MOLsphere and pulsations of the Galactic Center’s red supergiant GCIRS 7 from VLTI/GRAVITY^★

¹ LESIA, Observatoire de Paris, Universite PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
e-mail: gustavo.coira@obspm.fr
² Max Planck Institut fur Exterterrestrische Physik (MPE), Giessenbachstr.1, 85748 Garching, Germany
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ 1. Physikalisches Institut, Universitat zu Köln, Zülpicher Straße 77, 50937 Cologne, Germany
⁵ CNRS, IPAG, Univ.Grenoble Alpes, 38000 Grenoble, France
⁶ Universidade de Lisboa, Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
⁷ Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
⁸ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁹ European Southern Observatory, Santiago 19, Casilla 19001, Chile
¹⁰ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany,
¹¹ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹² Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
¹³ CENTRA, Centro de Astrofísica e Gravitação, Instituto Superior Técnico, Avenida Rovisco Pais 1, 1049 Lisboa, Portugal
¹⁴ Department of Astrophisical & Planetary Sciences, JILA, Duane Physics Bldg., 2000 Colorado Ave, University of Colorado, Boulder, CO 80309, USA
¹⁵ Department of Particle Physics & Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
¹⁶ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
¹⁷ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, Ciudad de México 04510, Mexico

Received: 23 September 2020
Accepted: 21 April 2021

Abstract

Context. GCIRS 7, the brightest star in the Galactic central parsec, formed 6 ± 2 Myr ago together with dozens of massive stars in a disk orbiting the central black-hole. It has been argued that GCIRS 7 is a pulsating body, on the basis of photometric variability.

Aims. Our goal is to confirm photospheric pulsations based on interferometric size measurements to better understand how the mass loss from these massive stars enriches the local interstellar medium.

Methods. We present the first medium-resolution (R = 500), K-band spectro-interferometric observations of GCIRS 7, using the GRAVITY instrument with the four auxiliary telescopes of the ESO VLTI. We looked for variations using two epochs, namely 2017 and 2019.

Results. We find GCIRS 7 to be moderately resolved with a uniform-disk photospheric diameter of θ_UD^* = 1.55 ± 0.03 mas (R_UD^* = 1368 ± 26 R_⊙) in the K-band continuum. The narrow-band uniform-disk diameter increases above 2.3 μm, with a clear correlation with the CO band heads in the spectrum. This correlation is aptly modeled by a hot (T_L = 2368 ± 37 K), geometrically thin molecular shell with a diameter of θ_L = 1.74 ± 0.03 mas, as measured in 2017. The shell diameter increased (θ_L = 1.89 ± 0.03 mas), while its temperature decreased (T_L = 2140 ± 42 K) in 2019. In contrast, the photospheric diameter θ_UD^* and the extinction up to the photosphere of GCIRS 7 (A_KS = 3.18 ± 0.16) have the same value within uncertainties at the two epochs.

Conclusions. In the context of previous interferometric and photo-spectrometric measurements, the GRAVITY data allow for an interpretation in terms of photospheric pulsations. The photospheric diameter measured in 2017 and 2019 is significantly larger than previously reported using the PIONIER instrument (θ_* = 1.076 ± 0.093 mas in 2013 in the H band). The parameters of the photosphere and molecular shell of GCIRS 7 are comparable to those of other red supergiants that have previously been studied using interferometry. The extinction we measured here is lower than previous estimates in the direction of GCIRS 7 but typical for the central parsec region.