Delta Scuti star
Delta Scuti stars are pulsationsveränderliche stars of population I . The Delta Scuti stars were also known as dwarf cepheids, AI Velorium stars or RRs stars.
properties
spectrum
The Delta Scuti stars are observed with spectral classes between A0 and F8 and belong to the luminosity classes III ( subgiants ) to V ( main sequence stars ). It is the area of the HR diagram in which the instability strip crosses the main series.
Pulsations
The periods are below 0.3 days and the amplitudes reach up to 0.8 size classes , whereby mostly only 0.02 may be reached. Both radial and non-radial pulsations have been detected. All Delta Scuti stars are multiperiodic and vibrate both in the fundamental as well as in various selected harmonics . They are therefore good targets for asteroseismological investigations in order to analyze the internal structure of the variable based on the vibration propagation . The restoring force for the oscillations is mostly the pressure, the so-called p-modes, and only rarely gravity in the g-modes. There is also a period-luminosity relationship for these pulsating variables , which applies to the period of the fundamental with an accuracy of 0.1 mag:
The pulsations are mainly caused by the kappa mechanism . In addition, a stochastic excitation by the convective movement in the photosphere can also play a role.
Cause of the pulsations
The kappa mechanism of the simply ionized helium zone is assumed to be the cause of the pulsations . The amplitude of the Delta Scuti stars is correlated with the duration of the rotation of the stars. The faster the star rotates, the better its mixing and there is enough helium in the He + zone. At low rotational speeds, the helium sinks deeper into the star, while the light-absorbing metals rise into the photosphere due to the radiation pressure . This means that the Delta Scuti star appears spectrally as a mild Am star and the amplitude is negligibly small due to the small amount of helium in the He + zone.
Subgroups
The GCVS systematic abbreviations for Delta Scuti stars are DSCT or DSCTC, whereby the DSCTC stars have smaller amplitudes and sinusoidal light curves . However, the two groups do not differ in any further physical properties. A separate group are the rare HADS, which have high amplitude delta scuti stars with amplitudes greater than 0.3. They rotate quite slowly with rotation speeds of less than 30 km / s. They pulsate in one or two dominant radial modes, but more precise photometric investigations show a large number of other non-radial oscillations. The HADS and the Cepheids pulsating in the first harmonic with the subtype DCEPS follow the same period-luminosity relationship, which is why the term dwarf cepheid is also used for the HADS. The Delta Delphini stars are Delta Scuti stars with mostly small amplitudes. The chemical composition of the star's surface corresponds to that of the Am stars with their increased frequency on selected metal lines. The chemically peculiar Lambda Bootis stars can also pulsate as Delta Scuti stars. The Ap stars also often show signs of pulsation light changes caused by Delta-Scuti and / or Gamma-Doradus variability .
Occurrence in star catalogs
The General Catalog of Variable Stars currently lists around 700 stars with the abbreviations DSCT or DSCTC , which means that a little over 1% of all stars in this catalog belong to the class of Delta Scuti stars.
development
The masses of the Delta Scuti stars are 1.5 to 2.5 solar masses and luminosity between 10 and 50 times that of the sun. The Delta Scuti stars can therefore be in different stages of development . You can be on your way to the main row, are in the quiet phase of hydrogen burning on the main row, or start moving from the main row towards the giant red branch .
SX Phoenix stars
In the metal-poor Population II there are stars that show the same vibrations as the Delta Scuti stars. They are called SX Phoenix stars after their prototype SX Phoenicis . Their physical properties hardly differ apart from the metallicity with the exception of the amplitude, which usually reaches a few tenths of a size class. Since the mass of the SX Phoenix stars is between 1.5 and 2.5 solar masses, these stars must be blue stragglers . This is due to the limited lifespan of stars with 1.5 solar masses of only one billion years, while Population II in the Milky Way is around 10 billion years old. In the case of the blue stragglers, on the other hand, part of the mass was only transferred later by a companion in a binary star system and only then does it reach the necessary mass to pulsate as an SX Phoenix star.
Occurrence in star catalogs
The SX Phoenix stars are significantly rarer than the Delta Scuti stars and the General Catalog of Variable Stars currently lists only about 40 stars with the abbreviation SXPHE , which means that only 0.1% of all stars in this catalog belong to the SX class - Phoenix stars are counted.
Examples
| Surname | Bayer name | Spectral class |
Apparent brightness (maximum) |
Amplitude (mag) | Period (days) |
|---|---|---|---|---|---|
| Vega | Alpha Lyrae | A0 V | −0.02 | 0.07 | 0.19 |
| Denebola | Beta Leonis | A3 V | 2.14 | 0.025 | - |
| Caph | Beta Cassiopeiae | F2 III-IV | 2.25 | 0.06 | 0.1043 |
| Rho Puppis | F6 IIp | 2.68 | 0.19 | 0.14088 | |
| Seginus | Gamma bootis | A7 III | 3.02 | 0.05 | 0.2903 |
| Pherkad | Gamma Ursae Minoris | A3 II-III | 3.04 | 0.05 | 0.1430 |
| Theta2 Tauri | A7 III | 3.35 | 0.07 | 0.0756 | |
| Dew Cygni | F0 IV | 3.65 | 0.10 | - | |
| Ypsilon Ursae Majoris | F2 IV | 3.68 | 0.18 | 0.1327 | |
| Gamma Coronae Borealis | A0 IV | 3.8 | 0.06 | 0.03 | |
| Rho1 Sagittarii | F0 IV-V | 3.9 | 0.04 | 0.05 | |
| Phicares | Epsilon Cephei | F0 IV | 4.15 | 0.06 | 0.0412 |
| Delta Serpentis | F0 IV | 4.23 | 0.04 | 0.134 | |
| Delta Delphini | F0 IV | 4.38 | 0.11 | - | |
| Delta Scuti | F3 IIIp | 4.6 | 0.19 | 0.19377 |
Individual evidence
- ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne . JA Barth Verlag, Leipzig 1990, ISBN 3-335-00224-5 .
- ↑ KM Hambleton et al .: KIC 4544587: an Eccentric, Short Period Binary System with δ Sct Pulsations and Tidally Excited Modes . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.1819v1 .
- ↑ C. Aerts (Author), J. Christensen-Dalsgaard (Author), DW Kurtz (Author): Asteroseismology (Astronomy and Astrophysics Library) . Springer Verlag, Berlin 2010, ISBN 978-1-4020-5178-4 .
- ↑ C. Ulusoy et al .: Mode identification in the high-amplitude delta Scuti star V2367Cyg . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.7147 .
- ^ Seo-Won Chang et al .: Statistical Properties of Galactic δ Scuti Stars: Revisited . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.1031v1 .
- ↑ a b Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved May 9, 2019 .
- ↑ Niu Jia-Shu, Fu Jian-Ning, Zong Wei-Kai: Pulsation Analysis of the High Amplitude δ Scuti Star CW Serpentis . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.4033v1 .
- ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
- ^ A. Arellano Ferro, R. Figuera Jaimes, Sunetra Giridhar, DM Bramich, JV Hernandez Santisteban, K. Kuppuswamy: Exploring the variable stars in the globular cluster NGC5024 (M53): New RR Lyrae and SX Phoenicis stars . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1106.1880 .
- ↑ Variables of δ Scuti type (Alcyone)