THE GOULD'S BELT VERY LARGE ARRAY SURVEY. IV. THE TAURUS-AURIGA COMPLEX

A large number of the sources detected here must correspond to background quasars seen in the direction of our targets. We estimate that number as follows. First, we take into account the fact that at the edges of the fields, the sensitivity is lower than at their centers. Following the Equation (A11) from Anglada et al. (1998), who discuss this effect, we obtain that the rough number of expected background sources with fluxes above S_{0, ν} in each field and for each sub-band are:

$$\begin{eqnarray*} \langle N\rangle _{4.5\ {\rm GHz}} &=& 1.21\left(\frac{S_{0,4.5\ {\rm GHz}}}{{\rm mJy}}\right)^{-0.75} \quad {\rm and} \\ \langle N\rangle _{7.5\ {\rm GHz}} &=& 0.35\left(\frac{S_{0,7.5\ {\rm GHz}}}{{\rm mJy}}\right)^{-0.75}. \end{eqnarray*}$$

Using these formulations and S_{0, ν} = 5 × σ_noiseν, we expect in our 127 fields a total of 731 ± 27 background sources at the 4.5 GHz sub-band and 271 ± 17 at the 7.5 GHz sub-band. The range of expected values was estimated assuming a Poisson distribution. Since in our survey we have considerable overlap of the fields, in particular at 4.5 GHz, the expected number of sources at 4.5 GHz can be considered as an upper limit. From our detected sources not related to YSO or stars we counted 530 and 316 in the 4.5 GHz and 7.5 GHz sub-bands, respectively. This strongly suggests that most of the unidentified sources reported here are background sources.

To analyze this possibility further, we note that extragalactic radio sources are expected to show little variability (typically ∼10% on a timescale of 1 yr) and to have negative spectral indices. We have used these characteristics to try and separate background sources from YSO candidates in previous studies (e.g., Dzib et al. 2013a; Kounkel et al. 2014 and Ortiz-León et al. 2014). However, there are well known exceptions of extragalactic radio sources that do not follow the typical behavior (e.g., Heeschen 1984 & O'Dea 1998) and that we now consider in more detail.

Lovell et al. (2008) found that around half of the flat spectrum extragalactic sources at centimeter wavelengths show significant variability on the scale of days to hours. These variations are attributed mainly to interstellar scintillation by material in our own Galaxy (e.g., Heeschen & Rickett 1987; Lovell et al. 2008 and Koay et al. 2011). We looked through the data of Lovell et al. (2008) and found that less than 0.6% of their sources exhibit high variability, using our variability criteria, between two of their four epochs separated from one another by four months. Assuming that our sources follow the same trend would indicate that only three of our background sources should be highly variable. Thus, high variability is a good indicator to separate the YSO population from the background sources.

On the other hand, Gigahertz Peaked-Spectrum (GPS) and High Frequency Peakers (HFP) are extragalactic sources that have convex radio spectra peaking at frequencies up to 5 GHz for GPS sources (see O'Dea 1998 for a review) and above 22 GHz for HFP (Dallacasa et al. 2000). Thus, the spectral index for these sources in the observed frequencies can be flat or positive. About 10% of the entire population of radio-bright background sources are GPS, while 20% of these have their peak above 4 GHz (see O'Dea 1998). Finally, the analysis by Dallacasa et al. (2000) shows that roughly 3% of their initial sources are HFP. Using these results we expect that around 21 of the background sources will have flat or positive spectral indices. Thus, spectral index is also a useful quantity to separate YSOs from background sources, but it is somewhat less reliable than variability.

In our sample of 530 possible background sources, 50 are either highly variable (3 × σ_Var. level) or have a flat or positive spectral index value (whitin the range given by α ± σ_α). Four of these were previously known to be extragalactic sources. The remaining 46 sources are listed in Table 5. From the previous arguments, we expect about 24 extragalactic sources with these characteristics. Thus, about half of the sources in Table 4 are most certainly extragalactic, while the nature of the other half is unclear. They might be previously unidentified YSOs, but (given the inherent uncertainties on the radio properties of different classes of active galactic nuclei) they could also be extragalactic background sources.

We mentioned earlier that 60 of the detected radio sources are related to objects previously classified as YSOs. Eleven of these were also identified as extragalactic in the Sloan Digital Sky Survey (SDSS Adelman-McCarthy et al. 2011), they are listed in Table 6. The radio properties of nine of these are consistent with being YSO (variability >50% or α flat or positive).

One way to distinguish Galactic from extragalactic sources is by measuring their proper motions. As extragalactic objects are far away we do not expect to measure proper motions in them. On the other hand, members of the Taurus-Auriga region exhibit proper motions of ∼20 mas yr⁻¹ (see, e.g., Torres et al. 2009). The proper motions of two of the sources in Table 6, have been measured at radio wavelengths, DG Tau (Rodríguez et al. 2012a) and DG Tau B (Rodríguez et al. 2012b), and were found to be consistent with those measured to other members in Taurus-Auriga.

By combining the position for Haro 6–10 S reported by Reipurth et al. (2004), with those obtained by us, we can roughly estimate the proper motions of this source. We obtained μ_αcos δ = 16.1  ±  7.9 mas yr⁻¹ and μ_δ = −33.8 ± 9.5 mas yr⁻¹, which is consistent with it being a member of Taurus-Auriga.

On the other hand, two of the sources in Table 6 (GBS-VLA J043229.46+181400.2 and GBS-VLA J041833.38+283732.2) have radio properties (low variability with negative spectral index) more typical of extragalactic objects than of YSOs.

GBS-VLA J043229.46+181400.2 has a radio flux of order ∼60 mJy which is large compared with most of the other YSOs in Taurus-Auriga. It was classified as a Class II YSO by Gutermuth et al. (2009) based on its infrared properties. We looked in the literature for a good determination of its position at radio frequencies, but we did not find any. Thus, we looked for a high resolution, good sensitivity observation in the VLA archive. We found one observation obtained in 1995 September as part of the project BB0051. After a standard calibration of these data we measured the position for this source to be R.A. = 04^h32^m29460 and decl. = 18°14'0019, with an error in position of ∼001. Combining this with our own observations, we obtain a rough estimate of the proper motions for this source: μ_αcos δ = 4.6 ± 3.3 mas yr⁻¹ and μ_δ = 5.2 ± 3.9 mas yr⁻¹. These results are consistent with no motion at all at the 1.4 × σ_μ level. This strongly suggests that GBS-VLA J043229.46+181400.3 is a background source rather than a YSO, and we will not consider it as a YSO in the rest of the paper.

The remaining source, GBS-VLA J041833.38+283732.2, was only observed in the 4.5 GHz sub-band and does not show significant levels of variability. At infrared wavelengths it was classified as a Class I YSO, and low variability is expected for most of the stars in this evolutionary status (e.g., Dzib et al. 2013a). Also, we note that all the YSOs in the evolutionary Class I that are detected by the SDSS, are classified as extragalactic by them, suggesting a bias in the SDSS identification criteria. Thus, we will consider it as a YSO. In summary, we consider that 10 of the 11 sources in Table 6 are counterparts of YSOs, and that 59 of the 60 sources previously classified as YSOs are in fact YSOs.

Sources identified as YSOs are listed in Table 7. From their radio properties, 33 objects show at least one clear indication that their emission is non-thermal in nature (high variability, circular polarized emission or negative spectral index). Another 22 objects are consistent with their emission being thermal free–free emission (positive spectral index and low variability). From the remaining sources three are non-variable, do not show signs of circular polarization, but they show a negative spectral index, that given the involved errors, the nature of their emissions remains uncertain. Finally, we do not have enough information to favor a thermal or non-thermal emission mechanism for GBS-VLA J041833.38+283732.2. Our results show that a significant population of the YSOs in Taurus-Auriga (56%) have radio emission that is non-thermal in nature. These stars can be used for Very Long Baseline Interferometry (VLBI) astrometric observations (e.g., Dzib et al. 2010, 2011; Torres et al. 2012).

The evolutionary status is known for 54 of the 59 YSOs detected in this radio survey. In addition, 53 have a known spectral type. Thus, we can compare their radio properties to their evolutionary status and spectral type.

We have found previously that, on average, more evolved stars show radio properties that resemble a non-thermal origin (i.e., more variable and with more negative spectral indices, Dzib et al. 2013a; Ortiz-León et al. 2014; Kounkel et al. 2014). This is in agreement with the idea that more evolved sources have already shed away most of their surrounding material and that we are detecting the non-thermal emission from their coronae instead of the thermal emission from the surrounding material (see Dzib et al. 2013a for a discussion). From Figures 2 and 3 (where we have also plotted our previous result in Ophiuchus) we see that the YSOs in Taurus-Auriga follow the same tendency. Note, however, that at each of the evolutionary stages we found both thermal and non-thermal radio emitters (see also Table 7) as also noted in Ophiuchus (Dzib et al. 2013a), Serpens and W40 (Ortiz-León et al. 2014) and Orion (Kounkel et al. 2014). The weighted average flux density seems also to be slightly higher for the more evolved YSOs (see Figure 4), but it is not as dramatic as in the Ophiuchus case (Dzib et al. 2013a; Figure 4).

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Finally, Figure 5 shows the flux density of the YSOs as a function of their spectral type. As in the Ophiuchus case, most of the YSOs sources detected at radio frequencies have a spectral type M and K, there are no detected sources of types G and F, and only one A-type and one B-type detection. In part, this presumably reflects the fact that the Taurus-Auriga star-forming region contains mainly low-mass stars. Contrary to the results in Ophiuchus, in Taurus-Auriga the averaged radio flux density for the low-mass members seems to be larger than the intermediate-mass members. We discuss the peculiarities of these stars in the next section.

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An interesting result that we have been noticing in our VLA surveys is the relation between the radio emission of YSOs compared to their previously reported X-ray emission. The recent results indicate that YSOs follow the so-called Güdel–Benz relation (Guedel & Benz 1993 and Benz & Guedel 1994) for magnetically active stars. The relation has the general form (Benz & Guedel 1994):

$$\begin{equation*} \frac{{L}_X}{L_{R,\nu }}=\kappa \cdot 10^{15.5\pm 0.5}\ {\rm [Hz]}. \end{equation*}$$

The value of κ varies for different kind of stars and fulfills κ ⩽ 1. Particularly, we proposed a κ = 0.03 for YSOs, based just in the positions on the L_X–L_R diagram of YSOs in the Ophiuchus star-forming regions (Dzib et al. 2013a), and also following the previous results obtained by Gagné et al. (2004) and Forbrich et al. (2010).

We searched for X-ray counterparts of the YSOs detected in Taurus-Auriga (see also Table 4) and plot them against the radio luminosities in Figure 6 with blue symbols. In Figure 6 we also plotted the results obtained in Ophiuchus (green symbols, from Dzib et al. 2013a), Serpens-W40 (red symbols; from Ortiz-León et al. 2014), and Orion (yellow symbols; from Kounkel et al. 2014). Most of the YSOs seem to follow, with ∼1 dex of dispersion, the Güdel–Benz relation with κ = 0.03. There are, however, some clear exceptions in the Orion region, where some sources are apparently X-ray underluminous by more than two orders of magnitude. We noticed that this trend does not depend on the evolutionary classification of the YSOs (squares for Class I/FS, rhombus for Class II/CTTS, circles for Class III/WTTS, and pentagons for unclassified sources in Figure 6) or if the radio emission is most likely to be thermal (open symbols) or non–thermal (filled symbols). Dzib et al. (2013b) has discussed that even when the gyrosynchrotron radio emission is hidden by free–free radio emission in upper layers of circumstellar material, the X-ray emission produced in the plasma of the corona could still be observed. Even more interesting is that, as noticed by Dzib et al. (2013b), independent of the dominant radio emission mechanism the YSOs seem to fit the Güdel–Benz relation, meaning that both radio emission mechanisms have similar levels of flux densities.

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Newly born low-mass stars (between 0.08 M_☉ to 3.0 M_☉) are magnetically active and could produce significant amount of gyrosynchrotron radio emission from their coronae. In the very young stars (i.e., Class I) this radio emission is expected to be hidden by the optically thick free–free radio emission from the surrounding material. There are a few cases, however, where non-thermal radio emission has been detected from Class I stars (e.g., Feigelson et al. 1998; Forbrich et al. 2007; Dzib et al. 2010, 2013a and Deller et al. 2013). Geometric effects or abnormally early removal of the surrounding material (e.g., in a tight binary system) have been invoked to explain these detections (e.g., Forbrich et al. 2007; Dzib et al. 2010). Thus, it is important to document more cases in order to identify which is the dominant mechanism that allows the detection of coronal emission in very young YSOs. Three Class I stars in our sample have properties that indicate their radio emission could be of a non-thermal nature: GBS-VLA J041354.72+281132.6, GBS-VLA J043232.07+225726.3 and GBS-VLA J043935.20+254144.3 are Class I stars that are highly variable, which is an indication that their emission has a non-thermal origin. In the case of GBS-VLA J043232.07+225726.3 this is also supported by the negative spectral index. The non-thermal origin of the emission from these very young stars must be confirmed using other techniques, e.g., VLBI.

Non-thermal radio emission from YSOs often exhibits high levels of variability (>50%) associated with magnetic activity of the star (Feigelson & Montmerle 1999). There are, however, some exceptions to this. The 6 M_☉ star S1 in the Ophiuchus core shows only modest levels of variability during a series of VLBI observations made by Loinard et al. (2008). This suggests a different origin with more organized magnetic fields. For a better understanding of this behavior, further studies must be carried with more objects. In the present report we detected two other sources which are likely non-thermal because they have detectable circular polarization, but which are only moderately variable.

GBS-VLA J041831.12+282715.9 = V 410 Tau, is a K4 star that shows variabilities below 30% during the three observed epochs. Circular polarized emission was found at a level of 8.7% (R) in the 7.5 GHz sub-band. Also, it shows a negative spectral index (α = −0.32 ± 0.17) which supports the non-thermal origin. Carroll et al. (2012) found that the magnetic topology in the visible pole of V 410 is dominated by a bipolar structure. These authors also argue that the absence of a radiative core and non-detection of differential rotation support the idea that a classical dynamo is not operating in V 410 Tau.

GBS-VLA J042203.15+282538.8 is an M3 star with measured variability below 16%. Its spectral index between the observed sub-bands is flat (α = 0.00 ± 0.28). Circular polarization is detected in both sub-bands at levels of 21.3% (L) and 30.8% (L) for 4.5 GHz and 7.5 GHz, respectively. These levels of circular polarization strongly suggest a non-thermal origin.

Only two sources have spectral indices that are consistent with large positive values (∼2). These sources are GBS–VLA J043540.95+241108.6 (with spectral index 2.38 ± 0.79) and GBS–VLA J044022.18+260515.2 (with spectral index >1.70 ± 0.14). The first source is associated with CoKu Tau 3, a well known Class II YSO and we attribute the spectral index to the presence of optically thick gyrosynchrotron emission. The second source, GBS–VLA J044022.18+260515.2, has been tentatively classified by us as a YSO candidate on the basis of its positive spectral index. An alternative interpretation is that it is a HFP. These are compact extragalactic radio sources with well-defined peaks in their radio spectra above 5 GHz and positive spectral indices at frequencies below their peak. Most of them are believed to be high redshift quasars (Orienti & Dallacasa 2014). The case of a possible HFP observed in projection in the star-forming region M17 is discussed by Rodríguez et al. (2014a).

Dzib et al. (2013a) show that the detection of YSOs near and beyond the brown dwarf boundary at distances farther than 100 pc is possible. They reported the radio emission of four very low-mass stars in the Ophiuchus core. Three of them show indications that the origin of their radio emission is non-thermal.

GBS-VLA J043158.46+254329.8 is a Class III YSO (Kenyon & Hartmann 1995), with a mass of 0.19 M_☉ and an M5.5 spectral type (Briceño et al. 2002). It was not detected in the 4.5 GHz sub-band on individual epochs at levels of 100 μJy nor in the image of the three concatenated epochs at a level of 63 μJy. On the other hand, in the 7.5 GHz sub-band it was not detected in the first two epochs at levels of 80 μJy, but in the third epoch it was detected with a total flux density of 165 ± 28 μJy. This variation (>45%) is just slightly below our adopted limit of high variability and suggests that GBS-VLA J043158.46+254329.8 could also be a very low-mass star with non-thermal radio emission.

Elias 1 is a well known Herbig AeBe (HAeBe)¹² star in the Taurus-Auriga cloud (Elias 1978). It has two companions, a T Tauri star at 4'' to its northeast (Leinert et al. 1997) and a close (50 mas) low-mass companion (Smith et al. 2005). It is variable at X-rays frequencies, and an X-ray flare was reported by Giardino et al. (2004). This is an indication that a corona is acting in at least one of the two components of the close binary. At radio frequencies, we detected a source related with the close binary (GBS J041840.62+281915.3). It is a highly variable star in both of the observed sub-bands (69.5% ± 16.7% and 51.5% ± 15.2% at 4.5 GHz and 7.5 GHz, respectively). This is another indication that a corona is involved.

The standard theory of intermediate-mass stellar evolution predicts that during the pre-main sequence (and, indeed, the main sequence), these stars of relative large masses are fully radiative. As a consequence, they are not expected to drive a dynamo, and should not have strong surface magnetic fields. Thus, they should not maintain a corona. X-ray observations have shown that some HAeBe stars have indeed signs that they can host coronae that resemble those of T Tauri stars (Damiani et al. 1994; Zinnecker & Preibisch 1994 and Stelzer et al. 2005, 2006, 2009). On account of that similarity, a popular interpretation was that the X-ray emission was in fact due to the presence of a young low-mass stellar companion (rather than to the HAeBe star itself). The hot X-ray component of the single HAeBe star HD 163296, on the other hand, suggested a coronal origin and is the most promising case of an HAeBe hosting a corona (Günther & Schmitt 2009). To our knowledge, non-thermal radio emission from young intermediate mass stars has only been detected directly, using the VLBI technique, from the B4V S1 star in the Ophiuchus cloud (Andre et al. 1991; Loinard et al. 2008) and from the proto-HAeBe binary star EC 95 in the Serpens cloud (Dzib et al. 2010). The high variability of EC 95 at radio frequencies strongly suggests that it has a coronal origin (see Dzib et al. 2010 for a discussion), while the low variability in S1 suggests a different origin. However, a corona has never been directly observed from a well established HAeBe star. Future VLBI observations of GBS-VLA J041840.62+281915.3 will establish if the non-thermal radio emission observed with the VLA comes from the HAeBe star itself or from its low-mass companion. Even the detection of the companion with the VLBI technique is of great interest because we will be able to estimate the dynamical mass of the HAeBe star.

The lack of variability and spectral index (1.27 ± 0.48) for this Herbig Ae star suggests that the emission is produced by a thermal jet. This suggestion is confirmed by the study of Rodríguez et al. (2014b) that combines additional VLA data at other wavelengths with our results.