Eruptive young stellar objects in general

Almost all young (pre-main sequence) stars exhibit semiregular or irregular brightness variations during the early phases of stellar evolution. For instance T Tauri stars show irregular light-variations of up to 3 mag, Herbig Ae/Be stars exhibit sporadic variations of about 1 mag. Light-variations during the pre-main sequence evolution thought to be connected to the circumstellar environment of the young star (e.g. accretion column/hot spot, variable accretion rate, variable extinction).

Certain objects, however, show not only these irregular variations but also eruptions. These are the EXors (named after the prototype EX Lupi) and the FUors (named after the prototype FU Orionis).

FUors in general

FU Orionis objects (FUors) are low-mass (< 2 solar masses) pre-main sequence stars undergoing outbursts in optical light of 3-5 mag. The timescale of the outburst varies from some months to some years, followed by a fading phase of several decades (in Fig. 1. the light curve of three FUor-type stars can be seen). FUors also have distinctive spectral characteristics (e.g. wavelength-dependent rotational velocity and spectral type; broad, blue-shifted optical absorption lines; strong CO absorption at 2.2 μm and water-bands between 1 and 2 μm many optical and infrared lines are double peaked). In some cases (e.g. Z CMa, L1551 IRS 5, BBW 76), eruption in the optical was not observed but the objects were identified as FUors on the basis of their spectral characteristics. Here can be found an up-to-date list of FUor-type and FUor candidate stars. According to the most widely accepted model, FUors are surrounded by an accretion disk from which material flows onto the star (L. Hartmann, S.J. Kenyon: The FU Orionis Phenomenon, Annual Review of Astronomy and Astrophysics, 34, 1996, pp. 207-240.). Fig. 3. shows a possible simple model of such a circumstellar disk, the so-called flared disk, whose thickness increases with the distance from the central star. The outburst is the consequence of a rapid temporal increase of the disk accretion rate. Before outburst, the accreting material accumulates close to the inner edge of the disk (within 0.25 AU from the star). At a critical column density of the disk, a thermal instability initiates the outburst and makes the accumulated material rapidly fall onto the star.

Figure 1: Light curve of three FUor-type object. (From L. Hartmann and S.J. Kenyon: The FU Orionis phenomenon, Annu. Rev. Astron. Astrophys. 34, 1996, pp. 211)

Figure 2: RGB colour-composite image of V1057 Cyg, V1515 Cyg and FU Ori (infrared (R), red (G) and blue (B) POSS2 images).

Figure 3: Schematic geometry of a flared disk model. The thickness of the disk is increasing with the distance from the star.

The inner part of the disk becomes very hot (ionized) which results in an optical outburst. During a century-long outburst 0.01 solar mass fall onto the star. When the disk has been depleted, the outburst declines: temperature and accretion rate decreases and hidrogen recombines. The circumstellar envelope that surrounds the star-disk system begins to refill the disk and the process starts again. The duration of the outburst is determined by the viscosity of the accreting material while the recurrence timescale (104 - 105 years) can be calculated from event statistics. (In fact it is not confirmed that FU Orionis phenomenon is repetitive).

See this page for an up-to date list of FUor-type and FUor candidate stars.

EXors in general

EX Lupi-type stars (or EXors) are eruptive T Tauri-type stars. EXors exhibit 1-4 mag optical outbursts having a characteristic timescale of some 10 - some 100 days, and separated by several months. (in Fig. 4. the light curve of the EXor-type star VY Tau can be seen). Their spectral type is K or M dwarf, and in maximum the spectrum is dominated by emission lines characteristics of classical T Tauri stars. Similarly to FUors, the eruptions of EXors are attributed to the enhanced accretion rate from the circumstellar disk to the stellar surface.

Figure 4: Light curve of the EXor-type star VY Tau. (From G.H. Herbig: Eruptive phenomena in early stellar evolution, Astrophysical Journal 217, 1977, pp. 712)

Studying eruptive young stellar objects at KISAG

• One part of the studies focuses on the infrared properties of FUor- and EXor-type young stellar objects. We collected and reduced multi-wavelength ISOPHOT data on FUors and EXors and combined these data with other infrared data sets (IRAS, MSX, 2MASS) in order to construct complete infrared SEDs. Our aim is to find out what role does the circumstellar material (especially the accretion disk) have in the outbursts.

• The other part of the studies aim the research of the temporal evolution of FUors and EXors both at optical and infrared wavelengths. Multi-epoch data enables us to study the geometry, density, temperature and energy budget of the circumstellar matter.

Related major topics at KISAG

FU Ori phenomenon

EXors