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

Observations and Results

We observed the 10 Cha T Tauri stars listed in MLF99 at 4.8 GHz and 8.6 GHz (6.2 and 3.5 cm) with the Australian Telescope Compact Array (Paul Wild Observatory) in 6-km C mode. The phase centres were positioned 10

north of the stars' optical positions to avoid any possible effects of DC offsets. With 20.5 hours of observing time allocated on 1999 Mar 7-8, each object was observed 9 times, with most of the observations taken in 10 minute blocks. Shorter blocks (8 minutes) were used near the end of the run to accommodate a ninth observation for all of the objects. The final 12 hours of observation were impaired due to the failure of antennae 6, hence roughly half of the observations employ only 5 antennae and have only 10 of the normal number of baselines (15). The antenna failure resulted in theoretical rms noise values increasing from 0.074 mJy to 0.11 mJy. The ATCA primary calibrator PKS B1934-638 was observed at the beginning of the run for 5 minutes. B1057-797 was used as a secondary phase calibrator, and was observed after every 2 x 10-minute block of on-source time. The weather was fair throughout the observations.

Data analysis of the radio fields was performed using the Miriad 1.0 package². Unusually high correlations (presumably due to radio interference) were clipped when examining the UV data, and by flagging extraneous correlations in the real versus imaginary amplitude diagram. The overall data quality was excellent, and among all of 4.8 and 8.6 GHz UV data sets, less than 0.6% of the correlations were clipped from each set. A ``dirty'' image of each field and frequency UV data set was made using the Miriad task INVERT in natural weighting mode. The average theoretical rms noise () for the Cha cluster member maps was 0.11 mJy beam^-1, and the measured background noise values for 14 of the 20 dirty images was just slightly higher than the theoretical values (usually around 0.13 mJy beam^-1). From these 14 featureless dirty maps (both frequencies for RECX 1, 7, 11, 12 and the 8.6 GHz maps for RECX 3, 4, 5, 6, 9 and 10), a search for detections was conducted immediately and no further processing was warranted. The other 6 dirty images had higher noise values (0.15 < < 0.5 mJy beam^-1) and required CLEANing to remove the effects of bright confusing sources within or near the primary beam.

Final maps were corrected for primary beam sensitivity. The radio position of the secondary calibrator B1057-797 was precisely as listed in the AT catalog of calibrator sources, indicating that no significant deviations in the optical-radio reference frame should exist in the synthesized maps. A 1 arcmin² square box map of each source at each frequency was made and examined in a preliminary search for detections. Miriad's statistical programs IMSTAT and HISTO were used to find the position and peak of the flux in a 20

x 20

 box coincident with the optical position.

No peaks were found within 2

of the optical positions. Only one target, RECX 10, had a radio source within 1 arcmin of the X-ray star: a 4.5 source with 1.1 mJy (0.7 mJy) at 4.8 GHz (8.6 GHz) was found 13.1

NW of RECX 10 at 8:44:30.65, -78:46:19.5 (J2000). The source has a double morphology with component separation of 4

, and is almost certainly an unrelated background double radio galaxy³. To investigate the possibility of discovering an Cha radio star that was not in the X-ray sample, we searched for optical counterparts of six bright sources within the primary beams. None were found within 5

of the radio sources. Finally, we searched unsuccessfully for radio emission from the three early-type members of the cluster: Cha (B8V), RS Cha (A7+A7IV) and HD 75505 (A1V). Table 1 contains stellar data on the Cha cluster WTTs (from MLF99) combined with the upper limit 4.8 and 8.6 GHz radio fluxes.

Table 1: Radio Observations of Cha Cluster T Tauri Stars

	Star	Optical Positions		Spectral	log(LX)	4.8 GHz	4.8 GHz	8.6 GHz	8.6 GHz
	RECX			Type	(erg s-1)	flux	log(LR)	flux	log(LR)
	#	h   m   s				(mJy)	(erg Hz-1 s-1)	(mJy)	(erg Hz-1 s-1)
	1	08 36 56.2	-78 56 45.7	K4	30.6	<0.34	<15.59	<0.37	<15.62
	3	08 41 37.2	-79 03 30.9	M3	29.1	<0.46	<15.71	<0.37	<15.62
	4	08 42 23.7	-79 04 03.6	K7	30.1	<0.31	<15.54	<0.34	<15.59
	5	08 42 27.3	-78 57 48.5	M5	29.0	<0.42	<15.68	<0.33	<15.57
	6	08 42 39.0	-78 54 43.5	M2	29.5	<0.52	<15.77	<0.37	<15.62
	7	08 43 07.7	-79 04 52.3	K3	30.3	<0.36	<15.61	<0.40	<15.66
	9	08 44 16.6	-78 59 08.9	M4	28.4	<0.40	<15.65	<0.36	<15.61
	10	08 44 32.2	-78 46 31.7	K7	29.8	<0.64	<15.86	<0.44	<15.70
	11	08 47 01.8	-78 59 35.2	K4	30.1	<0.45	<15.70	<0.38	<15.63
	12	08 47 56.9	-78 54 53.9	M2	30.1	<0.37	<15.62	<0.35	<15.60

© Copyright Astronomical Society of Australia 1997