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2795 2800 2805 Wavelength X (A)

AR Ai ir

AR Ai ir

Figure 18.14 Profiles of the Mg II h and k lines in AB Aur, taken on two successive nights. The continuum level is shown in each panel. Also indicated are the values of Vmax, the edge velocity in the absorption dip of the Mg II k line.

2795 2800 2805 Wavelength X (A)

Figure 18.14 Profiles of the Mg II h and k lines in AB Aur, taken on two successive nights. The continuum level is shown in each panel. Also indicated are the values of Vmax, the edge velocity in the absorption dip of the Mg II k line.

successive days. Both the Mg II k line (centered at 2796 A) and Mg II h (2803 A) again have P Cygni profiles, with strong absorption components. It is also clear that the detailed profiles change with time, even over this brief interval. In particular, the blueward edge of the Mg II k absorption trough has shifted to a less negative velocity.

The absorption edge traces the highest-speed material present in the wind at any time. If we plot this velocity, Vmax, over a more extended time, we find that it varies sinusoidally, as

AB Aur

Figure 18.15 Temporal variation of Vmax, the edge velocity of the Mg II k absorption dip in AB Aur. The smooth curve is the best-fitting sinusoid, with indicated period.

Figure 18.15 Temporal variation of Vmax, the edge velocity of the Mg II k absorption dip in AB Aur. The smooth curve is the best-fitting sinusoid, with indicated period.

seen in Figure 18.15. The derived period is 45 h. Note that the equatorial velocity of the star, as obtained from the broadening of optical absorption lines, is Veq sin i = 75 kms-1. The object's effective temperature and luminosity imply a radius of 3 Rq. Thus, the maximum rotation period, corresponding to an inclination i = 90°, is 49 h.

It therefore appears that the wind from AB Aur is being modulated in step with the star's rotation. The outflowing gas, at the point where Mg II absorption occurs, is clearly not spherically symmetric, but is somehow tracking the conditions with which it was launched at the stellar surface. One intriguing possibility is that the wind consists of high- and low-velocity streams, each emitted from a different portion of the surface. The velocity Vmax then peaks whenever the high-speed stream is directed toward the observer.

A handful of Ae/Be stars exhibit a wind signature in the ultraviolet: redshifted or blueshifted emission in Lya at 1216 A. There is also indirect evidence. Some sources appear to be driving CO outflows. Figure 16.2, which displayed a heterogeneous collection of intermediate-mass stars in the HR diagram, singled out those with outflows. We noted that this subclass is situated especially high up in the diagram, just below the birthline. We also cautioned that, at least in some instances, the star actually powering the fast-moving CO may be an unresolved neighbor or binary companion.

Such potential confusion of sources is a general problem for Herbig Ae/Be stars, arising both from their location in crowded regions and from their greater distance. The same issue enters any discussion of jets. Several dozen Ae/Be stars have, at one time or another, been identified as the sources of either continuous jets or chains of discrete, Herbig-Haro knots. An additional difficulty here is that the intermediate-mass object may create a reflection nebula whose continuum emission overwhelms the relatively weak jet. In any case, higher-resolution imaging has sometimes led to retraction of the original claim. A good example is the Herbig Be star LkHa 234 (Figure 18.2), which appears, from optical images, to drive a jet down the central axis of a broad reflection nebula. Additional, millimeter and mid-infrared studies show that the source of this jet, and of an oppositely directed CO outflow lobe, is more likely to be an even more embedded object located nearby.

The situation regarding molecular outflows and jets is thus less settled than one might hope. Nevertheless, the spectroscopic evidence, both from Ha and other lines, leaves no doubt that Ae/Be stars do power winds. But this fact itself presents a major conundrum. Our theoretical account of pre-main-sequence winds in § 13.4 focused on the mechanism of centrifugal ejection from magnetized objects. This picture, in turn, is motivated by our knowledge that T Tauri stars are at least partially convective and hence generate magnetic fields through dynamo activity. The situation is entirely different for intermediate-mass objects. Apart from transient effects during their thermal relaxation, these stars are stable against convection. Any surface magnetic fields that exist must have a different origin.

There is, in fact, scant empirical evidence for magnetic fields associated with Herbig Ae/Be stars. Measurements of Zeeman broadening have thus far been inconclusive. It is true that a dozen or so objects have radio flux at centimeter wavelengths. The emission, however, is generally unpolarized and appears to be thermal, presumably arising from the ionized component of the wind. Again, the situation is unlike that for T Tauri stars such as V773 Tau, whose polarized, compact radio emission indicates the presence of an underlying field (§ 17.1).

What, then, powers Herbig Ae/Be winds? Our previous, general discussion of wind origins further sharpens the dilemma. The earliest-type Herbig Be stars still have too small a luminosity to drive winds through radiative acceleration. Furthermore, even the crudest estimates of mass loss from Ae/Be stars (typically, M < 10-7 MQ yr-1) show that the wind cannot be driven by thermal pressure. The associated corona would be readily detectable through its powerful X-rays, contrary to observation. Speculations about the other driver traditionally considered, Alfven-wave pressure, would seem premature in the absence of good evidence for surface fields. For the time being, the origin of Ae/Be winds remains a mystery.

18.3.5 X-Ray Emission

Although X-rays are too weak to help explain the wind origin, this emission component does exist, with characteristics that are gradually becoming clear. A succession of spaceborne instruments, beginning with the Einstein satellite, have found X-rays in about a dozen stars, roughly half of the observed sample. While such numbers make generalizations difficult, the data reveal a luminosity LX that is a fraction 10-6 to 10-5 of the corresponding stellar value.

We may place this result into context by first recalling that LX/L* for T Tauri stars tends to be greater, of order 10-4. On the other hand, O and early-B stars, which also display X-rays, typically have LX/L* ~ 10-7. The physical origin of the emission in the two groups is also very different. In T Tauri stars, X-rays plausibly arise from reconnection of dynamo-generated magnetic loops near the stellar surface (§ 17.1). Massive stars, which lack outer convection zones, create this emission through shocks generated by instabilities within the stellar wind (§ 15.3).

Which of these two explanations, if either, applies to Herbig Ae/Be stars? Our question assumes, of course, that the X-rays do not stem from an unseen, lower-mass companion, a nontrivial assertion. Some Herbig Be stars have an associated LX that would be too large for a T Tauri star. For the others, the attribution remains uncertain. In any event, a solar-type dynamo mechanism is inapplicable to radiatively stable objects. We are left to consider the wind hypothesis. Until the origin and configuration of the flow itself is better understood, we cannot gauge the likelihood of internal shocks, arising either from the wind itself or from impact with stationary matter. Note that the temperature required for the X-rays (T ~ 107 K) can indeed be produced, at least in principle, by a shock speed comparable to the highest observed wind speeds, about 500 km s^1. As the star ages and its wind dies away, so does the X-ray flux. This picture is at least consistent with the fact that Lx falls to undetectable levels in main-sequence stars with spectral types from mid B to early F.

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