SMA SUBMILLIMETER OBSERVATIONS OF HL Tau: REVEALING A COMPACT MOLECULAR OUTFLOW

In Figure 1, we show the resulting 850 μm continuum image from HL Tau made with the SMA. We only detected a single source that is associated with HL Tau. This source has a position in the sky of α_J2000.0 = 04^h31^m3842, δ_J2000.0 = +18°13'5739, with a positional error of less than 10. This position coincides very well within the uncertainty with the position obtained by Mundy et al. (1996) at 2.7 mm with BIMA, discarding any proper motion of HL Tau in this interval of time.

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We measured a flux density and peak intensity for this compact source of 1.3 ± 0.3 Jy and 1.0 ± 0.2 Jy Beam⁻¹, respectively. These flux values are in good agreement with the SED for HL Tau obtained in Kwon et al. (2011). In Figure 1, HL Tau looks rounded; however, this is resolved as an elongated source using the MIRIAD task imfit and jmfit of AIPS. These two tasks use elliptical Gaussian fittings to estimate the source parameters. The deconvolved size that we get is that one for an elongated source with a northwest–southeast orientation (approximately 15 × 05 with a P.A. = +145°). This angular size corresponds to ∼200 × 70 AU with a P.A. = 145° at the distance of the Taurus molecular cloud. These size values also are in good agreement with those values already obtained by Kwon et al. (2011) using CARMA 1.3 and 2.7 mm observations. Given the high signal-to-noise ratio obtained from our observations, we tried to verify the results obtained by the tasks imfit and jmfit, and we then restored our original image with a slightly smaller beam (see Figure 1), revealing the elongated morphology obtained by the tasks.

Assuming optically thin isothermal dust emission, a gas-to-dust ratio of 100, a dust temperature of 30 K, a dust mass opacity κ_850 μm = 1.5 cm² g⁻¹, and an emissivity index β = 1.0 (see Mundy et al. 1996; Kwon et al. 2011), we estimate a total mass for the source of roughly 0.1 ± 0.02 M_☉. The uncertainty here is only the error in the flux measurement of HL Tau. However, we noted that the temperature for the disk used here could be higher, as shown in the model presented in Kwon et al. (2011).

This combination of properties suggests that the emission seen at 850 μm is dominated by the accretion disk, although a very small contribution from the inner envelope might also be present.

In Figure 2, we show the average spectrum of the ¹²CO(3–2) line toward HL Tau. The box length of the averaging spectra is about 5 arcsec, centered on the position of HL Tau. The spectrum has two strong peaks; the first appears to be at a velocity slightly smaller than 6.5 km s⁻¹, and the other at about 11.5 km s⁻¹. The systemic velocity of the cloud is ∼6.5 km s⁻¹ (Monin et al. 1996; Cabrit et al. 1996). The blueshifted peak is probably arising from the outflow and disk as revealed by Cabrit et al. (1996). The blueshifted peak is fainter than the redshifted one. There is one dip feature at about 8.5 km s⁻¹ that might have been caused by the missing flux at velocities close to systemic. In addition, there are some faint high-velocity features associated with the outflow emanating from HL Tau.

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In Figure 1, in addition to the 850 μm continuum image, we overlaid the integrated intensity (moment 0) high-velocity map of the ¹²CO(3–2) thermal emission. In the image, the blue and red colors correspond to the blueshifted and redshifted gas emission, respectively. To construct the moment 0 high-velocity maps, we integrated LSR velocities ranging from +1.88 km s⁻¹ to +5.1 km s⁻¹ for the blueshifted one, and LSR velocities ranging from +13.24 km s⁻¹ to +25.53 km s⁻¹ for the redshifted one. The emission at ambient velocities (+6 to +11 km s⁻¹) was clearly extended and poorly sampled with the SMA, and was suppressed in this moment 0 map. This extended emission is arising mainly from the molecular cloud, and thus the systemic velocity of the cloud and HL Tau should be placed within this range.

This image reveals a compact bipolar molecular outflow emanating from HL Tau. The bipolar outflow has a northeast to southwest axis with a position angle of about 50°. We thus noted that this bipolar outflow is the molecular counterpart of the optical and infrared jet that emanates from HL Tau with a similar position angle (Anglada et al. 2007; Takami et al. 2007; Krist et al. 2008). However, contrary to being collimated as the optical and infrared jet or other bipolar molecular outflows, e.g., HH 797 or HH 625 (Zapata et al. 2005; Pech et al. 2012), the molecular counterpart is a wide opening angle outflow and is only present very close to HL Tau. These physical properties have also been observed in other outflows, for example, in HH 30, and are attributed to the entrainment by the optical jet (Pety et al. 2006).

We note that both the blue and red northwest walls appear to be significantly more defined than the southeast ones. A possibility is that this effect could be created if very dense molecular material is present in the west part of the cloud. This condition will allow better entrainment of the molecular gas material.

The spatial structure of the moderate-/low-velocity gas of the bipolar outflow as a function of the radial velocities (position–velocity (PV) diagram) along the major axis is shown in Figure 3. This image shows how the redshifted component covers a much wider range of velocities than the blueshifted side. The redshifted lobe displays a high-velocity component with a low-velocity component extending far from the position of the star. On the other hand, the blue side of the outflow displays only a limited range of velocities close to the exciting source.

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Based on figures presented by Arce et al. (2007) and Lee et al. (2000) that show the molecular outflow properties predicted by different models and on the morphology and the PV diagram obtained here from the outflow (Figures 1 and 3), we suggest that this outflow is reminiscent of a wide-angle outflow. However, we noted that there is also a collimated jet emanating simultaneously from HL Tau that is mapped in the optical and infrared wavelengths (Krist et al. 2008). This is therefore one of the outflows that seems to present simultaneously a jet and a wide-angle outflow (see Arce et al. 2007). However, many more sensitive high angular resolution observations are needed to better resolve the putative wide-angle outflow and the jet.

Assuming that we are in local thermodynamic equilibrium, the ¹²CO(3–2) molecular emission is optically thin, a fractional abundance of 10⁴ between the carbon monoxide and the molecular hydrogen, a distance of 140 pc, and an excitation temperature T_ex = 50 K, we estimated a mass of 2.5 × 10⁻³ M_☉ for the outflow energized by HL Tau. This value for the outflow mass is expected for a flow energized by a young low-mass protostar; see Wu et al. (2004). The uncertainty in the outflow mass is on the order of a factor of two, mainly due to the ambiguity of the excitation temperature. We also estimate a kinematical energy of 3 × 10⁴² ergs and an outflow momentum of 0.03 M_☉ km s⁻¹. These estimates do not take into consideration the inclination of the outflow with respect to the line of sight and the amplitude calibration.

We found some faint molecular emission toward the southeast of HL Tau (about 10''), where the optical object HH 153 is located; see Figure 1. The emission revealed in Figure 1 seems to be marginal; however, this emission is present in a few velocity channels (−0.88 to +1.24 km s⁻¹), making our detection more reliable. HH 153 is a traverse outflow that appears to emanate from the HL Tau vicinity (Krist et al. 2008). However, we note that the measured proper motions of HH 153 (knots Halpha-B1, B2, B3, C1, C2; Anglada et al. 2007) point away from HL Tau also supporting an origin in this source. Future sensitive molecular observations of this region may help to reveal its energizing object.