Modeling of Hydrogen Emission Lines in the Spectrum of UX Ori in Its Bright State and during Eclipses

Abstract—

The hydrogen spectrum of the UX Ori-type star is modeled in a bright state and during an eclipse by an opaque dusty fragment of its own protoplanetary disk. The disk wind is considered as the main source of the emission spectrum. The radiation generated in the stellar magnetosphere is also taken into account. We showed that variations in the Hα line profile during eclipses depend sensitively on the wind opening angle. In models with a large opening angle, the emission line at the minimum brightness becomes single, asymmetric, and shifts towards the red end, which contradicts the observations made in 1992 during the deep minimum of UX Ori, when the asymmetric two-peak emission line turned into an asymmetric single and practically unbiased line. This indicates that one more source contributes to the emission spectrum, which is not occulted by an opaque screen at the moments of eclipses. As possible options, we considered (a) scattered radiation of a hypothetical dust halo in the polar region of the disk and (b) peripheral layers of the gas atmosphere of the disk, which are the source of the photoevaporation wind. To clarify the situation, new high-quality observations of the spectra of this type of star in deep minima are needed.

APPENDIX

In the Appendix, we offer additional material that may be useful for studying UX Ori-type stars.

1.1 Magnetospheric Accretion

As already mentioned, we assume that the use of the classical scheme of magnetospheric accretion for rapidly rotating HAEBE stars will be incorrect. However, this type of accretion can, in principle, be realized with a sufficiently strong intrinsic field of the star. In this case, the accreting material will be delivered to the poles of the star. For this case, we calculated the Hα line profiles for several models and some of them are presented in this Appendix.

A scheme of classical magnetospheric accretion, which is usually used for T Tauri stars, is shown in Fig. 11. It is assumed that the magnetosphere is formed by a dipole magnetic field, the axis of which coincides with the rotation axis of the star. The outer ${{r}_{{mo}}}$ and inner ${{r}_{{mi}}}$ boundaries of the magnetosphere exist, and the gas moves to the star along the magnetic lines only under gravity. The start region on the disk is between these terms. The magnetosphere rotates solidly at the speed of the star at the equator (150 km/s). Radiation transfer in spectral lines is considered in the Sobolev approximation taking into account the nonlocal radiation interaction. A detailed calculation algorithm is given in [39].

Fig. 11.

Scheme of classical magnetospheric accretion for the star UX Ori.

Two models of magnetospheric accretion with the inner $1.2{{R}_{*}}$ and outer $1.7{{R}_{*}}$ radius of the magnetosphere (model CMA1) and, respectively, $1.5{{R}_{*}}$ and $2.5{{R}_{*}}$ (model CMA2), are presented. Another parameter of the model was the gas temperature near the star’s surface, which was set in such a way that the maximum gas temperature in calculating the heat balance was 8000 K. The change in gas temperature with distance along the magnetic field lines is shown in Fig. 12. Both models were calculated for three values of the accretion rate: ${{\dot {M}}_{{{\text{acc}}}}} = 1 \times {{10}^{{ - 8}}}\,{{M}_{ \odot }}$, 3 × 10^–8 ${{M}_{ \odot }}$, and $1 \times {{10}^{{ - 7}}}\;{{M}_{ \odot }}$/year. The inclination is $i = 70^\circ$. The H$\alpha$ line profiles in the classical magnetospheric accretion models with the parameters CMA1 and CMA2 are shown in Fig. 13. When calculating the profiles with an accretion rate ${{10}^{{ - 7}}}\,{{M}_{ \odot }}$/year, the Stark effect was taken into account. It should be noted that with an increase in the accretion rate up to this value, the H$\alpha$ line takes the form of an inverse P Cyg profile.

Fig. 12.

Temperature variation with distance from the star in the models of classical magnetospheric accretion CMA1 and CMA2 with a maximum gas temperature of 8000 K.

Fig. 13.

Profiles of H$\alpha$ lines in the models of classical magnetospheric accretion CMA1 (a–c) and CMA2 (d–f). From top to bottom, line profiles for accretion rates are shown: ${{\dot {M}}_{{{\text{acc}}}}} = 1 \times {{10}^{{ - 7}}}\;{{M}_{ \odot }}$/year (top panel), $3 \times {{10}^{{ - 8}}}\;{{M}_{ \odot }}$/year (middle panel), and $1 \times {{10}^{{ - 8}}}\;{{M}_{ \odot }}$/year (bottom panel). The tilt angle is 70°.

1.2 The Influence of Moving Dust on Hα Line Profiles

Modeling of radiation scattering by stationary dust particles located in a continuously moving medium is a simplification of the problem. Dust can be on the walls of the cavity formed during the evolution of the system by outflows of matter in the region of the poles; this “wall” becomes the boundary of the disk wind zone, which rises above the disk surface and rotates. Therefore, dust particles also participate in complex motion. Therefore, we checked how the radial and tangential motion of dust affects the calculated Hα line profiles. The calculation algorithm is described in detail in [59]. Figure 14 shows the Hα line profiles from the last (lower) row in Fig. 7 (i.e., obtained at the moment of minimum star brightness) for the same hybrid models, and then light scattered by dust located in the polar region of 20° from the polar axis and moving at a speed of –100 km/s, i.e., to the star, and at a speed of 100 km/s, i.e., from the star. In the first case, besides the fact that the scattered light changes the shape of the line profile, it also shifts it entirely towards negative radial velocities. In the second case, the line profile shifts towards positive radial velocities. The amount of displacement depends on the speed of the dust and the inclination. The value of 100 km/s was chosen to better illustrate the effect. In Fig. 15, the same profiles of all the considered models obtained at the minimum brightness of the star are influenced only by the rotation of dust at a speed of 100 km/s. The light scattered by the spinning dust expands the line profiles and smooths their shape, making them single. The combined effect of dust motion thus leads to displacement, expansion and smoothing of the line -profiles.

Fig. 14.

Influence of the radial component of the dust velocity on the H$\alpha$ line profiles during the brightness minimum in models 1–5. “0” denotes the case of stationary dust; –100 and 100 denote dust approaching the star and receding from the star at 100 km/s, respectively.

Fig. 15.

Influence of the tangential component of the dust velocity on the Hα line profiles during the brightness minimum in models 1–5.