Astron. Astrophys. 349, 475-484 (1999)

1. Introduction

It is well established that young stars are often strong X-ray sources. The X-ray activity of weak-lined T Tauri stars is now regarded as a defining property of the class, and far more WTTs have been found in X-ray surveys than were known previously from other methods (Neuhäuser et al. 1995a). The Classical T Tauris (CTTS), thought to be younger than the WTTS, are also known to have associated X-ray emission, albeit less extreme. Recently, a number of younger, embedded objects have been detected in X-rays (Casanova et al. 1995, Grosso et al. 1997, Koyama et al. 1996, Carkner et al. 1998). The heating mechanism for the X-ray emitting plasma in WTTS is believed to be magnetic activity, similar to that seen on the Sun but many times stronger. This belief gains varying degrees of support from several directions. Young low-mass stars are rapidly rotating and convective, and so might be expected to have strong magnetic fields. This is borne out by direct measurements of the surface fields of several TTS (Guenther et al. 1999). Strong flaring activity is also often observed (Montmerle et al. 1983, Feigelson & Montmerle 1985, Guenther & Emerson 1997).

Preibisch (1998) (hereafter P98) reported the detection of a strong X-ray source in the Serpens star forming region. This X-ray source (hereafter Ser-X3) was identified with a group of optically invisible, IR stars collectively known as SVS4 (Strom et al. 1976). The SVS4 stars have been extensively studied in the NIR by, among others, Eiroa & Casali (1989), Eiroa & Casali (1992), Giovanetti et al. (1998) and Horrobin, Casali & Eiroa (1997). These sources are identified as pre-main sequence objects by Eiroa & Casali (1992) due to their being embedded in nebulous emission. P98 specifically associated the ROSAT source with the SVS4 member EC95, but could not exclude the possibility that nearby EC92 was the true counterpart. [Note: here, and henceforth, we use the numbering scheme of Eiroa & Casali (1992) (in their Table 1) to identify the IR sources.]. By deriving extinctions from the NIR colours, P98 calculated the hydrogen column density in the line of sight, and hence derived X-ray luminosities from the observed count rates of (2-7) erg/sec in the case of EC92, or 2 erg/sec in the case of EC95. These luminosities exceed those of the most extreme known coronal sources by an order of magnitude or more. Because there were X-ray observations at two times separated by days, and no significant variation was seen between these, P98 concluded that the emission was quiescent (i.e. non-flaring). On the basis of the extreme quiescent X-ray brightness, P98 speculated that the emission mechanism may be non-coronal.

Table 1. The observed radio flux associated with the point-like source S68-2 (EC95) at two wavelengths on 3 separate dates. The derived radio luminosity at the distance of the Serpens cloud (310 pc) is also given, as derived from the integrated flux.

Both IR sources are embedded in nebulosity linking them to neighbouring IR sources. This suggests they are an integral part of the SVS4 group and not background objects. Followup observations by Preibisch (1999, hereafter P99) indicate that EC95 has a total luminosity of 60. The lack of a UV-ionized HII region suggests that it is a late-type star (Zhang et al. 1988). P99's IR spectroscopy shows that EC95 is K-type star. Comparison with theoretical tracks indicates that EC95 is a very young, intermediate mass star, presumably a precursor of an HAeBe star, with a mass of 1.5-5.

IR photometry has been presented by Eiroa & Casali (1992) and Giovannetti et al. (1998), amongst others. 3mm observations by Testi & Sargent (1998) show no sign of a source at the position of EC95 (an upper limit of 2.7mJy beam^-1 for a beam of approximately 5"5"). This is consistent with the source being a HAeBe star rather than a low-mass protostar.

Preibisch (1997) (hereafter P97) used the ROSAT HRI detector to search for point sources in the NGC 1333 star-forming region. A number of X-ray sources associated with YSO's were detected. One of these X-ray sources, P97's source number 11 which we will refer to hereafter as NGC1333-X11, or just X11, was associated with the optically-invisible object SVS16. This object had a very large derived optical extinction () and so appeared to have an extreme X-ray luminosity.

Preibisch et al. (1998) (hereafter PNS98) obtained further NIR photometry and spectoscopy of this object. SVS16 consists of two widely separated components, SVS16-e and SVS16-w. As with EC95, IR spectra were used with an iterative fitting technique to obtain stellar parameters. For 16-w and 16-e, the spectral types are found to be M2 and M3, the luminosities 3.8 and 2.7 and 26.2 and 28, respectively.

The X-ray luminosity was measured to be 210³² erg/sec, confirming the previous detection by P97 and suggesting that this represents the quiescent X-ray emission. PNS98 concluded that, although the X-ray source was more extreme than the most active known T Tauri star, the emission could still be explained by a coronal model as the luminosity did not exceed that of the most active RS CVn systems. PNS98 determined =8, a high value for late-type stars. From a determination of the veiling, PNS98 determined that SVS16 had little circumstellar material, and is therefore likely to be a relatively evolved YSO.

The detection of these extreme sources poses interesting questions concerning the nature of X-ray emission from pre-main sequence objects. In particular, there is the question of whether any coronal model can explain such activity. A particular problem lies with the ratio , which is found to be very high in each case (several 10^-3 for EC95). Values of of more than approximately 10^-3 are not observed for dwarf coronal sources. The correlation between and rotation is instead seen to flatten off. This is attributed either to saturation of the dynamo mechanism which generates the underlying magnetic field, or to the stellar surface being completely filled with equipartition field.

There is no shortage of alternative candidates for modelling X-ray emission from a Young Stellar Object (YSO). The environments of YSO's are complex and poorly understood. Remnants of the parent cloud continue to accrete onto the star, either falling inwards in a spherically symmetric flow, or being channelled through an accretion disk. The disk may possess a magnetic field of its own, and in the case of Classical T Tauri stars (CTTS) stellar magnetic fields may play a role in channelling the material onto the stellar surface in the very inner regions (Königl 1991). At the centre of the system, the young star itself is often rapidly rotating, not yet having shed the angular momentum of its parent cloud (Bouvier et al. 1993). The interaction of various magnetic fields with the accreting material produces jets and outflows (e.g. Shu et al. 1994). Any or all of these types of processes could be invoked to produce significant X-ray flux.

The purpose of this paper is to investigate one of the empirical properties of coronae in general, that thermal X-ray emission from coronal plasma is correlated with synchrotron radio emission from accelerated electrons (Güdel & Benz 1993, Benz & Güdel 1994). We therefore search for radio counterparts to the two extreme sources. If radio luminosity of approximately the correct magnitude is found, this is evidence (although not conclusive evidence) that the source is coronal. Stronger evidence for magnetic activity (implying a corona) would include the detection of rapid flaring activity, circular polarization, or a synchrotron-type falling spectrum.

© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999
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