SU Ursae Majoris star

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The SU Ursae Majoris stars are a subgroup of dwarf novae , which partially or always show modulations in their light curves during their eruptions , which deviate by a few percent from the length of the orbital period. These modulations are known as superhumps .

properties

The SU Ursae Majoris stars belong to the short-period dwarf novae with periods of less than 2.1 hours and are therefore below or within the period gap . In addition to the normal outbreaks, they usually show supermaxima, which are about 1 mag brighter and last about twice as long. Superhumps only occur during the supermaxima. The super maxima are initiated by a normal eruption and then change to a super max after a short drop. The super cycle is the distance between the super maxima, which is approximately 10 times longer than the mean distance between normal maxima.

The SU Ursae Majoris stars are further divided into the

  • ER Ursae Majoris stars with extremely short supercycles of less than 60 days
  • and WZ Sagittae stars that have extremely long supercycles of more than 10,000 days

The WZ Sagittae stars, also known as TOADs (Tremendous Outburst Amplitude Dwarf Novae), do not show normal outbursts, but only super maxima with a super cycle on the order of decades. Only in this subgroup does a super maximum immediately follow a previous super maximum, while in the other SU Ursae Majoris stars there is always a number of normal eruptions between super maxima. Their bursts reach greater amplitudes of 6 to 8 mag. Furthermore, in contrast to the other SU Ursae Majoris stars, the WZ Sagittae stars show early superhumps and, after the eruptions, small maxima, which are referred to as rebrightenings. The early superhumps already appear as they rise to the maximum with a period that roughly corresponds to the later superhump period. The early superhumps are attributed to spiral arms in the accretion disks or an increased mass transfer between the components of the binary star system. An increased mass transfer could also be the cause of the rebrightenings.

It seems to be characteristic of the SU-Ursae-Majoris stars that the distance between the super maxima seems to increase with them over time.

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists around 120 stars with the abbreviation UGSU , which means that around 0.25% of all stars in this catalog belong to the class of SU Ursae Majoris stars.

Breakout model for the super breakouts

The normal eruptions of the SU Ursae Majoris stars correspond to those in other dwarf novae caused by a bistable state of the accretion disk due to magnetorotational instability . Like other cataclysmic variables , the SU Ursae Majoris stars consist of a red dwarf that transfers matter to a non-magnetic white dwarf via an accretion disk . The super-eruptions are either the result of feedback from the heated accretion disc to the red dwarf, which transfers more matter than usual, or the accretion disc expands during a normal eruption and comes into a 3: 2 orbit resonance with the orbit period. This resonance heats up the disk and more matter is transferred to the white dwarf. While in a normal eruption around 10 percent of the matter from the accretion disk flows onto the white dwarf, during supermaxima the disk is likely to lose around 50 percent of its matter.

Well-known SU Majoris stars

Web links

Individual evidence

  1. ↑ C. Hoffmeister, G. Richter, W. Wenzel: Veränderliche Sterne . 3. Edition. Barth, Leipzig 1990, ISBN 3-335-00224-5 .
  2. ^ John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  3. ↑ Shinichi Nakagawa et al .: Multi-Color Photometry of the Outburst of the New WZ Sge-type Dwarf Nova, OT J012059.6 + 325545 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.1855v1 .
  4. ↑ Chikako Nakata et al .: WZ Sge-type dwarf novae with multiple rebrightenings: MASTER OT J211258.65 + 242145.4 and MASTER OT J203749.39 + 552210.3 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1307.6712v1 .
  5. ↑ M. Otulakowska-Hypka, A. Olech: On super cycle lengths of active SU UMa stars . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.6248v1 .
  6. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved September 28, 2019 .
  7. ^ Brian Warner: Cataclysmic Variable Stars . Cambridge University Press, New York 2003, ISBN 0-521-54209-X .