Ray Emission from T Tauri Stars

Contrary to the cTTS, which are easily identified in the optical by their conspicuous spectral features related to the accretion process, wTTS are difficult to distinguish from more evolved main-sequence stars based on their optical spectra. Indeed, they were discovered and established as a class of premain sequence stars only, thanks to their strong X-ray emission. Einstein Observatory observations first revealed numerous unknown X-ray sources in nearby star forming regions, many of which were later confirmed to belong to the wTTS class by optical spectroscopy.

The X-ray emission from both cTTS and wTTS can be described as thermal emission from an optically thin plasma with temperatures up to 20MK and X-ray luminosities of 1029 "30 erg s^1. Their activity is often termed scaled-up solar-like, because it seems to share the same emission process but the energetics involved are 3-4 orders of magnitude above solar levels. Similar to cool main-sequence stars, the X-ray luminosity of T Tauri stars comprises between io~3-~5 of the bolometric luminosity, with the spread probably being due to the influences of various stellar parameters such as mass, age, and rotation.

10.4.2.1 Mass and Age-Dependence of T Tauri Star X-Ray Emission

Generally, the average X-ray luminosities of young stars increase continuously with mass for M < 3M0. The same holds also for Lx/Lbol as activity indicator. Here, the upper envelope corresponds roughly to the canonical saturation limit of 10~3, although some observations indicate a saturation limit somewhat higher than this value. Age may also play a role in determining the amount of X-rays liberated from magnetically active stars. Clearly in the course of stellar evolution (from the pre-main sequence to open clusters and field stars) the intensity of stellar X-ray emission declines (see Sect. 10.2.4). A similar effect is seen within a given star forming region if individual ages are assigned to the stars based on their position in the HR diagram in comparison with evolutionary isochrones. Figure 10.16 summarizes the mass and age dependence of T Tauri star X-ray emission derived from a recent Chandra observation of the Orion Nebula Cluster. For stars with M < 2M0 Lx decreases with age in each mass bin, but the Lx/Lboi ratio remains roughly constant, because of the simultaneous decrease of the bolometric luminosity as the stars approach the main sequence. A drastic drop in the fractional X-ray luminosity is seen for stars near 2M0. The exact location of this kink in terms of mass depends on the stellar age, and coincides roughly with the transition where the convective envelope disappears and the stars become fully radiative according to models of premain sequence evolution.

Fig. 10.16 The dependence of X-ray luminosity and Lx/Lbol ratio on stellar mass and age for a sample of premain sequence stars in Orion. On the right panel, masses at which the interior structure of stars is expected to become fully convective and fully radiative are indicated by leftward and rightward pointing arrows for each age bin, respectively (Figs. 10 and 11 from [10])

Fig. 10.16 The dependence of X-ray luminosity and Lx/Lbol ratio on stellar mass and age for a sample of premain sequence stars in Orion. On the right panel, masses at which the interior structure of stars is expected to become fully convective and fully radiative are indicated by leftward and rightward pointing arrows for each age bin, respectively (Figs. 10 and 11 from [10])

10.4.2.2 Rotation-Activity Connection in T Tauri Stars

For main-sequence stars, the most direct observational signature that X-ray emission is related to dynamo activity is the anticorrelation between X-ray luminosity and rotation period or Rossby number (see Sect. 10.1.6). For premain sequence stars, the situation is less clear. On the theoretical side, the importance of rotation in regulating the dynamo efficiency of fully convective objects is unknown. On the observational side, good samples for studies of the rotation-activity connection exist for only a few star forming regions, e.g., Taurus (which is little obscured) and Orion (which is compact). An anticorrelation between Lx and Prot like in main-sequence stars was found for premain sequence stars in Taurus, but derived on a relatively small sub-sample of X-ray detected T Tauri stars with known rotation periods. Recent studies in Orion, where about two fifths of the X-ray detected premain sequence sample have measured rotation periods, show no indication for any rotation-activity relation. A possible explanation is the large convective turnover time 100-1000 days) of premain sequence stars, which leads to a concentration of all premain sequence stars in the left side of the diagram of log (Lx/Lboi) vs. R0 (see Fig. 10.17), far off the linear regime.

Fig. 10.17 X-ray-rotation relationship for premain sequence stars in the Orion Nebula Cluster as a function of the Rossby number (R0). The main-sequence relationship from Pizzolato et al. (2003) is indicated by the solid line. Crosses denote the Orion stars without known rotation period (plotted arbitrarily at log R0 = 0) (Fig. 9 from [46])

Fig. 10.17 X-ray-rotation relationship for premain sequence stars in the Orion Nebula Cluster as a function of the Rossby number (R0). The main-sequence relationship from Pizzolato et al. (2003) is indicated by the solid line. Crosses denote the Orion stars without known rotation period (plotted arbitrarily at log R0 = 0) (Fig. 9 from [46])

10.4.2.3 X-Ray Emission from Classical and Weak-Line T Tauri Stars

Comparisons of the X-ray properties of cTTS and wTTS have led to controversial results. While in some star forming regions, cTTS and wTTS seem indistinguishable on basis of their X-ray luminosities, other studies - mainly those of the Taurus-Auriga complex - found that wTTS are on average brighter in X-rays than cTTS (see Fig. 10.18a). Incompleteness of the wTTS samples in some areas of star formation was put forth among the possible explanations. Since most wTTS are discovered through their X-ray emission, the known wTTS population is biased toward the brightest X-ray emitters if it is incomplete. This effect could produce artificial differences between the distributions of X-ray luminosities for the two samples.

Recent observations with Chandra and XMM-Newton suggest a different solution to the problem. They have shown that the outcome of such comparisons depends on the diagnostic used to distinguish the two groups of stars. Traditionally, cTTS and wTTS are distinguished on basis of the equivalent width of their H a emission, stars with WHa > 10 A being classified as cTTS and those with WHa < 10 A being considered wTTS; in more sophisticated studies, the spectral type dependence of the boundary between the two groups is also taken into account. This criterion is efficient in separating accreting from nonaccreting stars. Another possibility is to define T Tauri star subgroups based on the presence or absence of IR excess emission, separating disk-bearing from diskless stars. This method corresponds roughly to the IR classification of Class II (stars with disk) and Class III (stars without disk).

Concerning the X-ray luminosity, [38] found in a sample of premain sequence stars in the IC 348 cluster that IR excess and nonexcess stars are equally X-ray luminous (Fig. 10.18b). To summarize the available observations, pre-main sequence star

c rt

"a

0.0

: WTTS Pointings

: CTTS Pointings1—i

: WTTS RASS :

\ I

■ '

28.0 28.5 29.0 29.5 30.0 30.5 31.0 log (Lx [erg/sec])

28.0 28.5 29.0 29.5 30.0 30.5 31.0 log (Lx [erg/sec])

Fig. 10.18 (a) X-ray luminosity functions for cTTS and wTTS in the Taurus-Auriga star forming region observed with ROSAT. The pointed observations (solid lines) yield better sensitivity than the RASS (dotted lines). Both pointed and survey data demonstrate clear differences between the X-ray emission level of the two samples, with wTTS being brighter than cTTS (Fig. 3 from [49]). (b) X-ray luminosity functions for premain sequence stars in the IC 348 star forming region. The X-ray luminosities for the two subgroups of IR excess and nonexcess stars are indistinguishable (Fig. 12 from [38])

X-ray luminosities appear different if the stars are separated according to accretion signatures such as Ha or Ca II equivalent width, but they appear similar, if distinguished on basis of the presence or absence of a circumstellar disk. This suggests that the disk itself does not play a major role in regulating young star X-ray emission, but the accretion process influences the amount of observable X-rays, e.g., by changing the coronal geometry such as to decrease the fraction of the surface available for the allocation of closed magnetic structures.

10.4.2.4 X-Ray Variability on T Tauri Stars

Strong variability of the X-ray emission is characteristic of T Tauri stars, and has been used as an indicator of youth. Flares on T Tauri stars were long thought to be short (hour-long) events, reminiscent of solar impulsive flares and characterized by a fast rise of the intensity and subsequent exponential decay representing the cooling phase. However, continuous X-ray observations over many hours with the new generation of X-ray satellites have shown that T Tauri star flares comprise also long events that extend up to days. Prolonged flare decays may be interpreted as evidence for continued heating, similar to the case of main-sequence stars.

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