Non-LTE model atmosphere analyses of faint PN central stars observed with Keck HIRES

1997 ◽  
Vol 180 ◽  
pp. 120-121
Author(s):  
James K. McCarthy ◽  
Roberto H. Méndez ◽  
R.-P. Kudritzki
Keyword(s):  
High Resolution ◽  
Signal To Noise Ratio ◽  
Model Atmosphere ◽  
Helium Abundance ◽  
Line Profiles ◽  
Signal To Noise ◽  
Echelle Spectrograph ◽  
First Results ◽  
Central Stars ◽  
Temperature Surface

We are engaged in using the HIRES echelle spectrograph (Vogt et al. 1994) on the 10 m Keck I Telescope to significantly increase the number of central stars of planetary nebulae (CSPN) studied spectroscopically at high resolution and signal-to-noise ratio. With Keck we are able to extend our previous work (Méndez et al. 1988, 1992; McCarthy 1988) to much fainter magnitudes. In short, comparisons of the observed HI Balmer, HeI, and He II line profiles to the Munich grid of plane-parallel non-LTE model atmosphere line profiles provide distance- and nebula-independent determinations of CSPN effective temperature, surface gravity, and helium abundance. For CSPN showing wind emission, the comparisons are made to new “unified” models (reviewed by Kudritzki et al., this meeting) which include radiation-driven winds. The first results of this on-going program are shown below.

scholarly journals Non-LTE Model Atmosphere Analysis of Central Stars

1983 ◽  
Vol 103 ◽  
pp. 343-357 ◽  
Author(s):  
R.H. Méndez ◽  
R. P. Kudritzki ◽  
K. P. Simon
Keyword(s):  
Absorption Line ◽  
Model Atmosphere ◽  
Helium Abundance ◽  
Line Profiles ◽  
Atmospheric Parameters ◽  
Central Stars

This review will be concentrated on the determination of the main atmospheric parameters (Teff, log g, helium abundance) of PN nuclei, and of other subluminous objects, by fitting the observed absorption line profiles with theoretical profiles obtained from non-LTE model atmosphere calculations.

scholarly journals Unified Model Atmosphere Studies of Central Stars of Planetary Nebulae

1993 ◽  
Vol 155 ◽  
pp. 82-82
Author(s):  
R. Gabler ◽  
A. Gabler ◽  
R. H. Méndez ◽  
R. P. Kudritzki
Keyword(s):  
High Resolution ◽  
Absorption Line ◽  
Spectroscopic Analysis ◽  
Planetary Nebulae ◽  
Model Atmosphere ◽  
Unified Model ◽  
Parallel Model ◽  
Line Profiles ◽  
Atmospheric Parameters ◽  
Central Stars

A first step in the accurate quantitative spectroscopic analysis of central stars of PN has been based on fitting the results of NLTE, hydrostatic, plane-parallel model atmosphere calculations to the observed H and He absorption-line profiles in high-resolution spectra of bright central stars (Méndez et al. 1988, A&A 190, 113 and subsequent papers). Such analyses have provided very useful determinations of the basic atmospheric parameters: Teff, log g and He abundance.

scholarly journals Analysis of High Resolution Stellar Line Profiles

1980 ◽  
Vol 51 ◽  
pp. 75-84
Author(s):  
David F. Gray
Keyword(s):  
High Resolution ◽  
Spectral Line ◽  
Spectral Resolution ◽  
Signal To Noise Ratio ◽  
Physical Phenomenon ◽  
Velocity Fields ◽  
Line Profiles ◽  
Late Type ◽  
Signal To Noise ◽  
Line Shapes

High resolution implies that we obtain some information on spectral line shapes. In late-type stars, we need to measure velocities of a few km/sec to accomplish this. Increasing the spectral resolution and the signal to noise ratio allows us to progress step by step toward deeper physical understanding. The steps we take often lead to good debate and “stimulate” our lives. I am sometimes amused at the urgency we feel to press on to the next step. We rarely seem to pause and enjoy the completion of previous steps. Perhaps this is because we always see shortcomings in completed work. Quite typically one will “discover” the importance of some physical phenomenon (It makes little difference how many others already know about it.), and in his eyes everything done previously becomes wrong because this phenomenon was not included. We used to hear how Milne-Eddington or Schuster-Schwarzschild model atmospheres were inadequate -we had to use instead properly computed depth dependence. We used to hear how LTE models were no good - we had to use more detailed physics. Now we talk about line analyses being inadequate because it has not included velocity fields. The curious thing is that we believe that including our pet phenomenon gives the correct models. We ignore all those other phenomena as yet unseen! (Is this a mechanism for maintaining sanity?) I think it really amounts to a statement of what we are able to measure, compute, or understand.

scholarly journals On cross-correlations, averages and noise in electron microscopy

2019 ◽  
Vol 75 (1) ◽  
pp. 12-18 ◽  
Author(s):  
Michael Radermacher ◽  
Teresa Ruiz
Keyword(s):  
High Resolution ◽  
Cross Correlation ◽  
Signal To Noise Ratio ◽  
3D Structure ◽  
Signal To Noise ◽  
High Resolution Structure ◽  
Multiple Copies ◽  
High Level ◽  
2D Images ◽  
Resolution Structure

Biological samples are radiation-sensitive and require imaging under low-dose conditions to minimize damage. As a result, images contain a high level of noise and exhibit signal-to-noise ratios that are typically significantly smaller than 1. Averaging techniques, either implicit or explicit, are used to overcome the limitations imposed by the high level of noise. Averaging of 2D images showing the same molecule in the same orientation results in highly significant projections. A high-resolution structure can be obtained by combining the information from many single-particle images to determine a 3D structure. Similarly, averaging of multiple copies of macromolecular assembly subvolumes extracted from tomographic reconstructions can lead to a virtually noise-free high-resolution structure. Cross-correlation methods are often used in the alignment and classification steps of averaging processes for both 2D images and 3D volumes. However, the high noise level can bias alignment and certain classification results. While other approaches may be implicitly affected, sensitivity to noise is most apparent in multireference alignments, 3D reference-based projection alignments and projection-based volume alignments. Here, the influence of the image signal-to-noise ratio on the value of the cross-correlation coefficient is analyzed and a method for compensating for this effect is provided.

scholarly journals Recent Results on Abundance Determinations

1993 ◽  
Vol 138 ◽  
pp. 27-41
Author(s):  
Saul J. Adelman
Keyword(s):  
Signal To Noise Ratio ◽  
Model Atmosphere ◽  
Bulletin Board ◽  
High Signal ◽  
Rotating Stars ◽  
Signal To Noise ◽  
Peculiar Stars ◽  
Noise Data ◽  
Noise Ratio ◽  
Moving Groups

AbstractI review abundance determinations of normal B5-F4 and peculiar stars published since 1984. Several analyses performed with photographic spectrograms indicate interesting stars which should be analyzed with high signal-to-noise data. Studies of stars of known ages which belong to clusters, associations, and moving groups should led to the most direct confrontations with theory. The increase in signal-to-noise ratio provided by electronic detectors with respect to photographic plates should allow accurate analyses of moderating rotating stars. High resolution, high signal-to-noise ratio studies have revealed crucial information about the line profiles of Sirius, Vega, and other A stars. It would aid comparison of analyses if we could agree on a standard set of gf-values and line damping constants. A computer bulletin board would be a useful means to provide and maintain such data as well as model atmosphere codes.

scholarly journals Spectrophotometric studies of Planetary Nebulae with [WR] central stars

1997 ◽  
Vol 180 ◽  
pp. 269-269
Author(s):  
M. Peña ◽  
G. Stasińska ◽  
C. Esteban ◽  
R. Kingsburgh ◽  
L. Koesterke ◽  
...  
Keyword(s):  
Ionizing Radiation ◽  
Radiation Field ◽  
Planetary Nebulae ◽  
Model Atmosphere ◽  
The Other ◽  
Self Consistent ◽  
First Results ◽  
Photoionization Model ◽  
Central Stars

We present the first results of a project on PNe with [WR] nuclei whose aim is twofold. One is to search for possible spatial abundance variations inside the nebula. The other is to check whether, for each object, one can build a self-consistent photoionization model (with the code PHOTO, Stasińska 1990, A&AS, 83, 501) using, as an input, the ionizing radiation field from an expanding model atmosphere reproducing the observed stellar lines of He, C and O (Koesterke et al., these proceedings).

scholarly journals Temperature Scale and Iron Abundances of Very Hot Central Stars of Planetary Nebulae

2003 ◽  
Vol 209 ◽  
pp. 169-176 ◽  
Author(s):  
Klaus Werner ◽  
Jochen L. Deetjen ◽  
Stefan Dreizler ◽  
Thomas Rauch ◽  
Jeff W. Kruk
Keyword(s):  
Temperature Scale ◽  
Model Atmosphere ◽  
Uv Spectra ◽  
Stellar Surface ◽  
Line Profiles ◽  
Considerable Uncertainty ◽  
Metal Abundances ◽  
Effective Temperatures ◽  
Central Stars

The determination of effective temperatures of very hot central stars (Teff > 70 000 K) by model atmosphere analyses of optical H and He line profiles is afflicted with considerable uncertainty, primarily due to the lack of neutral helium lines. Ionization balances of metals, accessible only with UV lines, allow more precise temperature estimates. The potential of iron lines is pointed out. At the same time iron and other metal abundances, hardly investigated until today, may be derived from UV spectra. We describe recent HST spectroscopy performed for this purpose.A search for iron lines in FUV spectra of the hottest H-deficient central stars (PG1159-type, Teff >100000 K) taken with FUSE was unsuccessful. The derived deficiency is interpreted in terms of iron depletion due to n-capture nucleosynthesis in intershell matter, which is now exposed at the stellar surface as a consequence of a late He shell flash.

Development of a Philips cryo-TEM provided with a liquid helium cooled tilt stage and a vacuum transfer system

1998 ◽  
Vol 4 (S2) ◽  
pp. 400-401
Author(s):  
R. Wagner ◽  
A.F. de Jong ◽  
A.G. Koster ◽  
R. Morrison ◽  
F. Tothill ◽  
...  
Keyword(s):  
High Resolution ◽  
Liquid Helium ◽  
Signal To Noise Ratio ◽  
Transfer System ◽  
Helium Temperature ◽  
Single Particles ◽  
Signal To Noise ◽  
Specimen Holder ◽  
2D Crystals ◽  
Beam Damage

In order to reduce beam damage, biological TEM specimens are often observed at temperatures close to the boiling point of liquid nitrogen (77 K). Recently, encouraging results on single particles as well as on 2D crystals have appeared, derived from images taken near liquid helium temperature (4 K), in dedicated TEMs. At these temperatures the high resolution frequencies are much better preserved, increasing the allowable dose and thus the signal to noise ratio.4 Here we present the design of a new dedicated Philips He-TEM which combines the full functionality of a CM300 TWIN with a vacuum transfer system and a liquid helium cooled specimen holder.A schematic overview of the Cryo-TEM is shown in figure 1. The key differences compared to a standard CM microscope are: 1) The tip of the specimen rod is cooled below 10 K and the rod itself cannot be taken out of the goniometer (CompuStage). 2) The specimen enters the column on the opposite side.

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