UG U Geminorum variable stars

Observation

„/- large amplitudes n Mixed periods ^ Visual, CCD/PEP

- Quite often called dwarf novae. They are close binary systems consisting of a dwarf or subgiant K-M star that fills the volume of its inner Roche lobe and a white dwarf surrounded by an accretion disk. Orbital periods are in the range of 0i'05-0i'5. Usually only small, in some cases rapid, light fluctuations are observed, but from time to time the brightness of a system increases rapidly by several magnitudes and, after an interval of from several days to a month or more, returns to the original state. Intervals between two consecutive outbursts for a given star may vary greatly, but every star is characterized by a certain mean value of these intervals, i.e. a mean cycle that corresponds to the mean light amplitude. The longer the cycle, the greater the amplitude. These systems are frequently sources of X-ray emission. The spectrum of a system at minimum is continuous, with broad H and He emission lines. At maximum these lines almost disappear or become shallow absorption lines. Some of these systems are eclipsing, possibly indicating that the primary minimum is caused by the eclipse of a hot spot that originates in the accretion disk from the infall of a gaseous stream from the K-M star. According to the characteristics of the light changes, U Gem variables may be subdivided into three types: UGSS (subgroup) - SS Cygni type variables. They increase in brightness by 2'"-6'" in V in Id-2d and in several subsequent days return to their original brightness. The values of the cycle are in the range 10 days to several hundred. UGSU (subgroup) -SU Ursae Majoris type variables. These are characterized by the presence of two types of outbursts called "normal" and "super-maxima."Normal, short outbursts are similar to those of UGSS stars, while super-maxima are brighter by 2'", are more than 5 times longer (wider), and occur three times less frequently. During super-maxima the light curves show superposed periodic oscillations (super-humps), their periods being close to the orbital ones and amplitudes being about 0".'2-0"'3 in V. Orbital periods are shorter than 0.dl; companions are dM spectral type. UGZ (subgroup) - Z Camelopardalis type stars. These also show cyclic outbursts, differing from UGSS variables by the fact that sometimes after an outburst they do not return to the original brightness, but during several cycles retain a magnitude between maximum and minimum. The values of cycles are from 10 to 40 days, while light amplitudes are from 2'" to 5"' in V. GCVS

Dwarf novae outbursts are intrinsically much less luminous events than classical novae outbursts. Their peak absolute magnitudes are at least 100 times weaker. Dwarf novae are known to recur, with some recurring on time-scales as short as a few weeks. Dwarf novae also have short durations, lasting a few days. Dwarf novae can also exhibit a variety of unusual behaviors. SU UMa type sources occasionally exhibit extremely long outbursts known as superoutbursts. Z Cam stars will occasionally get stuck in standstills during which their brightness is both below outburst stage and well above quiescent levels. VY Scl stars, also known as anti-dwarf novae, will spend most of their time in an outburst state, with occasional dips into quiescence that last for a few days. Finally, there are novalikes which behave much like novae long after their eruptions, but which have never exhibited novae outbursts. They are also distinct from dwarf novae outbursts in that they have permanently high rates of mass transfer.

The principal source of electromagnetic radiation in a dwarf nova system is the accretion disk. The companion star to the white dwarf is a low-mass red dwarf star filling its Roche lobe with matter streaming onto the accretion disk through the inner Lagrange point. The gas stream from the LI point impacts the

Figure 5.7. Artist's conception of a dworf novae demonstrating H* formation of an accretion disk. CopyriO^ Gerry A Good.

Figure 5.7. Artist's conception of a dworf novae demonstrating H* formation of an accretion disk. CopyriO^ Gerry A Good.

accretion disk and creates a hot spot. Matter gradually transports through the accretion disk onto the surface of the white dwarf, generating temperatures which make the disk much hotter and brighter than either star. The dwarf nova outburst and other related phenomenon are believed to be caused by variations in the accretion rates through the disk. Material reaching the white dwarf surface through the disk must pass through a violent transition region, called the boundary layer: it is here that the X-rays in dwarf novae originate. This is shown dramatically by the recent observations of X-ray eclipses in HT Cas; the eclipse duration is the same as that of the white dwarf as determined by optical observations. The sharpness of the transitions into and out of the eclipse proves that the X-ray emitting region has a size comparable to that of the white dwarf.

Dwarf novae are generally believed to be semidetached binaries containing a white dwarf and low-mass main sequence stars. The Roche-overflow gas from the secondary star forms an accretion disk around the compact object. The chief source of the visual light of CVs is from the accretion disks. Several sorts of disk instabilities dramatically affect the luminosity of the disk, and then become observable as a variation in the optical flux. This feature not only provides us with one of the best opportunities in direct investigation of the physics of accretion disks through observations of CVs and XTs, but also enables us to reveal the nature of specific objects of astrophysical importance by applying the known physics of accretion disks.

Dwarf novae show semiperiodic outbursts with a typical amplitude ranging from 2 to 6 magnitudes, and with a recurrence period of 10 to 1000 days. In contrast, novalike variables do not show prominent outburst activities.

Dwarf novae are further subclassified according to their light behavior: SS Cyg stars display approximately regular, recurring outbursts, Z Cam stars display "standstills" during which the stars show little variation at brightness between maxima and minima, SU UMa stars display two distinct types of outbursts, short (normal) outbursts and super-outbursts that are brighter than the normal outbursts.

As stated in the first section, the visual light of CVs for the most part reflects the energy output from the accretion disk, hence the cause of variation should primarily be sought in the accretion process itself.

Figure 5.8. ug^ °f U Gem (dwarf ^ Data provided by 1 VSNET. Used will, permission.

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Figure 5.8. ug^ °f U Gem (dwarf ^ Data provided by 1 VSNET. Used will, permission.

Among several mechanisms to explain this rich variety of light variation of CVs, only two of them seems to have remained viable: mass-transfer instability and disk instability. The former paradigm primarily assumes that the changing mass-transfer rate from the secondary produces the luminosity variation of the accretion disk; the latter, in contrast, does not assume change of mass-transfer rate, but the intrinsic instability of the accretion disk produces temporal changes in mass-accretion rate in the disk which is observed as quiescent and outburst states. The discrimination of these two paradigms in variable accreting system has been, whether explicitly or implicitly posed, always one of the main goals of both observers and theoreticians.

After a long period of debate, a fairly good consensus in ordinary CVs seems to have been reached between most observers and theoreticians concerning the natural explanation of dwarf nova phenomenon: the disk instability model. The basic disk instability idea explains the dwarf nova phenomenon in the following way: the disk accumulates the accreted mass during quiescence and accretes it to the white dwarf during outburst. The nature of the disk instability which triggers such interchange of disk status was not known at that time. Subsequent theoretical studies finally discovered the thermal instability of the accretion disk due to the partial ionization of the hydrogen. This thermal instability has been shown to not only successfully reproduce the various light curves of SS Cyg-type dwarf novae but also gives a natural explanation of two basic types of CVs: dwarf novae and novalike variables.

The difference between dwarf novae and novalike variables is explained in the scheme of thermal instability theory, in that the higher mass-transfer rate in novalike variables produce thermally stable accretion disks.

% And showing a brightness a tar prolonged "standstill-

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The Z Cam stars have intermediate mass-transfer rates, and are believed to share properties of these two subclasses, that is, phases showing dwarf nova-type activity when the disk is thermally unstable, and standstills when the disk in thermally stable like novalike variables.

Observation Key

•fa Mixed stars

Mixed amplitudes ^^ Mixed periods <3> Visual, CCD/PEP

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