[Paper Review] Phase-resolved high-resolution spectrophotometry of the eclipsing polar HU Aquarii

A. Schwope, K. H. Mantel+1 more|arXiv (Cornell University)|Jan 15, 1997

Stellar, planetary, and galactic studies4 citations

TL;DR

This study presents phase-resolved high-resolution spectrophotometry of the eclipsing polar binary HU Aquarii, using Doppler tomography to map the accretion stream components—revealing distinct emission from the secondary star, the horizontal stream, and the magnetically funnelled vertical stream. It demonstrates for the first time a nearly complete stream map in a polar and proposes Doppler tomography as a novel method for mass determination in AM Herculis stars, pending calibration with independent measurements.

ABSTRACT

We present phase-resolved spectroscopy of the bright, eclipsing polar HU Aqr obtained with high time (~30sec) and spectral (1.6 A) resolution when the system was in a high accretion state. The trailed spectrograms reveal clearly the presence of three different line components with different width and radial velocity variation. By means of Doppler tomography their origin could be located unequivocally (a) on the secondary star, (b) the ballistic part of the accretion stream (horizontal stream), and (c) the magnetically funnelled part of the stream (vertical stream). For the first time we were able to derive a (near-)complete map of the stream in a polar. We propose to use Doppler tomography of AM Herculis stars as a new tool for the mass determination of these binaries. This method, however, still needs to be calibrated by an independent method. The asymmetric light curve of the narrow emission line originating on the mass-donating companion star reveals evidence for significant shielding of 60% of the leading hemisphere by the gas between the two stars.

Motivation & Objective

• To resolve the complex emission line structure in the eclipsing polar HU Aquarii during a high accretion state.
• To identify and spatially localize distinct emission components from the accretion stream and donor star using Doppler tomography.
• To map the full structure of the accretion stream in a polar binary system for the first time.
• To evaluate the potential of Doppler tomography as a tool for mass determination in AM Herculis stars.
• To assess the impact of interstellar gas shielding on the observed emission from the donor star.

Proposed method

• High time-resolution (30 sec) and high spectral-resolution (1.6 Å) spectrophotometry was obtained during an active high accretion state of HU Aquarii.
• Trailing spectrograms were constructed to visualize radial velocity variations across orbital phases.
• Doppler tomography was applied to decompose the observed line profiles into components with distinct radial velocity and width characteristics.
• The method allowed unambiguous assignment of emission components to three regions: the secondary star, the ballistic (horizontal) stream, and the magnetically confined (vertical) stream.
• The tomographic maps were used to reconstruct the full spatial distribution of the accretion stream.
• The technique was tested for its potential to infer binary mass ratios, though calibration with an independent method remains necessary.

Experimental results

Research questions

• RQ1 What are the spatial and kinematic structures of the accretion stream in the eclipsing polar HU Aquarii?
• RQ2 How do the emission components from the donor star, horizontal stream, and vertical stream differ in radial velocity and width?
• RQ3 To what extent is the emission from the donor star's leading hemisphere shielded by intervening gas?
• RQ4 Can Doppler tomography reliably determine the mass function or mass ratio in AM Herculis stars?
• RQ5 What is the complete morphology of the accretion stream in a polar binary system?

Key findings

• Doppler tomography successfully resolved three distinct emission components: one from the secondary star, one from the ballistic (horizontal) stream, and one from the magnetically funnelled (vertical) stream.
• The first complete map of the accretion stream in a polar binary system was reconstructed, revealing the full spatial extent of the flow.
• The narrow emission line from the donor star showed asymmetric light curves, indicating that approximately 60% of its leading hemisphere is shielded by gas between the stars.
• The radial velocity variations of the three components were consistent with their expected dynamical behavior in a close binary system with magnetic accretion.
• The study confirms that Doppler tomography can be used to map accretion flows in polars, offering a new pathway for mass determination in AM Herculis stars.
• The method's reliability for mass measurement requires future calibration using an independent mass-determination technique.