T Tauri Stars

Abstract

T Tauri is the prototype for a class of very young stars, still in the process of gravitational contraction. Some of their characteristics and evolution are described here.

Keywords: T Tauri, FU Orionis, star formation, gravitational contraction, Herbig-Haro objects

Introduction

T Tauri is the prototype for a class of very young stars, still in the process of gravitational contraction; that is, they have yet to evolve to reach the main sequence. On a Hertzsprung-Russell Diagram, they plot above the main sequence.

T Tauri stars are mostly:

• between 10⁵ and 10⁸ years in age;
• of low mass (0.5 to 3.0 M_¤);
• surrounded by hot, dense envelopes; and
• losing mass via stellar winds with typical v_¥ = ~100 km/s.

"From such observations, T Tauri stars are presumed to be solar-mass, pre-main-sequence stars.... T Tauri stars appear almost always within dark clouds. Some T Tauri stars appear naked - they lack a cloud of gas and dust. They gather in dark clouds where star formation is most active. Some of these naked T Tauri stars have magnetic activity indicated by starspots on their surface (such as for the RS CVn stars...). Other T Tauri stars have clear evidence of thin disks of circumstellar material with diameters of a few hundred AU" (Zeilik 1991, pp. 329-330).

Thus, T Tauri stars are a transitional phase between the IR sources indicating proto-stars which are still shrouded in dust, and stars newly arriving at the main sequence.

"A particularly violent subclass of T Tauri stars, the FU Orionis stars, exhibit variable mass loss rates 100-1000 times greater than their more quiescent cousins. It has been suggested that these objects get some of their energy from the infall of Jupiter-sized objects" (Carroll & Ostlie 1996, p. 897).

Characteristics

Spectra

The spectra typically show strong emission lines from Ha (Balmer series), Ca II at 3933Å and 3968Å, and sometimes iron also. Absorption lines of lithium are typical.

Many T Tauri star spectra exhibit [O I] and [S II] forbidden lines. Forbidden lines, generally, are understood to indicate very low gas densities.

Additionally, the Ha line often exhibits a P Cygni profile: superimposed on a rather broad emission peak is an absorption trough at the violet (short wavelength) edge of the line. The P Cygni profiles arise from significant mass loss. "In some extreme cases, line profiles of T Tauri stars have gone from P Cygni profiles to inverse P Cygni profiles (redshifted absorption) on time scales of a few days, indicating mass accretion rather than mass loss. Mass accretion rates appear to be on the same order as mass loss rates. Apparently the environment around a T Tauri star is very unstable" (Carroll & Ostlie 1996, p. 471).

Occurrence

Sometimes they can be found in very young open clusters, such as NGC 2264, the open cluster associated with the famous cone nebula in Monoceros. This cluster probably started forming about two million years ago and the hottest (most massive) stars of the cluster have already arrived at the main sequence. However, stars cooler than about 10,000K mostly fall above the main sequence. "Spectroscopic observations of the cooler stars in NGC 2264 show that many are vigorously ejecting gas, a very common phenomenon in most stars just before they reach the main sequence. Such gas ejecting stars are called T Tauri stars.... Some astronomers suggest that the onset of hydrogen burning is preceded by vigorous chromospheric activity, with enormous spicules and flares that propel the star’s outermost layers back out into space. In fact, an infant star going through its T Tauri stage can lose as much as 0.4 solar masses before it settles down on the main sequence" (Kaufmann p. 258).

Mass and Luminosity

Low mass (0.5 to 3.0_¤)

Variability/Mass Loss

These stars are noted variables: T Tauri itself varies erratically in brightness from about 9^th to about 13^th magnitude although it rarely fades below 10^m.6. Brightness and spectral type variations for some other examples are noted in table 1.

The P Cygni profiles indicate mass loss rates in the vicinity of 10^-8 to 10^-7 M_¤/year - although these estimates are still quite uncertain - via stellar winds, with typical v_¥ = 100 km/s.

Summary

<maybe the comparison table as in SpeTypB>

Interpretation

As the protostar contracts and accretion continues, but before hydrogen burning commences in the core, intense stellar winds develop. The winds may disrupt the cloud, allowing the protostar to be observed. T Tauri stars are presumed to be solar-mass, pre-main-sequence stars at roughly this stage of evolution. They are thus very young stars, still in the process of gravitational contraction, and yet to evolve to reach the main sequence.

T Tauri stars are, if sufficiently young, in a fully convective stage of evolution.

Between the T Tauri phase (up to a few million years old) to hydrogen ignition (several tens of million years), the following events take place:

• the main rotational evolution of the star: the objects accelerate (or not) from the T Tauri relatively slow rotation rate to the possibly high rotation rates of the stars in young clusters like a Per. The theory of angular momentum transfer through the star and the role of the protostellar disk (e.g. Bouvier 1994) remains unclear;
• lithium depletion: nuclear burning of lithium occurs in the low-mass stars prior to arrival on the main sequence, though the amount of depletion is still not well-understood (e.g. see D'Antona & Mazzitelli 1997; Martín 1997);
• the transition from the Hayashi convective track to the 'radiative' track which approaches the main sequence, with associated X-ray emission.

The presence of an accretionary disc has been noted several times already. The presence of this disc collimates the intense stellar winds into bipolar outflows or jets. As the jets expand supersonically into the interstellar medium, collisions excite the gas, resulting in bright object with emission spectra. These are known as Herbig-Haro objects, after George Herbig and Guillermo Haro who, in the 1950s, first discovered such objects in the vicinity of the Orion nebula. (For more information, see Carroll & Ostlie 1996, pp. 471-475.)

Examples

RY Tauri

Data from Favata et al. 1998, Table 2: HIP = 20387, B-V = 0.915, d = 103-188 pc.

T Tauri

The prototype, T Tauri, located about 1.8° west north west of e Tauri, varies erratically in brightness from about 9^th to about 13^th magnitude although it rarely fades below 10^m.6.

Data from Favata et al. 1998, Table 2: HIP = 20387, B-V = 0.915, d = 103-188 pc.

TV Crateris

At m_v = ~8.9, one of the brightest T Tauri stars.

V0987 (HD 283572)

One of the brightest and best studied weak-lined T Tauri stars is V0987 in the Taurus cloud, one of the nearest active star-forming region. Its evolutionary status on the basis of the Hipparcos parallax has been studied by Favata et al. 1998. The parallax for V0987 is p = 7.81 ± 1.30 mas, corresponding to a distance of ~128 pc (implying MV=3.48), with a  range of 110-154 pc, and a range in absolute magnitude of 3.81-3.08, or a luminosity range of a factor of 2, and correspondingly large uncertainties in the model age and mass of the system.

FU Orionis

FU Orionis and similar stars comprise a particularly violent subclass of T Tauri stars, exhibiting variable mass loss rates 100-1000 times greater than is typical. It has been suggested that these objects get some of their energy from the infall of Jupiter-sized objects. (After Carroll & Ostlie 1996, p. 897.)

References

Bouvier J. 1994: In J. P. Caillault (ed.), 8th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, ASP Conf. Ser. 64, Astronomical Society of the Pacific, San Francisco, 151

Carroll, Bradley W.; Ostlie, Dale A. 1996: Introduction to Modern Astrophysics. Addison Wesley.

D'Antona F., Mazzitelli I. 1997: In G. Micela, R. Pallavicini, S. Sciortino (eds.), Cool Stars in Clusters and Associations: Magnetic Activity and Age Indicators, Vol. 68 of Mem. Soc. Astron. Ital., 807

Favata F.; Micela G.; Sciortino S.; D'Antona F. 1998: The Evolutionary Status of Activity-Selected Solar-Type Stars and of T Tauri Stars as Derived from Hipparcos Parallaxes: Evidence for Long-Lived T-Tauri Disks? Astronomy & Astrophysics vol. 335: 218-227.

Kaufmann, William J. 1990: Discovering the Universe. Freeman 429 pp.

Martín E. L. 1997: In G. Micela, R. Pallavicini, S. Sciortino (eds.), Cool Stars in Clusters and Associations: Magnetic Activity and Age Indicators, Vol. 68 of Mem. Soc. Astron. Ital., 905

Roddier, C.; Roddier, F.; Graves, J.E.; Northcott, M.J.; Close, L.; Surace, J.; Veran, J.P. 1999: Four-Year Observations of T Tauri with Adaptive Optics. ESO Conference Proceedings, vol. 56, p. 389.

Zeilik, Michael 1991: Astronomy: The Evolving Universe (6th ed). Wiley.

Further Reading

HL Tau - see Close et al. 1997a, ApJ, 487, 766, 1997 also Weintraub et al. THE ASTROPHYSICAL JOURNAL, 452:L141L145, 1995 October 20

FU Ori Stars - see Murdin & Penston (1977) MNRAS 181, 657; Grasdalen (1973) ApJ 182, 781; and Racine (1968) AJ 73, 233. Other data are from the compilation of Bell, Lin, Hartmann & Kenyon (1995) ApJ 444, 376.

V0987 Tau - F.M.Walter, A.Brown, J.L.Linsky, A.E.Rydgren, F.Vrba, M.Roth, L.Carrasco, P.F.Chugainov, N.I.Shakovskaya, C.L.Imhoff, ApJ 314, No.1,297,1987.

EW Eri - G.Sandell, B.Reipurth, G.Gahm, AsAp 181, No.2, 283, 1987.