[Paper Review] Comparison of two different extinction laws with Hipparcos observations

B. Chen, Jean‐Luc Vergely+2 more|arXiv (Cornell University)|May 4, 1998

Economic Growth and Productivity3 citations

TL;DR

This study develops a new extinction law for the solar neighborhood using open cluster data and compares it with the AGG92 model using Hipparcos observations. By applying an inverse method to derive extinction maps from cluster distances and color excesses, the authors show their model provides a significantly better fit to Hipparcos data (χ² = 62.3 vs. 108.7), indicating reduced overestimation of extinction, though substructure in the galactic plane remains inadequately modeled.

ABSTRACT

Interstellar absorption in the galactic plane is highly variable from one direction to another. In this paper colour excesses and distances from a new open cluster sample are used to investigate the spatial distribution of the interstellar extinction. An inverse method (Tarantola & Valette, 1982) is used to construct the extinction map in the galactic plane below $|b| < 10^{o}$. The $A_{v} (r,l) $ diagrams are compared with those derived from individual stars (Arenou et al. 1992, Neckel & Klare 1980). An analytic expression for the interstellar extinction as a function of galactic longitude and distance in the solar neighborhood is given. The comparison of the model predictions with Hipparcos observations in the 4-dimensional space of ($V$, $B-V$, $H_v$, $r$) shows that our extinction model provides a better fit to the data. However, a new and more detailed extinction model is still lacking.

Motivation & Objective

• To develop a more accurate extinction model for the galactic plane using open cluster data.
• To address the limitations of existing extinction models at low galactic latitudes (|b| < 10°), where extinction is patchy and variable.
• To improve the accuracy of extinction corrections in stellar kinematics studies using Hipparcos and Tycho data.
• To test the predictive power of the AGG92 model and a newly constructed model against high-precision Hipparcos observations.

Proposed method

• An inverse method (Tarantola & Valette, 1982) is applied to derive extinction maps from color excesses and distances of 434 open clusters.
• The extinction model is constructed by combining the Sandage law for |b| > 10° with a new law derived from open cluster data in the galactic plane.
• Extinction predictions are compared with Hipparcos observations in the 4D space of (V, B−V, Hv, r) using χ² statistics.
• The model is validated using a well-selected sample of 3799 stars with Strömgren photometry and Hipparcos parallaxes.
• The AGG92 model is used as a reference for comparison, particularly its cell-based quadratic extinction relations.
• Statistical comparison is performed via χ² tests to evaluate model fit to observed star counts in 36 galactic longitude cells.

Experimental results

Research questions

• RQ1 Does a new extinction model based on open cluster data provide a better fit to Hipparcos observations than the AGG92 model?
• RQ2 How does the spatial distribution of interstellar extinction vary across the galactic plane at low latitudes?
• RQ3 To what extent do existing extinction models overestimate extinction in the solar neighborhood?
• RQ4 Why do both the AGG92 and new models fail to reproduce the observed star count distribution in the 50°–130° and 280°–290° longitude ranges?
• RQ5 Can an inverse method applied to open cluster data yield a more accurate extinction law for the galactic plane?

Key findings

• The new extinction model yields a χ² of 62.3 when compared to Hipparcos observations, significantly lower than the AGG92 model’s χ² of 108.7, indicating a better fit.
• The AGG92 model overestimates extinction, particularly at small distances, and predicts fewer stars than observed in several galactic longitude ranges.
• The new model shows lower extinction between galactic longitudes l = 210° to 240° and higher extinction between l = 20° to 40°.
• Both models fail to reproduce the observed star count peak at l = 280°–290° (152 stars) and the dip at l = 150°–160° (80 stars), indicating unresolved substructure.
• The results confirm that interstellar extinction in the galactic plane is highly variable and patchy, necessitating a more detailed extinction law at smaller spatial scales.
• Despite improved performance, the new model still lacks the resolution to fully capture extinction substructure, highlighting the need for a more refined model based on Hipparcos data.