Photospheres

Photo-physik, /. photophysics, -plstte, /. photographic plate, -praparat, n. photographic preparation. -sphMre,/. photosphere. -Strom,... 

Abstract. We present recent advances in the determination of chemical abundances of galactic Planetary Nebulae and discuss implications resulting from the comparison with theoretical predictions. From the analysis of diagrams of abundances of N/O vs He/H, N/O vs N/H and N/O vs O/H we argue that very likely the often used solar photospheric abundance of oxygen of 8.9, in usual units, is overestimated by a factor of 2-3, as suggested by very recent work in the Sun. This would solve an astrophysical problem with the measured abundances in planetaries. 

Considering the type II-III PNe in Fig. lb coming from the more common less massive progenitors, we have two evident naive interpretations either 1) the initial metallicity of the progenitors was lower than the solar one or 2) the solar photospheric abundance used here has been overestimated. 

A Comparison of Methods for Photospheric Abundance Determinations in K-Type Stars... 

Abstract. Coronal abundances have been a subject of debate in the last years due to the availability of high-quality X-ray spectra of many cool stars. Coronal abundance determinations have generally been compared to solar photospheric abundances from this a number of general properties have been inferred, such as the presence of a coronal metal depletion with an inverse First Ionization Potential dependence, with a functional form dependent on the activity level. We report a detailed analysis of the coronal abundance of 4 stars with various levels of activity and with accurately known photospheric abundances. The coronal abundance is determined using a line flux analysis and a full determination of the differential emission measure. We show that, when coronal abundances are compared with real photospheric values for the individual stars, the resulting pattern can be very different some active stars with apparent Metal Abundance Deficiency in the corona have coronal abundances that are actually consistent with their photospheric counterparts. 

The comparison of coronal and photospheric abundances in cool stars is a very important tool in the interpretation of the physics of the corona. Active stars show a very different pattern to that followed by low activity stars such as the Sun, being the First Ionization Potential (FIP) the main variable used to classify the elements. The overall solar corona shows the so-called FIP effect the elements with low FIP (<10 eV, like Ca, N, Mg, Fe or Si), are enhanced by a factor of 4, while elements with higher FIP (S, C, O, N, Ar, Ne) remain at photospheric levels. The physics that yields to this pattern is still a subject of debate. In the case of the active stars (see [2] for a review), the initial results seemed to point towards an opposite trend, the so called Inverse FIP effect , or the MAD effect (for Metal Abundance Depletion). In this case, the elements with low FIP have a substantial depletion when compared to the solar photosphere, while elements with high FIP have same levels (the ratio of Ne and Fe lines of similar temperature of formation in an X-ray spectrum shows very clearly this effect). However, most of the results reported to date lack from their respective photospheric counterparts, raising doubts on how real is the MAD effect. 

Photospheric abundances of stars within the solar neighborhood may be quite different [1], therefore it is necessary to compare coronal and photospheric abundances of the same star in order to understand this phenomenon. We have conducted a research on a sample of stars of different activity levels, from which we... 

High activity stars have no FIP effect, but when they are compared to their own photospheric abundances, the MAD effect is not clearly present in the two cases (A And and V851 Cen). 

There are very few cases of active stars with known coronal and photospheric abundances. Therefore a detailed assessment of the presence of FIP or MAD biases can only be done in a small number of stars. More work is clearly needed. 

The abundance ratios found in the photospheres of our target stars are imprints of the explosions of the first SNe II or even more massive stars. At very low metallicities there is a reasonable hope that the SNe which have polluted the environment were themselves primordial objects. In former papers on the chemical composition of very metal poor stars, some accent was put on trends of abundance ratios with metallicity ( McWilliam et al.[6], Norris et al. [8]). Such was the case for [Mn/Fe], or [Cr/Fe] decreasing with decreasing metallicity, or... 

Fig. 1. Evolutionary tracks (labelled in Mq) and isochrones (in Myr) for low-mass stars taken from two models [8,31]. The epochs of photospheric Li depletion (and hence Li-burning in the core of a fully convective star or at the convection zone base otherwise) and the development of a radiative core are indicated. The numbers to the right of the tracks indicate the fraction of photospheric Li remaining at the point where the radiative core develops and at the end of Li burning.

Theory doesn t tell us what initial Li a star has, only what depletion it suffers. An accurate estimate of the initial Li abundance is therefore a pre-requisite before observations and models can be compared. The Sun is a unique exception, where we know the present abundance, A(Li) = 1.1 0.1 (where A(Li)= log[AT(Li)/AT(H)] + 12) and the initial abundance of A(Li)= 3.34 is obtained from meteorites. For recently born stars, the initial Li abundance is estimated from photospheric measurements in young T-Tauri stars, or from the hotter F stars of slightly older clusters, where theory suggests that no Li depletion can yet have taken place. Results vary from 3.0 < A(Li) < 3.4, somewhat dependent on assumed atmospheres, NLTE corrections and TeS scales [23,33]. It is of course quite possible that the initial Li, like Fe abundances in the solar neighbourhood, shows some cosmic scatter. Present observations certainly cannot rule this out, leading to about a 0.2 dex systematic uncertainty when comparing observations with Li depletion predictions. 

Practically all sophisticated stellar evolution models predict the existence of processes altering photospheric abundances on long timescales (see e.g. Pinson-neault, these proceedings). For example, Richard et al. [6] predict iron abundances in turnoff stars of NGC 6397 to be lower by 0.2 dex than in red giants. 

The part of the Sun that we can see is called the photosphere and has a surface temperature of 5780 K. The solar flux from every square metre of the surface is then given by Equation 2.1 ... 

This quantity is the total amount of radiation at all wavelengths radiating through the surface of the sphere and is simply the Stefan-Boltzmann Law multiplied by the surface area of the photosphere. 

A similar temperature analysis is possible for the P-branch transitions to determine the temperature, and usually the R-branch and P-branch temperatures agree. It is from analysis of the relative intensities of transitions that temperatures for the interstellar medium and the photospheres of stars may be determined. 

The hottest stars have absorption features in the photosphere associated with lighter elements, some in highly ionised states, but the lower temperature stars have a more diverse atomic composition the coolest stars show molecular emission spectra. This suggests an evolution of stars that involves the formation of heavier elements and ultimately molecules. 

The matter that made up the solar nebula from which the solar system was formed already was the product of stellar birth, aging and death, yet the Sun is 4.5 billion years old and will perhaps live to be 8 billion years but the Universe is thought to be 15 billion years old (15 Gyr) suggesting that perhaps we are only in the second cycle of star evolution. It is possible, however, that the massive clouds of H atoms, formed in the close proximity of the early Universe, rapidly formed super-heavy stars that had much shorter lifetimes and entered the supernova phase quickly. Too much speculation becomes worrying but the presence of different elements in stars and the subsequent understanding of stellar evolution is supported by the observations of atomic and molecular spectra within the light coming from the photosphere of stars. 

When the atom is in thermal equilibrium with its surroundings in the photosphere of the star, the population of the n = 2 level is given by the Boltzmann law ... 

Herzprung-Russell diagram A graphical plot of stellar intensity versus photosphere temperature showing that observed stars fall into classes main sequence, red giants, supergiants and white dwarfs. 

Luminosity The photon flux from the photosphere of a star. 

The bulk of stellar radiation comes from the surface layers or atmosphere of a star, more particularly the photosphere , which is defined as the region having optical depths for continuum radiation between about 0.01 and a few. The optical depth ti is measured inwards from the surface and represents the number of mean free paths of radiation travelling vertically outwards before it escapes from the star. It is related to the geometrical height z above some arbitrary layer by... 

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