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Unveiling the nature of HE 0107-5240

E. Caffau, M. Steffen, P. Molaro, P. Bonifacio, N. Christlieb, D. S. Aguado, J. I. González Hernández, M. R. Zapatero Osorio, L. Monaco, M. Limongi, A. Chieffi, A. Falla, L. Roberti, A. J. Gallagher, M. Spite, P. François, H. -G. Ludwig, L. Sbordone, R. Lallement, C. Allende, R. Rebolo, S. Cristiani, G. Cupani, V. D'Odorico, C. J. A. P. Martins, D. Milaković, M. T. Murphy, N. J. Nunes, N. C. Santos, T. M. Schmidt

TL;DR

This study refines the orbital and chemical properties of HE 0107-5240, an ultra iron-poor, carbon-enhanced star in a long-period binary. By combining ESPRESSO radial-velocity monitoring with a co-added UVES spectrum and Gaia-based kinematics, the authors derive a revised orbital period of about $28.9$ years and a lower-limit companion mass, while placing the system on a prograde, highly eccentric halo orbit. A comprehensive abundance analysis using 1D LTE and 3D corrections confirms a CEMP-no signature with a high $^{12}$C/$^{13}$C ratio and non-detection of Sr and Ba, arguing against AGB pollution and favoring a primordial carbon origin. The work also challenges NLTE assumptions for Fe in such extreme metal-poor stars and highlights the sensitivity of carbon isotopic ratios to 3D/NLTE effects, underscoring the need for tailored 3D modelling to fully interpret the chemical fingerprint of the star.

Abstract

The vast majority of the most iron-poor stars in the Galaxy exhibit a strong carbon enhancement, with C/H ratios only about two orders of magnitude below solar. This unusual chemical composition likely reflects the properties of the gas cloud from which these stars formed, having been enriched by one, or at most a few, supernovae. A remarkable member of this stellar class, HE 0107-5240 with [Fe/H]=-5.56, has been identified as part of a binary system. To constrain its orbital parameters, radial velocity monitoring has been carried out using the ESPRESSO spectrograph. Radial velocities were derived using cross-correlation with a template, taking advantage of the strong G-band feature. Combining all observations yielded a high signal-to-noise spectrum, which has been used to refine our understanding of the stellar chemical composition. Additionally, a co-added UVES spectrum in the blue was used to complement the wavelength coverage of ESPRESSO. Observations of HE 0107-5240 over a span of more than four years have yielded a revised orbital period of about 29 years. Updated elemental abundances have been determined for Sc, Cr, Co, and, tentatively, Al, along with a new upper limit for Be. The iron abundance has been derived from ionised Fe lines. Significant upper limits have been established for Li, Si, and Sr. The star is confirmed to be a long-period binary. Iron abundances derived from neutral and ionised lines are consistent with local thermodynamical equilibrium (LTE) assumption, casting doubt on published deviation from LTE corrections for Fe for this star. The heavy elements Sr and Ba remain undetected, confirming the classification of HE 0107-5240 as a carbon enhanced metal-poor and non enhanced in heavy elements (CEMP-no) star and supporting the absence of an n-capture element plateau at the lowest metallicities.

Unveiling the nature of HE 0107-5240

TL;DR

This study refines the orbital and chemical properties of HE 0107-5240, an ultra iron-poor, carbon-enhanced star in a long-period binary. By combining ESPRESSO radial-velocity monitoring with a co-added UVES spectrum and Gaia-based kinematics, the authors derive a revised orbital period of about years and a lower-limit companion mass, while placing the system on a prograde, highly eccentric halo orbit. A comprehensive abundance analysis using 1D LTE and 3D corrections confirms a CEMP-no signature with a high C/C ratio and non-detection of Sr and Ba, arguing against AGB pollution and favoring a primordial carbon origin. The work also challenges NLTE assumptions for Fe in such extreme metal-poor stars and highlights the sensitivity of carbon isotopic ratios to 3D/NLTE effects, underscoring the need for tailored 3D modelling to fully interpret the chemical fingerprint of the star.

Abstract

The vast majority of the most iron-poor stars in the Galaxy exhibit a strong carbon enhancement, with C/H ratios only about two orders of magnitude below solar. This unusual chemical composition likely reflects the properties of the gas cloud from which these stars formed, having been enriched by one, or at most a few, supernovae. A remarkable member of this stellar class, HE 0107-5240 with [Fe/H]=-5.56, has been identified as part of a binary system. To constrain its orbital parameters, radial velocity monitoring has been carried out using the ESPRESSO spectrograph. Radial velocities were derived using cross-correlation with a template, taking advantage of the strong G-band feature. Combining all observations yielded a high signal-to-noise spectrum, which has been used to refine our understanding of the stellar chemical composition. Additionally, a co-added UVES spectrum in the blue was used to complement the wavelength coverage of ESPRESSO. Observations of HE 0107-5240 over a span of more than four years have yielded a revised orbital period of about 29 years. Updated elemental abundances have been determined for Sc, Cr, Co, and, tentatively, Al, along with a new upper limit for Be. The iron abundance has been derived from ionised Fe lines. Significant upper limits have been established for Li, Si, and Sr. The star is confirmed to be a long-period binary. Iron abundances derived from neutral and ionised lines are consistent with local thermodynamical equilibrium (LTE) assumption, casting doubt on published deviation from LTE corrections for Fe for this star. The heavy elements Sr and Ba remain undetected, confirming the classification of HE 0107-5240 as a carbon enhanced metal-poor and non enhanced in heavy elements (CEMP-no) star and supporting the absence of an n-capture element plateau at the lowest metallicities.
Paper Structure (40 sections, 5 equations, 15 figures, 4 tables)

This paper contains 40 sections, 5 equations, 15 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Galactic Kinematics
  4. Orbital parameters
  5. Chemical Analysis
  6. Stellar parameters
  7. Spectral energy distribution
  8. Abundances
  9. Lithium.
  10. Beryllium.
  11. Carbon.
  12. Nitrogen.
  13. Oxygen.
  14. Sodium.
  15. Aluminium.
  16. ...and 25 more sections

Figures (15)

  • Figure 1: Upper panels: HE 0107$-$5240 position (black filled star) in the Toomre diagram (left) and the orbital energy versus angular momentum (right). Bottom panels: square root of the radial action versus the azimuthal action (left) and action diamond (right). Coloured points are stars from the good-parallax sample of bonifacio21. The red and green shaded areas are the regions defined by feuillet21 to select candidate GSE and Sequoia stars.
  • Figure 2: Radial velocities (RV) of HE 0107$-$5240 versus heliocentric Julian date (HJD), together with the best RV model. The inner panel within the middle panel shows the ESPRESSO RV points after removing the best model. The RMS of the residuals of all RVs are provided within the middle panel and those from each spectrograph are given in the bottom panels.
  • Figure 3: BASTI isochrones Pietrinferni_2021 in a Gaia colour-magnitude diagram. The two green points correspond to HE 0107$-$5240, assuming surface gravities of $\log g = 2.20$ (upper point) and $2.46$ (lower point), respectively.
  • Figure 4: Observed residual flux of the spectrum (solid black) in the wavelength range of the 313.1 nm Beii resonance doublet, compared to synthesis (solid red). The position of the Beii 313.1 nm line is marked with a vertical red line.
  • Figure 5: Beryllium abundance in HE 0107$-$5240 (red symbol) compared to the Galactic behaviour Data from boesgaard2011 and smiljanic2009 and the upper limits for 2MASS J18082002--5104378 and BD+44 493 spite2019.
  • ...and 10 more figures