RV Tauri variable

RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima.

AC Herculis light curve
Light curve of AC Herculis, a fairly typical RV Tauri variable

History and discovery

German astronomer Friedrich Wilhelm Argelander monitored the distinctive variations in brightness of R Scuti from 1840 to 1850. R Sagittae was noted to be variable in 1859, but it was not until the discovery of RV Tauri by Russian astronomer Lidiya Tseraskaya in 1905 that the class of variable was recognised as distinct.[1]

Three spectroscopic groups were identified:[2]

  • A, GK-type with spectra unambiguously of type G or K
  • B, Fp(R), spectra are inconsistent, with features of F, G, and later classes found together, plus carbon (class R) features
  • C, Fp, peculiar spectra with generally weak absorption lines and without strong carbon bands

RV Tauri stars are further classified into two photometric sub-types based on their light curves:[3]

  • RVa: these are RV Tauri variables which do not vary in mean brightness
  • RVb: these are RV Tauri variables which show periodic variations in their mean brightness, so that their maxima and minima change on 600 to 1500 day timescales

The photometric sub-types should not be confused with the spectroscopic sub-types which use capital letters, often appended to RV: RVA; RVB; and RVC. The General Catalogue of Variable Stars uses acronyms consisting of capital letters to identify variability types, and so uses RVA and RVB to refer to the two photometric sub-types.[4]

Properties

RV Tau variables exhibit changes in luminosity which are tied to radial pulsations of their surfaces. Their changes in brightness are also correlated with changes in their spectral type. While at their brightest, the stars have spectral types F or G. At their dimmest, their spectral types change to K or M. The difference between maximum and minimum brightness can be as much as four magnitudes.The period of brightness fluctuations from one deep minimum to the next is typically around 30 to 150 days, and exhibits alternating primary and secondary minima, which can change relative to each other. For comparison with other type II Cepheids such as W Virginis variables, this formal period is twice the fundamental pulsation period. Therefore, although the approximate division between W Vir variables and RV Tau variables is at a fundamental pulsation period of 20 days, RV Tau variables are typically described with periods of 40–150 days.

The pulsations cause the star to be hottest and smallest approximately halfway from the primary minimum towards a maximum. The coolest temperatures are reached near to a deep minimum.[2] When the brightness is increasing, hydrogen emission lines appear in the spectrum and many spectral lines become doubled, due to a shock wave in the atmosphere. The emission lines fade a few days after maximum brightness.[4]

The prototype of these variables, RV Tauri is a RVb type variable which exhibits brightness variations between magnitudes +9.8 and +13.3 with a formal period of 78.7 days. The brightest member of the class, R Scuti, is an RVa type, with an apparent magnitude varying from 4.6 to 8.9 and a formal period of 146.5 days. AC Herculis is an example of an RVa type variable.

The luminosity of RV Tau variables is typically a few thousand times the sun, which places them at the upper end of the W Virginis instability strip. Therefore, RV Tau variables along with W Vir variables are sometimes considered a subclass of Type II Cepheids. They exhibit relationships between their periods, masses, and luminosity, although not with the precision of more conventional Cepheid variables. Although the spectra appear as supergiants, usually Ib, occasionally Ia, the actual luminosities are only a few thousand times the sun. The supergiant luminosity classes are due to very low surface gravities on pulsating low-mass and rarefied stars.

Evolution

Evolutionary track 1m
The evolutionary track of a solar mass, solar metallicity, star from main sequence to post-AGB

RV Tauri variables are very luminous stars and are typically given a supergiant spectral luminosity class. However they are relatively low mass objects, not young massive stars. They are thought to be stars that started out similar to the sun and have now evolved to the end of the Asymptotic Giant Branch (AGB). Late AGB stars become increasingly unstable, show large amplitude variations as Mira variables, experience thermal pulses as internal hydrogen and helium shells alternate fusing, and rapidly lose mass. Eventually the hydrogen shell gets too close to the surface and is unable to trigger further pulses from the deeper helium shell, and the hot interior starts to be revealed by the loss of the outer layers. These post-AGB objects start to become hotter, heading towards becoming a white dwarf and possibly a planetary nebula.

As a post-AGB star heats up it will cross the instability strip and the star will pulsate in the same way as a conventional Cepheid variable. These are theorised to be the RV Tauri stars. Such stars are clearly metal-deficient Population II stars since it takes around 10 billion years for stars of that mass to evolve beyond the AGB. Their masses are now less than 1 M even for stars that were initially B class on the main sequence.

Although a post-AGB crossing of the instability strip should happen in a period measured in thousands of years, even hundreds for the more massive examples, the known RV Tau stars have not shown the secular increase in temperature that would be expected. The main sequence progenitor of this type of star has a mass near to that of the sun, although they have already lost about half of that during red giant and AGB phases. They are also thought to be mostly binaries surrounded by a dusty disc.[5]

Brightest Members

There are just over 100 known RV Tauri stars.[6] The brightest RV Tauri stars are listed below.[7]

Star
Brightest
Magnitude[6]
Dimmest
Magnitude[6]
Period[6]
(days)
Distance[8]
(parsecs)
Luminosity[8]
(L)
Radius[8]
R
Temperature[8]
(K)
R Sct[a][5] 4.2 8.6 140.2 750±290 9,400±7,100 4,500
U Mon 5.1 7.1 92.26 1,111+137
−102
5,480+1,764
−882
100.3+18.9
−13.2
5,000
AC Her 6.4 8.7 75.4619 1,276+49
−44
2,475+183
−209
47.1+4.7
−4.1
5,900
V Vul 8.1 9.4 75.72 1,854+160
−140
2,169+504
−315
77.9+13.0
−10.1
4,500
AR Sgr 8.1 12.5 87.87
SS Gem[b] 8.3 9.7 89.31 3,423+836
−488
17,680+12,800
−6,400
150.6+41.7
−34.8
5,600
R Sge 8.5 10.5 70.594 2,475+353
−229
2,329+744
−638
61.2+12.4
−9.9
5,100
AI Sco 8.5 11.7 71.0
TX Oph 8.8 11.1 135
RV Tau 8.8 12.3 76,698 1,460+153
−117
2,453+605
−403
83.4+12.8
−12.8
4,500
SX Cen 9.1 12.4 32.967 4,429+1,071
−605
3,684+2,315
−842
61.1+14.7
−9.8
6,000
UZ Oph 9.2 11.8 87.44
TW Cam[c][9] 9.4 10.5 85.6 2700 ± 260 3000 ± 600 4,700
TT Oph 9.4 11.2 61.08 2,535+221
−172
714+131
−102
38.5+5.4
−4.5
5,000
UY CMa[5] 9.8 11.8 113.9 8400 ± 3100 4500 ± 3300 5,500
DF Cyg 9.8 14.2 49.8080 2,737+240
−186
815+155
−116
39.9+6.4
−4.5
4,840
CT Ori 9.9 11.2 135.52
SU Gem[5] 9.9 12.2 50.12 2110 ± 660 1200 ± 770 5,750
HP Lyr[9] 10.2 10.8 70.4 6700 ± 380 3900 ± 400 5,900
Z Aps 10.7 12.7 37.89
  1. ^ R Sct may be less luminous than given in the table. It may be a thermal-pulsing AGB star, observed in a helium-burning phase instead of a post-AGB star.[5]
  2. ^ SS Gem is likely to be a population I Cepheid[8]
  3. ^ TW Cam distance estimate may be too large.[5]

See also

References

  1. ^ Gerasimovič, B.P. (1929). "Investigations of Semiregular Variables. VI. A General Study of RV Tauri Variables". Harvard College Observatory Circular. 341: 1–15. Bibcode:1929HarCi.341....1G.
  2. ^ a b Rosino, L. (1951). "The Spectra of Variables of the RV Tauri and Yellow Semiregular Types". Astrophysical Journal. 113: 60. Bibcode:1951ApJ...113...60R. doi:10.1086/145377.
  3. ^ Oosterhoff, P. Th. (1966). "Resolutions adoptées par la Commission 27 (Resolutions adopted by Commission 27)". Transactions of the International Astronomical Union. 12: 269. Bibcode:1966IAUTB..12..269O.
  4. ^ a b Giridhar, Sunetra; Lambert, David L.; Gonzalez, Guillermo (2000). "Abundance Analyses of Field RV Tauri Stars. V. DS Aquarii, UY Arae, TW Camelopardalis, BT Librae, U Monocerotis, TT Ophiuchi, R Scuti, and RV Tauri". The Astrophysical Journal. 531: 521. arXiv:astro-ph/9909081. Bibcode:2000ApJ...531..521G. doi:10.1086/308451.
  5. ^ a b c d e f De Ruyter, S.; Van Winckel, H.; Dominik, C.; Waters, L. B. F. M.; Dejonghe, H. (2005). "Strong dust processing in circumstellar discs around 6 RV Tauri stars". Astronomy and Astrophysics. 435: 161. arXiv:astro-ph/0503290. Bibcode:2005A&A...435..161D. doi:10.1051/0004-6361:20041989.
  6. ^ a b c d "GCVS Variability Types". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. 12 Feb 2009. Retrieved 2010-11-24.
  7. ^ "List of the brightest RV Tauri stars". AAVSO. Retrieved 2010-11-20. (source article)
  8. ^ a b c d e Bódi, A.; Kiss, L. L. (2019). "Physical properties of galactic RV Tauri stars from Gaia DR2 data". arXiv:1901.01409 [astro-ph.SR].
  9. ^ a b Manick, Rajeev; Van Winckel, Hans; Kamath, Devika; Hillen, Michel; Escorza, Ana (2017). "Establishing binarity amongst Galactic RV Tauri stars with a disc⋆". Astronomy & Astrophysics. 597: A129. arXiv:1610.00506. Bibcode:2017A&A...597A.129M. doi:10.1051/0004-6361/201629125.

External links

AC Herculis

AC Herculis, is an RV Tauri variable and spectroscopic binary star in the constellation of Hercules. It varies in brightness between apparent magnitudes 6.85 and 9.0.

AC Her is an RVa star, meaning it is an RV Tauri variable whose maximum and minimum magnitudes do not slowly vary over hundreds of days. It also is a very clear example of a common type of RV Tauri light curve where the maximum following a deep minimum is brighter than the maximum following a shallow minimum. In each period of 75 days it has two maxima and two minima.AC Her is also a binary star, although the secondary can only be detected by its effect on the radial velocity of the primary. The invisible secondary is more massive than the supergiant primary, so the primary moves at relatively high velocity in its three years and three months orbit. The two stars are also surrounded by a dusty disc filling the region between 34 and 200 astronomical units (AU).Little is known of the secondary star except that its mass is around 1.2 M☉, deduced from the mass ratio of the binary system and the modelled mass of the primary star. The primary itself is calculated to have a mass of 0.6 M☉, but a luminosity of 2,500 L☉. It is slightly cooler than the sun, although this varies by over a thousand K as the star pulsates.The total system mass can be estimated from the dynamics of the disc, and this gives a value of 1.5 M☉, slightly lower than from other methods.

BH Crucis

BH Crucis, also known as Welch's Red Variable, is a star in the constellation Crux. A long period (Mira-type) variable, its apparent magnitude ranges from 6.6 to 9.8 over 530 days. Hence at its brightest it is barely visible with the unaided eye in a rural sky. A red giant, it had been classified by SIMBAD as ranging between spectral types SC4.5/8-e and SC7/8-e, but appears to have evolved into a C-type (carbon star) spectrum by 2011.Ronald G. Welch discovered the star while looking for new variables in October 1969. In the first thirty years since discovery, it has become redder and brighter (mean magnitude changing from 8.047 to 7.762) and its period lengthened by 25% from 421 to 530 days. Retrospective examination of photographic plates taken in South West Africa in 1937 and 1951 and stored at Sonneberg Observatory suggest the amplitude of variation might have been smaller in the earliest records. A study of the star published in 2011 found that the increase in period appeared to have stopped or even begun reversing (estimated at 524 days in 2011), and that the spectral class had changed from SC to C, with carbon emission becoming more prominent. Technetium was also recorded in the emission spectrum and its surface temperature deemed to have cooled to 3000 K. Unusually for a Mira variable, BH Crucis had a double maximum, with two peaks in brightness, reminiscent of an RV Tauri variable. However, with the lengthening of its period, this feature disappeared.Guandalini and Cristallo calculated the luminosity of Mira variables based on their periods. Using a period of 421 days, they calculated the absolute magnitude of BH Crucis to be -4.80. Uttenthaler and colleagues calculated a bolometric magnitude of -5.59. Gaia Data Release 2 gives a parallax of 0.9952 mas and a corresponding distance of around 1000 pc.

HP Lyrae

HP Lyrae (HP Lyr) is a variable star in the constellation Lyra, with a visual magnitude varying 10.2 and 10.8. It is likely to be an RV Tauri variable, an unstable post-AGB star losing mass before becoming a white dwarf.

HR 4049

HR 4049, also known as HD 89353 and AG Antliae, is a binary post-asymptotic-giant-branch (post-AGB) star in the constellation Antlia. A very metal-poor star, it is surrounded by a thick unique circumbinary disk enriched in several molecules. With an apparent magnitude of about 5.5, the star can readily be seen under ideal conditions. It is located approximately 1,700 parsecs (5,500 ly) distant.

HR 4049 has a peculiar spectrum. The star appears, based on its spectrum in the Balmer series, to be a blue supergiant, although in reality it is an old low-mass star on the post-AGB phase of its life. Its atmosphere is extremely deficient in heavy elements, over with a metallicity over 30,000 lower than the Sun. It also shows a strong infrared excess, corresponding closely to a 1,200 K blackbody produced by a disk of material surrounding the star. The star is also undergoing intense mass-lossHR 4049 has an unseen companion, detected from variations in the doppler shift of its spectral lines. The properties of the companion can only be estimated by making certain assumptions about the inclination of the orbit and the mass function. Given those assumptions, it is thought to be a low luminosity main sequence star.HR 4049 is an unusual variable star, ranging between magnitudes 5.29 and 5.83 with a period of 429 days. It has been given the variable star designation AG Antliae, but is still more commonly referred to as HR 4049. It has been described as pulsating in a similar fashion to an RV Tauri variable, although the preferred interpretation is that the variations are produced by variable extinction produced by the material around the star and that the period is the same as the orbital period.Although HR 4049 apparently has the spectrum of a blue supergiant, it is an old low-mass star which has exhausted nuclear fusion and is losing its outer layers as it transitions towards a white dwarf and possibly a planetary nebula. During this phase it has a luminosity several thousand times that of the Sun, although a mass around half that of the sun. The mass can only be guessed from the expected mass of the white dwarf that it is becoming.

IRAS 08544−4431

IRAS 08544−4431 is a binary system surrounded by a dusty ring in the constellation of Vela. The system contains an RV Tauri variable star and a more massive but much less luminous companion.

Lidiya Tseraskaya

Lidiya Tseraskaya (Russian: Лидия Петровна Цераская) (1855 - 1931) was a Soviet astronomer.

Born in Astrakhan. Graduated from the Teacher's Institute in Petersberg. Worked at the Moscow Observatory. Discovered 219 variable stars, among them (1905) RV Tauri variable and recognized its uniqueness. The Venusian crater Tseraskaya was named after her.Tseraskaya was married to Vitol'd (or Witold) Karlovich Tseraskiy (or Ceraski), who was Professor of Astronomy as Moscow University. Her academic papers were published as "W. Ceraski".

List of semiregular variable stars

This is a list of semiregular variable stars. Variability ranges are taken from the General Catalogue of Variable Stars (GCVS) where these are visual magnitudes, otherwise from the International Variable Star Index (VSX). Spectral types are taken from the GCVS, which may differ from more recent MK spectral types but often defines a range.

List of stars in Sagitta

This is the list of notable stars in the constellation Sagitta, sorted by decreasing brightness.

List of variable stars

There are over 41,638 known variable stars (2008), with more being discovered regularly, so a complete list of every single variable is impossible at this place (cf. GCVS). The following is a list of variable stars that are well-known, bright, significant, or otherwise interesting.

R Sagittae

R Sagittae is an RV Tauri variable star in the constellation Sagitta that varies from magnitude 8.0 to 10.5 in 70.77 days. It is a post-AGB low mass yellow supergiant that varies between spectral types G0Ib and G8Ib as it pulsates. Its variable star designation of "R" indicates that it was the first star discovered to be variable in the constellation. It was discovered in 1859 by Joseph Baxendell, though classified as a semi regular variable until RV Tauri variables were identified as a distinct class in 1905.R Sagittae is classified as an RV Tauri variable because of the distinctive regular variations with alternating deep and shallow minima. The period is conventionally quoted as the time between two deep minima and is the fundamental pulsation mode. The shallow minimum is the result of a first overtone pulsation. It is further classified as RVb since the average and maximum magnitude varies slowly over several years. The main period also varies over a period of decades. It has around 90% the mass of the Sun and an average effective (surface) temperature of around 5000 K. It is around 10,000 times as luminous as the Sun. Measurement of its parallax by the Gaia satellite yields a distance of around 8,100 light-years.Variable star observer David Levy recommends that amateur observers monitor it once a week to observe changes in brightness.RV Tauri variables are post-AGB stars, originally similar to the Sun but now in the last stages of their lives. They are crossing the Cepheid instability strip as they lose their outer layers on the way to becoming a planetary nebula. Although their spectra and luminosities resemble supergiants, they are old low mass population II stars. A period-colour-luminosity relationship has been derived from observations of RV Tauri variables in the Large Magellanic Cloud that is closely related to the relationship for type II Cepheid variables.

R Scuti

R Scuti (R Sct) is a star in the constellation of Scutum. It is a yellow supergiant and is a pulsating variable known as an RV Tauri variable. It was discovered in 1795 by Edward Pigott at a time when only a few variable stars were known to exist.

SX Centauri

SX Centauri is a variable star in the constellation Centaurus. An RV Tauri variable, its light curve alternates between deep and shallow minima, varying its apparent magnitude from 9.1 to 12.4. From the period-luminosity relationship, it is estimated to be around 1.6 kpc (5200 light-years) from Earth. Gaia Data Release 2 gives a parallax of 0.2175 mas, corresponding to distance of about 4,600 pc.RV Tauri variables like SX Centauri are supergiant pulsating stars and a subtype of the population II Cepheids. They are stars that have already passed the asymptotic giant branch (AGB) and are in the last stage of their evolution before becoming a planetary nebula. This transition phase is very fast, and may last for less than a thousand years. SX Centauri is in the beginning of this process and is estimated to be leaving the AGB right now, or to have left the AGB a few decades ago. Its pulsations are radial in nature and have a period of about 32.9 days (from deep minimum to deep minimum), causing the effective temperature of the star to vary between 5,000 and 6,500 K and the radius between 21 and 29 solar radii. The radius seems to have the same value in both the primary and secondary minima, while the temperature shows a 500 K variation between minima.The spectrum of SX Centauri shows infrared excess, indicating the presence of a circumstellar disk of hot dust around the star. interferometric observations constrained the diameter of the disk to less than 11 arcseconds (18 AU at the star's distance), indicating a very compact system. The infrared emission is consistent with a hotter component (715 K) corresponding to 4% of the dust, and a colder one (244 K) corresponding to 96% of the dust. This material is composed mainly of amorphous carbon and graphite (83%), with the remainder being pyroxene and olivine. The disk is related to a depletion of refractory elements (with high condensation temperature) in the star's photosphere; this is caused by separation of gas from dust rich in refractories, followed by accretion of the gas by the star.SX Centauri is a spectroscopic binary, having a companion star with an orbital period of 592 days and an orbital eccentricity of 0.16. This companion has a mass estimated between 1.4 and 1.9 solar masses and is probably an unevolved main sequence star. The system must have interacted in the past when the primary was a red giant, which is likely related to the formation of disk. All RV Tauri stars with dust disks are believed to be part of a binary system.Slow periodic variations in the mean brightness of SX Centauri have been detected, leading the star to be classified as an RV Tauri star of the photometric class b (RVb). The period of this variation is approximately equal to the orbital period of the system. This phenomenon can be explained as variation of the circumstellar extinction during the orbital motion of the disk.

S Vulpeculae

S Vulpeculae is a star located in the constellation Vulpecula. A supergiant star, it is around 382 times the diameter of the Sun.S Vulpeculae was first suspected of varying in brightness in 1836 and this was confirmed by 1862. A pulsating variable that grows and shrinks as it changes in brightness, it has been variously classified as an RV Tauri variable, a semiregular variable star, or a Cepheid variable.S Vulpeculae is now confirmed as a classical Cepheid variable with one of the longest known periods at 68 days, although the period has changed several times. As such, it is also one of the cooler and more luminous of the Cepheids, and it lies close to the zone where semiregular variable stars are found. The shape and amplitude of the light curve varies significantly from cycle to cycle and secularly. The apparent magnitude ranges from 8.69 to 9.42. The spectrum varies from early G to late K as it pulsates, with TiO bands typical of an M1 star when the star is coolest.

Sagitta

Sagitta is a dim but distinctive constellation in the northern sky. Its name is Latin for "arrow", and it should not be confused with the significantly larger constellation Sagittarius, the archer. Although Sagitta is an ancient constellation, it has no star brighter than 3rd magnitude and has the third-smallest area of all constellations (only Equuleus and Crux are smaller). It was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations defined by the International Astronomical Union. Located to the north of the equator, Sagitta can be seen from every location on Earth except within the Antarctic circle.

The red giant Gamma Sagittae is the constellation's brightest star, with an apparent magnitude of 3.47. Two star systems have been found to have planets.

Semiregular variable star

Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light curves may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes (usually 1-2 magnitudes in the V filter).

Sigurd Einbu

Sigurd Einbu (November 5, 1866 – May 10, 1946) (spelled Enebue at birth), was a Norwegian self-taught astronomer from Lesjaskog, known to have discovered a nova in the stellar constellation Gemini on March 12, 1912. The nova was named Nova Geminorum 2. Also in 1912 he introduced a new class of variable stars, the RV Tauri variable stars.

He spelled his family name Enebo during large parts of his scientific career, but in 1926 he changed it to Einbu according to the local pronunciation.

Einbu attended Hamar offentlige lærerskole, and worked as a teacher in Øyer, Vågå and Sel before returning to his home community Dombås. He took an interest in astronomy at an early age, but could not devote himself fully to astronomy until he was appointed statsstipendiat (Government scholar) in 1908. He was a founding member of Norsk Novaselskap. He established and ran a magnetic monitoring station at Dombås from 1916.

During his lifetime he was a people's educator and wrote frequently in newspapers and popular journals.

His childhood house Einbustugu has been moved from Lesjaskog to Dombås, and is today a museum in his memory.

T Centauri

T Centauri is a variable star located in the far southern constellation Centaurus. It varies between magnitudes 5.56 and 8.44 over 181.4 days. Pulsating between spectral classes K0:e and M4II:e, it has been classed as a semiregular variable, though Sebastian Otero of the American Association of Variable Star Observers has noted its curve more aligned with RV Tauri variable stars and has classified it as one.

U Monocerotis

U Monocerotis (U Mon) is a pulsating variable star and spectroscopic binary in the constellation Monoceros. The primary star is an RV Tauri variable, a cool luminous post-AGB star evolving into a white dwarf.

Z Apodis

Z Apodis (Z Aps) is a variable star in the constellation of Apus. It has an apparent visual magnitude which varies between 10.7 and 12.7, over a period of 39.37 days. Although described in the General Catalogue of Variable Stars as a cataclysmic variable star, it appears that it is a pulsating variable star, and has been classed as an RV Tauri variable star, type A.

Pulsating
Eruptive
Cataclysmic
Rotating
Eclipsing

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