Dredge-up

A dredge-up is a period in the evolution of a star where a surface convection zone extends down to the layers where material has undergone nuclear fusion. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere where they can appear in the spectrum of the star.

The first dredge-up occurs when a main-sequence star enters the red-giant branch. As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: the 12C/13C and C/N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced.

The second dredge-up occurs in stars with 4–8 solar masses. When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle.[1] This second dredge-up results in an increase in the surface abundance of 4He and 14N, whereas the amount of 12C and 16O decreases.[2]

The third dredge-up occurs after a star enters the asymptotic giant branch 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.[2]

The names of the dredge-ups are set by the evolutionary and structural state of the star in which each occurs, not by the sequence experienced by the star. As a result, lower-mass stars experience the first and third dredge-ups in their evolution but not the second.

References

  1. ^ Lambert, D. L. (1992). "Observational Effects of Nucleosynthesis in Evolved Stars". In Mike G. Edmunds and Roberto J. Terlevich (eds.). Elements and the Cosmos. University of Cambridge. pp. 92–109. ISBN 0-521-41475-X.CS1 maint: Uses editors parameter (link)
  2. ^ a b Kwok, Sun (2000). The origin and evolution of planetary nebulae. Cambridge University Press. p. 199. ISBN 0-521-62313-8.
104 Aquarii

104 Aquarii (abbreviated 104 Aqr) is a star in the equatorial constellation of Aquarius. 104 Aquarii is the Flamsteed designation, although it also bears the Bayer designation A2 Aquarii. Based on an annual parallax shift of only 3.89 ± 0.25 milliarcseconds, the distance to this star is about 840 light-years (260 parsecs). At that range, the brightness of the star in the V-band is reduced by 0.10 magnitudes as a result of extinction caused by intervening gas and dust.This is a double star and possible binary system. The primary component has a stellar classification of G2 Ib/II, which places it on the borderline between the bright giant and lower luminosity supergiant stars. It has passed the first dredge-up and may be undergoind Cepheid-like pulsations. With more than four times the mass of the Sun, this is an evolved star that has reached its current stage after only 135 million years. It has expanded to around 51–88 times the Sun's radius and is radiating 447–fold the luminosity of the Sun. This energy is being emitted from its outer atmosphere at an effective temperature of 5,478 K, giving it the golden-hued glow of a G-type star. It is a suspected variable star.The companion is a magnitude 7.9 star with an angular separation of 120.1 arcseconds from the primary.

33 Piscium

33 Piscium is a binary star system in the zodiac constellation of Pisces. It is visible to the naked eye with an apparent visual magnitude of 4.61. The distance to this system, as determined from an annual parallax shift of 25.32±0.53 mas, is about 129 light years. It is moving closer to the Sun with a heliocentric radial velocity of −6.6 km/s.This system was found to have a variable radial velocity by Leah Allen and Adelaide Hobe of Lick Observatory in 1911. It was identified as a single-lined spectroscopic binary, and the orbital elements were published by Canadian astronomer W. E. Harper in 1926. The pair have an orbital period of 72.93 days and an eccentricity of 0.27. This is a RS Canum Venaticorum variable, indicating a close binary system with active star spots, and has the variable star designation BC Psc.The primary, component A, is a first-ascent red giant with a stellar classification of K0 IIIb, having chemical abundances that match a first dredge-up mixing model. Pourbaix & Boffin (2003) estimated the mass of the primary as 1.7±0.4 M☉ and the secondary as 0.76±0.11 M☉. However, Feuillet et al. (2016) derived a much lower mass estimate of 0.83±0.22 M☉ for the primary. At the age of roughly five billion years, the star has expanded to 7 times the radius of the Sun. It is radiating 24 times the Sun's luminosity from its photosphere at an effective temperature of about 4,736 K.

46 Capricorni

46 Capricorni is a solitary star located around 790 light years away from the Sun in the southern constellation of Capricornus, near the northern border with Aquarius. It is visible to the naked eye as a dim, yellow-hued point of light with an apparent visual magnitude of +5.10. 46 Cap is also known by its Bayer designation of c Capricorni (c Cap), and occasionally as c1 Capricorni to distinguish it from the nearby star c2 Capricorni. It is moving closer to the Earth with a heliocentric radial velocity of −15.5 km/s.This star has received a stellar classification of G8Iab, which suggests it is a G-type supergiant star, as well as G7.5II-IIICN0.5, which instead indicates a luminosity class between a giant and a bright giant. Abundance analysis suggests the star has not yet passed the first dredge-up. It has 4.6 times the mass of the Sun and has expanded to 33 times the Sun's radius. The star is radiating 627 times the luminosity of the Sun from its photosphere at an effective temperature of 4,837 K.

61 Leonis

61 Leonis is a possible binary star system in the zodiac constellation of Leo. It is faintly visible to the naked eye, having an apparent visual magnitude of 4.73. The star is moving closer to the Sun with a heliocentric radial velocity of −12.7 km/s. It is located roughly 580 light years from the Sun, as determined from its annual parallax shift of 5.58 mas.This is an evolved red giant star with a stellar classification of M0 III that Eggen (1992) listed as being on the asymptotic giant branch (AGB). It is a marginal barium star, showing an enhanced abundance of s-process elements in its outer atmosphere. This material may have been acquired during a previous mass transfer from a now white dwarf companion, or self-enriched by a dredge-up during the AGB process. The measured angular diameter after correctly for limb darkening is 3.87±0.04 mas, which, at the estimated distance of this system yields a physical size of about 74.5 times the radius of the Sun.61 Leonis is a suspected variable star with apparent magnitude changing between 4.69 and 4.79. The variability was reported in a 1966 photometric survey, but has not been confirmed by more recent photometry.

83 Ursae Majoris

83 Ursae Majoris is a candidate binary star system in the northern circumpolar constellation of Ursa Major. This is a semiregular variable star, like Mira; for that reason it has been given the variable star designation IQ Ursae Majoris. It ranges in brightness from apparent visual magnitude 4.69 to 4.75. Percy and Au (1994) identified it as a small amplitude red variable with an irregular behavior, having a characteristic time scale of 20 days. Based upon an annual parallax shift of 6.23±0.22 mas, it is located roughly 520 light years from the Sun. The system is moving closer with a heliocentric radial velocity of −18.6 km/s.

The visible component is an evolved red giant with a stellar classification of M2 III. It is a marginal barium star, showing an enhanced abundance of s-process elements in its outer atmosphere. This material may have been acquired during a previous mass transfer from a now white dwarf companion, or self-enriched by a dredge-up during the asymptotic giant branch process.

Asymptotic giant branch

The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in their lives.

Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun. Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning), and a very large envelope of material of composition similar to main-sequence stars.

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

Carbon-burning process

The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in the cores of massive stars (at least 8 M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} at birth) that combines carbon into other elements. It requires high temperatures (> 5×108 K or 50 keV) and densities (> 3×109 kg/m3).

These figures for temperature and density are only a guide. More massive stars burn their nuclear fuel more quickly, since they have to offset greater gravitational forces to stay in (approximate) hydrostatic equilibrium. That generally means higher temperatures, although lower densities, than for less massive stars. To get the right figures for a particular mass, and a particular stage of evolution, it is necessary to use a numerical stellar model computed with computer algorithms. Such models are continually being refined based on nuclear physics experiments (which measure nuclear reaction rates) and astronomical observations (which include direct observation of mass loss, detection of nuclear products from spectrum observations after convection zones develop from the surface to fusion-burning regions – known as dredge-up events – and so bring nuclear products to the surface, and many other observations relevant to models).

Convection zone

A convection zone, convective zone or convective region of a star is a layer which is unstable to convection. Energy is primarily or partially transported by convection in such a region. In a radiation zone, energy is transported by radiation and conduction.

Stellar convection consists of mass movement of plasma within the star which usually forms a circular convection current with the heated plasma ascending and the cooled plasma descending.

The Schwarzschild criterion expresses the conditions under which a region of a star is unstable to convection. A parcel of gas that rises slightly will find itself in an environment of lower pressure than the one it came from. As a result, the parcel will expand and cool. If the rising parcel cools to a lower temperature than its new surroundings, so that it has a higher density than the surrounding gas, then its lack of buoyancy will cause it to sink back to where it came from. However, if the temperature gradient is steep enough (i. e. the temperature changes rapidly with distance from the center of the star), or if the gas has a very high heat capacity (i. e. its temperature changes relatively slowly as it expands) then the rising parcel of gas will remain warmer and less dense than its new surroundings even after expanding and cooling. Its buoyancy will then cause it to continue to rise. The region of the star in which this happens is the convection zone.

Giant star

A giant star is a star with substantially larger radius and luminosity than a main-sequence (or dwarf) star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.Giant stars have radii up to a few hundred times the Sun and luminosities between 10 and a few thousand times that of the Sun. Stars still more luminous than giants are referred to as supergiants and hypergiants.

A hot, luminous main-sequence star may also be referred to as a giant, but any main-sequence star is properly called a dwarf no matter how large and luminous it is.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

Nu Cephei

Nu Cephei (ν Cephei) is a class A2, fourth-magnitude supergiant star in the constellation Cepheus. It is a white pulsating α Cygni variable star located about 4,700 light-years from Earth.

ν Cephei is a member of the Cepheus OB2 stellar association, which includes stars such as μ Cephei and VV Cephei. It began life as an approximately 20 M☉ star around eight million years ago. It has now exhausted its core hydrogen and expanded and cooled into a supergiant. Elemental abundance analyses indicate that it has not yet spent time as a red supergiant, which would have brought about convection of fusion products to the surface in a Dredge-up.ν Cephei is currently about 15 times as massive as the sun, 190 times as large, and 100,000 times as luminous. Its large size and luminosity cause it to be somewhat unstable and produce irregular pulsations. This is a common feature of class A and B supergiants, which are grouped as α Cygni variable stars. The brightness changes by at most a tenth of a magnitude.

Pi Hydrae

Pi Hydrae (π Hya, π Hydrae) is a star in the constellation Hydra with an apparent visual magnitude of 3.3, making it visible to the naked eye. Parallax measurements put this star at a distance of about 101 light-years (31 parsecs) from the Earth.

The spectrum of this star shows it to have a stellar classification of K1 III-IV, with the luminosity class of 'III-IV' suggesting it is in an evolutionary transition stage somewhere between a subgiant and a giant star. It has a low projected rotational velocity of 2.25 km s−1. Pi Hydrae is radiating energy from its outer envelope with an effective temperature of 4,670 K, giving it the orange hue of a K-type star.Pi Hydrae is a type of giant known as a cyanogen-weak star, which means that its spectrum displays weak absorption lines of CN− relative to the metallicity. (The last is a term astronomers use when describing the abundance of elements other than hydrogen and helium.) Otherwise, it appears to be a normal star of its evolutionary class, having undergone first dredge-up of nuclear fusion by-products onto its surface layers. The measured angular diameter of this star, after correction for limb darkening, is 3.76 ± 0.04 mas. At its estimated distance, this yields a physical size of about 12–13 times the radius of the Sun. It has an estimated mass of 2.45 times the mass of the Sun.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Red giant

A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses (M☉)) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K (4,700 °C; 8,500 °F) or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.

The most common red giants are stars on the red-giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Other red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen burning shell just beyond that.

Rho Draconis

Rho Draconis (ρ Draconis) is a solitary star in the northern circumpolar constellation of Draco. It is faintly visible to the naked eye with an apparent visual magnitude is 4.52. Based upon an annual parallax shift of 7.61 mas as measured from Earth, it is located around 429 light years from the Sun. At that distance, the visual magnitude of the star is diminished by an extinction factor of 0.027 due to interstellar dust.With a stellar classification of K3 III, Rho Draconis is a normal giant star that is past the first dredge-up phase of its post-main sequence evolution. It has the peculiar spectrum of a CN star, showing abnormal line strengths for cyanogen and calcium. The star has expanded to around 28 times the Sun's radius and it is radiating 402 times the solar luminosity from its photosphere at an effective temperature of 4,370 K.

Tau Draconis

Tau Draconis, Latinized from τ Draconis, is an astrometric binary star system in the northern circumpolar constellation of Draco. The star is faintly visible to the naked eye, having an apparent visual magnitude of 4.45. Based upon an annual parallax shift of 22.28 mas as measured from Earth, it is located around 146 light years from the Sun. Its proper motion is propelling it across the sky at the rate of 0.176 arc seconds per year.This is a K-type giant star with a stellar classification of K2 III:, where the semi-colon indicates some uncertainty about its spectral value. It is considered metal-rich star and is past the first dredge-up phase of its post-main sequence evolution, although it shows under-abundances of carbon and oxygen in its spectrum. The star has 1.25 times the mass of the Sun and is an estimated 6.48 billion years old. It is radiating 48 times the solar luminosity from its enlarged photosphere at an effective temperature of 4,413 K.

Technetium star

A technetium star, or more properly a Tc-rich star, is a star whose stellar spectrum contains absorption lines of the light radioactive metal technetium. The most stable isotope of technetium is 98Tc with a half-life of 4.2 million years, which is too short a time to allow the metal to be material from before the star's formation. Therefore, the detection in 1952 of technetium in stellar spectra provided unambiguous proof of nucleosynthesis in stars, one of the more extreme cases being R Geminorum.Stars containing technetium belong to the class of asymptotic giant branch stars (AGB)—stars that are like red giants, but with a slightly higher luminosity, and which burn hydrogen in an inner shell. Members of this class of stars switch to helium shell burning with an interval of some 100,000 years, in "dredge-ups". Technetium stars belong to the classes M, MS, S, SC and C-N. They are most often variable stars of the long period variable types.

Current research indicate that the presence of technetium in AGB stars occurs after some evolution, and that a significant number of these stars do not exhibit the metal in their spectra. The presence of technetium seems to be related to the "third dredge-up" in the history of the stars.

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

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