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Appendix A: Results for individual objects

We start with L1551 IRS 5 because it is a good example to illustrate the procedure used to study the line emission. The rest of the objects are ordered as in Table 1.

L1551 IRS 5 is a Class I protostar driving a molecular outflow. High angular resolution observations in the radio have revealed a binary with separation of 03 (Bieging & Cohen 1985). One of the binary components is now known to be a binary itself with separation of 009 (Lim & Takakuwa 2006). We do not have any further information on the nature of the components of the system; Spitzer observations do not resolve the components (its spatial resolution is  ~ 4′′ at 13 μm).

This target is part of a GTO program. The observations were done with 6 different pointings offset in spatial direction. The pointings correspond to three sets of standard nod observations. Therefore we could obtain three average spectra located at three positions; one centered at the star position, and the two others at the offset positions. We can use these data to compare the continuum-subtracted flux at the different positions in order to have a rough estimate for the contribution of the extended emission, whenever the line is detected at multiple positions. In Fig. A.1 we show the spectrum of L1551 IRS 5 around the position of the [Ne II] line. Each plot in the left panel represents the average of the spectra between 2 nods at a given position. In this specific case, we have three different positions identified as “Offset 1”, “Offset 2”, and “Centered”. We have labeled each position in the upper left side of each plot. The maximum flux in the continuum is achieved for the “Centered” position. The line is detected in all three sets of pointings and we point out that it is shifted by 0.1 μm with respect to the expected wavelength of the [Ne II] line. Such a wavelength shift corresponds to a velocity shift of  ~ 230 km s^-1. This result is consistent with the velocity of the [Fe II] jet (285 km s^-1) derived by Davis et al. (2003) from near-IR observations. Pyo et al. (2009) have observed the [Fe II] jet in the near-IR and derived a position angle (PA) of 260° for the northern component and 235° for the southern component.  Spitzer observations have been obtained using a slit PA of  −56.7° and  −141.5° for the SH and LH modules respectively.

In the right panel of Fig. A.1 we have plotted the continuum-subtracted flux. In this way we could verify that although there is an extended component in the emission, the maximum, i.e., the highest line peak, corresponds to the position centered on the object. But we recall that extended emission close to the star can be present and not be resolved by Spitzer.

Fig. A.1 Spectrum of L1551 IRS 5 surrounding the position of the [Ne II] line at 12.8 μm. The plots on the left panel show the average of 2 pointings at each position. We have labeled the positions as Offset 1, Centered, and Offset 2. The spectrum obtained for the position “Centered” has the highest flux in the continuum so it was used for the line analysis. The expected position of the [Ne II] line is overplotted with a black dash-dotted line, we note that the line is shifted. The right panel shows the difference between the line flux and the continuum flux in order to show the real strength of the line.

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	Fig. A.2 Spectrum of L1551 IRS 5 surrounding the position of the [Fe II] line at 17.9 and 25.9 μm. The right panel shows the difference between the line flux and the continuum flux in order to show the real strength of the line. The plots on the left panel show the average of 2 pointings at each position. Labels follow the convention used for Fig. A.1.
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	Fig. A.3 Archival SH spectra of DK Tau and BP Tau showing a variety of emission lines from water and organic molecules. We present for comparison the spectrum of AA Tau previously presented in Carr & Najita (2008). The empty diamonds show the position of some water lines (see Pontoppidan et al. 2010). The spectra were background subtracted and shifted to allow comparison.
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In the case of [Fe II] at 17.9 and 25.9 μm, the lines are detected in all three pointings (Fig. A.2). Both lines are centered at the expected wavelength, in contrast to near-IR observations (Davis et al. 2003) and the [Ne II] line. In both cases, there is an important contribution from extended emission inferred from the high peak of the continuum-subtracted line in the position labeled Offset 2, suggesting diffuse emission as the main contributor to the line luminosity. However, the highest luminosity is still obtained from the pointing centered at the position of the star.

Davis et al. (2003) have reported a detection of an H₂ emission line at 2.12 μm, but we do not detect H₂ in the mid-IR spectra obtained with Spitzer.

The emission lines from L1551 IRS 5 are the brightest of the sample, with luminosities on the order of 10³⁰ erg s^-1. We have obtained VISIR/VLT observations in order to study the kinematics and profile of the [Ne II] line. We have not detected [Ne II] emission at the position of the star, which might suggest a jet origin for the [Ne II] emission, although we have detected the system in imaging mode using the [Ne II] filter (Baldovin-Saavedra et al. 2011, in prep.).

AA Tau is a Class II source for which [Ne II] is the only line we detected (of the lines we studied). However, Carr & Najita (2008) have presented the spectrum of AA Tau showing multiple emission lines from organic molecular species (HCN, C₂H₂, and CO₂), water vapor, OH, and [Ne II]. Follow-up observations (at R ~ 80   000, Najita et al. 2009) have confirmed that the [Ne II] emission is centered at the stellar velocity, which means that its origin lies in the disk rather than in a jet or outflow. Nevertheless, due to the high inclination of the system (~75°, Bouvier et al. 1999), the absence of velocity shifts in the emission line does not rule out a jet origin for the emission. Cox et al. (2005) have indeed reported a microjet detected using coronographic imaging with the Hubble Space Telescope (HST).

BP Tau is a Classical T Tauri star (CTTS) that was observed together with background observations. We detect [Fe II] at 25.99 μm, with a marginal detection at 17.93 μm. There is no record in the literature of previous [Fe II] detections. We have plotted in Fig. A.3 the SH spectra of AA Tau together with BP Tau and DK Tau. Both spectra show spectral features similar to the ones observed in AA Tau and reported by Carr & Najita (2008). The presence of molecular line emission in the spectra of young stars were recently reported in Pontoppidan et al. (2010). Although the spectra of BP Tau and DK Tau are quite similar to the spectrum of AA Tau, we do not detect [Ne II] emission from these two stars. Güdel et al. (2010) detected [Ne II] emission in BP Tau using the same data set as this study, although the line is weak (~10²⁷ erg s^-1). We observe an excess at the position of the [Ne II] line (see Fig. A.3), but below our 3σ detection threshold. A similar situation occurs for the H₂ line at 12.27 μm (see Fig. A.3). The presence of numerous faint emission lines likely makes our detection threshold slightly too high.

DG Tau is classified as both Class I and II in the literature. This young star drives a collimated Herbig-Haro (HH) jet observed in the near-IR. The Spitzer observations were made in the same way as L1551 IRS5, with 3 sets of 2 nod pointings. The [Ne II] line is detected in all of them. The observations were made using a slit PA of  − 57.1° and  − 141.9° for the SH and LH modules respectively. The jet PA is  − 138° (Lavalley et al. 1997), almost coincident with the LH module PA. When comparing the difference between the line and the continuum flux for each set of pointings, we note that the bulk of the emission comes from the position centered on the coordinates of the star, with the ratio being roughly 2:1 between the centered and the offset positions. A detection of [Ne II] has been previously reported in Güdel et al. (2010), based on a different data set than this study. We detect [Fe II] emission at 25.99 μm; this line is detected in all pointings, again with a higher contribution from the position centered on the coordinates of the star. Emission from [Fe II] in the near-IR has been previously reported (e.g., Bacciotti et al. 2002; Davis et al. 2003). In particular, Davis et al. (2003) do not detect any [Fe II] emission within 1′′ from the source, suggesting a jet origin for the [Fe II] emission.

DG Tau B has an edge-on disk and a bipolar jet detected in the optical (Mundt & Fried 1983; Eislöffel & Mundt 1998) and radio (Rodriguez et al. 1995). The jet is oriented perpendicular to the disk. The IRS spectrum has been obtained at only one position. We have detected [Ne II] emission towards this object.

DM Tau is a Class II object with [Ne II] line emission. The IRS observations were done with a background observation, so we are confident that the detected line comes from the source itself. Based on Spitzer observations made with the low-resolution modules (SL and LL), Espaillat et al. (2007) have derived a luminosity of the [Ne II] line of 1.3 × 10²⁸ erg s^-1. In contrast, we have obtained a luminosity of (8.0    ±    2.0) ×    10²⁷ erg s^-1, which is 40% lower than what is presented in Espaillat et al. (2007). This object is particularly interesting because it belongs to the category of transitional disks (Calvet et al. 2002). These objects show a lack of emission at wavelengths shorter than 10 μm, which is sometimes interpreted as an evidence of an inner hole and planet growth.

FS Tau A is a close binary with separation of 025 (Krist et al. 1998). The system is surrounded by a filamentary nebula. Spitzer observations have been done in 3 sets of 2 nod pointing; the [Ne II] line is detected in all of them. By analyzing the flux difference between the continuum and the line, we have found that the height of the line in the centered position is comparable to the height of the line at one of the offset positions, indicating the emission is likely extended. In addition we have detected H₂ at 12.28 μm, while there is no detection of the lines at 17.03 and 28.22 μm. At 28.22 μm, there could be a marginal detection, but only below the detection threshold of 3σ. In the case of the 12.28 μm line, it has been detected in 2 out of the 3 sets of pointings. Comparing the continuum-subtracted line flux at the different positions, we determine that the line flux is higher for a position shifted with respect to the star position, indicating an important contribution from extended emission.

FX Tau is a binary with 089 separation composed of a classical and a weak-lined T Tauri star (White & Ghez 2001). Assuming that the mid-IR emission is dominated by the CTTS, we have included this system in the Class II objects for our analysis. We have detected H₂ emission at 28 μm towards this object, and it is the only line detected. No previous detections have been reported.

IQ Tau is a CTTS showing [Ne II] line emission. There is no information in the literature on possible jet or outflow emission associated with this object. Carmona et al. 2011 (in prep.) detected [O I] emission towards this object employing optical spectroscopy (R ~ 40   000), observing a velocity shift of the line which indicates the presence of an outflow.

T Tau is a triple system visible in the optical and bright in X-rays. T Tau S is an infrared companion located 06 away (Dyck et al. 1982) not detected in X-rays. The southern component is a binary itself with a projected separation of (Köhler 2008). We have detected the [Ne II] line towards the T Tau system, consistent with previous studies (e.g., van den Ancker et al. 1999; Güdel et al. 2010). Based on the analysis of the continuum-subtracted line, there is a contribution from extended emission, in agreement with previous results. The first detections of [Ne II], [Fe II], and [O I], among other gas tracers, come from van den Ancker et al. (1999), based on Infrared Space Observatory (ISO) observations. However, we do not detect [Fe II] emission, which would be expected to be present in an outflow environment. Van Boekel et al. (2009) studied this system (R ~ 30   000) and have determined that the emission is extended and associated with an outflow from the North and South components. Only a small fraction of the emission is related to the X-ray bright T Tau N.

XZ Tau is a close binary system with separation of . Both stars have collimated, bipolar jets (Krist et al. 2008). Due to the close separation, we cannot resolve the system with Spitzer. Observations have been made in 3 sets of 2 nod pointings; we detect [Ne II] only at the pointing with the highest flux in the continuum, centered at the position of the stars. This is the first detection of [Ne II] reported toward this source.

CoKu Tau 1 is a Class II object. It is a binary with separation of (Robitaille et al. 2007) in which we have detected [Ne II] emission. Previous observations in Hα show evidence of an outflow (Eislöffel & Mundt 1998), but we do not detect the [Fe II] emission usually observed in outflows.

CoKu Tau 4 is a Class II object known to have a large excess at  ~ 30   μm and no excess at  < 8   μm (Forrest et al. 2004). This was interpreted as a clear signature of a transitional disk, but recent observations (Ireland & Kraus 2008) have shown that the clearing of the disk at shorter wavelengths is actually due to the presence of a stellar-mass companion. The binary projected separation is calculated to be  ~ 7.8 AU. We have detetected [Ne II] emission towards this binary.

GM Aur is a Class II source with [Ne II] emission; this object is also a transitional disk (Calvet et al. 2005). We have dedicated background observations, so we can discard the possibility of the emission coming from the background or a nearby source. The first detection of [Ne II] in the Spitzer spectrum was reported by Carr & Najita (2008). Follow-up observations of the [Ne II] line at R ~ 80   000 (Najita et al. 2009) have confirmed that the emission is from a disk rather than an outflow. Espaillat et al. (2007) have derived a 5σ upper limit for the luminosity of the [Ne II] line of 7.4 × 10²⁸ erg s^-1. This result is based on observations made with Spitzer IRS low-resolution modules (R ~ 60–120). Our calculations based on the high resolution module give a luminosity of (1.5    ±    0.3)    × 10²⁸ erg s^-1.

RW Aur is an optically bright T Tauri star; Petrov et al. (2001) suggested that this is a binary system. The secondary, RW Aur B, is located away from the primary. We have detected both [Fe II] lines (17.93 and 25.99 μm) in the spectrum of this system, which is not spatially resolved by Spitzer. The observations were made in 3 sets of 2 nod pointings. The line at 17.93 μm is detected in one of the offset positions, indicating that the emission is likely to be extended. The [Fe II] line at 25.99 μm is detected in all the three positions: centered on the position of the star, and two in positions displaced from the star position. The contribution from extended emission is important, but the line flux is higher at the position of the source. Previous observations have shown emission of [Fe II] in the near-IR from the jet associated with this system (Hartigan & Hillenbrand 2009).

UY Aur (A, B) is a binary system with separation (Hirth et al. 1997). Observations were made in 3 sets of 3 nod pointings. We have detected [Ne II] emission at the 3σ limit. The line is detected only in the pointing centered in the position of the system. We note that in this case the line is broad, with a FWHM 1.6 times higher than the IRS instrumental FWHM. This system is known to drive a high velocity (~300 km s^-1) [O I] bipolar outflow (Hirth et al. 1997). We do not observe any shift of the position of the line; the broadening could indicate either a strong wind or a jet with a blue and red-shifted component.

MHO 1/2 is a binary with a separation of 3′′ (Briceño et al. 1998). Since the Spitzer observations do not resolve the system, we cannot know in which star the emission originates. The system has been observed in 3 sets of 2 nod pointings. The [Ne II] line is detected only at one of the three positions.

V773 Tau is a multiple system. V773 Tau AB is a WTTS spectroscopic binary with a separation of 0003, i.e.,  ~ 0.37 AU at the distance of Taurus (Welty 1995). V773 Tau C is a visual companion at 017 from V773 Tau AB, classified as a CTTS. A fourth member of this system, V773 Tau D, has an angular separation of with respect to V773 Tau A (Woitas 2003) and it has been classified as an infrared companion due to the shape of its SED (Duchêne et al. 2003). These objects are believed to be T Tauri stars embedded in dusty envelopes and undergoing episodes of intense accretion. But their true nature is not yet understood. The Spitzer observations do not resolve the components of the system, but according to the SED presented in Duchêne et al. (2003) we assume that V773 Tau D is responsible for the bulk of the mid-IR emission and therefore for the level of continuum observed. However, we cannot be sure that the [Ne II] emission comes from it. In fact, it could also be produced in shocks from one of the components or in the interstellar environment in a similar way as the case of T Tau. The stellar properties for the system are compiled in Table 1. The Hα emission and X-ray luminosity probably come from V773 Tau AB, the WTTS binary, while the mass accretion rate value adopted is the one derived for V773 Tau C, which is likely similar to V773 Tau D. We assume that in the infrared regime, the emission is dominated by the CTTS, so we have included the system in the group of Class II sources. In the correlation tests presented in Sect. 5, we have not used the Hα equivalent width, which is derived for the WTTS binary.

HBC 366 is a WTTS. We have detected one of the H₂ lines (17.03 μm) in this spectrum. No other gas lines have been detected.

IRAS 04239+2436 is a binary of 03 separation, driving a highly collimated near-IR [Fe II] jet (Reipurth et al. 2000). The observations were done in three sets of two nod pointings. We detect both iron lines (17.93 and 25.99 μm) in the IRS spectrum. The two lines are detected in all three sets of pointings, but when analyzing the continuum-subtracted line, the highest contribution comes from the position centered on the star. The luminosity of the line at 25.99 μm is more than a factor of 3 higher than the luminosity at 17.93 μm. Previous observations (Davis et al. 2001; Davis et al. 2003) have detected emission of H₂ (2.122 μm) and [Fe II] (1.644 μm). The Spitzer spectrum of this source does not show any evidence of H₂ emission.

IRAS 04303+2240 is a CTTS. We have detected both [Fe II] lines (at 17.93 and 25.99 μm), with the line at 17.93 μm detected at the 3σ level. Although there is an excess at the wavelength of the [Ne II] line, it is formally not detected (at the 3σ detection limit). The ratio between the luminosity of both [Fe II] lines is 1:2. Observations were done in 3 sets of 2 nod pointings; the line at 17.93 μm is detected only in the position centered at the star. The line at 25.99 μm is detected in two out of three positions; the contributions of the centered position and the offset position are similar. Both lines are broadened by a factor of two compared to the intrinsic line width of the instrument. There is no information in the literature about previous detections of [Fe II] towards this source or about jets or outflows that could explain the [Fe II] emission, or winds explaining the broadening of the lines.

V710 Tau is a binary system formed by a CTTS and a WTTS. Observations in the X-rays made with Chandra (Shukla et al. 2008) and in the near-IR (White & Ghez 2001) have shown that the separation of the binary is . We detect H₂ at 17.03 and 28.22 μm. Considering that the star responsible for the continuum at mid-IR wavelengths is the CTTS, V710 Tau A, we have used the Hα equivalent width and X-ray luminosity of that star in the present study, and included the system in the Class II group, but the lines could also be produced by the WTTS.

SST 041412.2+280837 was included in a sample of the low-mass star population of Taurus for which we have obtained Spitzer-IRS spectroscopy. It was previously known as IRAS 04111+2800G and is classified as a Class I object (Rebull et al. 2010). This is the only object in our sample for which we have used low-resolution spectra (SL and LL) since we do not have the SH and LH spectra. We have detected [Ne II] and [Fe II] at 17.9 and 25.9 μm towards this object.

SST 042936.0+243555 is a newly discovered member of Taurus (Rebull et al. 2010). It belongs to the sample of low luminosity sources for which we have obtained IRS spectra. Rebull et al. (2010) and Luhman et al. (2010) both classify this object as a Class II. The spectrum shows [Ne II] emission.

SST 043905.2+233745 also belongs to the sample of newly discovered Taurus members (Rebull et al. 2010). It is a Class I object as classified by Luhman et al. (2010), but a Class II object according to Rebull et al. (2010). We have detected emission lines from [Ne II] and H₂ at 17.0 μm.

Appendix B: [Ne II] non-detections

	Fig. B.1 Spectra around the [Ne II] line of the targets in which we did not detect the line. The position of the line is plotted in dashed-dotted line and the 3σ noise level is plotted in dashed line. The name of each star is labeled in the top of each plot.
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