TTSs physics studied with IUE data

There are three main physical processes associated with the TTS that have been studied with IUE. These are:

Magnetic activity

The youth of the TTSs as well as their rapid rotation rates (e.g. Vogel & Kuhi 1981, Hartmann et al 1986) as compared with late type main sequence stars have been used to study the relevance of the dynamo effect and remnant primordial magnetic fields to the generation of nonradiative heating of the stellar atmosphere and the driving of stellar winds.

The emission-line fluxes have been used to derive emission measures and model the structure of the upper atmosphere (Jordan et al 1982; Brown et al 1984;Lago et al 1985). Brown et al (1984) found evidence in T Tau for a two-component density structure although the geometry involved was uncertain.

The TTS chromospheres produce strong Ca II and Mg II emission lines. The Mg II surface fluxes have been estimated for a number of TTS (Giampapa et al., 1981; Calvet et al., 1985). These surface fluxes are around 107 - 108 erg/cm2, roughly 50 times larger than for the Sun. The ratio of Ca II to Mg II emission is consistent with a normal chromosphere, and seems to be an extrapolation from the dwarf stars to higher activity levels for the wTTSs, however for the cTTSs the Mg II k-line seems to arise in a extended region probably associated with the Hα emission region (Calvet et al 1985). In fact, extended Mg II emission has been found in at least 2 cTTS: T Tau (Brown et al 1984) and CW Tau.

The correlations between the fluxes of lines formed in the chromosphere, transition region and corona (henceforth flux-flux correlations) and between these fluxes and the stellar rotation period (flux-period correlations) have been used to obtain a better understanding of the mechanisms responsible for the presence of activity. The TTSs extend these relations as defined by other cool stars towards larger flux densities typically by a factor of ~ 40. They deviate slightly from the flux-flux relations derived from the rest of the active stars: F, G, K dwarfs, dMe stars, RS CVn and even the wTTSs (Lemmens et al 1992; Gómez de Castro & Fernández 1996). Moreover, the scatter of the data points about the mean flux-flux relations for the TTSs is slightly larger than for other sources although this is probably related with the uncertainties in the conversion to surface flux densities (Lemmens et al 1992).

The majority of the cool stars obey a well-defined activity-rotation relation. There are some stars that are much more active than the rotation rate predicts, e.g. components in close binary systems. Among the overactive stars the cTTS form a special class: they are overactive in the chromospheric and transition region emissions but not so in X-rays (see e.g. Lago et al 1985). Bouvier (1990) reports evidence for an inverse correlation between x-ray surface flux and rotational period in a sample of 21 TTS (including both cTTS and wTTS) and proposes that this is caused by a solar-type magnetic dynamo; rotation is the primary parameter governing the level of magnetic dynamo activity in cool stars. However a comparison of the TTS with other active stars shows that the cTTS exhibit radiative losses in the lines that are up to 100 times larger than those measured in cool dwarfs and evolved binaries. This suggests that other effects are producing an anomalous enhancement of the radiative loses in the UV. Radiation from accretion disks is suggested as the main source.

The UV continuum and line emission of TTS are variable. The variations have been found to be periodic-like in some objects (Gómez de Castro and Fernández 1996, Gómez de Castro & Franqueira, 1997) but there are also rapid variations (in time scales of few hours) likely associated with the well known flaring activity of the TTS (see e.g. Montmerle et al 1993 for a review). A good example is the flare that occurred to BP Tau in February 1992. This flare lasted for few hours and was reported by Gullbring et al (1996) from a UBVRI monitoring campaign. The event was also detected with IUE as a fast increase in the Si II, Mg II and UV continuum light curves (Gómez de Castro & Franqueira 1997). The event could be peculiar in the sense that the optical light curves are very different than the observed in flare stars (it was cool with T=7000-8000 K and had light curves with similar rise and fall times ). Gullbring et al (1996) suggested that it was produced as a result of inhomogeneous accretion.

Accretion

The mechanism involved in the acceleration of the TTS winds is still unknown, but seems to be related with the presence of accretion disks around the TTS (e.g. Cabrit et al., 1990, Hartigan et al 1995). There are many indications of the presence of disks around TTS (Bastien,1987; Jankovics, Appenzeller and Krautter, 1983; Edwards et al., 1987; Beckwith et al., 1990). Disks around the TTS are accreting mass onto the star. However the details of this process are not known. For many years it has been assumed that the material from the viscous disk accretes steadily onto the star through the boundary layer between the disk and the star (Bertout et al 1988). This boundary layer is expected to be the responsible of the UV excess of the TTS with respect to main sequence stars of the same spectral types. Accretion rates of ~ 3 (10-7 - 10-9) M⊙/year are able to reproduce reasonably well the spectra of the TTS from the UV to the mid-infrared. FU Orionis variables have been also explained by accretion from protostellar disks (Kenyon et al 1989, Hartmann et al 1989). However there are some properties that cannot be explained by this simple boundary-layer model.

Some T Tauri Stars (TTS) have periodic photometric variability. The periods inferred are 2 - 10 days in agreement with those expected for rotational modulation. The periodicity has been explained as due to the presence of spots on the stellar surface. The analogy is normally made with the RS CVn systems and, in fact, the properties of many TTS can be explained in a similar way, i.e., by the presence of dark spots generated by an enhanced solar-like activity. However there are some objects whose properties cannot be explained by this mechanism. Some TTS have much stronger variations in the U than in the R or I bands, and in these cases the variability is best modelled by hot spots on the stellar surface. The detection of hot spots on the stellar surface of some CTTS has increased the suspicion that the infall material could be chanelled by strong dipolar fields on the stellar surface (Simon et al 1990, Koenigl 1991, Lamzin et al 1996). Some theoretical models have been developed in the last years mainly addressing the implications of magnetically channelled accretion in the spin-down of PMS stars and the generation of mass outflows (Tout and Pringle, 1992; Cameron and Campbell 1993; Shu et al 1994; Pearson and King 1995).

Key observational tests, however, have not been carried out until recently. If the material falls onto the stellar surface channelled by the field lines, the UV flux (the accretion flux) variation should be correlated with the optical variability. The correlation between the optical and the UV variability was first studied for BP Tau (Simon et al. 1990) and RU Lup (Giovannelli et al. 1990). A study of the correlation between optical and UV continuum variability has been also carried out for the TTS in Taurus using the IUE Archive Data. However these correlations could be just due to the well known flaring activity of the TTS (e.g. Joy 1945, Montmerle et al 1993) or to accretion instabilities. In general, the rotational period has not been well tracked. For instance, RU Lup was observed just 8 times in 5 years, and BP Tau has a photometric period around 7 days (Vrba et al. 1986) but only a half of it was well monitored in the 1200 -2000 Å    range where the most prominent resonance lines are observed by Simon et al (1990).

The presence of hot spots has been reported without ambiguity only for 11 CTTSs: DN Tau, GI Tau, GK Tau and BP Tau (Vrba et al 1986), DF Tau (Bouvier and Bertout 1989), DE Tau, DG Tau, IP Tau, GM Aur and TAP 57NW (Bouvier et al 1993) and DI Cep. Nine of these have been observed at least once with the International Ultraviolet Explorer (IUE) but only two have been properly monitored in the wavelength range between 1200 Å  and 2000 Å: DI Cep (Gómez de Castro & Fernández, 1996) and BP Tau (Gómez de Castro & Franqueira, 1997).

The UV monitoring of DI Cep showed that the light curves are similar in all the lines (O I, C IV, Si IV and Si II) suggesting that there is a broad range of temperatures in the hot spot (from 104 to 105 K). Variations in the spot properties during several years lead to the conclusion that rather than dealing with a strong dipolar field the material is channelled by a variable loop structure that could cause inhomogeneous accretion events.

The variations of the UV spectrum of BP Tau during 2 rotation periods show that lines that can be excited by recombination processes, such as those from O I and He II have periodic-like light curves, whereas lines that are only collisionally excited do not follow a periodic-like trend (Gómez de Castro & Franqueira 1997). These results agree with the expectations of the magnetically channelled accretion models. The kinetic energy released in the accretion shocks is expected to heat the gas to temperatures of ~ 106 K that henceforth produces ionizing radiation. The UV (Balmer) continuuum and the O I and He II lines are direct outputs of the recombination process. However, the C IV, Si II and Mg II lines are collisionally excited not only in the shock region, but also in inhomegeneous accretion events and in the active (and flaring) magnetosphere and therefore their light curves are expected to be blurred by these irregular processes.

Mass outflows from PMS stars

PMS stars are characterized by very high mass-loss rates of 10-6 - 10-8 M⊙/year. Outflows are detected over a wide range of scales from the parsec size of the molecular outflows and the optical jets to the some tens of AU scales where the optical forbidden lines or the P-Cygni profiles observed in Hα and NaD lines are formed. Strong P-Cygni profiles are also observed in the Mg II lines of the few TTS that have been observed in the high resolution mode. The lines have a P-Cygni profile with a redshifted asymmetric emission component and often a broad blueshifted absorption component characteristic of line formation in an expanding envelope. The terminal velocity of the wind inferred is of few hundreds of km/s. The low level continuum as well as the presence of some metallic lines makes difficult to determine the terminal velocities accurately. The comparison with the high signal-to-noise ratio profiles obtained with the Hubble Space Telescope of some of these sources suggest that some of them may have broad emission wings (see e.g. Gómez de Castro & Franqueira 1997). In some stars the absortion component is filled in with emission (see e.g. RW Aur in Imhoff & Appenzeller 1989).

There are few TTSs that are bright enough at short wavelengths to be observed in high dispersion. Penston and Lago (1982) analyzed the widths of the C IV, Si III] and C III] lines in RU Lup finding widths of ~ 170 km/s. These lines are formed at much higher temperature than the Mg II lines that are however broader.

The comparison between the Ca II and Mg II line fluxes in the nearly simultaneous observations carried out by Calvet et al (1985) suggests that the Mg II lines are formed in an extended region probably associated with the wind, in contrast with the Ca II lines that form closer to the surface. In fact, extended Mg II emission has been detected in the line by line images of some sources like T Tau or CW Tau. In the last years, high resolution imaging and long slit spectroscopy of some cTTS in optical bands (Solf 1989, Gomez de Castro 1993) have shown the presence of small scale jets unresolved in previous observations. The Mg II emission from the 4” scale jet of CW Tau has been detected with IUE (Gómez de Castro and Robles, in preparation).

Finally, the HH nebulosities produced by the interaction between the TTSs outflows and the environmental medium, have also been detected with IUE. The detection of HH objects with IUE by Ortolani & D’Odorico (1980) was a surprise. Their spectrum is characterized by the presence of strong emission lines that depend on the characteristics of the object. Low-excitation objects emission is dominated by the Lyman band of the H2. High excitation objects show C IV, C III, CII] and Mg II emission. The line emission is variable. There is also a far UV continuum centered at 1500 Angstroms that is usually interpreted as collisionally enhanced two-photon emission of hydrogen (Dopita et al 1982) although this interpretations seems quite uncertain.