RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima.
RV Tauri variables are luminous variable stars that have distinctive light variations with alternating deep and shallow minima.
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]
RV Tauri stars are further classified into two photometric sub-types based on their light curves: [3]
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]
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.
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]
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 [lower-alpha 1] [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 | 2,910 [9] | 1,368 [9] | 58 [9] | 4,627 [9] |
| SS Gem [lower-alpha 2] | 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 | 4,260 [9] | |||
| TX Oph | 8.8 | 11.1 | 135 | 5,368 [9] | 4,282 [9] | ||
| 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 | 6,676 [9] | 4,232 [9] | ||
| TW Cam [lower-alpha 3] [10] | 9.4 | 10.5 | 85.6 | 2,700±260 | 3,000±600 | 58 [9] | 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 | 8,400±3,100 | 4,500±3,300 | 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 | 4,822 [9] | |||
| SU Gem [5] | 9.9 | 12.2 | 50.12 | 2,110±660 | 1,200±770 | 5,750 | |
| HP Lyr [10] | 10.2 | 10.8 | 70.4 | 6,700±380 | 3,900±400 | 5,900 | |
| Z Aps | 10.7 | 12.7 | 37.89 | 3,600 [9] | 519 [9] | 31.5 [9] | 4,909 [9] |
| AF Crt [11] | 10.87 | 11.47 | 31.16 | 4,320±1,100 [12] | 1,700±750 [12] | 41.63 | 5,750 [13] |
A variable star is a star whose brightness as seen from Earth changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.
Red supergiants (RSGs) are stars with a supergiant luminosity class of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.
RV Tauri is a star in the constellation Taurus. It is a yellow supergiant and is the prototype of a class of pulsating variables known as RV Tauri variables. It is a post-AGB star and a spectroscopic binary about 4,700 light years away.
119 Tauri is a red supergiant star in the constellation Taurus. It is a semiregular variable and its angular diameter has been measured at about 10 mas.
The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars: Delta Scuti variables, SX Phoenicis variables, and rapidly oscillating Ap stars (roAps) near the main sequence; RR Lyrae variables where it intersects the horizontal branch; and the Cepheid variables where it crosses the supergiants.
A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class. They are stars that have evolved away from the main sequence, expanding and becoming more luminous.
V1401 Aquilae is a single, semi-regular pulsating star in the equatorial constellation of Aquila. It has the designation HD 190390 from the Henry Draper Catalogue, and was formerly designated 64 Sagittarii. The evolutionary status of the star is unclear, and it has been classified as a post-AGB object, a UU Herculis variable, or belonging to the W Virginis variable subclass of the type II Cepheids. It is dimly visible to the naked eye with an apparent visual magnitude that fluctuates around 6.38. Based on parallax measurements, it is located at a distance of approximately 2,380 light years. It lies 21.5° from the galactic plane.
Stellar pulsations are caused by expansions and contractions in the outer layers as a star seeks to maintain equilibrium. These fluctuations in stellar radius cause corresponding changes in the luminosity of the star. Astronomers are able to deduce this mechanism by measuring the spectrum and observing the Doppler effect. Many intrinsic variable stars that pulsate with large amplitudes, such as the classical Cepheids, RR Lyrae stars and large-amplitude Delta Scuti stars show regular light curves.
Type II Cepheids are variable stars which pulsate with periods typically between 1 and 50 days. They are population II stars: old, typically metal-poor, low mass objects.
R Scuti 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.
W Virginis is the prototype W Virginis variable, a subclass of the Cepheid variable stars. It is located in the constellation Virgo, and varies between magnitudes 9.46 and 10.75 over a period of approximately 17 days.
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 ranging between spectral types SC4.5/8-e and SC7/8-e, but appears to have evolved into a C-type spectrum by 2011.
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.
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.
HP Lyrae is a variable star in the constellation Lyra, with a visual magnitude varying between 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.
U Monocerotis 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.
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.
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 from Earth. Gaia Data Release 2 gives a parallax of 0.2175 mas, corresponding to distance of about 4,600 pc.
SZ Tauri is a variable star in the equatorial constellation of Taurus. The brightness of this star varies from an apparent visual magnitude of 6.39 down to 6.69 with a period of 3.149 days, which is near the lower limit of visibility to the naked eye. The distance to this star is approximately 2,070 light years based on parallax measurements. There is some indication this may be a binary system, but the evidence is inconclusive.
| Pulsating |
| ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Eruptive |
| ||||||||
| Cataclysmic | |||||||||
| Rotating |
| ||||||||
| Eclipsing | |||||||||
