Red clump

The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a more dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, but it is also noticeable in many intermediate-age globular clusters and in nearby field stars (e.g. the Hipparcos stars).

The red clump giants are cool horizontal branch stars, stars originally similar to the Sun which have undergone a helium flash and are now fusing helium in their cores.

HRDiagram
The red clump is the prominent group of red giant stars at about 5,000 K and 75 L.

Properties

Red clump stellar properties vary depending on their origin, most notably on the metallicity of the stars, but typically they have early K spectral types and effective temperatures around 5,000 K. The absolute visual magnitude of red clump giants near the sun has been measured at an average of +0.81 with metallicities between −0.6 and +0.4 dex.[1]

There is a considerable spread in the properties of red clump stars even within a single population of similar stars such as an open cluster. This is partly due to the natural variation in temperatures and luminosities of horizontal branch stars when they form and as they evolve, and partly due to the presence of other stars with similar properties.[2] Although red clump stars are generally hotter than red-giant-branch stars, the two regions overlap and the status of individual stars can only be assigned with a detailed chemical abundance study.[3][4]

Evolution

Open cluster HR diagram ages
Old open clusters showing barely detectable red clumps[5]

Modelling of the horizontal branch has shown that stars have a strong tendency to cluster at the cool end of the zero age horizontal branch (ZAHB). This tendency is weaker in low metallicity stars, so the red clump is usually more prominent in metal-rich clusters. However, there are other effects, and there are well-populated red clumps in some metal-poor globular clusters.[6][7]

Stars with a similar mass to the sun evolve towards the tip of the red giant branch with a degenerate helium core. More massive stars leave the red giant branch early and perform a blue loop, but all stars with a degenerate core reach the tip with very similar core masses, temperatures, and luminosities. After the helium flash they lie along the ZAHB, all with helium cores just under 0.5 M and their properties determined mostly by the size of the hydrogen envelope outside the core. Lower envelope masses result in weaker hydrogen shell fusion and give hotter and slightly less luminous stars strung along the horizontal branch. Different initial masses and natural variations in mass loss rates on the red giant branch cause the variations in the envelope masses even though the helium cores are all the same size. Low-metallicity stars are more sensitive to the size of the hydrogen envelope, so with the same envelope masses they are spread further along the horizontal branch and fewer fall in the red clump.

Although red clump stars lie consistently to the hot side of the red giant branch that they evolved from, red clump and red-giant-branch stars from different populations can overlap. This occurs in ω Centauri where metal-poor red-giant-branch stars have the same or hotter temperatures as more metal-rich red clump giants.[3]

Other stars, not strictly horizontal branch stars, can lie in the same region of the H-R diagram. Stars too massive to develop a degenerate helium core on the red giant branch will ignite helium before the tip of the red giant branch and perform a blue loop. For stars only a little more massive than the sun, around 2 M, the blue loop is very short and at a luminosity similar to the red clump giants. These stars are an order of magnitude less common than sun-like stars, even rarer compared to the sub-solar stars that can form red clump giants, and the duration of the blue loop is far less than the time spent by a red clump giant on the horizontal branch. This means that these imposters are much less common in the H-R diagram, but still detectable.[2]

Stars with 2 - 3 M will also pass through the red clump as they evolve along the subgiant branch. This is again a very rapid phase of evolution, but stars such as OU Andromedae are found in the red clump region (5,500 K and 100 L) even though it is thought to be a subgiant crossing the Hertzsprung gap.[2]

Standard candles

In theory, the absolute luminosities of stars in the red clump are fairly independent of stellar composition or age so that consequently they make good standard candles for estimating astronomical distances both within our galaxy and to nearby galaxies and clusters. Variations due to metallicity, mass, age, and extinctions affect visual observations too much for them to be useful, but the effects are much smaller in the infrared. Near infrared I band observations in particular have been used to establish red clump distances. Absolute magnitudes for the red clump at solar metallicity have been measured at −0.22 in the I band and −1.54 in the K band.[8] The distance to the galactic centre has been measured in this way, giving a result of 7.52 kpc in agreement with other methods.[9]

Red bump

The red clump should not be confused with the "red bump" or red-giant-branch bump, which is a less noticeable clustering of giants partway along the red giant branch, caused as stars ascending the red giant branch temporarily decrease in luminosity because of internal convection.[10]

Examples

Many of the bright "red giants" visible in the sky are actually early K class red-clump stars:

Arcturus has sometimes been thought to be a clump giant,[13] but is now more commonly considered to be on the red giant branch, somewhat cooler and more luminous than a red-clump star.[14]

References

  1. ^ Soubiran, C.; Bienaymé, O.; Siebert, A. (2003). "Vertical distribution of Galactic disk stars". Astronomy and Astrophysics. 398: 141. arXiv:astro-ph/0210628. Bibcode:2003A&A...398..141S. doi:10.1051/0004-6361:20021615.
  2. ^ a b c Girardi, Léo (1999). "A secondary clump of red giant stars: Why and where". Monthly Notices of the Royal Astronomical Society. 308 (3): 818. arXiv:astro-ph/9901319. Bibcode:1999MNRAS.308..818G. doi:10.1046/j.1365-8711.1999.02746.x.
  3. ^ a b Ree, C. H.; Yoon, S.-J.; Rey, S.-C.; Lee, Y.-W. (2002). "Synthetic Color-Magnitude Diagrams for ω Centauri and Other Massive Globular Clusters with Multiple Populations". Omega Centauri. 265: 101. arXiv:astro-ph/0110689. Bibcode:2002ASPC..265..101R.
  4. ^ Nataf, D. M.; Udalski, A.; Gould, A.; Fouqué, P.; Stanek, K. Z. (2010). "The Split Red Clump of the Galactic Bulge from OGLE-III". The Astrophysical Journal Letters. 721: L28. arXiv:1007.5065. Bibcode:2010ApJ...721L..28N. doi:10.1088/2041-8205/721/1/L28.
  5. ^ Sarajedini, Ata (1999). "WIYN Open Cluster Study. III. The Observed Variation of the Red Clump Luminosity and Color with Metallicity and Age". The Astronomical Journal. 118 (5): 2321. Bibcode:1999AJ....118.2321S. doi:10.1086/301112.
  6. ^ Zhao, G.; Qiu, H. M.; Mao, Shude (2001). "High-Resolution Spectroscopic Observations of Hipparcos Red Clump Giants: Metallicity and Mass Determinations". The Astrophysical Journal. 551: L85. Bibcode:2001ApJ...551L..85Z. doi:10.1086/319832.
  7. ^ d'Antona, Francesca; Caloi, Vittoria (2004). "The Early Evolution of Globular Clusters: The Case of NGC 2808". The Astrophysical Journal. 611 (2): 871. arXiv:astro-ph/0405016. Bibcode:2004ApJ...611..871D. doi:10.1086/422334.
  8. ^ Groenewegen, M. A. T. (2008). "The red clump absolute magnitude based on revised Hipparcos parallaxes". Astronomy and Astrophysics. 488 (3): 935. arXiv:0807.2764. Bibcode:2008A&A...488..935G. doi:10.1051/0004-6361:200810201.
  9. ^ Nishiyama, Shogo; Nagata, Tetsuya; Sato, Shuji; Kato, Daisuke; Nagayama, Takahiro; Kusakabe, Nobuhiko; Matsunaga, Noriyuki; Naoi, Takahiro; Sugitani, Koji; Tamura, Motohide (2006). "The Distance to the Galactic Center Derived from Infrared Photometry of Bulge Red Clump Stars". The Astrophysical Journal. 647 (2): 1093. arXiv:astro-ph/0607408. Bibcode:2006ApJ...647.1093N. doi:10.1086/505529.
  10. ^ Alves, David R.; Sarajedini, Ata (1999). "The Age-dependent Luminosities of the Red Giant Branch Bump, Asymptotic Giant Branch Bump, and Horizontal Branch Red Clump". The Astrophysical Journal. 511: 225. arXiv:astro-ph/9808253. Bibcode:1999ApJ...511..225A. doi:10.1086/306655.
  11. ^ a b Ayres, Thomas R.; Simon, Theodore; Stern, Robert A.; Drake, Stephen A.; Wood, Brian E.; Brown, Alexander (1998). "The Coronae of Moderate-Mass Giants in the Hertzsprung Gap and the Clump". The Astrophysical Journal. 496: 428. Bibcode:1998ApJ...496..428A. doi:10.1086/305347.
  12. ^ Sato, Bun'ei; et al. (2007). "A Planetary Companion to the Hyades Giant ε Tauri". The Astrophysical Journal. 661 (1): 527–531. Bibcode:2007ApJ...661..527S. doi:10.1086/513503.
  13. ^ Maeckle, R.; Holweger, H.; Griffin, R.; Griffin, R. (1975). "A model-atmosphere analysis of the spectrum of Arcturus". Astronomy and Astrophysics. 38: 239. Bibcode:1975A&A....38..239M.
  14. ^ Ramírez, I.; Allende Prieto, C. (2011). "Fundamental Parameters and Chemical Composition of Arcturus". The Astrophysical Journal. 743 (2): 135. arXiv:1109.4425. Bibcode:2011ApJ...743..135R. doi:10.1088/0004-637X/743/2/135.

External links

109 Tauri

109 Tauri, or n Tauri, is a single, yellow-hued star in the zodiac constellation of Taurus. It has an apparent visual magnitude of 4.96 and is faintly visible to the naked eye. The star has an annual parallax shift of 13.19±0.30 mas, putting it around 247 light years from the Sun. At that distance, the visual magnitude is diminished by an extinction of 0.24 due to interstellar dust. It is moving further from the Sun with a heliocentric radial velocity of +19 km/s.This is an evolved giant star with a stellar classification of G8 III, having consumed the hydrogen at its core and moved off the main sequence. At the age of 600 million years, it has become a red clump giant, indicating that it is on the horizontal branch and is generating energy through helium fusion at its core. The star has an estimated 2.47 times the mass of the Sun and has expanded to around eight times the Sun's radius. It is radiating about 60 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 5,035 K.

110 Virginis

110 Virginis is a star in the zodiac constellation Virgo, located 195 light years away from the Sun. It is visible to the naked eye as an orange-hued star with an apparent visual magnitude of 4.40. The star is moving closer to the Earth with a heliocentric radial velocity of −16 km/s.The stellar classification of 110 Virginis is K0.5 IIIb Fe–0.5, indicating that this is an evolved giant star with a mild underabundance of iron in its spectrum. At the age of 4.5 billion years old, it belongs to a sub-category of giants called the red clump, which means it is on the horizontal branch and is generating energy through the helium fusion at its core. Compared to the Sun, it has 167% of the mass but has expanded to 11 times the size. The enlarged photosphere has an effective temperature of 4,664 K and is radiating 76 times the Sun's luminosity.

15 Cygni

15 Cygni is a single star in the northern constellation Cygnus. With an apparent visual magnitude of 4.90, it is a faint star but visible to the naked eye. The distance to 15 Cygni can be estimated from its annual parallax shift of 11.0 mas, which yields a separation of some 296 light years. It is moving closer to the Sun with a heliocentric radial velocity of −23.6 km/s.This is an aging giant star with a stellar classification of G8 III, having consumed the hydrogen at its core and evolved off the main sequence. It is a red clump giant, which means it is generating energy via helium fusion at its core. The star is 1.50 billion years old with 2.3 times the mass of the Sun, and has expanded to 12 times the Sun's radius. It is radiating 93 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,920 K.

30 Aquarii

30 Aquarii is a single star located about 301 light years away from the Sun in the zodiac constellation of Aquarius. 30 Aquarii is its Flamsteed designation. It is visible to the naked eye as a dim, orange-hued star with an apparent visual magnitude of 5.56. The star is moving further from the Earth with a heliocentric radial velocity of 40 km/s.This object is an aging G-type giant star with a stellar classification of G8 III, although Houk and Swift (1999) found a class of K1 IV. It is a red clump giant, which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. The star is nearly two billion years old with a leisurely rotation rate, showing a projected rotational velocity of 1.6 km/s. It has double the mass of the Sun and has expanded to ten times the Sun's radius. The star is radiating 55 times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,944 K.

41 Aquarii

41 Aquarii (abbreviated 41 Aqr) is a double star in the equatorial constellation of Aquarius. 41 Aquarii is its Flamsteed designation. Its apparent magnitude is 5.354 and is located at a distance of around 230 light-years (71 parsecs) from Earth. The brighter component is a red clump giant star with a stellar classification of K0 III and a magnitude of 5.73. At an angular separation of 5.148 arcseconds, the fainter companion is an F-type main sequence star with a magnitude 7.16 and a classification of F8 V.

49 Andromedae

49 Andromedae (abbreviated 49 And) is a star in the constellation Andromeda. 49 Andromedae is the Flamsteed designation though it also bears the Bayer designation A Andromedae. It is visible to the naked eye under good viewing conditions with an apparent visual magnitude of 5.269. The distance to 49 Andromedae, as determined from its annual parallax shift of 10.4 mas, is around 314 light years. It is moving closer to the Sun with a heliocentric radial velocity of −11.5 km/s.With an estimated age of 1.75 Gyr years, this is an aging red clump giant star with a stellar classification of K0 III, indicating it is generating energy by helium fusion at its core. The spectrum displays "slightly strong" absorption lines of cyanogen (CN). It has 2.07 times the mass of the Sun and has expanded to 11 times the Sun's radius. The star is radiating 71 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,879 K.

49 Aquarii

49 Aquarii, abbreviated 49 Aqr, is a star in the zodiac constellation of Aquarius. 49 Aquarii is its Flamsteed designation. It is a dim star with an apparent visual magnitude of 5.53. The distance to 49 Aqr, as determined from its annual parallax shift of 12.28 mas, is 266 light years. It is moving closer to the Earth with a heliocentric radial velocity of −13 km/s.This is an aging K-type giant star with a stellar classification of K0 III CN II. It shows a spectral anomaly with the absorption lines of cyanogen (CN). This is a red clump giant, indicating that it is generating energy through the helium fusion at its core. It is around 950 million years old with 2.2 times the mass of the Sun and has expanded to nine times the Sun's radius. It is radiating 50 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,954 K.

68 Aquarii

68 Aquarii is a single star located 270 light years away from the Sun in the zodiac constellation of Aquarius. 68 Aquarii is its Flamsteed designation, though it also bears the Bayer designation of g2 Aquarii. It is visible to the naked eye as a dim, yellow-hued star with an apparent visual magnitude of 5.24. The object is moving further from the Earth with a heliocentric radial velocity of +24.5 km/s.This star is 3.79 billion years old with a stellar classification of G8 III, indicating the is a giant star that has exhausted the hydrogen at its core and expanded off the main sequence. It is a red clump giant, which means it is on the horizontal branch and is generating energy through helium fusion at its core. It has 1.39 times the mass of the Sun and 10 times the Sun's radius. The star is radiating 59 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 5,036 K.

69 Aquilae

69 Aquilae, abbreviated 69 Aql, is a star in the equatorial constellation of Aquila. 69 Aquilae is its Flamsteed designation. It is visible to the naked eye with an apparent visual magnitude of 4.91. Based upon an annual parallax shift of 16.2 mas, it is located 201 light years away. The star is moving closer to the Earth with a heliocentric radial velocity of −22.5 km/s.The stellar classification of 69 Aquilae is K1/2 III, which means this is an evolved giant star. It belongs to a sub-category called the red clump, indicating that it is on the horizontal branch and is generating energy through helium fusion at its core. The star is about 3.4 billion years old with 1.54 times the mass of the Sun and has expanded to 11 times the Sun's radius. It is radiating 45.7 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,529 K.

71 Cygni

71 Cygni is a star in the northern constellation of Cygnus, located 212 light years from the Sun. 71 Cygni is the Flamsteed designation; it has the Bayer designation g Cygni. It is visible to the naked eye as a dim, orange-hued star with an apparent visual magnitude of 5.22. The star is moving closer to the Earth with a heliocentric radial velocity of −21.5 km/s.At the age of one billion years, this is an evolved giant star with a stellar classification of K0− III, which means it has used up its core hydrogen and expanded. It is a red clump giant, indicating that it is on the horizontal branch of the Hertzsprung–Russell diagram and is generating energy by helium fusion at its center. The star has double the mass of the Sun and eight times the Sun's radius. It is radiating 45 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,983 K.

89 Virginis

89 Virginis is a single star in the zodiac constellation of Virgo, located 234 light years from the Sun. It is visible to the naked eye as a dim, orange-hued star with an apparent visual magnitude of 4.959. The star is moving closer to the Earth with a heliocentric radial velocity of −39 km/s.This is an evolved giant star with a stellar classification of K0 III, having exhausted the hydrogen at its core and expanded away from the main sequence. It is a red clump star, which indicates it is generating energy through helium fusion at its core. This object is 3.4 billion years old with 1.7 times the mass of the Sun and 12 times the Sun's radius. It is radiating 69 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4706 K.

HD 23319

HD 23319 (h Eridani) is an orange-hued star in the constellation Eridanus. It has an apparent visual magnitude of 4.59, which is sufficiently bright to make it visible to the naked eye. The distance to HD 23319 can be estimated from its annual parallax shift of 19.2 mas, which yields a range of about 170 light years. It is moving further away from the Earth with a heliocentric radial velocity of +9.9 km/s.This is an aging giant star with a stellar classification of K1.5 IIIb CN0.5, where the suffix notation indicates the spectrum shows a mild overabundance of the cyano radical. It is a red clump giant, indicating it is on the horizontal branch and is generating energy through helium fusion at its core. The star has 1.2 times the mass of the Sun and, at the age of 4.6 billion years, has expanded to 11 times the Sun's radius. It is radiating 63 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,581 K.

HD 85859

HD 85859 is a single star in the equatorial constellation of Hydra. It has an apparent visual magnitude of 4.88, which is bright enough to be visible to the naked eye. The distance to this star, as estimated from its annual parallax shift of 12.6 mas, is 259 light years. It is moving closer to the Earth with a heliocentric radial velocity of 50.5 km/s.The stellar classification of the visible component is K2+ III CN0.5, which matches an evolved K-type giant star with a mild overabundance of CN in the atmosphere. At the age of 4.34 billion years, it is a red clump star, which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. The star has 1.55 times the mass of the Sun and is radiating 178 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,415 K.

Horizontal branch

The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) and by hydrogen fusion (via the CNO cycle) in a shell surrounding the core. The onset of core helium fusion at the tip of the red giant branch causes substantial changes in stellar structure, resulting in an overall reduction in luminosity, some contraction of the stellar envelope, and the surface reaching higher temperatures.

Kappa Aurigae

Kappa Aurigae, Latinized as κ Aurigae, is the Bayer designation for a star in the northern constellation of Auriga. It is visible to the naked eye with an apparent visual magnitude of 4.3. Based upon an annual parallax shift of 19.31 mas, it is approximately 169 light-years (52 parsecs) distant from Earth.

This is an evolved giant star with a stellar classification of G8.5 IIIb. It is a red clump star that is generating energy through the fusion of helium at its core. Kappa Aurigae has expanded to 11 times the radius of the Sun and shines with 54 times the Sun's luminosity. This energy is radiated into outer space from the outer envelope at an effective temperature of 4,732 K. At this heat, the star glows with the orange-hue of a G-type star.

Mu Aquilae

Mu Aquilae (μ Aql, μ Aquilae) is the Bayer designation for a star in the equatorial constellation of Aquila. With an apparent visual magnitude of 4.45, it is visible to the naked eye. The measured annual parallax shift of 30.31 mas, which is equivalent to a distance of 107.6 light-years (33.0 parsecs) from Earth.

The stellar classification of Mu Aquilae is K3 III, indicating that this is an evolved giant star. It belongs to a sub-category called the red clump, indicating that it is generating energy through the fusion of helium at its core. Compared to the Sun, it has 92% of the mass but has expanded to eight times the size. This inflated outer envelope has an effective temperature of 4,467 K and is radiating 24.5 times the Sun's luminosity. At this heat, Mu Aquilae glows with the orange hue of a K-type star.

Nu Aurigae

Nu Aurigae, Latinized from ν Aurigae, is the Bayer designation for a star in the northern constellation of Auriga. It is visible to the naked eye with an apparent visual magnitude of 3.96 and is approximately 220 light-years (67 parsecs) distant from the Earth. This is an evolved giant star with a stellar classification of G9.5 III. It is a red clump star, which indicates that it is generating energy through the fusion of helium at its core. The outer envelope has expanded to 19 times the radius of the Sun and cooled to 4,571 K, giving it the characteristic yellow-hued glow of a G-type star. It shines with 135 times the luminosity of the Sun.This is an astrometric binary with a suspected white dwarf companion. A 10th magnitude star 54.6 arcseconds away is an optical companion.

Q Scorpii

Q Scorpii (Q Sco) is an orange giant star in the constellation Scorpius. Its apparent magnitude is 4.27. It lies in the tail of Scorpius, between the stars λ Scorpii and μ Scorpii, 7′ from the faint globular cluster Tonantzintla 2.

Q Scorii is a suspected astrometric binary, a star whose position is seen to oscillate although no companion has been detected. From Hipparcos data, its proper motion is seen to be discrepant and accelerating, although there is insufficient data to determine any orbit.Q Scorpii is cooler than the sun, but more luminous. It is a red clump giant, at the cool end of the horizontal branch, fusing helium in its core. Like all red clump giants, it has an effective temperature near 5,000 K and a bolometric luminosity of around 75 L☉.

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.

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