Near‐Infrared Spectropolarimetry of Three Prototype Low‐Mass Young Stellar Objects in the Taurus Dark Cloud

We present near-infrared spectropolarimetric data between 0.9 and 4.2 μm for three prototypes of low-mass young stellar objects (YSOs), L1551 IRS 5, HL Tau, and T Tau in the Taurus dark cloud. These sources are in different classes in the standard spectral classification scheme of low-mass YSOs by Lada. The polarization curves of the observed sources show distinct differences. The class I protostar L1551 IRS 5 shows a flat polarization curve with high polarization through the observed wavelengths. It also shows a polarization excess at the 3.1 μm ice band absorption feature. The "flat-spectrum source" HL Tau, which is thought to be in a transient phase from class I to class II, shows a steep decrease of polarization with increasing wavelengths from 1.0 to 2.5 μm, while it shows a flat polarization curve with high polarization in optical wavelengths and a slowly decreasing slope with small polarization in 3-4 μm. The class II source T Tau displays small polarization no more than 2% through the observed wavelengths; the polarization in the shorter wavelengths from optical to 1.3 μm decreases with increasing wavelengths. T Tau also shows an increasing polarization curve in the longer wavelengths over 1.6 μm, which is most likely to come from the infrared companion T Tau S (Kobayashi et al.). The prominent differences of the observed near-infrared polarization curves can be clearly understood in terms of the standard spectral classification scheme of low-mass YSOs. Thus near-infrared spectropolarimetry could serve as a potentially powerful diagnostic of circumstellar material, complementary to the standard spectral classification.