اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدWELS - Ultraviolet Spectra and Expanding Atmosphere Models
94
0
0.0
(
0
)
تأليف
W. L. F. Marcolino
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
The ultraviolet spectra of all weak emission line central stars of planetary nebulae (WELS) with available IUE data is analyzed. We found that the WELS can be divided in three different groups regarding their UV: (1) Strong P-Cygni profiles (mainly in C IV 1549); (2) Weak P-Cygni features and (3) Absence of P-Cygni profiles. We have measured wind terminal velocities for all objects presenting P-Cygni profiles in N V 1238 and/or C IV 1549. The results obtained were compared to the UV data of the two prototype stars of the [WC]-PG 1159 class, namely, A30 and A78. They indicate that WELS are distinct from the [WC]-PG 1159 stars, in contrast to previous claims in the literature. In order to gain a better understanding about the WELS, we clearly need to determine their physical parameters and chemical abundances. First non LTE expanding atmosphere models (using the CMFGEN code) for the UV and optical spectra of the star Hen 2-12 are presented.
قيم البحث
اقرأ أيضاً
We show that for any $lambda in mathbb{C}$ with $|lambda|<1$ there exists an analytic expanding circle map such that the eigenvalues of the associated transfer operator (acting on holomorphic functions) are precisely the nonnegative powers of $lambda
$ and $bar{lambda}$. As a consequence we obtain a counterexample to a variant of a conjecture of Mayer on the reality of spectra of transfer operators.
We investigate the physical characteristics of the Solar Systems proposed Planet Nine using modeling tools with a heritage in studying Uranus and Neptune. For a range of plausible masses and interior structures, we find upper limits on the intrinsic
Teff, from ~35-50 K for masses of 5-20 M_Earth, and we also explore lower Teff values. Possible planetary radii could readily span from 3 to 6 R_Earth depending on the mass fraction of any H/He envelope. Given its cold temperature, the planet encounters significant methane condensation, which dramatically alters the atmosphere away from simple Neptune-like expectations. We find the atmosphere is strongly depleted in molecular absorption at visible wavelengths, suggesting a Rayleigh scattering atmosphere with a high geometric albedo approaching 0.75. We highlight two diagnostics for the atmospheres temperature structure, the first being the value of the methane mixing ratio above the methane cloud. The second is the wavelength at which cloud scattering can be seen, which yields the cloud-top pressure. Surface reflection may be seen if the atmosphere is thin. Due to collision-induced opacity of H2 in the infrared, the planet would be extremely blue (instead of red) in the shortest wavelength WISE colors if methane is depleted, and would, in some cases, exist on the verge of detectability by WISE. For a range of models, thermal fluxes from ~3-5 microns are ~20 orders of magnitude larger than blackbody expectations. We report a search of the AllWISE Source Catalog for Planet Nine, but find no detection.
The interpretation of the expanding universe as an expansion of space has recently been challenged. From the geodesic equation in Friedmann universe models and the empty Milne model, we argue that a Newtonian or special relativistic analysis is not a
pplicable on large scales, and the general relativistic interpretation in terms of expanding space has the advantage of being globally consistent. We also show that the cosmic redshift, interpreted as an expansion effect, containts both the Doppler effect and the gravitational frequency shift.
We show that a consistent fit to observed secondary eclipse data for several strongly irradiated transiting planets demands a temperature inversion (stratosphere) at altitude. Such a thermal inversion significantly influences the planet/star contrast
ratios at the secondary eclipse,their wavelength dependences, and, importantly, the day-night flux contrast during a planetary orbit. The presence of the thermal inversion/stratosphere seems to roughly correlate with the stellar flux at the planet. Such temperature
PG1159 stars are hot, hydrogen-deficient (pre-) white dwarfs with atmospheres mainly composed of helium, carbon, and oxygen. The unusual surface chemistry is the result of a late helium-shell flash. Observed element abundances enable us to test stell
ar evolution models quantitatively with respect to their nucleosynthesis products formed near the helium-burning shell of the progenitor asymptotic giant branch stars. Because of the high effective temperatures (Teff), abundance determinations require ultraviolet spectroscopy and non-local thermodynamic equilibrium model atmosphere analyses. Up to now, we have presented results for the prototype of this spectral class and two cooler members (Teff in the range 85,000-140,000 K). Here we report on the results for two even hotter stars (PG1520+525 and PG1144+005, both with Teff = 150,000 K) which are the only two objects in this temperature-gravity region for which useful far-ultraviolet spectra are available, and revisit the prototype star. Previous results on the abundances of some species are confirmed, while results on others (Si, P, S) are revised. In particular, a solar abundance of sulphur is measured in contrast to earlier claims of a strong S deficiency that contradicted stellar evolution models. For the first time, we assess the abundances of Na, Al, and Cl with newly constructed non-LTE model atoms. Besides the main constituents (He, C, O), we determine the abundances (or upper limits) of N, F, Ne, Na, Al, Si, P, S, Cl, Ar, and Fe. Generally, good agreement with stellar models is found.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
