A Radio Survey of Older T Tauri Stars
in the Chamaeleontis Cluster

Introduction

Multiwavelength observations over the last two decades have shown magnetic activity in young pre-main sequence (pre-MS) stars or T Tauri stars (TTs) is greatly enhanced compared to the main sequence Sun. The enhanced solar-type activity in TTs likely results from the interaction between the young stars' rapid rotation and their deep convective envelopes through the dynamo mechanism. The enhanced stellar dynamo in T Tauri stars manifests itself through various diagnostics: active chromospheric emission (Ca II H and K, H, etc.), coronal thermal X-ray emission, and strong nonthermal radio emission (see reviews by Feigelson & Montmerle 1999, Montmerle et al. 1993, Bertout 1989). The majority of a star's pre-MS lifetime is spent as a weak-lined T Tauri (WTT) star, where the star-disk interaction has ceased, and the subsequent angular momentum evolution of the young star is no longer affected by the circumstellar disk.

In active late-type stars, microwave radio emission (L_R) is proportional to the soft X-ray emission (L_X) from high-energy stellar activity over 10 orders of magnitude in L_R ranging from solar microflares to RS CVn binaries (Benz & Güdel 1994). This remarkable power-law relation appears to become microwave-luminous for the very strongest stellar X-ray sources, an effect tied to the higher coronal temperatures among rapidly-rotating stars with convective envelopes. Stellar X-ray spectra are characterized by soft ( 1-2 MK) ``coronal'' components and hard ( 5-100 MK) ``flare'' components. The ``flare'' components decay with rotation and age such that their contribution is negligible for most stars older than 1 Gyr (Güdel, Guinan & Skinner 1997). Highly variable, circularly polarized gyrosynchotron radio emission is associated with hard X-ray emission in flares. The existence of magnetic fields up to thousands of Gauss in strength covering much of the surface of TTs is supported by Doppler imaging and Zeeman measurements (Feigelson & Montmerle 1999).

The emerging picture is that the vigorous magnetic dynamos of rapidly rotating young solar-type stars produce surface fields which in turn undergo violent reconnection to X-ray-bright plasmas and nonthermal electron populations that produce gyrosynchotron emission. Although stellar age was classically the principal variable upon which activity indicators hinged (Skumanich 1972), evidence has built that rotation, not age, is the key to determining a star's activity (see review by Simon 1992). Radio band studies of active stars with different ages and rotation rates can test this idea.

To date, radio surveys have been conducted for very young T Tauri stars (both classical and weak-lined TTs with ages of  2 Myr) in the nearby Taurus (O'Neal et al. 1990, Chiang et al. 1996), Ophiuchi (Leous et al. 1990), Chamaeleon and Corona Australis (Brown et al. 1996) and LkH 101 cluster (Stine & O'Neal 1998) star forming regions.¹ The two major studies of young WTTs in Taurus-Auriga found radio-detection rates of 10-25%. O'Neal et al. (1990) found 8% of 119 WTT stars had 6 cm (4.8 GHz) radio fluxes at the log(L_R) > 16.2 erg Hz^-1 s^-1 level. Chiang et al. (1996) found 7/28 (25%) of low-mass WTTs (K7-M4) were radio-emitting at 3.6cm (8 GHz) in the range 15.4 < log(L_R) < 16.7 erg Hz^-1 s^-1. The detected objects in the O'Neal sample ranged in spectral type from G2-M0.5, and most had log(L_X) > 30.0 erg s^-1.

Both homogeneous and inhomogeneous samples of older TTs have also been surveyed for radio emission. The WTTs in the Upper Sco subgroup of the Sco-Cen OB Association (Sco OB2) are the only radio-surveyed sample of older T Tauri stars (5 Myr; de Geus, de Zeeuw & Lub 1989). The Upper Sco WTTs were surveyed in radio by Brown et al. (1996), who examined 20 stars from the X-ray list of Walter et al. (1994). They detected 10/20 WTTs ranging in spectral type from G0-M2.5, and with a wide range of rotational velocities (< 15 < v sin i < 77 km ). It should be noted that 4/8 radio-detected, Upper Sco WTTs with known v sin i measurements had v sin i > 30 km , and all 10 of the detections had log(L_X) 30.0 erg s^-1. Widely dispersed X-ray-discovered active Li-rich stars found by the ROSAT All Sky Survey (RASS) contain both very young WTT stars dispersed from their star-forming regions, as well as older, low-mass ZAMS stars (e.g. Covino et al. 1997). The largest samples of RASS stars examined in radio are the surveys near the Taurus region by Carkner et al. (1997; C97) and Magazzu et al. (1999). The radio-detections of C97 had radio luminosity densities log(L_R) = 15.5-16.7 erg Hz^-1 s^-1. These stars have spectral types G1-M3.5, and strong X-ray emission with log(L_X) > 29.7 erg s^-1. Among the definite (> 4) detections in C97's sample whose rotational periods and velocities were determined photometrically by Bouvier et al. (1997), the radio-emitters were predominantly rapid-rotating G and K stars (<V_eq> = 55 km ). Another radio survey of an inhomogeneous sample of 32 X-ray selected young stars by White, Pallavicini & Kundu (1992) found many 8-20 Myr stars with log(L_R) = 15.8-16.2 erg Hz^-1 s^-1, whereas most of the older ZAMS-age stars were not detected. A number of nearby young, fast-rotating G-K ZAMS & WTT dwarfs like AB Dor, PZ Tel, and ``Speedy Mic'' (HD 197890) have also been found to be variable radio-emitters. These stars have a quiescent state at the 1-3 x 10<sup>15</sup> erg Hz<sup>-1</sup> s<sup>-1</sup> level, and flare at levels 10-50 x higher (Lim et al. 1992). Among older stars, coeval populations of 100-Myr-old ZAMS stars like the Pleiades have been observed with little success (Bastian, Dulk & Slee 1988) except for weak radio-detections of a few ``ultra-fast rotators'' (UFRs; Lim & White 1995).

The theme emerging from these radio surveys of young stars is that higher nonthermal radio emission exists amongst younger X-ray-luminous populations, and that typically only the fastest rotators amongst ZAMS groups are detectable radio-emitters.

We report here a survey of a unique group of WTTs with ages between that of the youngest T assoications (e.g. Taurus) and older ZAMS clusters like the Pleiades, to explore the association between rotation, youth, and radio emission in that age gap. Our target stars are the X-ray-luminous WTT members of the recently-discovered Chamaeleontis cluster (Mamajek, Lawson & Feigelson 1999; hereafter MLF99). The cluster's Hertzsprung-Russell diagram shows that these stars have ages of 8 4 Myr, and there is strong evidence that they were born from the same cloud and are codistant at d=97 pc. This sample alleviates uncertainties of estimating distances and ages that beset most studies of RASS young stars.

To predict the levels of radio emission expected from this older T Tauri population, we examined the L_X and L_R relations for active stars. Among the more active stars, log(L_R) = log(L_X) -  log() - 15.5  0.5, where L_X is measured in erg s^-1, L_R is in erg Hz^-1 s^-1, and  = 0.17 for RS CVn binaries, Algols, FK Comae stars, and WTTs (Benz & Güdel 1994). This relation should apply to the Cha WTTs as their L_X/L_bol ratios are near the activity saturation level of 10^-3 (MLF99). The L_X/L_R correlation has little dispersion among simultaneous and near-simultaneous observations (e.g. Güdel et al. (1993) for dMe stars, Fox et al. (1994) for RS CVn binaries) but a 0.5-1 dex spread is present in non-simultaneous data due to flares and rotational modulation of emitting structures (e.g. Carkner et al. 1997 for WTT stars). This dispersion can be expected for the Cha cluster WTTs, as eight stars exhibited X-ray variability ranging from factors of 2-7 over 20% of the HRI observations (Mamajek, Lawson & Feigelson 2000). From this analysis, we predict that a few of our brightest X-ray-emitting WTTs should be detectable radio-emitters at the log(L_R)  = 16 erg Hz^-1 s^-1 level at any given time.

© Copyright Astronomical Society of Australia 1997