Online Material

  Appendix A: The use of HCO⁺ as a dense gas tracer

  A.1 Outflow cavities in TMR1, TMC1A and GSS30-IRS1

The HCO⁺ 3-2 alignment with the near-infrared scattering nebulosities in the HST images for TMR1, TMC1A and GSS30-IRS1 (Fig. 10) is similar to what has been found in high angular resolution observations of the lower excitation 1-0 transition in both Class I (Hogerheijde et al. 1997a) and Class 0 (Jørgensen 2004) sources.

The patterns in the velocity maps (Fig. 11) support the interpretation that the HCO⁺ 3-2 emission has its origin in one-side of the outflow cone: in all maps the HCO⁺ emission is blue-shifted with respect to the systemic velocity with the most extreme velocities (most blue-shifted) narrowly confined around the continuum position and material closer to the systemic velocity extending around this. This signature can best be understood in a simple geometry where HCO⁺ probes material in a large infalling protostellar envelope being swept up by the outflow, as illustrated in Fig. A.1. If the velocities in the swept-up material are relatively small (comparable to the infalling velocities in the envelope) and HCO⁺ 3-2 optically thick, the front-side of the envelope (toward us) will be slightly red-shifted due to the infall and thus obscure the outflowing material in the red-shifted cone on the far-side of the envelope (Fig. A.1), which in turn is resolved-out. In contrast, the outflow cone pointed toward us will be unobscured by the envelope. Likewise, the near-infrared scattered light from the red-lobe would largely be blocked - in contrast to the scattered emission from the outflow cavity on the front-side. An alternative explanation is that all the sources are at the far-side of their respective clouds, the red-shifted part of the outflow cone breaking out into a low-density medium not observable in the line tracers. This would require a rather sharp density gradient, however.

  A.2 Limitations of HCO⁺ 3-2 as a tracer of disks

The results above suggest that HCO⁺ 3-2 is a good tracer of motions in disks for envelopes that have already been largely dispersed. For the more embedded YSOs, HCO⁺ 3-2 emission may be of limited use as a tracer of rotation in the disk due to its optical thickness. A simple estimate can be made of the typical envelope mass below which HCO⁺ 3-2 line emission is optically thin in the envelope and can be used as a tracer of the kinematics in the innermost regions of the protostar. First, for typical envelope parameters we expect temperatures in the range of 15-40 K and densities of 10⁴-10⁷ cm^-3. Using the Radex escape probability code (van der Tak et al. 2007) for these parameters, the HCO⁺ 3-2 line has an optical thickness 1 for an HCO⁺ column density of  cm^-2 and a line width of 1 km s^-1. Secondly, we can estimate what density, is required to reach that optical thickness in a free-falling envelope with a power-law density profile, at a size scale of 100 AU corresponding to the region where the disk potentially forms, i.e., by simply estimating from

with  cm^-2. Assuming a typical abundance of HCO⁺ of with respect to H₂(e.g., Jørgensen et al. 2004b), we find  cm^-3, which in turn corresponds to an envelope mass of 0.1 . This is the maximum envelope mass for which we can use the HCO⁺ 3-2 to trace scales smaller than about 100 AU without worrying about opacity effects in the larger scale envelope. It indeed corresponds to the estimate of the masses of the most deeply embedded Class I sources in our sample for which the outflow emission is seen. For these sources and the even more deeply embedded Class 0 sources a less abundant or more optically thin tracer is therefore required to trace the dynamical structure on less than 100 AU scales. Future observations with ALMA will provide the sensitivity to image, e.g., the optically thin 3-2 and 4-3 submillimeter transitions of H¹³CO⁺ in the disks around even more deeply embedded protostars and thus provide stellar masses for protostars in the deeply embedded stages.

 Appendix B: Summary of observed Class I sources

B.1 IRS 43

IRS 43 (YLW15; Young et al. 1986) has been the target of numerous studies across the full wavelength range. At radio wavelengths, IRS 43 is resolved into two separate components, YLW15-VLA1 and -VLA2 with a separation of 0.6 (Girart et al. 2000). From multi-epoch high angular resolution radio data, these binary components were found to show relative proper motions indicative of orbital motions in a 1.7  total mass binary (Curiel et al. 2003). Girart et al. found that one of the two components, VLA1, were resolved in observations at 3.6 and 6 cm with the VLA with a deconvolved size of about 0.4 along its major axis and suggested that it was associated with a thermal radio jet. In addition, IRS 43 is a peculiar X-ray emitter showing quasi-periodic X-ray flares (Tsuboi et al. 2000) consistent with magnetic shearing and reconnection between the central star and an accretion disk, with the period suggesting a mass of the central star in the range 1.8-2.2  (Montmerle et al. 2000).

The region was imaged in the near-infrared by Grosso et al. (2001) who discovered a set of ``embedded'' Herbig-Haro objects, well aligned with the thermal jet candidate from the centimeter observations of Girart et al. (2000). Spitzer Space Telescope images of the embedded Herbig-Haro objects (Fig. B.1) show the knots clearly along a line pointing directly back toward the group of objects where IRS 43 and another embedded protostar, IRS 44, is located. As also pointed out by Grosso et al., the association of the Herbig-Haro knots to IRS 43 is not unique: the nearby protostar, IRS 54 (see also Sect. B.5), also drives a large-scale, precessing outflow clearly seen in the Spitzer images in Fig. B.1.

The dotted lines in Fig. B.1 show the propagation of the IRS 54 outflow, following the H₂ emission in the eastern lobe and assuming that the precession of the IRS 54 outflow is symmetric around the protostar - thereby giving the expected location of the western lobe. The western lobe in this case passes through the location of the southern part of the near-infrared Herbig-Haro objects: based on their morphologies in the higher resolution near-infrared images, Grosso et al. (2001) suggested that these knots (crosses in the lower left panel of Fig. B.1) are indeed caused by the IRS 54 outflow with the northern knots (plus signs in the lower left panel of Fig. B.1 related to the IRS 43 outflow - the lower spatial resolution Spitzer images do not reveal this distinction clearly, though. Some support for the interpretation of Grosso et al. can be taken from the morphology of another bow-shock seen in the Spitzer images at to the west of the near-infrared Herbig-Haro knots. Together with the near-infrared Herbig Haro knots detected by Grosso et al., this bow-shock is on the southern side of the dashed-line suggesting that the dashed line indeed is delineating the northern edge of the western lobe (and the southern edge of the eastern lobe) of the IRS 54 outflow. It is also possible that another source in the group encompassing IRS 43 could be responsible for the Herbig-Haro knots: the main argument for the association to IRS 43 is the coincidence with the radio jet and the identification of a bow-shock south of IRS 43 in the near-infrared, possibly representing the counter-jet (Grosso et al. 2001; Girart et al. 2000,2004).

We detect IRS 43 in both HCO⁺ 3-2, HCN 3-2 and continuum. The HCN and HCO⁺ 3-2 emission is elongated in the W/NW-E/SE direction with a clear velocity gradient along its major axis. The major axis of the HCO⁺ and HCN 3-2 emission is close to the axis of the continuum emission - and perpendicular to the direction toward the Herbig-Haro objects (Fig. B.1; Grosso et al. 2001) and the thermal continuum jet (Girart et al. 2000).

B.2 TMR1 and TMC1A

Images of TMR1 using the HST/NICMOS at 1.6-2.1 m by Terebey et al. (1998) show s triple system consisting of a close binary (TMR-1A and TMR-1B) with a projected separation of 0.3 and a third source TMR-1C offset from the binary by about 10 but connected with this through an extended filament of near-infrared nebulosity. Terebey et al. (2000,1998) discuss the option that TMR-1C is a runaway giant planet, but conclude based on near-infrared spectroscopy that it is most likely is a background star. TMC1A is an embedded protostar with an associated scattering nebulosity (e.g., Chandler et al. 1996; Hogerheijde et al. 1998) aligned with blue-shifted outflow emission.

Both TMR1 and TMC1A are detected in continuum, together with HCO⁺and HCN 3-2. Both show clear evidence of the line emission being offset from the continuum peaks toward the outflow nebulosity observed in the near-infrared HST/NICMOS images - similar to the case for GSS30-IRS1. The offsets in the HCO⁺ 3-2 emission toward the two sources are similar to those observed in HCO⁺ 1-0 by Hogerheijde et al. (1998).

B.3 GSS30

The GSS30 region is the most complex of the studied fields associated with a clear near-infrared nebulosity. At least three young stellar objects (GSS30-IRS1, -IRS2 and -IRS3) are seen at infrared wavelengths, located within the SMA primary beam field of view, (Weintraub et al. 1993) and observed by Spitzer (e.g., Jørgensen et al. 2008). GSS30-IRS1 is located at the center of this outflow cone and has a mid-infrared SED slope consistent with a Class I protostar. GSS 30-IRS3 (LFAM1) was detected in high-resolution images at 2.7 mm (Zhang et al. 1997) and 6 cm (Leous et al. 1991). GSS30-IRS2 is likely a more evolved (T Tauri) young stellar object also detected at 6 cm.

The SMA continuum observations identify one protostar GSS30-IRS3 (LFAM1) whereas the two other protostars remain undetected down to the noise level of 3 mJy beam^-1. The same was the case for the similar in the 2.7 mm observations by Zhang et al. (1997). The single-dish continuum emission toward GSS30 is often attributed solely to GSS30-IRS1 (e.g., Zhang et al. 1997), but appears to be peaking at GSS30-IRS3 consistent with the suggestion that this source is more deeply embedded.

The HCO⁺ line emission shows a distinct peak at the location of GSS30-IRS1 with a narrow line detected toward GSS30-IRS2 (Fig. 4). HCN is also clearly detected toward GSS30, again toward GSS30-IRS1, but offset from the HCO⁺ 3-2 emission. Toward IRS1, both HCO⁺ and HCN appear to trace one side of the outflow cone directed toward us.

B.4 WL 12

WL 12 is a ``standard'' Class I YSO: it is neither a binary, nor does it have a spectacular outflow. It is clearly detected in the interferometric continuum data, but not in the HCO⁺ or HCN 3-2 maps. It is identified as a core in SCUBA maps and included in the list of embedded YSOs by Jørgensen et al. (2008) with red mid-infrared colors characteristic of those sources, as well as bright HCO⁺ 4-3 emission (van Kempen et al. 2009).

  B.5 IRS 54

IRS 54 is found to be a binary with a separation of about 7 in deep near-infrared wavelength observations (Haisch et al. 2004; Duchêne et al. 2004), possibly also responsible for the precession of the outflow observed in the mid-infrared Spitzer images (Fig. B.1). IRS 54 is the only source in our sample, which is not detected in either continuum and only shows a tentative detection in HCO⁺ 3-2. It is associated with a faint SCUBA core (peak of 0.1 Jy beam^-1; total integrated flux in a 40 radius aperture of 0.4 Jy) below the threshold for core detection in Jørgensen et al. (2008). It is also not identified as a separate millimeter core in the Bolocam maps of Young et al. (2006). Based on its HCO⁺ 4-3 line emission, van Kempen et al. (2009) classify it as a ``late Stage 1'' source in transition to the T Tauri stage.

B.6 IRS 63 and Elias 29

The results for IRS 63 and Elias 29 are discussed in more detail in Lommen et al. (2008). In summary, the two sourcesare detected in both continuum and HCO⁺ 3-2. IRS 63 shows the strongest continuum emission by about a factor 8 whereas the Elias 29 is stronger in HCO⁺ 3-2 also by about a factor 8. Where Elias 29 clearly is associated with strong extended continuum emission, witnessed both by its brightness profile in the SMA data and by comparison to single-dish observations, IRS 63 is dominated by emission from a compact, unresolved component, consistent with the suggestion that it is a source in transition between the embedded and T Tauri stages with .