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Massive Star Population in the Sextans A Dwarf Galaxy from HST UV Photometry

Roberto Flores, Elena Fantino, Giuseppina Battaglia, Antonio Aparicio

Abstract

We build a catalog of massive (M>$8~$M$_\odot$) main sequence stars in the \mbox{metal-poor} ($\sim0.1~$Z$_\odot$) dwarf irregular galaxy Sextans A. HST WFC3 UV photometry in the 275 and 336 nm wideband filters is arranged in a Color-Magnitude Diagram (CMD), and overlaid on top of stellar evolutionary tracks from the MIST library. The star properties (mass, age, etc.) are computed with a Finite Element (FE) interpolation of the stellar tracks. The FE method, originally developed for solid mechanics problems, provides a general framework for interpolating fields inside domains of complex geometry. Besides the interpolated properties, the algorithm computes their gradients with respect to the photometry. These sensitivities provide a direct an efficient estimate of the associated uncertainties. Our catalog contains 655 stars, with the most massive one estimated at $58\pm11~$M$_\odot$. A comparison with a ground-based spectroscopic census of OB stars yields only 8 matches, evidencing the minimal overlap between both datasets. The mass estimates derived from the UV CMD and the spectral classification are in good agreement for the majority of O-type stars found in both datasets. Our catalog provides an extensive list of candidates for followup spectroscopic observation, which could improve our understanding of the early evolutionary stages of massive \mbox{low-metallicity} stars.

Massive Star Population in the Sextans A Dwarf Galaxy from HST UV Photometry

Abstract

We build a catalog of massive (M>M) main sequence stars in the \mbox{metal-poor} (Z) dwarf irregular galaxy Sextans A. HST WFC3 UV photometry in the 275 and 336 nm wideband filters is arranged in a Color-Magnitude Diagram (CMD), and overlaid on top of stellar evolutionary tracks from the MIST library. The star properties (mass, age, etc.) are computed with a Finite Element (FE) interpolation of the stellar tracks. The FE method, originally developed for solid mechanics problems, provides a general framework for interpolating fields inside domains of complex geometry. Besides the interpolated properties, the algorithm computes their gradients with respect to the photometry. These sensitivities provide a direct an efficient estimate of the associated uncertainties. Our catalog contains 655 stars, with the most massive one estimated at M. A comparison with a ground-based spectroscopic census of OB stars yields only 8 matches, evidencing the minimal overlap between both datasets. The mass estimates derived from the UV CMD and the spectral classification are in good agreement for the majority of O-type stars found in both datasets. Our catalog provides an extensive list of candidates for followup spectroscopic observation, which could improve our understanding of the early evolutionary stages of massive \mbox{low-metallicity} stars.
Paper Structure (24 sections, 20 equations, 18 figures, 9 tables)

This paper contains 24 sections, 20 equations, 18 figures, 9 tables.

Table of Contents

  1. Introduction
  2. Materials and methods
  3. HST dataset
  4. Assumed properties for Sextans A
  5. Extinction
  6. Distance modulus
  7. Metallicity
  8. Extraction of stellar properties from the CMD
  9. Preparation of stellar tracks
  10. Finite element interpolation
  11. Selection of the interpolation domain
  12. Points falling outside the track set
  13. Estimation of uncertainties
  14. Limiting magnitude and photometric quality
  15. Reddening calibration
  16. ...and 9 more sections

Figures (18)

  • Figure 1: HST coverage of Sextans A (red square) and half-light circle (green circle). Background image credit: ESA/Aladin-Strasbourg Observatory.
  • Figure 2: Position of stars with valid UV photometry.
  • Figure 3: Color-age diagram highlighting the PMS (blue), MS (orange) and RGB (black) stages. Dashed lines mark the removed sections, segment between orange crosses is retained. Plot shows a track computed for 8.95 M$_\odot$, [Fe/H]=$-1$, $V/V_{crit}=0.4$.
  • Figure 4: CMD with HST data (blue markers) and MS tracks (red lines). [Fe/H]=$-1$, $V/V_{crit}=0.4$, $\mu^0=25.63$.
  • Figure 5: Triangular element in global (CMD) coordinates.
  • ...and 13 more figures