Delta Scuti variable
Delta Scuti stars are stars of spectral classes A2 pulsationsveränderliche to F8 and the luminosity classes III to V of the population I. The periods below 0.3 days and amplitudes reach up to 0.8 size classes, where usually only like 0.02 be achieved. The Delta Scuti stars were referred to as Zwergcepheiden, AI Velorium star or RRs stars.
Properties
The Delta Scuti stars are observed with spectral types between A2 and F8 and belong to the luminosity classes III ( sub- giants) to V (main sequence stars ). This is the area of the HR diagram in which the instability strip crosses the main sequence. Their masses are at 1.5 to 2.5 solar masses and luminosities between 10 and 50 times the sun. The Delta Scuti stars are therefore stars that are on the way to the main sequence, are in the quiet phase of hydrogen burning on the main sequence or begin to move from the main sequence towards the red giant branch.
It has been demonstrated in both radial and non- radial pulsations. All Delta Scuti stars are multi- periodic and oscillate in both fundamental as well as in various selected harmonics. They are so to analyze the internal structure of the variable on the basis of the vibration propagation good targets for asteroseismologische investigations. The restoring force for the vibration is usually the pressure to the so-called p- modes, and rarely the gravity g in the modes. Also for these pulsating variables, there is a period-luminosity relationship, which may for the period of the fundamental frequency with an accuracy of 0.1 applies:
The pulsations are caused mainly by the kappa mechanism. Addition, however, may also play a role, a stochastic excitation by the convective motion in the photosphere.
The GCVS nomenclature abbreviation for Delta Scuti stars are DSCT or DSCTC, the DSCTC - stars have smaller amplitudes and sinusoidal light curves. The two groups but did not differ in any other physical properties. A separate group are the rare HADS, the high- amplitude delta Scuti stars like with amplitudes of more than 0.3. They rotate very slowly with rotational speeds of less than 30 km / s They pulsate in one or two dominant radial modes, more accurate photometric studies but which show a variety of other non- radial oscillations. The HADS and the pulsating in the first overtone Cepheids with subtype DCEPS follow the same period-luminosity relationship, which is why the term Zwergcepheide is used for the HADS. The delta Delphini stars are Delta Scuti stars with mostly small amplitudes. The chemical composition of the stellar surface corresponds to that of Am stars with their increased incidence of selected metal lines. The chemically pekulären lambda Bootis stars can pulsate as delta Scuti stars. The A-stars often show signs of Pulsationslichtwechsel caused by Delta Scuti and / or gamma Doradus variability.
The cause of the pulsations of the kappa mechanism of singly ionized helium zone is suspected. The amplitude of the delta -star Scuti is correlated with the duration of rotation of the star. The faster the star rotates desso better is its mixing and there is enough helium in the He zone. At low rotation rates, the helium sinks deeper into the star as well the light-absorbing metals ascend by the radiation pressure in the photosphere. This means that the delta Scuti star appears spectrally as a mild Am star and the amplitude due to the small proportion of helium in the He zone vanishingly small.
SX Phoenix Star
In the metal-poor Population II, there are stars that show the same oscillations as the Delta Scuti stars. They are referred to by their prototype as SX- Phoenix - stars. Their physical properties differ to the metallicity hardly except for the amplitude, which usually reaches a few tenths of size classes. Since the mass of the Phoenix SX- star between 1.5 and 2.5 solar masses is work must always be blue stragglers, these stars. This is due to the limited lifetime of stars with 1.5 solar masses of only a billion years, while the population II is in the Milky Way about 10 billion years old. In the blue stragglers, however, a part of the mass has been transferred later by a companion in a binary star system and then it reaches the critical mass it needs to pulsate as SX- Phoenix Star.