STELLAR POPULATIONS. Erasmus Mundus Padova - Italy

Transcription

1 STELLAR POPULATIONS Erasmus Mundus Padova - Italy Prof. Antonio Bianchini Dipartimento di Astronomia Università di Padova antonio.bianchini@unipd.it

2 THE HR DIAGRAM - 5b Notes about stellar evolution The influence of metallicity Mass loss from stars Easmus Mundus Padova - Italy Prof. Antonio Bianchini

3 away from HB As we already said, the contraction of the He burning core continues, leading to a rise in surface temperature. The pressure increases on the hydrogen burning shell from the contracting envelope above, spurring on energy production and leading to an overall increase in luminosity. In addition, helium burning leads to an increasing molecular weight of the gas in the core. Eventually, the core must contract, similar to the way the H-burning core in a MS star must slowly contract, due to the ideal gas law. The contraction of the core leads to the expansion of the star's outer layers, causing its surface temperature to drop and it moves in the redward direction again. At the end of the HB phase, the star exhausts the helium in its core. The core collapses and the temperature rises enough to ignite a thick shell of He-burning. The layers above the He shell expand, which temporarily turns off the H-shell. Also, the contraction of the core produces neutrinos, which flow freely from the star, actually cooling the core somewhat. Electron degeneracy becomes important again. The expanding star cools enough for convection to deepen again, causing the second dredge-up. This leads to an observable change in surface abundances. The cooling, convective star once again approaches the Hayashi track, moving upwards onto the Asymptotic Giant Branch.

4 from the HB towards the AGB

5 The Asymptotic Giant Branch (AGB) When the C-O core contracts enough, the hydrogen shell reignites. The Asymptotic Giant Branch (AGB) is marked by a C-O core with two burning shells (H & He). The C-O shell continues to grow and contract as the He shell burns. As mentioned in the section above, the contracting C-O core becomes degenerate. With both shells burning, energy is used at an ever increasing pace and the star moves very quickly up the AGB. Stars develop periodic instabilities while on the AGB. Apparently, this part of evolution is not well understood. People generally agree that the source of the instability is in the helium-burning shell, but an interaction between the He and H burning shells is also suggested. The hydrogen-burning shell adds mass into the helium layer, causing it to become slightly degenerate. The temperature of the helium shell increases enough to lift the degeneracy and the shell experiences a flash, like the helium core flash. The hydrogen shell if forced to expand outwards and turns off for a time. The burning in the helium shell reduces and the H shell turns back on again. The process repeats itself.

6 Calculations show that these pulses may last for thousands of years for stars about 5 solar masses and hundreds of thousands of years for stars of 0.6 solar masses. Higher mass stars greater than about 2 solar masses will end up with their surface convection zone connecting with the convecting core, causing a third dredge-up, changing the surfaces abundances yet again. Stars on the AGB show enormous mass loss rates.

7 MASS LOSS IN AGB - Low mass stars lose about 0.1 M that can be derived assuming η = 0.3 in Reimers formula already used for RG stars. - Intermediate mass stars undergo Thermal Pulses for about 10 3 yr only with a modest increase of the CO nucleus. Observational data, however, suggest that the mass loss rate must be rather high. So, during a pulsational cycle of 10 yr the mass loss could be a superwind of 10-3 M yr -1 (Iben & Renzini 1984). The period of the pulsations can be given as log(p) = log(r) 0.9 log(m ) An empirical formula of the mean mass loss rate was provided by de Jager et al. (1988) However, RSG s (logl/l = 4-5.4) show mass loss rates 10 times larger. Salasnich, Chiosi et al. (1999) derived a new formula v exp = P

8 DREDGE-UP s The first dredge-up occurs when a main sequence star enters the red giant branch (RGB). As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: the 12 C/ 13 C and C/N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced. For stars with 4 8 solar masses of material, when helium fusion comes to an end at the core, convection mixes the products of the CNO cycle, resulting in the second dredge-up. This second dredge up results in an increase in the surface abundance of 4 He and 14 N, while the amount of 12 C and 16 O decreases. The third dredge-up occurs after a massive star enters the asymptotic giant branch (AGB) and a flash occurs along a helium burning shell. This dredge-up causes helium, carbon and the s- process products to be brought to the surface. The result is an increase in the abundance of carbon relative to oxygen, which can create a carbon star.

9 Low Mass Stars on the AGB For stars with masses less than about 8Mo, their cores never reach high enough temperatures to ignite their C-O and evolution stops here on the AGB. Stars less than 4Mo will never have enough mass to ignite the C-O core. We will just say here that low mass stars below 4MSun end their lives as white dwarfs and planetary nebula while slightly higher mass stars may or may not become supernovae. High Mass Stars on the AGB Stars greater than 10Mo will ignite carbon at several hundred million K before electron degeneracy becomes important. As these high mass stars evolve further, the models describe fuel exhaustion in the core, ignition of more burning shells, reignition of the core burning the next available fuel, and so on until the star represents something like a nuclear fueled onion. Each new burning sequence takes less and less time. Oxygen burning is completed in 200 days and silicon in only two days! This can't go on forever, of course. Once the nuclear reactions generate iron, the evolution has gone as far as it can go. Nuclear fusion of iron is not energetically favorable as it does not result in a net release of energy. However, the temperature is so high that nuclei are photo-disintegrated. Then, a number of complex passages lead to inverse beta decay that drastically reduce the number of particles so the final collapse occur. Because the core is so hot, a huge amount of neutrinos are produced in the core, escaping the star and causing the core to lose heat and contract even more quickly. This is where we get to the really neat part of the death of high mass stars. Basically, high mass stars will result in Type II and Type Ib Supernovae with remnant stars that have become neutron stars or black holes.

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11 Stars in the instability strip pulsate due to He III (doubly ionized helium). In normal A-F-G stars He is neutral in the stellar photosphere. Deeper below the photosphere, at about 25,000-30,000K, begins the He II layer (first He ionization). Second ionization (He III) starts at about 35,000-50,000K.When the star contracts, the density and temperature of the He II layer increase. He II starts to transform to He III (second ionization). Opacity increases and the energy flux from the interior of the star is effectively absorbed. The temperature of the layer increases and it starts to expand. After expansion, density and temperature decrease and He III begins to recombine into He II. The outer layers contract and the cycle starts from the beginning. The phase shift between a star's radial velocity pulsations and brightness variability depends on the distance of He II zone from the stellar surface in the stellar atmosphere.for low mass stars we have L=0.23M**2.3

12 Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cephei variables) undergo pulsations with very regular periods on the order of days to months. Classical Cepheids are population I variable stars which are 4 20 times more massive than the Sun, and up to 100,000 times more luminous. Cepheids are yellow supergiants of spectral class F6 K2 and their radii change by (~25% for the longer-period l Car) millions of kilometers during a pulsation cycle. Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established. Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.

13 Type II Cepheids (also termed Population II Cepheids) are population II variable stars which pulsate with periods typically between 1 and 50 days. Type II Cepheids are typically metal-poor, old (~10 Gyr), low mass objects (~half the mass of the Sun). Type II Cepheids are divided into several subgroups by period. Stars with periods between 1 and 4 days are of the BL Her subclass, days belong to the W Virginis subclass, and stars with periods greater than 20 days belong to the RV Tauri subclass.[13][14] Type II Cepheids are used to establish the distance to the Galactic center, globular clusters, and galaxies.

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24 BUUM!

25 MASS LOSS FROM LARGE MASS STARS Stars with masses larger than M UP (7-9 M ) explode as Supernovae type II. Ejecta from supernovae with masses M :

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33 Populations I and II Population I or metal-rich stars are those young stars whose metallicity is highest. The Earth's Sun is an example of a metal-rich star. These are common in the spiral arms of the Milky Way galaxy. Generally, the youngest stars, the extreme Population I, are found farther in and intermediate Population I stars are farther out, etc. The Sun is considered an intermediate Population I star. Population I stars have regular elliptical orbits of the galactic centre, with a low relative velocity. The high metallicity of Population I stars makes them more likely to possess planetary systems than the other two populations, since planets, particularly terrestrial planets, are thought to be formed by the accretion of metals.[9] Between the intermediate populations I and II comes the intermediary disc population.

34 Population II, or metal-poor stars, are those with relatively little metal. The idea of a relatively small amount must be kept in perspective as even metal-rich astronomical objects contain low quantities of any element other than hydrogen or helium; metals constitute only a tiny percentage of the overall chemical makeup of the universe, even 13.7 billion years after the Big Bang. However, metal-poor objects are even more primitive. These objects formed during an earlier time of the universe. Intermediate Population II stars are common in the bulge near centre of our galaxy; whereas Population II stars found in the galactic halo are older and thus more metal-poor. Globular clusters also contain high numbers of Population II stars.[10] It is believed that Population II stars created all the other elements in the periodic table, except the more unstable ones. Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of Timothy C. Beers et al. and the Hamburg-ESO survey of Norbert Christlieb et al., originally started for faint quasars. Thus far, they have uncovered and studied in detail about ten very metal-poor stars (as CS , CS , BD ) and two of the oldest stars known to date and also the oldest star: HE and HE and HE Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are HD (a red giant) and HD (a subgiant).

35 Population III or metal-free stars were a hypothetical population of extremely massive and hot stars with virtually no surface metals, except for a small quantity of metals formed in the Big Bang, such as lithium-7. These stars are believed to have been formed in the early universe. Their existence is inferred from cosmology, but they have not yet been observed directly. Indirect evidence for their existence has been found in a gravitationally lensed galaxy in the very distant part of the universe.[11] They are also thought to be components of faint blue galaxies. Their existence is proposed to account for the fact that heavy elements, which could not have been created in the Big Bang, are observed in quasar emission spectra, as well as the existence of faint blue galaxies. It is believed that these stars triggered a period of reionization. UDFy , a galaxy recently discovered, is believed to have been a part of this process. Current theory is divided on whether the first stars were very massive or not. One theory, which seems to be borne out by computer models of star formation, is that with no heavy elements from the Big Bang, it was easy to form stars with much more total mass than the ones visible today. Typical masses for Population III stars would be expected to be about several hundred solar masses, which is much larger than the current stars. Analysis of data on extremely low-metallicity Population II stars such as HE , which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 20 to 130 solar masses instead.[12] On the other hand, analysis of globular clusters associated with elliptical galaxies suggests pair-instability supernovae were responsible for their metallic composition.[13] This also explains why there have been no low-mass stars with zero metallicity observed, although models have been constructed for smaller Pop III stars.[14] Clusters containing zero-metallicity red dwarfs or brown dwarfs (possibly created by pair-instability supernovae[7]) have been proposed as dark matter candidates, but there is disagreement on this theory.[15][16] Confirmation of these theories awaits the launch of NASA's James Webb Space Telescope. New spectroscopic surveys, such as SEGUE or SDSS-II, may also locate Population III stars.

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