[Paper Review] First VLT spectra of white dwarfs in a globular cluster

S. Moehler, U. Heber+3 more|arXiv (Cornell University)|Feb 9, 2000

Stellar, planetary, and galactic studies3 citations

TL;DR

This study presents the first VLT spectra of white dwarfs in the globular cluster NGC 6397, confirming four hydrogen-rich DA white dwarfs via low signal-to-noise spectroscopy. It demonstrates that white dwarf atmospheric parameters—effective temperature and surface gravity—can be derived to estimate masses and absolute magnitudes, enabling an independent distance determination to the cluster that reveals systematic biases if white dwarf masses are assumed constant across the population.

ABSTRACT

We present the first spectra obtained with the Very Large Telescope for white dwarfs in a globular cluster. Estimates of atmospheric parameters are obtained and compared to evolutionary tracks. We discuss possible implications for the distance scale of globular clusters and white dwarf evolution and demonstrate how white dwarfs might be used to establish an independent distance scale to globular clusters.

Motivation & Objective

• To obtain the first high-quality spectroscopic data of white dwarfs in a globular cluster using the Very Large Telescope (VLT).
• To determine atmospheric parameters—effective temperature and surface gravity—for individual white dwarfs in NGC 6397.
• To test the feasibility of using white dwarfs as standard candles for independent distance determination to globular clusters.
• To investigate the implications of white dwarf masses and cooling tracks for the distance modulus and evolutionary models of globular clusters.
• To assess the consistency of white dwarf-derived distances with existing optical and Hipparcos-based distance estimates.

Proposed method

• Acquired low signal-to-noise (S/N ≈ 10) spectra of white dwarf candidates in NGC 6397 using the VLT's FORS1 multi-object spectrograph.
• Performed spectral fitting using model atmospheres to derive effective temperatures from Balmer line profiles.
• Estimated surface gravities (log g) by comparing observed spectra with theoretical models for hydrogen-rich (DA) white dwarfs.
• Interpolated derived parameters between evolutionary tracks of carbon-oxygen (C/O) and helium (He) white dwarfs to estimate masses.
• Calculated absolute magnitudes using photometric calibration from Bergeron et al. (1995a) for different log g and temperature combinations.
• Computed distance moduli for each star assuming different masses and surface gravities, comparing results to existing distance estimates.

Experimental results

Research questions

• RQ1 Can white dwarf atmospheric parameters be reliably derived from low S/N VLT spectra in a globular cluster field?
• RQ2 What is the range of white dwarf masses in NGC 6397, and how do they compare to theoretical expectations from horizontal branch morphology?
• RQ3 How do white dwarf-derived distance moduli compare to existing optical and Hipparcos-based distance estimates for NGC 6397?
• RQ4 Does assuming a constant white dwarf mass across a cluster introduce systematic biases in distance determination?
• RQ5 Are the coolest white dwarfs in the sample located near the ZZ Ceti instability strip, suggesting potential photometric variability?

Key findings

• Four white dwarf candidates in NGC 6397 were confirmed as hydrogen-rich DA white dwarfs via spectral analysis.
• The brightest star, WF4-358, has an estimated mass of (0.36 ± 0.12) M☉, consistent with a helium-core white dwarf, though a C/O white dwarf cannot be excluded due to large error bars.
• Effective temperatures derived from Balmer lines agree well with photometric estimates from (V-I)0 colors using Bergeron et al. (1995a) models.
• Distance moduli derived from the same log g values vary systematically across the sample: the brightest star (WF4-358) yields the smallest distance, the faintest (WF2-846) the largest.
• For a given log g, the mean distance modulus is 12.3 for 0.4 M☉, 12.0 for 0.5 M☉, and 11.6 for 0.6 M☉, with a r.m.s. error of 0.17 mag.
• The systematic variation in derived distance moduli suggests that assuming a constant white dwarf mass across a cluster may bias distance estimates, highlighting the need for individual mass determinations.