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 θUD* = 1.55 ± 0.03 mas (RUD* = 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 (TL = 2368 ± 37 K), geometrically thin molecular shell with a diameter of θL = 1.74 ± 0.03 mas, as measured in 2017. The shell diameter increased (θL = 1.89 ± 0.03 mas), while its temperature decreased (TL = 2140 ± 42 K) in 2019. In contrast, the photospheric diameter θUD* and the extinction up to the photosphere of GCIRS 7 (AKS = 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.
Highlights
The stellar population of the central parsec of the Galaxy has been widely studied (Genzel et al 2010; Morris et al 2012, and references therein), where the presence of a disk of young stars is well recognized (Genzel et al 2000, 2003; Paumard et al 2006; Lu et al 2009; Bartko et al 2009; Yelda et al 2014)
We find GCIRS 7 to be moderately resolved with a
We report on the spectro-interferometry of GCIRS 7 in the K band using GRAVITY at ESO/VLTI
Summary
The stellar population of the central parsec of the Galaxy has been widely studied (Genzel et al 2010; Morris et al 2012, and references therein), where the presence of a disk of young stars is well recognized (Genzel et al 2000, 2003; Paumard et al 2006; Lu et al 2009; Bartko et al 2009; Yelda et al 2014). GRAVITY is developed in a collaboration by the Max Planck Institute for extraterrestrial Physics, LESIA of Observatoire de Paris/Université PSL/CNRS/Sorbonne Université/Université de Paris and IPAG of Université Grenoble Alpes/CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the CENTRA – Centro de Astrofisica e Gravitação, and the European Southern Observatory These stars are massive O-type supergiants and Wolf-Rayet stars (Martins et al 2007; Bartko et al 2010; Sanchez-Bermudez et al 2014). The works of Yusef-Zadeh & Morris (1991), and Serabyn et al (1991) reported a cometary tail whose origin comes from GCIRS 7; more recently, Tsuboi et al (2020) revealed the presence of an ionised shell in the core of the cometary tail, estimating the mass loss of GCIRS 7 with ALMA observations This stellar population is permeated by the complex interstellar medium (ISM) environment and interacts with it.
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