Ontology of Astronomical Object Types
The Ontology of Astronomical Object Types is an ontology of classes of astronomical objects. For general discussion, see the IVOA Note at http://www.ivoa.net/Documents/Notes/AstrObjectOntology/ and the collection of use-cases at http://www.ivoa.net/Documents/Notes/AstrObjectOntologyUseCases/
HTML
RDF
accretion disk (SES) 1. Observational, signified via at least one of the following: (a) broad Halpha emission, which at appropriate resolution and S/N exhibits kinematic signatures of accretion (inverse P-Cygni); (b) ultraviolet excess emission (above photospheric levels), (c) gas in Keplerian rotation around the star; (d) 5 large amplitude and irregular photometric variability. Description: the excess UV is thought to arise from hot spots from accreting matter. It is often combined with emission lines (e.g. CO, [NeII], HCN, ...) that exhibit the characteristic double-horned profiles indicative of gas that could supply the accretion. 2. Physical, a disk – isolated or fed by material infalling from a protostellar envelope – which is transporting material inward toward the stellar surface.
Alpha2 Canum Venaticorum variables. These are main-sequence stars with spectral types B8p-A7p and displaying strong magnetic fields. Spectra show abnormally strong lines of Si, Sr, Cr, and rare earths whose intensities vary with rotation. They exhibit magnetic field and brightness changes (periods of 0.5-160 days or more). The amplitudes of the brightness changes are usually withine 0.01-0.1 mag in V. Sub-type: - Rapidly oscillating Alpha2 CVn variables. These are nonradially pulsating, rotating magnetic variables of Ap spectral type (DO Eri). Pulsation periods are in the range of 6-12 mmag (0.004-0.01 days), while amplitudes of light variation caused by the pulsation are about 0.01 mag in V. The pulsational variations are superposed on those caused by rotation.
Portion of an astronomical object which is not an astronomical object itself
BL Lacertae Object, Type1 Active Galactic Nucleus
BY Draconis-type variables, which are emission-line dwarfs of dKe-dMe spectral type showing quasiperiodic light changes with periods from a fraction of a day to 120 days and amplitudes from several hundredths to 0.5 mag in V. The light variability is caused by axial rotation of a star with a variable degree of nonuniformity of the surface brightness (spots) and chromospheric activity. Some of these stars also show flares similar to those of UV Cet stars, and in those cases they also belong to the latter type and are simultaneously considered eruptive variables.
Variables of the Beta Cephei type (Beta Cep, Beta CMa), which are pulsating O8-B6 I-V stars with periods of light and radial-velocity variations in the range of 0.1 - 0.6 days and light amplitudes from 0.01 to 0.3 mag in V. The light curves are similar in shape to average radial-velocity curves but lag in phase by a quarter of the period, so that maximum brightness corresponds to maximum contraction, i.e., to minimum stellar radius. The majority of these stars probably show radial pulsations, but some (V649 Per) display nonradial pulsations; multiperiodicity is characteristic of many of these stars. Sub-type: - A short-period group of Beta Cep variables. The spectral types are B2-B3 IV-V; periods and light amplitudes are in the ranges 0.02 - 0.04 days and 0.015 - 0.025 days, respectively, i.e., an order of magnitude smaller than the normally observed ones.
Variable stars showing outbursts caused by thermonuclear burst processes in their surface layers (novae) or deep in their interiors (supernovae). We use the term "novalike" for variables that show novalike outbursts caused by rapid energy release in the surrounding space (UG-type stars) and also for objects not displaying outbursts but resembling explosive variables at minimum light by their spectral (or other) characteristics. The majority of explosive and novalike variables are close binary systems, their components having strong mutual influence on the evolution of each star. It is often observed that the hot dwarf component of the system is surrounded by an accretion disk formed by matter lost by the other, cooler, and more extended component.
Cepheids. Radially pulsating, high luminosity (classes Ib-II) variables with periods in the range of 1-135 days and amplitudes from several hundredths to 2 mag in V (in the B band, the amplitudes are greater). Spectral type at maximum light is F; at minimum, the types are G-K. The longer the period of light variation, the later is the spectral type. The maximum of the surface-layer expansion velocity almost coinciding with maximum light. Sub-type: - Cepheids (TU Cas, V 367 Sct) displaying the presence of two or more simultaneously operating pulsation modes (usually the fundamental tone with the period P0 and the first overtone P1). The periods P0 are in the range from 2 to 7 days, with the ratio P1/P0 approx. 0.71.
Corresponds to an important chromosperic activity, usually a source of emission
circumstellar disk (LH) 1. Physical, any disk of gas and/or dust surrounding a star. 2. Physical, a disk surrounding only a single star, not a binary.
Classical T Tauri Star (CTTS) 1. (Original) Observational, a solar-type PMS star that exhibits at least one of the following: (a) broad, strong Halpha emission (Joy, 1945, ApJ 102, 168); (b) ultraviolet excess emission (above photospheric levels); (c) forbidden and in some cases permitted line emission; and (d) irregular optical variability. Description: Such stars can exhibit IR SEDs that span Class I-II, and in some cases, Transition Disk SEDs (see below). Typical examples are AA Tau; BP Tau (Class II); and HL Tau, DG Tau. 2. Physical, A solar-type PMS star surrounded by an accretion disk, and in some cases, an accretion disk and a ‘remnant’ envelope. An accretiondriven wind, manifest in emission line spectra, is also present. Description: The key distinguishing characteristics are the presence of an accretion disk and an accretion-driven wind. However, CTTS cover a ‘broad’ range of evolutionary stages.
A composite object is made of at least two distinct astronomical objects
Range allowing the description of the existence of a (noticeable) continuum for a given object
debris disk (LH) 1. Physical, circumstellar disk composed of rock, dust, and ice. 2. Physical, gas-poor circumstellar disk in which the primary physical process is the collisional grinding down of planetesimals into smaller particles, eventually dust which is known as “second generation” dust. 3. Physical, optically thin disk in which material is removed from the system via various drag or blowout scenarios on timescales much less than the age of the star. Description: Indicator of a planetary system given that large, planetary-mass bodies are required in order to induce and maintain the collisional cascade. In our Solar System, the Asteroid Belt and the Kuiper Belt are well-separated debris belts that would be considered a tenuous debris disk if viewed from afar.
These are the classical cepheids, or Delta Cep-type variables. Comparatively young objects that have left the main sequence and evolved into the instability strip of the Hertzsprung-Russell (H-R) diagram, they obey the well-known Cepheid period-luminosity relation and belong to the young disk population. Delta Cep stars are present in open clusters. They display a certain relation between the shapes of their light curves and their periods. Sub-type: - These are Delta Cep variables having light amplitudes <0.5 mag in V (<0.7 mag in B) and almost symmetrical light curves (M-m approx. 0.4 - 0.5 periods); as a rule, their periods do not exceed 7 days. They are probably first-overtone pulsators and/or are in the first transition across the instability strip after leaving the main sequence (SU Cas). Traditionally, both Delta Cep and W Vir stars are quite often called Cepheids because it is often impossible to discriminate between them on the basis of the light curves for periods in the range 3 - 10 days. However, these are distinct groups of entirely different objects in different evolutionary stages. One of the significant spectral differences between W Vir stars and Cepheids is the presence, during a certain phase interval, of hydrogen-line emission in the former and of Ca II H and K emission in the latter.
Variables of the Delta Scuti type. These are pulsating variables of spectral types A0-F5 III-V displaying light amplitudes from 0.003 to 0.9 mag in V (usually several hundredths of a magnitude) and periods from 0.01 to 0.2 days. The shapes of the light curves, periods, and amplitudes usually vary greatly. Radial as well as nonradial pulsations are observed. The variability of some members of this type appears sporadically and sometimes completely ceases, this being a consequence of strong amplitude modulation with the lower value of the amplitude not exceeding 0.001 mag in some cases. The maximum of the surface layer expansion does not lag behind the maximum light for more than 0.1 periods. Delta Sct stars are representatives of the galactic disk (flat component) and are phenomenologically close to the SX Phe variables. Sub-type: - Low amplitude group of Delta Sct variables (light amplitude <0.1 mag in V). The majority of this type's representatives are stars of luminosity class V; objects of this subtype generally are representative of the Delta Sct variables in open clusters.
Dwarf Nova (U Geminorum-type variable star)
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.
Set of ranges in the electromagnetic spectrum
Binary systems with orbital planes so close to the observer's line of sight (the inclination i of the orbital plane to the plane orthogonal to the line of sight is close to 90 deg) that the components periodically eclipse each other. Consequently, the observer finds changes of the apparent combined brightness of the system with the period coincident with that of the components' orbital motion.
Algol (Beta Persei)-type eclipsing systems. Binaries with spherical or slightly ellipsoidal components. It is possible to specify, for their light curves, the moments of the beginning and end of the eclipses. Between eclipses the light remains almost constant or varies insignificantly because of reflection effects, slight ellipsoidality of components, or physical variations. Secondary minima may be absent. An extremely wide range of periods is observed, from 0.2 to >= 10000 days. Light amplitudes are also quite different and may reach several magnitudes.
Beta Lyrae-type eclipsing systems. These are eclipsing systems having ellipsoidal components and light curves for which it is impossible to specify the exact times of onset and end of eclipses because of a continuous change of a system's apparent combined brightness between eclipses; secondary minimum is observed in all cases, its depth usually being considerably smaller than that of the primary minimum; periods are mainly longer than 1 day. The components generally belong to early spectral types (B-A). Light amplitudes are usually <2 mag in V.
W Ursae Majoris-type eclipsing variables. These are eclipsers with periods shorter than 1 days, consisting of ellipsoidal components almost in contact and having light curves for which it is impossible to specify the exact times of onset and end of eclipses. The depths of the primary and secondary minima are almost equal or differ insignificantly. Light amplitudes are usually <0.8 mag in V. The components generally belong to spectral types F-G and later.
Rotating ellipsoidal variables (b Per, Alpha Vir). These are close binary systems with ellipsoidal components, which change combined brightnesses with periods equal to those of orbital motion because of changes in emitting areas toward an observer, but showing no eclipses. Light amplitudes do not exceed 0.1 mag in V.
Eruptive variables are stars varying in brightness because of violent processes and flares occurring in their chromospheres and coronae. The light changes are usually accompanied by shell events or mass outflow in the form of stellar winds of variable intensity and/or by interaction with the surrounding interstellar medium.
Orion variables (eruptive) of the FU Orionis type. Characterized by gradual increases in brightness by about 6 mag in several months, followed by either almost complete constancy at maximum that is sustained for long periods of time or slow decline by 1-2 mag. Spectral types at maximum are in the range Ae(alpha) - Gpe(alpha). After an outburst, a gradual development of an emission spectrum is observed and the spectral type becomes later. These variables probably mark one of the evolutionary stages of T Tauri-type Orion variables, as evidenced by an outburst of one member, V1057 Cyg, but its decline (2.5 mag in 11 years) commenced immediately after maximum brightness was attained. All presently known FU Ori variables are coupled with reflecting cometary nebulae.
Fanaroff-Riley Type I, which is a Type2 Active Galactic Nucleus
Fanaroff-Riley type II, which is a Type2 Active Galactic Nucleus
Eruptive variables varying in brightness because of flares occurring in their chromospheres and coronae. Sub-types: - Orion variables of late (K-M) spectral types - Eruptive variables of the UV Ceti type, these are K Ve-M Ve stars sometimes displaying flare activity with amplitudes from several tenths of a magnitude up to 6 mag in V. The amplitude is considerably greater in the ultraviolet spectral region. Maximum light is attained in several seconds or dozens of seconds after the beginning of a flare; the star returns to its normal brightness in several minutes or dozens of minutes. - Flaring Orion variables of spectral types Ke-Me. These are phenomenologically almost identical to UV Cet variables observed in the solar neighborhood. In addition to being related to nebulae, they are normally characterized by being of earlier spectral type and greater luminosity, with slower development of flares (V389 Ori). - BY Draconis-type (rotating) variables, which are emission-line dwarfs of dKe-dMe spectral type showing quasiperiodic light changes with periods from a fraction of a day to 120 days and amplitudes from several hundredths to 0.5 mag in V. The light variability is caused by axial rotation of a star with a variable degree of nonuniformity of the surface brightness (spots) and chromospheric activity. Some of these stars also show flares similar to those of UV Cet stars, and in those cases they also belong to the latter type and are simultaneously considered eruptive variables.
Flat-Spectrum Radio Quasar, which is a Type1 active galactic nucleus
General Catalogue of Variable Stars code corresponding to a concept of the ontology
Gamma Doradus variables are variable stars which display variations in luminosity due to non-radial pulsations of their surface. The stars are typically young, early F or late A type main sequence stars, and typical brightness fluctuations are 0.1 magnitudes with periods on the order of one day. This is a relatively new class of variable stars, having been first characterised in the second half of the 1990s, and details on the underlying physical cause of the variations remains under investigation.
Herbig Ae/Be Stars (NC) 1. Observational, a category of spectral type A and B stars that exhibit properties similar to those of CTTS: strong optical emission lines and infrared to millimeter excesses consistent with disk emission. 2. (Orig.) Observational, Herbig (1962) first drew attention to a set of early type objects with emission lines, associated with localized nebulosity and larger scale dark clouds. As is the case for CTTS/WTTS, the original classification implied nothing about the presence of disks, though disks were later recognized as the physical driver for many of the observational characteristics. Description: These systems are interpreted as pre-main sequence and zero-age main sequence objects, with ages comparable to CTTS and WTTS but larger masses, 2 < M∗/M⊙ < 10. They are often referred to as just “Herbig Stars”.
A high-mass X-ray binary (HMXB) is a binary star system where one of the components is a neutron star or a black hole. The other component is a massive star, usually a Be star or a blue supergiant. A fraction of the stellar wind of the companion star is captured by the compact star, and produces X-rays as it falls onto the compact object. In HMXB, the massive star dominates the emission of optical light. The compact object is the dominant source of X-rays. The massive stars are very luminous and therefore easily detected. One of the most famous HMXB is Cygnus X-1 which was the first stellar-mass black hole discovered. Soft-gamma rays emitters.
galaxy in strong interaction with others (within a pair or a group)
Stars showing variations of irregular (non-periodic) type. Mostly (?) of eruptive type.
Gravitationnaly Lensed Image of a Galaxy
Gravitationnaly Lensed Image of a Quasar
Low-mass X-ray binaries (LMXBs) are binary stars where one of the components is either a black hole or neutron star. The other component (donor) is usually filling its Roche lobe and therefore transfers mass to the compact star. The donor is less massive than the compact object, and can be on the main sequence, a degenerate dwarf (white dwarf), or an evolved star (red giant). Found in the Milky Way, external galaxies, and in globular clusters. A typical LMXB emits almost all of its radiation in X-rays, and typically less than one percent in visible light, so they are among the brightest objects in the X-ray sky, but relatively faint in visible light. The apparent magnitude is typically around 15 to 20. The brightest part of the system is the accretion disk around the compact object. The orbital periods of LMXBs range from ten minutes to hundreds of days. Soft-gamma rays emitters.
any kind of comment on an element of the ontology
Text definition of a concept, as complete as possible
tests that should be taken care of outside of the ontology (e.g. check that a value is within sensible boundaries)
keywords for various purposes like plain text search
name in natural language for display purposes
non-standardized keywords used in registry subjects
Measured observational parameters/properties
Mira (Omicron) Ceti-type variables. These are long-period variable giants with characteristic late-type emission spectra (Me, Ce, Se) and light amplitudes from 2.5 to 11 mag in V. Their periodicity is well pronounced, and the periods lie in the range between 80 and 1000 days. Infrared amplitudes are usually less than in the visible and may be <2.5 mag. For example, in the K band they usually do not exceed 0.9 mag.
Geometry or morphology of astronomical objects (including components which are not astronomical objects)
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, slow, very slow, and recurrent categories.
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.
Orion variables. Irregular, eruptive variables connected with bright or dark diffuse nebulae or observed in the regions of these nebulae. Some of them may show cyclic light variations caused by axial rotation. In the Spectrum-Luminosity diagram, they are found in the area of the main sequence and subgiants. They are probably young objects that, during the course of further evolution, will become light-constant stars on the zero-age main sequence (ZAMS). The range of brightness variations may reach several magnitudes. This type may be divided into the following subtypes: - Rapid light variations observed (up to 1 mag in 1-10 days), - Orion variables of early spectral types (B-A or Ae). They are often characterized by occasional abrupt Algol-like fadings (T Ori); - Orion variables of intermediate and late spectral types, F-M or Fe-Me (BH Cep, AH Ori). F-type stars may show Algol-like fadings; K-M stars may produce flares along with irregular light variations; - Orion variables of the T Tauri type. Stars are assigned to this type on the basis of the following (purely spectroscopic) criteria: spectral types are in the range Fe-Me. The spectra of most typical stars resemble the spectrum of the solar chromosphere. The feature specific to the type is the presence of the flourescent emission lines Fe II 4046, 4132 A (anomalously intense in the spectra of these stars), emission lines [Si II] and [O I], as well as the absorption line Li I 6707 A. These variables are usually observed only in diffuse nebulae. - Some Orion variables (YY Ori) show the presence of absorption components on the redward sides of emission lines, indicating the infall of matter toward the stars' surfaces.
A planetary nebula is a nebula surrounding a stellar remnant (white dwarf)
Naked T Tauri stars are basically the same as Post-T Tauri stars.
Post-T Tauri star (PTTS) (SES) 1. Observational, a young star that exhibits a photospheric spectrum over a wavelength range from the NIR and longward. Description: Among solar-type PMS stars, such objects can be identified from their location in the HRD, from their x-ray properties supplemented by HRD location (confirmed from parallaxes; radial velocities; clear membership in a cluster.....) plus a robust SED. Among higher mass stars, such objectsmight be called “Post Ae/Be stars” and can be identified via their location in the HRD in a cluster/association plus robust SEDs. 2. Physical, a young star of any mass that no longeris surrounded by a circumstellar accretion disk. 3. Physical, an evolutionary step between T Tauri Stars and ZAMS, generally thought to refer to stars on the radiative track. Comments: In these definitions, there is no physical difference between a WTTS and a post-TTS. It’s just the choice of observational ‘tool’ for certifying that such an object is no longer accreting material. There is some sentiment on the committee for specifying that a Post-T Tauri star is older than either a CTTS or a WTTS (e.g., with ages of 5-30 Myr). See also naked T Tauri star. Naked T Tauri star (NTTS) (supplied by Scott Wolk and Fred Walter) 1. Observational, a low mass pre-main sequence star that exhibits no evidence of an optically thick circumstellar disk or on-going accretion. Description: In this picture, a NTTS differs from a post-T Tauri star in being an observational definition while the PTTS is described by definition 3 for post-T Tauri Star.
Pre-Main-Sequence Object (PMS) (NJE) 1. Observational, a star that lies above the main sequence.
protoplanetary disk (CL) 1. Physical, a primordial disk with the potential to form planets.
protostar (NJE) 1. Physical, an object that is still accreting mass and that will become or already is a main sequence star. 2. Physical, an object deriving most of its luminosity from accretion and that will become a main sequence star. 3. Physical, the entire structure of central star, disk, and envelope. Comments: The first definition allows the inclusion of massive protostars, which can be already burning nuclear fuel but still growing in mass. The second does not allow them above a certain mass where the nuclear-burning luminosity exceeds that from accretion. Both the first two apply to the central object, while the third refers to the entire structure. For the first two definitions, objects in Stages 0, I, or II could host protostars; For the third definition, only Stages 0 and I would be protostars.
Optically variable pulsars (CM Tau), which are rapidly rotating neutron stars with strong magnetic fields, radiating in the radio, optical, and X-ray regions. Pulsars emit narrow beams of radiation, and periods of their light changes coincide with rotational periods (from 0.004 to 4 s), while amplitudes of the light pulses reach 0.8 mag.
Pulsating variables are stars showing periodic expansion and contraction of their surface layers. The pulsations may be radial or nonradial. A radially pulsating star remains spherical in shape, while in the case of nonradial pulsations the star's shape periodically deviates from a sphere, and even neighboring zones of its surface may have opposite pulsation phases. Depending on the period value, on the mass and evolutionary status of the star, and on the scale of pulsational phenomena, different types of pulsating variables may be distinguished.
Variables of the R Coronae Borealis type. These are hydrogen-poor, carbon- and helium-rich, high-luminosity stars belonging to the spectral types Bpe-R, which are simultaneously eruptive and pulsating variables. They show slow nonperiodic fadings by 1-9 mag in V lasting from a month or more to several hundred days. These changes are superposed on cyclic pulsations with amplitudes up to several tenths of a magnitude and periods in the range 30-100 days.
Variables of the RR Lyrae type, which are radially-pulsating giant A-F stars having amplitudes from 0.2 to 2 mag in V. Cases of variable light-curve shapes as well as variable periods are known. If these changes are periodic, they are called the "Blazhko effect." Traditionally, RR Lyrae stars are sometimes called short-period Cepheids or cluster-type variables. The majority of these stars belong to the spherical component of the Galaxy; they are present, sometimes in large numbers, in some globular clusters, where they are known as pulsating horizontal-branch stars. Like Cepheids, maximum expansion velocities of surface layers for these stars practically coincide with maximum light. Sub-types: - RR Lyrae variables showing two simultaneously operating pulsation modes, the fundamental tone with the period P0 and the first overtone, P1 (AQ Leo). The ratio P1/P0 is approximately 0.745; - RR Lyrae variables with asymmetric light curves (steep ascending branches), periods from 0.3 to 1.2 days, and amplitudes from 0.5 to 2 mag in V; - RR Lyrae variables with nearly symmetric, sometimes sinusoidal, light curves, periods from 0.2 to 0.5 days, and amplitudes not greater than 0.8 mag in V (SX UMa).
Eruptive variables of the RS Canum Venaticorum type. This type is ascribed to close binary systems with spectra showing Ca II H and K in emission, their components having enhanced chromospheric activity that causes quasi-periodic light variability. The period of variation is close to the orbital one, and the variability amplitude is usually as great as 0.2 mag in V (UX Ari). They are X-ray sources and rotating variables. RS CVn itself is also an eclipsing system.
Variables of the RV Tauri type. These are radially pulsating supergiants having spectral types F-G at maximum light and K-M at minimum. The light curves are characterized by the presence of double waves with alternating primary and secondary minima that can vary in depth so that primary minima may become secondary and vice versa. The complete light amplitude may reach 3-4 mag in V. Periods between two adjacent primary minima (usually called formal periods) lie in the range 30-150 days. Two subtypes are recognized: - RV Tauri variables that do not vary in mean magnitude (AC Her); - RV Tauri variables that periodically (with periods from 600 to 1500 days and amplitudes up to 2 mag in V) vary in mean magnitude (DF Cyg, RV Tau).
Rapid irregular variables (class: eruptive) having no apparent connection with diffuse nebulae and showing light changes of about 0.5 - 1.0 mag within several hours or days. There is no strict boundary between rapid irregular and Orion variables. If a rapid irregular star is observed in the region of a diffuse nebula, it is considered an Orion variable. To attribute a variable to this type (R.I.), it is necessary to take much care to be certain that its light changes are really not periodic. Sub-types: - Rapid irregular variables of the early spectral types, B-A or Ae; - Rapid irregular variables of the intermediate and late spectral types, F-M and Fe-Me.
Variable stars with nonuniform surface brightness and/or ellipsoidal shapes, whose variability is caused by axial rotation with respect to the observer. The nonuniformity of surface brightness distributions may be caused by the presence of spots or by some thermal or chemical inhomogeneity of the atmosphere caused by a magnetic field whose axis is not coincident with the rotation axis.
Main part of the luminosity class in SIMBAD (letters only, found within SIMBAD's spectral type value after said spectral type)
morphological type for galaxies in SIMBAD
Standard name in Simbad's object classification
Short code in Simbad's object classification
Main part of the spectral type in SIMBAD (letters only, found at the beginning of SIMBAD's spectral type value)
The shockwave-generating process or the resulting Shockwave
Any class in Morgan Keeman's spectral classification
An individual star located in a star cluster
Symbiotic variables of the Z Andromedae type. They are close binaries consisting of a hot star, a star of late type, and an extended envelope excited by the hot star's radiation. The combined brightness displays irregular variations with amplitudes up to 4 mag in V. A very inhomogeneous group of objects.
VizieR keyword corresponding to an item of the ontology
Variables of the W Virginis type. These are pulsating variables of the galactic spherical component (old disk) population with periods of approximately 0.8 to 35 days and amplitudes from 0.3 to 1.2 mag in V. They obey a period-luminosity relation different from that for Delta Cep variables. For an equal period value, the W Vir variables are fainter than the Delta Cep stars by 0.7 - 2 mag. The light curves of W Vir variables for some period intervals differ from those of Delta Cep variables for corresponding periods either by amplitudes or by the presence of humps on their descending branches, sometimes turning into broad flat maxima. W Vir variables are present in globular clusters and at high galactic latitudes.
Weak-line T Tauri Star (WTTS) 1. (Original) Observational, a “solar-type” (M∗ ~0.5M⊙, K7-M0), age ~1 Myr, pre-main sequence star (from HRD location) that has W(Hα) < 10Å. Description: The limit was chosen in order to separate objects which showed the signatures of optically thick accretion disks (based on IRAS + NIR SEDs) from those that appeared to be photospheric. 2. (More modern) Observational, any pre-main sequence star that shows no evidence of accretion based on an age and spectral-type appropriate W(Halpha). Description: Firm inclusion in the class would require additional information from high resolution, high S/N spectra to test for the presence (CTTS) or absence (WTTS) of Hα profiles indicative of accretion, or signatures (e.g., forbidden [OI] or [SII]) of winds driven by accretion. 3. Physical, a young star that is no longer surrounded by an accretion disk. Comments: The definition of WTTS was introduced to link signatures of accretion (Halpha emission; forbidden line emission) diagnosed from the presence/kinematics of gas, with signatures of optically thick disks provided by IR SEDs. The definition that was applied to the initial Taurus sample is obsolete, in the sense that the W(Halpha) = 10Å limit is an appropriate (and only approximate) discriminant for Taurus-age stars with masses ~ 0.5 M⊙. However, the idea of using both gas and dust diagnostics to assess the physical state of the disk is still enormously valuable. In particular, it is by using both SEDs and gas diagnostics that we can probe the evolutionary state of transition/anemic disks and assess/constrain the likely physical cause for the observed SED signature.
Close binary systems that are sources of strong, variable X-ray emission and which do not belong to or are not yet attributed to any other type of variable stars. One of the components of the system is a hot compact object (white dwarf, neutron star, or possibly a black hole). X-ray emission originates from the infall of matter onto the compact object or onto an accretion disk surrounding the compact object. In turn, the X-ray emission is incident upon the atmosphere of the cooler companion of the compact object and is reradiated in the form of optical high-temperature radiation (reflection effect), thus making that area of the cooler companion's surface an earlier spectral type.
Young Stellar Object (YSO) 1. Physical, an object in any of the Stages defined above. (Strom et al. 1975, ARAA, 13, 187) 2. Observational, an object in any of the Classes but with an infrared excess detectable in Spitzer surveys (a definition used by the c2d team to distinguish YSOs from the larger class of pre-mainsequence objects (PMS)
features absorption spectral line(s) for
Indicates that the DOMAIN CompoundObject has the RANGE AstrObject among its direct physical components (the RANGE AstrObject cannot be a component of a component of the composite object)
Indicates that the DOMAIN AstrObject has the RANGE Measurement among its (observational) measurements
Indicates that the RANGE AstroPortion can be found whithin the DOMAIN CompoundObject or AstroPortion
Indicates that an astronomical object is the location of a Process/has an attached process
Indicates that the DOMAIN AstrObject is a direct physical component of the RANGE CompositeObject (not a component of a component of the RANGE CompositeObject)
Indicates what ionizes the gas of a given ISM