[Paper Review] Using Absorptive Linear Polarization Spectroscopy to Understand Imbedded Stars

J. R. Kuhn, Brian B. Geiss+1 more|arXiv (Cornell University)|Oct 4, 2010

Stellar, planetary, and galactic studies2 references3 citations

TL;DR

This paper demonstrates that absorptive linear polarization in stellar spectra—driven by optical pumping from anisotropic radiation—provides a powerful tool to infer the geometry of circumstellar environments around embedded stars. By analyzing high-resolution spectropolarimetric data, the authors show that polarization profiles in Balmer lines can reveal disk orientation, tilt, and spatial structure, enabling inversion to reconstruct projected images of obscuring gas with 180° ambiguity due to linear polarization symmetry.

ABSTRACT

Sensitive measurements of the linearly polarized spectra of stars can be used to deduce geometric properties of their otherwise unresolved circumstellar environments. This paper describes some of the evidence for optical pumping and absorptive linear polarization and explores some interesting applications of linear spectropolarimetry for obtaining spatial information from imbedded stars.

Motivation & Objective

• To understand the physical origin of linear polarization variability across stellar absorption lines in imbedded stars.
• To distinguish absorptive linear polarization due to optical pumping from scattering polarization mechanisms.
• To develop a method for inferring geometric properties of circumstellar disks using spectropolarimetric data.
• To demonstrate inversion of spectropolarimetric data into spatially resolved 'images' of intervening gas near stars.
• To validate the optical pumping model using non-hydrogenic lines and polarization ratios across hydrogen lines.

Proposed method

• Utilizes high-resolution linear spectropolarimetry with 0.1% accuracy and spectral resolution R > 50,000 from instruments like CFHT's ESPaDOnS and Haleakala's HiVIS.
• Applies radiative transfer models to simulate optical pumping effects in atomic levels, accounting for ground-state anisotropy due to anisotropic stellar radiation.
• Uses the Q and U Stokes parameters to derive polarization angle and amplitude across absorption lines, enabling geometric reconstruction.
• Employs inversion techniques on I, Q, U spectra to project optical depth and polarization direction onto the sky plane, assuming radial outflow or rotation profiles.
• Compares observed polarization profiles with modeled disk geometries to infer disk position angle, tilt, and orientation.
• Validates the optical pumping model by analyzing non-hydrogenic lines where scattering is unlikely, confirming absorptive polarization dominance.

Experimental results

Research questions

• RQ1 What causes the observed linear polarization variability across stellar absorption lines in imbedded stars?
• RQ2 Can absorptive linear polarization be distinguished from scattering polarization in high-resolution spectropolarimetry?
• RQ3 To what extent can optical pumping in Balmer lines constrain the geometry of circumstellar disks?
• RQ4 How can high-resolution spectropolarimetric data be inverted to reconstruct spatial structure of obscuring gas?
• RQ5 What role does optical pumping play in shaping the polarization profiles of hydrogen lines like Hα and Hβ?

Key findings

• Absorptive linear polarization in stellar spectra is primarily caused by optical pumping due to anisotropic radiation, not scattering.
• Polarization ratios across hydrogen lines (e.g., Hα, Hβ) can be modeled using simple optical pumping theory, supporting the mechanism's validity.
• The orientation and tilt of a circumstellar disk can be inferred from the polarization angle profile across an absorption line, with consistency observed in systems like ε Aur and 51 Oph.
• For MWC 361, the disk position angle is determined to be ~5° from symmetry analysis of Q and U data, matching extended IR observations but conflicting with Zeeman Doppler imaging results.
• High-resolution spectropolarimetry of Hα in AB Aur allows inversion to produce a projected optical depth map of intervening gas, revealing regions of strongest absorption.
• The inversion process yields a 180° ambiguity due to the two-fold symmetry of linear polarization, but still provides spatially resolved information on the distribution of obscuring material.