An historical perspective

The Astronomy Quarterly

IN SEARCH OF THE T TAum STARS An Historical Perspective

Catherine L. Imhoff

The T Tauri stars are a puzzling group of variable stars; they pose a real challenge to the astronomer who must try to explain and relate their myriad idiosyncracies. The significance of the T Tauri stars, however, goes far beyond their odd character traits - they are stars in the process of formation. Shrouded in natal clouds of gas and dust, their nuclear fires have not yet begun to burn and sustain the stars. Meanwhile they are still contracting and growing hotter. Thus the T Tauri stars occupy a key position in our understanding of how stars evolve. The theoreticians have given us an excellent idea of the processes involved in star formation; yet when we compare the theoretical model stars to the real T Tauri stars, we are struck by the profound differences. The observations indicate that processes such as mass loss, shock waves, rapid rotation, and stellar winds may be going on in young stars. Like childhood trauma;, these processes can affect the rest of the lives of the stars. We must unravel all the mysteries of these stars if we want to completely understand the formation of the stars, including the sun and its planetary system. Ideas on T Tauri stars have come and gone with great frequency in the last thirty years. To this date, no comprehensive picture of the stars which can explain the sum of their characteristics has arisen - at least not one agreed to by aU parties concerned. Perhaps it is time to step back and survey Our knowledge of the T Tauri stars, using the well-known benefits of hindsight. The intent of this informal review is to consider the major observations and ideas that have COntributed to our understanding of the T Tauri stars. It is hoped that this

213

historical perspective can then help us choose from among the diverging paths that the investigations of the T Tauri stars have taken. Discovery The story of the T Tauri stars goes back only to 1945. Alfred H. Joy, of Mt. Wilson Observatory, was examining a number of stars which have hydrogen Ha '" emission. Among these stars were several that stood out because of their many strong emission lines other than Ho:. Joy set aside eleven of these stars as a new class of peculiar variable star (1). Joy, who died in 1973, left behind a reputation as one of astronomy's foremost stellar spectroscopists. He worked for many years at Mt. Wilson Observatory, doing important research on spectroscopic parallaxes, radial velocities, Cepheids, M dwarfs, and, of course, T Tauri stars (2). T Tauri, a tenth-magnitude variable star, was chosen as the prototype for the new class of stars. In 1945 Joy described the characteristics of these few stars as the following (1): 1. They are irregularly variable, changing in brightness by as much as several magnitudes in a few days. 2. Their spectra roughly resemble that of the sun (spectral types F5 to G5) but include numerous emission lines similar to those seen in the solar chromosphere. 3. The stars appear to be of low luminosity, probably dwarfs on the main sequence. (This point is now known to be incorrect.) 4. The stars are found near bright and dark nebulae. The most striking characteristic of the T Tauri stars is the unusual and unique appearance of their spectra. Therefore it would be worthwhile to spend some time looking at their spectra in detail. There is a marked difference in the appearance of spectra of normal stars and those of the T Tauri stars, as may be seen in Figure 1. The first spectrum is that of (] Ophiuchi, a normal KO dwarf. It shows numerous lines (appearing as light lines in the figure) corresponding to the absorption of light by chemical elements in the outer, cool layers of the star's atmosphere. The particular behavior of the lines of the various elements depends upon the star's surface temperature, pressure, and chemical composition. In stars like the sun and a Ophiuchi, the spectral lines are fairly narrow and largely due to low excitation lines of iron, nickel, titanium, chromium, and other metals. The two strong lines at the left, called Hand K, are due to ionized calcium. One broad feature, known as the G·band, is primarily due to the molecule CH. The spectra of two T Tauri stars, T Tauri itself and RW Aurigae, include approximately the same assortment of absorption lines because they are roughly the same temperature as a Ophiuchi. However, the lines appear broadened or "washed out." In the spectra of some other T Tauri stars, the effect is so "'H 0: refers to the strong hydrogen line at 6563..\. Other lines in the series are designated by H {3 , H'Y , etc.

214

tv

......

CJl

Figure 1.

l CH

LH r

LH~

The wavelength increases from left to right. Emission lines due to ionized calcium and the Balmer series of hydrogen are indicated. The spectra were obtained at Perkins Observatory.

KOV) and two T Tauri star spectra (T Tauri and RW AurigaeL

A comparison between a normal stellar spectrum (a Ophiuchi,

CA][H+H.

CA][ K

II

LH3

RW AURIGA

T TAURI

KO V

extreme that no absorption lines can be found. In contrast, a host of bright emission lines (appearing as black lines in the figure) stand out. These include the Balmer lines of hydrogen (H f3 ' H'Y , H 0 , and H € ), the ionized calcium H and K lines, and many weak~r lines due to neutral and ionized helium, iron, titanium, chromium, and other metals, and forbidden'" ionized sulfur lines. Sometimes the combination of absorption and emission leads to asymmetric or complex profiles in the stronger lines (as seen in Call K, H'Y ' and H f3 for RW Aurigae in Figure 1). The detailed appearance of the emission lines varies from star to star, ranging from numerous strong lines, as in RW Aurigae, to a few weak lines. The emission lines seen in the T Tauri spectra indicate the presence of hot gases in the vicinity of the stars. Such lines are characteristic of the chromosphere of the sun, as noted by Joy. Bright lines are also seen in the spectra of many kinds of nebulae and peculiar stars. For instance, the Be stars are thought to be surrounded by rings of hot gases which add strong emission lines of their spectra. So the T Tauri stars began as simply a small group of peculiar variable stars with emission lines. Little was known (or guessed) about the nature of the stars or the source of their odd characteristics. Explanations

Soon after Joy delineated the class of T Tauri variable stars, the question arose concerning the reason for their peculiarities. An early speculation was that the T Tauri stars were ordinary stars moving through the surrounding dark nebulae and sweeping up material in the form of gas and dust. The impact of such material falling onto the surface of a star might account for its emission line spectrum and its variability. In the 1950's an alternative view of the T Tauri stars, fust set out by the Russian astronomer, Victor A. Ambartsumian (3), gained acceptance. He argued that the T Tauri stars are young stars in the process of formation and that whatever peculiarities they have are somehow connected with their youthfulness. Confumation of this view came in 1956 with the work of Merle F. Walker (4). It was known from the theoretical work by Louis G. Henyey and his co·workers (5) that when a star forms it is luminous but cool at first. As it contracts, its temperature increases until hydrogen fusion can begin at the center of the star. It can then become a stable, main sequence star. Young, newly formed stars are thus expected to fall to the right of the main sequence in the Hertzsprung-Russell diagram. Walker observed the colors and brightnesses of many of the stars in the very young stellar association NGC 2264 (4). He found "'''Forbidden'' lines do not appear in ordinary terrestrial or stellar conditions but can occur in very thin hot gases such as planetary nebulae and ionized hydrogen clouds.

216

that most of the stars, including many T Tauri-like stars, fell in a broad band to the right of the main sequence, the region where young stars were expected to lie. Walker's color-magnitude diagram is depicted in Figure 2. Walker's observations of this and other clusters showed that the T Tauri stars could be only a few million years old (the sun is nearly 5 billion years old)_ The stars therefore must be gravitationally contracting, still several million years away from the main sequence. Even with their youth assured, however, the peculiarities of the stars were not quickly explained. In fact there were more surprises to come. Description

By 1962 over one hundred new stars had been added to Joy's list, largely through the efforts of George H. Herbig, working at Uck Observatory. Herbig, both for his own work and for the encouragement he has given to others in the field, probablY more than anyone else deserves the title of "Mr. T Tauri Star." Due to their uniqueness, the identification of the T Tauri stars depends upon an examination of their spectra. Herbig gave the criteria needed to unambiguously assign a star to the class of T Tauri as the following (6): 1. The Balmer hydrogen lines and Call Hand K lines are in emission. 2. The' flourescent iron lines at 4063 and 4132A are present in emission. 3. The forbidden ionized sulfur lines at 4068 and 4076A are usually in emission. 4. The lithium absorption line at 6707.A is strong. H is necessary to give criteria of this sort because of the host of variable and emission-line stars found in the same regions as the T Tauri stars: the Herbig-Haro objects, the Herbig Ae and Be stars, the RW Aurigae variables (by coincidence RW Aurigae is also a T Tauri star), stars in cometary nebulae, stars with only HO:' emission, and eruptive young stars such as FU Orionis and VI057 Cygni. Many of these objects may indeed be related or similar to the T Tauri stars. A list of some typical T Tauri stars is given Table 1. Table 2 gives the characteristics of some of these possibly related "Orion population" objects for comparison. Herbig reviewed the status of the T Tauri stars in 1962 (6). By that time, several new items had been added to the list of characteristics of the T Tauri stars. For instance, more restraints were put to the location of the stars. The T Tauri stars are found only in the densest portions of dark clouds where other young stars are found. In Orion, for example, the T Tauri stars are found not in the bright emission nebula but in the very dark clouds surrounding it. The stars tend to occur in groups, known as T associations (in analogy with OB aSSOciations). Herbig investigated the problem of the unusual widths of the absorption lines in the spectra of several T Tauri stars. He showed that a possible explanation is that tl1e stars are rotating rapidly, with projected velocities of 20 to 65 km/sec

217

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Fig~re 2. Color-magnitude diagram for the young cluster NGC 2264. V is the visual magnitude and B-V is the color of the stars, with blue to the left and red to the right in the diagram. The line represents

v

5

TABLE 1 Some Typical T Tauri Stars

Star

Spectral type

Emission strength

V

B-V

U-B

BP Tauri

11.97

0.89

-0.18

dK5e

4

DE Tauri

13.05

1.33

-0.10

dMle

3-4

RY Tauri

10.79

1.04

0.56

G5e

2

T Tauri

10.37

1.24

0.74

Kle

2

DF Tauri

11.66

0.94

-0.22

dMOe

4

DG Tauri

11.62

0.97

-0.34

G:e

5

SU Aurigae

9.24

0.89

0.42

G2neIII

1

RW Aurigae

10.81

0.71

-0.07

dG5?e

5

GW Orionis

9.72

0.99

0.30

dK3e

2

YY Orionis

13.55

0.86

-0.39

G-Ke

2

Data from Herbig and Rao (17). Line emission strengths are assigned by Herbig on the basis ot the appearance of the star's spectrogram, ranging from 1 (only Ha emission) to 5 (many strong lines).

219

0

t-:) t-:)

X

X

X

X

X

X

X

X

X

Herbig-Haro objects

X

X

Variable

FU Orionis stars

X

Flare stars

X

UV

excess

X

X

Herbig Ae-Be stars

X

Infrared excess

RW Aurigae (Is) variables

X

Emission lines

T Tauri stars

Object

Characteristics of Some Orion Population Objects

TABLE 2

X

X

X

Dark nebula

A-F

F-M

M

B-F

F-M

Spectral type

(the sun's rotational velocity is only 2 km/sec). As th~ stars contract on their way to the main sequence, this rotation will speed up, approaching the large velocities observed for stars hotter than the sun (A and F spectral types). However, rotation is not the only mechanism that can account for the width of the lines, so the question of rotating T Tauri stars was left somewhat in doubt. The strength of the lithium absorption line is another characteristic of the T Tauri spectra. It is so strong that the stars must have over a hundred times the amount of lithium in their atmospheres than does the sun. This fact is now thought to be due to the stars' extreme youth. Lithium is an element which is destroyed by nuclear reactions at temperatures of about a million degrees. Although the surface of a star is never this hot, the interior can reach 15 million degrees (for a star like our sun). So convective motions in the envelope (outer layers) of a star can mix the cool lithium·rich gases near the surface with the hotter deeper layers in which the lithium has been destroyed. The process goes on gradually, and the amount of surface lithium decreases with time. As a result the strength of the lithium line diminishes. This process has been observed in stars of various ages, beginning with the youthful T Tauri stars and their strong lithium lines and ending with the 5 billion year old sun, whose lithium line is at the limit of detectability. As eadyas 1945, Joy noted that the ultraviolet light from the T Tauri stars seems especially strong compared to normal stars. This phenomenon is known as the "ultraviolet excess." This extra ultraviolet radiation appears around 3700A and increases rapidly to shorter wavelengths. In addition, there appears to be a SOurce of weak continuous emission, known as the "blue continuum," that affects some of the blue spectral lines. A number of explanations were advanced for the ultraviolet excess including synchrotron radiation, a leading contender at the time of Herbig's review. In retrospect some of those ideas seem farfetched, but at the time the ultraviolet excess was quite mysterious. The First Model By the mid 1960's, the T Tauri stars had accumulated quite a list of peculiarities. The established youth of the stars was very interesting but did not explain those peculiarities. On the other hand, the occurrence of strong emission lines, sometimes velocity shifted in wavelength, suggests that the stars could be lOSing mass. The emission lines could then be envisioned as arising from a shell of hot gases flowing away from the star. This hypothesis was explored by Leonard V. Kuhi (7) in 1964. Kuhi constructed models using this picture of the T Tauri stars to explain the shapes and velocity shifts of the emission lines. He found that the emission lines arise from a shell of hot gas expanding as fast as 300 km/sec away from the star. The gas slows as it expands outward to a region of cooler gas which absorbs Some of the light emitted by the inner shell. The two regions expanding at different velocities give rise to a characteristic line profile seen in several T Tauri stars. An illustration of Kuhi's model is seen in Figure 3.

221

COOL GAS

-

-

100 KM/SEC

obsorption

Figure 3. A simple representation of Kuhi's model for a T Tauri star. S is the star surrounded by expanding clouds of hot and cool gas. Not to scale.

222

In this way Kuhi showed that a T Tauri star could be losing up to 10-7 of a solar mass per year. If this mass loss continues over a million years or so, the star could lose 10 or 20% of its original material before it ever reaches the main sequence. This would slow down the star's evolution and probably slow down its rotation as well. Kuhi's picture of a T Tauri star as surrounded by expanding hot gases has had a strong effect on our conception of the stars. It appeared to solve the major problem of finding the source of the emission lines. The model was especially satisfying for its fit to the complex line profiles seen in some T Tauri stars. However, some T Tauri stars are not all T Tauri stars. Merle F. Walker (8) pointed out that there are other T Tauri stars whose line profiles differ from those studied by Kuhi, calling this subgroup the YY Orionis stars. In fact, their profiles appear to indicate mass inflow, not outflow. It seems odd that a class of stars would have members in which two completely opposite processes are going on. The picture is complicated by the fact that, in some stars, the line profiles have changed from one kind to another. Added to this problem is the question of why the cool T Tauri stars would be surrounded by large shells of hot, expanding gas.

A New Light In 1965 the Mexican astronomer Arturo Poveda (9) suggested that, ifT Tauri stars were indeed young stars, they might develop planetary systems like the sun's. In the early stages of the "solar system", however, the stars would be surrounded by dense clouds of dust, small particles of matter which would eventually condense into planets. The dust would be warmed by the nearby star and glow in the infrared. In 1966 a new discovery concerning the T Tauri stars was made that literally - shed a new light upon them. Eugenio E. Mendoza, Poveda's colleague in Mexico, observed a number of T Tauri stars with an infrared-sensitive photometer. Surprisingly the infrared radiation was not only present in the stars but extremely strong (10). The amount of infrared was far in excess of what would be expected from any normal star. Frank J. Low and Bruce J. Smith (11) and Mendoza (12) interpreted this to mean that there are extensive dust shells around the stars which give rise to the infrared radiation. An illustration of the energy distribution coming from a T Tauri star is given in Figure 4. The distribution of T Tauri is compared to that of a normal K2 dwarf. The wavelength scale has been compressed; somewhat concealing the fact that 2/3 of T Tauri's energy is emitted in the infrared. A normal star of moderate temperature puts out very little infrared compared to its visible radiation. Several points may be made concerning the unusual energy distributions of the T Tauri stars. First, the previous estimates of the luminosity (total energy per second) coming from the stars were far too low. Adding in the infrared energy raises the luminosities by factors of 3 or more. Second, the T Tauri stars are reddened; that is, their energy distributions have been affected by the

223

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TAURI

0.4

0.3 X ::l ..J

LL.

0.2

0.1

0.0 L.---L---L._....L.-J~..u..._---1-~---L--l-.L.L.J-'--~-30 0.1 0.3 I :3 10

WAVELENGTH (MICRONS)

Figure 4. Energy distribution of T Tauri compared to a normal K2 dwarf; note that most of T Tauri's energy is radiated in the infrared.

224

presence of dust. This well-known effect results in energy being absorbed in the blue portion of the spectrum and being reemitted into the infrared_ However, it is not immediately clear how much of the reddening is due to interstellar dust (dust clouds scattered between us and the star) and how much due to circumstellar dust (a shell of dust surrounding the star). A few years ago Mendoza and I tried to separate the interstellar from the circumstellar reddening affecting several T Tauri stars (10). The luminosities we derived for the T Tauri stars were a little lower than Mendoza's original estimates but they were still from 4 to SO times the luminosity of the sun. Knowledge of the T Tauri stars' luminosities allowed us to make rough estimates of their radii and masses, based on theoretical models. In general the stars are 2 to 6 times the solar radius in size and have from 1 to 3 times the mass of the sun. The models also indicate that the stars are from 1 to 5 million years old. Thus the stars have 99% of their lives yet to go. The locations of several of the brighter T Tauri stars (and some related hotter stars) in the Hertzsprung-Russell diagram are given in Figure 5. Our conception of the T Tauri stars now required at least two very different regions. One region, near the star, consists of hot (10,000"K) gas in emission. The other region contains the dust which must be much cooler, perhaps 700"K. The dust would presumably lie in a shell or disk at some distance from the star. It had become clear that the T Tauri stars could not be represented in any simple way. One Mystery Solved Leonard Kuhi continued to observe the T Tauri stars, using the Wampler Scanner at the Lick Observatory. This device allowed him to obtain photoelectric measurements of small portions of the spectrum, including individual strong emission lines and regions of the star's continuous spectrum, over the entire visible spectrum. These observations revealed a strong correlation between the ultraviolet excess and the strnegth of the Ha emission for a number of T Tauri stars (14), seen in Figure 6. Such a correlation between the ultraviolet and the hydrogen emission lines suggests a common source. Kuhi suggested in 1968 (IS) that the ultraviolet excess was largely due to Balmer continuum emission, arising from the capture of free electrons by ionized hydrogen atoms, in the same shell of hot gas giving rise to the emission lines. Furthermore, he showed from the strengths of the individual Balmer hydrogen emission lines that the temperature of the hot gas is around 1O,000"K and its density about 106 to 10 10 atoms/cm 3 . Balmer contiuum emission is a very reasonable explanation for the ultraviolet excess since strong Balmer line emission has long been observed. Thus the problem of the ultraviolet excess was quietly solved. In fact, of all the Conclusions drawn about the T Tauri stars in recent years, this is one of the most secure. Still, there remains one disquieting observation. The correlation between the ultraviolet excess and the H a emission is seen when comparing one star against another. However, if one looks at the variations of these quantities with time in

225

0.1

10,000

32,000

4,000

TEMPERATURE (OK) Figure 5. The theoretical H-R diagram for several T Tauri stars (filled circles) and a few related hotter stars (open circles). The luminosity in solar units is plotted against surface temperature. The lines represent the paths stars of certain masses are expected to take in the diagram as they evolve to the main sequence, moving from left to right. The numbers at the end of each path indicate the masses (in solar masses) of the stars.

226

25

en en 2.0

w

-

I-

/-

W

I-

1.5

w

-

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0

~ 1.0

1/

0:

~

:::> 05

I

.---

~

\

-\

0.0 l-

r I

0

5

10

15

STRi::NGTH OF Ha

Figure 6. The correlation between the ultraviolet excess and the strength of H a emission in T Tauri stars. Lines connect two observations of the same star on different dates.

227

a single star the correlation is not so good. If the same region gives rise to both the ultraviolet and line emission, one would expect their variations to occur in step with each other. This minor point may prove to be unimportant, or it may be symptomatic of problems yet undetected. Theory Confronts Observation

The problem of theoretically describing the collapse of a protostar is a difficult one. A commonly used simplification is to assume that the star contracts homologously; that is, all the parameters describing the star decrease in the same way, like the shrinking of a balloon. However, new calculations which did not make use of that assumption quickly showed that this view of the protostellar collapse is inaccurate. In 1969 Richard B. Larson (16), working on his doctoral dissertation at the California Institute of Technology, examined the gravitational contraction of a protostar in detail. He showed that the center of the protostellar cloud collapses quickly. TIlls dense core becomes a small central "star" surrounded by a large cloud of gas and dust. The core grows gradually as the gas and dust fall inward into the center. The infalling matter causes a shock wave to develop near the core, where the material is heated and compressed. Most of the time that this is going on the core is hidden from observation by the dense surrounding cloud. However, one might expect the dust to be warmed by the star and radiate in the infrared. Later, as the cloud thins, the star itself could finally be seen. By the time this happens the star may have evolved nearly to the main sequence. Very young stars should be observable only through their infrared radiation. In fact, such "infrared stars" have been detected. The T Tauri stars appear to fit into Larsons' scheme of evolution as the stars which are nearing the main sequence and whose dust clouds have thinned enough to reveal the star within. The remnant dust clouds are responsible for the infrared excesses, as described by Mendoza, Low, and Smith. Perhaps the shock wave produces some emission lines. At one point theory and observation come into sharp disagreement. larson's models showed that matter falls into the star during its entire collapse. The rate of infall might decrease with time but there is now theoretical indication that the flow of matter reverses and becomes mass loss. On the other hand, Kuhi's models based on the emission line profiles of several T Tauri stars showed large rates of mass loss (although recall Walker's stars which show infall). It was thus conceded that Larsons' models might be correct for the early development of the protostar, but at some time in its development something happens that brings about the outflow of material from the star. One basic question is at what stage the mass loss might begin in the T Tauri stars. Certainly early in their lives mass infall must have occurred or the starS would not have formed. What causes this to change and mass loss to occur? Perhaps the mass loss is responsible for sweeping away the remnants of the star~s natal cloud of gas and dust, revealing the new star within. One may note in passing the the loss of mass from stars is not an unusual

228

process. Some hot stars, late·type giants and supergiants, and even the sun undergo various rates of mass loss. In the case of the sun, the solar wind carries away only about 10,14 solar mass per year, a minute amount compared to that from the other stars. But what was the solar wind like 5 billion years ago, as the sun was forming? Might the present solar wind be a feeble remnant of a time when the sun was a protostar and perhaps losing mass like the T Tauri stars? Return to Joy

While astronomers debated the merits of mass loss versus mass infall, the number of T Tauri stars which had been discovered continued to grow. In addition, it had been increasingly recognized that other young stellar objects such as Herbig·Haro objects and flare stars might be related to the T Taud stars, differing in degree rather than in kind. So in 1972 George Herbig, assisted by N. Kamaswara Rao (17), published a list of 323 emission-line stars of the "Orion population," a term which includes a variety of such objects. Of these, 156 are listed as T Tauri (or probably T Tauri) stars. The "Herbig and Rao Catalogue" remains the most comprehensive list of T Tauri stars to date. At the same time, Herbig was inquiring into the nature of the peculiarities of theT Tauri stars. In 1970 (18) he suggested that the ultraviolet excess, the emission lines, and even a portion of the infrared excess might be due to the structure of the atmosphere of the T Ta uri star itself, rather than a phenomenon of some relatively distant envelope or shell. The presence of strong convection and mass loss might redistribute the change of temperature in the star's atmosphere so that the hot "chromospheric" layers occur relatively deep in the atmosphere, as compared to the sun. These deep chromo spheric layers would produce Balmer continuum radiation, emission lines, and some infrared radiation. This suggestion would fit together the observed correlation between the ultraviolet and emission lines, the mass loss, and many other of the observed peculiarities of the T Taud stars. A dust shell would still be required to explain most of the infrared excess. The idea of a chromospheric origin for the emission lines and ultraviolet excess has been pursued recently by Simone Dumont, Nicole Heideman, Leonard Kuhi, and Richard Thomas (19) at the Institut d'Astrophysique in Paris. They performed calculations which showed that a T Tauri chromosphere could account for the observed strengths of the hydrogen emission lines, the ultraviolet excess (Balmer continuum radiation), and some infrared radiation (paschen continuum radiation). To explain the line profiles of the emission lines, a mass flow out of the atmosphere is needed, so mass loss is an integral part of this model. A rough picture of the chromospheric model of a T Tauri star is given in Figure 7. An attractive feature of the postulated chromosphere is that it would be expected to be variable. Then the intensities and line profiles of the emission lines would also be expected to be variable, as has been observed. It would be difficult for a large expanding envelope, like that in Kuhi's earlier model, to exhibit such changes.

229

EXPANDING CHROMOSPHERE (10,000° K)

hydrogen lines and continuum

COOL GAS (3000° K)

low excitation and fluorescent lines

DUST (200-1000° K)

infrared excess

Figure 7. A simple chromospheric model of a T Tauri star. Not to scale.

230

The idea of a strong chromosphere also fits in with the known correlation between chromospheres and age. Olin C. Wilson (20) of Hale Observatories showed many years ago that the intensity of Call K emission, an indicator of chromospheric activity, increases as one looks at younger stars. One therefore might expect the younggest stars, the T Tauris, to have the strongest emission and large chromospheres. In fact the occurrence of Call Hand K emission in itself argues for the presence of a chromosphere, since those lines can be formed only in a somewhat dense gas such as a chromosphere and not in a thin, hot gas such as a planetary nebula or an ionized hydrogen cloud. It is somewhat ironic that after 25 years of research on T Tauri stars we have returned to Alfred Joy's original observation that the T Tauri emission lines resemble those of the solar chromosphere. His suggestion may indeed prove to be correct. A New Interpretation

It was generally accepted until a few years ago that the infrared excesses seen in the T Tauri stars were due to thermal dust radiation. However, another source was proposed in 1972 by Steven Strom, at the Kitt Peak National Observatory (21). He noted that there is a fairly good correlation between the intensity of Ha emission and the infrared excess from the T Tauri stars. Likewise, a correlation between the ultraviolet and infrared excesses exists. From this he reasoned that all the emission must arise from a single source, a gas envelope surrounding the star. It is possible for gas to radiate in the infrared through several processes. These include free-free emission (due to an electron passing near an ion) and free-bound emission (capture of an electron by an ion). In addition, free-free emission may occur from the interaction of a free electron with neutral hydrogen, as shown by Melvin Dyck and Robert W. Milkey (22). Strom's suggestion was explored fully by A. Eric Rydgren in his dissertation work at the University of Arizona. Rydgren fitted models including free-free and free-bound emission from a 20,000"1<. gaseous envelope to energy distribution observations of several T Tauri stars (23). For only one star, T Tauri, was it necessary to include a dust emission component in order to fit the observations. Some of the T Tauri stars show emission peaks at 10 and 20 microns*, which have commonly been ascribed to silicate dust grains at about 200"1<.. In addition, he argued that the variability observed in the T Tauri stars in entirely due to changes in the envelope emission. A few salient features of Rydgren and Strom's models should be noted. First, the models were calculated assuming peculiar extinction laws for the dark clouds near the T Tauri stars, as have been suggested for several such regions. The models are dependent upon the validity of this assumption. Second, the models require a very large contribution to the energy from the envelope to be five times the energy from the star in the blue part of the spectrum. *One micron is 1O,000A

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Thc Current Dichotomy

It is common to have several competing theories in the early investigation of astronomical objects. As time goes by, new observations arise which can be used to eliminate the contenders until only one theory - hopefully the correct one remains. For the T Tauri stars, the opposite process has been happening. Roughly speaking, the discussion of the T Tauri stars has fragmented into two camps. One school of thought ascribes the ultraviolet and line emission to a chromosphere and the infrared excess to dust emission. The other identifies all the excess emission as due to hot gas envelopes. (It is interesting to note that Leonard Kuhi, who originated the idea of gas envelope emission, is now involved in the chromospheric models.) There are basically two central questions: is the infrared excess primarily due to dust or gas, and what is the nature of the ultraviolet and line emitting region? What evidence is there for or against the opposing ideas? First one might note that there is good evidence that both gas and dust exist near the T Tauri stars. The ultraviolet and line emission must be due to hot hydrogen gas, whatever the nature of the emitting region. The evidence for mass loss indicates that some gas must also exist since the stars suffer from reddening and extinction. The stars are found to Occur in he darkest portions of the dust clouds. In addition, the silicate dust features at 10 and 20 microns have been observed for several of the stars. The observations suggest a range of conditions near the stars. The emission lines such as hydrogen and helium lines suggest gas at 10,000 to 20,000"K. The Balmer lines require fairly high density. The lower excitation lines suggest cooler temperatures. The Call Hand K emission lines require the dense regions of a chromosphere or shock wave. The forbidden emission lines require a hot but low density environment. The silicate dust emission requires about 200"1(. If the infrared excess is due to dust, that dust must be at about 500 to 1000"1( or so. Finally, in general one expects gas and dust to coexist unless the ambient temperature becomes too high for the dust to survive (about 1300"1( or so). The correlations observed between the infrared excess and the ultraviolet and line emission for a number of stars offer support for the gaseous envelope model. However, the lack of strong correlations for the variability of individual stars argues against the gaseous model and in favor of a several component model such as the chromosphere plus dust shell model. In general the models, both gas envelope and chromospheric, can more or less duplicate the observed energy distributions of the T Tauri stars. Both can produce some of the emission lines and have an explanation for the blue continuum. The gaseous envelope model requires peculiar extinction by the dark cloud dust and large amounts of free-free emission. The chromospheric model requires dust emission to explain the infrared excess. Both require mass loss. In both cases and mechanisms responsible for producing the emitting regions are not well-developed.

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A Pot Pourri

It is evident that careful, detailed work is required to find evidence that can clearly distingUish between the two schools of thought on the T Tauri stars. The arguments are complicated by the many peculiarities and several physical regions that are connected with the stars. In addition, it is always possible that the accepted course of research is on the wrong track and that a completely new concept of the stars is required. A summary of recent work on the T Tauri stars will show how current investigations are progressing. In 1970 the T Tauri star VI0S7 Cygni experienced a large and dramatic rise in brightness of about 6 magnitudes. Only once before had such a phenomenon been observed, when FU Orion is suddenly brightened in 1936. Fortunately VI0S7 Cygni had been well observed both before and after its outburst so that one might hope to explain such an unexpected brightening. Gary Grasdalen, at the University of California at Berkeley, made a thorough analysis of VI0S7 Cygni as his dissertation research (under Leonard Kuhi) (24). He rejected the idea that the dissipation of a dust shell could account for all the observed changes in the star, which had become an A supergiant. He postulated that the star had experienced a fundamental change in its structure which had not been predicted by evolutionary theory. Furthermore, all T Tauri stars might similarly be massive stars in such an unpredicted evolutionary stage and would be expected to experience outbursts in the future. Richard Schwartz, in his dissertation work at the University of W~shington, investigated the emission nebula which is associated with T Tauri (25). He was able to separate the emission line spectra due to an inner hot gas envelope, which gives rise to the Balmer lines and Call Hand K lines, and the surrounding nebula, which emits primarily forbidden line radiation. He suggested that the ultraviolet radiation produced in the hot gas in the envelope close to the star is sufficient to ionize the gas in the outer nebula and produce the forbidden emission lines. The fluorescent iron lines noted in T Tauri stars were investigated by Lee Anne Willson at the University of Michigan (26, 27). She showed that the iron lines at 4063 and 4132A are enhanced due to the coincidence in wavelength of Call H and the Balmer hydrogen line He with another line at 3969A, an effect originally noted by Herbig. Further analysis showed that the fluorscent lines arise perhaps five stellar radii away from the star, where the temperature is about 3000"1{. The profiles of the lines suggest that the- gas is not only expanding but rotating witth the star; in fact, at this distance rotation is more important than mass loss is describing the behavior of the gas. Martin Cohen, at the University of California at Berkeley, found an absorption band in the infrared spectrum of the T Tauri star HL Tauri (28). He argued that it is due to dust grains composed of ice at a temperature of perhaps 150"1{. The other T Tauri stars he examined do not have this absorption feature. He also showed that the T Tauri star infrared excesses could be explained by thermal emission from warm dust near the star, with temperatures of 700 to 1100"1{.

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Pui Kuan, at Kitt Peak National Observatory, investigated the possibility that shock waves in the atmospheres of the T Tauri stars are responsible for their mass loss (29, 30). His models predict a mass loss in fair agreement with Kuhi's early models of mass loss (7). The shock produces a region of high temperature, about 20,000"1(, which gives rise to the Balmer lines and continuous gas emission. In order to explain the strength of the gas emission, the emitting region must be in dense clumps. The variability of the stars may then be explained through the clumpy nature of the mass loss. The nature of the variability of the T Tauri is a basic problem. The correlations (or lack of them) among the variations of the ultraviolet excess, the line emission, and the infrared excess should prove to be a valuable tool in deciding the source (or sources) of the emission. Martin Cohen and Richard Schwartz made cooperative, simultaneous observations of several T Tauri stars in the visual and infrared regions (31). Their observations showed the behavior with time of the ultraviolet excess, the H Ci. emission, and the infrared. In a selection of 22 stars, they found nearly every possible behavior of variability. For instance, in RW Aurigae the ultraviolet, infrared and H Ci. varied in the same manner. However, in BP Tauri, HCi. was correlated with the ultraviolet but anti·correlated with the infrared. In Lk HCi. 198, the ultraviolet did not vary, but HCi. and the infrared both brightened. And so on. It is clear that no Simple, single-component model of the T Tauri stars can explain these variations. One additional suggestion made by Cohen and Schwartz is that scattering by dust may be at least partly responsible for the blue continuum and its veiling of the blue absorption lines. An interesting model for T Tauri stars based on mass infall was developed by Roger Ulrich, now at UCLA (32). In this model, matter falls toward the star under the influence of gravity, spiraling into the surface with a velocity of up to 100 to 300 km/sec. The impact produces a shock wave with a temperature of up to a million degrees. Balmer line and continuum emission is produced as the shock cools. The variability of the star may be accounted for by variations in the amount of matter falling into the star. The line profiles produced in this way resemble the line profiles observed by Kuhl (7). It is indeed surprising that two such opposite processes as mass loss and mass infall can produce similar line profiles. Back to Basics

There are many important questions concerning the T Tauri stars that remain unanswered; still, progress has been made in recent years that makes one optimistic about the future. Some of that progress seems to have occurred in a rather round·about way, with old discarded ideas resurrected and revamped. The return to the idea of chromospheres in the T Tauri stars is one; another is Ulrich's infall model, which brings to mind the early idea that the T Tauri stars swept up material in the dark clouds. It is clear that it would be worthwhile to reconsider some of our most basic ideas about the T Tauri stars. Let us stop and ask some very fundamental questions. 234

1. Is the T Tauri stage a normal phase of evolution? It is often assumed that all protostars, at least those in some mass range like 0.5 to 4 solar masses, go through a "T Tauri phase." We have little reliable statistical information to support this. In fact, in young clusters we see T Tauri stars, Ha emission stars, variable stars without emission, and non·variable stars intermingled. It is entirely possible that there is something quite unique about the T Tauri stars that sets them apart from other protostars. Such factors might be the occurrence of strong magnetic fields, rapid rotation, or the influence of surrounding dense gas and dust clouds. A related question asks how long such a T Tauri phase might last. Most assumptions are around 105 to 10 6 years. Clearly the stage cannot be very short or else very few stars could ever be seen as T Tauri stars. At the other extreme, the phase cannot last longer than the time it takes the star to evolve to the main sequence (l to 25 million years, depending on the mass of the star). 2. What is the evolutionary state of the T Tauri stars?

Evidence has been given by several authors that the T Tauri stars have evolved sufficiently to develop a structure approaching that of the main sequence stars, although nuclear reactions have not begun. This is largely based on the comparison of the stars' temperatures and luminosities with the theoretical predictions for protostars. However, if the FU Orionis and VI057 Cygni phenomenon is a common feature of the T Tauri stars, then all our assumptions must be carefully reexamined. 3. Is the T Tauri stage a unique phenomenon?

The variety of appearances and behaviors of the T Tauri stars makes one question whether they form a truly homogenous group. Isn't it possible that two or three different phenomena give rise to a "T Tauri" spectrum, and that these stars have been lumped into a single class? The grouping of stars into "in fall" and "mass loss" stars of ten years ago (the stars studied by Kuhi and by Walker) suggests that this may be so. Such a situation would make things unpleasant for those of us trying to explain the T Tauri phenomenon; one hopes that this is not so. However, you may recall the discovery that the Cepheid variables was composed of both Population I and Population II stars; that resulted in a change of a factor of two in the distance scale of the universe! So the possibility that the T Tauri group is heterogenous should not be ignored. 4. Where are the T Tauri binaries?

Among stars in general, there is a high proportion of binaries and other multiple star systems. It has been noted, however, that among the T Tauri stars there are few binaries. A handful of visual binaries are known, but no spectroscopic binaries. The peculiarities of the stars' spectra make it difficult to detect a spectroscopic binary, but one might expect a few to have been found. A

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suggestion has been made that all T Tauri stars are spectroscopic binaries, although this thought has not been explored. The interaction of two protostars, with perhaps a communal envelope, might then account for the emission lines and other peculiarities of the T Tauri stars. 5. Are the peculiarities of the T Tauri stars intrinsic or extrinsic?

In other words, does the excess radiation arise due to the nature of the stars themselves, as suggested by Herbig (18), or does it come from envelopes or shells, as proposed by Strom (21) and Rydgren (23)7 This comes back to the questions of the origin of the infrared excess and the location of the ultraviolet and line emission. A related question is the source of the stars' variability. In general, it is assumoo that the variations are due to changes in the dust shell or gas envelope, since there is no reason to suspect that the stars themselves are intrinsically variable. However, until we understand the exact evolutionary stage of the T Tauri stars, we cannot exclude the possibility that at least some of their variability is due to the star itself. The FU Orionis and V1057 Cygnus outbursts indicate that this may be so. 6. What kind of mass flow exists near the stars?

Both mass loss (outflow) and in fall have been postulated to explain the T Tauri stars. In addition, rotation of the envelope near the star has also been suggested. The answer to this question is fundamental to our understanding of the stars and how they react with their environments. 7. Will the T Tauri stars have planetary systems?

This question again returns to the evolutionary state of the stars. The observations, compared to the theoretical predictions in the usual way (ignoring mass loss, V1057 Cygni, etc.) indicate that the T Taud stars will eventually become A, F, and G main sequence stars. The A and early F stars rotate rapidly and are thus thought not to have lost any angular momentum to planetary systems. It seems possible that those which become G stars, like the sun, may develop planetary systems. One might argue that the planetary condensations must occur fairly early in the star's development. It seems unlikely that planetary formation could occur in a hot expanding gas envelope, although the envelope expected near aT Tauri star should be small in extent compared to the dImensions of the planetary system. However, mass loss could stir up the distant gas and dust, making condensation difficult. Presumably the regions of planet formation should be cool and dense, reSUlting in effective obscuration of the star. By the time the nebula thins to reveal the star, the planets might be largely formed. Later the nebular gas and dust (and the inner planets'lighter gases) will be largely swept away. A careful comparison between research on the early solar nebula and the observations of the T Tauri stars should be illuminating. Such a goal is behind a conference on "Protostars and Planets," to be held at the

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University of Arizona Lunar and Planetary Laboratory in January 1978. The conference seeks to bring together experts in both star formation and solar system studies in order to exchange ideas and answer some of the many questions we have in common. What kinds of investigations can shed some light onto these questions about the T Tauri stars? Here are just a few suggestions. A great deal remains to be done on the variability of the T Tauri stars. Cohen and Schwartz's observations (29) were made during the same night but not completely synchronously. The T Tauri stars can vary on time scales from months to minutes, so there is some question as to how simultaneously the observations must be made in order to be meaningful. The observations should also include spectroscopic information to monitor changes not only in line strengths but profiles and velocity shifts. If the profiles are largely constant, then the processes giving rise to the emission may be construed to be fairly steady. If they are greatly variable then the emitting regions must be reasonable small and unstable. The details of the variations could provide evidence for or against the various theories of mass flow. Observations of the T Tauri stars in the ultraviolet could answer many of the questions about the emitting region. The wavelength dependence of the ultraviolet excess beyond the atmospheric cutoff or the occurrence of chromospheric or coronal emission lines would be valuable information. Such observations would require a space telescope capable of examining faint stars. Such a telescope is the International Ultraviolet Explorer, due to be launched in January 1978. The author will attempt to observe several T Tauri stars and related objects, although they are so faint that they will be just detectable with the IDE. lt has regret ably not been possible to mention all the valuable work done on the T Tauri stars by many investigators at many observatories around the world. However, it is hoped that this historical perspective of the T Tauri stars has helped to illuminate the problems and possible solutions connected with these fascinating stars. It remains to be seen whether or not our endeavors will succeed in unlocking the secrets of the stars. The T Tauri stars may well prove to be a key.

1. Joy, A.H. 1945, Astrophys. J. 102, 168. 2. Abt, H.A. 1973, Mercury 2, 9. 3. Ambartsumian, V.A. 1947, Stellar Evolution and Astrophysics, Akad. Nauk Armen. SSR, Erevan. 4. Walker, M.F. 1956, Astrophys. J. Supp. 2, 365. 5. Henyey, L.G., LeLevier, R., and Levee, R.D. 1955, Pub. Astron. Soc. Pac. 67, 154. 6. Herbig, G.H. 1962, Adv. in Astron. and Astrophys. 1, 47. 7. Kuhi. L.V. 1964, Astrophys. J. 140, 1409. 8. Walker, M.F. 1966, Stellar Evolution (cd. R.F. Stein and A.G.W. Cameron), p. 405.

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