RADIO CONTINUUM DETECTION OF THE EXCITING SOURCES OF THE DG TAURI B AND. L1551NE OUTFLOWS. LUIS F. RODR´IGUEZ. Instituto de ...
THE ASTROPHYSICAL JOURNAL, 454 : L149 –L152, 1995 December 1 q 1995. The American Astronomical Society. All rights reserved. Printed in U.S.A.
RADIO CONTINUUM DETECTION OF THE EXCITING SOURCES OF THE DG TAURI B AND L1551NE OUTFLOWS LUIS F. RODR´ IGUEZ Instituto de Astronomı´a, UNAM, Apartado Postal 70-264, Me´xico, DF 04510, Mexico
GUILLEM ANGLADA Departament d’Astronomia i Meteorologia, Universitat de Barcelona, Avenida Diagonal 647, E-08028 Barcelona, Spain AND
ALEJANDRO RAGA Instituto de Astronomı´a, UNAM, Apartado Postal 70-264, Me´xico, DF 04510, Mexico Received 1995 September 1; accepted 1995 September 13
ABSTRACT The exciting sources of molecular outflows are characteristically associated with centimeter radio continuum emission, most probably originating in a partially ionized, collimated outflow. DG Tau B and L1551NE are two low-luminosity pre–main-sequence objects that have recently been found to be associated with molecular outflows. We have used 3.5 cm Very Large Array observations with angular resolution of 0"2 to detect compact radio continuum sources at both the positions of DG Tau B and L1551NE. The DG Tau B radio source has deconvolved dimensions (0"38 H 0"02) 3 (0"22 H 0"02), and its major axis is aligned along a position angle of 2988 H 58, coincident within a few degrees with the position angle of the axis of the optical and molecular outflows (12948). This result suggests that we are observing the base of the collimated jet that powers the region, on scales of tens of astronomical units. A comparison of the widths of the radio and optical jets in this source supports this interpretation. Subject headings: ISM: jets and outflows — radio continuum: ISM — stars: individual (DG Tauri B, L1551NE) — stars: pre–main-sequence cloud that was discovered by Emerson et al. (1984) from IRAS data. It is located at about 1490 to the east of L1551 IRS 5, the well-studied embedded source responsible for the spectacular bipolar outflow in the region. The detailed study of MoriartySchieven, Butner, & Wannier (1995) revealed that L1551NE has a molecular outflow of its own that is aligned approximately in the east-west direction. While the most luminous sources in these two regions of the sky (DG Tau and L1551 IRS 5) have received considerable attention, relatively little is known of DG Tau B and L1551NE. In this Letter we have used Very Large Array observations that were made with DG Tau and L1551 IRS 5 as phase centers to search and detect radio continuum emission associated with DG Tau B and L1551NE.
1. INTRODUCTION
Molecular outflows are believed to be driven by highly collimated jets that emanate from young stars (Raga et al. 1993; see review by Raga 1995). Within 10 of the exciting source, these jets can be detected and sometimes mapped in the radio continuum; the emission mechanism is most probably free-free radiation from the (partially) ionized jet. An important piece of evidence in favor of the radio jet–molecular outflow connection is the correlation found between the centimeter continuum luminosity and the momentum rate in the molecular outflow (Anglada et al. 1992; Cabrit & Bertout 1992; Anglada 1995). The continuum observations of these thermal radio jets have proved to be of significance in the detection of the exciting source of the outflows, in particular in complex fields, since the radio observations have good angular resolution, provide accurate positions, and are relatively unaffected by dust absorption. In some cases, it has been possible to image the jets, thereby providing strong evidence that the collimation processes are present on very small scales (#100 AU) and showing that the flux density and angular size dependences with frequency are in agreement (Rodrı´guez et al. 1994) with the predictions of thermal bipolar jet models (Reynolds 1986). DG Tau B is a low-luminosity pre–main-sequence star located about 550 to the southwest of the well-known T Tauri star DG Tau (Mundt & Fried 1983; Jones & Cohen 1986). DG Tau B has a narrow optical jet (Mundt, Ray, & Raga 1991) and a collimated, redshifted CO outflow recently discovered by Mitchell et al. (1994); both optical jet and molecular outflow are well aligned at a position angle of 12948. L1551NE is a young stellar object in the L1551 molecular
2. OBSERVATIONS
The 3.5 cm observations were carried out using the VLA of NRAO1 in the A configuration during 1994 April 17 (DG Tau B) and 1994 April 10 and 22 (L1551NE). The phase centers of the observations were at the positions of DG Tau and L1551 IRS 5, respectively. The 3.5 cm observations were made using the outer 18 antennas of the array, with the inner nine antennas being used at 43.3 GHz. The two sources discussed in this Letter fall outside the 43.3 GHz primary beam, and data at this frequency are not discussed here. The 3.5 cm observations have an angular resolution of 10"2 and are sensitive to structures smaller than 120. For all observations an effective bandwith of 100 MHz was employed. The absolute amplitude and phase calibrators were 13281307 and 04001258, respec1 NRAO is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.
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Vol. 454 TABLE 1
RADIO POSITIONS
AND
FLUX DENSITIES
Source
a(1950) a
d(1950) a
S 3.5 cm b
DG Tau B. . . . . . . . L1551NE . . . . . . . . .
04 h 23 m 58!91 04 28 50.54
125858949"2 118 02 09.7
0.36 H 0.02 0.18 H 0.03
a b
Positional error is estimated to be 10"05. Total flux density corrected for primary beam response. 3. DISCUSSION
FIG. 1.—Natural-weight VLA map at 3.5 cm wavelength of the exciting source of the DG Tau B molecular outflow. The cross marks the optical position of the source, located at a(1950) 5 04h 23m 59!0 and d(1950) 5 1258589490 H 10 (Mundt et al. 1991). The half-power contour of the beam (that includes the effect of bandwidth smearing) is shown in the top right-hand corner. Contour levels are 23, 3, 4, 5, 6, 7, and 8 times the rms noise of 15 mJy beam21 .
tively. The data were edited and calibrated using the software package AIPS of NRAO. Cleaned, natural-weight maps made at the positions of DG Tau B and L1551NE are shown in Figures 1 and 2.
FIG. 2.—Natural-weight VLA map at 3.5 cm wavelength of the exciting source of the L1551NE molecular outflow. The cross marks the optical position of the source, located at a(1950) 5 04 h 28 m 50!5 and d(1950) 5 1188029100 H 10 (Draper, Warren-Smith, & Scarrott 1985). The half-power contour of the beam (that includes the effect of bandwidth smearing) is shown in the top right-hand corner. Contour levels are 23, 3, 4, and 5 times the rms noise of 23 mJy beam 21 .
Radio sources were detected in association with the two objects studied. The sources are significantly displaced from the phase center of the observations (by 550 in the case of DG Tau B and by 1490 in the case of L1551NE), and, for a proper analysis of the maps, one has to take into account two effects: primary beam response correction and bandwidth smearing. The primary beam response correction is made by simply multiplying the map intensity by a factor to account for the decrease in sensitivity away from the primary beam center. This correction has been incorporated into Figures 1 and 2 and into Table 1, which lists the positions and total flux densities of the sources. Bandwidth smearing, the radio equivalent of optical chromatic aberration, produces a broadening of the source in the radial direction (the direction between the center of phase and the source). The half-power beam contours in Figures 1 and 2 have been calculated taking this effect into account; that is, they are the convolution of the unsmeared synthesized beam with a radially extended ‘‘beam’’ calculated following Condon et al. (1995). The source associated with DG Tau B is elongated, and its deconvolved dimensions (full width at half-maximum, as derived using the task IMFIT of AIPS) are (0"38 H 0"02) 3 (0"22 H 0"02) with its major axis aligned along a position angle of 2988 H 58, coincident within a few degrees with the position angle of the axis of the optical and molecular outflows (12948: Mundt et al. 1991; Mitchell et al. 1994). This result suggests that, as in other exciting sources of molecular outflows that have been studied with subarcsecond resolution (see review by Rodrı´guez 1995), we are observing the base of the collimated jet that powers the large-scale outflow. For a distance of 160 pc, our results suggest that collimation is already present at a scale of about 30 AU. In the case of L1551NE the source is detected only at the modest, but statistically significant, level of 5 s and appears unresolved for our elongated beam. There is a secondary source at the 4 s level about 0"6 to the east of the first source. New observations made with this source at the phase center are required to study its morphology and to test if it is multiple. Since the a priori probability of finding a 3.5 cm source with flux density greater than 0.1 mJy in a solid angle of 20 3 20 (the approximate error box of the optical positions) is only 16 3 10 25 , we consider both radio detections to be highly significant. The bolometric luminosities of the sources DG Tau B and L1551NE are about 1 and 6 L J , respectively (Jones & Cohen 1986; Emerson et al. 1984). For these low-luminosity objects, ionization by stellar photons appears to be insufficient to explain the observed radio continuum emission. An estimate of the expected free-free emission at 3.5 cm can be obtained from the Lyman-continuum ionizing fluxes calculated by Thompson (1984) for a zero-age main-sequence star. If we
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assume optically thin emission, a temperature of 10 4 K, and a distance of 160 pc for both sources, the expected flux density at 3.5 cm is only 9 3 10 214 mJy for DG Tau B and 8 3 10 210 mJy for L1551NE. These flux densities are many orders of magnitude below those observed (Table 1), which indicates that another mechanism should be producing the required ionization. An alternative possibility is that ionization is produced by shocks associated with the outflows. In the model of Curiel, Canto ´, & Rodrı´guez (1987) and Curiel et al. (1989), an initially neutral stellar wind is shock-ionized when it impacts with dense material surrounding the young star, and the expected 3.5 cm flux density in the optically thin case can be expressed ˙ in terms of the terminal velocity V and the momentum rate P of the wind as
S D
S DS
Sn V 5 3.2 3 10 3 mJy 4p 3
S
˙ P
M J yr
V 200 km s 21
21
km s 21
D
D S DS D 20.32
d kpc
22
T 10 4 K
0.45
,
(1)
where d is the distance to the source, T is the temperature of the ionized gas, and V/4p is an efficiency factor that can be taken to equal the fraction of the stellar wind that is shocked. The resulting flux density depends only weakly on the values of V and T, for which we will adopt typical values of 200 km s 21 and 10 4 K, respectively. Assuming that the momentum rate of the molecular outflow equals that of the wind, the expected flux density can be obtained from equation (1). The momentum rate in the DG Tau B molecular outflow is 7.6 3 10 26 M J km s 21 yr 21 , after proper correction for the orientation of the outflow (Mitchell et al. 1994; Mitchell 1995). For the L1551NE molecular outflow, the momentum rate is estimated to be 1.5 3 10 25 M J km s 21 yr 21 (Moriarty-Schieven et al. 1995). Using these values in equation (1), the observed 3.5 cm flux densities can be easily explained by shock ionization with an efficiency of 140% for DG Tau B and of 110% for L1551NE. These two sources fit well in the correlation between the observed centimeter radio continuum luminosity, S n d 2 , and the observed momentum rate in the molecular out˙ , which is found for more than 30 sources of low boloflow, P metric luminosity (Anglada 1995). A least-squares fit gives ˙ 5 10 22.5H0.3 (S n d 2 ) 1.1H0.2 , which is in agreement with the preP dictions of equation (1) with an average efficiency of 110%. These results corroborate the existence of a solid link between large-scale outflows and small-scale radio jets. It is interesting to note that even though the observed position of the DG Tau B radio source differs by 110 from the position of the optical source (see Fig. 1), the width of the radio source appears to coincide quite closely to what is expected from the optical observations. The 110 displacement between the optical and the radio sources could be due to errors in the relative registration of the radio and optical frames of reference. Figure 3 shows the FWHM of the DG Tau B jet and counterjet in the region within 200 from the source measured on a [S II] 6717131 frame by Mundt et al. (1991). Both the longer, redshifted jet and the shorter, blueshifted counterjet appear to have linear width versus position dependences, even though their opening angles are clearly
FIG. 3.—FWHM of the DG Tau B jet and counterjet as a function of position measured on a [S II] 6717131 image by Mundt et al. (1991). The top graph shows the width of the knots within 200 from the optical source, where we see that both the jet and the counterjet appear to have approximately conical morphologies. The origin of the position axis is placed on the projection of the position of the radio source along the axis of the optical outflow. A blowup of the region close to the source is shown in the bottom graph. The cones described by the jet and the counterjet meet at a position (black dot with error bars) that quite closely coincides with the optical source (whose position is labeled as ‘‘os’’ on the graph) and determine a width of 20"2 for the jet at the position of the source. The position and width of the radio source (‘‘rs’’) are indicated with a rectangle. The widths derived from the radio and optical data are consistent.
quite different from each other (see Fig. 3 and Raga & Canto ´ 1989). By carrying out linear fits to the jet and counterjet width versus position, we find that these lines intersect at a point (0"14 H 0"17) west of the optical source. This point of intersection obtained by extrapolating the linear width versus position dependences of the jet and counterjet toward the position of the source also results in a FWHM 0 5 (0"21 H 0"07) width (which is shown in Fig. 3) at intersection. This width coincides surprisingly well with the 20"22 deconvolved width of the radio source (see above). This result appears to imply that in the optical observations we see the same volume of emitting material that we observe at radio wavelengths after having gone through an expansion. Furthermore, we see that this expansion has taken place through a constant opening angle, approximately conical flow. This appears to be the first time in which it is possible to show that the radio emission from a thermal radio jet might correspond to the full extent of the material observed in the optical jets, and not only to some narrow, more energetic component of the outflow. The coincidence in the value of the FWHM 0 determined independently from the radio and optical data suggests that in this source the collimation of the jet is taking place on a scale of 30 AU. L. F. R. acknowledges support from DGAPA, UNAM, and CONACyT, Mexico. G. A. acknowledges support from DGICYT grant PB92-0900, Spain.
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RODRI´GUEZ, ANGLADA, & RAGA REFERENCES
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