SU Ursae Majoris

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Double star
SU Ursae Majoris
AladinLite
Observation
dates equinox :  J2000.0 , epoch : J2000.0
Constellation Big Bear
Right ascension 08 h 12 m 28.27 s
declination + 62 ° 36 ′ 22.4 ″
Apparent brightness (10.8 to 16.0) mag
Typing
Spectral class pec (basement)
Variable star type UGSU 
Astrometry
Radial velocity 27.0 km / s
parallax (4.53 ± 0.03)  mas
distance (718.9 ± 4.5)  Lj
(220,52 ± 1.39)  pc  
Proper movement 
Rec. Share: (5.7 ± 1.1)  mas / a
Dec. portion: (−20.3 ± 1.5)  mas / a
Physical Properties
Rotation time 109.98 min
Other names
and catalog entries
2MASS catalog 2MASS J08122826 + 6236224 [1]
Other names V * SU UMa, FBS B 662, RBS 694, 1RXS J081228.3 + 623627, AAVSO 0803 + 62, AN 5.1908, PG 0808 + 628, UCAC4 764-040076, X 08081 + 628

SU Ursae Majoris also SU UMa was discovered in 1908 by Lidiya Petrovna Tseraskaya (W. Ceraski) in Moscow. It is a cataclysmically changeable binary star system , consisting of a red dwarf and a white dwarf in the constellation Great Bear . The system wasclassifiedas a dwarf nova, issimilar interms of structure and composition to the subtypes U Geminorum , SS Cygni and Z Camelopardalis and forms the prototype of the so-called SU Ursae Majoris stars .

Observations

It is believed that the eruptions observed are the result of density fluctuations within the accretion disk that surrounds the white dwarf. In addition to the occurrence of normal dwarf nova outbreaks (which increase by 2 to 6 magnitudes from the resting phase and last between 1 to 3 days), SU UMa also shows super outbreaks.

Super outbreaks occur less frequently than normal outbreaks, last 10 to 18 days and can increase in brightness by at least 1 m . The beginning of a super-maximum cannot be distinguished from the beginning of a normal outbreak; during the outbreak, small periodic fluctuations known as superhumps of the order of several tenths of a magnitude are observed. What is unique about these superhumps is their cycle duration, which is 2 to 3% longer than the system's cycle time. The cycle time of the system can therefore be determined by observing the superhumps. Orbital times of less than 2 hours have been determined for almost all SU Ursae Majoris stars.

SU Ursae Majoris is an abundant source of observation because its brightness fluctuations occur over short periods of time. The normal outbreaks occur every 11 to 17 days and the super outbreaks every 153 to 260 days. The changes in visible brightness typically range from a minimum of 16 m to 10.8 m at super maxima and can be observed year-round in the northern hemisphere with telescopes with an objective diameter of approx. 150 mm (6 inches) and above.

Since the distinctive features of the SU Ursae Majoris stars are normal bursts, super bursts, and superhumps, it was an interesting finding that SU Ursae Majoris, as the prototype of this subgroup, showed no such behavior for almost three years in the early 1980s. A similar paradox in which no super-breakout was detected occurred between April 1990 and July 1991. This raised the question of whether this system even belonged to the SU Ursae Majoris classification.

Normal breakouts

Two competing theories are proposed to explain the outbreaks.

  • Mass transfer burst model:
the outbreak is the result of a sudden increase in mass transfer on the part of the companion. Such an increase can be triggered by an instability in the atmosphere of the main sequence star. The sudden mass transfer can then cause the accretion disk to collapse and, as a result, the accumulated matter is increasingly transferred to the white dwarf. which leads to a sudden increase in the brightness of the system.
  • Disc Instability Model:
the accretion disc oscillates between two states (the cause is assumed to be magnetorotational instability ).
A state with high viscosity, high internal friction and high accretion rate (> approx. 10 −7  M ☉ / year) in this case the disk heats up due to the high viscosity, which leads to a sharp increase in electromagnetic radiation, and a state with low viscosity and low accretion rate. The viscosity of the material in the disk changes by a factor of 10 between the two states.
With the help of this model, the eruptions can be described quite well, but so far no physical cause is known for the sudden change in viscosity.

Super breakouts and superhumps

While the normal bursts observed in SU Ursae Majoris are believed to be similar to the U Geminorum / SS-Cygni star- type bursts, super - bursts can be described by at least three possible mechanisms:

  • the improved mass transfer model,
  • the thermal limit cycle model
  • the thermal-gravitational instability model

In the case of the latter model, both the normal eruptions and the super-eruptions are determined by instability of the accretion disk. In addition to the thermal instability, there is also a tidal instability in which the disk radius expands to a critical point and a 3: 1 resonance can arise. This then results in another super-eruption, whereby the accretion disk regains its original size.

The occurrence of superhumps is only observed during super-eruptions, so it is reasonable to assume that these are naturally related. Superhumps can make up up to 30% of the total light output and thus make a significant contribution to the overall brightness of the system. They occur about a day after the start of a super-eruption and show a decreasing amplitude as the super-eruption comes to an end. Superhumps are also described with the thermal-gravitational instability model and are probably the result of an eccentric accretion disk. The occurrence of the fluctuations is independent of the inclination of the system towards the observer, since this aspect cannot be ascribed to an orbital effect.

See also

Individual evidence

  1. ↑ a b c d SU UMa. In: SIMBAD . Center de Données astronomiques de Strasbourg , accessed September 8, 2018 .
  2. ↑ a b c d e f SU UMa. In: VSX. AAVSO, accessed September 8, 2018 .
  3. ↑ a b c d Kerri Malatesta: SU Ursae Majoris . In: Variable Star of the Month . April 13, 2010.
  4. ↑ SN Shore, M. Livio, EPJ van den Heuvel: Interacting Binaries . Springer, Berlin 1994, ISBN 3-540-57014-4 .
  5. ↑ Iwona Kotko et al .: The viscosity parameter α and the properties of accretion disc outbursts in close binaries . In: A & A, 545, A115; . August 31, 2012. arxiv : 1209.0017 . doi : 10.1051 / 0004-6361 / 201219618 .
  6. ↑ a b Osaki, Yoji: Dwarf-Nova Outbursts . In: Publications of the Astronomical Society of the Pacific, v.108, p.39 . 1996. bibcode : 1996PASP..108 ... 39O . doi : 10.1086 / 133689 .