Stellar and wind properties of massive stars in the central parsec of the Galaxy

Abstract

Context. How star formation proceeds in the Galactic Center is a debated question. Addressing this question will help us understand the origin of the cluster of massive stars near the supermassive black hole, and more generally starburst phenomena in galactic nuclei.In that context, it is crucial to know the properties of young massive stars in the central parsec of the Galaxy. Aims. The main goal of this study is to derive the stellar and wind properties of the massive stars orbiting the supermassive black hole SgrA ⋆ in two counter-rotating disks. Methods. We use non-LTE atmosphere models including winds and line-blanketing to reproduce H and K band spectra of these stars obtained with SINFONI on the ESO/VLT. Results. The GC massive stars appear to be relatively similar to other Galactic stars. The currently known population of massive stars emit a total 6.0 × 10 50 s −1 (resp. 2.3 × 10 49 s −1 ) H (resp. He i) ionising photons. This is sufficient to produce the observed nebular emission and implies that, in contrast to previous claims, no peculiar stellar evolution is required in the Galactic Center. We find that most of the Ofpe/WN9 stars are less chemically evolved than initially thought. The properties of several WN8 stars are given, as well as two WN/C stars confirmed quantitatively to be stars in transition between the WN and WC phase. We propose the sequence (Ofpe/WN9 ⇋ LBV) → WN8 → WN/C for most of the observed GC stars. Quantitative comparison with stellar evolutionary tracks including rotation favour high mass loss rates in the Wolf-Rayet phase in these models. In the OB phase, these tracks nicely reproduce the average properties of bright supergiants in the Galactic Center.

KMOS view of the Galactic centre

“…This result agrees with the cluster total mass found by Lu et al (2013) = 32 000 M , but still in agreement with Lu et al (2013). We integrated up to stars with M = 150 M , but the most massive star in the Galactic centre has only M = 80 M (Martins et al 2007). When we integrate in the interval [1 M ; 80 M ], the cluster mass is only M α = 1.7 young, M ≤ 80 M = 16 000 M , for α = 1.7, and M α = 0.45 young, M ≤ 80 M = 12 000 M for α = 0.45.…”

“…Because of their broad emission lines (FWHM ∼ 700 km s −1 ) and the resemblance of the spectra of Id 144/AF and Id 1237/IRS 7E2 with the WN8 and WN9 spectra in our data set, we classify Id 144/AF and Id 1237/IRS 7E2 as broad emission-line stars, probably WN8 or WN9. Id 1237/IRS 7E2 is also classified as a WN8 star by Martins et al (2007). Martins et al (2007) used non-LTE atmosphere models to derive the properties of Galactic centre stars.…”

Clump formation through colliding stellar winds in the Galactic Centre

“…For most of the stars, the wind properties were taken from Martins et al (2007), who fitted stellar atmosphere models to spectra obtained with SINFONI at the VLT. For the rest, the wind properties were assigned by Cuadra et al (2008) based on their similarity to other stars whose spectra were properly modelled by Martins et al (2007). The typical uncertainty in the parameters derived by Martins et al (2007) is ∼ 100 km s −1 for the velocities and ∼ 0.2 dex for the mass-loss rates, small enough not to influence our conclusions.…”

“…Table 3. Galactic Centre mass-losing star sample taken from Martins et al (2007) and Cuadra et al (2008). Column 1: star ID.…”

“…As there is no agreement on the metallicity measurements for the massive stars in the Galactic Centre (see the review by Genzel et al 2010) we have set Z = 3Z⊙, following Cuadra et al (2005) and close to the values Martins et al (2007) studied. However, in order to quantify the sensitivity to our choice, we also produced models with metallicities of Z⊙ and 5Z⊙ in the analytical cooling function we use (see Equation 10).…”