CV News

Articles

What's In A Name? (Part Deux)

posted Oct 23, 2009, 10:12 AM by Mike Simonsen   [ updated Oct 28, 2009, 9:22 PM ]

Reproduced from Eyepiece Views
Mike Simonsen

The following is an updated, reorganized and more complete version of an article originally published in Eyepiece Views, July, 2002.

The conventional system for naming variable stars is archaic, but has served us for over 150 years now.

In order not to get variables confused with stars assigned Bayer lower case letters a-q, Friedrich Argelander began naming variables with the letters R-Z. After those were used up RR-RZ, SS -SZ, etc. were assigned. Then they start over with AA-AZ, BB-BZ, etc. all the way to QZ (skipping the J's). This allows for 334 names. After the letters are used up the stars are simply named V335, V336, V337 and on and on.

As if that weren't confusing enough, there are now a host of other prefixes and numbers assigned to variable stars and objects. The following is a guide to help the reader understand what these names mean and where they came from.

NSV xxxxx - These are stars in the Catalog of New and Suspected Variable Stars, produced as a companion to the Moscow General Catalog of Variable Stars (GCVS) by B.V. Kukarkin et al. All stars in the NSV have reported but unconfirmed variability, in particular, lacking complete lightcurves. Some NSV stars will eventually prove truly variable; others will be spurious. Information about this and the General Catalog of Variable Stars can be found at: http://www.sai.msu.su/groups/cluster/gcvs/gcvs/intro.htm

Many stars and variable objects are assigned prefixes based on astronomer, survey or project names. Many are temporary designations until they are assigned a conventional name in the GCVS.

3C xxx - These are objects from the Third Cambridge (3C) catalog (Edge et al. 1959), based on radio-wavelength observations at 158 MHz. There are 471 3C sources, numbered sequentially by right ascension. All 3C sources are north of -22 declination. The 3C objects of interest to variable star observers are all active galaxies (quasars, BL Lacs, etc.).

Antipin xx- Variable stars discovered by Sergej V. Antipin, a junior researcher working for the General Catalogue of Variable Stars Group.

HadVxxx - This represents variables discovered by Katsumi Haseda. Haseda's most recent discovery was Nova 2002 in Ophiuchus, V2540 Oph.

LD xxx - Variables discovered by Lennart Dahlmark, a Swedish retiree living in southern France are given this prefix. Dahlmark has been conducting a photographic search for new variable stars; discovering several hundred to date.

He-3 xxxx - Variables from Henize, K. G. 1976, "Observations of Southern Emission-Line Stars", Ap.J. Suppl. 30, 491.

HVxxxxx - Preliminary designations of variables discovered at Harvard Observatory.

Lanning xx - Discoveries of UV-bright stellar objects by H. H. Lanning from Schmidt plates centered primarily on the galactic plane. In all, seven papers entitled "A finding list of faint UV-bright stars in the galactic plane" were published.

Markarian xxxx - TThe widely used abbreviation for Markarian objects is Mrk. These are active galaxies from lists published by the Soviet Armenian astrophysicist B.E. Markarian. Markarian looked for galaxies that emit unusually strong UV radiation, which comes from either pervasive star-formation HII regions or from active nuclei. In 1966, Markarian published 'Galaxies With UV Continua'. Around that time, he started the First Byurakan Spectral Sky Survey (FBS), which is now completed. In 1975, Markarian initiated a Second Byurkan Survey (SBS). The SBS was continued by his collaborators after his death. For more information see 'Active Galactic Nuclei', by Don Osterbrock.

MisVxxxx - The stars are named MisV after MISAO Project Variable stars. The MISAO Project makes use of images taken from all over the world, searching for and tracking astronomically remarkable objects. The number of variables discovered so far reached 1171 on May 15, 2002. Few of these stars have lightcurves, and the type and range of many are still undetermined. The project website url is: http://www.aerith.net/misao/

S xxxxx - These are preliminary designations of variables discovered at Sonneberg Observatory.

SVS xxx - Soviet Variable Stars, indicates preliminary designations of Soviet-discovered variables.

TKx - TK stands for T.V. Kryachko. The TK numbers of new variables continue a numbering system first introduced in Kryachko and Solovyov (1996). This acronym was invented by the authors.

Another group of objects is labeled with the prefix O, then a letter, then a number (OJ 287 for example). These objects were detected by the Ohio StateUniversity radio telescope "Big Ear" in a series of surveys known as the Ohio Surveys.

Many variables are named with prefixes associated with surveys or satellites, combined with the coordinates of the object.

2QZ Jhhmmss.s-ddmmss - Objects discovered by the 2dF QSO Redshift Survey. The aim is to obtain spectra of QSOs out to redshifts so high the visible light emitted by these objects has shifted into the far infrared. The observations are actually of the ultra-violet part of the spectrum that has been redshifted into the visible. As with most QSO surveys, a serendipitous byproduct is the discovery of CVs and other blue stars. A description and awesome pictures of the equipment can be found here: http://www.2dfquasar.org/Spec_Cat/basic.html Home site: http://www.2dfquasar.org/index.html

ASAS hhmmss+ddmm.m - This is the acronym for All Sky Automated Survey, which is an ongoing survey monitoring millions of stars down to magnitude 14. The survey cameras are located at the Las Campanas Observatory in Chile, so it covers the southern sky from the pole to about +28 degrees declination.

FBS hhmm+dd.d - Stands for First Byurakan Survey and the coordinates of the object. The First Byurakan Survey (FBS), also known as the Markarian survey, covers about 17,000 square degrees.

EUVE Jhhmm+ddmm - These are objects detected by NASA's Extreme Ultraviolet Explorer, a satellite dedicated to studying objects in far ultraviolet wavelengths. The first part of the mission was dedicated to an all-sky survey using the imaging instruments that cataloged 801 objects. Phase two involved pointed observations, mainly with the spectroscopic instruments. One of the highlights of the mission was the detection of Quasi Periodic Oscillations (QPOs) in SS Cyg.

FSVS Jhhmm+ddmm - Discoveries from the Faint Sky Variability Survey, the first deep wide-field, multi-color, time-sampled CCD photometry survey. It was specifically aimed at detecting point sources as faint as 25th magnitude in V and I and 24.2 in B. Targets were faint CVs, other interacting binaries, brown dwarfs and low mass stars and Kuiper Belt Objects.

HS hhmm+ddmm- The Hamburg Quasar Survey is a wide-angle objective prism survey searching for quasars in the northern sky, avoiding the Milky Way. The limiting magnitude is approximately 17.5B. The taking of the plates was completed in 1997.

PG hhmm+DDd- Palomar Green Survey conducted to search for blue objects covering 10714 square degrees from 266 fields taken on the Palomar 18-inch Schmidt telescope. Limiting magnitudes vary from field to field, ranging from 15.49 to 16.67. The blue objects detected tend to be quasars and cataclysmic variables. The CVs were documented in Green, R. F., et al. 1986, "Cataclysmic Variable Candidates from the Palomar Green Survey", Ap. J. Suppl. 61, 305.

PKS hhmm+ddd - This was an extensive radio survey (Ekers 1969) of the southern sky undertaken at Parkes (PKS), Australia, originally at 408 MHz and later at 1410 MHz and 2650 MHz. These sources are designated by their truncated 1950 position. For example 3C 273 = PKS 1226+023. This is still the most common, and useful, system of naming quasars.

ROTSE1 thru 3 Jhhmmss.ss+ddmmss.s - The Robotic Optical Transient Search Experiment (ROTSE) is dedicated to the observation and detection of optical transients on time scales of seconds to days. The emphasis is on gamma-ray bursts (GRBs). Objects detected by this survey are designated with positions to 0".1 precision.

ROSAT is an acronym for the ROentgen SATellite. ROSAT was an X-ray observatory developed through a cooperative program between Germany, the United States, and the United Kingdom. The satellite was designed and operated by Germany, and was launched by the United States on June 1, 1990. It was turned off on February 12, 1999.

Prefixes for x-ray sources detected by ROSAT include, 1RXS, RXS and RX. The J2000 coordinates for the source are then stated according to the accuracy of the X-ray position and the density of stars in the field.

arcsecond accuracy ---> RX J012345.6-765432
tenth-arcmin accuracy ---> RX J012345-7654.6
arcmin accuracy ---> RX J0123.7-7654

Distressingly, these can all refer to a single object!

Rosino xxx or N xx - Variables discovered by Italian astronomer L. Rosino, primarily in clusters and galaxies through photographic surveys.

SBS hhmm+dd.d - Indicates objects discovered by the Second Byurakan Sky Survey, plus the coordinates of the object.

SDSSp Jhhmmss.ss+ddmmss.s - These are discoveries from the Sloan Digital Sky Survey. The positions of the objects are given in the names. SDSS- (Sloan Digital Sky Survey), p- (preliminary astrometry), Jhhmmss.ss+ddmmss.s (the equinox J2000 coordinates). In subsequent papers on CVs detected by SDSS (Szkody et al) the p was dropped and the names became simply SDSS Jhhmmss.ss+ddmmss.s.

TAV hhmm+dd - The Astronomer Magazine, in England, has a program that monitors variable stars and suspected variable stars. TAV stands for The Astronomer Variable, plus the 1950 coordinates.

TASV hhmm+dd - TASV stands for The Astronomer Suspected Variable, plus the 1950 coordinates. The Astronomer Variable star page can be found at this url: http://www.theastronomer.org/variables.html

XTE Jhhmm+dd - These are objects detected by the Rossi X-Ray Timing Explorer Mission. The primary objective of the mission is the study of stellar and galactic systems containing compact objects. These systems include white dwarfs, neutron stars, and possibly black holes.

With more and more surveys being conducted, and more new variables being discovered, this list of non-conventional names will undoubtedly grow. I hope this explanation has helped to demystify the existing names and prepares you for the onslaught of names yet to come.

There is a CDS Web page where you can find details about specific acronyms. The GCVS also has a list of catalog abbreviations.

CV Types and Abbreviations

posted Mar 27, 2009, 2:09 PM by Mike Simonsen

This first group of types and definitions is from the General Catalog of Variable Stars

http://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt

(N) Novae

Close binary systems with orbital periods from 0.05 to 230 days. One of the components of these systems is a hot dwarf star that suddenly, during a time interval from one to several dozen or several hundred days, increases its brightness by 7-19 mag in V, then returns gradually to its former brightness over several months, years, or decades. Small changes at minimum light may be present. Cool components may be giants, subgiants, or dwarfs of K-M type. The spectra of novae near maximum light resemble A-F absorption spectra of luminous stars at first. Then broad emission lines (bands) of hydrogen, helium, and other elements with absorption components indicating the presence of a rapidly expanding envelope appear in the spectrum. As the light decreases, the composite spectrum begins to show forbidden lines characteristic of the spectra of gas nebulae excited by hot stars. At minimum light, the spectra of novae are generally continuous or resemble the spectra of Wolf-Rayet stars. Only spectra of the most massive systems show traces of cool components.

Some novae reveal pulsations of hot components with periods of approximately 100 s and amplitudes of about 0.05 mag in V after an outburst. Some novae eventually turn out to be eclipsing systems. According to the features of their light variations, novae are subdivided into fast (NA), slow (NB), very slow (NC), and recurrent (NR) categories.

(NA)

Fast novae displaying rapid light increases and then, having achieved maximum light, fading by 3 mag in 100 or fewer days (GK Per);

(NB)

Slow novae that fade after maximum light by 3 mag in >= 150 days (RR Pic). Here the presence of the well-known "dip" in the light curves of novae similar to T Aur and DQ Her is not taken into account: The rate of fading is estimated on the basis of a smooth curve, its parts before and after the "dip" being a direct continuation of one another;

(NC)

Novae with a very slow development and remaining at maximum light for more than a decade, then fading very slowly. Before an outburst these objects may show long-period light changes with amplitudes of 1-2 mag in V (RR Tel); cool components of these systems are probably giants or supergiants, sometimes semiregular variables, and even Mira variables. Outburst amplitudes may reach 10 mag. High excitation emission spectra resemble those of planetary nebulae, Wolf-Rayet stars, and symbiotic variables. The possibility that these objects are planetary nebulae in the process of formation is not excluded;

(NL)Nova-like

Novalike variables, which are insufficiently studied objects resembling novae by the characteristics of their light changes or by spectral features. This type includes, in addition to variables showing novalike outbursts, objects with no bursts ever observed; the spectra of novalike variables resemble those of old novae, and small light changes resemble those typical for old novae at minimum light. However, quite often a detailed investigation makes it possible to reclassify some representatives of this highly inhomogeneous group of objects into other types;

(NR) Recurrent Novae

Recurrent novae, which differ from typical novae by the fact that two or more outbursts (instead of a single one) separated by 10-80 years have been observed (T CrB).

(SN) Supernovae

Supernovae (B Cas, CM Tau). Stars that increase, as a result of an outburst, their brightnesses by 20 mag and more, then fade slowly. The spectrum during an outburst is characterized by the presence of very broad emission bands, their widths being several times greater than those of the bright bands observed in the spectra of novae. The expansion velocities of SN envelopes are in the thousands of km/s. The structure of a star after outburst alters completely. An expanding emission nebula results and a (not always observable) pulsar remains at the position of the original star. According to the light curve shape and the spectral features, supernovae are subdivided into types I and II.

(SNI)

Type I supernovae. Absorption lines of Ca II, Si, etc., but no hydrogen lines are present in the spectra. The expanding envelope almost lacks hydrogen. During 20-30 days following maximum light, the brightness decreases by approximately 0.1 mag per day, then the rate of fading slows and reaches a constant value of 0.014/day;

(SNII)

Type II supernovae. Lines of hydrogen and other elements are apparent in their spectra. The expanding envelope consists mainly of H and He. Light curves show greater diversity than those of type I supernovae. Usually after 40-100 days since maximum light, the rate of fading is 0.1 mag per day.

(UG) U Geminorum

U Geminorum-type variables, 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 0.05-0.5 days. 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: SS Cyg, SU UMa, and Z Cam.

(UGSS) U Gem, SS Cygni sub-type

SS Cygni-type variables (SS Cyg, U Gem). They increase in brightness by 2-6 mag in V in 1-2 days and in several subsequent days return to their original brightnesses. The values of the cycle are in the range 10 days to several thousand;

(UGSU) U Gem, SU Urase Majoris sub-type

SU Ursae Majoris-type variables. These are characterized by the presence of two types of outbursts called "normal" and "supermaxima". Normal, short outbursts are similar to those of UGSS stars, while supermaxima are brighter by 2 mag, are more than five times longer (wider), and occur several times less frequently. During supermaxima the light curves show superposed periodic oscillations (superhumps), their periods being close to the orbital ones and amplitudes being about 0.2-0.3 mag in V. Orbital periods are shorter than 0.1 days; companions are of dM spectral type;

(UGZ) U Gem, Z Camelopardalis sub-type

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 mag in V.

(AM) AM Herculis- also called polars

AM Her type variables; close binary systems consisting of a dK-dM type dwarf and of a compact object with strong magnetic field, characterized by variable linear and circular polarization of light. The total range of light variations may reach 4-5 mag V.

Although not officially recognized by the GCVS these types are commonly in use:

(UGWZ) U Gem, WZ Sagittae sub-type

A variety of UGSU in which the interval between super-outbursts is unusually long, measured in decades, while normal outbursts are few and far between. Observations of the 1978 outburst revealed super-humps in WZ Sge's light curve, which are the defining characteristics of SU UMa type dwarf novae; thus WZ Sge is now considered the prototype for a subset of the SU UMa class. Other WZ Sge stars include AL Com and EG Cnc, which have super-outburst intervals of approximately 20 years.

The approximately 30-year supercycle length that WZ Sge displays is the most inactive group of the SU UMa type stars. The factor determining the different timescales appears to be mass-transfer rate. WZ Sge stars have a very low mass-transfer rate, perhaps only 1012 kg/s. Given the slow rate of mass-transfer, it would then take decades to accumulate enough material for a super-outburst. The puzzle of these stars, however, is why they show few or no normal outbursts during this interval. Even with a low mass-transfer rate, material should accumulate, drifting viscously into the inner disc, and trigger an outburst. One suggestion for why this does not occur is that the disk viscosity is very low. The material would then remain in the outer disc, where much more can be stored before an outburst is triggered. The problem with this idea, however, is to explain the extremely low viscosity level. Another possible explanation involves the removal of the inner disc, to prevent outbursts starting there. This could occur through siphons or because of a magnetic field on the white dwarf.

UGSU(ER) UGSU, ER UMa sub-type

A variety of UGSU in which the interval between superoutbursts is unusually short. ER UMa stars typically spend a third to a half their time in super-outburst, with a super-cycle (the interval between super-outbusts) of only 20 to 50 days. When not in super-outburst these stars show frequent normal outbursts - about one every 4 days.

(DQ) DQ Herculis- also known as IP, intermediate polars

In the intermediate polars (which are thought to have lower magnetic fields than the polars), the spin period of the white dwarf is shorter than the orbital period. The accretion process in the intermediate polars is through a disc with a disrupted inner radius (where the magnetic field is powerful enough to influence the flow of the gas), or an accretion stream as in the polars. The intermediate polars with the shortest spin periods are the DQ Herculis stars.

Additional notes:

Systems exhibiting eclipses are commonly indicated as UG+E, UGSU+E, UGSS+E, etc.

If the type is uncertain, or there is uncertainty between two possible types, this is indicated by a colon following the type or types (UG:, UGSU:, UGSU:/UGWZ:, N:/UGWZ:, NR:/UGWZ:, etc.)

Introduction to Cataclysmic Variables

posted Mar 27, 2009, 12:49 PM by Mike Simonsen


Of the (approximately) four hundred thousand million stars in our Galaxy, more than half are not single stars like our Sun, but occur in binary or multiple systems. Binary star systems come in many flavours: red stars orbiting blue stars, huge stars orbiting tiny stars, black holes orbiting blue giants, red stars orbiting neutron stars, and so on. In a CV binary, one star is a white dwarf: a collapsed star with the mass of the Sun in the volume of the Earth (our Sun will become a white dwarf in about four and a half billion years). The other star is a red dwarf rather like our Sun, but redder and less massive.

Dwarf Nova


Cataclysm III


The red dwarf and the white dwarf orbit each other once every few hours: they are so close together that the average CV system would fit comfortably into our Sun. When we observe CVs, we can't resolve the two stars: they appear on the sky as a point source.

The red star in a CV is so close to the white dwarf that it becomes tidally distorted --- gas is stripped off the red star and falls towards the white dwarf. Because of conservation of angular momentum, the infalling gas can't plunge directly onto the surface of the white dwarf. In systems where the white dwarf doesn't have an appreciable magnetic field, the infalling gas forms a disc -- an accretion disc -- with the white dwarf at its centre. The gas in the disc spirals down towards the white dwarf, radiating its gravitational potential energy away as it goes. The accretion disc usually outshines both the red star and the white dwarf in visible light. If the white dwarf is strongly magnetic, other interesting things happen: more about that in the section on magnetic CVs.

The CVs with accretion discs come in several flavours. The first to be discovered were the novae, originally novae stella: new stars. They draw attention to themselves by their stupendous amplitude of variation --- 6 to 19 magnitudes (that's factors of about 100 to several million in brightness) over a period of months or years. The novae with the largest outburst amplitudes fade the fastest. Nova outbursts are due to thermonuclear runaways of the hydrogen-rich material that has accreted onto the white dwarf. Most novae known have only been observed to undergo one nova outburst, but several are recurrent novae. The recurrent nova T Pyx is particularly interesting: it was recently observed to possess collimated jets emanating from the central regions of the accretion disc (Shahbaz et al. 1997, Astrophysical Journal, 484, 59). It is the first CV system in which jets have been observed.

Another group of non-magnetic CVs is the dwarf novae. Their outbursts are not quite as spectacular as those of the novae (in outburst, dwarf novae are a mere factor of 6 to 100 brighter than in quiescence), but the outbursts occur more often. In general, the more frequent the outburst, the smaller the amplitude of outburst. Examples of the extremes of the dwarf nova phenomenon are V1159 Ori, which has outbursts once every four days with an amplitude of about two magnitudes; and WZ Sge, which shows outbursts once every thirty years, and the outburst ampitude rivals that of a classical nova outburst. Both these extremes of behaviour occur in the dwarf novae with the shortest orbital periods. A dwarf nova outburst is thought to be a large release of gravitational potential energy caused by a temporary enhancement of the rate of mass transfer through the disc.

An interesting subset of the dwarf novae are the SU Ursa Majoris stars. They show two distinct kinds of outburst: normal dwarf nova outbursts, and superoutbursts, which last 5-10 times longer and are slightly brighter than the usual dwarf nova outbursts. Most superoutbursting dwarf novae have short orbital periods (less than 2 hours). Some of the SU UMa stars with long outburst intervals show interesting rebrightenings on the way back to quiescence after a superoutburst.

During a superoutburst, a SU UMa star shows an additional modulation of the light curve, a superhump, which is caused by precession of the accretion disc. Superhumps show up in the light curve as a modulation with a period slightly longer (a few percent) than the orbital period.

Another group of CVs with accretion discs are the nova-like variables. The difference between the nova-like variables and dwarf novae is that nova-like variables don't undergo dwarf nova outbursts. This is because the rate of transfer of matter in their discs is (by-and-large) stable, and the overall brightness varies only slightly about its mean level. In addition, the rate of mass transfer in the discs of nova-like variables is much higher than that in quiescent dwarf novae, and the accretion discs are thus very bright.

The nova-likes with the highest mass transfer rates are the SW Sextantis stars. This intriguing group of stars has many unusual properties. For example, the emission lines in their spectra behave in an unexpected way. If you observe a CV system edge-on, so that your line of sight is along the plane of the disc, you expect to see double-peaked emission lines. This is because the gas in one half of the disc is moving towards you, and its emission is thus blue-shifted, while the gas in the other half is moving away from you, and its emission is red-shifted. Despite being edge-on systems, the SW Sextantis stars have single-peaked emission lines. For more details of the other weird properties of SW Sextantis stars, and some recent models that explain them, see e.g. Hellier (1996, Astrophysical Journal, 471, 949) and Dhillon, Marsh & Jones (1997, Monthly Notices of the Royal Astronomical Society).

Some nova-like variables show superhumps, like those seen in the SU UMa stars in superoutburst. Unlike the superoutbursting systems, however, these nova-likes have superhumps in their light curves all the time. They are the permanent superhumpers. An interesting group of CV systems that show permanent superhumps is the AM Canum Venaticorum stars. They are helium-rich CVs: no hydrogen has ever been detected in any of them. They have much shorter orbital periods than the hydrogen-rich systems: the two stars of AM CVn itself whizz around each other every quarter of an hour. It seems that the two crucial criteria for a system to show superhumps are the following: first, there must be a high rate of mass transfer through the accretion disc; and secondly, the mass-donating red star must be much less massive than the accreting white dwarf (its mass should be no more than a quarter of the white dwarf's mass). Interestingly, the superhump phenomenon is not confined to CVs: they have also been observed in the weird neutron star binary SS433 and in black hole soft X-ray transients (see e.g. O'Donoghue & Charles, Monthly Notices of the Royal Astronomical Society, 282, 191).


Magnetic CV


Magnetic Accretion



Magnetic CVs are systems where the white dwarf has an appreciable magnetic field (several tens of millions of Gauss). Because the matter in the accretion stream is partially ionized, it can't form a disc (because charged particles can't cross field lines, only spiral around them). Instead, the gas is threaded onto the field lines and plunges straight down onto the magnetic poles of the white dwarf. Because the accretion occurs almost perpendicularly to the surface of the white dwarf, copious X-ray and EUVB emission is liberated at the poles.

In systems where the magnetic field of the white dwarf is strong enough to synchronize the rotation of the white dwarf with that of the binary, the red star and the white dwarf rotate essentially as rigid bodies. These are the polars, or AM Herculis stars. The magnetic field in these stars is so strong (it's about fifty million times the strength of the Earth's magnetic field) that no accretion disc can form.

One of the most distinctive properties of the light emitted by polars is that it is both linearly and circularly polarized (this is one of the reasons why polars are called polars. The other reason is that the name was recommended by the Polish astronomers Krzeminski & Serkowski). By observing changes in the intensity of polarized emission over time, we can discover the magnetic field strength of the white dwarf and the magnetic field geometry.

In the intermediate polars (which are thought to have lower magnetic fields than the polars), the spin period of the white dwarf is shorter than the orbital period. The accretion process in the intermediate polars is through a disc with a disrupted inner radius (where the magnetic field is powerful enough to influence the flow of the gas), or an accretion stream as in the polars, or either, or both (just to prevent you from falling asleep). The intermediate polars with the shortest spin periods are the DQ Herculis stars.

New CV systems are being discovered all the time. Each new member of the CV zoo presents us with new properties that add to our knowledge of the CV phenomenon. From radio to TeV gamma rays, cataclysmic variables continue to provide fascinating new areas of research.

Adopted from an article written by Kate Harrop-Allin, 22 March 1998


'Cataclysm III' and 'Magnetic Accretion' copyright Mark A. Garlick
Unauthorized use strictly prohibited

CV Watch List

posted Mar 27, 2009, 10:01 AM by Mike Simonsen   [ updated Apr 4, 2009, 8:16 AM ]

Please report outbursts or unusual activity of these CVs as soon as possible.

AAVSO Observing Campaigns-


An observing campaign is defined as a request for observations to achieve a specific goal, usually over a finite amount of time. Long-term monitoring campaigns for specific objects and research efforts will be listed here.

AAVSO CV Campaigns

Monitoring of the neutron star binary SDSS J102347.68+003841.2

Request for observations of N LMC 09


Monitoring of YZ Cnc, Z Cam, and EM Cyg for radio observations

Long Term Monitoring of U Sco


BAAVSS Polar Programme-

The BAAVSS Long Term Polar Monitoring Programme has been set up to monitor over a period of years a selection of AM Her stars (see list below), which are in need of further investigation. 

The objective is to observe on a nightly basis both visually and with CCD's, and to report any change in high/low state activity.  The programme is supported by Dr Boris Gaensicke, Warwick University, whose article on Polars appeared in the September 2006 issue of the BAAVSS Circular (No. 129), and was the catalyst for this programme to be launched. You can read the article here.

A new picture of polar accretion is evolving

Image credit: S. Howell / P. Marenfeld/NOAO

TA/BAA Recurrent Objects Programme
-

The main aims of the programme are...

  • To  monitor for outbursts or unusual behaviour either visually or with CCD's, in order to determine an outburst cycle, and with the help of CCD photometry, the orbital period.  
  • To obtain high quality photometry during outburst.
  • To raise the profile of the objects on the programme in order that greater observational coverage will be obtained.
  • To provide Professional astronomers with much needed data on these stars.

At present there are 79 objects on the programme (see below).  Most are Dwarf Novae, but several Novae have been included where more than one outburst has been detected, or is suspected of being recurrent from professional literature.  Other interesting NL objects appear too!


Hamburg Survey CVs-


Most cataclysmic variables (CVs) have been discovered either because of their variability, strong X-ray emission, or very blue colours. We have started a program to find CVs because of their peculiar emission line spectra, selecting candidates from the Hamburg Quasar Survey.

The motivation of this project has been to test whether this selection would unravel new types of CVs (which it did, e.g. the low accretion rate polars such as HS1023+3900), and we had the hope to find the many short-period CVs that CV evolution theory predicts.

Interestingly enough, most of the new CVs have no or very rare outbursts, but instead of numerous short-period systems, we found a large fraction of long-period CVs, some of them clear members of the SW Sex class, but in some cases it is very difficult to unambiguously identify the CV subtype. While some systems appear to be rather normal dwarf novae, such as the recently outbursting HS0417+7445, it it is very important for the majority of the systems to gather detailed long-term light curves of these systems to better understand their nature.

UGZ or not UGZ, that is the question...

posted Mar 27, 2009, 9:40 AM by Mike Simonsen



By Mike Simonsen SXN

The old adage "you can’t believe everything you read" was never truer than for cataclysmic variables of the type UGZ. Named after the prototype Z Camelopardalis, the most distinguishing property of UGZ type CVs are the "standstills" they occasionally undergo in their cycles. This is described in the definition from the General Catalog of Variable Stars (GCVS):

"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 mag in V."

Do all UGZs exhibit standstills? Are objects that don’t exhibit standstills therefore, not UGZs? It would seem fairly straightforward from the GCVS definition, but things are seldom as simple as they seem in the world of variable stars.

Since this article is intended for visual observers, we will discuss only objects that have outburst magnitudes of 13.5V or brighter. Our primary sources for information on type, position, magnitude range, period, etc., are the GCVS http://www.sai.msu.su/groups/cluster/gcvs/gcvs/ and the Catalog and Atlas of Cataclysmic Variables http://icarus.stsci.edu/~downes/cvcat/index.html. As you will see, even these two well-respected sources don’t always agree on the facts, and in some cases both are wrong!



RX And- (0058+40) With a normal range of 10.3-14.0V, this star is easy to follow at all times during its cycle in modest sized telescopes. A quick look at the AAVSO light curve for this star for the last 700 days will show two obvious standstills after outbursts. ( http://www.aavso.org/data/lcg/ ) The cycles are fairly short and the amplitude of the curve is 3-4 magnitudes. This is a typical UGZ, showing all the normal characteristics defined in the GCVS.



TW Tri- (0130+31) Outbursting to 13th magnitude a little less often than the 40 days defined in GCVS, the case for TW Tri’s UGZ-ness is less clear. There are no obvious standstills in the AAVSO light curves, possibly because the magnitude at which it might rest before returning to quiescence (15th mag?) is too faint for most visual observers to record. Another very similar star is VW Vul (2053+25). Outbursting as bright as 13.1 at times and having a minimum in the 16th magnitude range, short standstills occur in the mid-14’s and can be difficult to follow visually.

KT Per- (0130+50) Listed as UGZ+ZZ in the GCVS, this suffix to the classification is explained in GCVS as "ZZ Ceti variables. These are nonradially pulsating white dwarfs that change their brightnesses with periods from 30 s to 25 min and amplitudes from 0.001 to 0.2 mag in V. They usually show several close period values. Flares of 1 mag are sometimes observed; however, these may be explained by the presence of close UV Ceti companions." Another explanation for dwarf nova oscillations (DNOs) and quasi-periodic oscillations (QPOs) observed in cataclysmic variable stars is proposed by Brian Warner in 2004PASP..116..115W - Publ. Astron. Soc. Pac., 116, 115-132 (2004) - February 2004, Rapid oscillations in cataclysmic variables.

http://simbad.u-strasbg.fr/cgi-bin/cdsbib?2004PASP..116..115W

The interpretation of these modulations is that they are "magnetically channeled accretion from the inner accretion disk for DNOs" and "magnetically excited traveling waves in the disk for QPOs".

TT Ari- (0201+14) GCVS lists this star as UGZ. Downes online CV catalog lists the type as vy/dq: In other words, they are not sure if it is a VY Scl type, which exhibit sudden fades, or a magnetic variable, specifically an intermediate polar, whose accretion disk is interrupted by the presence of a strong magnetic field.

Most other references describe it as nova-like (NL).

As far back as 1979 this star is referred to as a NL object. An early IBVS, from 1979, IBVS 1622, TT Ari; describes fast photoelectric photometry on this NL.

http://www.konkoly.hu/cgi-bin/IBVS?1622

More recently, IBVS 5664, December 2005, describes the recent fading of this star. Again TT Ari is described as a NL variable, in spite of the VY Scl-like fading.

TT Ari: Out from the Positive Superhump State

http://www.konkoly.hu/cgi-bin/IBVS?5664

This fading episode is apparent in the ASAS light curve for this star.

http://www.astrouw.edu.pl/cgi-asas/asas_variable/020653+1517.7,asas3,%20%20%204.292588,0,1000,0

It is also visible in any recent light curve from AAVSO data. No obvious outbursts are shown in the long-term light curve of TT Ari, certainly not every 40 days or less, so why is this one listed as UGZ in GCVS?

TZ Per- (0206+57A) Plot a light curve for this star going back 500 days and you’ll see a standstill episode centered on magnitude 13.5, beginning around JD 2453375. It appears to last at least 100 days, but the end of the standstill is ambiguous due to the seasonal gap in the curve. This is pretty typical for TZ Per, making it a great star for visual observers to monitor. Varying between 12.0 and 15.6, it is visible more often than not.

AQ Eri- (0501-04) This variable is listed as a suspected UGZ in GCVS, however IBVS 5107 describes AQ Eri as a UGSU based on superhump observations. ‘Superoutburst Observation of AQ Eri: Evidence for an Anomalous Superhump Excess?’ http://www.konkoly.hu/cgi-bin/IBVS?5107. The Downes catalog does not include AQ Eri in its listings of UGZs. Is this merely outdated information in GCVS? Does the presence of superhumps preclude classification as a UGZ? Maybe not; read on.

CN Ori- (0547-05) This is a very active star, with a range of 11.0-16.2, outbursting about every other week. But the light curve looks very much like a UGSS to me. Try as I may, I don’t see evidence of standstills in the data. Perhaps, as with TW Tri, the standstills occur below the threshold of most visual observers. But I would expect to see gaps in the outburst frequency if this were happening, and I just don’t see it.

What magnitude does it park at when in standstill? Are the standstills short-lived, or do they exist at all? This star also seems to be at or beyond the amplitude limit described in GCVS. Is this a UGZ?

Two more variables fit this mold SV CMi (0725+06) and AB Dra (1953+77). They are both active stars and interesting to follow visually, but they appear to be more UGSS-like than UGZ. AB Dra in particular, is so active I can’t believe it has time to go into standstill between its frequent outbursts.

Z Cam- (0814+73) This is the prototype of this class, and a great star for visual monitoring. It ranges from 10.0-14.5V and will sometimes get stuck on the way down to minimum at or around 11.5. The last standstill was relatively short, and not very stable. However, the one before that lasted almost a year!

AT Cnc- (0822+25) AT Cnc is included as UGZ in Downes et al, but not GCVS. This unusual variable has been spending more time in standstill than in outburst or quiescence in recent times. It is therefore not surprising to find a great number of papers on AT Cnc in standstill, including this recent paper co-authored by AAVSO’s Elizabeth Waagen.

2005PASP..117..931S - Publ. Astron. Soc. Pac., 117, 931-937 (2005) - September 2005

A recurrence time versus orbital period relation for the Z Camelopardalis stars.

http://simbad.u-strasbg.fr/cgi-bin/cdsbib?2005PASP..117..931S

It was surprising to find this paper describing superhumps in AT Cnc in standstill. Apparently, superhumps do not preclude inclusion in the UGZ classification!

2004A&A...419.1035K - Astron. Astrophys., 419, 1035-1044 (2004) - June(I) 2004

Detection of superhumps in the Z Camelopardalis-type dwarf nova AT Cnc at standstill.

http://simbad.u-strasbg.fr/cgi-bin/cdsbib?2004A%26A...419.1035K

More papers on AT Cnc in standstill seem to make it clear that this variable is a UGZ.

Unusual Slow Fading of Standstill in AT Cnc

http://www.konkoly.hu/IBVS/5001.html#5099

1999PASJ...51..115N - Publ. Astron. Soc. Jap., 51, 115-125 (1999)

Spectroscopic and photometric observations of a Z Cam-type dwarf nova, AT Cancri, in standstill.

http://simbad.u-strasbg.fr/cgi-bin/cdsbib?1999PASJ...51..115N

SY Cnc- (0855+18) Varying between 10.6 and 14.0, this active UGZ is always visible in modest telescopes. Lying so near the ecliptic presents problems as the moon passes through Cancer each month, and occasionally a bright planet, like Jupiter, will plant itself right in the field, making observations interesting and a bit difficult.

AH Her- (1640+25) This is another fairly typical UGZ. The range from outburst to quiescence is 10.6-14.7, with occasional standstills around 12.5-13.0 after outbursts.

UZ Ser- (1805-14) UZ Ser is misclassified in both GCVS and Downes et al. As early as 1987 standstill behavior was observed in UZ Ser. 1987JAVSO..16...91D - J. Am. Assoc. Variable star obs., 16, 91-93 (1987) Unusual behavior of UZ Serpentis. http://simbad.u-strasbg.fr/cgi bin/cdsbib?1987JAVSO..16...91D

More recently, this star was included as one of 16 examples of UGZ.

2005PASP..117..931S - Publ. Astron. Soc. Pac., 117, 931-937 (2005) - September 2005

A recurrence time versus orbital period relation for the Z Camelopardalis stars.

http://simbad.u-strasbg.fr/cgi-bin/cdsbib?2005PASP..117..931S

It seems to be general knowledge amongst many that this star is a UGZ, but it is not included as such in GCVS or Downes et al.

V1504 Cyg- (1925+42) GCVS lists this star as a suspected UGZ. Downes et al lists it as UGSU. Considering the fact that both normal and super outbursts have been observed and superhumps have been detected in outburst, this star must be considered a UGSU. This is in fact the subject of IBVS 4532, November 1997, Confirmation of the SU UMa nature of V1504 Cyg http://www.konkoly.hu/cgi-bin/IBVS?4532

EM Cyg- (1934+30) This is a very active UGZ in a beautiful star field in Cygnus. It ranges from 12.5-14.4, with short standstills in the low 12th mag range. This is also the only known case of an eclipsing UGZ, so the accurate type is UGZ+E.

FY Vul- (1937+21) Listed as UGZ in Downes and GCVS, this CV has an outburst cycle between 30 and 50 days, but also shows some quasi-periodic variation on shorter time scales, perhaps 15-20 days. The amplitude of variation is rather small for a UGZ type dwarf nova. It has been suggested that this star and V1101 Aql may actually represent a previously unrecognized group of low-amplitude dwarf novae (IBVS 4766, 1999). http://www.konkoly.hu/cgi-bin/IBVS?4766 Ranging from 13.4-15.3V this very active star is doing something every night.

V1285 Cyg- (1941+35) Listed in GCVS as a suspected UGZ, this star is actually a SR as described in the paper 1987A&A...185..203B - Astron. Astrophys., 185, 203-205 (1987)

The reclassification of the supposed dwarf nova V1285 Cygni as a semiregular variable.

http://simbad.u-strasbg.fr/cgi-bin/cdsbib?1987A%26A...185..203B

EV Aqr- (2101+00) Listed as UGZ in both GCVS and Downes et al, here is another case where both catalogs have it wrong. EV Aqr is obviously a SR with a period of roughly 124 days, ranging from 11.2-14.0V. This can be seen clearly in the ASAS data for this star.

http://www.astrouw.edu.pl/cgi-asas/asas_variable/210618+0052.7,asas3,%20124.919464,0,1000,0

HX PEG- (2335+12) Although not classified as UGZ in either GCVS or Downes’ catalog, HX Peg exhibits obvious standstills as well as the outbursts and quiescent periods that generally describe UGZ-like behavior. It is variously described as UGZ in other sources (Honeycutt et al. 1998 and the aforementioned 2005PASP..117..931S - Publ. Astron. Soc. Pac., 117, 931-937 (2005), but not the two catalogs we have examined.

Another puzzling fact seems to contradict the accepted normal behavior of Z Cam variables. It is generally accepted that standstills are triggered by outbursts, and that standstills always end with a fade to quiescence. This is stated plainly in Cataclysmic Variable Stars, How and Why They Vary by Coel Hellier (pp. 73-74). However, both AT Cnc and HX Peg have been known to go into outburst from a standstill. Does this fact make them non-UGZ, does it make them a special sub-group of UGZ or does this throw a wrench into the current models of enhanced mass-transfer sustained by enhanced irradiation? How do these systems ramp back up to outburst levels after entering a standstill? I don’t know, but clearly the observational evidence does not always agree with the theories.

So, where does this leave the visual observer interested in Z Cam type variables? I’d say it puts you in great demand. Obviously, there is much to learn about these CVs and there are plenty of examples bright enough to follow and active enough to keep your interest for years to come. Unfortunately, there is no one definitive list of UGZ type CVs from which to pick your targets. Don’t waste your precious observing time on EV Aqr and V1285 Cyg if you want to observe CVs. Do keep an open mind and don’t accept everything you read as gospel.

Your observations may help shine a light on a previously suspected UGZ, or help to classify a new type of variable, or just add to the important data and general knowledge of CVs. It is your data that researchers use to create models of CVs and try to untangle the mysteries of their behaviors. And it is the unpredictable nature of CVs that keeps observers observing them night after night, year after year. Who knows, maybe tonight AT Cnc will go into outburst from a standstill. We’ll never know if you don’t get out there and make the observations.

Good luck and clear skies to you all.

Small Scope Sampler

posted Mar 27, 2009, 9:37 AM by Mike Simonsen   [ updated Mar 2, 2011, 11:24 AM ]

by Mike Simonsen, C. E. Scovil Observatory, Imlay City, MI, USA
and Rod Stubbings, Tetoora Observatory, Tetoora Rd, Vic, Australia

While it is true that most cataclysmic variables are quite faint in quiescence, there are some that can be observed throughout their cycle, and a great number of them that are bright enough in outburst to detect visually with a 6-8" telescope.

In hopes of encouraging new observers, and those who thought these stars were beyond the reach of their aperture, we have put together a list of CVs suitable for small telescopes. They are all brighter than 13th magnitude in outburst and are active enough that you won't have to wait long to make your first outburst detection.

Name RA(2000) Dec Type Range

WW Cet 00:11:24.77 -11:28:42.7 UGZ: 9.3 V - 15.7 V

RX And 01:04:35.55 +41:17:58.0 UGZ 10.9 v - 14.6 v

TY Psc 01:25:39.35 +32:23:09.7 UGSU 11.7 V - 16.3 V

KT Per 01:37:08.72 +50:57:20.0 UGZ 10.6 V - 16.1 V

AR And 01:45:03.27 +37:56:33.3 UG 11.0 V - 17.6 V

WX Hyi 02:09:50.65 -63:18:39.9 UGSU 11.4 V - 14.8 V

TZ Per 02:13:50.99 +58:22:52.7 UGZ 12.3 v - 15.6 v

FO Per 04:08:35.03 +51:14:48.8 UG 11.8 v - 16.2 p

VW Hyi 04:09:11.34 -71:17:41.1 UGSU 8.5 V - 13.8 V

V1159 Ori 05:28:59.52 -03:33:52.8 UGSU 11.2 V - 15.1 V

CN Ori 05:52:07.77 -05:25:00.7 UGZ 11.9 v - 16.3 v

SS Aur 06:13:22.44 +47:44:25.7 UG 10.5 v - 14.5 v

CZ Ori 06:16:43.23 +15:24:11.7 UG 11.2 V - 17.0 V

HL CMa 06:45:17.22 -16:51:34.5 UG/UGZ: 11.7 V - 14.5 V

IR Gem 06:47:34.58 +28:06:22.7 UGSU 11.2 V - 17.0 V

U Gem 07:55:05.29 +22:00:05.7 UG 9.1 V - 15.2 V

YZ Cnc 08:10:56.62 +28:08:33.6 UGSU 10.5 V - 15.5 V

SU UMa 08:12:28.20 +62:36:22.6 UGSU 11.2 V - 15.0 V

Z Cam 08:25:13.20 +73:06:39.4 UGZ 10.5 v - 14.8 v

AT Cnc 08:28:36.92 +25:20:02.6 UGZ 12.7 B - 16.2 B

VZ Pyx 08:59:19.89 -24:28:55.1 UGSU 11 V - 17 V

SY Cnc 09:01:03.35 +17:53:56.1 UGZ 11.1 V - 14.5 V

X Leo 09:51:01.51 +11:52:31.1 UG 12.4 V - 16.5 V

V383 Vel 10:21:41.71 -49:49:24.3 UGSS 12.5 p - 17. p

V442 Cen 11:24:51.92 -35:54:37.7 UGSS 11.9 V - <16.5 v

TW Vir 11:45:21.13 -04:26:05.9 UG 12.1 v - 16.3 v

MU Cen 12:12:53.86 -44:28:16.3 UGSS 11.8 V - 14.9 V

V803 Cen 13:23:44.51 -41:44:30.4 IBWD 13.2 V - 16.8 V

TT Boo 14:57:44.74 +40:43:42.2 UGSU 12.7 v - 19.2 V

T CrB 15:59:30.19 +25:55:12.1 NR 2.0 p - 11.3 p

HP Nor 16:20:49.59 -54:53:22.0 UGZ 12.8 v - 16.4 V

AH Her 16:44:09.99 +25:15:02.1 UGZ 11.3 v - 14.7 v

AT Ara 17:30:33.77 -46:05:58.5 ugSS 11.5 v - 14.9 v

RS Oph 17:50:13.12 -06:42:28.2 NR 4.3 v - 12.5 v

V426 Oph 18:07:51.71 +05:51:48.5 UGZ/DQ: 11.5 V - 19.4 V

UZ Ser 18:11:24.90 -14:55:33.9 UGSS 11.9 v - 16.0 v

AM Her 18:16:13.33 +49:52:04.2 AM 12.0 V - 15.5 V

AY Lyr 18:44:26.73 +37:59:51.8 UGSU 12.3 V - 18.0 V

AB Dra 19:49:06.50 +77:44:23.5 UGZ 12.3 V - 15.8 V

UU Aql 19:57:18.68 -09:19:20.8 UG 11.0 V - 17.0 V

RZ Sge 20:03:18.49 +17:02:52.6 UGSU 12.2 B - 17.4 B

SS Cyg 21:42:42.66 +43:35:09.5 UGSS 8.2 v - 12.1 v

RU Peg 22:14:02.58 +12:42:11.4 UGSS 9.0 V - 13.1 V

IP Peg 23:23:08.60 +18:24:59.4 UGSS+E 10.5 V - 17.8 V

References:

Downes, R., et al, The Catalog and Atlas of Cataclysmic Variables, (2001, PASP 113, 764)

Recurrent Novae

posted Mar 27, 2009, 8:15 AM by Mike Simonsen

By Mike Simonsen


These cataclysmic variables grab our attention
and spark our imaginations because of the incredible amplitude of their
outbursts, typically 8-12 magnitudes, and the rarity of these spectacular
events. Many of these outbursts are once-in-a-lifetime events. Like an
apparition of Halley's comet, witnessing an outburst of T CrB twice in a
lifetime would be a matter of uncommon luck, longevity or both.

In the General Catalog of Variable Stars (GCVS) recurrent novae are included
in the same category as novae, with the main distinction being the features
of their light curves.

"According to the features of their light variations, novae are subdivided
into fast (NA), slow (NB), very slow (NC), and recurrent (NR) categories.

NR Recurrent novae, which differ from typical novae by the fact that two or
more outbursts (instead of a single one) separated by 10-80 years have been
observed (T CrB)."

This implies that the outburst mechanism, orbital periods, spectra and the
nature of the components of these close binaries are the same or very
similar. To understand recurrent novae we need to understand novae first,
and then make distinctions.

Novae are close binary systems with orbital periods from 0.05 to 230 days.
The primary of the system is a hot white dwarf star while the cooler
secondary components may be giants, subgiants, or dwarfs of K-M type.

Although few novae have been caught in the very act of rising to eruption,
it is generally accepted that the time it takes to go from restless
quiescence to full max is 1- 3 days. The same is probably true for recurrent
novae.

The cause of a nova eruption is a thermonuclear reaction on the surface of
the white dwarf. After years of mass exchange between the binary pair,
temperature and pressure at the surface of the white dwarf build
sufficiently to cause the layer of accreted material to explode like a
hydrogen bomb. This bomb, however, can have the mass of 30 Earths! Once the
temperature becomes high enough, this layer begins to expand. Minutes into
the process the shell can be radiating at 100,000 solar luminosities and
expanding outwards at 3000 km/s. Eventually the shell envelopes the entire
binary and the orbital motion of the pair acts like a propeller to whip
things up. After 1000 days or so the envelope expands to the point it can be
seen as nebulosity surrounding the pair. Over hundreds of years the shell
dissipates into the interstellar medium.

Most novae probably erupt more than once in their lifetime, with the mass of
the white dwarf determining the amount of accreted material that needs to
accumulate before triggering on outburst. Systems with a white dwarf of 0.6
solar masses might take as long as 5 million years between eruptions. A
system with a 1.3 solar mass white dwarf might only take 30,000 years
between eruptions.

So are recurrent novae simply the same type systems with even more massive
white dwarfs? The accretion rate of a system with a 1.4 solar mass white
dwarf could have a recurrence time of less than 100 years. T Pyx may be one
such system, but it is unclear at present if the outburst mechanism for all
recurrent novae is the same as novae, or if some are the result of accretion
by Roche-lobe overflow or stellar winds, or a result of disc instabilities.

Even more interesting is the possibility that recurrent novae may actually
be progenitors of Type Ia supernovae. Observations of novae eruptions and
the resulting nebulae indicate the mixing of the accreted layer with the
outer layers of the white dwarf may cause the white dwarfs to lose mass over
time and repeated eruptions. The heaviest white dwarfs, with their higher
accretion rates, may actually gain mass over time! Although a large part of
the envelope mass is blown away in the wind, these primaries may retain a
substantial part of the envelope mass after hydrogen burning ends. The white
dwarfs in some recurrent novae have now grown up to near the Chandrasekhar
mass limit and might soon explode as a Type Ia supernova.

With so few known examples and the rarity of these events it is no wonder
that recurrent novae eruptions are extremely interesting to astronomers.
Monitoring these stars for outbursts over decades of relative inactivity is
still one of the extremely valuable contributions visual observers can
provide to science.

Finding the stamina and determination to follow such stars is no small task.
Even Leslie Peltier, one of the greatest AAVSO observers of all time, had an
"unhappy affair" with T CrB that can serve as a lesson to us all. In
'Starlight Nights' he writes:

"From 1920 on I watched it closely at every opportunity. For more than
twenty-five years I looked in on it from night to night as it tossed and
turned in fitful slumber. Then one night in February 1946 it stirred, slowly
opened its eyes, then quickly threw aside the draperies of its couch and
rose!
Full eighty years had passed since the star had shattered the symmetry of
the Northern Crown. And where was I, its self-appointed guardian on that
once-in-a-lifetime night when it awoke? I was asleep!"

Peltier had set the alarm for 2:30 AM to observe morning variables. When he
got up the sky was clear and the stars were shining, but feeling he might
have a cold coming on he decided to go back to bed. He goes on to describe
his personal relationship with the star, one that many of us feel for our
favorite variables, and how it changed after that.

"I alone am to blame for being remiss in my duties, nevertheless, I still
have the feeling that T could have shown me more consideration. We had been
friends for many years; on thousands of nights I had watched over it as it
slept and then, it arose in my hour of weakness as I nodded at my post. I
still am watching it, but now it is with wary eye. There is no warmth
between us any more."

In more recent times, CI Aql had been suspected of being a recurrent nova
even though only one recorded outburst had occurred in 1917. As such it was
included in the BAAVSS Recurrent Objects Programme for many years. For
reasons he still will not discuss with even the best of friends, Gary
Poyner, coordinator of the program, decided to drop CI Aql from the list in
2000; literally weeks before it erupted again for the first time in over 80
years! Sorry Gary, but its just too good a story not to recount.

Below is a table of known recurrent novae.
Try not to sleep through the next eruption of any of these unpredictable
stars.

Name
RA/Dec (2000)
Magnitude range
Years of known outbursts
Chart availability

T Pyx
09 04 41.53 ?32 22 47.2
6.5 v - 15.3 v
Outbursts in 1890, 1902, 1920, 1944 and 1966
AAVSO charts

IM Nor
15 39 26.47 ?52 19 18.0
7.8 V - 22.0 j
Outbursts in 1920 and 2002
AAVSO charts

T CrB
15 59 30.19 +25 55 12.1
2.0 p - 11.3 p
Outbursts in 1866 and 1946
AAVSO charts

U Sco
16 22 30.80 ?17 52 44.0
8.8 V - 19.5 V
Outbursts in 1863 and 1999
AAVSO charts

RS Oph
17 50 13.12 ?06 42 28.2
4.3 v - 12.5 v
Outbursts in 1898, 1933, 1958, 1967, 1985, 2006
AAVSO charts

V745 Sco
17 55 22.27 ?33 14 58.5
11.2 p - 21 j
Outbursts in 1937 and 1989
No AAVSO charts

V394 CrA
18 00 25.97 ?39 00 35.1
7.2 V - 18.8 V
Outbursts in 1949 and 1987
No AAVSO charts

V3890 Sgr
18 30 43.32 ?24 01 08.6
8.4 p - 17.2 p
Outbursts in 1962 and 1990
Very poor AAVSO chart; lettered sequence only

CI Aql
18 52 03.57 ?01 28 39.4
8.8 V - 15.6 p
Outbursts in 1917 and 2000
AAVSO charts

1-7 of 7