A short history of flare starsAlthough flare stars may have been detected as early as 1924, the earliest confirmed observations are attributed to W.J. Luyten, who discovered strongly variable spectra in two high proper-motion dwarf stars now known as V1396 Cyg and AT Mic. In particular, Luyten noted that the emission lines of hydrogen were first observed in a very bright state, but then rapidly faded. Several similar stars were found in short order: V371 Ori, WX UMa, YZ CMi, and DO Cep were all discovered in the late 1930's and early 1940's.
However, the field of flare star research really took off with the discovery of flares on Luyten 726-8, another high proper-motion binary, in September of 1948 (Joy & Humason 1949). Astronomers observing this star at Mount Wilson discovered a huge increase in brightness over a very short time. Later analysis of the spectra taken during the observation revealed a change in brightness of over four magnitudes, and a rise in effective temperature to well over 10,000 K. These incredible changes faded just as rapidly, with the star returning to its cool, quiescent state in less than a day. Today, the star is known as UV Ceti, the class prototype for the flare stars.
Since their initial detection and characterization in visible light, the UV Ceti stars have also been detected over a wide wavelength range, from X-rays to radio. The coincidence between radio and optical flaring in the UV Ceti stars was noticed as early as 1966 (Lovell & Solomon), and X-ray flares were first detected in 1975 (Heise et al.). Many multiwavelength campaigns have been conducted on various flare stars, and as a result, we now have a reasonably good physical picture of how flares work. The number of known flare stars is also increasing with time: the GCVS currently lists 1620 stars of UV Ceti (UV) or UV Ceti + Nebular (UVN) type. Recently, variable emission lines have been detected in young brown dwarf stars (Liebert 2003), raising the exciting possibility that brown dwarfs may also exhibit flaring activity.
The Characteristics and Physics of Flare starsAs a class, the known flare stars have spectral types of late M through late-K, corresponding to temperatures between about 2500 to 4000 K. Often, they have detectable emission lines of hydrogen and calcium in their spectra, indicating chromospheric activity. They have masses between 0.1 and 0.6 times that of the Sun; some brown dwarfs may exhibit flaring activity, though the study of these stars is still very much in its infancy. Many of the known flare stars are members of young stellar associations (e.g. the Orion and Taurus star-forming regions), though some older flare stars are known. Many are also known to be binary stars, and this may correspond to an increased likelihood of activity. Some of the UV Ceti flare stars are also members of the BY Draconis class of spotted variables.
Variability in the flare stars is characterized by rapid, irregular, large-amplitude increases in stellar brightness, followed by a much slower decay (from minutes to hours) back to a quiescent level. The strongest variations occur in the blue end of the optical spectrum: a flare may cause a one-magnitude change in brightness in the V-band, but five magnitudes in the U-band. Flares are typically accompanied by brightening of the emission line spectra of the star, particularly of the Balmer series of hydrogen, and the appearance of ionized helium lines as well. Flares have also been observed in the radio and X-ray regions of the spectrum, though they are not necessarily coincident with optical flares.
The flare stars are known to be bright at X-ray (Figure 4) and radio wavelengths as well. The physics of flare star radio flares are likely the same as those on the Sun: a magnetic event accelerates charged particles that interact with magnetic fields to produce cyclotron and synchrotron radiation. X-ray flares have also been observed, but the flare stars are also known to have very large quiescent X-ray luminosities, most likely from a large, bright corona. Their X-ray luminosities can be on the order of one percent of the total bolometric luminosity -- far, far greater than would be expected for a low-mass, non-interacting star.
Observing flare stars
The flare stars have been a part of the AAVSO observing program for nearly as long as the stars have been known. The earliest observations of UV Ceti in the AAVSO International Database date to January of 1950, less than 18 months since its discovery in late 1948. Observations commenced on several other stars in the early and mid-1950's. One of the earliest high time-resolution lightcurves of a flare was made by Thomas Cragg on November 14, 1952. Cragg caught UV Ceti in the midst of a flare (mvis = 10.4), and made observations once every few minutes until the star faded to its quiescent level (mvis = 12.2). He recorded another flare in UV Ceti on January 23, 1959, shown in Figure 5. Many other flares have been caught and followed by AAVSO observers since then, both in UV Ceti and in other flare stars, like EV Lac and V371 Ori. Flare stars have also been subject to collaborative efforts between the AAVSO community and professional astronomers, particularly for the purpose of multiwavelength campaigns.
The morphology of a flare event you are likely to see is much like the January 7-8, 1982 flare of V371 Ori (observed by Lew Cook) shown in Figure 6. The star will brighten from its quiescent level by (up to) a few magnitudes over the space of just a few minutes. After the peak is reached the visible light fades back to quiescence over a period of tens of minutes to a few hours. Depending upon the strength of the flare and the color sensitivity of the observer, the peak brightness may be between one and several magnitudes above the quiescent level. Flare stars can undergo much smaller flares of a few tenths of a magnitude or less, though these may be difficult to measure by visual observers. Sometimes, the star may reach a plateau, superimposed with several short flares -- the high point near 06:50 in Figure 6 may be an example of such a flare.
The flare stars spend relatively little time at their brightest -- perhaps a few minutes per flare -- and the occurrences of flares are unpredictable. Therefore, flares are difficult to catch. The best way to observe one is to devote one night to a single flare star, and monitor it once every few minutes, much like one would observe a short-period eclipsing binary. In the ideal (but unlikely) case, these observations would allow you to catch the quiescent pre-flare brightness level, the rapid rise of a flare, and its decay to quiescence. Unfortunately, flares are unpredictable, so it may be awhile before you catch a flare star in the act. Flare stars are ideally suited to CCD and (especially) photoelectric observations because of the high time-resolution needed to catch all phases of the flare, and photomultipliers have the additional benefit of being very blue-sensitive. If you are using a CCD or photomultiplier, U, B, or V filters or their equivalent are recommended, since the flare amplitudes are larger at bluer wavelengths. However, many flare events have been detected by visual observers (including the two shown above), so persistent visual observers should have no problem observing flare stars.
When submitting observations to AAVSO, it is easiest to use the following procedure:
Postscript: a note on charts
Many of the flare stars were first discovered during studies of high proper motion stars, and some have proper motions of several arcseconds per year. Because of this, many of the charts for flare stars are out of date, especially those made during the 1960's and 1970's.
We are in the process of generating new charts for these objects, but until they are available, we have assembled "blinking" images from the first and second editions of the Digital Sky Survey, with the time between images ranging between seven years (V371 Ori) and 43 years (AD Leo). You can use the blinking images to help you estimate the position of the star in the current field.
The blinking images may be found here, and DSS2 images (ca. 1997) indicating the star's current position may be viewed individually through the following links: WX UMa, UV Cet, EV Lac, AD Leo, YZ CMi, and V371 Ori. All images are 30 arcminutes on a side, and the flare star is located within the square aperture in the static DSS2 images. The approximate direction of the proper motion (if any) is indicated with an arrow. We recommend that you use the blinking images and the existing AAVSO charts together -- use the blinking images as finder charts to determine the current position of the variable star, and the AAVSO chart for the sequence.
This Variable Star of the Season was prepared by Dr. Matthew Templeton, AAVSO.
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