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R Scuti

RV Tauri a Variable in Scutum

Right Ascension 18 : 47 : 28.9 (h:m:s)
Declination -05 : 42 : 18 (deg:m:s)
Distance 2.5 (kly)
Visual brightness 4.45 .. 8.20 (mag)
Period 140.05 (days)
Spectra type K0Ibpv

This variable star has been discovered by Edward Pigott in 1795 (Pigott & Englefield, 1797). It is a yellow giant variable star of the RV Tauri type, and varies semi-regularly between 5th and 8th magnitude.

Various observers have determined primary periods between 140 and 146 days. Normally, the star's brightness varies between mag 4.8 and mag 6.0, but every fourth or fifth minimum drops to mag 8.0 or fainter. Superimposed over the primary period is at least one other period. According to Burnham, this star was in vestigated by McLaughlin who found that the star's spectrum is normally about G2, and it oscillates similar to a Cepheid variable, besides that the radial velocity of its oscillation is much more irregular. In particular, near the deep minimum reddens to about M3, and shows titanium-oxide bands, which are typical for red-giant spectra. During the consequent rise, hydrogen emission lines appear in the spectrum, which gradually turn to absorption lines as the star reaches its maximum. Spectral studies indicate that the various layers of the star ar expanding and contracting at different rates, and that the star is really huge in linear dimension: at least about 100 times the diameter of our sun. Absolute magnitude was estimated at -4.5 to -5.0. This results in a distance estimate of 2,500 to 3,000 light-years. Burnham quotes a mass estimate of 20-25 times that of our sun, based on mass-luminosity relations, but it is more probably low, of order of one solar mass only.

Other spectral classifications have been G0e Ia near its maximum, K0p Ib near the minimum, in the Moscow General Catalogue of Variable Stars of 1970, and G8 to M3 by J.S. Glasby (1969).

The star shows a proper motion of about 0.06 arc seconds per year, and is receding from us at about 26.5 miles per second (42.4 km/s).

From their spectral classification, e.g. in the Hertzsprung-Russell Diagram (HRD), i.e. their temperature and luminosity, RV Tauri stars seem to lie between Cepheid variables and Mira-type stars. Consensus has been achieved that they are most probably of stars low mass in advanced state of evolution. More acurately, these stars are thought to have developed from Red Giant Mira stars and are just going to produce a planetary nebula, eventually leaving behind a White Dwarf star. In their RV Tauri stadium, their mass is usually below one solar mass.

RV Tauri stars are further classified on their variability behaviour: RVa if their mean magnitude is constant (as for R Sct), RVb if that value varies with longer period (typically 600-1,500 days). Other wellknown RV Tauri stars include RV Tauri and U Monocerotis.

R Scuti is situated near the northern edge of a rich Milky Way star cloud, the Scutum Cloud. It lies just about 1deg south of Beta Scuti, and about 1deg NW of bright open star cluster M11.

Recent and Upcoming Maxima of R Scuti:



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Hartmut Frommert
Christine Kronberg