IMAGING MWC 349 FROM 7 MILLIMETERS TO 90 CENTIMETERS

The Astrophysical Journal, 610:827 834, 2004 August 1 Copyright is not claimed for this article. Printed in U.S.A. IMAGING MWC 349 FROM 7 MILLIMETERS TO 90 CENTIMETERS D. Tafoya, Y. Gómez, and L. F. Rodríguez Centro de Radioastronomía y Astrofísica, UNAM, Apartado Postal 3-72 (Xangari), 58089 Morelia, Michoacán, Mexico; d.tafoya@astrosmo.unam.mx, y.gomez@astrosmo.unam.mx, l.rodriguez@astrosmo.unam.mx Received 2004 January 28; accepted 2004 April 7 ABSTRACT MWC 349A is the brightest radio continuum star in the centimeter domain. The thermal radio continuum emission is believed to originate in an ionized bipolar flow that photoevaporates from the surfaces of a neutral Keplerian disk. In this work we present high angular resolution observations taken with the Very Large Array (VLA) at wavelengths from 7 mm to 90 cm that allow the study of structures over 2 orders of magnitude in size in this object. The 7 mm image shows an intermediate equatorial region 0B04 wide, with no free-free emission, that could be the neutral photoevaporating disk around MWC 349A. Combining these data with archival observations at 1.3, 2, 3.6, 6, and 20 cm, we estimate that the flux increases with frequency as 0:670:03 and the angular size decreases with frequency as 0:740:03, confirming the presence of a biconical thermal wind that expands at constant velocity. We also report the marginal detection of MWC 349A at 90 cm. At the wavelengths of 3.6, 6, and 20 cm, we image and model the interaction zone between the winds of MWC 349A and MWC 349B, supporting the physical association of these components. Finally, by comparing 6 cm data taken in 1982 and 1996 we find evidence of variability in MWC 349A that is interpreted as a decrease of 2% in the mass-loss rate over the time interval of the observations. Subject headings: radio continuum: stars stars: individual (MWC 349A) stars: winds, outflows 1. INTRODUCTION MWC 349 is a peculiar emission-line system discovered by Merrill et al. (1932). MWC 349 has been studied extensively in a wide range of wavelengths, particularly in the radio regime, by several authors (e.g., Braes et al. 1972; Olnon 1975; Altenhoff et al. 1976; White & Becker 1985; Cohen et al. 1985; Rodríguez & Bastian 1994). Its primary component, MWC 349A, is a very hot luminous star that exhibits an unusually slow and dense ionized stellar wind with a velocity of 50 km s 1 (Hartmann et al. 1980; Altenhoff et al. 1981; Tanaka et al. 1985), making it the brightest radio continuum star in the centimeter domain. A tentative companion star called MWC 349B, located 2B4 0B1 west from the primary star and classified as a B0 III type, seems to be interacting with the MWC 349A wind (Cohen et al. 1985). MWC 349A is the only source that shows high-gain natural hydrogen maser emission (Martín-Pintado et al. 1989b; Planesas et al. 1992; Gordon 1992; Thum et al. 1992; Ponomarev et al. 1994; Strelnitski et al. 1996). These masing lines are believed to arise from the inner region of a near edge-on disk with the eastern region redshifted with respect to the western one and separated by 0B065 (80 AU at a distance of 1.2 kpc) along the equator (Planesas et al. 1992). The radio continuum emission from MWC 349A shows an extended, ionized bipolar structure (White & Becker 1985; Cohen et al. 1985). The total continuum flux density of MWC 349A varies with frequency as S / þ0:6 (Olnon 1975; Chiuderi & Torricelli Ciamponi 1977; Dreher & Welch 1983), consistent with the behavior expected for an ionized stellar wind. Observations of the H92 recombination line made by Rodríguez & Bastian (1994) resolved spatially for the first time the kinematic structure of the MWC 349A envelope, finding clear evidence of rotation, with the eastern region blueshifted with respect to the western region, the opposite orientation to that found by Planesas et al. (1992) in the maser 827 lines. Rodríguez & Bastian propose the presence of a neutral photoevaporating disk in rotation and expansion around the star, whose symmetry axis has an inclination angle of 15 with respect to the plane of the sky. Assuming Keplerian rotation, they estimate the mass of the star to be 30 M. Even though the presence of a neutral disk in MWC 349A has been inferred by several authors, there is little conclusive, direct imaging evidence for a neutral disk, although there is spectroscopic evidence for this in the double-peaked optical and IR emission lines discussed by Hamann & Simon (1986). Recent observations in the near-infrared of MWC 349A show images in which an east-west elliptical structure is observed, consistent with the shape expected from a nearly edge-on disk (Danchi et al. 2001; Hofmann et al. 2002). In particular, Danchi et al. (2001) show a 1.65 m maximum entropy image reconstructed from aperture-masking interferometry on Keck I. This elongated structure is perpendicular to the bipolar outflow main axis and has been interpreted as an edge-on disk with a major axis length of 36 1:9 mas at a position angle (P.A.) of 100, consistent with the P.A. of the dark lane observed in high angular resolution radio continuum maps. Other observations at the same frequency by Hofmann et al. (2002) did not show so clearly this enlargement in the equatorial direction. In this work we attempt to present direct evidence for the disk by making a very high angular resolution (36 mas) image ofmwc349aat7mmwiththeverylargearray(vla). With these observations, we clearly detect the dark lane produced by the neutral disk. The model that proposes an ionized wind from the stellar surface predicts homologous images; in other words, in going to higher frequencies one should see similar images, with the angular size of the source decreasing as 0:7 (Panagia & Felli 1975). We note that in the 3.6 cm map of Rodríguez & Bastian (1994), the contrast between the peak of the lobes and the region between them is only 15% but that this contrast has increased to 25% in the 2 cm map of

828 TAFOYA, GÓMEZ, & RODRÍGUEZ Vol. 610 TABLE 1 Observational Parameters Synthesized Beam a Frequency (GHz) Phase Calibrator Bootstrapped Flux (Jy) Half-Power Beamwidth (arcsec) P.A. (deg) Epoch 0.330... 2037 + 511 10.62 0.12 5.38 ; 5.15 3 1996 Dec 05 1.425... 2005 + 403 2.60 0.02 1.33 ; 1.15 +13 1996 Dec 05 4.860... 2005 + 403 3.17 0.01 0.38 ; 0.36 21 1996 Dec 05 8.310... 2005 + 403 3.14 0.01 0.22 ; 0.20 +86 1988 Dec 31 14.940... 2005 + 403 2.54 0.17 0.13 ; 0.11 29 1996 Dec 05 22.367... 2005 + 403 3.45 0.10 0.085 ; 0.077 62 1990 Mar 29 43.340... 2005 + 403 1.56 0.14 0.038 ; 0.034 71 1996 Dec 16 a For ROBUST ¼ 0 u-v weighting. The robustness is a new form of visibility weighting that varies smoothly from natural to uniform weighting as a function of a single real parameter ( Briggs 1995). The ROBUST ¼ 0 weighting gives the optimal compromise between sensitivity and angular resolution. White & Becker (1985). Of course, the actual reason for this decrease could be attributed to different angular resolution (the size of the source scales as 0:7, while the angular resolution of a given interferometer configuration goes as 1 ), but it suggests that a dark or near-dark region could be finally detected at very high angular resolution. We also analyzed archival VLA data at various wavelengths to better understand this remarkable source. 2. OBSERVATIONS The 1.3 cm and 7 mm continuum observations were made on 1996 December 16 with the VLA of the NRAO. 1 The array was in the A configuration, giving an angular resolution of 0B1 at 1.3 cm and 0B04 at 7 mm. The array was divided in two subarrays observing simultaneously at these wavelengths. The phase center of the 1.3 cm and 7 mm observations was set to (1950) ¼ 20 h 30 m 56 s :85 and (1950) ¼ 40 29 0 20B4, the position of the MWC 349A star. The absolute amplitude calibrator was 1328 þ 307 (3C 286), for which flux densities of 2.55 (1.3 cm) and 1.31 Jy (7 mm) were adopted. The phase calibrator was 2005 þ 403, for which flux densities of 2:20 0:21 (k ¼ 1:3 cm)and1:56 0:14 Jy (k ¼ 7 mm) were obtained. Additional observations were taken from the VLA archive database at 1.3, 2, 3.6, 6, 20, and 90 cm in order to constrain the spectral index and angular size of MWC 349A. A summary of these observations is given in Table 1. Since the u-v coverage of our 1.3 cm data was not as good as that obtained in the 1.3 cm VLA archive database, we decided to use the archive data at 1.3 cm in this paper. The continuum data at 3.6 and 1.3 cm have been already published by Rodríguez & Bastian (1994) and Martín-Pintado et al. (1993), respectively. All data were edited and calibrated following standard procedures and images were made using the NRAO software AIPS. 3. RESULTS AND DISCUSSION 3.1. The Spectra of MWC 349A It is known that in the case of expanding envelopes around hot stars where the density distribution decreases with distance (n e / r 2 ), the radio spectrum changes with frequency 1 The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. as þ0:6 and the size of the source as 0:7 (Panagia & Felli 1975; Olnon 1975; Wright & Barlow 1975; Schmid-Burgk 1982). This behavior has been observed in several sources, such as P Cyg ( Panagia & Felli 1975), Cep A HW2 ( Rodríguez et al. 1994), and in particular in MWC 349A (Olnon 1975; Dreher & Welch 1983; White & Becker 1985). In this work we used observations made from 7 mm to 90 cm to study these frequency dependences. The MWC 349A flux density has been measured from radio continuum data at 0.7, 1.3, 2, 3.6, 6, 20, and 90 cm (see Table 2). The source appears resolved at these frequencies except at 90 cm, where an upper limit for the angular size is given (see x 3.4). The total flux densities and angular sizes listed in Table 2 were determined directly from the u-v data. Escalante et al. (1989) have shown that in the case of a wind source, for short baselines the observed flux density depends linearly on the projected baseline separation. Thus, it is possible to determine the flux density and source size by making a linear fit to the data in the u-v space. A least-squares fit was applied to the real part of all our high angular resolution data using the equation V(b) ¼ S (1 Ab); where b is the projected baseline separation, given in wavelengths, S is the total flux density, i.e., the flux density at a zero projected baseline (b ¼ 0), and A is the fitted slope. Then the angular diameter of the source within which half of the flux density is originated can be obtained from the equation =arcsec ¼ 1:19 ; 10 5 A: TABLE 2 Total Flux and Angular Sizes for MWC 349A Frequency (GHz) Total Flux (mjy) Angular Size (arcsec) 0.330... 30 10 <10 1.425... 76.4 6.4 1.89 0.09 4.860... 154.8 8.5 0.74 0.04 8.310... 183.5 9.5 0.58 0.03 14.940... 380.0 21.2 0.30 0.02 22.367... 446.2 44.8 0.24 0.03 43.340... 635.0 95.6 0.17 0.03

No. 2, 2004 IMAGING MWC 349 829 Fig. 1. Total flux (top) and angular size (bottom) as a function of frequency for MWC 349A. The dashed lines are the power-law fits where S / 0:670:03 and / 0:740:03, respectively. The 90 cm ( lowest frequency) data are not used for the fits. Figure 1 shows the flux density and angular size distribution as a function of frequency where a power-law fit to the data (excluding the 90 cm point, which has a low signal-to-noise ratio), using a least-squares method, gives and S v =mjy ¼ 54 4ðv=GHzÞ þ0:670:03 v =arcsec ¼ 2:5 0:1ðv=GHzÞ 0:740:03 : The spectral index for the flux density derived by us is consistent with previous values given by Dreher & Welch (1983) of +0:65 0:02 and Altenhoff et al. (1981) of +0:67 0:05, which are very close to the theoretical value of +0.6, expected for a density distribution of r 2 for an isotropic mass-loss rate at constant velocity. The determination of the angular size in MWC 349A from the images at different frequencies is not straightforward, since we do not have a spherical source but an object whose morphology changes with frequency. However, when we fit a line to the short spacings of the visibility function this gives a reliable value for the angular size such that the spectral index derived is very close to the expected value of 0.7. This is the first time that this spectral index for the angular size has been derived directly from the observations of MWC 349A. 3.2. The 7mm, 1.3 cm and 2cmContinuum Emission MWC 349A is characterized by a low collimation bipolar morphology that is better appreciated in the high angular resolution images. Figure 2 shows a panel of three high angular resolution images at 2 cm, 1.3 cm, and 7 mm wavelengths (left, from top to bottom, respectively). The inner cross in these images marks the central position of our 7 mm source in the image made with an angular resolution of 36 mas, (1950) ¼ 20 h 30 m 56:846 s and (1950) ¼ 40 29 0 20B26. This 7 mm image shows clearer than ever before the inner structure of MWC 349A. The source was spatially resolved at these three frequencies and the dark lane is better appreciated in the images at 1.3 cm and 7 mm, with the two lobes of ionized material arising from both sides of the equator. The presence ofthedarklaneinthe1.3cmimagehasbeendiscussedby Martín-Pintado et al. (1993). It is observed that the angular size of the source decreases as the frequency increases. After rotating these three images by 10 clockwise in order to have the dark lane in the left-right direction, we averaged the flux along a slice made in the top-bottom direction to see the contrast between the emission regions and the dark lane. The slices, in the righ-hand side of Figure 2, show the normalized flux versus the angular displacement scaled as 0:7 to better appreciate the differences. The 7 mm slice shows the highest contrast between the emission lobes and the dark lane, with a value of 3.6, followed by the 1.3 cm slice where the contrast is 2.1, and finally in the 2 cm slice the contrast is only 1.1. The average separation between the two lobes in the 7 mm image is 0B04, which at a distance of 1.2 kpc is equivalent to 50 AU. The average width of the dark lane increases as we observe at longer wavelengths (lower frequencies) and at 1.3 cm the average separation is 0B1 (120 AU at a distance of 1.2 kpc). This effect is clearly seen in the overlay shown in Figure 3. It is evident from the 7 mm image that the dark lane is due to the absence of free-free emission. It could be argued that the dark lane is a region free of gas, either ionized or neutral. However, assuming that the ionized gas from the lobes has an electron temperature of 10 4 K and a sound speed of 10 km s 1, then if the source is stationary the dark lane should be filled by the ionized gas in a time scale of 10 yr; we need the neutral disk in order to avoid this filling from taking place. Also, it can be noted from the 7 mm and 1.3 cm overlay shown in Figure 3 that the width of the dark lane does not remain constant but clearly flares (i.e., becomes wider in its outer parts) as we displace from the central star. The flaring of disks in T Tauri stars has been discussed by Kenyon & Hartmann (1987). Very close to the star the emission from the two lobes is no longer separated, since the dark lane becomes narrower. It is unclear if this emission around the central star is simply due to the lack of angular resolution (in the sense

830 TAFOYA, GÓMEZ, & RODRÍGUEZ Vol. 610 Fig. 2. Left: VLA images of MWC 349 at 2 cm (top), 1.3 cm (middle),and7mm(bottom). Contours are 5, 4, 4, 5, 6, 8, 10, 12, 14, 16, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, and 110 times the rms noise of the images (218, 727, and 859 Jy beam 1 for the 2 cm, 1.3 cm, and 7 mm images, respectively). All images were made with ROBUST ¼ 0 and their beams are shown in the bottom right corner. Right: Averaged slices made at P:A: ¼ 10. Note the increasing contrast between the peak (lobes) and valley (dark lane) values as we go to shorter wavelengths. that radio observations of even higher angular resolution will finally separate the emission from both lobes at all radii) or to the presence of an inner photoionized region around it (as required by the models of Hollenbach et al. 1994 and Lugo et al. 2004). If the latter explanation is correct, we can estimate a lower limit for the electron density in the inner edge of the disk. The angular diameter of this ionized waist can be obtained from a slice along the equatorial direction on the 7 mm image (Fig. 2). Using a Gaussian fit to this slice, we obtain a deconvolved angular diameter of 0B08 0B01 (100 AU at a distance of 1.2 kpc). On the other hand, using the total flux at 7 mm (635 mjy) we obtain that the total number of ionizing photons required to produce this flux is 1:0 ; 10 47 s 1 and consequently the electron density needed to maintain the inner edge of the disk neutral is 1:4 ; 10 7 cm 3. Since the angular radius for the inner region could be smaller, this means that the electron density obtained here is a lower limit. This lower limit is consistent with the predicted electron density necessary to explain the H30 recombination line maser emission (10 7 10 8 cm 3 ;Martín-Pintado et al. 1989b; Gordon 1992; Ponomarev et al. 1994). The ionized inner diameter in the equator, with a size of 100 AU, is similar to the separation of the H30 recombination line maser emission components of 80 AU found by Planesas et al. (1992), suggesting that the maser emission could be arising in the interface region that separates the ionized inner region from the outer neutral disk and not from the surface of the photoionized disk, as is the

No. 2, 2004 IMAGING MWC 349 831 Fig. 3. VLA images at 1.3 cm (gray scale) and7mm(white contours) of MWC 349A. The grey scale goes from 3 to 25 mjy beam 1 and the contours are as given in Fig. 2. Note the flaring of the dark lane with distance from the central position. case for the centimeter recombination lines (Rodríguez & Bastian 1994). 3.3. The 20, 6, and3.6 cm Continuum Emission Figure 4 shows images of MWC 349 at 20, 6, and 3.6 cm that sample larger scales and confirm the presence of a wind interaction between the A and B components, represented by the arc emission region. Cohen et al. (1985) were the first to interpret the arc structure observed at 6 cm as the interaction of the winds from the binary system and used the spectral classification of the B component (B0 III) to estimate a spectroscopic distance to the system of 1.2 kpc. The existence of a near companion had been reported previously by several authors (Merrill et al. 1932; Brugel & Wallerstein 1979; Herzog et al. 1980). Recently, from interstellar polarization observations Meyer et al. (2002) propose that MWC 349A could be part of the Cyg OB2 complex, placing it at a larger distance. Meyer et al. (2002) argue that the weak radio arc structure observed by Cohen et al. (1985) does not unambiguously link the two stars. However, our observations clearly reveal the presence of the interacting wind at the three wavelengths and make their interpretation unlikely. A simple model for a two-wind interaction of isotropic stellar winds given by Cantó et al. (1996) has been used to model the shape of the thin interacting shell in the binary system MWC 349 (see Fig. 4), assuming that the line that joins the two stars is in the plane of the sky. We follow the next analytical relation described by Cantó et al. (1996), R ¼ D sin 1 csc ( þ 1 ); where D is the separation between the two stars and 1 and are the angles between the line that joins the two stars and the lines that connect the stars A and B with a given point in the interacting arc structure, respectively. These two angles are related with the expression 1 csc 1 ¼ 1 þ ( cot 1); where (Ṁ B v B )=(Ṁ A v A ), Ṁ A and Ṁ B,andv A and v B are the mass-loss rates and wind terminal velocities for the A and B components, respectively. Equation (1) can be solved in an approximate way by using the following expansion for 1 (assuming small values for 1 ; see Cantó et al. 1996): ( " 1 15 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi# ) 1 þ 1 þ 4 1=2 (1 cot ) : ð3þ 2 5 We have used our 7 mm flux density and image to estimate a mass outflow for MWC 349A, following Reynolds (1986). We assume an ionized bipolar outflow with constant opening angle (120 ) and terminal velocity (50 km s 1 ), as well as constant electron temperature, taken to be 10 4 K. For the ð1þ ð2þ

832 TAFOYA, GÓMEZ, & RODRÍGUEZ Vol. 610 for the MWC 349B component of 1600 km s 1 (Abbott 1978), the best fit for the arc structure is obtained for ¼ 0:04, which implies a mass-loss rate for the MWC 349B component of 6:3 ; 10 9 M yr 1. Recently, Gordon (2003) has proposed that the terminal velocity of the wind from MWC 349A is only 32 km s 1. If this proposal is correct, the mass-loss rates of components A and B will turn out to be smaller by a factor of 32/50. Figure 4 shows the result of this fit to the arc structure observed at 20, 6, and 3.6 cm, supporting the physical association of components A and B. We also estimated the spectral index of the interaction arc between components A and B, which turns out to be flat, since its flux densities at 20, 6, and 3.6 cm are all consistent with 8 1 mjy. This suggests that we are observing optically thin free-free emission. Other interaction arcs seen in massive binary systems have been found to have a nonthermal spectrum, indicating optically thin synchrotron emission (e.g., Moran et al. 1989; Contreras et al. 1997). However, in the case of MWC 349 the wind velocity of the main component is very low and this may inhibit the acceleration of relativistic electrons and favor free-free emission. In jets from young stars, free-free emission is found to be dominant, except in the cases where fast (more than several hundred km s 1 ) flows are present (Curiel et al. 1993; Martí et al. 1993; Garay et al. 1996; Wilner et al. 1999), where synchrotron dominates. It is somewhat perplexing that a structure appears to the southeast of MWC 349A in the 1996 images at 6 (see Fig. 4, middle panel )and2cm(seefig.2,top panel ). The fact that the structure appears at two different wavelengths seems to imply that it is real and not the result of the data processing. It is not observed in the images made at other epochs, suggesting a transient nature. 3.4. The 90 cm Continuum Emission MWC 349A was detected at 90 cm (see Fig. 5). Before our detection, Setia Gunawan et al. (2003) had set a 4 upper Fig. 4. VLA images of MWC 349 at 20 (top), 6 (middle), and 3.6 cm (bottom). The dashed line is the model described in the text. Contours are 5, 4, 4, 5, 6, 8, 10, 12, 15, 20, 30, 40, 60, 80, 120, 160, 200, 300, 400, 600, 800, and 1000 times the rms noise of the images (91, 35, and 38 Jy beam 1 for the 20, 6, and 3.6 cm images, respectively). The 20 cm image was made with ROBUST ¼ 0, while the 6 and 3.6 cm images were made with ROBUST ¼ 5. The beam of each image is shown in the bottom right corner. adopted distance of 1.2 kpc, we obtain a mass-loss rate of 5:0 ; 10 6 M yr 2. This value is a factor of 2 smaller than that given in the literature by Cohen et al. (1985). This difference is due to the fact that Cohen et al. (1985) assume the outflow to be isotropic, while we take into account the modest collimation present. Adopting our mass-loss rate estimate for the wind of MWC 349A, v A 50 km s 1 (Hartmann et al. 1980; Altenhoff et al. 1981; Tanaka et al. 1985), and a wind terminal velocity Fig. 5. VLA image of MWC 349A at 90 cm. The cross marks the position of the source as determined from the shorter wavelength images. Contours are 2, 2, 3, and 4 times 2.6 mjy beam 1, the rms of the image. This image was made with ROBUST ¼ 0. The beam is shown in the top left corner.

No. 2, 2004 IMAGING MWC 349 833 limit of 28 mjy for MWC 349A at 350 MHz (86 cm). The peak flux density measured by us is 10:6 2:6 mjy (see Fig. 5). Since the image of an ionized wind has very weak and extended emission wings, we integrated the flux density as a function of distance from the center of the source and crudely estimate that the total flux density at 90 cm is 30 10 mjy. This value is in agreement with the upper limit of Setia Gunawan et al. (2003) and is also consistent with the value expected from the power-law extrapolation of the measurements made at shorter wavelengths. 4. VARIABILITY AT 6 cm MWC 349A is well known to be variable in the visible and near-infrared (Andrillat et al. 1982; Jorgenson et al. 2000) as well as in the millimeter and submillimeter maser recombination lines (Martín-Pintado et al. 1989a; Gordon et al. 2001). We searched for variability in the 6 cm continuum, comparing the data taken on 1996 December 5, discussed above, with a similarobservationtakenon1982june4,14.5yrbefore.in the 1982 observations (published by Cohen et al. 1985), we obtain a bootstrapped flux for 2005 þ 403 of 5:39 0:08 Jy and a synthesized beam of 0B038 ; 0B034, P:A: ¼þ17. The two data sets have similar u-v coverage and were cross-calibrated in amplitude and phase using the technique developed by Masson (1986). The 1982 data had only one 50 MHz bandwidth centered at 4.885 GHz, and the comparison was made with the 50 MHz bandwidth of the 1996 data that was centered at the same frequency. After cross-calibration we made images with natural weighting, using as a restoring beam a circular beam with a diameter of 0B46, the average size of the beams of the individual images. We assumed that the emission of the center of the source is optically thick and has not changed between epochs. This condition was enforced by multiplying the 1982 image by 1.01. These images were then subtracted to produce a difference image (see Fig. 6). This difference image clearly shows a negative broken ring at a radius of 0B5. This negative ring has an average intensity of about 4% of that of the 1982 image, i.e., I=I 0:04. Since at the projected radius where the negative ring appears the emission is approximately optically thin and goes as the electron density squared, we have I I (n2 e ) n 2 e 2(n e) n e 0:04: We further assume that this decrease in the electron density is produced by a decrease in the mass-loss rate. We then interpret this intensity decrease to imply that over the period of 14.5 yr separating the observations, the mass-loss rate decreased by 2%. If this trend is maintained in the future, we crudely estimate a lifetime of 700 yr for the massive mass loss stage now present in MWC 349A. It is important to clarify that the decrease in intensity is observed at 0B5 from the star, and thus refers to gas that left it some 50 yr ago. Similar two-epoch observations at different wavelengths will sample different regions in the wind (and thus different times of ejection) and may clarify the behavior of the variability over larger scales of time. 5. CONCLUSIONS We present high angular resolution VLA observations of the continuum emission from 7 mm to 90 cm toward the MWC 349 system. Our main conclusions are summarized as follows. Fig. 6. Cross-calibrated VLA 6 cm images of MWC 349 for epochs 1982.4 (top) and 1996.9 (middle), as well as the difference image 1996.9 1982.4 (bottom). The contours for the 1982.4 and 1996.9 epochs are 4, 4, 5, 6, 8, 10, 15, 20, 40, 80, 160, and 320 times 0.058 mjy beam 1,theaverage rms of the two images. The contours for the difference image are 15, 12, 10, 8, 6, 5, 4, 4, 5, and 6 times 0.082 mjy beam 1. 1. The spectra of MWC 349A derived from our continuum observations show that the total flux densities increase with frequency as 0:670:03 and the angular sizes decrease with frequency as 0:740:03, confirming the presence of a biconical mass-loss rate at constant velocity. This is the first time that a reliable value for the angular size dependence has been derived directly from the observations of MWC 349A.

834 TAFOYA, GÓMEZ, & RODRÍGUEZ 2. The 7 mm image shows clearer than previously the inner structure of MWC 349A. In this image, we obtain the highest contrast yet observed between the emission lobes and the dark lane. The average separation of the lobes is ~50 AU, and this dimension may be related to the width of the neutral disk. The weak free-free emission observed toward the inner region could be due to photoionization from the central star, and if this is the case a lower limit for the electron density of 1:4 ; 10 7 cm 3 is derived. 3. Observations at 20, 6, and 3.6 cm show the presence of the interaction zone between the winds of the MWC 349A and B components, the shape of which has been modeled following Cantó et al. (1996) using the physical parameters Ṁ A 5:0 ; 10 6 M yr 1, v A 50 km s 1, Ṁ B 6:3 ; 10 9 M yr 1, and v B 1600 km s 1. This result supports the physical association between the A and B components. 4. 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