide for sizes smaller than some value, and the contrast of the lines decreases with increasing burst size for greater ones.
相似文献4.
The question of which progenitor channel can reproduce the observed rate of Type Ia supernovae (SNe Ia) remains unresolved, the two leading models being the so-called single and double degenerate scenarios. The former implies a large population of accreting, nuclear-burning white dwarfs with photospheric temperatures T～105–106 K during some part of their accretion history. Recently, we demonstrated that a population of accreting white dwarfs large enough to reproduce the observed SN Ia rate would contribute significantly to the ionizing radiation expected from the stellar population in early-type galaxies, now commonly observed to host spatially extended regions of neutral and ionized gas. From our photoionization calculations, we show that one can constrain the contribution of the single degenerate channel to the SN Ia rate in early-type galaxies from upper limits on the luminosity of a number of emission lines characteristic of ionization by high-temperature sources. Detection (or strong upper limits on) He II 1640 Å and [C II] 1335 Å, expected to be overluminous in these galaxies if the single-degenerate channel holds true, can strongly constrain the total luminosity of nuclear-burning white dwarfs in these populations. In the near-UV, our photoionization calculations demonstrate that the EW of the [O II] 3727 doublet and the [Ne III] 3869/[O II] 3727 ratio can also provide a powerful diagnostic, particularly in post-starburst galaxies. Together with the He II 3203 Å (5 → 3) recombination line, these lines present an excellent opportunity for strongly constraining the population of accreting, nuclear-burning white dwarfs, and in general the available ionizing continuum, at relatively short delay-times (time from initial starburst). 相似文献
5.
We present three sets of observations of n = 1 to n = 2 lines due to helium-like aluminium (Alxii), made during two solar flares (25 August, 1980 and 19 October, 1986), using the X-Ray Polychromator on the SMM satellite. The observed temperature-sensitive line ratio G is shown to be consistent with the close-coupling calculations of Keenan and McCann (1987), although the ratio R, which is both temperature and density-sensitive for lower-Z elements, is not sufficiently well determined from these data to say more than that the observed values of R are not inconsistent with the theoretical calculations. This region of the spectrum also includes the helium-like magnesium (Mgxi) 11
S - 31
P line, and it is shown that the ratio of this line to the Alxii resonance (11
S - 21
P) line is a more sensitive indicator of electron temperature than are the Alxii
G and R ratios. We demonstrate that the three ratios may be used together in order to derive values of emission measure, electron temperature and electron density during these flares. 相似文献
6.
K. G. Strassmeier M. Weber T. Granzer L. Schanne J. Bartus I. Ilyin 《Astronomische Nachrichten》2014,335(9):904-934
We present continuous and time‐resolved R = 55 000 optical échelle spectroscopy of ε Aurigae from 2006–2013. Data were taken with the STELLA Echelle Spectrograph of the robotic STELLA facility at the Observatorio del Teide in Tenerife. Contemporaneous photometry with the Automatic Photoelectric Telescopes at Fairborn Observatory in Arizona is presented for the years 1996–2013. Spectroscopic observations started three years prior to the photometric eclipse and are still ongoing. A total of 474 high‐resolution échelle spectra are analyzed and made available in this paper. We identify 368 absorption lines of which 161 lines show the characteristic sharp disk lines during eclipse. Another 207 spectral lines appeared nearly unaffected by the eclipse. From spectrum synthesis, we obtained the supergiant atmospheric parameters Teff = 7395 ± 70 K, log g ≈ 1, and [Fe/H] = +0.02 ± 0.2 with ξt = 9 km s–1, ζRT = 13 km s–1, and v sin i = 28 ± 3 km s–1. The residual average line broadening expressed in km s–1 varies with a period of 62.6 ± 0.7 d, in particular at egress and after the eclipse. Two‐dimensional line‐profile periodograms show several periods, the strongest with ≈110 d evident in optically thin lines as well as in the Balmer lines. Center‐of‐intensity weighted radial velocities of individual spectral lines also show the 110‐d period but, again, additional shorter and longer periods are evident and are different in the Balmer lines. The two main spectroscopic Hα periods, ≈ 116 d from the line core and ≈ 150 d from the center‐of‐intensity radial velocities, appear at 102 d and 139 d in the photometry. The Hβ and Johnson V I photometry on the other hand shows two well‐defined and phase‐coherent periods of 77 d and 132 d. We conclude that Hα is contaminated by changes in the circumstellar environment while the Hβ and V I photometry stems predominantly from the non radial pulsations of the F0 supergiant. We isolate the disk‐rotation profile from 61 absorption lines and found that low disk eccentricity generally relates to low disk rotational velocity (but not always) while high disk eccentricity always relates to high velocity. There is also the general trend that the disk‐absorption in spectral lines with higher excitation potential comes from disk regions with higher eccentricity and thus also with higher rotational velocity. The dependency on transition probability is more complex and shows a bi‐modal trend. The outskirts of the disk is distributed asymmetrically around the disk and appears to have been built up mostly in a tail along the orbit behind the secondary. Our data show that this tail continues to eclipse the F0 Iab primary star even two years after the end of the photometric eclipse. High‐resolution spectra were also taken of the other, bona‐fide, visual‐binary components of ε Aur (ADS 3605BCDE). Only the C‐component, a K3‐4‐giant, appears at the same distance than ε Aur but its radial velocity is in disagreement with a bound orbit. The other components are a nearby (≈ 7 pc) cool DA white dwarf, a G8 dwarf, and a B9 supergiant, and not related to ε Aur. The cool white dwarf shows strong DIB lines that suggest the existence of a debris disk around this star. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
Numerical solutions of the structural equations for rapidly rotating white dwarfs are tabulated for values of the central density parametery
–2
o ranging from 0.0001 to 0.80. The physical parameters of white dwarfs rotating at the break-up velocity are also tabulated. Further, the massradius relations for both the equatorial and the polar radii are derived. These two relations are found to differ from other recent theoretical calculations by less than 10 and 4%, respectively. The use of uniformly rotating white dwarf models to account for the observational data on Sirius B is discussed. It is concluded that Sirius B may be rotating rapidly and near its break-up velocity. Further it is suggested that the radius and Einstein red shift for this star are 0.01135 R and 59 km/sec. 相似文献
8.
The radiation transfer at the gyrofrequency in the coronae of magnetic white dwarfs is considered. The electron distribution over Landau levels, taking both radiative and collisional transitions into account, is obtained. The emissivity and absorption coefficients of extraordinary radiation at the gyrofrequency are calculated. The ranges of parameters where cyclotron lines are observed in emission or absorption are found. The upper limit on coronal plasma density (2×1011 cm–3) for isolated magnetic white dwarfs with absorption lines in the spectrum is specified. 相似文献
9.
The purpose of this paper is to report on some intensity measurements of the Fe xiii lines at 10 747 Å and 10 798 Å made during the total eclipse of 12 November, 1966. Infrared spectra were taken of the solar corona at a dispersion of 90 Å per mm, using an RCA image converter and spectrograph aboard the NASA CV 990 aircraft off the coast of southern Brazil. The spectra have been reduced to equivalent width in terms of the coronal continuum and values derived for different points in the corona.The observed equivalent widths of the lines lie in the range 10 to 30 Å for the 10 747 line and 5 to 12 Å for the 10 798 line. The ratio of these equivalent widths is found to vary from 2.3 in the inner corona to 6 at a point 1.36 solar radii from the center of the Sun.The above results are discussed in terms of the excitation mechanisms involved in producing the lines. In particular, the results are compared with the recent theoretical calculations of Chevalier and Lambert, who are the first to include the effects of proton collisions in the excitation of the 3p
2
3
P levels of Fexiii. Our observations are consistent with an electron density of 4 × 108 in the inner corona; a value which compares favorably with those derived by other observers from the strength of the K continuum. These are, to our knowledge, the first eclipse observations of the infrared Fe xiii lines which indicate that proton collisions are important in the excitation of the coronal lines. The coronal abundance of iron is estimated from the equivalent width of the 10 747 line, and in common with other observers we find an overabundance as compared to the photospheric abundance by a factor of 10. 相似文献
10.
F.P. Keenan M. Mathioudakis A.C. Katsiyannis C.A. Ramsbottom K.L. Bell R.J. Thomas J.W. Brosius 《Solar physics》2002,205(2):265-277
R-matrix calculations of electron impact excitation rates in N-like Mgvi are used to derive theoretical electron-density-sensitive emission line ratios involving 2s
22p
3–2s2p
4transitions in the 269–403 Å wavelength range. A comparison of these with observations of a solar active region, obtained during the 1989 flight of the Solar EUV Rocket Telescope and Spectrograph (SERTS), reveals good agreement between theory and observation for the 2s
22p
3
4
S–2s2p
4
4
Ptransitions at 399.28, 400.67, and 403.30 Å, and the 2s
22p
3
2
P–2s2p
4
2
Dlines at 387.77 and 387.97 Å. However, intensities for the other lines attributed to Mgvi in this spectrum by various authors do not match the present theoretical predictions. We argue that these discrepancies are not due to errors in the adopted atomic data, as previously suggested, but rather to observational uncertainties or mis-identifications. Some of the features previously identified as Mgvi lines in the SERTS spectrum, such as 291.36 and 293.15 Å, are judged to be noise, while others (including 349.16 Å) appear to be blended. 相似文献
11.
We demonstrate, using a sample of high quality spectra, that the average equivalent width ratio of the unsaturated R(0) and R(1) lines of the CN B2Σ+ – X2Σ+ (0, 0) band near 3875 Å equals π instead of 3.73 – calculated for the CMBR temperature T = 2.725 K and the already published oscillator strengths of both transitions, i.e. the excitation temperature of CN is slightly higher than it should be expected if CMBR is the only excitation mechanism. Taking into account the saturation effects by means of profile fitting during the column density calculations does not remove the revealed temperature excess. Possible interpretations of the effect are discussed (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
12.
We present calculated Stark-polarized line profiles for a number of H i lines observed in the visible and infrared emission spectrum of solar prominences and other limb activity. For use in measurements of possible electric fields in these structures, we also calculate curves giving the difference in line width between the 1/2 (I ± Q) profiles as a function of electric-field intensity. Our calculations take into account magnetic fields in these structures, and incorporate typical observed values of Doppler broadening. These calculations explicitly consider the H i fine structure neglected in previous work, and thus are more accurate in the range of low to intermediate electric-field intensity likely to be encountered in solar plasmas (E < 103 V cm–1). Our results enable us to compare behavior when E and B are parallel, or perpendicular. We draw particular attention to the high electric-field sensitivity of the transitions between high levels such as 12–8 and 15–9 in H i, observed in prominences at wavelengths around 11. Their sensitivity is roughly an order of magnitude larger than that of the high Paschen-series lines used in solar plasma electric field studies so far. 相似文献
13.
Results of evolutionary calculations for a close binary system with a central helium burning He-star filling its Roche lobe and an accreting white dwarf are presented. Values for the mass of the components and the degree of central helium exhaustion before the filling of the Roche lobe are varied. It is shown that in such a system the mass accretion rate will remain for a long time (2–4) Ö 10–8
yr–1 ifq<1 (q=M
He,2/M
CO,1). The obtained results are discussed in connection with pre-supernova I phenomenon. 相似文献
14.
The forbidden transitions within 3s
2 3p
5 3d of Fe ix and Ni xi and 3s
2 3p
4 3d of Fe x and Ni xii
Improved level values for 3s
2 3p
5 3d and 3s
2 3p
4 3d of Fe ix and Fe x are derived from the observed forbidden lines with special regard to recent accurate measurements in the vacuum-ultraviolet part of the coronal spectrum. A procedure for estimating expected relative intensities is proposed and used for an additional check on the consistency of the identifications, which now comprise 12 lines of Fe IX and 11 lines of Fe x. Finally, by use of a suitable extrapolation technique the wavelengths of corresponding lines in nickel are predicted, which leads to some new identifications and a total of 6 identified lines of Ni xi and 4 of Ni xii. 相似文献
15.
Takara Nishikawa 《Solar physics》1984,93(1):37-51
The weakening of EUV line emission due to continuum absorptions of neutral hydrogen and neutral helium is investigated to examine its wavelength-dependence. After convolving the intensities predicted from multilevel calculations over an instrumental profile, we found a systematic, linear weakening for lines shortward of 912 Å, which can be attributed to Lyman continuum absorption in the cool chromospheric cloud. The degree of the weakening at the quiet Sun seems to be constant in the temperature range of 4.3 < log T < 5.4. We also find that the lines shortward of 504 Å are somewhat weakened by He i continuum absorption. From the comparison of both weakenings the temperature of the absorber is estimated to be rather low (T
e 7 × 103 K). 相似文献
16.
Keenan F.P. Katsiyannis A.C. Ramsbottom C.A. Bell K.L. Brosius J.W. Davila J.M. Thomas R.J. 《Solar physics》2004,219(2):251-263
Recent R-matrix calculations of electron impact excitation rates in N-like Si VIII are used to derive theoretical emission line intensity ratios involving 2s 22p 3–2s2p 4 transitions in the 216–320 Å wavelength range. A comparison of these with an extensive dataset of solar active region, quiet-Sun, sub-flare and off-limb observations, obtained during rocket flights of the Solar EUV Research Telescope and Spectrograph (SERTS), indicates that the ratio R 1= I(216.94 Å)/I(319.84 Å) may provide a usable electron density diagnostic for coronal plasmas. The ratio involves two lines of comparable intensity, and varies by a factor of about 5 over the useful density range of 108–1011 cm?3. However R 2= I(276.85 Å)/I(319.84 Å) and R 3=I(277.05 Å)/I(319.84 Å) show very poor agreement between theory and observation, due to the severe blending of the 276.85 and 277.05 Å lines with Si VII and Mg VII transitions, respectively, making the ratios unsuitable as density diagnostics. The 314.35 Å feature of Si VIII also appears to be blended, with the other species contributing around 20% to the total line flux.
相似文献17.
A high-resolution spectrum clearly shows the presence of A
6Σ+–X
6Σ+(0,0;0,1;1,0;2,0;2,1) and (2,2) bands system of CrH molecular lines in the spectral range 10,000 cm−1 to 14,050 cm−1. At least 3928 lines of the six bands, accounting for 57% of the lines registered in the laboratory can be identified with
certainty in the sunspot spectrum. Most of the lines are found blended with TiO, CaH, MgH and other atomic species. These
molecular lines are typically much more temperature sensitive than atomic lines, which make them ideal, complementary tool
for studying cool stellar atmospheres as well as the internal structure of sunspots. Equivalent width for an adequate number
of well identified molecular lines of these bands using the Gaussian-profile approximation method versus rotational quantum
number J has been used to determine the effective rotational temperature of the CrH molecule. The range of effective rotational temperature
value obtained from these bands is 1766 K to 2442 K. This range agrees well with the effective rotational temperatures derived
for other molecules in sunspot umbrae. 相似文献
18.
The expected equivalent widths of individual rotational lines of the most intense Q
2 branch of the 0-0 band of the A
2-X2i; system of S32H and S34H have been calculated in the umbral spectrum for five disk positions using Zwaan's (1974) sunspot model. Percentage abundance of S34 in the terrestrial case has been considered valid in our calculations.Strong lines of S32H and S34H of the A-X band system should be detectable in the sunspot spectrum. The molecule SH may play a possible role as a major opacity source in the ultraviolet spectrum of sunspots along with the molecule OH in the upper layers (up to
0.5m = 1.0) wherefrom most of the continuum arises. Study of this molecule in the umbral spectrum may also provide the solar isotopic abundance ratio N(S32)/N(S34). 相似文献
19.
Stuart D. Jordan 《Solar physics》1977,51(1):51-59
Calculations performed with several models of the solar chromosphere support Ulmschneider's conclusion that relatively short period acoustic waves heat the low chromosphere in the region just above the temperature minimum. However, these same short period waves (10 period P80 s) are not able to maintain chromospheric temperatures at heights where 5000Å(normal) < 10-6. The calculations also show that an earlier conjecture stating that the H2 population might influence the non-LTE chromospheric H- population is probably not correct, due to lower values of the ratio n
e/n
H inferred from more recent observations. Finally, the calculations support Athay's contention that the Cayrel mechanism alone cannot produce the observed temperature rise, because the magnitude of the radiative cooling in the lines is too great. 相似文献
20.
Model calculations for laser action in Hei are carried out, when helium plasma is rapidly cooled by expansion. Results are presented for four transitions, two of which, 31
S21
P
0 (7281) and 31
D21
P
0 (6678), show strong population inversion. Available observational evidence for possible laser action in these two lines in Wolf-Rayet and emission-line stars is summarized and discussed. 相似文献