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

Abstract

GCIRS 7, the brightest star in the Galactic central parsec, formed $6\pm 2$ 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. 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. We find GCIRS 7 to be moderately resolved with a uniform-disk photospheric diameter of ${\theta }_{\text{UD}}^{\ast }=1.55\pm 0.03$ mas ($R_{\text{UD}}^{\ast }=1368\pm 26$ $R_{\odot }$) in the K-band continuum. The narrow-band uniform-disk diameter increases above 2.3 $\mu$m, with a clear correlation with the CO band heads in the spectrum. This correlation is aptly modeled by a hot ($T_{\text{L}}=2368\pm 37$ K), geometrically thin molecular shell with a diameter of ${\theta }_{\text{L}}=1.74\pm 0.03$ mas, as measured in 2017. The shell diameter increased (${\theta }_{\text{L}}=1.89\pm 0.03$ mas), while its temperature decreased ($T_{\text{L}}=2140\pm 42$ K) in 2019. In contrast, the photospheric diameter ${\theta }_{\text{UD}}^{\ast }$ and the extinction up to the photosphere of GCIRS 7 ($A_{{\mathrm{K}}_{\mathrm{S}}}=3.18\pm 0.16$) have the same value within uncertainties at the two epochs. 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 (${\theta }_{\ast }=1.076\pm 0.093$ 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.

12 pages, 11 figures, 3 tables. Accepted for publication in Astronomy and Astrophysics (A&A)

References in corpus (21)

• The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation
• Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole
• Stellar and wind properties of massive stars in the central parsec of the Galaxy
• A new mass-loss rate prescription for red supergiants
• What causes the large extensions of red-supergiant atmospheres? Comparisons of interferometric observations with 1-D hydrostatic, 3-D convection, and 1-D pulsating model atmospheres
• A population of dust-enshrouded objects orbiting the Galactic black hole
• The Period-Luminosity Relation of Red Supergiant Stars in the Small Magellanic Cloud
• First proper motions of thin dust filaments at the Galactic Center
• Vigorous atmospheric motion in the red supergiant star Antares
• Properties of the CO and H${}_2$O MOLsphere of the red supergiant Betelgeuse from VLTI/AMBER observations
• The molecular and dusty composition of Betelgeuse's inner circumstellar environment
• Infrared Spectra and Visibilities as Probes of the Outer Atmospheres of Red Supergiant Stars
• CHARA/MIRC observations of two M supergiants in Perseus OB1: temperature, Bayesian modeling, and compressed sensing imaging
• The inhomogeneous sub-millimeter atmosphere of Betelgeuse
• A Stellar Wind Origin for the G2 Cloud: Three-Dimensional Numerical Simulations
• Properties of bow-shock sources at the Galactic center
• Multi-epoch VLTI-PIONIER imaging of the supergiant V766 Cen: Image of the close companion in front of the primary
• VLTI/AMBER spectro-interferometry of the late-type supergiants V766 Cen (=HR 5171 A), sigma Oph, BM Sco, and HD 206859
• Single-mode interferometric field of view in partial turbulence correction. Application to the observation of the environment of Sgr A* with GRAVITY
• GCIRS 7, a pulsating M1 supergiant at the Galactic centre. Physical properties and age
• First VLTI infrared spectro-interferometry on GCIRS 7 - Characterizing the prime reference source for Galactic center observations at highest angular resolution

Cited by in corpus (4)

• The Great Dimming of Betelgeuse seen by the Himawari-8 meteorological satellite
• Mid-Infrared studies of dusty sources in the Galactic Center
• The K-band highest-resolution images of the Mira star R Car with GRAVITY-VLTI
• VLTI-GRAVITY measurements of cool evolved stars: II. Pulsation properties and mass-loss process of the Mira star R Car and the red supergiant VX Sgr