Light curves of variable stars : a pictorial atlas

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Lipunova, G. Medvedeva, E. Pastukhova, N. Perova, G.

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Ponomareva, Yu. Pskovsky, A. Rastorguev, S. Shugarov, T. Detre,t B. Szeidl, L. Szabados, K. Olah of the Information Bulletin on Variable Stars deceased Contents List of contributors Preface Acknowledgements Journal abbreviations Acronyms and abbreviations page xi xiii xvi xvii xix 1 Introduction 1 1.

Duerbeck Astronomisches Institut, Westf. And since the TYCHO experiment yields only a scanty number of scattered measurements of each star during the life time of the satellite, it is immediately evident that one will encounter the problem of recognising the type or class of variability to which the variable star belongs. Such classification is - even with abundant data - not a trivial task, since many variable stars have light curves which, at first sight, look very similar.

In addition, proper classification needs much more than a good-looking light curve, since luminosity and effective-temperature photometric indices also play a role, as well as miscellaneous data obtained with apparatus that are complementary to photometric instruments. We thought to get some help by looking for standard light curves of typical variable stars that would be used as a template during the process of classification. We discovered then, with some surprise, that a compilation of typical photoelectric light curves of variable stars has never been published, nor does there exist a concise compendium of photometric properties of groups and classes of variables.

What can be found, instead, is a large number of detailed morphological descriptions and numerous photometrically-incompatible photographic and visual light curves, scattered over many books and journals. So, we decided to fill this gap and we started the compilation of typical light curves in a format that enables quick recognition of the pattern of variability. And we looked for a concise description explaining the physical processes that cause the observed variability, or an indication of which phenomena are not well understood.

It was immediately obvious to us that no single person would be able to complete such a job, so the help of half a dozen experts was sought, and this book is the result of our joint efforts. We first intended to present all data light and colour curves in one single Xlll xiv Preface photometric system, and to represent the data with a single graphical software package. However, as the light curve data were coming in, we realised that it was just impossible to combine all photometric data into one single homogeneous data set. Moreover, we learned with dismay that for many light curves published less than a decade ago, the only remnant data are the graphs themselves, the original data having been lost forever - indeed a vivid demonstration that archiving of astronomical data is still in its infancy.

We have thus chosen the most representative photoelectric light and colour curves that could be found, and we have reproduced them, together with photographic light curves, where necessary. For some stars with large-amplitude light variations on very long time scales, we have illustrated the photoelectric light curve with a visual light curve, based mostly on data that were kindly supplied by the AAVSO. All these visual-magnitude graphs are displayed on approximately the same magnitude scale along the Y -axis. The book is organised into seven chapters. Chapter 1 provides a general introduction.

Chapters 2 to 7 deal with the subclasses and light curve properties of each of the six main groups of variables as defined in the Fourth edition of the General Catalogue of Variable Stars GCVS. Each GCVS group is subdivided in several subclasses, and these subclasses are the topic of a separate Section. Each Section consists of a short description of the type of variability that is discussed, a representation of the historical background, and finally of a number of light and colour curves for several notorious stars member of the class.

We are convinced that this book will not only be useful during the analysis of TYCHO data, but also that other projects will profit from it. In particular, the growing number of Automatic Photometric Telescopes APTs being commissioned worldwide, the increasing number of serendipitous discoveries of variable stars in all kinds of CCD imaging projects, and the searches in data banks and archives continuously yield light curves to non-specialists. In addition to being a guide on how to read light curves, this book will also be a useful tool for the astronomy student and researcher, a textbook for astronomy 1.

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The search for specific variable stars, types and classes of variables, and astrophysical keywords, is made very easy through a most detailed name and subject index. We also believe that this book comes at the right time. Variable-star research has been done so far using 'small' telescopes - that is, telescopes with apertures from about cm down to 50 cm, and even below. Such research, for reasons of the photon budget, was mostly confined to our own galaxy.

For some years, galactic work, in general, has become less popular and extra-galactic research is becoming the fashion. Variable-star work, in particular, is looked at with some disdain, and the 'small-telescope' tool is mentally associated with small science.

Today it is mostly forgotten that the persistent study of variable stars has led us to answers on many questions on stellar structure and on stellar-atmosphere physics. A very important task, carried out almost invisibly through nearly half a century, has been the bookkeeping and the classification of newly-discovered variables, along with the dissemination of information, both in catalogue form GCVS and in information bulletins IBVS. Compilations that have taken man-centuries of labour in very difficult circumstances by dozens of colleagues, almost exclusively at the Sternberg Astronomical Institute in Moscow and at the Konkoly Observatory in Budapest, for no other return than the satisfaction in serving the whole astronomical community.

It is for this reason that we dedicate this work to all those who have contributed and are still contributing to this task. Sterken C. Jaschek Acknowledgements We wish to thank a number of persons without whose help this book could not have been completed. We are indebted to the Staff of Sonneberg Observatory for many years of collaboration on the matter of bibliography of variable stars.

Special thanks go to Dr J. We thank Dr M. We emphasize the support by Messrs C. Schohn and J. Marcout, who made the photographic reproductions of a number of light curves, and also the help by Mr Jan Van Mieghem of the University of Brussels, who edited and upgraded some graphs and figures. We are grateful to Mrs M. Wagner and the Strasbourg Librarian Mrs M. Hamm who helped on the bibliography. Bibliography not available at Strasbourg was kindly provided by Mrs J. Vin, Librarian of the Haute Provence Observatory. We very much appreciate the helpful support by Mrs M.

Tokyo Obs. AAS Bull. CDS Cape Ann. GCVS Fund.

Cosmic Phys. Harvard Bull. Hvar Obs. IAU Circ. Bull Var. Notes Phys. Lick Obs. Bull Mem. Messenger Mitt. Zvezdy Proc. Arts Sci. Japan Publ Astron. SAAO Circ. Sky Telesc. Space Sci. IAU Zeitschr. Jasniewicz All stars display variations of brightness and colour in the course of their passage through subsequent stages of stellar evolution.

As a rule, however, a star is called variable when its brightness or colour variations are detectible on time scales of the order of the mean life time of man. The variations may be periodic, semi-periodic or irregular, with time scales ranging from a couple of minutes to over a century. It is this kind of variable star which is the topic of this book. The typical time scale, the amplitude of the brightness variations, and the shape of the light curve can be deduced from photometric observation, and those quantities place the star in the appropriate class.

For example, a star of the UV Ceti type typically has brightness variations the so-called flares of several magnitudes in an interval of time as short as a few minutes, whereas a Cepheid shows periodic variations of about one magnitude in a time span of several days.

However, spectral type, luminosity class and chemical composition are complementary important spectroscopic parameters that are needed for classifying variable stars according to the origin of their variations. Due to stellar evolution, a variable star discloses long time-base variations of its amplitude and of its typical duration of the cycle of variability: for a short-period binary star, variations of the period and of the times of minimum or maximum of the light curve can be explained by modifications in the mass transfer between both components; for a 8 Scuti type star, a change of the period can be due to a change of the radius by the evolution of the star from the dwarf stage to the giant stage.

On the other hand, for the RV Tauri type stars, the origin of the observed abrupt changes of period is still not understood. Therefore it is of crucial importance that astronomers undertake long-term observing programs of variable stars and also archive all photometric measurements done in the past and today.

Archiving of variable-star photometric data Proper archiving is crucial for a complete understanding of the mechanisms of variability. Archives can also help to solve ambiguities: it can happen that period analysis on a set of data yields a period different than a previously published one, simply because the methods and the understanding of statistics of period analysis have evolved. Unfortunately, access to original data today is not a simple issue in our international astronomical community.

Indeed, if a list of data is very extensive, the majority of the astronomical journals will not accept it for publication, not even in their Supplement Series. Therefore, observers themselves do archive their own data and, occasionally, make them available for public use. Thus, in order to prepare the light curves for this book, we have asked authors for their unpublished data. Not entirely to our surprise, no more than one out of every five responded positively. We emphasize here that, in order to avoid the definitive loss of unpublished observations of variable stars, the International Astronomical Union IAU has asked its Commissions 27 Variable Stars and 42 Close Binary Systems to archive the photometric observations of variable stars.

These Archives replace lengthy tables in scientific publications by a single reference to the archival file number, and they also contain many valuable observations which have never been used for scientific publication. Schmidt of the University of Nebraska at Lincoln.

Electronic storage and retrieval of photometric data from the Archives can be done by communicating with the Centre de Donnees de Stellaires CDS ; somefilescan be electronically obtained free of charge from CDS. Copies of all existingfilescan be obtained in paper form from three archives, viz. Romanov Odessa Astronomical Observatory. Detailed reports on the file contents are published by Breger Variable-star monitoring programs A large variety of observing programs of variable stars is presently in progress all over the world.

An important part of all major astronomical journals is devoted to individual stars which display regular or erratic variations of brightness. Announcements of newly discovered novae and supernovae or of drastic changes in cataclysmic stars are published by the Central Bureau for Astronomical Telegrams Brian G. Marsden that depends on IAU Commission 6. The importance of studies of stellar variability on long time scales has been emphasized above. Long-term observing programs of variable stars are in progress in major observatories. More than a few advantages and problems of coordinated campaigns and of long-term monitoring have been elucidated by Sterken , The benefits of ground-based support of space observations of variable stars are obvious: in the case of very hot stars white dwarfs, central stars of planetary nebulae, etc.

For example, simultaneous IUE and groundbased photometric observations of the binary central star of the planetary nebula Abell 35 have strongly contributed to clarify the nature of the socalled 'Abell 35'-type objects: such binaries experience chromospheric activity as RS CVn-like binaries Jasniewicz et al b ; besides, the late-type giant stars in the nuclei display characteristics common to the FK Comae stars Jasniewicz et al , a.

The advantages of observations collected at discrete geographical longitudes are also clear because quasi-contiguous sequences of measurements can in 4 Introduction principle be collected, and then the amplitudes of the alias periods resulting from the period searches can be significantly reduced.

The contributions by amateur variable-star observers also play an important role in the continous monitoring of variable stars; that is especially the case for cataclysmic stars and long-period variables such as the Mira Ceti type stars. In this book the behaviour of variable stars on very-long time scales is illustrated by means of light curves kindly provided by the AAVSO and by individual amateurs. Searching for new variable stars Several ground-based and space projects will, in the coming years, considerably increase the number of known variables.

We focus on two examples, viz. The TYCHO program involves photometric observation in two colours of about stars across the sky. Photometry is based on the star-mapper signal and two detectors at effective wavelengths and nm. Several thousand confirmed standard stars were used for calibrating the photometric system. Every program star was observed about times during the satellite's life time January to March ; thus the TYCHO project will yield a substantial number of new variable stars.

A methodological strategy for analyzing the photometric data of the TYCHO space experiment has been announced by Heck et al In order to identify new variable stars, various statistical algorithms will have to be applied, especially algorithms 1. The next step will be to assess the type of variability of each newly-discovered variable star. The present book can be a helpful tool for this task because it brings an exhaustive and up-to-date compilation of light curves of all known types of variable stars. A second key project is the ground-based microlensing experiments dedicated to the search for massive compact objects black holes, brown dwarfs, etc.

According to Paczynski massive dark bodies might act as gravitational microlenses, temporarily amplifying the apparent brightness of background stars in nearby galaxies. An EROS catalogue of new eclipsing binary stars in the bar of the Large Magellanic Cloud has already been published Grison et al, and many other papers are in preparation. A lot of insights into stellar evolution should follow from these newly-discovered variables, and it will be a very rewarding enterprise to store the entire resulting data set in various locations for archival purposes.

Introduction Table 1. Sterken For a working definition of the concept of variable star, we refer to Section 1. If one of the brighter objects of a constellation turned out to be variable, its Bayer notation Greek or Roman letter followed by the genitive of the Latin constellation name was employed as the name of the variable. The visual observations leading to the compilation of the Bonner Durchmusterung BD by Argelander in the middle of the 19th century led to the discovery of a substantial number of new variable stars.

Thus, Argelander introduced a specific nomenclature for those variables that had no Bayer number in a constellation by designating these stars by the Roman capital letters R, S, T, U, V, W, X, Y and Z attached to the Latin name of the constellation. With the introduction of the photographic plate, the number of variable stars in each constellation quickly exceeded the capacities of Argelander's scheme, and the Astronomische Gesellschaft AG extended the scheme by adopting double 1.

That extension brought the number of possible variable star names per constellation from 9 to That second extension brought the number of stars to be assigned a name to per constellation. According to Hoffmeister , the Dutch astronomer Nijland proposed that a new and uniform system of nomenclature be introduced, and suggested that R, S, QZ be replaced by V1, V 2, V with subsequently discovered variables being labeled V, V, etc.

In principle, this system can be used for any possible number of variables that are discovered in a single constellation. However, with the advent of the discovery of larger and larger numbers of variables by space- and ground-based surveys see Section 1. For novae a special nomenclature was used for a long time: older novae are designated by the label Nova followed by the constellation and the year in which they appeared to the observer , for example Nova Cygni When necessary, the running number of the nova that erupted in this constellation that year is added.

This system of labeling is still used as a provisional designation until the nova has been assigned a definite variable-star designation thus, for example, Nova Vul , number 2 has become LU Vul. A similar nomenclature is used for supernovae, where the year of appearance and a sequel character is used, for example, Supernova a. Alternative designations for specific stars used in this book are, besides the 1. See also Andre As in the case of supernovae, the main reason for such alternative nomenclature is that these stars are extragalactic variables, to which no variable star designation is assigned.

The specific problem of assigning a unique name to a variable star is, of course, only part of the more universal problem of labeling celestial objects in a clear and internally consistent system. Variable-star observers using twodimensional detectors quite often discover numerous variable stars to which neither a variable-star name, nor another designation had been given previously.

This, especially, is the case when observing open clusters using such detectors with high quantum efficiency. In addition, such investigations necessitate the earmarking of quite a number of non-variable comparison and standard stars. Instead of extending existing schemes, some authors, unfortunately, find it necessary to replace previously assigned designations even HD and HDE numbers allocated more than half a century ago by private numbering schemes and acronyms that involve the initials of their own names.

In order to avoid the practice of creating new appellations out of thin air spreading like wildfire, the IAU has issued a public notice 'Specifications concerning designations for astronomical radiation sources outside the solar system'1 that shows how to refer to a source or how to designate a new one. For general information, in particular about existing designations, one should consult Fernandez et al. Sterken Variable-star specialists know that it is a very difficult enterprise to construct a physically sound and consistent taxonomy of classes and types of variable stars.

One of the very first attempts at classification of variable stars was the scheme of Pickering , who recognised five classes, viz. Classification schemes based on a physical mechanism e. Accessible on ftp anonymous at node cdsarc. From then on, three phyla were always recognised: irregular variables, pulsating variables, and binary systems.

Meanwhile, the classification of variable stars has undergone a change that, as Cecilia Payne-Gaposchkin puts it 'recalls the suppression of the Linnaean system by the modern system of botanical classification' Payne-Gaposchkin A first classification outline was published in Trans. Since then, this classification has frequently been reworked and extended.

Not surprisingly, GCVS is somehow considered as the bible of systematic placement in variable star knowledge. Kholopov et al. Compilation and publication of GCVS started in under the auspices of IAU, with an intended frequency of publication of a new edition every 10 years. Figure 1. Users of GCVS and NSV should know that these works are not just a mere compilation, but are based on a critical evaluation of all underlying data, as expressed by Samus' in the case of an announced nova V Aql 'Nova' Aql , which was proven to be an asteroid during the preparation of GCVS.

The 5th volume of GCVS is the first systematic catalogue of extragalactic variables, listing data on variables in nearby galaxies, viz. The majority of extragalactic variables belong to Cepheid types - Fig. Astronomers who are familiar with the above-mentioned works must notice that every newly-published catalogue or review paper introduces new classes or subclasses of variable stars, and that the classification in itself, at the same time, not only refines, but also becomes, occasionally, less and less consistent, including even the classification of the same object in different classes,1 or the transfer of an object from one class to another and back as time passes.

Such changes, of course, reflect the progress and understanding in the field - a progress that is not only due to more and more sophisticated detection techniques, but also to the increasing time baseline along which data are being collected. An illustrative example of the latter point is the case of the 'ex-constant star' - that is, a star that turns out to be a variable only after many years of measurements become available for analysis.

Another aspect of published classification schemes is the pronounced vagueness with which some classes are being defined; the GCVS, for example, lists several subclasses where members are characterised by the label 'poorly studied', a fact that eventually will lead to more refinement and to the definition of new classes and subclasses. Looking over the past decade, we have seen new classes like the rapidly-oscillating Ap stars and the slowly-pulsating B stars, to name only two.

The actual classes do not have clear-cut borders, since these borders directly depend on the parameter that is being used for the classification see also M. Feast's remarks on the classification of Cepheids in Section 3. If that parameter is directly related to the wavelength range in which the data are 1. Not by error, but by the fact that some variable stars are polymorphic - see, for example the Be stars Section 3. Variable stars are, traditionally, classified in two main families: intrinsic variable stars, and extrinsic variables, respectively for stars that vary due to physical processes in the the star itself, and for stars that vary due to processes external to the star such as rotation combined with a physical process in the star that, for example, produces spots and orbital motion - the latter case would not lead to observed variability if the observer were not at a vantage point that allows, for example, eclipses of a companion to be seen.

Intrinsic variables are the pulsating variables Chapter 3 , and the eruptive 12 Introduction and explosive variables Chapters 2 and 5. Among the intrinsic variables, the most difficult to classify are the explosive and symbiotic variables studies over limited wavelength regions have often resulted in symbiotic stars being misclassified as something else, most often as peculiar planetary nebulae, see P.

Whitelock in Section 5. Classification is often based on the most characteristic features of the light and colour curves, and this inevitably leads to a non-homogeneous result, sometimes with very limited value for a real physical classification see, for example, J. Krautter's remarks in the introduction to Section 2. Smith , on the other hand, appeals to the photometric community to avoid using physical mechanisms in defining variable-star classes, and to fall back on a prototypical star's name in constructing schemes of classification.

Not only physical mechanisms, but also the position of a variable star in the H-R diagram is used as an index for classification, especially when the position of a star falls in a region that is occupied by one of the many pulsating-variable families that delineate a socalled 'instability strip'. Another element that is often used in classification is the presence of specific emission lines in the spectrum: whereas for some classes the presence of such emission lines is a necessary condition for membership WR stars, see Section 2.

It should be clear for any one attempting to label a variable that the distinction between one class and another is not always very obvious and that taxonomy of variable stars is not a trivial pursuit. Few attempts towards a revolutionary new design of variable-star classification have been made. For the apprentice taxonomist, the reading of Douglas 1. He illustrates this scheme with several examples, some of which are discussed in this book. In this work we do not make any attempt to change or to improve the existing classification, instead we follow the gross classification of variables in six families as defined in the GCVS and in its Supplements as a kind of ground plan.

We have confined ourselves to describing the types of variables along the main branches, overlooking the many twigs and sprigs - which often appear on different limbs in parallel - that account for the more than different star types to be found in the GCVS. In this Introduction wefirstgive an overview of the different types of variables which, according to GCVS, belong to each of these six families.

For each such group we start with the definition taken verbatim from GCVS, followed by an overview table that is structured in exactly the same way as the description in GCVS is conceived. In addition, each table is followed by an indication about where we do not follow the GCVS classification for example, by introducing a class of variables not described in GCVS. For more detailed information on each type of variable, we refer the reader to the corresponding sections of this book. In the subsequent chapters and sections of this book we consistently indicate a class of variable stars either by the abbreviated name or by the full name, for example, either W UMa, or W Ursae Majoris stars.

Lyr or the notation in the Argelander scheme W UMa. The remainder of this Section reproduces the classification schemes of the GCVS their six natural groups along with some concise information on how this book either follows the GCVS scheme, or deviates from it. Eruptive variables GCVS: 'We call eruptive variables the stars varying their brightness because of violent processes and flares taking place in their chromospheres and coronae. Krautter points out in Sec- 13 14 Table 1.

Hall, in Section 6. The first attachment may be misleading because the mechanism for the variability is actually rotational modulation, with the surface brightness non-uniform as a result of cool spots distributed unevenly in longitude, yet it does not appear as one of the types of rotating variable stars Table 1. The second labeling also may be misleading because more than half of the variables classified in the GCVS as RS are not eclipsing.

Before their first observed eruption, such stars are normally classified as a Cyg 1. In addition, we describe the y Cas stars as Be stars Section 3. Pulsating variables GCVS: 'One calls pulsating variables the stars showing periodic expansion and contraction of their surface layers. Pulsations may be radial or non-radial.

Feast in Section 3. In Section 3. Lipunova mentions a specific new type of variable in the 5th volume of GCVS, viz. First discovered by N. Kurochkin in , it is now known to be an anomalous Cepheid having too great a luminosity for its period or too small a period for its luminosity - even for the continuation of the Population I period-luminosity relation.

These stars are found, in particular, in dwarf spheroidal galaxies. Therefore, Section 3. A few RR Lyrae stars show variations in the shape and amplitude of their light curves from cycle to cycle due to the simultaneous excitation of two periods see Section 3.

Period Analysis of light curves of MoA database: finding periods of variable stars

Feast, in Section 3. This usage is not recommended, because of possible confusion with the RV Tau subtypes A and B that are recognised on the basis of their spectra see footnote by P. Whitelock in Section 3. For irregular variables slowly varying with no evidence of periodicity , the GCVS uses the classifications Lb and Lc for giants and supergiants, respectively.

Whitelock, in Section 3. Rotating variables GCVS: 'We call rotating variables the stars with non-uniform surface brightness or ellipsoidal shape, their variability being caused by their axial rotation with respect to an observer. The non-uniformity of surface brightness distributions may be caused by the presence of spots or by some thermal or chemical inhomogeneity of stellar atmospheres caused by magnetic field, its axis being not coincident with the star's rotation axis.

In the roAp stars Section 4. This only illustrates once more the difficulty of finding a consistent classification scheme for variables of truly polymorphic nature, even if a most objective and neutral approach is being followed. Yet the three different sets of astronomers working on these three different sets of stars have three different sets of explanations for the similar behaviour.

That bears some thought. Cataclysmic explosive and nova-like variables GCVS: We call explosive variables the stars showing outbursts caused by thermonuclear burst processes in their surface layers Novae or deep in their interiors Supernovae. Eclipsing variables GCVS: 'We adopt a triple system of classifying eclipsing binary systems: according to the shape of the combined light curve, as well as to physical and evolutionary characteristics of their components. Another type to draw attention to are the W Ser stars. The W Serpentis group was defined by Plavec , but is not part of the GCVS classification scheme, and they can be described as a group of long-period Algollike mass-transferring binaries, which are characterised by very substantial discs around the more massive components, strange and poorly-repeating light 1.

X-ray sources GCVS: 'Close binary systems, sources of strong variable X-ray radiation, which do not belong to any of the above types of variable stars. Krautter's remark in Section 4. Duerbeck The growing number of variable stars and the growing number of publications containing observations of a single variable star, or an interpretation of the observed phenomenon, or publications mentioning a variable star among others, from the very beginning led to bibliographical compilations.

Changing trends in the type of publication are obvious: while the first bibliographies appeared in printed form, the next generation was distributed as microfiche, or even on magnetic tape, while the most recent compilation of references is available by anonymous ftp from an astronomical data centre. We would like to give a 'historical' overview of the bibliographies, because the more recent ones do not repeat the information which is contained in the older printed versions.

In Table 1. As can be seen, the location of information for a given variable star is not obvious from Table 1. However, if the user has access to a computer connected to a network, the most convenient procedure to obtain bibliographic information on a variable star is to consult electronically the SIMBAD database. This database also offers a listing of the bibliography from about 1. Contents literature of stars named until the end of , R. After uncompressing, the interesting sections can be copied e. It is trivial to note that all these bibliographies have deficiencies. Sterken Throughout this book we refer, in general, to photometric measurements obtained differentially - that is, using a nearby suitable comparison star of the same colour and of similar magnitude as the variable star.

In order to facilitate literature searches, we have included when possible the name of the comparison star in the figure captions, and also in the Object Index. Instead, the reader should go back to the original sources of data. In doing this, users of data catalogues should carefully read the introductory texts to catalogues, and not only take the listed data at their face value. We refer to Figs. The study of the photometric variability of stars is intimately related to the precision and accuracy of photometric magnitude and colour measurements. Variable-star observers, through history, contributed a great deal towards the improvement of photometric techniques and to the establishment of adequate photometric standards.

The literature often gives impressively small formal errors on photometric measurements, but it is rather difficult to assess the real uncertainty of the results, and to assign a reliable error bar to absolute 1. The shaded areas indicate the wings cut off by the slots of the spectrometer. Many users of a photometer are not aware of the existence of systematic errors that are introduced during the photometric reduction process.

Even when obtaining a fairly good internal precision, one often has no idea what the real accuracy of a measurement is. There are many reasons for this: the most important ones are the problems of atmospheric extinction correction, and the complications associated with the transformation of the data from a local instrumental system to a universal standard system, and these steps depend on 24 Introduction Geneva photometry passbands 4S00 Wavelength [A] Figure 1.

Problems with standardisation are as old as the craft of photometry. But for variable star observing, the existing problems are much aggravated by the ever present restriction of phase coverage, leading too often to observing at excessively high air masses. Moreover, for short-period variables, there is the need for observation of many contiguous cycles of variation, in turn leading to instrumental incompatibilities in multi-site observing campaigns.

On top of all this, variable stars of most exotic spectral types are being observed, and the most elementary conditions for transformability of these measurements are constantly violated for example, in the case of objects with strongly-variable emission lines, like LBV and WR stars, but also symbiotic stars, see P. Whitelock's warning in Section 5.

In the past the wavelength ranges of photometric systems have been dictated by the means available rather than by the astrophysical problem to be studied. But today, with most photometric systems being established, the application of a specific photometric system to a particular type of variable star is, again, mainly dictated by the availibility of the photometric system at the observing facility where the data are collected. Sometimes, a specific photometric system is available on a given site, but the photometer is mounted on a telescope too small to yield a favourable photon budget, forcing the observer to work with a less-optimal instrumental configuration.

Open circles indicate the position of the central wavelength of the indicated systems in nm, increasing along the vertical direction. Sterken The four main branches visual, photographic, photomultiplier tube and CCD are detector branches, from which the individual systems have grown.

Note that this representation is an idealised situation in the sense that it depicts the view as seen by the designer of each system. For an illustration of the transmission differences between such 'clone' systems, we refer to Fig. The former arise from the fact that the photometric systems used have mutually different pass bands, and that there is no way to evaluate the corrections needed to properly transform data from one such system to another.

The latter are of a purely methodological nature and are due to the reduction procedure. As said already, conformity errors are often unavoidable, since prescriptions of a purely practical origin such as the availability of a given photometric system at one telescope may force the investigators to rely on data coming from different systems. Reduction errors can arise between batches of data reduced separately, as is the case in long-term and network projects.

Some of the parameters in the reduction schemes have larger errors than others, and these uncertainties on the colour coefficients translate into errors in the colour indices themselves; the resulting errors are appreciably 27 2. For the sake of clarity, consecutive points for SNa have been connected by a line. The diagram is based on uvby data taken from Manfroid et al. Such effects are random shifts that affect all measurements of a given star by the same amount during a specific observing run. For a more detailed account, see Sterken HD is a a highly-reddened LBV located in the direction of the galactic 1.

Introduction 28 Note that, because of the large changes in apparent magnitude, two different sets of comparison stars have been used with each a different position in the given diagrams. The situation for novae is similar: the colour indices of novae are largest near maximum light and become bluer when the photosphere contracts and heats up, see H. Duerbeck's remark in Section 5. When data out are, RSI can make. The spectrum will provide Started to Green time text. It may takes up to Informatics before you was it. The publishing will adjust obtained to your Kindle IL.

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Light Curves of Variable Stars: A Pictorial Atlas

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It presents a wealth of typical light-and color-curves to allow identification, together with a detailed and up-to-date description of each subclass. The editors, together with seven other world experts, have created a unique pictorial atlas of variable stars. In the first chapter they give a clear introduction to the nomenclature and classification of the light curves of variable stars, and to photometric systems. In the remaining chapters They provide a detailed account of each subclass: from eruptive, pulsating, rotating and cataclysmic variables, eclipsing-binary systems through to and X-ray binaries.

Specific variable stars, types and classes of variables, together with key astrophysical terms can be quickly and easily located in the book by means of detailed object-name and subject indexes. This comprehensive and up-to-date volume provides essential reference for all of those interested in variable stars - from researchers and graduate students through to dedicated amateurs. To extremely useful reference guide to both graduate students and the well informed amateur.