THE ASTROPHYSICAL JOURNAL, 481:447È451, 1997 May 20 ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A. HUBBL E SPACE T EL ESCOPE WFPC2 IMAGES OF EMISSION NEBULOSITY NEAR XZ TAURI JOHN E.KRIST,1 CHRISTOPHER J.BURROWS,1,2 KARL R.STAPELFELDT,3 GILDA E. BALLESTER,4 JOHN T.CLARKE,4 DAVID CRISP,3 ROBIN W.EVANS,3 JOHN S.GALLAGHER III,5 RICHARD E. GRIFFITHS,6 J. JEFF HESTER,7 JON A.HOLTZMAN,8 JOHN G.HOESSEL,5 JEREMY R.MOULD,9 PAUL A. SCOWEN,7 JOHN T.TRAUGER,3 ALAN M. WATSON,8 AND JAMES A. WESTPHAL10 Received 1996 October 2; accepted 1996 December 19 ABSTRACT XZ Tauri is a M3 V T Tauri star with a preÈmain-sequence binary companion detected by infrared speckle interferometry at a projected separation of0A.3 (about 40 AU). Previous ground-based obser- vations of the system have shown bipolar emission nebulosity, including a possible jet knot. Images obtained with the Wide Field and Planetary Camera 2 on the Hubble Space Telescope show an elon- gated, Ðlled bubble of emission nebulosity extending over 4A to the north of the system which contains two or three compact knots. The source of the nebulosity is undetermined. For the Ðrst time, the com- ponents are separated in visible light, and the southern component (XZ Tau S) dominates the integrated light of the system. The reverse has been found to be true in the near-infrared. Thus we conÐrm that XZ Tauri belongs to the small class of young binaries with cool infrared companions. Subject headings: ISM: jets and outÑows È stars: individual (XZ Tauri) È stars: preÈmain-sequence 1. INTRODUCTION sical T Tauri stars that drive outÑows. The radio continuum emission must originate from only one of the two XZ Tau XZ Tauri (HBC 50) is located at an estimated distance of components, as this emission was unresolved in a0A.3 VLA 140 pc(Elias 1978) in the L1551 dark cloud and was once beam. Unfortunately, the existing astrometry is not suffi- thought to be a single T Tauri star. It was shown by near- ciently accurate to show which star corresponds to the VLA infrared speckle interferometry to be a0A.3 binary (Haas, source. Previous and recent centimeter-range measure- Leinert, & Zinnecker1990). The southern component ments are summarized byWilner, Ho, & Rodriguez (1996), (hereafter referred to as XZ Tau S), which we will show to be which agree with those of Rodriguez et al. dominant at visible wavelengths, is an M3 V T Tauri star Ground-based CCD imaging and spectral measurements (Cohen& Kuhi 1979). The northern one (XZ Tau N) is a (Mundt,Ray, & Buhrke 1988; Mundt et al. 1990) show a possible protostellar object with an estimated color tem- bipolar emission outÑow and a nebular condensation at a perature of 1800È1900 K(Haas et al. 1990), and it domi- position angle of about 20¡. This orientation is nearly per- nates in the infrared. The system has a signiÐcant IRAS pendicular to the 100¡È112¡ polarization angle of the excess(Weaver & Jones 1992), but the small 1.3 mm Ñux system as measured byBastien (1982). Gledhill & Scarrott density ofF \ 17 ^ 3 mJy (Osterloh & Beckwith 1995) l (1989) give a polarization angle of 76¡, though they indicate indicates a circumstellar mass of only about 10~3 M_. that this is similar to other stars in the periphery of the Jensen,Mathieu, & Fuller (1996) demonstrate that this may L1551 cloud.Mundt et al. (1990) traced nebulosity out to be due to clearing of the disk by the gravitational inÑuence 10A on either side of the stars and reported radial velocities of the companion. VLA continuum observations by on the order of 40È80 km s~1 with the northern component Rodriguezet al. (1994) provide a Ñux density measurement blueshifted. of 0.23 mJy at 3.6 cm, which they interpret as free-free emis- sion from an ionized wind close to the starÈtypical of clas- 2. OBSERVATIONS XZ Tau was observed with the Wide Field and Planetary Camera 2 (WFPC2;Burrows 1995) of the Hubble Space 1 Space Telescope Science Institute, Baltimore, MD 21218; krist=stsci.edu. Telescope (HST ) on 1995 January 5 (HST program 5768). 2 Astrophysics Division, Space Science Department, European Space The primary target was HL Tau, which was placed near the Agency. center of the Planetary Camera (PC). XZ Tau was located 3 MS 183-900, Jet Propulsion Laboratory, 4800 Oak Grove Drive, toward the edge of the PC Ðeld at the pixel coordinates Pasadena, CA 91109. (x, y) (563, 682). The scale of the PC is0A.0455 pixel . 4 Department of Atmospheric, Oceanic, and Space Sciences, University \ ~1 of Michigan, 2455 Hayward, Ann Arbor, MI 48109. The observations consist of two 400 s exposures through 5 Department of Astronomy, University of Wisconsin, 475 N. Charter Ðlter F675W (WFPC2 R band) at a gain of 7 e~ per analog- St., Madison, WI 53706. to-digital converter unit (ADU), and two 600 s exposures 6 Department of Physics, Carnegie-Mellon University, Wean Hall, 5000 and one 3 s exposure in F814W (WFPC2 I band), all at a Forbes Ave., Pittsburgh, PA 15213. 7 Department of Physics and Astronomy, Arizona State University, gain of 14 e~ ADU~1. The images were calibrated with the Tyler Mall, Tempe, AZ 85287. standard HST pipeline, and the duplicate frames were 8 Department of Astronomy, New Mexico State University, Box 30001, summed, rejecting cosmic rays. In the F675W and long- Department 4500, Las Cruces, NM 88003-8001. exposure F814W frames the star images are saturated and 9 Mount Stromlo and Siding Springs Observatories, Australian National University, Weston Creek Post Office, ACT 2611 Australia. bleed up and down the columns. 10 Division of Geological and Planetary Sciences, MS 170-25 Caltech, The reduced images are shown in the left-hand column of Pasadena, CA 91125. Figure 1 (Plate 16). The small inset image is the unsaturated 447 448 KRIST ET AL. Vol. 481 3 s F814W image of the binary stars themselves and is the Ðlter, at a variety of positions within about 30 pixels of the Ðrst time the binary has been resolved at visible wave- objects and with a similar target color and less critically at a lengths. In the F675W and long F814W frames, a slightly similar epoch (within a few days for blue passbands and a defocused ghost image of the binary is seen about 5A east of few weeks otherwise). Even then, experience shows that the the saturated primary image and is about 8.5 magnitudes result is an rms noise level of about 15% of the local target fainter. It lies on the line from the CCD center through the PSF level. A nonoptimal PSF can easily lead to RMS errors stars and is known to be caused by a reÑection from the at the 30%È50% level. cameraÏs Ðeld Ñattener (Burrows 1995; Krist 1995). An extensive review of archived HST images showed no similarly saturated PSFs near the XZ Tau position. The 3. RESULTS closest useful PSFs were near the center of the PC, about 3.1. Unsubtracted Images 11A away. These were images of the naked T Tauri stars SAO 76411A in F675W (HST program 5204) and HDE As shown in the lower row ofFigure 1, the F675W image 283572 in F814W (HST program 6223). These two stars shows nebulosity forming an elongated bubble extending were subtracted by each other, as well as by other stars, and approximately4A.3 (600 AU) to the north at a position angle revealed no evidence for circumstellar material. of 15¡, with a maximum width of about2A.8 (390 AU). An alternative to using an observed PSF is to create a Within the bubble is a bright, compact source (knot a), 2A.6 simulated one using Tiny Tim(Krist 1996). A model can be from XZ Tau S. The nebula edges appear well deÐned. The generated for any given position, Ðlter, object color, and long F814W image shows no evidence for the bubble but spatial sampling. However, because of uncertainties in the does show knot a faintly. optical parameters of the telescope (i.e., focus, Ðeld depen- F675W contains the most signiÐcant HH object emission dent aberrations, and scattering properties), signiÐcant mis- lines ([S II] 6717, 6731A ,A Ha 6563,A and [O I] 6300 ). matches between a model PSF and an observed one are F814W contains weaker lines, including [Fe II] 8617A . It is expected. For our purposes, the most signiÐcant model therefore possible to distinguish reÑection nebulosity, which errors occur between0A.4 and 3A from the star, where the tends to have neutral to red colors, from emission nebulae, lack of large angle scattering in the Tiny Tim models results which are apparently very blue in these two passbands. The in undersubtraction of the wings. We examined subtrac- absence of the bubble in the F814W image (I[R [ 2) indi- tions using both the observed PSFs and the Tiny Tim cates that it is unlikely to be reÑection nebulosity. A pho- models to ensure that real structures are identiÐed instead toionized nebula is ruled out by the fact that XZ Tau S is a of PSF artifacts. late-type star and thus is a negligible source of ionizing Given that the brighter component, XZ Tau S, is an M3 photons. The emission-line spectrum of a Herbig-Haro star, models for each Ðlter were computed using Tiny Tim object can reproduce the broadband colors of the bubble. V4.1 for the reddest object color standard to the software Therefore the bubble is probably an emission nebula (B[V \ 1.59).