Physics Gifts

- Art Gallery -

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 denser region of the red giant branch or a bulge towards hotter temperatures. It is prominent in many galactic open clusters, and 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.

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
Main article: Horizontal branch
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:

Capella Aa[11]
ε Tauri[12]
β Ceti[11]

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

Soubiran, C.; Bienaymé, O.; Siebert, A. (2003). "Vertical distribution of Galactic disk stars". Astronomy and Astrophysics. 398: 141–151. arXiv:astro-ph/0210628. Bibcode:2003A&A...398..141S. doi:10.1051/0004-6361:20021615.
Girardi, Léo (1999). "A secondary clump of red giant stars: Why and where". Monthly Notices of the Royal Astronomical Society. 308 (3): 818–832. arXiv:astro-ph/9901319. Bibcode:1999MNRAS.308..818G. doi:10.1046/j.1365-8711.1999.02746.x.
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.
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 (1): L28–L32. arXiv:1007.5065. Bibcode:2010ApJ...721L..28N. doi:10.1088/2041-8205/721/1/L28.
Sarajedini, Ata (1999). "WIYN Open Cluster Study. III. The Observed Variation of the Red Clump Luminosity and Color with Metallicity and Age". The Astrophysical Journal. 118 (5): 2321–2326. Bibcode:1999AJ....118.2321S. doi:10.1086/301112.
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 (1): L85. Bibcode:2001ApJ...551L..85Z. doi:10.1086/319832.
d'Antona, Francesca; Caloi, Vittoria (2004). "The Early Evolution of Globular Clusters: The Case of NGC 2808". The Astrophysical Journal. 611 (2): 871–880. arXiv:astro-ph/0405016. Bibcode:2004ApJ...611..871D. doi:10.1086/422334.
Groenewegen, M. A. T. (2008). "The red clump absolute magnitude based on revised Hipparcos parallaxes". Astronomy and Astrophysics. 488 (3): 935–941. arXiv:0807.2764. Bibcode:2008A&A...488..935G. doi:10.1051/0004-6361:200810201.
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): 1093–1098. arXiv:astro-ph/0607408. Bibcode:2006ApJ...647.1093N. doi:10.1086/505529.
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 (1): 225–234. arXiv:astro-ph/9808253. Bibcode:1999ApJ...511..225A. doi:10.1086/306655.
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 (1): 428–448. Bibcode:1998ApJ...496..428A. doi:10.1086/305347.
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.
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.

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

Stanek's page on red clumps used for distance measurement

vte

Stars
Formation

Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track

Evolution

Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]

Remnants

White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster

Hypothetical

Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

Stellar nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova

Structure

Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism

Properties

Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system

Earth-centric
observations

Sun
Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard

Lists

Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

Physics Encyclopedia

World

Index

Hellenica World - Scientific Library

Retrieved from "http://en.wikipedia.org/"
All text is available under the terms of the GNU Free Documentation License