Cool stars

Our sun is, of course, a star. It is a relatively cool star and, as such, contains a number of diatomic molecules (see Figure 25-3). There are many stars, however, with still lower surface temperatures and these contain chemical species whose presence can be understood in terms of the temperatures and the usual chemical equilibrium principles. For example, as the star temperature drops, the spectral lines attributed to CN and CH become more prominent. At lower temperatures, TiO becomes an important species along with the hydrides MgH, SiH, and A1H, and oxides ZrO, ScO, YO, CrO, AlO, and BO. 

The Hubble mid-UV echelle spectra extend blueward to 2130 A for hot stars, or to 2380 A or 2885 A for cooler ones. Redward they extend to 3120 A for faint stars, to 2885 A for bright hot stars, and to 3150 A for bright cool stars. These spectra are flux-calibrated, which fixes the mid-UV continuum normalization. 

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. 

R.Pallavicini, S. Randich, P. Sestito Lithium abundances in intermediate age and old clusters . In 13th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, ed. by F. Favata et al. (ESA, Special Publication), in press (2004)... 

The art of computing model stellar atmospheres has progressed rapidly with increases in computer power. A guide to atmospheres of cool stars is given by B. Gustafsson, Chemical Analyses of Cool Stars , Ann. Rev. Astr. Astrophys., 27, 701, 1989. 

The lithium resonance doublet line X 6707 is fairly easy to observe in cool stars of spectral types F and later, and it has also been detected in diffuse interstellar clouds. There is thus an abundance of data, although in the ISM the estimation of an abundance is complicated by ionization and depletion on to dust grains. The youngest stars (e.g. T Tauri stars that are still in the gravitational contraction phase before reaching the main sequence) have a Li/H ratio that is about the same as the Solar System ratio derived from meteorites, Li/H = 2 x 10-9, which is thus taken as the Population I standard. 

Stars further along in their life cycle are often cooler and redder than younger stars. Shells of dust that has been condensed from material ejected from these cool stars often surround them. Such circumstellar dust shells, heated by the stars, emit strongly in the infrared with a spectrum characteristic of absorption bands in the dust the emissivity of a small particle is equal to its absorption efficiency (see Section 4.7). An excellent review of circumstellar dust has been given by Ney (1977). 

Treffers, R., and M. Cohen, 1974. High-resolution spectra of cool stars in the 10- and 20-micron regions, Astrophys. J., 188, 545-552. 

Of a special astronomical interest is the absorption due to pairs of H2 molecules which is an important opacity source in the atmospheres of various types of cool stars, such as late stars, low-mass stars, brown dwarfs, certain white dwarfs, population III stars, etc., and in the atmospheres of the outer planets. In short absorption of infrared or visible radiation by molecular complexes is important in dense, essentially neutral atmospheres composed of non-polar gases such as hydrogen. For a treatment of such atmospheres, the absorption of pairs like H-He, H2-He, H2-H2, etc., must be known. Furthermore, it has been pointed out that for technical applications, for example in gas-core nuclear rockets, a knowledge of induced spectra is required for estimates of heat transfer [307, 308]. The transport properties of gases at high temperatures depend on collisional induction. Collision-induced absorption may be an important loss mechanism in gas lasers. Non-linear interactions of a supermolecular nature become important at high laser powers, especially at high gas densities. 

K. H. Hinkle and W. W. G. Scharlach 1985, in "Cool Stars with Excesses of Heavy... 

Johnson, H. R. 1985, Cool Stars with Excess of Heavy Elements, ed. M. Jaschek and P. C. Keenan (Reidel Dordrecht), 271. 

But such behaviour is not really restricted to the Luminous Blue Variables. Humphreys (1987) described a cool star ("variable A") that shows the same behaviour, and so does the cool hypergiant HR 8752 (Piters et al., 1987) here an episodical mass ejection started around 1968 the star obtained a later spectral type the expelled gas remained detectable till 1980-1982. It would make sense to include such variables in the sample and to speak just of Very Luminous Variables, hence adding the word "Very" and deleting "Bright". 

Mass loss and stellar instability of cool stars... 

Mr. Plex nods. Sir, the electromagnetic spectrum includes lots of wavelengths that we can t see. Do stars exist that radiate mostly in wavelengths that we can t see Yes, I think of them as ghost stars. For example, we can t see very cool stars with very long wavelengths. ... 

Mass determines a star s luminosity and temperature. You can estimate a star s relative luminosity Lr (with respect to the Sun) from the star s relative mass Mr (with respect to the Sun) by L = Mf5. Very small, cool stars would be below the chart. For example, a brown dwarf is an astronomical object with characteristics that are intermediate between a planet and a star—and it is 50 times smaller than our Sun. The biggest stable stars have masses about 70 times the Sun s mass. ... 

Sensitive observations enable comparative surveys of silicate emission features from disks around low-mass, intermediate-mass, and Sun-like stars. While no strong correlations have been found with disk properties, flatter disks and disks around the coolest stars more often show crystalline silicate features. Cool stars and very low-mass disks display prominent crystalline silicate emission peaks (Apai et al. 2005 Merfn et al. 2001 Pascucci et al 2009). Thus, whatever processes are responsible for the presence of crystals around Sun-like stars must be capable of very efficiently producing crystals around low-mass stars, too. Interferometric measurements suggest that the amorphous/crystalline dust mass fraction is higher in the inner disk than at medium separations (van Boekel et al. 2004 Ratzka et al. 2007). The surveys also show that amorphous silicate grains frequently have similar magnesium and iron abundances in protoplanetary disks. In contrast, those with crystalline silicates are always dominated by Mg-rich grains (e.g. Malfait et al. 1998 Bouwman et al. 2008). 

From theoretical considerations it was concluded in the 1960s that carbon (graphite) or silicate dust can condense in carbon-rich or oxygen-rich1 cool stars and that this dust can be driven out from the stars by radiation pressure (Hoyle ... 

The superior sensitivity of the Spitzer Space Telescope also opened a new window to the study of silicate emission features around cool stars - too faint to be studied by other instruments. With typical luminosities of 1% and masses of 5% of... 

The abundance of silicate crystals in these disks provides strong constraints on thermal processing whatever processes are responsible for the presence of crystals around Sun-like stars must also be capable of very efficiently producing crystals around cool stars and in very low-mass disks. 

According to a tentative pha% diagram (Whittaker, 1978), carbynra are the stable forms of carbon above 2600 K, and should therefore condense in place of graphite from a carbon gas. Calculations by Clegg (1980) show, however, that the atmospheres of cool stars are too tenuous to allow carbon to condense above 2600 K by the time the vapor becomes saturated with C, temperatures are well below 2600 K, in the stability field of graphite. 

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