[Paper Review] A revised calibration of the Mv-W(OI 7774) relationship using Hipparcos data: Its application to Cepheids and evolved stars

A. Arellano Ferro, Sunetra Giridhar+1 more|Redalyc (Universidad Autónoma del Estado de México)|Oct 31, 2002

Stellar, planetary, and galactic studies1 references11 citations

TL;DR

This paper revises the $M_V$-$W$(OI 7774) relationship using improved Hipparcos-based distances and reddenings for 27 A-G supergiants, achieving ±0.38 mag accuracy. It introduces a phase-dependent correction for random-phase OI 7774 measurements in Cepheids, extending the calibrator sample to 58 and enabling reliable mean absolute magnitude estimates from single-epoch observations.

ABSTRACT

A new calibration of the Mv-W(OI 7774) relationship has been calculated using better estimates of reddenings and distances to a sample of 27 calibrator stars of A-G spectral types, based on accurate parallaxes and proper motions from the Hipparcos and Tycho catalogues. The present calibration predicts absolute magnitude with accuracies of $\pm$ 0.38 mag. for a sample covering a large range of Mv, from -9.5 to $+0.35$ mag. The colour term included in a previous paper has been dropped since its inclusion was not bringing any significant improvement to the calibration. The variation of the OI 7774 feature in the classical cepheid SS Sct has been studied. We calculated a phase-dependent correction to random phase OI feature strengths in Cepheids, such that it predicts mean absolute magnitudes using the above calibration. After applying such correction, we could increase the list of calibrators to 58 by adding Mv and OI triplet strength data for 31 classical Cepheids. The standard error of the calibration using the composite sample was comparable to that obtained from the primary 27 calibrators, showing that it is possible to calculate mean Cepheid luminosities from random phase observations of the OI7774 feature. We use our derived calibrations to estimate Mv for a set of evolved objects to be able to locate their positions in the H-R diagram.

Motivation & Objective

• To improve the accuracy of the $M_V$-$W$(OI 7774) relationship by using more precise distances and reddenings for calibrator stars.
• To eliminate the color term in the calibration, which was found not to significantly improve accuracy.
• To develop a phase-dependent correction for OI 7774 equivalent width in Cepheids to allow luminosity estimation from random-phase observations.
• To extend the calibrator sample by applying the corrected method to 31 classical Cepheids, increasing the total number of calibrators to 58.
• To apply the revised calibration to evolved stars, including post-AGB candidates, to estimate their absolute magnitudes and locate them in the H-R diagram.

Proposed method

• Utilized high-precision trigonometric parallaxes and proper motions from the Hipparcos and Tycho catalogues to derive improved distances and reddenings for 27 A-G supergiant calibrators.
• Measured the equivalent width $W$(OI 7774) at high resolution ($R \sim 18,000$) to avoid overestimation errors common at lower resolution.
• Re-calibrated the $M_V$-$W$(OI 7774) relationship using a linear fit to the improved data, excluding the color term after statistical testing showed no significant improvement.
• Studied the pulsational variation of the OI 7774 triplet in the classical Cepheid SS Sct to derive a phase-dependent correction factor for random-phase observations.
• Applied the phase correction to 31 classical Cepheids, combining their $M_V$ and $W$(OI 7774) data with the original 27 calibrators to form a composite sample.
• Derived two final calibrations: one for the blue component at $\lambda 7771.954$ ($W71$) and one for the blended triplet ($W74$), both validated on the composite sample.

Experimental results

Research questions

• RQ1 Can the $M_V$-$W$(OI 7774) relationship be improved using more accurate distances and reddenings from Hipparcos data?
• RQ2 Is the inclusion of a color term necessary for the $M_V$-$W$(OI 7774) calibration, or does it degrade accuracy?
• RQ3 Can the OI 7774 equivalent width measured at random phases in Cepheids be corrected to yield reliable mean absolute magnitudes?
• RQ4 Does the OI 7774 feature in Cepheids exhibit the same luminosity sensitivity as in non-variable supergiants?
• RQ5 Can the revised calibration be reliably applied to evolved stars, including post-AGB candidates, to estimate their absolute magnitudes and evolutionary status?

Key findings

• The revised $M_V$-$W$(OI 7774) calibration achieves a precision of ±0.38 mag over the absolute magnitude range from $-9.5$ to $+0.35$ mag.
• The color term previously included in earlier calibrations was found statistically insignificant and was therefore excluded, improving robustness.
• The phase-dependent correction for random-phase OI 7774 measurements in Cepheids successfully enabled the inclusion of 31 additional calibrators, expanding the total sample to 58.
• The standard error of the calibration using the composite sample (58 stars) was comparable to that of the primary 27 calibrators, confirming the method's reliability.
• The calibrations for both the blue component ($W71$) and the blended triplet ($W74$) predict $M_V$ with accuracies of ±0.42 mag and ±0.43 mag, respectively.
• The application of the calibration to evolved stars, including five post-AGB candidates, allowed their placement in the H-R diagram, though luminosity estimates for stars with non-solar oxygen abundances carry larger uncertainties.