[Paper Review] HST and VLT observations of the Symbiotic Star Hen 2-147

M. Santander-García, R. L. M. Corradi+6 more|arXiv (Cornell University)|Jan 14, 2007

Stellar, planetary, and galactic studies4 references3 citations

TL;DR

This study combines Hubble Space Telescope and VLT integral field spectroscopy to model the 3D kinematics and geometry of the nebula around symbiotic star Hen 2–147. It finds that the expansion parallax method yields a distance of 1.5 ± 0.4 kpc if matter expansion is assumed, but this discrepancy with the Mira period-luminosity relation (3.0 ± 0.4 kpc) is resolved only when the observed expansion is attributed to a shock front, not ionized gas, leading to a revised distance of 2.9 ± 0.4 kpc.

ABSTRACT

We investigate the dynamics of the nebula around the symbiotic star Hen 2-147, determine its expansion parallax, and compare it with the distance obtained via the Period-Luminosity relation for its Mira variable. The geometry of the nebula is found to be that of a knotty annulus of ionized gas inclined to the plane of sky and expanding with a velocity of ~90 km/s. A straightforward application of the expansion parallax method provides a distance of 1.5+-0.4 kpc, which is a factor of two lower than the distance of 3.0+-0.4 kpc obtained from the Period-Luminosity relationship for the Mira (which has a pulsation period of 373 days). The discrepancy is removed if, instead of expanding matter, we are observing the expansion of a shock front in the plane of the sky. This shock interpretation is further supported by the broadening of the nebular emission lines.

Motivation & Objective

• To determine the true distance to the symbiotic star Hen 2–147 using nebular dynamics.
• To resolve the discrepancy between distance estimates from the expansion parallax method and the Mira period-luminosity relation.
• To investigate whether the observed nebular expansion in the plane of the sky arises from ionized gas motion or shock front propagation.
• To construct a 3D spatio-kinematical model of the nebula using multi-epoch HST imaging and high-resolution VLT spectroscopy.
• To assess the validity and limitations of the expansion parallax method in systems with shock-dominated kinematics.

Proposed method

• Multi-epoch Hubble Space Telescope (HST) imaging was used to measure the apparent expansion of the nebula in the plane of the sky over 3 years.
• High-resolution integral field spectroscopy from the VLT was used to measure radial velocities and derive 3D kinematics of the nebula along the line of sight.
• A 3D spatio-kinematical model was constructed to describe the nebula's geometry and velocity field, assuming a knotty annular structure inclined to the sky plane.
• The expansion parallax method was applied to estimate the distance based on the observed angular expansion and radial velocity.
• The Mira pulsation period (373 days) was derived from 25 years of SAAO near-infrared photometry to estimate distance via the period-luminosity (PL) relation.
• The shock origin of the expansion was tested by analyzing line broadening and comparing kinematical models with and without shock effects.

Experimental results

Research questions

• RQ1 Does the observed expansion of the nebula in the plane of the sky correspond to the motion of ionized gas or to a shock front?
• RQ2 Why is the distance derived from the expansion parallax (1.5 ± 0.4 kpc) significantly lower than that from the Mira period-luminosity relation (3.0 ± 0.4 kpc)?
• RQ3 Can the discrepancy between the two distance estimates be reconciled by accounting for shock-driven expansion rather than gas motion?
• RQ4 What is the true 3D geometry and kinematic structure of the nebula around Hen 2–147?
• RQ5 How reliable is the expansion parallax method in symbiotic nebulae where shocks may dominate the observed kinematics?

Key findings

• The nebula has a knotty annular structure inclined to the plane of the sky, expanding at approximately 90 km s⁻¹.
• A straightforward application of the expansion parallax method yields a distance of 1.5 ± 0.4 kpc, which is inconsistent with the Mira period-luminosity relation.
• The discrepancy is resolved when the observed expansion is attributed to a shock front rather than ionized gas motion, yielding a distance of 2.7 ± 0.5 kpc.
• The Mira period-luminosity relation gives a distance of 3.0 ± 0.4 kpc, consistent with the shock-corrected expansion parallax result.
• The final adopted distance to Hen 2–147 is 2.9 ± 0.4 kpc, reconciling both methods when shock effects are included.
• The nebula was likely ejected approximately 200 years ago, likely coinciding with a nova outburst of the white dwarf companion.