UX Orionis Stars - Variation gone mad

As we saw, stars of fairly low mass like the Sun are what T Tauri stars eventually become. If, however, there is more mass available, we end up with a much hotter and more powerful star that will use up its nuclear fuel much faster. The Gods clearly love these stars, for they die young. Everything about them is a speeded-up version of lower mass stars; they form in a shorter time and go through their lives in less time. However, because the temperatures in their cores are so much greater, the latter parts of their evolution are different from those of low-mass stars since this greater temperature allows more elements to burn. When the star eventually scatters itself through interstellar space in a supernova explosion, those extra elements come in very handy - they can eventually turn into you and me, and hopefully E.T. versions of us too!

Looking pole-on: Basic variation (if any) derives from Starspots only:
T Tau / Herbig Star type (depending on spectrum)
Oblique view: Basic variation plus some light reduction from transit of disc clumps
Edge-on view: Deep, more frequent fades from transit of disc clumps

The Herbig stars, like the T Tauri objects, are surrounded by swirling discs (or probably in some cases, shells) of gas and dust grains, which contain significant amounts of common terrestrial substances such as Silicon and Graphite. Like T Tau stars, they may also show continuous gentle variations which may be caused by starspots. But a subgroup of these stars, named after UX Orionis, show an additional important characteristic. From time to time, they undergo eclipse-like fades by as much as three magnitudes. Why these stars should show sudden deep fades while the structurally-similar T Tau stars tend not to is intriguing, though some of these stars, such as T and RY Tau, do undergo deep fades from time to time which are less eclipse-like. Are the eclipse-like fades caused by the higher temperature of the area of the star changing the chemistry or the constituents of the circumstellar disc, producing larger dust grain sizes so that more light is absorbed? Do the UX Ori stars represent a different stage in a star's evolution, where the dust and grain concentrations are much larger, having started to coalesce into planetesimals? Is it a contrast effect - since the star itself is brighter, more light is seen to be cut off? Are these stars born under different initial conditions to begin with? Or are there several of these factors at work? One star, RZ Piscium, which is classed as a UX Ori star by Guertler et al is of spectral type K, that is, it should be a T Tauri object but for the Algol-like fades it exhibits. It is also anomalous for other reasons, and this points the way to a distinction between the circumstellar environment of the two types. Why - with the exception of RZ Piscium - are there no T Tau stars with UX Ori type minima? We have to bear in mind that T Tau stars congregate in large numbers and many of them are faint objects whose light variations are not well studied. It is possible that there are in fact many stars of this sort but we simply have a lack of suitable data on them. For instance, several years ago, a whole population of T Tau stars was found in the far-south constellation of Chamaeleon, like RZ Psc not close to the main concentrations of star-forming clouds along the galactic equator.

Something else to consider here is the system geometry, shown in the diagrams. Our view in space relative to each star and its attendant disc will determine what sort of variation is seen. The deep fades caused by edge-on systems have led some astronomers into classifying them in other completely different types. BO and IL Cephei for instance are both UX Orionis systems which were formerly classed as Algol stars. VX Cas, considered to be a UX Orionis star, may instead be a VY Scl type cataclysmic binary. Add to the periodic fades the fact that the stars may also be spotted to varying extents and you have a group of stars with interesting lightcurves - though as a purely visual observer I naturally prefer the "more bang for your buck" antics of the UX Orionis stars. Studies have noted a correspondence between the angle of inclination and the depth of fades. The very active star CQ Tauri shows fades of about 2 magnitudes, with an inclination of 66 degrees (90 representing an edge-on view) whereas AB Aurigae, a much less drastic variable with fades of about 0.3m, only has a disc inclination of probably 30 degrees.

As you are probably expecting by now, there are complications! First of all, about 60% of stars are binary systems. T Tauri itself is one (probably triple, in fact) and this will of course affect the shape of the circumstellar environment, the evolution of the disc (or shell) and thus the observed light variations. What we are probably seeing is an evolutionary effect - the Herbig stars typified by UX Ori are very young, with large circumstellar shells or discs, with a lot of material to obscure the star, whereas those that are less active represent the star at a later stage of its life, when more of the potentially obscuring material has been used up or dispersed in some way. By the time our Herbig star has reached the ZAMS (zero-age main sequence) its circumstellar material has been dispersed into the manufacture of planets, comets and suchlike (maybe comets are produced from the shell rather than the disc, giving rise to their divergent inclination from the system's invariable plane) or has been ejected into space, where of course it can precipitate further bouts of star formation. One could draw a graph showing amplitude of variation against age, such as is shown below.

1. Young star: frequent and large variation due to obscuration 2. Older star: less variation due to dispersal of circumstellar material 3. Star on Main Sequence: now constant, due to absence of obscuring matter
Some stars, such as BO Cephei and XY Persei show a periodicity in their fades, superimposed on smaller "background" variation which we can attribute to intrinsic variation (the "star-spotting" phenomenon) or occultations by lesser dust clouds, so it seems that in these cases eclipses are caused not only by clumping in the circumstellar environment but also by another star in the system. Others show no periodicity, or simply lack observational evidence over a sufficiently long period. Guertler's paper stresses the need for long-term coverage of these stars - coverage that amateur observers, whether visual, CCD or multiwavelength, are well placed to provide. Have a look at some of the individual stars under the Nebular Stars section.

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