Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userElemental Abundances in NGC 3516
53
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present RGS data from an XMM-Newton observation of the Seyfert 1 galaxy NGC 3516, taken while the continuum source was in an extreme low state. The spectrum shows numerous emission lines including the H-like lines of C, N and O and the He-like lines of N, O and Ne. These data show that the N lines are far stronger than would be expected from gas of solar abundances. Based on our photoionization models, we find that N is overabundant compared to C, O and Ne by at least a factor of 2.5. We suggest this is the result of secondary production of N in intermediate mass stars, and indicative of the history of star formation in NGC 3516.
rate research
Read More
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric elemental abundances of these stars allow to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted elemental abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars. PG1159 stars appear to be the direct progeny of [WC] stars.
X-ray spectra of AGN often contain signatures indicative of absorption in multiple layers of gas whose ionization-state and covering fraction may vary with time. It has been unclear to date how much of the observed X-ray spectral and timing behavior in AGN can be attributed to variations in absorption, versus variations in the strengths of emission or reflection components. Diagnostics of the inner regions of AGN cannot be reliably performed until the origin of observed effects is understood. We investigate the role of the X-ray absorbers in the Seyfert 1 galaxy NGC 3516. Time-averaged and flux-selected spectroscopy is used to examine the behavior of NGC 3516 observed in Chandra HETG and XMM data from Oct 2006. New H-like and He-like emission and absorption features discovered in the Fe K regime reveal a previously unknown zone of circumnuclear gas in NGC 3516 with log xi ~ 4.3 and column density 1E23 cm^-2. A lower-ionization layer with log xi ~2 and of similar column density is confirmed from previous observations, this layer has a covering fraction around 50%, and changes in covering provide a simple explanation of a deep dip in the light curve that we interpret as an eclipse of the continuum due to passage of a cloud across the sight line within half a day. These inner zones of absorbing gas are detected to have outflow velocities in the range 1000-2000 km/s, this, and constraints on radial location are consistent with an origin as part of a disk wind in NGC 3516.
Using synthetic spectra to gauge the observational consequences of altering the abundance of individual elements, I determine the observability of new Lick IDS style indices designed to target individual elements. Then using these new indices and single stellar population models, I investigate a new method to determine Balmer series emission in a Sloan Digital Sky Surveys grand average of quiescent galaxies. I also investigate the effects of an old metal-poor stellar population on the near ultra violet spectrum through the use of these new indices and find that the presence of a small old metal-poor population accounts for discrepancies observed between index trends in the near UV and optical spectral regimes. Index trends for 74 indices and three data sets are presented and discussed. Finally, I determine the near nuclear line-strength gradients of 18 red sequence elliptical Virgo cluster galaxies for 74 indices.
We test the reprocessing paradigm of the optical -- UV AGN variability by detailed modeling of the correlated X-ray -- optical (3590 and 5510 Angstrem) variability of the recent multiwavelength campaign of NGC 3516. We produce model optical light curves by convolving the observed X-ray flux with the response function of an infinite, thin accretion disk, illuminated by a point-like X-ray source at a given height above the compact object (the lamp-post model) and compare their properties to those observed. Special attention is given to the correct computation of the X-ray albedo of the disk. We further compute the X-ray reflection response at two energies (E = 1 and 20 keV) and argue for the possibility of hard lags in their cross spectra. We also compute the continuum Optical -- UV and the X-ray reflection spectra as well as the Fe K-alpha fluorescent line profiles which we also compare to observations. Despite the large (~50 percent) amplitude excursions of the X-ray flux, the model optical light curves exhibit variability amplitudes of 3 -- 4 percent, not unlike those observed. However, the model CCF between the X-ray and the model optical variations show clear lags of 0.1 and 0.25 days for black hole masses M = 10^7 and 10^8 Solar masses, respectively, not apparent in the data. The synchrony of X-ray - optical variations points toward the BH mass < 10^7 Solar masses, whereas the X-ray spectra prefer BH mass > 10^8. Our conclusion is that the combination of the observed optical/UV/X-ray spectral and timing observations are inconsistent with the lamp-post model geometry for NGC 3516.
The aim of this paper is to provide the fundamental parameters and abundances for a large sample of local clump giants with a high accuracy. The selection of clump stars for the sample group was made applying a color - absolute magnitude window to nearby Hipparcos stars. The abundances of carbon and nitrogen were obtained from molecular synthetic spectrum, the Mg and Na abundances were derived using the non-LTE approximation. The classical models of stellar evolution without atomic diffusion and rotation-induced mixing were employed. The atmospheric parameters (Teff, log g, [Fe/H], Vt) and Li, C, N, O, Na, Mg, Si, Ca and Ni abundances in 177 clump giants of the Galactic disc were determined. The underabundance of carbon, overabundance of nitrogen and normal abundance of oxygen were detected. A small sodium overabundance was found. A possibility of a selection of the clump giants based on their chemical composition and the evolutionary tracks was explored. The theoretical predictions based on the classical stellar evolution models are in good agreement with the observed surface variations of the carbon and nitrogen just after the first dredge-up episode. The giants show the same behavior of the dependencies of O, Mg, Ca, Si (alpha-elements) and Ni (iron-peak element) abundances vs. [Fe/H] as dwarfs do. This allows one to use such abundance ratios to study the chemical and dynamical evolution of the Galaxy.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
