HIGH-ANGULAR-RESOLUTION OBSERVATIONS AT 7 MM OF THE CORE OF THE QUADRUPOLAR HH 111/121 OUTFLOW

In Figure 1 we show the 7 mm image of the core of the HH 111/121 quadrupolar outflow. An unusual structure is evident in the image. The structure can be described as two overlapping, elongated sources aligned in the plane of the sky and approximately perpendicular to each other. We have fitted the emission with two Gaussian ellipsoids using the task JMFIT of the AIPS package (see Table 1). The data image and the model image are shown in Figure 2. In what follows, we will interpret this 7 mm structure, as well as that seen at 3.6 cm, in terms of combinations of ionized-jet/dusty-disk scenarios. However, other possibilities, such as the presence of non-thermal components, should be recognized.

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Table 1. Decomposition of the 7 mm Continuum Emission in Two Components

Component	α(J2000)a	δ(J2000)a	Total Flux Density (mJy)	Deconvolved Angular Sizeb
North–South	05 51 46.2545	02 48 29.660	2.2 ± 0.3	015 ± 002 × 005 ± 001; + 23° ± 6°
East–West	05 51 46.2521	02 48 29.659	1.4 ± 0.3	020 ± 005 × ⩽004; + 116° ± 7°

Notes. ^aUnits of right ascension are hours, minutes, and seconds and units of declination are degrees, arcminutes, and arcseconds. The absolute positional accuracy is estimated to be 001. ^bMajor axis × minor axis; position angle of major axis.

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For the purpose of comparison, in Figure 1 we also show the 3.6 cm continuum image obtained from the VLA data presented by Reipurth et al. (1999). These data have been recalibrated and the position of their phase calibrators updated (by displacements of order 016 to the most recent values in the VLA calibrator catalog) to allow accurate astrometric comparison with the 7 mm image. As first discussed by these authors, the 3.6 cm VLA image suggests a quadrupolar jet with a common origin within 01 (∼40 AU). The main jet is aligned approximately in the east–west direction (that with a P.A. of 97° ± 1° is taken to be the exciting agent of the extended optical HH 111 flow), while the second jet is aligned approximately in the north–south direction (that with a P.A. of 4° ± 4° is taken to be the exciting agent of HH 121). The error in the position angle of the north–south jet was obtained from our reanalysis of the Reipurth et al. (1999) data. The quadrupolar structure of the 3.6 cm emission is more evident in the maximum entropy reconstruction of the image shown by Reipurth et al. (1999). From this image we crudely estimate that about 70% (0.7 ± 0.2 mJy) of the total 3.6 cm emission (1.0 mJy; see Table 2) comes from the east–west jet, while the remaining 30% (0.3 ± 0.2 mJy) comes from the weaker north–south jet.

We are then faced with a question: since the position angles of the two nearly perpendicular structures observed at 7 mm are similar to those observed at 3.6 cm, at 7 mm are we seeing two jets, two disks, or a jet and a disk? To address these interpretations, we first discuss additional continuum observations of the region.

We searched in the literature and the VLA archives for additional data points of the continuum flux density of this source. In Table 2 we present a summary of the total flux densities obtained for the region. The observations are not taken simultaneously and, if time variations are present, this will lead to an erroneous determination of the spectral indices. We also note that the observations are taken at different angular resolutions and that this may affect the comparison. The VLA is sensitive only to emission in angular scales smaller than or comparable to ∼25 times its angular resolution, while for the Owens Valley Radio Observatory (OVRO), the largest detectable angular scale is about 10 times the angular resolution. In particular, the 7 mm data were taken with the highest angular resolution (and thus the smallest detectable angular scale) and we may be underestimating the total flux density at this wavelength in comparison with the other observations, which are all taken at lower angular resolutions. Lower-angular-resolution VLA observations at 7 mm are needed to determine the complete flux density of HH 111.

The total continuum flux density at 7 mm is about 40% larger than the addition of the individual flux densities given in Table 1. This difference results from the presence of faint, extended 7 mm emission that is not accounted for in the individual Gaussian fits. Analysis of the residual (with the Gaussian fits removed) image suggests that this faint emission seems to extend over a diameter of ∼03, but its nature is unclear. As can be seen from Table 2 and in Figure 3, the flux density is relatively flat in the centimeter range and rises rapidly above ∼30 GHz. We have fitted the data points with the sum of two power laws of the form

$$\begin{eqnarray*} S_\nu ({\rm total}) &=& S_{8.4\,{\rm GHz}}({\rm free-free}) (\nu /8.4\,{\rm GHz})^{0.3}\nonumber\\ &&+ S_{43.3\,{\rm GHz}}({\rm dust}) (\nu /43.3\, {\rm GHz})^{2.5}, \end{eqnarray*}$$

where the first term is intended to represent the contribution of free–free emission from the ionized outflow and the second term is intended to represent the contribution of dust emission from the disks. Since we have four parameters to fit and only five data points, this fit is only intended to show that the data can be reasonably fitted with the sum of two power laws. In Figure 3 we show the fit for S_8.4 GHz(free–free) = 0.93 mJy and S_43.3 GHz(dust) = 3.6 mJy. Under this interpretation, the 8.44 GHz emission is clearly dominated by free–free, with only ∼6% of dust contribution. On the other hand, the 43.34 GHz emission is dominated by dust, with a ∼30% contribution from free–free. Following Rodríguez et al. (2007) and Hunter et al. (2006), we estimate the total mass in the dust to be on the order of 0.1 M_☉. For this estimate, a dust temperature of 45 K was used, following the assumptions of Stapelfeldt & Scoville (1993).

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The excess of 7 mm emission with respect to the value expected from the extrapolation of the centimeter measurements rules out the possibility that at 7 mm we are simply observing two jets, since we have estimated that a significant fraction (∼70%) of the 7 mm emission is due to dust. However, we cannot fully rule out the possibility that we are seeing a core dominated by dust emission with the extended emission dominated by free–free emission.

The two-disks interpretation is suggested by the fact that at 3.6 cm the structure with the larger flux density is the east–west one, while at 7 mm this is true for the north–south structure (see Figure 1 and Table 1). Rodríguez et al. (2008), from a comparison between the momentum rate in the molecular outflow and the radio continuum emission from the jet, have argued that high-mass young stars can be understood as scaled-up versions of low-mass young stars. Assuming that this conclusion is also valid for the parameters of jets and disks, and with the tentative assumption that the flux densities of disks and jets are correlated, this favors matching the east–west structure at 3.6 cm (the jet) with the north–south structure at 7 mm (the disk), and the north–south structure at 3.6 cm (the second jet) with the east–west structure at 7 mm (the second disk), forming a binary disk–jet system similar to that observed in L1551 IRS5 (Rodríguez et al. 2003).

This interpretation faces, however, three difficulties. The first difficulty is that the axes of the proposed disk–jet structures are not, as expected, nearly perpendicular. The east–west structure at 3.6 cm has a P.A. of 97° ±  1°, while the north–south structure at 7 mm has a P.A. of 23° ±  6°, having a relative angle of 74° ±  6° between them. Furthermore, the north–south structure at 3.6 cm has a P.A. of 4° ±  4°, while the east–west structure at 7 mm has a P.A. of 116° ±  7°, having a relative angle of 112° ±  8° between them.

The second difficulty is related to the stability of the proposed binary disk system. From Table 1, we estimate that the centroids of the two 7 mm structures are separated by 0036 ± 0014 or ∼15 ± 6 AU at a distance of 414 pc. We expect the disks in a binary system to be truncated at radii on the order of a fraction of the binary separation (Armitage et al. 1999; Papaloizou & Pringle 1987; Pichardo et al. 2005). We then expect the radii of the disks to be compact, ⩽15 AU (assuming that the observed separation is comparable to the true separation). In contrast, the observed radii (see Table 1) are significantly larger, on the order of 35 AU.

The final difficulty is that, if we assume masses of order 1 M_☉ for the stars in the binary system, an orbital period of order 40 years is expected for a separation of ∼15 AU. The expected timescale in which strongly misaligned binary disks should be brought into rough alignment by tidal torques is about 20 orbital periods (Bate et al. 2000), which is only 800 years if the 15 AU separation is real. With this relatively short timescale it is difficult to explain the lifetime of 10⁴ years of the HH 111 jet and its remarkable stability over this period. On the other hand, it should be noted that Reipurth et al. (1992) have suggested that the separation of bright knots in the optical flow are consistent with a timescale of variation of the central source of 40 years.

These last two difficulties are mitigated if we consider that the real separation is much larger than the projected one. A physical separation of 105 AU would be consistent with the values of 35 AU for tidally truncated radii. The orbital period would now be about 800 years and the timescale for alignment ∼1.6 × 10⁴ years, consistent with the lifetime of the system. However, if we define the physical separation as r, the probability of observing it at a projected separation of r' or smaller is

$$$ P({\le} r^{\prime }) = 1 - [1 - (r^{\prime }/r)^2]^{1/2}, $$$

which, for r' = 15 AU and r = 105 AU, gives P(⩽r') ≃ 0.01.

We conclude that either we are observing the binary at an unlikely orientation, that disks in binary systems are more stable and independent than previously thought, or that the interpretation is incorrect.

This final interpretation implies that at 7 mm we are seeing a north–south structure dominated by dust emission that traces a disk and an east–west structure that could be related to the jet that powers the optical HH 111 outflow. A major advantage with this interpretation is that the structures have a relative angle of 93° ±  9°, very close to perpendicularity. The east–west structure has a flux density of 0.7 ± 0.2 mJy at 3.6 cm and of 1.4 ± 0.3 mJy at 7 mm. This results in a spectral index of 0.4 ± 0.2, which is consistent with the value expected for a thermal jet (e.g., Anglada 1996; Eislöffel et al. 2000).

However, this interpretation also presents some difficulties. The east–west structure at 3.6 cm has a position angle of 97° ±  1°, while the east–west structure at 7 mm has a position angle of 116° ± 7°. In other words, the two structures are not nearly parallel, as would be expected if the 3.6 cm emission were the larger-scale counterpart of the 7 mm jet emission. Under this interpretation, all the 7 mm emission is related to the HH 111 disk–outflow system and there is no evidence at this wavelength of the HH 121 system. Of course, it should be pointed out that the dust and free–free emission from the HH 121 system are probably present in the image at a low level, but that we cannot disentangle their presence given the modest signal-to-noise ratio of the data.