Sky & Telescope October 1981 [antikvár]

Are We Beginning To Understand T Tauri Stars?Martin Cohen, NASA-Ames Research CenterAstronomy changes rapidly in L response to new technology. In this era of satellites and large ground-based observatories it is no longer sufficient to be interested in a single spectral region or technique. The "big picture" is crucial. Only by collating observations made across the spectrum and by diverse techniques can we hope to gain major new insights. At a meeting in April, astronomers glimpsed for the first time the full observational complexities of T Tauri stars and were confronted by a major dilemma regarding the possible phenomena likely to occur in them.It was a very comfortable and productive gathering: 16 of us, drawn mostly from European institutions, in a hotel by a long windy beach at Ofir. north of Porto, in Portugal. In sharp contrast to giant international meetings or three-week workshops, our group was small enough for fruitful yet informal exchanges and diverse enough to consider the whole range of phenomena associated with T Tauri stars. Within the past two years X-ray, ultraviolet, and radio studies have taken their place alongside earlier optical and infrared observations of these objects. The opportunity to stack up all these different data side by side is what rendered this workshop so stimulating, exciting. and topical.T Tauri stars all vary irregularly in brightness, occur in intimate association with dusty molecular clouds, and exhibit bright optical emission lines, principally of hydrogen, helium, and iron. The youth of these stars is indicated by their environments. by the presence of often substantial excess infrared emission, and (now that a large survey of the class is available) by their location well above the main sequence in the Hertzsprung-Russell diagram. Stars with masses roughly 0.2 to three limes the Sun's and with ages from 100,000 to 1,000,000 years typify the T Tauri regime. A feeling prevails that the often rich emission-line spectra seen at visible wavelengths indicate the presence of a stellar chromosphere, where the temperature rises steeply from the usual cool pho-tospheric values around 4.000 to 5.000° K.What are the consequences of this chromosphere? Normally we resort to a study of the shapes of emission lines to answer this question. But the vast majority (70 percent) of T Tauri line profiles involving hydrogen-alpha (at 6563 angstroms wavelength and the strongest optical line) are symmetric and contain no clue as to whether an infall or outflow of material is taking place. Only five percent of the profiles exhibit unequivocal P Cygni shapes.whereby a blue-shifted absorption component indicates outflowing gas.In some stars the profiles change along the lines of the hydrogen series, from a complex double-peaked structure to an apparent inverse P Cygni shape (with a red-shifted absorption that sugggests outflow). We even know of cases where the same line alters in time from P Cygni to inverse P Cygni and back again. The safest and most logical model is one in which material can be recycled; that is. it leaves the stellar surface, moves out. cools, and then falls backlike the giant arched prominences so spectacularly tossed up by the Sun.Yet the interpretation of the profiles is apt to lead to arguments as heated as the chromospheres themselves. For example, imagine that the maximum radial velocity indicated by the hydrogen-beta line represents a physical velocity of some sort; then, in perhaps half of the well-studied stars, it exceeds the likely escape velocity. That is. half of the stars seem to be blowing off material by means of a wind.The true situation probably defies simple modeling, as the series of spectra for DR Tauri shows (below). Here on five nights spanning four months we see the region between hydrogen-beta (the strong line at left) and singly ionized iron (Fell. 5018 ang-