High–resolution spectroscopic monitoring of the exceptionally active classical T Tauri star ( CTTS ) RW Aur A was carried out in three seasons of 1996 , 1998 and 1999 with simultaneous B , V photometry .
The high quality spectra revealed a multicomponent structure of the spectrum , which includes : 1 ) a veiled photospheric spectrum of a K1–K4 star , 2 ) broad emission lines of neutrals and ions , 3 ) narrow emission lines of He i and He ii , 4 ) red–shifted accretion features of many lines , 5 ) shell lines at about the stellar velocity , 6 ) blue–shifted wind features and 7 ) forbidden lines . Periodic modulations in many spectral features were found .
The photospheric absorption lines show sinusoidal variations in radial velocity with an amplitude of \pm 6 km s ^ { -1 } and a period of about 2 \aas@@fstack { d } 77 .
The radial velocities of the narrow emission lines of He vary with the same period but in anti–phase to the photospheric lines .
The equivalent widths of the narrow emissions vary with a phase–shift with respect to the velocity curve .
The strength of the red–shifted accretion components of Na D and other lines is also modulated with the same period .
The broad emission lines of metals vary mostly with the double period of about 5 \aas@@fstack { d } 5 . One unexpected result is that no correlation was found between the veiling and the brightness , although both parameters varied in wide ranges .
This is partly due to a contribution of the shell absorption to the photospheric line profiles , which make them vary in width and depth thus simulating lower veiling . The spectral lines of the accreting gas show two distinct components : one is formed at low velocity at the beginning of the accretion column , and the other at high velocity near the stellar surface .
The low velocity components are strong in low excitation lines of neutrals , while the high velocity components are strong in high excitation lines of ions , thus showing the gradients of temperature and density along the accretion column . Most of the observed features can be interpreted in the framework of non–axisymmetric magnetospheric accretion , but the origin of this asymmetry can be explained in different ways .
We consider two possible models .
The first model suggests that RW Aur A is a binary with a brown dwarf secondary in a nearly circular orbit with a period of 2 \aas@@fstack { d } 77 .
The orbiting secondary generates a moving stream of enhanced accretion from one side of the disk towards the primary .
The other model assumes that RW Aur A is a single star with a rotational period of 5 \aas@@fstack { d } 5 and with two footpoints of channeled accretion streams within a global magnetosphere which is tilted relative to the rotational axis or otherwise non–axisymmetric .
Both models can explain qualitatively and quantitatively most of the observed variations , but there are some details which are less well accounted for .