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Abstract

A precise distance measurement to the RR Lyrae variable star BN CVn is presented based on multi-band CCD observations. The study involved obtaining apparent magnitudes in the Bessel B and V, Sloan i, and Pan-STARRS z bands and correcting them for interstellar extinction. Absolute magnitudes were calculated by applying a metallicity–luminosity relation in the V filter and period–luminosity–metallicity relations in the i and z filters, using a pulsation period of 0.56501 ± 0.01319 day and adopting [Fe/H] = –2.0 ± 0.2 from recent spectroscopic studies. The resulting weighted mean distance was found to be 2105 ± 29 pc. This result shows excellent agreement (within 3.3%) with previous RR Lyrae photometric studies, while being 6.9% lower than Gaia EDR3’s parallax measurement, reflecting remaining systematic uncertainties in astrometric solutions for pulsating variables. The findings demonstrate that carefully calibrated photometric methods can provide reliable distances and support the continued importance of RR Lyrae stars for Galactic structure studies, particularly when combined with improved Gaia astrometry.

Introduction

Aim

Methodology Cont’d

Metallicity

Results Cont’d

Discussion and Conclusion

  • RR Lyrae stars are old, low-metallicity Population II pulsating variables used as standard candles.
  • Their brightness follows the Period–Luminosity–Metallicity (PLZ) relation.
  • BN CVn (RRab-type) is a bright field RR Lyrae star, not part of any globular cluster.
    • Determine a precise photometric distance to BN CVn using multi-band CCD observations.
    • Compare the results to Gaia and other measurements.
  • Adopted [Fe/H] = –2.0 ± 0.2 (Wang et al. 2019, LAMOST-II).

[M / H] = [Fe / H] + Log (0.638f + 0.362)| Log Z = [M / H] – 1.765

  • Converted to Log Z = –3.551 ± 0.20 for PLZ equations.
  • Differential aperture photometry performed using AstroImageJ (AIJ) to extract instrumental magnitudes of BN CVn and nearby comparison stars.
  • Color-term calibration coefficients derived from observations of the standard star PG1323–086, ensuring transformation to standard photometric systems across B, V, i, and z bands.
  • A custom Python program was developed to:
    • Compute magnitude-weighted means for each filter and propagate photometric uncertainties.
    • Generate light curves and phase plots for each band after applying the derived transformation coefficients.
    • Combine results into a unified dataset for period analysis.
  • Distance: 2105 ± 29 pc
  • Method validity: High consistency with literature values
  • Confirms PLZ-based distances as reliable within a few percent of Gaia.
  • Shows BN CVn is a nearby field RR Lyrae suitable for Galactic structure calibration.
  • Demonstrates the effectiveness of small telescopes for science contributions 36

BN Canum Venaticorum Distance Determination Using

Period-Luminosity-Metallicity Relation

Author: Miracle Chibuzor Marcel�Affiliation: Pan-African Citizen Science e-Laboratory (PACS e-Lab)

Email: miracle.c.marcel@gmail.com | Website: www.pacselab.space

Figure 1. Schematic of RR Lyrae location on the HR diagram (horizontal branch).

Photometry & Calibration

Figure 2. Image taken in the V band with AIJ, showing comparison stars (red) and BN CVn (green).

Figure 3. Phase-folded light curves in the B, V, i, and z bands. These phase light curves for each of the bands are similar and perfectly show the characteristics of an RRab-type star.

Figure 4. V-band Periodogram.

Source

Method

Distance (pc)

Difference

This work

PLZ photometry

2105 ± 29

—

Liu et al. (2020)

RR Lyrae PL

2037 ± 133

+3.3 %

Gaia EDR3 (2020)

Parallax

2260 ± 80

–6.9 %

Gaia DR2 (2018)

Parallax

2605 ± 178

–19.2 %

Table 2. Comparison with Previous Studies

Methodology

Target & Data

  • Selected from AAVSO Variable Star Index (VSX).
  • Observed June 8–21, 2025 using 0.4 m Las Cumbres Observatory telescopes.
  • Filters: Bessel B, V, SDSS i, Pan-STARRS z.
  • Cadence: 4.5-hour intervals over 2 weeks.

Results

 

Band

Period (days)

​

​

​

Distance (pc)

B

0.56507± 0.01878

12.704 ± 0.002

0.2011

-------

 

V

0.56495±0.01864

12.398 ± 0.003

0.1519

0.642 ± 0.065

2093 ± 63

I

0.56495±0.18424

11.934 ± 0.017

0.0732

0.527 ± 0.044

1848 ± 40

Z

0.56571±0.47190

12.810 ± 0.015

0.0530

0.545 ± 0.042

2770 ± 57

Weighted mean

0.56501 ±0.01319

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2105 ± 29

 

Abstract

Methodology Cont’d

Metallicity

Results Cont’d

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